Indian Journal of Chel\lical Technology Vol. 7, March 2000, pp. 55-60

Studies on fouling and gel polarisation aspects of polyvinyl butyral blended cel­lulose acetate ultrafiltration membrane by resistance model approach

A K Ghosh, V Rarn achandhran, M S Hanra • & B M Misra Desa lination Di vis ion. Bhabha Atomic Research Cen tre, Mumbai 400 085. Indi a

Received 31 AlIglls11 999 : accepled(J8 IJecember 1999

Ultrafiltration (UF) process is ga ining in creasing attention fo r process ing a va ri ety of chemical enl uent streams. A large number of po lymeric candidates have been identified as poten ti all y useful UF me mbrane candidates. A series of UF membranes fro m polyvi nyl butyral blended cellu lose acetate were sy nthesized and their fou ling charac teri sti cs were investi­gated with respect to changing polarity of the polymer candidate. The results are ana lysed using resistancc model approach using po lyethylene glyco l and polye thyleneimine so lutes. Membrane fo uling and gel polarisation aspec ts arc studi ed from the observed changes in water permeation rates when po lar and nonpo lar macromolecu lar solutes are employed. The effect of operating pressure, time and conccn tration were stud ied and th e va ri ous resistances are quant ifi ed . Attempts were made to pred ict ge l polarisation resistance using a si mplified model.

Concentration and puri fication of macromolecu lar species in aqueous so lutions by ultra filtration (UF) process is a we ll recogni sed unit operati on. UF process has a large num ber of proven applications in the food, pharmaceut ica l and chemica l industries t

•

The usefulness of UF process is large ly being fe lt in the industrial waste water reusez and recovery due to the hi gher so lvent permeati on flu xes obtainabl e at low pressures as we ll as to the ava ilab ili ty of membranes with better chemica l stability as compared to Nanofiltration (NF) and Reverse Osmos is (RO) processes. The use of U F process in water trea tment has been in the remova l of turb idi ty , co lour and microorganism. Remova l of tox ic or troub lesome metal ions and water so fteni ng by polymer bindi ng cum ultrafi ltration using water so lub le macromolecules is one of the recent deve lopments reported in literature]. Synthesis of water so luble polymers with specific che lating properties.! , foul ing phenomena S occurring due to the adsorption, deposition and pore pl ugging by macromolecular so lutes on membrane surface and the deve lopment of asymmetric ultrafiltration membranes are a lready reported 6. However informat ion ava ilable on the choice of suitable polymeric membrane substrate with respect to their susceptibility to fouling by polymeric complex ing agents as well as tailoring of membrane molecul ar we ight cut-off are not adequate.

The polar and hydrogen bonding character of the polymer materials and their affinity to polar organic solutes can be quantitative ly represented by so lubility

parameter (asp) and ~ - parameter which are extensive ly di scussed elsewhere 7 . The so lu bili ty parameter (asp ) and its hydrogen bonding (Oh) and di spersion (ad) components of a polymer represent ing polar and nonpolar characters have a bearing on the preferenti al sorpti on characteristics of a polymer. The

~-pa ramete r is generated from I iqu id chromatography data to characteri ze polymer membrane materials in terms of their affiniti es to nonioni sed polar organic so lutes re lative to ionized inorganic so lutes using su ita bly chosen reference solutes. The reported 7 asp, bh, bd and ~ va lues for cellu lose acetate (C A) and polyv inyl butyral (PVB) polymcrs are given in Table I . Thc lower va luc of bh for PVB polymer indicates thcir weak hydrogen bonding capac ity and lower sorpti on fo r water as we ll as hydrogen bond ing so lutcs . Lower [3 va lucs for PVB as compared to CA denotes weaker a ffinity for organic so lutes . l-I ence CA and PVB polymers ,,"crc taken up for study as possible ca ndidates fo r blendi ng to generate F membrancs. PVB blended CA membranes are prepared for a systematic study on the fouling effect of polyethylene imine (PEl), a po lar water so lub le macromolecule which is frequently uscd for its chelating properti es in UFo

Theory

Us ing the resistance model approach ~ the permeate flu x for a dilute so lution of macromolec ular so lu te of high molecular we ight is given in the Eq.( I)

56 INDI AN 1. CHEM. TEC HNOL, MARCH 2000

tlP Jllf (t) = ·rrl

pJRm + R" + R,. (t )] ... ( I)

where Jllf (t) is permeate flux at any time f , ~llV is the pure so lvent viscos ity. Rill is the membrane hydraul ic resistance which is a funct ion of pore size, tortuos ity, membrane thickness and poros ity. R" is res istance due to irreversible adsorptive fouling of so lute on the membrane. Rp (t) is the res istance due to ge l polari sation at time I.

It can be noted that J ill reaches an almost constant final flux Jllf (F) and the time corresponding to thi s Jill

(F) is I(F) . At this stage Rp(t) becomes constant Rp(F) .

6.P. Jllf (F) = arrl

... (2) Jiw[Rm + R" + R,,(F) ]

Further there also ex ists a concentration polari sation resulting from the relative rate of so lute transport to the membrane surface by convection and the back diffusive so lute flu x. Whil e both concentration polari sation and fouling reduce flux , they have opposing effects on the observed rejection. Another way to distingui sh the two phenomena is through their time dependence. Concentration polarisation is dependent on operating parameters such as pressure, temperature, feed concentration and ve loc ity but is not a funct ion of time. Foulin g is partially dependent on these variables, particularly feed concentration but is also a function of time.

The mass transfer co-efficient, k , can be ca lculated from the equation 7,

k = (PR) / Ms xSx3600[( I +m( I :f)MAII OOO] x c x (1-X ,). ln {(XrX,)/(XJ-X,)} .. . (3)

--------. 70 - 0--_. ______ . 60

";.c

i'~ 50 - e ___ . ___ . ______ . ~ 40

J ~ 20

()

lime. nin

Fig. I- Permeate flu x behav iour with time for PEG (20,000) at 50 psi pressure

120 _________ •

-----.---.------. ~£. 1.5 60

~. ~ 60 " ~ ----.- . I • 40

20 ~~ ----=:::::. --.. ~ .- . •

0 0 20 40 60 eo 00 120 140 tiO 1!0 200 220

Torre, rrin

Fig. 2- Permeate flu x behav iour with time for PEl at 100 psi pressure

(PR) refers to the product rate and f refers to the so lute separation with reference to the chosen reference so lute . MA and 1'V/13 refer to molecular weight of so lute and water. S refers to membrane area, m is the so lute molarity and C is the molar density of feed.

Table I----Osr' 0h, Od and P - parameters for ce llulose acetate (CA) and polyvinyl butyral (PV13) po lymers

Polymeric materi al

CA 395

PV13

12.7

9.8

6.33

3.25

7.60

7.67

Table 2 - PWP, PR and % solute separation of Ufo membranes for PEG and PEl so lutes

Membrane PWP' (I.nf2 h· l) PEG' PEl

PR (I .m·2 h· l ) f(%) PR (I.m-2 h· l )

CA 160.0 11 6.4 93 .6 53.7

CA-PVB I 101.0 67.4 60 .3 3 1.5

CA-PVB II 60 .5 2 1.7 34 .9 9.2

CA-P VB III 20.7 7.6 20.2 3.2

• Data co llected at 100 psi pressure f'eed temperaJure : 26°C •• Data co llected at 50 psi pressure

13

137

1.05

f(%)

813

68.8

68 .8

62 .5

GHOSH et al. . STUDIES ON POLYVINYL BUTYRAL BLENDED CELLULOSE ACETATE UF MEMBRANE 57

130

110

90

'" ':~ 70

~ u: 50

30

10

•

1::3:;::: V - . -cA-PVB III

/ / " . / e e / e

A _ A .--­A--- - y_ . ,, - "

40 50 60 70 80 90 100 110 1L'O 130

Pressure . psi

Fig. 3-PR for PEG (20,000)-500 ppm as a function of prcssurt: for differen t UF membranes

ISO

130

110

7" 90

N ·

'§ 70

.2 "-

50

30

10

I -·~ I / . ----e -cA-PV8 I - A.----CA.PVB 11 ..

-"~-PVB"'/

/ / " ________ e e _______ e _A

A _ A .-- --. ... ".--Y

40 50 60 70 00 90 100 110 120 130

Pressure . pSI

Fig. 4---PR for PEI-500 ppm as a fu nction of prcssu n: tllr ditfa­en: UF mcmbrallt:s

Xl, X2, and XJ refer to so lute mole fraction at bulk, membrane so lution interface and mem bra ne permeated prod uct respective ly.

Experimental Procedure

Polyethylene glycol (PEG-20000) hav ing molecular weight in the range of 20000 - 27S00 was obta ined from M is. Aldr ich Chemical Company, USA. SO% aqueous so luti on of polyethyle ne imine (PEl) of molecular we ight - SOOOO vvas al so procured from the same source and diluted with delllinerali sed water to get the desired concentrat ions. Polyv inyl butyral (locally obta ined) blended ce llulose acetatc

3 5

3.0

7 5 2.5

... ;:" . tt

2.0

15

<0 60 ao tJO 120 140

Fig. 5- Rp data as a functi on of pressure for CA memhrane

membranes were prepared by phase inversion. The dope solution were prepared by dissolving both polymers in glacia l acetic acid . Membranes were cast with a thi ckness of 2S0 ).1 . Three different U F membrane samples were prepared by varying the ratio of PVB I CA as 0.2S, O.S and 0.7S . Pure CA membrane was prepared under identical conditions for the sake of compari son.

A constant recircu lation type UF cell was used in a test loop described prev iously. Feed was constantly recircu lated at a fl ow rate of 3 I Imin and the desired pressure was set using a back pressure regu lati ng va lve. The permeate was recyc led back into the feed tank for keeping the bulk concentration constant.

The moist membrane was placed on a hi gh porosity polyester suppor1 fabric wi th air exposed surface fac ing the feed . Flux was ca lcul ated at an interva l of I S min for each run by not ing the time taken to co llect 10m L of permeate. After each run the whole ce ll was rinsed thoroughly wi th deminerali sed water. The membrane was washed and rin sed with dell1inera lised water fo r 2 h to remove any depositio n. PEG was analysed spectrophotoilletrically. PEl was vo lumetri ca ll y anal ysed.

Results and DisclIssion

The pure water permeabi lity (PWP). prod uct rate CPR) and so lute separation for PEG-20000 and PEl 'o lut es in case of different ur membranes ai"e given in Tab le 2. The membrane samples. de ignated as CA. CA- PVB I. C!\- PVB II and C!\-PVI3 III refer to mcmbranes obtain ed by va rying CA/PVB weight ratios, namely. 7S12S. SOISO and 2S175 respecti ve ly. It ca n be seen that th e PWP va lues of CA. CA- PVB 1. C!\-PVI3 II and C!\-PVB III membranes decrease in that order. The progress ive dec line in the PWP values

58 INDI AN J. CHEM. TECHNOL.. MARCH 2000

Table 3- PWP restoration fouling suscept ibility data before and alier use

Membrane (PWP) I (I.m-2.h-l ) (PWP)~

PEG( l.m-2 If l) PEl (I.m-2 If l

)

CA 160.0 120.5 122 . 1

CA-PY I1I 10 1.0 72.9 75.6

CA-PYB II 60.5 40.5 48.0

CA-PYB III 20 .7 12.0 13.1

Appl ied pressure: 100 psi. Feed temperature : 26° C

Table 4-- R", and R, values for diffcrent UF membranes

CA CA-PYB I CA-PYB II CA-PYB III

R",(c nf l) x 10-" 1.55

1?,(cm-l) x I(r" (PEG) OS I

R,(cnf l) x I(r" (PEl) 0.48

Tahle 5--Mass transkr co-e ffi cient va lues for PEG and PEl sol­utes lor CA-PYB I membrane

I' F.G

PEl

1ass Transk r Co-efficient x 107 ( em/sec)

2.3 20

0.903

Feed : 50 [l[lm at SO [lsi pressure

of CA. CA-P VB I, CA- PVB II and C;\-PV B III va lue·) indicate th e effect o f dec reas in g hydrogen bondil ,g component of the res ultant po lymer blend as ex pected fro m th e 8 11 va lues o f the co mponent polymers. Increase in the PVB blending decreases the PWP as we ll as rejecti on o r PEG so lute. PVB blended membrane show better separati on of PEl as compared to PEG. The lower separati on of PEG is due to the favourabl e interacti on between the PEG so lute and the PVB blended polymer as could be expected from the so lu bi lity parameter (8,1') data. The PR va lues for a ll the membranes are lower than PWP va lues pa rticul arly in the case of PEl so lutes. The separation data for C;\ me mbra nes fo r PEl so lute is s ignifIca ntly lower as compared to PEG so lutes eventbough the ave rage molec ul ar we ight of PEl is hi gher than that of PEG . Thi s coul d be due to it s sma ller hydrodynami c vo lume ari sin g beca use of its hi ghly branched structure as aga inst the linea r structure of PEG as we ll as the favourable interacti on between PEl so lute and the po lar nature of the polymer. The product rate data fo r a ll the membranes are given as a fun cti on o f time with respect to PEG and PEl so lu tes in Fig.l and 2. The PR va llie decrease and reach a limiting va lue in a ll the cases as genera lly ex pected. The effect of

2. -15 4. I () II .m

095 2.02 8.68

(LX] 1.06 6.95

pressure on the product rate va lues fo r both the so lutes are given in Fig.3 and 4. The PR va lues are found to increase with increase in pressure. The membranes after test were thoroughl y washed with deminerali sed water several times and PWP was checked. The initial (PWP)1 va lues as we ll as the fin al (pWp)F values obtained after use with PEG and PEl so lutes are given in Tab le 3. The (PWP)" values are found to be lower than (PWP)' va lues .

The intrinsic hydraulic resistance of di fferent membranes for the permeation of pure water (RIlJ as we ll as the hydraulic resistance ari sin g out of adsorptive fouling (Ra) with reference to both the so lutes are given in Table 4. It can be seen that the hydraulic res istance of membrane samples due to adsorpti ve fouling is less than the in trin sic res istance va lues for pure water permeati on. R" va lues reflects the extent of irrevers ibl e fo uling on the membranes e ither by adsorption or by pore plugging. Compari son of R" values for CA-PVB blended UF membranes indicate th at R" va lues increase with increas ing component of PV B. It appears that the increase in Ra va lues with increase in PV B content is due to the favorable nonpolar - nonpolar interacti on whi ch is a lso reflected in the lower separation observed fo r PEG so lute. It can also be seen that Ra va lues with reference to PEl are lower as compared to PEG fo r PVB blended membranes. The lower Ra va lues observed for PEl so lute for PVB blended membranes as we ll as observed higher separa tion refl ect the unfavorable interaction between the polar PEl so lutes and the nonpolar PVB component.

GHOSH el al.: STUDIES ON POL YVrNYL BUTYRAL BLENDED CELLULOSE ACETATE UF MEMBRANE 59

Membrane

Table 6---Gel polarisation resistance data for different feed concentrations

Rp (em-I) x 10- 11

PEG ' PEl"

CA

CA-PVB I

CA-PVB II

CA-PVB III

50 ppm

2.13

3.68

1140

3249

500 ppm

2.9 1

3.99

25 .20

44 .07

50 ppm

4.62

7.87

24.70

78.20

500 ppm

5.20

11.28

26.95

89.50

* Data collected at 100psi pressure ** Data collected at 50psi pressure

'5 5

- e --'-C A-PVB II for PEl - . --cA-PVB U for PEG - T---CA-PV8 I for PEl Feed SOOppm

--'-CA-PV8 I for PEG

- + --- +-----+ + --.. -------- .. .. -------- .. -----.-------. . --.--+ + .------.

60 eo tlO t10

F\"esscre • psi

Fig. 6--Rp data as a lunction of pressu re for CA-PVB blend membranes

" • - . - Experimental value for PEG

11 - e - EJpenmenlal value for PEl • • Calculated value for PEG • Calculated value lor PEl

I~· .----. ------.,.,----=:::::.::::: .... -:::: ..... -:::: ..... :

Fig. 7--Z:a lcu lated and experimenta l Rp va lucs as a func ti on of time for (C!\-PVB I) mcmhrane

The res istance due to ge l layer formati on (Rp) was obtained from the corresponding PR va lues in the case of PEG and PEl so lutes for a ll the membranes. The effect of operating pressure on the ge l po lari sation res istance is g iven in Fig.S and 6 . It can be seen that Rr values are hi gher for PEl so lute for a ll the membranes tested as compared to PEG solute . The

higher values of PEl could be due to the slow diffusivity of the high molecular weight PEl solute relative to PEG. The mass transfer coefficient (k) on the feed side of the membrane obtained using Eq. (3) is given in Table 5. The lower mass transfer coefficient values obtained for PEl solute also refl ects the observation. Also with increasing PYB content in CA-PYB blend membranes, thi s difference is found to increase again reflecting the trend expected from the

lower 13- parameter of CA and PYB polymer materials. The effect of time on ge l po lari sat ion res istance for both the so lutes indicate that the Rp va lues increase and reach a steady va lue indicat ing the limiting flux. The effect of feed concentration on the gel polarisation res istance is g iven in Table 6. As can be expected the Rp values increase with increase in feed concentration.

The permeate flu x data obta ined versus time was represented by the following non-linear mode l.

... (4)

In thi s equation a and 13 va lues are fixed by va lues

of J"f (0) and J"f (E) respective ly. -The a and 13 va lues are obtained for all the membranes us ing the experimental va lues of J"f (0) and J"I" (E). T he ge l po lari sat ion resistance Rp (t) at different times dllrin g the experimental run were predicted us ing Eq.(4) and (I). The predicted buildup of ge l po lari sation res istance in th e case of PEG and PEl so lutes are g iven as continuous line in Fig_7 w ith actua l ex perimenta l data represented by po ints fo r CA-PYB I me mbrane. There ex ists a reasonab le agreement between the experimenta l data and the model prediction . It is true fo r a ll the membranes.

Conclusions

The increase in the polyvinyl butyral content in the

60 INDIAN J. CHEM. TECHNOL.. MARCH 2000

PYB-CA blend reduces the PWP as well as separation of both the solutes. PYB blended CA membranes show higher separation for PEl as compared to PEG -20 000 whereas CA membranes show the opposite behaviour.

The drop in the PWP values as a result of irreversible fouling with PEG -20000 and PEl solutes are comparable.

The intrinsic hydraulic resistance and resistance due to adsorptive fouling were found to increase with increase in the PYB content for different membranes. The resistance due to adsorptive fouling was observed to be lower for PEl solute.

The effect of pressure on gel polarisation resistance was found to be higher for PEl solute as compared to PEG solute. This trend was found to increase with increase in PYB content.

The gel polarisation resistance for both the solutes followed the nonlinear negative power law with respect to time for all the membranes.

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4 Chaut"cr Bernard & Deratani Andre. Nuclear and Chemical Waste Management. (1988) 175

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tion . (National Research Council. Canada). 1985