Equipment, continuous countercurrent

Disadvantages of these continuous countercurrent systems are associated primarily with the complexity of the equipment required and with the attrition resulting from the transpoiT of the ion exchanger. An effective alternative for intermediate scale processes is the use of merry-go-round systems and SMB units employing only packed-beds with no movement of the ion-exchanger. 

Such an assembly of mixing and separating equipment is represented in Figure 14.3(a), and more schematically in Figure 14.3(b). In the laboratory, the performance of a continuous countercurrent extractor can be simulated with a series of batch operations in separatory funnels, as in Figure 14.3(c). As the number of operations increases horizontally, the terminal concentrations E1 and R3 approach asymptotically those obtained in continuous equipment. Various kinds of more sophisticated continuous equipment also are widely used in laboratories some are described by Lo et at. (1983, pp. 497-506). Laboratory work is of particular importance for complex mixtures whose equilibrium relations are not known and for which stage requirements cannot be calculated. 

The variety of extractors used in liquid-solid extraction is diverse, ranging from batchwise dump or heap leaching for the extraction of low grade ores to continuous countercurrent extractors to extract materials such as oilseeds and sugar beets where problems of solids transport have dominated equipment and development. 

This type of design procedure is based on two additional assumptions which may not always be true in practice. First, it is assumed that equilibrium is completely attained in each stage. In practice this is usually not the case because mass transfer is a first-order rate process [Eq. (8)] and complete equilibrium is only reached asymptotically. A second factor is that in many types of continuous countercurrent equipment, some reverse flow (backmixing) occurs for example in Fig. 5b, a small portion of stream R2 may find its way back into stage 1. 

The basis of the process is to use activated carbon to recover SO2 in a salable form. The process flowsheet and chemistry are summarized in Figure 1. All steps of this dry, cyclic process are performed in continuous, countercurrent, multi-stage fiuidized-bed equipment. In the S02-removal step the carbon catalyzes the reaction of the SO2 with oxygen in the flue gas to form SO3, which is hydrolyzed to sulfuric acid that remains sorbed... 

Back mixing, slurry transport, moving parts in the equipment, and scaling problems are the biggest difficulties in continuously running plants. Therefore, a truly ideal continuous countercurrent melt crystallizer that is flexible enough to handle a variety of substances with high efficiency is not, unfortunately, found to date. Due to problems, the semibatch-type of processes dominates in most of the recently built plants. 

Those which provide continuous countercurrent contact between the insoluble phases. The equivalent of as many theoretical stages may be built into a single piece of equipment as is desired or is practical, without intermediate removal of the phases. Design calculations are described in Chap. 8. 

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