3146
Ind. Eng. Chem. Res. 1994,33,3146-3153
MATERIALS AND INTERFACES Preparation of Carbon Molecular Sieve Membrane and Diffusion of Binary Mixtures in the Membrane Y. D. Chent and R. T. Yang' Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260
Crack-free carbon molecular sieve membrane supported on a macroporous substrate was formed by coating a layer of poly(furfury1 alcohol) followed by controlled pyrolysis. Molecular sieving membranes of desired thicknesses can be formed by repeating this procedure. Steady-state diffusion fluxes of single-component and binary mixtures of CHdCzHs through the membrane were measured. Although large overall concentration gradients were used (to achieve accuracy), concentration-dependent diffusivities were obtained through integral analyses of the flux data. From the concentration dependence of the single-component diffisivities, it is possible to predict binary diffusivities using a simple theory developed by the authors. Good agreement was obtained between theoretical predictions and experimental data for binary diffusion.
Introduction Inorganic membranes have received much attention because of their potential advantages for high-temperature gas separations and membrane-reactor applications (Armor, 1988; Hsieh, 1989; Zaspalis and Burggraaf, 1991; Itoh, 1987; Ioannides and Gavalas, 1993; Falconer et al., 1994). For gas-separation purposes in particular, the membranes need to have molecular sieving abilities, i.e., with pores near the dimensions of the gas molecules to be separated. Zeolites and carbon molecular sieves are two potential candidates for making these membranes. However, to form large, crackfree zeolite membranes remains a most difficult challenge, while it appears more feasible to form carbon molecular sieve membranes. Carbon molecular sieves can be prepared by controlled pyrolysis of natural and synthetic precursors, such as coal, coconut shell, pitch, phenol formaldehyde resin, polyfiurfuryl alcohol, poly(acrylonitrile), and poly(vinylidene chloride). When natural precursors are used, controlled activation (i.e., carbon gasification) by oxidation is also needed (Walker, 1972; Juntgen, 1977; Yang, 1987). Pyrolysis of polymeric precursors has been widely studied (Fitzer and Schafer, 1970; Walker, 1972 and 1990; Kitagawa and Yuki, 1981; Foley, 1988). The ultramicroporosity is initiated by small gaseous molecules channeling out of the solid matrix during pyrolysis. Supported and unsupported carbon molecular sieve membranes by pyrolysis of polymers has been studied by Bird and Trimm (1983) and by Hatori et al. (1991; 1992). However, the considerable shrinkage during carbonization caused cracking and deformation, which made it impossible to form a continuous membrane as concluded by Bird and Trimm (1983). Koresh and Soffer (1983) prepared apparently crack-free molecular sieving hollow fiber membranes by carbonizing polymer hollow fibers. These membranes would likely be lack of mechanical strength for practical application, hence it would be beneficial to form the carbon membranes on Current address: Arbor Research Corp., 806 Airport Blvd., Ann Arbor, MI 48108.
macroporous supports (Gavalas, 1993). Most recently, Rao and Sircar (1993) reported a supported carbon membrane by pyrolyzing polyvinylidene chloride coated on a macroporous carbon disk. This membrane had pores larger than that needed for molecular sieving and was hence used for permeation under a pressure gradient. Despite the practical importance of binary and multicomponent diffusion in molecular sieves, our knowledge (particularly theoretical knowledge) on the subject remains very limited. From pure component diffusion rates, one needs to be able to predict mixture diffusion rates. Krishna (1990) developed a model in which the cross-term diffusion coefficients are calculated from the main-term coefficients using an empirical formula obtained by Vignes (1966) for correlating diffusion data of binary hydrocarbon liquid mixtures. Wei and coworkers (Tsikoyiannis and Wei, 1991; Qureshi and Wei, 1990) formulated both single- and multicomponent theories based on a stochastic (micropore) and computational approaches, and these theories have predictive capabilities. A basic assumption in the theories of Wei et al. is pore blocking which applies,to channel-type zeolites. More recently, a theory was developed for binary diffusion with equal single-component diffusivities by employing a correlation factor used in the physics of self-diffision (Nelson and Wei, 1992). This approach appears to be simple and potentially useful. Concurrently, a predictive theory based on irreversible thermodynamics (Yang et al., 1991; Chen et al., 1993) and one based on kinetic theory (Chen and Yang, 1992) have been developed. The kinetic theory is particularly simple to use and has been used with good results (Chen and Yang, 1992; Chen et al., 1994). The purpose of this study was 2-fold. First, it was t o demonstrate that large, crack-free carbon molecular sieve membranes could be formed on macroporous supports and the suitable support was to be identified. Second, diffusion of binary mixtures was to be measured using this membrane, and the results were to be compared with our kinetic theory for predicting binary diffusivities from pure-component diffusivities (Chen and Yang, 1992).
0888-588519412633-3146$Q4.5QlQ 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 12,1994 3147
Experimental Section Preparation of Carbon Molecular Sieve Membrane. The carbon molecular sieve was formed by pyrolysis of polyfurfuryl alcohol (a liquid at room temperature) following a well established procedure (Walker et al., 1966;Foley, 1988;Mariwala and Foley, 1994). The final temperature in the heating procedure was 500 "C. For this reason, the compatibility in thermal expansion (or contraction) between the carbon and the substrate was an important fador in selecting the macroporous support. A macroporous electrode graphite (from Great Lakes Carbon Corp.) yielded the best results. The following properties pertained to the graphite: void fraction = 0.228,true density = 2.15 g/cm3,impurity -3%. The use of a Vycor glass support did not give satisfactory results. A further study is needed t o understand the effects of using different supports on the membrane. The carbon molecular sieve membrane was formed by coating polyfurfuryl alcohol on the graphite support, followed by pyrolysis. This procedure was repeated until a film of the desired thickness was reached. The graphite disk was 1.75 in. (4.45cm) in diameter and 3/16 in. (0.476cm) thick. The polyfurfuryl alcohol (Durez Resin) was supplied by the Occidental Chemical Corp. Before coating poly(furfury1 alcohol) (PFA) on the graphite, the surface of the graphite was fine polished and was cleaned in ultrasonic distilled water followed by drying in air. A thin layer of PFA was coated on one face of the graphite disk. The coated graphite was subjected to the following heating procedure: 90 "C for 3 h in air, heating to 300 "C a t 1.5 "C/min in Nz flow, and held a t 300 "C for 2 h, further heating to 500 "C a t the same heating rate in N2 and held at 500 "C for 6 h. The sample was then cooled to room temperature. The coating-pyrolysis procedure was repeated five times until a uniform carbon molecular sieve layer of 15 pm was formed. Equilibrium Isotherm Measurement. Equilibrium isotherms of CH4 and C2Hs were measured by the gravimetric method using a Cahn Vacuum Microbalance by following the weight changes of a small sample of carbon molecular sieve when subjected to small step changes in partial pressure. The carbon molecular sieve sample for the isotherm measurements was prepared by pyrolyzing pure PFA with the same heating procedure described above. Hence, the sample for the isotherm measurements did not contain graphite. After pyrolysis, the sample was ground and sieved to 50 mesh size for adsorption studies. Before isotherm measurements, the sample was degassed by heating at 423 K in helium flow until a constant weight was reached. The equilibrium isotherms of CH4 and C2H6 at three different temperatures (297,323, and 353 K) were measured. Diffusivity Measurement. The Wicke-Kallenbach cell steady-state technique was employed. The carbon membrane/graphite disk was mounted between two stainless steels cells (upper and lower). Two Viton O-rings (1.75in. 0.d. and 1.5 in. i.d.1, stable t o 473 K, were used to seal between the cells and the carbon sample. The edges of the sample were also sealed from the ambient so the flux was restricted between the two cells. Special care was taken to avoid any pressure difference across the membrane so that permeation of gases across the membrane was negligible. This was accomplished by keeping the manometer liquid levels equal between the outlets from the two cells. The diffusion cell was placed in a constant temperature bath
and kept a t 297,323,and 353 K. Before each experiment, the carbon molecular sieve membrane was outgassed by heating at 423 K i n a helium flow for a t least 2 h. The gas concentrations at the inlets and outlets connected to the upper and lower cells were analyzed with a gas chromatograph (Varian, Model 3700, with FID detector). The FID detector was capable of detecting concentrations well below 1 ppm. Both purecomponent diffusion and binary mixture diffusion experiments were performed. Helium was used as the inert carrier. By changing the concentrations, the concentration dependence of the diffisivities were determined. The flux data were taken after at least 8 h to ensure that steady state was reached. The flux across the membrane was calculated from mass balance at the steady state. The flow rates through both cells across the membrane were kept the same. In all experiments, pure He was fed into the lower cell, whereas He carrying different concentrations of CHI, C2&, or CHdCzHs mixtures was fed to the upper cell. The outlet concentration from the lower cell was then used to obtain the total flux. This was the most accurate way to measure the total flux. A control experiment was performed with a bare graphite disk (with the same thickness) and using the same flow rates. The flux in the control experiment was several orders (22)of magnitude greater than those measured with the carbon molecular sieve membrane. Consequently, the resistances in the macroporous graphite and in the gas films were negligible.
Results and Discussion Carbon Molecular Sieve Membrane Supported on Macroporous Graphite. Figure 1 shows the SEM photomicrographs of the carbon molecular sieve membrane on the graphite support, formed by the controlled pyrolysis procedure. The thickness of the membrane was calculated from the net weight gain upon completion of the membrane preparation and also from the SEM pictures. The SEM pictures showed that the membrane was of uniform thickness and was crack-free. (Further evidence of the film being crack-free was provided by comparing the dfisivities measured using this film and the literature data on molecular sieve carbons, as to be discussed.) The thickness of the membrane was 15 pm. Figure 2 shows SEM micrographs of the surfaces of the graphite and the carbon molecular sieve membrane. The surface of the membrane was relatively smooth, with a roughness within 0.02 pm. The pore sizes in the graphite support were of the order of 5-10 pm. Pure-Component Equilibrium Isotherms. The equilibrium isotherms were measured on carbon molecular sieve formed by the same procedure for pyrolysis of polyfurfuryl alcohol but without graphite support, as described in the foregoing. Isotherms of CH4 and C2H6 were measured at three temperatures: 297,323,and 353 K,as shown in Figures 3 and 4. The equilibrium data were fitted to the Sipps-type or loading ratio correlation isotherm:
Lbpn 4s
1
+ bp"
Values of the three fitting parameters for CH4 and CZH6 are given in Table 1. Isotherm data for CHI and C2H6 are available from the literature on the carbon molecular sieve, CMS 5A
3148 Ind. Eng. Chem. Res., Vol. 33, No. 12,1994
Figure 1. Right: SEI1 phiitomicrop.nplr ( 1 1 the cross s r c t i o n (of a 15 p m carbon molecular sieve layer liuilt on a macropomus graphite support. Lek SEM p h o t o m i c w k ~ a p l of i the cross section of the carbon molecular sieve and i t s surface.
Figure 2. SEM pliiitomicrugrrrplrs nf thr surface of graphite Iright) and the surface of thc carbon IIIOI~~CUIRI sieve film (IeR).
by Takeda Co. (Nakahara and Toshiaki, 1974), and for CH4 on the Bergbau Forschung carbon molecular sieve (Kapoor and Yang, 1989). The pores in the Takeda CMS 5A were larger than that in the Bergbau-Forschung carbon, while the equilibrium amounts adsorbed were also higher in the CMS 5A. Comparing our isotherm data with the literature data, our amounts adsorbed fell in between these two sets of data, i.e., approximately 20% lower than the CMS 5A and approximately 20% higher than the Bergbau-Forschung carbon. The isosteric heats of adsorption (Hs') at the average surface coverage values (over the experimental ranges) were calculated from the temperature dependence and listed in Table 2, along with the heats of vaporization (AH).The heats of adsorption were approximately 1.4AH. Single-Component Diffusion. The flux (J) for diffusion through the molecular sieve membrane can be expressed in the Fickian form:
where D is the diffusivity, q is the amount adsorbed, and x is distance. Diffusion in molecular sieves is strongly concentration ( 9 )dependent and has been well described by Yang et al. 1974 (Chen and Yang, 1992):
(2)
where Ax is the thickness of the molecular sieve carbon
J=-D@/&
(3) where Q is fractional saturation; Q = q/q. and q s is the saturated amount adsorbed, D Ois the diffusivity a t zero adsorption, and 1 is an interaction parameter, the meaning of which will be further discussed. For a steady-state operation, the gas flux J across membrane is constant. By substituting eq 3 into eq 2 and integrating from qH to q L , one gets
Ind. Eng. Chem. Res., Vol. 33, No. 12,1994 3149 0.8
1 0
K K
0 297 V 323
,
I
0
200
0
600
400
800
Pressure ( T o r r ) Figure 3. Equilibrium isotherms of C& on carbon molecular sieve.
-
2.0,
,
,
,
I
,
,
,
.
i
0 '
V-
0
200
400
600
800
Pressure (Torr) Figure 4. Equilibrium isotherms of C2H6 on carbon molecular sieve. Table 1. Single-ComponentEquilibrium and Diffusivity Parameters on Carbon Molecular Sieve (D Was Based on constant Diffusivity Regression) sorbate K CH4 297 323 353 CzHs 297 323 353
45, mmoYg 1.145 1.051 0.797 2.072 1.821 1.549
Do x lo8, b, lmorr 4.91 x 4.51 x 4.21 x 2.00 x 5.30 x 4.94 x
n
1.24 1.21 1.22 0.84 1.03 0.99
cm2/s 1.695 2.429 3.438 0.146 0.188 0.315
D x 108, cm2/s 0.056 2.379 0.195 3.047 0.280 4.445 0 0.293 0 0.343 0 0.524
A
Table 2. Energetic Parameters and Preexponential in Carbon Molecular Sieve Factor for Diffusivity (0;) sorAH, Do* x lo6, Hat, E,, kcdmol kcaYmol bate kcallmol cm% 2.51 2.13 1.22 CH4 3.01 2.97 3.74 0.21 CZH6 5.46 I
layer. Its value was 15 pm. The subscripts H and L denote high and low concentrations on the two sides of the diffusion cell, respectively. Since q~ >> q ~ the , amount adsorbed on the low concentration side was negligible. Equation 4 can be further expressed in terms of pressure by substituting eq 1 into eq 4:
where p is the partial pressure of the diffusing gas on the high concentration side.
Table 3. Steady-State Flux of CI& across Carbon Molecular Sieve Membrane temp, mole fraction mole fraction flux x 108, K YH yL 103 moY(cm2s) 294 0.270 2.2 0.863 0.280 2.2 0.868 0.390 3.6 1.211 0.453 3.6 1.434 0.459 3.8 1.477 0.651 5.3 2.038 0.883 6.5 2.536 0.887 6.6 2.563 0.959 7.0 2.713 323 0.308 2.7 1.027 0.340 3.0 1.160 0.452 3.8 1.452 0.622 5.2 2.001 0.759 4.1 2.350 0.963 4.9 2.799 353 0.342 2.0 1.170 0.419 3.3 1.479 0.463 2.7 1.591 0.656 3.9 2.272 Table 4. Steady-State Flux of C2€& across Carbon Molecular Sieve Membrane temp, mole fraction mole fraction flux x 109, K YH yL 103 g moY(cm2s) 294 0.412 1.0 4.104 0.549 1.5 5.920 1.8 6.756 0.659 0.866 2.1 8.252 0.987 2.3 9.037 323 0.202 0.8 2.468 0.322 0.7 3.772 0.333 0.8 3.958 0.496 1.3 6.262 0.529 1.5 6.763 0.587 1.3 6.561 0.747 1.5 7.637 0.977 2.1 10.45 353 0.212 0.5 2.973 0.295 0.7 3.982 0.412 1.0 5.413 0.502 1.2 6.529 0.544 1.3 7.003 0.713 1.7 9.102 1.8 9.759 0.787 0.993 2.2 12.39
The experimental data for CH4 and C2H6 fluxes at various upstream concentrations and at three temperatures are given in Tables 3 and 4. The fluxes are expressed by eq 5. Since the equilibrium isotherms are known, eq 5 contains only two unknowns: DOand 1.The values of DOand A were obtained from the flux data by regression. The regressed values are given in Table 1 for CHI and C2H6 a t three temperatures. The flux data are also plotted as functions of upstream partial pressure, shown in Figures 5 and 6. The flux data were also treated assuming that the diffisivities were not concentration dependent. The flux equation for constant diffusivity can be shown as
The best-fitted curves using eq 5a are also shown in Figures 5 and 6. Comparing the fittings by the two flux equations, it is clear that the diffusivities were concentration dependent. The parameter 1 is an important one in our theoretical treatment for activated diffusion in molecular sieves. The parameter 1 reflects the difference between the
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