Analysis using Constant Pressure Phase Diagram: Benzene-Toluene System (ideal solution) at 1 atm

See the Figure below. Consider a container whereby a fixed amount of liquid benzene-toluene mixture is gradually heated.

Heating of Benzene-Toluene Mixture

We will study the changes in composition (mole fraction benzene) and temperature as distillation progresses from point (a) to point (e) using the constant pressure phase diagram as shown in the Figure below. The original concentration in the benzene-toluene mixture is 0.40 mole fraction benzene (i.e. 0.60 mole fraction toluene).

Phase Diagram for Benzene-Toluene

Point (a): Liquid-phase; no vapour Temperature 86 oC Concentration of benzene Liquid: x = 0.40; Vapour: y = 0.00

Point (b): Liquid-phase; first bubble of vapour produced Temperature 95.2 oC (Bubble Point - solution about to boil) Concentration of benzene Liquid: x = 0.40; Vapour: y = 0.61

Point (c): Vapour-Liquid Mixture; continued vaporization of liquid as heat is added Temperature 98.0 oC Concentration of benzene Liquid: x = 0.31; Vapour: y = 0.52

Point (d): Vapour-phase; last droplet of liquid remains Temperature 101.6 oC (dew point) Concentration of benzene Liquid: x = 0.21; Vapour: y = 0.40

Point (e): Vapour-phase only, no liquid Temperature 108 oC The process is unsteady-state in nature.


NOTE: from the above Figure

As heating continues, vaporization occurs over a range of boiling points.

At any time when 2 phases - vapour and liquid - are present, the concentration of benzene (more volatile component) is the vapour (y) is always higher than that in the liquid (x). The reverse is true for toluene (less volatile component).

For a fixed original amount of benzene-toluene mixture; the temperature, mole fractions in the vapour phase and liquid phase change continuously over the entire distillation process: both x and y decreases as temperature increases.

An important observation here that has direct impact on the analysis of distillation process is that for separation by distillation to be useful, 2 phases must be present - vapour and liquid. The more volatile component will distribute itself more in the vapour phase than the liquid phase. No separation is achieved if all the liquid is vapourised.

Analysis of concentration changes using phase diagram does not take into consideration the quantity changes: as heating continues, the quantity of liquid remaining in the container gradually decreases as more is turned into vapour. The quantity of vapour produced increases.

Another important observation is that as the quantity of vapour obtained increases, the concentration of the more volatile component in the vapour decreases: see for examples, changes from point (b) to (d). This is because as heating continues and temperature in the residual liquid increases, more of the less volatile component is being vapourized as well.

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