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Extractive distillation is proving a viable, safe alternative to azeotropic distillation
Manufacturers in the chemical process industries (CPI) are under increasing pressure to minimize the discharge of organic compounds. As a result, more companies are turning to onsite purification and recycling systems. However, recovering low-boiling organic compounds, such as ketones or alcohols, poses significant challenges, since these chemicals form singlephase azeotropes with water.
The traditional way to break these azeotropes has been through azeotropic distillation, in which a water-immiscible, low-boiling compound or "entraîner," such as benzene, cyclohexane or isopropyl ether, is added to the mixture to form a ternary, two-phase azeotrope. However, many of these entrainers are either proven or suspected carcinogens, or classified as hazardous pollutants. In addition, since the entrainers are low-boiling, they are likely to contaminate the solvent - a very great risk when the final product is a pharmaceutical. As a result, more CPI manufacturers are seeking safer alternatives to azeotropic distillation for recovering and recycling low-boiling chemical solvents.
Extractive distillation offers a safer way to break azeotropes to recover low-boiling chemicals. Like azeotropic distillation, the process makes use of a third component to break the waterethanol binary azeotrope. However, instead of raising water's activity coefficient as azeotropic distillation does, extractive agent lowers water's activity coefficient so that it approaches 1.0 throughout most of the column. In the presence of the extractive agent, the solvent-water system acts ideally. Ethanol, for example, shows an approximate ethanol-to-water relative volatility range of 2.6 to 3.2. Since the extracting agent boils at a much higher temperature than the solvent to be removed, virtually all of the extractant is removed from the solvent.
In addition, extractive distillation allows a wider range of chemicals, including some food-grade additives, to be used as extractive agents, minimizing contamination and safety concerns. For instance, glycols' high boiling points and water solubility make them an attractive choice for extraction agents. The glycol molecule's hydroxyl groups bond, weakly, to water molecules, raising the solvent's relative volatility with respect to water, and laying the foundation for simple, cost effective dehydration.
Dry ethanol recovery
This article examines the requirements and costs, as well as the relative benefits, of extractive distillation. Dry ethanol recovery and recycling, using propylene glycol as the extraction agent, are...