Liquid Sidestream Column

Liquid sidestream columns are frequently used when the feed stream is a ternary mixture in which the concentration of the lightest component is small. This lightest component is removed in the distillate product. The intermediate component is removed in a sidestream that is withdrawn as a liquid from a tray higher up in the column above the feed tray. A liquid sidestream is used because the concentrations of the lightest component in the liquid phase on all the trays above the feed tray are smaller than the concentrations of the lightest component in the vapor phase. This indicates a limitation in the application of a liquid sidestream column; the volatility between the lightest and intermediate components must be fairly large. If this volatility is too small, a high-purity sidestream cannot be produced.

10.1.1 Steady-State Design

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 flowrate is 100kmol/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 The liquid sidestream is withdrawn from stage 12.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 specifications:

1. Distillate impurity is set at 2 mol% MeOH by varying the distillate flowrate.

2. Sidestream impurity is set at 2 mol% water by varying the sidestream flowrate.

3. Bottoms purity is set at 2 mol% MeOH by varying the reflux flowrate.

Note that the other impurity in the sidestream (DME) cannot be specified with a fixed column configuration because there are no remaining degrees of freedom. However, the number of stages and the locations of the feed and the sidestream can be changed to alter the sidestream DME composition.

The key separation in this liquid sidestream column is between DME and methanol in the section above the feed tray. Since all 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 methanol 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 ^24. Thus the vapor composition of 4.04 mol% DME has a liquid in equilibrium with it that is only 0.16 mol% DME. The column diameter is 0.61 m. The reboiler heat input is 1.346 MW.

Figure 10.1 gives the flowsheet and steady-state parameters for the process. Note that the distillate flowrate is quite small because of the low concentration of DME in the feed. This gives a very high reflux ratio (RR = 41), which means that the reflux drum level will have to be controlled by manipulating the reflux flowrate.

323 K 11 atm

323 K 11 atm

Sidestream Reflux

Figure 10.1 Liquid sidestream.

Water Product 44.90 kmol/h 0.02 MeOH 0.98 Water

Figure 10.1 Liquid sidestream.

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