Figure 7. Nusselt number profiles along the length of the heat integrated reactor under nominal operating conditions for thermochemically producing hydrogen from methanol by steam reforming.
The Sherwood number profiles along the length of the heat integrated reactor are presented in Figure 8 under nominal operating conditions for thermochemically producing hydrogen from methanol by steam reforming. 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. Another deficiency of traditional heat transfer equipment is start-up time and thermal response to transients. As reactors are traditionally large and heavy, they have significant thermal inertia. Therefore, the system takes significant time to re-equilibrate from any change in load or process operating conditions. Therefore, a reactor with enhanced response characteristics particularly for rapid start up is desired. The design consists of multiple packed tubes, of small diameter, being placed in intimate contact with a heat generating flame. The arrangement leads to improved heat transfer and therefore chemical conversion. However, the packed tube results in a significant pressure drop and the author states the process is still heat transfer limited. Therefore, a reactor design which minimizes the process side pressure drop and does not suffer from heat transfer limitation is required. It is proposed that the arrangement can either be used as a heat exchanger, where energy is transferred from one stream to another via conduction through the wall or it is suitable as a chemical reactor where the second set of channels allow the introduction of a heat transfer fluid [41, 42]. It is noted that the reaction can be a catalytic process and the catalytically active material can be coated onto the monolith passage walls to minimize pressure drop [43, 44]. In this arrangement the heat transfer from the process catalyst to the dividing wall will be highly efficient, however, the uptake of the energy by the heat transfer fluid will suffer from all of the limitations of traditional heat transfer operations [45, 46]. In this case the boundary layer will provide a significant resistance to heat transfer and will severely limit the rate of the process [47, 48]. The high velocities will reduce the characteristic thickness of the boundary layer and ensure that a sufficient mass of heat transfer fluid is available to absorb the heat of reaction without significantly changing temperature. These requirements will lead to excessive pressure drop through the coolant channels. Therefore, a reactor design which minimizes the heat transfer fluid side pressure drop is required. Combining endothermic and exothermic reactions on opposing sides of dividing walls of adjacent channels can serve as an efficient method of heat transfer. It is proposed that steam reforming of a hydrocarbon be performed by one layer and the energy for this process be supplied by a hydrocarbon oxidative process being promoted in the subsequent layer. Various hybrids of this theme are proposed. However, as the heat is supplied by an autothermal reaction, oxygen must be supplied along with the fuel stream. As well as the oxygen, associated nitrogen is present. This nitrogen acts to absorb process energy which lowers the thermal efficiency of the process as well as diluting the desired product, hydrogen. The presence of the nitrogen increases the load on downstream partial oxidation units which act to oxidize carbon monoxide to carbon dioxide. The nitrogen also reduces the streams suitability for use in fuel cells. Therefore, a reactor which can supply sufficient energy to an endothermic reaction without mixing the streams is needed. It should be noted that an advantage 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, the gaseous components from a Fischer-Tropsch synthesis. It should be further noted that 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. Suitable metals include copper, aluminum, stainless steel, iron, titanium, and mixtures or alloys thereof.