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Evaporators Evaporators Presentation Transcript

  • Evaporation Evaporation is the removal of solvent as a vapor from a solution or slurry Evaporation is used for concentration of aqueous solutions, it involves removal of water from solution by boiling the liquor in suitable vessel called evaporator and withdrawing the vapor
  • Valuable Product In evaporation thick liquor is a valuable product and vapor is usually wasted e.g. concentration of fruit juices or urea concentration In rare cases, vapor is valuable product, e.g. Mineral-bearing water evaporation to get boiler feed water, or sea water evaporation to get portable water
  • Evaporation vsboiling/drying/distillation/crystilization  Evaporation occurs at liquid-vapor interface when vapor pressure is less than saturation pressure of the liquid at a given temperature  Boiling occurs at solid-liquid interface when liquid is brought into contact with a surface at temp Ts
  • Cont…… Evaporation differs from drying b/c residue is always a liquid It differs from distillation b/c vapor is a single component and even if it is a mixture, no attempt is made to split it into fractions It differs from crystallization b/c we want to concentrate the solution rather than forming and building crystals
  • Classification of liquids to beEvaporatedA) Those which can be heated to high temp without decomposition and those which can only be heated to 330 KB) Those which yield solids on concentration and those which do notC) Those which, at a given pressure, boil at same temp as water, and those which have much higher boiling point
  • Evaporator Exchanger which is used to concentrate a solution consisting of a non-volatile solute and a volatile solvent, is called an evaporator Evaporator Main Parts Feed Inlet, Concentrate outlet, Steam jacket, steam condensate outlet, sight glass, vapor head & vapor outlet, heating section etc
  • Evaporator Types/Classification A) based on heating media
  • Cont…..B) Based on Effect 1. Single effect evaporators 2. Multiple effect evaporatorsC) Based on circulation of liquor over heating surface 1. Natural Circulation Units 2. Forced Circulation Units 3. Film type (wiped film) evaporators
  • Different Evaporators
  • Batch Pan Oldest type and used for limited application Either jacketed/internal coils/heaters Product residence time is many hours Boil at low temp and high pressure for heat sensitive materials heat transfer area and coefficients are low under natural circulation Evaporation capacities are low Large temp differences not achieved b/c of rapid fouling of heating surface
  • Cont….. Used for concentration of Jams and Jellies, also for some pharmaceutical products
  • Natural Circulation Units Circulation is obtained by convection currents arising from heating surfaceA) Tubes are horizontal with steam insideB) Tubes are vertical with steam outsidea. solution to be evaporated boils outside the tubes and steam condenses inside of tubes. These tubes interfere with natural circulation and minimize liquid agitation Overall heat transfer coefficients are lower
  • a. Tubes are horizontal with steaminside
  • Cont….. Cylindrical section above heating portion is used to separate vapor from liquid vapor leaves through de-entraining device to prevent carry over of liquid droplets with vapor Steam enters through one side chest and leaves through opposite chest Steam condensate out via steam trap
  • Cont….Advantages1. Cheap2. Easy to install3. Require less space for installation4. Suitable for liquids that not crystallize5. Can be used for batch/continuous operationDisadvantages1. Not suitable for viscous liquids b/c of poor circulation
  • B) Tubes are vertical with steamoutside Solution boils inside vertical tubes with heating media, usually steam, held in steam chest, through which tubes pass. Boiling of liquid in tubes, causes liquid flow upward through tubes and un- evaporated liquid flows downward through central hole
  • Cont…..
  • Cont….. These overcome disadvantages of horizontal tube evaporators 1. natural circulation is promoted(1-3 ft/s) 2. heat transfer coefficients are higher 3. solid built inside tubes is removed by mechanical cleaning4.Viscous liquids can be used, but circulation is slow5. Used in sugar and salt industries These are impractical when solution is very viscous or form foams or is heat sensitive
  • Cont…
  • Thank You forYour Attention