Pervaporation of alcohols from hydrocarbon mixtures

Pervaporation of alcohols from hydrocarbon mixtures

Separation HPurification Technology Separation Pervaporation and Purification Technology 11 ( 1997) 113-l 18 of alcohols from hydrocarbon mixtur...

539KB Sizes 0 Downloads 64 Views

Separation HPurification Technology Separation

Pervaporation

and Purification

Technology

11 ( 1997) 113-l 18

of alcohols from hydrocarbon

mixtures

Ha.bib I. Shaban *, Mohammad R. Riazi, Raheela Sahar Chemical Engineering Department, Kuwait University, P. 0. Box 5969, Safat 13060, Kuwait Accepted 13 January 1997

Abstract In this work, the characteristics and enhancement of pervaporation for the separation of various alcohols from hydrocarbon mixtures are demonstrated. Experiments were performed through a composite plate-and-frame type hydrophilic PVA (polyvinyl alcohol) membrane at a constant permeate vacuum pressure of 6.5 mmHg and at temperatures of 30, 45 and 60°C. The Wilson equation was used to describe the activity coefficients for the system. The analysis is presented in terms of the variations in permeation flux, permeability and selectivity. The results show that as the concentration of alcohols in the feed increases, the permeability, selectivity and total flux also increase. By increasing the temperature, the permeabilities increase while selectivity decreases. However, the experimental results show that at temperatures of 30 and 45°C a good degree of separation of alcohols from a hydrocarbon mixture is possible by the pervaporation technique using a PVA membrane with a selectivity of greater than 10. 0 1997 Published by Elsevier Science B.V. Keywords: Alcohols; Hydrocarbons;

Pervaporation;

Polyvinyl alcohol membrane

1. Introduction

The pervaporation membrane process has been found to be effective for the separation of liquid mixtures, especially azeotrope mixtures and mixtures with close boiling points. The pervaporation process depends not only on the physicochemical properties of the polymeric materials and the structure of the membrane, but also on the operating conditions (i.e. temperature and downstream pressure). The use of polyvinyl alcohol (PVA) membranes in the separation of some organic compounds from water by the pervaporation method was demonstrated elsewhere [ 1,2]. The process is based on * Corresponding

author.

Fax: + 965 4839498.

1383-5866/97/$17.00 0 1997 Published PZZ S1383-5866(97)00005-l

selective transport across a composite polymeric membrane, followed by evaporation of the permeate. As described by Baker [3], in the pervaporation process a liquid mixture contacts one side of a membrane and the permeate is removed as a vapor from the other side. Transport through a membrane is induced by a difference in partial pressures between the liquid feed solution and the permeate vapor. For a binary system of A and B, the fluxes are given by:

JA = QA(YAXAP; -YAP)

(1)

JB = QB([email protected]‘~-_y,d’)

(2)

in which J is the partial flux, Q is the permeability, “Jis the activity coefficient, P* is the vapor pressure, P is the vacuum pressure in the permeate side of

by Elsevier Science B.V. All rights reserved

H.I. Shaban et al. /Separation and Purification Technology II (1997) 113-118

114

the membrane, x is the mole fraction in the feed and y is the mole fraction in the permeate side. Permeability is defined as the product of the diffusion coefficient and the gas solubility in the membrane per unit thickness of the membrane. Since the diffusion coefficient is a function of temperature, permeability is also a function of temperature. The ratio of permeabilities for a binary mixture is the membrane selectivity CI, which is given by: a=

QA/QB

(3)

A high selectivity can be obtained from either a favorable diffusivity ratio or a large difference in solubilities. A selectivity of 4 or greater is generally needed for good separation. For most gases, the permeability increases with temperature, and it is often correlated in the following form [4]:

Q = Q, exp bWW

(4)

membrane, supplied by Mitsui Shipping and Building Co., Japan. The effective membrane surface area was 0.5 m2. The feed solution was circulated at a flow rate of 10 1 h-’ from the feed reservoir. Experiments were conducted at three different temperatures of 30, 45 and 60°C. For each set of experiments, the temperature was controlled by a thermostat under a constant vacuum pressure of 6.5 mmHg and an upstream pressure of 1.8 kgf cmm2. A flow diagram of the pervaporation process is shown in Fig. 1. The sample is introduced to the feed tank, the components to be removed are vaporized through a plate-and-frame type composite PVA membrane with a support of PAN (poly-acronitrile), which is connected at very low pressure in the vacuum vessel. The vapor in the permeate side is then condensed and purged out. A vacuum pump maintains the required vacuum. This vaporization cools down the processed fluid, which must be reheated to maintain

in which E is the activation energy, R is the gas constant and Q, is a pre-exponential constant. The main objective of this work is to study the separation of alcohols from hydrocarbon compounds by the pervaporation process. The ffects of temperature and feed composition on flux, permeability and selectivity are studied through extensive experiments with a PVA membrane.

2. Experimental For the present study, a mixture of various hydrocarbons with alcoholic compounds mainly found in petroleum fractions was prepared as the initial feed. The feed contained 25 wt.% alcohols, mainly ethanol, isopropanol, 2-butanol and y1butanol. The hydrocarbons were mainly C5 and C6 from the paraffins, naphthenes and aromatics groups found in a petroleum mixture. The average boiling points of these alcohols and hydrocarbons are sufficiently close that separation by distillation is not appropriate. The pervaporation of mixtures through the PVA membrane was carried out by an ordinary pervaporation technique. The separation of alcohols from hydrocarbons was carried out using a flat, hydrophilic polyvinyl alcohol (PVA) composite

W

permeateO”t

Fig. 1. (a) Schematic (b) Fluid circulation

of the pervaporation and assembly.

separation

system.

115

H.I. Shaban et al. /Separation and Purification Technology 11 (1997) 113-118

the highest flux through the membranes. The weight of the condensed vapor obtained in the permeate over a given time was used for the calculation of the total permeation flux. The ermeate was analyzed by gas chromatography using a Porapack-Q column in order to obtain the concentration of organic contents. From the measured permeate flux and the permeate composition, the partial permeate fuxes of the alcohols and hydrocarbon compounds were also calculated. As the experiments began, because of recycling the compositions of the mixture in the feed tank and in the permeate changed with time. At different feed compositions, samples of liquid from the mixtures in the feed and permeate tanks were taken and analyzed.

Table 1 Some estimated

Property

Alcohols

M Tb (K) T,(K) P, (atm) 0

66.1 367.4 529.8 46.4 0.607

of alcohols

(A) and hydrocarbons

Hydrocarbons

(A)

Temperature

3. Calculations and results 30 45 60

(B)

70 321 531.7 51.0 0.115

Table 2 Liquid molar volumes and vapor pressures hydrocarbons (B) at various temperatures

(“C)

The feed in this study was a mixture of different hydrocarbon compounds and different alcohols. A hydrocarbon mixture or a narrow boiling-point range petroleum mixture can be considered as a single ‘pseudocompound’ [5]. For relatively narrow boiling-point range mixtures, this assumption does not cause any serious error in property calculations. We also consider all alcohols as one pseudocompound designated ‘A’, and all hydrocarbons are grouped as another pseudocompound designated ‘B’. Therefore, the multicomponent mixture of alcohols and hydrocarbons can be treated as a binary mixture of A and B. The properties of A can be determined from the properties of pure alcohols [6] and their composition in the feed. The general physical properties of the hydrocarbon mixture (pseudocompound B) are calculated using methods given by Riazi and Daubert [ 71. The calculated basic properties necessary to estimate the thermodynamic properties of pseudocompounds A and B are given in Table 1. These properties of pseudocompounds depend on the composition of the mixture, and change slightly as the compositions of individual alcohols vary in the feed. Vapor pressures and liquid molar volumes at three different temperatures are given in Table 2. The vapor pressures have been calculated by the Lee-Kesler correlation, while the Racket equation

properties

(B)

of alcohols

(A) and

Liquid molar volume, V (cm3 mol ‘)

Vapor pressure, P* (atm)

A

B

A

B

72.6 75.9 77.6

93.4 95.1 97.0

0.051 0.113 0.236

0.616 0.950 1.502

was used for calculatios of the liquid molar volume [ 61. One important parameter needed to calculate the permeability from Eqs. (1) and (2) is the activity coefficient y. From various models available to describe the relationship between activity and composition, method of Wilson is the most suitable model for alcohol-hydrocarbon systems. As explained by Prausnitz [8], for systems of polar (alcohol) and nonpolar (hydrocarbon) mixtures, the Wilson equation best represents the activity coefficients. The equations for yA and yB are: In yA = - ln(x, + C,,x,)

c AB

c BA

+xB

(5) xA+CABXB

ln

YB =

C13AXA+XB

-ln(xB + CBAXA) CAB -xA XA+CABXB

-

CBA (6) cBAxA

+xB

in which the parameters CABand CBAare given as

116

H.I. Shaban et al. /Separation

a function of temperature C,, = 2 exp (- a,,/RT)

and Purijication

Technology II (1997) 113-118

only: (7)

“A

CBA= “A exp (-aBA/RT)

(8)

“B

where “A and vg are the liquid InOlar VOh.UIIeS Of A and B at temperature T. The beauty of these equations is that the energy parameters CRAB and UgA are not temperature-dependent. Therefore, Wilson’s equation gives an expression for both composition and temperature. Alcohols existing in the mixture studied here are ethanol, propanol and butanol. A binary mixture with any of these alcohols and one hydrocarbon compound has energy parameters in the range 6200-6300 J mol - ’ for CRAB and 900-920 J mol - ’ for alga [ 6,8,9]. For these changes in values of ClABand a&.,, there is little change in the estimated values of YAand YB.For our calculations, average values of CRAB = 6250 and aBA= 910 J mol - ’ have been used for the alcoholhydrocarbon system. At each temperature, activity coefficients were calculated for different alcohol compositions using Eqs. (5) and (6). Then the permeabilities QA and QB were calculated from Eqs. ( 1) and (2), and finally Eq. (3) was used to calculate the selectivity. The partial flux of alcohol JA, versus alcohol composition in the feed at various temperatures is shown in Fig. 2. The total flux (J= JA + JB) versus alcohol composition in the feed at three different temperatures is shown in Fig. 3. The fluxes decrease with feed composition, but increase with temperature at a fixed composition. The permeability QA versus the composition of alcohols in the feed is shown in Fig. 4. The permeability of alcohols increases with increasing feed composition. At a hxed composition of 16.8 wt.% alcohol in the feed, the permeabilities of A and B (QA and QB) versus the reciprocal of temperature are shown in Fig. 5. As shown in Fig. 5, the permeability increases with temperature. The data shown in Fig. 5 for QA and QB versus temperature satisfy the exponential form of Eq. (4) for the relationship between permeability and temperature. For alcohol, the activation energy E was found to be

MM

Fig. 2. Partial flux of alcohols versus feed composition.

15 wt.%

20

25

Alcohol

Fig. 3. Total permeation flux versus feed composition.

37.54 kJ mol -I, and the pre-exponential factor was 53 263. Selectivity a versus composition at various temperatures is shown in Fig. 6. An increase in temperature decreases the selectivity, while selectivity increases with alcohol concentration. As the temperature increases, the permeability and flux also increase, while selectivity decreases. Therefore if very high selectivity is not required, a higher

H.I. Shaban et al. /Separation and Purification Technology I1 (1997) 113-118

117

??

A 0

I

I

II...

,n ,

Pnm

0; I

’ ,

a

15

10

5

P

wt.%

I

I

3

I

I

20

I

25

Alcohol

Fig. 6. Selectivity versus feed composition and temperature.

Fig. 4. Alcohol permeability versus feed composition. 0.1

,

,

,

,

( ,

, , ,

, , , ,

an appropriate vacuum pressure can be obtained. For example, for a feed composition of 16.3 wt.% alcohol at a temperature of 30°C and a vacuum pressure of 30 mmHg, the permeate composition can be determined from the information given here. From Figs. 4 and 6, QA=0.01825 and QB=0.00042 kg m-2h-‘atm-‘. Using the molecular weights given in Table 2, we can calculate the mole fractions of alcohol in the feed (x*=0.165). From Eqs. (5) and (6), the activity coefficients can be calculated (yA=3.36 and yB=1.12). For our calculation parameters, RA, RB and cr* are defined in the following forms:

RB = Fig. 5. Permeabilities of alcohols and hydrocarbons versus the reciprocal of temperature.

temperature is appropriate for better flux. However, this is a matter of the optimum operation conditions of a given pervaporation unit. The information on permeability presented here can be used to estimate the permeate composition at vacuum pressures other than 6.5 mmHg. Also, if a certain composition in the permeate is required,

P

YEA

(10)

where MA and MB are the molecular weights of A and B, respectively. The mole fraction of alcohol in the permeate (yp3 can be estimated via solution of the following quadratic equation [lo]: ay2,+by,+c=O

(12)

118

H.I. Shaban et al. /Separation and Purification Technology II (1997) 113-118

where a = a*RA -R,, b = R, + xA - 1 - LY*(R*+ xA) and c = LX*X*. For this particular example, the mole fraction of alcohol in the permeate is yA =0.465, which corresponds to 45.3 wt.%. The experimental value is 47.2 wt.%. At a pressure of 6.5 mmHg, the permeate composition is 64.5 wt.%.

5.1. Greek letters

selectivity as defined in Eq. (3) (dimensionless) fxAB parameter in the Wilson activity coefficient correlation defined in Eq. (7) (J mol - ‘) activity coefficient (dimensionless) Y acentric factor (dimensionless) w U

5.2. Subscripts 4. Conclusions In this work a pervaporation unit with a PVA membrane has been used to separate alcohols from a hydrocarbon mixture. Experimental results are presented in terms of partial fluxes, permeability and selectivity. As the temperature increases, the flux and permeability also increase while selectivity decreases, and an optimum temperature condition should be obtained. The relation between permeability and temperature for an alcohol-hydrocarbon system has also been determined. From the results presented here, the composition of the permeate can be estimated at various operating conditions for alcohol-hydrocarbon systems.

Acknowledgment This research was supported financially by the Research Administration of Kuwait University.

References [l] H.I. Shaban, Separation of binary, ternary and multicom-

[2]

[3]

5. Nomenclature [4]

a, b, c parameters E J A4 P P* PC

Q

R T

TC V X

Y

alcohols hydrocarbons

A B

defined in Eq. (12) (dimen-

sionless) activation energy (J mol - ‘) mass flux (kg m-’ h-l) molecular weight (g mol - ‘) pressure in the permeate side (atm) vapor pressure (atm) critical pressure (atm) mass permeability (kg mm2 h atm-l) gas constant, 8.314 J mol-’ K-l) temperature (K) critical temperature (K) liquid molar volume (cm3 mol - ‘) mole fraction in the feed (dimensionless) mole fraction in the permeate (dimensionless)

[5]

[6]

[7] [8]

[9]

[lo]

ponent organic/water mixtures, Gas Sep. Purificat. 9 (2) (1995) 69. HI. Shaban, Removal of water from aroma aqueous mixtures using pervaporation processes, Sep. Technol. 6 (1996) 69. W. Baker, in: E.L. Cussler, R.L. Riley, W. Eykamp (Eds.), Pervaporation, Membrane Separation Systems, Recent Development and Future Direction, 1991, p. 151. C. Zhu, M. Liu, W. Xu, W. Ji, A study on characteristics and enhancement of pervaporation-membrane separation process, Desalination 71 (1989) 1. M.R. Riazi, T.E. Daubert, Characterization parameters for petroleum fractions, Ind. Eng. Chem. Res. 26 (1987) 755. R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, 4th ed., McGraw-Hill, New York, 1988. M.R. Riazi, T.E. Daubert, Simplify property predictions, Hydrocarb. Process. 59 (3) ( 1980) 115. J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1986. J.M. Smith, H.C. Van Ness, Introduction to Chemical Engineering Thermodynamics, 4th ed., McGraw-Hill, New York, 1987. W.L. McCabe, J.C. Smith, P. Harriot, Unit Operations of Chemical Engineering, 5th ed., McGraw-Hill, New York, 1993.