COUNTERCURRENT MULTISTAGE EQUIPMENT

Your objectives in studying this section are to be able to  

Estimate the number of real trays required for a sieve-tray absorber or stripper by graphical stepwise construction on an xy diagram, incorporating in the analysis the Murphree tray efficiency. 

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Cuprous ammonium acetate extraction. Butadiene is purified by aqueous CAA extraction in a liquid-gas countercurrent process developed by Exxon (67-69). The cuprous salt forms a soluble addition complex with butadiene, which is decomposed by heat thus the process is adaptable to countercurrent multistage equipment. Typically, the C4 hydrocarbon mixture with a butadiene content of 30-40% contacts the CAA solution in a countercurrent fashion in a series of mixer-settlers. Cooling to ca -15°C is required to promote complex formation. The more saturated hydrocarbons, butanes, and butenes are first removed by distillation. Butadiene is released from the complex by further heating to 80° C. After ammonia is removed by washing with water, distillation produces butadiene that is 98-99% pure. Acetylenes and allenes are extracted with the butadiene but must... 

To illustrate the principle of an equilibrium-stage cascade, two typical countercurrent multistage devices, one for distillation and one for leaching, are described here. Other types of mass-transfer equipment are discussed in later chapters. 

Equipment in this category is usually arranged for multistage countercurrent contact of the insoluble hquids, without repeated complete separation of the hquids from each other between stages or their equivalent. Instead, the liquids remain in continuous contact throughout their passage through the equipment. 

The equipment used for the different steps consists of mixer-settlers in a two-stage or multistage mode comprising countercurrent flow of aqueous and organic phases, and with partial internal circulation in each step to achieve the retention time required. Also, static mixers combined with mixing pumps and settlers can be used in extraction and scrubbing, where only short retention time is necessary. 

Multistage countercurrent separation is usually considered for laboratory applications. As the principles apply to laboratory size equipment as well as to large-scale installations, the procedure given below should also be used for laboratory experiments. 

For an earlier introduction to column crystallization vis-a-vis the variety of equipment and patents, see Albertins et al. (1967). Ulrich (1993) provides a more modern version of melt crystallization devices, plants and processes. For mathematical modeling and optimization of multistage crystallization processes, see Gilbert (1991). A more Involved model of continuous countercurrent contacting with axial dispersion in melt crystallization has been illustrated in Hemy and Moyers (1984). 

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