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Academic year 2011-‐2012
Erasmus Mundus Master in Membrane Engineering Semester S1
Report on the six-‐month project
A review of Alumina: Most abundant and productive material of the
mother nature.
KAYAALP Umay PAPOUTSOGLOU Dimitra
January 2012
Supervisor:
AYRAL Andre; [email protected]‐montp2.fr
BACCHIN Patrice ; [email protected]‐tlse.fr
2
TABLE OF CONTENT
1. INTRODUCTION ........................................................................................................................... 3 2. ALUMINA AS A MATERIAL ....................................................................................................... 4 2.1. NOMENCLATURE ..................................................................................................................... 4 2.2. STRUCTURE AND MINERALOGICAL PROPERTIES ........................................................ 8 2.2.1. STRUCTURE OF ALUMINA PHASES ..................................................................................................... 8 2.2.2. PSEUDOMORPHOSIS .............................................................................................................................. 9 2.2.3. SURFACE AREA OF ALUMINA ............................................................................................................. 12 2.2.4. POROSITY .............................................................................................................................................. 13 2.2.5. SORPTIVE CAPACITY ........................................................................................................................... 15
2.3. MECHANICAL PROPERTIES OF ALUMINA ..................................................................... 16 2.4. THERMAL PROPERTIES OF ALUMINA ............................................................................ 20 2.5. CHEMICAL PROPERTIES OF ALUMINA ........................................................................... 22 2.5.1. WET CHEMICAL REACTIONS OF SINTERED ALUMINA .................................................................. 22 2.5.2 REACTION OF CHEMICAL ELEMENTS WITH ALUMINA ................................................................... 23
1.8. COLLOIDAL PROPERTIES OF ALUMINA ......................................................................... 24 3. ALUMINA MEMBRANES .......................................................................................................... 26 3.1. INTRODUCTION ..................................................................................................................... 26 3.1 PREPERATION OF ALUMINA MEMBRANES .................................................................... 29 3.2.1. MACROPOROUS ALUMINA MEMBRANE PREPARATION ................................................................ 29 3.2.2.MESOPOROUS ALUMINA MEMBRANES ............................................................................................. 31 3.2.3 MICROPOROUS ALUMINA MEMBRANES ........................................................................................... 35
4.DESIGN OF THE MEMBRANE MODULES ............................................................................. 38 4.1 DIFFERENT TYPES OF MODULES ...................................................................................... 38 4.1.1 ALUMINA MEMBRANE MODULES ....................................................................................................... 39 4.1.2. COMMERCIALIZED MODULES OF MEMBRANE ALUMINA ............................................................... 42
4.2. SEPARATION CHARACTERISTICS FOR ALUMINA MEMBRANES ............................. 45 5. APPLICATIONS ........................................................................................................................... 49 5.1. CERAMIC MEMBRANES ........................................................................................................ 49 5.2. ALUMINA MEMBRANES APPLICATIONS ........................................................................ 52 5.2.1. LIQUID PHASE SEPARATION APPLICATIONS ............................................................................... 52 5.2.2. GAS PHASE SEPARATION ................................................................................................................. 63
6.SUMMARY AND CONCLUSIONS .............................................................................................. 74 APPENDIX A : MEMBRANE MATERIAL SHEET ..................................................................... 75 APPENDIX B: CHEMICAL INTEREST OF ALUMINA .............................................................. 77 REFERANCES ................................................................................................................................... 85
3
1. INTRODUCTION
Ceramists are not close agreement as to the substances included in the term of
‘’ceramics’’, nor do they seem to have devised as simple, consistent definition of
the term that is entirely satisfactory. Kingerly defined it as ‘’the art and science of
making and using solid articles which have as their essential component, and are
composed in large of inorganic nonmetallic materials.’’ L. Mitchell defined
ceramics as ‘’all high-‐temperature chemistry and physics of nonmetallic materials,
and the techniques of forming products at high temperatures.’’ The first definition
allows inclusion of materials having melting points below room temperature, as,
for example ice; while the second does not exclude certain organic substances
that may been produced at high temperatures, such as carbon disulfide. Although
materials of all kinds, including organic substances, are involved in the
preparation of ceramics, it is believed that these definitions are too broad to
cover the ceramic applications of alumina.
The investigation of alumina as a ceramic material was undertaken to provide
information under the following specifications: a review dealing with alumina
both from a theoretical and a practical point of view, and including information
on the nomenclature, properties of alumina, alumina as a membrane material
and finally industrial alumina membrane applications.
The following information has been gathered:
• occurrence in nature
• crystal or mineralogical characteristics
• mechanical, thermal, chemical and colloidal properties.
• alumina membranes fabrication, modules and industrial applications.
A general review tried to be gathered to understand deeply about alumina
material and also alumina membranes.
4
2. ALUMINA AS A MATERIAL
2.1. NOMENCLATURE
De Morveau suggested the word ‘alumine’ in 1786 as the proper name for the
basic earth of alum. This was Anglicized to ‘Alumina’ in England while Germany
‘tornerde’ is still used, meaning clay earth. The term ‘alumina’ is presently used
rather indefinitely in ceramic literature to denote;
1. aluminous material of all types taken collectively
2. the anhydrous and hydrous aluminum oxides taken indiscriminately
3. the calcined or substantially water free alimunium oxides without
distinguishing the phases present
4. corundum or alpha alumina, specifically. It is often used interchangeably
with the molecular formula Al2O3.
The true meaning is sometimes hard to determine from the context.
More than 25 alumina solid phases is defined in recent years. But it is doubtful if
all of them really exist or not. These phases includes, amorphous hydrous and
anhydrous oxides, crystalline hydroxides and oxides, and aluminas containing
small amounts of oxides of alkalies or alkaline earths, designated as beta
aluminas.
The phases found in the nature, and few of artificial types, have common or
mineralogical names. Most of them also are defined by greek letter formulas.
Corundum, emery, sapphire, and ruby are more or less pure forms found in the
nature and known for antiquity as abrasives and gem stones. All consist of the
the phase designated alpha alumina (! – Al2O3). Figure 1 shows the dehydration
sequence of alumina hydrates in air.
Diaspore
Another native mineral, described by Haüy in 1801, was named ‘diaspore’ by
him, from the Greek for ‘scatter’ because it flew apart upon heating.
5
Figure 1: Dehydration Sequence Of Alumina Hydrates In Air
Gibbsite
Vaquelin in 1802 gave its formula as Al2O3 ∙ 3H2O. Dewey named a well-‐
crystallized mineral ‘gibbsite’ for G. Gibbs an American mineralogist. It
corresponded with the formula Al(OH)3 or Al2O3 ∙ 3H2O. It is the principal phase
of the ‘trihydrate’ bauxites.
Bauxites
Berthier examined a mineral from Les Baux in southern France, containing about
%52 Al2O3 and 20% bound water, from which it was supposed that the mineral
was Al2O3 ∙ 2H2O. The mineral was named ‘bauxite’ by St. Clair Deville.
Bohemite
After X-‐ Ray diffraction became generally used for analysis of chemical
components a new pattern of bauxite is discovered. This bauxite has %15 bound
6
water Al2O3 ∙ H2O. This component named ‘bohemite’ for both natural and
artificial products. These phases are the izomers of diaspore.
In USA, the word ‘bauxite’ has come to mean any highly aluminous ore composed
mainly of one or more of phases, gibbsite, boehimite and diaspore. In reality no
phase corresponding to Al2O3 ∙ 2H2O has been found.
In 1925, Haber devised a system of nomenclarature for trivalent alumina phases
the known. The ‘alpha’ series included diaspore and corundum (alpha alumina);
the ‘gamma’ series included hydrargillite (gibbsite), bauxite (bohemite), and
gamma alumina. The classification was obviously based on the end product of
calcination, but this is somewhat arbitrary because gamma alumina also
transforms to alpha alumina.
Bayerite
A new phase is identified by Böhm in aluminum hydroxide precipitates that had
aged moist for several months at room temperature. The water content was
about that of gibbsite, but the X-‐Ray pattern was different, indicating an isomer
of gibbsite. This phase called ‘Bayerite’ by Fricke in 1928 on the erroneous
supposition that it was normal product of the Bayern process. L. Milligan had
already shown in 1922 that the Bayer product is predominantly gibbsite.
Bayerite was claimed to have been found in nature by Gedeon, and more recently
(1963) by Gross and Heller.
The failure of Haber classification system to distinguish between bayerite and
gibbsite prompted the devising of the Alcoa system of nomenclature (Frary). In
this system, the choice of Greek letters was initially based on the relative
abundance of phase in nature. Gibbsite was called alpha alumina trihydrate;
bohemite, alpha alumina monohydrate; bayerite beta alumina trihydrate; and
diaspore, beta alumina monohydrate. Gamma alumina and beta alumina had the
same significance as in the Haber system.
7
Table 1: Nomenclature of Crystalline Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 5)
Mineralogical Name
Phase of Form Name
Symposium (1) Alcoa (2) Haber (3) British (4) French (5) Other
Hydroxides
Gibbsite (6) Hydrargillite Al(OH)3
! − !"!!! ∙3!!!
! − !"(!")!
! − !"!!! ∙3!!!
Bayerite (7) Al(OH)3 ! − !"!!! ∙3!!!
!− !"(!")!
! − !"!!! ∙3!!!
Nordstrandite (1) Randomite (8) Bayerite II
(8)
Al(OH)3
Bauxite (9) Al(OH)2
Bohemite (10) AlOOH ! − !"!!! ∙!!!
! − !"(!")!
! − !"!!! ∙ !!!
Diaspore AlOOH ! − !"!!! ∙!!!
! − !"(!")!
Tohdite (11) AlOOH 5 !"!!! ∙ !!! ! − !"!!! (12)
Aluminas Rho
Chi Chi Chi + Gamma Chi + Gamma
Eta Eta Gamma Eta Gamma Gamma Gamma Delta Gamma Xi1, Xi2 (13)
Kappa Kappa Ioata (17) Kappa + Theta Kappa +
Delta
Corundum, Sapphire Alpha Alpha Alpha Alpha Alpha
AlO Al2O M2O ∙ 11 Al2O3 (14) M2O ∙ 6 Al2O3 (15) MO ∙ 6 Al2O3
Zeta Alumina Li2O ∙ 5 Al2O3 (1) Ginsberg, Huttig, Strunk-‐Lichtenberg (2) Edwards, Frary, Stumpf, et al. (3) Haber, Weiser, and Milligan (4) Rooksby, Day, and Hill (5) Thibon, Tertian, and Papee (6) Dewey (7)Fricke (8)Teter, Gring, and Keith (9)Böhm
(10)de Lapparent (11)Yamaguchi (12)Steinheil (13)Cowley (14)Rankin and Merven (15)Scholder (16)Barlett (17)P.A. Foster
8
Standardization of the nomenclature for aluminas is very desirable particularly
to avoid the confusion in the hydrous phases. Gingsberg reported the
conclusions of a symposium held in 1957, in which an attempt was made to
devise a universal standard nomenclature. Some features of the proposed system
are improvements, for example the substitution of ‘hydroxide’ instead of
‘hydrate’, namely, aluminum trihydroxide for alumina trihydrate; aluminum
oxide hydroxide for alumina trihydrate; aluminum oxide hydroxide for alumina
monohydrate. Also, it was agree to use the Alcoa nomenclature for the transition
aluminas, but to designate some of them as ‘forms’ rather than ‘phases’, to imply
the present uncertainty about them. The confusion in naming the hydroxide
phases has not been resolved, however.
2.2. STRUCTURE AND MINERALOGICAL PROPERTIES
The remarkable range of properties of the hydrous and non hydrous crystalline
properties of alumina has been interesting for researchers, and its structure has
been induced much scientific curiosity.
Examples of these structural peculiarities are the factors determining the
phenomena of transition phases, and the exceptional strength and hardness of
corundum. Besides the ideal crystal structures, which are a rarity in actual
ceramic systems, defect crystal structure and microstructure are significant.
Gross structure beyond the crystal lattice beyond the crystal lattice is also of
ceramic interest.
2.2.1. Structure of Alumina Phases
Crystal structure is the main factor controls the properties of aluminas. In
general, the phases of most significance in alumina are those produced by
pseudomorphic dehydration.
The crystal structures of alumina phases are shown in Table 2. Mineralogical
properties of the various phases are shown in Table 3.
9
2.2.2. Pseudomorphosis Achenbach (1931), Damerell (1932), and Teritan (1950) have shown that the
dehydration of gibbsite crystals is pseudomorphic, that is, external shape of the
crystals is retained and there is an orientation relationship of the crystal axes of
the new phases to those of the original. The crystals lose transparency and
smoothness upon heating, and fine-‐grained fibers develop parallel to the
hexagonal surface. Void space resulting from the loss of water from the gibbsite
and increasing density of the transition phases is distributed in microporosity of
very high surface area in the porous skeleton (Weitbrecht and Fricke).
Pseudomorphosis is of considerable importance because of its effect on surface
area of the intermediate phase structures, and on crystal size and size
distribution of the fully calcined aluminas for ceramic forming processes.
With the basis of alumina structure, all the transition aluminas have oxygen ions
in approximately cubic close packing. The differences in their patterns represent
changes in intensities of reflections resulting from differences in distribution of
the aluminum ions. The initial cationic disorder of the low-‐temperature phases
depends upon the source of the alumina. The transitions become more ordered
with increasing heat treatment.
10
Table 2: Crystal Structure of the Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 30)
Phase Formula Crystal System Space
Group
Mole-‐cules
Unit Cell Parameters Angle Ref. Angstroms
a b c Hydrated Aluminas Gibbsite ! − !"!!! ∙ 3!!! Monoclinic !!!ℎ 4 8.641 5.07 9.72 1
Bayerite ! − !"!!! ∙3!!! Monoclinic !!!ℎ
2 4.716 8.679 5.06 3 Nordstrandite !"!!! ∙ 3!!! Monoclinic 8 8.63 5.01 19.12 4 Bohemite ! − !"!!! ∙ !!! Orthorhombic !!!"ℎ 2 2.868 12.227 3.7 5,12 Diaspore ! − !"!!! ∙ !!! Orthorhombic !!!"ℎ 2 4.396 9.426 2.844 5 Transition Aluminas
Chi Cubic 10 7.95 9 Eta Cubic (Spinel) !!! 10 7.9 9,8 Gamma Tetragonal 7.95 7.95 7.79 6,7 Delta Tetragonal 32 7.967 7.967 23.47 10,11 Iota Orthorhombic 4 7.73 7.78 2.92 9 Theta Monoclinic !!!ℎ 4 5.63 2.95 11.86 13 Kappa Orthorhombic 32 8.49 12.73 13.96 9 Corundum ! − !"!!! Rhombohedral !!!! 2 4.758 12.991 2 Al2O Cubic 4.98 20 !"# ∙ !"!!! Cubic (Spinel) !!! 7.915 22 Beta Aluminas (21) !"!! ∙ 11!"!!! Hexagonal !!!ℎ 1 5.58 22.45 14 !!! ∙ 11!"!!! Hexagonal !!!ℎ 1 5.58 22.67 14 !"# ∙ 11!"!!! Hexagonal !!!ℎ 1 5.56 22.55 16 !"# ∙ 6!"!!! Hexagonal !!!ℎ 2 5.54 21.83 15 !"# ∙ 6!"!!! Hexagonal !!!ℎ 2 5.56 21.95 15 !"# ∙ 6!"# Hexagonal !!!ℎ 2 5.58 22.67 17 Zeta Alumina !"!! ∙ 11!"!!! Cubic !!! 2 7.9 18,19
(1) Megav (2) Swanson, Coo Isaacs, & Evans (3) Unmack (4) Lippens (5) Swanson & Fuyat (6) Saalfeld (7) Brindey & Nakahira (8) Verwey
(9) Stumpf (10) Teritan & Papee (11) Rooymans (12) Reichertz & Yost (13) Kohn (14) Beevers & Brouhult (15) Laderqvist (16) Bragg (17) Adelsköld
(18) Kordes (19) Braun (20) Hoch & Johnson (21) Scholder & Mansmann (22) Filonenko, Larov, Andreeva & Prevszer
11
Table 3 : Mineralogical Properties of the Alumina (WALTER , Alumina as a Ceramic Material, 1970, page 31)
Phase Index of Refraction nd Cleav-‐
erge Mohs Hard
Micro Hard
kg/mm2
Dens. Measured
g/ml Ref.
!
Hydrated Aluminas Gibbsite 1.568 1.568 1.587 (100) 2.5 -‐3.5 2.42 1,2 Bayerite 1.583* 2.53 3,2 Bohemite 1.649 1.659 1.665 (010) 3.5 -‐ 4 3.01 4,5,14 Diaspore 1.702 1.722 1.75 (010) 6.5 -‐ 7 3.44 1,14
* Transition Aluminas
Chi 3.0 19 Eta 1.59 -‐1.65 2.5-‐3.6 6
Gamma 3.2 20 Delta 3.2
Iota 1.604 3.71 7 Theta 1.66-‐1.67 3.56 6 Kappa 1.67-‐1.69 3.3 6
Corundum Al2O 1.7604 1.7686
none 9.0 2150 3.96-‐3.98 21,15
AlO Al2O3
1.77 -‐ 1.80
2070 3.84 13
Beta Aluminas
Sodium Beta 1.635 -‐1.650 1.676
8,10,18
Potassium Beta
1.642 1.675
17,18
1.640 1.668 Magnesium
Beta 1.629 1.665-‐1.680
16,8 Calcium Beta 1.752 1.759
1.754 1.763
3.731 11,12 Barium Beta 1.694 1.702
3.69 10
Lithium Zeta
1.735
3.61 9 *Average Estimate (1) Dana (2) Roth (3) Montoro (4) Ervin (5) Bonshtedt Kupletskkaya (6) Thibon (7) Foster, P. A.
(8) Rankin & Mervin (9) Kordes (10) Toropov (11) Filonenko (12) Wisnyi (13) Filonenko, Larov, Andreeva & Pevzner (14) Fricke & Severin
(15) Coble (16) Bragg, Gottfried, West (17) Kato & Yamauchi (18) Beevers & Brohult (19) Stumpf (20) Gingsberg (21) Biltz & Lemke
! !
! !
!
!!!
!
!
12
2.2.3. Surface Area of Alumina
Alumina is widely used as a catalyst or catalyst support in many heterogeneous
catalytic processes owing to its high surface area, superior chemical activity and
low cost. In order to prepare the thermal-‐stable alumina with high surface area
and large pore volume, two ways have been adopted. One is that some additives
including silica, phosphoric oxide, barium oxide, cerium oxide and lanthanum
oxide have been added to alumina. But the presence of these additives will
modify the original properties. The other way is using some new methods and
techniques, such as sol-‐gel method and supercritical drying techniques.
Table 4 lists the specific surface areas, pore volumes, and pore diameters
measured for samples of Al2O3. The results show that the alumina samples
prepared using β-‐cyclodextrin template had the higher surface areas (124-‐484
m2/g), larger pore volumes (0.7-‐1.27 mL/g) and more thermal stability than
samples prepared without using β-‐cyclodextrin. The sample A-‐773 exhibits the
highest SBET (484 m2/g) among the alumina samples calcined at different
temperatures. When the temperature exceeds 773 OK, the SBET decreases rapidly,
but the pore volume changes a little. After calcination at 1273 OK, the A-‐1273
maintains surface area of 124.2 m2/g and pore volume of 0.70 mL/g. However,
B-‐773 and B-‐1273 have the surface areas of 348 and 98.3 m2/g, respectively.
The pore volume of B-‐1273 is only 0.54 mL/g.
Table 4: Specific surface areas, pore volumes, and pore diameters measured for samples of Al2O3
Sample SBET (m2/g) Pore Volume (mL/g) Pore Diameter (A)
A-‐773 484.34 0.98 60.85 A-‐923 285.27 1.27 24.41 A-‐1073 220.86 0.87 20.49 A-‐1273 124.22 0.7 72.23 B-‐773 348.01 1.08 24.54 B-‐923 228.28 1.14 28.15 B-‐1073 201.2 0.74 24.67 B-‐1273 98.3 0.54 72.16
Harris and Sing study on gels formed by hydrolysis of aluminum isopropoxide.
1100 m2/g of surface area is detected. Storage of these unstable gels in the
13
presence of water vapor caused loos in surface area to about 500 m2/g. The
adsorption isotherms of nitrogen, determined at -‐196 OC on the outgassed
products were of the reversible S-‐type, characteristic of physical adsorption on
nonporous solids. Gels that have been dehydrated at room temperature
approximately to the formula Al2 . 3H2O showed no X-‐ray evidence of crystalline
structure.
2.2.4. Porosity Porosity, and special case, permeability significantly affect the properties of
alumina ceramics, and in a wide range of magnitude. Porosity is generated in
sintered alumina structures for various reasons, some of which are to improve
permeability to gases and liquids for porous diaphragms and diffuser plates, to
increase the thermal insulation of refractories, and to improve the fuel
combustion in radiant heaters. Volatile or combustible burn-‐outs (sawdust,
naphthalene) have been used to generate pores. Gas generators include:
hydrogen peroxide and aluminum powder with acids or alkalies.
Calcining mixtures of ground and unground Bayer alumina at high temperatures
can develop gross porosity beyond 50% by volume. Uniformly distributed
porosity is attained by ‘bisque’ firing fine-‐ground alumina in the undersintered
range 1000 to 1400 OC.
Barrett, Joyner and Hallenda applied adsorption -‐ desorption on sintered
alumina to have further information about pore shape and pore distribution. In
literature five general types of hysteresis loops are defined, from which fifteen
capillary shapes could be deduced. The adsorption isotherms of the activated
forms of alumina fit the three main types, A, B and E, all of which have steep
desorption curves. Type A has a steep sorption branch, type B a gradual sorption
branch, with a broad hysteresis range, and type E a gradual sorption branch with
a narrow hysteresis range. The pore shapes are mainly open and closed tubular
capillaries, ink bottle shapes, and slit shapes. In figure 2 the pore size
distribution curves were derived from the N2 physisorption isotherms according
to the B-‐J-‐H method, Figure 2 a and b show, respectively, the pore size
14
distributions for the commercial Al2O3; and the ceramic foams obtained from the
same aluminas.
Figure 2: a) N2 physisorption isotherm of commercial -‐alumina; (b) Pore size distribution of commercial -‐alumina; (c) N2 physisorption isotherm of commercial -‐alumina ceramic foam; (d) Pore size distribution of commercial ?-‐alumina ceramic foam.
Hayes, Budworth, and Roberts; investigated the permeability of sintered
aluminum tubes (total porosity 4 to 9 %, purity 99.3 to 99.8% Al2O3). These
tubes at first were impermeable to oxygen, nitrogen and argon at temperatures
below 1500 OC. At 1500 to 1750 OC, the specimens showed appreciable
permeation to oxygen, presumably by a surface diffusion process. The diffusion
coefficient was about 100 !"2/sec. very slight or no permeation was found for
nitrogen, and none for argon. After continued exposure for 100 hours at 1700 OC,
permeation by normal channel-‐flow developed suddenly and swamped the
earlier phenomena. The permeation of nitrogen through hot-‐pressed sintered
alumina (4 to 14% total porosity) was predominantly by Knudsen flow.
15
2.2.5. Sorptive Capacity
The strong desiccating action of activated alumina has been known at least since
1879. The properties that make the activated aluminas particularly suitable for
desiccant use are: the ability to develop high surface area during formation of
dehydration; a high degree of chemical inertness; resistance to softening,
swelling, and disintegration when immersed in water or other liquids; high
resistance to shock and abrasion; and the ability to return to the original highly
adsorptive from by a suitable thermal regenerative treatment.
Alumina is also use for special type of adsorption called chromatography, in
which the identification and separation of adsorbed ions are usually based on a
visual, spatial order of adsorption. Typical properties of desiccant, commercial
alumina samples are shown in Table 5.
A partial list of gasses and liquids that can be dried by activated alumina (Alcoa
brochure, June 1, 1967) includes the following.
Gasses
Acetylene, air, ammonia, argon, carbon dioxide, chlorine, cracked gas, ethane,
ethylene, freon, furnace gas, helium, hydrogen, hydrogen chloride, hydrogen
sulfide, methane, natural gas, nitrogen, oxygen, propane, propylene, and sulfur
dioxide.
Liquids
Benzene, butadiene, butane, butene, butyl acetate, carbon tetrachloride, chloro
benzene, cyclohexane, ethyl acetate, freon, gasoline, heptane, n hexane, jet fuel,
kerosene, lubricating oils, naphtha, nitrobenzene, pentane, pipe-‐line products,
propane, propylene, styrene, toluene, transformer oils, vegetable oils and xylene.
16
Table 5: Typical Properties of Desiccant, Chromatographic And Catalytic Aluminas (WALTER , Alumina as a Ceramic Material, 1970, page 39)
F-‐1 H-‐151 F-‐20 T-‐71 F-‐110 F-‐7 Typical properties Al2O3 % 92.00 90.00 92.00 99.5 + 92-‐94 84.00 Na2O % 0.90 1.60 0.90 0.01 0.08 0.90 Fe2O3 % 0.08 0.13 0.08 0.06 0.03 0.08 SiO2 % 0.09 2.20 0.09 0.04 0.01 0.09 Loss on ignition (1100 OC) % 6.50 6.00 6.20 0.00 6.0-‐8.0 12.10 SO3 % 0.09 CaO % 0.06 Nickel Formate %
2.50
Form Granular Ball Granular Granular Ball Granular Surface area m2/g 210.00 390.00 210.00 0.50 180-‐280 Bulk density, loose kg /m3 832.9620 816.9435 929.0730 1217.4060 800.9250 832.9620 Bulk density, packed, kg/m3 881.0175 848.9805 1089.2580 1361.5725 881.0175 881.0175 Spesific Gravity 3.30 3.1-‐3.3 3.30 Static sorption at 60% RH 14-‐16 22 -‐ 25 Crushing Strenght 55.00 75.00 Pore volume ml/gm 0.15-‐0.20 0.38 pH 9.00 Sieve Analysis On 80 mesh % 2 max Through 270 mesh % 5 max
2.3. MECHANICAL PROPERTIES OF ALUMINA Alumina has remarkable mechanical properties in comparison with conventional
porcelains and other single oxide ceramics. None of the likely refractory single
oxide contenders approaches pure sintered alumina in bending and tensile
strength, and is exceed only by stabilized ZrO2 in compressive strength. Many if
the advantageous strength characteristics are retained to lesser extent by the
high and low alumina porcelains.
The interest in mechanical properties stems from several applications such as
possible substitution of alumina ceramics for refractory metal parts in air-‐bone
equipment, or fabrication forms in which high mechanical strength, membrane,
hardness or thermal shock resistance is important.
17
The mechanical tests of particular significance include: flexural, compressive,
tensile torsional, and impact strengths; modulus of elasticity and rigidity;
Poisson’s ratio and bulk modulus; fatigue, creep, internal friction, thermal shock
resistance, and flaw detection; and hardness.
Structural applications of aluminum oxide in the high temperature field require
knowledge of the effect of temperature on mechanical properties. Data on
mechanical properties of alumina is collected in Table 6 and 7. The data in the
table include information taken from ceramic literature as well as average values
for commercial production, taken from the standards of alumina ceramic
manufacturers association and the literature of several studies.
Table 6 is belonging to typical properties and specifications of commercial
grades of 3 kinds of alumina. Hydrated aluminas, prepared in a modern Bayer
plant, is indicated the specimen of C. And specimen A belongs to calcined
aluminas, which specimen T indicates tabular alumina.
Data on the mechanical properties of alumina is collected in Table 7. The data in
the table include information taken from the ceramic literature, as well as
average values for commercial production, taken from the standarts of the
Alumina Ceramic Manufacturers Association.
Table 6: Typical Properties and Spesifications of Hydrated Aluminas-‐Series C-‐30 (WALTER , Alumina as a Ceramic Material, 1970, page 21)
Typical Properties C A T Al2O3 % 65 98.9 99.5+
SiO2 % 0.01 0.02 0.06
Fe2O3 % 0.003 0.03 0.06
Na2O % 0.16 0.45 0.20
Moisture (110 OC) % 0.04 1.0 Specific Gravity 2.42 3.6-‐3.8 3.65 -‐3.8 Sieve Analysis on 100 mesh % 0-‐1 4-‐15 on 200 mesh % 5-‐10 50-‐75 on 325 mesh % 30-‐55 88-‐98
through 270 mesh % 45-‐70 2-‐12
18
Table 7: Mechanical Properties of Alpha Alumina Oxide (WALTER , Alumina as a Ceramic Material, 1970, page 45)
Bending Strength (Modulus of Rupture) Temp OC MPa
(1)
Sapphire Flame-‐fused,
oriented 0O between optic axis and bar axis a
25 680 600 180
1000 300
Sapphire Flame-‐fused, oriented 45 O between optic axis and
bar axis a ,b
25 480 600 313
1000 567
Ruby Flame-‐fused, oriented 45 O
between optic axis and bar axis a ,b
25 333 600 220
1000 567 Polycrystalline Alumina
S25OC=142500 e-‐11.83PG-‐0.60+3.33P
(18) S1200OC=73000e
-‐11.33PG-‐0.60+3.33P Polycrystalline Alumina (99.9% Al2O3 98% theoretical density, hot pressed) (2)
Crystal Size (microns) 1 – 2 10 – 15 40 – 50 Temperature OC
25 OC 447 320 233 400 OC 347 247 227 1000 OC 327 247 207 1350 OC 247 107 93
Commercial Grades of Polycrystalline Alumina Nominal % Al2O3 %99.9 (5) %99 (3) %94 (3) %85 (3)
25 OC 413 347 307 307 980 OC
153 113 80
Compressive Strength (MPa) Sapphire Polycrystalline Temperature OC 100 % Al2O3 (7) 99% Al2O3 (3) 94 % Al2O3 (3) 85 % Al2O3 (3)
25 OC 295 3733 2000 2000 1600 25 OC 3300 2840
400 OC
1420 800 OC
1233
1000 OC
853 1200 OC
473
1400 OC
237 1600 OC
47
Hardness on the Mohs Scale 9
19
Table 7 continued
Tensile Strength MPa
Temperature OC
Single Crystal Filaments Polycrystalline Orientation 45 O to optic axis
(9) Uncoated (10) Coated (10) (e) 94 % Al2O3 (3) 85 % Al2O3 (3)
30 OC 473 467 1400 251 173 117 300 OC 350 243 800 OC 350 227 1050 OC 225 1100 OC 587 209 1200 OC 123 63 57 1400 OC 28
Modulus of Elasticity (E) X 108 MPa E (polycrystalline)=59.49 X 106 e -‐3.95P, where P = fractional pore volume (8) Single Crystal (12) Polycrystalline
Temperature OC Sapphire Ruby
(0.75%Cr2O3)
(11) 94 % Al2O3 (3) 85 % Al2O3 (3)
25 OC 35.07 36.07 39.53 26.80 21.27 500 OC 32.07 33.00 38.18
1000 OC 29.00 30.07 36.59 1200 OC 27.93 29.00 35.77 Elastic constants Elastic Compliances
( X108 MPa) ( X108 MPa) (13)
48.03 2.35 48.16 2.17 14.27 6.94 15.82 -‐0.72 10.72 -‐0.36 -‐2.27 0.49
Modulus of Ridity(G) X 108 MPa Polycrsytallined Alumina
Hot-‐Pressed Cold-‐Pressed 94 % Al2O3 (3) 85 % Al2O3 (3)
Temperature OC Sapphire (13) Zero Porosity (14) Zero Porosity (14) Density 3.62 (3) Density 3.42 (3)
25 OC 15.53 (Reuss) 15.51 15.93 11.33 8.67
16.05 (Vogit)
a Loading rate 94.7 MPa/minute; b minimum creep at 45O; c zero porosity; d less than 5% porosity; e rupture time less than one minute. (1) Wacthman &Maxwell (1959) (2) Springs Mitchell & Vasilos (1964) (3) Coors Prorcelain Data Sheet 0.001, August 1964 (4) Diamonite Products Manufacturing Company (1963) (5) Frenchtown Porcelain Co., Bull. 5462 (7) Ryshewitch (1941) (9) Wachtman & Maxwell (1954) (8) Knudsen
(10) Berezhkova &Rozhanskii (11) Crandall, Chung, & Gray (1961) (12) Wachtman &Lam (1959) (13) Wahtman, Tefft, Lam & Stinchfield (1960) (14) Lang (1960) (15) Ryshewitch (1951) (16) Spriggs & Brisette (17) Kingery & Pappis (18) Passmore, Spriggs & Vasilos (1965)
20
2.4. THERMAL PROPERTIES OF ALUMINA Chemical and thermal stability, relatively good strength, thermal and electrical
insulation characteristics combined with availability in abundance have made
alumina attractive for engineering applications.
Thermal properties of alumina are listed below on the Table 8.
Table 8: Thermal properties of alumina. (WALTER , Alumina as a Ceramic Material, 1970, page 64)
Melting Point !Al2O3 2051.0 ± 9.7 ℃ (1) Boiling Point !Al2O3 3530 ℃ (3800 ± 200 OK) (2) Vapor Pressure T OK Atm
(2)
2309 8.7 X 10 -‐6 2325 1.03 X 10 -‐6 2370 1.66 X 10 -‐6 2393 1.68 X 10 -‐6 2399 2.15 X 10 -‐6 2459 3.78 X 10 -‐6 2478 5.81 X 10 -‐6 2487 9.1 X 10 -‐6 2545 2 X 10 -‐6 2565 1.29 X 10 -‐6 2605 1.91 X 10 -‐6
Heat of Formation at 298.16 OK (kcal/mole)
Entrophy at 298.16 OK (kcal/mole)
! Al2O3∙ 3H2O -‐612.8 33.51 (5,3) ! Al2O3∙ 3H2O -‐609.4
(4)
Amorphous -‐304.2
(3) ! Al2O3∙ H2O -‐417.8 23.15 (4) ! Al2O3∙ H2O
8.43 (3)
! Al2O3 -‐400.4 12.16 (7,6) AlO -‐138 48.967 (2,14) Al2O -‐248 59.75 (2,14) Enthalpy (E+PV) in kcal/mole ! Al2O3 (∆!!"#.!" °!
! ) 0.03550922 ! − 4.0884 10!! !! − 11.23206 !"!!" ! + 19.63341 (a) (9)
0.035549846 ! − 3.9085 10!! !! − 11.2306 !"!!" ! + 17.23778 (b) (16)
0.03031602 ! + 8.3979 10!! !! − 2.81406 ! 10!/ ! − 12.87764 (c) (15)
Specific Heat (cal/g K) ! Al2O3∙ 3H2O 0.2694 + 6.43 X 10!! t (d) (8)
0.2855 at 25 ℃ (9)
0.348264 – 8.019 X 10-‐6 T – 47.8423/T (d,e) (9)
21
Table 8: continued
! Al2O3 OK cal/g OK OK cal/g OK 400 0.22545 900 0.28789
(9) 500 0.24857 1000 0.2924 600 0.26372 1100 0.29595 700 0.27431 1200 0.29877 800 0.28205
1318 0.2783 1787 0.4196 (10)
1510 0.3364 1660 0.3814 2575 0.468 (11)
Thermal Expansion, Linear (X 10 -‐6 /OC) ! Al2O3 Temperature Range Single Crystal (12) Polycrystalline
OC Orientation O0 C-‐axis 90O (12) -‐273.16 to 0 1.95 1.65 1.89 -‐73 to 0 4.39 3.75 4.1 0 to 127 6.26 5.51 6.03 327 7.31 6.52 6.93 527 7.96 7.15 7.5 927 8.65 7.8 8.08 1127 8.84 7.96 8.25 1327 8.98 8.12 8.39 1527 9.08 8.2 8.49 1727 9.18 8.3 8.58
Thermal Conductivity (cal/sec cm O C)
Temperature OC cal/sec cm O C -‐263 3
(13)
-‐253 9 -‐233 14 -‐223 12 25 0.086 100 0.069 300 0.038 500 0.025 700 0.018 900 0.015 1100 0.014 1300 0.014 1500 0.013 1700 0.014 1900 0.015
22
(a) Range 400 OK to 1200 OK; (b) Range 678 OK to 1330 OK; (c) Range 1290 OK 1673 OK; (d) T= OKelvin; (e) Range of equation 400
to 1200 OK.
(1) Schneider, Nat. Bur. Stds. Private communication, Nov , 1968 (2) Brewer & Searcy (3) Rossini, Wagman, et. Al. N. B. S. Circ.
500, 1952 (4) Russel et. Al. (1955) (5) Barany & Kelley (6) Kerr, Johnson, & Hallett (7) Mah (8) Roth, Wirths, & Berendt (1942)
(9)Furukawa et. Al. (10) Shomate &Naylor
(11) Sheindlin(1964) (12) Wachtman, Scuderi, & Cleek (13)Coors Porcelain Co. (AD 995)
(14) Panyushkin &Maltsev (15) Banashek, Sokolov, Rubinchik & Fomin (16) Sokolov, Banashek, & Rubinc
2.5. CHEMICAL PROPERTIES OF ALUMINA
Chemical reactions of alumina of general ceramic interest include the resistance
to attack of sintered alumina by various reagents, particularly at high
temperatures. High temperature chemistry includes those chemical phenomena,
which occur above 1000 OC. Such temperatures are attained by combustion, by
electrical heating, or by chemical explosions and nuclear reactions (Margrave,
1962). Some example studies are listed below due to understand further
chemical behavior of alumina.
2.5.1. Wet Chemical Reactions of Sintered Alumina In aqueous solution, aluminum oxide exhibits an amphoteric behavior. It can be
expressed by the following equilibra:
!!!! + 3 !!! ↔ !"(!")! ↔ !"!!! + !!!!
Impermeable alumina has been marked resistance to wet chemical corrosion. As
a rule the lower phases of alumina and the hydrous aluminas show increasing
chemical reactivity as they decrease in density. Early experiments on the
resistance of sintered alumina to attract were intended to demonstrate its
suitability as a container, crucibles, etc., for thermal reactions (Winzer, 1932).
Concentrated H2SO4, HCl, HNO3, H3PO4, and 20% NaOH dissolved no more than
0.02% of a 30 X 35 mm crucible within six hours, as indicated by loss in weight of
the crucible. This is not necessarily indicative of chemical inertness, as for
example; phosphoric acid readily reacts even with coarsely crystalline tabular
alumina to form slowly soluble phosphate bonds at temperatures below the
23
boiling point of the acid. Dawihl and Klingler (1967) state that sintered alumina
containing 3% silicates is far more resistant to corrosion by HCl, HNO3, and
H2SO4 in concentrations from 10 to 95% acid and up to 100 OC than titanium,
cast silicon and Cr – Ni steels.
Finely divided alumina is rapidly dissolved by HF, hot concentrated H2SO4,
mixtures of these acids, ammonium fluoride, molten alkali bisulfates or
pyrosulfates, and by concentrated HCl, especially when under pressure. All these
reagents have been used to dissolve alumina in analytical procedures. Sintered
alumina dissolves in concentrated H2SO4 faster than some high alumina
porcelains containing clay binders (85% Al2O3). Karpacheva and Rozen found
that the densest sintered alumina reacts with heavy water, H218O, the following
reaction rates, as percent reaction, within 80 minutes, were observed.
Temp °C 200 400 600 900 Rate (% 80 min) 20 30 50 97
Hot water solutions of the free alkali hydroxides and carbonates cause
perceptible reaction, the rate being correspondingly faster and higher
temperatures and under pressure.
2.5.2 Reaction of Chemical Elements with Alumina
Some reactions of interest between the chemical elements and alumina are
shown in the Table 9. Reaction of molten alumina with some refractory metals
and allots are included. These are presented in alphabetical order, in general, for
convenience of reference. Detailed information is given with references in
Appendix A related to element interactions with alumina. For references see
Appendix A.
24
Table 9: Interest Between The Chemical Elements And Alumina
Aggressively attraction Molten Lithium
No Attraction
Antimony
Attraction
Potassium Arsenic Carbon Beryllium Florine Chlorine Copper Cobalt
Limited attraction
BaO Gallium SRO Lead
Bishmut Mercury Cerium Nitrogen
Chromium Phosphorus Iron Silver
Manganse Sulfur Molybdenum Selenium
Nickel Tellurium Niobium Tin Palladium Uranium Platinum
Tantalum Titanium
Zirconium Tungusten Sodium
1.8. COLLOIDAL PROPERTIES OF ALUMINA Surface of pure alumina is defined alkaline. By electrophoretic methods Fricke
and Keefer (1949) determined the isoelectric point of zero point charge (zpc) of
gamma alumina to be at 9.0 pH, that of amorphous Al(OH)3 at pH 9.4, that
gibbsite at pH 9.20, and that of bohemite at pH 9.40 to 9.45. The potential
determining ions were considered to be H+ and OH-‐, which enter into
electrochemical reaction at the surface in the case of aluminum oxide. The
essential part of the surface reaction schematically is as follows:
OHH (+)+H2O
H3O+
! "## OH OH $
! "## O($)+H2O
Positive Surface Uncharged surface Negative Surface
Isoelectric point of hydrated alumina is listed in Table 10:
25
Table 10 : isoelectric point of hydrated alumina (O’Conor 1956)
Aging Conditions Precipitation Fresh Preparation Rapid Aging by Heating Suspension Aged 4 Months Excess Alkali 5.08 6.79 5.78 Equivalent Alkali 6.63 7.28 7.06 Deficent Alkali 7.29 7.43 7.32
Zeta potential of natural corundum reported positive in water 17-‐20 OC, but it
changes to negative on heating to 1000 OC. (O’Conor 1956)
The interaction between the liquid and the surface can be estimated by contact
angle (q) measurements. The details of the interactions between the surface and
the solvents were explored by analysis of the effect the properties of the liquid
on varying the equilibrium contact angle with water (!!") with the different
anodic alumina surfaces. These values were determined from the intersection of
the fits for advancing and receding angles versus contact angle hysteresis with
ordinate at ! q=0, one finds the equilibrium angle !!" . For different samples
results vary between 62.6 -‐ 73.3. Results obtained Wihelmy Method. (Redon et.
al., 2005)
26
3. ALUMINA MEMBRANES
3.1. INTRODUCTION In recent years there has been a growing interest in utilizing inorganic
membranes to address a variety of separation problems in many industries
Various inorganic membranes made from metals, inorganic polymers and
ceramics have been proposed for liquid and gas separation applications.
Membrane technologies play an increasingly important role in pollution
prevention, resource recovery and waste treatment activities. Due in large part
to cost considerations, polymeric membranes dominate these applications,
however, the use of polymeric membranes in separations involving aggressive
materials such as many organic solvents, acids, bases and oxidants is often
limited by the tolerance of the polymeric material to extreme conditions. The
interest in utilizing such membranes in separations has increased since the
advent of consistent-‐quality, commercially available ceramic membranes with
narrow pore size distributions.
Ceramic membranes are noted for their excellent mechanical strength and
tolerance to solvents, as well as pH, oxidation, and temperature extremes. For
example, they can be used at significantly higher temperatures, have better
structural stability without the problems of swelling or compaction, generally
can withstand more harsh chemical environments, are not subjected to
microbiological attack, and can be backflushed, steam sterilized or autoclaved.
An ideal ceramic membrane must be highly selective, permeable and durable.
For aqueous applications, or aqueous/organic separations it is desirable for the
ceramic to be hydrophilic to maximize flow and minimize fouling. The membrane
selectivity is primarily dependent upon the pore size distribution; the narrower
the pores size distribution, the more selective the membrane. Mechanical
integrity is, enhanced in such applications by slip-‐casting a relatively thin
selective membrane onto a thicker, more permeable support yielding an
asymmetric membrane.
27
The former forms of dense (or nonporous) membranes of palladium or its alloys,
silver and zirconia have shown to be permeable only to certain gases (e.g.,
hydrogen and oxygen). These materials are used in sensors, electrodes and
coatings. Their industrial use as a separation tool, however, is limited primarily
by low permeability compared to microporous metal or ceramic membranes.
Currently, microporous stainless steel, silver and ceramic membranes such as
alumina, zirconia and glass are available commercially. In Table 11 the selected
commercial alumina membranes are listed. Data collected from web based
information. Among the ceramic membranes, alumina is gaining acceptance for
liquid phase separations. Like zirconia membranes, alumina membranes for
liquid phase separations came to commercial fruition from uranium isotope
enrichment work. (Hsieh, Bhave, and Fleming, 1988)
Table 11 : Selected commercial Alumina Membranes
Manufacturer Trade Name Application Membrane Material
Support Material
Membrane pore
diameter
Membrane support
configuration
Alcoa / SCT Membralox UF ! − !"!! ! Al2O3 40-‐100 ! Tube and multichannel element
MF ! Al2O3 ! Al2O3 0.2-‐5 µ m
Norton Ceraflo MF ! Al2O3 ! Al2O3 0.2-‐1µ m Tube
NGK MF Al2O3 Al2O3 0.2-‐5µ m Tube and Plate
Alcan/Anotec Anopore UF Al2O3 Al2O3 250 ! Disk
MF Al2O3 Al2O3 0.2µ m
Alumina membranes are constantly growing area. In the Figure 3, it can be
seen that, the publication numbers are highly increasing parallel with the
membrane research especially during recent years.
This data is collected from sciencedirect.com.
28
Figure 3 : Number of publications on membranes and alumina membranes based on years.
The main reasons why alumina is still trending research topic can be explained
by following reasons. During the last several decades, many investigations have
been focused on porous anodic alumina (PAA) due to its benefits of tunable
nanopore diameter and long-‐range ordered feature of the porous nanochannels
in macroscopic domain. The pore diameter of the PAA nanochannel can precisely
be controlled from a few nanometers to several hundreds of nanometers by
applying regarding electrolyte, voltage (or current), and reaction temperature
during the electrochemical anodization reaction of aluminum substrate.
Moreover, reaction time provides a tunability in the thickness of the porous
nanochannels from a hundred of nanometers to a hundred of micrometers. Such
easy control ability of the pore diameter and the thickness makes the PAA one of
the interesting materials, which are frequently being applied in nanoscience. So
In the first figure publications
on alumina membranes is
evaluated.
Second figure evaluates the
trend of membrane
publications during years.
It can be concluded that;
trends of alumina membranes
Second figure evaluates the
trend of membrane
publications during years.
It can be concluded that;
trends of alumina membranes
are rising correlated with
general trends with
membranes. Alumina
membranes are promising
and developing area in
membrane society.
29
far, the studies to utilize the PAA have been performed in a wide range of
research fields such as nanomaterial design, molecular sieving, photonic and
optical device, and catalysis.
3.1 PREPERATION OF ALUMINA MEMBRANES 3.2.1. Macroporous Alumina Membrane Preparation
Porous ceramic membranes with pore size ranging from 0.1 to 50 mm and
porosity above about 40% are used for filtration (e.g., hot gas filtration),
diffusion (e.g., waste water treatment), dispersion rolls, inkpads for
fingerprinting and numerous other applications. In particular, filtration is
important for the petrochemical, mining and chemical industries.
The anodizing of pure aluminum in various acidic electrolytes has attracted
considerable attention in recent years since it opens up the possibility of
preparing films with well-‐controlled, uniform pores from tens to several
hundreds of nanometers in diameter. The basic cell structure containing
cylindrical pores has long been known and methods for preparing regular pore
arrangements in a hexagonal pattern with interpore distances of between 50 nm
and 500 nm have been reported. The preparation of regular pore arrays typically
involves electrolytic polishing and multiple anodising steps or even mechanical
pre-‐texturing. As-‐prepared porous anodic alumina (PAA) membranes are
amorphous to X-‐ray diffraction (XRD). Their chemical composition is not
stoichiometric Al2O3 but incorporates a considerable quantity of anion impurities
and hydroxyl groups incorporated from the electrolyte into the alumina
structure or bound to the alumina surface. (Kirchner, et. al)
PAA has proved to be useful for fabricating many materials, especially as a
template for the synthesis of metallic or semiconductor nanometer -‐scaled wires
and particles. After chemical functionalizing, PAA membranes can be utilized for
catalytic or optical purposes. Further, the porous film itself may be employed for
filtration, gas separation or as a photonic crystal. Because it is a ceramic oxide,
30
PAA has considerable potential in high-‐temperature applications, although
severe problems can occur when certain types of PAA membranes are heated.
Macroporous alumina membranes also can be made from particles or
discontinuous fibers by the use of a binder or by sintering. The sintering of
ceramic particles is perhaps the simplest approach to forming a porous ceramic
filter, however, the sintering of bulk ceramics is a very energy expensive process
due to the high temperatures required. The pore size is controlled by the starting
particle size, sintering time and temperature. This method is generally used to
produce alumina microfiltration filters, which contain larger pores and supports
for ultrafiltration membranes, which contain smaller pores. (Avci et. al. )
Binders are most commonly in the form of fine particle dispersions (colloid). The
silica colloid is an example. After application by wet forming (as in paper
making), drying and appropriate heat treatment is needed. The binder
technology is central to the technology of membrane fabrication. A binder is
necessary to hold the ceramic particles or fibers together to form a membrane. It
must be used in a sufficient quantity in order for the membrane to have
acceptable mechanical strength. However, it must not block the pores in the
membrane, as is the case if it is used excessively. Thus, an effective binder should
be able to bind the particles or fibers together and result in a mechanically
strong membrane, even when it is used in a very small proportion. In addition,
the binder must be able to withstand high temperatures, as encountered in hot
gas filtration.
Silica and vitreous glass are widely used binders in the refractory and ceramic
industry. It is often used in the form of an aqueous dispersion. The silica colloid is
particularly attractive due to the high temperature resistance of silica compared to
vitreous glass and the good binder dispersion enabled by the small particle size of the
silica in the colloid. The average size of the silica particles in the colloid can range
from under 10 nm to over 80 nm. The silica binder is easy to use, but it tends to fill
the open or continuous porosity of the filter membrane.
31
Phosphate has been utilized as a binder in the refractory industry for many years.
Pirogov et al. used a phosphoric acid (H3PO4) binder in a mullite-corundum body in
their study to determine the optimum content of graphite and SiC additives. Birchall
et al. studied the mechanical properties of an unsintered SiC compact bonded by
aluminum phosphate (AlPO4) glass. Toy and Whittemore evaluated the reactivities of
several calcined aluminas with phosphoric acid and demonstrated that a glassy AlPO4
phase and aluminum metaphosphate (Al(PO3)3) are effective bonding phases.
3.2.2.Mesoporous Alumina Membranes
Of the present technologies, sol–gel is the best method for making ceramic
ultrafiltration membranes. However, the pore size is generally limited to the
sizes of the ceramic precursor particles prior to sintering. For sol–gels, the
particle size distribution is difficult to control, and they must be used
immediately after preparation to avoid aggregation or precipitation. (Johns et. al)
Commercially available membranes are currently ! -‐Al2O3 membranes are
currently thermally stabilized at 650 OC, with a mean pore size equal to 5 nm.
Such membranes can be used for Nanofiltration of aqueous solutions containing
inorganic salts or amino acids. They can also be used for gas permeation
applications. However these membranes present a rather low chemical stability
in aqueous media at very high pH values. More over, their structural evolution
has to be taken into account for high temperature applications. (Ayral)
Sol-‐gel process can be applied generally;
• Solution must be prepared An example of Boehmite (! -‐AlOOH) sol from
an inorganic precursor was prepared as in shown figure 4.
• Pretreatment alumina supports (synthesized or provided). Heating, or
solution applications.
• Membrane solution contacts with support.
• Penetration of the solution into the voids of the support occurs by
capillary action.
• The support was then shaken to remove any excess solution, and dried at
room temperature.
32
• The coated support was heated up to 600-‐1000 OC step by step and held
remained in temperature range for stabilization.
• Multiple coatings were obtained by treating a previously coated filter.
It was reported the unsupported membrane was obtained by dying the sol layer:
!"#$% ! − !"##$ !"# !"#$%&
!ℎ!" ! − !"##$ !"# !"#$% !"#$%&
!ℎ!" ! − !"##$ !"# !"#$% !"#$%&
!ℎ!" !"#$! ! − !"##$ !"#$%"& !"#$% !!"#℃! !!"#$%!"&
! − !!!!! !"#!$$%&'() !"!#$%&"
the sol layer was formed by dipping the layer was dried at room temperature,
the drying step removes excess water, cracks formed easily during drying step.
Fig.4. Preparation procedure of boehmite sol
In order to obtain the Al2O3 membrane without crack, a method, rapid gelation
processing for preparation of gel layer can be used. The sols of ! − !"##$ are
atomized firstly then sprayed onto a substrate. The gel layer was obtained from
sols directly. No crack was observed.
33
Figure 5 is the schematic diagram of the rapid gelation processing. The sols are
atomizing by a high-‐velocity gas, and sprayed into the substrate (glass or
ceramics plate coated with cellulose acetate film. After drying, the resulting gel is
placed in acetone to dissolve the cellulose acetate and to obtain the unsupported
gel film). The 0.1-‐ 200 µm of thickness of the membrane can be obtained
without cracking at conventional drying rate. When the thickness of membrane
is higher than 200 ~ m, cracking can be prevented at controlled drying rate.
The transparent boehmite gel membranes obtained by rapid gelation were fired
at a temperature (>350°C) to form ! -‐alumina membrane.
Figure 5: Schematic drawing of the rapid gelation processing, 1 -‐ nozzle, 2 -‐ atomizing sol and 3 -‐substrate.
Mesoporous γ-‐alumina membranes are formed by dip-‐coating a porous substrate
in a Boehmite (γ-‐AlOOH) precursor sol, will be treated by heat and sintering
steps. The quality and properties of the membrane depend on the dispersion
rheology and quality of the Boehmite sol and the dip-‐coating process as such. A
high quality Boehmite sol is prepared by hydrolysis and condensation of an
aluminum alkoxide: ATSB ([C4H9O]3Al), followed by one or more purification
µ
34
steps. Five steps in total are necessary for a single layer mesoporous γ-‐alumina
membrane preparation:
1. Boehmite sol preparation
2. Boehmite sol purification
3. Dip-‐coating sol preparation
4. Dip-‐coating procedure
5. Heat treatment/sintering
In the Figure 6 an example of dip coating process is shown.
Figure 6: Filter ring assembly for mesh filtering: top filter ring (left), bottom filter ring (middle) and assembly diagram with nylon mesh (right). The thickness of the top filter clamp ring can be increased to increase the filtration volume. Currently it can hold ~10 ml.
In laboratory scale experiments to the synthesis of mesoporous alumina
membranes using slip-‐casting technique has been tried. Catalytic membranes
with precise active layer width and location were prepared by sequential slip
casting of Pt/Al and alumina sols. Different distributions of catalyst within the
membrane can be easily obtained by varying the thickness of the slip-‐cast layers
as well as their arrangement. The layer widths are controlled by the slip-‐casting
parameters, including slip-‐casting time, alumina content and particle size. The
catalyst loading and dispersion of the active layer can be controlled precisely.
The effects of sintering temperature (600-‐1200°C) on the membrane's pore
35
structure, morphology, phase structure, surface area and gas permeability are
observed. ( Yeung et. al; 1996)
3.2.3 Microporous Alumina Membranes
In the slipcasting method, a porous support is usually made first by conventional
ceramic processing techniques to provide rigid structure with relatively large
pore size for slip deposition. Since particle size directly related with pore size,
the slip used, as the membrane precursor needs to contain well-‐dispersed
particles of uniform size. Depending on the desired pore size of the membrane,
the membrane precursor particles may be prepared by precipitation,
classification, sol-‐gel method, etc. In the sol-‐gel techniques, ultrafine particles of
a few nm in diameter can be prepared by polycondensation or redox reactions of
aluminum salts or hydrolysis and condensation of aluminum alkoxides.
After treatment with a peptizing agent such as an acid and optionally with a
viscosity modifier, the slip is deposited on the porous support by the dipping or
slipcasting procedure. This procedure for filtration based on capillary pressure
drop created by the contact of the slip with the support. (Leenaars, Burggraaf
1985) This pressure drop forces the dispersion medium (e.g water to flow into
the dry pores of the support while the slip particles are retained and
concentrated at the surface forming a thin membrane. The membrane precursor
is then dried and calcined to provide the required pore size for specific
applications and the needed bonding between the membrane and the support.
These steps must be done with a great sensibility to avoid cracks, which can
occur due to shrinkage upon drying and/or calcining. (Hsieh, Bhave, and
Fleming, 1988)
The slipcasting method is commonly used for making commercial alumina
membranes. Method can be replied to organize intermediate layers between
thin, permeable, selective membrane and the thick, porous support, which
provides the needed mechanical strength. T Thus, alumina membranes, like
many other porous inorganic membranes, are composite in nature. The cross-‐
36
section of a multi-‐layered alumina membrane composite can be exemplified by
the scanning electron micrograph of Figure 7. In general, the selective membrane
layer has a thickness of 2-‐10 !m. This thickness is a trade-‐off between high flux
and mechanical stability. The intermediate layers are generally 10-‐20, !m thick.
The bulk support constitutes the majority of the pore volume and thickness. If
the membrane precursor particles are so fine that they penetrate the relatively
large pores in the support, the permeability of the membrane/sup port
composite will decrease due to partial or complete blockage of the pores in the
support. To prevent fine particles from penetrating into the support, one or more
intermediate layers of graded particle size are used.
Figure 7: Cross-‐sectional scanning electron micrograph of a three-‐layered alumina membrane/support composite (pore diameter of 0.2, 0.8 and 10 μm, respectively, in each layer)
The ability to consistently produce high quality alumina membranes on a
commercial scale has been the key to wider acceptance of ceramic membranes as
a separation tool. The surface morphology of a high quality alumina membrane,
shown in Figure 8, displays uniform particle size.
37
Figure 8: Top-‐view scanning electron micrograph of an alumina membrane (pore diameter of 0.2 !m)
Typically, membranes and supports have a porosity of 30-‐60% by volume. This
range provides a good compromise between permeability and strength.
Membrane science has a leading role in innovative processes and is considered
one of the main strategic axes of research activities in all developed countries.
Advanced technology programs in the USA or Japan involve them, and with an
annual growth rate of 10 to 20 %, and a total world market above 10 billion
Euros around 2010, membranes are likely to become more and more important
in the future. Besides flat membranes as motivated by literature, anisotropic
membranes consist of an extremely thin surface layer supported on a much
thicker, porous substructure.
The surface layer and its substructure may be formed in a single operation or
separately. Multi layer membranes and their production will be motivated in
next chapter.
38
4.DESIGN OF THE MEMBRANE MODULES
The separation science and technology using all the kinds of different
membranes is highly influenced by the membrane module configuration as far as
the efficiency of a membrane process depends on the design of its module. The
cost reduction of membrane module has led to the commercialization of
membrane process some decades before depends on the sort of application and
the different materials used. In practice, the commercial available membrane
modules are assembled in units consisting usually of several membranes,
sometimes from many thousands of them.
4.1 DIFFERENT TYPES OF MODULES
Currently, four main types of membranes modules are in the market: (a) planar,
(b) spiral, (c) tubular (d) hollow fibers modules and (e) honeycomb. The
modules are closely related to the geometry of the membranes and therefore of
the materials. Each commercial module is consisted of a number of membranes,
placed in different ways. The structure of membrane can be (a) symmetric, (b)
anisotropic and (c) composite and according to the sort of use, also two main
categories can be distinguished: (a) vibrating membrane and (b) submersible
membranes. Besides, in terms of geometry, two types of different geometry can
exist: (a) planar and (b) tubular [Remigy, 2007]. From these basic configurations,
many other secondary modes can also occur such as vibrating hollow fiber
modules and many others.
In Table 12, main parameters of membrane modules are gathered. The Table 1 is
presented in “Filtration Membranaire” published by CNRS in 2007 and it is only
representative for actual membrane technology trends. All the information
concerning membrane technology needs frequent updating because membrane
engineering is a state-‐of-‐the-‐art research and market field.
39
Table 12 Comparison of performances for different modules and membranes [Remigy, 2007]
Different perfomance of membranes modules
Geometry Planar Spiral Tubular Hollow
fibers
Cost of inventissement
(U$$/m2, 2000)
50 to 200 5 to 100 50 to 200 5 to 20
Energy cost Moyen Medium Important Low
Hydraulic diameter 1 to 5 0,8 to 1,2 12 to 20 0,1 to 1
Comptability 100 to 400 300 to 1000 10 to 300 1000 to
15000
Membranes replacement Membrane by
membrane
Entire
module
Tube by
tube
Entire
module
Pretreatment Moyen Medium Low Medium
(entire
filtration
internal/exte
rnal)
Medium to
low
(external/int
ernal)
Cleaning Good Difficult to
medium
Excellent Medium
Material All the materials Polymers All the
materials
Polymer
4.1.1 Alumina membrane modules
Alumina membranes characterized as inorganic, ceramic membranes are usually
fabricated in tubular or less often planar module, composite or anisotropic, in
some cases immerged, monolith, disk mode. On the other side, vibrant or
immerged modules in hollow fibers and planar or spiral geometry are commonly
polymer (organic) membranes modes. The most recent trend appeared in
alumina membrane market is honeycomb module which combines some of the
most important advantages of previous modes.
40
Planar module: In general, membranes of planar module are less met in current
market trends and represent either older membrane systems either pilot or
patent scale applications. They are based on conventional filter press design and
membrane feed spacers and product spacers are layered together between two
end plates. The most common uses of plate and frame modules concerns
electrochemical applications as ion exchange, electrodialysis, pervaporation
systems, electrosensors and membrane electrode assembly. These applications
concern all the different kinds of membranes as metal or polymer membranes.
Figure 9 a planar aluminium module is given as a patent for an improved proton
exchange membrane. The membrane could be used in membrane module unit as
electrochemical cells having internal passages parallel to the membrane surface.
The passages in the membrane extend from one edge to another and allow fluid
flow through the membrane and give access directly to the membrane. The
invention, awarded by NASA in 1997, is related to applications in as electrode
assemblies for fuel cells, electrolyzes, electrochemical hydrogen and oxygen
pumps, and related devices [Gonzalez-‐Martin et al., 1997].
Figure 9 Membrane electrode assembly containing internal passages [Gonzalez-‐Martin et al., 1997]
Tubular module: Alumina membranes most of the times have tubular mode
which is composed of a number of tubular membranes with an internal relatively
41
small diameter. They may be unique tubes assembled in a module using joints or
monolithic composed of several tubes (Figure 10 and 11). The selective part is
located inside the tube or tubes. Feed is made at one end of the module and the
fluid flows inside the tubes or, the retentate springs at the other end. The
permeate pass through the body of the tube or out of the monolith to the
external part.
Figure 10 Schematic side-‐view of membrane module consisting of multi-‐channel elements
Figure 11 Cross-‐section of a monolithic multi-‐channel membrane element [Hsieh et al., 1998]
In general, the main advantages and disadvantages of tubular noticed in the
literature are because of ceramic material properties, the most common
substance for which they are made of.
(a) Advantages:
42
• No pre-‐treatment, accepts slurries in large particles (about one-‐tenth the
diameter of the tube)
• Easy to clean, use of mechanical cleaning
(b) Disadvantages:
• Very low compactness of the order of 10 to 300 m2/m3
• Relatively high investment
• Energy costs can be significant, due to a high rate of circulation
• Fragility in the case of ceramic membranes
As an example for diameter difference, in laboratory scale of Stoitsas et al.
(2005) research work, alumina and silica membrane specimens for gas
separation are of tubular geometry with a length 340∙103m, an internal diameter
of 8·103m and an external diameter of 14·103m when the membrane surface per
specimen is 8.5·103m2. In this case, the ceramic membrane is an asymmetric 4-‐
layer system. The first layer operates as a support, the third as a microfiltration
layer (pore sizes of 100 or 200nm depending on the firing temperature) and the
fourth as an ultrafiltration layer with a pore size of 3–5nm. The second layer
(pore size of 500nm) serves to bridge the gap between the macroporous support
and the microfiltration layer.
Honeycomb: Honeycomb pore size membrane is part of the new generation
membranes for microfiltration and ultrafiltration applications. It is available for
ceramic membranes either of alumina or carbon supports. The term honeycomb
concerns the microstructure of porous in porous layer and because of this
geometry, the membrane is suitable for special applications.
4.1.2. Commercialized modules of membrane alumina
Tubular mode
The figure below (Figure 12) is from Veolia Water Solutions & Technologies
(VWS) manual for new ceramic membrane modules.
43
Figure 12 Large diameter monolith membrane element with permeate conduits
The industry utilizing the CeraMem® technology provides a variety of inorganic
microfiltration (MF) and ultrafiltration (UF) membranes. In CeraMem®
technology, typical multi-‐channel ceramic membrane has multiple parallel
passageways that run from a feed inlet end face to an opposing outlet end face.
The surfaces of the passageways are coated with permselective membrane. A
feed stream is introduced under pressure at the inlet end face, flows through the
passageways over the membrane, and is withdrawn at the downstream end face
as retentate. Permeate flux passes through the membrane flows into the porous
monolith material. Under an applied pressure, the combined permeate from all
the passageways flows through the porous monolith support to the periphery of
the monolith and is removed at the monolith exterior surface
[veoliawaterst.com/ceramem].
Honeycomb module
The Anopore™ inorganic membrane (Anodisc™) in honeycomb module is a novel
material with precise, no deformable honeycomb pore structure (Figure 13) with
no lateral crossovers between individual pores that filters at precisely the stated
cut-‐off, allowing no larger sized particles to pass through the membrane.
44
Figure 13 AnoporeTM alumina membrane with honeycomb pore size distribution [whatman.com]
The AnoporeTM inorganic membrane is composed of a high purity alumina matrix
that is manufactured electrochemically [whatman.com]. This kind of membrane
shows a high thermal permeability and selectivity and it can be applied for a
wide range of special applications as pharmaceutical or laboratory ultrafiltration
process.
Generally, commercial alumina membranes have an asymmetric structure with
membrane deposition on the inside surface of a tube (or channel). The diameter
of module depends on the kind of application. Large scale units can be used eg. in
wastewater treatment plants and small scale units can be used eg. in gas
separation. A typical industrial installation will have several of these modules
arranged in series and/or parallel configuration [Sondhi et al., 2007].
45
4.2. SEPARATION CHARACTERISTICS FOR ALUMINA MEMBRANES
For future looking through membrane uses it is important to summarize the
most crucial separation characteristics as “crossflow mode”, “flux “ or “rejection”
properties.
Crossflow
Crossflow filtration is a pressure-‐driven separation process in which a stream
flows parallel to the filter surface. For microfiltration applications such a parallel
flow creates shear forces and/or turbulence to sweep away particulates and
prevent blinding of the filter surface. The feed flows past the inside surface of the
tube (or channel) and the permeate (filtrate) flows through the porous support
to the shell side of the module. This is generally true for all crossflow membrane
modules such as tubular, hollow fiber, spiral wound, or plate and frame modules.
Inorganic membranes used in commercial large-‐volume applications are,
however, available only in single tubes or multi-‐channel elements as described
earlier. Such membrane-‐based separation devices have to satisfy certain
performance cri-‐membranes used in commercial large-‐volume applications are,
however, available only in single tubes or multi-‐channel elements as described
earlier. Such membrane-‐based separation devices have to satisfy certain
performance criteria to justify a cost-‐effective process as flux or filtration rate,
long term flux stability and separation or rejection properties of the membrane.
Flux
This is one of the most important performance criteria for cost effective
membrane technology, particularly for large-‐scale separations. In membrane
separation processes the (permeate) flux may be influenced by such factors as
crossflow velocity, transmembrane pressure differential, temperature and feed
characteristics.
46
Crossflow velocity
This is the average rate at which the process fluid flows parallel to and across the
membrane surface. Due to continuous removal of filtrate (permeate) through the
membrane, the crossflow velocity is somewhat higher at the inlet than at the
outlet of the membrane element. For many particulate filtration applications (e
.g., MF) it is generally recommended to be in the velocity range of 1-‐5 m/sec
depending on process stream properties such as viscosity, particulate loading,
etc ., and on constraints imposed by pressure drop limitation In MF with an
alumina membrane, for instance, a crossflow velocity in the range 3-‐5 m/sec is
recommended. An increase in crossflow velocity generally results in an increase
in flux, since at higher shear rates the removal of particulates at the membrane
surface is more effective. A higher crossflow velocity, however, results in a
substantially higher tube side pressure drop which may pose a problem to the
membrane material. For most polymeric hollow fibers, the allowable
transmembrane pressure difference is limited to about 30 psig, since the typical
fiber burst pressure is only about 50-‐60 psig. In contrast, alumina membranes do
not suffer from this limitation and usually have a pressure rating of 250 psig or
higher. In Figure 14, a simplified circuit of crossflow membrane is designed, with
feed and retentate and pressure application points illustrated.
Figure 14: Simplified schematic of crossflow membrane filtration [Hsieh et al., 1998]
47
Feed
It has been found that the flux behavior for pure water (e.g., filtered tap or
deionized water) through a single tube is similar to that through a multi-‐channel
alumina membrane element (such as Membralox®). However, for scale-‐up
studies, alumina membranes with tube bundles or multi-‐ channel monolithic
elements should be used over single tubes to obtain hydrodynamic and fluid
management data.
Flux stability
Critical factor determining process and economic viability of separation
applications with membranes is also flux stability. In MF and OF applications,
flux decay can be a serious problem. Flux decay is usually a direct result of an
increase in the hydraulic resistance of the membrane due to fouling, eg. excessive
accumulation of debris/particulates at the membrane surface or in the pores.
The phenomenon of membrane fouling is only partially understood. In general,
fouling is more severe with symmetric membranes. The majority of alumina
membranes have an asymmetric pore structure with a pore size variation across
the entire thickness of membrane/support composite. Such asymmetric
membranes are thus expected to be less susceptible to internal pore fouling.
Many liquid streams contain extraneous matter in the form of small particles,
macromolecules, etc. During the filtration operation, these substances are often
concentrated near the membrane surface and can precipitate or agglomerate on
the surface as well as in the pores. Flux decay was observed in the filtration of
tap water with a single-‐tube alumina membrane when the backflushing
technique for cleaning was not employed. This may be due to membrane fouling
by trace quantities of particulate and colloidal matter, since prefiltered water
showed a higher flux in comparison to unfiltered tap water. A stable flux for
extended operation can be obtained if flux decay due to fouling can be controlled.
48
Rejection characteristics
One of the important parameters determining membrane rejection
characteristics is the mean pore diameter of MF and OF membranes having
narrow pore size distributions. In general, the smaller the pore diameter (i.e., the
tighter the membrane), the higher will be the rejection coefficient to
particulates/ solutes of greater nominal size. However, as pore size decreases,
permeability to liquid also decreases rapidly.
49
5. APPLICATIONS
5.1. CERAMIC MEMBRANES There are a variety of industrially important liquid or gas phase systems where
the general membrane technology has been successfully applied for separation
purposes. Such membrane processes as MF, UF, and reverse osmosis (RO) using
polymeric membranes have been in commercial practice for over two decades.
However, in recent years ceramic membranes are also being considered for
crossflow MF and OF applications and also gas separation application [Laitinen,
2004].
Ceramic membranes are usually composed by metal oxides as aluminum,
titanium or zirconium oxide as they are or mixed. They are formed from
different layers of particular length and the support is consisted of porous α-‐
alumina in tubes unit. The active layer is consisted from alumina (Al2O3, α or γ),
from zirconium (ZrO2) or titanium oxide (TiO2). The main advantages or ceramic
membranes can be listed as:
• sufficient resistance at high temperature ( to 300 ˚C)
• chemical resistance
• pH range from 0 to 14
• compatibility of organic solvents and ionizing radiation
• support of sterilization
Organic membranes are hardly resistant to solvents eg. polymeric membranes
including PTFE used in wastewater treatment process and on the other side
inorganic membranes are more readily used for such kind of application [Remigy
et al., 2007]. In applications of industrial wastewater treatment as solvent
recycling from polluted solution, conventional reclamation technologies, such as
distillation and standard filtration, suffer significant limitations in terms of
technical viability, cost, and user friendliness when ceramic membrane
technology performs important advantages, as listed in Table 13 [Ciora et al.,
2003].
50
Table 13 Advantages of ceramic membrane technology for the recovery of spent high flash solvents [Ciora et al., 2003]
M&P Ceramic Membrane Technology
Advantage Comments
1. Excellent solvent
resistance
Can be used to treat entire range of high flash solvents
2. Excellent recoverd
product quality
Finished product quality similar to virgin material
3. Low temperature
operation
No thermal degradation of solvent
4. Good product recovery
ratios
>90% solvent recovery can be aschieved
5. No additional waste
disposal problem
Waste volume necessary for disposal is <10% of original volume
6. Low tech Technology is easily implemented. No special training required.
Minimal maintenance etc.
7. Implemented on small
scale
Most high flash solvent waste is highly segmented with numerous
small-‐scale generators of waste solvent
At the same moment, considerable disadvantages often limit ceramic
membranes in pilot or small scale applications and render polymeric membranes
as the most widely applied membranes. Ceramic membranes are related to high
cost of preparation, fragility and high cost of installation and as also limited
variety of modules (only tubular and more rarely planar). In addition, some
ceramic active layers are sensitive to basic attacks when they are deemed to be
generally very resistant to alkaline pH [Remigy et al., 2007].
Inorganic membranes can be classified according to the principal component
they consist of apart from other layers or additives to carbon, metal and
composite membranes [Remigy et al., 2007].
Analytically, the most important applications of these categories are gathered
below:
51
• Carbon
If the membranes are absolutely consisted of carbon they are used for gas
separation. For the filtration field, carbon membranes comprise a carbon porous
layer on which a metal oxide layer is deposited (zirconium or alumina).
• Metals
A wide variety of metals can be used for the manufacture of filtration
membranes. Two metals, however, stand out: steel (including stainless steel) and
aluminum. The stainless steel membranes are composed of sintered particles
while those of aluminum are formed of a film of anodized aluminum. The metal
membranes are exclusively microfiltration membranes or high UF with a pore
size of not less than 20 nm.
• Combination of different materials
In Table 13, the most important advantages and drawbacks are revised for the
different kinds of materials. Note that the comparisons in terms of thermal and
chemical resistance ignore the other materials in the manufacture of modules
(seals, adhesives).
52
5.2. ALUMINA MEMBRANES APPLICATIONS In the recent years of membrane technology development, alumina ceramic or
composite membranes are of great interest for a wide range of applications and
new structures compositions. They offer a largely independent control of pore
size, pore topology, pore orientation, molecular functionalization and pore wall
composition.
A developing research field focuses on nanosize of porous alumina membranes
creating a potential for molecular separators, materials for the inclusion of
conducting or semiconducting nanostructures. The use of porous ceramic
membranes often offers a solution for aggressive environments as corrosive
solutions or high temperature process [Levanen et al., 2004].
The following sections will give more information on applications or possible
application areas of alumina ceramic membranes divided in two main categories:
1. Liquid phase separation
2. Gas phase separation
Remarkable is that in the analysis of applications below, alumina is one the
components of membranes. In many cases composite membranes are studied
with many different layers and at least one alumina coat.
5.2.1. Liquid phase separation applications Most of the commercial liquid phase applications for ceramic membranes are
either for microfiltration or ultrafiltration. The first large-‐scale commercial
success of ceramic membranes has been in the food and beverage industries.
However, significant applications are found also in other areas, such as
biotechnology, pharmaceutical, petrochemical, and other process industries as
well as in environmental control. In recent years also ceramic nanofiltration
membranes have been developed. Microfiltration is mostly applied in cases
53
where liquid streams contain particulates, and ultrafiltration is used when
smaller molecules are removed.
Some of the important liquid phase applications where alumina membranes are
employed are: treatment of industrial and municipal water, sterilization of
liquids in the pharmaceutical industry, clarification and sterilization of
beverages, e .g. wine, beer and fruit juices, cell harvesting and sterilization,
cheese-‐making by ultrafiltration and wastewater treatment. A number of studies
on crossflow filtration can be found in the recent literature [Laitinen et al., 2004].
Although many of them deal specifically with polymeric membranes, the
principles of crossflow filtration, in general, can also be applied to membranes
made of inorganic materials such as alumina. In all these application areas the
ceramic membranes have to compete with the polymeric membranes which are
becoming more and more stable and which have an advantage of much lower
prices. The use of ceramic membranes can only be justified in cases, where they
give much better performance, or in cases, where there are no suitable polymeric
membranes available [Hsieh et al., 1998].
In the following part, the applications of alumina membranes in gas phase
separation process are gathered in Table 14.
i. Food processing
Generally, the major advantages of ceramic membranes in the area of food and
beverages are their resistance to alkaline cleaning solutions, their stability in
steam sterilization, better capability to withstand higher operating pressures,
and a consistent pore size. In crossflow microfiltration applications, the major
advantage of the ceramic membranes has been the possibility to use uniform
transmembrane pressure, which for example in dairy industry has enabled high
fluxes and low fouling. The market share of inorganic membranes in the food and
dairy industry was about 10% at 1990s and tomorrow this percentage is
54
Table 14 Liquid phase separation process of alumina membranes and composite alumina membranes
Porous alumina membrane appications
Liquid phase separation applications
Food processing
Clarification of natural fruit juices fruit juice industry (orange, apple, grape etc.)
Filtration of sugar cane juice sugar concenration
Alcoholic beverages breweries, wineries
Environmental applications
Petrochemical industry wastewater treatment petrochemical industry, oil recovery
Pulp and paper industry wastwater treatment water reuse in production line
Municipal wastewater treatment municipal wastewater treatment plants, water
reuse
Activated sludge treatment depollution of wastewater sludge, compost
Nuclear industry wastewater treatment
Fluoride removal municipal wastewater, chemical industry
wastewater
Arsenic removal from water resources depollution of water sources and groundwater
Arsenic and fluoride removal depollution of water sources and groundwater
Chromium and arsenic removal metal industry wastewater, valuable metals
recovery
Biotechnology and Pharmaceutical
applications
Fermentation broths recovery of valuable antibiotics
Fungal cells separation diafiltrationm polysaccharide production
Penicillin recovery
Lysozyme ultrafiltration recovery of a water-‐soluble
supposed to be doubled [Bhave et al., 1991]. The main applications can be
roughly divided into (a) concentration of soluble molecules and suspended
solids and (b) clarification by removing suspended solids. As already mentioned,
the main usage of inorganic membranes is in the dairy industry, in which the key
to success has been the invention of transmembrane pressure mode. Some
studies have also been made concerning the usage of inorganic membranes for
the concentration of other vegetable and animal proteins as well as for the
processing of starch and sugar.
55
(a) Clarification of natural fruit juices
Clarification or fruit juice such as apple, cranberry and grape is one of the most
successful and widely practiced industrial applications of ceramic membranes. Ceramic
membrane filtration provides a particularly attractive alternative, replacing such
conventional treatments as gelatin addition, holding/decanting, diatomaceous earth,
cake filtration, and polishing, with a single unit operation. Membranes produce superior
clarity juice and deliver higher yields compared with conventional clarification
processes.
(b) Filtration of sugar cane juice
In the filtration of sugar cane juice, ceramic membranes as carbon/carbon or
alumina/alumina [Jiratananon et al., 1997] can be used in several different stages in the
raw and refined sugar production. One interesting opportunity is in the
microfiltration/ultrafiltration of clarified juice (7–14 Brix) and/or pre-‐evaporated juice
(20-‐25 Brix) as a pretreatment prior to ion exchange or chromatographic separations.
Pretreated and filtered juice is softened, evaporated and purified using ion exchange
and chromatographic processes leading to a better quality refined sugar. Typical
operating conditions include feed temperatures of 90–100oC, a high crossflow velocity
(4–7 m/s) and transmembrane pressures up to 5bar. The need to purchase, use and
dispose of filter aids is eliminated. In some applications, the cost of equipment necessary
to dewater filter-‐aid sludge may be comparable to the cost of the membrane system.
[Sondhi et al., 2005]
(c) Alcoholic beverages
In the area of alcoholic beverages, alumina membranes are used for the
clarification and sterilization of the final products. Composite multilayer ceramic
membranes membranes applications exist for the filtration of wine and beer
[Gillot et al., 1986] as well as for sake and vinegar [Hagasewa et al., 1991]. By
membrane filtration the amount of waste generated and chemicals used during
the production can be considerably decreased. The use of membrane filtration
instead of a conventional process is advantageous also since membrane filtration
produces a sterile product and further sterilization is not necessary. For
Membralox® membranes these applications have existed since 1988 [Gillot et
al., 1990]. In breweries as well as in wineries membrane filtration can be used to
56
replace clarification, stabilization and sterilization steps. The alumina membrane
Membralox® is for example used to further concentrate tank bottoms or
centrifuge concentrates [Gillot et al., 1990].
ii. Environmental applications
The increasing concern for the environment and tighter legislation on emissions are
increasing widely the applications of ceramic membranes. In many chemical process
applications there is a growing need to treat not only the waste streams, to meet the
increasingly stringent environmental regulations, but also to recover and reuse the
chemicals. The nature of these process streams can vary and in some cases the process
may require aggressive operating and/or cleaning conditions. Examples of such
applications include the filtration of chemical solvents, dye and pigment wastewater
from dye processing and colouring plants, and highly variable wastewater containing
detergents, polymers and organic solvents [Sondhi et al., 2007]. Environmental
applications can be roughly divided into wastewaters from various sources, as well as
oily wastes and sludges. In the following part the process industry wastewaters from the
chemical, textile, and pulp and paper industry are discussed as well as wastewaters
from the nuclear industry and sludge from municipal wastewater dehydration.
(a) Petrochemical industry wastewater
The wastewater applications in the petrochemical industry include the treatment of
acidic and alkaline process wastewaters from a vinyl chloride monomer manufacturing
facility. Lahier and Goodboy (1993) have shown that the high content of heavy metals
from acidic wastewater can be removed by using two-‐stage membrane filtration with
alumina microporous membranes. As a pretreatment the pH of the wastewater was
adjusted from 0.7 to 12. In the first filtration stage, the precipitated metal hydroxides
were separated from the water and in the second filtration stage the filtered metals
were concentrated to 17-‐20 w.w%. The water recoveries from the filtrations were 80%
and 99.5%. In Lahier and Goodboy research work, alumina microfiltration membranes
were evaluated for treatment of 3 aqueous streams containing heavy metals, oils, and
solids at petrochemical manufacturing facilities. They have also shown that an alkaline waste stream containing dichloroethane, water, and calcium/iron hydroxides can be
treated with ceramic membranes.
57
(b) Pulp and paper industry
In pulp and paper industry the main interest in studying the possible applications of
ceramic membranes has been in bleach plant effluent treatments. The wastewater sources in paper industry comes from disk filtration, flotation and close loop water
circulation as well as the clarification of coating colour basin. In Laitinen et al. (1998) as
well as in many 90s references [Petterson et al., 1988; Gaddis et al., 1995], alumina
membranes are used for wastewater ultrafiltration for suspended solids (SS), COD, color
and colloidal material removal. Laitinen tried flat sheet α-‐ and γ-‐alumina membranes
and modified α-‐alumina membranes and concluded in quite low COD removal is
achieved (under 22%), iron and odor from color wastewater treated sufficiently and SS
and colloids was almost totally remover. Fouling of membrane seems to be the most
considerable problem for large scale application of the method. Great advantage of
wastewater treatment of pulp industry without using chemical compounds is recycling
feasibility, In some cases [Laitinen et al., 2001] existing results have showed that
ultrafiltration permeate was satisfactory for continuous recycling and the pulp
produced with recycling had better strength properties than when recycling was not
used. Hence, toxic discharges were reduced, waste management became more effective,
and the need for maintenance was reduced.
(c) Municipal wastewater
Ultrafiltration/microfiltration is suitable for tertiary treatment of municipal wastewater
after standard treatment steps. The final product can be reused as the level of pollutants
and microorganisms falls short of environmental legislation limits. In Vera et al. (1997)
a study on the filtration of the secondary clarified suspension is studied using alumina
microfiltration membranes of 0.14μm pore size. The membrane system proved to serve
as a total barrier for suspended solids, total coliform, fecal coliform and fecal
streptococci. This fact together with the reductions of turbidity, COD and phosphorus
removal from secondary treatment made the microfiltered water perfectly adapted to
irrigation. A critical flux of 100 l/(m²h) was achieved at 1bar pressure and 3 m/s
crossflow velocity. Other possible application studied is ceramic membrane
microfiltration of hotel wastewater in order to reuse it in secondary purposes, such as
toilet flushing [Laitinen, 2004]. Nevertheless, Because of the high cost of membrane
installation and performance, the wide scale application seems to be not yet necessary
in this kind of municipal wastewater and activated sludge treatment.
58
(d) Activated sludge
Xing et al. (2001) has studied the use of a bioreactor combined with γ-‐alumina and
zirconia ceramic ultrafiltration membranes for the reclamation of urban wastewater.
According to results, the treated water could be reused directly for municipal purposes,
such as toilet flushing and car washing, and after softening treatment for industrial
purposes such as cooling supply and process water. Removal of ammoniac nitrogen and
suspended solids was product of membrane separation however COD removal efficiency
was attributed more than 90% by bioreactor and no by ceramic membranes.
(e) Nuclear industry
In the nuclear industry, composite alumina ceramic membranes have been studied for
the treatment of radioactive waste streams. AEA Technology in England has done
excessive pilot plant studies with ceramic membranes and the results have shown that,
for a low-‐level radioactive general site waste, good alpha and beta/gamma removal can
be achieved. [Laitinen,2004]. Cadmium adsorption from silica-‐alumina membrane is
simulated by Pacheso et al. (2006) from nuclear and chemical wastewater treatment.
Sol-‐gel structured nanoparticles of silica and alumina are used for multilayer composite
alumina membranes fabrication.
(f) Fluoride removal
Nanofiltration (NF) is an attractive technique for reducing fluoride anions (F-‐)
concentrations to acceptable levels in drinking water, however commercial
membranes show minimal chloride of fluoride selectivity. In laboratory scale,
multilayer organic membranes (polystyrene sulfonate) with porous alumina
supports exhibited Cl, F and Br high selectivity, three times more than the
commercial available membranes [Pagana et al., 2007]. Moreover,
chloride/fluoride selectivity is essentially constant over Cl-‐/F-‐ feed ratios from 1
to 60, so these separations will be viable over a range of conditions [Seong et.al,
2007].
59
(g) Arsenic removal from water resources
Alumina ceramic membranes can be used for As(V) from water resources – as
contaminated water. For such kind of application, γ-‐Al2O3/α-‐Al2O3 can be used.
The concentration of arsenic ions decreased from 1ppm, which was originally to
power in 5ppb. Therefore, the concentration of ions As (V) in output was lower
than the maximum permissible limit set for drinking water [Pagana et al.,
2009].In Laitinen’s doctorate thesis, published in 2004 from Finland Technical
University, from all the membrane separation process studied applied in a wide
range of wastewaters, alumina membranes assumed to be suitable for arsenic
removal from bore well water if flocculation was used as a pretreatment as well
as for the treatment of the stone cutting wastewater. The fluxes achieved in
short-‐term experiments for the bore well water were promising and the
optimization of operation conditions should be considered [Laitinen, 2004].
(h) Arsenic and fluoride removal
In a similar recent study, Wu et al. (2011) highly ordered mesoporous aluminas
and calcium-‐doped aluminas were synthesized. Their fluoride adsorption
characteristics, including adsorption isotherms, adsorption kinetics, the
effect of pH and co-‐existing anions were investigated. These materials
exhibited strong affinity to fluoride ions and extremely high defluoridation
capacities. The highest defluoridation capacity value reached 450 mg/g. These
materials also showed superb arsenic removal ability: 1 g of mesoporous
alumina was able to treat 200 kg of arsenic contaminated water with a pH
value of 7, reducing the concentration of arsenate from 100 ppb to 1 ppb.
60
(i) Chromium and arsenic removal
In laboratory scale, separation processes have been developed for ion Cr (III)
removal from water using γ-‐alumina membranes [Pagana et al., 2000; Sklari et
al., 2005]. In Pagana et al. (2000) is described how two pilotic system have been
developed with concentration of chromium ions. In the conclusion of this study,
the concentration of ions Cr (III) in output is assumed to be significantly lower
than the maximum permissible limit set for drinking water (greek standars). By
the same research group, a combined process of removing ions As (V) and Cr(III)
from simulated wastewater using nanostructruded alumina membranes
provided also sufficient results. The concentrations of arsenic and chromium
ions decreased from 1ppm and 0.5ppm, which was originally to power in 5ppb
and 3ppb, respectively. Therefore, the concentrations of ions As (V) and Cr (III)
in output was lower than the maximum permissible limits set for drinking water.
In the figure below (Figure 15) the pilot system is illustrated [Pagana et al.,
2008].
Figure 15 Flow diagram of the Cr (III) removal process [Pagana et al., 2008]
In this work, asymmetric multilayer porous ceramic membranes are developed.
Composite γ-‐Al2O3 membranes made by sol–gel method are prepared for
chromium and aersinc ions removal from water solutions. As(V) removal is
achieve d by a two stage adsorption – ultrafiltration processes in series.
Moreover, Cr(III ) removal is achieve d by an adsorption–ult rafiltrat ion parallel
61
process (Figure 15) using membrane ultrafiltration in a pilot system (see
characteristics inTable 15 ).
Table 15: Basic characteristics of membrane ultrafiltration process applied in Pagana et al., 2007 pilot system for chromium removal (Pagana et al., 2007)
Chromium removal: basic characteristics of the membrane ultrafiltration process
Feed volume 100%
Final permeate volume 100%
Average permeate flux 60ml min-‐1
Pressure difference 3.105 Nm-‐2
Membrane surface A (A=2πrl) 0.05m2
In general, the adsorption-‐ultrafiltration ion process using ceramic membranes
may offer a low cost effective alternative arsenic and chromium purification
technology basically in terms of membrane stability, applied pressure and
product flux with the additional advantage of being suitable for small local and
decentralized units for point-‐of-‐use. However, further experiments should be
carried out in order to provide more accurate an d widely accepted conclusions
[Pagana et al., 2007].
iii. Biotechnology and Pharmaceutical applications
In general framework, in biotechnology and pharmaceutical applications, the
biocompatibility of the membranes is important [Shackleton et al., 1987]. In such
kind of applications, the main interest has been on the filtration of fermentation
broths. In biotechnology, ceramic membranes have been used for primary
extraction, purification, and concentration of biomass, antibiotics, vitamins,
amino acids, organic acids, enzymes, biopolymers, and biopesticides [Cueille et
al., 1990]. There are also applications in the treatment of vaccines, recombinant
proteins, cell cultures, and monoclonal antibodies as well as in continuous
fermentation.
The alumina membranes have several features that indicate the practical
benefit in analytical and diagnostic separations. They are transparent,
allowing to view the retained materials from either side of the membrane.
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The capillary pore structure allows no lateral diffusion of liquids along the
membrane. Nanoporous alumina membranes are usually prepared
electrochemically in sulfuric or phosphoric acid bath. After preparation they
are used for microfiltration of biological media, such as human red blood cells
and bovine serum albumin [Laitinen, 2004].
(a) Fermentation broths
Alumina membranes are used for fermentation broth clarification at numerous
installations worldwide, successfully competing with other technologies such as
polymeric membranes, vacuum filtration and centrifugation. These systems are
often used for the recovery of valuable antibiotics from dilute solutions. Typical
operating conditions include feed temperatures of 30–60°C, high crossflow
velocity (5–8 m/s) and trans-‐membrane pressures up to 6bar [Laitinen, 2004].
(b) Fungal cells separation
Haarstrick et al. [1990] have studied microfiltration using alumina microporous
membrane in the separation of fungal cells and the purification of the produced
polysaccharide. They concluded that crossflow microfiltration could be an
alternative to conventional separation including purification processes and
suggested that a diafiltration mode should be used in order to avoid the
problems arising from the high viscosity of the broths.
(c) Penicillin recovery
An alumina microfiltration membrane has been used in laboratory scale
[Adikane et al., 2001] to optimize penicillin G recovery from fermentation broth.
Reduction over 40% has been achieved in processing time by optimizing the
crossflow velocity. Moreover, it is showed that the operating fluxes could be
regenerated and a 98% recovery of penicillin G could be obtained over 12 cycles.
63
(d) Lysozyme ultrafiltration
Group of Baudry et al. (2001) has studied the use of modified ceramic
membranes by alumina and zirconium oxide used for lysozyme and lactoferrin
ultrafiltration. It is concluded that as other studies had prouved, adsorbed
protein on the membrane surface was able to highlight UF mechanism. The
enhanced selectivity was observed when the protein to be retained had
additional interactions with the membrane surface. Conrad et al. (1998) studied
microfiltration for the recovery of a water-‐soluble, chiral compound of
therapeutic interest from a bioconversion broth of whole cells and soybean oil. It
is assumed that that it was possible to use microfiltration to harvest an aqueous
product stream from the oil-‐water bioconversion broth.
5.2.2. Gas phase separation
The use of membranes in gas separations has grown at a very rapid pace in
recent times. One particularly interesting application of gas separation with
membranes is the removal of dilute heavy organics from light gas streams such
as the removal of solvents from the exhaust of different process industries
[Javaid et al., 2005]. Gas mixtures can be separated by either dense or porous
membranes. Microporous inorganic membranes have often been modified using
in many cases different types of alumina and studied as possible materials for
achieving solubility based separation.
The dense ceramic membranes are mostly impermeable to all other gases, giving
extremely high selectivity towards oxygen or hydrogen. Oxygen permeation
through a dense ceramic membrane is due to a large number of oxygen vacancies
that are generated by doping and the electron holes produced by the defect
reaction exist in the solid electrolyte. Under a gradient of oxygen partial pressure
imposed on the membrane at a high temperature, the oxide ions are transported
along with holes from the high partial pressure side to the low partial pressure
side. Similarly, when hydrogen is exposed to a mixed proton conducting
membrane, it may be transferred through the membrane under a hydrogen
partial pressure gradient. Again, apart from the membrane bulk diffusion, the
64
surface reactions are also important and need to be taken into consideration for
the hydrogen permeation. In microporous ceramic membranes, the gas
permeation behaviour may be dominated by Knudsen diffusion, surface
diffusion, multilayer diffusion, capillary condensation or molecular sieving (i.e.
configurational diffusion) and is strongly dependent on the pore size and pore
size distribution of the membrane, operating temperature and pressure, and the
nature of the membrane and the permeating molecules [Laitinen, 2004]. Gas
separation using porous ceramic membranes is one of the important research
topics [Wiley, 2007].
Recently, most of the development membrane separation activities have been
focused on gas separations, particularly as ionic conductors for oxygen transport
and as molecular sieve membranes for hydrogen separations. Their use in
environmental applications has been very limited due to cost considerations,
although they offer several unique advantages in this area, such as chemical and
thermal stability and rugged structural stability [Sondhi et al., 2007].
Table 16 : Gas separation by porous alumina membranes [Kaldis et al., 2004]
Pore diameter (Å) Operating temperature
(oC)
Applied pressure
differenece (kPa)
Gas mixture
~100 25-‐75 3.03 -‐ 16.2 He/N2
~100 25-‐75 3.03 -‐ 16.2 He/C2H6
~100 25-‐75 3.03 -‐ 16.2 He/Kr
1020 800 343 H2/H2S
200-‐400 10 152-‐252 H2/H2S
~10 31-‐77 4.04-‐39.4 H2O/air
~10 65-‐70 H2O/CH3OH
~10 78-‐84 H2O/C2H5OH
~10 82-‐88 H2O/(CH3)2CHOH
~30 65-‐70 H2O/CH3OH
~30 78-‐89 H2O/C2H5OH
~30 82-‐88 H2O/(CH3)2CHOH
100-‐200 196-‐197 23.2-‐106 H2/He
100-‐200 196-‐197 23.2-‐106 H2/CO
65
100-‐200 196-‐197 23.2-‐106 He/N2
100-‐200 196-‐197 23.2-‐106 H2/CO2
100-‐200 196-‐197 23.2-‐106 H2/CO
100-‐200 196-‐197 23.2-‐106 He/N2
100-‐200 196-‐197 23.2-‐106 H2/CO2
100-‐200 196-‐197 23.2-‐106 H2/C2H6
100-‐200 196-‐197 23.2-‐106 H2/C3H8
100-‐200 20 H2/He
100-‐200 20 He/N2
100-‐200 20 H2/CO2
300-‐400 65 13.1-‐92.9 235U/238U
Sustainable membranes in high temperatures are strongly needed for energy
production application as biomass burning (organic waste gasification, coal
gasification etc.). Ceramic membranes are appropriate for such kind of
applications because of their high thermal stability and corrosion resistance. In
installations as coal gasification plants, gas separation is needed for emission
treatment after combustion or other processes. Usually, carbon dioxide is the
substance which must be isolated in order to fit environmental requirements.
Membrane technology is one of the promising tools in these applications and
many research works has been realized in recent years.
In the following part, the applications of alumina membranes in gas phase
separation process are gathered in Table 17.
Table 17 : Gas phase separation process of alumina membranes and composite alumina membranes
Porous alumina membrane appications
Gas phase separation applications Industrial/Scientific applications
Carbon dioxide capture
CO2/N2 separation Nox control, gas phase streams from energy plants
H2/CO2 separation gazification, combustion emissions
Hydrocarbons separation
Acetone recovery paint, ink, oil industry
Propane separation natural gas processing, petroleum refining
Catalytic reactors
VOCs oxidation air pollution, VOCs recovery
Methane to ethane reaction gas treatment
66
i. Carbon dioxide capture
There is growing consensus among the scientific community that the rising
atmospheric levels of CO2 as a result of human activities (e.g., fossil fuel burning,
cement production). Membrane processes appear to be an attractive option to
carry out gas separations in terms of their lower environmental impact and
energy costs compared to more conventional separation technologies (e.g.,
distillation, absorption, adsorption, crystallisation). On the guidance of a recent
study recently published by Lito et al. (2011), different adsorption models have
been screened to account for CO2 and N2 adsorption in MFI-‐alumina samples [C.-‐
H. Nicolas et al., 2011].
(a) CO2/N2 separation
In Sang H. Hyun et al., (1995) top layers of γ-‐Al2O3 composite membranes have
been modified for improving the separation factor of CO2 to N2. The separation
factor through the TiO2 supported γ-‐Al2O3membrane was found to be fairly
enhanced by silane coupling, but in case of the α-‐Al2O3 supported membrane was
not. The CO2/N2 separation factor through the modified γ-‐Al2O3 / TiO2 composite
membrane is 1.7 at 90°C. The separation factor is proportional to the CO2
concentration in the gas mixture, and the modified membrane is stable up to
100°C. The main mechanism of the CO2 transport through the modified γ-‐Al2O3
layer is known to be a surface diffusion.
(b) H2/CO2 separation
Coal gasification is called any process of converting coal into gas for many different uses as illuminating, heating etc. In the figure above (
Figure ) an integrated gasification combined cycle is shown (IGCC). In IGCC (a)
air separation and gasification, (b) gas cleaning, (c) gas conditioning and (d) gas
separation are combined for total air treatment after coal gasification are aligned
(Figure 16). Using conventional technology, it is possible to build the IGCC plant
67
above, technologies such as cryogenics, solvent extraction, adsorbents in
pressure or temperature swing adsorption, and low temperature polymeric
membranes. Due to compliance with sorption mechanisms, these technologies
operate best at low temperatures (<50oC). The problem here is that gas
separation follows downstream from gas conditioning. The gasification of coal
predominantly produces syngas (CO and H2) with some remaining
hydrocarbons, CO2 and water. Hence, the syngas requires further processing.
Figure 16: A conventional air treatment process with carbon capture [Diniz et al., 2007]
The syngas requires further processing through the WGS reaction (see equation
below), in order to maximise H2 production.
CO + H2O à CO2 + H2 ΔH =-‐41. 2 kJ.mol-‐1
The WGS reaction is exothermic and the conversion is limited by thermodynamic
equilibrium as the conversion to H2 and CO2 decreases with increasing
temperature. The reaction is therefore carried out in two stages, in high (350-‐
400oC) and low temperature (250-‐300oC) shift reactors with interstage cooling.
In addition, further cooling to <50oC is required to reduce the temperature to the
temperature acceptable by conventional gas separation technologies.
Cooling large stream of hot gases is capital intensive, and incurs a loss of power
production; furthermore, these processes are likely to deliver CO2 at reduced
pressures, which will have to be re-‐compressed to >100atm for transportation
and storage. Hence, conventional processes will attract large energy penalties.
In order to reduce efficiency losses, an alternative is to separate gases at higher
temperatures. In this case, inorganic membranes derived from ceramics, silica,
68
metal and by further doping or alloying showed preferential H2 selectivity over
CO2 at high temperatures (>200oC) [see e.g. recent work of Joe Da Costa in
Australia]. These technologies can also operate in membrane reactor
arrangements for the water gas shift reaction, which allow for shifting the
reactions to higher conversions due to the extraction of hydrogen from the
reaction chamber. The advantage here is two:
1. CO2 is kept at high pressure thus reducing requirements for CO2 compression
downstream.
2. As H2 is selectivity taken, the syngas stream is reduced by up 30-‐35%
depending on the recovery rate.
Although CO2 will have to be cooled down prior to compression, the volumes are
reduced thus requiring a lower cooling duty. Therefore, inorganic membranes
and their incorporation in MRs are foreseen to be the technology of choice for
advanced IGCC plants as depicted in Figure 17.
Figure 17: An Advanced scheme for IGCC with carbon capture [Diniz et al., 2007]
The scheme shown above (Figure 18) for gas separation and conditioning is
being developed in Australia by the University of Queensland as the research
provider under the R&D program directed by the Centre for Low Emission
Technology (www.clet.net). The use of a higher temperature membrane allows
further simplification of the gas cooling, conditioning and separation step, but
this scheme requires a high temperature membrane tolerant to the syngas.
69
Figure 18: Schematic of a multi tube membrane module for H2 and CO2 separation [Diriz et al., 2007]
In Diriz et al. (2007) for high temperature gas separation membrane, commercial
D-‐alumina tubes coated with a top γ-‐alumina layer, dip-‐coated with cobalt and
selective silica layer. The best membrane performance delivered H2 purity in
excess of 98% at 300o C. The positive energy of activation for H2 permeation
coupled with the negative energy of activation for CO2 permeation of metal
doped silica membranes provide a favourable fundamental property for
engineering design of gas separation at higher temperatures (up to 500o C) such
as those required in a coal gasification process. The process integration provided
by Diriz et al. is potentially beneficial for the next generation of high temperature
processing unit operations in low emissions coal gasification.
70
ii. Hydrocarbons separation
(a) Acetone recovery
Huang et al. (1997) modified alumina membranes for recovery of acetone from
nitrogen by reducing the pore size to enhance multilayer diffusion and capillary
condensation transport mechanisms [Huang et al. 1997]. Depending on the
temperature and the feed composition, the separation factor varied from being
less than 10 to as high as 200 is dominant. The modified membranes showed
higher acetone permeability and higher separation factors as compared to
polymeric membranes. However, the performance was strongly influenced by
temperature and feed composition.
(b) Propane separation
Hydrocarbons as propane or butane are usually by-‐products of natural gas
processing and petroleum refining and they are commonly used as a fuel for
engines, heating appliances etc. In Randon et al. (1995) alumina membrane are
modified and gas permeation experiments were conducted using nitrogen and
propane. The modified membrane exhibited significantly high propane
permeance. The modification had made the membrane hydrophobic and
improved the membrane’s solubility-‐based separation characteristics. Other
attempt for alumina membrane modification using different alkyl
trichlorosilanes showed a significant increase in propane/nitrogen selectivity
accompanied, by a loss in permeance, after modification. In comparison to PDMS,
one of the best-‐known polymers for solubility-‐based separations, the hybrid
membranes exhibited equal or greater propane/nitrogen selectivity although at
lower propane permeance [Javaid et al., 2005]. McCarley and Way (2001), in a
similar study, modified 5nm alumina membranes with C18 trichlorosilane. They
conducted both single gas and mixed gas permeation experiments at fixed
transmembrane pressures. The treated membrane showed a significant increase
in ideal selectivity for heavier gases (n-‐butane) over lighter gases (nitrogen and
methane).
71
iii. Catalytic reactors
The application of porous ceramic membranes as catalytic reactors starts in the 1980s.
The driving force for this change was the possibility of integrating reaction and
separation, which had already been achieved in the field of biochemical reaction
engineering using polymeric membranes. These, however, were not applicable at the
temperatures used in most of the processes of interest in the chemical process industry
[Lafarga et al., 1998]. Since the materials used in the manufacture of ceramic
membranes are also commonly used as conventional catalyst supports (as alumina),
there has been a strong interest in the development of membrane reactors by
researchers with previous experience on heterogeneous catalysis, who adapted many of
the preparation and characterization techniques used in this field. The most important
and valuable fact is that membrane itself is the catalyst [Coronas et al., 1999] and it can
be used for a wide range of applications. Catalytic membranes are used to improve
contact efficiency with the objective of attaining higher conversions by decreasing mass
transfer resistances. Membrane reactor with catalytic surface aims to improve the
contact in gas–liquid–solid systems by providing a well-‐defined contact region: the
liquid-‐filled pores of the catalytic zone of the membrane in close proximity to the gas
interphase. This does not require a permselective membrane and avoids the problem of
catalyst recovery that appears in slurry reactors.
Poorly permselective inert porous membranes such as mesoporous alumina,
mesoporous glass etc. can reveal attractive for a number of dehydrogenation
reactions. Infiltrated MFI/alumina composite membranes have been used as O2
distributors but also as barriers for dehydrogenation reactions in MRs [Julbe et
al., 2001].
(a) VOCs removal
Volatile Organic Compounds (VOCs) are among the most common air pollutants
emitted from chemical, petrochemical, and allied industries. VOCs are one of the
main sources of photochemical reaction in the atmosphere leading to various
environmental hazards and on the other hand, these VOCs have good commercial
value. Hence, growing environmental awareness has put up stringent regulations
to control the VOCs emissions. In such circumstances, it becomes mandatory for
72
each VOCs emitting industry or facility to opt for proper VOCs control measures.
There are many techniques available to control VOCs emission (destruction
based and recovery based) with many advantages and limitations [Khan et al.,
2000]. One of these methods for removal and no so often for VOCs recovery is
gas separation with ceramic membranes -‐ zeolithe and alumina membranes in
most of the cases. By the University of Zaragoza in Spain team and other research
groups this concept has already been demonstrated eg. using hydrogenation
reactions over Pt/Al2O3 catalysts in membrane module.
In Figure 19, it is illustrated a flow-‐through membrane in which the permeation
of a premixed feed stream takes place. This was the approach employed in works
by Saracco et al. (1999), who used catalytically modified fly ash filters for alcohol
dehydration and for the reduction of nitrogen oxides with NH3. Pina et al. (1999)
used Pt/Al2O3 and perovskite-‐containing membranes operating in the Knudsen
regime for the purification (by catalytic combustion) of air streams containing
volatile organic compounds (VOCs) in low concentrations. According to this
study, since in the Knudsen diffusion regime the probability of collisions
between the molecules and the wall of the pores is maximised, this type of
membrane would be expected to give high contact efficiency in the reaction of
diluted streams, such as those commonly encountered in VOC removal. The
results of Pina et al. (1999) showed that the membrane could perform very
efficiently in the combustion of VOCs at low temperatures, although at the
expense of a significant pressure drop.
Figure 19: Applications of membrane reactors [Coronas et al., 1999]
73
The industrial application of this type of reactor surely requires optimization of
the membrane structure aimed to reducing the pressure drop. Otherwise, its use
would be restricted to applications involving reaction simultaneous with gas
filtration, where the pressure drop is already present.
(b) Methane to ethane reaction
In Lafarga D. and Santamaria J. projects published in international literature
[Lafarga et al., 1994; Sebastian et al., 2009] a new reactor concept has been
developed and tested for the conversion of methane into ethane, ethylene and
higher hydrocarbons via oxidative coupling. The basic idea consists of providing
a low oxygen concentration in the reactor in order to increase hydrocarbon
selectivity. To this end, oxygen is supplied to the reacting mixture by permeation
through a porous wall as a modified alumina membrane, which is capable to
withstand the temperatures and pressures involved, and at the same time give
adquate values of oxygen flux [Lafarga et al., 1994].
Concluding, despite the plethora of studies appeared in the open and patent
literature focusing on the development of ceramic materials for gas mixture
separations [Koutsonikolas et al., 2010] no many industrial processes have been
commercialized. Although alumina membrane is the most used in large scale,
polymer membranes are still dominating in commercial field of separation
process. The lack of selectivity combined with their high unit cost most often
ascribed to the support, as well as their lack of hydrothermal stability in some
cases, act as key economical and technological barriers for the industrial
implementation.
74
6.SUMMARY AND CONCLUSIONS The term alumina includes large number of chemical compounds: amorphous,
crystalline trihydroxides, monohydroxides, transition aluminas and stable
anhydrous alumina.
Alumina takes attention with good thermal, mechanical and chemical stabilitiy,
as a material for industrial applications. With wider applications of membrane,
alumina is trending topic. Alumina pore size distributions can be controlled
easily modifications on operation conditions from a hundred of nanometers to a
hundred of micrometers.
Besides traditional pathways like sintering and anodic oxidation, alternative
methods -‐ sol-‐gel methods, dip coating, rapid gelation-‐ are introducing with
alumina membrane synthesis. The optimum range that will provide a good
compromise between permeability and strength is being improved also with
additives.
According to literature overview, alumina membranes are applied especially in
the areas where the advantages of ceramic membranes can be exploited. In
biotechnology and pharmaceutical industry their resistance to microbial attack
and biological degradation is really very important and their good thermal and
chemical stability are often the factors that have made them beneficial in
wastewater treatment and gas separation applications. However, they suffer
from poor steam stability, fouling and cracking and because of this alumina is
often mixed with additives such as metals or oxides to improve such kind of
drawbacks [Laitinen, 2004]. Modifications can also be used to increase
selectivity by surface adsorption or molecular sieving effects and a wide range of
research field in ceramic membrane is oriented to this direction.
75
APPENDIX A : Membrane Material Sheet MEMBRANE MATERIAL NAME Alumina 1. INTRINSIC PROPERTIES Nature High Low 1. 1. General physicochemical properties Bulk density at RT kg/m3 1217.406 800.925 1. 2. Mechanical properties Tensile Strength (MPa) 173 a 117 b Bending Strenght Mpa 413 c 307 d Modulus of Elasticity (E) X 108 MPa 26.8 a 21.27 b Compressive Strenght Mpa 3733 e 1600 b Modulus of Ridity(G) X 108 MPa 11.3 a 8.67 b Hardness on the mohs scale 9 1. 3. Thermal properties Melting point OC 2051.0 ± 9.7 Boiling Point OC 3530 ± 200 1. 4. Surface properties Zero point of charge 9.45 f 9 g Water contact angle 73.3 62.6 SBET (m2/g) 484.34 98.3 Pore Diameter (A) 72.23 20.49 Pore Volume (mL/g) 1.27 0.54
1. 5. Chemical properties Operating pH range
Compatibility with solvent 1 phosphoric acid -‐ HF well
Compatibility with solvent 2 HCl-‐ HNO3 weak a polycrystalline alumina 94% e polycristalline alumina 100% b polycrystalline alumina 85% f gamma alumina c polycristalline alumina 99.9% g bohemite d polycristalline alumina 85.5%
76
2. CHARACTERISTICS OF COMMERCIAL MEMBRANES BASED ON THIS MATERIAL Membrane name Memrbalox Membrane manufacturer Pall Membrane shape capillary tube, multichannel, monolithic Membrane structure symmetric Support nature ultrapure µ-‐alumina, zirconia, titania Membrane type porous Average pore size 5μm Molecular weight cut-‐off 1-‐5kg/mol Water permeance at RT 300 l/(m²h bar) Membrane name Anopore Membrane manufacturer Myriad Membrane shape disc Membrane structure symmetric Support nature polypropylene Membrane type porous Average pore size 0.02-‐0.1μm Molecular weight cut-‐off 0,1μm Water permeance at RT 1 m³/m² Membrane name Céram inside Membrane manufacturer TAMI Membrane shape Disc Membrane structure Porous, plate, tubular, multichannel Support nature Alumina/titania/zirconia Membrane type dense, porous Average pore size 0,14-‐1,40μm Molecular weight cut-‐off 1-‐300kg/mol Water permeance at RT 800 -‐1200 l/(m²h bar)
77
APPENDIX B: Chemical Interest Of Alumina
Alkali Metals
Ryshkewitch (1960) stated that molten lithium attracts alumina aggressively,
potassium to lesser degree, and sodium the least. Reed claimed that the
corrosion resistance of sintered alumina to both liquid and vaporized sodium is
good at 900 OC. A commercial sintered alumina article disintegrated within 168
hours at 940 OC, however. A synthetic sapphire cylinder (1/4 in. diameter by 1
in. length lost about 1% in weight in 168 hours ay 900 OC but remained clear.
Kelman, Wilkinson, and Yaggee found that sodium potassium did not attract
below 500 OC, but caused corrosion at 600 OC. Kolosova et al. found a weight loss
of only 0.01 to 0.06% in 35 hours at 400 OC for sintered alumina immersed in
molten alkali (79%K, 21%Na).
The behavior of vaporized alkali metals on oxides and other dielectric materials
has been of interest for thermoelectric converters. Wagner and Coriell (1959)
tested Al2O3, BN, ZrO2, MgO, HfO2, ThO2, CaO, and NbC to exposure to cesium
vapor at temperatures as high as 1475 OC. Only fused alumina remained
unaffected by the test. It was concluded that cesium probably reacts with
impurities in the contacting material, but the effect can ben observed only when
the surface areas available for reaction are relatively large, as is the case with
sintered specimens. Higgings attributed pitting of single crystals by cesium
vapor at 600 OC to silicon. And barium impurities; neutron irradiation increased
the attack. C. E. Addams (1959) observed that rubidium effectively condensed at
high temperatures only on oxides with which it could form stable complex
compounds. Cowan and Stoddard (1964) stated that glasses could not be used in
thermionic converter seals because of alkali metal (cesium) corrosion.
Alkaline Earth Metals
78
Jaeger and Krasemann (1952) observed no reaction of calcium, barium, or
strontium to their boiling points (about 1150 OC). This is likely, since BaO and
SRO can be reduced to the metals (thermite reaction) by metallic aluminum at
1100 OC in a vacuum (Gvelisiani and Pazukhin). Magnesium also shows no
attract to its boiling point (1100 OC).
Aluminum
Aluminum reacts with alumina at 1100 OC to form Al2O, and at 1600 OC to form
AlO (Hoch and Johnston).
Antimony, Arsenic
Jaeger and Krasemann claimed no attract of sintered alumina.
Beryllium
Beryllium (melting point 1500 OC) shows no attract of sintered alumina. A slight
darkening of the Alumina, caused by formation an interfacial layer of
chrysoberyl, BeO ∙ Al2O3 (melting point 1870 OC), and Be ∙ 3Al2O3 (melting point
1910 OC) are identified compounds (Lang, Fillmore, and Maxwell, 1952;
Galakhov, 1957).
Bishmut
The corrosion resistance is good to 1400 OC (Reed). The reaction product is
Bi2O3∙2Al2O3 (Levin and Roth, 1964).
Carbon
Carbon reduces alumina to normal carbide Al4C3 (Prescott and Hinckle) at above
1700 OC (Jaeger and Kresemann), initiating at as low as 1310 OC (Komarek et al.),
79
but requiring about 2400 OC for complete conversion (Kohlmeyer and
Lundquist). Two oxicarbides also exist at about 2030 OC, Al4O4C (Foster, Long
and Hunter; Cox and Pidgeon). The presence of Fe2O3, SiO2, TiO2, and V2O5 as
impurities in sintered alumina might induce deterioration at lower temperatures
(Stroup), as low as 1380 OC (Kroll and Schlechten). The reduction to metallic
aluminum occurs at about 2000 OC (Miller, Foster, and Baker). Graphite does not
wet molten alumina, but severely pitted in contact with it in water-‐free, inert
atmosphere or vacuum (Barlett and Hall).
Cerium
Ceric oxide, CeO, forms no compounds with alumina (Wartenberg and Eckhardt).
Cerous oxide, Ce2O3, forms Ce2O3 ∙ 11 Al2O3 and Ce2O3 ∙ Al2O3, both of which
decompose in air to form Al2O3 and CeO2 at temperatures above 800 OC (Leonov
and Keler).
Chlorine
Chlorine does not attack, except in the presence of carbon (Singer and
Thurnauer).
Chromium
Chromium wets alumina at 1650 OC in a reducing atmosphere (Blackburn,
Shelvin and Lowers).
Cobalt
Cobalt neither wets nor reacts with sintered alumina to above its melting point
(1480 OC) in a reducing atmosphere (Sieverts and Moritz; Tumanov et. al.).
Copper
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Copper reacts with the transition aluminas at 800 OC in air to form CuAl2O4,
which is stable to 1000 OC. It then converts to CuAlO2, which is stable in air to
about 1260 OC (Hahn and de Lorent, 1955; Misra and Chaklader, 1963).
Fluorine
Winzer claimed attack of sintered alumina at 1700 OC by dry fluorine.
Gallium
Sintered alumina is inert to gallium to 1000 OC (Kelman et al.). Wartenberg and
Reusch (1932) found solid solutions above 810 OC with Ga2O3.
Hydrogen
Jaeger and Krasemann observed no reduction of alpha alumina in hydrogen up to
the melting point, only a surface darkening L. J. Trostel, Jr. (1965) noted that
hydrogen attacks alumina refractories below 1600 OC, however in the presence
of water vapor.
Iron
Iron can be melted in sintered alumina under reducing conditions, but wets
about 1600 OC (Blckburn et. al.). The spinel FeO ∙ Al2O3, dissolves to about 6%
alumina at 1750 OC (McIntosh, Rait, and Hay; Fischer and Hoffman).
Muan and Gee (1956), and Muan (1958) found limited solubility for Fe2O3 in
corundum. Richards and White; Atlas and Sumida; and Turnoc and Lindsley
investigated the spinel reactions; Fe2O3 ∙ Al2O3 has a structure similar to kappa
alumina, and requires above 1320 OC prepare.
Lead
81
No reaction occurs with alumina at the melting point of lead (327 OC). In lead-‐
bismuth eutectic alloy (44.5% Pb), Gangler found 0.000 mils/year loss at 1090 OC. Lead aluminate is unstable above 970 OC (Geller and Bunting).
Lithium (See alkali metals)
Manganese
Manganese does not attack sintered alumina to above the melting point (1260 OC) in a reducing atmosphere. Although it is more active than iron, cobalt or
nickel it can be distilled in sintered alumina to give a spectroscopicially pure
product (Sieverts and Moritz).
Mercury
Kelmann et al., and Hahn, Frank, et. al. found no reaction with alumina at 300 OC.
Molybdenum
Alumina is not reduced by molybdenum even above the melting point of
alumina. Discoloration may occur at 2100 OC in a dry, inert atmosphere (He).
Nickel
No attack occurs in dry inert atmosphere Nickel can be melted in sintered
alumina in hydrogen atmosphere (Economos and Kingery). Wetting occurs at
1800 OC.
Niobium
82
Mass spectrometric and thermogravimetric analysis at 1800 to 2200 OC indicates
the principal reaction is
!!!!! + 3 !" → 2 !" ! + 3!"# ! .
Secondary reactions under neutral conditions are:
!!!!! + !" → !!!! ! + !"!! !
!!!!! + 2 !" + 1 2!! → !!!! ! + !"!! + !"# (!)
(Grosmann, 1966).
Nitrogen
Nitrogen does not attack (Jaeger and Krasemann).
Palladium, Platinum
Both metals can be handled in the molten conditions in sintered alumina. (Jaeger
and Krasemann)
Phosphorus
No attack was observed in moderate temperatures (Jaeger).
Silver
No compounds can be prepared with silver and alumina (Hahn, Frank et. al.).
Sulfur, Selenium, Tellurium
These elements do not attack alumina.
Tantalum
Tantalum reacts only slightly with molten alumina in water free inert
atmosphere, H2, N2, CO, or in vacuum (Bartlett and Hall). Alloy 90 Ta-‐10W shows
83
slight reaction. Tantalum carbide and 4 TaC ∙ ZrC react only slightly (Bartlett and
Hall).
Tin
No reaction occurred in molten tin at 1000 OC.
Titanium, Zirconium
No reaction occurred in an inert atmosphere below 1800 OC, at which
temperature black discoloration of the grains and corrosion occurred (Economs
and Kingery). Titanium nitride wets molten alumina with negligible corrosion in
water free inert atmosphere (Barlett and Hall).
Titanium + Aluminum-‐Hardened Nickel-‐Base Alloy
Decker, Rowe and Freeman found that trance amounts of zirconium or boron
picked up from zirconia or magnesia crucibled reduced cracking of the hardened
nickel-‐base alloys during hot-‐working and increased their rupture strength and
ductility. It was desirable to compensate for this effect when making heats in
sintered alumina crucibles.
Tungusten
Jager and Kresemann observed no reaction between tungusten and alumina.
Wallace et. al. (1961) investigated the reactions in a Knudsen cell-‐oven operable
at 2500 OC in conjunction with a Nier-‐type mass spectrometer. Jacodine (1961)
observed a growth of nodules or hillocks of alumina in the investigation of
heater-‐cathode breakdown of alumina-‐coated heaters operated at 1200 OC and
180 volts dc. Tungusten wets molten alumina with negligible corrosion in water-‐
free inert atmosphere (Barlett and Hall).
84
Uranium
Jaeger and Krasemann found no reaction between uranium and sintered alumina
to 1200 OC. Dykstra (1960) found no solid solution between Al2O3 and uranium
oxide.
Vanadium
Burdese obtained reaction between V2O5 and gamma alumina at 500 OC to form
Al2O3 ∙ V2O5.
Zirconium Boride (ZrB2) and Zirconium Carbide (ZrC)
Molten alumina wets and reacts moderately in water free, inert atmosphere
(Barlett and Hall).
85
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