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presentation on evaporators

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  1. 1. Khalid nawaz cell no: 0300-70244 Institute Of Chemical Engineering And Technology Lahore Pakistan
  2. 2. Evaporation  Evaporation is a unit operation that consists of the elimination of water of a fluid food by means of vaporization or boiling.  Evaporation is the removal of solvent as a vapor 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 vapor
  3. 3. Evaporator  The equipment used to remove water from the food product is called evaporator  Evaporators are used to separate materials based on differences in their boiling temperatures.  Its purpose is to concentrate nonvolatile solutes such as organic compounds, inorganic salts, acids or bases. Typical solutes include phosphoric acid, caustic soda, sodium chloride, sodium sulfate, gelatin, syrups and urea.
  4. 4. Why we need evaporation Reduces transportation cost Storage costs  Prepare for the next Unit operation – drying, crystallisation etc.  Reduces deteriorative chemical reactions  Better microbiological stability  Recovery of solvent
  5. 5. Driving force&Result  Driving force: temperature difference in between steam chest temperature and product temperature  Result: volatile solvent is removed from the feed  Solution(high volatile solvent+non volatile solute)  Concentrate(non volatile solute)
  6. 6. Examples  Concentration of milk to produce condensed milk  Concentration of juices  Concentration of NaOH , NaCl from aqueous solutions to produce salt.  Ether recovery from fat extraction
  7. 7. Principles  Steam heat is used for transfer of heat for subsequent vessels. Steam has a very high heat content Heat is given up at constant temperature.  It can be used at high pressure to generate electric power and low-pressure exhaust steam is used for process heating.  Evaporation is a process of vaporizing large quantities of volatile liquid to get a concentrated product.  Evaporation is a surface phenomenon, i.e., mass transfer takes place from the surface.
  8. 8. Basic Parts of an Evaporator  Heat-exchanger  Vacuum  Vapour separator  Condenser m
  9. 9. Vaccum for non condensable Evaporator Coolant In Vapor out Condensor unit Coolant out Vapor Separator Feed in Steam in (Saturated vapor) Heat Exchanger Condensate out (Saturated Liquid) Product out
  10. 10. Evaporator  An evaporator is used to evaporate a volatile solvent, usually water, from a solution. Its purpose is to concentrate non-volatile solutes such as organic compounds, inorganic salts, acids or bases. Typical solutes include phosphoric acid, caustic soda, sodium chloride, sodium sulphate, gelatine, syrups and urea.  In many applications, evaporation results in the precipitation of solutes in the form of crystals, which are usually separated from the solution with cyclones, settlers, wash columns, elutriating legs, filters or centrifuges. Examples of precipitates are sodium chloride, sodium sulfate, sodium carbonate and calcium sulphate. The desired product can be the concentrated solution, the precipitated solids, or both.
  11. 11. Evaporator Type  Batch Pan  Rising Film  Falling Film  Multiple Effect Evaporators m
  12. 12. Heat Transfer Coefficient, h  A coefficient which indicates the amount of heat flow that is exchanged across a unit area of a medium or system in a unit amount of time with a unit of temperature difference between the boundary of the system.  S.I. unit W m-2 K-1.
  13. 13. Overall Heat Transfer Coefficient, U  In cases of combined heat transfer for a heat exchanger, there are two values for h.  Convective heat transfer coefficient for the fluid film inside the tubes  Convective heat transfer coefficient for the fluid film outside the tubes.  The thermal conductivity (k) and thickness (Dx) of the tube wall must also be accounted for.  So an additional term (Uo), called the overall heat transfer coefficient, must be used instead.
  14. 14.
  15. 15. Batch Pan  Oldest type and used for limited application  Either jacketed/internal coils/heaters  Product residence time is many hours  Boil at low temp and high pressure for heat sensitive materials  heat transfer area and coefficients are low under natural circulation  Evaporation capacities are low  Large temp differences not achieved b/c of rapid fouling of heating surface
  16. 16. Cont…..  Used for concentration of Jams and Jellies, also for some pharmaceutical products
  17. 17. 3) Calandria type Evaporator:  Commonly it is known as short tube or single effect evaporator. Features:    Vertical type of evaporator. Tubes sheets extending across the body and central down take. Material which has to be evaporated is introduced in tubes. The tube may be about 1.2m long and 5 cm in diameter, but the size varies with the nature of the substances.  There is space below containing a steam coil to give extra heating capacity and large enough to afford circulation of liquid.
  18. 18. Disadvantages: *Large floor space and weight *Poor heat transfer at low temperature differences. *Not use for thermo-labile products
  19. 19. Natural Circulation Units Circulation is obtained by convection currents arising from heating surface A) Tubes are horizontal with steam inside B) Tubes are vertical with steam outside a. solution to be evaporated boils outside the tubes and steam condenses inside of tubes. These tubes interfere with natural circulation and minimize liquid agitation Overall heat transfer coefficients are lower
  20. 20. a. Tubes are horizontal with steam inside
  21. 21. Cont…..  Cylindrical section above heating portion is used to separate vapor from liquid  vapor leaves through de-entraining device to prevent carry over of liquid droplets with vapor  Steam enters through one side chest and leaves through opposite chest  Steam condensate out via steam trap
  22. 22.  The major use is for making distilled water for boiler feed. Horizontal tube evaporators are used in the pharmaceutical industry, pulp and paper industry.  They are relatively low cost. They have very low headroom. Horizontal tube evaporators are not suitable for salting or scaling liquids, and they have smaller capacity than other evaporators. 11.01.2014 26
  23. 23. Cont…. Advantages 1. Cheap 2. Easy to install 3. Require less space for installation 4. Suitable for liquids that not crystallize 5. Can be used for batch/continuous operation Disadvantages 1. Not suitable for viscous liquids b/c of poor circulation
  24. 24. outside  Solution boils inside vertical tubes with heating media, usually steam, held in steam chest, through which tubes pass.  Boiling of liquid in tubes, causes liquid flow upward through tubes and un-evaporated liquid flows downward through central hole
  25. 25. Cont…..
  26. 26. Cont…..  These overcome disadvantages of horizontal tube evaporators 1. natural circulation is promoted(1-3 ft/s) 2. heat transfer coefficients are higher 3. solid built inside tubes is removed by mechanical cleaning 4. Viscous liquids can be used, but circulation is slow 5. Used in sugar and salt industries  These are impractical when solution is very viscous or form foams or is heat sensitive
  27. 27. Cont…
  28. 28.  Climbing film Evaporators: It is also known as long tube or rising film evaporator.  The theory of climbing film evaporator’s working is that the ascending force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At the same time the production of vapor increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards. This co-current upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid. This is advantageous during evaporation of highly viscous products and products that have a tendency to foul the heating surface.   Usually there must be a rather high temperature difference between the heating and boiling sides of this type of evaporator. Otherwise the energy of the vapor flow is not sufficient to convey the liquid and to produce the rising film. The length of the boiling tubes will typically not exceed 23 ft (7m).
  29. 29.           Advantages: High heat transfer rates at high temperature differences Ease of cleaning Relatively inexpensive Disadvantages: Large floor space and weight Poor heat transfer at low temperature differences Not use for thermolabile products
  30. 30.  Horizontal Film Evaporator: Also known as falling film evaporator.  This type of evaporator is generally made of long tubes (4-8 meters in length) which are surrounded by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which flows downward as well. This evaporator is applicable to highly viscous solutions so it is frequently used in the chemical, food, and fermentation Falling Film Evaporator with boiling inside the tubes. The feed to the evaporator is fed to the top of the Calandria through efficient liquid distributors. Steam is given on the shell side. The concentrate is collected at the bottom. The same operation is repeated in multiple effects to achieve steam economy.
  31. 31.  ) Rising-falling film Evaporator: Rising and falling film evaporators are some time combined in to a same unit. When a high ratio of evaporation to feed is required and concentrated liquid is viscous, a tube bundle can be divided in to two sections with the first section functions as a rising film evaporator and the second section serves as falling film evaporator.
  32. 32.  Advantages:  Low cost and small floor space  Suitable for viscous material  Large units for handling high evaporation load  Disadvantages:  High headroom required  Recirculation is frequently required
  33. 33. Multiple-effect Evaporator  Water is boiled in a sequence of vessels, each held at a lower pressure than the last.  Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next  Generally the first vessel (at the highest pressure) requires an external source of heat
  34. 34. Construction:  A multiple effect evaporator system for concentrating a process liquid comprises: (a) a plurality of evaporator effects arranged in series, each effect including a process liquid inlet and a process liquid outlet; a heating fluid inlet and heating fluid outlet; (b) heat exchange means in each effect for passing said process liquid in heat exchange relationship with heating fluid for evaporating water out of said process liquid; and wherein evaporated water from one effect serves as heating fluid for an adjacent effect; and (c) an evaporative condenser provided with liquid inlet means for receiving process liquid from one of said evaporator effects, and liquid outlet means for transmitting said process liquid to another of said evaporator effects; and means for receiving heating fluid vapor and for passing said heating fluid vapor in heat exchange relationship with cooled process liquid in a cooling circuit, for condensing said heating fluid vapor.
  35. 35. Equipment description (1) Thermal recompression unit, (2) Steam for heating (3) Feed in (4) Calandria (5) Feed out (6) Vapour Separator (7) Pre-heater (8) Condenser (9)Cooling water in, (10) Cooling water return
  36. 36.
  37. 37. Working:  Multiple effect evaporator Due to heat transfer, the liquid       temperature increases & reaches the B.P. during this process, vapour well be generated from the liquid feed. So, formed vapour displaces air in the upper part of 1st evaporator. Moreover, the vapour also displaces the air in the steam space of the 2nd evaporator. After complete displacement of air by vapour in the steam compartment of 2nd evaporator, the second valve is closed. The vapour of 1st evaporator transmits its heat to the liquid of 2nd evaporator & gets condensed. Condensate is removed through the second condensate valve. These steps continue in the 3rd evaporator also.
  38. 38. Working:  As the liquid in 1st evaporator gains temperature the difference in temperature between the liquid & steam decreases, hence, the rate of condensation decreases.  As a result, the pressure in the vapour space of 1st evaporator gradually increases to P1 by increasing temperature to T1 , which is the B.P. of the liquid in first evaporator & decreasing the temperature difference (t0-t1).  A similar change takes place in the 2nd evaporator & the liquid reaches the B.P.  similarly, the process will be repeated in 3rd evaporator. Finally 3 evaporators come to a steady state with the liquid boiling in all the 3 bodies.
  39. 39. Working:  As boiling proceed, liquid level in 1st evaporator comes down. Feed is      introduced through the feed valve to maintain the liquid level constant. Similarly evaporation of liquid takes place in 2nd & 3rd evaporators. To maintain the liquid levels constant, feed valves F2 & F3 are used for 2nd & 3rd evaporator respectively. This process is continued until the liquid in all the evaporators reaches the desired viscosity. Now the product valves are opened to collect the thick liquid. Thus in this evaporators, there is continuous supply of feed, continuous supply of steam & continuous withdrawal of liquid from all 3 evaporators. Hence, evaporators work continuously.
  40. 40. Efficiency of multiple effect evaporator  It is the quantity of vapour produced per unit steam admitted.  Feed is admitted at its B.P. so it does not require any more heat to raise its temp.  Hence, the supplied steam is condensed to give heat of condensation. This heat will then transferred to the liquid.  The heat transferred now serves as latent heat of vaporization, i.e. liquid undergoes vaporization by receiving heat. Loss of heat by means is negligible.
  41. 41. Feeding of Multiple Effect Evaporators  There are three types of feeding methods: Forward feeding Backward feeding Parallel feeding 1)Forward feeding: The pressure in the second effect must be reduced below that in the first. In some cases, the first effect may be at a pressure above atmospheric; or the first effect may be at atmospheric pressure and the second and subsequent effects have therefore to be under increasingly lower pressures. Often many of the later effects are under vacuum. Under these conditions, the liquid feed progress is simplest if it passes from effect one to effect two, to effect three, and so on, as in these circumstances the feed will flow without pumping. This is called forward feed
  42. 42. Forward feeding: In the case of a forward feet operation, the raw feed is introduced in the first effect and is passed from effect to effect parallel to steam flow. The product is withdrawn from the last effect. This procedure is highly advantageous if the feed is hot. The method is also used if the concentrated product may be damaged or may deposit scale at high temperature
  43. 43. Backward feed: Alternatively, feed may pass in the reverse direction, starting in the last effect and proceeding to the first, but in this case the liquid has to be pumped from one effect to the next against the pressure drops. This is called backward feed and because the concentrated viscous liquids can be handled at the highest temperatures in the first effects it usually offers larger evaporation capacity than forward feed systems, but it may be disadvantageous from the viewpoint of product quality.
  44. 44. Backward feeding: In the backward operation, the raw feed enters the last (coldest) effect and the discharge from this effect becomes a feed for the next to last effect. This technique of evaporations is advantageous, in case the feed is cold, as much less liquid must be heated to the higher temperature existing in the early effects. The procedure is also used if the product is viscous and high temperatures are required to keep the viscosity low enough to produce good heat transfer coefficients.
  45. 45. Parallel feeding:  Parallel feed :
  46. 46.  A hot saturated solution of the feed is directly fed into each of the three effects in parallel without transferring the material from one to another. This is commonly used in the concentration of the salt solution, where the solute crystallizes on concentration without increasing the viscosity.  Operations :-The equipment is at room temp. & at atm. Pressure at the beginning. The liquid feed is introduced into all the 3 evaporators up to the level of upper tube sheets.
  47. 47. Multiple effect evaporator Advantages  Suitable for large scale & for continuous operation.  Highly economical when compared to single effect.  Multiple effects, or stages, are now used to minimize the energy input required to evaporate or boil off undesirable water content.  The total evaporation achieved in these systems is approximately the number of effects times the energy input to the first effect.
  48. 48. Typical materials of construction for a number of evaporator applications are shown below: PRODUCT MATERIAL OF CONSTRUCTION Most dairy and food products 304/316 stainless steel Most fruit juices 316 stainless steel Sugar products Carbon steel /304/316 Foods containing high salt (NaCl) Titanium/Monel High alloy stainless steels Duplex stainless steels Caustic soda < 40% Stress relieved carbon steel Caustic soda high concentration Nickel Hydrochloric acid Graphite/Rubber lined carbon steel
  49. 49. Design criteria and processing factors:         Must prevent entrainment due to product loss Contamination of the vapour phase (polution) Condensation of vapour onto surfaces (corrosion and fouling) Overhead mist or spray may cause troublesome deposits Vortices increase pump head requirements and therefore equipment Configuration Short circuiting a big problem as it presents the problem of cavitation (there must be a net positive suction head)
  50. 50. Other factors:     Liquid concentration -> relates to viscosity and heat transfer Temperature and Pressure Boiling temperature is inversely proportional to pressure. Boiling points may increase as solution get concentrated (boiling point rise)  Foaming -> will determine the height of your freeboard in the Design  Solubility of materials -> May be the limit to the concentration that you can achieve.  Scale deposits -> decrease your heat transfer coefficient.
  51. 51. Criteria for selection of Multiple Effect Evaporator plant:  During the design of evaporation plants, numerous, sometimes contradictory, requirements have to be considered. They determine which type of construction and arrangement is chosen, and the resulting process and economic data. The most important requirements are as follows:  Capacity and operational data, including quantities, concentrations, temperatures, annual operating hours, change of product, controls automation, etc.  Product characteristics, including heat sensitivity, viscosity and flow properties, foaming tendency, fouling and precipitation, boiling behaviour, etc.  Required operating media, such as steam, cooling water, electric power, cleaning agents, spare parts, etc.
  52. 52.  Capital and other financial costs  Personnel costs for operation and maintenance  Standards and conditions for manufacture, delivery, acceptance, etc.  Choice of materials of construction and surface finishes  Site conditions, such as available space, climate (for outdoor sites), connections for energy and product, service platforms, etc.  Legal regulations covering safety, accident prevention, sound emissions, environmental requirements, and others, depending upon the specific project.
  53. 53. Possible Causes Possible Remedies Drive motor trips § The product may be backing up in the evaporator body due to choking. § Open discharge lines to check and eliminate cause of choking. § The evaporator body may be heavily scaled. § Dismantle the unit and check for scaling. § The feed rate is excessive. § Control the feed rate. § There might be a problem with the drive motor. § Check for malfunctioning of the drive motor. System starts vibrating § There may be heavy scaling. § Go through the cleaning procedure. § The product is backing up in the body due to choking. § Check and eliminate the cause of product backup. § Uneven heating of evaporator shell causing thermal stresses on its body. § Check the steam pressure of thermocompressor.