Conversion of Crosscurrent into Countercurrent Process

Example 1 In an API manufacture, a stirred tank of 5 m capacity is used as a reactor. It is charged with a solvent and cooled to 5 °C from the ambient temperature. The reactants are added and the reaction is carried about for 8 h under constant stirring. At the end of the reaction, the reaction mixture is washed with the same volume of water at 10 °C and then the wash water is decanted. The washing is repeated 6 times. Then, the vessel is cleaned for the next operation to meet regulatory requirements. The total batch time is about 40 h. Thus the vessel is occupied for 40 h. These washings amount to batch crosscurrent multistage extraction. There is a loss of the API as well as of the solvent (in which the API is held) in each of these washing. The yield is only about 60%. 

Alternative The washing can be done in a Podbielniak centrifugal extractor, which is continuous countercurrent extraction. Alternately, the same can be done in a cascade of annular centrifugal extractors configured in the countercurrent mode. It is known that the amount of cold water required and the loss of the API would be far less compared with values for the crosscurrent process [18]. 

Washing and adsorption The belt filters are used for washing and dewatering of fine solids in the manufacture of catalysts, zeolites, alumina and other crystalline substances, and so on. The belt filter requires the associated equipment — several vacuum pumps and separation drums. Through-circulation tray driers and through-circulation rotary driers are used to dry solids. All these are continuous crosscurrent processes. These could be replaced with a moving-bed, Hildebrand (screw) conveyer or Kennedy extractor [18], each of which is a countercurrent process. Their potential for PI has not been explored. 

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