Ind. Eng. Chem. Res. 1994,33, 3209-3216
3209
SEPARATIONS Silver-FacilitatedOlefiflaraffin Separation in a Liquid Membrane Contactor System Dean T. TSOU; Marc W. Blachman,. and James C. David BP Research, 4440 Warrensville Center Road, Cleveland, Ohio 44128-2837 Silver-facilitated ethylene/ethane separation was successfully accomplished using supported liquid membrane in a membrane contador configuration. Ethylene flux increased with increasing driving force. The effect, however, gradually leveled off a t higher driving forces ( > l o 0 psi) due to the limitation of the ethylendsilver complexation equilibrium. Increases in liquid side pressure resulted in a slight initial drop in ethylene flux that quickly leveled off to a constant value as more liquid pressure was applied. The reason for this drop may be a combination of two-phase flow effects and a change in the gas-liquid interface area. Varying liquid circulation rate showed two distinct regions of permeation behavior. At low liquid flow rate, the ethylene permeation is limited by the rate at which the carrier solution is delivered to the flash pot. In this region, ethylene flux increases with increasing liquid flow rate. At high liquid flow rates the ethylene flux is at a maximum and is limited by diffusion through the membrane wall. Ethylene transport is also affected by the fiber morphology (porosity and tortuosity) resulting from different spinning conditions.
Introduction Simple olefins are produced in great quantities as important petrochemical feed stocks. These olefins are usually produced by dehydrogenation of the corresponding alkanes. Their separation, a difficult process due to the similarity in molecular sizes and physical properties, is currently carried out by distillation, a highenergy intensive process (Eldridge, 1993). Hence, there is a strong incentive to develop new processes for their separation with lower energy cost. To improve the energy efficiency of this separation process, absorption using chemical complexing agents such as copper or silver salts for selective olefin removal has been investigated (Krekeler et al., 1963). However, separation using packed absorber columns has not been used extensively due probably to the high contactor cost associated with this process. Separation using membranes provides high contact surface area per equipment volume and greatly improves the efficiency and economics of the process. Incorporation of a solution of chemical complexing agent in the membrane (facilitated transport membranes) produces very high selectivity and flux (Schultz et al., 1974; Goddard et al., 1974). Facilitated transport has been applied to the separation of olefins from paraffins. Koval and Spontarelli (1988) reported the facilitated transport of hexene and hexadiene between two decane phases separated by thin, hydrated AgCloaded Nafion membranes. Steigelmann et al. (1973) and Hughes et al. (1986) showed that Millipore cellulose acetate filters impregnated with aqueous solutions of AgNO3 can selectively transport ethylene. Teramoto et al. (1986) presented the experimental results and theoretical
* To whom correspondence should be addressed.
Present address: Gelman Sciences, 8780 Ely Road, Pensacola, FL 32514-7010. Present address: Whatman, Inc., 4440 Warrensville Center Road, Cleveland, OH 44128-2837.
*
analysis for the facilitated transport of ethylene through supported liquid membranes containing silver nitrate as carrier. These systems began to show commercial promise with the development of high pressure resistant polysulfone hollow fibers that are capable of functioning under 200 psi transmembrane pressure differential (Valus et al., 1991). These factors, combined with the high surface areas afforded by the hollow fiber configuration, resulted in the pressure resistance and fluxes needed to make a commercially sized separation units economically feasible (Blachman et al., 1992). An inherent problem with polysulfone hollow fibers is their hydrophobicity. For the aqueous AgNO3 solution-based system, drying of the liquid membrane during operation causes irreproducible olefidparaffin separation performance and early membrane failure. Regeneration of the module is required frequently. In a typical experiment the module will be run under the appropriate humidified conditions until the olefin flux and/or purity drop below some acceptable level. The module will then be regenerated, which usually involves the pumping of a AgNO3 solution through it long enough t o refill the support membrane’s pore structure. The module can then be put back on stream. The time a module performs adequately on its initial start up can vary between several hours to a few days, and the effectiveness of subsequent regeneration is even more unpredictable. There are many literature reports on the instability of supported liquid membranes (Neplenbroek et al., 1992a; Gavach, 1987) and methods to improve their stability (Neplenbroek et al., 1992b). The problem caused by the incompatibility between the hydrophobic polysulfone hollow fiber and the aqueous AgNO3 solution can be minimized by making the membrane more hydrophilic or by making the solution more organic-like. Previous research results have indicated that a class of organic solvents, alkyl carbonates, can be used as cosolvents with water in these AgNO3-based olefin transport systems without ad-
0888-588519412633-3209$04.50/0 @ 1994 American Chemical Society
3210 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Table 1. Hollow Fiber Preparation dope quench Nzcore pump godet fiber temp temp pressure speed speed no. (“C) (“C) (psig) (rpm) (Wmin) 91 3.6 12 1 40 40 4.0 15 172 2 50 24 9 194 3 50 24 4.0
fiber 0.d. @m) 415 356 260
fiber fiberwall i.d. thickness (pm) k m ) 206 104.5 70 216 170 45
versely affecting AgN03 solubility or olefin flux (Tsou et al., 1992). Another approach to solve the drying problem was to use the hollow fiber module in a membrane contactor configuration. In this membrane contactor configuration the facilitator solution is continuously circulated over the permeate side of the membrane module, picking up olefins and transporting them to a flash pot where the solution is degassed and recirculated back to the membrane module. There are several important advantages to a membrane contactor system. One is that the membrane is in constant contact with the aqueous solution and is therefore continuously being “regenerated”. Another is that the module and flash pot are two distinct units and can therefore be controlled independent of one another, thereby allowing greater flexibility in system optimization. The idea of using constant contact of the membrane with the liquid carrier solution t o reduce drying problems and to improve stability can also be found in the “flowing liquid membrane” setup reported by Teramoto et al. (1989) and the “hollowfiber contained liquid membrane” setup studied by Sirkar (1988) and Majumdar et al. (1988). The increased amount of facilitator also provides a significant (‘buffer” against varying conditions and impurity effects. The theoretical aspect of the gaslliquid membrane contactor has been studied by Cussler and Qi (1985). We have carried out an extensive study using our high-pressure hollow fiber modules for the separation of ethylene and ethane in a membrane contactor system. Our goal was to obtain a basic understanding of how liquid membrane contactor performance responds to variations of parameters significant in a large scale industrial operation. The results are presented in the present paper and discussed on the basis of the theory of facilitated transport.
Experimental Section All fibers were examined under a Leitz Secolux optical microscope equipped with a high performance JAVELIN CCD monochrome camera. A Boeckeler video measurement system was used to obtain physical dimensions of all hollow fibers. All gas analyses were carried out on a Varian 3700 gas chromatograph using a 50 m x 0.32 mm i.dJ0.45 mm 0.d. Chrompak PLOT fused silica (Al203/KClliquid phase) capillary column at an isothermal 80 “C. All integrations were done on a HewlettPackard 3393A integrator. Since the amount of ethane from the GC analysis is usually very small and includes more experimental error, the results on ethane (and on separation factor) should be interpreted accordingly. The usually much larger peaks for ethylene GC analysis put the experimental error for ethylene at -5%. The hollow fiber used for all experiments was prepared using in-house pilot plant spinning equipment. By varying the pumping rate of the dope solution and the drawing rate (godet speed), hollow fibers of varying dimension were obtained. During and after spinning, the fibers were washed repeatedly with distilled water and stored after soaking in a 30% glycerol in water solution. Table 1 listed the spinning conditions and the
Fred Gar Ln
Comrollrr
He Sweep
3L Mass R o w cootrouer
Reject G u Out
a
a
B.& P r e l a w Regulator
b
TaGC
Membrmc Module
W
P
Figure 1. Liquid membrane contactor configuration.
dimensions of the resulting fibers used in this study. A casing was prepared out of l/4 in. stainless steel tubing and Swagelok fittings. The necessary number of fibers to yield the desired surface area was drawn through the casing and potted at the ends with epoxy (DOW D.E.R. 332 epoxy resin 10 parts by weight, CIBA GEIGY HY 837 hardener 4 parts by weight). The epoxy was allowed to set for a t least 5 h. The modules, including all the fittings, were generally about 26 cm long with about 17 cm usable length of fibers in it. The module was flushed with about 1.0 L of distilled water immediately before use. Excess water was blown out of the module, and the module was then connected to the testing system. A diagram of the automated testing system used is shown in Figure 1. The feed gas mixture (74%ethylene/ 26%ethane) was passed over one side of the membrane (active area of membrane = 20-50 cm2, cell temperature = 25 “C) at a flow rate of 50 cm3/min. Feed gas pressure can be varied from 0 t o 200 psig. A silver nitrate solution (0.5-5.0 N) is circulated through the inner bore of the fiber (countercurrent to the test gas) at a rate of 0.0-10.0 cm3/min. A back-pressure of 0-200 psig can be applied to the liquid side using a back-pressure regulator placed between the module outlet and the flash pot. The flash pot is swept with helium (20 cm3/min),and the permeate (sweep stream) is periodically analyzed for ethylene and ethane. The 75 cm3 flash pot was kept at atmospheric pressure and filled about half-full with AgNO3 solution for all experiments. The fluxes of both ethylene and ethane were obtained from the permeate gas composition as determined by GC and the supplied He gas sweep rate. Many factors affect the olefin separation capability of the system. System behavior was monitored as these factors were systematically varied.
Results and Discussion A. Estimation of Equilibrium Constant and Henry’s Coefficient. The equilibrium constant between Agf ions and ethylene and the free ethylene concentration in aqueous salt solution are estimated. The free ethylene concentration in solution was taken from the work of Morrison (1952): log S = -69.6
+ 3900/T + 23.7 log T
(1)
where S is the absorption coefficient at 1 atm of ethylene
Ind. Eng. Chem. Res., Vol. 33,No. 12, 1994 3211 partial pressure per 100 g of water and Tis the absolute temperature (K). The salting out effect due to the presence of AgN03 was estimated using the modified Setschenow equation:
1M ..""
-
log(--) cc, = kl 0.20
where a, is the Bunsen absorption coefficient (cm3 of gas/cm3 of solution) of ethylene in pure water and a is that in salt solution. The salting out parameter k is the sum of constants that are characteristic of gases, cations, and anions as follows (van Krevelen et al., 1948):
(3) and Z is the ionic strength:
As the value of X A ~ +was not reported, X A ~ +was approximated by XK+because the solubility of N2 in the aqueous solution is approximately equal to that in the aqueous KNO3 solution (Tret'yakov et al., 1973). The values of X reported by Onda et al. (1970) were used. Onda also reported values of X c 2 h for temperatures of 15 and 25 "C. Thus free ethylene concentrations at various &No3 concentrations can be estimated reasonably well. Henry's coefficients were then calculated from the resulting solubilities (see the supplementary material). The interaction of silver(1) cations with olefins has been studied extensively (Bennett, 1962; Hartley, 1973; Quinn et al., 1969; Winstein et al., 1938). Since the equilibrium constant is defined in terms of activities rather than concentrations, and activities usually depend markedly on concentrations, the equilibrium constants reported in the literature are usually determined a t very dilute aqueous solutions of AgNO3 where the activity coefficients approach unity. For the olefin separation application, concentrated solutions of silver(I) salts are needed to maximize the olefin absorption in a given volume. The reported equilibrium constants, therefore, cannot be used adequately for the analysis of facilitated olefin transport. There are, however, some data of olefin solubilities in various concentrations of aqueous silver nitrate solutions that are available in the literature and can be used to calculate an equilibrium concentration quotient suitable for use in the analysis of facilitated olefin transport performance. Bertsil B. Baker reported in 1964 that the amount of ethylene absorbed per mole of silver nitrate decreased with increasing silver nitrate concentration. His data, together with the free ethylene solubilities in aqueous salt solutions of the same concentrations and temperatures, were used to calculate the ethylene-Ag+ complexation equilibrium concentration quotient. This equilibrium concentration quotient increased a t higher concentrations of silver nitrate. Similar studies were also reported by Featherstone et al. (1964) and Crookes et al. (1973). Their results were similarly used t o calculate the equilibrium concentration quotients for the ethylene-&NO3 complexation interaction. Although different values of equilibrium concentration quotient were obtained from these calculations, they agree with each other reasonably well. A quadratic fit was per-
1
0.00
0.0
100.0
50.0
150.0
200.0
250.0
Ethylene Partial Pressure (psia)
Figure 2. Ethylene uptake at 25 "C.
formed on these data obtained between 20 and 25 "C. These fitted equilibrium concentration quotients were used for the analysis of ethylene transport in the facilitated membrane separation carried out a t ambient temperature. The temperature dependence of these equilibrium concentration quotients for temperatures other than 25 "Ccannot be obtained. (These calculated equilibrium concentration quotients and their quadratic fit are listed in the supplementary materials.) Once the AgNO3-olefin equilibrium concentration quotients and Henry's constants for free olefin in aqueous salt solutions are obtained, it is possible to calculate the olefin uptake in terms of olefin/Ag ratio at various olefin partial pressures and AgNO3 concentrations. Figure 2 shows the ethylene uptake curves for three solutions of silver nitrate with different concentrations. These calculations provide an estimate of the maximum thermodynamically allowed driving force for facilitated ethylene transport when the operating conditions of the system are specified. For example, when the average ethylene partial pressure is 110 psia at the membrane module and 10 psia at the flash pot, a 5.0 N AgN03 facilitator solution can be cycling between an ethylene/Ag ratio of 0.66 a t the module and 0.15 at the flash pot (from the 5.0 N curve of Figure 2) if thermodynamic equilibrium is reached at both locations and no kinetic limits exist in the system. However, because of the actual diffusion limitations through the membrane wall, the ethylene transport usually does not enjoy the full thermodynamically allowed driving force (vide infra). B. Facilitated Ethylenemthane Separation Using Liquid Membrane Contactor. Using a feed gas stream at 125 psig and circulating AgN03 solution a t 25 psig, good transport of ethylene and high permeate side ethylene purity was observed. The system was very stable, and no sign of drying or failure of any kind was observed during the month long experiments. The flux, reported in (STP) cm3/(crn2/s),is proportional to the driving force of the system:
flux = -(driving Q force) 1
(5)
In this contactor configuration the driving force for each component was taken as the differencebetween the logmean average of the membrane side feed and reject partial pressures and the partial pressure of the component in the flash pot. The permeability coefficient (Q/ I), reported in (STP) cm3/(cm2/s)per cmHg, is the flux of each component normalized by that component's driving force. The dimensionless separation factor is the ratio of the two component's permeability coefficients. The typical performance of our ethylene transport system together with some literature reported gas
3212 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Table 2. Ethylene Transport Performance membrane ion-exchange cellulose polysulfone h
3.OOE-03 2SOE-03
v
2.00E-03
1 L
1.50E-03
flux x lo3 (cm3/(crn2/s))
0 0 0 100
gas phase gas-liquid receiving phase gas phase gas-liquid contactor
R.O.
“ae 2
Atransmembrane pressure (psig)
configuration
3 1
1.7
1.1 12.0
Q/1 (cm3/(cm2/s)per c d g )
reference
2.3 x 10-7 4.6 x 3.0 10-5 5.0 10-5
LeBlanc (1980) Hughes (1986) Teramoto (1986) this work
1.60505 1.40505 P8 1.20505 1.ooE-05 8.00E06 0 6.00506 4.00506 w 2.00E06 O.OOE+OO 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
3a
;
5
o,183
$ 0.00
2.00
4.00
6.00
8.00
Liquid Flow Rate (cc/min)
Figure 3. Effect of liquid flow rate on ethylene transport (fiber no. 1, 125 psig feed gas pressure, and 1.0 N AgN03 circulating a t 25 psig).
phase systems are listed in Table 2 for comparison. Our high-pressure polysulfone hollow fiber in a membrane contactor configuration compares favorably to those literature reported gas phase systems. It should be noted that the permeability coefficient (Q/O for a given membrane varies somewhat depending on membrane morphology and operating conditions. The latter dues primarily to the presence of a chemical equilibrium (vide infra). The pressure resistance nature of our hollow fiber allows our system to be operated a t a much higher feed pressure and hence, substantially higher flux. The dependence of the performance of this liquid membrane contactor system for ethylene/ethane separation on varying operation parameters was systematically studied. (1) Effect of Varying Liquid Recycle Rate. The first series of experiments was studied using both 1.0 and 5.0 N AgNO3 in which the liquid recycle rate was varied from 0.3 to 8.0 cm3/min. In these experiments the feed gas and liquid pressures were held constant at 125.0 and 25.0 psig, respectively. The observed ethylene flux showed a dramatic increase with increasing liquid flow rate but rapidly leveled off. This behavior for 1.0 N AgN03 facilitator solution is shown in Figure 3. The same behavior was observed for 5.0 N AgN03 solutions. The region of rapidly changing flux with liquid flow rate can be described as the range where olefin transport is limited by the rate of liquid circulation from the membrane module to the flash pot. An increase in this rate will be reflected in an increase in ethylene flux. The plateau region is the range at which the system’s performance is not limited by the rate of liquid transport. Instead the rate-limiting step has been shifted to ethylene permeation through the membrane wall. Thus, further increase in liquid flow rate will have little or no effect on ethylene flux. At the plateau region more liquid is circulated through the system than necessary, a situation where AgN03 utilization becomes low. A way of measuring the efficiency of liquid circulation is the ratio of transported ethylene molecules to circulated silver ions. This ratio quickly decreases as the liquid flow rate is increased at the plateau section. These ethylene to silver ratios for each of the liquid circulation rates are shown above each data point on the curves in Figure 3.
Liquid Flow Rate (cclmin)
Figure 4. Effect of liquid flow rate on ethane transport (fiber no. 1, 125 psig feed gas pressure, and 1.0 N &No3 circulating at 25 psig).
For the set of experiments with 5.0 N &No3 as carried solution, the olefin flux plateau has a value of about 5.0 x cm3/(cm2/s).For 1.0 N AgNO3 carrier solution, the plateau ethylene flux is about 2.5 x cm2/(cm2/s). In both cases, even though the flux drops off a t low liquid circulating rate, the olefin-Ag ratio continues to increase. Thus there is a trade off between silver utilization efficiency and olefin flux. The most significant point on these graphs is the “elbow“ where the plateau begins. Even though higher silver utilization can be obtained a t lower flow rates, these points correspond to maximum silver utilization without sacrificing flux. It should be noted that even at the lowest flow rates the ethylene t o silver ratio was never higher than the theoretical maximum silver utilization. The theoretical maximum silver utilization was calculated from the difference in the thermodynamically allowed silver-olefin complexations under the conditions for the membrane module and the flash pot, i.e., 0.6 for 1.0 N &No3 solution and 0.5 for 5.0 N AgNO3 solution as determined from Figure 2. (The silver-ethylene complexation at the module was not really a t equilibrium with the feed gas ethylene partial pressure. The actual ethylene partial pressure was lower than the feed gas ethylene partial pressure due to permeation loss of ethylene to the carrier solution. These values represent only the maximum possible if ethylene partial pressure remains constant in the membrane module.) Thus there is still capacity left in the &NO3 solution to accept more ethylene from the gas phase before the liquid leaves the membrane module and returns to the flash pot. It should also be noted that to achieve maximum silver utilization one has to increase the “contact time” of the AgNO3 solution inside the fiber bore. This can be attained by either slowing down the liquid flow rate or using a longer module. The ethane results as shown in Figure 4 are unusual. In both the 1.0 and 5.0 N AgNO3 cases the ethane flux vs liquid circulating rate curves show parallel behavior to the ethylene results except at very slow liquid flow rates. The ethane flux increased suddenly to above plateau values at these very slow liquid flow rates. This is attributed to the consequence of two-phase ethane flow. When the ethane partial pressure corresponded to the dissolved ethane became grater than the limiting liquid pressure, ethane started to degas. When two-
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3213 4.00E-05
h
Y
1.4OE-02 1.20E-02
w
1.00E-02
I
8
3 8.00E-03 L*
I
6.OOE-03 4.00E-03 2.OOE-03 O.OOE+OO 0
50
100
I50
200
Liquid Pmssure (psig) Figure 6. Effect of liquid back pressure on ethylene transport (fiber no. 2, 125 psig feed gas pressure, and 5.0 N AgN03 circulating at 8.0 cm3/min).
and thus this condition is instantly met at the bore side of the liquid filled fiber. Degassing of both ethylene and ethane means their concentrations are relatively low in the circulating solution compared with those in the presence of liquid pressure. The presence of gas in the membrane bore could also result in a "thinning" of the liquid membrane that would result in a faster olefin permeation rate. Increasing the liquid pressure subdues this tendency to degas and results in the initial drop in olefin permeation. The initial decrease of ethylene flux with increasing liquid back pressure can also be interpreted as a result of changing liquid level inside the asymmetric micropores of the membrane wall. As the liquid pressure is increased, the gas-liquid interface is forced further into the membrane and correspondingly smaller pores. This may result in a significantly smaller gas-liquid contact area (possibly a greater liquid diffusion path) and hence a lower flux for all components. Once a certain pressure is reached this boundary could become stable, which corresponds t o the leveling off area of the graph (Figure 6). Further increases in liquid back pressure would have no effect. Another possibility is some compression effect of the fiber that we do not yet understand. For whatever reason the effect was minimal and disappeared when reasonable liquid pressure was applied. (3) Effect of Feed Gas Pressure. The effect of changing feed gas pressure was studied with no liquid back pressure a t the membrane module and a constant liquid flow rate of 8.0 cm3/min. The total feed gas pressure was systematically varied. Increasing feed gas pressure increases the partial pressure of all components on the gaseous side of the liquid membrane. An increase in fluxes of all components is expected and indeed observed as shown in Table 3. Increasing feed gas pressure increases the available driving force for the two permeating species. For ethane the flux correlates linearly with the increasing driving force. The linear correlation can be also seen from the constant Q/1 for ethane. The ethylene response, however, was different. Increasing the feed pressure resulted in an increase in flux that gradually leveled off at higher ethylene partial pressures. This is also reflected in the decreasing ethylene Q/l with increasing feed pressure. Similar behavior was observed for all &No3 concentration levels studied. This leveling is caused by the olefin-silver complex formation equilibrium that limits the amount of silverolefin complex a t higher olefin pressure (i.e., olefin concentration) that can be present in solution.
3214 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Table 3. Facilitated Ethylenathane Transport-Membrane Contactor Configurationa feed gas Q/Z ethylene crn3/(crn2/s) flux ethylene flux ethane [-%NO31 pressure (crn3/(crn2/s)) per cmHg (wig) (cm3/(cm2/s)) (M) 1.013-05 3.733-06 1.0 25 1383-03 8.573-06 6.743-06 1.0 50 1.943-03 7.563-06 8.743-06 1.0 75 2.393-03 1.03E-05 6.743-06 1.o 100 2.753-03 5.933-06 2.973-03 1.113-05 1.o 125 4.473-06 1.123-05 2.0 50 2.473-03 9.673-06 8.503-06 2.0 100 3,803-03 4.213-06 1.553-05 3.5 50 3.303-03 1.203-05 5.393-06 3.5 75 3.623-03 1.413-05 7.883-06 3.5 100 5.303-03 1.333-05 6.023-03 1.043-05 3.5 125 1.953-06 1.833-05 5.0 25 2.353-03 3.333-06 1.573-05 5.0 50 3.343-03 1.373-05 4.083-03 4.473-06 5.0 75 1.373-05 5.0 100 5.173-03 5.763-06 5.0 125 5.513-03 7.343-06 1.183-05 a
Q/l ethane cm3/(cm2/s) per cmHg
separation factor
7.193-08 7.783-08 7.163-08 6.483-08 5.673-08 4.933-08 5.043-08 4.533-08 4.083-08 4.443-08 4.653-08 3.573-08 3.563-08 3.303-08 3.263-08 3.373-08
140 110 106 104 105 228 192 341 294 317 285 49 1 443 416 420 350
Experiments carried out using fiber 1 with AgN03 solutions circulating a t 8 cm3/min and 0 psig, feed gas flow rate was 50 cm3/min.
Ag+
+ olefin
-
Ag'(o1efin)
In other words, the Ag+ becomes the limiting agent in the formation of silver-olefin complex that is the carrier responsible for the observed high olefin flux. This effect is not observed with the ethane since it does not coordinate with the silver and therefore involves no complex formation equilibrium. The presence of the effect of shifting chemical equilibrium on ethylene transport with increasing feed gas pressure and the absence of this effect on ethane transport was also evident in the observed trend of separation factors. As the feed gas pressure was increased the observed separation factor for ethylene1 ethane separation decreased. The nonlinear behavior of the ethylene flux and consequently the separation factor when feed gas pressures were systematically varied is the result of the nonlinear relationship between the olefin partial pressure and the Ag(ethylene)+ complex in solution. With the equilibrium concentration quotients and ethylene solubilities estimated in section A above, it is possible to calculate the concentration of the Ag(ethylene)+ complex concentrations in solution at both the flash pot and the membrane module. The Ag+lethylene equilibrium is assumed to be established instantaneously. The concentration of Ag(ethylene)+complex at module inlet is assumed to be the same as that in the flash pot. The concentration of Ag(ethylene)+ complex at module out is the sum of the inlet concentration and that gained through the observed flux. The free ethylene concentration, although small compared to Ag(ethylene)+ complex, was also included in the calculation. Thus the driving force is the concentration differential of all the ethylene containing species. When the ethylene transport is related to this concentration differential, a linear Qll with feed gas pressure is obtained. (A table containing calculated Adethylenel+ complex concentrations at both the inlet and outlet of the membrane module as well as at the flash pot is included in the supplementary material). -4linear correlation between the &/I based on concentration unit for the 5.0 N AgN03 carrier solution is shown in Figure 7. Thus the performance behavior of this membrane contactor for facilitated ethylenelethane separation is as expected from the basic theory of facilitated transport. The slightly upward trend of the ethylene &I1 calculated in concentration
I
E 2.5OE-03 I
-
I
0
I
20
40
60
80
100
120
140
Feed Pmssure (pig) Figure 7. Effect of feed gas pressure on ethylene transport (fiber no. 1, 5.0 N &No3 solutions circulating at 8.0 cm3/min).
terms may be due to errors in estimated Henry's coefficients and equilibrium concentration quotients used. (4) Effect of Varying Silver Nitrate Concentration. Results summarized in Table 3 also show the effect of increasing AgNO3 concentration on the performance of the membrane module for olefin separation. Increasing AgN03 concentrations actually lowered the ethane flux due to the lower ethane solubility a t high salt concentrations. Olefin flux, in general, increased with increasing AgN03 concentration. This is in line with the facilitation effect provided by the silver salt. However, this flux increase begins to level off a t higher AgNO3 concentrations to the point where there is less than a 10% difference between the performance of 3.5 and 5.0 N silver nitrate. The leveling off of ethylene flux at high AgNO3 concentration is probably due to a combination of salting out and chemical equilibrium effects. Other effects, such as rate limitation of ethylene transport at the gas-liquid interfacial boundary, may also play a role. The free olefin concentration a t higher salt (AgN03) concentration should be lower than that at lower salt concentration due to the salting out effect. The lowering of free olefin concentration at high AgNO3 concentration tends to shift the complexation equilibrium toward the left (eq 61, i.e., less silver-olefin complex formation. This prevents a linear increase in silver-olefin complex concentration as the total AgN03 concentration is increased and results in a leveling off effect on ethylene flux a t high AgN03 concentration. However, the ethylene permeability (QIZ),when calculated in concentration terms, also varied with the increasing AgN03 concentration. This is shown in
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3216
1 0'005
5Y
0.0°4
8 0.003 v c 0 0.002
1
0.001
Y 0.000
0
1
2
3
4
5
6
[AgNo~l(M) Figure 8. Ethylene Q/Z vs &No3 concentrations (fiber no. 1,100 psig feed gas pressure, and AgNO3 solutions circulating a t 8.0 cm3/ mid.
Figure 8. The absence of a linear relationship between ethylene QIZ expressed in concentration terms and the AgNO3 concentration indicates that other factors are involved in the ethylene transport. The permeability constant (Q/Z) includes factors such as the diffusion coefficient and the solubility of the permeating species. This decreasing trend of the ethylene Q/Z on AgNO3 concentration is probably caused by the increased fictional coefficients of the permeating species at higher salt concentrations. (5) Effect of Membrane Wall Thickness. Several different hollow fibers were prepared with varying wall thickness. The ethylenelethane separation performance behavior of these fibers followed the general trend observed for the one discussed above. A comparison of the ethylene transport for the three fibers is listed in Table 4. As expected, changing fiber wall thickness has a profound effect on the flux of all permeating species. Higher fluxes were observed with the thinner walled fibers. The effects are the same for both permeating ethylene and ethane. When the permeability is normalized with the wall thickness ( Q ) ,it can be seen that the fibers with wall thickness of 45 and 75 pm have very similar ethylene permeabilities. However, their Qs are slightly different from that for the fiber with wall thickness of 104.5 pm. The thickness normalized Q usually contains the porosity and tortuosity factors of the membrane that can be different when the fibers were produced under different conditions. Fibers 2 and 3 were produced a t the same dope solution temperature of 50 "C and quench temperature of 24 "C. The larger temperature difference during fiber spinning may have caused a faster membrane coagulation and a more open morphology resulting in higher Qs for the ethylene transport. Fiber 1 was produced at the same dope solution and quench temperatures of 40 "C, and the resulting skin morphology can be very different. Detailed investigation t o better correlate fiber preparation conditions with performance and fiber characteristics and morphology can lead to better performing fibers for this type of separation.
Conclusions The membrane contactor configuration provided continuous regeneration of the liquid membrane and resulted in a stable facilitated ethylene transport system. (1) Varying liquid recycle rate results in two distinct regions of behavior. At low liquid flow rate olefin flux increases rapidly with increasing liquid recycle rate where olefin permeation is limited by olefin transport from the membrane module to the flash pot. At higher
liquid flow rate, diffusion through the membrane wall becomes the limiting factor and olefin flux levels off. Ethane flux follows the same behavior as that of the ethylene, with indications of two-phase flow at low liquid pressure and recycle rates. (2) At higher driving forces the ethylene flux begins to level off due to the olefin transport becoming increasingly limited by the silver-ethylene complex equilibrium. The calculated ethylene permeability (Q/Z), when expressed in pressure units, varied as the ethylene partial pressure changed. The calculated ethylene permeability (Q/Z) becomes independent of ethylene partial pressure when it is expressed in concentration units. Ethane flux is directly and linear proportional to ethane driving force as expected since no facilitator equilibrium is involved. (3) Applying liquid pressure results in an initial decrease in flux which then levels off to a constant value. The initial high ethylene flux at no liquid back pressure may be a consequence of two-phase phenomenon. The initial increase in liquid pressure can also result in liquid being forced into the membrane skin, resulting in a decrease in the gas-liquid contact area. Membrane compression may also play a role. (4)Ethylene flux increased as the silver nitrate carrier became more concentrated. This increase, however, levels off at high AgNO3 concentration due probably to a combination of salting-out and chemical equilibrium effects. Other factors, such as gas-liquid interfacial transport of the ethylene and increased frictional coefficient of the diffusion process at high salt concentration, may also contribute to this change. Ethane permeation is subject solely to the salting-out effect, and flux decreases as AgN03 concentration increases. (5) Ethylene transport is inversely proportional to the membrane wall thickness. The wall thickness normalized permeability ( Q ) is the same for fibers produced under the same spinning conditions. When the fibers were prepared under different spinning conditions, different Q s were observed. The different fiber morphology (porosity and tortuosity) resulted from different spinning conditions presumably caused this change. The membrane contactor configuration provided a way to minimize the liquid membrane stability problem. Ethylendethane was easily separated using this AgN03based facilitated membrane contactor system. Sufficiently high flux (throughput) and separation efficiency were achieved. This system can be used economically for the separation and/or purification of other olefins. In fact we have successfully operated a pilot plant based on this technology for the separation of propylene from propane in late 1990. It has reliably produced purified propylene from refinery grade propylene at a rate of half a barrevday. While this technology is being refined and improved, there are still some aspects about this system that need to be further clarified. The effect of poisons and impurities that are commonly present in industrial gas streams on the carrier performance and lifetime needs to be studied. There is a need to better correlate the fiber performance with its preparation conditions, characteristics, and morphology. Accurate measurement of fiber dimensions as well as strength parameters, porosities, and skin thickness would make the correlation between membrane characteristics and transport performance much easier. Better understanding through this kind of correlation could lead t o improved olefin/ paraffin separation performance with optimized fiber.
3216 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 Table 4. Effect of Fiber Wall Thicknessa wall ethane flux fiber thickness ethylene flux no. bm) (crnY(cm2/s)) (cm3/(cm2/s)) 3.36e-5 3 45 1.87e-2 1.76e-5 2 75 1.06e-2 4.73e-3 8.41e-6 1 104.5 a
ethylene &I1 (cm3/(cm2/s) per cmHg)
ethane &/I (cm31(cm2/s) per cmHd
ethylene QI1 (cm3/(cm2/s)per M)
ethylene Q (cm3cd(cm2/s)per M)
4.41e-5 2.81e-5 9.97e-6
1.37e-7 6.33e-8 4.00e-8
7.75e-3 4.35e-3 1.78e-3
3.18e-5 3.27e-5 1.86e-5
Experiments carried out with 125 psig feed gas pressure and 50 psig 5.0 N AgNO3 carrier solution circulating a t 8 cm3/min.
The issue of membrane treatment, both immediately after spinning and just prior to using in the separation system, has not been adequately investigated. Different handling techniques (varying wash and storage solvents, membrane drying techniques, etc.) could lead to improved ease in processing, enhanced membrane performance and longevity, and better cost-effectiveness.
Acknowledgment The hollow fibers tested in this study were prepared by Mr. Alex E. Velikoff. Helpful discussions with him and other members of the BP Advanced Separation Group are gratefully acknowledged.
Supplementary Material Available: Tables of ethylene solubilities, ethylene-&NO3 equilibrium concentration quotients, ethylenelethane separation performances at varying carrier solution flow rates, carrier solution pressure, and calculated &I1 based on Ag(ethylene)+ complex concentrations at both the inlet and outlet of the membrane module as well as at the flash pot (6 pages). Ordering information is given on any current masthead page. Literature Cited Baker, B. B. The Effect of Metal Fluoroborates on the Absorption of Ethylene by Silver Ion. Znorg. Chem. 1964, 3, 200-202. Bennett, M. A. Olefin and Acetylene Complexes of Transition Metals. Chem. Rev. 1962, 62, 611-652. Blachman, M. W.; Velikoff, A. E.; Davis, J. C.; Valus, R. J. High Pressure Facilitated Membranes for Selective Separation and Process for the Use Thereof. U.S. Patent 5,131,928, 1992. Crookes, J. V.; Woolf, A. A. Competitive Interactions in the Complexing of Ethylene with Silver(1) Salt Solutions. J . Chem. SOC.,Dalton Trans. 1973, 1241-1247. Cussler, E. L.; Qi, Z. Microporous Hollow Fibers for Gas Absorption I. Mass Transfer in the Liquid. J . Membr. Sci. 1985,23,321332. Eldridge, R. B. Olefiflaraffin Separation Technology: A Review. Znd. Eng. Chem. Res. 1993,32, 2208-2212. Featherstone, W.; Sorrie, A. J . S. Silver-Hydrocarbon Complexes. J . Chem. SOC.1964, 5235-5242. Gavach, C. Ion Transport in Liquid Membranes. Proceedings of the 4th Summer School in Membrane Science, Sept 13-18, 1987, Chester, England. Goddard, J. D.; Schultz, J. S.; Suchdeo, S. R. Facilitated Transport via Carrier-Mediated Diffusion in Membranes, Part 11. Mathematical Aspects and Analyses. AZChE J . 1974,20, 625-642. Hartley, F. R. Thermodynamic Data for Olefin and Acetylene Complexes of Transition Metals. Chem. Rev. 1973, 73, 163190. Hughes, R. D.; Mahoney, J. A.; Steigelmann, E. F. Olefin Separation by Facilitated Separation Transport Membranes. AIChE National Meeting, April 5-9, 1981, Houston TX. Hughes, R. D.; Mahoney, J. A.; Steigelmann, E. F. In Recent Developments in Separation Science; Li, N. N., Calo, J. M., Eds.; CRC Press: Boca Raton, FL, 1986; Vol. 9, pp 173-195. Koval, C. A.; Spontarelli, T. Condensed Phase Facilitated Transport of Olefins Through a n Ion Exchange Membrane. J . Am. Chem. SOC.1988,110, 293-295.
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Abstract published in Advance ACS Abstracts, October 1, 1994. @
