Desalination 160 (2004) 263-270

Pervaporation of MTBE/methanol mixtures through PVA membranes

Nilufer Durmaz Hilmioglu*, Sema Tulbentci Chemical Engineering Department, Chemical and Metallurgical Faculty, Istanbul Technical University,

Maslak, 34469, Istanbul, Turkey Tel. +90 (212) 2856369; Fax i-90 (212) 285-2925

Received 10 February 2003; accepted 17 June 2003

Abstract

Sorption and pervaporation experiments were carried out with PVAIPAA cross-linked membranes for the separation of azeotropic methyl tert-butyl ether and methanol mixtures. The influence of the PVA/PAA ratio and liquid mixture composition were investigated. With increasing PA4 content in the membranes, solubilities and fluxes decreased and selectivities increased. Total sorption and fluxes increased with increasing concentration of MeOH. Increasing the concentration of MeOH resulted in decreasing selectivities. Because of polarity, MeOH permeated selectively through the membranes. Sorption results showed the same tendency with pervaporation results.

Kqworak Pervaporation; Sorption; MTBE; Methanol; PVA; PAA

1. Introduction

Azeotropic mixtures have conventionally been separated by distillation, extraction and adsorp- tion processes; but there are high capital invest- ment and energy consumption for these separation technologies. Recently, pervaporation

*Corresponding author. Present address: Chemical Engineering Department, Engineering Faculty, Kocaeli University, Veziroglu Campus, 41040, Kocaeli, Turkey. Fax +90 (262) 3355241; email: durmaz@itu.edu.tr, niluferh@kou.edu.tr.

has gained much attention to separate organic mixtures due to its high separation efficiencies coupled with energy savings, especially for the close boiling point and azeotropic mixtures [ 11.

Gasoline lead phase-out legislation has prompted the use of compounds like methyl tert- butyl ether (MTBE) as alternative octane en- hancers for motor oil. Blended in a gasoline pool, these octane enhancers ‘increase the octane number of gasoline and are excellent oxygenated fuel additives that decrease carbon monoxide emissions. Therefore, MTBE has been one of the

001 l-9164/04/$- See front matter 0 2004 Elsevier Science B.V. All rights reserved PII: SO01 1-9164(03)00666-O

264 A? D. Hilmioglu, S. Tulbentci / Desalination 160 (2004) 263-270

fastest growing chemicals of the past decade. MTBE is produced by reacting methanol (MeOH) with isobuthylene from mixed-C4 stream in a liquid phase over a strong acid ion-exchange resin as catalyst. An excess of MeOH is used in order to improve the reaction conversion. The excess MEOH then has to be separated from the products by distillation. The operation requires a high capital expenditure and is not energy efficient because ofthe azeotrope of MeOH and MTBE. For this reason the possible application of pervaporation, which is a more energy-efficient and lower cost process, has been considered as an alternative to distillation [2-6].

The studies in the literature investigating the separation of MTBE/MeOH by pervaporation are given in Table 1 [3-131. A complete survey of the literature for the preparation of PVA mem- branes are represented in Tables 2 [ 14-351 and 3 [36-541.

PVA can be cross-linked by heat treatment or chemically. In this study, PVA is cross-linked both chemically and thermally to make the PVA membranes stronger. Poly(viny1 alcohol) was blended with a low-molecular weight of poly

Table 1 Membranes in literature for separation of MTBE/MeOH mixtures by pervaporation

Membranes used in the literature

CA PPO (polyphenylene oxide) PVA (commercial, Mitsui Eng.) PAA (blend) PELC (polyelectrolyte sodium

cellulose sulphate) CTA Chitosan PVA (cross-linked) Agarose PVP (polyvinyl pyrolidone)

Reference

3 7 5 6 8

9 10 11 12 13

(acrylic acid) and then cross-linked by heat treatment. In the other study, PVA was cross- linked chemically with two different cross- linking agents: glutaraldehyde and formaldehyde and thermal cross-linking [55].

In this study equilibrium sorption experiments and pervaporation experiments were carried out at different PVA/PAA ratios and different liquid feed mixture compositions. Solubilities, sorption selectivities and pervaporation fluxes and per- vaporation selectivities were also estimated.

Table 2 PVA-based membranes for pervaporation applications

Polymer Preparation method

References

PVA PVA PVA, AN PVA, AA, MA PVA, MA, MMA PVA, latex PVA, poly(N-vinyl-

2-pyrolidone) PVA, CS PVA, poly (AMcoA

ANa) PVA PVA PVA, PAN, PSSA PVA PVA PVA PVA, PAN PVA, PSSA PVA, PAN PVA, PVAc PVA PVA, poly(iso-

butylene-co- maleic anhyride)

PVA, chitosan

Solution-casting 14 Photocross-linking 15 Grafting 16 Grafting 17 Grafting 18 Grafting 19 Blending 20

Blending Annealing,

blending Annealing Heat treatment Heat treatment Freezing-thawing Freezing-thawing Composite Composite Composite Composite Composite Irradiation Ion exchange

Ion exchange 35

21 22

23 24 25 26 27 28 29 30 31 32 33 34

N. D. Hilmioglu, S. Tulbentci / Desalination 160 (2004) 263-2 70

Table 3 Methods of WA cross-linking

265

Cross-linking agent

Amic acid Maleic acid Glutaraldehyde

Formaldehyde” Dicarboxyilic acid&r (III) solution” Ketonesa Radiation” Sodium hydroxide -

Cross-linking method

Heat treatment Heat treatment; triethanol amine/water HCl, H,SO,/CH,COOH/CH,OH Heat treatment, Na.$0,/H2S0, H$O,Ma$O, CH,COOHMaOH H,S0,MA2S0,/H,0 - - - -

Heat treatment

Reference

36-40 41 4248

49 50 50 51 52 53,54

“Used for ters osmosis.

2. Experimental

2.1. Material

Methyl tert-butyl ether, methanol, poly (vinyl alcohol), poly(acrylic acid) were analytical grade. Membranes were prepared by the cross-linking technique in the laboratory.

2.2. Membrane preparation

PVA (hydrolyzed 99%) with a molecular weigth of 89,000-98,000 and PAA with a molecular weigth of 2000 from Aldrich were used for producing the membranes.

PVA was dissolved in water by refluxing and stirring for 8 h at 100°C. An homogeneous solution of 10% by polymer weight in water was obtained. PAA was dissolved in water by stirring for 1 h at room temperature. Then the two polymer solutions were mixed together by varying composition to form an homogeneous solution for 24 h at room temperature. The blended homogeneous membranes were cast onto a glass plate. Then the membranes were

allowed to dry in the air at room temperature. The dried blended membranes were peeled off after 2-3 days and crosslinked in a nitrogen convection oven at 150°C for 1 h by heating [14,37,49,52,56-591. The PVAIPAA ratio was changed from 95/5 to 80/20. Because of brittle- ness, the membrane with a PVARAA ratio of 75/25 could not be tested.

2.3. Sorption experiments

Dry strips of polymer films were immersed in a closed bottle containing either methanol, MTBE or a mixture of them both. The bottle was placed in a thermostated oven at 25OC. After the swelling, the equilibrium state was reached, the strip was removed from the bottle, and put into a closed tube after the surface liquid was quickly removed with tissue papers; the swollen mem- brane was then weighed. The sorbed liquid was distilled out of the sample by a vacuum appa- ratus. The composition of the distilled sorbate was analyzed by gas chromatography [52,60].

The swelling ratio was calculated using

266 N.D. Hilmioglu, S. Tulbentci / Desalination 160 (2004) 263-270

Eq. (1) [14]:

DS = (ws - yf)/ w, Wg) (1)

where W, is the weight of the dry membrane and W, is the weight of the membrane swollen in the solution.

The sorption selectivity clsorp is defined by

%xp = (3 k) (3 4) (2)

where x and z represent the weight fractions in the feed and in the membrane, respectively. Indices 1 and 2 refer to the component which has more sorption capacity, and the component which has less sorption capacity, respectively.

2.4. Pervaporation experiments

The pervaporation cell is given in Fig. 1, and the pervaporation apparatus is shown in Fig. 2. The pervaporation experiments were performed with a continuous steel set-up at different temperatures and different liquid feed mixture concentrations for two types of membranes. The membrane was installed in the pervaporation cell. The effective membrane diameter was 6 cm. The feed temperature was kept constant at i0. 1°C. The feed liquid was circulated through the pervaporation cell from a feed tank by a pump with a rate of 2 l/h. The pressure at the down- stream side was kept approximately 10 mbar within hl mbar by a vacuum pump. Permeate was condensed in liquid nitrogen traps. The composition of the collected permeate was determined by GC (TCD Chromosorb 101).

Detailed information about the system was given in a previous study [55].

Pervaporation properties are characterized by the flux J and the selectivity a. Pervaporation flux was determined by weighting of the per- meate. After the 8-h runs permeate samples were taken and analyzed every hour. Steady-state was obtained after 3 h throughout the experiments.

2

1 1 3 n 3

5 6 I

Fig. 1. Pervaporation cell. 1 feed pipe, 2 retentate pipe, 3 bolt, 4 O-ring, 5 filter paper, 6 membrane, 7 supporting layer, 8 permeate pipe, 9 nut.

Cimulation Pump

Fig. 2. Pervaporation apparatus.

The pervaporation selectivity apV is defined by

where x and y represent the weight fractions in the feed and permeate, respectively. Indices 1 and 2 refer to the more permeable component, and the less permeable one, respectively.

N. D. Hilmioglu, S. Tulbentci / Desalination 160 (2004) 263-2 70 267

3. Results and discussion

3.1. Sorption results The PVAIPAA ratios were chosen as 9515,

90/10, 8505, and 80/20. Since the membrane with a ratio of 75/25 was very brittle, it was not tested.

The degree of swelling for pure methanol, MTBE and their mixtures in the membranes at 25OC are given in Fig. 3, where it can be seen that the degree of swelling of methanol decreases with increasing PAA content in the membranes. This can explain the affinity of the membrane and solvent decreases with increasing PAA content. From the pure solvent solubilities, it can be confirmed that decreasing affinity can result in decreasing solubility.

The influence of the PVA/PAA ratio and liquid mixture composition on the sorption selectivity are given in Fig. 4 where it can be seen that the sorption selectivity increases with increasing PAA content. The increasing sorption selectivity can be explained by methanol being sorbed preferentially by the membrane because of lower MTBE solubilities of the membranes.

From these figures it can be observed that the degrees of swelling increase and the sorption selectivities decrease with increasing methanol concentration of the liquid mixture in the all PVARAA ratios of the membranes. Since methanol has a better affinity towards the mem- branes than MTBE, overall solubilities increase, and sorption selectivities decrease as the metha- nol concentration increases.

3.2. Pervaporation results Pervaporation data for the MTBE/MeOH

binary system at 25°C for the four membranes are given in Figs. 5 and 6. The flux and pervapo- ration selectivity values were presented as a function of the MeOH concentration in the feed. Fluxes were divided by membrane thicknesses. As the MeOH concentration increases, the flux

0 20 40 60

MeOH wt. %

80 100

Fig. 3. Degrees of swelling of pure MeOH and MTBE and their mixtures at 2S’C.

18

16 x 4 95/s

n 90110

A 85llS . x

x 80/20 *

x 1,

x

. * l

l .

. . f

0 20 40 60 80 100

MeOH wt%

Fig. 4. Sorption selectivities ofMeOWMTBE mixtures at 25’C.

increases but the selectivity decreases. With increasing MeOH concentration in the feed mixture, because of a strong interaction between MeOH and membranes, the membranes become more swollen and as a result polymer chains become more flexible and transport becomes easier.

In Figs. 5 and 6 the PVAIPAA ratio was from 9515 to 80120. As the PAA content in the membrane increases, the flux decreases but the pervaporation selectivity increases. The decreas- ing flux with increasing PAA content can be caused by a lower swelling of the membranes. In other words, permeabilities decrease with increasing PAA content. This showed that the cross-linking portions in the membranes increase with an increase in the PAA cross-linking agent.

268 N. D. Hilmioglu, S. Tulbentci /Desalination 160 (2004) 263-2 70

l l

I l 956

’ 90110

A 85/15

n W20

[ , ~~)“““~~~-~.~-~~~~~.~~. r-‘” _ . . . . . - . __._.” . . . . . . - _- - - -_. ._ ~ - . - . . . . - - . . - “ . ,

0 20 40 60 80 100 MeOH wt.%

35 0 > 30 XX

l 95!5 = 90/l 0

xx X A 85115

AA x so/20

‘A A X

.

0 20 40 60 80 100

MeOH wt. %

Fig. 5. Influence of the feed composition on the per- vaporation flux at 25°C.

Fig. 6. Influence of the feed composition on the per- vaporation selectivity at 2YC.

Table 4 Summary of seiectivities of PVA membranes for MTBE/MeOH mixtures

Membrane MeOH, wt% in the feed Temp., ‘C Separation factor (MeOH/MTBE) Reference

PAAIPVA: 80120 20 20 40 6 (blended) PVAfPAA: 75125 20 30 1250 11 (cross-linked) PVAIPAA: SO/20 5 25 30 This work (blended and cross-linked)

This can be proof that the cross-linking reactions progressed properly.

Since PAA can preferentially interact with MeOH, it has more polarity. The pervaporation selectivities increase with the increasing PAA cross-linking agent.

Table 4 presents a summary of the selecti- vities of PVA membranes for the separation of MTBEA4eOH azeotropic mixtures reported in the literature and tested in this work. Park et al. [6] investigated PAA membranes blended with PVA. The highest selectivity was obtained by Rhim et al. [l 11. In that study, fully hydrolized PVA with a molecular weight of 50,000 was cross-linked with PAA with a molecular weight of 2000 (25 wt% in water) [49]. In this study, the molecular weight of PVA and some operating conditions are different from Rhim’s and the PAA is solid, but the selectivity was higher than the level of commercial scale, which is about 10.

4. Conclusions Tested membranes are MeOH selective. Sorp-

tion and pervaporation experiments show that pervaporation characteristics can be controlled with adjusting PVA/PAA ratio in the membranes.

Sorption may increase with higher MeOH concentration and this causes higher permea- bility, but because of the decline of the selective sorption, pervaporation selectivity decreases also.

As the cross-linking agent in the membrane increases, cross-linking degree also increases. Therefore, swelling decreases. Because the membrane acts as a perm-selective medium, preferential sorption increases. Since polymer segment mobility decreases with cross-linking, the diffusion of penetrants through the mem- branes also decreases. Lower diffusion and lower sorption cause lower flux and higher selectivity.

Pervaporation has been considered as an alternative separation technique. It may not be

N. D. Hilmioglu, S. Tulbentci /Desalination 160 (2004) 263-270 269

practical to separate completely any entire reactor effluents or it may not replace distillation columns in the petrochemical industry. Pervapo- ration should be used in combination with a conventional separation technique such as a hybrid distillation-pervaporation system to break the azeotropy economically.

5. Symbols

DS - Wd - WY -

XI -

x2 -

Yl -

Y2 -

21 -

z2 -

Greek

a SOlQ - a -

P"

References

Degree of swelling, g/g Weight of the dry membrane, g Weight of the swollen membrane, g Weight fractions of the component with more sorption capacity in the feed Weight fractions ofthe component with less sorption capacity in the feed Weight fractions of the component with more sorption capacity in the pervaporate Weight fractions of the component with less sorption capacity Weight fractions of the component with more sorption capacity in the membrane Weight fractions of the component with less sorption capacity in the membrane

Sorption selectivity Pervaporation selectivity

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