Evaporation Process

Evaporation

• Evaporation is the removal of liquid from a solution by boiling the solution in a suitable vessel and withdrawing the vapour, leaving a concentrated liquid residue.
• Heat is necessary to provide the latent heat of vaporization, and the rate of evaporation is controlled by the rate of heat transfer.

Mechanism of Evaporation

• When heat is applied to a solution, the motion of molecules increases, and molecules on the surface overcome the surface tension of the liquid and evaporate.
• Surface molecules have less cohesive force than other molecules.

Applications of Evaporation

• Evaporation is used in the manufacture of pharmaceuticals, including:
  • Liquid extracts, soft extracts, and dry extracts
  • Concentration of blood plasma and serum
  • Manufacture of drugs containing antibiotics, enzymes, hormones, and other substances
  • Purification of vitamins
  • Concentration of proteins and biological products
  • Stripping of solvents from vegetable and plant or herbal extracts
  • Removal of water and solvents from fermentation broths
  • Concentration of penicillin and related products

Solution Properties

• Knowledge of solution properties is important for the design of evaporation equipment.
• Important properties of solutions include:
  • Concentration
  • Viscosity
  • Boiling point
  • Foaming
  • Degradation due to high temperature
  • Scaling

Factors Affecting Evaporation

• There are seven factors that affect evaporation:
  • Temperature
  • Surface area
  • Agitation
  • Atmospheric aqueous vapour pressure
  • Atmospheric pressure on the liquid under evaporation
  • Type of product required
  • Economic factors

Evaporation under Reduced Pressure

• Evaporation occurs at a lower temperature with reduced pressure, which reduces the risk of damage to heat-sensitive materials.
• Advantages of evaporation under reduced pressure include:
  • Lower operating temperature
  • Higher temperature gradients
  • Lower steam pressure

Equipment Used for Evaporation

• Evaporators are classified into three main groups:
  • Natural circulation evaporators
  • Forced circulation evaporators
  • Film evaporators

Natural Circulation Evaporators

• Types of natural circulation evaporators include:
  • Evaporating pans
  • Evaporating stills
  • Short tube evaporators
• Working principle: convection currents set up by the heating process
• Advantages: simple in construction, easy to operate, clean, and maintain
• Disadvantages: whole of the liquid is heated all the time, which may lead to decomposition of components

Forced Circulation Evaporators

• Construction: short tube calendria and a large cylindrical vessel
• Working principle: rapid liquid movement improves heat transfer
• Advantages: suitable for operation under reduced pressure, rapid evaporation rate, and suitable for thermolabile materials
• Disadvantages: corrosion-erosion can occur, and salt deposits can detach and accumulate at the bottom

Film Evaporators

• Types of film evaporators include:
  • Wiped film evaporators
  • Long tube evaporators
    • Climbing film evaporators
    • Falling film evaporators
• Advantages: good heat transfer, and suitable for viscous liquids
• Disadvantages: potential instability of the falling film, and difficult to clean and maintain

Material of Construction

• Mostly made of steel
• Special materials are used for highly corrosive fluids, such as copper, nickel, stainless steel, and aluminum

Method of Feeding

• Variations in the multiple effect have come from the mode of feed supply
• There are four possible feeding arrangements:
  • Forward feed
  • Backward feed
  • Mixed feed
  • Parallel feed

Types of Evaporators

• Evaporators with heating medium in the jacket
• Vapor heated evaporators with tubular heating surfaces
• Steam jacketed kettles/batch type pan evaporators
• Horizontal tube evaporators
• Vertical tube evaporators
• Climbing film evaporators
• Falling film evaporators