Two Pressure Operation

The location of the azeotropic point depends on pressure, its position in the equilibrium diagram shifts to the right with a decrease in pressure. A higher fraction of more volatile component is then found in the azeotropic mixture. The separation of such a liquid mixture by distillation is based on this fact. Components are completely miscible and form a minimum or maximum azeotrope without a third component. 

In Fig. 2-30, this rectification separation process, in two columns operated at two different pressure levels, is explained as a tv/o pressure process for a binary mixture. The binary mixture consists of components 1 and 2, with mole fraction Xp of the low-boiling component 1. In the first column, operated at a lower pressure Pqj, the binary mixture is separated into component 2 as the bottom product, and an azeotropic mixture of composition, as an overhead product. In the second column, operated at a pressure Pg2 Pgi l he azeotropic mixture is separated into component 1 (at the bottom) and azeotropic mixture x 2 the top). The azeotropic mixture of the second column is then fed into the side of column 1 at an appropriate location. 

The two pressure process is particularly economic if the reboiler of the first column is used to condense the top vapor of the second column, with the overhead product acting as the heating medium. This operating mode is possible if there is a large enough difference in the pressure levels of the two columns and if the boiling points of both components 1 and 2 are not too far apart. 

RC2 Rectification columns Condenser Reboiler Phase separator Feed of component 1 and 2 Liquid phase 1 with a mole fraction Xpi of component 1 Liquid phase 2 with a mole fraction Xf2 of component 2 A Azeotrope 

The shifting of A] A2 is of practical use in separating water rich isopropanol/water mixtures (Fig. 2-31 b). A feed/ is almost completely separated into water flow rate JV leaving column DCl and a isopropanol flow rate P leaving column DC2. Both columns are operated at ambient pressure. The overhead products AT, and JC2 are practically of azeotropic composition (point Aj, Fig. 2-31 a). In the condenser C, most of the condensed overhead products form the reflux / , or / 2, to the columns DCl or DC2, respectively. The remainder is then separated into a water rich fraction i and an alcohol rich fraction 2 in the diffusion distillation unit DA (Fig. 2-31 c shows the diffusion separation unit). Some of the overhead product fluxes Kj and K2 are heating the unit while water is used for cooling. 

---