Figure 4. Effect of wall thermal conductivity on the average Nusselt number in the endothermic process of the combined parallel plate heat exchanger-reactor.
The temperature contour plots in the combined parallel plate heat exchanger-reactor are illustrated in Figure 5 for hydrogen production by steam-methanol reforming. The process engineer is often caused to compromise between the pressure drop within the tube reactor with the overall heat transfer and catalytic effectiveness. The inner heat transfer coefficient can be effectively increased by raising the superficial velocity of the process gas. The higher gas velocity therefore improves the thermal effectiveness of the system. However, higher gas velocities increase the system's pressure drop and results in increased compressor sizes and associated operating costs. A reactor must be of sufficient length to allow a reaction to proceed to the required conversion. Utilizing high gas velocities typically results in reactors with large length to width ratios which again results in systems with high pressure drops. The smaller the characteristic dimension of the catalyst particle the higher is the utilization of the catalyst. This is sometimes expressed as a higher effectiveness factor. However, beds formed from small particles exhibit higher pressure drops than similar beds formed from larger particle. Consequently, an engineer designs a system with expectable compromises between heat transfer, catalyst utilization, system conversion, and pressure drop. Therefore, a reactor for conducting catalytic processes which can promote overall heat transfer and levels of conversion whilst minimizing pressure drop is desired. The interior walls of the channels of either or both groups of channels may be coated with a catalytic material converting that group of channels into a catalytic reactor. Consequently, this reactor group is in intimate contact with the separately manifolded channels of the other group because of the common wall design and as a result, highly exothermic or endothermic reactions may be carried out under much greater temperature control than was possible in fixed bed or fluidized bed or countercurrent flow reactors. This greater temperature control can result in much greater product selectivity and yield for a wide variety of chemical reactions. Now heat can be removed or added to the reactor as required the temperature being regulated merely by careful control of the flow of reactants through the reactor channels and control of coolant or heat source flow through the coolant channels. The temperature control possible with this system is extremely efficient because all the catalytic material in the reactor channels, where heat is either absorbed or liberated due to chemical reactions taking place, is on the surface of the walls that can transmit heat through the walls directly to the thermal control channels. Most of the heat liberated or absorbed in the chemical reaction is conducted through the walls which support the catalyst and thereby eliminating the necessity for heat conduction through the gas phase which is a much less efficient way to cool a catalytic particle. In addition, this configuration allows not only most of the heat to be conducted through solid walls but also allows all the walls on which the catalysts reside to have all common walls with the coolant channels. Consequently, one has an extremely uniform and precise method of controlling the catalyst temperature because no two catalytic surfaces share a common wall, Also, it is possible to fabricate a structure with very thin walls and this too contributes to greater efficiency and ease of temperature control. In traditional systems, the catalyst is on particles randomly packed into a reactor tube. Exothermic reactions occurring generate heat at the catalytic particles even in the center of the tube. This heat must migrate through the gas phase and through the particles in the reaction tube to the wall of the tube and only then pass through the wall to be dissipated in a coolant. This passage of heat through the reactor tube gas and particle phase before finally contacting the wall is responsible for the difficulty encountered in controlling the temperature in the reactors traditionally used.