Evaporation - Pharmaceutical Engineering B.Pharmacy 3rd Semester 2022 (2021-2022) Previous Year's Question Papers/Notes Download - HK Technical PGIMS

​Pharmaceutical Engineering Theory (BP304T) Evaporation

Evaporation
 Evaporation is a process of vaporizing large quantities of liquid to get a concentrated
product.
 Equipments used for evaporation are known as evaporators.
 Heat is supplied to the evaporator, which transmits it to the evaporating liquid so as to
provide latent heat of evaporation.
 Steam is normally used as a source of heat.
 Evaporation is a surface phenomenon. This means mass transfer takes place from the
surface. No boiling occurs.
 In practice, surface evaporation is slow; so the practical definition of evaporation is the
removal of solvent from the solution by boiling the liquor in a suitable vessel and
withdrawing the vapor, leaving a concentrated liquid residue in the vessel.
Applications
 Manufacture of bulk drugs:
 In pharmaceutical industries, chemical industries etc.
 Manufacture of biological products:
 Evaporation is used in the manufacture of certain biological products such as
insulin, biochemical products like penicillin, and plant products.
 Preparation of blood products such as blood plasma and serum also involves
evaporation.
 Enzymes, Hormones and Antibiotics are also some of the products which are
manufactured by evaporation.
 Miscellaneous:
 Water containing minerals is subjected to evaporation to get demineralized water after
condensation.
Factors Affecting Evaporation
 The rate of evaporation depends on several factors.
 The relationship may be expressed mathematically as:
Factors Affecting Evaporation are:
1. Temperature
2. Vapor Pressure
3. Surface Area
4. Moisture Content of the Feed
5. Type of Product Required
6. Time of Evaporation
7. Film and Deposits
8. Economic Factors
1)Temperature
 Higher the temperature→ higher the vapor pressure → higher will be the term (b-b’) →
higher the evaporation.
 At a given temperature, some molecules have higher kinetic energy than average, while
others have lower that average kinetic energy.
 Fast moving molecules escape from the surface of liquid into vapor, while slow moving
molecules remain behind.
 When the temperature of the liquid is raised, more molecules acquire sufficient
kinetic energy and escape from the surface to vapor state. This is the situation below
the boiling point of liquid.
2)Vapor Pressure
 Rate of evaporation ∝ the vapor pressure of the liquid.
 As per the equation 1, the lower the external pressure value → the higher the evaporation.
 This condition is achieved by applying vacuum.
 Liquids with low boiling points → have higher high vapor pressure even at low
temperatures → so they evaporate quickly.
3)Surface area
 From equation 1, it is clear that, the grater the surface area of the liquid → the
greater will be the evaporation.
 This is the reason why evaporation is conducted in evaporators with larger surface area.
4)Moisture Content of the Feed
 Some drugs easily degrade by hydrolysis in presence of moisture at higher temperature.
 To prevent this → material is exposed to lower temperature initially, and then
exposed to higher temperature for final concentration.
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Evaporation
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 Example: Dry extract of Belladonna is prepared by this method
5)Type of Product Required
 It sometimes decides the apparatus used for evaporation.
 Evaporating pan produces liquid or dry concentrate.
 Film evaporator gives liquid concentrate.
 Vacuum evaporator gives porous products suitable for conversion to granules.
6)Time of Evaporation
 Longer time of exposure → higher evaporation. For thermostable constituents.
 Exposure of a drug to a relatively high temperature for a short period of time may be less
destructive than a lower temperature with long exposure period.
 This is why film evaporators are used.
7)Film and Deposits
 When vegetable extracts are concentrated in steam pan, a film may be formed on the
surface and/or precipitated matter may deposit on the heating surface.
 Film reduces the evaporating surface and precipitated matter disturbs the transfer of heat.
 To avoid this, efficient stirring is necessary.
8)Economic Factors
 Economies of Labor, Fuel, Floor Space and Materials → primary considerations.
 Recovery of solvents and utilization of waste heat are also important because they can
reduce the cost.
 Rate of evaporation depends on rate of heat transfer.
 Therefore, evaporator is designed to give maximum heat transfer to the liquid.
Difference Between Evaporation and Other Heat Processes
Evaporation Other Heat Processes
 Residue is concentrated liquid.
 Evaporating liquid is only one
component in most of the cases.
 No attempt is made to separate the
mixture even if there is any mixture.
 The purpose of evaporation is to get a
concentrated liquid only, not to get
crystals.
 Drying: Residue is Solid
 Distillation: Evaporating liquid is
a combination of two or more
components
 Distillation: It is compulsory to
separate each component.
 Crystallization: The purpose of
concentrating the solution is to
get crystals.
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Classification of Evaporators
1) Evaporators with Heating Medium in Jacket
 Example: Evaporating Pan (Steam Jacketed Kettle)
2) Vapor Heated Evaporators with Tubular Heating Surfaces