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DKK3433UNIT OPERATION

Dr. Syed Mohd Saufi2012/2013-I1

Evaporation+Falling Film Type EvaporatorDr SMS 2012/20132http://www.youtube.com/watch?v=3T8Km9BYHeg&playnext=1&list=PL4ECF5DA8511498E6&feature=results_main

+3ContentsIntroductionProcessing Factor in EvaporationType of EvaporationMethod of Operation of EvaporatorsCalculation Method for Single Effect EvaporatorCalculation Method for Multiple Effect Evaporator

+4IntroductionEvaporation is achieved by adding heat to the solution to vaporize the solvent.Vapor (usually water) from a boiling liquid solution is removed and a more concentrated solution remains.Heat is provided by the condensation of a vapor (such as steam) on one side of a metal surface with the evaporating liquid on the other sideThe normal heating medium is low pressure exhaust steam from turbines, special heat transfer fluids or flue gases.Example: concentration of aqueous solutions of sugar, sodium chloride, glue, milk and orange juice.In some case, the purpose of evaporation is to concentrate the solution so that upon cooling, salt crystal will be formed and separate

+Basic Operation of EvaporatorThe typical evaporator is made up of three functional sections: the heat exchanger, the evaporating section, where the liquid boils and evaporates, and the separator in which the vapour leaves the liquid and passes off to the condenser or to other equipment. In many evaporators, all three sections are contained in a single vertical cylinder. In the center 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.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 resistance to heat flow is imposed by the steam and liquid film coefficients and by the material of the tube walls. Dr SMS 2012/20135

http://www.nzifst.org.nz/unitoperations/evaporation1.htm+Basic Operation of EvaporatorThe circulation of the liquid greatly affects evaporation rates, but circulation rates and patterns are very difficult to predict in any detail. With dissolved solids in increasing quantities as evaporation proceeds leading to increased viscosity and poorer circulation, heat transfer coefficients in practice may be much lower than this.As evaporation proceeds, the remaining liquors become more concentrated and because of this the boiling temperatures rise. The rise in the temperature of boiling reduces the available temperature drop, assuming no change in the heat source. And so the total rate of heat transfer will drop accordingly. Also, with increasing solute concentration, the viscosity of the liquid will increase, often quite substantially, and this affects circulation and the heat transfer coefficients leading again to lower rates of boiling. Yet another complication is that measured, overall, heat transfer coefficients have been found to vary with the actual temperature drop, so that the design of an evaporator on theoretical grounds is inevitably subject to wide margins of uncertainty.

Dr SMS 2012/20136http://www.nzifst.org.nz/unitoperations/evaporation1.htm+Processing Factor in Evaporation7Concentration in liquidSolubilityTemperature sensitivity of materialsFoaming or frothingPressure and temperatureScale deposition and materials of construction

+8Concentration in liquidUsually liquid feed to evaporation is relatively dilute and has a lower viscosity and higher heat transfer coefficient, hAs evaporation proceeds, the solution become more concentrate and high viscosity, then will drop the heat transfer coefficient value.Therefore, adequate circulation and turbulence must be present to keep the h value becoming too low.SolubilityAs solutions are heated, the concentration of solute increase and solubility is decrease and can be exceed the solubility limit of the solution, then the crystal formed.Solubility is increase as temperature increase. This means when hot concentrated solution from evaporation is cooled to room temperature, crystallization may occur. Temperature sensitivity of materialsMany food products or biological materials may be temperature sensitive and degrade at higher temperatures or after prolonged heating.Must be considered in the operation of evaporation.Processing Factor in Evaporation+9Processing Factor in EvaporationFoaming and frothingCaustic solutions, some food solutions such as milk, some fatty acid solutions form foam/froth during boiling.This foam will losses from the solution by the vapor comes out from the evaporation.

Pressure and temperatureHigher operating pressure, higher boiling temperature of the solutionAs concentration of the solution increased by evaporation, the temperature of boiling may rise- called boiling point rise (BPR)To keep the temperatures low in heat sensitive materials, it is often necessary to operate under 1 atm (i.e under vacuum)

Scale deposition and materials of constructionSome solid material can be deposit on the heating surface of the evaporation, this will reduce the overall heat transfer coefficient and cleaning is necessary.Material for construction of evaporation must be minimize corrosion phenomena.

+Rate of Evaporation The basic factors that affect the rate of evaporation are the:rate at which heat can be transferred to the liquidquantity of heat required to evaporate each kg of watermaximum allowable temperature of the liquidpressure at which the evaporation takes placechanges that may occur in the foodstuff during the course of the evaporation process.Important practical considerations in evaporators are the:maximum allowable temperature, which may be substantially below 100C.promotion of circulation of the liquid across the heat transfer surfaces, to attain reasonably high heattransfer coefficients and to prevent any local overheating,viscosity of the fluid which will often increase substantially as the concentration of the dissolved materials increases,tendency to foam which makes separation of liquid and vapour difficult.Dr SMS 2012/201310http://www.nzifst.org.nz/unitoperations/evaporation1.htm+Type of EvaporatorOpen kettle or panHorizontal-tube natural circulation evaporatorVertical-type natural circulation evaporatorLong-tube vertical-type evaporatorFalling-film type evaporatorForced-circulation-type evaporatorAgitated-film evaporatorOpen-pan solar evaporator

11+Open Kettle/Pan Evaporatorheat is supplied by condensation od steam in a jacket or in coils immersed in the liquidin some cases, kettle is direct firedinexpensive and simple to useheat economy is poorin some cases, paddles or scrapers are used for agitationDr SMS 2012/201312

http://rpaulsingh.com/animated%20figures/fig8_4.htm+Horizontal Tube Natural Circulation EvaporatorThe horizontal bundle of heating tubes similar to heat exchanger is usedThe steam enters the tubes, where it condenses, leaves at the other end of the tubes.The boiling liquid solution covers the tubes. The vapor leaves the liquid surface, often goes through some de-entraining device such as baffle to prevent carryover of liquid droplets, and leaves out the top.Relatively cheap, used for nonviscous liquids with high heat-transfer coefficient and liquid that do not deposit scale.Dr SMS 2012/201313

+Vertical Type Natural Circulation EvaporatorThe liquid is inside the tubes and the steam condenses outside the tubesBecause of boiling and decreases in density, the liquid rises in the tubes by natural circulation, and flows downward through a large, central open space or down comer.Often called as short-tube evaporator

Dr SMS 2012/201314

+Long Tube Vertical Type EvaporatorThe tubes are 3 to 10 m long and the formation of vapor bubbles inside the tubes causes a pumping action, which gives quite high liquid velocitiesLiquid passes through the tubes only once and is not recirculates. Contact time can be quite low in this type of evaporator.In some cases, as when the ratio of feed to evaporation rate is low, recirculation is made by adding large pipe connection between the outlet concentrate line and the feed line

Dr SMS 2012/201315

http://rpaulsingh.com/animated%20figures/fig8_5.htm

http://rpaulsingh.com/animated%20figures/fig8_6.htm+Falling Film Type EvaporatorLiquid is fed to the top of the tubes and flows down the walls as thin filmV-L separation take place at the bottom widely used for concentrating heat sensitive materials such as fruit juicesDr SMS 2012/201316

http://rpaulsingh.com/animated%20figures/fig8_7.htm

http://www.niroinc.com/evaporators_crystallizers/falling_film_evaporators.asp+Falling Film Type EvaporatorDr SMS 2012/201317http://www.youtube.com/watch?v=3T8Km9BYHeg&playnext=1&list=PL4ECF5DA8511498E6&feature=results_main

+Forced Circulation Type EvaporatorUsed pump to circulate the liquidIncrease liquid-film heat transferUse for viscous liquidsDr SMS 2012/201318

http://www.niroinc.com/evaporators_crystallizers/forced_circulation_evaporator.asp+Forced Circulation Type EvaporatorDr SMS 2012/201319

http://www.youtube.com/watch?feature=endscreen&list=PL4ECF5DA8511498E6&NR=1&v=22W753joAnA+Agitated Film EvaporatorMechanical agitation of liquid film to increase turbulence in this film, and hence the heat transfer coefficient Modification of falling film evaporator with only a single , large, jacketed tube containing an internal agitator.Liquid enters at the top of the tube and as it flows downward, it is spread out into a turbulent film by vertical agitator blades.The concentrated solution leaves at the bottom and vapor leaves through a separator and out the top.Dr SMS 2012/201320

http://www.technoforce.net/agitated-thin-film-evaporators.html

http://distilleryplants.tradeindia.com/agitated-thin-film-evaporator-355261.html+Method of Operation of EvaporatorsSingle effect evaporatorsForward feed multiple effect evaporatorsBackward feed multiple effects evaporatorsParallel feed multiple effect evaporators

Dr SMS 2012/201321+Single Effect EvaporatorsThe solution in the evaporator is assumed to be completely mixed, the concentrated product and the solution in evaporator have the same composition and temperature T1, which is the boiling point of solution at P1.The temperature of the vapor is also at T1, since it is equilibrium with the boiling solution.The pressure is P1, which is the vapor pressure of the solution at T1.Often used when the required capacity of operation is relatively small and the cost of steam is relatively cheap compared to the evaporator costHowever, energy utilization is poor since the latent heat of the vapor leaving is not used but is discarded.Dr SMS 2012/201322

The rate of heat transfer (q : W, btu/h)

U : overall heat transfer coefficient, W/m2.K; btu/h.ft2.FA : heat transfer area, m2; ft2Ts, T1 : in K; FTs is temperature of condensing steam

+Forward Feed Multiple Effect EvaporatorsThe fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow.Used when the feed hot or when the final concentrated product might be damaged at high temperature.At steady-state operation, the flow rates and the rate of evaporation in each effect are constant.The boiling temperature decrease from effect to effect, cause pressure also decrease (e.g. if first evap is at 1 atm the last evap. will be under vacuum).Dr SMS 2012/201323

1 kg of steam will evaporate 1 kg of water in each evaporationThe 1st evap. operates at a T high enough that the evaporated water serves as the heating medium to the 2nd evap.Very rough estimation, 3kg water will be evaporated for 1 kg steamSteam economy (kg vapor evaporated/kh steam used) is increased+Backward Feed Multiple Effect EvaporatorsFresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect.Advantageous when the fresh feed is cold or when concentrated product is highly viscous.Liquid pump are used in each effects, since the flow is from low to high pressure.The high temperature in the first effect reduce the viscosity and give reasonable heat-transfer coefficient.

Dr SMS 2012/201324

+Parallel Feed Multiple Effect EvaporatorsInvolves the adding of fresh feed to each effect and the withdraw of concentrated product from each effect.However, the vapor from each effect is still used to heat the next effectMainly used when the feed is almost saturated and solid crystal are the product, as in the evaporation of brine to make salt

Dr SMS 2012/201325+Overall Heat Transfer Coefficients in EvaporatorComponents contribute to the overall heat transfer coefficient , U in evaporatorsteam-side condensing coefficient can be predicted using Eqs 4.8-20 to 4.8-26.metal wall resistance usually negligible due to high thermal conductivity of metal; increase velocity to decrease the rate of scale formationresistance of the scale on the liquid side cannot be predictedliquid film coefficient, h - usually inside the tube - can be predicted using various eq depend on type of tubes configuration/evaporator type

Dr SMS 2012/201326

+Calculation Method for Single Effect EvaporatorhF and hL often not available, enthalpy-concentration data are available for only few substance, some approximation are made:Using latent heat of evaporation of 1 kg water from from steam table at solution boiling temperature, T1Calculate using heat capacity, cpF and cpL if availableDr SMS 2012/201327

+Example 8.4-1Heat-Transfer Area in Single-Effect Evaporator.A continuous single-effect evaporator concentrates 9072 kg/h of a 1.0 wt % salt solution entering at 311.0 K (37.8 C) to a final concentration of 1.5 wt %. The vapor space of the evaporator is at 101.325 kPa (1.0 atm abs) and the steam supplied is saturated at 143.3 kPa. The overall coefficient U = 1704 W/m2 .K. calculate the amounts of vapor and liquid product and the heat-transfer area required. Assumed that, since it its dilute, the solution has the same boiling point as water.

Dr SMS 2012/201328

+Effect of Processing Variables on Evaporator Operation.Feed temperature, TFTF < Tbp, some of latent heat of steam will be used to heat up the cold feed, only the rest of the latent heat of steam will be used to vaporize the feed. feed is under pressure & TF > Tbp, additional vaporization obtained by flashing of feed.

Evaporator pressure, P1desirable T [q = UA(TS T1)], A & cost .T1 depends on P1 - will P1 T1 then T (e.g under vacuum) .

Steam pressure, PS PS will TS but high-pressure steam is costly.Optimum TS by overall economic balances are need.

Dr SMS 2012/201329+Boiling Point Rise & Heat of SolutionMajority cases, solutions in evaporator are not dilute, thus thermal properties of the solution being evaporated may differ considerably with water.Dhrings rule a straight line of solution boiling point against water boiling point at the same pressure for a given concentration at different pressuresHeat of solution must be considered in heat balance for the substance that give a considerable temperature rise during dissolve in water.

Dr SMS 2012/201330+Duhrings PlotDr SMS 2012/201331

+Example 8.4-2Dr SMS 2012/201332+Enthalpy-Concentration ChartDr SMS 2012/201333

+Example 8.4-3An evaporator is used to concentrate 4536 kg/h of a 20 % solution of NaOH in water entering at 60 C to a product of 50 % solid. The pressure of the saturated steam used is 172.4 kPa and the pressure in the vapor space of the evaporator is 11.7 kPa. The overall heat-transfer coefficient is 1560 W/m2.K. calculate the steam used, the steam economy in kg vaporized/kg steam used, and the heating surface area in m2

Dr SMS 2012/201334

+Solution Example 8.4-3Refer to Fig. 8.4-4, for flow diagram for this solution.For the total balance, F = 4536 = L + VFor the balance on the solute alone, F xF = L xL 4536 (0.2) = L (0.5) L = 1814 kg/h of liquidSubstituting into total balance and solving, V = 2722 kg/h of vapor

To determine T1 = Tsat + BPR of the 50 % concentrate product, first we obtain Tsat of pure water from steam table. At 11.7 kPa, Tsat = 48.9 C. From Duhring chart (Fig. 8.4-2), for a Tsat = 48.9 C and 50 % NaOH , the boiling point of the solution is T1 = 89.5 C. From the enthalpy-concentration chart (Fig.8.4-3), forTF = 60 Cand xF = 0.2gethF = 214 kJ/kg. T1 = 89.5 C and xL = 0.5gethL = 505 kJ/kg.

Dr SMS 2012/201335+Solution Example 8.4-3For saturated steam at 172.4 kPa, from steam table, we getTS = 115.6 C and = 2214 kJ/kg.To get HV for superheated vapor, first we obtain the enthalpy at Tsat = 48.9 C and P1 = 11.7 kPa, get Hsat = 2590 kJ/kg. Then using heat capacity of 1.884 kJ/kg.K for superheated steam. SoHV = Hsat + cP BPR = 2590 + 1.884 (40.6) = 2667 kJ/kg. (alternatively read superheated table)Substituting into heat balance equation and solving for S,F hF + S = L hL + V HV4535 (214) + S (2214) = 1814 (505) + 2722 (2667)S = 3255 kg steam /h.The heat q transferred through the heating surface area, A isq = S () = 3255 (2214) (1000 / 3600) = 2 002 000 WSolving for capacity single-effect evaporator equation;q = U A T= U A (TS T1)2 002 000 = 1560 A (115.6 89.5)Solving,A = 49.2 m2. Steam economy = 2722/3255 = 0.836

Dr SMS 2012/201336+Calculation Method for Multiple Effect EvaporatorThe calculation are done using material balance, heat balance and heat capacity equation (q=UAT) for each effect. Normally using trial and error method.Objective to calculateArea (A) in each effectAmount of steam (S) needAmount of vapor (V) leaving each effect

Usually given or known valueSteam pressure in first effect Final pressure in the vapor space of last effect (P3)First condition and flow to first effect (F, XF)Final concentration of the liquid leaving on the last effect (X3)Physical properties such as enthalpies or heat capacity of the liquid and vaporOverall heat transfer coefficient on each effect, normally the value is same in each effect, U

37Dr SMS 2012/2013

+Calculation Method for Multiple Effect EvaporatorAssumption made in operation;no boiling point rise.no heat of solution.neglecting the sensible heat necessary to heat the feed to the boiling point.Heat balances for multiple/triple-effect evaporator.Heat is same in all effect: q = U1 A1 T1 = U2 A2 T2 = U3 A3 T3 Areas in all effects are equal,: q/A = U1 T1 = U2 T2 = U3 T3 The temperature drops in evaporator (no BPR),T = T1 + T2 + T3 = TS T3 The temperature drops in evaporator (with BPR),T = T1 + T2 + T3 = TS Tsat@P3 (BPR1+BPR2+BPR3) hence we know that T are approximately inversely proportional to the values of U,

similar equations can be written for T2 and T3if we assumed that the value of U is the same in each effect, the capacity equation,q = U A (T1 + T2 + T3 ) = UA T

Dr SMS 2012/201338

+Calculation Method for Multiple Effect EvaporatorDr SMS 2012/201339+Dr SMS 2012/201340Find T3, BPR3 and TS3Assume V1=V2=V3Calc. L1,L2,L3,X1,X2,X3 from MBCompare A1,A2,A3 with AmCalc. q1, q2, q3 and solve A1,A2,A3Find AmCompare V1,V2,V3from MB with V1,V2,V3 from EBFind H1,H2,H3, s1,s2,s3Find T1,T2,T3,Ts1,Ts2,Ts3Calc. T, T1, T2, T3Adjust for cold feedCalc. BPR1, BPR2, BPR3From EB, calc. new V1,V2,V3, L1,L2,L3,STOP>10%>10%+Example 8.5-1A triple-effect forward-feed evaporator is being used to evaporate a sugar solution containing 10 wt% solids to a concentrated solution of 50 %. The boiling-point rise of the solutions (independent of pressure) can be estimated from (BPR C = 1.78x + 6.22 x2 ), where x is wt fraction of sugar in solution. Saturated steam at 205.5 kPa and 121.1C saturation temperature is being used. The pressure in the vapor space of the third effect is 13.4 kPa. The feed rate is 22 680 kg/h at 26.7 C. the heat capacity of the liquid solutions is cP = 4.19 2.35x kJ/kg.K. The heat of solution is considered to be negligible. The coefficients of heat transfer have been estimated as U1 = 3123, U2 = 1987, and U3 = 1136 W/m2.K. If each effect has the same surface area, calculate the area, the steam rate used, and the steam economy.

Dr SMS 2012/201341

+Dr SMS 2012/201342

A triple-effect forward-feed evaporator is being used to evaporate a sugar solution containing 10 wt% solids to a concentrated solution of 50 %. The boiling-point rise of the solutions (independent of pressure) can be estimated from (BPR C = 1.78x + 6.22 x2 ), where x is wt fraction of sugar in solution. Saturated steam at 205.5 kPa and 121.1C saturation temperature is being used. The pressure in the vapor space of the third effect is 13.4 kPa. The feed rate is 22 680 kg/h at 26.7 C. the heat capacity of the liquid solutions is cP = 4.19 2.35x kJ/kg.K. The heat of solution is considered to be negligible. The coefficients of heat transfer have been estimated as U1 = 3123, U2 = 1987, and U3 = 1136 W/m2.K. If each effect has the same surface area, calculate the area, the steam rate used, and the steam economy.

+SummaryIntroductionProcessing Factor in EvaporationType of EvaporationMethod of Operation of EvaporatorsCalculation Method for Single Effect EvaporatorCalculation Method for Multiple Effect EvaporatorDr SMS 2012/201343Any Question?+