Control process for simulated moving bed para-xylene separation

A process of continuously controlling the value of a characteristic in the separation of para-xylene from a stream whose composition contains at least the components consisting of para-xylene, ortho-xylene, meta-xylene, and ethylbenzene, and recovering a para-xylene product stream, said characteristic selected from the group consisting of the purity of the para-xylene product stream, the recovery of the para-xylene in the product stream, and the combination thereof;

• the separation using a simulated moving bed, having multiple input and output streams each with individual flow rates, a step time, multiple valve positions, and the components in a pumparound or pusharound stream, where the initial value of and characteristic is V.sub.1, and the final value of the characteristic is to be within .+-..delta. of a target value V.sub.T, said process comprising:
• a. determining by on-line or in-line sampling the current concentrations of at least para-xylene, ortho-xylene, meta-xylene, and ethylbenzene, in said pumparound or pusharound stream at a minimum of three valve positions within one valve cycle;
• b. determining, from the aggregate of the current concentrations within one valve cycle, a current value of the characteristic;
• c. adjusting the step time and flow rates according to an algorithm, said algorithm relating a change in the current value of the characteristic to changes in the step time and the flow rates to afford a new concentration of at least one of the components and an associated new value of the characteristic, V.sub.N, where .vertline.V.sub.T -V.sub.N .vertline..ltoreq.V.sub.T -V.sub.I .vertline.;
• and
• d. repeating steps (a)through (c) until .vertline.V.sub.T -V.sub.N .vertline..ltoreq..delta..