(761b) Modeling and Simulation of an Industrial Scale SMB Unit for the Separation of p-Xylene

The Simulated Moving Bed (SMB) process, developed by Universal Oil Products (UOP) in the 1960s, has been widely used for the separation of p-xylene with high purity (99.7 %) in industrial scale (Parex). Prior to the application of optimization techniques to an industrial scale unit, an accurate model has to be developed.

A detailed analysis of the construction of an industrial scale plant shows the existence of three types of dead volumes which interfere in the performance of the unit, and therefore should not be neglected. The Parex process has 24 adsorption columns divided into two adsorbent chambers, which are connected by two circulation lines. The internal piping and the support grids create dead volumes between adsorption columns. And finally, each bed is connected to the rotary valve by means of a bed line.

A mathematical model of the Parex unit accounting for the three types of dead volumes was developed. The mathematical model has proven to be capable of satisfactorily predicting the performance of the Parex unit in terms of purity and recovery.

The simulations were used to analyze the cyclic steady state of the simulated moving bed unit with 7 zones, and it was made a comparison with the 4 zones operation mode. For a chosen case, the effect of the dead volumes in the internal concentration profiles of the unit and in the extract history was studied. The analysis of the performance of the unit shows that the size of the each type of dead volume influences the performance of the Parex unit and consequently they have to be taken into account in the design and optimization of an industrial scale plant.