Intrinsic kinetics and design simulation in a complex reaction network; steam-methane reforming

Abstract

A programme was developed by which the intrinsic rate equations (YY) can be determined separating mass and heat transfer resistance from the integral data in the complex reaction network. The steam-methane reforming process was studied using this programme in an integral flow reactor (60 ID × 1,800) over a nickel catalyst at temperatures varying from 674 K to 1160 K, pressure raning from 1.2 to 25.5 bar and S/C in the feed between 1.44 and 4.50. The reactor was considered to be a fixed-bed model with axial pseudo-one dimensional temperature profile. The intrinsic rate equations for steam-methane reforming (RF) and shift (SF) reaction were determined by the calculation of mass and heat balances considering interphase and intraphase diffusions using a numerical solution of the non-linear two-point boundary value problem. Assuming the competitive production model of CO and CO 2, in the hybrid expression of the Langmuir-Hinshelwood expression and the power law expression, the following values were derived for the activation energies (E) and the effectiveness factors (η); E R = 106.87 kJ/g-mol, E S = 54.531 kJ/g-mol, η R, η S = 0.05 - 0.065. The apparent rate equations (NN) estimated without separating mass and heat transfer influences were also obtained. The relation of RF and SF rate constants in each case was as follows: in YY case, 700 K < t < 900 K: k R < k S, 900 K < t < 1185 K: k S < k R, while in NN case, 700 K < t < 1185 K: k R < k S. In the design simulation the excess ratio of the catalyst volume required with NN to that with YY increases with lower pressure, lower SVo, higher S/C and higher inlet temperature under the same heat flux profile. For example at P = 2.0 bar, SVo = 5,000 h −1, S/C = 4.5 and T in = 823 K, the excess ratio was 11.9%. This deviation could be mainly ascribed to the difference of activation energy between YY and NN.