Intercooling, recycle

Kokossis and Floudas (1994) extended the MINLP approach so as to handle nonisothermal operation. The nonisothermal superstructure includes alternatives of temperature control for the reactors as well as options for directly or indirectly intercooled or interheated reactors. This approach can be applied to any homogeneous exothermic or endothermic reaction and the solution of the resulting MINLP model provides information about the optimal temperature profile, the type of temperature control, the feeding, recycling, and by-passing strategy, and the optimal type and size of the reactor units. 

In this examination, the cathode recycle case will have no recuperator and the recuperator case will have no cathode recycle. However, it is quite possible that a hybrid system could use both. The anode exhaust is combusted at the turbine inlet in all cases. This could be done in other locations, such as the recycle loop, which would reduce the recycle required. It is apparent that a number of other configurations could be imagined (such as intercooling) however, each will likely be a modification of one of the base configurations given here. 

Use of multiple beds/reactors in series with intercooling or quenching is a method which can be used to reduce the amount of recycle and its associated costs. Multiple catalyst beds reduce costs by using the recycle material several times before it is separated from the reaction products. 

Fig. 1-3 Typical reactors (a) tubular-flow recycle reactor, (b) multi tube-flow reactor, (c) radial-flow catalytic reactor, (d) stirred-tank reactor with internal cooling, (e) loop reactor, (f) reactor with intercoolers (opposite)...

Accurate temperature control is essential to avoid undesirable reactions which occur at elevated temperature. The exothermic hydrogenation reaction increases reactor temperature. At higher temperatures, thermodynamic equilibrium favors dehydrogenation of cyclohexane to benzene, as well as isomerization to methylcyclopentane. Both of these reactions reduce overall yield. Several measuies are typically used to limit temperature excursions. They are the use of multistage reactors with intermediate cooling, recycle of cyclohexane, stepwise introduction of benzene feedstock, and reactor intercooling. 

With higher MAPD content, np to the maximum of about 6-7%, three beds are used with a combination of intercooling and cold gas recycle to control the temperature rise. 

---