Znd. Eng. Chem. Res. 1991,30,591-594 Levich, V. G.Convective Diffusion in Liquids. In Physicochemical Hydrodynamics; Prentice-Ha& Englewood Cliffs, NJ, 1962;pp 39-138. McGregor, R.; Etters, J. N. Transitional Kinetics in Disperse Dyeing. Text. Chem. Color. 1979,11, 202159-206163. Newman, A. B. The Drying of Porous Solids: Diffusion and Surface Emission Equations. Trans. Am. Znst. Chem. Eng. 1931, 27, 203-220. Wilson, A. H. A Diffusion Problem in Which the Amount of Dif-
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fusing Substance is Finite. Philos. Mag. 1948, 39, 48-58.
Jamee N.Etters Textile Sciences, Dawson Hall, The University of Georgia Athens, Georgia 30602 Received for review July 30, 1990 Revised manuscript received January 7, 1991 Accepted January 17,1991
Development of a Composite Palladium Membrane for Selective Hydrogen Separatioq at High Temperature A method is described for development of a composite palladium membrane for selective hydrogen separation a t high temperature. Electroless plating is used to form a thin palladium film on a silver porous substrate. The composite formed showed excellent mechanical strength and very large selectivity for hydrogen. The studies performed so far suggest that electroless plating can be utilized in making a metal composite membrane that can be used at high temperatures. Composite properties seem fairly constant at high temperatures. The permeability of the composite membrane is comparable to theoretical permeabilities for pure palladium. Introduction Recently there has been increased interest in applying inorganic membranes for in situ separation of product species-particularly hydrogen-to achieve equilibrium shift. From thermodynamic considerations, in a chemical reaction, if one of the reaction products that slows down the reaction rate can be continuously removed, the equilibrium state of the reaction can be shifted in the direct of forward reaction, thereby increasing the conversion. Experimentalisb have shown that it is possible to remove products selectively through pores of thermally stable Vycor glass (Shinji et al., 1982). Itoh (1987) obtained enhanced conversion of cyclohexane to benzene from equilibrium conversion of 14% at 473 K and 1-atm pressure to 100% by removing hydrogen selectively through a thin (25 pm) palladium membrane from the reaction mixture. Recently, Zhao et al. (1990) have presented similar experimental results for dehydrogenation of 1butene to butadiene using a palladium membrane reactor. However, the productivity of these membrane reactors is severely limited by the poor permeability of the membrane. Commercially available membranes are either thick films or thick-walled tubes. Since the permeability is inversely proportional to the film thickness, a thick membrane acts as a poor separator. However, the thermal stability and mechanical strength of a film is directly proportional to its thickness. Hence, we need to provide the necessary mechanical strength to the thin film. Thus a major challenge lies in developing a permselective thin solid film, without compromising the integrity as well as the desired properties of the film. Availability of such a membrane for high-temperature application will open a new area of research in membrane reactor technology and gas separation. Metals like palladium can be used as membranes at high temperatures owing to their selectivity toward certain gases like hydrogen. However, to obtain high flux we need thin membranes. These thin membranes cannot withstand high pressure differentials,hence we need to provide mechanical strength to these membranes. This can be achieved if a thin film of metal can be supported by a thermally stable porous substrate. The composite thus formed will act as a membrane with high selectivity and high flux. Electroless plating can be used to plate a thin film of metal on any porous substrate. It involves reduction of 0888-5885/91/2630-0591$02.50/0
Table I. TyDical Platina Bath ComDosition component concentration palladium chloride 0.375g/L ammonium hydroxide 30.0 mL/L ammonium chloride 4.5 g/L 10.0g/L sodium hypophosphite monohydrate
a metal salt by a reducing agent like hypophosphite preferentiallyon a catalytic surface. Once plated the metal on the surface acts as a catalyst for further reaction. The metal forms a thin uniform film on the surface. Although relatively expensive, electroless plating is superior to electrolytic plating because of the following reasons (Lowenheim, 1978): 1. Nonconducting (ceramics, Vycor glass, polymeric) surfaces can also be coated by use of electroless plating. 2. The deposits are thin, more dense, and uniform. 3. Complicated apparatus like power supply and electrical contacts are not needed. 4. The throwing power of electrolss plating is nearly perfect. 5. There is no formation of projections or buildup on the edges in electroless plating. The objective of this paper is to describe the methods and procedures used to achieve the goal of forming a composite membrane using electroless plating and to describe the characterization of the composite formed. Experimental Section The experiments were divided into two parts: plating of metal on a porous substrate and characterizing the composite formed. Plating. Porous silver disks (Poretics Corporation, Livermore, CA; 47-mm diameter, 0.2-pm pores, 0.5-mm thickness) were used as porous substrate. These disks were cleaned in acidified boiling water for 10 min to remove organics and dirt. One surface was activated with a sensitizing solution consisting of tin chloride and palladium chloride. This activated disk was then plated with palladium by electroless plating. The plating bath consisted of a palladium-amine complex and sodium hypophosphite as reducing agent. The pH of the bath was maintained at 10.2 by using an amine buffer (Athaval and Totiani, 1989). Table I shows typical plating bath composition and
b 1991 American Chemical Society
592 Ind. Eng. Chem. Res., Vol. 30, No. 3, 1991
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Mass flow meter
pressure gauge
' - - - - -7-- -
Fesd in
Figure 1. (Top) Experimental setup for permeability testing. (Bottom) Schematic of membrane cell.
conditions. Plating was carried out a t three different temperatures (23,40,and 48 "C).The amount of palladium deposited was calculated from the weight difference in the sample after and before plating. The plated samples were then analyzed for composition with EDAX (energy-dispersive analysis). The thickness of plating was calculated from the plating density and the weight gain. Plating thickness was independently calculated by using an SEM (scanning electron microscope). Permeability Studies. The composite membrane formed by electroless plating was subjected to permeability studies. The membrane was tested for permeability of hydrogen and argon at various temperatures and pressure differentials. A schematic of the experimental setup used for testing permeability of the composite membrane is shown in Figure 1 (top). The permeability testing assembly consisted of the gas source, a membrane cell, a flowmeter, and optionally a gas chromatograph. The schematic of the membrane cell is shown in Figure 1 (bottom). The membrane cell consisted of a feed chamber and permeate chamber. The composite membrane was held between the chambers by use of graphite gasketa. The membrane area was estimated from the area not covered by the gaskets. The membrane cell was heated in a furnace to the desired temperature. Pure gas was fed to the feed chamber at different pressures, and permeate gas flow rates were measured with a bubble flowmeter on the downstream and a mass flowmeter on the upstream of the membrane cell.
Rssults and Discussion Plating. The rate of electroless plating depends on factors like activation of surface, concentrations of metal salt and reducing agent in the plating bath, and temperature and pH of bath.
0
1000
2000
IhN(W
Figure 2. Amount of palladium plating on silver disk. a, 48 OC; 40 O C ; W, room temperature (23 "C).
*,
Figure 2 shows plating data for different temperatures. The amount of palladium plated is plotted versus time. This graph shows that plating is slow initially and attains a constant rate after some time. The constant rate of plating is plotted against the temperature of plating in Figure 3. As can be seen, the rate of plating increases with temperature. The initial slow rate of plating and a constant plating rate later can be explained by using mixed potential theory
Ind. Eng. Chem. Res., Vol. 30,No. 3,1991 593
4.004 0.0031
0.0033
0.0032
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1TT
Figure 3. Effect of temperature on rate of plating.
(Paunovic and Ohno, 1988). Mixed potential theory interprets the electroless plating reaction in terms of partial half-anodic and half-cathodic reactions. These half-reactions are striving to reach respective equilibrium potentials at the surface. The resultant potential on the surface at steady state if E,,, (steady-state mixed electrode potential). The time requires to reach the Em,, is called induction time (Paunovic, 1977). The initial slow rate of plating may be attributed to the induction time. Once the steady-state potential is established, the rate of plating remains constant. The increase in plating rate as temperature increases suggests an Arrhenius type of relationship. However, this explanation is rather an oversimplificationof the mixed potential theory and more work is needed to analyze the factors affecting induction time and the rate of plating. Plated samples were observed under a scanning electron microscope. A typical SEM photograph of plated sample, unplated sample, and cross section of plated sample is shown in Figure 4. Figure 4 (middle) shows clusters of palladium-phosphorus alloy on the surface indicating the possibility of a nucleation and growth mechanism for the deposition process. It can be seen that plating is more or less uniform. From the cross section of the composite (Figure 4 (bottom)) it can be seen that the plating does not penetrate the pores. The plating thickness was found to be about 5 pm. EDAX analysis of plating composition shows about 6 wt 70phosphorus in the plating (Figure 5). This suggests that the hypophosphite half-reaction not only reduces the metal but also forms some elemental phosphorus. Such large amount of phosphorus may be due to nonoptimal plating Conditions. We need to study the effect of phosphorus on the performance of plating. Furthermore, on heating the palladium-phosphorus film to 450 "C and then conducting an EDAX analysis, we found that there was apparently no change in phosphorus content. This suggests that the palladium-phosphorus may have formed an allay stable at 450 "C. Permeability. Permeability studies were conducted for two gases: hydrogen and argon. Experiments were performed at two temperatures (407 and 370 "C)and four different pressures. Fluxes of hydrogen and argon under different conditions were plotted against driving force of (ph1I2 - pI1l2) for hydrogen. No flux of argon was detected by the bubble flow meter for a period of 6 h for any of the pressure differentials studied. Figure 6 shows a typical
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A.
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Figure 4. (Top) SEM photograph of unplated silver disk (magnification 1200). (Middle) SEM photograph of Pd-plated silver disk (magnification 1200). (Bottom) SEM cross section of Pd-plated silver disk (magnification 2400).
Table 11. Tmical Permeability Data permeability, barrers temD. o c obsd calcd 370 6.5 X 5.5 x 6.2 X lo* 7.2 X lo4 407
graph of flux of hydrogen against driving force. From the graph it can be seen that the flux follows Siewert's law, and hence, it may be concluded that the porous substrate may not be offering any resistance to the flow. The theoretical permeability for the actual plating thickness (5 pm) was calculated (Ackerman and Koskinas,
Ind. Eng. Chem. Res. 1991,30,594-600
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with the theoretical value. Furthermore, the activation energy for palladium agrees with the theoretical value. The composite was found to be mechanically and thermally stable. The effect of thermal cycling and pressure cycling during the experimental runs did not result in changing the hydrogen permeability, and there was no physical peeling of the palladium film.
Pd
Registry No. Pd, 7440-05-3;Ag, 7440-22-4;HB,1333-74-0.
Literature Cited Ackerman, F. J.; Koskinas, G. J. Permeation of hydrogen and deuterium through palladium-silver alloys. J. Chem.Eng. Data 1972, 1 (171,51-55. Athavale, S. N.; Totlani, M. K. Electroless plating of palladium. Met. Finish. 1989,1,23-27. Itoh, N. Development of a novel oxidative palladium membrane reactor. AZChE J. 1987,33,23-27. Lowenheim, F. A. Deposition of inorganic films from solution. In Thin Film Processes; Vossen, J. V., Kern, W., E&.; Academic: New York, 1978;Chapter 111-1. Paunovic, M. Ligand effects in electroless copper deposition. J. Electrochem. SOC.1977, 127, 349-354. Paunovic, M., Ohno, I., Eds. Proceedings of the Symposium on Electroless Deposition of Metals and Alloys; ProceedingsElectrochemical Society 88-12;Electrochemical Society: Pennington, NJ, 1988. Shinji, 0.;Mieono, M, Yonedo, Y.Dehydration of cyclohexaneby use of a porous glass reactor. Bull. Chem. SOC.Jpn. 1982, 55, 2760-2764. Zhao, R.; Itoh, N.; Govind, R. Novel oxidative membrane reactor for dehydrogenation reactions; Baker, R. T. K., Murrell, L. L., Eds.; Novel Materials in Heterogeneous Catalysis; ACS Symposium Series 437;American Chemical Society: Washington, DC, 1990; pp 216-230.
ENERGY (KeV)
Figure 5. EDAX curve.
* Author to whom correspondence should be addressed.
0.0 1.2
1.6
2.0
2.4
2.8
Rakesh Govind,* Devendra Atnoor drmnJbrcs(pril'1nI
[email protected]%
Figure 6. Graph of hydrogen flow vs driving force. 0,407 OC; 370 'C.
*,
1972) and compared with experimentally obtained permeability. Table 11gives permeability data for observed and calculated permeability at different temperatures. From the table it can be seen that the data match very well
Department of Chemical Engineering University of Cincinnati Cincinnati, Ohio 45221-01 71 Received for review September 6 , 1990 Revised manuscript received December 3, 1990 Accepted December 13, 1990
Computer Simulation of an Industrial Calciner with an Improved Control Scheme Raw sodium carbonate (soda ash) is produced by calcining natural trona ore (sodiumsesquicarbonate). The calcination process is carried out in rotary calciners, where a typical retention time of the solid particles is 20-25 min. On the other hand, the hot combustion gas has a much shorter retention time. A problem is encountered with the degree of calcination, overcalcination, or undercalcination, when the throughput varies from a design rate. This can be avoided if the product temperature is substituted as the controlled variable. The computer simulation of the calciner confirms these problems, which were encountered in the industry. A better way to control the product quality is presented using a combined feedback control scheme: calciner off-gas temperature control modified with the product temperature control.
Introduction Direct-fired rotary calciners are heterogeneous reactors with continuous exchange of heat and mass between the gas and the solid phase. As a result, the equations needed to define the state of the calciner are large in number and 0888-5885/91/2630-0694$02.50/0
varied in nature, A mathematical model of an industrial soda ash calciner is briefly described in the ensuing d o n . On the basis of maes- and heat-balance considerations, the model is designed for the simulation of steady-state and dynamic behavior of the calciner. In this paper, the 0 1991 American Chemical Society
