Liquid Sidestream Column with Stripper

The flexibility of a sidestream column is greatly increased if additional separation columns are added. This can extend their economic applicability to feeds that have significant amounts of all components and to systems in which relative volatilities are not large. If the sidestream is a liquid, a stripper can be added that removes some of the light impurity in the liquid sidestream coming from the main tower. 

To illustrate this configuration, we take the same DME/MeOH/water system studied in Section 10.1, but increase the feed concentration of DME from a small 5 mol% up to a significant 35 mol%. 

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Sidestream column with stripper. Higher-purity sidestream products can be obtained if a stripping column is used in conjunction with a sidestream column. The liquid drawoff stream from the main column is fed onto the top tray of a stripper. The stripper has a reboiler, which produces vapor to strip out most of the light component A. 

The side-stripper can be modeled as two columns in the thermally coupled indirect sequence, as shown in Figure 11.13. The first column is modeled as a conventional column with a partial condenser and partial reboiler. The second column is modeled as a sidestream column with a liquid sidestream on stage above the feed stage4. The vapor entering the condenser and liquid leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). This allows... 

This configuration adds two degrees of freedom to the conventional column (the liquid flowrate to the stripper and the heat input to the stripper reboiler), so two compositions in the sidestream product from the stripper can theoretically be controlled. The control system shown in Fig. 6.23a uses stripper reboiler heat input to control the impurity of A in the sidestream product. The impurity of C in the sidestream product is controlled by manipulating the flowrate of liquid to the stripper. This system presents a highly interacting 4X4 multivariable control problem. Therefore in practice it may be more effective to control only one composition (or temperature) in the stripper and one temperature in the main column, with the flowrate of reflux and liquid to the stripper flow controlled. 

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. 

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