Q & A > Question Details
After cracking at VDU Heater, part of the Sufur on the AR(ATM Residue) is changed to H2S or Mercaptans and the stream flow to VDU Column.
Part of the H2S and Mercaptan go to the VDU overhead.
To catch the Sulfur (Specifically, H2S or Mercaptan) at Off gas Stream, Amine adsorber is installed at that line.
However, when I review the Heat and Material balance (HMB), only H2S is considered at the stream and Amine cannot catch the mercaptan well. It is useful to catch the H2S only.
Why the designer normally do not consider mercaptan catcher on the off gas stream?
Is there any specific reason?
For example, mercaptan cannot go to the VDU column overhead.
 
Answers
30/08/2018 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The overhead drum from the educator system should have 3 exit streams:
• off gas to flare or fire box
• liquid to slop
• sour water to SWS
H2S is expected to be in the off gas. Mercaptans and disulfides in the liquid to slop. H2S and NH3 in the sour water.
Assuming you are treating the off gas with amine. It is not expected to absorb any mercaptans or disulfides.