In our everyday lives we come across the process of 'Evaporation'. This phenomenon is a property possessed by all liquids and is the ability of a liquid to change to vapour at a temperature well below its boiling point at atmospheric pressure.
We have all seen how a pool of rain water disappears (evaporates) after some time has passed. The rate of evaporation depends upon the 'ambient' temperature around the liquid and its volatility - the high
er the temperature and the more volatile the liquid, the faster the evaporation rate.
The evaporation rate also depends on the air movement across the pool of liquid in that, evaporation will be slow when there is no wind and faster as the wind increases.
Also in our everyday life, the use of evaporation plays a part in 'Refrigeration', which is common for keeping food fresh as in the Household Fridge, in air-conditioning for keeping buildings cool and, in freezers used to provide us with ice for cold drinks and to preserve food for long periods of time.
In industry, the principles of evaporation and refrigeration are used to great extent in the treatment, separation, handling and storage of materials in any of the three states of matter - Solid, Liquid or Gas. Distillation concerns the separation of liquid mixtures by evaporation processes.
As previously stated, evaporation of liquids depends on the temperature and, as temperature is increased, evaporation becomes faster. At some particular temperature, depending upon the liquid composition, bubbles of gas begin to appear and the liquid movement becomes violent as the gas breaks the surface of the liquid in larger and larger amounts. At this point the liquid is said to be 'BOILING'.
The addition of more heat energy, increases the boiling rate and the evaporation rate, but the temperature does not increase further. This condition continues until ALL of the liquid has evaporated - now referred to as 'Vaporised' - i.e. changed to vapour (or gas). Still more addition of heat energy will now begin to increase the temperature of the gas. This is called SUPERHEATING'.
The temperature at which a pure liquid changes to gas, is called the 'BOILING POINT' (BP) of the liquid - for example, the BP of pure water is 100 °C (212 °F).
The boiling point of a liquid refers to its vaporisation temperature at atmospheric pressure. If the pressure above the liquid is increased, the boiling temperature also increases due to the vapour molecules having more difficulty in leaving the liquid, and, a pressure decrease will decrease the BP.
(The principle of the increase in boiling temperature of water at higher pressure, is used in a 'Pressure Cooker' which speeds up cooking time and also helps to retain the vitamins and minerals in the food being cooked).
Conversely, a decrease in pressure to below that of the atmosphere, will decrease the boiling temperature of a liquid due to the molecules having easier access to the vapour space above the liquid. As an example of this, at the top of Mount Everest, water boils at about 70 °C (158 °F). (At this temperature a good cup of tea or a well-boiled egg isn't possible).
Condensation is the opposite of evaporation and has the same temperature as the BP of the liquid. When heat energy is removed from a gas or vapour, it begins to condense. Further heat removal does not change the temperature until all of the gas has changed to liquid. More cooling will then decrease the temperature of the liquid.
However, if the vapour or gas is superheated, the first effect of heat removal is to decrease the temperature until the gas is at the boiling temperature of the liquid from which it was formed, it will then begin to condense.
The principles of evaporation and condensation are used extensively in industry.
EVAPORATION IN INDUSTRY
Evaporation is a unit operation in which a solvent is removed from a solution in order to concentrate the dissolved solids. The solvent, usually water, is boiled off by the addition of heat energy, often under reduced pressure. The concentrated solution may be the final product or feed for further processing, such as crystallisation.
There are three MAIN reasons for the use of evaporation in the Chemical Industry :
- To remove excess solvent in order to produce a more concentrated solution as a sellable product.
- To concentrate dissolved solids to produce an almost saturated solution from which crystallisation will readily take place.
- To produce a more concentrated 'Raw Material' for further processing.
EVAPORATORS: Types & Applications
These pieces of equipment, generally vertical, cylindrical vessels, are designed to concentrate weak solutions in order to satisfy any of the above listed purposes.
Basically, the concentration is carried out by heating, with steam as the thermal fluid which boils off the excess, unwanted solvent to leave a solution at the required strength. The solvent vapour may be disposed of or condensed for other uses as required.
Figure: 1 shows a type of evaporator which may be used in the batch production of a concentrated solution of sodium hydroxide.
The concentrated liquor may then be utilised as it is or go on to be separated by crystallisation.
The process consists of an evaporator vessel containing a steam heated element made up of a vertical or horizontal tube bundle. The weak solution is fed into the mid-section of the vessel to a pre-set level. Steam is then passed through the tube bundle and its heat transferred to the liquid.
As the liquid is heated, natural convection currents are set up which imparts circulation within the liquid. When the liquid reaches its boiling point, the produced water vapour is taken off the top of the evaporator and condensed or vented as required.
The steam, as it passes through the tubes, gives up its latent heat to the solution and condenses to water. The condensate is then discharged through a steam trap.
As water is driven off the solution, the concentration of the solution increases until, by testing, it reaches the desired concentration. The solution is then drawn off from the vessel bottom and discharged to storage via a cooler or sent direct to other processes.
Later systems use a pump to speed up the circulation of liquid in the evaporator and thereby speed up the evaporation process.
These are referred to as 'Forced Circulation Evaporators'.
THE EFFECT OF VACUUM ON EVAPORATION
When the pressure on a liquid varies, the boiling point temperature of the liquid also varies;
i.e. Pressure increases - BP increases. Therefore, if a liquid is heated while under vacuum, the boiling point of the liquid will be decreased which will also reduce the amount of heat energy needed to boil the liquid.
Figure: 2 - shows the effect of vacuum on the boiling point of pure water. The first vessel is at atmospheric pressure and the water is boiling at 100 °C (212 °F). In vessels 2 and 3, different levels of vacuum have been applied, resulting in lower BP's of the water.
From this, it can be seen that, if a solution in an evaporator is heated under reduced pressure, the evaporation process will proceed at a reduced heat input. This effect of vacuum is particularly useful when heat-sensitive liquids are being processed.
MULTIPLE EFFECT EVAPORATORS
The design and development of multiple effect evaporators, has resulted in a continuous process of solution concentration and increased thermal efficiency, by operating under increasing vacuum conditions, thereby reducing heat input requirements.
Figure : 3 - shows three evaporators in operation.
This system is called a 'Triple-Effect Evaporator'.
In this system, evaporator #1 is at atmospheric pressure (101 kPa or 14.7 psia) and at 100 °C (212 °F).
#2 is under vacuum at 58 kPa (8.4 psia) and about 85 °C (185 °F).
#3 is at 16 kPa (2.3 psia) and about 55 °C (131 °F).
The weak solution feed enters evaporator #1 and the concentrated liquor is passed into # 2 vessel. The steam produced in #1, is used to heat the liquor in # 2 evaporator.
This liquor then passes into #3 evaporator. In order to create the vacuum in vessels 2 and 3, the vapours are drawn off through condensers by ejector systems.
The steam heating medium, as it passes through the heating coils, gives up its latent heat and the resulting condensate is piped away through steam traps.
The pressures and temperatures indicated do not apply to any particular process but only to illustrate the principles involved. Pressures and temperatures of the system will depend on the nature of the solution being processed.
The following illustrates the effect on the boiling temperature of a liquid when lower pressure is exerted on it. This will also decrease the amount of heat energy required to vaporise the liquid. Conversely, when pressure acting on a liquid is increased, the boiling point of the liquid is also increased. E.g. Water under a pressure of 600 Psi will not boil until its temperature reaches 486 °F requiring much more heat energy.
EXAMPLE OF TRIPLE EFFECT EVAPORATION
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.