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Thermal Unit Operation (ChEg3115) Lecture 6: Evaporator Instructor: Mr. Bassazin Ayalew and Mr. Tesfa Nega By Bassazin Ayalew & Tesfa Nega Topic Outline Introduction Types of evaporators Processing Factors Operation methods of evaporators Overall heat transfer coefficient in evaporators Calculations in single effect evaporator Calculations in single effect evaporator Evaporation- Introduction Evaporation is the process used to concentrate a solution by removing the solvent (mainly water) in a purified form by the application of heat. Evaporation is the unit operation by which solvent is evaporated from the solution by boiling the liquid in suitable vessel and withdrawing the vapor, leaving a concentrated liquid residue. Evaporation is a type of vaporization of a liquid that only occurs on the surface of a liquid. The other type of vaporation is boiling, which, instead, occurs within the entire mass of the liquid. Evaporation is the removal of a solvent by vaporisation, from solids that are not volatile. It is normally used to produce a concentrated liquid, often prior to crystallisation, but a dry solid product can be obtained with some specialized designs. Evaporation is a surface phenomenon, i.e., mass transfer takes place from the surface. Evaporation- Introduction Evaporation like drying, removes volatile substances from a solution but the two processes differ in the following, Evaporation Removal of most water from solution Normally takes place at boiling point of water Drying Removal of small amount of water from solid material (moisture) Occurs at temperature below boiling point and is typically influenced by humidity cont.…Evaporation- Introduction Evaporation VS. The process by which a liquid changes to its vapors at a temperature below its boiling point. It takes place at all temperatures. The temperature may change during evaporation. Vaporizer The process by which a liquid changes to its vapors at its boiling point. It takes place only at fixed temperature called boiling point. The temperature during vaporization does not change. It is a slow and silent process. It is a fast and violent/ intense process. It takes place only at the surface of the liquid. It takes place over the entire mass of the liquid. The rate of evaporation depends upon the The rate of vaporization does not depend upon surface area of the liquid, wind speed, humidity, the surface area, wind speed, humidity, and and temperature. temperature cont.…Evaporation- Introduction Evaporation differs from the other mass transfer operations such as distillation and drying. In distillation, the components of a solution are separated depending upon their distribution between vapor and liquid phases based on the difference of relative volatility of the substances. Figure. Distillation column cont.…Evaporation- Introduction Removal of moisture from a substance in presence of a hot gas stream to carry away the moisture leaving a solid residue as the product is generally called drying. Evaporation is normally stopped before the solute starts to precipitate in the operation of an evaporator. In chemical industries, the manufacture of heavy chemicals such as caustic soda, table salt, and sugar starts with dilute aqueous solutions from which large quantities of water must be removed using evaporators before final crystallization take place in suitable equipment. cont.…Evaporation- Introduction Why we need evaporation Reduce transportation cost Storage costs Prepare for the next operation-drying, crystallization, etc. Recovery of solvent. Reduces deteriorative chemical reactions Better microbiological stability 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. Processing Factors Some of the properties of evaporating liquid that influence the process of evaporation are Concentration in the liquid Low viscosity: high heat transfer coefficient High viscosity: low transfer transfer coefficient Adequate circulation and/or turbulence must be present to keep the coefficients from becoming too low Solubility Solubility increases with temperature Crystallization may occur when a hot concentrated solution is cooled to room temperature Temperature sensitivity of materials Food and biological materials may be temperature sensitive and degrade at higher temperature or after prolonged heating. Cont.. Processing Factors Foaming or frothing Food solution such as skim milk and some fatty-acid solution form a foam or froth during boiling. The foam is carried away along with vapor leaving the evaporator, thus losses might occur. Pressure and temperature High operating pressure: high boiling point As the concentration of the dissolved material in solution increases by evaporation, the temperature of boiling may rise To keep the temperatures low in heat-sensitive materials : operate under atmospheric pressure (under vacuum). Scale deposition and materials of construction Some solutions deposit solid materials called scale on the heating surfaces (fouling) May cause a drastic decrease of the heat-transfer coefficient (U) and the evaporator must be cleaned. The materials used in construction of the evaporator should be chosen to minimize corrosion. Type of evaporators Basic Parts of an Evaporator Heat-exchanger Vacuum Vapour separator Condenser Cont.… Type of evaporators Horizontal tube evaporator: The steam condensate leaves at the other end of the tube. The horizontal bundle of heating tubes is similar to the bundle of tubes in a heat exchanger. The steam enters the tube, condense and leaves at the end of tube. The boiling solution covers the tube and the vapor leaves the liquid surface, goes through the baffle to prevent carry over of liquid droplets. Used for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales Advantage- relatively cheap, easy to install, requiers less space for installation, can be used for batch /or continous operation. Disadvantage - poor liquid circulation (and therefore unsuitable for viscous liquids) Cont.… Type of evaporators Horizontal tube evaporator: The major use is for making distilled water for boiler field. Horizontal tube evaporatores are used in the pharmaceutical industry, pulp and pare industry, etc. Horizontal tube evaporators are not suitable for salting or scaling liquids, and they have smaller capacity than other evaporators. Used for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales Advantage- relatively cheap Disadvantage - poor liquid circulation (and therefore unsuitable for viscous liquids) Cont.… Type of evaporators Horizontal tube evaporators Cont.… Type of evaporators Short-tube vertical evaporators the oldest but still widely used in sugar industry in evaporation of cane-sugar juice. also known as calandria or Robert evaporators. consist of a short tube bundle (about 4 to 10 ft. in length) enclosed in a cylindrical shell The feed is introduced above the upper tube sheet and steam is introduced to the shell or steam chest of the calandria. The solution is heated and partly vaporized in the tubes. Typically it’s downcomer area is taken as 40 to 70% of the total cross sectional area of tubes. Not suitable for viscous liquid Cont.. Type of evaporators Short-tube vertical evaporators Advantage High heat transfer coefficents at high temperature difference. Easy mechanical descalling Relatively inexpensive Disadvantage Poor heat transfer at low temperature difference. High floor space and weight Relatively high holdup Poor heat transfer with viscous liquids. Cont.. Type of evaporators Short-tube vertical evaporators Calandria type evaporator. Cont.. Type of evaporators Long tube- vertical type evaporator Heat transfer coefficient on the steam side is very high compared to the evaporating-liquid side – thus, high liquid velocities are desirable. The liquid is inside the tubes (3-10m long) The formation of vapor bubbles inside the tubes causes a pumping action which gives quite high liquid velocities. The liquid pass through the tube only once and is not recirculated. Widely used in producing condensed milk. Cont.. Type of evaporators Falling Film Evaporators In a falling film evaporator, the liquid is fed at the top of the tubes in a vertical tube bundle. The liquid is allowed to flow down through the inner wall of the tubes as a film. As the liquid travels down the tubes the solvent vaporizes and the concentration gradually increases. Liquid is fed to the top of tubes and flows down the walls as a thin film. Vapor-liquid separation usually takes place at the bottom. Has small holdup time (5-10s or more) and high heat transfer coefficients Widely used for concentrating heat-sensitive materials such as fruit juices Cont.. Type of evaporators Falling Film Evaporators Cont.. Type of evaporators Forced-circulation type evaporator The heat transfer coefficient can be increased by pumping to cause forced circulation of the liquid inside the tubes. This could de done in the long vertical or horizontal tubes -type evaporator by adding a pipe connection shown with a pump between the outlet concentrate line and the feed line However, the vertical tubes used are usually shorter than in the long-tube type evaporator Very suitable for viscous liquids. Cont.. Type of evaporators Forced-circulation type evaporator Cont.. Type of evaporators Agitated-film evaporator Agitators used to agitate the liquid to induce turbulence in film and hence, the heat transfer coefficient will be increase. This is done in modified falling film evaporator with only single, large, jacketed tube containing an internal agitator. Liquid enters the top of the tube and as it flows downwards, it is spread out into a turbulent film by the vertical agitator blades. Concentrated solution leaves at the bottom and vapor leaves out the top. Suitable for highly viscous and heat-sensitive material- e.g. rubber latex, gelatin, antibiotics and fruit juices Disadvantage-high cost and small capacity Cont.. Type of evaporators Agitated-film evaporator application include Starch Industry Dairy Industry Food Industry Pulp and Paper Textile Industry Beer and Beverages Edible Oil Industry Specialty Chemical Dyes and Pigments Soap and Biofuels Alcohol Industry Pharmaceutical Industry Natural Products Chlor-Alkali Petrochemical and Polymer Industry Design Principle of Single Effect and Triple Effect of Evaporator Performance of evaporators The performance of a steam-heated evaporator is measured in terms of its capacity and economy. Capacity of an evaporator is defined as the number of kilogram of water evaporated per hour which is directly proportional to :The difference between the condensing temperature of the steam supplied and the temperature of the boiling solution in effect(in case of multiple effect it is the last one). The overall coefficient of heat transfer from steam to solution. Capacity=V Economy (or steam economy) is the number kilogram of water vaporized from all the effects per kilogram of steam used ; which it depends on the number of effect and influenced by the temperature of the feed. V Economy= S Mechanisms of Increasing Steam Economy Pre-heating the feed Using vapors generated in evaporator Using a non-steam source Steam generation is an expensive operation Use of multiple-effects Vapor recompression Thermal (steam jet booster) Mechanical (compressor) Note: The above techniques increase steam economy, but NOT capacity (throughput) Single-effect Evaporator Evaporators are classified by the number of effects as single and multiple effect. In case of a single-effect evaporator, the vapor from the boiling liquor is condensed and the concentrated product is withdrawn from the bottom of the evaporator. Nearly always the material to be evaporated flows inside the tubes. The boiling liquid is subjected under moderate vacuum Reducing the boiling temp of the liquid increases the temperature difference b/w the steam and the boiling liquid and thus increases the heat transfer rate in the evaporator. When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. Single-effect Evaporator Typically 1.1 to 1.3 kg of steam is required to evaporate 1 kg of water. The steam consumption per unit mass of water evaporated can be decreased by putting more than one evaporator in series such that the vapor from one evaporator is used in the second evaporator for heating. The vapor from the second evaporator is condensed and the arrangement is called double-effect evaporators. Single-effect Evaporator A single-effect evaporator is wasteful of energy because the latent heat of the vapor leaving is not used but is discarded. It is simple but utilizes steam ineffectively. Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost. Single-effect Evaporator Based on figure: The feed enters at TF Saturated steam at TS enters the heat- exchange section. Condensed steam leaves as condensate or drips. The solution in the evaporator is assumed to be completely mixed Hence, the concentrated product and the solution in the evaporator have the same composition. Temperature T1 is the boiling point of the solution. The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution. The pressure is P1, which is the vapor pressure of the solution at T1. Multiple-effect Evaporator Increasing the evaporation rate by using a series of evaporators between the steam supply and condenser. The latent heat, can be recovered and reused (effective steam utilization) For large capacity operations Types of multiple-effect evaporator; Forward-feed multiple effect evaporator The 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 is hot or when the final concentrated product might be damaged at high temperatures. The boiling temperature decrease from effect to effect. If the first effect is at P1 = 1 atm, so the last effect will be under vacuum. Multiple-effect Evaporator Multiple-effect Evaporator Forward feed arrangement Advantages: Feed moves from high pressure to low pressure, so pumping of feed is not requited. Product is obtained at lowest temperature. This method is suitable for scale forming liquids because concentrated product is subjected to lowest temperature. Disadvantages: It is not suitable for cold feed ,because the steam input in effect-1 raises the temperature of feed, and a small amount of heat is supplied as latent heat of vaporization. Therefore the amount of vapor produced will be less than the amount of steam supplied. Lower amount of vapor in effect-1 produces lower amount of vapor in subsequent effect. Therefore overall economy is lower. Multiple-effect Evaporator Backward-feed multiple effect evaporator The fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. Used when the fresh feed is cold However, liquid pumps must be used in each effect, since the flow is from low to high pressure. This reverse-feed method is also used when the concentrated product is highly viscous. Temperature increase from effect to effect The high temperatures in the early effects reduce the viscosity and give reasonable heattransfer coefficients. Multiple-effect Evaporator Multiple-effect Evaporator Backward Feed arrangement Advantages: It is suitable for cold feed. It will give more economy. This method is suitable for viscous products, because highly concentrated product is at highest temperature, hence lower viscosity. Disadvantages: As liquid moves from low pressure side to high pressure side, so pumping is required. Multiple-effect Evaporator Parallel-feed multiple effect evaporator Involves the adding of fresh feed and withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt. Multiple-effect Evaporator Mixed Feed arrangement In the mixed feed operation, the dilute feed liquid enters at an intermediate effect and flows in the next higher effect till it reaches the last effect of the series. In this section, liquid flows in the forward feed mode. Feed is introduced in intermediate effect moves forward and then backward to effect-1 and steam introduced in the first effect. Partly concentrated liquor is then pumped back to the effect before the one to which the fresh feed was introduced for further concentration. Mixed feed arrangement eliminates some of the pumps needed in backward configuration as flow occurs due to pressure difference whenever applicable. Multiple-effect Evaporator Mixed Feed arrangement Multiple-effect Evaporator Mixed Feed arrangement ► Advantages: Pumping of liquid 'requires only where liquid moves from low pressure to high pressure. Product is obtained from highest temperature, hence lowest viscosity. ► Disadvantages: As liquid moves from low pressure side to high pressure side, so pumping is required. Thermal/ Process Design Considerations Many factors must be carefully considered when designing evaporators. The type of evaporator or heat exchangers, forced or natural circulation, feeding arrangement, boiling point elevation, heat transfer coefficient, fouling, tube size and arrangement are all very important. Thermal/ Process Design Considerations Guidelines for selection of most suitable evaporator Tube size, arrangement and materials The selection of suitable tube diameter, tube length and tube –layout is determined by trial and error calculations like in design of shell and tube heat exchangers. If the pressure drop is more than the allowable pressure drop further adjustments in tube diameter, tube length and tube-layout is required. A variety of materials including low carbon steel, stainless steel, brass, copper, cupronickel etc. are used. However the selection of tube materials depends on the corrosiveness of the solution and working conditions. Thermal/ Process Design Considerations Heat transfer coefficients The heat transfer coefficient of condensing steam in shell side is normally very high compared to the liquid side. Therefore tube side (liquid side) heat transfer coefficient practically controls the rate of heat transfer. The overall heat transfer coefficient should be either known/ calculated from the performance data of an operating evaporator of the same type and processing the same solution. Typical values of overall heat transfer coefficient are given in Table below. Thermal/ Process Design Considerations Heat transfer coefficients Thermal/ Process Design Considerations Boling-Point Rise (BPR) of Solution Most evaporators produce concentrated liquor having a boiling point considerably higher than that of pure solvent (or water). This phenomenon is called boiling point elevation (BPE/R). BPE occurs as the vapor pressure of a solution (usually aqueous solution) is less than that of pure solvent at the same temperature. Boiling point of a solution is a colligative property. It depends on the concentration of solute in the solution for a pair of solute and solvent. BPE of the concentrated liquor reduces the effective temperature driving force compared to the boiling of pure solvent. The boiling-point-rise, BPR is usually given in terms of fraction of solute, x or by using empirical law such as instance Dühring’s rule for sodium hydroxide. Thermal/ Process Design Considerations Boling-Point Rise (BPR) of Solution In the majority of cases in evaporation, the solutions are not assumed to be dilute enough to be considered to have the same thermal properties as water. In most cases, the thermal properties (heat capacity and the boiling point) of the solution being evaporated may differ considerably from those of water. For strong solutions of dissolved solutes the boiling-point rise due to the solutes in the solution usually cannot be predicted. However, a useful empirical law known as Duhring’s rule can be applied. According to this rule, a straight line is obtained if the boiling point of a solution in °C or °F is plotted against the boiling point of pure water at the same pressure for a given concentration at different pressures. Example: Duhring plot for boiling point of NaOH Thermal/ Process Design Considerations Boiling Point Elevation (Boling point Rise) Determination of BPE: For strong solutions, the BPE data is estimated from an empirical rule known as Dühring rule. It states that the boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure. Thus if the boiling point of the solution is plotted against the corresponding boiling point of pure water at the same pressure, a straight line is generated. Different lines are obtained if such plots made for solution of different concentrations. The main advantage is that a Dühring lines can be drawn if boiling points of a solution and water (read from steam table) at two different pressures are known. This line can be used to predict boiling point of a solution at any pressure. Thermal/ Process Design Considerations The temperature of boiling liquor depends on :When Pressure in the evaporator. Solute concentration. Liquid head (also called hydrostatic head). When the solution has the characteristics of pure water, its boiling point can not be read from steam tables . Thermal/ Process Design Considerations Boling point Rise. The boiling point rise must be subtracted from the temperature difference that is predicted from steam table. The boiling point rise for common solution can be estimated from Dühring chart by using the boiling point of pure water at a given pressure P1 and the concentration of product solution as shown in figure(7a and b) Thermal/ Process Design Considerations Boling point Rise Thermal/ Process Design Considerations Hydrostatic head. Hydrostatic Head :- The effect of liquid head above the tube bundle is to add to pressure and rise the boiling temperature ∆P= hydrostatic head(liquid head) = (1/2) actual liquid head P’ = ρ.g.h Where ρ density of solution h liquid head (level) P’= pressure due to hydrostatic head of liquid. Thermal/ Process Design Considerations Selection of suitable evaporator The selection of the most suitable evaporator type depends on a number of factors. Mainly these are: Throughput, Viscosity of the solution (and its increase during evaporation), Nature of the product and solvent (such as heat sensitivity and corrosiveness), Fouling characteristics and, Foaming characteristics. A selection guidelines based on these factors is given in Table below: Thermal/ Process Design Considerations Selection of suitable evaporator Mechanical Design Considerations Temperature and pressure are the two important factors that affect the mechanical design of evaporator systems. Many other like startup, shutdown, external loading from supports, wind loading, earthquake load etc. also significantly affect the evaporator operation. Operating temperature and pressure: The operating temperature is the temperature that is maintained for the operation of the metal vessel suitably selected during design. The operating pressure is the pressure at the top of a pressure vessel specified Mechanical Design Considerations Design temperature and pressure: It is important to determine both minimum and maximum anticipated operating temperature and pressure in order to obtain the design temperature and pressure. The design pressure is generally the sum of the maximum allowable pressure and the static head of the fluid in the pressure vessel. Maximum allowable working pressure: The maximum allowable working pressure is the maximum pressure to which the equipment can be safely operated. Generally, it should not be less than the maximum anticipated operating pressure divided by a factor of 0.90. Mechanical Design Considerations Thermal expansion: Differential thermal expansion between various parts of equipment has a significant effect on the mechanical design. There may be a significant difference of expansion between the shell and the tube side because of temperature difference of two fluids. Thermal expansion may also determine the way in which tubes are fixed to the tube sheet. Usually a suitable expansion joint is centrally placed between two segments of the shell when the differential expansion may be large Heat Transfer in Evaporator Heat transfer is the most important single factor in evaporator design, since the heating surface represents the largest part of evaporator cost. The type of evaporator selected is the one having the highest heat transfer coefficient. The rate of heat transfer is expressed as :- Heat transfer coefficient is a strong function of temperature difference ∆T. Where U overall heat transfer coefficient. A heat transfer area. ∆T temperature difference between the heating medium and the boiling liquor ( Teffect Tsolution ). Thermal Design Calculation Single effect calculations: Single effect evaporator calculations are pretty straight forward. The latent heat of condensation of the steam is transferred through the heating surface to vaporize water from a boiling solution. Therefore two enthalpy balance equations are required to in order to calculate the rate of solvent vaporization and the rate of required input heat. Generally it is possible to solve the energy and the material balance equations analytically by a sequential approach. Thermal Design Calculation Single effect calculations: The following assumptions are made to develop the mass and energy balance equations:- there is no leakage or entrainment the flow of non-condensable is negligible heat loss from the evaporator system is negligible From the enthalpy data of the solutions, steam and condensate, the rate of heat input or the rate of steam flow can be calculated. The overall heat transfer coefficient UD (including dirt factor) is should be either known from the performance data of an operating evaporator of the same type and processing the same solution or a reasonable value can be selected from the standard text books Thermal Design Calculation Single effect calculations: Thermal Design Calculation Single effect calculations: Thermal Design Calculation Single effect calculations: Thermal Design Calculation Single effect calculations: Additional information: λ = latent heat of steam (obtained from the steam table in appendix at Ts (saturated steam temperature) An approximation for the latent heat of evaporation of 1 kg mass of water from aqueous solution can be obtained from steam table using temperature of the boiling solution, T1. If the heat capacity of the liquid feed (CpF) and the product (CpL) are known, they can be used to calculate the enthalpies. Effect of Processing Variables on Evaporators Effect of feed Temperature (TF) • The inlet temperature of the feed has a large effect on the evaporator operation. • When feed is not at its boiling point, steam is needed first to heat the feed to its boiling point and then to evaporate it. • Thus, feed must be at temperature greater or equal to the boiling point of the solution to improve the efficiency of evaporator • Preheating the feed can reduce the size of evaporator heat-transfer area. Effect of Pressure (P) • Pressure in the evaporator determine the boiling point of the solution (T1). • Steam pressure determine the steam temperature (Ts). • Since q = U A (Ts – T1), larger values of (Ts – T1) is desirable since as (Ts – T1) increases, the heating surface, A and cost of the evaporator decrease. Effect of Processing Variables on Evaporators • Thus to obtain larger values of (Ts – T1), lower T1 is needed. • To obtain, lower T1 the pressure in the evaporator can be reduced by operating under vacuum using a vacuum pump. Effect of Steam Pressure (Ps) • Another alternative to obtain larger values of (Ts – T1), is using higher Ts • To obtain, higher Ts , high pressure steam can be used. • However, high pressure steam is more costly as well as often being more valuable as a source of power elsewhere. • Therefore, overall economic balances must be considered to determine the optimum steam pressure. Thermal Design Calculation Multiple effect calculations: Material Balance. Overall M.B. F = L3 +V………………1 Solute Balance F.Xf = L.XL3……………..2 Material Balance for Each Effect. 1st effect F = V1 + L1 2nd effect L1 = V2 + L2 …………6 3rd effect L2 = V3 + L3 V =V1 + V2 + V3………..7 Thermal Design Calculation Multiple effect calculations: Where hf and hL enthalpy of feed and product (this depend on solute concentration and temperature )kJ/kg. λs latent heat of steam kJ/kg . Hv enthalpy of vapor kJ/kg. hs enthalpy of condense steam kJ/kg. Enthalpy of feed hf and product hL can be estimated from:i. h =Cp ∆T ii. Dühring chart see figure(8). Enthalpy-concentration chart for the system NaOH-water Thermal Design Calculation Multiple effect calculations: Heat Balance for Each Effect. 1st effect Fhf +S s =V1Hv1+L1hL1........8 2nd effect L1hL1 +V1 2 =V2Hv2+L1hL2........9 3rd effect L2hL2 +V2 2 =V3Hv3+L3hL3........10 λ1 ,λ2, λ3 from steam table at P1 ,P2 , P3 respectively. Thermal Design Calculation Multiple effect calculations: Calculation step Considering for three evaporators arranged as shown in the above figure, in which the temperatures and pressures are T1, T2, T3, and P1, P2, P3, respectively, in each unit, then the heat transmitted per unit time across each effect is: Effect 1 Q1 = U1A1∆T1, where ∆T1 = (Ts − T1), Effect 2 Q2 = U2A2∆T2, where ∆T2 = (T1 − T2), Effect 3 Q3 = U3A3∆T3, where ∆T3 = (T2 − T3). If the liquor has no boiling point rise, and neglect the effect of hydrostatic head, therefore; ∆T1 = (Ts −. T1) T1 = T1 boiling point of pure water at P1 (operating pressure ) . . This is also for ∆T2 and ∆T3 , T1 =( T1 -T2 ), T3 =( T 2 -T 3 ) Thermal Design Calculation Multiple effect calculations: Calculation step If the liquor has boiling point rise then :- ∆T1 = (Ts − T1) . T1 = T1 + BPR or T1 from Dühring chart The heat required Q1 transfer across A1 appear as latent heat in the vapor V1 and is used as steam in the second effect and so on. Hence, Q1 = Q2 = Q3 = Q…………………………. ..11 U1A1∆T1 = U2A2∆T2 = U3A3∆T3………………..…….12 Where A1 =A2 = A3 Therefore; U1∆T1 = U2∆T2 = U3∆T3……………………………....13 Q =U1T1= U2T2 U3T3 ....... 14 A Thermal Design Calculation Multiple effect calculations: Calculation step On this analysis, the difference in temperature across each effect is inversely proportional to the heat transfer coefficient. T =T1+T2 T3 = Ts -T3....... 15 1 U1 ΔT1= ΔT .............. 16 1 1 1 + U2 U3 U1 1 U2 ΔT2= ΔT .............. 17 1 1 1 + U2 U3 U1 Thermal Design Calculation Multiple effect calculations: Calculation step 1 U3 ΔT3= ΔT = ΔT ΔT3 +ΔT 3 1 1 1 + U2 U3 U1 .............. 18 The water evaporated in each effect is proportional to Q , so to estimate the capacity of three effect by adding the value of Q for each effect :Q = Q1 + Q2 + Q3 ………………………….19 = U1A1∆T1 + U2 A2∆T2 + U3 A3∆T3……….20 Assume U are equal for each effect and A thus :Q = U A(∆T1 + ∆T2 + ∆T3)…………………....21 = UA∆T…………………………………….22 Thermal Design Calculation Multiple effect calculations: Procedure Steps :- 1. The outlet concentration and pressure of the last effect are known, if BPR present, this can be determined from relation given or Dühring chart. 2. Determine the total amount evaporated by overall material balance and then calculated the concentration for each effect. 3. Estimate the temperature drops ∆T1 , ∆T2 ,∆T3 from equations 16,17,18 . 4. Calculate the amount vaporize , the concentration of liquid and value of Q transferred in each effect from material balance and heat balance, if the amount vaporize differ appreciably from those in step 2 then step 2,3,4 repeated using amount of evaporation calculated in step 4. 5. Using equation Q =UA∆T to calculate A1 , A2 , A3 if the area are not equal, then step6. Thermal Design Calculation Multiple effect calculations: Procedure Steps :- 6. calculate the average value of A as follows :- Am =A=( A1 + A2 + A3)/3 then calculate a new values of . . . T1 , T2 , T3 :ΔT1'= ΔT1A1 ΔT2A2 ΔT3A3 , ΔT2'= , ΔT 3'= Am Am Am ΔT'=ΔT1' ΔT2' +ΔT3' 7. Use the new ∆T from step 6 in the calculation. The calculation with step 4 will continued until the area are equal. If the BPR is present then using the concentration from step 4 to determine the BPR and boiling point in each effect, this gives a new value of summation of ∆T by subtracting the summation of all BPR from the overall ∆T. Calculate new value of ∆T from equation Q = UA∆T ΣBPR = BPR1 + BPR2 + BPR3, ∆T = Σ∆T- ΣBPR Backward feed The surface requirement for both forward and backward feed is same. From material and heat balance, there are five equations and five unknowns (Ws,w1,w2,w3,w4), so these may be solve simultaneously. THERMAL DESIGN CALCULATION The surface and steam requirements for multiple effect chemical evaporation can be computed by imposing a heat balance across each effect individually and a material balance over the whole system. The following nomenclature will be employed for a quadruple effect: CF: specific heat of feed tF: temperature of feed wF: feed rate Ts: saturation temperature of steam to the first effect λS is latent heat of steam introduced in the 1st effect at Ps Ws: steam rate to the first effect w1-4: total water removed by evaporation c1,c2,c3,c4: specific heat of liquor in effect 1 to 4 t1,t2,t3,t4: boiling points of liquor in effect 1 to 4 w1,w2,w3,w4: water removed in effect 1 to 4 U1,U2,U3,U4: design overall coefficient in effect 1 to 4 λ1, λ2, λ3, λ4 : latent heat of steam introduced in the 1st effect at P1, 2nd effect at P2, 3rd effect at P3, and 4th effect at P4 Assume that there are no chemical heat effects as a result of the concentration (i.e. negative heat of solution) and that there is no BPR. The surface in each of the bodies will be identical. It is shown by experience that under tis condition the pressure difference between each effect will be approximately equal. The assumption of an equal division of the total pressure difference not valid particularly when overall coefficients in the different effect vary greatly or when there is considerable flashing in the first effect. To equalize the surface in each body the temperature difference in the individual effects can be adjusted so that a larger temperature difference will be employed in the effect having the lowest heat transfer coefficient, the heat load in all effects remaining nearly equal. Multiple effect evaporator may be designed for a minimum surface or minimum initial cost. Example 1: Calculation of a single effect forward-feed evaporator A single effect evaporator is used to concentrate 7kg/s of a solution from 10 to 50wt% of solid. Steam is available at 205kN/m2 ,and evaporation takes place at 13.5kN/m2 ,if the overall heat transfer coefficient is 3205kW/m2.k.Calculate the heating surface required and the amount of steam used if the feed to the evaporator is at 294k. The specific heat of solution is :10% solution = 3.7kJ/kg.k 50% solution = 3.14kJ/kg.k Example 2: A single effect evaporator is used to concentrate 5000 kg/h of solution of sodium hydroxide from 10% to 25% solids. Steam is available at 143.27KPa absolute; the vapor space is maintained at 62.745KPa gage pressure. The feed enters at its boiling point corresponding to the vapor space pressure. Calculate: 1.The steam consumption per hour. 2.If the available heat transfer area is 35m2 estimate the heat transfer coefficient. Example: Calculation of a triple effect forward-feed evaporator It is desired to concentrate 50,000lb/hr of a chemical solution at 100F and 10.0% solids to a product which contains 50% solids. Steam is available at 12psig, and the last effect of a triple effect evaporator with equal heat transfer surfaces in each effect will be assumed to operate at a vacuum of 26.0 in. Hg referred to a 30 in. barometer. Water is available at 85 F for use in barometric condenser. Assume a negligible BPR, an average specific heat of 1.0 in all effects, the condensate from each effect leaves at its saturation temperature and that there are negligible radiation loss. Calculate a) steam consumption b) heating surface required for each body c) condenser water requirement. The accepted overall coefficients of the heat transfer for the different effect will be U1=600, U2=250, U3=125Btu/hr ft2F Question 1: It is desired to design a double effect evaporators for concentrating a certain caustic soda solution from 12.5wt% to 40wt%. The feed at 50oC enters the first evaporator at a rate of 2500kg/h. Steam at atmospheric pressure is being used for the said purpose. The second effect is operated under 600mmHg vacuum. If the overall heat transfer coefficients of the two stages are 1952 and 1220kcal/m2.h.oC. respectively, determine the heat transfer area of each effect. The BPR will be considered and present for the both effect. Question2: Determine the heating surface area required for the production of 2.5kg/s of 50wt% NaOH solution from 15 wt.% NaOH feed solution which entering at 100oC to a single effect evaporator. The steam is available as saturated at 451.5K and the boiling point rise (boiling point evaluation) of 50wt% solution is 35K. the overall heat transfer coefficient is 2000w/m2.K. The pressure in the vapor space of the evaporator at atmospheric pressure. The solution has a specific heat of 4.18kJ/kg.K. The latent heat of vaporization under these condition is 2257kJ/kg. Question3: A forced circulation triple-effect evaporator using forward feed is to be used to concentrate a 10wt% NaOH solution entering at 37.8oC to 50wt%. The steam used enter at 58.6KPa gage. The absolute pressure in the vapor space of the third effect is 6.76KPa. The feed rate is 13608kg/h ,heat transfer coefficient are U1=6246,U2=3407 and U3=2271 W/m2.K.All effect have the same area ,Calculate the surface area required and steam consumption. Design problem A 5% aqueous solution of a high molecular weight solute has to be concentrated to 40% in a forward-feed double effect evaporator at the rate of 8000 kg.h-1. The feed temperature is 40°C. Saturated steam at 3.5 kg.cm2 is available for heating. A vacuum of 600 mm Hg is maintained in the second effect. Calculate the area requirements, if calandria of equal area are used. The overall heat transfer coefficients are 550 and 370 kcal.h- 1m-2 °C-1 in the first and the last effect respectively. The specific heat of the concentrated liquor is 0.87 kcal.kg-1°C-1.