MUTLIPLE EFFECT EVAPORATORS

1. MUTLIPLE EFFECT EVAPORATORS PROJECT REPORT DONE BY: KAVITA YADAV 2009305060 ABHISHEK SINGH 2009305061 PRIYANKA RAVIKUMAR 2009305062 SURIYASRI.S 2009305063 SATYANARAYANAN 20086242

2. INTRODUCTION: • An evaporator is essentially a heat exchanger in which a liquid is boiled to give a vapour, which also acts as a low pressure steam generator. • An evaporator is hence treated as a low pressure boiler, and the steam thus produced is used for further heating in another following evaporator called another effect. • A multiple-effect evaporator is an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. • The multiple-effect evaporator was invented by the African-American engineer Norbert Rillieux.

3. Norbert Rillieux design

4. Multi Effect Evaporators are the equipment in which steam from an outside source is condensed in the heating element of first effect. The boiling temperature at which the first effect operates is high enough so that the evaporated water can serve as the heating medium for second effect. The vapors so formed are then sent to a condenser if it is a double effect evaporator.

6. PRINCIPLE: • Water is boiled in a sequence of vessels, each held at a lower pressure than the last. • Generally the first vessel (at the highest pressure) requires an external source of heat • Because the boiling point of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next.

7. 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. Process outline: • Driving force: Temperature difference in between steam chest temperature and product temperature. • Result : Volatile solvent is removed from the feed. Solution (volatile solvent + non volatile solute) Concentrate (Higher solute Conc.)

9. Vaccum for non condensable Evaporator Coolant In Vapor out Coolant out Condensor unit Vapor Separator Feed in Steam in (Saturated vapor) Heat Exchanger Condensate out (Saturated Liquid) Product out

10. Why do we need a multiple effect evaporator? • Need • Reduces transportation cost • Storage costs • Prepare for the next Unit operation – drying, crystallisation etc. • Reduces deteriorative chemical reactions • Better microbiological stability • Recovery of solvent

11. Evaporation • Evaporation is basically a separation step which uses heat transfer to separate products presenting differences at boiling point.  This technology results in several different downstream processes such as concentration, crystallization and drying. • Multiple effect evaporation: The energy consumption to evaporate an aqueous solution is fairly significant; therefore, in order to reduce the energy cost, systems such as multiple effect evaporation and thermal vapour recompression are often used. The steam consumption of the evaporator unit can be reduced by using the vapour from the first chamber to heat the second one.

12. 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 sulfate, gelatin, 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 sulfate. The desired product can be the concentrated solution, the precipitated solids, or both. 

13. Types of evaporators Single effect evaporator Double effect evaporator

14. Triple effect evaporator Multiple effect evaporator

15. Operation It may be possible to make use of this, to treat an evaporator as a low pressure boiler, and to make use of the steam thus produced for further heating in another following evaporator called another effect. Consider two evaporators connected so that the vapour line from one is connected to the steam chest of the other as shown in making up a two effect evaporator

16. If liquid is to be evaporated in each effect, and if the boiling point of this liquid is unaffected by the solute concentration, then writing a heat balance for the first evaporator: q1 = U1A1(Ts - T1)=U1A1DT1 where q1 is the rate of heat transfer, U1 is the overall heat transfer coefficient in evaporator 1, A1 is the heat-transfer area in evaporator 1, Ts is the temperature of condensing steam from the boiler, T1 is the boiling temperature of the liquid in evaporator 1 and DT1 is the temperature difference in evaporator 1, = (Ts - T1). Similarly, in the second evaporator, remembering that the "steam" in the second is the vapour from the first evaporator and that this will condense at approximately the same temperature as it boiled, since pressure changes are small, q2 = U2A2(T1 - T2) = U2A2 DT2

17. If the evaporators are working in balance, then all of the vapours from the first effect are condensing and in their turn evaporating vapours in the second effect. Also assuming that heat losses can be neglected, there is no appreciable boiling-point elevation of the more concentrated solution, and the feed is supplied at its boiling point, q1 = q2 Further, if the evaporators are so constructed that A1 = A2, the foregoing equations can be combined. U2/U1 = dT1 / dT2  The above equation states that the temperature differences are inversely proportional to the overall heat transfer coefficients in the two effects. This analysis may be extended to any number of effects operated in series, in the same way

18. 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.

19. 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

20. Thermal recompression unit TVR = Thermal Vapour Recompression uses a sonic nozzle jet and high pressure steam to recompress a lower pressure steam/vapour. In the live steam nozzle (1) the pressure of the in-flowing steam is converted into velocity. A jet is created which draws in the low pressure vapour. In the diffuser (2) a fast flowing mixture of live steam and vapours is formed, the speed of which is converted into pressure (temperature increase) by deceleration.

21. Calandria evaporator • The Calandria Evaporator has a heat exchanger (with tubes usually less than six feet long) integral with the vapour body.  The level is maintained in the upper portion of the tubes or above the top tubesheet and the circulation pattern is up through the tubes and down through a central pipe called a "downcomer".  Circulation is created by the difference in specific gravity between the body liquor and the heated liquor and vapor generated inside the tubes, plus a vapour lift effect. 

22. Vapour separator A vapour-liquid separator is a vertical vessel used in several industrial applications to separate a vapour-liquid mixture. Gravity causes the liquid to settle to the bottom of the vessel, where it is withdrawn. The vapour travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapour as it exits the top of the vessel.

23. Pre heater Pre heater is a device for preliminary heating of a material, substance, or fluid that will undergo further use or treatment by heating. Preheating in stages increases efficiency and minimises thermal shock stress to components.

24. Condenser It is an apparatus in which vapour is condensed within tubes that are cooled by the evaporation of water flowing over the outside of the tubes. 

25. Cut section of multiple effect evaporator

26. Working • Multiple effect evaporator The vent valves are kept open & all other valves are closed. • Now high vacuum is created in the liquid chambers of evaporators. The steam valve & condensate valve of the first evaporator are opened. • Steam is supplied. Steam first replaces cold air in the steam space of 1st evaporator. The supply of steam is continued until the desired pressure P0 is created in the steam space of 1st evaporator. • At this pressure, the temperature of the steam is T0. Steam gives its temperature to the liquid feed in the 1st evaporator and gets condensed. • Condensate is removed through the condensate valve.

27. 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.

28. 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.

29. 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.

30.  Forced-Circulation NaCl Evaporator

31. Efficiency: • So efficiency of evaporator may be expressed as: • efficiency of evaporator=total mass of vapour produced/total mass of steam supplied. • In single-effect evaporator, steam produces vapour only once. • So efficiency of single effect evaporator=N units of vapour produced/N units of steam supplied=1 • In multiple effect evaporator, one unit of steam produces vapour many times, depending on the no. of evaporators connected. • So efficiency of multiple effect evaporator=N units of vapour produced/1 unit steam supplied=N • Therefore, efficiency of multiple effect evaporator is N times the economy of the single effect evaporator.

32. 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.

33. Cut sections of various equipments of multiple effect evaporator Evaporator

34. Evaporator blower

35. Water heat exchanger

36. Pre heater

37. Overview of thermal recompressor

38. Evaporator condenser

39. Feeding of Multiple Effect Evaporators • There are three types of feeding methods: Forward feeding Backward feeding Parallel feeding 1)Forwardfeeding: 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

40. 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

41. 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.

42. 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.

43. Parallel feeding: • Parallel feed :

44. 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.

45. 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.

46. Typical materials of construction for a number of evaporator applications are shown below:

47. 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)

48. 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.

49. STEAM CONSUMPTION AND RUNNING COSTS OF EVAPORATORS

50. 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.