![ Assume area of all calendria same as A , & if no heat loss:
Steam Economy: [ Mv1 + Mv2 + Mv3 ] / Ms
Typical value of steam economy for a single effect
evaporator is between to 2-3.
Q = Q1
= Q2
= Q2
Q = U1A1ΔT1 = U3A3ΔT3 = U3A3ΔT3
Q = U1AΔT1 = U3AΔT3 = U3AΔT3
Q/A = U1ΔT1 = U3ΔT3 = U3ΔT3
{ΔT = ΔT1 + ΔT2 + ΔT3 = Ts- T3](https://image.slidesharecdn.com/evaporators-171128165404/85/Multi-Effect-Evaporators-21-320.jpg)
Multi Effect Evaporators
- 1. Multiple Effect Evaporators By: Nishant Arora nishantarora213@gmail.com
- 2. Definition 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.
- 3. Multi Effect Evaporator Multiple Effect Evaporation remains one of the popular methods used for the concentration of aqueous solutions. The chief factor influencing the economy of an evaporator system is the number of effects. By increasing the number of effects we can increase the economy of an evaporator system. The first effect of a multiple effect evaporator is the effect to which the raw steam is fed. Vapors obtained from first effect act as a heating medium for another effect. TVR or MVR is used in Multiple Effect Evaporators to use the dead vapors. For an evaporator maximum boiling temp is 70C in first stage and 40C in the last
- 5. Multi Effect Evaporator In a single effect evaporator steam is supplied for heating the liquid. The total heat is not transferred from the steam. So the rest of the heat is wasted. To use the heat efficiency , connections are made so that vapor from one effect can serves as the heating medium for the next effect. The dilute feed enter the first effect where it is partially concentrated; then it flows to second effect for additional concentration and soon. The final concentration is done is last effect. In the first effect raw steam is fed in which the vapor pressure is highest,p1. The second effect has p2 & soon. p1.>p2. >p3. >……pn..n is the number of effects. This pressure gradient is maintained by the vacuum pump and the condenser. Depending on lower in vapour pressure, boiling point of liquid also lowered as liquid move. Suppose T is the boiling point of liquid then T1>T2>T3………>Tn.
- 6. Different types Feed arrangement .Forward Feed arrangement. Backward Feed arrangement. Mixed Feed Parallel Feed arrangement.
- 7. Forward Feed arrangement .Both Feed and Steam introduced in the First effect.
- 8. Forward Feed arrangement Advantages: Disadvantages: ‒ Feed moves from high pressure to low pressure, so pumping of feed is not required. ‒ Product is obtained at lowest temperature. ‒ This method is suitable for scale forming liquids because concentrated product is subjected to lowest temperature. ‒ 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.
- 9. Backward Feed arrangement Feed is introduced in last effect and steam is introduced in first effect.
- 10. Backward Feed arrangement Advantages: Disadvantages: ‒ 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. ‒ As liquid moves from low pressure side to high pressure side, so pumping is required.
- 11. Mixed Feed arrangement Feed is introduced in intermediate effect moves forward and then backward to effect-1 and steam introduced in first effect.
- 12. Mixed Feed arrangement Advantages: Disadvantages: ‒ Pumping of liquid requires only where liquid moves from low pressure to high pressure. ‒ Product is obtained from highest temperature, hence lowest viscosity. ‒ As liquid moves from low pressure side to high pressure side, so pumping is required.
- 13. Parallel Feed arrangement Fresh Feed is introduced in every effect and steam is introduced in first effect. Advantage: — It is suitable where feed has to be concentrated slightly.
- 14. Applications of Multi-Effect Evaporators: Advantages: ‒ Product Concentration . ‒ Solvent Volume Reduction is a solution. ‒ Water/Solvent Recovery. ‒ Crystallization ‒ Advanced System. ‒ Easy Operation and Maintenance. ‒ Cost Effective Operations(less steam requirement). ‒ In seven stage evaporator, it is possible to evaporate 12kg of water with 1kg steam. No of effects Steam used /kg water evaporated Steam used /kg water evaporated (if TVR used) Single 1.2kg ------------- Double 0.6kg 0.45kg Triple 0.4kg 0.25kg
- 15. Design Principles of Single Effect and Triple Effect Evaporators By: Nishant Arora
- 16. Single Effect Evaporator Feed(Tf ) Steam (Ts ) Vapors (T1) Concentrate (T1) Condensate (Ts) Mf =Mass flow rate of feed, kg/sec. Ms = Mass flow rate of steam MV = Mass flow rate of vapors Mp= Mass flow rate of concentrate Mc =Mass flow rate of condensate Xf = Solid fraction in feed, dimensionless. Xp= Solid fraction in concentrate Hf = Enthalpy of feed, KJ/kg. Hs = Enthalpy of saturated Steam (at Ts ). Hv = Enthalpy of saturated Vapors (at T1). Hp = Enthalpy of concentrate Hc = Enthalpy of condensate As the heat transfer is indirect , so Ms=Mc . Assume that , vapours and condensate contains no totals solids. Ts = Saturation temp. & T1 = Boiling Point of liquid feed. Assume that, Condensate leaves the evaporator at T= Ts . Also, it is assumed that the steam gives off only its latent heat (λ), where λ= Hs – Hc . (T1)
- 17. Mass Balance: (Rate of mass in=Rate of mass out) So, Solids Balance given by: Similarly Enthalpy Balance : (Total heat entering=Total heat leaving) Ms + Mf = MV + Mp + Mc & Mf = MV + Mp ( ; Ms = Mc ) Mf * Xf = Mp * Xp Mf* Hf + Ms * Hs = MV *Hv + Mp * Hp + Mc* Hc & Mf* Hf + Ms * Hs = MV *Hv + Mp * Hp + MS* Hc ( ; Ms = Mc ) Substitute, λ= Hs – Hc ; we get Mf* Hf + Ms * λ = MV *Hv + Mp * Hp Now , Hf = Cpf (Tf – 00C) {Cpf = Specific heat of feed(KJ/KG0C)} Hs = From steam table at Ts temp. Hv = From steam table at T1 temp. Hp = Cpp (T1 – 00C) {Cpp = Specific heat of conc.} Hc = From steam table at Ts temp.
- 18. Now , area for heat transfer can be obtained in calendria, Performance of evaporator is expressed in terms of steam economy. Typical value of steam economy for a single effect evaporator is close to 1. Q= UA(Ts- T1) = Ms * Hs - MS* Hc = Ms * λ U=Overall heat transfer coefficient, W/M2K q=rate of heat transfer, W Steam Economy= MV/ Ms
- 19. Multiple Effect Evaporator Feed Steam (T1) (T2) (T3) Condensate Condensate Condensate Vapors(Mv1) Vapors(Mv2) Vapors(Mv3) Conc.(Mp1) Conc. (Mp2) Concentrate(Mp3) Feed(Mf ) A Triple Effect Evaporator
- 20. Mass Balance: (Rate of mass in=Rate of mass out) And Solid Balance is given by: For Enthalpy Balance: For Heat Transfer area: Mf =Mv1+Mv2+Mv3+ Mp Mf * Xf = Mp * Xp Ist Effect Mf* Hf + Ms * Hs = Mv1* Hv1 + Mp1* Hp1 + Ms* Hc1 2nd Effect Mp1* Hp1 + Mv1* Hv1 = Mv2* Hv2 + Mp2* Hp2 + Mv1* Hc2 3rd Effect Mp2* Hp2 + Mv2* Hv2 = Mv3* Hv3 + Mp3* Hp3 + Mv2* Hc3 Ist Effect Q1 =U1A1ΔT1 = U1A1(Ts- T1) = Ms * (Hs - Hc1 ) 2nd Effect Q2 = U2A2ΔT2 =U2A2(T1- T2) = Mv1 * (Hv1- Hc2) 3rd Effect Q3 = U3A3ΔT3 =U3A3(T2- T3) = Mv2 * (Hv2- Hc3)
- 21. Assume area of all calendria same as A , & if no heat loss: Steam Economy: [ Mv1 + Mv2 + Mv3 ] / Ms Typical value of steam economy for a single effect evaporator is between to 2-3. Q = Q1 = Q2 = Q2 Q = U1A1ΔT1 = U3A3ΔT3 = U3A3ΔT3 Q = U1AΔT1 = U3AΔT3 = U3AΔT3 Q/A = U1ΔT1 = U3ΔT3 = U3ΔT3 {ΔT = ΔT1 + ΔT2 + ΔT3 = Ts- T3
- 22. Thanks