Evaporation
Unit Operations of Chemical Engineering
EVAPORATION Evaporation (1)
Evaporation
General concepts • Evaporation refers to the removal of a solvent (usually water) from a solution by means of boiling the liquor in a vessel (the evaporator) which has been designed for eliminating partially the water and for withdrawing the vapour. An evaporator can be considered an heat exchanger modified, and may be studied as a heat transfer unit because of depending on the heat speed transferred to the solution to be concentrated. • The aim is to concentrate a solute, which always must be considered as non-volatile, by eliminating the solvent. If the evaporation is not stopped when the solute begins to precipitate, the operation can be considered a crystallisation. • A lot of products can not stand high temperatures, according to it sometimes the evaporation is carried out by using vacuum inside the evaporation chamber which allows the boiling point of the mixture to decrease, therefore it is necessary a vacuum system. It is very important to bear in mind the advantages of the evaporation using pressures below atmospheric pressure. • The presence of a solute in the solution to be concentrated causes the solution boiling point to increase (compared to the corresponding temperature at the pressure in the evaporation chamber), this is known as boiling point rise, (BPR). • Evaporators are used either in inorganic chemistry and organic chemistry, in paper pulp, in feeding industry, especially in the concentration of sugar. Two typical examples are the concentration of fruit juices and the concentration of the caustic soda. • To supply the necessary heat to concentrate a solution the normal heating medium is steam which condensates in the evaporator and transfers its latent heat. • Applying methods to recover the transfer heat from the vapour makes possible to achieve great economy in heat utilisation. Heat transfer in evaporators The equation for heat transfer takes the form: where:
Q is the heat transferred per unit time U is the overall coefficient of heat transfer A is the heat transfer surface ∆T is the temperature difference between the two streams
On applying the previous equation, there is some difficult in deciding the correct value for the temperature difference because of the BPR. If water is boiled in an evaporator under a given pressure, then the temperature of the liquor can be determined from steam tables and temperature difference is readily calculated. At the same pressure a solution has a higher boiling point than that of water, and the difference between its boiling point and that of water is the BPR. Evaporation (2)
Evaporation
Types of evaporators (I)
Feed
Vapour
Steam condensate
Steam condensate
Steam
Concentrated solution
Evaporation chamber is made up by a vertical cylinder body, concentrated solution leaves the system by the bottom. Conic shapes may be favourable for the cleaning if crystallization appears. Steam cross through the tubes and condenses inside them. This system is advisable for not viscous and without crystallizing.
Evaporator with horizontal tubes
Evaporation (3)
Evaporation
Types of evaporators (II) vapour
feed
vent Steam
Steam condensate
Concentrated solution
There is an annular chamber crossed by bundles of vertical tubes with steam condensing on its outside surface. In the heat exchanger section, called calandria in this type of evaporator, steam condenses in the outer jacket and the liquid being evaporated boils on the inside of the tubes and in the space above the upper tube plate. Values of overall heat transfer coefficients that have been reported for evaporators are of the order of 1800-5000 J m-2 s-1 °C-1 for the evaporation of distilled water in a vertical-tube evaporator with heat supplied by condensing steam. However, with dissolved solids in increasing quantities as evaporation proceeds leading to increased viscosity and poorer circulation, heat transfer coefficients in practice may be much lower than this. At the top of the cylinder, there are baffles, which allow the vapours to escape but check liquid droplets that may accompany the vapours from the liquid surface.
Evaporator with vertical tubes Evaporation (4)
Evaporation
Boiling point of solutions (ºC)
Dühring plots for boiling point solutions (Dühring’s rule)
To calculate the BPR it is useful the use of Dühring charts where the boiling point of the solution is plotted against that of water at the same pressure and a straight line is obtained. The below figure shows the Dühring chart corresponding with solutions of caustic soda. The boiling point rise is much greater with strong electrolytes such as salt, and caustic soda. The effect of hydrostatic head may be considered by supposing the solution to be boiling at the top of a tube. Then the pressure of the liquid which is just at the top of the tubes is that in the vapour space. The solution at the bottom of the tubes is at a higher pressure due to hydrostatic head and the solution must be heated to the higher temperature corresponding to the increased pressure. This effect is not very deep if the temperature difference is not very high.
Schematic representation of an evaporator Vapour Evaporation chamber Solution Steam
Heating chamber
Feed (Diluted solution)
Concentrated solution
Condensed steam
Boiling point of water (ºC)
Two essential parts must be emphasized: the heating chamber and the evaporation chamber and both are separated by a heat transfer surface. Evaporation (5)
Evaporation
Mass and heat balances
Overall mass balance: Balance in relation to the solute to be evaporated:
Heat balance: From here it may be known the amount of steam necessary to achieve the evaporation process:
The heat transferred across the heating surface will be: Assuming steam is saturated vapour when enters the evaporator and saturated liquid when leaves it gives: Evaporation (6)
Evaporation
Evaporators characteristics It may be defined two characteristics (among others) which allow evaluating the behaviour of an evaporator. Economy:
Capacity:
Finally, to connect the two above equations it may be defined the Steam consumption:
The economy of only one evaporator is always less than one (100%). When a evaporators system (multiple-effect) is used, the economy increases with the number of effects. When feed enters the evaporator as a liquid below the boiling point, an amount of the heat supplied must be used to warm up the feed until its boiling point. When the feed is above its boiling point, a flash evaporation can happen. Usually, the steam which leaves the evaporation chamber is overheated and its enthalpy can be calculated by using the following expression: When the streams only have one component, usually water or steam, it is easy to obtain their enthalpy values from charts. When there are more than one component in the solution streams, it is necessary to turn to experimental data such as graphics where may be found the relations among enthalpies, concentrations and temperatures (enthalpy-concentration charts). If the latent heat for water must be calculated, the Regnault equation may be applied:
Evaporation (7)
Evaporation
Enthalpy-concentration charts
Enthalpy (kcal/kg solution)
To know the stream enthalpies of the feed and concentrated solution, which respectively enters and leaves the evaporator, the enthalpy-concentration charts are useful and they must be calculated for each solute. These diagrams have the information about the mixing heat. Here it can be seen the chart corresponding with the solution of NaOH in water at different concentrations. To use them it must be read the concentration on the xaxis, the solution temperature must be found and the enthalpy will be read on the y-axis. When dealing with solutions with small mixing heat, the enthalpies may be calculated from their specific heats.
Concentration (kg NaOH/kg solution)
Evaporation (8)
Evaporation
Properties of saturated steam (I) Absolute pressure [kN/m2]
Temperature
Enthalpy per unit mass
Absolute pressure
[kJ/kg]
[kN / m2]
[ºC]
[K]
ΘS
TS
water
latent heat
steam
1.0
6.98
280.13
29.3
2485.0
2514.4
2.0
17.51
290.66
73.5
2460.2
3.0
24.10
297.25
101.0
4.0
28.98
302.13
5.0
32.90
6.0
Temperature
Enthalpy per unit mass
[ºC]
[K]
[kJ/kg]
ΘS
TS
water
latent heat
steam
40.0
75.89
349.04
317.7
2319.2
2636.9
2533.6
45.0
78.74
351.89
329.6
2312.0
2641.7
2444.6
2545.6
50.0
81.35
354.5
340.6
2305.4
2646.0
121.4
2433.1
2554.5
60.0
85.95
359.10
359.9
2293.6
2653.6
306.05
137.8
2423.8
2561.6
70.0
89.96
363.11
376.8
2283.3
2660.1
36.18
309.33
151.5
2416.0
2567.5
80.0
93.51
366.66
391.7
2274.0
2665.8
7.0
39.03
312.18
163.4
2409.2
2572.6
90.0
96.71
369.86
405.2
2265.6
2670.9
8.0
41.54
314.69
173.9
2403.2
2577.1
100.0
99.63
372.78
417.5
2257.9
2675.4
9.0
43.79
316.94
183.3
2397.9
2581.1
101.325
100.00
373.15
419.1
2256.9
2676.0
10.0
45.83
318.98
191.8
2392.9
2584.8
105.0
101.0
374.15
423.3
2254.3
2677.6
12.0
49.45
322.60
206.9
2384.2
2591.2
110.0
102.32
375.47
428.8
2250.8
2679.6
14.0
52.58
325.73
220.0
2376.7
2596.7
115.0
103.59
376.74
434.2
2247.4
2681.6
16.0
55.34
328.49
231.6
2370.0
2601.6
120.0
104.81
377.96
439.4
2244.1
2683.4
18.0
57.83
330.98
242.0
2363.9
2605.9
125.0
105.99
379.14
444.4
2240.9
2685.2
20.0
60.09
333.24
251.5
2358.4
2609.9
130.0
107.13
380.28
449.2
2237.8
2687.0
25.0
64.99
338.14
272.0
2346.4
2618.3
135.0
108.24
381.39
453.9
2234.8
2688.7
30.0
69.13
342.28
289.3
2336.1
2625.4
140.0
109.32
382.47
458.4
2231.9
2690.3
35.0
72.71
345.86
304.3
2327.2
2631.5
145.0
110.36
383.51
462.8
2229.0
2691.8
Evaporation (9)
Evaporation
Properties of saturated steam (II) Absolute pressure [kN/m2]
Temperature
Enthalpy per unit mass
Absolute pressure
[kJ/kg]
[kN/m2 ]
[ºC]
[K]
ΘS
TS
water
latent heat
steam
150.0
111.37
384.52
467.1
2226.2
2693.4
155.0
112.36
385.51
471.3
2223.5
160.0
113.32
386.47
475.4
165.0
114.26
387.41
170.0
115.17
175.0
Temperature
Enthalpy per unit mass
[ºC]
[K]
[kJ/kg]
ΘS
TS
water
latent heat
steam
280.0
131.21
404.36
551.5
2170.1
2721.5
2694.8
290.0
132.39
405.54
556.5
2166.6
2723.1
2220.9
2696.2
300.0
133.54
406.69
561.4
2163.2
2724.7
479.4
2218.3
2697.6
320.0
135.76
408.91
570.9
2156.7
2727.6
388.32
483.2
2215.7
2699.0
340.0
137.86
411.01
579.9
2150.4
2730.3
116.06
389.21
487.0
2213.3
2700.3
360.0
139.87
413.02
588.5
2144.4
2732.9
180.0
116.93
390.08
490.7
2210.8
2701.5
380.0
141.79
414.94
596.8
2138.6
2735.3
185.0
117.79
390.94
494.3
2208.5
2702.8
400.0
143.63
416.78
604.7
2132.9
2737.6
190.0
118.62
391.77
497.9
2206.1
2704.0
420.0
145.39
418.54
612.3
2127.5
2739.8
195.0
119.43
392.58
501.3
2203.8
2705.1
440.0
147.09
420.24
619.6
2122.3
2741.9
200.0
120.23
393.38
504.7
2201.6
2706.3
460.0
148.73
421.88
626.7
2117.2
2743.9
210.0
121.78
394.93
511.3
2197.2
2708.5
480.0
150.31
423.46
633.5
2112.2
2745.7
220.0
123.27
396.42
517.6
2193.0
2710.6
500.0
151.85
425.00
640.1
2107.4
2747.5
230.0
124.71
397.86
523.7
2188.9
2712.6
520.0
153.33
426.48
646.5
2102.7
2749.3
240.0
126.09
399.24
529.6
2184.9
2714.5
540.0
154.77
427.92
652.8
2098.1
2750.9
250.0
127.43
400.58
535.4
2181.0
2716.4
560.0
156.16
429.31
658.8
2093.7
2752.5
260.0
128.73
401.88
540.9
2177.3
2718.2
580.0
157.52
430.67
664.7
2089.3
2754.0
270.0
129.99
403.14
546.2
2173.6
2719.9
600.0
158.84
431.99
670.4
2085.0
2755.5
Evaporation (10)
Evaporation
Properties of saturated steam (III) Absolute pressure [kN/m2]
Temperature
Enthalpy per unit mass
Absolute pressure
[kJ/kg]
[kN/m2]
[ºC]
[K]
ΘS
TS
water
latent heat
steam
620.0
160.12
433.27
676.0
2080.8
2756.9
640.0
161.38
434.53
681.5
2076.7
660.0
162.60
435.75
686.8
680.0
163.79
436.94
700.0
164.96
720.0
Temperature
Enthalpy per unit mass
[ºC]
[K]
[kJ/kg]
ΘS
TS
water
latent heat
steam
980.0
179.01
452.16
758.7
2016.7
2775.4
2758.2
1000.0
179.88
453.03
762.6
2013.6
2776.2
2072.7
2759.5
1100.0
184.06
457.21
781.1
1998.6
2779.7
692.0
2068.8
2760.8
1200.0
187.96
461.11
798.4
1984.3
2782.7
438.11
697.1
2064.9
2762.0
1300.0
191.60
464.75
814.7
1970.7
2785.4
166.10
439.25
702.0
2061.1
2763.2
1400.0
195.04
468.19
830.1
1957.7
2787.8
740.0
167.21
440.36
706.9
2057.4
2764.3
1500.0
198.28
471.43
844.6
1945.3
2789.9
760.0
168.30
441.45
711.7
2053.7
2765.4
1600.0
201.37
474.52
858.5
1933.2
2791.7
780.0
169.37
442.52
716.3
2050.1
2766.4
1700.0
204.30
477.45
871.8
1921.6
2793.4
800.0
170.41
443.56
720.9
2046.5
2767.5
1800.0
207.11
480.26
884.5
1910.3
2794.8
820.0
171.44
444.59
725.4
2043.0
2768.5
1900.0
209.79
482.94
896.8
1899.3
2796.1
840.0
172.45
445.60
729.9
2039.6
2769.4
2000.0
212.37
485.52
908.6
1888.7
2797.2
860.0
173.43
446.58
734.2
2036.2
2770.4
2200.0
217.24
490.39
930.9
1868.1
2799.1
880.0
174.40
447.55
738.5
2032.8
2771.3
2400.0
221.78
494.93
951.9
1848.5
2800.4
900.0
175.36
448.51
742.6
2029.5
2772.1
2600.0
226.03
499.18
971.7
1829.7
2801.4
920.0
176.29
449.44
746.8
2026.2
2773.0
3000.0
233.84
506.99
1008.3
1794.0
2802.3
940.0
177.21
450.36
750.8
2023.0
2773.8
3500.0
242.54
515.69
1049.7
1752.2
2802.0
960.0
178.12
451.27
754.8
2019.8
2774.6
4000.0
250.33
523.48
1087.4
1712.9
2800.3
Evaporation (11)
Evaporation
E-1. A solution containing 10% of caustic soda has to be concentrated to a 40% solution at the rate of 5000 kg/h. The below figure shows the layout where one single-effect evaporator, one direct contact condenser and two heat exchangers are arranged to carry out this operation. Saturated steam at 3.2 atm (absolute pressure) leaves the evaporator as saturated liquid and leaves the exchanger at 40ºC. Absolute pressure in the evaporation chamber: 560 mmHg. Liquid temperature at the outlet of the direct contact condenser: 92 ºC. Feed enters the evaporator at 20 ºC. Concentrated solution leaves the exchanger at 45 ªC. Cooling water enters the exchanger at 20 ºC at leaves it at 50 ºC. Uevaporator: 1800 kcal/m2hºC Uexchanger (heating the diluted solution): 500 kcal/m2hºC Uexchanger (cooling the concentrated solution): 300 kcal/m2hºC a. Calculate the boiling solution temperature and all the data needed. b. Calculate the saturated steam consumed. c.
Calculate the heat transfer surface of the evaporator.
d. Calculate the cooling water needed. e. Calculate the heat transfer surface of each exchanger.
Evaporation (12)
Evaporation
Multiple effect evaporators The basic concept is consider that the vapour produced is suitable for passing to another similar unit and for using it as heating system in this unit and son on.
Forward feed arrangement for a triple-effect evaporator aigua
Condensador
P1
P2
T1
T2
P3 T3
Condensat
Vapor condensat
Condensat
Concentrat
Evaporation (13)
Evaporation
E-2. A solution containing 15% solids is to be concentrated to 55% solids in a double-effect evaporator operating at a pressure in the second effect of 18 kN/m2. The feed is 2.5 kg/s at a temperature of 375 K ad its specific heat is 3.75 kJ/kg K. The boiling point rise of the concentrated solution is 6 K and the steam fed to the first effect is at 240 kN/m2. The overall heat transfer coefficients in the first and second effects are 1.8 and 0.63 kW/m2 K respectively. If the heat transfer area is to be the same in each effect, determine its value.
Evaporation (14)
