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Evaporadores de simple y múltiple efecto. Concepto y generalidades.

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  1. 1. Evaporator <ul><li>Equipment used in evaporation, the process of boiling a liquid in order to reduce its volume </li></ul><ul><li>Need </li></ul><ul><ul><ul><li>Reduces transportation cost </li></ul></ul></ul><ul><ul><ul><li>Storage costs </li></ul></ul></ul><ul><ul><ul><li>Prepare for the next Unit operation – drying, crystallisation etc. </li></ul></ul></ul><ul><ul><ul><li>Reduces deteriorative chemical reactions </li></ul></ul></ul><ul><ul><ul><li>Better microbiological stability </li></ul></ul></ul><ul><ul><ul><li>Recovery of solvent </li></ul></ul></ul>
  2. 2. Evaporator <ul><li>Driving force: </li></ul><ul><li>Temperature difference in between steam chest temperature and product temperature. </li></ul><ul><li>Result : </li></ul><ul><li>Volatile solvent is removed from the feed. </li></ul><ul><li>Solution (volatile solvent + non volatile solute) </li></ul><ul><li>Concentrate (Higher solute Conc.) </li></ul>
  3. 3. Examples <ul><li>Concentration of milk to produce condensed milk </li></ul><ul><li>Concentration of juices </li></ul><ul><li>Concentration of NaOH, NaCl from aqueous solutions to produce salt. </li></ul><ul><li>Ether recovery from fat extraction </li></ul>
  4. 4. Basic Parts of an Evaporator <ul><li>Heat-exchanger </li></ul><ul><li>Vacuum </li></ul><ul><li>Vapour separator </li></ul><ul><li>Condenser </li></ul>
  5. 5. Evaporator Vapor out Feed in Steam in (Saturated vapor) Product out Condensate out (Saturated Liquid) Vapor Separator Heat Exchanger Vaccum for non condensable Condensor unit Coolant In Coolant out
  6. 6. Evaporator Type <ul><li>Batch Pan </li></ul><ul><li>Rising Film </li></ul><ul><li>Falling Film </li></ul><ul><li>Multiple Effect Evaporators </li></ul>
  7. 7. Processing Factors <ul><li>Conc of solute in feed </li></ul><ul><li>Conc of solute in product/concentrate </li></ul><ul><li>Pressure and Temp. of the system </li></ul><ul><ul><ul><ul><li>Depends on temperature sensitivity of material. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Boiling point elevation </li></ul></ul></ul></ul>
  8. 8. Processing Factors <ul><li>Steam pressure and temperature </li></ul><ul><li>Material of construction </li></ul><ul><li>Foaming </li></ul><ul><ul><li>Low heat transfer rate </li></ul></ul><ul><ul><li>Entrainment loss </li></ul></ul>
  9. 9. Heat Transfer Coefficient, h <ul><li>A coefficient which indicates the amount of heat flow that is exchanged across a unit area of a medium or system in a unit amount of time with a unit of temperature difference between the boundary of the system. </li></ul><ul><li>S.I. unit W m -2 K -1. </li></ul>
  10. 10. Overall Heat Transfer Coefficient, U <ul><li>In cases of combined heat transfer for a heat exchanger, there are two values for h. </li></ul><ul><li>Convective heat transfer coefficient for the fluid film inside the tubes </li></ul><ul><li>Convective heat transfer coefficient for the  fluid  film  outside  the  tubes.    </li></ul><ul><li>  The  thermal  conductivity  (k)  and thickness (Dx) of the tube wall must also be accounted for.   </li></ul><ul><li>So an additional term (Uo), called the overall heat transfer coefficient, must be used instead.   </li></ul>
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  16. 16. Steam Trap <ul><li>Function </li></ul><ul><ul><li>Discharge condensate, air and other incondensable gases from a steam system while not permitting the escape of live steam </li></ul></ul>
  17. 17. Ball Float Steam Trap
  18. 18. Reduces the ferocity of discharge and sound. Sound levels can be reduced by up to 80%.
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  21. 21. Vapor Out m v h v Steam in m s h s Condensate out m c h c Water in m w h w Q E Q L
  22. 22. Lab Calculation <ul><li>Calculate U, overall heat transfer coefficient for the evaporator </li></ul><ul><li>Perform a mass balance and energy balance </li></ul><ul><li>Calculate discrepancy </li></ul>
  23. 23. Mass Balance <ul><li>Feed Side: </li></ul><ul><li>m (kg/s) </li></ul><ul><li>m (feed in) = m (Vapor) </li></ul><ul><li>Steam Side </li></ul><ul><li>m (steam)= m (condensate collected) + m (flash) </li></ul>
  24. 24. Flash Steam Loss <ul><li>The amount of flash steam produced during the pressure reduction can be expressed as: </li></ul><ul><li>w = (H il - H fl ) / H fe </li></ul><ul><li>where </li></ul><ul><li>w = ratio of flash steam generated (kg flash steam / kg condensate) </li></ul><ul><li>H il = initial liquid enthalpy (kJ/kg) </li></ul><ul><li>H fl = final liquid enthalpy (kJ/kg) </li></ul><ul><li>H fe = enthalpy of evaporation (kJ/kg) </li></ul>
  25. 25. Energy Balance <ul><li>H (kJ/kg) </li></ul><ul><li>Energy from the steam to the water Q E </li></ul><ul><li>The Heat Loss from the vessel to the room, Q L </li></ul><ul><li>Amount of heat in condensing steam Q S </li></ul><ul><li>Q S = Q E + Q L </li></ul>
  26. 26. Measurements <ul><li>Q S and Q E </li></ul><ul><ul><li>Mass Flow rate- condensate, feed water </li></ul></ul><ul><ul><li>T,P will give H Specific Enthalpy </li></ul></ul><ul><ul><li>Q = mH </li></ul></ul><ul><li>Q L </li></ul><ul><ul><li>Surface area </li></ul></ul><ul><ul><li>Surface temperature </li></ul></ul><ul><ul><li>Q L =hA ∆ T </li></ul></ul><ul><li>Q E = UA ∆ T </li></ul>
  27. 27. <ul><li>Further Reference </li></ul><ul><li>Transport Processes and Separation process principle by: </li></ul><ul><li>Christie. J. Geankoplis </li></ul><ul><li>Animations on evaporator visit: </li></ul><ul><li> and click animations then evaporator </li></ul>
  28. 28. Multiple-effect Evaporator <ul><li>Water is boiled in a sequence of vessels, each held at a lower pressure than the last. </li></ul><ul><li>Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next </li></ul><ul><li>Generally the first vessel (at the highest pressure) requires an external source of heat </li></ul>
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  30. 30. Thank You