Pervaporation of methanol-MTBE mixtures through modified poly(phenylene oxide) membranes

journal of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 91 ( 1994) 283-29 1

Pervaporation of methanol-MTBE mixtures through modified poly ( phenylene oxide ) membranes’ F. Doghieri”, A. Nardellab, G.C. Sartia,*, C. Valentinib ‘Dipartimento di Ingegneria Chimica e di Processo, Universitbdi Bologna, Viale Risorgimento 2, 40136 Bologna, Italy bEniricerche S.p.A., Monterotondo Scala, Roma, Italy (Received October 22, 1993; accepted in revised form February 16, 1994)

Abstract Modified poly(phenylene oxide) membranes were used to separate methanol from methyl tert-butyl ether solutions through a pervaporation process. The process characterization was performed considering the influence of both the permeate side pressure and the methanol concentration in the feed mixture. The membrane performance has been expressed in terms of transmembrane flux and overall separation factor. Experimental results are discussed in terms of solubility, diffusivity and driving force for the diffusion process of each component through the polymeric membrane. Keywords: Pervaporation;

Poly(phenylene

oxide); Methyl tert-butylether; Methanol; Membrane selectivity

1. Introduction Pervaporation is an attractive separation technique which has been the object of numerous experimental and theoretical investigations [ l-91. In most cases the main concern was the applicability of the process to the separation of aqueous solutions containing organic compounds; both hydrophilic and hydrophobic membranes were used in order to obtain either the dehydration of the stream or the permeation of the organic components. A widely studied application is the dehydration of alcoholic mixtures, e.g., for the production of absolute ethyl alcohol [ 10-141. On the other hand, hydrophobic membranes were ‘Paper presented at ICOM-93, Heidelberg, Germany, August 30-September 3, 1993. *Corresponding author.

used for the treatment of eflluents [ 15- 181, for the recovery of valuable organic substances from sidestreams of industrial processes [ 19-22 ] and for the harvesting of fermentation products [ 23, 241. More recently the interest rose on the application of pervaporation for the separation of organic mixtures in industrial processes [ 25-281. In this work the separation of methanol (MeOH ) from methyl tert-butyl ether (MTBE ) solutions is studied in view of a possible performance improvement of the industrial production of MTBE as a high octane enhancer in gasoline. To that aim, a pervaporation process is considered which uses poly (phenylene oxide) (PPO ) membranes modified through the introduction of alcoholic groups in the chain. Details on the polymer structure and properties are given in refs. 29 and 30.

0376-7388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO376-7388(94)00051-Y

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At first, a feasibility study for the pervaporation process was performed and the necessary membrane pretreatments were determined. Then the characterization of the process was considered with regard to variations of feed composition and permeate side pressure. In view of practical application and also for the sake of comparison with the data reported in ref. 25, our attention was mainly confined to feed mixtures with low MeOH concentrations, up to 2 1% by weight, and downstream pressures in the range 1.3-60 mbar. The influence of the process parameters on the separation performance was determined by measuring the flux and selectivity under different conditions. After suitably introducing the membrane permeability for a pervaporation process, the results are discussed in terms of permeability, solubility and diffusivity resulting from the different activities of the swelling agent, methanol, on both membrane surfaces. Simple relations have been obtained which express the dependence of the separation factor on the permeate pressure and feed composition. Those relations, which are shown to predict in a satisfactory way the experimental results, are based on physically sound ideas and appear to be of a more general applicability, beyond the present case. The results are compared with those for the pervaporation of MeOH-MTBE mixtures using CA membranes [ 25 1.

2. Experimental A pervaporation cell containing a flat circular membrane with an area of 43.3 cm2 was used. A centrifugal pump was used for the recirculation of the retentate to the 2 1capacity feed tank. The vacuum system used on the permeate side of the plant allowed for measure and control of the downstream pressure. Finally, the permeate was collected by condensation in a liquid nitrogen trap and its composition was determined with a gas chromatograph. MeOH RS for HPLC and RS UV grade MTBE

provided by Carlo ERBA were used to prepare the feed mixtures. PPO-OH dense membranes, 40 pm thick, were obtained through solvent evaporation from chloroform solutions. Before the pervaporation runs, the membranes underwent a thermal treatment for 1 h at 190°C and a conditioning process in a MeOH-MTBE mixture ( 7 wt% MeOH ) for 10 days. The thermal treatment produced a partial crosslinking of the polymer which was shown to be essential in order to preserve the mechanical strength of the membrane when it was placed in a swelling environment. On the other hand, the relaxation process of the polymer matrix, obtained through the conditioning treatment, allowed for the production of a homogenous set of membranes whose behavior under permeation conditions showed good stability. The effect of the thermal and conditioning treatments on the mechanical properties of the polymer and on the separation performance of the membrane was not optimized and for a better understanding further investigations are needed. The flow measurements for the MeOH and MTBE species across the membrane which are reported below refer to the steady state conditions reached after a transient period of a few hours.

3. Results The influence of both the feed composition and downstream pressure on the transmembrane flux and selectivity was analyzed. For the sake of comparison with the results obtained using asymmetric CA membranes, the same operating conditions were used as the ones considered by Schell et al. [ 251; in particular, the temperature was kept constant at 22 ‘C. For a downstream pressure of 1.3 mbar increasing MeOH concentrations in the feed mixture were considered, up to 2 1 wt%. For the constant mass fraction of MeOH in the feed mixture, ~~,o~(~) =2 l%, different pressures on the permeate side of the membrane from 1.3 to 60 mbar were studied. As a reference, pervaporation runs

285

F. Doghieri et al. /Journal ofMembrane Science 91(1994) 283-291

0

1000

2000

3000

4000

5000

6000

Permeate side pressure (Pa) Fig. 1. Transmembrane

fluxes as function of downstream

pressure, methanol weight fraction in the feed bran

= 2 1%.

500

450 400 350 Flux

300

(@h m2) :z 150 100 50 0 0

1000

2000

aW0

4WO

Pemwate side pressure

5000

6000

0

(Pa)

Fig. 2. Methanol selectivity as function of downstream pressure, methanol weight fraction in the feed c~~~n(~) = 2 1O/o.

0.05

MeOHmassfmctioninthefeed

0.05

0.1

MOHmass

Fig. 3. Transmembrane flux of methanol as function of methanol feed concentration, downstream pressure P,= I .3 mbar.

were performed for both pure MeOH and pure MTBE as the feed, for the same downstream pressures. When the downstream pressure increases an

0.25

Fig. 4. Transmembrane fluxes as function of methanol feed concentration, downstream pressure P, = I .3 mbar.

0

MeOH mass fraction In the feed

0.2

0.15

0.1

0.15

0.2

0.25

fraction in the feed

Fig. 5. Methanol selectivity as function of methanol feed concentration, downstream pressure P,= 1.3 mbar.

approximately linear decrease of the flow of MeOH from 460 to 3 16 g/h m2 is observed while correspondingly a linear increase from 200 to 240 g/h m2 is obtained for the MTBE transmembrane flux (Fig. 1). Consequently, MeOH selectivity cv calculated as

F. Doghieri et al. /Journal qf Membrane Science 91 (I 994) 283-291

286

(Y=

(%kOH/%4TBE 1

permeale

( Wh4~OH/%4TBE

) feed

decreases linearly in the range investigated, from an average value of 7.7 at 1.3 mbar to an average value of 4.9 at 60 mbar (Fig. 2 ) . At 1.3 mbar downstream pressure, the MeOH flux is proportional to the feed alcohol concentration (Fig. 3 ) and reaches a value of 2000 g/h m2 for the pervaporation of pure MeOH. The MTBE transmembrane flux initially decreases from 232 to 120 g/h m2 when the alcohol concentration in the feed mixture, uMeOHcF), increases from 0 to 3.2%; then it increases up to (F)=21% (Fig. 4). Con210 g/h m2 for wM,=~H sequently, as the MeOH content in the feed increases, a minimum value is also shown by the total mass flow across the membrane, which is dominated by the MTBE flux for low values of @MeoH(F)and by MeOH flux for higher alcohol contents in the feed. The resulting selectivity at 1.3 mbar downstream pressure shows a monotonous decrease as the alcohol concentration in the feed mixture increases, ranging from or = 23.4 for mMuleoHcF) = 1% down to a=7.7 forw,,0H’F’=21% (Fig. 5). 4. Discussion The main aspects that need to be considered are the effects of downstream pressure and feed composition on the partial fluxes and consequently on the separation factor. In particular we refer to two relevant points: (i) with increasing the downstream pressure, there is an increase in MTBE flux while MeOH flux decreases; (ii) with increasing the MeOH content in the feed, MTBE flux initially decreases and then increases after a minimum is reached. The same tendency was also observed for the same system using CA membranes [ 25 ] under the same operating conditions; a comparison between the pervaporation performance of PPOOH membranes and CA membranes is thus possible and helpful. The definition of membrane permeability $

usually applied to describe the transport of gases [ 3 1 ] is here extended to the case of liquid phases, thus generalizing what appeared in a recent work by Wijman and Baker [ 32 1, as:

where f ,’ and f p, respectively, indicate the fugacity of the species i on the feed and permeate sides of the membrane, N, is the transmembrane flux of species i and 6 is the membrane thickness. For gaseous phases, the species fugacities reduce to partial pressures at limited pressure values, so that Eq. (1) reproduces the well-known relationship. In our calculations, we assume that the conditions in the bulk liquid and gas phases surrounding the membrane represent the conditions of the respective fluid-membrane interfaces. Indeed concentration polarization effects proved to be negligible as the partial fluxes were insensitive to the recirculation rate on the feed side of the membrane. In absence of specific experimental data, the overall membrane thickness to be introduced in Eq. ( 1) was assumed to be well approximated by the value measured for the dry sample in all cases. Under the usual pressure conditions, for the cases in which the feed to the membrane cell is a pure liquid, f 7 may be evaluated with good approximation by the value of the vapor pressure P: at the same temperature; analogously when a mixture is fed to the cell the fugacity f f: may be calculated as follows:

fr=P:w,

(2)

The activity coefficient yi may be evaluated according to usual thermodynamic techniques (see, e.g., ref. 33). In the present work, activity coefficients yi for MeOH and MTBE in the liquid feed were calculated, for all the process conditions, using the Redlich-Kwong-Soave equation of state. On the other hand, for the gas phase downstream the membrane, the fugacity of component i may be approximated by the value of its partial pressure in all cases, due to the low value of the total pressure.

287

F. Doghieri et al. /Journal ofMembrane Science 91(1994) 283-291

In Table 1 the permeability values obtained from Eq. ( 1) are reported for the case of 0~~~~~) = 0.2 1 at different downstream pressures; for sake of completeness, the fugacity values and fugacity differences are also included. In the liquid feed, the corresponding activity coefficients are ~~M~0r-r = 1.732 and YMTBE = 1.3 14. Apparently, when the downstream pressure is raised from 130 to 1000 Pa, MeOH fugacity increases from 110 to 840 Pa, while no permeability changes are measured for both components. On the contrary, when the downstream MeOH fugacity is 4700 Pa a significant permeability increase is obtained of the order of 25% for MeOH and 20% for MTBE. Since MeOH is a plasticizing if not swelling agent, the above increase may be easily attributed to the increase of the diffusivity in the membrane layers close to the gas phase due to the higher content of MeOH in the permeate side. In particular we observe, on the basis of permeability data, that an appreciable plasticization has already taken place when the MeOH fugacity is around 4700 Pa. In the same range of downstream pressures, the driving force for MTBE transport,i.e., its fugacity jump across the membrane, is rather constant so that MTBE flux variations are only associated with permeability changes. For the MeOH species, on the contrary, the fugacity jump greatly decreases when the permeate pressure increases and that causes the flux reduction which is shown in Fig. 1, despite the positive variation of MeOH permeability. In Table 2 permeability values at 130 Pa downstream pressure are reported as computed for the case of different values of MeOH concen-

tration. In the range considered no substantial variation appears for the permeability up to cF)=0.032; for larger MeOH contents we OMeOH observe an increase in MeOH permeability, although not as high as one would expect based on the usual exponential dependence of diffusivity upon the concentration of the plasticizing agent. Therefore we recognize that the MeOH partition coefficients SMeoH= cMeOH/fhleOH, relating the equilibrium MeOH concentration cMeOH in the polymeric membrane to the corresponding value of the MeOH fugacity, is not a constant and decreases as the MeOH content increases. We now turn our attention to the non-monotonous behavior of MTBE flux with the feed composition. In general, MTBE diffusivity is highly affected by the concentration of the plasticizing agent MeOH. However, when the feed MeOH content increases from %&HCF) = 0 to 0.032, the of MTBE decreases from permeability 0.551 x lo-l3 to 0.471 x lo-l3 s, thus indicating that MTBE solubility is somewhat inhibited by MeOH so that the MTBE partition coefficient S MTBEdecreases as the MeOH content increases. When the feed MeOH content further increases beyond 3.2%, the MeOH fugacity exceeds 4490 Pa, i.e., sufficiently high to produce a significant plasticization, which results in an increase in the permeability of MeOH itself; consequently, the values of MTBE diffusivity and permeability increase as well. Finally, let us consider explicitly the separation factor cy and its variations with both feed composition and downstream pressure. In order to obtain quantitative estimates we remind that, according to its definition, we can write

Table 1 Permeabilities

PV (Pa)

130 1000 6000

and selectivity data as function of downstream

MeOH o,(F)

0.209 0.208 0.205

pressure MTBE

L-x

f'

f'

Af

9

f'

fP

Af

9

(Pa)

(Pa)

(Pa)

(10-l’s)

(Pa)

(Pa)

(Pa)

(lo-‘3s)

10520 10480 10370

110 840 4660

10410 9640 5710

4.86 4.89 6.15

22420 22480 22650

20 160 1340

22400 22320 21310

1.05

1.06 1.21

exp.

Eq. (6)

7.1 1.2 4.9

7.1 4.4

F. Doghieri et al. /Journal of Membrane Science 91(1994) 283-291

288 Table 2 Permeabilities MeOH m(F)

CY=

MTBE Y

0.011 0.017 0.032 0.066 0.100 0.209

downstream pressure P,= 1.3 mbar

and selectivity data as function of methanol feed concentration,

4.33 4.14 3.13 3.03 2.56 1.73

f’ (Pa)

S’ (Pa)

.‘p (10-13s)

Y

$a)

1860 2720 4490 7130 8640 10520

54 64 83 98 105 110

1800 2650 4400 7030 8530 10410

2.21 2.02 1.99 2.46 3.10 4.86

1.001 1.003 1.01 1.04 1.08 1.31

NMeOHINMTBE

((iJMMeOH/ WMTBE

(3)

) feed

(Y

I

S’ (Pa)

f’ (Pa)

‘$a)

(10-l’s)

28620 28220 27280 25660 24490 22420

76 65 47 32 25 20

28550 28150 27230 25630 24460 22400

0.54 0.51 0.48 0.59 0.70 1.05

of zero downstream we can write:

exp.

Eq. (9)

23.4 20.9 19.3 15.7 13.4 7.7

22.3 20.0 15.8 12.7 7.1

pressure, then from Eq. ( 5 ) (x

PV

and then, after Eqs. ( 1) and (2):

(Y=a!O l(

The ratio between the membrane permeabilities of the two components is expected to be weakly dependent on temperature or composition of the feed and on permeate pressure. In order to investigate the influence of the downstream pressure on cy it appears thus convenient to consider a reference case in which the permeate pressure is an arbitrarily fixed reference value PvcR) at the same temperature and feed composition; by indicating with a(R) the corresponding separation factor, from Eq. (4) we obtain:

For those processes in which very high selectivities are obtained ((YX= 1)) an approximately linear dependence of the separation factor (Yfrom downstream pressure is predicted by Eq. (5 ) or (6) in accordance with the experimental results shown in Fig. 2. For sake of comparison, the experimental values of the separation factors for the process investigated are reported in Table 1, as well as the values predicted by Eq. (6 ) . In this case, a! ’ was approximated by the experimental value at P,= 1.3 mbar. The comparison shows a rather satisfactory agreement. From Eq. (4) a simple relation may be derived to evaluate the influence of the feed composition on the separation factor. When the downstream pressure is low enough to make the permeate side fugacities negligible with respect to the corresponding values in the feed, Eq. (4) reduces, as also indicated by Wijman and Baker [ 321, to:

(4)

a

cp*yx ) MeOH-

a(R)-

Nxz;;y;;TBE Pan

(p*Yx)MeOH

-

MeOH

(5)

(y ( ;) XMeOH

+

XMTBE

Eq. (5 ) was derived by considering the usual case in which for the component preferentially retained by the membrane the downstream fugacity is negligible with respect to the fugacity in the liquid feed. In addition, the ratio between the two permeabilities was considered constant with the downstream pressure. If a0 refers to the separation factor in the limit

(y=p

9

M~OH

9 MTBE

(6) (P*Y)M,oH

(P*Y)

M~OH

(P*Y)MTBE

aXMeOH

MMTBE

+

XMTBE

>

(7)

MM~OH

In order to investigate the influence of the feed composition it appears convenient to consider a reference case in which the feed composition is an arbitrarily chosen value x’ , and the corresponding separation factor is (x’ . From Eq. ( 7 ) , since the ratio between the membrane permea-

F. Doghieri et al. /Journal of Membrane Science 91(1994) 283-291

bilities of the two components a constant, we obtain:

is expected to be

(8) YMTBEY~OH

where Y’ stand for the activity coefficients in the reference state. If the latter is chosen to be the infinitely dilute solution, then Eq. (8) reduces to

YMTBE

YE&OH

(9)

In Table 2 separation factor data are reported as predicted on the basis of Eq. (9) when cuoois approximated as the experimental value obtained for o&&HcF) = 0.0 11. Comparison between the experimental and calculated separation factors reported in Tables 1 and 2 demonstrates that Eqs. (6) and (9 ) satisfactorily predict the effects induced by the variations of permeate pressure and feed composition, respectively. The above result is indeed very simple, although it appears rather general if applied to the separation of organic mixtures in the dilute mixture range. As far as the pervaporation performance is concerned in the concentration range of practical interest, i.e., for MeOH content smaller than N 6 wt%, we note that by using dense symmetric membranes made of modified PPO, 40 pm thick, the separation factors range between 16 and 23, while the total permeate flux ranges between 190 and 300 g/h m2, at 1.3 mbar downstream pressure. On the other hand, the membrane thickness may be dramatically reduced rather easily by a factor of 100 and consequently the total flux may increase by the same factor accordingly. Therefore, the membranes under consideration may give rise to fluxes lo- 100 times larger than the fluxes observed in asymmetric CA membranes by Schell et al. [ 25 1. The separation factor here obtained is 5-20 times smaller; however, we can see that the smaller separation factors observed are more than counterbalanced by the very high transmembrane fluxes obtained.

289

5. Conclusions The feasibility of a pervaporation technique for the separation of MeOH-MTBE mixtures has been investigated using modified poly (phenylene oxide ) membranes; particular attention has been given to mixtures with low alcohol contents. For sake of reference, symmetric membranes were used, 40 ,um thick, suitably crosslinked through a thermal pretreatment and conditioned in a MTBE/MeOH mixture. Selectivity as well as transmembrane fluxes of each species changes with both the alcohol content of the feed and the downstream pressure. Selectivity decreases as the downstream pressure of the MeOH concentration in the feed increases. As the permeate pressure increases MeOH flux decreases, contrary to what happens to the flux of MTBE. Our calculations demonstrate that the permeabilities of both components increase with the downstream pressure; this is associated with the higher fugacity of the plasticizing agent, MeOH, in the membrane layers. This justifies the flux trend of MTBE, for which an increase in the downstream pressure does not alter the overall fugacity difference across the membrane. On the other hand, due to the low MeOH content in the feed, the overall driving force for MeOH is significantly reduced by an increase in the downstream pressure which ultimately leads to smaller MeOH fluxes. The MeOH flux monotonously increases with the alcohol concentration in the feed while a minimum for the MTBE flux is obtained, located in the low alcohol concentration range. This behavior is attributed to two different effects associated with an increase in the MeOH content in the feed. One effect is the higher plasticization induced in the membrane by higher MeOH contents which results in an increase in the permeability of both species contained in the feed. The other effect is the change in the driving force for the mass transport across the membrane; as the MeOH content increases in the feed, the driving force for MeOH permeation increases while the driving force for MTBE correspondingly decreases. In order to estimate the overall pervaporation

F. Doghieri et al. /Journal

290

of Membrane Science 91 (I 994) 283-291

performance we can consider to a first approximation the selectivity and the permeabilities as constants with the membrane thickness and use the values obtained for the 40 pm membranes also for asymmetric membranes, the thickness of which can be safely taken as 0.4 pm; then MeOH flux as high as 4700 g/h m2 may be expected for feed mixtures with 1.7% of methanol in the feed, with a separation factor in excess of 20. These considerations make the process quite attractive when compared with both flux and separation factors measured for the pervaporation process using asymmetric CA membranes, albeit in that case the separation factor is much higher than for the membranes used in the present work

List of symbols equilibrium concentration of species i in the polymer membrane fugacity of species i in the feed fugacity of species i in the permeate molecular weight of species i transmembrane flux of species i vapor pressure of species i downstream pressure permeability of species i molar fraction of species i in the feed mixture molar fraction of species i in the permeate methanol selectivity membrane thickness activity coefficient of species i fugacity jump across the membrane for species i mass fraction of species i in the feed

Acknowledgments The fruitful assistance of Dr. D. Fajner and Miss S. Parmeggiani in performing the experiments is gratefully acknowledged.

References

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