Crystallization of Organic Compounds, 2nd Edition

Chapter 11Filtration

11.1 INTRODUCTION

Filtration is an important unit operation which separates the desired species from the undesirable species by their sizes through a filter medium. It is a standard unit operation employed in many industries, including pharmaceutical, chemical, medical, biological, waste‐water treatment, air purification, etc.

In filtration operation, there can be three different cases. In the first case, the retentate is the desired species. The filtrate is the undesired species. This applies to the isolation of active pharmaceutical ingredients (APIs) solids and any solid intermediates. In this context, the slurry resulting from the crystallization is a mixture of the suspended solids of valuable product and the solvents or mother liquor containing undesired impurities together with some dissolved valuable product. After the filtration, the cake is retained and filtrate is discarded. The cake is further washed with pure solvents to wash off residual mother liquor from the cake. Complication can arise when the cake traps non‐negligible amount of undesired species and cannot meet the desired purity specifications. Other complications can arise as well, such as too much loss of desired product in the mother liquor and washes due to particle breakthrough the filter medium. Also, cake can be compressed or filter medium is (partially) blocked by fine particles in the slurry. Both factors can hinder the filtration operation.

In the second case, the opposite ...