Since the reflux ratio is 1.07, the reflux drum level can be controlled by either distillate or reflux. We select distillate flow to control level to gain the advantage of proportional-only control smoothing out the disturbances to a downstream process. The reflux is manipulated to control the temperature on stage 3 at 339.5 K, which maintains the purity of the DME product. Figure 10.10 gives the temperature and composition profiles in the column. Note that the BuOH vapor composition yBuOH is smaller than the BuOH liquid composition xBuOH on all trays below the feed tray. This illustrates why the sidestream is removed as a vapor instead of a liquid.
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