Extractive Distillation: Separating Close-Boiling-Point Components: Adding cosolvents can make tough separations technically and economically feasible

Illustration

Illustration: Graph: FIGURE 1 (left). When evaluating different solvents for extractive distillation, engineers must reconcile tradeoffs between selectivity and solvency

Photograph

Photograph: During extractive distillation, the addition of a polar, non-volatile solvent increases the relative volatility of close-boiling-point components

Illustration

Illustration: Graph: FIGURE 2 (above). The addition of a cosolvent can markedly boost mixed-solvent performance. MIST is a proprietary solvent mixture of Phillips Petroleum Co. It is used to recover cyclohexane from naphtha or natural gas liquids by extractive distillation

Illustration

Illustration: Graph: FIGURE 3 (right). Water, when added as a cosolvent, improves the properties of the solvent 3-methyl sulfolane during toluene separation

Using conventional distillation techniques, the separation of components with very close boiling points is both impractical and uneconomical, because it often requires large numbers of trays and a high reflux ratio (i.e., high energy consumption) [1-3]. An alternative method for separating close-boiling components is through the use of extractive distillation (ED). During ED, a polar, non-volatile solvent, introduced near the top of the distillation tower, preferentially associates with the more-polar, key components in the feed mixture. The addition of such a solvent can greatly increase the relative volatility of close-boiling-point components, making the separation economically and technically feasible.

Extractive distillation has been used in the chemical process industries (CPI) for many years, although not as widely as traditional distillation and azeotropic distillation. This article focuses on the selection of cosolvents for various hydrocarbon separations using extractive distillation.

Cosolvents: Key ingredients

Proper solvent selection is crucial to the success of any ED operation. During ED, the addition of a polar, non-volatile cosolvent improves solvent selectivity and solvency (solubility) for difficult separations. It also improves both the overall operation, and the effectiveness of the primary solvent.

The important characteristics to consider when selecting a solvent for use in a commercial ED process are:

-- Selectivity (i.e., the solvent's ability to enhance the separation of key components in the mixture)

-- Solvency (i.e., the solubility of the solvent for hydrocarbon mixtures)

-- Boiling point

-- Thermal stability

-- Freezing point

-- Reactivity with the feedstock

-- Corrosivity

-- Toxicity

-- Price

Among these, solvent selectivity and solvency are the primary factors that alter the relative volatility of the key components to be separated....