Pervaporation of low volatility aromatics from water

143

Journal of Membrane Sczence, 53 (1990) 143-158 Elsevler Science Publishers B V , Amsterdam

PERVAPORATION WATER*

OF LOW VOLATILITY

AROMATICS FROM

K W BODDEKER, G BENGTSON GKSS Research Center, 2054 Geesthacht (F R G) andE BODE Fachhochschule Coburg, 8630 Coburg (F R G) (Received January 12,1989, accepted m revlsed form July 25,1989)

Summary Pervaporatlve ennchment of high boiling aromatic hydrocarbons from ddute aqueous solution through elastomenc polymer membranes 19investigated Selectlvlty m favor of the organic. solution component, m addition to preferential membrane permeablllty, requires the aqueous-organic feed solution to be non-ideal with positive deviation from Raoult’s law The observed effects of downstream pressure and membrane thickness on pervaporatlon performance are ratlonahzed by a transport model plcturmg the membrane as freely permeable to the organlcs while typically posing a resistance to water The model predicta organic flux density and ennchment to be proportional to the activity coefficient of the organic solutes m water, and mversely proportional to downstream pressure m pervaporatlon

1. Introduction Separations of liquid mixtures by pervaporation, according to the sorptiondiffusion concept, rely on specific interactions between the solution constituents and the membrane polymer. Selectivity is due to differences in membrane permeability irrespective of pure component vapor pressures. Depending on the membrane material used, pervaporation of aqueous-organic solutions may be directed at selective removal of water (dehydration), or at selective removal and concomitant enrichment of the organic component [ 11. By tendency, glassy (amorphous) polymers preferentially permeate water, the smallest of liquid molecules; the sorption isotherm is of the Flory-Huggins type as described by a dual sorption model. Elastomeric polymers, on the other hand, interact preferentially with the organic solution component; the sorption isotherm is of the Henry type, i.e., linear, and is described by a free volume model. A further type of pervaporation membrane is ion exchange membranes, which may be viewed *Paper presented m part at IMTEC ‘88, Sydney, N S W., Australia, November 15-17,1988

0376-7388/90/$0350

0 1990 -

Elsevler Science Pubhshers B V

144

as crosshnked electrolytes and thus are anticipated to relate to water; with polar species, however, such as phenol, specific interactions are additionally to be expected. This commumcation illustrates the conditions under which high boiling organic species are selectively pervaporated from their aqueous solutions. 2.Conditions for pervaporability High boiling (low volatility) organics can be pervaporated from water provided that the following conditions are met: (a) The orgamcs to be separated form non-ideal solutions with water, deviatmg from Raoult’s law in a positive manner, i.e., showing activity coefficients > 1. (b) The pure organic species have a low, although not vanishing, vapor pressure. (c ) The membrane polymer exhibits preferential permeability for the orgamc solution component. Pervaporation under these conditions yields a depleted aqueous retentate and an enriched organic permeate, either of which may be the target product of the process Implications of concern to pervaporation of a “positive” non-ideal solution behavior, all reflecting a low degree of molecular interaction, are: (a) Limited miscibility of the organic species with water, resulting m phase separation when enriched beyond the respective solubility limit. (b) Volatility with steam, implying that, in the vapor phase, the partial pressures add up to the sum of the pure component vapor pressures. It follows that the mole fraction of the low volatility organic species in the equilibrium vapor phase is:

(1) (c) Another corollary of “positive” non-ideality is the occurrence of positive azeotropes, i.e., of constant boiling compositions of higher vapor pressure than either of the pure solution components. Permeabihty m diffusion-controlled membrane processes is the product of solubility and diffusivity of the permeating species in the membrane polymer. Selectivity m favor of the organic species may be interpreted in terms of preferential sorption of the organics by the membrane, and/or hindered mobility of water in the membrane. Boundary effects, except for concentration polarization, are usually considered negligible. 3.Organic compounds examined The organic compounds examined are aromatic hydrocarbons, listed in Table 1 in order of increasing enrichment on pervaporation from dilute aqueous

145

TABLE 1 Physical constants and pervaporatlon data of the organic compounds examined, arranged m order of increasing organic ennchment from dilute aqueous solution Organic component

Resorcmol Hydroqumone Phloroglucmol Benzolc acid Phenol 2-Methylphenol 2,5-Dlmethylphenol p-t-Butylphenol o-Chlorophenol o-Nltrophenol Toluene Nltrobenzene Thymol Styrene o-Dlchlorobenzene Water

Phyalcal constanta

Correlation data

MW

Kp, “C

Vapor pressure at 5O”C, mbar

110 110 126 122 94 108 122 150 128 139 92 123 150 104 147

281 286 sub (d) 249 182 191 2115 237 175 215 110 6 210 233 145 (PI 180 5

<02 3 25 12 03 13 13 128 2

100

123

11 11 11 12 11 13 16 21 56 11 13 11 21 14 01

18 02

“PEBA 40, membrane thickness 46 p,

-

(3:; 85

Pervaporatlon”

Solublhty Ennchment in water factor, j? at 2O”C, mmol/l

Flux density of organic, g/m2-hr

-

-

11170 726 79 3 23 7 892 231 18 53 218 144 0 54 15 4 53 2 88 0 34 -

20 75 150 210 250 265 270 275 295 380 690 1000 -

04 13 28 35 54 51 45 56 84 51 -

50 ’ C

solution. Pervaporatron performance is correlated with pure component vapor pressure and water solubllity, the latter reflecting the degree of non-ideality of the respective aqueous-organic feed solution (see below). Solubllity in water ranges from practically insoluble (o-dichlorobenzene) to almost completely miscible (resorcinol), the low solubility of the highest-valent phenol, phloroglucmol, being accounted for by the symmetry of the molecule. The investigation concentrated on phenol-water because of the relatively wide concentration range accessible, but also because of the practical significance of this system. The most extensive documentation of liquid-vapor eqmlibrium data on phenol-water refers to a temperature of 44.4”C [ 21; pure component vapor pressures at this temperature are: water, 92.7 mbar; phenol, 1.86 mbar. The phase diagram and activity coeffkients of the phenol-water system are shown in Fig. 1. The coexisting phases at somewhat higher than room temperature are referred to as “phenol in water”, approximately 10 wt% phenol, and “water in phenol”, approximately 70 wt% phenol. The phenol-water axeotrope has a normal boiling pomt of 943°C at 9.2 wt%

146

60

0

01

Weight

02

03

Fraction

04

06

06

07

of Phenol

06

09

10 WI

Fig 1 Actlvlty coefficients at 44 4°C [2] and phase diagram of the phenol-water

system

of phenol; on lowering the pressure, the azeotropic composition is shifted to lower phenol concentrations. In terms of the above conditions, phenol occupies an intermediate position between the aromatic hydrocarbons considered here and aliphatic alcohols, such as butanols [ 31. 4. Polymer and membranes

The elastomeric polymer used in this study are polyether-block-polyamides (PEBA), which are found to exhibit exceptional selectivity for aromatic hydrocarbons in pervaporation. PEBA formulations are segment-elastic, consisting of thermoplastic polyamides made flexible by elastomeric polyether links; the ratio of the two determines the Shore hardness of the polymer [ 41. Commercially available granulates with the trade name Pebax (Atochem) were used, designated PEBA 35,40 and 55 according to their Shore hardness. Homogeneous PEBA membranes covering a range of thickness of 2 to 200 pm were prepared by solution casting. As indicated in Table 2 by pervaporation results on phenol-water, the solvent system used has little effect on pervaporation performance. Membranes thicker than 25 pm were made by casting directly onto glass plates, wlnle below 25 p a microporous polyetherimide (PEI ) support was used. PEBA membranes are also made by melt extrusion. The sorption isotherms of phenol and water m PEBA 40 at 44.4’ C are pre-

147 TABLE 2 Pervaporatlon of phenol-water through PEBA 35 cast from different solvent systems (feed concentratlon 100 mg/kgphenol, 50°C) Solvent system

1-Butanol-1-propanol (4 1) Cyclohexanone-1-butanol(6 1) Toluene-1-propanol (12 1) Chloroform-formic acid ( 13 1)

Membrane thickness,

Flux density, g/m2-hr

p

Phenol

Water

90 95 95 85

152 155 132 152

122 0 118 5 1155 134 5

Ennchment factor, /?

121 124 111 108

105

104

s : t8 s 0

i

103

1

H b

in 102

10"

10'

Equlllbrwm

Concentration

10‘

Fig 2 Sorption isotherms of phenol and water m PEBA 40 at 44 4” C

sented in Fig. 2, covering equilibrium phenol concentrations from 10 mg/kg to near the solubility limit of phenol in water. The sorption isotherm of phenol is linear up to an external concentration of 5 X lo4 mg/kg, at which point the polymer absorbs its own weight in phenol; beyond this, swelling becomes excessrve Water sorption is not affected by the presence of phenol up to a concurrent phenol sorption of comparable magnitude (about 1.5 wt% ); thereafter it increases slowly as the polymer softens. Sorption of the aromatrc hydrocarbons in PEBA inversely parallels their solubility in water, cf. Table 1, as is shown in Fig. 3 by the relative rates of depletion of dilute aqueous-organic solutions on contact with the polymer. Again, phenol 1s in a borderline position; all other organics considered (in-

148

5

;;i-

06

-

-..sno,_

~-Chh7phe”o,

-v 0

20

40

60

P-kft-BUt~lthhs”o, 60

100

120

140

160

Time

160

200

220

I minutes

1

Fig 3 Ftelatwe rate of sorption of aromatw hydrocarbons from aqueous solution by PEBA 40 granulate

cludmg benzoic acid, but excepting the higher-valent phenols) are more readily sorbed, suggesting that they conform to the conditions for pervaporability more closely than phenol itself. 5. Pervaporation: experimental Pervaporation experiments were conducted in a test apparatus with contmuous feed circulation and adjustable downstream pressure, as represented in Fig 4. The test cell, providing radial membrane coverage through a peripheral feed inlet and central feed outlet, 1sshown in Fig. 5; membrane area is 100 cm’. Standard vacuum fittings are used in the downstream manifold. Downstream (permeate) pressure is monitored by an expansion gauge located underneath the test cell. Downstream pressure is regulated by an adjustable valve actuated by the pressure monitor. The feed is circulated from a thermostated reservoir at a flow rate of 2000 ml/min. Permeate withdrawal at < 1 ml/min is small by comparison, minimizing the effects of concentration polarization and temperature loss due to pervaporation action. The permeate is collected in weighed traps at liquid nitrogen temperature and, after appropriate dilution, is analyzed by W spectroscopy or gas chromatography. Except for the vacuum pump (including a safety trap not shown in Fig. 4) and the pressure monitor, the entire apparatus is enclosed in a thermostated cabinet. Pervaporation effects, in the following sections, are recorded in terms of the enrichment factor of the preferentially permeatmg organic species (/3+/c: = w:‘/w: using mass concentrations), and the total or partial flux density J in units of g/m’-hr. Transformation from molar units, where desirable, 1sindicated.

r

--- -----

149

1 -_---

Fig 4 Test loop for pervaporatlon experunents under controlled condhons Q= feed flow rate, P=permeate pressure

Fig 5 Pervaporatlon test cell (GKSS)

T=temperature,

Effectwe membrane area 100 cm2

6. Pervaporation: effect of downstream pressure

Pervaporation of low volatility compounds requires the permeate pressure to be correspondingly low in order to estabhsh activity gradients across the membrane. It 1s anticipated, therefore, that the pervaporatlon effects under consideration will strongly depend on permeate pressure (as downstream pressure, p” ) and, moreover, that the observed pressure dependence will provide inside mto the transport mechanism.

150 250

P

E ;:

200 ”

200 -

150

2 e E

100

5 ii

Enrrctnnent - celwleted +

Permeate

Pressure

experiment

(mbar]

Fig 6 Pervaporatlon of a ddute phenol-water solution as a function of permeate pressure, companson of observed and calculated phenol ennchment

The dependence of pervaporation performance of aqueous-organic solutions of low volatility organic compounds is illustrated in Fig. 6, again using phenol as a representative example (feed concentration 100 mg/kg; 44.4’ C; PEBA 40, membrane dry thickness 46 p). It is observed that water permeability (as total flux density) depends little on downstream pressure, whereas phenol enrichment increases strongly as the pressure is lowered (experimental points in Fig. 6). Estimating the activity gradient for phenol across the membrane by comparing the partial pressure of phenol in the permeate (p:’ ) from the molar fluxes according to

with the vapor pressure of phenol in the feed @: ) reveals that, as long as the total permeate pressure (p” ) is substantially higher than the partial pressure of phenol therein, phenol is permeating through the membrane apparently without an activity gradient. For water, by contrast, a steep activity gradient does exist. These observations form the basis for a transport model describing, as it seems, a limiting situation of pervaporative separations. 7. Pervaporation: transport model The transport model for the pervaporation situation under consideration is based on the assumption that the organic permeant has very nearly constant activity throughout the system, implying that the permeability of the mem-

151 feed

( 11

permeate

(vI

Fig 7 Dmgmm of the pervaporation model 1=orgenlc permeant,

J=

water

brane for the organic substance is extremely high. Membrane permeability for water, on the other hand, is typically restricted. In terms of the pertinent pressures, the model is depicted in Fig. 7. The membrane is visualized as being soaked with the organic permeant. Water vapor, adding up to the total permeate pressure, aids in the transfer of the permeant to the condenser. Reduction of permeate pressure, by affecting the partial pressure of water only, increases the separation effect. The model is verified by examining selectivity and flux in terms of model conditions. When using molar concentrations, the separation factor is readily obtained as follows. The basic assumption of the model is expressed as (3)

P:‘=Pi

The molar concentration of the organic compound in the permeate vapor is (4)

x:‘=p:‘/p”

The partial pressure of the organic component in the liquid feed is (5)

P:=YP%

Combining eqns. (3)- (5) yields the separation factor a! as: =ypp(l-r:)

x:‘(l-4) a=(1

-x:‘)

x:

p” --yp$c:

(6)

The separation factor approaches infinity as the total permeate pressure is reduced to the vapor pressure of the organic permeant (p” +p: ). An expression for the enrichment factor of the organic component, using mass concentration units, is obtained as follows. By definition, /3=c:l/c;=w:l/w:

(7)

The mass fractions w, of the organic component in permeate ( n) and feed ( ’ ), respectively, are

152

(8) (9) By making use of the model assumptions, eqns (3) and (5), w:’ m eqn (8) is expressed as a function of xi, which m turn is substituted by xi from eqn. (9)) leading to the following expression for the enrichment factor of the organic component on pervaporation under the conditions of the model:

(10) The enrichment factor approaches l/w: as the permeate pressure is reduced to the vapor pressure of the organic permeant (j?+ l/w: for p” +p: ) . For dimuushing feed concentration of the orgamc substance, eqn. (10) reduces to: ASPS

(11)

showing enrichment to be proportional to the vapor pressure of the orgamc species times its activity coefficient at high dilution. The flux density of the organic permeant, m molar concentration units, is obtained from the relation

J, -=- P: 4 PS

(12)

Incorporating the model presumptions, eqns. (3) and (5)) leads to (13) Transformation from molar to mass flux using J (mass) =MJ (molar) and eqn. (9) yields

J,=J

P%&w: ‘p”

[M,(l-w:)+M,w:]

-p:M,yw:

(14)

which, for small feed concentration of the organic, is approximated by J

I-

_JpLyw’ I

P”

1

(15)

Water flux is nearly constant at the dilutions in question, and is independent of the nature of the organic solute. The two model statements of eqns. (11) and (15) are examined in the light

153

of experimental evidence m the following sections. Returning first to the effect of downstream pressure (Section 6), eqn. ( 10) was used to calculate the pressure dependence of pervaporative phenol enrichment shown m Fig. 6. Comparison of the calculated curve with the experimental points shows reasonable agreement. Departure at the elevated pressure side is attributed to non-condensable compounds (air) introduced with the feed Towards low pressure, phenol flux lags behmd that predicted on the basis of zero resistance to the organic component, as presumed, it is suggested at this pomt that the resistance to phenol noticed at low partial pressure of water is at and/or in the membrane after all. 8. Pervaporation: effect of membrane thickness Enrichment of the organic component is a function of total permeate pressure, eqns. (10) and (11)) being the sum of the partial pressures of the permeants. Since permeate pressure results from rates of permeation, eqn. (2)) membrane thickness affects pervaporatlve enrichment through its Influence on flux. In the model approximation, the membrane is freely permeable to the organic species; any effect of membrane thickness on flux is thus governed by the resistance of the membrane towards water permeation, presumably decreasing with decreasing thickness. Confirmation of this simple picture is presented in Table 3 by pervaporation data of dilute phenol solutions using membranes of graded dry thickness (feed concentration 100 mg/kg, 50” C; PEBA 35). It should be noted that the enrichment factor according to eqns. (10) and (11) does not explicitly depend on membrane thickness, implying that an increase in water flux will not affect the separation as long as the total permeate TABLE 3 Pervaporatlon of phenol and water at varymg membrane thickness (membranes below 30 pm are composites) Membrane thickness, ,um

Flux density, g/m’-hr Phenol

Water

3 6 10 15 30 70 100 115

13 17 18 16 16 16 1.5 13

2200 2400 2300 1750 510 140 125 100

Ennchment factor, B 58 70 77 94 31 112 121 137

154

.

_I“,;/;”

P’

Enrrchmsnt

o# 1600

l/ --I:,;;,--,,

100

1400

I

80

1200

1000

60

BOO

Y \rk .

40

20

LAP

600

Flux

u_

osnr,ty

400

20

Membrane

40

.G 5

0

200

--a-Z-

40

0

0

0

)‘

60

80

100

120

140

160 110

Dry Thickness

g.

LL

Iw I

Fig 8 Enrichment factor and total flux density m phenol-water membrane dry thickness

pervaporatlon as fun&on

of

pressure is kept at a constant level. In real pervaporatlon, however, it is not possible to maintain a constant permeate pressure at varying flux density, higher fluxes invariably leading to lower enrichment effects. The opposing tendency of enrichment and flux as a function of membrane thickness in the pervaporation of phenol-water is illustrated in Fig. 8 on two related polymers, PEBA 35 and 40, at a feed concentration of 100 mg/kg of phenol at 50’ C. Considering the extremes of membrane thickness, it is obvious that thick membranes will present a resistance to the organic permeant as well, and that thin membranes will eventually cease to pose a resistance to water permeation. At some lower limit of membrane thickness, then, pervaporation approaches evaporation except for possible membrane boundary effects. Equilibrium enrichment of phenol from &lute solution, to which the curves of Fig. 8 should converge at zero membrane thickness, is about 1.5 at 50” C. 9. Pervaporation: effect of feed concentration Inspection of eqn. (10) shows the enrichment factor to be relatively msensltive to the feed concentration of the organic species, w :, increasing as depletion of the feed solution progresses. The individual flux of the organic, meeting essentially no resistance accordmg to the model, is directly related to feed concentration, eqn. (15). Both predictions are borne out by the pervaporation results on phenol-water presented in Table 4, which cover feed concentrations up to 1000 mg/kg (PEBA 40; membrane thickness 46 ,um; 50°C). Downstream pressure was set at 2 mbar. As a control, the permeate pressure was calculated from eqn. (10) using available data (which pertain to activity coefficient and vapor pressure at 44.4”C) and included in Table 4. Observed again is the in-

155 TABLE 4 Pervaporatlon of phenol-water as function of feed concentration, mcludmg trend of calculated permeate pressure Feed concentration, mg/kg

Flux density, g/m’-hr Phenol

Water

31 48 85 153 324 585 994

04 06 11 19 41 75 12 7

173 a 174 4 177 7 177 2 180 9 1846 189 4

Nt

E

Ennchment factor, B

Permeate pressure, mbar

74 73 73 68 68 67 63

174 1 76 176 188 186 186 194

16-

0

Feed

100 Concentration

200

300

460

560

600

1 pm 1

Fig 9 Pervaporatlon of aromatic hydrocarbons from dilute aqueous solution organic flux density vs feed concentration Numbers alongsIde compound names are Kp values ( ‘C)

C ). With water flux and permeate pressure being reasonably constant, the slopes of the lines, according to eqn. (15)) are proportional to pure component vapor pressure times activity coefficient, pf y. Since the activity coefficient of the organics increases with dilution, so does the enrichment factor; cf. eqn. (11).

156

phenol

enrrchment

6-o

0

01

Electrolyte

10

Fig 10 Salting-out m pervaporatlon of phenol

10 0

[ wt-%

Concentration

NaCl

1

Effect of electrolyte concentration on enrichment and flux

The activity of nonelectrolytes in aqueous solution is increased upon addition of a salt (salting-out effect); at the same time the solvent activity is lowered In keeping with the observed influence of the activity coefficient on pervaporatlon performance, salting out is expected to enhance both organic enrichment and orgamc flux density. Confirmation is presented in Fig. 10, showing phenol pervaporation at increasing electrolyte concentration in the feed (concentration of phenol, 400 mg/kg; PEBA 40; membrane thickness 46 pm; 50°C). 10. Pervaporation: effect of pH

Phenol, being a weak acid, dissociates in water, the degree of dissociation depending on the pH value. Smce only undlssociated phenol pervaporates, whereas phenolate ion does not, phenolic enrichment depends on pH precisely as the concentration of undissociated phenol in equilibrium with phenolate ion does. By contrast, phenolate ion is rejected by desalination membranes in reverse osmosis, whereas undissociated phenol is not. In either mode, the major change occurs at a pH value of ca. 10. To predict the effect of pH on the pervaporative enrichment of phenol, the ratio of undissociated phenol (cph) to total concentration (& ) is calculated in terms of pH and the dissociation constant, pK,, as cPh -= c;,,

1 (l+lO-pX”+pH)‘l

157

PH value

pH = PK.

= 104

Fig 11 Pervaporatlon (ennchment) and reverse osmous (relectlon) of phenol as function of pH

Actual enrichment as a function of pH is obtained by reducing the enrichment factor of undissociated phenol according to: (17) In Fig. 11, the calculated pH dependence of phenol enrichment is compared with experimental data (feed concentration 300 mg/kg; PEBA 40; membrane thickness 46 pm, 50’ C ), and contrasted with the reJection of phenol by cellulose acetate membranes [ 51. At pH = pK,= 10 4 (50 oC ) the reduction is 0.5 in both instances. 11. Concluding remarks It is concluded from the foregoing that pervaporation against the vapor pressure relation is a viable alternative to extraction in depleting aqueous feed streams from low volatility organic compounds, and/or concentrating low volatility organic compounds from dilute aqueous solution. A useful parameter m assessing pervaporability appears to be the activity coefficient of the organic solutes in the aqueous feed. When transferring the results of this study to practical separations, a number of features are worth mentioning. One is the fact that, given the limited solubility of the organic compounds m question, pervaporative enrichment leads to phase separation of the condensed permeates, pointing to the composition of the organic-rich phase as the natural enrichment of the process. According to another observation, optimum separations are obtained with relatively thick membranes of the order of 100 pm, obviating the need for composite membrane structures. Finally, increasing downstream pressure (p” ) adversely affects en-

158

rxhment and flux of the organic permeants; cf. eqns. (11) and (15). Since downstream pressure is largely determined by the resistance to vapor transfer between membrane and condenser, a sufficiently open apparatus design is mandatory in order to realize the observed separation effects. List of symbols

i

M n P PO W

x ; Y

mass concentration (mg/kg = ppm) flux density (g/m’-hr ) molar mass (molecular weight) number of moles pressure (mbar = 0.1 kPa) equilibrium vapor pressure weight fraction ( w x 100 =wt% ) mole fraction separation factor enrichment factor activity coefficient

Irdzces 1 organic permeant water J I feed; feed side of membrane I, permeate; permeate side of membrane

References 1 K W Boddeker, Pervaporatlon durch Membranen und rbre Anwendung sur Trennung von Flusslggemlschen, VDI-Verlag, Dusseldorf, 1986 2 R Weller, H Schuberth and E Lerbmtz, Die Phasengleichgewmhte dampffornug/flussrg des Systems Phenol/n-Butylacetat/Wasser be1 44,4”C, J Prakt Chem ,21 (1963) 234-249 3 G Bengtson and K W Boddeker, Pervaporatron of low volatlles from water, m R Baklsh (Ed ), Proc Third Int Conf Pervaporatron Processes, Nancy, 1988 4 J R Flesher, Jr, Pebax polyether block amide - a new family of engmeermg thermoplastic elastomers, m R B Seymour and G S. Knshenbaum (Eds ), High Performance Polymers, Then Origm and Development, Elsevler, New York, NY, 1986, pp 401-408 5 T Matsuura and S SounraIan, Reverse osmosis separation of phenols m aqueous solutions using porous cellulose acetate membranes, J. Appl. Polym Scl ,16 (1972) 2531-2554