Evaporation

EVAPORATIONEVAPORATION

By: Dr. Tahseen IsmailBy: Dr. Tahseen Ismail

EVAPORATIONEVAPORATION `̀

Evaporation means the free escape of vapours from Evaporation means the free escape of vapours from the surface of a liquid below its boiling point. It is a the surface of a liquid below its boiling point. It is a slow process that can take place even at room slow process that can take place even at room temperature. temperature.

Evaporation will be maximum at the boiling point of the Evaporation will be maximum at the boiling point of the substance.substance.

Evaporation differ from boiling in that, evaporation Evaporation differ from boiling in that, evaporation takes place at all temperatures where as boiling takes takes place at all temperatures where as boiling takes place only at one temperature at a given pressure.place only at one temperature at a given pressure.

EVAPORATION EVAPORATION

Evaporation occurs only at the surface of a Evaporation occurs only at the surface of a liquid, but in boiling evaporation takes place liquid, but in boiling evaporation takes place from whole of the liquid. Boiling occurs from whole of the liquid. Boiling occurs when the vapors pressure of the liquid when the vapors pressure of the liquid becomes equal to the atmospheric becomes equal to the atmospheric pressure.pressure.

Evaporation differ from distillation, that in Evaporation differ from distillation, that in evaporation the solvent which is generally evaporation the solvent which is generally water is not condensed and collected where water is not condensed and collected where as in distillation the solvent vapors are as in distillation the solvent vapors are condensed and are collected in a receiver. condensed and are collected in a receiver.

FACTORS AFFECTING FACTORS AFFECTING EVAPORATIONEVAPORATION

TemperatureTemperature Surface areaSurface area AgitationAgitation Atmospheric aqueous pressureAtmospheric aqueous pressure Atmospheric pressure on the liquid under Atmospheric pressure on the liquid under

evaporationevaporation Type of product requiredType of product required Economic factorsEconomic factors

TemperatureTemperature The rate of evaporation is directly proportional to the The rate of evaporation is directly proportional to the

temperature.temperature. Higher the temperature higher will be evaporation but Higher the temperature higher will be evaporation but

evaporation is at its maximum at the boiling point of the evaporation is at its maximum at the boiling point of the liquid. Thermostable preparations can be evaporated at liquid. Thermostable preparations can be evaporated at higher temperature but the thermo labile preparations will higher temperature but the thermo labile preparations will have to be evaporated at low temperature.have to be evaporated at low temperature.

Many substances like glycosides, alkaloids, hormones, Many substances like glycosides, alkaloids, hormones, enzymes and antibiotics may get decomposed even at enzymes and antibiotics may get decomposed even at temperature below 100°C and hence they should be temperature below 100°C and hence they should be evaporated at low and careful temperature. Extremely heat evaporated at low and careful temperature. Extremely heat sensitive substances are evaporated under reduced sensitive substances are evaporated under reduced pressure.pressure.

TemperatureTemperature The rate of evaporation is directly proportional to the The rate of evaporation is directly proportional to the

temperature.temperature. Higher the temperature higher will be evaporation but Higher the temperature higher will be evaporation but

evaporation is at its maximum at the boiling point of the evaporation is at its maximum at the boiling point of the liquid. Thermostable preparations can be evaporated at liquid. Thermostable preparations can be evaporated at higher temperature but the thermo labile preparations will higher temperature but the thermo labile preparations will have to be evaporated at low temperature.have to be evaporated at low temperature.

Many substances like glycosides, alkaloids, hormones, Many substances like glycosides, alkaloids, hormones, enzymes and antibiotics may get decomposed even at enzymes and antibiotics may get decomposed even at temperature below 100°C and hence they should be temperature below 100°C and hence they should be evaporated at low and careful temperature. Extremely heat evaporated at low and careful temperature. Extremely heat sensitive substances are evaporated under reduced sensitive substances are evaporated under reduced pressure.pressure.

TemperatureTemperature The rate of evaporation is directly proportional to the The rate of evaporation is directly proportional to the

temperature.temperature. Higher the temperature higher will be evaporation but Higher the temperature higher will be evaporation but

evaporation is at its maximum at the boiling point of the evaporation is at its maximum at the boiling point of the liquid. Thermostable preparations can be evaporated at liquid. Thermostable preparations can be evaporated at higher temperature but the thermo labile preparations will higher temperature but the thermo labile preparations will have to be evaporated at low temperature.have to be evaporated at low temperature.

Many substances like glycosides, alkaloids, hormones, Many substances like glycosides, alkaloids, hormones, enzymes and antibiotics may get decomposed even at enzymes and antibiotics may get decomposed even at temperature below 100°C and hence they should be temperature below 100°C and hence they should be evaporated at low and careful temperature. Extremely heat evaporated at low and careful temperature. Extremely heat sensitive substances are evaporated under reduced sensitive substances are evaporated under reduced pressure.pressure.

TemperatureTemperature

Time required for evaporation is very critical. Time required for evaporation is very critical. Exposure to a higher temperature for a shorter Exposure to a higher temperature for a shorter period of time may be less destructive than a lower period of time may be less destructive than a lower temperature with exposure for a longer period of temperature with exposure for a longer period of time. Therefore as for as possible evaporation time. Therefore as for as possible evaporation should be carried out for short time at low should be carried out for short time at low temperature.temperature.

Therefore evaporation in some cases may be Therefore evaporation in some cases may be carried out at a low controlled temperature till a carried out at a low controlled temperature till a concentrated product is obtained. concentrated product is obtained.

Surface area:Surface area:

The rate of evaporation is directly proportional to The rate of evaporation is directly proportional to the surface area of the vessel exposed to the surface area of the vessel exposed to evaporation. Greater the surface exposed to evaporation. Greater the surface exposed to evaporation higher will be the rate of evaporation higher will be the rate of evaporation.evaporation.

AgitationAgitation

During evaporation the upper layers of the liquid to During evaporation the upper layers of the liquid to be evaporated has a tendency to form a scum or be evaporated has a tendency to form a scum or layer which lowers the rate of evaporation. Therefore layer which lowers the rate of evaporation. Therefore it becomes necessary to agitate or stir the solution it becomes necessary to agitate or stir the solution under evaporation. This will also prevent the under evaporation. This will also prevent the decomposition of the preparation at the bottom due decomposition of the preparation at the bottom due to excessive heat and will also prevent the settling of to excessive heat and will also prevent the settling of the solids at the bottom of the containerthe solids at the bottom of the container..

Atmospheric aqueous vapour Atmospheric aqueous vapour pressurepressure

If the atmospheric aqueous vapour pressure If the atmospheric aqueous vapour pressure or moisture contents in the air are high, the or moisture contents in the air are high, the rate of evaporation will be slow but if it is less rate of evaporation will be slow but if it is less then evaporation will be fast. Therefore rate then evaporation will be fast. Therefore rate of evaporation can be increased by free of evaporation can be increased by free circulation of warm air over the evaporating circulation of warm air over the evaporating pan.pan.

Atmospheric pressure on the Atmospheric pressure on the liquid under evaporationliquid under evaporation

The rate of evaporation is inversely proportional to the The rate of evaporation is inversely proportional to the atmospheric pressure on the surface of the liquid. If atmospheric pressure on the surface of the liquid. If the atmospheric pressure on the liquid is reduced to the atmospheric pressure on the liquid is reduced to half then the rate of evaporation will be doubled. Due half then the rate of evaporation will be doubled. Due to this reason in many cases the evaporation is done to this reason in many cases the evaporation is done under reduced pressure i.e. by producing vacuum under reduced pressure i.e. by producing vacuum over the surface of the liquid.over the surface of the liquid.

Type of product requiredType of product required

The selection of the method and apparatus to be used The selection of the method and apparatus to be used for evaporation depends upon the type of product for evaporation depends upon the type of product required. Evaporating pans or stills may be used to required. Evaporating pans or stills may be used to produce liquid or dry products.produce liquid or dry products.

Economic factorsEconomic factors

While selecting the method and apparatus While selecting the method and apparatus employed for evaporation, due consideration employed for evaporation, due consideration must be given to economy of labor, fuel, floor must be given to economy of labor, fuel, floor area and materials. The recovery of solvents area and materials. The recovery of solvents and the utilization of "waste" heat are also and the utilization of "waste" heat are also important factors, which contribute significantly important factors, which contribute significantly in the reduction of overall costs.in the reduction of overall costs.

A) Small scale methodsA) Small scale methods

Small quantities of liquids can be evaporated in a glass dish. Small quantities of liquids can be evaporated in a glass dish. Generally direct heating by burner or electric hot plate is used Generally direct heating by burner or electric hot plate is used for evaporation. But these should not be employed because for evaporation. But these should not be employed because direct heating leads to the decomposition of the substances direct heating leads to the decomposition of the substances towards the end of evaporation.towards the end of evaporation.

So for this purpose different types of bathes are used as a So for this purpose different types of bathes are used as a source indirect heating. A water bath is most suitable when the source indirect heating. A water bath is most suitable when the liquids are to be heated up to100ºC.Glycerine bath is used to liquids are to be heated up to100ºC.Glycerine bath is used to attain a temperature up to 150°C. Sand bath or oil bath attain a temperature up to 150°C. Sand bath or oil bath containing liquid paraffin or soft paraffin may be used when containing liquid paraffin or soft paraffin may be used when higher temperature up to 300°C is required.higher temperature up to 300°C is required.

To prevent decomposition, whole of the liquid should not be To prevent decomposition, whole of the liquid should not be evaporated to dryness. The last traces of the solvent from the evaporated to dryness. The last traces of the solvent from the concentrated liquid may be removed under controlled concentrated liquid may be removed under controlled temperature.temperature.

B) Large Scale MethodsB) Large Scale Methods

Evaporating pansEvaporating pansEvaporating stillsEvaporating stills1) Evaporating pans1) Evaporating pans.. On large scale, liquid extracts containing water as a On large scale, liquid extracts containing water as a

menstrum are evaporated in open pans called evaporating menstrum are evaporated in open pans called evaporating pans. These consist of hemispherical or shallow pans made of pans. These consist of hemispherical or shallow pans made of copper, stainless steel, aluminum, iron or other metals and copper, stainless steel, aluminum, iron or other metals and surrounded by a steam jacket. Pans made from stainless steel surrounded by a steam jacket. Pans made from stainless steel and jacket from iron is most suitable.and jacket from iron is most suitable.

The hemispherical shape is most suitable because it provides The hemispherical shape is most suitable because it provides the best surface to volume ratio for heating.the best surface to volume ratio for heating.

The pans may be fixed or made to tilt to remove the product.The pans may be fixed or made to tilt to remove the product.

Evaporating pansEvaporating pans

Evaporating pansEvaporating pansAdvantagesAdvantages They are simple, easy and cheap to construct.They are simple, easy and cheap to construct. Easy to use and clean.Easy to use and clean. Stirring of the evaporating liquid can be done easilyStirring of the evaporating liquid can be done easily

DisadvantagesDisadvantages The whole of the liquid is heated all the time, which may lead to The whole of the liquid is heated all the time, which may lead to

the decomposition of the component.the decomposition of the component. Solids may be deposited at the bottom, which makes the stirring Solids may be deposited at the bottom, which makes the stirring

necessary.necessary. These pans can only be used for evaporating aqueous and These pans can only be used for evaporating aqueous and

thermostable liquid extracts.thermostable liquid extracts. The room in which evaporating pans are used for evaporating the The room in which evaporating pans are used for evaporating the

liquids must be fitted with efficient exhaust fans otherwise the liquids must be fitted with efficient exhaust fans otherwise the room will be filled with dense fog of condensed vapours.room will be filled with dense fog of condensed vapours.

2) Evaporating stills2) Evaporating stills Evaporating stills are quite similar to evaporating Evaporating stills are quite similar to evaporating

pans. The vessel is made of copper stainless steel, pans. The vessel is made of copper stainless steel, with a cover, which is fitted with a water condenser so with a cover, which is fitted with a water condenser so that the solvent is condensed and collected in a that the solvent is condensed and collected in a receiver. receiver.

At the lower point of the pan, a tube fitted with a tape At the lower point of the pan, a tube fitted with a tape is connected for the removal of the product from the is connected for the removal of the product from the pan. A steam jacket surrounds the pan.pan. A steam jacket surrounds the pan.

For cleaning the pan or easy removal of the product For cleaning the pan or easy removal of the product the cover of the vessel may be fitted with quick the cover of the vessel may be fitted with quick release system of clamps.release system of clamps.

Evaporating stillsEvaporating stills

Evaporating stillsEvaporating stills Advantages Advantages

They are very simple to construct and easy to They are very simple to construct and easy to clean.clean.

A vacuum pump can be fitted to the apparatus by A vacuum pump can be fitted to the apparatus by which evaporation under reduced pressure can be which evaporation under reduced pressure can be donedone

Since the cover can be removed easily therefore Since the cover can be removed easily therefore the still may be conveniently used for evaporating the still may be conveniently used for evaporating extracts to dryness.extracts to dryness.

DisadvantagesDisadvantages The whole of the liquid is to be heated all the time, The whole of the liquid is to be heated all the time,

which may lead to deterioration of the product.which may lead to deterioration of the product. This method is used only for the small batches of This method is used only for the small batches of

liquids which are to be evaporated.liquids which are to be evaporated.

Applications of EvaporationApplications of Evaporation

This process is used in the preparation of liquid This process is used in the preparation of liquid extracts, dry extracts and in the concentration of extracts, dry extracts and in the concentration of blood serum.blood serum.

It s also used in the manufacture of drugsIt s also used in the manufacture of drugs