1. Evaporation

2. EVAPORATION Definition:- Theoretically,
“Evaporation means simply vaporization from the surface of a liquid. Vaporization of a liquid below its boiling point is called evaporation.”
Thus, no boiling occurs and the rate of vaporization depends on the diffusion of vapour through the boundary layers above the liquid.

3. Thus a practical definition of 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.”
This means that heat will be necessary to provide the latent heat of vaporization and, in general the rate of evaporation is controlled by the rate of heat transfer.
Evaporators are designed, therefore, to give maximum heat transfer to the liquid, with the largest possible area, a suitable temperature gradient.

4. Mechanism
When heat applied in solution the motion of molecules increase and molecules present in the surface overcome the surface tension of the liquid and it evaporates because surface molecules have less cohesive force than others.

5. Evaporation Introduction
Vaporization of a solvent from a solution to make it concentrated.
Solvent volatile
Solute non-volatile in nature.
We are left with thick liquor
It is different from drying only, as in drying the solvent is vaporized to have a solid end product
Distillation and Evaporation

6. APPLICATION OF EVAPORATION
Evaporation is one of the most important processes in the manufacture of pharmaceuticals.
It is used in the preparation of
Liquid extracts, soft extracts & dry extracts.
In the concentration of blood plasma & serum.
It is also used in the manufacture of drugs containing, antibiotics, enzymes, hormones & many other substances.
Hajvery University (Faculty of Pharmacy)

7. 4) Used in purification of vitamins. 5) Concentration of proteins
4) Used in purification of vitamins. 5) Concentration of proteins. 6) Concentration of biological products. 7) Stripping of solvents from vegetable & plant or herbal extracts. 8) Removal of water & solvents from fermentation broths. 9) Concentration of penicillin & related products
Hajvery University (Faculty of Pharmacy)

8. Solution properties 
Knowledge of solution properties is important for the design of the equipment for evaporation. Some of the important properties of the solution are given below,
Concentration  Initially, the solution may be quite dilute and the properties of the solution may be taken as the properties of solvent. As the concentration increases, the solution becomes viscous and heat transfer resistance increases. The crystal may grow on the heating coil or on the heating surface. The boiling points of the solution also rise considerably. Solid or solute contact increases and the boiling temperature of the concentrated solution became higher than that of the solvent as the same pressure (i.e. elevation in boiling point).
Foaming  Many of the materials like organic substance may foam during vaporization. If the foam is stable, it may come out along the vapor known as entrainment. Heat transfer coefficient changes abruptly for such systems.
Degradation due to high temperature The products of many chemical, food, pharmaceutical industries etc. are very temperature sensitive and they may get damaged during evaporation. Thus special case or technique is required for concentrating such solution.
Scaling  Many solution have tendency to deposit the scale on the heating surface, which may increase the heat transfer resistance. These scales produce extra thermal resistance of significant value. Therefore, scaling in the equipment should not be ignored thus de-scaling becomes an important and routine matter.
Equipment material  The material of the equipment must be chosen considering the solution properties so that the solution should neither be contaminated nor react with the equipment material.

9. Factors affecting Evaporation
There are seven factors to affect evaporation.
Temperature
Surface area
Agitation
Atmospheric aqueous vapour pressure
Atmospheric pressure on the liquid under evaporation
Type of product required
Economic factors

10. Temperature: The rate of evaporation is directly proportional to the temperature.
Surface area: The rate of evaporation is directly proportional to the surface area of the vessel exposed to evaporation.
Agitation: is necessary for evaporation.
Atmospheric aqueous vapour pressure: The rate of evaporation is inversely proportional to the atmospheric aqueous vapour pressure.

11. 5) Atmospheric pressure on the liquid under evaporation: The rate of evaporation is inversely proportional to the atmospheric pressure on the liquid under evaporation. 6) Type of product required: The selection of the method and apparatus to be used for evaporation depends upon type of product required. 7) Economic factors: When selecting the method and apparatus the economic factors are important

12. Boiling point elevation

18. EVAPORATION UNDER REDUCEDE PRESSURE
At reduced pressure evaporation increases. Advantage:
Evaporation occurs at low temperature, with less risk of damage to heat sensitive materials.
A lower operating temperature gives higher temperature gradients, without the need of excessive steam pressures.
The lower the operating, temperature the lower the steam pressure that can be used.

19. EQUIPMENT USED FOR EVAPORATION
Equipments which are used for evaporation are called Evaporators.
Types of Evaporator: 
Evaporators are divided mainly into three groups.
Natural circulation evaporator
Types:
i) Evaporating pans
ii) Evaporating stills
iii) Short tube evaporator.

20. 2) Forced circulation evaporator
2) Forced circulation evaporator. 3) Film evaporator Types: i) Wiped Film evaporator ii)Long Tube Evaporator a) Climbing film evaporator b) Falling film evaporator

21. 1) NATURAL CIRCULATION EVAPORATORS
Working principle:
The movement of the liquid results from convection currents set up by the heating process.
Convection currents: The process in which heat moves through a gas or liquid as the hotter parts rises and the cooler part sinks.

22. Salt production

23. Lithium production

24. Evaporating Pans
“On a manufacturing scale, liquid extracts containing water are evaporated in open pans called evaporating pans.”
Construction:
The evaporating pan consists of
Hemispherical shallow made of
Copper
Stainless steel
Alluminium
Enameled iron
Steam jacket
The hemispherical shape gives the best surface\volume ratio for heating and the largest area for the disengagement of vapour.
machines/steam-jacketed-kettle-or-evaporating-pan

25. Working:
The dilute solution is taken in the pan. Steam is introduced through the steam inlet into the jacket to heat the pan. In these evaporators the movement of the liquid results from convection currents set up by the heating process. The concentrated liquid is collected through the outlet placed at the bottom of the pan.

27. Advantages
Evaporating pan or steam jacketed kettle is constructed both for small scale and large scale batch operations.
Evaporating pan or steam jacketed kettle is a simple in construction and easy to operates, clean and maintain.
Its cost of installation and maintenance is low.
For the construction of the evaporating pan materials such as copper, stainless steel and aluminum so on wide variety of materials can be used.
Stirring of the contents or materials and removal of the products is easy.

28. Disadvantage
The whole of the liquid is heated all the time which may lead to decomposition of the components.
On the evaporating surface foam is rapidly formed which decreases evaporation. Solids may be deposited at the bottom which make the stirring necessary.
This pan can only be used for evaporating aqueous and thermostable liquid extracts.
They can not be used for evaporating extracts containing organic solvents like alcohol etc.

29. ii) Evaporating still Construction
It consists of a jacketed-evaporating pan with a cylindrical cover that connects it to a condenser. The over all assembly is called still. The cover is clamped with the evaporating pan.

30. Working :
The dilute liquid is fed into the still, the cover is clamped. Steam is introduced into the jacket. The liquid is evaporated and condensed in the condenser and collected. The product (i.e. concentrated liquid) is collected through the product outlet.

31. Advantages:
Easy to clean and maintain.
Allow the equipment to be used for slovents other than water. E.g. ethanol.
Disadvantages:
All the liquor is heated all the time.
The heating surface is limited.

32. Horizontal tube evaporation

33. In this type of evaporator, several parallel 8 feet longtubes are joined together in series.
Each tube is enclosed in the opposite direction to the feed liquid in the inner tube.
The liquid passes through the tubes, gets evaporated as it moves along the tube and finally passes to a separator.
The concentrated liquid is collected at the bottom while the vapour is led off to the atmosphere or used for some heating purpose.

34. iii) Short tube evaporator
Construction:
The evaporator is a cylindrical vessel. The lower portion of the vessel consists of a nest of tubes with the liquor inside and steam outside– this assembly is called calendra.
The specifications of calendria are as follows:
Tube length:                       1 – 2 m
Tube diameter:                  40 – 80 mm
Diameter of evaporator:   2.5 m
Number of tubes:                             1000
The feed inlet is at the top of the calendra. The product outlet is placed at the bottom of the evaporator. Steam inlet and outlet is placed from the side of the calendria.

35. Working:
The liquor in the tubes is heated by the steam and begins to boil, when the mixture of liquid and vapor will shoot up the tubes (in a similar manner to that of a liquid that is allowed to boil to vigorously in a test-tube).
The product is collected through the product outlet.

36. Advantages
1.      Use of tubular calendria increases the heating area, possibly by a factor of 10 to 15 compared to that of an external jacket.
Increasing the rate of heat transfer.
3.      Condenser and receiver can be attached to run the evaporation under vacuum with nonaqueous solvents.
Disadvantages
1.     Since the evaporator is filled to a point above the level of the calendria, a considerable amount of liquid is heated for a long time. The effect of this continual heating can be reduced to some extent by removing concentrated liquor slowly from the outlet at the bottom of the vessel.
2.      Complicated design, difficult for cleaning and maintenance.

37. 2) Forced circulation evaporators
Construction:
The evaporator consists of a short tube calendria and a large cylindrical vessel (body of the evaporator) for separation of vapor and liquid takes place.
The liquor inlet is provided at the side of the cylindrical vessel.
A pump is fitted in between the calendria and the body of the evaporator.
A tangential inlet for liquid under high pressure is placed at neck of the body of the evaporator.
The vapor outlet is placed at the top of the body and it may be passed through a condenser to collect the condensed liquid.

38. Working principle:
Feed is introduced through the liquor inlet. Pump will force the liquid through the calendria. Steam heats the liquid inside the calendria. As it is under pressure in the tubes the boiling point is elevated and no boiling takes place. As the liquor leaves the tubes and enters the body of the evaporator through the tangential inlet there is a drop in pressure and vapor flashes off from the superheated liquor. The concentrated liquid is pumped out through the product outlet and the vapor is collected through the vapor outlet.

39. Advantages
These have an great advantage over natural circulation evaporators in that the rapid liquid movement improves heat transfer, especially with viscous liquids or materials that deposit solids or foam readily.
The equipment is suitable for operation under reduced pressure due to the capacity of the process to overcome the effect of greater viscosity of liquids.
These evaporators possess rapid evaporation rate.

40. 4) These evaporators are mainly used for thermolabile materials.
5) These can also be used in practice for the concentration of insulin and liver extracts.
Disadvantage:
Corrosion-erosion can occur, due to high circulation velocities.
Salt deposits detach and accumulate at the bottom.

41. 3) FILM EVAPORATORS
Film evaporators spread the material as a film over the heated surface, and the vapor escapes the film.
Following are the types of film evaporators.
i) Wiped Film evaporator
ii)Long Tube Evaporator
a) Climbing film evaporator
b) Falling film evaporator

43. i) Wiped film evaporators
Construction:
A form of film evaporator coming into increasing use is the wiped film evaporator or rotary film evaporator, which contains of a single, short tube of wide diameter, better described as a narrow cylindrical vessel,1 or 2 meters in length.
A section across the evaporator is shown here where it will be seen that the vessel is surrounded by a heated jacked. Through the vessel is a bladed rotor, with a clearance of the order of 1mm between the tips of the rotor blades and the wall of the vessel.

44. Working:
The liquor is introduced at the top of the vessel and spread as a film over the heated wall by the action of the rotor.
Evaporation occurs as the liquor passes down the wall, vapour is taken to a condenser and the concentrated liquor withdrawn at the bottom of the vessel.
The evaporator is therefore a form of single tube, falling film evaporator in which the film is formed and agitated mechanically.

45. ii) Long tube evaporators (Climbing film evaporators)
Construction:
The heating unit consists of steam-jacketed tubes, having a length to diameter ratio of about 140 to 1, so that a large evaporator may have tubes 50 mm in diameter and about 7 m in length.
The liquor to be evaporated is introduced into the bottom of the tube, a film of liquid forms on the walls and rises up the tubes, hence it is called climbing film evaporator.

46. Working:
At the upper end, the mixture of vapor and concentrated liquor enters a separator, the vapor passes to a condenser, and the concentrated liquid to a receiver.
Cold or pre heated liquor is introduced into the tube.
Heat is transferred to the liquor from the walls and boiling begins.
Ultimately sufficient vapor has been formed for the smaller bubbles to unite to a large bubble,
filling the width of the tube and trapping a ‘slug’ of liquid above the bubble.
As more vapor is formed, the slug of liquid is blown up the tube, the tube is filled with vapor, while the liquid continues to vaporize rapidly, the vapor escaping up the tube and, because of friction between the vapor and liquid, the film also is dragged up the tube upto a distance of 5 to 6 metres.

47. ii) Long tube evaporators (Falling film evaporators)
Construction:
The heating unit consists of steam- jacketed tubes, having a length to diameter ratio of about 140 to 1, so that a large evaporator may have tubes 50 mm in diameter and about 7 m in length.

48. Working principle
The liquor to be evaporated is introduced at the top of the evaporator tubes and the liquor comes down due to gravity.
The concentrate and vapor leaves the bottom. They are separated in a chamber where the concentrate is taken out through product outlet and vapor from vapor outlet.

49. Expense to manufacture and install the instrument is high.
Advantages:
Because of obtaining good heat transfer the method being especially useful with liquids that are too viscous to be processed in units in which the film is formed naturally.
Disadvantages:
A major disadvantage of falling film evaporators is the potential instability of the falling film
Expense to manufacture and install the instrument is high.
Difficult to clean and maintain.
Hajvery University (Faculty of Pharmacy)

50. Feed characteristics Influencing Evaporation
Concentration of solution
Temperature sensitivity
Foaming
Scale formation
Other properties like freezing point, specific heat, gas liberation, toxicity level, radioactivity etc

51. Evaporation Material of Construction Mostly made of steel
For highly corrosive fluids, special materials are used like, Cu, Nickle, Stainless steel, aluminium etc

52. Capacity of Evaporator
Economy of Evaporator
Economy of evaporator is the total mass of water vaporized per unit mass of steam input to the evaporator.
Capacity of Evaporator
Capacity of an evaporator is the amount of water vaporized in the evaporator per unit time.
Ratio of capacity to economy is called the steam consumption per hour.

53. Evaporation Single effect Evaporation
Nearly always the material to be evaporated flows inside the tubes.
The boiling liquid is subjected under moderate vacuum
Reducing the boiling temp of the liquid increases the temperature difference b/w the steam and the boiling liquid and thus increases the heat transfer rate in the evaporator.
When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded.
Simple but does not use steam effectively.

54. Evaporation Multiple effect Evaporation
If the vapor from one evaporator is fed into the steam chest of a second evaporator and the vapor from the second is sent to the condenser, the operation becomes double effect.
The heat in the original steam is reused in the second effect and the evaporation economy increased.
Also useful one the feed temp is very low, preheating

55. Evaporation Method of feeding Forward feed Backward feed Mixed feed
Variations in the Multiple effect has come from the mode of feed supply
There are four possible feeding arrangements
Forward feed
Backward feed
Mixed feed
Parallel feed

56. Figure: (a) Forward feed (b) Backward feed flow patterns in four effect evaporator.

57. Figure: (c) Mixed feed (d) Parallel feed flow patterns in four effect evaporator.

58. Evaporation Boiling point elevation
Boiling point of solvent increases when some solute is added, this phenomenon is called boiling point elevation.
Duhring’s rule states that a linear relationship exists b/w the temperatures at which two solutions exert the same vapor pressure. The rule is often used to compare a pure liquid and a solution at a given concentration.

59. Duhring’s rule is a graphical representation of such a relationship

60. Evaporation Types of Evaporators
Evaporators with heating medium in jacket
Vapor heated evaporators with tubular heating surfaces
Horizontal tube evaporators
Evaporators with tubes placed vertically
Evaporators with short tubes
Single effect evaporators
Multiple effect evaporators
Evaporators with long tubes
Climbing film evaporators
Falling film evaporators
Forced circulation evaporators

61. Evaporation Steam jacketed kettles/Batch type pan evaporators
Simplest and oldest type
Steam supplied in the jacket gives it’s heat content and condensate leaves through the outlet.

62. Evaporation Advantages
Used for both small scale and large scale operations
Simple in construction and easy to operate
Low maintenance and installation
Disadvantages
Heat economy is less
Not suitable for heat sensitive materials
Heat transfer rate decreases drastically when the conc. increased

63. Evaporation Horizontal Tube evaporators
Steam is passed through the tube which are immersed in the pool of liquid to be evaporated.
Feed is introduced into the evaporator until the steam compartment is immersed. The feed absorbs heat and solvent is evaporated.
Use: Best suited for non-viscous liquids because of poor circulation

64. Evaporation Vertical Tube evaporators
Liquid is passed through the vertical tubes and steam is supplied from outside the tubes
It consists of short vertical tubes, typically 1-2 m long and 50 – 100 mm in diameter are arranged inside the steam chest.
The tube bundle is located in the bottom of the vessel
Provides more heat transfer surface area
Liquid level is to be maintained above the tubes/calandria
Not suitable for solution which have solid particles

65. Evaporation Climbing Film evaporators
Liquid is passed through the vertical tubes and steam is supplied from outside the tubes
It consists of short vertical tubes, typically 1-2 m long and 50 – 100 mm in diameter are arranged inside the steam chest.
The tube bundle is located in the bottom of the vessel
Provides more heat transfer surface area
Liquid level is to be maintained above the tubes/calandria

66. Evaporation Heat transfer coefficient
The heat flux and the evaporator capacity are affected by the overall heat transfer coefficient.
The heat transfer coefficient is influenced by the design and method of operation of the evaporator.
Coefficient is a reciprocal of five individual resistances: the steam film resistance, the two scale resistances, the tube wall resistance, and the resistance from boiling liquid.
Tube side solution, shell side steam

67. Evaporation Steam film coefficient
Steam will have high heat transfer coefficient.
Depends upon the type of condensation.
No scaling as there is no solute
The presence of noncondensable gas seriously reduces the steam film coefficient.
Tube side high possibility of scaling, thus often cleaning is necessary

68. Evaporation Liquid-side coefficient
Liquid side coefficient depends upon the velocity of liquid.
In case of viscous materials, the resistance of the liquid side controls the overall rate of heat transfer to the boiling liquid.
Forced circulation gives higher heat transfer coefficient.
Tube side high possibility of scaling, thus often cleaning is necessary

69. Because of the difficulty of measuring individual coefficients in an evaporator, experimental results are usually expressed in terms of overall coefficients.

70. Evaporation Enthalpy balances for single effect evaporator
Latent heat of condensation of the steam is transferred to vaporize water.
Two enthalpy balances are needed, one for steam and one for the vapor or the liquid side.

71. Evaporation Industrial applications In sugar industry
In dehydrating milk, which is then used in many food products
In fertilizer plants
Evaporation is not only removing water
In the production of refined petroleum products, more volatile compounds are evaporated off to separate the more crude components