Types of evaporators

By Engnr. Zarnab Ashraf

1. TYPES OF EVAPORATORS PRESENTED BY:- ENGNR. ZARNAB ASHRAF

2. Table of contents Introduction Why we need evaporation? Principle Basic parts of evaporator Properties on which evaporation greatly depend Driving force Heat transfer coefficient Types of evaporators Applications & working

3. introduction 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 vapour from a solution or slurry. Evaporation is used for concentration of aqueous solutions. Equipment which is used for evaporation is called Evaporator. Evaporators are used to separate materials based on differences in their boiling temperatures . .

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. principle 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.

6. Properties on which the evaporation greatly depends Concentration of the liquid : by time liquid becomes concentrated. Heat transfer coefficient becomes adequately low so the circulation or the turbulence must be there to prevent the coefficient becomes sufficiently low. Solubility : by the variation in temperature effects the solubility of the solution either condensation of the liquid from cold liquid causes crystallisation of the substance .effects the evaporation. Temperature sensitivity of the material : some material degrade due to prolong heating. Foaming and frothing: foam and froth forms during boiling accompanies the vapour coming out of the evaporator due to which entrainment loss occurs

7. CONT… Pressure and temperature : B.P. of the solution is related to pressure of the system . The higher the operating pressure of the evaporator the higher the temp at boiling . By increase in the concentration temperature of boiling may rise. Scale deposition and material of construction : by these factors heat transfer coefficient decreases causes the decrease in heat transfer.

8. 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 ). Examples: Concentration of milk to produce condensed milk Concentration of juices Concentration of NaOH , NaCl from aqueous solutions to produce salt.

9. 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. 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, which is reciprocal of all the resistances. REFRENCE:- http :// avibert.blogspot.co m

10. Basic parts of an evaporator Heat-exchanger Calandria Vapor separator Condenser

11. TYPES OF EVAPORATORS Batch–type pan evaporator Natural circulation evaporator Rising-film evaporator Falling-film evaporator Rising/falling film evaporator Forced circulation evaporator Agitated thin film evaporator

12. Batch–type pan evaporator 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

13. CONT…. Used for concentration of Jams and Jellies, also for some pharmaceutical products. T he residence time of the product is usually very long. REFRENCE:- Book introduction to food engeenring

14. Natural circulation evaporator In natural circulation evaporators, short vertical tubes, typically 1–2 m long and 50–100 mm in diameter, are arranged inside the steam chest. The whole calandria (tubes and steam chest) is located in the bottom of the vessel. The product, when heated, rises through these tubes by natural circulation while steam condenses outside the tubes. Evaporation takes place inside the tubes, and the product is concentrated.

15. cont… The concentrated liquid falls back to the base of the vessel through a central annular section.

16. Rising-film evaporator A low-viscosity liquid food is allowed to boil inside 10–15 m-long vertical tubes. The tubes are heated from the outside with steam . The tubes are heated from the outside with steam . The liquid rises inside these tubes by vapors formed near the bottom of the heating tubes . The upward movement of vapors causes a thin liquid film to move rapidly upward. A temperature differential of at least 14°C between the product and the heating medium is necessary to obtain a well-developed film.

17. Falling-film evaporator In contrast to the rising-film evaporator, the falling-film evaporator has a thin liquid film moving downward under gravity on the inside of the vertical tubes. The falling-film evaporator allows a greater number of effects than the rising-film evaporator. Vapour liquid separation usually takes place at the bottom. Used widely for concentrating heat sensitive materials such as orange juice and other fruit juice. Holdup time is very small(5-10). High heat transfer coefficient (due to high velocities ).

18. Rising/Falling-Film Evaporator In the rising/falling-film evaporator, the product is concentrated by circulation through a rising-film section followed by a falling-film section of the evaporator. The product is first concentrated as it ascends through a rising tube section, followed by the pre concentrated product descending through a falling-film section, there it attains its final concentration . http://foodtechinfo.com/foodpro/index_gas_technologies/concentration_-_rise-fall_plate/

19. Forced circulation evaporator We use pump in film type evaporator Increase heat transfer coefficient Useful for viscous fluid External heating provide more ease to cleaning to tubes more complicated piping is used For viscous we use positive displacement pumps and for colloidal we use low power pumps. Heating element is placed at the lower level to avoid boiling on heating surface .This reduces the rate of deposition of solids.

20. Agitated thin film evaporator Main resistance of the heat transfer is on evaporator side. Heat transfer coefficient and turbulence is increased by mechanical agitation of this liquid film. Modifying falling film evaporator with a single large jacketed tube containing internal agitator. Used for viscous fluids , high sensitive materials like rubber, latex, gelatine, antibiotics and fruit. Heat transfer coefficient increases High cost Small capacity

21. Table 8.1 Types of Evaporators Employed in Concentrating Liquid Foods a     Evaporator type     Tube Depth       Circulation     Viscosity capability (× 10 —3 Pa s)   Able to handle suspended solids   Applicable to multiple effects Applicable to mechanical vapor decompression     Heat transfer rate     Residence time     Capital cost       Remarks Vertical tubular Long Natural Up to 50 Yes Yes No Medium High Low   Vertical tubular Long Assisted Up to 150 Yes Yes No Good High Low- medium   Vertical tubular Short Natural Up to 20 Yes Yes No Medium High High Calandria usually internal to separator Vertical tubular Short Assisted Up to 2000 Yes No No Low Very high High Calandria usually internal to separator Vertical tubular Long Forced- suppressed boiling Up to 500 Yes Limited No Medium Very high Very high Used on scaling duties Plate heat exchanger N/A Forced- suppressed boiling Up to 500 Limited Yes No Good Medium Medium Used on scaling duties Vertical tubular rising film Long None or limited Up to 1000 Not desirable Yes No Good Low Medium   Vertical tubular rising/falling film Medium None or limited Up to 2000 Not desirable Yes No Good Low Medium   Plate rising/ falling film N/A None or limited Up to 2000 Very limited Yes No Good Low Low- medium   Vertical tubular falling film Long None or limited Up to 3000 Not desirable Yes Yes Excellent Very low Medium Used on heat- sensitive products Vertical tubular falling film Long Medium Up to 1000 Yes Yes Yes Good Low Medium   Plate falling film N/A None or limited Up to 3000 No Yes Yes Excellent Very low Medium Used on heat- sensitive products Swept surface N/A None Up to 10,000 Yes No No Excellent Low Very high   a Courtesy of APV Equipment, Inc.

22. Method of operation of evaporation Single effect evaporator Forward feed multiple effect evaporator Backward feed multiple effect evaporator Parallel feed multiple effect evaporator Mixed feed multiple effect evaporator

23. Parallel feed multiple effect evaporator Adding and withdrawal of concentrated product from each effect. Feed almost saturated and solid crystals are the product. E.g.- evaporation of brine to make salt

24. Single effect evaporator Required capacity of operation is relatively small

25. Forward feed multiple effect evaporator Feed should be near the B.P. of the solution at the pressure in the 1 st effect. Previous unit vapour serves as a heating medium for the next effect. Here latent of the vapour can be reused and recovered again. Uses when feed is hot or when the concentration product might be damaged at high temperature B.P decreases from effect to effect Pressure is 1 atm at 1 st effect and under vacuum in other effects

26. Backward feed multiple effect evaporator Uses when fresh feed is cold. Flows from low to high pressure for this to happen we apply pumps at certain places. Temperature increases from effect to effect. Used when concentrated product is highly viscous High temperature and low viscosity gives high heat transfer coefficient

27. Mixed feed multiple effect evaporator When feed moves forward with fresh feed is entering at the 2 nd or 3 rd effect is known as mixed type. Used in solutions having considerable change in viscosity with temperature over concentration range.

28. Effects of processing variables on evaporator Effect of temperature : feed must be at temp greater or equal to the b.p of the solution. Preheating the feed can reduce the size of evaporator heat transfer area. Effect of pressure : a pressure of 101.32 kpa is used in the vapour space which gives the b.p of the feed . I f the rise in temperature increases means large difference in temperature is desirable heating surface area and the cost of the evaporator decreases . If pressure decreases under vacuum condenser and pump are used. Effect of stream pressure :with increase in pressure of the saturated stream difference in b.p increases giving small size and hence cost decreases.