Characteristic of reduction processes by syngas in catalyst bed

The above discussion was only for the reduction of the iron oxide catalyst in pure hydrogen. In industrial devices, usually the mixture gas of H2 + N2 is used as reducing reagent. Therefore, the actual reduction process is also accompanied by the formation of ammonia, and the process will become more complex. 

The characteristic of this process is that several processes are parallel or cross-carried out i.e., the reduction process for the each catalyst particle proceeds from surface to core step-by-step and the reduction process of whole catalyst in reactor (bed) proceeds from the top (outside) down (internal) step by step, and also reaction process of H2 with N2 forms ammonia on reduced catalysts. Therefore, temperature (t) of catalyst bed, the reduction degree (i ) of catalyst and water vapor concentration (y ) are changing. Trends of different types of reactors at different reduction stages are shown in Fig. 5.26. 

In Fig. 5.26 (b) state, the hotspot temperature locates at the inside in bed (adiabatic bed), and gradually moves towards the outlet. The water vapor concentration increases gradually and flows through most of the unreduced catalyst. This stage is the main stage of the process and it continues until the end of the process [Fig. 5.26(c)]. Therefore, whether it is the axial or radial adiabatic bed of catalyst, it does not need to worry about the bottom or the outlet part catalyst whether or not is reduced completely i.e., generally they can totally be reduced because of high temperatures. 

At this time, the temperature at outlet of the catalyst bed is higher than that of inlet so that the reduction of catalyst is forwarded from outlet to the inlet, or the reduction rate at outlet is higher than that of inlet. Therefore, the reduction of the catalyst at outlet would be completed earlier than that of inlet, namely, the pulsive direction of the reduction is opposite to the water vapor flow direction. As a result, the reduced catalyst at the outlet will be serious poisoned by the water vapor. So it is negative to reduce the catalyst during production in the production conditions. The reduction time cannot be shortened optionally and the production cannot prematurely begin before the catalysts is not reduced completely and every inch. It is not proper to consider that the catalyst could always be totally reduced during the long production run. 

For the large particle of catalysts used in industry, it is worth noting for the following characteristics. 

---