Steam distillation is used where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed. In this distillation type, steam is passed directly into the liquid in the still, but it should be kept in mind that the solubility of the steam/water must be very low otherwise it would contaminate the product and one will have to bear more cost to separate water from the product. Steam disillation is perhaps the most common example of differential distillaion.
Explanation:
Two cases may be considered in distillation using steam. The steam may be superheated and so provide sufficient heat to vaporize the material concerned, without itself condensing. The second case will exist when some of the steam may condense, producing a liquid water phase.
If there is no liquid phase present, then from the phase rule there will be two degrees of freedom,
F = C - P + 2
=> F = 2 - 2 + 2
=> F = 2
F = degrees of freedom (tells about the number of parameters to be essentially specified to specify a system)
C = no. of components of the solution (binary solution considered here, hence 2 components)
P = no. of phases (2 in this case i.e, steam and liquid solution phase)
Both the total pressure and the operating temperature can be fixed independently, and partial pressure of high volatile component, which must not exceed the vapor pressure of pure water, if no liquid phase is to appear (since at higher partial pressure than water will cause steam to condense, as at high pressure boiling point increases. See Effect of pressure on boiling point of liquid )
When a liquid water phase is present (3 phases will exist , there will be only one degree of freedom),
F = C - P + 2
=> F = 2 - 3 + 2
=> F = 1
here, water will form a separate phase since being polar it is not miscible with the organic solution which is a non-polar one.
Since, degree of freedom for this case is 1, therefore, selecting one variable (temperature or pressure) fixes the system, with the water and the other component each exerting a partial pressure equal to tis vapor 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 temperature below 373K at atmospheric pressure. By using reduced operating pressures, the distillation temperature may be reduced still further, with a consequent economy of steam.
Comparison of the two cases:
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 partial pressure of the component 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 the 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.
Explanation:
Two cases may be considered in distillation using steam. The steam may be superheated and so provide sufficient heat to vaporize the material concerned, without itself condensing. The second case will exist when some of the steam may condense, producing a liquid water phase.
If there is no liquid phase present, then from the phase rule there will be two degrees of freedom,
F = C - P + 2
=> F = 2 - 2 + 2
=> F = 2
F = degrees of freedom (tells about the number of parameters to be essentially specified to specify a system)
C = no. of components of the solution (binary solution considered here, hence 2 components)
P = no. of phases (2 in this case i.e, steam and liquid solution phase)
Both the total pressure and the operating temperature can be fixed independently, and partial pressure of high volatile component, which must not exceed the vapor pressure of pure water, if no liquid phase is to appear (since at higher partial pressure than water will cause steam to condense, as at high pressure boiling point increases. See Effect of pressure on boiling point of liquid )
When a liquid water phase is present (3 phases will exist , there will be only one degree of freedom),
F = C - P + 2
=> F = 2 - 3 + 2
=> F = 1
here, water will form a separate phase since being polar it is not miscible with the organic solution which is a non-polar one.
Since, degree of freedom for this case is 1, therefore, selecting one variable (temperature or pressure) fixes the system, with the water and the other component each exerting a partial pressure equal to tis vapor 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 temperature below 373K at atmospheric pressure. By using reduced operating pressures, the distillation temperature may be reduced still further, with a consequent economy of steam.
Comparison of the two cases:
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 partial pressure of the component 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 the 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.
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