A catalytically active membrane reactor for fast, exothermic, heterogeneously catalysed reactions

A membrane reactor with separated feed of reactants is demonstrated as a promising contractor type when dealing with heterogenously catalysed, very fast and exothermic gas phase reactions. Due to the separation of reactants a good control of the system is obtained, because process variables can be varied independently from each other. Transport of reactants is the rate governing process and because this is only slightly temperature dependent a thermal runaway will not occur. When dealing with e.g. combustion process no explosive mixtures will build up and safety is increased. Based on the dusty-gas model, the concentration profiles of components inside the membrane can be calculated together with the fluxes. However this is a calculation time consuming process and not necessary in all cases. In absence of a pressure drop and no slip of reactants to the opposite side a linearisation is possible leading to a simplified expression for the interfacial flux of a reactant and a criterion to evaluate the possibility of slip of reactants. Using the oxidation of carbon monoxide catalysed by platinum as a model reaction this approximation was experimentally verified by comparison of measured fluxes with the calculated results. Apart from flux measurements exploratory overall conversion measurements were carried out with the membrane reactor module in order to demonstrate its operation performance. From these studies it was concluded that conversion levels up to 90% carbon monoxide could easily be achieved

An experimental study of diffusion and convection of multicomponent gases through catalytic and non-catalytic membranes

Diffusion of binary and ternary gases through catalytic and non-catalytic membranes has been studied experimentally at atmospheric pressure. These experiments were conducted in a modified Wicke-Kallenbach diffusion cell consisting of two continuously stirred gas volumes separated by a membrane. The equipment was suitable to measure fluxes of components through the membrane in the absence of gas-to-membrane mass transfer limitations.\ud \ud Transport through a porous membrane has been measured and compared with the results of the dusty-gas model, which has been used to predict transport through a membrane. With independently determined input parameters this model turned out to be able to predict the transport of a multicomponent gas mixture through a membrane within a few percent (< 5%). The Fick model extended with a convective trnasport contribution was not able to produce similar results as obtained from the dusty-gas model, especially when an overall pressure gradient was present over the membrane.\ud \ud In order to demonstrate the occurrence of surface effects, dynamic transport of a binary gas was studied in a similar experimental setup as described by Novák et al. In this setup the transport of gas mixtures containing helium, argon and nitrogen was in good agreement with the model simulations. For transport of carbon dioxide and propane through a γ-Al2O3 coated membrane, adsorption phenomena were observed, but no substantial surface mobility was detected at temperatures ranging from 293 to 433 K

An experimental study of diffusion and convection of multicomponent gases through catalytic and non-catalytic membranes

Diffusion of binary and ternary gases through catalytic and non-catalytic membranes has been studied experimentally at atmospheric pressure. These experiments were conducted in a modified Wicke-Kallenbach diffusion cell consisting of two continuously stirred gas volumes separated by a membrane. The equipment was suitable to measure fluxes of components through the membrane in the absence of gas-to-membrane mass transfer limitations. Transport through a porous membrane has been measured and compared with the results of the dusty-gas model, which has been used to predict transport through a membrane. With independently determined input parameters this model turned out to be able to predict the transport of a multicomponent gas mixture through a membrane within a few percent (< 5%). The Fick model extended with a convective trnasport contribution was not able to produce similar results as obtained from the dusty-gas model, especially when an overall pressure gradient was present over the membrane. In order to demonstrate the occurrence of surface effects, dynamic transport of a binary gas was studied in a similar experimental setup as described by Novák et al. In this setup the transport of gas mixtures containing helium, argon and nitrogen was in good agreement with the model simulations. For transport of carbon dioxide and propane through a γ-Al2O3 coated membrane, adsorption phenomena were observed, but no substantial surface mobility was detected at temperatures ranging from 293 to 433 K

A catalytically membrane reactor for fast, highly exothermic, heterogeneous gas reactions : a pilot plant study

Membrane reactors have been frequently studied because of their ability to combine chemical activity and separation properties into one device. Due to their thermal stability and mechanical strength, ceramic membranes are preferred over polymeric ones, but small transmembrane fluxes obstruct a widespread industrial use of a membrane reactor. Consequently, a bench-scale membrane reactor with a tubular, macroporous membrane (d, = 700 nm) was developed in order to attain increased fluxes. A cooling pipe was concentrically placed inside the tubular membrane to remove heat from the membrane surface, so the present membrane reactor was suitable to conduct exothermic reactions. As a model reaction, the heterogeneous oxidation of carbon monoxide over platinum, with separated feed of carbon monoxide and oxygen, was performed in the present setup. First, the present membrane reactor was characterized by the determination of the transport parameters, structure parameters of the membrane, and the external transfer coefficients. Subsequently fluxes of the reactants and products were measured over a wide range of process conditions. Especially the influence of a transmembrane pressure difference was studied extensively. Furthermore overall conversion of carbon monoxide was measured under various process conditions, and the results were compared with the simulations of a simplified, overall reactor model. From the results of the present investigation, it could be concluded that the application of a pressure difference over the membrane turned out to be a major process control parameter. It increases the product yield and preferentially directs the fluxes toward one side of the membrane. It was shown that even for macroporous catalytic membranes substantial pressure differences are allowed without any slip of unconverted reactants through the membrane. Furthermore, high degrees of conversion were observed in the present setup, and the simulations of the overall reactor model were in reasonable agreement with the experimental data. The overall model contained no adjustable parameters. From this study, the catalytically active, ceramic membrane reactor with separated feed of reactants turned out to be highly flexible and easy to control

Intrinsic kinetics of the oxidation of methane over an industrial copper II oxide catalyst on a gamma-alumina support

The kinetic rate of the heterogeneously catalysed methane oxidation reaction was studied at temperatures ranging from 723 to 923 K and atmospheric pressure. A commercially available CuO catalyst supported by thermally stable γ-Al2O3 was used as the catalyst. This kinetic study was carried out in order to evaluate whether this reaction can be suitably handled in a novel type of membrane reactor (J.W. Veldsink et al., Chem. Eng. Sci., 47 (9–11) (1992) 2939–2944). The equipment in which the experiments were conducted consisted of a differentially operated fixed-bed reactor. For this set-up heat and mass transport limitations have been checked and shown to be absent. The influence of the concentrations of the products and reactants on the reaction rate was studied over a wide range of conditions. Fractions of methane ranged from 1 to 6 vol.%, oxygen between 600 ppm and 21 vol.%, carbon dioxide between 0 and 20 vol.%, and water from 0 to 8 vol.%. All experimentally observed reaction rates could be represented within 30% by an Eley-Rideal type reaction rate expression