Quench Exchanger Cleaning. Olefins furnace effluent temperature leaving the
radiant coil is in the range of 1475–1575 °F. Heat is recovered from this stream in quench
exchangers by producing high-pressure steam. Gas crackers do not produce much fuel
oil, which can condense and foul at low temperatures, and hence their quench exchangers
(TLEs) are typically designed for a final outlet as low as 400 °F. As the furnace run
progresses, the outlet temperatures remain fairly steady.
Liquid crackers produce a large amount of fuel oil, which will condense and foul the
quench exchangers. The heavier the liquid feedstock, the higher will be the fouling rate of
the quench exchangers. Liquid feedstock with a heavy tail (high boiling point portion),
such as condensate, will also cause increased fouling of TLEs. For example, light
naphtha will tend to foul exchangers less than heavy gas oil. A liquid cracker is typically
designed for a clean quench outlet temperature of 650–800 °F and an end-of-run
condition of 1200 °F. While an online decoke will partially clean the TLEs, outlet
temperatures will never be restored to clean condition without an off-line mechanical
cleaning. The higher the quench outlet temperature, the lower will be the overall energy
efficiency of the plant. The loss in heat recovery steam must be made up by other means
such as external boilers. Depending on steam demand and furnace availability, some
plants clean the TLEs every three furnace decoke cycles, while others may choose to
clean once a year. In order to maintain energy efficiency, quench exchanger cleaning
frequency needs to be taken into account in furnace availability calculations.