Figure 6. Contour plots of water mole fraction, oxygen mole fraction, and carbon dioxide mole fraction under nominal operating conditions for the heat integrated reactor.
The Nusselt number profiles along the length of the heat integrated reactor are presented in Figure 7 under nominal operating conditions for thermochemically producing hydrogen from methanol by steam reforming. Many chemical processes utilize catalysts to enhance chemical conversion behavior. A catalyst promotes the rate of chemical conversion but does not affect the energy transformations which occur during the reaction. Often catalytic processes are conducted within tubes which are packed with a suitable catalytic substance. The process gas flows within the tube and contacts the catalytic packing where reaction proceeds. The tube is placed within a hot environment such as a furnace such that the energy for the process can be supplied through the tube wall via conduction. The mechanism for heat transfer with this arrangement is rather tortuous as heat must first be transferred through the outer boundary layer of the tube, conducted through the often-heavy gauge wall of the tube and then pass through the inner boundary layer into the process gas. The process gas is raised in temperature and this energy can be utilized by the process for chemical reaction. 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 [33, 34]. Utilizing high gas velocities typically results in reactors with large length to width ratios which again results in systems with high pressure drops [35, 36]. The smaller the characteristic dimension of the catalyst particle the higher is the utilization of the catalyst [37, 38]. This is sometimes expressed as a higher effectiveness factor [39, 40]. However, beds formed from small particles exhibit higher pressure drops than similar beds formed from larger particle. So, 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 dividing walls must be of sufficient strength to maintain the integrity of channels. The minimum wall thickness therefore depends upon material of construction. The wall thickness is in the range of about 0.5 millimeter to 5 millimeters and more particularly in the range of about 0.5 millimeter to 2 millimeters. The wall will act as a thermal barrier to heat transfer, however, as the wall is very thin its resistance is small. The heat for the reaction is supplied directly through the wall from the oxidation channels occurring on the opposing side of the dividing wall. As the heat transfer characteristics are highly independent of the bulk reactants velocity, a velocity can be chosen to ensure that the reactants exiting the reactor has attained the desired level of conversion or indeed reached any equilibrium. It is interesting to note that in such an arrangement it is desirable to operate the reactants in a co-current flow arrangement. This ensures that the area with the greatest heat generation is adjacent to the area with the greatest heat requirement. However, cases may exist where a countercurrent flow arrangement is desirable. The system can be used to for a number of reactions as a wide range of process conditions are possible. A number of techniques are available in which to deposit an active catalyst onto the wall of the monolith. One such technique is that of the washcoat as is used in catalytic converters. Others include the sol-gel technique, metal sputtering, or the grinding of commercial catalyst pellets followed by attachment through the use of a cement or sol-gel. Many of the coating techniques allow different thicknesses of coating to be applied. It may also be possible to increase or decrease the thickness of the coating along the channel length. This technique can be used enhance the kinetics in the downstream sections of the channel. The thickness of the catalyst coating depends upon the process proceeding within the catalyst matrix. The products of some processes are highly dependent upon the catalyst thickness. In this case, the thickness should be no larger than the characteristic length beyond which the product spectrum degrades. For some processes the catalyst thickness has no effect on the product spectrum, an example of which is the steam reforming of methane. In this case the catalyst thicknesses can be of any dimension. However, excessively thick coatings are avoided as the catalyst interior performs little reaction due to diffusion limitations and acts as a thermal barrier. Many catalysts are prone to deactivation due to diffusion of an impurity into the catalyst. In cases where the catalyst is supported on a metallic surface, the source of the impurity is often the metal surface itself. Metals have low diffusion coefficients, however, as the catalyst is in intimate contact with the support over extended periods and at elevated temperature, small amounts of the metallic substrate will diffuse into the catalyst structure. A common example of this effect is the poisoning of nickel-based steam reforming catalysts with iron. It is possible to minimize this effect by using a dense and nonporous barrier coating located between the metal surfaces and the active catalyst. However, this problem can be circumnavigated through the use of ceramic structures.