Using the figure, give the approximate composition of the distillate that initia

Question

Using the figure, give the approximate composition of the distillate that initially would be obtained from the simple distillation of a solution containing 50% A and 50% B. Explain how you arrived at your answer.

As a general rule a mixture of any two substances which boil at least 80degree apart may be separated by a single simple distillation. The lower boiling or more volatile component distills more rapidly and is richest in the first fraction of the condensate. The higher boiling or less volatile component is the chief constituent of the last fraction of the condensate In a good fractional distillation, we can distill off and collect pure A, switch receiving flasks and then collect pure B. A convenient separation can be achieved by means of a fractionating column (Figure 2). This is an ingenious device which accomplishes a whole series of miniature separations in a single, continuous operation. In principle, a fractionating column provides an extensive surface for heat interchange between ascending vapor and descending condensate. This makes possible a whole series of partial vaporizations and partial condensations (called reflux) along the length of the column. A simple column can be packed with a wide variety of inert materials with extensive surface area: glass beads or helices, metal helices or gauze etc. As the hot vapor rises through the packing, it condenses in stages throughout the column. The condensate trickles down over the packing. As it does so, there is a continuous exchange of heat between the cooler descending condensate and the warmer ascending vapor of the surface of the packing. As the condensate at any point in the column accepts heat from the vapor, part is revaporized and the vapor formed is richer in the more volatile component than the condensate. At the same time, as the vapor loses heat to the condensate part of it condenses. The condensate is richer than the vapor with respect to the high boiling component. This process, repeated many times throughout the entire length of an efficient column, finally produces vapor of the lower boiling component which passes through the side arm of the distilling head on top of the column into the condenser. The residue in the distilling flask meanwhile becomes continuously richer in the higher boiling component. Throughout the length of the column there is a temperature gradient, from the boiling point of the volatile component at the top of the column, to the boiling point of the mixture in the flask, at the bottom of the column (Figure 3). Figure 4 is a phase diagram that explains fractional distillation of an ideal two component system.

Explanation / Answer

From the fig. 4 it is seen that at initial 50% A and 50% B (L3), the vapour phase composition is V3 ( 80% A and 20% B). Re-vaporization of this at 55 oC wil give 95% A and 5% B (V4). this will be the requird composition

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