2024.9.04
Chemistry at pirika.com > Chemstry > Chemical engineering > Reprint: gas-liquid equilibrium estimation by ASOG. > Chapter 1: Basic equations of solution theory
Between the temperature T, pressure P and volume V of an ideal gas,
the relation PV = nRT (n: number of moles, R: molar gas constant) is established.
In the case of real gases, this relationship does not hold due to the volume of molecules in space (correction for V is necessary) and the interaction between molecules (correction for P is necessary). You may have learnt Boyle’s law and Charles’ law in high school chemistry. The ideal gas is the one in which the Boyle-Charles law holds.
A variant of the ideal gas equation of state is the number of moles, n = PV/RT, so the molar ratio of alcohol and water in the gas phase is the ratio of pressure (since volume and temperature are common).
Vapour pressure of water at 87.135 °C = 481 mmHg, vapour pressure of ethanol = 1038.9 mmHg, so the molar ratio is 1038.9/(481+1038.9) = 68.4 mol%.
This means that there is a lot of ethanol with a low boiling point in the gas phase.
As a thought experiment, consider extracting only this gas phase. On cooling, a 68.4% molar solution of ethanol is obtained. At what temperature will this boil and what will the composition of the gas phase be at that temperature?
It will boil at a temperature lower than 87.135°C because the proportion of ethanol with a lower boiling point is increasing, and the proportion of ethanol in the gas phase will be even higher.
If this is repeated many times, the mole fraction of ethanol will become higher and higher.
In practice, the ethanol/water system cannot be concentrated beyond the azeotropic point at which the liquid and gas phase compositions are equal.
This is the basic concept of distillation separation. It is the basis for separation processes that are essential for manufacturing processes, such as the separation of petrol and paraffin components from crude oil, and the distillation separation of impurities from chemicals.
So far, this assumes that the ethanol/water mixture is an ideal solution. In reality, an ideal solution is formed only by homologues that are similar in structure and size, such as benzene-toluene and chloroform-carbon tetrachloride.
Next section: 1.3 To real solutions
Copyright pirika.com since 1999-
Mail: yamahiroXpirika.com (Replace X with @.)
The subject line of the email should start with [pirika].