2013 akhilesh m.tech print final

8/12/2019 2013 Akhilesh M.tech Print Final

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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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15

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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17

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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18

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

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