Water desalination by pervaporation with hollow fiber membranes

DESALINATION ELSEVIER

Desalination 107 (1996) 121-129

Water desalination by pervaporation with hollow fiber membranes E. Komgold a*, E. Korin b, I. Ladizhensky a ~The Institutes for Applied Research; bDepartment of Chemical Engineering, Ben-Gurion University of the Negev, PO Box 653, Beer Sheva 84105, Israel Tel. +972 (7) 646-1940; Fax +972 (7) 647-2969 Received 10 March 1996; accepted 21 April 1996

Abstract

A new desalinationprocess consisting of air humidification by pervaporation through hydrophilic or microporous hydrophobic hollow fibers followed by dehumidification by cooling water was investigated. In this system hot water is passed through hollow fibers in a recycled air-sweep pervaporation process. The water is heated by waste heat or solar energy or by any other cheap source of energy. The flux of water through the hollow fibers is in the range of 1.5-3.0 l/m2h, when water temperature is 45-65°C. The energy requirement for recycling hot water depends on water temperature as well as on the diameter and length of the hollow fibers. The energy requirement for air recycling depends on the air temperature and on the pressure drop of the system. The calculated energy requirement for pumping air and water in a pilot plant unit of a capacity of 6.3 l/h with 4 m2 of anion-exchange hollow fibers was about 2 kWh/m3, when hot water temperature was 60°C. Keywords: Water desalination; Pervaporation; Hollow fibers

1. Introduction

distillation processes. The energy requirements

Total world water desalination capacity by means of the various processes used is as high as 15 million m3/d. More than 65% of this capacity is accounted for by reverse osmosis (RO), about 5%

by

electrodialysis and

*Corresponding author,

about 30%

by

vary between 6 and 10 kWh/m 3, and investment costs vary between 600 and 2000 US$/m3/d, according to the size of the unit and the type of process used. New desalination processes are being sought in which energy requirements and the investment costs are reduced. One of the old techniques uses solar energy directly in a conventional solar still. It is distinguished in its

0011-9164/96/$15.00 Copyright© 1996 Elsevier Science B.V. All rights reserved. PII S0011-9164(96)00157-9

122

E. Korngold et al. / Desalination 107 (1996) 121-129

simplicity of construction, operation, and maintenance. The output of a solar still does not exceed 4--6 1 of fresh water per day per square meter of basin; hence, a large surface area is needed for a large desalination capacity [1,2]. Recently, a new separation technology called membrane distillation (MD) has been developed for water desalination of brackish or seawater by solar energy [3-6]. This process is based on the use of hydrophobic microporous membranes that separate two liquid streams, the hot stream of brackish or seawater (heated by solar or other energy sources) and the cold stream of the distilled water. The hydrophobic membrane prevents passage of the liquid phase because of its small pore size (in the range of 0.05-0.15 ktm), but allows passage of water vapors. The pressure gradient of the water vapors produced by the temperature difference between the hot and the cold sides is the driving force of the desalination process. The water is evaporated at the hot side of the membrane, diffuses through it and condenses on the cold side of the distilled water stream. The MD separation process can be carried out at 50-90°C and at near atmospheric pressure; hence, it is possible to apply conventional solar water-heating technologies to this process. Preliminary cost evaluation of the MD method [6] indicates that for a production rate in the range of 50 l/d, it is economic compared with the conventional RO method powered by electrical energy. This advantages makes it suitable for small isolated communities that are short of conventional energy resources. However, h ydrophob ic membranes are relatively expensive and prone to leakage between the hot stream (brackish water) and the cold water stream (distilled water) flowing on either side of the membrane. Another disadvantage is the heat flux through the membrane from the hot water to the cold water, which reduces the efficiency of the process. In addition, the pores of the microporous membrane may become filled with water during prolonged use, as a result of a decreased

hydrophobicity of the membrane followed by a decrease in the flux of water vapors. In our work a new desalination system consisting of air humidification by pervaporation and dehumidification by cooling was investigated. This system applies salty water at temperature of 45-80°C to obtain fresh water by pervaporation. The salty water, either brackish or sea water, is heated to the desired temperature by various means, e.g., solar energy, waste heat from industry, geothermal water. Our method uses a polyethylene ion-exchange membrane [7] in the form of hollow fibers to separate the salty water stream from the air stripping stream. Water diffuses from the water side of the membrane and evaporates at the air stream side of the membrane (Fig. 1). The condensation of the water vapors is carried out in a separate unit (Fig. 2). Thus, there is virtually no risk of contamination of the distilled water. This setup also ensures good thermal efficiency due to recovery of the latent heat released during vapor condensation. The advantage of using hollow fibers for water pervaporation as compared with direct contact Hot water I~ !

L..._

ii~iiliii[~iiii~iiiiiiii~iiiiiiiii ili::: ::~ ..... _ _ Air ÷ ::ii!i HzO ii!i w a t e r vapors Hollow ~1.0 iii ~ iiii-1,0 fibers ~lii: ~. i?i{ ::::~,,~ ::iii~ HaP iii! "~' :.ii~ ilii,..b° 4~o :i:, :::: -,,~ii! HtO i il Z" ili::iiiii!~i ~ Air . ~ :iii!!ii!ii!i! ....,...... :i:: iiliiii!! I i~ I Hot water Fig. 1. Scheme ofthe air sweep pervaporation process.

123

E. Korngold et al. / Desalination 107 (1996) 121-129

/ hot cold

cold water

air

::~.::::::::::::;:::

iiiiii~iiiiiiii~iiiii ::ii!iiiiiiiii::g~'~::::::~ii~::i~:?:

_..._.....J...

water

!~'1

::. ~i.:

blow

i:::~ ',;i h • a t 1n 9

OW fibers

; .~:;~;~. ~.~.:::

ii!i

feed

water

i~.~i:i~.:.~::!~?.:i:i :::i:': ::::.::~i: ::i ::~::i:N ~.~ :':g.~:': :~::::1 i i~:i~i ~: i~:~:~:i~:~:~l : :.,: : ::::~:R::::::: ::::::::::::::::::::::::::::: ho L a I r • w a t er co I d w a tel"

i:~i~:~:~:~:~:!

~ \ ~ot water distilled

water

between water and air in absorption towers is the ability to control all process parameters and optimize pressure drop and consequently pumping energy of the recirculated water and air. Moreover, anion-exchange hollow fibers do not allow passage of volatile organic materials present in the water [9]; hence the desalinated water is free from organic substances. The anionexchange hollow fibers made by us were successfully tested for water desalination by the Donnan ion-exchange process [8]. The effect of temperature, air velocity and concentration of solutes on water vapor flux was previously studied with hydrophilic anion-exchange hollow fibers and microporous hydrophobic hollow fibers [10,11]. In this study we examined different types of hollow fibers in the new desalination system. Their dimensions (wall thickness and diameter) and chemical properties were optimized for use in water desalination by pervaporation. The efficiency of the anionexchange hollow fibers manufactured by us was compared with that of commercial hollow fibers made from microporous hydrophobic polypropylene. Data of energy requirements for pumping (air and hot water) with this new

Fig. 2. Scheme of water desalination by pervaporation with air recirculation.

process at different water temperatures are reported. 2. Experimental The water desalination system by pervaporation with hollow fibers (hydrophilic or microporous hydrophobic) is described in Figs. 1 and 2, while desalination system without air circulation is described in Fig. 3. Description of desalination system with heat recovery is given in Fig. 4. The hot water that passes through several modules is cooled and used for condensation of the water present in the saturated hot air. The anion-exchange hollow fibers used in our experiments are self made by sulfochlorination, amination, and quaternization of polyethylene hollow fibers, according to a process developed by de KfrSsy at our institute for production of fiat polyethylene ion-exchange membranes [7]. They have an exchange capacity of 0.8-1.1meq/ g dwt, swelling in water of 25-36%, outside diameter of 0.4-1.5 mm and wall thickness of 50-180 /am. The microporous hollow fibers, Celgard 2500 (Celanese Co.), have an ouside diameter of 0.4 mm and wallthicknessof25/am.

E. Korngoldet el. / Desalination 107 (1996) 121-129

124 ,,,at+,

.or

Total specific pumping energy requirements was calculated according to the following equation: E

Hollow fibers

I

H0

I

-

~i

2

Air

:.i:.::ili::i ~10 :,i::iiiiii: 2

I

,,~

fl

~i 2

+!+::+;~ o

+

P2Q2

(1)

water

+

I

...... ~. i~i-" ~i++~+°

H 0

PIQt

I

I.-..-=a.--!111; i]i~ ~ ~. .+" t. ++++ii I~ ;ii!i "- iiii!i!.,~,-\~: I/~H? ~ ..r~ , --Jiii!i........+o iii::i!i:::: I

-,at .... ,~e,sat,*,

27.2

-

coo,,+

l

~

where E is the specific pumping energy requirement in kWh/m 3 ofdesalinatedwater, P I the pressure of cycled hot water (arm), P2 the pressure of cycled air (atm), Q~ the flow rate of hot water (m3/h), Q2 the flow rate of air (m3/h), w the flow rate of desalinated water (m3/h), and r I1, rlz the efficiency of water and air pumps.

~

-Distilled water

3. Results

t

3.1. Effect of hollow-fiber thickness on waterflux The ion-exchange capacity of thin (140 /Jm) anion-exchange polyethylene mem-

Hot water

Fig. 3. Scheme of water desalination by pervaporation without air circulation. 47

I

-

C

--

"~

- a lT" +

hollow fibers m o d u l ~ - ~

heating reed w a t e r

-

--

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r

_

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6"0 C-

air

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-- --3

I

~o c -a,T -

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+ ° 1/ |i~i:~! ii"~,~: ~ --m,-

25 C

t+c

iiiii

distilled w

cooling w a t e r ~lower

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=---

r

I _

_

50C "-niT"

~

-- -

_

L

and discussion

air

30C

-'J

Fig. 4. Scheme of water desalination by pervaporation with heat recovery.

E. Korngold/Desalination 107 (1996) 121-129 a.o,

2s.

~

2.°.

~

3.2. Comparison offlux through microporous and anion-exchange hollowfibers

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-

0"020

, 30

•

,

, 50

400 Temperature, C

60

Fig. 5. Water vapor flux vs. water temperaturewith two types of anion-exchange hollow fibers. (1) Capacity: 1.1 meq/g; swelling: 31.5%, thickness: 0.07 mm. (2) Capacity: 0.81 meq/g; swelling: 36.6%, thickness: 0.17 mm. ,.,

t ._30 =,2~ ]~2:] ~" • ,.o4

._.4_ Hydrophilicfibers ~ ~

Porous fibers

~,.~I

~

/

05!

45 ° C 5o s5 ,o ,s Temp., Fig. 6. Water vapor flux vs. water temperature w i t h microporous hollow fibers (thickness: 0.025 mm) and anion-exchange hydrophilic hollow fibers (thickness: 0.07 mm). 25

30

125

35

40

branes is in the range of 0.6-0.8 meq/g. In an ionexchange membrane diffusion of water is carried out by means of a continuous pathway of water shells around charged groups, which is faster than the diffusion by means of clusters of free water inside the membrane [9]. Therefore the capacity increase in thin ion-exchange membranes leads to a high water flux through the membrane, beyond the value calculated from Fick's equation• Water flux through two types of ion-exchange hollow fiber is given in Fig. 5. The figure shows a significant increase of water flux when the thickness of membrane decreases,

Water vapor flux through the microporous Celgard 2500 membrane in comparison with our hydrophilic anion-exchange membrane is given in Fig. 6. The two types of membrane differ in the way water diffuses in them: in the microporous membrane water diffuses as vapors, while in the hydrophilic membrane as liquid, the diffusion coefficient of the vapors being much higher than that of the liquid. Despite this fact, water flux through the two membranes was found to be similar.

3.3. Effect of hollow-fiber diameter on water pumping energy As shown before, the flux of water through the hollow fibers increases as wall thicknenss decreases (Fig. 5). However, in order that the mechanical properties of the hollow fiber is not impaired, a decrease of wall thickness must be associated with a similar decrease in the fiber diameter (D). Since the flow inside the hollow fibers is laminar, the pressure drop along the fiber is proportional to D 4. The surface area of hollow fibers is proportional to their diameter, and the flow rate inside hollow fibers is proportional to their surface area. Therefore, the pumping energy of water inside a hollow fiber will be proportional to D 3. If the flow rate inside the hollow fibers is kept too low, the pumping energy will indeed decrease, but the temperature of water flowing inside the hollow fiber will also decrease, causing a decrease in air temperature and in water vapor concentration in the air. The result will be an increase in the specific air pumping energy requirement and a decrease in the capacity of the desalination unit. Therefore, the optimal conditions must be carefully determined. The water pumping energy requirement for water temperature drop of 5.5°C and 11 °C with

E. Korngold / Desalination 107 (1996) 121-129

126

hollow fibers differing in their diameter was calculated from data given in Fig. 7 and from the specific water flux in hollow fibers. The results given in Fig. 8 show the strong influence of hollow-fiber diameter on the circulated water pumping energy calculated for 1 ton of desalinated water,

,~o1.5 mm ,~

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0.9 mm f.___.__.......~

0.7 mm

100

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0.35 mm

3.4. Effect o f air velocitiy on water flux The effect of air velocity on water flux in hydrophilic membranes is given in Fig. 9. At low air velocities, below 1.5 m/s, water flux depends on air velocity. High concentration o f water vapors in the boundary layer near the membrane causes an increase of water concentration in the membrane on the downstream side, and therefore, according to Fick's equation, the flux decreases [10]. When air velocity approaches 1.5 m/s, water vapor pressure in the boundary layer near the membrane approaches water vapor pressure in the bulk of the air stream. Hence, above this velocity water flux is independent o f the air velocity, and dependent only on water diffusion flux within the membrane.

-

3.5. Effect o f air temperature requirements of a i r recirculation 0.0

0.~

0,2

0.4

o.3

o.5

on

energy

0.8

Pressure (arm) Fig. 7. Flow rate vs. pressure drop with 1-m hollow fibers of different internal diameters.

,v

Maximal water vapor concentration in the air is dependent on air temperature, as shown in Fig. 10. In our process air absorbs water vapors at a high temperature. The vapors condense upon cooling the air at a low temperature. The specific energy requirement is inversely proportional to the amount of water condensed per a unit o f

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Intemaldiameter Fig. 8. Water pumping energy vs. hollow fiber diameter at two water temperature differences between incoming and outcomingwater (incoming water temperature: 60°C, hollow fiber length: 1 m).

o~

;

" ;

" ~ " ; " ~ " d Air velocity (m/s)

" -'

Fig. 9. Water pervaporation flux vs. air velocity. Wall thickness: 0.07 mm; outer diameter: 0.7 mm. Water temperature: 42°C.

E. Korngold et al. /Desalination 107 (1996) 121-129

127

1000 '"

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f

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NN,x ~

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~ Input air temperature • 20 C

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~.~

100

120

Fig. 10. Water vapor concentration in air vs. temperature.

the air energy requirement for air circulation becomes less important, when the temperature of saturated hot air increases.

2,5-

] '.0] ~o.s = [ ' ~ 0.0 3o

so Temperature, ° C

\

• 30c ~

;

40C

~

;s " ,'o ,'s " 5'0 " ,'~ " ;0 " ;5 ' ,0 Outputair temeprature, °C Fig. 1I. Specificenergy requirements for air recirculation vs. temperatureof the hot air which is cooled to different temperatures. Air blower pressure: 15 mm water, efficiency: 70%. recycled air. On the basis of Fig. 10 and Eq. (1), the energy requirement for recycled air is calculated as a function of the temperature of hot air saturated with water vapors and cooled to different temperatures when the pressure drop of recycled air is 15 mm of water and blower efficiency is 70% (Fig. 11). The results show that

3.6. Water desalination without air circulation

In this system the hot water is passed through the hollow fibers and water vapors condense on the wall of the module (made from copper), which is cooled by water (Fig. 3). The advantage of this system is that there is no need for circulation of air; thus, the system becomes simpler. However, the specific water flux through the hollow fibers is much lower as compared with the flux in the system with air circulation. The main resistance to vapor water transport by diffusion is dependent on the distance between the hollow fibers to the wall of the module. The driving force is the difference between water vapor concentration (or pressure) near the hollow fibers to that near the cold wall of the module. Results of water pervaporation flux at different water temperatures with and without air circulation with anion-exchange hollow fibers (outside diameter: 1.3 mm, wall thickness:

128

E. KorngoM et al. / Desalination 107 (1996) 121-129

3.0'

s. 20

Coolingwater temp.

,.

' •

.

/

/7

20oc

5-

~

~. 1.o.

,r.T.d

20.5

I 0 ~5

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40

4'5

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5'5

6'0

6'5

"

,'o

"

,,'~

"

5'o

;~

o.0

"

o'~

"

",'o

Water tem eprature, ° C

70

Water temperature, °C Fig. 12. Water flux vs. temperature with (1) and without (2) air circulation. Cooling water temperature: 21 °C. 0.1 mm) is given in Fig. 12. The omission of air circulation reduces pumping energy requirements, but this advantage is counteracted by a larger surface area of membranes.

Fig. 13. Pilot plant capacity vs. water temperature at different cooling water temperatures. Membrane surface area: 4 m2. ,o. ,-, ,. ~ ,.

Xx

7'

3. 7. Water desalination system with heat recovery

~ 6.

When hot water passes through one or several modules of hollow fibers arranged in series at low velocity, its temperature significantly decreases. It can be used for condensing water vapors, thus saving the amount of heat needed for desalination. Example of such a system is given in Fig. 4. Such a heat recovery system requires, on the other hand, a large surface area of the hollow fibers and a large pumping energy

~ ~ ' "~

requirement. Therefore, the use of this system depends on the availability of heat at low temperatures,

3.8. Unit operation A desalination unit consists of two modules in series, each containing 2 m 2 with hollow fibers of wall thickness 0.1 mm, condenser of 3 m 2 surface area, hot water pumps of 1.1 m3/h at 0.15 atm and blower of 80 m3/h at pressure of 17 mm of water, The energy requirement for air and water

" N

~

"~ ~......_~

Cooling water temp. 20°C "

29°C

~ Oo

;5

5'0 ,'s 6'0 6's Hot water temperature, °C

70

Fig. 14. Energy requirements for hot water and air recirculation in a pilot plant unit vs. water temperature. circulation according to Eq. (1), taking into account 70% efficiency, is 11.7 W. The capacity of water desalination in this unit as a function of the tempeature of hot and cooling water is given in Fig. 13. The specific pumping energy requirement, calculated on the basis of the results in Fig. 13, is presented in Fig. 14. It shows that the desalination energy requirement is reduced proportionally to tempeature of the hot water.

E. Korngold et al. / Desalination 107 (1996) 121-129

129

4. Conclusions

Acknowledgements

In this study a new desalination process was developed which can use for water heating a lowcost energy, such as waste heat disposed from an industrial plant, solar energy, geothermal energy or any other source of heat above 45 °C. If such a source of heat is available, the electrical energy requirements are only for pumping and for recycling air and water. The desalination system,

This work was supported by the Office of Science and Technology, Ministry of Science and Technology. We thank Mr. J. Freiman for excellent technical assistance and Ms. Dorot Imber for editorial help.

which was built and put in operation in this study, enables to circulate air and water at very low pressures, and hence the pumping energy requirement for circulation of air and hot water is low. When the temperature of hot water is higher than 60°C, the circulation energy requirement is lower than 2 kWh/m 3 (on the basis of pumping efficiency of 70%, not taking into account pumping energy requirements for cooling water). In this study the efficiency of commercial

[1] K.S. Spiegler, in: Solar Distillation, Academic Press, London, 1966, pp. 151-198. [2] S.A. Lawrence and G.N. Tiwari, Int. J. Solar Energy, 1 (1989) 215. [3] E. Driolo, Y. Wu and U. Calabro, J. Memb. Sci., 33 (1987) 227. [4] S. Kobuta, K. Ohta, and I. Hayano. Desalination, 69 (1988) 19. [5] D.W. Gore, Proc. 10th Annual Convention Water Supply Improvement Assoc., Honolulu, 1982. [6] P.A. Hogan, Sudjito, A.G. Fane and G.L. Morrison, Desalination, 84 (1991) 81. [7] F. de K~Sr6sy and Y. Shorr, U.S. Patent No. 3,388080,1963. [8] E. Korngold and D. Vofsi, Desalination, 84 (1991) 123. [9] I. Cabasso, E. Komgold and L. Zhong-Zhou, J. Polym. Sci. Polym. Lea., 23 (1985) 577. [10] E. Komgold and E. Korin, Desalination, 91 (1993) 187. [11] I. Ladizhensky, E. Korin and E. Korngold, Water desalination by means ofhydrophilic hollow fibers, Annual Report July 1993-June 1994, Beer-Sheva, Institutes for Applied Research, Ben-Gurion University of the Negev, Report No. BGUN-ARI59-94, July 1994 (in Hebrew).

microporous hollow fibers (Celgard 2500) (internal diameter 0.35 mm) and of our hydrophilic anion-exchange hollow fibers (internal diameter 0.35-1.5 mm) was investigated. The flux of water through microporous hollow fibers and thin anion-exchange hollow fibers was similar: 1.5-3.0 1 of water per 1 m 2 of membranes at 45-65 °C. Practically, the commercial microporous hollow fibers cannot be used, because they are manufactured only with a small internal diameter which causes a high water pumping energy requirement. The anionexchange hollow fibers, which were most suitable for this purpose, have an internal diameter of 1.2 mm, wall thickness of 0.1 mm and ionexchange capacity of 1.0 meq/g.

References