Equipment, countercurrent distillation

The equipment for continuous distillation can only separate one stage in the equilibrium diagram. Countercurrent distillation, also called rectification, has found widespread application with normal pressure and coarse vacuum distillation when complex mixtures or components with small relative volatility factor are to be separated. The fundamentals are discussed above (2.1.3.3.2) the technical side will be dealt with here [41-45]. 

Equipment Absorption, stripping, and distillation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow countercurrently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place. Background material on this mass transfer process is given in Sec. 5. 

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

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. 

Since most continuous extraction methods use countercurrent contacts between two phases, one a light liquid and the other a heavier one, many of the fundamentals of countercurrent gas absorption and of rectification carry over into the study of liquid extraction. Thus questions about ideal stages, stage efficiency, minimum ratio between the two streams, and size of equipment have the same importance in extraction as in distillation. 

A plant requires 15 kg/s of cooling water to flow through its distillation-equipment condensers. The water will leave the condensers at 318 K. It is planned to cool the water for reuse by contact with air in a countercurrent adiabatic cooling tower. The dry-bulb temperature of the entering air is 303 K and its wet-bulb temperature is 297 K. The wet-bulb temperature approach must be 5 K. The air rate to be used is 1.5 times the minimum. Specialized packing will be used for which Kya is expected to be... 

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... 

Gas scrubbers are operated in countercurrent flow and are typically equipped with structured packings or trays. The degree of removal of gaseous components depends on their solubility in the liquid absorption agent and thus on pressure and temperature, and on the liquid-to-gas ratio, which is defined as the ratio of the carrier fluid flow to the gas flow. For the design of an absorption column, the McCabe-Thiele approach is used, as already examined for distillation in Section 3.3.2.2. 

Most mass transfer equipments consist of gas (vapor) and liquid two-phase flow, for instance, vapor-liquid two-phase cross-current flow is undertaken in tray distillation column gas-liquid two-phase countercurrent flow is taken place in packed absorption column. Some processes may also include solid phase, such as adsorption or catalytic reaction. Thus, the fluid system may contain gas and liquid two phases, or gas, liquid single phase besides solid phase. 

Distillation normally involves three pieces of equipment a column, a condensor on top of the column, and a reboiler at the bottom of the column. The reboiler, often a steam-jacketed kettle, is heated so that much of its contents evaporate and flow upwards through the column. The vapors passing out of the column are liquefied in the condensor and much of this condensate is sent back downwards through the column. This countercurrent flow of vapor and condensate is common to all forms of distillation. 

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