Equivalent true moving bed

In this approach, an equivalent countercurrent movement of solid is assumed instead of the SMB process. This equivalent true moving bed (TMB) neglects the d3mamics associated with periodic switching and produces mean concentration profiles over a switching period. 

The principle of SMB operation can be easily understood by analogy with the equivalent True Moving Bed (TMB) process. The TMB unit (Fig. 3.4-8) is divided into four sections section 1, between the eluent and extract ports section 2, between the extract and feed ports section 3, between the feed and raffinate points and section 4, between the raffinate and the eluent inlet. In the ideal TMB operation, liquid and solid flow in opposite directions, and are continuously recycled the liquid flowing out of section 4 is recycled to section 1, while the solid coming out of section 1 is recycled to section 4. The feed is continuously injected between sections 2 and 3, and two product lines can be continuously collected the extract, rich in the more-retained species A, and the raffinate, rich in the less-retained species B, which move upwards with the liquid phase. Pure eluent or desorbent is injected at the beginning of section 1, with the liquid recycled from the end of section 4. 

It can be shown that the performances of simulated moving beds are equivalent to those of true moving beds [11]. Also, SMB process has a lot of advantages compared to classical elution chromatography. With a continuous process, purified products are recovered at 100% with a low dilution and a low eluent consumption. Moreover, SMB process are not very sensitive to column efficiency and we can obtain very good product purities with even low column efficiencies [11],... 

A first model is used to compute the flowrates allowing to perform the separation with the greatest productivity. Then, the "mixed cell in series" model takes into account thermodynamic, hydrodynamic and kinetic properties of the system and compute the concentration profile inside the columns [14], In this model, we make the assumptions that the pressure drop inside the column is negligible compared to the pressure drop realized and controlled with the analogical valves, and we model the true moving bed assuming that the performance of SMB and TMB are equivalent. A mass balance equation is written for each stage and a classical Newton Raphson numerical method is used to solve the permanent state of the process [14],... 

The True Moving Bed (TMB) approach, assumes that the SMB process is equivalent to the true counter-current moving bed (TMB). This TMB model neglects the dynamics associated with the periodic switching of the columns and gives the mean concentration profiles over a switching period. 

The principle of operation can be best described with reference to the equivalent true countercurrently moving bed (TCC) configuration. Since the two configurations are equivalent, i.e they achieve the same separation performance provided the geometric and kinematic conversion rules are fulfilted, the simpler model of the equivalent TCC unit can be used to predict the steady state separation performances of SMB units, in particular, for design purposes. 

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