evaporation

Question 1

definition

Answer 1

 Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid.
 The other type of vaporization is boiling, which instead occurs on the entire mass of the liquid.

Question 2

intro

Answer 2

 Theoretically evaporation means free escape of vapours from the surface of a liquid below its boiling
point.
 As evaporation is a very slow process, therefore a liquid is usually boiled / heated to speed up this
process.
 So, practically evaporation may be defined as the removal of liquid from a solution, by boiling the
solution in a suitable vessel from where the vapors are withdrawn and a concentrated liquid is left
behind.
 The Evaporation is maximum at the boiling point of a substance.
 For molecules of a liquid to evaporate, they must:
o Be located near the surface
o Be moving in the proper direction
o Have sufficient kinetic energy to overcome liquid-phase intermolecular forces
 Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited.
 Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more
quickly at higher temperatures.
 As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy,
and the temperature of the liquid, thus, decreases.

Question 3

factors effecting evaporation

Answer 3

 Temperature
 Surface Area
 Agitation
 Atmospheric Aq Vapor Pressure
 Type of Product required
 Economic Factors

Question 4

temperature

Answer 4

The rate of evaporation is directly proportional to the temperature. The higher the temperature, the
higher will be evaporation but evaporation is maximum at the boiling point of the liquid.

Question 5

surface area

Answer 5

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

Question 6

agitation

Answer 6

During evaporation a layer or scum is formed at the surface. Therefore it is necessary to agitate and
stir the solution during evaporation. Agitation also increases the rate of evaporation.

Question 7

atmospheric aqeous vapor pressure

Answer 7

If atmospheric moisture contents in air are high, rate of evaporation will be slow but if less then
evaporation will be fast. Rate of evaporation can be increased by circulation of warm air over the evaporating
pan.

Question 8

atmospheric pressure

Answer 8

Rate of evaporation is inversely proportional to the atmospheric pressure on the surface of the liquid.
Rate of evaporation become doubled by reducing the atmospheric pressure on the liquid to one half. Due to
this reason, in many cases evaporation is done under reduced pressure.

Question 9

type of product required

Answer 9

Selection of method and apparatus depends on the type of the product required.

Question 10

economic factors

Answer 10

They contribute significantly in selecting the method and the type of apparatus to be used for
evaporation.

Question 11

types of evaporators

Answer 11

1. Small Scale Evaporators
2. Large Scale Methods
   a) Evaporating Pans
   b) Evaporating Stills

Question 12

small scale evaporators

Answer 12

 Small quantity of liquids may be evaporated in porcelain or glass dish.
 Direct heat by Bunsen burner or electric hot plate may be applied, but direct heat leads to
decomposition of the substances towards the end of the evaporation
 A fixed maximum temperature can easily be attained by employing different types of baths as a source
of indirect heating
 Water bath is most suitable when liquids are to be heated up to 100oC. These are simple and cheap
 Sand bath or oil bath containing liquid paraffin or soft paraffin may be used when higher temperatures
upto 300oC are required.
 Glycerin bath is used to attain a temperature upto 150oC
 To prevent decomposition, whole of the liquid should not be evaporated to dryness; instead the last
traces of the solvent from the concentrated liquid should be removed under controlled temperature.
 In case of large quantities of liquids or solutions having costly solvents, the evaporation should be
carried out by distillation under reduced pressure.

Question 13

evaporting pans (Large)

Answer 13

A. Evaporating Pans:
 On large scale, liquid extracts containing water as a menstruum are evaporated in large open pans
called evaporating pans.
 They consist of hemispherical or shallow pans, made of copper, stainless steel, aluminum, enameled
iron or other metal and surrounded by a steam jacket.
 The hemispherical shape is most suitable because it gives the best surface/volume ratio for heating and
the largest area for disengagement of vapours.
 The pans may be fixed, or have a mounting, permitting it to be tilted to remove the product.

Question 14

advantages of evaporating pans

Answer 14

a) They are simple, easy and cheap to construct.
b) Easy to use, clean and maintain.
c) Stirring of the evaporating liquid can easily be done.

Question 15

disadvantages of evaporating pans

Answer 15

a) On the evaporating surface a scum is rapidly formed which decreases rate of evaporation.
b) Solids may be deposited at the bottom, which makes stirring necessary.
c) Cannot be used for extracts containing organic solvents like alcohol etc.
d) The rooms, in which the evaporating pans are used, must have an efficient exhaust system. Otherwise
the room will be filled with dense fog of condensed vapors and water will start falling from the roof
and along the sides of the wall

Question 16

evaporating stills

Answer 16

 These are similar to pans, and consist of a vessel made of copper or stainless steel.
 They are used in large scale pharmaceutical manufacturing

Question 17

applications of evaporation

Answer 17

1. One of the most important methods in manufacture of pharmaceuticals.
2. Used for preparation of different type of extracts.
3. In the manufacture of drugs containing antibiotics, hormones, enzymes etc.