2094
Ind. Eng. Chem. Res. 2006, 45, 2094-2107
Consecutive-Parallel Reactions in Nonisothermal Polymeric Catalytic Membrane Reactors Jose´ M. Sousa†,‡ and Ade´ lio Mendes*,‡ Departamento de Quı´mica, UniVersidade de Tra´ s-os-Montes e Alto Douro, Apartado 202, 5001-911 Vila-Real Codex, Portugal, and LEPAEsDepartamento de Engenharia Quı´mica, Faculdade de Engenharia, UniVersidade do Porto, Rua Roberto Frias, 4200-465 Porto, Portugal
This work reports the development of a nonisothermal and nonadiabatic pseudo-homogeneous model to study a completely back-mixed membrane reactor with a polymeric catalytic membrane, for conducting the consecutive hydrogenation of propyne to propene and then to propane. The performance of the reactor is analyzed in terms of the propyne concentration in the permeate stream (the only outlet stream from the reactor), the conversion of propyne and hydrogen, and the selectivity and overall yield to the intermediate product propene. The operating and system parameters considered are the Thiele modulus, the dimensionless contact time, the Stanton number, and the effective hydrogen sorption and diffusion coefficients. To define the regions where the catalytic membrane reactor may perform better than a conventional reactor, a comparison between both reactors is made. For the range of parameter values considered, the reactor model in this study demonstrates that the catalytic membrane reactor performs better than the conventional catalytic reactor in some regions of the Thiele modulus parametric space, for medium to high Stanton number values and for the total flowthrough configuration (total permeation condition). Concerning the effective sorption and diffusion coefficients of hydrogen, they shall be higher than the ones of the hydrocarbons. 1. Introduction Theoretically speaking, the combination of a chemical reaction and separation modules in a single processing unitsa catalytic membrane reactorsshould have several advantages over the conventional arrangement of a chemical reactor followed by a separation unit. Among other advantages that can be explored in this new configuration,1 two main ones can be identified. In one hand, membrane reactors can be used to increase the overall conversion above the theoretical thermodynamic value for equilibrium-limited reactions, by a selective product removal.2,3 On the other hand, a segregated feed of the reactants can be used to improve the selectivity and/or overall yield to an intermediate product in complex reactive systems and/or to control the reactor temperature, thus improving operation safety.4-7 Most of the potential applications for this new technology refer to the ensemble of processes conducted at high temperatures, from 300 to 1000 °C.1,8 As a consequence, only inorganic ceramic or metallic membranes, able to operate in such harsh conditions, can be considered. Nevertheless, beyond the applications in the field of biocatalysis,1,9 catalytic polymeric membranes can also be integrated into membrane reactors to be used in specific areas where processes are conducted in mild conditions, namely, in fine chemical synthesis10,11 and partial hydrogenation of alkynes and/or dienes to alkenes,12-14 among others.15-17 The focus of this study is on the performance of a catalytic polymeric membrane reactor, by comparing it with a conventional reactor, when conducting a consecutive-parallel reaction system given by
Reaction 1: A + B f C
Reaction 2: B + C f D
Many commercially important chemicals are intermediate * To whom correspondence should be addressed. Tel.: +351 22 5081695. Fax: +351 22 5081449. E-mail:
[email protected]. † Universidade de Tra ´ s-os-Montes e Alto Douro. ‡ Universidade do Porto.
products in consecutive-parallel reactions, which, in this case, is product C. The interest in using catalytic membrane reactors to study its overall yield and/or selectivity has been increased among the scientific community, according to the number and diversity of papers available on the open literature.1,4,7,18-21 Some of such studies focus on the improvements that can be achieved with a distributed feed of a reactant alongside a tubular reactor,4,19,20 while others analyze the impact of a strategy based in a segregated feed of the reactants on the overall yield to the intermediate (desired) product.7 All these works consider inorganic membranes, catalytic7,18 or inert.4,19-21 They report modeling,4,7 experimental,18,21 or both modeling and experimental works.19,20 Another important class of consecutive-parallel reactions that have been studied in polymeric catalytic membrane reactors are selective hydrogenations.12-14,22-24 For example, the selective hydrogenation of impurities such as propyne and propadiene in an industrial propene stream is an important reaction in the petrochemical industry.12,25 As a monomer for the industrial production of polypropylene, the purified propene stream should contain
