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Sulfur In Vdu Overhead Stream (Off Gas)

sulfur vdu off gas cracking

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#1 kim tae hyoung

kim tae hyoung

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Posted 30 August 2018 - 03:37 AM

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.

#2 gegio1960


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Posted 30 August 2018 - 04:21 AM

I've performed several process designs for systems to remove H2S from the VDU overhead gases and I'll try to provide comments and answers on the base of this experience...

1) my company started to provide this design during the '90s, when the enviromental regulations constrained the contents of SOX of flue gases at very low levels. before that time the VDU off gases were simply burnt in dedicated low pressure burners, usually placed in the VDU heaters.

2) so, the scope of the amine washing is the reduction of the S content in the gases before the combustion, the presence of R-SH is negligible, up to levels that don't violate the S specification in the fuel gas. this happened in 100% of the cases I've examined in my professional life.

3) the composition shown by a process simulation is not useful for the design: it has to be measured by a lab analysis.

4) VDU off gases contain O2 and so the amine utilized for the washing is subject to degradation and, preferably, it is segregated from the other amine systems of the refinery.

good luck! 

#3 kim tae hyoung

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Posted 30 August 2018 - 08:19 PM



Thanks for your support. it is very helpful to understand VDU design. 

The mercaptan concentration of off gas is around 400 ppm ~ 1200 ppm mol. 

Is it negligible parameter on VDU operation? 


The main fuel source of VDU Heater is fuel gas. So, the sulfur concentration of the mixed fuel source ( Fuel gas + off gas ) is lower than regulation. 

But, i want to know the normal ratio of H2S and Mercaptan at off gas  or how much mercaptan concentration is acceptable/negligible at off gas based on experience. 



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