ENCH 445: Lecture 6 Staging of Separation Processes dfrey1/ench445/ENCH445-Lecture6c.pdf ENCH 445:...

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Transcript of ENCH 445: Lecture 6 Staging of Separation Processes dfrey1/ench445/ENCH445-Lecture6c.pdf ENCH 445:...

  • Consider the fractionation of a benzene (B) and toluene (T) mixture:

    ♻ Recycle intermediate products from previous stage

    Points to remember:

    Intermediate heating and cooling are only needed on the last stages when recycle is used 1. Recycle to previous stage is easiest in practice and the most "reversible" since there is 2. relatively little mixing

    Staging of Separation Processes ENCH 445: Lecture 6

    1

  • Multistage Distillation Column

    2

    Thermosiphon Reboiler:

    Types of trays:

    Sieve tray:

    Down position Up position

    Valve tray:

  • Can also employ "packed" columns. External piping is the same as a plate column.

    Why use counter current contacting?

    Co-current Absorber

    Assume A

    is dilute

    Counter-current Absorber

    3

  • Methods to produce counter current flow: Convert a product stream into the counter current stream. This usually requires energy 1. consumption, e.g. distillation

    Add a mass separating agent e.g. liquid-liquid extraction 2.

    A second reason for staging is to increase energy efficiency, e.g. multi effect evaporation to product fresh water from sea water

    4

  • Heating methods in the chemical process industry:

    Why is steam heating most often used?

    Because of control, safety and economic considerations

    Electrical Heating: constant heat source

    Steam Heating:

    Steam Trap:

    Steam Heating Electrical Heating Direct Fire Using a Burner

    Float Trap:

    5

  • 📝 Calculation: amount of heat transfer when using steam

    6

    Pressure-Enthalpy Diagram for Steam

    Approx. and assumes working steam trap Relation between Tcoil and Pcoil from steam phase diagram

    Single Stage Flash

    Liquid product