Separation Tower Design

After studying this chapter and the materials on distillation on the multimedia CD-ROM that accompanies this book, the reader should 

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Absorption and stripping are technically mature separation operations. Design procedures are well developed for both packed columns and tray towers, and commercial processes are common. In most applications, the solutes are contained in gaseous effluents from chemical reactors. Passage of strict environmental standards with respect to air pollution by emission of noxious gases from industrial sources has greatly increased the use of gas absorbers (also known as scrubbers) in the past decade. 

The above [ventuil+packed tower] design was preceded by a humidifier section. The packed tower was followed by a demister pad/candle type demister in a separate vessel or in a separate attachment above the packed tower. 

Kister, H. Z., and K. F. Larson, Packed Distillation Tower Design, in Schweitzer, P.A. (Ed.), Handbook of Separation Techniques for Chemical Engineers, 3 ed., McGraw-Hill, New York, 1997, section 1.6. 

In atmospheric and some low- to medium-pressure processes, one or more separate oxidation-cooling units are often included prior to gas absorption. These units are built in the form of vertical towers which are cooled with external water curtains, shell-and-tube units, and also drum and cascade coolers. Excess air in the gas promotes initial oxidation, and some of the water vapor also present condenses to form weak nitric acid, which is later concentrated in the absorption section. Additional air for oxidation usually is injected at some point in the process, often in the absorption tower. In some plants, the gas is rapidly cooled to condense the water vapor without forming much weak acid, thereby helping to increase final acid concentration. A special condenseix yclone separator unit designed for this purpose is described by.Graham etal..[10]. 

In tower design, the reflux ratio is determined based on the trade-off between operating cost in the reboiler and capital cost for the tower. In other words, use of more separation stages requires less reflux rate and thus less reboiling energy but at the expense of additional capital cost. 

There are several types of continuous treater tower designs used in conventional lube plants. These include trayed towers, packed towers and rotating disc contactors (see Figure 13). The treater tower internals are designed to promote contact and separation of the oil and the solvent phases. 

In this chapter we will discuss single theoretical stage Bubble-Point and Dew Point calculations and give examples as to how these can be used in our work. Then weTl discuss the use of the Absorption Factor and Stripping Factor chart, which can be used to design distillation towers. In order to use this chart in that way it is necessary to consider a distillation tower as two separate towers. The top of the tower is considered an absorber. The chart is then used in a different way to design the lower part of the distillation tower as a stripper. 

The lye boHer is usuaHy steam heated but may be direct-fired. Separation efficiency may be iacreased by adding a tower section with bubble-cap trays. To permit the bicarbonate content of the solution to buHd up, many plants are designed to recirculate the lye over the absorber tower with only 20—25% of the solution flowing over this tower passiag through the boHer. Several absorbers may also be used ia series to iacrease absorptioa efficieacies. 

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