Precipitation processes semi-batch precipitator

Phillips, R., S. Rohani, and J. Baldgya (1999). Micromixing in a single-feed semi-batch precipitation process. AIChE J. 45, 82-92. 

The aim of this work is to study the semi-batch precipitation of the polymer poly (2,6-dimethyl-1,4-phenylene ether) from a solution in toluene, using high-pressure CO2 as an anti-solvent. The influence of the process conditions on the degree of crystallinity, and the particle size and size distribution has been determined. 

A precipitation process can be operated under batch, semi-batch or continuous conditions, each of which will have its own distinct influence on the product crystal size distribution in addition to the combined influences of feedstock entry positions, variations in the reactant addition rate profile and mixing intensity (Tavare and Garside, 1990 Sohnel and Garside, 1992). 

A number of laboratory studies have been recorded recently aimed at characterizing the kinetics of both the chemical reaction and crystallization steps in a reaction crystallization process. Examples of liquid phase reactions studied for this purpose are the crystallization of salicylic acid from aqueous solutions of sodium salicylate using dilute sulphuric acid (Franck et al, 1988) and the crystallization of various calcium phosphates by reacting equimolar aqueous solutions of calcium nitrate and potassium phosphate (Tsuge, Yoshizawa and Tsuzuki, 1996). Several aspects of crystal size distribution control in semi-batch reaction crystallization have been considered by Aslund and Rasmuson (1990) who studied the crystallization of benzoic acid by reacting aqueous solutions of sodium benzoate with HCl. An example of crystallization arising from a gas-liquid reaction in an aqueous medium is the precipitation of calcium carbonate from the reaction between calcium hydroxide and CO2 (Wachi and Jones, 1995). 

Zauner, R. and Jones, A.G. (2000b) Scale-up of continuous and semi-batch precipitation processes. Industrial and Engineering Chemistry Research, 39, 2392-2403. 

The ratio of API/polymer solution (solvent) to antisolvent is one of the most important factors for the success of the precipitation process. For continuous process, it is controlled by the feed rates of the APFpolymer solution and the antisolvent into the high-shear mixing chamber and is generally constant for the process. For semi-batch... 

Efficient and rapid removal of solvent and antisolvent is a vital component of the successful MBPprocess. Once the precipitation has been completed (concurrently for continuous process and at the end of the solvent addition for the semi-batch process), the MBP is filtered from the suspension. This is generally performed by any of the standard filtration processes such as vacuum filtration ranging from Buchner funnel to Nutsche filter or centrifugal filter. Centrifugal filters are preferred for filtration of MBP suspension because of the particle size, the hydrogel nature of the polymer, and the effectiveness in solid/liquid separation... 

The industrial process [23, 212, 213] of ethoxylation and propoxylation is usually a semi-batch process. The starter alcohol and KOH are mixed and water is removed by distillation. In a second step, monomers are fed into the reactor, where the feed rate is chosen so as to be able to remove the heat of polymerization and to keep the latent heat of polymerization of unreacted monomers in a safe state. By this process, homopolymers and random copolymers are accessible. Block copolymers are produced by successive feeds of the respective monomers. Catalyst is removed by addition of acids and subsequent crystallization and filtration of precipitated salts. An optional fourth step is the removal of volatile compounds by distillation. 

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