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Quench Tower/Settler

  • Feedstock comes into the plant and goes through the ethylene furnaces (pyrolysis)
  • Once cracked into a variety of hydrocarbons and hydrogen, it immediately begins to recombine into larger molecules
  • To prevent these reactions, the cracked vapor goes through the quench towers to cool using oil or water
  • The largest hydrocarbons are carried with the water into the quench settler or the quench water separation drum (QWSD)


  • Loss of control of interface will reduce ethylene production due to inefficient quench tower operation
  • As feedstock is increased, more cooling fluids are required
  • The residence time in the settlers needs to be minimal to maintain composition and prevent secondary reactions
  • A quench settler or QWSD that catches the hydrocarbon liquids and water from the quench tower creates an interface in the QWSD and possibly an emulsion layer if too much caustic is added
  • The interface is important to recirculate the quench water from the settler to the tower and not recirculate the hydrocarbons which will reduce productivity and potentially cause fouling of equipment; also applies to quenching oil versus tars and other heavy  hydrocarbons
  • If fluid composition negatively changes in the quench tower, less ethylene is produced from the feedstock reducing efficiency and productivity.
  • Regulating an interface level can also aid in using less caustic if quench water is used