Liquid Sidestream Column

The specific numerical case used is a ternary mixture of dimethyl ether (DME), methanol (MeOH), and water. The feed composition is 5 mol% DME, 50 mol% MeOH, and 45 mol% water. The feed flow rate is lOOkmol/h, and the feed is fed on Stage 32 of a 52-stage column. The liquid sidestream is withdrawn from Stage 12. The column pressure is set at 11 atm so that cooling water can be used in the condenser (reflux-drum temperature is 323 K with a distillate composition of 98 mol% DME and 2 mol% MeOH). The NRTL physical property package is used. 

The presence of a sidestream provides an additional degree of freedom. Three purities can be set. We use three Design Spec/Vary functions to achieve the following 

Distillate impurity is set at 2 mol% MeOH by varying the distillate flow rate. 

Bottoms purity is set at 2mol% MeOH by varying the reflux flow rate. 

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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. 

Figure 10.5 Liquid sidestream column feed rate changes.

Figure 10.8 Liquid sidestream column feed composition changes.

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 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... 

Figure 6.19 Single liquid and vapor sidestream columns.

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. 

Another entire column with a partially vaporized feed, a liquid side-stream rate equal to D withdrawn from the second stage from the top, and a total condenser is shown in Fig. 13-27. The specified compositions are Zy = 0.40, x = 0.05, and Xd = 0.95. The specified L/V ratio in the top section is 0.818. These specifications permit the top operating line to be located and the two top stages stepped off to determine the liquid sidestream composition x, = 0.746. The operating line below the sidestream must intersect the diagonal at the blend of the side-stream and the overhead stream. Since S was specified to be equal to D in rate, the intersection point is... 

Figure 5-3 Jacobian matrix for a complex column with a liquid sidestream withdrawn from plate p.

Modifications of the equations for conventional columns which are needed in order to obtain approximate solutions for complex columns are presented below for the special case of a column having one liquid sidestream withdrawn from stage number (kx + 1). 

For all examples column pressure = 400 lb/in2 abs, partial condenser, thermal condition of the feed is boiling-point liquid. Initial temperature profile linear between the pinches (252-282°F) for all examples. For Examples 11-7 and 11-8 the initial temperature profile was taken to be linear between 227 and 242°F for plates 1 through 5 and the temperature of the distillate was taken to be 217°F. Initial vapor rates V — V2 for all j. In Example 11-7 the liquid sidestream is withdrawn from plate 6 (the accumulator is assigned the number 1), and in Example 11-8, it is withdrawn five plates above the feed plate. Use the equilibrium and enthalpy data given in Tables B-2 and B-24 of the Appendix. 

Solution llx It is first important to note that the benzene// -xylene/toluene system may be approximated quite well with constant relative volatility behavior. Furthermore, the given product specifications are all sharp. It is therefore appropriate to use the TT approach delineated in this section. For simplicity, we shall also assume that the liquid sidestream is removed as a liquid product. It has also been shown in this section that an entire Petlyuk column operating under the aforementioned conditions can be represented with a region of feasibility in the domain. This condition... 

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. 

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 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. 

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