A Catalytically Active Membrane Reactor for Fast, Highly Exothermic, Heterogeneous Gas Reactions. A Pilot Plant Study

Abstract

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.

04/01861 Feasibility of the direct generation of hydrogen for fuel-cell-powered vehicles by on-board steam reforming of naphtha

“…Compactness is also enhanced because conversion levels in such membrane reactors are increased. A significant amount of research work on membrane reactors is underway [4,5] and there are also signs of successful technological developments [6].…”

“…The gas mixture is then cooled down by the incoming process water and directed to the low temperature (LT) shift reactor, where CO is converted catalytically to CO 2 . The remaining traces of CO is further converted to CH 4 in the methanator and the gas mixture at this point contains about 53% H 2 , 17% CO 2 , 28% H 2 O, and some traces of CH 4 . Both the shift and methanation reactions are exothermic.…”

“…Catalysts stabilized by cement may show shrinkage and decrease in strength by frequent exposure to high temperature. Silica is avoided as a support in this case because of its volatility (as Si(OH) 4 ). Catalysts based on magnesia, although resistant to high temperature steaming, are sensitive to steaming at LT because of the risk of hydration of MgO to produce Mg(OH) 2 , which has a higher molar volume, thus leading to catalyst break down.…”

“…In this case part of the product gas is cooled and recycled to control the temperature rise. On the other hand, care must be taken during heating up and cooling down to avoid the formation of nickel carbonyl (Ni(CO) 4 ), which is extremely toxic, almost odorless gas, favored below 150 8C. The operating temperature range in the methanator is 230 -350 8C and over this temperature range the equilibrium constant of CO methanation varies from 0.13 £ 10 11 to 0.48 £ 10 6 [9].…”

Modeling and optimization of the combined carbon dioxide reforming and partial oxidation of natural gas

“…However, detailed mechanistic models are used much more often than empirical models for interpretation of catalytic processes. In this case, the mechanistic models are used as a basis for investigation of reaction mechanisms, for the estimation of adsorption and activation energies [7,9,10,20,21] and for evaluation of the mass and heat transfer rate constants in the reactor [22][23][24]. Process optimization can be accomplished after determination of the kinetic, mass transfer and heat transfer mechanisms and estimation of the model parameters.…”