Challenges of Minor Impurities in Process Development

Impurities bring additional complexities to separations, often beyond what is normally considered during conceptualization. Process design needs to take these complexities into account.

When developing new chemical processes, the initial focus is typically to explore the main reaction and separation systems. The hierarchical design process (1) begins with evaluating if the end products are worth more than the reactants; if not, then the process development should stop, or additional details of the process should be defined and the costs should be reassessed. At some point, it is important to look at impurities that are relatively minor. The ideal time to start considering minor impurities is in the conceptual design phase before detailed engineering increases costs and would require rework as more information is known.

Impurities are a significant driver in the development of pilot plants, which serve to reduce risk. Failure to consider impurities is a well-known risk in chemical processes. Books on process development discuss recycle loops (2) and emphasize the need to close them to determine if an impurity will build up. No matter how low the concentration of an impurity is, if it has no outlet, then it will eventually build up and dominate the process, potentially making the process infeasible. In many cases, once an impurity in a stream becomes large enough, chemical equilibrium with other components may allow the exit purge to increase enough to equal the generation rate or feed concentration of the impurity. These recycle streams can grow quite large, possibly many times larger than the flow of the final product.

Past work has asserted (3) that pilot plants should be scaled down from full-sized operations, not scaled up from the lab. In other words, pilot operations must mimic the industrial scale as closely as possible. The scale itself can lead to different mixes of impurities for some processes, which may be due to the type of equipment, differences in mass and heat transfer, etc. Residence time can also lead to differences in the type and quantity of impurities generated. Therefore, care must be taken in designing each unit operation for the pilot plant to ensure that it adequately represents the commercial-scale facility.

This article covers some topic areas that are not often addressed in process development, sometimes due to the scale or characteristics of a particular reaction or separation system. Focusing on distillation, this article discusses unknown impurities, impurity generation, and other techniques for impurity removal.

Intermediate impurities

One of the main areas of concern is intermediate-boiling impurities...