Armin Rix

Distillation of Specialty Chemicals

Abstract Most specialty chemicals are multifunctional molecules with low vapor pressure and high reactivity, requiring gentle operating conditions in separation processes. To accommodate these needs, considerable effort is put into the development of alternative separation processes such as crystallization, extraction, membrane separations, adsorption, and/or hybrid combinations thereof. Nevertheless, distillation is—and for the foreseeable future will remain—the workhorse in special chemistry separation technology. Although many specialty chemicals are solids, fluid processing is an important part of the manufacture of intermediates. In this chapter, distillation problems typical to special chemistry are addressed. The first part covers applications with low liquid load. In packed columns operating in a deep vacuum, the determination of minimum liquid load required for efficient separation is important, and appropriate design of liquid distributors is essential. To minimize pressure drop, the free area of distributors and liquid collectors is an important design variable. Trayed columns with a low reflux ratio often operate in the spray regime, where reasonable efficiency can be ensured by optimized designs. Chemical reactions frequently occur in special chemistry distillation columns. In reactive distillation, chemical reaction and distillation are intentionally combined inside a column. Reactive distillation offers potentially great economic advantages by lower investment and higher selectivity or conversion, but it poses tough design problems. Unwanted reactions may cause fouling of distillation equipment, for example, the plugging of trays, packing, or metering orifices of distributors, resulting in loss of capacity and/or separation efficiency. Aqueous systems frequently occur in special chemistry processes. They are especially challenging because the column profile often shows considerable changes of flow rates, flow regimes, and physical properties. The efficiency of aqueous separations is difficult to estimate, and considerable changes in surface tension may induce foaming. The formation of a second liquid phase inside the column requires internals with a tailor-made design.