Figure 6. Hydrogen molar fraction contour plots in the combined parallel plate heat exchanger-reactor for hydrogen production by steam-methanol reforming.
The Sherwood number profiles in the exothermic process are presented in Figure 7 along the length of the combined parallel plate heat exchanger-reactor. The combined parallel plate heat exchanger-reactor system can be used to for a number of reactions as a wide range of process conditions are possible. It is inevitable that the catalyst coating will eventually deactivate to the point where economics drive for its replacement. It is sometimes possible to extend a catalyst life and reclaim some activity by techniques such as hydrogen treatment or methods to remove carbon buildup. These techniques can be readily applied to the combined parallel plate heat exchanger-reactor. The combined parallel plate heat exchanger-reactor also allows the body to be removed and monolith replacement performed. This is simply achieved by removing the relevant inlet manifold and outlet manifold and removing any monolith supports or containment structure. A new monolith can be inserted and reverse procedure applied. If the endothermic catalyst requires high temperature hydrogen activation, the heat can be supplied via the exothermic channels. The spent monolith can be recycled after recovery of any of the precious metal components of the catalyst. 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 methanol. 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. An advantage of the arrangement is the low thermal inertia of the system. This allows the reactor to operate with inherently fast thermal response and is particularly advantageous during startup. The low thermal inertia will minimize startup time to the order of minutes from the order of hours, which is typical for large packed tube technology. With suitable ancillary equipment, the system can be operated with a level of control and operating flexibility not encountered in traditional steam reformers. An advantage of the reactor system is the ability to use low calorific fuel for the exothermic reaction. Such fuel is not ideally suited to homogeneous combustion and results in a highly unstable flame. Heterogeneous combustion aids in spreading the heat generation along the length of the channel and helps prevent hotspot formation. The use of low caloric value gas allows the use of certain waste streams as the fuel to supply the heat. Examples of such streams include the off-gas stream from a fuel cell, and the stream remaining after hydrogen removal from a membrane gas shift reactor. The heat generation rate per unit area is approximately matched to the heat requirement in the adjacent channel. This can be achieved by controlling the catalyst thickness in each channel. A trial-and-error process may be required to obtain the optimum catalyst thicknesses for some processes. If the processes are not thermally matched, the overall efficiency of the reactor will be reduced. When using large channels, it is possible that the reaction will become diffusion limited, such that the rate of reaction is dictated by the rate at which unreacted molecules can diffuse from the center of the channel into the catalyst matrix. In this case, it is possible to add flow disturbance elements in the channel or emanating from the wall. These elements will produce a degree of convective mixing by forming local flow disturbances in an otherwise laminar environment. If a heat transfer fluid is used to remove the heat of reaction from a reaction occurring in any adjacent channel, then these flow disturbance elements would provide a useful and low pressure drop method of enhancing thermal performance.