Multistage countercurrent operations

In most industrial applications, multistage countercurrent contacting is required. The hydrodynamic driving force necessary to induce countercurrent flow and subsequent phase separation may be derived from the differential effects of either gravity or centrifugal force on the two phases of different densities. Essentially there are two types of design by which effective multistage operation may be obtained ... 

In leaching processes, finely divided solids are contacted with solvents to remove soluble constituents. Usually some kind of multistage and countercurrent operation is desirable. The most bothersome aspect is handling of the wet solids. 

Needless to say, kinetic parameters established in this way are empirical factors dependent on the definition of other variables,e.g., averaging of particle size distribution to determine R. Nevertheless, the model provides a rational approach to the complex physical and chemical phenomena of multistage, multireaction leaching. It is intended to expand the model to include countercurrent operations. 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

The main objective of absorption processes is the removal of one or more components from a gas stream using selective solvents. Figure 3 shows a typical absorption process in which, with the help of a selective solvent (absorbent), the undesired compounds (in this case, HjS, COj) are removed from the raw gas (in this case, natural gas) in a multistage countercurrent process. While the purified gas leaves the absorber (saturated with the selective solvent), the absorbent is regenerated in a second column (desorber) and is recycled to the absorber. In the case of absorption, one can distinguish between physical and chemical absorption processes. In the case of physical absorption, the absorber is operated at high pressures and low temperatures while for the desorber the opposite conditions are used. 

Continuous distillation, or fractionation, is a multistage, countercurrent distillation operation. For a binary solution, with certain exceptions, it is ordinarily possible by this method to separate the solution into its components, recovering each in any state of purity desired. 

We wish to remove acetic acid from water using pure isopropyl ether as solvent. The operation is at 293 K and 1 atm (see Table 7.2). The feed is 45 wt% acetic acid and 55 wt% water. The feed flow rate is 2000 kg/h. A multistage countercurrent extraction cascade is used to produce a final extract that is 20 wt% acetic acid and a final raffinate that is also 20 wt% acetic acid. Calculate how much solvent and how many equilibrium stages are required. 

Kremser originated the group method. He derived overall species material balances for a multistage countercurrent absorber. Subsequent articles by Souders and Brown, Horton and Franklin, and Edmister improved the method. The treatment presented here is similar to that of Edmister for general application to vapor-liquid separation operations. Another treatment by Smith and Brinkley emphasizes liquid-liquid separations. 

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