water solutions by using a spiral wound polyamide membrane

Desalination, 99 (1994) 159-168 Elsevier Science B .V. Amsterdam-Printed in The Netherlands

159

Assessment of thermal effects on the reverse osmosis of salt/water solutions by using a spiral wound polyamide membrane N .M. Al-Bastaki and H .I. AI-Qahtani Department of Chemical Engineering, University of Bahrain, P0 Box 32038 (Rahrain) Tel. 973-681234 (441), -688322 ; Fax 973-686300 (Received April 18, 1994; in revised form June 5, 1994)

SUMMARY

The effect of varying the temperature of the feed water for reverse osmosis was studied for two aqueous solutions, one with a relatively high concentration of NaCl (5 gll) and the second with a low salt concentration (0.8 g/1) . The feed temperature was varied between 12-30°C and pressures of 20, 40 and 60 bars were employed . It was found that increasing the temperature resulted in an increase in both the salt passage and the permeate flow rate with increasing temperature . A sieve type mechanism can be used to qualitatively explain these results in terms of an increase of the pore size with temperature, thus allowing for more salt passage and permeate flow rate . INTRODUCTION

Reverse osmosis (RO) has recently become one of the major methods for water desalination, particularly in the GCC (Gulf Cooperation Council) region . A better understanding of the mechanisms and various factors controlling and influencing the RO process has therefore become necessary . Many attempts have been made to explain and model the salt rejection in 0011-9164/94/$07.00 © 1994 Elsevier Science B .V. All rights reserved . SSDI0011-9164(94)00125-1



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the RO of water/salt systems . Among the recent ones are those by Johnson [1], Punzi and Hunt, Punzi and Muldowney [2-4] . The focus in these works was on diffusion and convection type models, whereas Brusilovsky and Hasson [5] introduced modifications to model for multi-ion solutions . Temperature is one of the important factors which influence the salt rejection in the RO process [6,7] . Temperature difference alone can act as a driving force as is the case with nonisothermal transport or thermoosmosis, in which case there is a temperature gradient across the membrane and a temperature difference between the feed and the permeate [8,9] . The present work is concerned with the effect of varying the feed temperature on the water flux and salt rejection at various pressure levels . Isothermal temperature conditions were assumed and hence temperature was the same for all the solutions in the process, namely, the feed, permeate and concentrate .

EXPERIMENTAL SET-UP

The RO unit used in this work was a laboratory-scale unit type CH87 supplied by ISI Impianti, Italy . The unit, as can be seen in Fig . 1, includes feed, permeate and concentrate PVC storage tanks with sizes of 340 1, 45 I and 340 1, respectively . A 1 .1 kW pump is used to deliver water at high pressures to a FilmTec membrane unit . Feed tank heating element

flow meter r ~,I

membrane

L

v ' -~ Pump ) filter ~. /

flow meter

con cent tank

Fig. 1 . Schematic diagram of the experimental apparatus .

>

perm . tank

161

Temperature, pressure and flow rate of the feed water and the product were measured in each test . The conductivity of the feed, concentrate and permeate solutions were measured by taking samples and using a portable conductivity meter . The membrane used was a FilmTec type BW30-2521, which is a spiral wound thin film composite type placed in a GRP shell with a diameter of 61 mm (2.4 inches) and a length of 457 mm (18 inches) . The membrane is housed inside a hoop wound GRP tube with a cylindrical plug inserted at each end. As indicated in Fig . 2, in spiral wound membranes a corrugated spacer is employed to support the clearance which forms a channel for the flow of the feed and eventually the concentrate . The top and bottom walls of this channel are, of course, the membrane . The permeated water flows in another channel supported by a corrugated spacer, where the two walls of this channel are the back sides of the membranes . The permeated water is eventually collected in a plastic tube placed in the center of the unit . The membrane material consists of three layers, which are a polyester support

Carrier In

Concert

Corrugated Spacer

e

Permeate Feed Fig . 2 . Schematic diagram showing the construction of a spiral wound membrane [10] .

162

Ultra-thin Polyamide Layer

0.2

µm

40µm

120

µm

Microporous Ploysulfone

Polyester Support Web

Fig . 3 . Schematic cross section of thin-film composite reverse osmosis membrane .

web (120 µm), a microporous polysulfone interlayer (40 µm) and an ultrathin polyamide barrier (metaphenylene diamine) with a thickness of 2 µm (see Fig . 3) . Two aqueous solutions were employed in conducting the experiments . The first solution was prepared by mixing 750 g of NaCl in 150 1 of water resulting in a concentration of 5 g/l . This solution will be referred to as solution 1 . The second solution was the local tap water, which has a salt concentration of .8 0 gll with NaCl being one of the main salts present . This solution will be referred to as solution 2 .

RESULTS AND DISCUSSION

In the following results the salt passage, SP, is defined as follows : SP = Cplcf where Cf and C are the feed and permeate concentrations (g/1), respectivep ly. Fig . 4 shows the variation of salt passage with temperature at a constant pressure of 40 bars for solution 1 (high concentration of NaCI) . It can be seen that as the temperature is increased, the SP increases . A similar



163

1 .12 1 .1 1 .08 0 . to 0

1 .06 1 .04 1 .02-

0 .98 10

1 12

I 14

I 16

I 18 Temp .

T 20

1 22

1 24

26

(C)

Fig . 4 . Variation of salt passage with temperature at P=40 bar for solution 1 .

0 .6

1

0 .58 0 .56 0 . er

e

0 .54 0 .52 0 .5 0 .48 -,r L 0 .48 ~ 1 18 20

1

22

24

26

28

Temp . (C) Fig. 5 . Variation of salt passage with temperature at P=40 bar for solution 2 .

30



1 64

4 - --O 25 C

0 .98 ~ ° 1 -10 20

- C

17 C

b

12 C

-I i

I

30

40 50 Pressure (bar)

Fig . 6 . Variation of salt passage with pressure at several temperatures for solution 1 . 0 .6

` ' x---` `---x ----` x

3o c

0 .58 0 . 56'1 0,54 26 C 0 .52 -0 .5 -

23 C

0 .48 -

20 C

0 .46

10

I

I

20

30

1

so 40 (bar) Pressure

--, I

s0

Fig . 7 . Variation of salt passage with pressure at several temperatures for solution 2 .

70

165 behavi r is also observed for solution 2 (low concentration of NaCI) in Fig . 5 . On the other hand, the influence of pressure on the salt passage is found to be very little as can be seen in Figs . 6 and 7 for solutions 1 and 2, respectively . These figures show that at each of the temperatures, examined, the salt passage remains constant as the pressure is increased from 20 bars to 60 bars in the case of the low concentration solution (Fig . 7) while it decreases slightly with pressure in the case of the high concentration solution, particularly at higher temperatures (Fig . 6) . On the other hand, it can be clearly seen that by increasing the temperature from 20°C to 30°C, a significant increase in the salt passage results . To explain the relationship between the salt passage and temperature, a sieve-type mechanism is assumed here in which the membrane acts as sieve rather than a barrier . In this case, the influence of temperature is to enlarge the membrane pore sizes . As a result, more salt as well as more water are expected to pass through the membrane at higher temperatures . This idea agrees with the results presented above for salt passage . Figs. 8 and 9 show that the permeate flow rates increase with increasing temperature for solutions 1 and 2, respectively, for pressures of 20, 40 and 60 bars . These results are also in general agreement with the sieve mechanisms approach . Generally in RO a temperature correction factor of 3 % per °C is used . This value seems to be a reasonable estimate compared to the results shown in the above figures . For example, an increase of 23% in each of the salt passage and permeate flux can be observed in Figs . 8 and 9, respectively, for solution 2 at a pressure of 40 bars by increasing the temperature from 20°C to 30°C. To have a better insight into the sieve-type mechanisms, the related . equations will be examined . The sieve mechanism for a steady-state solvent flux, N. in random isotropic microporous membrane can be defined as follows [101 : No =Kw AP/q,l K~

= E ?_'

/ 20

where KW is the hydraulic permeability, rlw the viscosity of the solvent, l the membrane thickness, Ap the pressure drop across the membrane, a the porosity of the membrane, and r the hydraulic mean pore radius .



1 66 70

6

6 _ _ - {>

60 bar

0 -

40 bar

20 bar

20

1 24

22

1

1

26

30

28 Temp. (C)

32

Fig . 8 . Variation of permeate flow rate with temperature for solution I at different pressures .

70 60 bar 60 50 ~-

40 bar

40 3020 bar

20 10 18

I

I

I

-

20

22

24 Temp . (C)

I

26

28

30

Fig . 9 . Variation of permeate flow rate with temperature for solution 2 at different pressures,



1 67 I

1 .26

I

I

I

1 .241 .224

W a

1 .2

z 1 .18Y 1 .161 .14 1 .12

1

10

Fig . 10 . Variati solution 1 .

1

I

14

12

16

18

20

22

24

26

Temp . (C) of permeability coefficient with temperature at different pressures for

1 .4 20 bar

a

1 2 _

40 bar

e

Ir Y

i

1

0 .8 18

1

I

I

f

I

20

22

24 Temp. (C)

26

28

-

30

Fig . 11 . Variation of permeability constant with temperature at several pressures for solution 2 .

1 68

According to the above equation, the hydraulic permeability is proportional to the square of the mean pore radius . Therefore, it is anticipated that an increase in the temperature can result in an increase in the mean pore radius and hence an increase in the salt and water permeability . The above results, as mentioned earlier, have shown this behaviour . In Figs . 10 and 11 the water permeability (calculated by dividing the water flux, N., by the pressure drop across the membrane, 4) is plotted versus temperature for solutions 1 and 2, respectively, at different pressures . It is shown in these results that the water permeability increases with temperature and that the lower pressure results in a higher permeability compared to the higher pressures .

CONCLUSIONS

For the solutions studied in this work (one with a high concentration of NaCI and the other with a low concentration of salts, mainly NaCt, in water), the salt passage increased with increasing the temperature from about 12°C to 30°C . However, changing the pressure from 20 bars to 60 bars did not affect the salt passage for the low concentration solution and only resulted in a small decrease in the salt passage for the high concentration solution, particularly at higher temperatures . The permeate flow rates and the water permeability also increased with increasing the temperature . A sieve-type mechanism seems to be adequate to qualitatively explain this temperature effect in terms of an increase in the mean pore size with temperature.

REFERENCES I G.E. Johnson and C .E . Boesen, Desalination, 17 (1975) 145 . 2 V .L. Punzi and G .P . Muldowney, Rev . Chem . Eng ., 4(1-2) (1987) 1 . 3 V .L. Punzi, G .P . Muldowney and K .B . Hunt, AIChE J, Ivol ., year, pp .]? 4 G .P . Muldowney and V.L . Punzi, Ind . Eng . Chem . Res ., 27 (1987) 2341 . S M . Brusilovsky and D . Hasson, Desalination, 71 (1989) 355 . 6 S .D . Freeman and R .J . Petersen, Reverse osmosis at rigorous temperature and pH conditions, Annual Meeting, American Institute of Chemical Engineers, New York, 1984 . 7 P .J . Connell and J .M . Dickenson, J . Appl . Poly . Sci ., 35(5) (1988) 1129 . 8 J .I . Mengual and F . Garcia-Lopez, J . Non-Equilib . Thermodyn., 13(4) (1988) 385 . 9 J .1 . Mengual, F. Garcia-Lopezand C . Fernandez-Pineda,I, Membr . Sci ., 26 (1986) 211 . 10 R .H . Perry and D . Green, Perry's Chemical Engineering Handbook, 6th ed ., McGraw Hill, New York, 1984, pp . 17-23 .