Make the Most of Separation Power

The concept of separation power can be used to characterize any binary separation process.

Separation process technology is a key area of expertise for many chemical engineers and process chemists involved in chemical manufacturing, whether that involves process development, equipment design and fabrication, or process operation and improvement. Nearly all chemical products and the raw materials used to manufacture them must be separated and purified to some extent for commercial success. This basic need for chemical separation has motivated the development and continuous improvement of many separation technologies over many years, an engineering tradition that continues today.

With this in mind, the authors recently published a book reviewing the history and design of popular separation methods, including distillation, extraction, adsorption, crystallization, and the use of membranes (1). In it, we review historical developments, discuss key fundamentals and process features, and offer insights from our experience working in industry with many talented engineers and chemists.

In looking back at the historical record, it is apparent that many separation techniques such as countercurrent flow, use of reflux, and middle-fed processing were first introduced in the course of improving distillation processing. Distillation provided a working example of how these techniques could be used to improve separation performance for other kinds of separations as well. In fact, many of the basic separation process concepts first developed for distillation have proved universal (1).

This article explores the concept of separation power, which is most associated with distillation operation (2), and demonstrates how this simple concept may be used to characterize the performance of any binary separation process. Here, we define a binary separation process as any process that separates a feed stream into two outlet streams or fractions (for either continuous or batchwise operation). Typically, this produces a product-rich stream (or the product-rich fraction from a feed batch) and a stream rich in unwanted impurities.

Determining separation power for a binary process provides a measure of its actual separation capability, and this allows a quick assessment of the opportunity for further improvement. It allows the operator to quickly determine whether a unit is performing as designed, and it may allow quick estimation of the tradeoff that exists between achieving high product purification and high product recovery. It is a shortcut method that, while only approximate, can be quite useful in engineering practice. It is particularly helpful in operations where quick assessments are needed and day-to-day responsibilities often do not allow time to prepare and run simulations, which are preferred when time allows but also can involve significant uncertainties in certain cases.