Simulated moving beds (SMB):
In practice it is very difficult and not efficient to move the solid phase. Therefore, the counter-current movement of the adsorbed phase is emulated by moving the in- and outlets simultaneously in direction of the gas flow.| Figure 1: Scheme for the separation of i-pentane (red) and 22DMB (blue) using a simulated moving bed in a TMB unit. The coloring represent the composition distribution and was obtained by simulation; see also Figure 1 in TMB site. |
where L is the length of a single fixed bed. Figure 1 shows a schematic drawing of the SMB configuration that corresponds to the TMB unit presented in the previous section. The unit has 8 chromatographic columns. Two columns between in- and outlets correspond to a section of a TMB unit. Comparing the results of a simulated moving bed with the ones for an equivalent true moving bed shows that the composition profiles are closely matched although the emulated movement of the adsorbed phase is discrete in time; for further detail see later discussion.
Notes on the implementation:
Analogously to the TMB model we adapted the dynamic model of a fixed bed chromatographic column in order to simulate a SMB unit. The in and outlet of the original fixed bed are connected and nodes are added in order to describe the feeds and withdrawals. The conventions for the nodes are the same as the ones described for a TMB unit. Each time a node has to be moved an event is triggered and the simulation is stopped. The nodes are relocated and the simulation is restarted. CSAD also allows moving each node individually. This allows definitions of more complex switching strategies.
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