Separation of ethylene from ethane by a flowing liquid membrane using silver nitrate as a carrier

Journal of Membrane Science, 45 (1989) 115-136 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

115

SEPARATION OF ETHYLENE FROM ETHANE BY A FLOWING LIQUID MEMBRANE USING SILVER NITRATE AS A CARRIER*

MASAAKI TERAMOTO, HIDETO MATSUYAMA, TAKUMI YAMASHIRO and SUEAKI OKAMOTO Department of Industrial Chemistry, Kyoto Institute of Technology, Kyoto 606 (Japan) (Received August 15,1988; accepted in revised form November 4,1988)

Summary A new type of liquid membrane, designated as a “flowing liquid membrane” is proposed, in which a liquid membrane solution flows in a thin channel between two microporous membranes. Separation of ethylene from ethane by flowing liquid membrane modules was carried out, in which silver nitrate was used as a carrier of ethylene. It was found that, compared with the usual immobilized liquid membranes, the flowing liquid membrane has higher permeability and stability. The mechanism of the facilitated transport of ethylene through the flowing liquid membrane was elucidated, and this successfully predicted the permeation flux as well as the selectivity. The performance of the membrane module was satisfactorily explained by the proposed design equations.

Introduction

High permeability, selectivity and stability are the fundamental properties required for membrane separation processes. Although gas separations by polymer membranes have been performed industrially, high selectivity cannot be obtained in systems where gas permeation occurs by the mechanism of simple dissolution-diffusion or Knudsen diffusion. Gas separations by immobilized liquid membranes using carriers which can selectivity and reversibly bind certain permeant species have been attracting attention, because very high selectivity, due to the specificity of the reaction between carriers and permeant species, as well as high permeability, due to the facilitation effect, can be achieved. The investigation of facilitated transport of gases through liquid membranes actually started from models of physiological processes; see the review by Schultz et al. [ 11.For example, Scholander [ 21 reported that the permeation rate of oxygen through a supported liquid membrane was markedly enhanced by incorporating hemoglobin as an oxygen carrier into the aqueous membrane *Presented at ICOM ‘87, Tokyo, Japan, June 8-12,1987.

0376-7388/89/$03.50

0 1989 Elsevier Science Publishers B.V.

116

solution absorbed in a Millipore filter. Ward and Robb [ 31 demonstrated that an immobilized membrane of an aqueous bicarbonate-carbonate solution was 4100 times more permeable to carbon dioxide than to oxygen, and was suitable for the removal of carbon dioxide in a closed life-supporting environment. This study led to work aimed at the application of immobilized liquid membranes to industrial separations of such gases as carbon dioxide, hydrogen sulphide [ 4,5 1, carbon monoxide [ 6,7], ethylene, propylene [ 81 and oxygen [ 9,101. Most of the liquid membranes used in these studies were immobilized liquid membranes, consisting of thin porous filters such as cellulose acetate impregnated with carrier solutions. These liquid membranes have the disadvantage that they are apt to degrade because the solution absorbed in the pores of the support membrane evaporates in the feed and sweep gas phases, even when the feed gas and/or the sweep gas is passed through vapor saturators. Also, even though thinner membranes are desired in order to obtain higher permeation fluxes, it is very difficult to make this type of membrane extremely thin and stable. Teramoto et al. [ 111 investigated the time course of the permeation rates of ethylene and ethane through an immobilized liquid membrane containing silver nitrate as a carrier of ethylene, and found that, if the saturation of gas streams with solvent vapor was insufficient, some parts of the membrane dried up and the selectivity was almost lost in about 200 minutes. In a long-term test on the separation of oxygen from nitrogen by a liquid membrane consisting of a transition metal complex dissolved in basic solvents, Johnson et al. [ 91 found that initially there was a gradual decline in the oxygen permeability for several weeks. Thus, the degradation of immobilized liquid membranes is one of the most serious problems which must be solved in order to use such membranes commercially. Another type of liquid membrane that has been used in laboratory research to obtain reproducible permeability data is a thin-layer liquid membrane, in which a membrane solution is held stationary between two microporous membranes, such as polyethylene, or nonporous polymer membranes, such as silicone rubber [ 12-141. Although the stability of this type of membrane seems to be better than that of immobilized liquid membranes in which the carrier solution is absorbed in the pore of the microporous support membranes, the mass transfer resistance through the layer of the membrane solution is large because the layer is thick and stationary. In order to overcome these inherent problems of these two types of liquid membrane, a new type of liquid membrane for gas separation is proposed here, and is called the “flowing liquid membrane”. The result of its application in the separation of ethylene from ethane is reported. The schematic diagram of the flowing liquid membrane is shown in Fig. 1. A membrane solution (in this case aqueous silver nitrate) is circulated through a thin channel between two hydrophobic microporous membranes separating

117 microporous membrane

Fig. 1. Schematic diagram of the flowing liquid membrane. F: feed gas channel, S: sweep gas channel, M: flowing liquid membrane channel.

feed gas and sweep gas phases from the flowing liquid membrane phase. The mass transfer rate through the liquid membrane is enhanced by the turbulence of the membrane solution flowing through the channel, in which a mesh spacer is inserted. Moreover, the resistances to permeation through the two hydrophobic microporous membranes are very low, probably because the aqueous membrane solution does not wet the membranes. Thus, the pores of membranes are available as the path for gas-phase diffusion, and the permeation rate is much faster than that through pores filled with aqueous solution. Another advantage of this membrane is that, even if the membrane solvent evaporates, degradation of the membrane can be prevented by supplying new solvent to the membrane solution phase. Therefore, the flowing liquid membrane is expected to have higher permeability and stability compared with conventional liquid membranes. In this work, a series of experiments on the separation of ethylene from ethane was carried out by using small modules of the flowing liquid membrane, and permeability and stability characteristics of the modules were investigated. Further, design equations for the module were developed on the basis of the proposed mechanism of facilitated transport through the flowing liquid membrane, and the experimental data were simulated by the design equations. Analysis of flowing liquid membrane Analysis of permeation rate In this separation system, only ethylene can react with Ag+, as expressed by eqn. (1)) whereas ethane does not react. A (gas)+B

(carrier) * C

(CJL +Ag+

= complex)

(I)

The concentration profiles of the chemical species in the following liquid membrane are shown in Fig. 2. It is assumed that the reaction rate is so fast that the chemical equilibrium described by eqn. (2) is established throughout the membrane solution [ 8,111.

Fig. 2. Concentration profiles of the chemical species in the flowing liquid membrane. 1: bulk phase of feed gas, 2: boundary layer of feed gas, 3: hydrophobic microporous membrane, 4: boundary layer of membrane solution, 5: bulk phase of membrane solution, 6: boundary layer of sweep gas, 7: bulk phase of sweep gas.

To analyze the permeation rate of ethylene, the following four elementary steps are considered. 1. Diffusion of A from the bulk phase of the feed gas to the feed gas-membrane interface Fi, which consists of diffusion through the boundary layer of the feed gas phase and subsequent diffusion through the pores of the hydrophobic microporous membrane. 2. Diffusion of A and C in the feed side boundary layer of the membrane solution, accompanied by chemical reaction (absorption of ethylene into the membrane solution). 3. Diffusion of A and C in the sweep side boundary layer of the membrane solution, with chemical reaction (desorption of ethylene from the membrane solution). 4. Diffusion of A from the sweep gas-membrane interface Si to the bulk phase of the sweep gas, which consists of diffusion through the pores of the support membrane and subsequent diffusion through the boundary layer of the sweep gas phase. The permeation fluxes in steps 1 and 4 are expressed as follows: J*F = kCAF(P*F JAS=~GAS (PAsi

P.4Fi

-PAS

1

(3)

1

(4)

where JAF and JAs are absorption flux and desorption flux, respectively, kc is the mass transfer coefficient in the gas phase, and P is partial pressure. The absorption flux in the membrane solution (step 2) is analyzed as follows, based on the fi.lm theory, by an approach similar to that of Teramoto et al. [ 111. The steady-state differential mass balance of each species in the boundary layer of the membrane solution are represented as: DA(d2A/ti2)

=kAB-k’C

(5)

119

Ds(d’B/d.r’)

=kAB-k’C

Dc(d2C/dr2)=

(6)

-IzAB+k’C

(7)

where k and k’ are the forward and reverse reaction rate constants of eqn. (1)) respectively. The boundary conditions are as follows: A=AFi,B=BFi,

x=0:

JAF = &AF

X=XMF: AcAM,

(PAF

B=BM,

C=CFi ) =

-PAR

(8)

-Dc(dC/dr)

-DA(dA/dx)

C=CM

(9) (10)

Absorption rate J AF can be derived from eqns. (5) and (7): JAF = (DA/XMF

) (AFi -AM)

+

(WXMF)

(CFI

-G)

(11)

From eqns. (6) and (7)) the following equation is obtained by taking account of the equimolar counter-diffusion of B and C. Dg(BM--BFi)+DC(CM-CFi)=O

(12)

Equations (13) and (14) are derived from eqns. (2) and (12): BFi=

(DBBM

+&CM)/(%

CFi=~ri(Dn&

+DCmFi)

+DcCM)/(Dn

+Dcmri)

(13) (14)

Substitution of eqn. (14) into eqn. (11) gives the following equation: JAF=z

(AFi-AM)

(15) = bAF

(AFi -AM)

Here kMAFis the mass transfer coefficient of ethylene in the boundary layer of the membrane solution and r, and rc denote DB/DA and DC/DA, respectively. It is often assumed that DB is equal to DC, that is, rB is equal to rc [ 121. This assumption is reasonable when the molar volume of the carrier is considerably larger than that of the permeant species. However, in the present case this assumption can not be made because the molar volume of Ag+ is similar to that of ethylene. Since the liquid flows along the support membranes in the channels, the mass transfer rate through the flowing liquid membrane is considered to be proportional to D * [ 151. It was recognized that the enhancement factor in the gas absorption, accompanied by an instantaneous irreversible reaction derived on the basis of penetration theory, is approximated by that based on the film

120

theory if the diffusivity ratio term is replaced by its square root [ 161. Thus, it seem reasonable to replace rs and rc in eqn. (15) by rk and r& respectively. for the present membrane system, the ratio of r$-&/ (rBg +rc’KAFi)

to rBrc/

(rB + r,-&dFi) is 1.04 when rs >> rc&Ari and I.19 when rs -K rc&Ari. Therefore, eqn. (16) gives a higher value than eqn. (15). However, the difference between them is not large.

JAF=IZILIAF(AFi-A~){l+r~r~I(B~/(r~

+r&KAri)}

(16)

The conservation of the carrier in the bulk phase of the membrane solution is represented as: B()=B&,l+c,

(17)

From eqns. (2) and (17)) AM is expressed in terms of ATM, the total concentration of ethylene (A, + CM), as follows: AM= [-{1+K(B,-ATM)}+~{1+K(Bo-ATM)}2+4KATM]/2K

(13)

Since BM and CM are expressed by eqns. (19) and (20), respectively, BM and CMcan be calculated from the value of ATM: %l==Bol(l+=M

(19)

CM=~M&I(l+~M)

(20)

Therefore, if ATM and PAF are given, J AFcan be calculated by using eqns. (16) and (9). In a similar manner, the permeation rate of ethylene from the bulk phase of the membrane solution to the interface Si (step 3) can be derived as follows, and JAs can be calculated for given values of ATM and PAS. JAs=kMAs(AM-Asi){l+r~r~~KB~/(r~C+r~KAsi)}

(21)

It should be noted that, in the case of the flowing liquid membrane, the desorption flux JAs is not necessarily equal to the absorption flux JAF, as mentioned below. On the other hand, physical absorption and desorption fluxes of ethane, JA.F and Jats, are expressed as follows: JA’F = kGA’F(PA’F-PA’Fi) =kMA,F(A;“i-Ah)

(22)

= KGAsF(PAsF-H,.Ah) JAss = kMA.S(Ah-

A~i)=k,,,s(P*,si-P*,~)

= KGA.S(HA,A;VI -PASS)

(23)

1IKx-i~ = 1Ikca.F + H.zt,Ikiw,,F

(24)

lIKx,s=

(25)

1Ikca.s +K,,Ikm,s

121

where subscript A’ indicates ethane, kMAsFand kMAsSare mass transfer coefficients in the feed side and sweep side boundary layers of the membrane solution, respectively, and H is a Henry constant. Design equations of the flowing liquid membrane module The variations of the mole fractions of ethylene and ethane in the module and the differential mass balances are shown in Fig. 3 for the case that the feed gas, the sweep gas and the membrane solution flow in parallel. In the feed gas phase, the mole fraction of ethylene, YAF,decreases and that of ethane, YA*F, increase conversely because ethylene is selectively transported. If the flow patterns of both gas and liquid phases are assumed to be of plug flow type, the differential mass balance equations of ethylene and ethane are expressed as follows: -dGAr=J&S

(26)

-dGA,r=JA&

(27)

aAT=(JAF-JAS)dS

(28)

d.LA, =

(JA*F- JA,s)dS

dGAs=JAsds

(29)

____~~

(30)

microporous membrane feed gas .. mmembrane .

sweep=

Fig. 3. Variation of the mole fractions of ethylene and ethane in the module and differential mass balances.

122

dGAcS=J,s,ds

(31)

The boundary conditions are: s=O: GAF=GAFi,,

GAS=GASin=O, GA.F=GA'Fin,

GA,S=GA,Sin=O

(32)

LAT= LAT,, LA’ = LA’in 9

s=S:GAF=GAF,,~,GA’F=GA’F,~~,G~s=G~so,,t, GA's=G~s,ut LAT=

(33)

LATout, LA' =LA'out

where GAand GA,are molar flow rates of ethylene and ethane in the gas phases, respectively an d LAT and LA, are the total molar flow rate of ethylene (sum of flow rates of A and C) and the molar flow rate of ethane in the liquid membrane phase, respectively. In order to solve eqns. (26-31), even numerically, JAF, JAsF, JAs and JA.s must be expressed as functions of GA=,GA,r, GAS,GA’s, LAT and LA,. The relations between the molar flow rate G and the partial pressure are expressed as: PAF={GAF/(GAF+GA,F)}PF

(34)

PA,F={GA,F/(GAF+GA,F)}PF

(35)

PAS={GAS/(GAS+GA'S+GIS))%

(36)

~A,s={GA~s/(GAS+GA,S+GIS))~S

(37)

where GIs is the molar flow rate of the sweep gas, and PF and Ps are total pressures of the feed and sweep gas phases less the vapor pressure of water, respectively. The relationship between LAT and ATM, and that between LA and the concentration of ethane in the membrane solution A ;M, are represented as follows: ~&M=LAT/UM

(33)

A~=LA~/vM

(39)

where vM is the volumetric flow rate of the membrane solution. As mentioned above, if PAF, PA,F, PAS, PA’S, ATM and AL are given, JAF, JA,F,JA,F, JAS and JAss can be calculated. Therefore the right-hand side of eqns. (26-31) can be expressed in terms of GAr, GASF,GAS,GA.s, LAT and LA,, and these equations can be numerically solved. The following equations hold at steady-state operation where the membrane solution is circulated. LATi,=LATout

(46)

LA'in=LA'out

(41)

123

Although these values are not given explicitly, LATin and LAfi,, can be determined by repeating the calculation until the assumed values of LATin and LAfi, agree with LAToutand LAfaut , respectively, calculated by the numerical integration of eqns. (28) and (29). In the flowing liquid membrane, JAF and JArF are not necessarily equal to JASandJA,s,respectively, at each location of the membrane, and therefore LAT and LAPare not necessarily constant in the module. The conditions that must be satisfied are: josJ,,~~=j~JAs~~ S

s

0

JA,Fds=

(42)

sJ,s.ds s0

(43)

Experimental The schematic diagram of the module of the flowing liquid membrane used in the present work is shown in Fig. 4. The module is made of acrylic acid resin, and a feed gas channel F and a sweep gas channel S are separated from the flowing liquid membrane channel M by two hydrophobic microporous membranes. Duragard@ 2502 (Polyplastics Co. Ltd., thickness 50 pm, porosity 0.45, pore diameter 0.04x0.4 pm; polypropylene) or Fluoropore@ FP-010 (Sumitomo Electric Co. Ltd., thickness 60 ,um,porosity 0.57, mean pore diameter 0.1 pm; polytetrafluoroethylene) were used as the porous membranes. In each channel, a mesh spacer (Dainippon Plastic Co. Ltd., KDN-4@, thickness 0.9 mm, opening about 0.73) was inserted. Most of the experiments were carried out with a module having a membrane area of 352 cm2 (width 8 cm, length 44 cm).

microporous

membrane

Fig.4. Schematicdiagramof themoduleof theflowingliquidmembrane. F: feed gas channel, S: M: flowing liquid membrane channel.

sweep gas channel,

124

The module was operated in a once-through mode for the feed and sweep gases and in a recycle mode for the membrane solution, and the gases and liquid were allowed to flow concurrently. The flow rate of the membrane solution was varied in the range from about 20 to 180 cm3/min by a roller pump (Furue Science Co. Ltd., RP-LV3) and those of the feed gas (ethylene 49 mol%, ethane 51 mol%) and sweep gas (nitrogen) were usually 300 cm3/min at 298 K and 0.1013 MPa. Both feed and sweep gases were passed through wash bottles filled with water to saturate them with water vapor. The pressure of the membrane solution phase was kept slightly higher than atmospheric to prevent feed and sweep gases from entering the membrane phase. The sweep gas leaving the module was analyzed with a gas chromatograph equipped with a thermal conductivity detector (Shimadzu GC-SA). The module was placed vertically in an air bath maintained at 298 K. Results and discussion Mass transfer characteristics

of the flowing membrane phase

The mass transfer coefficient in the membrane solution was calculated from eqns. (44) and (45 ) from data obtained by physical transport experiments of ethylene and ethane in which deionized water was passed through channel M: JA~~=~MA’ (AFi-Asi),,=hMAO

(PAF-PAs)~~I~~A

(PA’F-PA.s)~~/HA’ J,bv= km, O(Aki-AQi)av=kMA.”

(44) (45)

where J,, is the average permeation flux in the module. As described below, the resistances in the gas phases were found to be negligibly small in this case. The relationships between the mass transfer coefficient kh?Oand the volumetric flow rate of the membrane solution, UM,are shown in Fig. 5. kMo is proportional to uMo.8. The dotted line in Fig. 5 indicates the mass transfer coefficient of ethylene through an immobilized liquid membrane (170 pm thick cellulose filter impregnated with water) reported by Teramoto et al. [ 111. The mass

:o

----_-__

50 100 200 vM Ccm3/minl

Fig. 5. Mass transfer coefficients for ethylene and ethane in a module with flowing deionized water; dotted line: immobilized liquid membrane (Teramoto et al., Ref. [ 111) .

125

transfer coefficient of the flowing liquid membrane is large compared with that of the immobilized liquid membrane, about five times that of the latter, at the highest values of uM.The ratio of kMAoto kMfAois about 1.1, which agrees with the value calculated from the above-mentioned relationship of kM oc D% by using 1.87x10-g/sec [ 171 and 1.52~lo-' m”/sec [ 181 as DA and DA,, respectively. Physical mass transfer coefficient kM when an aqueous silver nitrate solution was used as the membrane solution was estimated from the following equation: k&&A’=

(~“/~)%W’)~

(46)

where ,M’and p are the viscosities of water and a silver nitrate solution, respectively, and p” and p are the respective densities. In deriving eqn. (46), it was assumed that eqn. (47) and the Stokes-Einstein equation applied. Sh CCRe’.‘Sc f

(47)

Equation (47 ) was derived by assuming that Sherwood number Sh is proportional to SCj [ 151. The overall mass transfer coefficient of ethane, KG*,, through a silver nitrate membrane solution was determined experimentally from the following equation:

JA’~V=KGA’(PA,F-PA,S)~~

(48)

Figure 6 shows the effects of UMand silver nitrate concentration on KG*,. Similarity to the relationship shown in Fig. 5, KGA, is proportional to UM0.8W As the silver nitrate concentration increases, K GAsdecreases because the viscosity of the solution increases and the solubility of ethane decreases due to a saltingout effect.

n

2 10

I 20

I I I1111 50 100 vM km3/min3

I 200

Fig. 6. Mass transfer coefficients for ethane in a module with flowing solutions containing various AgNO, concentrations, B,.

126

Mass transfer characteristics of the gas phase In order to measure the mass transfer coefficient in the gas phase, experiments were performed in which an aqueous sodium sulphite solution (1 X lo” mol/m3) and sulphur dioxide diluted with nitrogen to a concentration of 0.947 mol% were supplied to the membrane solution channel and the feed gas channel, respectively, and the absorption rate of sulphur dioxide was determined from the gas flow rate and the difference between the inlet and outlet concentrations. The concentration of sulphur dioxide in the gas phase was determined spectrophotometrically (Hitachi UV3200 instrument). It was confirmed that the liquid flow rate had no influence on the absorption rate and the diffusion in the gas phase was the rate-determining step. Gas-phase mass transfer coefficient kGsoz was calculated by the following equation: k GS02

=

(G/S& ) ln (YSO~~~/YSO~~~~ 1

(49)

where GF is the total molar gas flow rate, Pr is the total pressure and ysoz is the mole fraction of SO,. The effect of GF on kGSOzis shown in Fig. 7, which indicates that in the range of GF studied, kGSOzwas almost independent of GF and approximately equal to 1.55 x 10W6mol/ ( m2-set-Pa). It was therefore inferred that the resistance to diffusion through the pores of the hydrophobic support membrane was the predominant factor limiting gas transport. Permeation of ethylene and ethane Figure 8(a)-(c) shows the effect of volumetric flow rate of the membrane solution on the outlet mole fractions of ethylene and ethane in the feed gas and the sweep gas for various carrier concentrations. The permeation rates of ethylene and ethane are enhanced by the increase in uMbecause the membrane mass transfer coefficient kM increases. As the concentration of silver nitrate increases, the permeation rate of ethylene increases due to the facilitation effect, while that of ethane decreases because of the increase in the viscosity of the membrane solution and a decrease in solubility in the membrane phase due I

’

’

“E s 1.6

’

’

I

’

’

I

0

1

0

0

0

0

0 w:

,.r, n 51

zl.2

I 0

0

5.0 G~x104Cmollsl

Fig. 7. Effect of gas flow, GF, on the gas-phase mass transfer coefficient of SOP, kcSOz.

50.42

Bo=1.25x102mol/m3

i0.3-

Bo=1x103 mol/m3

2

-

talc.

"

"

- 0.2 t!,.,I o-’

vM Ccm?minl

0.7 o,6_‘L)

’

>

h’Foul I 1

*:z:;

0

I

"

"

100 vM Ccmhminl

2000

.

I7

0.5-

A

6014

x10”mol/m3

! 0.4-

2 0.3g

Fig. 8. Effect of flow rate of the membrane solution, uM, on the outlet mole fractions of ethylene and ethane. The parameter B,, is the concentration of AgNOB.

to the salting-out effect. Therefore, the selectivity for ethylene relative to ethane increases with an increase in the concentration of AgN03, and yAsJyA,sout reaches a value as high as 460. The effect of carrier concentration on yAsoUt, yA’soutandyAsouJyA,s,utis shown in Fig. 9, in which the tendency mentioned above can be seen clearly. The solid lines in Figs. 8 and 9 are the results calculated by using the design equations. The physical parameters necessary for the calculations were estimated as follows. The solubility of ethylene in the AgN03 solution (HA) was estimated by the following equation [ 19,111:

1% (HAIHAW)

= kS1

(50)

128 1000

A3 100 0 ; j

-

10

s -B

talc.

4 = 100 cm?min 1

0

1

2 BOXlci3

3

4

Fig. 9. Effect of AgN03 concentration, YASmt

and

5

Cmol/m31

B,, on the outlet mole fractions of ethylene and ethane,

YA'S,,t ’

ks=i,(C,H,)+i+(Ag+)+i_(NO,-)

(51)

Here i9( CzH4), i, (Ag+ ) and i_ (NOB- ) are the contributions of solute gas, cation and anion, respectively, to the salting-out effect parameter ks, and HAw is the solubility in water. The values of i reported by Onda et al. [191 and the value of HAw given by Morrison and Billett [20] were used, i.e.: ig(C2H4)=-1.95x10-4, i+(Ag+)=-3.62x10-5, i_(NOB-)=3.23~10-4 m3/mol, and HAw = 2.12 x lo4 Pa-m3/mol. The solubility of ethane in AgN03 solution was estimated as the ratio of IZMAS to the observed value of KG*,, shown in Fig. 7. The value of IRMA.was estimated from the value of kMulASo for pure water using an equation similar to eqn. (46). The diffusivities of ethylene and the ionic species, and equilibrium constant K, were discussed in the previous paper [ 111. The diffusivities of ionic species were estimated from those at infinite dilution by the method of Sherwood and Wei [ 211. It was assumed that the mass transfer coefficients of ethylene and ethane in the gas phase are equal to that of sulphur dioxide. As shown in Figs. 8 and 9, the agreement between calculated results and observed data is satisfactory, which indicates that the permeation model and the design equation presented in this work are adequate to analyze the transport of ethylene and ethane. In the present experiments, the contribution of the diffusional resistance in the gas phases to the total resistance is small; the former is at most about 2% of the latter when the carrier concentration is 4 x lo3 mol/m3 and UMis 116 cm3/min. Therefore, gas-phase resistance can be ignored in the physical transport experiments of ethylene and ethane, as mentioned above. The variations of LAT and LA8with membrane area, s, calculated by the design equations were negligibly small, suggesting that the relationships JAF= JAs and JAsF= Jn,s approximately hold.

129 1d71_

,

I I , I I, I,

I J

- BoxlO% 4 mall m3

ys,$ _ 1 -

talc. 0 0

0 -10

‘OlO

Oo r 0 -

I , ,111, 20

50 100 200 vM Ccm3/minl

Fig. 10. Effect of membrane solution flow rate, IIM,on overall mass transfer coefficient of ethylene, K GA.

1 I lo3 :

I

Bglo’= n

,

I

I

I

4 mol/m3 4 .

m JJ

I

2

A

[email protected] Cl 0 ”

10’ T 3 _ x _ 3 r” -

0.125 P A

*

-

lo’

0

’ ’ ’

’

A

talc.

’ ’ ’ ’ ’

100 vM tcm3/min3

200

Fig. 11. Effect of membrane solution flow rate, uM, on the ratio of the ethylene/ethane overall mass transfer coefficients, K&KG*, . The parameter B, is the .&NO, concentration.

Figure 10 shows the effect of uM on the overall mass transfer coefficient of ethylene &A as defined by the following equation: JA.,=KGAV’AF

-PAS)av

(52)

130

When the concentration of AgN03 was 4 x lo3 mol/m3, the permeation of ethylene was facilitated markedly, and KoA was about 100 times larger than the coefficient of the physical permeation. The relationships between K&K oA, and uM are shown in Fig. 11. As the carrier concentration B,, increased, KGA/KGAcincreased, and reached more than 600 when B0 was 4 x lo3 mol/m3. The solid lines in Figs. 10 and 11 are also the calculated results, and agree satisfactorily with the observed values. Stability of flowing liquid membrane The comparison of stability of the flowing liquid membrane with that of the immobilized liquid membrane reported by Teramoto et al. [ 111 is shown in Fig. 12. It is clearly seen that the flowing liquid membrane is more stable than the immobilized liquid membrane, and yAsuutof the former is almost constant for more than 15 h. However, after repeated use of this liquid membrane for about 100 h, a decrease of the permeation rate was recognized. Figure 13 shows the experimental results of the simultaneous permeation of ethylene and ethane obtained with a new membrane and the degraded membrane. The average permeation rate of ethylene through the degraded membrane was considerably lower than that through the new membrane. On the other hand, with respect to the permeation rate of ethane, which is about two orders of magnitude lower than that of ethylene, no difference could be seen between the permeation rates through the two membranes. Brown spots were observed on the surface of the hydrophobic microporous membrane in the degraded module. The analysis of the brown spots with a Shimadzu ESCA 750 spectrometer suggested the formation of Ag and Ag,O. It was deduced that Ag was formed by the reduction of Ag+ 0.3

I I I I , I I I I , I I I I

0.003

immobilized liquid membrane

o-o 0

5

10 t

15

Ehl

Fig. 12. Comparison of stability of the flowing liquid membrane with that of the immobilized liquid membrane. The mole fraction of ethylene in the sweep gas, yAsoUt,measured as a function of time, t. Flowing liquid membrane; feed gas =pure ethylene, AgN03 concentration B,= 1 x 10” mol/m”, membrane flow rate uM= 94 cm3/min. Immobilized liquid membrane; feed gas = ethylene, 33.3 mol%, and ethane, 66.7 mol%, B,=2.5 x 10’ mol/m”.

131

qlb’: fiT4g 9

A:Czl-k

(new membrane)

A:Cz&

-(degraded membrane)

_

1

Bo=1x103 mol/m3

vM Ccm3/min3 Fig. 13. Simultaneous permeation of ethylene and ethane obtained with new and degraded membranes.

by some organic compound which could not be identified, and that Ag was oxidized to Ag,O by oxygen during the period of exposure of the membrane to air. The cause of membrane degradation can thus be deduced as follows: the surface of the support membrane was gradually covered by Ag and Ag,O during repeated experiments, and these clogged some pores of the membrane, which increased the diffusional resistance through the pores. The increase in the gasphase diffusional resistance generally lowered the permeation rate. However, the permeation rate of ethane was little influenced by such membrane fouling because the contribution of the gas-phase resistance of ethane permeation to the overall resistance was negligibly small. On the other hand, in the case of ethylene permeation, the contribution of the diffusional resistance in the gas phase was much larger than that for ethane, because in the former case the resistance of the liquid membrane was much reduced by the facilitation effect. In order to confirm the above deduction, the permeation rates of ethylene and ethane through the degraded membrane were simulated by the design equations using a value of Izothat was 2.5% of that of the new membrane, and are shown by dotted lines in Fig. 13. The calculated results are in good agreement with the experimental data. The solid lines in Fig. 13 are the calculated values for the new membrane. It was anticipated that the degraded module should be regenerated by passing an aqueous nitric acid solution, which can dissolve Ag and Ag,O on the membrane surface, through each channel. In fact, all brown spots vanished

132

HNOB. The experimental results for the single permeation of ethylene through the regenerated membrane are shown in Fig. 14. Experimental data fall on the same line as is obtained with the new membrane, indicating that the degraded membrane was completely regenerated. This finding suggests that addition of nitric acid to the carrier solution should be very effective in preventing membrane degradation. A long-term stability test was performed with a smaller module (membrane area 3 cmx 20 cm=60 cm’). Fluoropore@ FP-010 was used, which is chemically more stable and more hydrophobic than Duragard@ 2502. The carrier concentration was 4 x lo3 mol/m3, and pure ethylene and nitrogen were introduced into the module as the feed and sweep gas at a flow rate of 50 cm3/min. The result is shown in Fig. 15, where the periods when the experiment was interrupted with the channel of the membrane solution filled with the aqueous silver nitrate solution are shown by the dotted lines. The flowing liquid membrane using Fluoropore@ FP-010 as the support was stable for over 11 days, and no brown spot appeared on the support membranes. Thus, it was con-

upon washing the module with 4 x lo3 mol/m3

0.3

7

0.2

;0 30.1

0

Fig. 14. Regenerationof a degraded membrane with HNO,.

I

I

I

I

I 100

I

6-

0

0

t

I

I 200

I

I 300

Chl

Fig. 15. Long-term stability test. Permeation of ethylene as a function of time.

133

firmed that the use of Fluoropore @ FP-010 was very effective in preventing degradation of the flowing liquid membrane. Enrichment of ethylene In the experiments described above, the total pressure of the feed gas phase was equal to that of the sweep gas. In order to produce enrichment by membrane separation, it is necessary to keep the partial pressure of the feed gas phase, PAF, higher than that of the sweep gas phase, PAS, either by increasing the total pressure of the feed gas or reducing that of the sweep gas. Under such condition, enrichment is possible even if the mole fraction in the feed gas, YAF, is smaller than that in the sweep gas, YAs. Experiments of ethylene enrichment were carried out under the condition that the mixture of ethylene (49 mol% ) and ethane (51 mol% ) was introduced into the feed phase at atmospheric pressure and the receiving vessel was evacuated. In the experiment, two modules (A and B ) with 51 cm2 membrane area (width 3 cm, length 17 cm) were used. The microporous membranes used were Duragard@ 2502 for module A and Fluoropore@ FP-010 for module B. The compositions of the receiving phase are shown in Table 1. When module A was used, more than 98 mol% of ethylene was obtained and the molar ratio of ethylene to ethane was about 120. On the other hand, when module B was used, although the mole fraction of ethylene in the receiving phase was similar to that with module A, the ratio of ethylene to ethane reached more than 520 because of the higher concentration of AgN03 in the membrane solution. With module A, unless HNO, was added to the membrane solution, the module was soon degraded. However, module B was stable without adding HNO,. The better chemical stability of polytetrafluoroethylene than polypropylene towards AgN03 may be the reason for the different experimental results. TABLE

1

Composition of the receiving phase in the enrichment experiments with ethylene (water vapour is not included)

Module A

Module B

Membrane solution

Composition of the receiving phase

1 X lo3 mol/m3 AgNOB and 3.32 X lo3 mol/m3 HN03

C,H,

3.87 kPa

6.66 kPa

4 x lo3 mol/m3 AgN03

Microporous membrane

Pressure of the receiving phase

Duragard@ 2502

Fluoropore@ FP-010

&Ha “Air C&H,

a Air was introduced in the receiving phase due to leakage of the vacuum line.

CxHs “Air

98.31 mol% 0.80 mol% 0.89 mol% 98.51 mol% 0.19 mol% 1.30 mol%

134

Conclusion

A new type of liquid membrane called a flowing liquid membrane, in which the membrane solution flows in a thin channel between two hydrophobic membranes, was proposed for the purpose of overcoming the instability of the immobilized liquid membrane and the low permeability of the thin-layer liquid membrane. 1. The mass transfer coefficient in the membrane solution phase was increased with increasing volumetric flow rate of the membrane solution, and it was found to be large compared with that of the immobilized liquid membrane reported by Teramoto et al. [ 111. 2. Permeation experiments with ethylene and ethane were carried out by using flowing liquid membrane modules containing silver nitrate as carrier of ethylene. Ethylene was transported selectively compared with ethane, and the molar ratio of ethylene to ethane in the sweep gas reached as much as 460 when the carrier concentration was 4 x lo3 mol/m”. 3. The rates of facilitated transport through the flowing liquid membrane were analyzed theoretically. The outlet concentrations of ethylene and ethane and the selectivity could be satisfactorily simulated by the proposed design equations. 4. The flowing liquid membrane was much more stable than the immobilized liquid membrane. With repeated use of the module, solids, which appeared to be Ag and Ag,O, covered the hydrophobic support membrane and caused a decrease in the permeation rate of ethylene, but the module could be regenerated easily by washing it with nitric acid solution. Less degradation was observed with microporous polytetrafluoroethylene membranes than with polypropylene membranes. 5. By reducing the total pressure of the receiving phase, more than 98 mol% of ethylene was obtained from an approximately equimolar mixture of ethylene and ethane. Acknowledgement

This work was partially supported by a Grant-in-Aid for Scientific Research (No. 60550666,1985) from the Ministry of Education, Science and Culture of Japan. List of symbols

A A’ AT B

concentration of ethylene, mol/m3 concentration of ethane, mol/m3 total concentration of ethylene ( =A + C), mol/m3 concentration of AgN03, mol/m3

135

&I C D G H

. . .

$,Z+ 4 -

J K

k k’ KG k kiv ks

L P Re rJ S ;c Sh UM XMF

Y

initial concentration of AgN03, mol/m3 concentration of complex, mol/m3 diffusivity, m”/sec molar flow rate of gas, mol/sec Henry constant, Pa-m3/mol contributions of gas, cation and anion to ks, m3/mol permeation flux, mol/m2-set chemical equilibrium constant of eqn. (1)) m3/mol forward reaction rate constant of eqn. (1)) m”/ (mol-set ) reverse reaction rate constant of eqn. (1)) set-l overall mass transfer coefficient in gas phase, mol/ ( m2-set-Pa) mass transfer coefficient in gas phase, mol/ ( m2-set-Pa) mass transfer coefficient through membrane solution, m/set salting-out parameter, m3/mol molar flow rate in liquid membrane phase, mol/sec partial pressure, Pa Reynolds number DJ/DA(J=B,C) total membrane area, m2 membrane area, m2 Schmidt number Sherwood number volumetric flow rate of membrane solution, cm3/min thickness of boundary layer of membrane solution, m mole fraction

Greek letters viscosity, Pa-see F density, kg/m3 P Superscript value of water 0 Subscripts ethylene A ethane A’ average av B AgNO3 complex C feed gas F feed gas-membrane solution interface Fi gas phase G inlet in

136

M out S Si

membrane solution phase outlet sweep gas sweep gas-membrane solution interface

References 1

2

3 4 5

6 7 8

9

10 11

12 13 14 15 16 17 18 19 20 21

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