(115c) Process Approaches to Eliminate a Difficult Filtration

Particle formation and subsequent solid / liquid separations are prevalent in batch processes to produce fine and specialty chemicals. These unit operations often pose some of the more difficult challenges to scale up and often are the focus of significant plant trouble shooting efforts upon start up of a new chemical process. Predictive models are more difficult to develop than for other unit operations like distillation and extraction, and often fall back to empirically derived correlations. Scale down experiments to the lab bench and mini-plant scales often do not adequately capture scale sensitive aspects such as mixing and heat transfer. Halide exchange reactions which introduce the fluorine atom into organic molecules are important industrial reactions in the agricultural chemical arena. Typically these reactions employ anhydrous potassium fluoride in a polar, aprotic solvent system. The byproduct of these reactions is potassium chloride which is formed by reactive precipitation. Often very small particles are produced which are difficult to filter. Common solvents are high boiling, water-miscible solvents such as DMF, DMSO, NMP, and sulfolane. Removal of residual solvent from the filter cake also poses difficulty. One of the focuses of process development research is to build in ?manufacturing scale robustness? and to that end our research has been to devise alternate separation processes to avoid the difficult filtration step or to improve the filtration operation. Several process approaches will be discussed to avoid or modify the salt filtration step described above. These include examples of a solvent-free reaction scheme, a low-boiling solvent concept, a flux oil option, and finally a novel common ion separation technique. While these examples are from a particular type of reaction encountered in many agricultural product syntheses, the principles of process conception and development have broad application.