Evaporation is a special case of heat transfer to a boiling liquid.

Transcription

1 Evaporator

2 Evaporation is a special case of heat transfer to a boiling liquid. Evaporation occurs at the liquid vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature.

3 Evaporation is a unit operation in which solvent (water) is removed by means of vaporization or boiling Evaporation is the removal of solvent as a vapour from a solution or slurry. Evaporation is used for concentration of aqueous solutions, it involves removal of water from solution by boiling the liquor in suitable vessel called evaporator and withdrawing the vapour.

4 Objectives of Evaporation: To concentrate a non-volatile solute (solute has negligible volatility) such as organic compounds, inorganic salts, acids or bases from a solvent. Common Solutes: Caustic soda, Caustic potash, Sodium sulfate, Sodium chloride, Phosphoric acid and Urea Common Solvent: Water

5 Examples of Evaporation: 1. Concentration of cane sugar juice in a sugar plant 2. Concentration of an aqueous solution of ammonium sulphate in a fertilizer plant 3. Concentration of dilute recycled sodium hydroxide in an alumina plant and many others

6 Distillation: Solution containing more than one volatile compound is vaporized (in a reboiler) and the components are separated thereafter in distillation column. Drying: Entire solvent is vaporized out from a solution leaving a solid residue as the product, the operation is called drying. Evaporation: Evaporation of solution is an essential step in the operation of a crystallization unit. In crystallization, the solution is evaporated to make it supersaturated. Crystal grow in the supersaturated solution.

7 Evaporation Vaporizing solvent is the main function Crystallization Crystalline product and crystal growth is main function Food, pulp and paper, pharmaceuticals, fine chemicals, organic and inorganic chemicals, polymer etc.

8 Evaporator

9 Invention of Evaporators Norbert Rillieux is famous for his invention of the multiple effect pan evaporator for sugar refining process in Rillieux was born in New Orleans, Louisiana in He used the steam generated from one pan to heat the sugar juice in the next pan for energy efficient means of water evaporation.

10 Equipment, in which evaporation is performed, is known as evaporator. The evaporators used in chemical process industries are heated by steam and have tubular surface. An adequate number of tubes are provided through which the solution is circulated and the tubes are heated by steam. In general the steam is the saturated steam and thus it condenses on the outer tube surface in order to heat the tube.

11 Classification of Evaporators The velocity of circulation of the solution through the tubes should be reasonably high so that (i) a high inside heat transfer coefficient is attained and (ii) formation of deposits or scales on the inner surface is reduced. Circulation may be caused by density gradient of the solution in the vertical tubes or by an external mechanical means like a pump. Accordingly, most evaporators are broadly classified as: (1) Natural circulation and (2) Forced circulation

12 Classification of Evaporators Evaporation can be divided into three categories on the basis of boiling phenomena (i) Pool boiling: In this phenomena bulk or pool of liquid boils. Examples are kettle boiling, natural circulation boiling units, thermo siphon reboilers in distillation (ii) Convection heating and boiling: example is forced circulation boiling units (i) Film evaporation: In film evaporation, a thin liquid film is maintained on the heating surface

13 Evaporators can be classified as: Solar Evaporator Natural Circulation Evaporator Batch Pan Evaporator Forced circulation Evaporator Horizontal Tube Evaporator Basket Type Evaporator Long Tube Vertical Evaporator Short Tube Vertical Evaporator Rising Film Evaporator Agitated Thin Film Evaporator Plate Type Evaporator Falling Film Evaporator Horizontal Spray Film Evaporator Vapor Compression evaporator

14 Classification of Evaporators Evaporators are classified by the number of effects. In a single-effect evaporator, steam provides energy for vaporization and the vapor product is condensed and removed from the system. In a double-effect evaporator, the vapor product off the first effect is used to provide energy for a second vaporization unit.

15 Classification of Evaporators This cascading of effects can continue for many stages. Multiple-effect evaporators can remove much larger amounts of solvent than is not possible in a single effect.

16 Single Effect Evaporators In single effect evaporator, the steam is fed to the evaporator which condenses on the tube surface and the heat is transferred to the solution. The saturated vapor comes out from the evaporator and this vapor either may be vented out or condensed. The concentrated solution is taken out from the evaporator. Single effect Evaporator

17 Single effect evaporator Evaporator is made up of three functional sections: (1) Heat exchanger, (2) Evaporating section, where the liquid boils and evaporates, and (3) Separator in which the vapour leaves the liquid and passes off to the condenser or to other equipment Conventional Evaporator

18 In many evaporators, all three sections are contained in a single vertical cylinder. In the centre of the cylinder there is a steam heating section, with pipes passing through it in which the evaporating liquors rise. 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. A diagram of this type of evaporator, which may be called the conventional evaporator.

19 In the heat exchanger section, called a 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. The circulation of the liquid greatly affects evaporation rates, but circulation rates and patterns are very difficult to predict. The resistance to heat flow is imposed by the steam and liquid film coefficients and by the material of the tube walls.

20 Values of overall heat transfer coefficients that have been reported for evaporators are of the order of J m -2 s -1 C -1 for the evaporation of distilled water in a vertical-tube evaporator with heat supplied by condensing steam.

21 Multi Effect Evaporators The saturated vapor coming out from the evaporator-1 is used as steam in the second evaporator. Partially concentrated solution works as a feed to the second evaporator. This arrangement is known as double effect evaporator in forward feed scheme. Multi effect Evaporator

22 Multi Effect Evaporators Note: The vapour leaving evaporator-2 is at the boiling temperature of the liquid leaving the first effect. In order to transfer this heat from the condensing vapor from the evaporator-1 to the boiling liquid in evaporator-2, the liquid in evaporator-2 must boil at a temperature considerable less than the condensation temperature of the vaporization, in order to ensure reasonable driving force for heat transfer.

23 Multi Effect Evaporators A method of achieving this is to maintain a suitable lower pressure in the second effect so that the liquid boils at a lower temperature. Therefore, if the evaporator-1 operates at atmospheric pressure, the evaporator-2 should be operated at same suitable vacuum. The benefit of the use of multiple effect evaporators is that in this arrangement multiple reuse of heat supplied to the first effect is possible and results in improved steam economy.

24 Solar Evaporator In this evaporator, solar energy is the heating source. Production of sodium chloride from seawater or brine by concentration in large ponds has been practiced all around the world. Crystallization is done in large open tanks. The process depends upon solar radiation intensity, weather, humidity and wind velocity. It is the cheapest evaporation process because solar energy is free of cost. When small quantities of solutions are to be concentrated, batch pan evaporator is the choice.

25 Batch Pan Evaporator Production of jams and jellies, fruit juice concentration, production of some pharmaceutical products is done in these unit. The pan can be provided with a jacket or coil for circulating heating medium. This unit can be provided agitation. These are small units having restricted heat transfer area. In these units high temperature difference cannot be used due to possibility of degradation of product and fouling of the heat transfer surface. These are useful for small capacity batch operation.

26 Natural Circulation Evaporator As the name indicates, the circulation of the solution is natural and the density difference derives it. The solution gets heat up and partially vaporized as it flows up the tubes. The heated liquid flows up because of the density difference. Vapor-liquid disengagement occurs above the tube. Thick liquor comes down from this down comer and withdrawn from the bottom. The natural-circulation evaporators may be used if the solution is quite dilute.

27 Natural Circulation Evaporator In the dilute solution the natural circulation will be at sufficient speed. It may also be used when the solution does not have suspended solid particles. As the solution stays in the tube for larger time, the solution should not be heat sensitive. The Calandria type or short-tube evaporators have short tubes as compared to the long tube evaporators. The short-tube evaporation uses circulation and solution flows many times in the evaporators. However, in case of the long tube evaporator the flow is once through.

28 Natural Circulation Evaporator Calandria type Evaporator

29 Short tube Vertical Evaporators It is called as a calandria or standard evaporator. It consists of short tubes 1-2 m in length and mm diameter attached in two tube sheets. The entire assembly is called the calandria of the evaporator. The tube bundle has a large down comer at the center that helps for circulation of liquid in the evaporator. The driving force for the fluid flow is the density difference between the liquid in the down comer and in the tubes. In the tubes there is a two-phase mixture.

30 Short tube Vertical Evaporators A short tube vertical evaporator has a short tube bundle enclosed in a shell. This is called a calendria. The calendria is of annular construction, i.e. there is an open region at the center. The liquid flows down through the central open space of the calendria called down take or down comer. Thus, a continuous natural recirculation of the solution occurs. Thick product liquor is withdrawn from the bottom. Their STV evaporators are also known as standard evaporates.

31 Natural Circulation Evaporator Long-tubes vertical Evaporators

32 It is used as re boilers for distillation column and can be used for boiler feed water preparation. Horizontal tube Evaporators In horizontal tube evaporator, the tubes re horizontal. Example is kettle reboilers in distillation column. In horizontal standard evaporator, the process liquid is on the outer surface of the tube and the heating medium is inside the tubes. The unit is relatively cheaper and offers moderate to high heat transfer coefficient. Horizontal tube unit is not suitable where fouling problem is serious, because scale build up reduces the performance drastically.

33 Basket Evaporators In basket evaporator the tube bundle can be removed from the main body. Circulation of liquid occurs in the space between the shell and the tube bundle. Its advantage is that cleaning of tube is easy because the bundle is removable. In these units thermal expansion problem does not arise. The concentrated liquor leaves through an outlet pipe at the conical bottom of the evaporator. A basket evaporator uses tubes similar to those of the calandria type.

34 Falling Film Evaporators Highly heat sensitive materials are processed in falling film evaporators. They are generally once-through evaporator, in which the liquid enters at the top, flows downstream inside the heater tubes as a film and leaves from the bottom. The tubes are heated by condensing steam over the tube. As the liquid flows down, the water evaporates and the liquid gets concentrated. To have a film inside of the tube, the tube diameter is kept high whereas the height low to keep the residence time low for the flowing liquid.

35 Falling Film Evaporators Therefore, these evaporators, with non-circulation and short resistance time, handle heat sensitive material, which are very difficult to process by other method. The main problem in falling film evaporator is the distribution of the liquid uniformly as a thin film inside the tube.

36 Falling Film Evaporators

37 Forced Circulation Evaporator Natural circulation evaporators have many limitations (as mentioned earlier) through they are economical as compared to forced circulation evaporator. A forced circulation evaporator has a tubular exchanger for heating the solution without boiling. The superheated solution flashes in the chamber, where the solution gets concentrated. In forced circulation evaporator horizontal or vertical both type of design is in- practice. The forced circulation evaporators are used for handling viscous or heat sensitive solution.

38 Forced Circulation Evaporator Vertical tube forced-circulation Evaporators

39 Agitated Thin Film Evaporators Agitated Thin Film Evaporators

40 Plate Evaporators Plate Evaporators

41 Characteristics of different types of Evaporators Evaporator Typical products handled comments Calandria Forced circulation Falling film Salt, glycerin from spent soap lye Salting or scale-forming materials depending on steam-chest configuration; caustic soda solution, sodium sulphate, tomato juice to 30% concentration etc. Low to medium viscosity materials, heat sensitive products, fruit juices and pharmaceuticals Suitable for batch or continuous operation in single or multiple effects Available with: (1) horizontal steam-chest with external vapor separator (less used now) (2) Vertical steam chest with external separator (3) vertical steam-chest with integral vapor head. Operates with either submerged or partially filled tubes in single or multiple effect Single or multiple effects; can be operated on single pass or with partial recycle of concentrated products

42 Characteristics of different types of Evaporators Evaporator Typical products handled comments Natural circulation (Thermosyphon) Agitated film Foaming liquids, less viscous materials, black liquor from the pulp industry, spent soap lye, electroplating solutions, spin bath liquid Handles the full range of feed viscosities, gelatin, fruit puree, glue Rising Film Caprolactum; ammonium nitrate, fruit juices, for crystal producing solutions with suspended solids External separator provides some holding time adjustment, integral vapor head type with down comer gives minimum hold up Available (1) vertical with integral vapor separator (2) vertical with external separator, co-current flow (3) horizontal with tapered shell counter current flow Allows single pass operation with high liquid and vapor velocities; minimum liquid hold up

43 Characteristics of different types of Evaporators Evaporator Typical products handled comments Plate type Fruit juices, extracts, gelatin, condensed and whole milk Liquid and vapor flow essentially as in rising and falling film evaporators without liquid distribution problems

44 Typical overall heat transfer coefficient in evaporator

45 Methods of Feeding of Evaporators Evaporator trains may receive their feed in several different ways. The feed order is NOT related to the numbering of effects. Effects are always numbered according to decreasing pressure (steam flow). 1. Forward feed 2. Backward Feed 3. Mixed Feed 4. Parallel Feed

46 Methods of Feeding of Evaporators Forward feed arrangement in triple-effect evaporator (dotted line: recycle stream)

47 Forward Feed Forward Feed arrangements follow the pattern I, II, III. These require a single feed pump (reduced fixed costs). They typically have reduced economy (higher operating costs) since the cold feed must be raised to the highest operating temperature. These also tend to have the most concentrated liquour, which tends to be the most viscous, in the lowest temperature effects, so their may be difficulties getting a good overall heat transfer coefficient.

48 Methods of Feeding of Evaporators Backward feed arrangement in triple-effect evaporator (dotted line: recycle stream)

49 Backward Feed Backward Feed arrangements go III, II, I. These need multiple pumps to work against the pressure drop of the system; however, since the feed is gradually heated they usually have better economies. This arrangement also reduces the viscosity differences through the system and so is better for viscous solutions.

50 Methods of Feeding of Evaporators Mixed feed arrangement in triple-effect evaporator (dotted line: recycle stream)

51 Mixed Feed Mixed Feed arrangements offer a compromise, with the feed entering in the middle of the system (i.e. II, III, I). The final evaporation is done at the highest temperature so economies are still better than forward feed, but fewer pumps are required than in a backward feed arrangement.

52 Methods of Feeding of Evaporators Parallel feed arrangement in triple-effect evaporator

53 Parallel Feed Parallel Feed systems split the feed stream and feed a portion to each effect. This is most common in crystallizing evaporators where the product is likely to be a slurry.

54 Advantages and limitations of different modes of feed supply to multiple effect evaporator Mode of Feed supply Forward Feed Backward Feed Advantages Simple to operate; less expensive; the liquor flows from one effect to the next driven by the pressure differential between successive effects and hence no pump is required to transferring the liquor; less chance of deterioration of heat sensitive materials because the more concentrated liquor is vaporized at a lower temperature The most concentrated liquor is in contact with the highest temperature steam and thus lower viscosity and higher heat transfer rate in the first effect as a result. Limitations Reduced rate of heat transfer in the second and higher effects; feed should not be below the boiling point because this reduces steam economy by consuming external steam to supply sensible heat. Inter-effect pumps are necessary; higher risk of damage of the viscous product subjected to a higher temperature; risk of fouling.

55 Advantages and limitations of different modes of feed supply to multiple effect evaporator Mode of Feed supply Advantages Mixed feed Combines the simplicity of forward feed and economy of backward feed; useful for concentration of a highly viscous feed. Parallel feed More suitable for use with crystallizers; allows better control. Limitations More complex, piping instrumentation which make arrangement more expensive. More complex arrangement; pumps generally required for each effect.

56 The crystal may grow on the heating coil or on the heating surface. The boiling points of the solution also rise considerably. There is wide variation in characteristics of liquor to be concentrated that requires judgment and experience in designing and operating evaporators. Some of the properties of evaporating liquids that influence the process of evaporation are: 1) 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.

57 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). 2) 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 (carry over of a portion of liquid by rising vapour is called as entrainment). Heat transfer coefficient changes abruptly for such systems.

58 3) Scale: Many solution have tendency to deposit the scale on the heating surface, which may increase the heat transfer resistance that results in reduction of heat transfer coefficient and hence heat transfer rate. 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. It is therefore necessary to clean the tubes at definite intervals.

59 4) Temperature sensitivity: The products of many chemical, food, pharmaceutical industries etc. are very temperature sensitive and they may get damaged during evaporation (when heated to moderate temperature even for short times). For concentrating such materials special techniques are to be used that reduce temperature and time of heating.

60 5) Materials of Construction: 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. Generally evaporators are made of mild steel whenever contamination and corrosion is a problem. Special materials such as copper, nickel, stainless steels may be used. Other liquid characteristics that must be considered in design are specific heat, freezing point, toxicity, explosion hazards and radioactivity.

61 Criteria for selection of evaporator The selection of evaporator is done on the basis of 1. Factor related to process 2. Factor related to mechanical design Factors related to process are Viscosity Thermal sensitivity Heat transfer coefficient Vapour-liquid separator Fouling Crystallization Evaporator load Temperature driving force Foaming properties

62 Viscosity: It is the highly influencing parameter. The higher the viscosity the lower is heat transfer coefficient and larger the heat transfer surface area. Fouling: In most evaporators fouling is due to sedimentation, crystallization, chemical reaction, corrosion, polymerization etc. Fouling tendency is considerably reduced as velocity increases

63 Crystallization: For solutions which have a tendency to crystallize during evaporation, tabular heating surface is the best choice. Thermal sensitivity: Food, dairy, brewery and pharmaceutical products are temperature sensitive; Film type evaporators are best for such operations; usually operated under vacuum.

64 Evaporator load: When heat load is high or heat transfer coefficients are low, large heating surface area is required. In such operations multi effect evaporation is a good choice. Heat transfer coefficient: Heat transfer coefficient is a function of fluid velocity, viscosity, density, specific heat and thermal conductivity. Higher heat transfer coefficient needs smaller heat transfer surface area for a given duty. Fouling of the heating surfaces, reduces heat transfer coefficient and increases pumping cost.

65 Heat transfer coefficient: Film type evaporators are attractive in such cases. The thin film offers less resistance to vapour-liquid separation. Separation of entrained liquid is carried out in the vapour head to avoid excessive entrainment, product loss and environmental pollution. Foaming: If foam is formed during evaporation. It can cause large material loss due to entrainment. Either antifoaming agents has to be used or if it is not possible special arrangement has to be made for vapour liquid separation.

66 Selection guide of evaporator

67 Performance Measures: There are three main measures of evaporator performance: 1.Capacity (kg vaporized / time) 2.Economy (kg vaporized / kg steam input) 3.Steam Consumption (kg / hr) Steam Consumption = Capacity/Economy Capacity: Capacity of an evaporator is defined as the number of kilogram of water vaporized or evaporated per hour. Evaporator economy: Economy of an evaporator is defined as the number of kilogram of water evaporated per kilogram of steam fed to the evaporator. It is also called as steam economy.

68 Performance Measures: Economy calculations are determined using enthalpy balances. In single effect evaporator the amount of water evaporated per kg of steam fed is always less than one and hence economy is less than one.

69 The methods of increasing the economy are: (i) Use of multiple effect evaporation system (ii) Vapour recompression In multiple effect evaporation system, the vapour produced in first effect is fed to the steam chest of second effect as heating medium in which boiling takes place at low pressure and temperature and so on. Thus in triple effect evaporator, 1 kg of steam fed to first effect evaporates approximately 2.5 kg of water

70 Another method to increase the economy of an evaporator is to use principle of thermo compression. Here the vapour from the evaporator is compressed to increase its temperature so that it will condense at a temperature higher enough to permit its use as heating media in the same evaporator.

71 Boiling Point Elevation As evaporation proceeds, the liquor remaining in the evaporator becomes more concentrated and its boiling point will rise. The extent of the boiling-point elevation depends upon the nature of the material being evaporated and upon the concentration changes that are produced. In actual practice, boiling point elevation and liquid head affect the boiling point of a solution. As vapour pressure of most aqueous solutions is less than that of pure water at a given temperature; the boiling point of the solutions is higher than that of pure water at a given pressure.

72 Boiling Point Elevation The difference between the boiling point of a solution and that of pure water at any given pressure is known as boiling point rise/elevation of a solution will be higher than that of the water at a given pressure is known as Boiling Point Rise/Elevation(BPE) or vapour pressure lowering.

73 Boiling Point Elevation The boiling point of a solution is a colligative property -- it depends on the concentration of solute in the solution, but not on what the solute and solvent are. Boiling point elevation is small for dilute solutions and large for concentrated solution of inorganic salts. Boiling point elevation of strong solution can be obtained from an empirical rule known as Duhring s rule. It states that the boiling point of a given solution is linear function of the boiling point of pure water at the same pressure. Hence when the boiling point of the solution plotted against the boiling point of the water, straight line results.

74 Duhring plot for boiling point of sodium chloride solutions Vapour pressure/temperature curve for water

75 Duhring plot for boiling point of sodium hydroxide solutions

76 To use a Duhring plot: 1. For a particular system pressure, determine the boiling temperature of pure water. This can be done from a vapor pressure equation or steam table. 2. Enter the plot from the bottom (the water boiling point), trace up to the diagonal line representing the NaOH fraction, then trace left to read the solution boiling point from the vertical axis. 3. The boiling point elevation is the difference between the two temperatures.

77 Vacuum Evaporation For the evaporation of liquids that are adversely affected by high temperatures, it may be necessary to reduce the temperature of boiling by operating under reduced pressure. The reduced pressures required to boil the liquor at lower temperatures are obtained by mechanical or steam jet ejector vacuum pumps, combined generally with condensers for the vapours from the evaporator.

78 Vacuum Evaporation Mechanical vacuum pumps are generally cheaper in running costs but more expensive in terms of capital than are steam jet ejectors. The condensed liquid can either be pumped from the system or discharged through a tall barometric column in which a static column of liquid balances the atmospheric pressure. Vacuum pumps are then left to deal with the noncondensibles, which of course are much less in volume but still have to be discharged to the atmosphere.