1. Chapter 16
Evaporation

2. The objective of evaporation is to concentrate a consisting of a nonvolatile solute and a volatile solvent. Evaporation is conducted by vaporizing a portion of the solvent to produce a concentrated solution or thick liquor.

3. Evaporation differs from distillation in that the vapor usually is a single component, and even when the vapor is a mixture ,no attempt is made in the evaporation step to separate the vapor into fractions .

4. concentrating a solution rather than forming and building crystals.
Evaporation differs from crystallization in that emphasis is placed on
concentrating a solution rather than
forming and building crystals.

5. Normally ,in evaporation the thick liquor is the valuable product and the vapor is condensed and discarded.

6. Liquid characteristics
The practical solution of an evaporation problem is profoundly affected by the character of the liquor to be concentrated.
Some of the most important properties of evaporating liquids are as follows.

7. Concentration
As the concentration increases, the solution becomes more and more individualistic. The density and viscosity increase with solid content.

8. Continued boiling of a saturated solution causes crystals to form; these must be removed or the tube clog.

9. The boiling point of the solution may also rise considerably as the solid content increases, so that the boiling temperature of a concentrated solution may be much higher than that of water at the same pressure.

10. Foaming
A stable foam accompanies the vapor out of the evaporator, causing heavy entrainment. In extreme cases the entire mass of liquid may boil over into the vapor outlet and be lost.

11. Temperature sensitivity
Many fine chemicals , pharmaceutical products, and foods are damaged when heated to moderate temperatures for relatively short time .

12. scale
Some solutions deposit scale on the heating surfaces. The over-all coefficient then steadily diminishes, until the evaporator must be shut down and the tubes cleaned.

13. Single- and multiple-effect operation
Most evaporators are heated by steam
condensing on metal tubes. Nearly always the material to be evaporated flows inside the tubes.

14. Reducing the boiling temperature of the liquid increasing the temperature difference between the stream and the boiling liquid and thereby increases the heat-transfer rate in the evaporator

15. single-effect evaporation.
When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded, the method is called
single-effect evaporation.

16. If the vapor from one evaporator is fed into the stream chest of a second evaporator and vapor from the second is then sent to a condenser, the operation become
double-effect.

17. Additional effects can be added in the same manner.
A series of evaporators between the stream supply and the condenser is called
multiple-effect evaporator.

18. Types of evaporators
The chief types of stream-heated tubular evaporator in uses today are:
1）Short-tube evaporators
2）Long-tube vertical evaporators
forced-circulation
Upward-flow (climbing-film)
down-ward-flow (falling-film)

19. Once-through and circulation evaporators
Evaporators may be operated either as once-through or circulation units.
In once-through operation the feed liquor passes through the tubes only once, releases the vapor, and leaves the unit as thicker liquor. All the evaporation is accomplished in a single pass

20. In circulation evaporators a pool of liquid is held within the equipment. Incoming feed mixes with the liquid from the pool, and mixture passes through the tubes.

21. Unevaporated liquid discharged from the
tubes returns to the pool
All short-tube and forced-circulation evaporators are operated in this way.

22. Short-tube evaporators
In the older types of evaporators the tubes are “short”.
In the short-tube vertical evaporator shown in Fig

23. vapor
Central downcomer
feed
Stream inter
condensate
concentrate

24. The stream condenses outside the tubes
The stream condenses outside the tubes. The tube bundle contains a large central downcomer, the cross-sectional area of which is 25 to 40 percent of the total cross-sectional area of the tubes

25. Most of the boiling takes place in the smaller tubes, so that the liquid rises through these tubes and returns through the downcomer.

26. In this evaporator the driving force for flow of liquid through the tubes is the difference in density between the liquid in the downcomer and the mixture of liquid and vapor in the tubes.

27. Circulation is by natural convection but at much less rapid rate than in long-tube natural-circulation evaporators; the heat-transfer coefficients, therefore, are fairly high with thin liquids but low when the liquid is viscous.

28. Long-tube evaporators with upward flow
A typical long-tube vertical evaporator
with upward flow of the liquid is shown in Fig.

29. Vapor out
Stream in
Concentrate out
feed
condensate

30. Forced-circulation evaporators
Higher coefficients are obtained in forced-circulation evaporators, an example of which is shown in fig.

31. Vapor out
Deflector plate
Stream in
feed
Concentrate out
Condensate

32. Here a centrifugal pump forces liquid through the tubes at an entering velocity of 2 to 5.5m/s

33. The tubes are under sufficient static head to ensure that there is no boiling in the tubes; the liquid becomes super heated as the static head is reduced during flow from the heater to the vapor space, and it “flashes” into a mixture of vapor and spray in there

34. Film evaporators
Concentration of highly heat-sensitive materials such as orange juice requires a minimum time of exposure to a heated surface
This can be done in once-through film evaporators

35. Vapor out
Concentrate out
Stream in
Condensate
Feed in

36. There are two types of film evaporators:
climbing-film evaporator
falling-film evaporator

37. The chief problem in a fall-film evaporator is that of distributing the liquid uniformly as a film inside tubes.

38. Feed in
Stream in
Vapor out
condensate
Concentrate out

39. This is done by inserts the metal plate in the tube ends to cause the liquid to flow evenly into each tube, or by spraying the feed on the inside surface of each tube with the “spider” distributors

40. Still another way is to use an individual spray nozzle inside each tube

41. For good heat transfer the Reynolds number
of the falling film should be greater than 2000
at all point in the tube.
During evaporation the amount of liquid is continuously reduced as it flows downward.

42. Too great a reduction can lead to dry spots near the bottom of the tube.
Falling-film evaporators can be used in
concentrating sensitive products. They are also well adapted to concentrating viscous liquids.

43. Performance of tubular evaporators
The principal measures of the performance of a steam-heated tubular evaporator are capacity and the economy

44. Capacity
is defined as the number of kilograms of water vaporized per hour.
Economy
is the number of kilograms vaporized
per kilogram of stream fed to the unit.

45. Evaporator Capacity
The rate of heat transfer q through the heating surface of an evaporator is the product of three factors:
The area of the heat-transfer surface A
The overall heat-transfer coefficient U
The overall temperature drop Δt

46. (16-1)

47. If the feed to the evaporator is at the boiling temperature corresponding to the absolute pressure in the vapor space.
All the heat transferred through the heating surface is available for evaporation.
The capacity is proportional to q

48. If the feed is cold, the energy is required for heating it to its boiling point.
The capacity for a given value of q is reduced accordingly , as heat used to heat the feed is not available for evaporation.

49. If the feed is at the temperature above the boiling point, a portion of the feed evaporates spontaneously.
The capacity is greater than that corresponding to q. this process is called flash evaporation.

50. Temperature difference
The actual temperature drop across the heating surface depends on :
The solution being evaporated
The difference in pressure between the stream chest and the vapor space above the boiling liquid, and depth of liquid over heating surface.
The friction loss in the tubes

51. In actual evaporators, however, the boiling
point of a solution is affected by two factors:
The boiling point elevation
And liquid head

52. Boiling-point elevation and Dühring’s rule
For a given pressure the boiling point of the aqueous solutions is higher than that of pure water.

53. The increase in boiling point over that of water is known as the boiling-point elevation (BPE) of the solution.

54. (BPE) is best found from an empirical rule known as Dühring’s rule
(BPE) is best found from an empirical rule known as Dühring’s rule. Which states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure.

55. If the boiling point of the solution is plotted against that of water at the same pressure, a straight line results.
Different lines are obtained for different concentrations

57. Effect of liquid head and friction on temperature drop
If the depth of liquid in an evaporator is appreciable, the boiling point corresponding to the pressure in the vapor space is that of the surface layer of liquid only.

58. At the distance Z m below the surface is under a pressure of the vapor space plus a head of Z m of liquid.

59. In an evaporator, therefore, the average
boiling point of the liquid in the tubes is
higher than the boiling point in the vapor space.

60. This increase in boiling point lowers the average temperature drop between the steam and the liquid and reduces the capacity.

61. The true temperature drop, corrected for both boiling elevation and static head, is represented by the average temperature drop between the saturation temperature of stream and the variable liquid temperature.

62. Heat-transfer coefficient
The overall coefficient is strongly influenced by the design and method of operation of the evaporator.

63. In most evaporators the fouling factor of the condensing steam and resistance of the tube wall are very small, and they are usually neglected.

64. Steam-film coefficients
The steam-film coefficient is high. Since the presence of non-condensable gas seriously reduces the film coefficient.

65. Provision must be made to vent noncondensables from the steam chest and to prevent leakage of air inward.

66. Liquid-side coefficients
The liquid-side coefficient depends to a large extent on the velocity of the liquid over the heated surface.

67. The resistance of the liquid side controls the overall rate of heat transfer to the boiling liquid for viscous fluid.
Forced circulation gives high liquid-side
coefficient.

68. Because of the difficulty of measuring the high individual film coefficients in an evaporators, experimental results are usually expressed in terms of overall coefficients

69. If one resistance( say, that of the liquid film) is controlling ,large changes in the other resistances have almost no effect on the overall coefficient.

70. Typical overall coefficients for various types of evaporators are given in table
W/m2ºC
Long-tube vertical evaporator
Natural circulation
Force circulation

71. Evaporator Economy
The chief factor influencing the economy of an evaporator system is the number of effects.
The economy also is influenced by the
temperature of the feed.
Quantitatively, evaporator economy is
entirely a matter of enthalpy balance.

72. Enthalpy balances for single-effect evaporator
In single-effect evaporator, the latent heat of condensation of the team is transferred through a heating surface to vaporize water from a boiling solution.

73. Two enthalpy balances are needed, one for the team and one for the vapor or liquid side.

74. Fig. shows a single-effect Evaporator.
The rate of steam and of condensate is ms
Feed is mf and that of the concentrate is m
The rate of vapor flow to the condenser is mf -m

75. Vapor out
Stream in
condensate
feed
Concentrate out

76. It is assumed that there is no leakage or
entrainment,
That the flow of noncondensable is negligible, and that heat losses from the evaporator need not be considered.

77. Both the superheat of the steam and the subcooling of the condensate are small, however, and it is acceptable to neglect them in making an enthalpy balance.

78. Under these assumptions the difference between the enthalpy of the steam and that of the condensate is simply λs .

79. The enthalpy balance for the steam side is
(16-2)
The enthalpy balance for the liquor side is
(16-3)

80. In the absence of heat losses, the heat transferred from the steam to the tubes equals that transferred from the tubes to the liquor.

81. Thus , by combing Eqs. (16-2) and(16-3)
(16-4)

82. The liquor-side enthalpies depend upon the characteristics of the solution being concentrated.
Most of solutions when mixed or dilute at constant temperature do not give much heat effect.

83. Some of solutions when mixed or dilute evolve considerable heat effect.

84. An equivalent amount of heat is required, in addition to the latent heat of vaporization, when dilute solutions of these substances are concentrated to high densities.

85. Enthalpy balance with negligible heat of dilution
For solutions having negligible heats of dilution, the enthalpy balances over a single-effect evaporator can be calculated from the specific heats and temperatures of the solutions.

86. The heat-transfer rate q on the liquor side includes:
qf, the heat transferred to the thin liquor to change its temperature from tf to the boiling temperature t
qv, the heat to accomplish the evaporation

87. If the specific heat of the thin liquor is assumed constant over the temperature range , then
(16-5)
and
(16-6)

88. If the boiling-point elevation of the thick liquor is negligible, λv=λ, the latent heat of vaporization of water at the pressure in the vapor space.

89. When the boiling-point elevation is appreciable, λv differs slightly fromλ.
In practice, however, it is nearly always sufficiently accurate to use λ.

90. The final equation for the enthalpy balance can be gotten from Eqs
The final equation for the enthalpy balance can be gotten from Eqs. (16-5) and (16-6)when the heat of dilution is negligible.
(16-7)

91. Equation (16-7) states that the heat from the condensing steam is utilized
To vaporize water from the solution
To heat the feed to the boiling point

92. If the feed enters above the boiling point in the evaporator, part of the evaporation is from flash.

93. Enthalpy balance with appreciable heat of dilution
If the heat of dilution of the liquor being concentrated is too large to be neglected, an enthalpy-concentration diagram is used for the values of H in Eq. (16-4)

94. Figure is an enthalpy-concentration diagram for solution of sodium hydroxide and water.

96. Single-effect calculations
The use of material balances , enthalpy balances, and the capacity equation (16-1) in the design of single-effect evaporation is shown in example16.1

97. Multiple-effect evaporators
Figure shows a triple-effect system
feed
Steam in

98. Connections are made so that the vapor from one effect serves as the heating medium for the next.

99. A condenser and air ejector establish a vacuum in third effect in the series and withdraw noncondensables from the system.

100. The first effect in the series is the effect to which the raw steam is fed and in which the pressure in the vapor space.
The last effect is that in which the vapor-space pressure is minimum.

101. In this manner the pressure difference between the steam and the condenser is spread across two or more effects in the multiple-effect system.

102. The pressure in each effect is lower than that in the effect from which it receives steam and higher than that of the effect to which it supplies vapor.

103. Each effect has a temperature drop across its heating surface corresponding to the pressure drop in that effect.

104. In figure, dilute feed enters the first effect, where it is partly concentrated;
It flows to the second effect for additional concentration and then to the third effect for final concentration.
Thick liquor is pumped out of the third effect.

105. In steady operation all internal concentrations, flow rates, pressures, and temperatures are kept constant.

106. The heating surface in the first effect will transmit per hour an amount of heat given by the equation
(16-8)

107. If the part of this heat that goes to heat the feed to the boiling point is negligible for the Moment.

108. The temperature of the condensate leaving the second effect is very near the temperature t1 of the vapors from the boiling liquid in the first effect.

109. In steady operation the heat that was expanded in creating vapor in the first effect must be neglected when this same vapor condenses in the second effect.

110. The heat transmitted in the second effect, however, is given by the equation
(16-9)

111. As has just been shown, q1 and q2 are nearly equal, and therefore
(16-10)

112. This same reasoning may be extended to show that, roughly
(16-11)

113. In ordinary practice the heating areas in all the effects of a multiple-effect evaporator are equal.

114. Therefore, from Eq. (16-11)it follows that since q1=q2=q3=q

115. Methods of feeding Forward feed
The usual method of feeding in a multiple-effect evaporator system is forward feed.

116. The feed is pumped into the first effect and send it in turn through the other effects. As shown in fig.

118. The concentration of the liquid increases from the first effect to the last.
The transfer from effect to effect can be done with pumps, since the flow is in the direction of decreasing pressure.

119. Backward feed
Another common method is backward
feed. As shown in figure.

121. Dilute liquid is fed to the last effect and then pumped through the successive effects to the first.

122. This method requires a pump between each pair of effects, since the floe is from low pressure to high pressure.

123. Backward feed often gives a higher capacity than forward feed when the thick liquor is viscous.
It may gives a lower economy than forward feed when the feed liquor is cold.