EVAPORATION and VAPOUR PRESSURE PDF Print E-mail
Written by Norrie   
Tuesday, 16 March 2010 12:47
Article Index
EVAPORATION and VAPOUR PRESSURE
THE BOILING POINT OF A LIQUID
VAPOUR PRESSURE of LIQUID MIXTURES
DISTILLATION PROCESSES
BATCH DISTILLATION
FRACTIONAL DISTILLATION
All Pages

'Evaporation' is the process of changing liquid to vapour. This process can be used to separate a mixture of liquids which have different boiling point temperatures. It is known that a liquid produces vapour at any temperature.



This is due to the molecular motion which occurs at any temperature above absolute zero. The motion causes some of the molecules to 'jump out' of the liquid into the space above the liquid. The higher the temperature, the more molecules enter the vapour space.

The number of molecules in the vapour space cause a 'Vapour Pressure' above the liquid and, again, the higher the temperature, the greater the vapour pressure. The vapour pressure is referred to as the 'VP' of the liquid at a specific temperature. For any given liquid at a specific temperature, the number of molecules in the vapour space is constant, thereby giving a constant VP

If we consider a pure substance like water at a constant temperature, the motion of the molecules causes some to leave the liquid and enter the vapour space creating a vapour pressure. For example, the vapour pressure of water at 32 °F (0 °C) is 0.1 psia, well below atmospheric pressure. At 100 °F

(37.8 °C) the VP is 1.0 psia and at 212 °F (100 °C) it is 14.7 psia (101 kPa). The point at which a vapour pressure of a liquid is at its maximum for the specific temperature, the VP is called the Saturated Vapour Pressure (SVP) and, at this point the liquid and its vapour are said to be at 'Equilibrium'. (See Figure: 1)


Figure : 1

At equilibrium, the number of molecules entering the vapour space is equal to the number returning to the liquid. This condition remains constant as long as the temperature remains constant.

Increase or decrease in temperature will result in an increase or decrease in the number of molecules in the vapour phase and therefore an increase or decrease in the VP - but, when the temperature stabilises, the liquid and vapour will again be at equilibrium and show the VP for that temperature.


THE BOILING POINT OF A LIQUID

When the VP of a liquid is equal to the pressure exerted on the liquid, the liquid is said to be boiling. For example, as previously mentioned, the VP of water at 32 °F, is 0.1 psia. As temperature is increased, the VP increases. At 100 °C the VP is equal to atmospheric pressure.

The Boiling Point of pure water is therefore 212°F @ 14.7 psia, (100 °C @ 101 kPa). Different liquids have different boiling points to that of water. Light or volatile liquids will vaporise easily and will boil at a lower temperature producing a higher VP. Heavy, less volatile liquids will have higher BP's and lower VP's.

If the external pressure acting on a liquid is increased, as in a closed container, the molecules have more difficulty entering the vapour phase. More energy (Heat) is needed to vaporise the liquid. This means that an increase in pressure on the liquid results in an increase in the Boiling Point of the liquid. Conversely, a decrease in pressure on the liquid will make vaporisation easier and therefore the boiling temperature will be decreased.

Table 1 shows the effects of external pressure on the BP of pure water.

From the above table we can say that:

  • The boiling temperature of water varies with changes in pressure.

Each external pressure given in the table is also the VP of the water at the specific temperature given. The table illustrated above, refers only to the effects of pressure on the BP of water. The boiling temperature and therefore the VP of other liquids are affected in the same way when pressures are increased or decreased.

Table 2, shows the effect of pressure on the BP's of other liquid substances.

  • The boiling temperature and the VP of a liquid varies with changes in external pressure.

TABLE : 2


The principles discussed above, have many applications in the Oil and Chemical Industries. One important application is that of Vacuum Distillation (Separation of a Mixture), under vacuum conditions. This will be dealt with a little later in the discussion.

The examples given up to this point, are for pure substances. In the distillation of Crude Oil for example, the products are generally simpler mixtures of hydrocarbons extracted from the complex mixture in crude oil.


VAPOUR PRESSURE of LIQUID MIXTURES

The vapour pressure of a liquid mixture depends upon the percentage of each component of the mixture. As a simple example, if a 50% / 50% mixture of butane and pentane is heated to 100 °F, the vapour pressure will consist of 50% of the Butane VP plus 50% of the Pentane VP. Because each component exerts only a PART of the vapour pressure, the total VP is made up of the SUM of the PARTIAL PRESSURES of each of the components as follows:

VP of Butane @ 100 °F = 51.5 psia
VP of Pentane @ 100 °F = 15.5 psia
Partial pressure of Butane = 50% of 51.5 = 25.75 psia
Partial pressure of Pentane = 50% of 15.5 = 7.75 psia
∴ VP of the mixture = 25.75 + 7.75 = 33.50 psia

Butane, being lighter, (more volatile), than Pentane will provide more of the vapour than the Pentane. The actual proportion of Butane and Pentane making up the vapour, can be calculated as follows:


The above calculations can be used for any mixture of a number of different liquids, provided that their VP's at a standard temperature and their percentage of the mixture, are known. It should be noted that, when the mixture boils, the lightest component will actually boil first while the other components produce their partial pressures at that specific temperature. The continued addition of heat will cause a gradual increase in boiling temperature as each component vaporises out of the mixture.

The mixture will therefore have a 'Boiling Range' which has 'Initial' and 'Final' Boiling Point temperatures (IBP and FBP). The graph produced from this boiling range is very important in the quality control of products from a distillation unit.


DISTILLATION PROCESSES

'Distillation' is a process used to separate the components of a mixture of liquids into other, simpler mixtures like - Naphtha, Gasoline, Diesel Fuel .. etc, or into pure compounds -such as Methane, Ethane, Propane, Butane .. etc.
Distillation processes are accomplished by the controlled addition of heat energy, system pressure, (or vacuum) and other process parameters as required.

A very simple distillation process is that of producing pure water from sea water. The sea water is heated to boiling point (which, due to the dissolved solids, will be slightly higher than 100 °C (212 °F)). The vapour produced will be pure steam (water vapour).

If we now condense the steam we will have pure water. When all of the water has been vaporised from the mixture, a solid deposit of salts will remain in the distillation apparatus. (This method of producing potable water is used in areas where good, clean water cannot be obtained - the only difference is, that the mixture is only partly vaporised in order to prevent the solids blocking the system. If the process is carried out under vacuum, the amount of heat energy required is much reduced due to the decrease in the boiling temperature of the water.

Figure: 2 - shows a simple distillation process as may be used in a laboratory.
Figure: 3 - shows the same simple operation that may be used commercially to produce potable water.
Figure: 4 - Take a 50:50 mixture of Benzene and Toluene in a distillation unit.

Benzene is the more volatile of the two components. If we bring the mixture to boiling point, the vapour will be richer in Benzene than in Toluene. If we now condense the vapour, we will have a liquid richer in Benzene than the original mixture. The first step in the separation of the two components has been taken.

The degree of separation of the mixture depends upon the volatility of each of the components. In general we can say that, for any mixture of liquids:

  • If the volatility of the components is similar, their boiling points are close together and their separation will be difficult.
  • When the boiling points of the components are much greater, separation will be easier due to a greater difference in volatility.

Distillation of liquid mixtures is referred to as 'Simple Distillation' or 'Fractional Distillation'.

Figure: 2


Figure: 3


Figure: 4


BATCH DISTILLATION

In industry, particularly many years ago, distillation was carried out as a 'Batch' process. This means that a quantity of mixture was placed in a 'Still' or Boiler, and heated to boiling point by steam or by a fire under the boiler. As in the above method, the vapour was condensed and the liquid collected in a receiver. When the process was finished, the still was washed out and another batch of mixture put in. The process was repeated until all of the mixture was processed.

During the above process, a temperature gauge in the vapour line will remain at a fairly constant temperature while the lightest, most volatile component is coming off the mixture. When the temperature begins to rise significantly, the next lightest component is beginning to boil off and the receiver can be changed to collect the second product .. etc. or, if only the one product is required the unit can be shut down.

(See Figure: 5)

If the boiling points of the components in the mixture are close together, the product in the receiver will contain some of the heavier component and will need to be re-run again as many times as necessary to produce the purity required.

Later developments of batch distillation were made where a number of stills were operated in series with the heavier components being returned (refluxed) back to the preceding still. This method resulted in a purer product from the final still. (This will be discussed a little later in the lesson).

Figure: 5

During the separation of the mixture, the lowest Boiling Point component will be collected first. when the Temperature Indicator (T.I.) begins to increase, the condensate will be switched to receiver #2 to collect the next lowest B.P. product.

If purer products are required, they can be re-run.

SUMMARY

  • Distillation is a process used in industry to separate and purify liquid mixtures. The mixture is heated in a still and the vapour containing more of the most volatile component is condensed and collected in a receiver.
  • The degree of separation achieved, depends on the relative volatility of each component of the mixture.
  • The further apart the boiling points of the components, the purer the products.
  • Simple distillation can only be used to separate components with widely different BP's.
  • If the BP's are close together, then a process called 'Fractional Distillation' must be used. This is the next topic of discussion.

FRACTIONAL DISTILLATION

As an example of fractional distillation, we will take a 50:50 solution of Methanol and Water. At atmospheric pressure, methanol boils at 64 °C (142 °F) and water boils at 100 °C (212 °F). The methanol is the more volatile of the two and will vaporise much easier.

Figure: 6 shows how we can try to separate the mixture into its components - refer to the diagram as you read on.

We are showing three simple distillation units connected in series. Each unit has its own still, heater and condenser.

  1. The 50:50 mixture is fed into the first still and heated to produce a vapour rich in the lighter component -Methanol. The mixture is shown to be boiling at 77 °C (170 °F). The vapour consists of 70% methanol and 30% water giving an initial, partial separation.
  2. The methanol-rich vapour is condensed and passed into still No. 2 where an even richer vapour is produced at 69 °C (156 °F), consisting of 85% methanol and 15% water.
  3. Further enrichment of the condensate from still No. 2 can be achieved in still No. 3 where at 66 °C (151 °F), the vapour consists of 95% methanol and 5% water. If more stills are added, virtually pure methanol can be produced from the mixture.


Figure: 6

An improvement on the above system can be seen in Figure: 7, where the hot vapour from one still is used to heat the liquid in the following still the temperature of which is lower than that of the preceding still. Also, the return (reflux) of the liquid from a still to the still before, ensures that the maximum separation for the number of stills can take place.

Further purity of the product can only be achieved by the addition of more stills.

As stated earlier, the examples of distillation processes given up to now are old methods in use long ago. Modern distillation processes use 'Distillation Columns' or 'Towers' which, in effect, are a series of stills built vertically within a single tower.


Figure: 7

Figure: 8, Shows the components of Figure: 7 arranged vertically in a single column.

Figure: 8


Figure: 9



About the Author

Norrie is a retired professional who has been working in Oil and Gas and LNG production in Marsa-el-Brega, Libya for 30 years.

Norrie used to be in the Training Dept. and prepared Programmes for Libyan Trainees.

Last Updated on Tuesday, 16 March 2010 12:54