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Is there any relation between Velocity steam and Heater Outlet temperature?. Normally Velocity steam is adjusted for heater tubes residence time.
04/02/2011 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
Velocity steam is adjusted to reduce the residence time of vacuum feed in vacuum heater. A thin layer of vacuum feed moving close to the boundary of the heater tube will be in the velocity of the heater tube to which it stays closer. That is, it is almost zero. The boundary layer is exposed to the skin temperature of the tube for long time and it leads to thermal cracking which is an unwanted reaction takes place in the heater. To avoid this phenomenon, velocity steam is introduced and this helps to create a turbulence and hence the boundary layer comes to the free stream region and the change is a continuous one. This helps to avoid coke formation in the heater tube and increases the run length of the heater.
Also the velocity steam helps to strip off lighterends by reducing the partial pressure of hydro carbon based on Dalton's law of partial pressure.
Higher rate of velocity steam than the design flow will affect heater outlet temperature as part of the heat is taken by the steam itself.
Also, higher rate of velocity steam will affect vacuum in the vacuum column affecting the distillation.