Ms. Kulkarni et Mp. Dudukovic, PERIODIC OPERATION OF ASYMMETRIC BIDIRECTIONAL FIXED-BED REACTORS WITH TEMPERATURE LIMITATIONS, Industrial & engineering chemistry research, 37(3), 1998, pp. 770-781
Possible ways of coupling a solid-catalyzed endothermic reaction with
an exothermic reaction in a bidirectionally fed fixed-bed reactor, ope
rated in a periodic steady state, when the maximum allowable temperatu
re is limited by either process, catalyst, or materials constraints, a
re discussed. Steam reforming of natural gas coupled with methane comb
ustion is considered as an example. The catalyst bed is heated by the
combustion reaction during the exothermic semicycle, while the endothe
rmic reaction, with reactants fed from the opposite end, cools the bed
during the endothermic semicycles. It is shown that two modes of peri
odic operation are possible. In the wrong-way process, reactants are f
ed at temperatures below the initial bed temperature, which results in
maximum temperatures that can exceed the allowable limits. To suppres
s excessive temperature overshoots the fuel feed concentration must be
very low, which leads, due to the creeping temperature hot zone, to o
nly a small fraction of the heat produced during the exothermic semicy
cle being available for the endothermic reaction. Thermal efficiency a
nd the reactor productivity are low. In the normal process, the inlet
reactant temperature is above the ignition temperature, leading to a s
tationary spreadout temperature profile, high thermal efficiency, and
high reactor productivity, as well as to controllable maximum temperat
ure. Simulations for the wrong-way and normal processes are described
as well as the possibilities of achieving very high thermal efficienci
es in a process that integrates the reactor with heat recovery units.