ABSTRACT/ SUMMARY.

Vapour-Liquid Equilibrium Unit is specially designed to investigate the relationship

between vapour and liquid at equilibrium for any binary system as well as for multipurpose

component system. This lab was performed primarily to investigate the relationship between

vapour and liquid by constructing an equilibrium curve for the methanol-water system at

atmospheric pressure. This experiment was started by feeding 0.1 L methanol and water into

the evaporator through valve V1. The V1 was immediately closed. The quantity of methanol

was added until its volume was equal to the volume of water which was 3.0 L. Then, the

temperature controller TIC-01 is set to about 100°C. The mixture of methanol and water was

heated until boiled. The vapour mixture was raised up and it was cooled down by the

condenser at the top of evaporator. When all the temperature and pressure had been stabilized,

it was then collected from the unit. The collected samples were analyzed by digital

refractometer in order to determine its composition. The equilibrium curve was plotted. The

result showed that at atmospheric pressure, the vapour and liquid possessed linear

relationship. The objective of this experiment was finally achieved and this experiment was

considered success.

INTRODUCTION.

Vapour-Liquid Equilibrium Unit is a unit that is specially designed so as to investigate

the relationship between the vapour and liquid at equilibrium for any binary system and also

for any multi-component system.The unit may be run at atmospheric pressure or at any

elevated pressure. The unit consists of a stainless steel evaporator that is fitted with electrical

heater and electronic sensors for the measurement of pressure and temperature of liquid and

vapour (Solution Engineering, n.d).

Moreover, a stainless steel coil type heat exchanger is mounted at the top of the

evaporator. It is also equipped with digital displays that mainly aimed at assisting in

monitoring the temperature and pressure of the vapour in vessel. in addition, the equilibrium

temperature is primarily adjusted by using the temperature controller (Solution Engineering,

n.d). This appliance is suited with pressure relief valve for safety purpose. This unit also is

well insulated in order to minimize the heat losses (Solution Engineering, n.d)..

Apart from that, vapour-liquid equilibrium is important for many engineering

application (Foxit Reader, 2010). Moreover, the thermodynamic basis for phase equilibrium is

quite similar to the chemical equilibrium, as claimed by Foxit Reader’s report concluded in

2010.

AIM.

This experiment is mainly aimed at finding the relationship between vapour and liquid by

constructing an equilibrium curve for methanol-water system at atmospheric pressure.

THEORY.

In this experiment, the concept of vapour-liquid equilibrium is recognized by

generating vapour-liquid equilibrium data for methanol-water system at constant pressure. By

definition, an ideal solution is one by which the vapour pressure of a particular component is

said to be proportional to the mole fraction of that component in the liquid phase over the

entire range of mole fraction (http://www.colby.edu).

The compositions are presented in mole fractions of the more volatile component.

Equilibrium compositions are functions of temperature and pressure. Hence, the data are

obtained based on the isothermal or isobaric condition.

Mole fraction= Mole of componentTotal number of moles present

It has been found that in a very dilute solution, the vapour pressure of solvent (major

component) is proportional to the mole fraction, X of the solvent (Foxit Reader, 2010).

Let we consider;

Xi- mole fraction of component i in the liquid phase

Yi - mole fraction of component i in the vapor phase

According to the Raoult’s Law, it states that for any component i, the partial pressure pi=yip

equals the vapour pressure of pure component i multiplied by its mole fraction xi in the liquid

phase, that is,

Raoult’s Law : yip=xipsat(T) eq.1

In Raoult’s law, the thermodynamic deviation is important as it can be generalized to the non-

ideal case (Foxit Reader, 2010). We may start from the general VLE condition µg,i = µl,i in

(eq. 1), which states that the chemical potential (which is equal to Gibbs energy) for each

component is the same in both phases at the given p and T. Now, the Gibbs energy is a state

function, and another route for taking component i from the liquid to the vapour phase can be

considered, consisting of four steps (all at temperature T): First, take the component I out of

liquid mixture. Then, the change in chemical potential for the “unmixing” is ∆µi,1 = −RT ln ai

where the activity is ai = γixi. For an ideal liquid mixture, the activity coefficient is 1, γi = 1.

Secondly, take the pure component as liquid from pressure p to the saturation pressure

xipsat(T). Since the liquid volume is small, this gives a very small change in chemical potential.

It is known as the Poynting factor, which we neglect, where ∆µi,2 ≈ 0.

Thirdly, evaporate the pure component at T and xipsat(T). Since we have equilibrium

(∆G = 0), there is no change in the chemical potential, ∆µi,3 = 0. In the gas phase, go from

pure component at pressure xipsat(T) to a mixture at p where the partial pressure is pi. Since

the initial and final state are in equilibrium, the sum of the change in chemical potential for

these four steps should be zero the formula derive

−RT ln xi + RT ln(pi/pisati (T)) = 0

Then, the Raoult’s Law follows.

The relationship between the vapour pressure and the temperature of pure substance can be

represented by the Antoine Equation (Darren Duncan, 2014). The expression is shown as

follows:

Log P =A− BC+T

And it can be transformed into this temperature explicit form;

T= B

A−log P−c , where P is the absolute vapour pressure of the substance.

T is the temperature of the substance. A, B, C are substance-specific coefficient. While Log is

typically Log10.

A simpler form of equation with only two coefficients is sometimes used

log P=A−BT which can be transformed into T= B

A−log P .

References

SOLUTION ENGINEERING. (n.d). Vapour Liquid Equilibrium Unit: Solution Engineering

Sdn. Bhd. Retrieved from http://www.solution.com.my/pdf/BP16(A4).pdf

Foxit Reader. (2010). VLE for Ideal Mixture: Raoult’s Law. Retrieved from

http://www.nt.ntnu.no/users/skoge/septek/lectures/flash_from_skogestad_book.pdf

Darren Duncan. (2014). Antoine Equation: Retrieved from

http://en.citizendium.org/wiki/Antoine_equation