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%.
The feed flowrate is 100 kmol/h, and the feed is fed on stage 32 of a 52-stage column. A liquid sidestream is withdrawn from stage 12 and is fed into a small six-stage stripping column. The flowsheet is given in Figure 10.15. The bottoms from the stripper is the MeOH product. The vapor from the stripper is fed back to the main column.
The presence of a stripper provides additional degrees of freedom. In addition to being able to adjust the liquid sidestream drawoff rate from the main column, the heat input to the stripper reboiler can also be adjusted. Of course, the stripper can do nothing to change
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