Otimização da produção de hidrogênio pela reforma a vapor do metano em reator com membrana laboratorial

Abstract

Hydrogen is used as fuel and as a feedstock in important processes such as ammonia and methanol production and Fischer-Tropsch synthesis, besides the increasing use in fuel cells. Methane steam reforming is the main route for hydrogen production and its predominant reactions are globally endothermic and reversible, which forces the reactor to operate at high temperatures, in conventional technology, in order to get reasonable conversions. Reactors with membranes that allows selective hydrogen permeation have been proposed as an alternative to conventional reactors, since with them it is possible to achieve high conversions at lower temperatures. Nevertheless, the economic viability of membrane reactors to hydrogen production depends on their operating efficiency. Therefore, to make this technology usable, it is necessary to find the best operating conditions to membrane reactors. In this work, a laboratory-scale membrane reactor to hydrogen production from methane steam reforming, was modeled and optimized. The full proposed model, constituted by mass, energy and momentum equations was validated with experimental date from the literature. Two intrinsic kinetics for methane steam reforming were evaluated during the model validation. The full model was compared with another model constituted by mass balance only (isothermic model) and, in general, their discrepancy were negligible. Another model, refereed as simplified model, was obtained though the response surface technique using the full model. Five important parameters, namely: inlet reactor pressure (P0 r ), methane feed flow rate (FCH0 4 ), sweep gas flow rate (FI), external reactor temperature (Tw) and steam to methane feed flow ratio (m) was used as decision variables in hydrogen production optimization. Three optimization strategies were used: (1) parametric analysis, using the full model; high methane conversions and hydrogen recoveries (99.99% and 99.01%) were reached, however, disdaing the interaction among the variables; (2) constrained optimization, using the simplified model and NPSOL code; the objective function was defined as the summation of both XCH4 and YH2 ; the optimized codified values related with P0 r and FI reached the maximum value while m reached the minimum value; the resulting XCH4 and YH2 were 93.85% and 92.09% respectively; (3) constrained optimization, using the isothermic model and DIRCOL code; it was used the previous objective function, but, it was not possible to solve the optimization problem in all operating conditions desired. In this case, it was possible to reach a methane conversion around 96% and a hydrogen recovery of 91%.