Investigation of a Thermally Regenerative Reactor System

A novel cyclic reactor system is proposed for heterogeneous,
catalytic, gas-phase reactions. This system utilises the inherent
characteristics of the thermal regenerator to impose favourable
reaction temperature profiles along the catalyst bed without setting
up radial temperature gradients. This control of the longitudinal
profile enables higher conversions to be obtained than those from
steady state reactors. The reactor system is investigated by computer
simulation using the endothermic, reversible dehydrogenation of ethylbenzene
to styrene in the presence of steam as an example. The higher
conversions obtained from the proposed system produce utility cost
savings in this process.
Kinetics presented in the literature for this reaction are compared
and assessed. None of these is entirely satisfactory and a more representative
set is derived. Models for the reacting and regenerating
bed are discussed and suitable models are presented. A comparative
study of solution methods for these models is carried out in order to
determine one which gives an accurate solution and also minimises
computing requirements.
The most suitable operating policy lor the system, with an endothermic
reaction, is the use of constant heat inputs with constant flow
during each period of operation, This allows the bed inlet temperature
to vary with time, but it seems likely that the damping effect of the
system will be large and the inlet temperatures may be assumed constant
Counter-current, rather than co-current, operation of the system is
preferred,
A simple design procedure, which does not require the solution of
the cyclic model, is described. This is found to give good predictions
of the cyclic steady state performance of the system.
Che effect of the various system parameters on the performance
is investigated. The major parameters for a given bed size are the
period time, reactor and regenerator steam flows and regenerator
inlet temperature. It is shown that the system can give higher
conversions than a steady state reactor but i.t may be desirable to
operate at lower conversions to reduce the operating cost. Guidelines
for optimising the system are discussed.