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CHAPTER 1
INTRODUCTION
1.1 Overview
Desalination technologies are of great importance in much water scarcity around the world.
The brackish water and sea water desalination is recognized as viable alternative for potable
water production due to localized scarcity and quality deterioration of the water sources. The
ground water also have high salt concentrations in different area the world. The increasing
demand of fresh water around the world leads to the desalination of water. However,
desalination practices have been challenged by increasingly stringent product water quality
standards, as knowledge on the occurrence and subsequent environmental and human healthimpact of natural and anthropogenic compounds such as boron expands.
Various water desalination technologies have been used to effectively remove the salts from
salty water, producing a stream of water with minimum concentration of salt. The most
common methods are distillation (thermal) and membrane process. In recent years the
membrane process such as reverse osmosis and membrane distillation have become more
attractive drinking water process as compared to conventional processes. Among these
various membrane desalination processes that is membrane distillation and reverse osmosis
processes have been recognized as greater potential for drinking water production from salty
ground water, seawater and brackish water (Mohammadi and Safavi, 2009). The reverse
osmosis is a membrane separation process which the salty water is pressurized through
membrane at a high pressure and the salt is retained by the membrane. This reverse osmosis
process requires high amount of energy for creating a high up to 60 bar during the
desalination of water.
1.2 Removal by membrane distillation and other processes
The membrane distillation (MD) is a combination of thermal and membrane processes where
driving force for desalination is the difference in vapour pressure of water across the
membrane. The process is thermally driven transport of water vapour through a porous
hydrophobic membrane. The one side of the membrane barrier is always in contact with the
feed water (feed side) and other side (permeate side) is may be in contact either with an
aqueous solution gives a configuration called direct contact membrane distillation (DCMD),
with a sweeping gas, this is process is termed as sweeping gas membrane distillation
(SGMD), with a gap plus cold plate called air gap membrane distillation (AGMD) or with a
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vacuum, the process in this case called vacuum membrane distillation (VMD) (Mengual,
Khayet and Godino, 2004). Since it operates on the principle of vapour-liquid equilibrium,
100% (theoretical) of ions, colloids, macromolecules and non-volatile components are
rejected while reverse osmosis can only reach a desalting efficiency of 96% to 98%
(Mohammadi and Safavi, 2009).
The membranes used in MD are hydrophobic and micro-porous which allow the water
vapour (volatile component), not the liquid, to pass through it. The membranes are made up
of the polymers such as Polyvinyldenefluoride (PVDF), Polytetrafluoroethylene (PTFE),
polyethylene (PE) and polypropylene (PP) (Alklaibi and Lior, 2004). Some main advantages
of membrane distillation are: complete separation (in theory) of ions, macromolecules
colloids etc., it produces high quality of distillate, Low grade heat (solar, industrial waste heat
or desalination waste heat) can be used for the heating the feed and water can be distilled at
low temperatures (Mohammadi and Safavi, 2009).
1.3 About Boron
With the rapid growth of desalination by these technologies, the issue of boron removal came
in under scientific spotlight. Boron is generally found in natural water in the form of boric
acid and ionic form as borate salts. The concentration of boron in ground water is depend
upon the surrounding geological characteristics (Ferreira, Moraes and Alves, 2006) and its
presence in surface water is due to discharge of treated sewage effluents. Boron concentration
in the surface water and ground water including wastewater ranges between 5-100 mg/l
(Ozturket al., 2008) while seawater boron concentration ranges from 0.5 to 9.6 mg/l (Ali et
al., 2012).
World health organization (WHO) have established regulatory guidelines for boron drinking
water, where maximum permissible limit is 0.3 mg/l in 1993 (Melniket al., 1999) which is
revised to a maximum permissible limit of 0.5 mg/l in 1998 (WHO, 1998). Beside this,
different countries have set their own guidelines. For example,BIS limit of boron in drinking
water is 0.5 mg/l (BIS IS 10500, 2009), Europe set the limit at 1.0 mg/l, Singapore set at 1.5
mg/l and Canada has this limit at maximum 5.0 mg/l (Ali et al., 2012). While the 4thedition
of the guidelines for drinking water quality published by the WHO in 2011, the maximum
permissible limit of the boron was set at a concentration 2.4 mg/l (WHO, 2011) from the 0.5
mg/l which was set earlier edition of the guidelines this was done lack toxicity data on
humans but this permissible limit have many harmful effects on the plants cucumber, potato,
onion garlic, carrot, wheat , sunflower, cherry etc. which are very sensitive to the boron level
above the 2.0 mg/l (Hilal, et al.,2011) which results in premature ripening of fruits, yellowing
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of leaves, spots on the fruits and lower yields (Peinemann, and Nunes, 2010). Hence some
countries and water treatment utilities still maintains their maximum permissible limit at 0.5
mg/l for agricultural purposes also (Ali et al., 2012).
Therefore, the boron concentration of 0.5 mg/l can be easily achievable by the use of vacuum
membrane distillation. The mass transfer through the membrane in vacuum membrane
distillation can be increase by the application vacuum in the permeate side in which the main
advantage is very low conductive heat loss with a reduced mass transfer resistance and this
process allows higher partial pressure gradients and hence high flux (Lawson and Lloyd,
1997).
1.4 Vacuum membrane distillation
In the VMD process, the evaporation occurs at feed side, and the never interfere with the
selectivity associated with the vapour- liquid equilibrium which occurs in pervaporation. In
this process actually the downstream pressure is dropped down to equilibrium vapour
pressure hence the convective mass transfer occurs and due to low pressure in the permeate
side, molecular mean path of the permeate (gas) is much larger than the pore diameter of the
membrane as a result mass transfer dominated by the Knudsen mechanism (Bourawi and
Khayet, 2006; Gil and Jonsson, 2003).
1.5 Objective of this study
Aim of this study to produce potable water, using vacuum membrane distillation to remove
boron, which is recently considered as contaminant in the drinking and irrigation water. This
study was focused on the performance of VMD which was evaluated by the response
parameters like permeate flux, percentage removal of boron.
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CHAPTER 2
LITERATURE REVIEW
2.1.Past studies on boron removal2.1.1 Boron removal by reverse osmosis
With the development of the cellulose acetate membrane, the large scale application of the
reverse osmosis systems have been installed at different places around the world. Since the
simplicity and flexibility, the reverse osmosis systems have been extensively used for water
desalination from 1970s (Hunt and Song, 2009). A procedure developed by Taniguchi et al
(2001 and 2004) to estimate boron in the RO permeates in relation to measured salt
permeability. That analysis was based on the concentration polarization model (Kimura,
1995). The RO process is one of the most used treatment option for the water desalination.
Despite the capacity of RO to remove ionic species to 98%, the RO process is not been very
effective process for the removal of boron (Hyung and Kim, 2006). The boron rejection by
the eight RO plant in Japan with various design options have reported that the rejection
ranges between 43 to 78% (Magara et al, 2004). The full scale study of the single pass RO
shows removal of maximum 80% at neutral pH (Tanguchi et al., 2001). A Study on new
generation seawater reverses-osmosis (SWRO) membranes and found that boron rejection on
Asian seawater desalination could achieve about 95%. However they concluded that SWRO
followed by brackish water reverse osmosis (BWRO) at greater pH (Tanguchi et al., 2004).
Study of investigating the effects of RO operating parameters on boron rejection via
numerical analysis as noted that boron removal could be improved theoretically by lowering
the operating temperature, increasing the applied pressure and raising pH of RO feed (Sagiv
and Semiat, 2004). The poor boron removal at neutral pH is due to uncharged boric acid
diffused through the membrane, forming hydrogen bridges with the active groups of
membranes while at higher pH, they suggested that borate ions were hydrated by dipolar
water molecules that lead to an increased molecular size which in turn enhanced the rejection
by RO membrane.(Pastor et al.,2001).
Ludwig (2004) analysed the hybrid systems in seawater desalination with different aspects of
power plant design, RO plant configuration, resource conservation, environmental impacts,
and water quality and product capacity. Prats et al., (2000) investigated the effects of pH and
recovery rate on boron removal by various RO membranes. The study was conducted using a
7.2 m3
/d plant with brackish water reverse osmosis (BWRO) membranes from Hydranautics
and Toray. Boron removal was 4060% at pH 5.58.5 and it increased to >94% at pH 10.5
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and when permeate recovery was increased from 10 to 40%, boron removal improved from
44% to 59%. That is, 4 times higher in recovery could only increase boron rejection by 1.5
2 times. While stretching the permeate recovery to 40% might be workable only for short-
term purpose.
While membrane manufacturers normally indicate nominal rejection of 85 90% in their
membrane specification sheets, actual rejections in commercial systems typically fall within
the range of 78 80%. For advanced SWRO, nominal and actual rejections could be
estimated at 92 94% and 8587%, respectively. However, pilot tests in their study could
obtain only 82 85% boron removal under field operating conditions (Glueckstern et al.,
2003). Thus, it is necessary to consider a safely margin for boron removal in designing a
desalination system. If time and budget are permitted, a study with a testing period of about 6
months in the field should always be conducted before finalizing the design of a large-scale
desalination plant. System installation at a place with high energy cost should also consider
the merit of incorporating ion exchange process for boron removal and to achieve maximum
water production rate at the expense of a slight increase in product salinity. However, ion
exchange process is not environmentally friendly as it requires the use of significant amount
of chemicals to regenerate the exhausted resins. Boron-selective resin would not improve the
product salinity, too. Sustainability of operating a RO system at very high pH is still a
questionable debate for most membrane practitioners.
2.1.2 Boron removal by ion exchange and adsorption
In Ion exchange methods of removal, resins leads to the impression that boron-selective
resins work on chelating of boron through a covalent attachment and formation of an internal
coordination complex and these resins are classified as macro-porous cross-linked poly-
styrenic resins, functionalized with Nmethyl-D-glucamine (NMG). While fixed bed ion
exchange systems are still more practical, there are studies on using resin in suspension
followed by micro- or ultra-filtration. These arrangements are referred to as adsorption-
membrane filtration (AMF) process (Kabay et al., 2010). Their advantages are the better
sorbent capacity and lower power consumption.
Boron removal increased with higher salt concentration for RO membranes but decreased
with higher salt concentration for NF membranes (Sarpet al., 2008). Removal of trace
elements by membrane could be affected by electrolytes, pH and conductivity of the solution
and solute permeability decreased with increased pH and decreased conductivity (Yoon et al.,
2005).
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The boron removal by RO membrane using polyol as the complex-forming compounds to
enhance boron removal as the use of mannitol at molar ratios of 510 (approximately 500
1000 mg/L of mannitol to remove 5 mg/L of boron) to achieve better boron removal (Geffen
et al., 2006).
Membrane surface protein also affects the mass transfer coefficients of water, vapour and
solute as membrane with increasing hydrophobic property enhance the mass transfer
coefficient of the water (Zhao et al., 2005).Physical properties and thermodynamic
parameters of solution could also affect mass transfer in RO membranes. Smaller ions with
larger hydrated radii would be rejected at a higher rate (Ghiuet al., 2003). Borate ion at
higher pH also possess larger hydrated radii and this could account for the observation that
borate ion could be retained easier than boric acid by membrane.Regarding zeta potential,
one of the studies put focus on the impact of different cations and humic acid on membrane
surface potential and hence on membrane fouling (Elimelech and Childress, 1996).
Bryjaket al., (2008) studied the removal of boron from seawater using adsorption membrane,
a hybrid process. They used boron-selective ion exchange resin for adsorption and a
microfiltration membrane. They found that it would take 30 minutes contact time to reduce
boron from 10 mg/L to 2 mg/L. When they take initial boron concentration of 2 mg/L, it took
approximately 3 minutes to bring boron down to less than detection limit. However, the use
of 1 g/L resin in the suspension requires a higher operating pressure for microfiltration
membrane. In addition, resins used in continuous suspension and turbulence may become
powder and shorten the life span. Organic fouling was another detrimental impact on ion
exchange resin for wastewater application.
Okay et al.,(1985) investigated and evaluated the two methods viz. ion exchange and
adsorption using a high concentration of boron 100-500mg/l from a mine drainage in Turkey.
The removed about 85% of boron using magnesium oxide and they found temperature as a
significant affecting factor and the optimum temperature was 400C.the contact time was also
a factor and tine of contact was 2 hour for removal of boron for more than 85%.According to
Choi and Chen (1979) boron removal by MgO adsorption could be lower at lower initial
concentration.
Liu et al., (2009) explored the boron adsorption by composite magnetic particles. They used
the pure Fe3O4 and composite magnetic particles derived from Fe3O4 and
bis(trimethoxysilylpropyl)-amine (TSPA). Adsorption of boron was about 50% better with
magnetic particles TSPA and adsorption was better at pH 2.26.0 than that at pH 11.7. They
also found that adsorption of boron on fly ash decreased at higher ionic strength, similar to
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that reported in other studies on adsorption process.Adsorption process takes place on both
boric acid and borate by either hydrogen bonding, electrostatic and hydrophobic attractions
depending on solution pH (Liu et al.,(2009). Boron adsorption was reported to be highest at
neutral pH and lowest at alkaline pH, possibly due to electrostatic repulsion. Their illustration
of adsorption on iron particle might be one of the reasons for enhanced boron (Qin et al.,
2005).
The technique of boron removal by the ion exchange method in relation to ionic strength and
pH of the solution as boron was removed by resin; adsorption of other ions also took place at
negligible amount when the feed water salinity was more than 5 milliequivalent or being
gasified with carbon dioxide at 0.74 bar (Simonnotet al., 2000). Boron-selective resin
IRA743 of Rohm and Haas used by Simonnotet al.,(2000) could adsorb boron as well as
other ions and thus it would be necessary to elute the exhausted resin with caustic for
regeneration. Since ion exchange resins are sensitive to impurities present in the water, this
method is normally suitable only for boron removal of relatively clean water to produce
ultrapure water (UPW). Hydrodynamics is not favourable for small column due to poor
distribution too. Other limitation is the need to handle substantial amount of regeneration
chemicals for final disposal. Reuse of acid for regeneration was tested and reported to be
possible. However, there was no indicative data in their study for the amount of acid which
could be saved. Besides, the process was not authorized as drinking water process in France
at the time when the study was conducted.
Melnik et al., (1999) studied the boron behaviour and removal by electrodialysis. Their study
used different types of ion exchange membranes to determine the optimum electrodialysis
conditions for removing boron from seawater and ground water. It was noted that 0.3 0.5
mg/L boron in dialysate was obtained at a pH range of 2 8 using homogeneous ion
exchange membrane when the feed boron is 4.5 mg/L. The study pointed out that a minimum
NaCl concentration of 0.2 g/L must be maintained to efficiently operate electrodialysis.
Nadav (1999) introduced the effect of boron in water on agricultural products. He noted that
deficiency in boron could result in poor budding, excessive branching and retarded growth. In
contrast, a high boron level may cause boron poisoning, yellowish spots on the leaves,
accelerated decay and plant expiration.
2.1.3 Boron removal by electrodialysis
Melnik et al., (1999) studied the removal of boron and behaviour by electrodialysis. The
study used various types of ion exchange membranes find out electrodialysis conditions for
removing the boron from seawater and ground water. They noticed that 0.3 0.5 mg/L boron
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in dialysate at a pH range of 2 8 using homogeneous ion exchange membrane when the
feed boron concentration is 4.5 mg/L. Their study says that a minimum NaCl concentration of
0.2 g/L must be maintained to efficiently operate electrodialysis. By adding anionite in
desalination chamber, applied voltage was reduced and energy consumption was cut down by
30%. When the feed boron concentration was 40 mg/L in a sample of seawater at Kamchatka
in Russia, boron in dialysate was 27 mg/L, which corresponds to a removal efficiency of only
32%. No reason was given for the low rejection when feed boron concentration was high. It
might be due to long contact time of fluid with ion exchange membrane causing more boron
transport into the dialysate. Since electrodialysis process is an energy intensive method, the
study tried to find the optimum pH for different type of membrane pairs. The optimal values
were reported to be pH 2 8 and >10 for homogeneous and heterogeneous types,
respectively. However, there was no explanation or suggestion to further improve efficiency
at different desalination capacities. When conventional electrodialysis would be terminated at
a minimal salt concentration of 1 g/L, they managed to set-up the arrangement of ion
exchange membranes to operate the system until salinity went down to 0.2 g/L
2.1.4 Boron removal by membrane distillation
Hou et al.,(2010) studied the boron removal by direct contact membrane distillation. Their
results indicated that boron removal is less dependent on pH and salt concentration by
membrane distillation process. When the system was operated at a temperature gradient of 30
0C between feed and permeate streams at pH 3 11, boron removal was reported to be stable
at >99%. Boron removal efficiency was also found to be stable at a temperature gradient of
up to 600C. This observation should be verified as higher temperature could theoretically
encourage diffusion and hamper the rejection. They also reported that boron removal in
membrane distillation process was not sensitive to salt types with a concentration of up to
5000 mg/L. This result is more comprehensive since water permeation occurs through
membrane as evaporation process. Unless waste heat is available, membrane distillation
process will require substantial amount of energy to raise the temperature of feed solution to
maintain a temperature gradient between feed stream and stripping stream
2.2.Membrane distillation (historical prospective)Membrane distillation is relatively new process that is being investigated worldwide as low
cost, energy saving alternative to the conventional separation processes such distillation and
reverse osmosis (Lawson and Llyed, 1997).Membrane distillation (MD) is a thermally driven
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process, in which only vapour molecules are transported through poroushydrophobic
membranes (Bourawi et al,2006). The benefits of MD from other more used methods are;
100% theoretical rejection of ions, macromolecules, colloids and other non-volatile
components, lower operating temperature than conventional distillation, lower pressure
requirement the conventional methods and low chemical reaction with the membrane and
solutions (Lawson and Llyed, 1997). Membrane distillation was first conceived as a process
that would "operate with a minimum external energy requirement and a minimum
expenditure of capital and land for the plant (Weyl, 1967).The large vapour space required by
conventional distillation column is replaced in MD by the pore volume of a microporous
membrane. Conventional distillation relies on high vapour velocities to provide intimate
vapour-liquid contact, MD employs a hydrophobic microporous membrane to support a
vapour-liquid interface (Lawson and Llyed, 1997).
The feed temperature used in membrane distillation is generally rages between 60 to 900C
but may be low as 300C. Therefore low grade waste energy, solar or geo thermal energy van
be used to heat the feed to required temperature (Morrison et al, 1992). The advantage of
lower operating temperatures made MD more attractive in the food industry where
concentration of various fruit juices can be prepared with better colour and flavour (Calabro,
1994).
The MD is found to be safer and more efficient than RO for removing the non-volatile and
ionic compounds from the water. Since MD is a thermally driven process, operating pressures
are generally on the order of zero to a few hundred kPa, relatively low compared to pressure
driven processes such as RO. Lower operating pressures translate to lower equipment costs
and increased process safety. An- other benefit of MD stems from its efficiency in terms of
solute rejection. Since MD operates on the principles of vapour-liquid equilibrium, 100%
(theoretical) of ions, macromolecules, colloids, cells, and other non-volatile constituents are
rejected; pressure-driven processes such as RO, UF, and MF have not been shown to achieve
such high levels of rejection (Lawson and Llyed, 1997).
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S.No. Author / Year Title Component Parameter study Types of
Membrane
1. A.M. Urtiaga,G. Ruiz,I. Ortiz
2000
Kinetic analysisof the vacuummembrane
distillation ofchloroform fromaqueous solutions
Chloroform (i) initial chloroform concentrationin the feed (2122012mg/l),(ii) feed flow rate in the laminar flow
regime (0.230.98 l/min) and in thetransition to the turbulentflow regime (2.78 l/min),(iii) Temperature (547.5C) and vacuum pressure in thepermeate side (7 and 14mm Hg).
Micro porouspolypropylenehollow fiber
membranes
2. A.M. Urtiaga,E.D. Gorri, G.Ruiz, I. Ortiz2001
Parallelism anddifferences ofpervaporation andvacuummembrane
distillation in the
removal of VOCsfromaqueousstreams
Chloroform The separation of chloroform fromaqueous solutions in the range ofconcentrations 200
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M.P. Godino2004
vacuummembranedistillation
capillary membrane module and theheat transfer coefficients have beenevaluated in both the lumen and theshell side of the membrane module
module
8. MohamedKhayetTakeshiMatsuura2004
Pervaporation andVacuumMembraneDistillationProcesses:Modelling andExperiments
chloroformwatermixtures
In the VMD process, a more generaltheoretical model that considers theporesize distribution, the solutiondiffusion contribution throughnonporous membrane portion, and thegas transport mechanisms throughmembrane pores was developed basedon the kinetic theory of gases. Thecontribution of each mechanism wasanalyzed
Polyvinylidenefluoride(PVDF) flat-sheetmembranes
9. B. Wu,W.K. Teo2004
Preparation and
application ofPVDF hollow
fiber membranesfor TCA removalfrom aqueous
solutions byvacuummembranedistillation
volatile
organiccompounds
The effects of various operational
parameters in VMD such asdownstream pressure, feed solution
temperature, feed flow rate and feedconcentration on the permeation fluxof water/VOC, VOC removal
efficiency and mass transfercoefficient were investigated anddiscussed.
Asymmetric
micro porouspolyvinylidene
fluoride(PVDF)hollow fiber
membranes
10. Rico Bagger-Jorgensen,Anne S.Meyer,Camilla
Varming,GunnarJonsson
2004
Recovery of
volatile aromacompounds fromblack currantjuiceby vacuum
membranedistillation
volatile
aromacompoundsfrom blackcurrantjuice
The recovery of seven characteristic
black currant aroma compounds byvacuum membrane distillation (VMD)carried out at low temperatures (1045C) and at varying feed flow rates (100
500 l/h) in a lab scale membranedistillation set up
Micro porous
hydro-phobicmembrane
11. LIU Zuohua,LIU Renlong ,DU Jun,TAOChangyuan,LI Xiaohong2004
Removal ofaqueous phenolcompound byvacuummembranedistillation
Phenolcompound
It is found that the optimal feedtemperature for PVDF membrane is 50
; and for PTFE membrane, 60 .The pH value of the feed has littleinfluence on the membrane fluxes andion rejection ratios, while it influencedconsiderably on the selectivity.
Micro porousmembranes ofpolyvinylidenefluoride(PVDF) andploy-tetra-fluoro-ethylene(PTFE) withnominalaverage poresizes 0.22 mand 0.20 m,respectively.
12. Baoan Li,Kamalesh K.Sirkar2005
Novel membraneand device forvacuummembrane
distillation-baseddesalination
Waterdesalination
A large number of rectangular moduleshaving the hot brine in cross flow overthe outside of the fibers and vacuumon the fiber bore side have been
investigated for their VMDperformances to hot brine (1% NaCl)
poroushydrophobicpolypropylenehollow
fibers
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process over a brine temperature range of 6090C. Studies were carried out with hotwater as well; further hot water flow inthe tube side and vacuum in the shell
side were also implemented.
13. T.MohammadiM.Akbarabadi2005
Separation ofethylene glycolsolution byvacuummembranedistillation
Ethyleneglycol
The membrane almost completelyrejects ethylene glycol and the desiredconcentration was achieved
Pollypropylenemembrane
14. F. Banat,S. Al-Asheh,
M. Qtaishat2005
Treatment ofwaters colored
with methyleneblue dye by
vacuummembrane
distillation
Methyleneblue dye
The concentration of MB dye withinthe feed reservoir was monitored over
time. The impact of operatingvariables such as feed temperature,
flow rate and initial dye concentrationwas investigated.
A mathematical model incorporatingtemperature and concentrationpolarization effects was developed and
validated on the experimental data.
Pollypropylene
membrane
15. S. Al-Asheha,F. Banat,M. Qtaishat,M. Al-Khateeb
2006
Concentration ofsucrose solutionsvia vacuummembranedistillat
ion
Sucrose The effect of several parameters,including feed temperature, flow rate,and initial sucroseconcentration on theflux quality and quantity was studied
Hydrophobicmicro-porousmembrane
16. YING XU,BAO-KUZHU,
YOU-YI XU2006
Pilot test ofvacuummembrane
distillation forseawaterdesalination on aship
SeawaterDesalination
This device could reach a desaltingdegree of 99.99% and a membraneflux of 5.4 kg/m2h at 550C and -
0.093Mpa
Polypropylenehollow fibermembrane
17. M.S. EL-Bourawi,M. Khayet,
R.Ma,Z. Ding,Z.Li,
X. Zhang2007
Application of
vacuummembrane
distillation forammonia removal
Ammonia The effects of different operating
parameters on ammonia removal fromaqueous solutions of different
concentrations have been investigatedExperimental results showed that highfeed temperatures, low downstream
pressures and high initial feedconcentrations and pH levels enhance
ammonia removal efficiency.The pH is found to be the mostdominant factor. Temperature andconcentration polarizations within feedboundary layer proved to have a
significant influence on mass transport.Ammonia removal efficiencies higher
than 90% with separation factors ofmore than 8
hydrophobic
microporousmembrane
18. Na Tang,PenggaoCheng
a,XuekuiWanga,
Study on theVacuum
MembraneDistillation
AqueousNaCl
solution
The effect of concentration of aqueousNaCl solution, feed flow rate and
temperature were studied.The VMD performance of 6wt.%
hydrophobicmicroporous
PVDF HollowFiber
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Huanju Zhang2007
Performances ofPVDF HollowFiber Membranesfor Aqueous NaCl
aqueous NaCl solution showed that thepermeation flux reached 25.171 kg/(m2h) and the salt rejection was as highas 99.8%, the pressure on the vacuum
side was 3kPa, the feed temperaturewas 343.15K and the feed linear flow
rate was 0.461m/s.
membranes
19. Zhi-PingZhao,Fang-Wei Ma,Wen-FangLiu,Dian-ZhongLiu2008
Concentration ofginseng extractsaqueoussolutionbyvacuummembranedistillation. 1.Effectsofoperating
conditions
concentrateGinsengextractsaqueoussolution
The experimental results showed thatboth initial flux and flux decline wereaffected obviously by the selectedoperating variable, i.e. temperature,vacuum pressure and concentration.High operating temperature and highvacuum tightness resulted in highpressure difference and hence highinitial flux.
A fouling layer on membrane surface
was observed
Polyte-trafluoroethylene (PTFE)micro-porousplatemembranes,with averagepore size of0.2 mm and
thickness of 50
mm20. Zanshe
WANG,
Shiyu FENG,XiangfengSHI,Zhaolin GU2008
ExperimentalStudy on
Concentration ofAqueous LithiumBromide SolutionbyVacuumMembraneDistillation
Process
LithiumBromide
This study aims to investigate theapplicability of vacuum membrane
distillation for concentrating aqueouslithium bromide solution, and toanalyze the feasibility of applyingvacuum membrane distillation processto the absorption refrigeration system.Commercial aqueous lithium bromidesolution with 50% concentration flows
through the inner side of membranethread while the vacuum degree in
outer side retain constant from0.09Mpa to 0.095MPa, the feedtemperature varied from 650C to 900C
Hydrophobicpolyvinylidene
fluoride(PVDF)
21. V. Soni,J. Abildskov,G. Jonsson,R. Gani2008
Modelling andanalysis ofvacuummembranedistillation for the
recovery ofvolatile aroma
compounds fromblack currantjuice
recovery ofvolatilearomacompoundsfrom black
currant juice
The VMD process and is able topredict the effects of concentration andtemperature polarization on the overallprocess performance.
Micro poroushydrophobicPTFE
22. A. Criscuoli,J. Zhong,A. Figoli,M.C.Carnevale,R. Huang,E. Drioli
2008
Treatment of dyesolutions byvacuummembranedistillation
Dyes The influence of operating parameters,as feed temperature, feed flow rate,feed concentration, on the permeateflux and on rejection has beeninvestigated. In all experimental tests,a complete rejection has been achievedand pure water has been recovered at
the permeate side.
PolypropyleneHydrophobicInnerdiameter:1.79mmThickness:0.51mm
Pore size:0.20mmPorosity: 75%
Effectivemembrane
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area, 0.0028m2
23. TorajMohammadi,MohammadAli Safavi2009
Application ofTaguchi methodin optimization ofdesalination byvacuummembranedistillation
Distillatewater
Taguchi method was used to plan aminimum number of experiments.The optimal levels thus determined forthe four factors were: temperature 55C, vacuum pressure 30 mbar, flowrate 30 mL/s and concentration 50 g/L
hydrophobicmicro-porous,polypropylene(PP)Pore size, 0.2mPorosity, 75%Thickness, 163m
24. Yajing LiKunpengTian2009
Application ofVacuumMembrane
Distillation inWater Treatment
Waterdesalination
Compared to other MD processes,operation temperature of VMD processcould be lower, and at the same
temperature, its flux would be larger
Hydrophobicmicro-porous,polypropylene
(PP)Pore size, 0.2
mPorosity, 75%
Thickness, 163m
25. Guy Ramon,YehudaAgnon, CarlosDosoretz2009
Heat transfer invacuummembranedistillation: Effectof velocity slip
A two-dimensional, boundary layermodel is presented, for describing theheat transfer in the feed channel of avacuum membrane distillation (VMD)module.The model solution provides the
temperature field in the feed channeland its dependence on the bulkvelocity and temperature, as well asthe vapor mass flux across the
membrane.
Hydrophobicmicro-porousPTFEmembrane
26. J.-P. Mericq,S. Laborie,C. Cabassud2009
Vacuummembranedistillation for anintegratedseawaterdesalinationprocess
Seawater VMD can be used for concentratedsalty solutions, because concentrationpolarization and temperaturepolarization might be non-limitingeven for quite high salt concentrations
Hydrophobicmicro-porousPTFEmembrane
27. NazelyDiban,Oana Cristina
Voinea,AneUrtiaga,
InmaculadaOrtiz2009
Vacuummembrane
distillation of themain pear aroma
compound:Experimentalstudy and masstransfer modelling
The recoveryof the main
pear aromacompound,
ethyl 2,4-decadienoate,
The effect of the operating variables,aroma feed concentration, feed flow
rate, temperature and downstreampressure onto the process performance
was analysed.Aroma enrichment factors up to 15were experimentally obtained.A mathematical model able to predictthe kinetics of the components
separation and the partial componentfluxes and enrichment factors was
developed
Hollow fibermodule of
polypropylene(PP) micro
porousmembranes
28. Bhausaheb L.Pangarkara,Prashant V.
Thorat, SarojB. Parjane,
Performanceevaluation ofvacuum
membranedistillation for
AqueousNaClsolution
VMD performance was investigatedfor aqueous NaCl solution.The influence of operational
parameters such as feed flow rate, feedtemperature, feed salt concentration
Flat sheethydrophobicmicro porous
PTFEmembrane
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Rajendra M.Abhang2010
desalinationby using a flatsheet membrane
and permeate pressure on themembrane distillation (MD)permeation flux have beeninvestigated.
The VMD performance showed thatthis device could reach a desalting
degree of 99.99% which was notaffected by feed concentration.The membrane distillation flux
reached 14.62 kg/m2h at 333 K bulkfeed temperature, 1.5 kPa permeate
pressure, 54 l/h feed flow rate, and30,000 mg/l feed concentration.
29. Mericq JP,Laborie S,Cabassud C.
2010
Vacuummembranedistillation of
seawater reverse
osmosis brines
ConcentratedRO BrineSolution
Vacuum Membrane Distillation(VMD) is considered as acomplementary process toRO to
further concentrate RO brines and
increase the global recovery of theprocess.Operating conditions such as a highlypermeable membrane, high feed
temperature, low permeate pressureand a turbulent fluid regime allowedhigh permeate fluxes to be obtainedeven for a very high salt concentration(300 g/ L).
Flat sheethydrophobicmicro porous
PTFE
membrane
30. Na Tang,Huanju Zhang,Wei Wang
2011
Computational
fluid dynamicsnumerical
simulation ofvacuummembranedistillation foraqueous NaClsolution
Aqueous
NaClsolution
The simulation has studied the mass
transformation and heat transformationof VMD process in the porous media,
in which aqueous NaCl solution wasseemed as an incompressible andsteady fluid.In the processes of mass transfer, heattransfer and phase transition, allparticles were supposed in localthermodynamic balance. By means ofFLUENT, one of the software aboutcomputational fluid dynamics (CFD),the numerical simulation of the two-dimensional model of VMD foraqueous NaCl solution was established
under the steady state.
Hydrophobic
micro porousPVDF
membrane
31. Jean-PierreMericqa,
StphanieLaborieb, CorinneCabassud2011
Evaluation ofsystems coupling
vacuummembranedistillation andsolar energy forseawaterdesalination
Seawaterdesalination
Vacuum membrane distillation (VMD)is a hybrid membrane-evaporative
process which has been shown to be ofinterest for seawater desalination.The main drawback of this process isthe relatively high energy requirementlinked to the need to heat the feedwater.A way to solve this problem could bethe use of a renewable source such as
solar energy to provide the heat energyrequired.
Hydrophobicporous
membrane
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Findley (1960) uses the membranes made up of different materials (paper, gum, glass fibres,
cellophane, nylon) treated with silicon and used Teflon to create water hydrophobicity and
concluded If low cost, high temperature, long-life membranes with desirable characteristics
2.3. Types of membrane distillation processesThis is a thermally driven process in which only vapour molecules are transported through
the pores of hydrophobic porous membrane. The liquid to be treated with MD is direct
contact with the one side of the membrane but do not penetrate inside the membrane pore as
liquid due to the surface tension of the membrane surface. The driving force is the
Abhang,MahendraGuddad2011
Desalination were investigated.The permeate flux was stronglyaffected by the feed inlet temperature,feed flow rate, and boundary layer heat
transfer coefficient.
36. Bhausaheb L.Pangarkar,M. G. Sane,Saroj B.Parjane,Rajendra M.Abhang,MahendraGuddad2011
VacuumMembraneDistillation forDesalinationof Ground Waterby using FlatSheet Membrane
aqueousNaClsolution andnaturalground water
The influence of operationalparameterssuch as feed flow rate (30 to 55 l/h),feed temperature (313 to 333 K), feedsalt concentration (5000 to 7000 mg/l)and permeate pressure (1.5 to 6 kPa)on the membrane distillation (MD)permeation flux have beeninvestigated.
A flat sheethydrophobicmicro porousPTFEmembrane
37. G Z Tong2012
Simulation ofVacuum
MembraneDistillationProcess and
Optimization ofMembraneModule
sea water andthe brackish
water
It is meaningful to develop and use themembrane distillation technology to
realize the desalination of sea water,energy-saving and emission reductionand comprehensive utilization of waste
water.The large-scale commercialcomputationalfluid dynamics software FLUENT wasapplied to numerically simulate theprocess of VMD
hollow fibermembrane
38. G Z Tong2012
Study on ProcessEnhancementandSimulation of
VacuumMembraneDistillation
NaClaqueoussolutions
The influences of operating conditionssuch as feed liquid temperature, feedliquid flow and permeate-side vacuum
degree on permeation flux areinvestigated.Two methods are performed
respectively for membrane distillationprocess enhancement. One is withturbolator, and the other with two-phase flow
Hydrophobicporous carbonmembrane
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transmembrane vapour pressure difference that maintain by one of these methods which are
applies on the permeate side (Melnik and Pena, 1997; Khayetet al., 2000; Bandiniet al.,
1992)1.
An aqueous solution which is colder than the feed solution is in direct contact with the
permeate side of the membrane giving rise to the configuration known as direct contact
membrane distillation (DCMD). The transmembrane temperature difference induces a vapour
pressure difference. Consequently, volatile molecules evaporate at the hot liquid-vapour
interface, cross the membrane in vapour phase and then condense in the cold liquid-vapour
interface inside the membrane module.
An air gap is maintained between the membrane and a condensation surface. In this case, the
evaporated volatile molecules cross both the membrane pores and the air gap to finally
condense over a cold surface inside the membrane module. This MD configuration is called
air gap membrane distillation (AGMD).
A cold inert gas sweeps the permeate side of the membrane carrying the vapour molecules
and condensation takes place outside the membrane module. This type of configuration is
termed sweeping gas membrane distillation (SGMD).
Vacuum is applied in the permeate side of the membrane module by means of a vacuum
pump. The applied vacuum pressure is lower than the saturation pressure of volatile
molecules to be separated from the feed solution. In this case, condensation occurs outside of
the membrane module. This MD configuration is termed vacuum membrane distillation
(VMD).Various studies with different membrane and conditions are in chronological order:
2.4.Vacuum membrane distillationVacuum membrane distillation (VMD) is a variant of MD. In this configuration vacuum isapplied on the permeate side of the membrane module by means of vacuum pumps. The
applied pressure is lower than the saturation pressure of volatile molecules to be separated
from the feed solution and condensation takes place outside the membrane module at
temperatures much lower than the ambient temperature. In VMD, the feed solution in direct
contact with the membrane surface is kept at pressures lower than the minimum entry
pressure (LEP); at the other side of the membrane, the permeate pressure is often maintained
below the equilibrium vapor pressure by a vacuum pump.The total vapor pressure difference
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between the two sides of the membrane causes a convective mass flow through the pores that
contributes to the total mass transfer of VMD.
This configuration has the following two advantages:
A very low conductive heat loss: This is due to the insulation against conductive heat loss
through the membrane provided by the applied vacuum. The boundary layer in the vacuum
side is negligible, which implies a decrease in the heat conducted through the membrane and
enhancement of the VMD performance.
A reduced mass transfer resistance: The diffusion inside the pores of the evaporated
molecules at the liquid feed/membrane interface is favoured.
It is to be noted that VMD process is mistakenly thought to be the same as pervaporation
(PV) process. The same systems can be applied for experiments. In both the processes, the
upstream side of the membrane is in contact with feed liquid while vacuum is applied on the
downstream side of the membrane. The fundamental difference between them is the role that
the membrane plays in the separation. VMD uses a porous and hydrophobic membrane,
whereas PV requires dense and selective membranes and the separation is based on solubility
and diffusivity of each feed component in the membrane material (Khayet et al, 2004).
2.5.Membrane material and VMD applicationsVacuum membrane distillation (VMD) uses a micro-porous hydrophobic membrane that acts
as a physical barrier to prevent the aqueous feed phase passing through and creates a liquid
vapour interface at the membrane pores. The most suitable material for VMD membranes
includes polymers such a PTFE, PVDF and PP (Zquierdoet al, 2004).
Khayet and Matsuura (2004) used water as non-solvent additive to improve the VMD
permeability of PVDF membranes and to reduce their cost. They fabricated both supported
and unsupported membranes using 15 wt% of PVDF in the solvent dimethyl acetamide
(DMAC). They observed the porosity was (26.8-79.6%) and pore size of (0.02-0.7 m) both
increased with increasing water content in casting solution but LEP decreased. The VMD
permeate flux increase exponentially with water content in the casting solution for both
supported and unsupported membranes.
Li et al (2003) prepared polypropylene and polyethylene hollow fiber membranes for VMD
desalination by Melt-extrude/ cold stretching method. Higher water permeate fluxes were
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obtained for the PE membranes than for the PP membranes and the result was attributed to
the larger pore size of the PE membranes. They also used DCMD and the highest permeate
flux reported was 0.8 l/m2.h in DCMD and about 4.0 l/m2.h in VMD.
The external surface of commercial PP hollow fibres (Accurel Membrana, Wuppertal,
Germany) coated with thin micro-porous silicon-fluoro-polymer layer (Li and Sirkar, 2005)
and they increased the hydrophobicity of the PP membrane. The fibres were arranged in a
rectangular cross-flow module for the hot feed to flow over the outside surface of the fibres
and to reduce the temperature polarization effect (Li and Sirkar, 2005). VMD experiments
done at feed temperature ranges from 60-900C with 1% NaCl solution, no pore wetting and
high temperature polarization coefficients and higher fluxes (41-79 kg/m2) were found.
Wu et al, (2004 and 2007) used various asymmetric micro-porous membrane PVDF hollow
fiber membranes with different pore sizes ranging from 0.031 to 0.068m and different
effective porosities. They fabricate these membranes for VMD by the wet spinning technique
using the DMAC and non-solvent additive lithium chloride and water. They removed TCA
from aqueous solution of various TCA concentrations and also remove toluene and benzene
from water. They found that porosity and permeability of PVDF hollow fiber membrane
increased when the membrane was treated with ethanol.
Hydrophobic poly phthalazinone ether sulfone ketone hollow fiber composite membranes
were prepared by coating of silicon rubber in the internal surface of the hollow fiber
membrane by Zin et al (2007 and 2008). They used the membrane for continuous run and
observed that the coating temperature influence the permeate flux, higher coating temperature
had low flux while low coating temperature had high flux.
Recently, comparisons of hydrophobic Zirconia (50 nm pore size) and Titania (5 nm pore
size) tubular ceramic membranes used in different MD configurations (VMD, DCMD and air
gap membrane distillation, AGMD) have been carried out (Cerneaux et al 2009). The
grafting of perfluoroalkylesilane on the internal surface of the tubular membranes was done.
The salt rejections of higher than 99% were found for all the configuration of MD. However
highest flux was obtained with VMD.
Surface-modified flat sheet polyethersulfonemembrane with surface modifying
macromolecules has been used in the VMD for the treatment of ethanol aqueous solution
(Suk et al, 2010).
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Isotactic polypropylene hydrophobic micro porous flat sheet membranes were fabricated via
thermally induced phase separation for desalination by VMD process (Tang et al, 2010).
These membranes exhibited a small pore size distribution and an asymmetric structure with
cellular pores on the dermal layer throughout the cross-section. It was reported that the
formation of these membranes is not very sensitive to the quench bath temperature, whereas
the diluent agent has a strong effect on the morphology and performance of the membranes.
For a feed temperature of 700C and a downstream pressure of 4kPa, a VMD permeate flux of
28.92 kg/m2.h was obtained when pure water was used as feed and 24.81 kg/m2.h when 0.5
mol/l NaCl aqueous solution was used as feed with a salt rejection factor of 99.9%.
The different membrane materials are widely used in the food industries for the concentration
of fruit juice (Schneider, 1988).Vacuum membrane distillation has been evaluated recently
for its application to the concentration of sucrose solutions duringbeverage production
(Calabroet al.,1990; Gil and Jonsson, 2004).
Banat and Qtaishat (2005) demonstrated the ability of VMD to treat wastewaters
contaminated with dyes such as Methylene Blue (MB). The concentration of MB in the feed
reservoir (18.5 mg/l) and the permeate flux were monitored as feed temperature, feed flow
rate, initial NaC1 salt concentration were varied. The permeate flow rate decreased
exponential with time. The dye was concentrated in the feed reservoir and was not found in
permeate. However they found some decay in the flux to some extent. Criscuoli (2008), also
performed the feasibility of VMD for dye concentration ranges 25 to 500 mg/l while
recovering pure water at permeate side. The experimental tests carried out showed that the
permeate flux increases with feed temperature and flow rate, due to the higher vapour
pressure and to the higher heat transfer coefficient, respectively, and that it has close relation
with the chemical properties of dyes..
Vora et al (1983) used Valencia and Midseason orange oil to concentrate the juice using
VMD to 10 and 25 fold at temperature of 57 and 620C at vacuum of 10 mmHg. They
compare the results with original oil and found the different flavour and colour characteristics
that was totally different.
Lifshitz et al(1962) have concentrated orange oil by VMD upon 10-fold at 45-50C and 3-5
mm Hg. Analytical examination of various steps in the process of concentrations showed that
the specific gravity, refractive index, optical rotation, aldehyde and ester values changedlinearly with the concentration.
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Pino et al(1992) have verified that the content of total aldehydes and alcohols (measured by
GC) increased linearly with the concentration of cold pressed Valencia orange oil from six to
10-fold. The concentration was achieved at 58-64C and 10 mm Hg.
Gill and Jonssen (2003) and Banat and Simondl (1999) reported about the separation of
mixture of Ethanol and water while Couffint et al (1988) and Jorgensen et al (2004) were
reported about the removal of the traces of the gasses and VOCs from the water using VMD.
Asheh et al(2005) were used the VMD for the concentration of the sucrose and found VMD
as very effective process for the concentration and getting pure water as permeate. Increasing
the feed flow rate increase the permeate flux but increase in water flux become insignificant
when Reynolds number becomes greater than 5000 and on increasing the sucrose
concentration have marginal effect on the permeate.
Current research status on VMD References
1) The separation of chloroformfrom aqueous solutions
A.M. Urtiaga et al. 2000; A.M. Urtiaga et al.
2001; Mohamed Khayet et al.2004
2) The separation of the Aromacompounds/Volatile organic
compounds
Serena Bandini et al. 2002; B. Wu et al. 2004;
Rico Bagger-Jorgensen et al. 2004; V. Soni et al
2008; NazelyDiban et al 2009; Rico Bagger
Jorgensen et al. 2011
3) Water Desalination andsensitivity analysis
Fawzi Banat et al. 2003; Baoan Li et al. 2005;
TorajMohammadi et al 2009; Yajing Li et al
2009:Na Tang et al. 2009 ; Pangarkar et al. 2010;
Xu et al. 2009
4) The separation of the Ethanol M.A. Izquierdo-Gil et al. 20035) The separation of the Phenol
compounds
LIU Zuohua et al. 2004
6) Separation of the Ethyleneglycol
T. Mohammadi et al. 2005
7) Separation of the dye(Methylene blue)
Van der Bruggen et al. 2004; F. Banat et al. 2005;
Mozia et al. 2006; A. Criscuoli et al 2008;
Table 2.2 Current works on VMD
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2.6.Transport through the membraneThe VMD process involves two types of transfers namely mass transfer and heat transfer.
These transports take place through the hydrophobic membrane used in the membrane
module (Khayet and Matsuura 2003).
2.6.1.Mass transferIn general, the mass transfer in MD consists of two steps: one is across the boundary layer at
the feed side and other is across the membrane. If a binary mixture of one non-volatile
component and at least one volatile component, is used as the feed in VMD, then within the
mixture the mass transfer resistance to volatile component exists. Due to this resistance, the
evaporation of volatile component at the membrane surface will result in its depletion and the
build-up of the non-volatile component near the membrane surface. The region near the
membrane surface, where the concentration profile of volatile and non-volatile components is
established, is called concentration boundary layer. As shown in Fig. the thickness of
concentration boundary layer is c (m). In this layer the concentration of non-volatile
8) Concentration of sucrosesolutions
S. Al-Asheha et al. 2006
9) SeawaterDesalination/concentrated RO
Brine solution
YING XU, et al. 2006; J.-P. Mericq et at 2009;
Mericq JP et al 2010; Jean-Pierre Mericqa et al
2011;G Z Tong 2012
10)Removal of ammonia R. Ma, et al. 200711)Separation of the aqueous NaCl
solution
Na Tang et al.2007;Zhi-Ping Zhao et al 2008;
Bhausaheb L. Pangarkara et al 2010;Na Tang et
al 2011; SushantUpadhyaya et al 2011;
Bhausaheb L. Pangarkar et al. 2011; G Z Tong
2012
12)Concentration of AqueousLithium Bromide Solution
Zanshe WANG et al 2008
13)Brackish water desalination Adel Zrell et al. 2011; G Z Tong 201214)Desalination of the natural
ground water
Bhausaheb L. Pangarkar et al. 2011
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component increases from Cb to Cm (kmol m-3). The buildup of component in the
concentration boundary layer due to the mass transfer resistance is referred to as
concentration polarization. For a given bulk concentration, the presence of concentration
boundary layer reduces the driving force for the volatile component to pass through the
membrane, and thus decreases the transmembrane mass flux.
The mass transfer theory is applied across the concentration boundary layer;
= ln
(1)
Where,NA(kmol m-2s-1) is mass flux of volatile component across the boundary layer, k (m s -
1) is mass transfer coefficient of boundary layer and c (kmol m-3) is total concentration at the
feed.The temperature difference between the two sides of the membrane, TfmTpm, gives rise
to a vapor pressure difference of volatile component across the membrane, P (T fm)P (Tpm),
which acts as the driving force for mass transfer through the membrane. This transfers are
governed by Knudsen diffusion.Two important factors affecting mass transfer are the mean
free path of the gas molecule transferred (m) and the mean pore diameter of the membrane
d (m). In accordance with the physical quantity, /d, defined as Knudsen number (Kn).
(n) = ()
() (2)
2.6.2.Heat transferA thermal boundary layer is also formed when the hot feed is in direct contact with
membrane.The thermal boundary layer is adjacent to the solid surface of the membrane, and
it is assumed that only in this region does the fluid exhibit its temperature profile.Within the
VMD module, liquid and vapor with different temperatures are separated by a microporous
membrane (with the thickness of ), so the thermal boundary layer appears at the feed side
(with the thickness of f) of the membrane, as shown in Fig.. In the boundary layer,the feed
temperature decreases from the value of Tb (K) at its bulk to the value of Tm (K) at the
surface of the membrane. Since VMD relies on phase change to realize separation, the latent
heat for evaporation of feed must be transferred from the feed bulk, across its thermal
boundary layer, to the membrane surface at the feed side.
The heat flux is given;
= ( ) (3)
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Fig. 2.1 Mass and heat transfer through the membrane (Lawson et al.,1997)
2.7.Boron2.7.1.Boron chemistryBoron is never found in elemental form in the nature. It exists as a mixture of the10B
(19.78%) and 11B (80.22%) isotopes (Budevariet al,1989). Boron at low concentration in
aqueous solution is known to exist mainly as boric acid. Molecular weight of H3BO3 or
B(OH)3 is 61.83. The unit cell is normally triclinic, containing four molecules of boric acid
(Adams, 1965). Boron in nature generally found in the form of boric acid and borates ions
and in combination of both metals and non-metals. Boric acids and its salts are widely used in
industries such as leather, carpets, cosmetics, antiseptics, pesticides, agricultural fertilizers,
photographic chemicals, detergent industries, neuron absorber for nuclear installations, and
glass industries (WHO, 1998). Since its ability to stand with high temperature, other forms of
boron are widely used in welding, cutting fluids high energy fuels and microchips. Boric acid
and borates are released into the environment by human activities including the use of borate
salt laundry products, coal burning, power generation, chemical manufacturing, copper
smelters, rockets, mining operations andindustries using boron compounds in the
manufacture of glass, fiberglass, porcelain enamel, ceramic glazes, metal alloysand fire
retardants (US department of health and human services 2007). Boric acid and borate salts
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exist naturally in rocks, soil, plants and water as forms of the naturally occurring element
boron (WHO, 1998)
Fig. 2.2Molecular structure of Boric acid
Boron is present in many foods and drinking water supplies. Estimated human consumption
of boron in the U.S. diet ranges from 0.02 mg boron/day with an estimated average intake of
1.17 mg boron/day for men and 0.96 mg boron/day for women (Rainey et al, 1999; Hunt,
2007).
Being the only non-metallic element in group 13 of the periodic table, the chemistry of boronand its compound boric acid is unique. With the (1s)2(2s)2(2p)1 valence electron
configuration, boron is electron deficient. As a result, boric acid can act as a weak acid.
However, with only 3 electrons in the valance shell, boron cannot comply with the Octet rule
and therefore boric acid is not a proton donor. Instead, the dissociation of boric acid can only
occur via a hydrolysis process:
B(OH)3+2H2O B(OH)4+ H3O
+; pKa = 9.23 (1)
At relatively low concentrations, only the mononuclear species B(OH) 3 and B(OH)4 are
present. However, at higher concentrations and with increasing pH, especially above pH 10,
poly-nuclear ions such as [B3O3(OH)5]2 and [B4O5(OH)4]2
would be formed (Woods et
al.,2010). The formation of these rings is attributed to the interaction of boric acid molecules
and borate ions in solution (Khaet al, 2010):
B(OH)3+2B(OH)4 [B3O3(OH)5]2+ 3H2O (2)
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At pH below this pKa value, boric acid exists in an undissociated form. Since boron is an
electron deficient element, the crystal radius of boric acid is quite large, in the range 0.244
0.261nm .However; boric acid is poorly hydrated and is therefore expected to have a small
hydrated radius (Khaetal. 2010).
Fig. 2.3Schematic drawing of the bicyclic [B4O5(OH)4]2,(Kha et al, 2010).
Thus, the effect of pH can be illustrated by the dissociation equilibrium given by the
following equation.
H3BO3 + 2H2O B(OH)4 + H3O
+ (3)
When pH increases, H3BO3, which is a Lewis acid reacts with water resulting in the
production of B(OH)4and H3O
+. Especially, B(OH)4becomes the dominant species at pH
between 9.5and 11.
Boric acid can also form complex with organic diols such as mannitol, sorbitol, ribitol,
erythritol and glycerol according to Raven (1980).
2.7.2.Health and Ecological implicationsAcute ingestion of boric acid or borate salts in humans has led to severe toxicity. Commonlyreported symptoms include nausea, vomiting (often with blue-green coloration), abdominal
pain and diarrhoea (which may contain blood or have a blue-green colour) and oedema and
congestion of the brain and meninges. The liver enlargement, vesicular congestion, fatty acid
changes, swelling and granular degeneration also occurs. Other less commonly reported
symptoms include headaches, lethargy, weakness, restlessness, tremors, unconsciousness,
respiratory depression, kidney failure, shock and death (Litovitz et al, 1988); (Reigart et al,
1999; Wong et al,1964).Large oral exposures have resulted in an intense red skin rash within
24 hours of exposure, followed by skin loss in the affected area 1-2 days after the skin
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coloration first appears. These skin rashes typically affect the face, palms, soles, buttocks and
scrotum (Reigart et al, 1999).Infants ingesting small amounts of boric acid in acute exposures
displayed irritability, vomiting, erythema, exfoliation, diarrhoea and nervous system affects
(Strong et al, 2001). Dermal exposure to borax has resulted in redness or inflammation of the
skin (Campbell et al, 2000).
Chronic exposure to borax in infants has led to seizures, vomiting and diarrhoea. Severe cases
of chronic exposure have caused coma, seizures, circulatory collapse, liver and kidney
dysfunction, anaemia and death. Seizures and death are more commonly reported in infants
chronically exposed to boric acid than adults (Stronget al, 2001; Linden et al, 1986).
Boron is an essential nutrient for plant growth, but too much boron is toxic to plants (WHO,
1998; Wood, 1994). Symptoms of excess boron uptake include cessation of root and leaf
growth and yellowing of the leaf tip. Bark splitting and necrosis at the tips of roots and leaves
may also occur. Damage from excess boron can reduce the overall productivity of the plant
and lead to death (WHO, 1998). Although boron is a micronutrient for plants, there is certain
level of tolerance table 2.3.
Table 2.3 Relative tolerance of agricultural crops to boron (Gupta et al., 1987;
Canadian environmental quality guideline, 1999
Boron tolerance
(mg/l)
Agricultural crops
Sensitive (2.0) Lettuce, cabbage, celery, turnip, Kentucky bluegrass, corn, artichoke,
tobacco, mustard, clover, squash, muskmelon, sorghum, lucerne, Purple
vetch, parsley, red beet, sugar beet, asparagus, cabbage.
Plant roots take up soil boron mainly as dissociated boric acid through active transport when
soil boron levels are low. Passive diffusion occurs at higher soil boron levels. Boron is
transported unchanged to the leaves where water evaporates, leaving the boron behind to
accumulate in the leaves. Because boron is virtually immobile in the phloem of plants, little
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moves to other tissues, such as the stems and fruits (WHO, 1998).In general, most vegetable
crops are fairly tolerant of high concentrations of boron in soils or irrigation water. However,
tuber and cereals crops are considered semi-tolerant. Citrus, stone fruits and nut trees are
most sensitive to boron (WHO, 1998).
2.7.3.Regulatory guidelinesGuideline values or standards for boron in drinking water varies widely around the world,
table 2.4. As per WHO (1998) guidelines, boron concentration of 0.5 mg/l is a guideline
value and not mandatory, different countries have set their own guideline levels. BIS limit of
boron in drinking water is 0.5 mg/l (BIS IS 10500, 2009). For example, the limit was set at
1.0 mg/l in the Europe and Singapore, 4.0 mg/l in Australia, and 5.0 mg/l in Canada. In the
4th edition of the Guidelines for Drinking Water Quality published by WHO in 2011, the
boron guideline value was revised to 2.4 mg/l from 0.5 mg/l, due to the lack of toxicity data
on humans (Farhatet al., 2012).
Table 2.4 Regulations and guidelines for boron in drinking water (Kha et al .,2010).
R
egulationandguidelines(mg/l)
Time of
issuing
1990 1997 1998 2000 2001 2004 2005 2007 2008 2009 2011
WHO 0.3 - 0.5 - - - - - - - 2.4
European
union
- - - 1.0 - - - - - - -
Canada - - - - - - - 5.0 - - -
New
Zealand
- - - - 1.4 - - - - - -
Israel - - - - - - - 0.3 - - -
Singapore - - - - - 1.0 - - - - -
Abu
Dhabi
- - - - - 1.5 - - - - -
Japan - - - - 1.5 - - - - - -
Australia - - - - - - - 4.0 - - -
US - - - - 1.5 - - - - - -
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2.8.Boron Analysis2.8.1.SpectrophotometryAbsorption Spectroscopic methods of analysis rank among the most widespread and powerful
tools for quantitative analysis. The use of a spectrophotometer to determine the extent of
absorption of various wavelengths of visible light by a given solution is commonly known as
colorimetry. This method is used to determine concentrations of various chemicals which can
give colours either directly or after addition of some other chemicals.
Absorption Spectroscopic methods of analysis are based upon the fact that compounds absorb
light radiation of a specific wavelength. In the analysis, the amount of light radiation
absorbed by a sample is measured. The light absorption is directly related to theconcentration of the coloured compound in the sample.
The wavelength () of Maximum Absorption is known for different compounds. For example
the colored compound formed for analysis of boron has maximum light absorption at = 540
nm conversely, a minimum amount of light is transmitted through the compound at = 540
nm.Visible light (400-700 nm) constitutes only a small portion of the spectrum that ranges
from gamma rays (less than 1 pm long) to radio waves that are thousands of meters long.
2.8.2.Spectrophotometer InstrumentAll spectrophotometer instruments designed to measure the absorption of the radiant energy
has the basic components as follow:
A stable source of radiant light A wavelength selector to isolate a desired wavelength from a source (filter or mono-
chromator)
Transparent container (cuvette) for sample and the blank A radiation detector (phototube) to convert the radiant energy received to a
measurable signal
And a readout device that display the signal from the detectorThe energy source is used to provide a stable source of light radiation, whereas the
wavelength selector permits the separation of radiation of the desired wavelength from the
other radiation. Light passes through a glass container sample. There is a relationship
between concentration and absorbance. This relationship is expressed by the Lambert-Beer
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Spectrophotometers are instruments equipped with monochromators that permit the
continuous variation and selection of wavelength. (Diana et al.,)
Fig. 2.5Finding the concentration of unknown sample from the standard curve
2.9.Analytical methods for Boron determinationCurcumin Method
Boron has to be transferred to boric acid or borates on reaction with curcumine
(diferuolylmethane C21H20O6) in acidic solution after which a red color boron chelate
complex is formed Rosocyanine [B(C21H19O6)2Cl] is formed.
Carmine Method
It involves the combination with carmine or carminic acid in acidic medium (sulphuric acid)
which followed by photometric measurement.
Other Methods for boron analysis
Spectrophotometric determination with azomethine-H olumetric determination following distillation, for waters that contain 0.2 mg/l and
that are colored.
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
Absorbance
Concentration (mg/l)
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2.10. The Curcumin Spectrophotometric Methods ((standard methods,2012)
Apparatus
Spectrophotometer, or photometer with a green filter for use at 540 nm High silica glass or porceline evaporating dishes, 100-150 ml Water bath set at 550c glass stoppered volumetric flasks, 25-50 ml containers (boron free)Reagents
Stock boron solution: dissolve 571.6 mg anhydrous boric acid in distilled water dilute itto 1 liter
Standard boron solution: take 10 ml of stock boron solution and dilute it to 1L withdistilled water
Curcumine reagent: dissolve 40 mg finely ground curcumine and 5 g oxalic acid 80 mlof ethyl alcohol, add 4.2 ml of conc. HCl and make it to100 ml with ethyl alcohol- stor itin refrigerator(stable for several days).
Hydrochloric acid Ethyl alcohol
Oxalic acidProcedure
Preparation of calibration curve: take 0.0, 0.1, 0.2, 0.4, 0.6, 0.8 &1.0 ml boron standardsolution into same size evaporating dishes, make its volume to 1 ml with distilled water,
add 4 ml of curcumin reagent to each, mix slowly. Heat the dishes on water bath at 550c
for 80 mins, cool, add 10 ml of ethyl alcohol and mix the red product with polyethylene
rod.
Transfer the dish contents into 25 ml volumetric flasks, make up to the mark with alcoholand mix.
Make photometric measurements at 540 nm i.e. maximum absorbance for rosocyanin.
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CHAPTER 3
MATERIALS AND METHODS
3.1.
Reagents
The chemicals used in the experiments were of analytical grade. Curcumin powder of LOBA
Chemie used for the analysis of boron. The other chemicals like H3BO3,Sodium chloride,
Sodium sulphate, Sodium hydroxide, Hydrochloric acid, Oxalic acid and Ethanol were of
CDH/RENKEM make. Membrane (Fluoropore) used in the membrane module was from
Millipore. Boric acid and salts (NaCl and Na2SO4) were directly dissolved into distilled water
according to respective testing conditions while 0.1 M hydrochloric acid and 0.1 M caustic
soda were prepared for pH adjustment during the study.
3.2. Apparatus UV-1800 Spectrophotometer pH meter (LAB INDIA) High silica glasses evaporating dishes, 100-150 ml Water bath set at 550C Glass stoppered volumetric glasses, 25ml Pipettes Sample containers (boron free) A glass rod
3.3. Experimental setupA lab scale vacuum membrane distillation setup was used in the study. The schematic
diagram of the setup is shown in the Fig. 3.2. A flat sheet hydrophobic microporous PTFE
membrane with an effective area of 0.00212 m2was use for the experiments. The feed water
was heated to required temperature using a heating apparatus (heater) at base of the feed tank.
The feed solution then pumped from the feed tank by a feed pump (037/0.50 from crompton)
to the membrane unit. The one more thing in the setup was that the feed flow was not able to
flow the small required flow rates so the excess flow should be bypassed to keep the feed
flow rate constant. The feed was circulated through the lumen of hollow fibers. The feed flow
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rate was maintained using rotameter installed before the membrane unit. The temperature of
both fluids of inlet and outlet were monitored by using digital thermometers. A vacuum pump
was (FRACOVAC) installed at base to create the required vacuum at permeate side by which
the partial pressure difference across the membrane was maintained.
Fig. 3.1Schematic representation VMD setup
A distillation unit was used at permeate side to condensed the vapor coming from the
membrane unit. A cold water reservoir was housed in the setup to supply cold water to the
distillation unit. A permeate receiver was used to collect the water comes from distillation
unit. A pressure gauge was used to measure and maintain the vacuum created. The membrane
properties are given in the table:
Table 3.1Membrane Properties
S. No. Properties Specifications
1. Membrane material PTFE
2. Surface property Hydrophobic
3. Diameter, mm 90
4. Pore size, m 0.22
5. Thickness, m 175
6. Porosity, % 70
7. Effective membrane area, m2 0.00212
8. Maximum operating temperature, 0C 130
9. Producer Millipore
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Fig. 3.2 Experimental setup of VMD in the laboratory
3.4. Experimental ProceduresFeed was prepared by adding boric acid into distilled water. To obtain various concentration
of feed solution the required amount of boric acid added to measured volume of distilled
water. For the study of effects of salts on boron removal, appropriate amount of NaCl and
Na2SO4were added to the feed for various run of experiments. The effect of pH (3-11) on the
rejection was done by adding require amount of 0.1 M NaOH and 0.1 M HCl to the feed.
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During the experiment feed solutions pumped through the module for 150 minutes for each
run and after intervals of 30 minutes permeate samples were collected for analysis. The
different temperature (40-600C) of feed solution was maintained by thermocouple installed at
the base feed tank. A pressure of 17 kPa was applied at permeate side for all the experiments.
The feed flow rate was maintained at 1 LPM for all the parameter except the effect of feed
flow rate.
Fig. 3.3 Membrane module of VMD setup
Boron analysis was done by Curcumin method using UV spectrophotometer (shimadzu UV-
1800). Curcumin reagent made by dissolving 40 mg of Curcumin powder and 5 g of oxalic
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acid in 80 ml of ethyl alcohol (95%) plus 4.2 ml of conc. HCl followed by addition of ethyl
alcohol (95%) up to 100 ml. It can be stored for several days in refrigerator.
3.5. Preparation of calibration curve: Taken 0.0, 0.25, 0.50, 0.75 & 1.0 ml boron standard solution into same size
evaporating dishes, made its volume to 1 ml with distilled water, added 4 ml of
curcumin reagent to each, and mixed slowly.
Heated the dishes on water bath at 55 0C for 80 minutes, cooled, added 10 ml of ethylalcohol and mixed the red product with polyethylene rod.
Transfered the dish contents into 25 ml volumetric flasks, made up to the mark withalcohol and mixed.
Made photometric measurements at 540 nm i. e. maximum absorbance forrosocyanine.
Fig. 3.4 Different standard solutions (rosocyanine) with a blank
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CHAPTER 4
RESULTS AND DISCUSSIONS
4.1.Standard curve preparation for boron analysisCurcumin spectrophotometric method was used for the determination of boron in the water.
This method is one of the most sensitive methods of boron analysis. We used the procedure
as mentioned before and determined the variation of Absorbance with Boron concentration.
Boron concentration (mg/l) Absorbance at 540 nm
Blank 0.0
0.5 0.2
1.0 0.41
1.5 0.67
2.0 0.97
2.5 1.24
Table 4.1Absorbance value at different boron concentration
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Fig. 4.1 Calibration curve of boron
According to procedure we have prepared standard boron solution of concentrations 0.5, 1.0,
1.5, 2.0, and 2.5 with a blank and after the treatment with Curcumin reagent and 95% ethanol,
a product called Rosocyanin (red coloured compound) was formed. The redness of solutionwas increased with increase in boron concentration. The absorbance was taken at 540 nm
using 1800-UV spectrophotometer and curve obtained was cross checked twice. Calibration
was the used to find out the boron concentration of the permeate sample by measuring there
absorbance values using the 1800-UV spectrophotometer.
4.2.Effect of feed pH on boron rejection and permeate fluxThe effect of pH on boron rejection and permeate flux has been studied in feed solution pH
range of 3-11. The results are given in Table 4.2. As shown in the table, pH does not have
any effect on boron rejection and permeate flux. Similar results have been reported by Hou et
al., (2010) for boron removal by DCMD process. In case of boron removal by RO process,
pH of feed solution has been reported to have significant effect on boron rejection and
permeate flux because at pH higher than 9.0 the boric acid is dissociated into ions and bipolar
borate ions which is larger in size and retained by the reverse osmosis membrane.
R = 0.9905
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.5 1 1.5 2 2.5 3
Absorbance
Boron concentration (mg/l)
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.
Table 4.2 Effect of feed pH on boron rejection and permeate flux
(Conditions: Feed temperature: 60 0C, Permeate temperature: 25 0C, Feed flow rate:1
LPM, Vacuum pressure: 17 kPa).
S. No. Time
(min)
pH Permeate
flux
(kg/m2.h)
Permeate
boron
concentration
(mg/l)
%
Boron
rejection
1. 30 3 23.461 0.201 99
2. 90 3 23.461 0.166 99.17
3. 150 3 23.461 0.231 98.84
4. 30 5 24.029 0.164 99.18
5. 90 5 22.898 0.209 99.01
6. 150 5 23.461 0.239 98.804
7. 30 7 24.029 0.204 99
8. 90 7 23.461 0.170 99.15
9. 150 7 23.461 0.135 99.35
10. 30 9 24.029 0.186 99.07
11. 90 9 22.616 0.145 99.27
12. 150 9 22.616 0.145 99.27
13. 30 11 24.029 0.24 98.8
14. 90 11 22.616 0.066 99.67
15. 150 11 23.461 0.251 98.84
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In case of membrane distillation process feed solution does not have any effect on boron
removal and permeate flux because the permeate flux is depend on the temperature of the
feed and applied vacuum on the permeate side. The temperature gradient is responsible for
the driving force. The flux is due to vapour pressure difference across the membrane. Since
pH has no interference with the temperature gradient and transmembrane vapour pressure
difference this explains why pH does not affect the permeate flux.
.
Fig. 4.2Effect of feed pH on boron rejection
The above results shows there was no significant effect of any pH on the percentage removal
of boron in vacuum membrane distillation process because the process is independent of the
composition of non-volatile components in solution and the chemical reactions at any pH
within operating range.
While it is impossible to get this much high rejection with reverse osmosis especially at pH
lower than the pKa (9.24) value of boric acid. With reverse osmosis process this much
rejection only obtained at a pH higher than the 9.0 because the boric acid is dissociated into
ions and bipolar borate ions which are larger in size and retained by the reverse osmosis
membrane. Being different from reverse osmosis the VMD uses temperature gradient created
on the membrane surfaces is its driving force which is independent of pH. These are the
reason behind the higher boron rejection using the VMD process whether it is acidic or basic.
97
98
99
100
0 30 60 90 120 150 180
%B
oronrejection
Time (min)
pH=3
pH=5
pH=7
pH=9
pH=11
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Effect of pH on the permeate flux
The effects of pH of feed boron solution on permeate flux shown in Fig. 4.3. There was no
significant effect on the permeate flux as we increase the feed solution pH from 3-11. Feed
solution with pH 3.0, 5.0, 7.0, 9.0 &11 were used to study the influence on permeate flux, in
all the cases of feed pH, the permeate flux is almost constant. The permeate boron
concentrations of the collected samples are analysed, the concentrations were 0.031, 0.069,
0.062, 0.145 & 0.066 respectively (Fig. 4.7) which were under the permissible limit of the
boron in drinking water
Fig. 4.3 Effect of pH on permeate flux
Fig. 4.4Variation of permeate boron concentration as a function of feed pH
0
5
10
15
20
25
1 3 5 7 9 11 13
permeateflux(kg/m2.h
pH
0
0.1
0.2
0.3
0.4
0.5
1 3 5 7 9 11 13
Permeateboronconc.(mg/l)
pH
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4.3.Effect of the presence of salt on boron rejection4.3.1.Effect of the presence of NaCl on boron rejectionTo investigate the effect of NaCl on the rejection of boron a series of experiments were done.
Feed solution of 20 mg/l boron concentration mixed with NaCl concentrations of 500, 1000,
1500, 2000, 2500 & 3000 mg/l at pH=7 with a feed temperature of 60 0C and feed flow rate
of 1 LPM, fed to membrane module using the feed pump and permeate samples were
collected at every 30 minutes till the 150 minutes of the run as shown in Table 4.3. The
samples were analysed and permeate concentration measured. It was found that no significant
effect on the rejection of boron as no any trend was obtained as shown in Fig.4.5. The
permeate concentrations were 0.121, 0.138, 0.114, 0.076, 0.114 & 0.170 mg/l which are
below the maximum permissible limit of drinking water.
Table 4.3 Effect of the presence of NaCl on boron removal
(Conditions: Feed flow rate: 1 LPM, Feed temperature: 60 0C, Vacuum pressure: 17
kPa, Permeate temperature: 250C, Feed boron concentration: 20 mg/l, pH: 7)
S. No. NaCl(mg/l)
Permeateboron
concentration
(mg/l)
% Boronrejection
1. 0 0.067 99.67
2. 500 0.121 99.39
3. 1000 0.138 99.31
4. 1500 0.114 99.43
5. 2000 0.076 99.62
6. 2500 0.114 99.43
7. 3000 0.170 99.30
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Fig. 4.5 Effect of feed NaCl on boron rejection
These results show the rejection of boron is hardly affected by the feed NaCl concentration in
the studied range. This result supports the findings of Alklabi and Lior (2006).
4.3.2. Effect of the presence of Na2SO4 concentration on boron removalTable 4.4 Effect of presence of Na2SO4on boron rejection
(Conditions: Feed flow rate: 1 LPM, Feed temperature: 60 0C,Vacuum pressure: 17
kPa, Permeate temperature: 250C, Feed boron concentration: 20 mg/l, pH: 7)
S. No. Na2SO4
(mg/l)
Permeate
boron
concentration
(mg/l)
%
Boron
rejecti
on
1. 0 0.067 99.67
2. 500 0.167 99.16
3. 1000 0.200 99.01
4. 1500 0.210 98.95
5. 2000 0.06 99.75
6. 2500 0.235 98.82
7 3000 0.114 99.43
97
98
99
100
0 500 1000 1500 2000 2500 3000 3500
%Rejctionofboron
NaCl concentration (mg/l)
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To investigate the influence of feed Na2SO4concentration on the rejection of boron, various
boron removal experiments were carried out Fig. 4.6. The feed temperature was fixed at 60
0C and the concentration of boron in the feed solution was 20 mg/l. The feed flow rate was
kept at 1 LPM. The different feed Na2SO4 concentrations 500, 1000, 1500, 2000, 2500 &
3000 were fed to the membrane