To illustrate the latter limitation, suppose that we take the same ternary mixture considered in the previous section. But now the feed composition is 45 mol% DME, 50 mol% MeOH, and 5 mol% water. Can a vapor sidestream column be used effectively? 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 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 5.4 mol% water, the vapor composition is 3.2 mol% 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 flowrates of the vapor and liquid in the column, specifically, increasing the reflux ratio. This renders the sidestream configuration uneconomical for this chemical separation.
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