10 replies to this topic

[Aranga]

Hai All,

I have worked with Diesel Hydrotreator process. We used NiMox catalyst.
I have two doubts.. Please reply me.

1)why we are having catalyst as oxide form, and why it cant come as sulphided form?
2) why it is necessary to sulphide immediately after we reduced the catalyst from oxide form by hydrogen? what will happen if we dont sulphide immediately?

Thanks,
Renga

[karguvelayyanar]

Hi Renga, 

        I'm sorry to say this i have little bit English communication skill. please try to understand. 

           In my knowledge,

1. all catalyst are formed in oxide form. why? because maximum of catalysts are pyrophoric nature. That means easily react with oxygen. when oxidation is happens more amount of heat will be liberated so we have to prevent that reaction.

2. if the catalyst is oxide form, it is inert for our reaction. so we reduce the catalyst for our reaction.

3. simply i say our catalyst is nickel sulphide  not nickel oxide. 

nickel oxide is easy to handle compared to nickel sulphide as well as cheaper than nickel sulphide. 

[karguvelayyanar]

Hi Renga, 

        I'm sorry to say this i have little bit English communication skill. please try to understand. 

           In my knowledge,

1. all catalyst are formed in oxide form. why? because maximum of catalysts are pyrophoric nature. That means easily react with oxygen. when oxidation is happens more amount of heat will be liberated so we have to prevent that reaction.

2. if the catalyst is oxide form, it is inert for our reaction. so we reduce the catalyst for our reaction.

3. simply i say our catalyst is nickel sulphide  not nickel oxide. 

nickel oxide is easy to handle compared to nickel sulphide as well as cheaper than nickel sulphide. 

[Aranga]

Hai Karguvelayyanar,

 

Thanks for your reply. I have heard the Iron suiplhide is Pyrophoric in nature. But not other compounds. This is a new information.

 

  During the time of reduction and sulphiding, (for all hydrogenation catalysts Ni-Mox, C0-Mox, Cu-O) we have to sulphide it soon as possible after reduction. why is that so? I guess the the catalyst will be degraded in the pure mettallic form.

 

 moreover oxidation of a elemental metal, to bring back in oxidised form may be difficult, and need high pressure temperature conditions.

 

Your english is so good.

 

Thanks.

 

 

[thorium90]

Its active form is its sulphided state, so naturally you want to have it sulphided just before using it. As to why sulphide immediately after reduction, there is no reason other than why wait? If you were starting up a plant, if there is no reason to delay start up, then why delay it? If you already have the reducing gas go through the beds for the required time, why is there a need to suddenly go on holiday or pack the catalyst back into the drum and wait for something to happen? Does it make sense to just start the suphiding immediately since the unit is already running, there is reducing gas going through and you are already waiting for it, operators are standing by in the field waiting by the feed valves for instruction, and waiting, and waiting...? Sure you could wait, but for what? Plant startup means there is no real production and is a loss making time for the plant, so why wait?

 

Perhaps there might be other areas of concern in your place like if air was used to purge the catalyst previously instead of nitrogen and therefore suphiding needs to be done immediately after reduction. Otherwise, both steps are generally done together

Edited by thorium90, 20 February 2013 - 03:45 AM.

[Aranga]

Its active form is its sulphided state, so naturally you want to have it sulphided just before using it. As to why sulphide immediately after reduction, there is no reason other than why wait? If you were starting up a plant, if there is no reason to delay start up, then why delay it? If you already have the reducing gas go through the beds for the required time, why is there a need to suddenly go on holiday or pack the catalyst back into the drum and wait for something to happen? Does it make sense to just start the suphiding immediately since the unit is already running, there is reducing gas going through and you are already waiting for it, operators are standing by in the field waiting by the feed valves for instruction, and waiting, and waiting...? Sure you could wait, but for what? Plant startup means there is no real production and is a loss making time for the plant, so why wait?

 

Perhaps there might be other areas of concern in your place like if air was used to purge the catalyst previously instead of nitrogen and therefore suphiding needs to be done immediately after reduction. Otherwise, both steps are generally done together

 

Dear Thorium,

 

  Thanks for your Reply. I am not telling about waiting for some time. But Incase( Very less possible to happen) unfortunately, the Recycle gas compressor shows any serious mechanical problem during the time of sulphiding, can we stop the sulphiding process at  half way?  what will happen to catalyst, if we stop it half way?

 

  This question is asked by one of my senior colleague. He says catalyst will be degraded. But How? by what?

 

Thanks again.

[thorium90]

I can think of a few reasons why your senior thinks that way.

The support for the catalyst has very high surface areas and can readily absorb moisture which will then interfere with presulphiding. However, if the nitrogen you used previously to purge the beds are dry enough, the catalysts would already be pretty dried out.

It is possible to stop the presulphiding halfway, If somehow during the procedure something happens and you need to stop work, you can do the following. Purge the beds with nitrogen, then close all the outlet and inlet valves and leave the nitrogen pressurized inside to box up the unit.

If it is because you have lost H2 flow, then it would be best to stop the presulphiding. Depending on the sulphiding agent used and the temperature it was at, carbon formation and laydown can occur.

Presulphiding is to increase the catalyst activity and is normally done until H2S breakthrough at the outlet. However, its still possible to run with the normal feed even if the presulphiding is not complete (No guarantees on performance). Depends on how long has already passed since the start, the bed size, how much sulphur in the feed etc

Edited by thorium90, 21 February 2013 - 01:08 AM.

[Aranga]

Thank you Mr. Thorium.

 

As you have mentioned, This is may be reasoned because of carbon formation from DMDS.

 

Thanks for your Kind knowledge sharing.

[karguvelayyanar]

dear Aranga,

             Thank you for your kind reply.

      My thinking is,

      If we did not done the sulphiding of catalyst, our reactor temperature will decreases. At that time hydrogen embritlement will happens.

That may the main reason 

     

Thank you 

 

[thorium90]

The occurrence of hydrogen embrittlement depends on many factors, like material, temperature, pressure etc. Have you seen it happen on a HDS vessel before?

[Aranga]

I have read about Temperature embrittlement in HDS Reactor,  But Never Seen it. My opinion about Hydrogen embrittlement at the time of Sulphiding is not possible to occur. Because we inject very less amount of hydrogen (Our case 19Kg/Hr only) with larger amount of nitrogen (1218Kg/Hr) at the time of Sulphiding by DMDS. And the Temperature is 200C only.

 

   During the time of normal operation, it is much more higher. And Temperature also over 320C. So Reactors must be designed to withstand for atleast higher than normal operation conditions. I beleive Hydrogen embrittlement is not the reason.

 

Thanks for your replies.