13

CHAPTER 1

Membrane Separation Process

1.6 Introduction

1.7 Membrane Separation technology

1.8 Membrane Structure and morphology

1.4 Membrane processes that separate primarily based on size.

1.5 Membrane processes that separate based on principles other than size.

1.6 Various configurations of operating a filtration process

1.7 Membrane modules

1.8 Membrane characterization

1.9 Fundamental of membrane permeation

1.10 Mechanism of Separation through membranes

1.11 The phenomenon of liquid permeation

1.12 The phenomenon of gas permeation

1.13 Applications of Membrane

1.14 Reference

14

1.1 Introduction :

Membranes are the selective barriers normally used to separate two phases and restrict

the transport of various chemicals in a selective manner. (Fig.1.1). Membranes can be

homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, porous or

non porous. In case of porous membrane pore, size is the key parameters which determine the

effective ness and efficiency of the membrane. It can carry a positive or negative charge or be

neutral or bipolar. Transport through a membrane can be affected by convection or by diffusion

of individual molecules, induced by an electric field or concentration, pressure or temperature

gradient. Normally, separation occurs under a pressure gradient or sometimes under an electrical

potential gradient, associate with or without a catalytic reaction. System which can be separated,

are solid particles suspended in a fluid medium and mixture of two different liquids or gases.

Separation through a membrane is schematically shown in Fig 1.1. Porous membranes are

typically classified according to their pore sizes in the following manner:

Fig . 1.1 : A membrane as a selective barrier between two homogeneous phases [1]

1.2 Membrane Separation technology:

A membrane separation system separates an influent stream into two effluent streams known

as permeate and the concentrate. Permeate is the portion of the fluid that has passed through the

semi-permeable membrane. Whereas the concentrate stream contains the constituents that have been

Permeate (fluid)

Mixed phase feed

Porous

Membrane

15

rejected by the membrane. Membrane separation process enjoys numerous industrial applications

with the following advantages:

1. Appreciable energy savings

2. Environmentally benign

3. Clean technology with operational ease

4. It replaces the conventional processes like filtration, distillation, ion-exchange and

chemical treatment systems.

5. It produces high, quality products.

6. Greater flexibility in designing systems.

1.3 Membrane Structure and morphology: From a structural point of view membranes are broadly divided into two types as shown in Fig.

1.2:

a. Symmetrical, and

b. Asymmetrical (or anisotropic)

Symmetrical membrane has similar structural morphology at all positions within it. An

anisotropic membrane is constituted of two or more structural planes of non-identical

morphologies. From a morphological point of view, membranes can be porous or dense. Porous

membranes have been tiny pores or pore networks within themselves (see Fig. 1.3). On the other

hand, dense membranes do not have any pores and solute or solvent transport through these takes

place by a solubilization mechanism.

1.4 Driving force in membrane separation In order to drive the solutes and solvents through a membrane driving force is necessary. These

include:

1. Transmembrane (hydrostatic) pressure (TMP)

2. Concentration or electrochemical gradient

3. Osmotic pressure

4. Electrical field

5. Partial pressure

16

6. pH gradient

Fig. 1.2: Symmetrical and asymmetric membranes [2]

Fig. 1.3: Porous membranes (2)

1.5 Membrane processes that separate primarily based on size:

Membrane processes are divided into four types based upon the size of component in the feed

solution that is allowed to pass. With some overlap, the categorization, from largest to smallest

permeable species, is microfiltration, ultrafiltration, nanofiltration and reverse osmosis (see Fig.

1.4). The different applications are listed in Table 1.1. A fifth type of size based membrane

17

separation process called dialysis allows solutes similar those in reverse osmosis to pass through.

However, unlike reverse osmosis, which is a pressure driven process, dialysis is a concentration

gradient driven process.

Size, Microns 0.001 0.01 0.1 1 10

Microfiltration

Ultrafiltration

Nanofiltration

Reverse Osmosis

Salts

Virus PP Proteins

Flour Bacteria

Fig.1.4: “Size based” fractionation processes [3]

1.5.1 Microfiltration (MF)

Micro filtration (MF) is the process of removing particles or biological entities in the

0.025 µm to 10.0µm range from fluids by passage through a microporous medium such as a

membrane filter. Transmembrane pressures ranging from 1 to 50 psi are used as the driving

force. If the pore sizes of the membrane are smaller than the particles in the solution, surface

filtration results. Although micron-sized particles can be removed by use of non-membrane or

depth materials such as those found in fibrous media, only a membrane filter having a precisely

defined pore size can ensure quantitative retention. Membrane filters can be used for final

filtration or prefiltration, whereas a depth filter is generally used in clarifying applications where

quantitative retention is not required or as a prefilter to prolong the life of a downstream

membrane. Membrane and depth filters offer certain advantages and limitations. They can

complement each other when used together in a microfiltration process system or fabricated

Human Hair

Metal Ions

18

device. The retention boundary defined by a membrane filter can also be used as an analytical

tool to validate the integrity and efficiency of a system. For example, in addition to clarifying or

sterilizing filtration, fluids containing bacteria can be filtered to trap the microorganisms on the

membrane surface for subsequent culture and analysis. Microfiltration can also be used in sample

preparation to remove intact cells and some cell debris from the lysate. Membrane pore size cut-

offs used for these types of separation are typically in the range of 0.05 µm to 1.0 µm. [3]

1.5.2 Ultrafiltration (UF)

Ultrafiltration (UF) is the process of separating extremely small particles and dissolved

molecules from fluids. The primary basis for separation is the molecular size, although in all

filtration applications, the permeability of a filter medium can be affected by the chemical,

molecular or electrostatic properties for the sample. Ultra filtration can only separate molecules,

which differ by at least an order of magnitude in size. Molecules of similar size cannot be

separated by ultra filtration. Normal transmembrane pressure ranges from 10 to 100 psi. The

product can be the permeate, the retentate, or both Materials ranging in size from 1K to 1000K

molecular weights (MW) are retained by certain ultrafiltration membranes, while salts and water

will pass through. Colloidal and particulate matter can also be retained. Ultrafiltration

membranes can be used both to purify material passing through the filter and also to collect

material retained by the filter. Materials significantly smaller than the pore size rating pass

through the filter and can be dehydrogenated, clarified and separated from high molecular weight

contaminants. Materials larger than the pore size rating are retained by the filter and can be

concentrated or separated from low molecular weight contaminants. Ultrafiltration is typically

used to separate proteins from buffer components for buffer exchange, desalting, or

concentration. Ultrafilters are also ideal for removal or exchange of sugars, non-aqueous

solvents, the separation of free from protein-bound ligands, the removal of materials of low

molecular weight, or the rapid change of ionic and/or pH environment (see Figure 1.1).

Depending on the protein to be retained, the most frequently used membranes have a nominal

molecular weight limit (NMWL) of 3 kDa to 100 kDa. Ultrafiltration is far gentler to solutes

than processes such as precipitation. UF is more efficient because it can simultaneously

concentrate and desalt solutes. It does not require a phase change, which often denatures labile

species, and UF can be performed either at room temperature or in a cold room [3,4].

19

1.5.3 Nanofiltration (NF)

Nanofiltration is a liquid separation membrane technology positioned between reverse

osmosis (RO) and ultrafiltration. While RO can remove the smallest of solute molecules, in the

range of 0.0001 micron in diameter and smaller, nanofiltration (NF) removes molecules in the

range 0.001 micron. NF refers to a membrane process that rejects solutes approximately 1

nanometer (10 angstroms) in size with molecular weights above 200. Because they feature pore

sizes larger than RO membranes, NF membranes remove organic compounds and selected salts

at the lower pressure than RO system. It separates at the molecular level, removing all suspended

solids and most dissolved solids. Transmembrane pressures range from 40 to 200 psi (2.0 kg/cm2

to 14 kg/cm2. NF essentially is a lower-pressure version of RO where the purity of product water

is not as critical as with pharmaceutical grade water, or the level of dissolved solids to be

removed is less than what typically is encountered in brackish water or seawater. [3, 4]

1.5.4 Reverse Osmosis (RO)

It is used to remove dissolved solids from solvents. By applying transmembrane pressure

to concentrated solutions, it is possible to force the solvent through the RO membrane towards

the lower concentration. Hence the terms reverse osmosis. It is a separation process of small

(monovalent) ions and molecules (M < 300 Da) on so called “dense” membranes. The range of

the sizes of molecules that are separated with RO is 1–10 Å. The range of the transmembrane

pressure applied is 10–100 bars depending upon the concentration difference of the separated

species on both sides of the solution. Reverse osmosis (RO) is increasingly used in chemical,

textile, petrochemical, electrochemical, food, paper and tanning industries, as well as in the

treatment of tap water and wastewaters. Reverse osmosis is mainly used for water purification,

including ultrapure water production, desalination, water treatment, wastewater treatment and

landfill leachates treatment. After RO purification, pure water may be easily recovered from

wastewater and subsequently reused in various production steps. RO is used for industrial

effluents treatment, for water reuse, and for concentration of valuable products, for TDS and

COD removal from wastewater [2,3,5].

20

1.6 Membrane processes that separate based on principles other than size 1.6.1 Pervaporation (PV)

It is a process to separate a volatile or low-boiling-point liquid from a non-volatile liquid.

The driving force is a vacuum on the gaseous side of the membrane. It is a tool for separation of

liquid mixtures, especially dehydration of liquid hydrocarbons.

1.6.2 Dialysis

It is a membrane separation process in which one or more dissolved species flow across a

selective barrier in response to a difference in concentration. It is the earliest membrane based

molecular process to be developed. The mode of transport is diffusion, and separation occurs

because small molecules diffuse more rapidly than larger ones, and also because the degree to

which the membrane restricts the transport of molecules usually increases with solute size.

1.6.3 Electrodialysis (ED)

It is an electrochemical process used to separate charged particles from an aqueous

solution or from other neutral solutes. A stack of membranes is used, half of them passing

positively charged particles and rejecting negatively charged ones; the other half doing the

opposite. An electrical potential is imposed across the membranes, and a solution with charged

particles is pumped through the system. Positively charged particles migrate toward the negative

electrode, but are stopped by a positive-particle-rejecting membrane. Negatively charged

particles migrate in the opposite direction with similar results. Both types migrate in opposite

directions out of one set of cells and collect in the remaining cells. The result is a concentrated

solution of both positively and negatively charged particles in every other cell and a low

concentration (the product) in the remaining cells [2,3,5].

1.7 Various configurations of operating a filtration process:

1.7.1 Dead-end Filtration

The most basic form of filtration is dead-end filtration. The complete feed flow is forced

through the membrane, and the filtered matter is accumulated at the surface of the membrane.

The dead-end filtration is a batch process as the accumulated matter in the filter decreases the

21

filtration capacity, due to clogging. A next process step to remove the accumulated matter is

required. Dead-end filtration can be a very useful technique for concentrating compounds.

Table 1.1: Represents the characteristics of membranes used in different membrane separation processes, process driving forces and applications of such processes [3] Process Membrane Type

and Pore Radius Membrane Material

Process Diving Force

Applications

Microfiltration Symmetric microporous, 0.1-10 microns

Cellulose nitrate or acetate, Polyvinylidene difluoride (PVDF), Polyamides, Polysulfone, PTFE, Metal Oxides etc.

Hydro-static pressure difference at approx. 10-500 kPa

Sterile filtration, Clarification

Ultrafiltration Asymmetric microporous, 1-10 nm

Polysulfone, Polypropylene, Nylon 6, PTFE, PVC, Acrylic Copolymer

Hydrostatic pressure difference at approx. 0.1-1.0 Mpa

Separation of macromolecular solutions

Reverse Osmosis Asymmetric skin-type, 0.5-1.5 nm

Polymers, Cellulosic acetate, Aromatic Polyamide

Hydrostatic pressure difference at approx. 2-10 Mpa

Separation of salts and microsolutes from solutions

Electrodialysis Cation and anion exchange membrane

Sulfonated cross-linked polystyrene

Electrical potential gradient

Desalting of ionic solutions

Gas Separation Asymmetric homogeneous polymer

Polymers & copolymers

Hydrostatic pressure and concentration gradients

Separation of gas mixtures

Pervaporation Asymmetric homogenous polymer (A non-porous membrane)

Polyacrylonitrile, Polymers

Vapour pressure gradient

Separation of azeotropic mixtures

Nanofiltration Thin-film membranes

Cellulosic Acetate and Aromatic Polyamide

9.3-15.9 bar Removal of hardness and desalting

22

a) Co-current flow

b) Completely mixed flow

c) Counter current flow

d) Cross flow

.

e) Dead end flow

Fig. 1.5 : Types of ideal continuous flow used in membrane based separation [4]

R

P

F

S

R F

P

R

R F

S

P

P

P

F

F

23

1.7.2 Cross-flow Filtration

With cross-flow filtration, a constant turbulent flow along the membrane surface prevents

the accumulation of matter in the membrane surface. The membranes used in this process are

commonly tubes with a membrane layer on the inside wall of the tube. The feed flow through the

membrane tube has an elevated pressure as the driving force for the filtration process and a high-

flow speed to create turbulent conditions. The process is referred to as "cross-flow", because the

feed flow and filtration flow direction have a 90 degrees angle. Cross-flow filtration is an

excellent way to filter liquids with a high concentration of filterable matter.

1.7.3 Hybrid-flow Filtration

The hybrid flow process combines the dead-end and the cross-flow principle. As in the

cross-flow filtration tubular membranes are with the filtration layer on the inside wall are used.

The filtration process has two phases: the production phase and the flushing phase. During the

production phase, the tubes are closed at one side, and a dead-end filtration is performed. During

the flushing phase, the tube is open to both sides and the fraction that did not pass through the

membranes is removed in order to clean the membrane surface as in cross-flow filtration. This

filtration technique is especially suitable for treating water streams containing suspended solids in

low concentrations (polishing).

1.7.4 Submerged Filtration

With submerged membrane filtration, the membranes are submerged in the liquid that has

to be filtered. The filtration is performed from the outside to the inside of the membrane

(filtering layer is on the outer side of the tube or plate). Sheer forces along the membrane surface

are created by a flow of air bubbles on the surface. In some cases, the airflow also results in a

liquid flow created by the airlift principle. The driving force is a vacuum applied on the inner

side of the membrane [2,3,5].

1.8 Membrane modules

A key characteristic of membrane processes is the membrane geometry in the actual

equipment, which provides the membrane housing and the desired hydrodynamic conditions.

24

Membrane processes can be classified upon the basis of the flow pattern within the membrane

module. These are shown in Fig 1.5

Membrane modules can be of different types (see Table1.2 for comparison). Some of these are:

1. Stirred cell module

2. Flat sheet tangential flow (TF) module

3. Tubular membrane module

4. Spiral wound membrane module

5. Hollow fibre membrane module

1.8.1 Stirred cell modules

Stirred cell modules are useful for small scale and research applications. These are more

commonly used for UF and MF. Stirred cell modules provide uniform transmembrane pressure

and hydrodynamic conditions at all points of the membrane surface. The effects of process

parameters on process efficiency can be very easily determined using stirred cell units. These are

therefore very useful for small-scale process development work. However, these are not of much

use in intermediate and large-scale operations.

Fig.1.6 Stirred Cell module (9)

25

1.8.2 Flat sheet tangential flow (TF) module

The flat sheet TF design is similar to that of a plate and frame filter press. It consists of

alternate layers of membranes, support screens (corrugated structural sheets) and distribution

chambers for feed and permeates. The units may have square, rectangular or oval cross-section.

These units can be easily disassembled for cleaning and for replacement of defective membranes.

Other advantages include the ability to accommodate low levels of suspended solids and viscous

fluids. Disadvantages include relatively low packing density. TF designs are commonly used for

UF, MF and NF. Electrodialysis and electrochemical membranes use only this configuration.

The liquid flow pattern in a flat sheet TF unit can be quite complex. The pressure drop

distribution is also quite difficult to characterize. Consequently, design and analysis of such

devices are largely based on empirical methods.

Fig 1.7 Schematic Flat sheet tangential flow (TF) module [4]

26

1.8.3 Spiral wound membrane module

The spiral wound membrane module is like a huge envelope made of membrane and

containing a feed spacer. Feed flowing around the envelope at high pressure goes across the

membrane and is collected inside the envelope. Permeate flows inside the envelope into the roll

and then runs out the end of the module. The feed and permeate flow patterns are usually quite

complex. Design and analysis is therefore largely empirical. Advantages include high packing

density and relatively low cost. Disadvantages include problems handling suspended solids,

difficulty in cleaning and, in high-temperature applications (plastic components may deform).

Another major problem is that these devices cannot usually handle high transmembrane pressure.

Fig. 1.8 Schematic of Spiral wound module [4]

1.8.4 Tubular membrane module

The tubular membrane module has a cylindrical geometry with the wall acting as the

membrane. The tubes are generally more than 3 mm in diameter. Normally, a tubular membrane

module is made up of several tubes arranged as in a shell and tube type heat exchanger. The feed

stream enters the lumen of the tubes, the permeate passes through the walls and is collected in

the shell while the retentate passes out the other end of the tubes. Advantages include turbulent

flow (providing good membrane/solution contact and removing retentate film build-up),

27

relatively easy cleaning, easy handling of suspended solids and viscous fluids and ability to

replace or plug a failed tube while the rest of the system runs. The feed flow pattern is easy to

characterize and therefore design and analysis based on fluid dynamic principles is possible.

Tubular membrane modules can handle reasonably high transmembrane pressure. Disadvantages

include high capital cost, low packing density, high pumping costs, and limited achievable

concentrations.

Fig. 1.9: Schematic drawing illustrating the tubular membrane module [4]

1.8.5 Hollow fibre membrane module

The hollow fibre membrane module is similar in design to the tubular membrane module

except in terms of scale and number of tubes. The tubes or rather fibres are typically 0.25 to 2.5

mm in diameter. A hollow fibre membrane module usually consists of a bundle of several

hundred fibres. The hollow fibre membranes are spun separately, bundled, and potted into

permeate

feed

concentrate

porous tube

membrane

permeatefeed solution

28

cartridge housings. The fibres may be bundled together into either a U-shape or have a straight-

through configuration. Tube bundles are inside a pressure vessel and feed material normally

flows inside the tubes. Fibres in the straight-through design are somewhat larger and allow low

levels of suspended solids. The finer strands in the U-tube cannot tolerate suspended solids. U-

tubes tend to be used for reverse osmosis, and the straight-through design for ultrafiltration.

Advantages include low pumping power, very high packing density; cleaning can be

accomplished with back-flushing, and ability to achieve high concentrations in the retentate.

Disadvantages include the fragility of the fibres, inability to handle suspended solids well,

difficult cleaning and, in the straight-through design, damage of one fibre requires replacement

of the entire module [2,3,5,7].

Fig. 1.10 Schematic drawing illustrating the construction of a hollow fiber module [4]

epoxy resin

hollow fiber

permeate

feed solution

concentrate

shel l tube

29

Table 1.2: Comparison of different membrane modules

Type Fluid flow regime

Membrane area/module volume

Mass transfer coefficient

Hold-up volume

Special remarks

TF flat sheet

Laminar- turbulent

Low Low to moderate

Moderate Can be dismantled and cleaned easily.

Spiral wound

Laminar Moderate Low Low High pressures cannot be used.

Hollow fibre

Laminar-turbulent

High Low to moderate

Low Susceptible to fibre blocking.

Tubular Turbulent Low Moderate to high

Moderate to high

Flow easy to characterize. Excellently suited for basic membrane studies.

1.9 Membrane characterization

The performance of a membrane process depends on the properties of the membrane. Thus

membrane characterization is an important exercise form membrane developers and membrane

users. Some of these include:

1. Mechanical strength e.g. tensile strength, bursting pressure

2. Chemical resistance e.g. pH range, compatibility with solvents

3. Permeability to different species e.g. pure water permeability, gas permeability

4. Average porosity and pore size distribution

5. Sieving properties e.g. Nominal molecular weight cut-off

6. Electrical properties e.g. membrane zeta potential

30

1.10 Fundamental of membrane permeation: When two multi component mixtures containing atleast one fluid are separated by a permeable

membranous barrier (e.g. a solid porous film) such that the partial molar free energy of one or

more components common to both the mixtures differs across the barrier, there will generally be

a selective transfer of components through the barrier in the derection of the declining chemical

potential for each. A few different situations may arise: (i) If the upstream and down stream

fluids are gas or vapour mixtures at differing hydrostatic pressures, for example, components

pass through the barrier in the direction of declining partial pressure by a process termed as “gas

permeation”; (ii) If the upstream fluid is liquid mixture, and the downstream fluid is a gas in

which the partial pressure of the permeating component is lower than the vapour pressure over

the component of the upstream liquid, transmembrane permeation occurs by a process usually

termed as “pervaporation”. (iii) If the liquid phases in contact of the membrane are in the same

hydrostatic pressure, component can transfer across the barrier under the action of concentration

difference between the contacting liquids by the process of “dialysis”. (iv) If a liquid mixture is

confined at a higher hydrostatic pressure from one side of the barrier compared to that of the

other, certain component of the upstream liquid will permeate through the membrane by the

process of “ultrafiltration or reverse osmosis”[2,3,5,7].

All these membrane transport process have the common features of (a) transport the mass across

the membrane under a specific driving force e.g. pressure difference, concentration diference,

applied e.m.f. etc. (b) the capacity to alter mixture composition by virtue of the ability of the

membrane barrier to pass one component more rapidly than another despite equality of driving

potential. It is this unique characteristic of membrane separation which differentiates the process

from the common separation techniques. Various driving forces involved in different separation

processes (including membrane separation) and their useful size ranges are presented in Fig. 1.10

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Fig. 1.11 : Different membrane separation process relative to size [8]

1.11 Mechanism of Separation through membranes

In a membrane separation process involving a mixture of gas or liquid and solids there are five

separation mechanisms which can be described from micro porous membrane.

i) Geometric exclusion

ii) Donan Exclusion

iii) Adsorption / Difusion

iv) Knudsen diffusion

v) Capillary condensation

vi) Surface Flow

32

1.11.1 Geometric exclusion (Sieving)

Generally, geometric exclusion mechanism takes place on the basis of the size effect. The

pore size of the membrane should be smaller than the particle size which is to be separated. Most

of the membrane based on the geometric exclusion principle. This is a very similar phenomenon

which occurs in the normal filters. However, pore sizes in this case, are much finer than that in

common filters. Obviously it increases the complexity of the process. Pore size of reverse

osmosis or gas separation membrane is less than 1 nm and ultrafiltration has pore size in the

range of 2 – 100nm, microfiltration membrane has pore size in the range of 100 – 5000nm and

the membrane with pore size greater than 5000nm are used as particulate filter. As the pore size

becomes finer, the problem of clogging and fouling is much of serious nature than in the

common filter and since the mass transport takes place through much finer constrictions, fluid

dynamic also become complex in nature. The pore surface also plays an important role in such

membranes. Some of the specific situations with reference to the liquid and gas separation

membrane are discussed later.

1.11.2 Donan Exclusion

The separation of charged particle takes place by the Donana exclusion process. In this

process soluble ions are separated from a solution. One familiar example of such process is

dialysis. For example, when we dialyse a colloidal electrolyte e.g. sodium salt of dye acid against

the pure water, then sodium ion penetration through the membrane, where is the colloidal –

dimensioned dye stuff anion is retained. However , it is not possible for a large amount of

sodium ion to be separated from dye stuff anion as this would result in an excess positive ion at

one side of the membrane and an excess negative ion on the other side. This would be against the

rule of electrochemistry. To retain the electrical neutrality, water molecule must be splited in a

quantity equal to the amount of sodium ion that passes through the membrane. The OH- ion will

migrate with Na+ ion and equivalent number of H+ ions associating with dye anion. The result is

high concentration dye acid in one compartment and that of sodium hydro-oxide I the other. This

phe omenon is known as membrane hydrolysis and is a special case of Donnan equilibrium and

the process is called Donnan exclusion.

33

1.11.3 Adsorption / Diffusion

In case of adsorption / diffusion mechanism, firstly molecules are absorbed on the surface

of the membrane so that interfacial fluid layer is enriched in one of the constituents of the bulk

solution. A continuous removal of this interfacial layer by flow under pressure through the

capillary of te membranes results in a product solution whose composition is different from the

bulk solution. In this capillary flow mechanism it is very important that the thickness of the

sorbed layer and the capillary diameter bear a critical relationship, Dp = 2t where Dp is the

capillary diameter and t is the absorbed layer thickness. If the capillary diameter is much larger

than the sorbed layer, separation can not occur efficiently. Thus depending on the layer thickness

and the pore size, adsorbed molecules are separated from the non absorbed component. The

schematic representation of the process is illustrated in Fig 1.5.

++ t ++ + ++ ∆P ++ + ++++ ++++++ + ++ + +++++++ ++ +++ t ++++ Dp < 2t Dp = 2t Dp>2t Fig. 1.12 An example of diffusion of adsorbed particles (o) through a membrane of varying pore (Dp) sizes. Effective separation for Dp < 2p (2) 1.11.4 Knudsen diffusion

For gases, pure Kudsen diffusion takes place if the mean free path of molecules is much

larger than the average pore size (50 to 100 times). In such cases total diffusion is determined by

Kundsen diffusion. Separation efficiency is proportional to the ratio of the square root of the

mass of the molecule as mean free path is related to the mass of the molecules. High separation

efficiency is obtained from mixture of gases like H2, He with heavy gases like CO2, SO2 etc. A

combined transport by Knuundsen diffusion and adsorption / diffusion is possible for gases [1-

5,7].

DP

34

1.11.5 Capillary condensation

Preferential sorption-capillary flow mechanism was the fundamental approach for the

practical development of reverse osmosis process for converting sea water into potable water. If

the surface of the membrane in contact with the solution is of such a chemical nature that it has

preferential sorption for water or preferential repulsion for solutes then a multimolecular layer of

preferentially sorbed pure water could exist at the membrane surface. Continuous removal of this

interfacial water can then be effected by letting it flow under pressure through membrane

capillaries.

Capillary condensation can occur according to the Kelvin equiation [3-5].

4γV RgT ln (p / p0 ) = ─ ------------ cos φ (1) Dp Where p is the critical vapour pressure at pore diameter Dp

P0 is the saturated vapour pressure at a temperature T

Rg is the gas constant

The γ is the surface energy of the condensed vapour

V is the molar volume of the condensed vapour

Φ is the angle of contact between the liquid qnd pore wall.

Separation occur in such cases by the difference between the vapour pressure of the

component in the mixture. For example in Fig.1.7 one of the gas in the mixture condensed at

high pressure side (p2). And then the condensd liquid evaporated through the membrane to the

lower gas pressure side (p1). Because of the difference in vapour pressure the other component

does not condense and the separation efficiency increases [5].

35

Fig. 1.13 Schematic representation of vapour condensation and transport through a Microsporus membrane (2). 1.11.6 Surface Flow

Permeation rates of gaseous molecules through the membranes may be enhanced in some

cases by surface diffusion mechanism. In surface diffusion, gas molecules adsorb to the surface

of the pores of the membranes and then diffuse along the pore walls. High selectivity can be

achieved in cases where preferential adsorption of one of the components occurs. Surface flow is

the result of solution diffusion mechanism and is used to describe the preferential permeation

rate of some gaseous components through membranes from a given feed mixture and is similar

to preferential sorption capillary flow mechanism described previously for liquid feed systems.

Separation of condensable gases and vapours from noncondensible permanent gases through

membranes is often explained by surface flow. Such a separation is carried out at a suitable

temperature and pressure near or below the critical temperature of the gas. At high pressures,

multilayer adsorption of gas molecules occurs at the pore walls reducing the size of the pores. At

a particular pressure, the entire pore volume is filled with the permeating gas and the gas

molecules are said to have condensed inside the membrane pores, effectively blocking the

passage of permanent gases. Such a phenomenon is known as capillary condensation [8- 12]

PpP p2<p1

p2

p1

membrane

36

1.12 The phenomenon of liquid permeation: The principle of separation by ultrafiltration and microfiltration, is based on the sieving

concept. The mean diameter of the pore for ultrafiltration membrane is in the range 20 A° to

1000 A° and the range is 0.1 µm to 10 µm. for the microfiltration membrane.

For membrane separation, the permeate flow rate is directly proportional to the

permeability of the membrane which in turn is related to the membrane pore diameter., the

transmembrane pressure differential and the effective membrane area. With reference eto a cross

flow membrane module, the transmembrane pressure differential (∆p) is schematically

represented by Fig.

For many MF application with membrane pore diameters in the range 0.2 – 1 µm and all UF

applications where pore diameters are typically in the range 40 – 500 A°, it can be shown that the

permeate flow through the membrane is laminar (13).

PERMEATE

Recirculative Loop

Fig.1.14 Simplified schematic of cross flow a membrane filtration module.

1 2

3

P + P Δp = - P2

(2)

Where P1 = tube side (feed) inlet pressure

P2 = tube side (retenate) pressure

P3 = permeate outlet pressure

RETENATE

Cross Flow Membrane

P1 P3 P2

FEED

37

The flow of liquid through the pore in a filter can be analysed by considering the

laminar flw through a capillary. The flow through such a system is described by the Hagen –

Poiseuille equation (14).

4π r g V pQ = 8 η l

(3)

where, Q is the flow rate of the fluid

r is the radius of a capillary

g is the acceleration due to gravity,

Δp is the pressure drop over the cylinder

η is the viscosity of the fluid

l is thwe length of each capillary.

If the process in a filter are considerd to be uniform and cylindrical, the porosity e of

the filter havig an over all surface area A is related to the number of cylindrical pores (N) in this

area and the pore radius r by the following equation :

e A = N π r2 (4)

For this system equation ( 4) becomes

2Nπr gVpQ =

8ηl (5)

Combination of equations ( 4 ) and ( 5) yields

2er g pQ = 8ηl

(6)

38

The so called Kozeny – Carman equation which can be derived from equation (6),

describes laminar flow through less uniform porous media (13).

3

2 20

V 1 t A 1 (1 )t v

pek k S e

(7)

Here A is the external surface area of the porous medium exposed to the high or

low pressure side of the permeant, l is the thickness of the porous medium, e is its volume

fraction composed of pores and Sv is the internal surface area of the porous medium per unit

volume of the non porous medium. The product of k0 kt is define as K, which is the so called

Kozeny – Carman constant. The numerical value of K depends on the shape of the pores and the

tortuosity of the membrane. If the porous medium is composed of a regular packing of more or

less isomatrically shaped particles, then K is close to 5 (15). So it can be concluded that the

permeability of a porous material of certain pore size and porosity can give an idea about the

structure of the porous medium [13].

Shimizu et al. [16] calculate the pore radius and tortuosity of the membrane from the

flux through the membrane and the rates of capillary rise of several solvents. Tortuosity values

by this method agreed well with that obtained by measurement of ionic conductivity. However,

pore radius obtained by this method was 1.17 times larger than that obtained by Hg intrusion

method. Pores in the membrane are irregular in shape because the membrane were prepared by

packing particles where as by Hg intrusion methods measured pores are considered to be

cylindrical.

The permeate flux through a membrane increases with Δp upto a point which is

generally termed as threshold pressure. Beyond the threshold value he flux becomes independent

of Δp. This phenomenon is often known as concentration polarosation [17,18] Below the

threshold pressure range, the Kozeny – Carman equation shows a linear relationship with the

tranmembrane pressure drop and flow rate is inversely proportional to the viscosity of the

permeating fluid and also the thickness of the membrane. Thus for a given size of the membrane

the permeability will reduce for a higher viscous fluid at a constant temperature and membrane

thickness.

39

Such membrane based separation devices have to satisfy certain performance criteria

to justify the cost effectiveness of the process. There are (1) Flux of filtration rate (2) long term

flux stability and (3) separation and rejection properties of membrane [19].

Flux stability is perhaps the most critical factor determining the efficiency of the

separation process and also its economic viability. In MF and UF applications, flux decay can be

a serious problem [18,20]. Flux decay is usually a direct result of an increase in the hydraulic

resistance of the membrane due to fouling i.e. excessive accumulation of debris / particulates at

he membrane surface or in the pores. Such clogging of the membrane can be prevented by

adding certain flocculating agents which absorb or flocculate the particles which are responsible

for clogging [21].

Fouling can be somewhat minimize by cross flow filtration. Due to continuous

removal of filtrate / permeate through the membrane, the cross flow velocity is somewhat higher.

An increase in cross flow velocity generally is due to higher shear rate, he removal of

particulates at the membrane surface is more effective [22].

In the filtration process, interaction takes place between the membrane surface and

the suspended particle. When a mixture is brought in contact with membrane surface by any

driving force, some molecule permeate through the membrane while the other are retained. This

leads to an accumulation of the retained components and a polarosation layer appear at the solid

liquid interface with a higher concentration of the retanate near the membrane surface. During

filtration process, a steady state is reached where the transport of the solutes towards the

membrane surface is counter balance bya diffusive flow of the rejected and accumulated

materials from the membrane surface back in to the bulk solution. Therefore a constant

concentration profile of the rejected materials is obtained in the boundry layer. Concentration of

the solute at the surface (Cw) is always higher than I the bulk solution (CB). However, interaction

phenomenon of the solution or the solute with materials of the pore wall, is called the

concentration – polarosation of the solute in the vicinity of the separating layer. The

concentration profile of a solute I a polarosation surface is schematically describe in Fig.

The membrane resistance to the flow can be described by the following equation

which is derived from the Hagen – Poiseuille equation [14].

40

m

1 pR = = Q J

(8)

where, Rm is membrane resistance

Q is permeability

Δp is transmembrane pressure

J is permeate flux

One of the most important parameter determining membrane rejection coefficient is

the mean pore diameter of UF and MF membrane having narrow pore size distribution. In

general smaller the pore diameter, higher is the rejection co-efficient to particulate / solutes of

greater normal size.

Membrane Concentration Boundry Layer

Cw

:

Flux

Cn

Distance

Fig. 1.15: Schematic representation of concentration polarization and the concentration profile during membrane filtration [22]

41

1.13 The phenomenon of gas permeation :

For non sorbable gases the flow of gas through the media is governed by the Knundsen

and Poiseuille equation. For a homogeneous media the total permeability Fₒ (mol / m² -sec-Pa)

i.e. given by [24,25].

Fₒ = Fₒk + F1op . p (9)

Where Fₒk is the permeability due to Knundsen diffusion and F1op the contribution due to laminar

kok

2εμ rvF = 3RTL

(10)

2

1 µ r8

popF

RTL

(11)

Where ε is the porosity, r is the modal pore radius of the medium, µk and µp shape

factor, which are both, in general, assumed to be equal to the reciprocal tortuosity of the medium,

R the gas contant, T the temperature, p the mean pressure, η the viscosity of the gas and v the

molecular velocity expressed as

128 R Tv = ( )

η M (12)

Where M is the molecular mass of the gas.

The relative contribution of laminar flow to the total flow is determined by the ratio of

the mean free path of the gas molecules to the modal pore size. In case of small pore, at low

pressure and high temperatures, Knundsen diffusion is the predominant transport mechanisim.

Pure Knundsen diffusion appear if the mean free path of the molecules is much larger

than the average pore size of the membrane [26]. In this case the total diffusion is determined by

Knundsen diffusion according to equation described below:

42

AAD KA

dCN DdX

(13)

where NAD is the molar diffusive flux of component A

CA is the molar concentration of component

DKA is the Knundsen diffusion co-efficient and can be expressed as

12

KA PA

8RT1 2-fD = D 3 ηM f

(14)

Where MA is the molecular mass of component A

R is the gas constant

T is the temperature

f is the fraction of molecules with diffuse reflection on the pore wall.

Separation of gases is possible by Knundsen diffusion process. The separation efficiency

is proportional to the ratio of the square root of the mass of the molecule. High efficiency factor

can be obtained only with light gases in mixture with heavier gases.

43

1.14 Applications of Membrane

The membrane processes are used for separation of chemical substances and are of immense

interest for a wide range of commercial applications. The analytical applications of membranes

are grossly over shadowed by the industrial applications of membranes. A complete and

comprehensive discussion of all membrane based applications would be very exhaustive and is

beyond the scope of this unit. However, a few major large scale applications of membranes will

be discussed here.

1.14.1 Desalination and Water Treatment

Desalination of sea water (containing approximately 35000 to 40 000 milligrams of dissolved

salts per litre) and brackish waters (containing approximately 5000 to 10000 milligram of

dissolved salts per litre) to produce potable water is one of the widely known industrial

applications of membrane processes. Reverse osmosis processes to a large extent has been

successfully used for the last two decades in many parts of the world. In addition to desalination,

reverse osmosis process is also used for the treatment of municipal waste waters and effluents

from the various Separation chemical industries.

1.14.2 Protein Recovery

The most prominent use of ultrafiltration process is in the food industry, where it is used ,for

example to recover proteins from cheese whey, to concentrate milk before cheese making and for

fruit juice clarification.Other important applications of UF process is in the removal of colour ,

odour and bacteria from the surface waters for drinking water needs. The pore size of most of the

UF membranes used for such applications are such that they remove virtually all the

microorganisms present in the water and viruses to a large extent.

1.14.3 Production of Table Salt

Electrdialysis was first developed for the desalination of saline waters, in particular brackish

water. The production of potable water is currently the most important industrial application of

44

electrodialysis. One significant feature of electrodialysis is that the salts can be concentrated to

comparatively high values (in excess of 18 to 20 %) . The production of table salt from sea water

by the use of electrodialysis to concentrate sodium chloride up to 200 grams per litre prior to

evaporation is a technique developed extensively and used in Japan.

1.14.4 Hemodialysis

The most important application of dialysis process is in the artificial purification of blood using a

dialysis membrane and this process is called hemodialysis. In hemodialysis, the impure blood

from the patients flows across a dialysis membrane and a physiological saline solution flows

along the other side of membrane. This process replaces kidney function in three principal areas,

namely, removal of waste metabolites, removal of excess body water and restoration of acid-base

and electrolyte balances. The waste metabolites include urea, uric acid, the end product of

protein metabolism and creatinine, the end product of muscle metabolism.

1.14.5 Ion Selective Membrane Electrode

The analytical applications of membranes are largely in the area of ion selective membrane

electrodes and particulate analysis. For most of the ion selective membrane electrodes, the role of

membranes is not to transport specific ions but to selectively adsorb on either side giving rise to

measurable electrical potential difference. A particular membrane permits only a particular kind

of ion to penetrate and adsorb. Ion selective membrane electrodes have many applications in

water analysis and environmental monitoring.

1.14.6 Specific Gas Probes

Membranes are also used in specific gas probes for measuring dissolved gases and gas phase

partial pressures. The specific gas analysis is based on the electrochemical oxidation or reduction

of the gas at the appropriate electrode of an electrochemical cell giving rise to a current whose

magnitude depends on the concentration of the gas consumed in the electrochemical reaction.

The whole detection system is protected by the membrane which is permeable to the particular

gas component of interest. The membrane can be polytetrafluoro ethylene, polyethylene,

polypropylene, nylon or cellophane. The membranes typically have pore sizes of several

45

microns. Portable devices for specific gas analysis are available for detection of gaseous oxygen,

hydrogen sulphide, carbon dioxide, sulphur dioxide, nitrogen dioxide and hydrogen cyanide.

1.14.7 Detection and Analysis of Particulate Contamination

Microfiltration membranes offer a general way of removing particulate material from fluid

streams and are routinely used in a range of analytical procedures to determine particulate

contamination in gases and liquids. In general, these methods involve passing a representative

sample of liquid or gas through a suitable membrane. All particulate matter which exceeds the

membrane pore sizes are retained on the surface where the contaminants may then be analysed.

1.14.8 Microbiological Analysis

Microporous membranes are important for the detection of microorganisms in foods, beverages

pharmaceutical products and potable water sources. The technique involves the filtration of the

samples through a microfiltration membranes to trap the microorganism, then culturing the

microorganism on the membrane and then counting the grown colonies [27].

46

1.15 Reference :

1. Keizer K and Burggraaf A. J., Science of ceramics, vol 14 etd. by Dereck Taylor(1991).

2. Ghosh R K, Membrane separation processes, J Chem. Engg, McMaster,page 3(2005-06).

3. Porter C. Mark, Handbook of Industrial Membrane Technology (1990)

4. Sirkar K K, Membrane Handbook, Van Nostrand Reinhold, New York(1992).

5. Coulson and Richardson’s Chemical Engineering Volume 2 (4th edition), J.M. Coulson

etal. Butterworth-Heinemann, Oxford(1991).

6. Keith Scott, Handbook of Industrial Membranes, (1995) Elsevier Advanced Technology,

Oxford,

7. Keizer K., Leenaars A.F.M.and Burgraff. Ceramic in Advance Energy Technologies,

Edited by H. Krockel, M. Merz, O Van der Biest, (Proceeding of European Colloquium,

20 – 22 Sept. 1982,, Petten, NetharlandD. Radial Publishing Co. (1984)..

8. Balakrishnan M., Agarwal G.P. and Coony C.P., Membrane Sci. 3, 85, 112(1993).

9. Van Halle E., Gaseous Diffusion for Aerodynamic Separation of gases and isotop, Van

Carman Instt. For Fluid Dyanamics, 8th Lecturer Series(1978).

10. Aeaeda M, L., Du D. and Usijima M., Proceedings 4th Int. Drying Sym., Edited by R.

Toel A.S. mazumdar, 2, 472(1984).

11. Aeaeda M, L., Du D. and M. Fuji, J. Chem. Eng., Jpn, 19, 84(1986).

12. Aeaeda M and L., Du D., J. Chem. Eng., Jpn, 19, 72(1986).

13. Leenaars A.F.M. and Burggraaf A.J, J Membrane Sci. 24, 245(1985).

14. McCabe W L and Smith J C, Unit operation of Chem. Engineering, 3rd editon, (1976).

15. Carman P C, Trans. Inst. Chem. Eng. 15, 150(1987).

16. Porter M. C. in Microfiltration in Bungay et al. (eds), Synthetic Membrane : Science,

Engineering and publication, Reidel, Dordrecht, 225(1986).

17. Lee S., Aurelle Y and H., Roques J Membrane Sci., 19, 23(1984).

18. Hsish H.P., Bhave R.R. and Fleming H.L., J Membrane Sci., 39,221(1998).

19. Cheriyan M and Kuo K. P., J Dairy Sci. 67, 1406 (1984).

20. Paul Hurly and High J, J Technol, 7(8), 21, (1987),.

21. L.Cot, British ceramic Proceedings, edt by B.C.Steele and D.P.Thomson, no. 43,

December p 111(1988).

22. Paulson D. J., Wilson R. L.and D. D. Spatz, J Food Technol, 38, 77(1984).

47

23. Tamon H, Kyotani S, Wade H, Okazaki M and Toei R, J Chem. Eng. Jpn, 14, 136(1981).

24. Rowell R. L., Carrano S. A., Bethune A. J. de and Malinauskas A. P. , J Colloid

Interface sci., 37, 242(1971).

25. Keizer K, Ulhorn R.J.R., Van Varun R. J. and Burggraaf A. J., J Membrane Sci39,

285(1988).

26. Korngold E., in “ Synthetic Membrane Processes” Edt. By George Delfort, Academic

Press. Inc. (1984).