Distillation columns

Distillation columns can have trays or packing. The advantage of packing is that the column can be smaller in diameter and marginally shorter. 

Distillation is an integral component in the chemical process industry. About 90-95% of all separations are done by distillation. It is estimated that there are 40,000 distillation columns in operation in the United States alone, representing a capital investment of 8 billion (Humphrey 1995). 

The graphical method for analyzing distillation columns is similar to the graphical method for analyzing multistage countercurrent absorbers Find a reference point. 

Let s start with the stripping section. The reference point is the composition of the bottoms stream. Note that the bottoms composition is the same as the composition of the liquid entering the evaporator. And because the vapor stream is the only stream that leaves the evaporator, its composition must be the same as the entering liquid. Our reference point is thus on the diagonal of the vapor-liquid equilibrium map. 

As stated above, we consider in this text only feeds in which the vapor and liquid are in equilibrium. Thus the feed point will lie on the equilibrium line. The more general cases are treated in the chemical engineering course on separation processes. In the examples shown here, we have started counting trays at the bottom (or top) and the feed composition conveniently matches one of the intersections with the equilibrium line. In other cases (such as the exercises at the end of this chapter) the feed composition is less conveniently matched to the diagram. In these cases, you might consider starting at the feed composition and stepping off trays upward toward the condenser and downward toward the evaporator. 

Processes can be analyzed using experimental data. Because experimental data are often presented graphically, analysis based on empirical data often uses graphical methods. 

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These reactions can be reversed in a distillation column. This releases the hydrogen sulfide and carbon dioxide for further processing. The monoethanolamine can then be recycled. 

Another variable that needs to be set for distillation is refiux ratio. For a stand-alone distillation column, there is a capital-energy tradeoff, as illustrated in Fig. 3.7. As the refiux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required (see Fig. 3.7). If the capital... 

Figure 3.7 The capital-energy tradeoff for stand-alone distillation columns.

Probably the most common method used for sequence selection for simple distillation columns is heuristic. Many heuristics have been proposed, but they can be summarized by the following four ... 

Heat Integration of Sequences of Simple Distillation Columns... 

Figure 5.10 Distillation columns with three products. (From Smith and Linnhoff, TVans. IChemE, ChERD, 66 195. 1988 reproduced hy permission of the Institution of Chemical Engineers.)...

This remixing which occurs in both sequences of simple distillation columns is a source of inefficiency in the separation. By contrast. 

Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegrated sequences and (2) study... 

Porter, K. E., and Momoh, S. O., Finding the Optimum Sequence of Distillation Columns—An Equation to Replace the Rules of Thumb (Heuristics), Chem. Engg. J., 46 97, 1991. 

Triantafyllou, C., and Smith, R., The Design and Optimization of Fully Thermally Coupled Distillation Columns, Trans. IChemE, Part A, 70 118, 1992. 

Glinos, K., and Malone, M. F., Optimality Regions for Complex Column Alternatives in Distillation Columns, Trans. IChei lE ChERD, 66 229, 1988. 

Kaihel, G., Distillation Columns with Vertical Partitions, Chem. Eng. TechnoL, 10 92, 1987. 

Kaibel, G., Distillation Column Arrangements with Low Energy Consumption, IChemE Symp. Ser., 109 43, 1988. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

The effluent from the reactor contains both PRODUCT and unreacted FEED which must be separated in a distillation column. Unreacted FEED is recycled to the reactor via a pump if the recycle is liquid or a compressor if the recycle is vapor. 

Distillation. There is a large inventory of boiling liquid, sometimes under pressure, in a distillation column, both in the base and held up in the column. If a sequence of columns is involved, then, as discussed in Chap. 5, the sequence can be chosen to minimize the inventory of hazardous material. If all materials are equally hazardous, then choosing the sequence that tends to minimize the flow rate of nonkey components also will tend to minimize the inventory. Use of the dividing-wall column shown in Fig. 5.17c will reduce considerably the inventory relative to two simple columns. Dividing-wall columns are inherently safer than conventional arrangements because they lower not only the inventory but also the number of items of equipment and hence lower the potential for leaks. 

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

Wastewater leaves the process from the bottom of the second column and the decanter of the azeotropic distillation column. Although both these streams are essentially pure water, they will nevertheless contain small quantities of organics and must be treated before final discharge. This treatment can be avoided altogether by recycling the wastewater to the reactor inlet to substitute part of the freshwater feed (see Fig. 10.36). 

As shown in Fig. 10.6, the vapor from the reactor flows into the bottom of a distillation column, and high-purity dichloroethane is withdrawn as a sidestream several trays from the column top. The design shown in Fig. 10.6 is elegant in that the heat of reaction is conserved to run the separation and no washing of the reactor... 

The dominant heating and cooling duties associated with a distillation column are the reboiler and condenser duties. In general, however, there will be other duties associated with heating and cooling of feed and product streams. These sensible heat duties usually will be small in comparison with the latent heat changes in reboilers and condensers. 

The consequences of placing distillation columns in different locations relative to the pinch will now be explored. There are two possible ways in which the distillation column can be integrated. The reboiler and condenser can be integrated either across the pinch or not across the pinch. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Evolving the Design of Simpie Distillation Columns to Improve Heat Integration... 

If the distillation column will not fit either above or below the pinch, then other design options can be considered. One possibility is... 

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