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We maintain 180-200 ppm H2S gas (measured by Drager Tube CH29101) at the outlet of NHT recycle gas compressor discharge to keep the HR 506 (Co Mo) catalyst in its active form or sulphided form.FYI, our HN feed contain very low sulfur which is Doctor test negative. For this purpose,we are adding 2.8-3 kg pure DMDS with the feed which can be able to produce more or less 0.095 kg H2S in the recycle Gas but from the design data book calculated value is 21.99 Kg H2S (provided recycle gas flow 467 kg/hr) which is 47,087 ppm.So, how the calculation has done and how it becomes 180-200 ppm H2S in the recycle gas. Please show me the basis of calculation.
 
Answers
23/03/2017 A: Feroz Ahmed, Petromax Refinery Ltd, feroz_erl94@yahoo.com
In fact 2.8-3 kg DMDS/ Day consumption. Therefore, if i multiply with 0.68 then i will get approx 2 kg/ Day or 0.09 kg/ day S ( Sulphur) injection rate. Now, 0.09 Kg/hr S will produce =34*0.09/32=0.095 Kg H2S/hr which in wt ppm= 0.095/467*10^6=203 wt ppm, but in volume=0.095/34*98.55=28.35 vol ppm [ RG flow=98.55 kmol/hr]
So, please correct me if i am wrong.
23/03/2017 A: Feroz Ahmed, Petromax Refinery Ltd, feroz_erl94@yahoo.com
Thanks Mr. Virendra Kapoor, we have calculated by multiplying 0.68 and found the wt ppm is close to 200 .
20/03/2017 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
The calculation is simple. Multiply the flow of DMDS by 0.68 to produce & calculate H2S. It may be about 2kg/hr. Convert it into kg- mole / hr. It may be about 0.05 or so kg-mole/hr. Convert the RG flow in kg-mole/hr. Remove the amount of H2S going to separator bottom with liquid product. Thus you get the flow of H2S to RG. Thus you get ppm of sulfide gas in RG. It may be approximately equal to what you get by Drager tube approximately 200- 300 ppm or so.
20/03/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
It sounds like your current feed has a much lower sulfur content than the design feed. Most hydrotreaters have more than enough sulfur in the feed to keep the catalyst fully sulfided. You only add DMDS when feed sulfur is very low.
One other thing to keep in mind is that the amount of H2S in the recycle gas is primarily dictated by the amount of sulfur in the feed and the purge gas rate and has nothing to do with the recycle gas rate. If your purge gas rate is low compared to the recycle gas rate, most of the H2S will recycle back to the front end of the system and cause the H2S concentration in the reactor effluent side to increase until the amount of the H2S in the purge gas equals the amount produced in the reactor.
If there is a water wash, some of the H2S will end up in the sour water produced. Some will also end up in the cold separator liquid. Any H2S that leaves via these two routes decreases the steady state amount leaving with the purge gas by the same amount.