Vapour Liquid Equilibrium

Aim
To determine the vapour-liquid equilibrium data for the given binary system. (A = more volatile component. B = less volatile component). 1. To check the thermodynamic consistency of the data. 2. To present the thermodynamic characteristics and constants of Margules equation for the given system.

Theory
Vapour-liquid equilibrium data are the basic information of the system required for the design of equilibrium stages of a vapour-liquid separation equipment like distillation. Equilibrium data represent the composition of the mixture in the vapour phase (Y ) and that in the corresponding equilibrium liquid phase (X) at equilibrium. The compositions are presented in mole fractions of the more volatile component (A). Equilibrium compositions are functions of temperature and pressure. Therefore the data are reported under isothermal or isobaric conditions. However, isobaric data are required more often because industrial separations are carried out at constant pressure. 1. The vapour-liquid equilibrium data are presented as equilibrium compositions:
YA = PA γA XA = KA XA πA YA /(1 − YA ) XA /(1 − XA )

(33.1) (33.2)

2. Relative Volatility: αAB =

3. The Gibb's-Duhem equation for the binary system for checking thermodynamic consistency of the data by the integral test: 1 γ1 ln dXA = 0 (33.3) γ
0 2

4. The Van Laar equations are log γA = a 1+ aXA bXB 2;

log γB =

b 1+ bXB aXA

2

(33.4)

Many systems form an azeotrope where the equilibrium vapour and liquid compositions are the same i.e. YA = XA

Experimental setup
The apparatus shown in Figure 33.1 is known as a vapour-liquid equilibrium still and has two essential parts: (1) Vaporizer, (2) Flash chamber or Equilibrium chamber. The binary liquid mixture is vapourized by an electrical heating coil in the vaporizer at a slow rate. The vapour from this rises to the equilibrium chamber. Vapour and liquid equilibrium at the temperature and pressure 118

Figure 33.1: Schematic diagram of VLE setup

119

of the chamber is established. The temperature or the chamber is recorded by the thermometer which extends well inside the equilibrium chamber. The pressure of the chamber may be regulated by connecting the vapour line to vacuum or pressure header. The vapour and liquid phases separate from each other in the ash chamber. The liquid ows down the liquid line through a water cooled joint to the feeder line, through which the mixture again goes back to the vaporizer. The still may be drained through the feeder line and may be charged through either the vapour or the liquid line.

Procedure
1. Prepare a calibration chart of composition vs. refractive index (R.I.) or density, for the two pure components A and B and their mixtures with XA varying from 0 to 1. Prepare the mixture of various compositions and determine the R.I. or density. 2. Prepare about 200 ml of mixture of A and B. The experiment should be started with the mixture richer in more volatile component (say 160 ml of A and 40 ml of B) so that in subsequent readings temperature increases. This mixture should be lled in the still through either of the limbs. 3. Check carefully that the electrical coil is completely submerged in