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.
01/12/2018 A: Egbert van Hoorn, Hocon B V, Egbertvh@hotmail.com
Removal of H2S can be achieved by amine treating but mercaptan removal is very difficult.
Even removal of H2S is a challenge at the low pressure of the VDU gas because the partial H2S pressure is very low and therefore the driving force is low.
Mercaptans are very weak acids and are not efficiently removed by amine solutions.
Mercaptans can be removed by caustic/MEROX treatment because caustic is a stronger base.
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.