Vapor Sidestream Column

Vapor sidestream columns are frequently used when the feed stream is a ternary mixture and the concentration of the heaviest component is small. The other criterion is that the relative volatility between the intermediate component and the heaviest component must be fairly large. 

To illustrate the latter limitation, suppose we take the same ternary mixture considered in Section 10.1. However, now the feed composition is 45 mol% DME, 50mol% MeOH, and 5 mol% water. Can a vapor sidestream column be effectively used The normal boiling point of MeOH is 337.7 K. The normal boiling point of water is 373.2 K. This is a much smaller difference than is the case for DME and MeOH (248.4 K vs. 337.7 K). The result is a relative volatility between MeOH and water of about 1.73. 

All the water in the feed must flow down past the vapor sidestream drawoff tray. If the liquid composition on this tray is 5.4mol% water, the vapor composition is 3.2mol% water. Thus, the purity of the sidestream is low. The only way to reduce the impurity of water in the sidestream is to reduce the water concentration in the liquid by drastically increasing the internal flow rates of the vapor and liquid in the column, that is, increase the RR. This makes the sidestream configuration uneconomical for this chemical separation. 

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Figure 6.19 Single liquid and vapor sidestream columns.

In order to consider a reasonable system to illustrate a vapor sidestream column, we change the feed stream to contain -butanol (BuOH) instead of water. The normal boiling point of n-butanol is 390.8 K compared with 337.7 K for MeOH. This produces a relative volatility of about 4.4 thus, a vapor sidestream product with only 1 mol% BuOH can be produced with an RR of 1.07. The composition of the liquid on the sidestream drawoff tray is 4.3 mol% BuOH. 

The column has 51 stages, and the feed is introduced on Stage 21. The vapor sidestream is withdrawn from Stage 41. The column operates at 11 atm. Figure 10.9 gives the flowsheet with stream conditions and design parameters for this vapor sidestream column with a ternary feed stream of composition 45 mol% DME, 50 mol% MeOH, and 5 mol% BuOH. The column diameter is 0.635 m. The reboiler heat input is 1.075 MW. 

Figure 10.13 (a) Vapor sidestream column feed rate changes, (b) Feed rate +20% change with and without ratio. 

Figure 10.14 Vapor sidestream column feed composition changes.

The development of a control structure for this complex system turned out to be more difficult than for the stripper flowsheet. The initial control scheme evaluated is shown in Figure 10.21. It is a logical extension of the control structure used for the vapor sidestream column in which the vapor sidestream is ratioed to the reboiler heat input. As we will demonstrate below, this structure worked well for some disturbances, but it could not handle decreases in the composition of MeOH in the feed, resulting in a shutdown of the unit. 

FIG. 13-33 Typical construction for a sidestream showing the intersection of the two operating lines with the q line and with the x — q diagonal, (a) Liquid sidestream near the top of the column, (h) Vapor sidestream near the bottom of the column. 

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... 

Avoid nesting control loops. Control loops are nested if the operation of the external loop depends on the operation of the internal loop. Figure 8.11 illustrates a nested loop. A vapor sidestream is drawn off a column to hold the column base level, and a temperature higher up in the column is held by heat input to the reboilcr. The base liquid level is ouly affected by the liquid stream entering and the vapor boiled off, and therefore it is not directly influenced by the amount of vapor sidestream withdrawn. Thus the base level... 

The distillation column sketched in Fig. P8.ll has an intermediate leboiler and a vapor sidestream. Sketch a control concept diagram showing the following control objectives ... 

The conventional flowsheet to separate a ternary mixture uses two distillation columns in series. It is sometimes more economical to use a single distillation column with a sidestream. This is particularly true when product purities are moderate to low. Consider the case where the ternary mixture contains components A. B, and C, with decreasing relative volatilities. Figure 6.19 shows two common situations a liquid sidestream is withdrawn from a tray somewhere above the feed tray, or a vapor sidestream is withdrawn from a tray somewhere below the... 

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. 

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. 

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