Sidestream column

Liquid sidestreams are used when there is less A in the feed than B or C. The vapor passing up through the sidestream drawoff tray contains all the A that is in the feed. The concentration of A in the liquid phase on this tray is smaller than its concentration in the vapor phase since A is the most volatile component. Therefore we withdraw a liquid sidestream. In the case where there is less C in the feed than A or B, we use a vapor sidestream below the feed. The concentration of the sidestream impurity C is less in the vapor phase than in the liquid phase since C is the heaviest component. 

Similar problems can occur with vapor sidestreams, but the solution is not as easy because we cannot provide vapor holdup in the system. One approach is to use an internal vapor controller. The flowrate of the vapor sidestream and the flowrate of the steam to the reboiler are measured. The net flowrate of vapor up the column above the vapor sidestream drawoff tray is calculated. This flow is then controlled by manipulating the vapor sidestream drawoff rate. 

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. 

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. 

This process adds additional degrees of freedom, which we may want to utilize. The control scheme shown in Fig. 6.23c is simple and practical. Reflux flowrate in the first column is flow-controlled and heat input prevents A from dropping out the bottom of the first column. Heat input to the second column controls the impurity of B in the bottoms product. Sidestream flowrate controls the purity of the sidestream product. Reflux flowrate in the second column controls the impurity of B in the distillate product. 

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In summary, single-column sidestream arrangements can be attractive when the middle product is in excess and one of the other components is present in only minor quantities. Thus, the sidestream column only applies to special circumstances for the feed composition. More generally applicable arrangements are possible by relaxing the restriction that separations must be between adjacent key components. 

A simple model for side-rectifiers suitable for shortcut calculation is shown in Figure 11.12. The side-rectifier can be modeled as two columns in the thermally coupled direct sequence. The first column is a conventional column with a condenser and partial reboiler. The second column is modeled as a sidestream column, with a vapor sidestream one stage below the feed stage4. The liquid entering the reboiler and vapor leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). The vapor and liquid streams at the bottom of the first column can then be matched with the feed and sidestream of the second column to allow the calculations for the second column to be carried out. 

The optimization can be carried out using nonlinear optimization techniques such as SQP (see Chapter 3). The nonlinear optimization has the problems of local optima if techniques such as SQP are used for the optimization. Constraints need to be added to the optimization in order that a mass balance can be maintained and the product specifications achieved. The optimization of the side-rectifier and side-stripper in a capital-energy trade-off determines the distribution of plates, the reflux ratios in the main and sidestream columns and condition of the feed. If a partitioned side-rectifier (Figure ll.lOd) or partitioned side-stripper (Figure 11.lid) is to be used, then the ratio of the vapor flowrates on each side of the partition can be used to fix the location of the partition across the column. The partition is located such that the ratio of areas on each side of the partition is the same as the optimized ratio of vapor flowrates on each side of the partition. However, the vapor split for the side-rectifier will only follow this ratio if the pressure drop on each side of the partition is the... 

Figure 17.3 gives some comparisons of the performance of the multivariable DMC stmctuie with the diagonal stmcture. Three linear transfer-function models are presented, varying from the 2 x 2 Wood and Berry column to the 4 x 4 sidestream column/stripper complex configuration. The DMC tuning constants used for these three examples are NP = 40 and NC = 15. See Chap. 8, Sec. 8.9. 

Figure 6.19 Single liquid and vapor sidestream columns.

Figure 6.20 gives a control scheme for a single sidestream column. The flowrate of the sidestream can be manipulated, so we have an additional control degree of freedom. Three compositions can be controlled the impurity of B in the distillate (xDiS). the purity of B in the sidestream (xsS), and the impurity of B in the bottoms xBS). Note that we cannot control the two impurity levels (A and C) in the sidestream xsa and Xs,c because there are not enough degrees of freedom. 

Sidestream columns are also used in combination with other columns. Figure 6.23 gives three common configurations in which sidestream columns are linked to strippers, rectifiers, or prefractionators. 

Figure 6.23 Sidestream column with other columns, (a) With stripper (b) with rectifier (c) with prefractionator.

Sidestream column with rectifier. Figure 6.236 shows a process where a vapor sidestream is fed into a rectifying column to remove some of the C impurity in the vapor stream. A 4 X 4 multivariable control strategy... 

Sidestream column with prefractionator. Figure 6.23c illustrates a complex configuration in which a prefractionator column is used to perform a preliminary separation of the ternary feed. The idea is to produce a distillate from the first column that contains very little of the heaviest component C. When this distillate is fed into the second column at a location above the sidestream drawoff, there will be only a small amount of C that must flow down past the sidestream tray. This permits the production of high-purity sidestream product. Similarly the prefractionator should let very little of the lightest component A drop out the bottom so that there is little A in the vapor stream flowing past the sidestream tray. This lets us achieve high sidestream purities. 

Prefractionator reverse (ternary). Figure 6.24c shows a third alternative flowsheet that combines heat integration with a complex configuration. This system can be used to separate a ternary mixture. In the system shown, the sidestream column is run at high pressure and the prefractionator at low pressure. 

Reflux flowrate in the prefractionator is manipulated to keep the temperature profile where it should be, so that very little of the heaviest component goes out the top and very little of the lightest component goes out the bottom. Product purities in the sidestream column are controlled by reflux flowrate, sidestream flowrate, and heat input. 

Most sidestream columns nave a small flow dedicated to removing an off-key impurity entering the feed, and that stream must be manipulated to control its content in the major product. For example, an ethylene fractionator separates its feed into a high-purity ethylene sidestream, an ethane-rich bottom product, and a small flow of methane overhead. This small flow must be withdrawn to control the methane content in the ethylene product. The key impurities may then be controlled in the same way as in a two-product column. 

Doukas, N., and W. L. Luyben, "Control of Sidestream Column Separating Ternary Mixtures, Instrum. Technol. 25(6), 1978, p. 43. 

The sidestream column can be broken into a series of CSs in an analogous manner to Figure 6.1. Since sidestream withdrawal is essentially the reverse of multiple feed additions, the difference points obey the same linear mixing laws as shown in Figure 6.19. Therefore, all difference points and compositions are linked to each other via straight lines in composition space. 

Beneke, D.A. and A.A. Linninger, Graphical design and analysis of thermally coupled sidestream columns using colunm profile maps and temperature collocation. AlChE Journal, 2010. 

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 colmnn 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 medimn-purity propylene, and the distillate is high-purity polymer-grade propylene. 

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. 

The key separation in this liquid sidestream column is between DME and MeOH in the section above the feed tray. Because aU the DME in the feed must flow up the column past the sidestream drawoff tray, the concentration of DME in the vapor phase is significant. The liquid-phase concentration, however, is smaller if the relative volatility between DME and MeOH is large. The normal boiling points of these two components (DME = 248.4 K and MeOH = 337.7 K) are quite different. This gives a relative volatility at the sidestream drawoff tray of about 24. Thus, the vapor composition of 4.04 mol% DME has a liquid in equilibrium with it that is only 0.16mol% DME. The column diameter is 0.61 m. The reboiler heat input is 1.346 MW. 

The major control structure issues of this sidestream column are how to manipulate the sidestream flow rate and how to manipulate the distillate flow rate. They cannot be fixed or just ratioed to the feed flow rate because changes in feed composition require that the flow rate of the distillate and the sidestream change to achieve the desired purities. Figure 10.2 shows a control structure that provides effective control of this complex column. Figure 10.3 gives the controller faceplates. Note that two of the flow controllers are on cascade (remote set points). 

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