Optimal heat-flux profiles in side-fired primary steam reformers

PIÑA, JULIANA; SCHBIB, SUSANA NOEMÍ; BUCALÁ, VERÓNICA; BORIO, DANIEL OSCAR

Conferencia; Chemical Reaction Engineering VIII-Chemical Reactor Engineering 2000: Novel Engineering for the New Millennuim; 2001

In non-adiabatic tubular reactors, the products distribution at the reactor outlet is strongly influenced by the evolution of the temperature along the reaction path. For the particular process of steam reforming of natural gas, the axial temperature profile depends on the heat-flux across the catalyst tubes. In side-fired furnaces, the burners are placed in horizontal rows on each side of the radiation chambers. The heat-flux profiles can be influenced by controlling the fuel-gas flowrates at each row of the burners. This action affects directly the maximum tube skin temperature, which is a key variable. Even a slight increase in the maximum tube wall temperature may result in a serious decline in the expected tube lifetime (Dybjkaer, 1995). The simulation and optimization of a large-scale side-fired primary reformer is presented, aiming to determine the heat-flux profiles which minimizes the maximum external tube skin temperature, for a fixed methane slip (constant reactor conversion). Based on the kinetic model proposed by Xu and Froment (1989), the catalyst tubes are simulated by means of a one-dimensional heterogeneous model. The tube wall temperatures are calculated from the heat fluxes, using the expressions reported by Froment and Bischoff (1990). The catalyst pellet, with a complex geometry, is represented through an equivalent annular model. At all the reactor positions, the strong influence of the internal mass-transfer resistance is taken into account by means of effectiveness factors for the three reactions considered. The calculated optimal heat-flux profiles are a useful tool to define and control the heat input along the length of the tube, thus ensuring an optimal temperature profile at all operating conditions.