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Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 517-521 Natta, G.; Baccaredda, M.; Tranina, F.; Pergollni, R.; Balcet, E.; SoIdano, U. U.S. Patent 3 152997, 1964. Nlwa, M.; Mlzutani, M.;Takahashi, M.; Murakaml, Y. J . Catal. 1901, 70, 14. Pernicone, N.; Lazzarln. F.; Liberty, 0.;Lanzavecclla, G. J . Catal. 1969, 14, 293. Peterson, T. I. Chem. Eng. Sci. 1962, 17, 203. Popov, B.; Osipova, K.; Malakhov, V. K.; Kolchin, A. Kinet. Katai. 1971, 72, 1464. Reddy, P. J.; Murti, P. S. Indian J . Technol. 1971, 9 , 161. Reid, V. W.; Solomon, D.G. Analyst (London) 1955, 8 0 , 704.
Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. Sundararaman, N. Ph.D. Thesis, Indian Instltutes of Science, Bangalore, 1983. Trifiro, F.; Pasquon, I. Chim. Ind. (Milan) 1971, 53, 577. Chem. Process Eng. 1970,51(4), 100; US. Patent 31 52997, 1967.
Received for review November 18, 1985 Accepted March 31, 1986
Separation of Syngas Components Using a Dissolved Palladium Complex Stephen E. Lyke, Michael A. Lllga, Richard M. Ozanlch, and David A. Nelson’ Chemical Technology Department, Pacific Northwest Laboratory, Richland, Washington 99352
Brlan R. James. and Chung-Li Lee Department of Chemistry, University of British Columbia, Vancouver, B.C. V6T 7Y6, Canada
A process for the separation of carbon monoxide from gas mixtures has been developed based upon the selective The process was reversible binding of CO by the solventdissolved complex Pd,(dpm),Br, (dpm is Ph,PCH,PPh,). tested in a continuous bench-scale apparatus with absorber and thermal stripper columns. Separation of CO/N2 and CO/H2/COz/CH,/N, mixtures was examined by using 1,1,2-trichIoroethane both with and without the palladium complex. With the multicomponent mixture, both the solubility of COOand CH, in the pure solvent and the chemical binding of CO by the complex were signlficant. A process scheme using the combined effects to produce hydrogen from oxygen-blown coal gas is proposed.
Introduction carbon monoxide. These include CH3Mn(CO),(Calderazzo and Cotton, 1962), IrC1(CO)(PPh3)2(Vaska, 1966), Over the last decade, interest in the separation of gases (phthalocyanato)iron(II) (Ercolani et al., 1981),and various has increased as coal gasification technologies have become iron(I1) prophyrin systems (James et al., 1977; Hashimoto commercialized. Many procedures have been examined et al., 1982; Collman et al., 1983). The greatest difficulty for separation of individual components from gas mixtures. encountered in using metal complexes such as these for Although cryogenic rectification is predominantly used, gas separations is selectivity. For example, most complexes pressure-swing adsorption (PSA) (Wall, 1975) and memthat bind hydrogen also bind other gases, especially carbon brane technologies (Henis and Tripodi, 1983) appear to monoxide, often irreversibly, and are often 02-sensitive. have surpassed cryogenics for several specialized areas of Other requirements that are not always met are long-term gas separation. The use of coordination complexes offers stability and high solubility. unique possibilities for gas separation that may be ecoFor rapid and reversible CO binding, promising comnomically competitive with PSA and could technically plexes are those in the series Pdz(dpm),X2, where X is augment membrane separations. The COSORB process, halogen or NCO and dpm is PhzPCH2PPh2(Benner and based on dissolved CuA1C14,effectively removes CO from Balch, 1978) (eq 1). We initiated this research to detergas mixtures but is sensitive to water (Haase and Walker, 1974). Transition-metal hydride systems, such as MgHz CHz (BogdanoviE, 1985), have attractive properties for the PhzP’ ‘PPhz separation and storage of HF Further, coordination comI I plexes of cobalt have been examined in the past as a means X-Pd-Pd-X + CO to provide oxygen aboard ships (Fogler, 1947) and aircraft I I k-1 IIC‘’ (Adduci, 1976). PhP\ RPPhz (K) PhZP, ,PPhz CH. Our attention has been directed toward the syngas CHZ components, carbon monoxide and hydrogen. Many mine the feasibility of separating CO from gas mixtures transition-metal complexes in solution have been reported with concentrated Pd2(dpm),Xz solutions under continuto bind hydrogen reversibly. Noteworthy among these ous bench-scale conditions. In particular, mixtures similar complexes are IrC1(CO)(PPh3)2(Ph = phenyl) (Vaska and to those obtained from air-blown or oxygen-blown coal DiLuzio, 1962), RhC1(PPh3)3(Halpern and Wong, 1973), gasifiers were of considerable interest. The results disdecamethyltitanocene (Bercaw et al., 1972),W(CO)3(PCy3)2 (Cy = cyclohexyl) (Kubas, 1980), and R U ( C O ) ~ ( P P ~ ~ )cussed ~ below indicate that the complex successfully sep(Porta et al., 1978). Many other complexes have been arates CO from binary CO/N2 mixtures and serves well as a hydrogen separator when used with a quinary gas investigated previously for their ability to reversibly bind
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Table I. Approximate Solubilities of Synthesis Gas Components in TCE at 1 atm solubility, M X lo3 temp, “C HP CO CH, CO? NB 25 2.1 5.2 16 72 3.4 50 6.1 14 50 4.3 14 40 5.0 75 6.8
,
______
vent ---Gas -Liquid
Streams Loop
mixture. Also discussed are the process applications of this system.
Experimental Section General. The primary palladium source was PdC12 (Johnson Matthey). The dpm ligand (Strem) and the gases (Matheson C. P. grade CO, C02, H2, CH4, and N2) were used without further purification. Dichloromethane and 1,1,2-trichloroethane (TCE) were distilled prior to use. The Pd2(dpm)2C12complex was synthesized from Pd(PhCN)2C12,Pd2(dba)3CHCls (dba = dibenzylideneacetone), and dpm, and the Pd2(dpm),X2(X = Br, I, NCO) complexes were synthesized from the chloride precursor by the published methods (Benner and Balch, 1978; Olmstead et al., 1977). The solubilities of various gases in trichloroethane were measured with a constant-pressure apparatus described elsewhere (James and Rempel, 1968). Results showed that Henry’s law was followed up to 1 atm. The data were combined with a corresponding states correlation (Prausnitz, 1969) to describe solubilities over the temperature range used in the experiments (Table I). Bench-Scale Apparatus. Bench-scale experiments were performed in an absorber-stripper system constructed of glass, Teflon, and stainless steel (Figure 1). Solution containing the dissolved complex circulated between the absorber and stripper columns while absorbing CO or other gases and transferring them to the stripping gas. Each column had a 30-mm-inside diameter and a 25-cm packed depth. Both columns were packed with 3-mm borosilicate glass beads providing a surface area of 2400 cm2per column. The dry packing had a void fraction of 0.4. The system included provisions for heating and cooling the circulating solvent with minimum holdup. Approximately 300 mL of 0.03 M Pd2(dpm)2Br2in TCE was used within the apparatus. Gases from cylinders were metered by Matheson 600 series flowmeters. Samples taken at inlets and outlets were analyzed with a Hewlett-Packard multicolumn gas chromatograph. The concentrations of complexes in solution were estimated by UV-vis spectrophotometry (Lee et al., 1984). In the bench-scale experiments, nitrogen stripped CO from the solution, while a commercial system would use a pressure change and/or steam for stripping. The use of a noncondensable stripper gas for these experiments allowed for reliable metering and analyses of both inlet and outlet streams. Details of the bench-scale apparatus and its operation have been described elsewhere (Nelson et al., 1986). Mathematical Model. Estimation of the film conversion parameter (Levenspiel, 1972) showed that the intermediate regime of absorption with reaction should apply. Reaction kinetics should be fast enough so that mass-transfer limitations are significant, but slow enough so that transfer and reaction can happen in series. Mass transfer should occur through a liquid film in which a negligible amount of reaction occurs, while most reaction should occur in the bulk liquid between dissolved gas and complex. The dissolved gas concentration in the bulk liquid at any point in an absorber or stripper is bounded between saturation at the gas-phase partial pressure and
PUAP
Cooler
stripper
Figure 1. Bench-scale apparatus.
chemical equilibrium with the complex. The gas concentration in solution balances the rates of mass transfer and reaction. The mass-transfer process can be represented by Kla(x, - X ) = -G dy/dh (2) where K 1is the overall liquid-phase mass-transfer coefficient (cm/s), a is the interfacial area per unit volume (cm-l), x is the dissolved gas concentration (mol/cm3), x, is the dissolved gas concentration in physical equilibrium with gas, G is the superficial molar velocity (mol/(cm2.s)), y is the mole fraction of transferring component in the gas phase, and h is the distance through the column (cm). All of the terms in eq 2 can be related to controlled or measured quantities except for Kl and a. Once a model has been constructed, the product K p can be adjusted to fit experimental data. If the model is successful, Kla determined under one set of conditions should be capable of predicting system performance under a variety of conditions. Because Kl depends on diffusion rate, the product is expected to be proportional to absolute temperature. The physical equilibrium expression for Henry’s law coefficient H ((atm.cm3)/mol) at total pressure P (atm) is y P = Hx,
(3)
The concentration of complex-bound gas in the liquid can be represented as w (mol/cm3), while the superficial velocity of the circulating liquid is L (cm/s). If the subscript “0” indicates conditions at a column terminus, an overall material balance expression can be written as follows: L ( x + W ) + G o o = Lo(x0 + w,) + Gy (4) For an absorber-stripper operating at steady state, the rates of mass transfer and chemical reaction in any volume element can be related by a balance on dissolved CO K ~ c I (-x X~) = E[k,(z - W ) X - k , ~ -] L dx/dh (5) where E is the volume fraction liquid holdup and z is the concentration of dissolved complex (mol/cm3). The forward and reverse reaction rate coefficients kf (cm3/(mol.s)) and k, ( s - ~ )are functions of temperature as shown in Table 11. Equations 3-5 were solved simultaneously by eliminating x i and obtaining x w and x as functions of y, G, and L. This solution was incorporated into a model that also allowed for rate-limited vaporization of solvent and physical absorption-desorption of nitrogen. The model was implemented by using a Lotus spreadsheet, with
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Table 11. Thermodynamic and Kinetic Data for the Reaction of Pdz(dpm)zXzwith CO in dma Solution" AS,b J AS:," J X 10-4K,M-' lo-%
