High-boiling azeotrope

Note that a high-boiling azeotrope is formed between acetone and chloroform. The above problem description and mixture analysis helps us to define the CAMD problem. 

For a high boiling azeotropic system, the bubble and dew point temperature curves meet at a maximum in the Txy diagram. Mixtures of nitric acid and water form exhibit a high boiling azeotrope this system is shown in Fig. 3.6. 

Figure 3.6 Phase diagram for mixtures of nitric acid and water, which has a high boiling azeotrope (a) temperature-composition diagram at 1 atm, and (b) pressure-composition diagram at 25°C. Data taken from MA Yakimov and VYa Mishin, Radiokhimiya 6. 543 (1964).

As slated above, a separation by distillation involves differing volatilities of the components to be separated. If the volatility difference is so small diet an adequate separation cannot be made, it may be possible to increase the volatility difference by the addition of an extraneous material that can be separated later this process is known as extractive distillation if the added material is relatively nonvolatile and leaves the column with die residue. If the added material forms an azeotrope with one or mote of the components df the mixture and in so doing enhances the separability of the original mixture, azeotropic distillation results. In this last-namad mode, the extraneous material, or azeotropic agent, may lenve the column in the distillate (low-boiling azeotrope) or in the residue (high-boiling azeotrope). 

A low-boiling azeotrope (much the more common) will occur if 72 > a, vvhile a high-boiling azeotrope will occur if 1/a >... 

In this case, the dimethylamine is very much the most volatile component of the system and under fractionation rapidly moves up a column. DMF forms a high-boiling azeotrope with formic acid and this moves to the column base. 

DMAc is very similar in properties to DMF but is 17 times more stable in alkaline conditions and 2 times more stable in acidic. Like DMF, DMAc forms a high boiling azeotrope with its constituent acid (acetic acid in this case) during distillation from water and the dimethylamine formed, being very much the most volatile constituent in the system, moves quickly to the top of the fractionating column. The column tops tend to be very evil smelling and may need to be treated to make their discharge acceptable but the recovered distillate can be taken as a side stream if its odour is important. DMAc is appreciably more expensive than DMF but it is an example of a solvent which is overall more economical because its recovery in many applications is cheaper. 

Figure 9.16 Different types of liquid-vapor phase diagrams for a binary liquid mixture of component A and B as functions of the mole fraction of the component with the higher boiling temperature, (a) The phase diagram for a system with a low-boiling azeotrope (minimum boiling point) and (b) the phase diagram for a system with a high-boiling azeotrope (maximum boiling point). The arrows show how the paths for various distillation processes depend upon the position of the initial composition relative to the azeotrope.

Figure 9.16(a). Solutions with a high-boiling azeotrope [Figure 9.16(b)] can be purified by distillation, but the component that is separated depends upon which side of the azeotrope the initial concentration lies. For example, following the fractional distillation pathway a-d in Figure 9.16(b) will eventually lead to a solution that is nearly pure component B and the pathway e-h will lead to nearly pure component A. 

Explain in terms of molecular interactions why the temperature-composition phase diagram for chloroform and acetone possesses a high-boiling azeotrope. 

Figure 3.3.9 Isobaric liquid-vapor equilibrium of a low-boiling azeotrope (a) and a high-boiling azeotrope (c) at 1.013 bar (b) an almost ideal solution for comparison.

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