Q & A > Question Details
We planned to carry out top layer catalyst skimming in our Naphtha Hydro-Desulfurization Treatment (NHT) reactor. The skimming amount will be about 25% of total reactor volume. For this purpose, we have purchased fresh UN-SULFIDED Co-Mo catalyst, that will be loaded on the top of old sulfided catalyst during the near future skimming activities.
In order to anticipate the future unit re-start up with PARTIALLY UN-SULFIDED catalyst, we consulted the catalyst manufacturer how to carry out the IN-SITU PRESULFIDING for this new unsulfided catalyst with the presence of old sulfided catalyst underneath . But the recommendations were not convincing.
** The presulfiding will be "liquid phase presulfiding" where the Sweet ( Treated ) Naphtha is circulated through the NHT reactor under H2 environment, before DMDS injection commences at reactor temperature of 180 degC. The required amount of DMDS for new unsulfided catalyst will be injected at the rate of 0.2%-wt-S of the circulating Naphtha. During this DMDS injection, reactor inlet temperature will be raised up gradually to 270 degC where the H2S breakthrough will happen, and will be on hold at this temperature till all required DMDS is injected. The Unit will then be adjusted to get on specification Stabilizer bottom product before being put on once through operation**
Please advise regarding to below questions :
1. Will the old catalyst --which had been sulfided in the past -- get the adverse effect during this future presulfiding, such as washed out sulfur from old catalyst surface ?
2. With the presence of old sulfided catalyst under the new un-sulfided one, we are not sure whether the measured exotherm across the reactor, and also the H2S breakthrough ,will be representing the actual presulfiding progress. Because old catalyst will enhance the exotherm and will advance the DMDS breakdown into H2S.
In order to protect the new catalyst from thermal damage, what can we do to minimize the exotherm effect from the old catalyst to the new one ?
In order to determine the end of presulfiding, can we rely on the total amount of required DMDS that has been injected during the presulfiding ?
18/06/2014 A: rvidner chib, RIl, rschib@rediff.com
Mr Ralph is exactly right. just Sulfid eusing DMDS from a good vendor , may be Arkema and keep the temp low.
08/07/2013 A: Saleh Daryabari, Senior Process Engineer of RCD and RFCC, Arak Refinery, daryabari@gmail.com
The old catalyst will not make problem during sulfiding. If you have small amount of H2S in recycle gas at temperature higher than 180C, sulfur wash out or catalyst reduction will not happen.
For sulfiding you need H2S in recycle gas, so if DMDS breakdown to H2S in the lower catalyst, H2S goes to recycle gas and sulfide upper catalyst.
Yes, one of the ways to determine end of sulfiding is injecting calculated (+10%) DMDS.
I think the procedure is generally OK but at this situation you should pay too much attention to operating temperature of catalysts and temperature profile of reactor, if you are using tow kind of catalyst from different catalyst manufacturer.
11/06/2013 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
Have you considered a preventative approach to prevent the reactor fouling in the first place?
I presume you are having issues with pressure drop build up in the NHT reactor hence the need to shutdown to skim the top of your NHT reactor. Rather than having a shutdown to skim the top of the reactor and replace part of the catalyst, you might want to consider installing an external feed filter to remove the particulate contaminants in the feed blocking the reactor. It's a lot easier to replace disposable cartridge filter elements installed in a feed filter external to the reactor than it is to have to replace the graded bed and/or the top section of NHT catalyst. Installing a feed filter also provides the benefit of protecting the feed exchanger from fouling also which is an additional benefit as approximately 20% of NHT operators report shorter than expected NHT runtime due to feed exchanger fouling. Having an external feed filter avoids a shutdown too which would typically be 5-6 days in my expereice.
10/06/2013 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Ralph 'hit the nail on the head.' Patience in raising the heater outlet temperature IS a very important part of the process.
10/06/2013 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
No problem. But you left out the important part to avoid excessive temperature: Gradually increase the furnace outlet temperature over a period of at least 4 hours.