What would be the good choice as an absorbent in Sponge absorber? Either Light cycle oil or heavy cracked naphtha (Lean Oil) from main fractionator.
Answers
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28/08/2014
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A:
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The 2-stage absorption process works as follows:
1. Naphtha is used as lean oil in the primary absorber because the lighter the material, the higher the absorption factor, allowing for a lower flow rate. However, some naphtha is lost to the off-gas by equilibrium.
2. A heavier absorption oil, with a lower absorption factor is, used as absorption oil in the 2nd stage (sponge absorber) simply to absorb the naphtha from the 1st stage. “Entrainment” is not the correct word to use, as some do, as the naphtha in the feed to the sponge absorber is in equilibrium, not entrained, unless the operation calculates to be above 90% of flood. LCO, being heavier, is the better choice for the sponge absorber.
The combination described above provides the lowest liquid rates for a given overall result. That is why it is used in that way.
If the system is overloaded, and a debottlenecking project is not possible, the LCO sponge oil can be replaced with naphtha to improve the cut between ethane and propylene. When this is done, the operation becomes a 2-stage absorber with no sponge absorber. More lean oil is lost to the off-gas but propylene recovery becomes maximized.
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