Single and multiple effective evaporator (mee)

A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water.[1] In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat. While in theory, evaporators may be built with an arbitrarily large number of stages, evaporators with more than four stages are rarely practical except in systems where the liquor is the desired product such as in chemical recovery systems where up to seven effects are used.

The multiple-effect evaporator was invented by an African-American inventor and engineer Norbert Rillieux. Although he may have designed the apparatus during the 1820s and constructed a prototype in 1834, he did not build the first industrially practical evaporator until 1845. Originally designed for concentrating sugar in sugar cane juice, it has since become widely used in all industrial applications where large volumes of water must be evaporated, such as salt production and water desalination.

Multiple effect evaporation commonly uses sensible heat in the condensate to preheat liquor to be flashed. In practice the design liquid flow paths can be somewhat complicated in order to extract the most recoverable heat and to obtain the highest evaporation rates from the equipment.

Multiple-effect evaporation plants in sugar beet factories have up to eight effects. Six effect evaporators are common in the recovery of black liquor in the kraft process for making wood pulp.

1. Single & Multiple Effective Evaporator (MEE) Presented By – Sumer Pankaj Class – M.Sc. EST Semester – 3 rd Roll no. - 41 Institute of Science and Technology for Advanced Studies and Research Affiliated to Sardar Patel University Recognized under section 2(f) and 12 (B) of UGC act 1956 Mota Bazaar, Vallabh Vidyanagar, Anand , Gujarat 38812058 PG Department of Environmental Science and Technology

2. Content What are Evaporators Basic mechanism of evaporators Principle of Evaporators Application of Evaporators in Industrial sector Application of evaporators Factors to be taken care of Types of Evaporators Single Effective Evaporates (SEE) Multiple Effective Evaporator (MEE) Mechanism of MEE Construction of MEE Working of MEE Efficiency of MEE Types of MEE Advantages of MEE Cost reduction in MEE plants Criteria for selection of MEE plant Checklist for designing and operation of MEE plant Construction material of MEE plants in various industries Safety measures during designing of MEE plan

3. What are Evaporators ? Equipment used in evaporation, the process of boiling a liquid in order to get a concentrated solution or in to recover maximum percentage of water. Driving force: Temperature difference in between steam chest temperature and product temperature. Result : Volatile solvent is removed from the feed. Feed = Solution (volatile solvent + non volatile solute) Concentrate (Higher solute Conc.)

4. Basic Mechanism of an Evaporator Vapour out Feed in Steam in (Saturated vapour ) Product out Condensate out (Saturated Liquid) Vapor Separator Heat Exchanger Condenser unit Vacuum for non condensable Coolant In Coolant out

5. Principle of 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. 

6. Applications of Evaporates in Industrial sector 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. Basic Parts of an Evaporator

8. Factors to be taken care of - Concentration of solute in feed. Concentration of solute in product/concentrate. Pressure and Temp. of the system Depends on temperature sensitivity of material. Boiling point elevation Steam pressure and temperature Material of construction Foaming Low heat transfer rate Entrainment loss In the U.S. MEE plant

9. Types of Evaporators

10. Single Effective Evaporator

11. Multiple Effective Evaporator A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water. 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 vapour 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 Multiple Effect Evaporation remains one of the popular methods used for the concentration of aqueous solutions. Water is removed from a solution by boiling the liquor in an evaporator and withdrawing the vapour.

12. Mechanism of Multiple Effective Evaporator

13. Construction on MEE 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.

14. Thermal recompression unit Steam for heating Feed in Calandria Feed out Vapor Separator Pre-heater Condenser Cooling water in Cooling water return

15. Working of MEE Multiple effect evaporator Due to heat transfer, the liquid temperature increases & reaches the B.P. during this process, vapor well be generated from the liquid feed. So, formed vapor displaces air in the upper part of 1st evaporator. Moreover, the vapor also displaces the air in the steam space of the 2nd evaporator. After complete displacement of air by vapor in the steam compartment of 2nd evaporator, the second valve is closed. The vapor 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 .

16. Working of MEE cont… 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 vapor 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 (t -t 1 ). 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.

17. Working of MEE cont… 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.

18. Efficiency of MEE It is the quantity of vapor 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.

19. Types of MEE There are four types of MEE based upon feeding methods:

20. Forward Feed MEE 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 vaccum in other effects 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

21. Forward Feed MEE Steam Feed

22. Backward Feed MEE 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 Feed Steam

23. Backward Feed MEE

24. Parallel Feed MEE 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. Eg - evaporation of brine to make salt Feed Feed Feed Steam

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

26. Mixed Feed MEE Feed Steam

27. Advantages of MEE 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 .

28. Cost reduction of MEE plants Effect of temperature : feed must be at temp greater or equal to the boiling point 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 boiling point of the feed .if 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 boiling point of increases giving small size and hence cost decreases.

29. Criteria for selection of MEE plant

30. Checklist for designing of MEE plant Must prevent entrainment due to product loss Contamination of the vapor phase ( pollution ) Condensation of vapor 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) 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 .

31. Construction material of MEE in various industries INUSTRIES/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

32. Safety measures in MEE plant Possible Causes Possible Remedies 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.

33. Thank you …