In this chapter we study distillation columns that have more than the normal two product streams. These more complex configurations provide savings in energy costs and capital investment in some systems. Sidestream columns are used in many ternary separations, and the examples in this chapter illustrate this application. However, a sidestream column can also be used in a binary separation if different purity levels are desired. For example, two grades of propylene products are sometimes produced from a single column. The bottoms stream is propane, the sidestream is medium-purity propylene, and the distillate is high-purity polymer-grade propylene.
The most widespread use of sidestream columns occurs in petroleum fractionators, which have multiple sidestream products that are complex mixtures of many components. The sidestreams have progressively higher boiling point ranges as we move down the column. The top sidestream may be kerosene. The next may be a diesel cut or jet fuel. The next is a light gasoil. The final may be a heavy gasoil. The liquid streams withdrawn from the main column are each fed to a small stripping column. Open steam is fed at the bottom of each sidestream stripper to strip out light components from the liquids withdrawn from the main column. We discuss these types of sidestream columns in Chapter 11.
Sidestream columns come in several flavors. Both liquid and vapor sidestreams are used. Sometimes the sidestream is a final product. Because the purity attainable in a sidestream is limited, the sidestream from the main tower is sometimes fed to a second column (usually a stripper or a rectifier) for further purification with a recycle stream back to the main column. Several examples are studied in this chapter.
Distillation Design and Control Using Aspen™ Simulation, By William L. Luyben Copyright © 2006 John Wiley & Sons, Inc.
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