Thermal regimes in cocurrently cooled fixed-bed reactors for parallel reactions: Application to practical design problems

The conditions of existence of the seven feasible thermal regimes for exothermic first-order parallel reactions occurring in cocurrently-cooled fixed-bed reactors are derived here. The diversity of thermal regimes offered by the cocurrent scheme was applied to solve practical optimization problems. Theoretical optimal temperature profiles inside the reactor tubes which maximize the outlet yield of the desired product were determined. These ideal thermal trajectories were very well tracked by attainable thermal profiles generated by simulation of a cocurrently-cooled fixed-bed reactor represented by a plug-flow pseudo-homogeneous model. In fact, the objective function values found at feasible operation conditions were very close to the optimal ones. These results indicate that the conventional designs such as constant coolant temperature or the countercurrent scheme (where only hot-spot or decreasing profiles can be generated) could not handle the optimization problems as efficiently as the cocurrent configuration. Moreover, the cocurrent design allows, among other advantages, to reach the specified level of conversion (or yield) with higher selectivity and lower maximum temperature values than the common constant coolant temperature configuration. These findings imply the possibility of a significant improvement in the economics of the plant. (C) 1997 Elsevier Science Ltd.