According to Dalton's law of partial pressure, when a material immiscible with water coexists with water, the vapor pressure of the whole system should be the sum of the vapor pressures of the components, i.e.
p= pA+ pB
Where p is the total vapor pressure, pA is the vapor pressure of water, and pB is the vapor pressure of immiscible material with water.
The temperature at which the vapor pressure of each component of the mixture is equal to the atmospheric pressure is the boiling point. This boiling point is lower than the boiling points of the components. Thus, by applying water vapor distillation at atmospheric pressure, high boiling point components can be steamed with water all the way out at an environment below 100℃. Since the total vapor pressure is independent of the relative amount of the two in the confound, the temperature does not rise to the boiling point of the liquid remaining in the flask until one of the components is almost completely removed. We know that the ratio of the partial pressures of each gas (pA, pB) in the vapor of a confount is equal to the ratio of their substance quantities (nA, nB), i.e.
The nA = mA/mA; NB = mB/mB. Among them
mA and mB are the masses of the vapor of each substance in A definite volume, and MA and MB are the masses of substances A and B with respect to the molecules. Based on:
It is clear that the quantities of these two substances with respect to the distillate (that is, their quantities with respect to the vapor) are proportional to their vapor pressures and to their molecular masses.
Aniline, for example, has a boiling point of 184.4℃ and is immiscible with water. When water and aniline are heated all the way to 98.4℃, the vapor pressure of water is 95.4 kPa and that of aniline is 5.6 kPa. Their total pressure is close to the atmospheric pressure, so the liquid begins to boil, and aniline is distilled along the way. The relative mass of water and aniline are 18 and 93 respectively.
That is, 3.3 g of water can be steamed to hold or carry out 1 g of aniline. The composition of aniline in solution is 23.3%. The amount of water distilled in the test is often more than the calculated value, because aniline is slightly soluble in water, some water vapor in the test is not very recent to contact with aniline and then leaves the distillation beaker.
When water vapor distillation is used to disperse and purify a substance, the vapor pressure of the substance at 100 ° C must be at least 1.33 kPa. If the vapor pressure is between 0.13 and 0.67 kPa, its content in the distillate is only 1% or even lower. In order to increase the content of the distillate, a way must be found to increase the vapor pressure of the substance, that is, to raise the temperature of the vapor above 100℃, which is to use superheated water vapor distillation. For example, benzaldehyde (boiling point 178℃), water distillation, boiling at 97.9℃, at this time pA=93.8 kPa, pB=7.5 kPa, then:
Benzaldehyde accounted for 32.1% in the distillate at this time.
If the introduction of 133℃ superheated steam, benzaldehyde vapor pressure can reach 29.3kPa, so as long as there is 72 kPa water pressure, can make the system boiling, then:
Thus the benzaldehyde content in the distillate was increased to 70.6%.
The application of superheated water vapor also has the advantage of less condensation of water vapor. In order to prevent condensation of superheated steam, it can be kept warm under the distillation bottle or even heated.
From the above analysis, it can be seen that the application of water vapor distillation of this dispersion is conditional, the purified material must have the following conditions: (1) insoluble or insoluble in water; (2) coexist with boiling water without chemical reaction; (3) at 100℃ must have a positive vapor pressure (generally not less than 1.33 kPa).
