Chapter 20
Novel Oxidative Membrane Reactor for Dehydrogenation Reactions Experimental Investigation Renni Zhao, N. Itoh, and Rakesh Govind
Downloaded by UNIV LAVAL on April 23, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch020
Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221
The use of a membrane reactor for shifting equilibrium controlled dehydrogenation reactions results in increased conversion, lower reaction temperatures and fewer by-products. Results will be presented on a palladium membrane reactor system for dehydrogenation of 1-butene to butadiene, with oxidation of permeating hydrogen to water on the permeation side. The heat released by the exothermic oxidation reaction is utilized for the endothermic dehydrogenation reaction. Application of membranes in reactors to enhance the reaction yield and conversion has been recognized in recent years.Various configurations for membrane reactors have been proposed and studied in the literature(l). One of these configurations involves a permselective membrane which selectively separates the reaction product(s) to enhance the conversion of equilibrium limited reactions, such as hydrogénation or dehydrogenation of hydrocarbons. Other potential advantages include lower reaction temperature thereby reducing effect of by-product reactions resulting in fewer by-products, and lower separation and recycle costs. In this paper experimental studies have been conducted on an oxidation palladium membrane reactor for dehydrogenation of 1-butene to butadine . Since only hydrogen permeates through a palladium membrane, separation and and hence shifting of the reaction to the products is achieved without any loss of the reactant. Furthermore, palladium is a good conductor of heat, thereby allowing the heat evolved from the exothermic oxidation reaction (permeating hydrogen to water) on the permeation or separation side to flow across the membrane to the reaction side . This heat coupling between an exothermic oxidation reaction and an endothermic dehydrogenation reaction allows the reactor to operate adiabatically. Background The use of palladium-based membranes results from the 1866 discovery by Thomas Graham(2) that metallic palladium absorbs an unusually large amount of hydrogen. Hydrogen permeates through Pd-based membranes in the form of highly active atomic hydrogen which can react with other 0097-^156/90/0437-0216$06.00/0 © 1990 American Chemical Society Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV LAVAL on April 23, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch020
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compounds adsorbed on the catalyst surface. The use of palladium membranes gained importance from increased application of membrane based separation in the field of chemical processing, biotechnology, environmental control and natural gas and oil exploration(3). It is reported that palladium-based membrane reactors have been used to some extent in the industrial production of chemicals and pharmaceuticals in the Soviet Union(4). One of the earliest applications of membrane to shift equilibrium was developed by Wood(5) (1960). He showed that by imposing a nonequilibrium condition on a hydrogen-porous palladium silver alloy membrane, an otherwise stable cyclohexane vapor is rapidly dehydrogenated to cyclohexene. The idea of conducting hydrogénation and dehydrogenation reactions simultaneous on opposite surfaces of a membrane, which is selectively permeable to hydrogen, was first presented by Grgaznov et α(6)(7). Itoh et ai(8). studied dehydrogenation of cyclohexane in a palladium membrane reactor containing a packed bed of Pt/A^Os catalyst. The removal of hydrogen from the reaction mixture using the palladium membrane increased the conversion from the equilibrium value of 18.7% to as high as 99.5%. It was shown that for given rates of permeation and reaction, there is an optimum thickness of membrane, at which maximum conversion is obtained. In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. Ilias and Govind(lO) have reviewed the development of high temperature membranes lor membrane reactor application. Hsieh(4) has summarized the technology in the area of important inorganic membranes, the thermal and mechanical stabilities of these membranes, selective permeabilities, catalyst impregnation, membrane/reaction considerations, reactor configuration, and reaction coupling. Recently, Itoh and Govind(n) have reported a theoretical study of coupling an exothermic hydrogen oxidation reaction with dehydrogenation of 1-butene in an isothermal palladium membrane reactor. Other studies on dehydrogenation reaction, such as decomposition of hydrogen sulfide, have been conducted with porous membranes such as Vycor glass, and alumina(Fukada et α/.(12), Kameyama et α/.(13)(14) ), Raymont(15) has suggested the decomposition of abuntantly available hydrogen sulfide as a possible means for generating hydrogen. It is known that a palladium membrane can enhance the conversion of a thermodynamically limited dehydrogenation reaction. Model Development A schematic of a palladium membrane reactor is shown in figure 1. The reversible reaction of 1-butene dehydrogenation occurs on the reaction side of the membrane in which the chrome-alumina catalyst is uniformly packed. The oxidation of hydrogen with oxygen in air occurs in the permeation or separation side on the palladium membrane surface. The
Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Downloaded by UNIV LAVAL on April 23, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch020
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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
palladium membrane acts as a catalyst for the oxidation reaction. The hydrogen produced by the dehydrogenation reaction permeates through the palladium membrane and then reacts with the oxygen on the permeation side. The dimensionless model equations governing the membrane reactor are essentially differential material and energy balances for each side of membrane and are summarized in Table I. In this model the follow simplifying assumption have been made: 1. 2. 3. Negligible radial gradients of temperature and concentration; 4. Negligible pressure drop on either side of membrane; 5. The heat and mass transfer resistances, aside from the permeation process itself, are negligible. Plug flow conditions exist at high Reynolds numbers typically found in flow through packing or small radius tubes. In a permeator, the convection flow dominates over the diffusion flow i.e. a high Peclet number can be expected in a reactor with a membrane. Moreover, the pressure drop in a packed bed is usually a small fraction of the total pressure and can be neglected without significant error. Experimental Method In the experimental study, the reactor, schematically shown in Fig.2, consisting of two separate rectangular parts with a 90 mm length, 25 mm width and 25 mm depth groove. The palladium membranedOO mmX33 mm and 0.025 mm thickness) held in place by two pieces of gasket which have a 80 mmX 20 mm rectangular hole in the center, is sandwiched between the two reactor parts. Six thermocouples were located along the length of reactor to determine the temperature profile inside the reactor. The reversible reaction of 1-butene dehydrogenation occurred on the reaction side of membrane where chrome-alumina catalyst was uniformly packed. The oxidation of hydrogen with air occurred on the palladium surface on the other side of membrane, referred to as the permeation (or separation) side. The palladium membrane acts as the catalyst during the oxidation. This surface reaction of oxidation decreases the hydrogen concentration on the separation side , thereby increasing the permeation of hydrogen through the membrane. Further, heat liberated by the exothermic oxidation reaction on the separation side flows across the membrane and facilitates the endothermic dehydrogenation reaction, thereby increasing the reaction rates. (See Table II.) A schematic of the experimental apparatus is showed in figure 3. The feeds of 1-butene, argon and 10% oxygen and nitrogen mixture were supplied from gas cylinders. The flow rates are measured by mass flow meters individually. The down stream flow rate on the permeation side chamber is open to the atmosphere, thereby maintaining the permeation side at atmospheric pressure. The products of dehydrogenation and oxidation were analyzed correspondingly using FID or TCD detector. The detector signal was monitored by an on-line microcomputer. Results and Discussion Measurement of reaction rate constant. The disappearance rate of 1-butene can be expressed by the following rate expression(16):
Baker and Murrell; Novel Materials in Heterogeneous Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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Table I Basic Equations Developed for the Membrane Reactor System Mass Balance Reaction Side (L>0) c —
l « - f t ^ U - g - » / ,
du
Λ
(1)
Γ Jr
Ρ
τ
dL
= D o » e r p { e ( l - l ) } / -1% exp{t (l-^ r
r
p
Ρ
)>0)
^
= 1* ^ { ε , Π - i )}( Sf-
V = V» + l 0
0
(
+
U
H+
V -U H
C
(5)
- V ^ i » - 2 i ) a > < e , ( l - 1 )>f.
U°„- V° )I2 H
(6)
V„ = 2
