Alternative designs for a catalytic converter operating under autothermal conditions

Abstract

This paper focuses on the study of two different autothermal designs of a catalytic combustor, based on natural gas oxidation for heat generation. The feed is preheated by means of two different alternatives: (a) indirect heat exchange between the products and feed streams (Design I), (b) direct heat exchange by recycling a fraction of the hot gases leaving the catalytic combustor (Design II). Steady-state and dynamic results have demonstrated that autothermal catalytic units, after an appropriate start-up procedure, can operate with high methane conversion, without exceeding the maximum allowable temperature and minimizing harmful emissions such as NO(x) (to preheat the reactants, the flame of conventional units is replaced by a heat feedback of a fraction of the total heat generated by the catalytic combustor). Due to the dilution originated by the recycle stream (which admits the use of higher methane molar fractions in the fresh feed stream) Design II appears as the most convenient alternative because it can provide significantly higher heat fluxes than Design I for the same process constraints. Moreover, Design II can provide an exit stream at the highest thermal level (for a given maximum allowable temperature) feasible for an autothermal converter.