Continuous countercurrent operations

The McCabe-Thiele approach has been developed to describe the Sorbex process (76). Two feed components, A and B, with a suitable adsorbent and a desorbent, C, are separated ia an isothermal continuous countercurrent operation. If A is the more strongly adsorbed component and the system is linear and noninteracting, the flows ia each section of the process must satisfy the foUowiag constraints for complete separation of A from B ... 

Continuous countercurrent operation in a single vertical column where longitudinal concentration gradient could be established... 

In the chemical industry, chromatographic separation process is an emerging technology for the separation of pharmaceutical products, food and fine chemicals. To improve the economic viability, a continuous countercurrent operation is often desirable but the actual movement of the solid leads to a serious operating problem. Therefore, the simulated moving bed (SMB) process is an interesting alternative option. 

Multi-stage extraction is used to achieve a higher efficiency of separation, in which the product is almost completely removed from the raffinate. The solvent is split up into several portions and fed to a series of mixers and settlers. The disadvantage of this method is the need to use large volumes of solvent. A more complicated system, called countercurrent, multi-stage extraction, uses a series of mixers and settlers arranged as before, but the feed liquid and pure solvent are passed through the system in opposite directions, that is counter-currently. Continuous countercurrent operation may be carried out by means of spray columns, packed columns (similar to those used in distillation), plate columns, or, sometimes, bubble cap columns. 

There is complete formal analogy between continuous countercurrent adsorption and other continuous countercurrent operations such as distillation or gas absorption. Such processes are normally operated at steady state and, for an isothermal system containing only a single adsorbable species, the analysis is straightforward. 

Liapis, A.I., and Rippin, D.W.T., Simulation of binary adsorption in continuous countercurrent operation and comparison with other operating modes. AlCliE J., 25(3), 455-460 (1979). 

Techniques and applications All the operational techniques ordinarily used for adsorption are used also for ion exchange. Thus we have batch or stagewise treatment of solutions, fluidized- and fixed-bed operations, and continuous countercurrent operations. Fixed-bed percolations are most common. Chromatographic methods have been used for fractionation of multicomponent ionic mixtures. Applications have been made in the treatment of ore slurries ( resin-in-pulp ) for collection of metal values. 

To obtain a quasi-continuous operation, a series of extractors (Figure 8.5) is operated in quasi-continuous countercurrent operation. Fresh CO2 is always switched to the extractor that is longest in line and consequently a sufficient concentration difference is maintained and the highest possible loading with soluble substances is ensured. 

Fig. 17. The two basic modes of operation for an adsorption process (a) cycHc batch system (b) continuous countercurrent system with adsorbent...

Solvent Extraction. Solvent extraction has widespread appHcation for uranium recovery from ores. In contrast to ion exchange, which is a batch process, solvent extraction can be operated in a continuous countercurrent-fiow manner. However, solvent extraction has a large disadvantage, owing to incomplete phase separation because of solubihty and the formation of emulsions. These effects, as well as solvent losses, result in financial losses and a potential pollution problem inherent in the disposal of spent leach solutions. For leach solutions with a concentration greater than 1 g U/L, solvent extraction is preferred. For low grade solutions with <1 g U/L and carbonate leach solutions, ion exchange is preferred (23). Solvent extraction has not proven economically useful for carbonate solutions. 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

In fluidized-bed adsorbers, the combination of high gas rate and small adsorbent particle size results in suspension of the adsorbent, giving it many of the characteristics of a fluid. Fluidized bed adsorbers, therefore, lend themselves to truly continuous, countercurrent, multistage operation. Adsorbent inventory is minimized. 

Capacity of a column operating as continuous countercurrent unit... 

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