System approach modelling applied to the Eluxyl process

The Eluxyl process achieves paraxylene separation to other xylenes and ethylbenzene. Eluxyl is a simulated moving bed process that requires a fine tuning to produce paraxylene at a very high level of purity, up to 99.90%. A numerical model has been developed in parallel with the process. Objectives are to help tuning, to help design of additional equipment such as valves and mixing chambers and to test the so-called "what if cases". According to these objectives, the model is planned to be modified many times in order to test different equipment, to evaluate "what if cases" or even to choose among different configurations. To match these requirements the numerical model has been del eloped under system approach. This paper presents the Eluxyl process and its mathematical model. Then, the ability of such a model to represent real units is illustrated through four examples. At first, the paper describes how slow opening and closing valves were modelled. The objective was to evaluate the requirement valves should meet about their time spent before total opening. Secondly, to emphasise the advantage of system approach modelling, the paper presents how a missing (under maintaining) valve could be modelled. In a third example, it is shown how a system approach model can help the design of real units. This paper explains how the different designs for the mixing chambers that equip the Eluxyl process have been modelled and tested. The key point is that the best design is selected on the simulated resulting performances of the unit-purity and yield. Finally a "what if case" is presented. Here, a leaking valve is assumed and the net effect on purity and yield shows that requirement about valve leaking should be severe. All along this paper, we show how system approach modelling is powerful, fast, convenient and almost bug free compared with standard language programming.