Eleven mixtures of the three isomers of xylene and ethylbenzene were ilromerized over a sika-alumina catalyst in a difkrential tubular reactor a t temperatures from 700" to 900°F. and pressures from 0 to 100 Ib./sq. in. gauge. Analysis showed that meta-xylene isomerizes reversibly to ortho-ond para-xylene, but the direct interconcersion of ortho-xylene to mqtaxylene does not occur. Further, it appears that the isomerization reactions are first order and, by the Hougen-Watson technique, follow a single-site reaction model.

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A mathematical study was carried out for a stirred reactor in which the feed and product streams were periodically interrupted. The cycled reactor proved superior to a continuous stirred-tank reactor for first-and second-order reactions, and surpassed a plug-flow reactor for those cases in which the reaction rate passes through a maximum, such as autocatulytic and exothermic adiabatic reactions. Temperatureprogrammed cycled reactors were also studied, but appeared to have l i t t l e advantage over conventional reactors.Cyclic processes have stirred up great interest among chemical engineers because of their ability to improve the performance of various conventional unit operations such as distillation, absorption, and extraction, However, cyclic operation of reactors has led to some confusion with some reports indicating process improvement while others indicate the opposite.The confusion regarding cyclic reactors s t e m s entirely from a problem of nomenclature. The terms pulsed, cycled, and periodic have been applied to such systems, sometimes having similar and sometimes quite different meaning. Those cyclic or pulsed operations in which sinusoidal or square-wave disturbances are superimposed on the steady feed rate to a continuous stirred-tank reactor (CSTR) ( 5 ) are to be differentiated from those treated here, in which the constant feed rate is periodically interrupted, resulting in short batch reaction steps (6). The two systems present e'ntirkly hiffdrent exberihent'al dnd hathematical problems and cannot be compared in any simple way. A third type of system, a backmix reactor with steady feed and naturally occurring internal oscillations (chemical oscillators), should also be clearly differentiated.The present study i s an elaboration of that by Fang and Engel ( 6 ) in which the feed and product streams of a stirred reactor were periodically turned on and off, resulting in cyclic behavior (controlled cycling). GENERALOne of the chief advantages of a backmix reactor over batch and plug-flow reactors i s its ability to avoid sensitive reactant and product concentrations lying between the initial and final concentrations. The controlledcycled stirred-tank reactor (CCTR) has this same capability because it operates in a series of short batch reaction steps under such desired operating conditions that the initial and final concentrations during a batch step will meet the required concentration criteria. At the end of each batch reaction, part of the reactor contents i s dumped and fresh feed i s added to reestablish the normal reactor volume. After several such operating cycles, the CCTR will achieve pseudo steady state conditions where each reaction cycle is a duplicate of the previous one.In general, a distributed feed plug-flow reactor can also avoid certain critical concentration ranges, and there i s a Superficial similarity between distributed feed reactors and the CCTR. However, it should be clear that although adjacent segments of a distributed feed reactor may have identical concentration patterns...

Steady state multiplicity in exothermic adiabatic reaction systems oreviouslv been demonstrated bv a number of authors. SDecificallv. has the hork of Root and Schmitz provid the existence of multipll steady'states in a loop reactor, but difficulty was encountered in controlling such a reaction system at the intermediate steady states.Since a close analogy can be demonstrated between loop reactors and controlled cycled reactors (CCTR), it appeared reasonable to attempt to achieve steady state multiplicity in a CCTR. When the chemical reaction of Root and Schmitz was carried out in a CCTR, it proved relatively easy to obtain experimental values of intermediate steady states and to control the reactor at these conditions by means of a simple on-off temperature controller.This work suggests that chemical systems displaying steady state multiplicity can be experimentally investigated in a CCTR and that cyclic operation may be the best way for controlling such reactions systems.

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