Periodic counter-current process

Counter-current processes involving, for example, the use of spinning disc or pulse column techniques do not require the same degree of premixing and achieve separation in a shorter period of time. However, as a continuous process it becomes essential to correctly size the plant required in order to maintain an adequate stock. Unlike the batch process, counter-current techniques are not always easily able to accommodate a sudden escalation in product order requirements. 

Description The process simulates a moving bed of adsorbent with continuous counter-current flow of liquid feed over a solid bed of adsorbent. Feed and products enter and leave the adsorbent bed continuously, at nearly constant compositions. A rotary valve is used to periodically switch the positions of the feed-entry and product-withdrawal points as the composition profile moves down the adsorbent bed. 

Figure 5.18 illustrates the principle of SMB processes. The mobile phase passes the fixed bed columns in one direction. Counter-current flow of both phases is achieved by switching the columns periodically upstream in the opposite direction of the liquid flow. Of course, in a real plant the columns are not shifted but all ports are moved in the direction of the liquid flow by means of valves. The counter-current character of the process becomes more obvious when the relative movement of the packed beds to the inlet and outlet streams during several switching intervals is observed. After a number of switching or shifting intervals equal to the number of columns in the system, one so-called cycle is completed and the initial positions for all external streams are re-established. 

The simulated moving bed reactor (SMBR) based on the simulated moving bed (SMB) process is a practical alternative for implementing counter-current continuous reactors. Counter-current movement of the phases is simulated by sequentially switching the inlet and outlet ports located between the columns in direction of the liquid flow (Fig. 8.4). As with the SMB process, two different concepts are known to realize the counter-current flow. One is based on switching the ports and the other on the movement of columns. However, both require elaborate process control concepts to realize the movement. Owing to the periodical changes of the set-up the pro-... 

Two different modeling approaches are used for simulated moving bed reactors. The first approach combines the model of several batch columns with the mass balances for the external inlet and outlet streams. By periodically changing the boundary conditions the transient behavior of the process is taken into account. The model is based on the SMB model introduced in Chapter 6 and is, therefore, referred to as the SMBR model. The second approach assumes a true counter-current flow of the solid and the liquid phase like the TMBR. Therefore, this approach is called the TMBR model. 

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. 

Figure 5.15 illustrates the principle of SMB processes. The mobile phase passes the fixed bed columns in one direction. Counter-current flow of both phases is achieved by switching the columns periodically upstream in the direction opposite to the liquid flow. Alternatively, not the columns are shifted but all ports are moved... 

Periodic countercurrent systems are widely used in ion exchange and water purification systems for the removal of trace concentrations of organic components with beds of activated carbon.At low sorbate concentrations the adsorption isotherms for many organic pollutants are linear or only slightly favorable so that the mass transfer zone is wide, and in a simple two-bed cyclic batch system the LUB would therefore be uneconomically large. For such systems, in order to obtain an economic process, some form of counter-current operation is necessary in order to reduce the adsorbent inventory. 

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