Steam Distillation

Where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed, the process of steam distillation may be used. Steam is passed directly into the liquid in the still and the solubility of the steam in the liquid must be very low. Steam distillation is perhaps the most common example of differential distillation.

Two cases may be considered. The steam may be superheated and so provide sufficient heat to vaporise the material concerned, without itself condensing. Alternatively, some of the steam may condense, producing a liquid water phase. In either case, assuming the ideal gas laws to apply, the composition of the vapour produced may be obtained from:

where the subscript A refers to the component being recovered, and B to steam, and: m = mass,

M = molecular weight, PA, PB = partial pressure of A, B, and P = total pressure.

If there is no liquid phase present, then from the phase rule there will be two degrees of freedom, both the total pressure and the operating temperature can be fixed independently, and PB = P — PA, which must not exceed the vapour pressure of pure water, if no liquid phase is to appear.

When a liquid water phase is present, there will be only one degree of freedom, and selecting the temperature or pressure fixes the system, with the water and the other component each exerting a partial pressure equal to its vapour pressure at the boiling point of the mixture. In this case, the distillation temperature will always be less than that of boiling water at the total pressure in question. consequently, a high boiling organic material may be steam-distilled at temperatures below 373 K at atmospheric pressure. By using reduced operating pressures, the distillation temperature may be reduced still further, with a consequent economy of steam.

A convenient method of calculating the temperature and composition of the vapour, for the case where the liquid water phase is present, is by using the diagram shown in Figure 11.47 which is due to Hausbrand(52) , where the parameter (P — PB) is plotted for total pressures of 101.3, 40 and 9.3 kN/m2, and the vapour pressures of a number of other materials are plotted directly against temperature. The intersection of the two appropriate curves gives the temperature of distillation, and the molar ratio of water to organic material is given by (P — PA)/PA. Thus, if nitrobenzene is distilled at atmospheric pressure with live saturated steam, the boiling point will be about 372 K and the mass-ratio of water to nitrobenzene in the vapour will be:

Where there is no liquid water phase present, the steam consumption will be high unless the steam is very highly superheated. With a water phase present, the boiling point of the mixture will be low, and consequently PA will have a low value. Thus, on a molar basis, the steam consumption will again be high, although due to the relatively low molecular weight of steam, the consumption may not be excessive. Steam economy may be effected by using indirect heating of the still, having no liquid water phase present, or by operating under reduced pressure.

In an operation of this kind, illustrated in Figure 11.48, it is essential that the separation of the material being distilled from the water should be a relatively simple operation.

where the subscript A refers to the component being recovered, and B to steam, and: m = mass,

M = molecular weight, PA, PB = partial pressure of A, B, and P = total pressure.

Steam Distillation
Figure 11.47. Vapour pressure curves for steam distillation calculations
Benzene Steam Destilllation

Figure 11.48. Steam distillation

In determining the number of stages required to effect a steam distillation, the steam flow must be included in the operating-line equation for the lower part of the column. Using indirect heating and assuming constant molar overflow, the lower operating line for the organic material is given by:

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Responses

  • lavinia
    When Nitro benzene is vapour refulxe?
    4 years ago
  • MEDORO
    How is the molecular weight of an organic liquid determinedby steam distillation?
    2 years ago

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