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Questions and Answers
What is the construction of a wiped film evaporator?
A single, short tube of wide diameter with a bladed rotor inside.
Which type of evaporator is also known as a climbing film evaporator?
Falling film evaporators are known for their good heat transfer capabilities.
False
In falling film evaporators, the major disadvantage is the potential _______ of the falling film.
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Define evaporation.
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What is a practical definition of evaporation?
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Evaporation requires the latent heat of vaporization.
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Evaporators are designed to give maximum heat transfer to the liquid, with the largest possible __________.
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Match the application with the corresponding use of evaporation:
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How does agitation affect evaporation?
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What is the effect of atmospheric aqueous vapor pressure on evaporation?
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Evaporation under reduced pressure increases the operating temperature.
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What are the three types of evaporators mentioned in the content?
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Film evaporators spread the material as a film over the heated surface, allowing the __________ to escape the film.
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Study Notes
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
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Description
Learn about the process of evaporation, including the role of heat and the mechanism of how it occurs. Understand the concept of latent heat of vaporization and how it affects the rate of evaporation.
