Batch processes countercurrent

Industrial-scale adsorption processes can be classified as batch or continuous (53,54). In a batch process, the adsorbent bed is saturated and regenerated in a cychc operation. In a continuous process, a countercurrent staged contact between the adsorbent and the feed and desorbent is estabhshed by either a tme or a simulated recirculation of the 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. 

The three main modes of chromatographic operation are elution chromatography, selective adsorption/desorption, and simulated countercurrent chromatography. Of these, elution chromatography, used as a cyclic batch process, was the first to be developed for large-scale separations. 

Industrial-scale adsorption processes can be classified as batch or continuous. In a batch process, die adsorbent bed is saturated and regenerated in a cyclic, operation. In a continuous process, a countercurrent staged contact between lire adsorbent and die feed and desorbent is established by cidier a true or a simulated recirculation of die adsorbent. The efficiency of an adsorption process is significantly higher in a eoiuinuous mode of operation than in a cyclic batch mode. For difficult separations, batch operation may require 25 times more adsorbent inventory and twice die desorbent circulation rate than does a continuous operation. In addition, in a batch mode, the four functions of adsorption, purification, desorption, and displacement of the desorbent from the adsorbent are inflexibly linked, wtiereas a continuous mode allows mure degrees of freedom with respect to these functions, and thus a better overall operation. 

Fig. 4.1. Flash cascades [13] and batch processes (reactive condenser/reactive reboiler, [14]) being used to predict the top and bottom products of a countercurrent reactive distillation column.

In order to increase the efficiency of such a batch process and to reduce the inconvenience of discontinuous operation when the extractor is decompressed and opened for replacing the spent material against fresh one the extraction volume is spread over three or four vessels. These are switched into the gas circulation in a battery-type sequence utilizing the countercurrent principle. This means the extractor containing already depleted material is first contacted with the fresh gas and the extractor with the fresh material containing the full extract concentration is contacted in the second or last position in order to benefit as much as possible from the dissolving capacity of the gas. 

The third aspect of improvement may mean an entirely different process for making an old substance. For many decades soap has been manufactured by caustic soda saponification of fats in the batch process. In recent years plants have been installed for the high pressure, high temperature rapid hydrolysis of fats in the countercurrent liquid mixed phase using water and a catalyst. In this case the sodium fatty acid or the soap may be prepared by neutralizing the free fatty acid hydrolysis product with caustic soda or soda ash depending upon convenience and the market. Naturally the unit consumption factors changed when the improved process was used. 

Slurry approach use if the carbon usage is <180 kg/day. Mix and suspend powdered adsorbent and then filter exit line. Often uses up to three stages of countercurrent contacting. Used for continuous bleaching of edible oils. Batch process is simple, flexible, and easy to change feedstocks. Continuous operation offers better protection against oxidation, provides shorter holdup, and has the potential of heat recovery. Bleach time 25 min. A related topic is transfer line reactor (Section 16.11.6.9). 

In continuous distillation the feedstock — usually preheated to a certain degree — is uninterruptedly passed into the apparatus and is separated into an overhead distillate and a bottom product, both of which are continuously removed from the process. After the ratio and compositions of these two products, and other predetermined conditions, have become established, they should not change during the further course of the operation (see chap. 4.7.2). Continuous countercurrent distillation, like the corresponding batch process, is carried out with a column, and the feed is usually fed into the column at some point between its base and top (Fig. 162). 

Analysis of complex mixtures often requires separation and isolation of components, or classes of components. Examples in noninstrumental analysis include extraction, precipitation, and distillation. These procedures partition components between two phases based on differences in the components physical properties. In liquid-liquid extraction components are distributed between two immiscible liquids based on their similarity in polarity to the two liquids (i.e., like dissolves like ). In precipitation, the separation between solid and liquid phases depends on relative solubility in the liquid phase. In distillation the partition between the mixture liquid phase and its vapor (prior to recondensation of the separated vapor) is primarily governed by the relative vapor pressures of the components at different temperatures (i.e., differences in boiling points). When the relevant physical properties of the two components are very similar, their distribution between the phases at equilibrium will result in shght enrichment of each in one of the phases, rather than complete separation. To attain nearly complete separation the partition process must be repeated multiple times, and the partially separated fractions recombined and repartitioned multiple times in a carefully organized fashion. This is achieved in the laborious batch processes of countercurrent liquid—liquid extraction, fractional crystallization, and fractional distillation. The latter appears to operate continuously, as the vapors from a single equilibration chamber are drawn off and recondensed, but the equilibration in each of the chambers or plates of a fractional distillation tower represents a discrete equihbration at a characteristic temperature. 

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. 

In the Molex process the Sorbex system is used for the separation of linear and branched hydrocarbons, using as adsorbent 5A molecular sieve. This is the same adsorbent as is used in the cyclic batch processes such as Ensorb and Isosiv. In contrast to the situation with the Cg aromatics, the separation factor is very large and it seems unlikely that for such an easy separation the increased cost of a Sorbex unit is justified by the reduction in adsorbent inventory and/or desorbent circulation rate. Detailed cost comparisons do not appear to have been published, but it seems likely that any economic advantage which the Molex process may have arises more from the energy savings associated with liquid phase operation than from the intrinsic advantages of a countercurrent adsorption system. 

The polymer batch process is comprised of a Continuous Stirred Tank Reactor (CSTR) and a countercurrent heat exchanger for cooling (Figure 1). The reaction can be represented as a simple two component exothermic polymerization of the following form ... 

This example shows the distinct advantage that tiie countercurrent process holds, not only over the single-stage operation, which was expected, but also over its crosscurrent counterpart Consumption by the latter is higher by a factor of 2.2/0.87 = 2.5. The principal advantage of the crosscurrent cascade is that it is more easily adaptable to batch processing, whereas the counter-current cascade is by necessity continuous. 

Both continuous and batch diffusers are used, but batch processes largely are being displaced. Continuous diffusers come in a variety of forms and shapes, but all of them employ the same principle, that is, movement of juice countercurrently to the movement of the cossettes. In batch diffusers, the cossettes are held stationary, and the diffusion juice is moved from cell to cell. A few of the well-known types of continuous diffusers are the RT, BMA, DDS, DeSmet, Buckau-Wolf, and Olier. 

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

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. 

The gas/liquid process may be batch liquid, continuous liquid, or continuous multistage liquid. If the process is multistage liquid flow, the gas flow must be specified as cocurrent or countercurrent. 

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