Application of heat flux as a control variable in small-scale packed-bed steam reforming

In steam reformation, high thermal resistance and poor heat transfer of the packed catalyst bed can create time-lag between the moment when the heat is applied and the corresponding rise in temperature. Thus, problems arise from the dynamic requirements of the system, which can create a time-lag in the reactor's performance and also induce temperature oscillations resulting in a degrading catalyst. Lag compensation is necessary if one uses temperature feedback control to maintain the reactor temperature. A better solution is to recognize that heat flux is more suitable as a control variable, since available heat is what sustains the chemical reaction inside the reactor. Thus, controlling heat flux can directly influence the reaction and the resultant temperature inside the reactor. A heat flux controller is implemented for two small-scale, packed-bed, steam reformers. A standard temperature feedback controller is also implemented. The two systems are compared in their transient response. Temperature and reformate gas concentrations are measured to evaluate the performance of the two controller topologies. The heat flux based controller significantly outperforms the temperature feedback controller in both geometries tested.