Continuous countercurrent processe

The need for a continuous countercurrent process arises because the selectivity of available adsorbents in a number of commercially important separations is not high. In the -xylene system, for instance, if the Hquid around the adsorbent particles contains 1% -xylene, the Hquid in the pores contains about 2% xylene at equiHbrium. Therefore, one stage of contacting cannot provide a good separation, and multistage contacting must be provided in the same way that multiple trays are required in fractionating materials with relatively low volatiHties. 

Since the 1960s the commercial development of continuous countercurrent processes has been almost entirely accompHshed by using a flow scheme that simulates the continuous countercurrent flow of adsorbent and process Hquid without the actual movement of the adsorbent. The idea of a simulated moving bed (SMB) can be traced back to the Shanks system for leaching soda ash (58). 

In the mid-1960s liquid—liquid extraction processes were introduced and today all large-scale commercial production is done in this way. An aqueous solution of the Ln3+ ions is extracted in a continuous countercurrent process into a nonpolar organic liquid containing tri-n-butylphosphine oxide or bis(2-ethylhexyl)phosphinic acid (DEHPA). Typical separation factors for adjacent rare earths using DEHPA are 2.5 per extraction step so that under automatic multistep or countercurrent conditions purities of 99 to 99.9% are routinely achieved. 

Both solvent sublation and bubble fractionation are viable as continuous countercurrent processes for the removal of hydrophobic compounds from water. Both processes are primarily dependent on the size of air bubbles introduced into the column as well as the extent of axial dispersion in the aqueous phase. The fractional removal in solvent sublation is less dependent on the column diameter. 

Figure 9.3 Principle of a continuous countercurrent process (true moving-bed process) with the different functions of the four zones.

After the switching time all inlets and outlets are shifted into the direction of the fluid flow. In this way the solid bed seems to move in a countercurrent way to the fluid in discrete steps. If all inlets and outlets are located at their primary positions, and so Nc switches were done, one cycle is finished. As in a continuous countercurrent process the inlet and outlets divide the process into four zones, which perform the functions as described above. 

Many model approaches assume a continuous countercurrent process neglecting the mixing of streams at the switching point. In contrast to these are the models that take into account the sectioning in discrete colunms and thereby the switching procedure. 

Steady-state operation of a continuous countercurrent adsorption system may be conveniently discussed in terms of the simple McCabe-Thiele analysis. The simplest type of continuous countercurrent process is illustrated in Figure 12.2, and it is evident that any of the separation processes discussed in Chapter 11 could, in principle, be carried out in such a system, rather than in a cyclic batch process. 

Chromatographic and continuous countercurrent processes are competitive in that both these types of process are applicable to difficult separations where the separation factor is small. In order to make clear the similarities and differences between these proce,sses we introduce, following Valentin, the idea of the number of equilibrations between fluid and solid phases. In a chromatographic column it follows from the definition of a theoretical plate that each sorbate molecule is equilibrated between fluid and solid on average just once for each plate. The number of equilibrations is therefore simply the number of theoretical plates to which the chromatographic column is equivalent. In a countercurrent system the situation is more complex because of the internal reflux. For simplicity we consider a system with linear equilibrium. At... 

A gas or vapor in a gas can be adsorbed on a solid which is slurried in a liquid. For example, sulfur dioxide can be adsorbed from a mixture with air on activated carbon slurried in water [8]. This procedure was suggested as early as 1910 [25], but interest in it has revived only recently. The reported details thus far are confined to laboratory studies of sparged vessels [73, 88] operated semibatch (gas flow continuous, sluny batch) or cocurrent flow or both [66]. A continuous countercurrent process has been described [4, 27]. The slurry is of course much easier to handle than dry solid, and it has been shown that the capacity of the slurried adsorbent is about the same as for dry solid [73], much larger than that of the liquid solvent alone [96]. 

Colgate-Emery A process for hydrolyzing natural triglycerides (fat splitting). It is a continuous countercurrent process operated at high pressure. Widely used in the United States. 

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