Hydrogen Purification with PSA

[dimitris]

I am a process engineer woring at an Ammonia plant.
The necessary H2 for the NH3 production is performed with Catalytic Steam Reforming and the purification of the gas stream is done in a Pressure Swing Adosrption multivessel Unit. Lately the performance of the adsorbers has decreased. We suspect tha gaseous ammonia entered the vessels. The adsorbents used are activated carbon at the bottom part and synthetic zeolites at the top part of the vessel. (Gas enters the bottom of the vessel and exits from the top). Does anybody know if gaseous ammonia can cause permanent deactivation on synthetic zeolites adosorbing ability? What about liquid ammonia too?

 

[whylie]

Dear dimitris,

I don't have the operation experience of PSA units,but I was searching information on the net,and it is clearly stated that liquids have severe effect on performance.as yopu know that the feed stream (impure H2),ensuing from the reformer has to free of moisture..a good reference is an ICI company called Syntix,or go to yahoo,hotmail search and put Pressure Swing Absorption..

Now I need your immediate help for a detailed operation question..as I was studying Aa P&ID of the unit I see a small compressor with suction from the final gases going to the PSA(with Nitrogen N2,written on the discharge at 7 atm ) thev discharge joins the heat exchange feed preheating train up to the reformer reactor..Can you pls explain the purpose of this compressor..is it for purging trhe system during shutdown ,or is it part of the normal operation..I think I have some P&ID's with mistakes..

Thank you for any clarification..

Reagards,

Whylie

Wayil@hotmail.com

 

[dimitris]

Dear Whylie,
I am not pretty sure that I got your question right, but I suspect that you are talking about a compressor that recirculates part of the PSA feed stream to the reformer preheating section.
Such a system is usually incorporated in hydrogen production units so as to feed hydrogen in the sulphur removal section of the whole unit.
Hydrogen is required to react with the atomically bonded Sulphur with the Hydrocarbon feed. The product is H2S which is then removed and discharged.
Usually though, the compressor is put at the exit of the PSA unit so as to circulate impurities free Hydrogen.
I hope that I am helpful enough,
Dimitris...

 

[whylie]

Dear dimitris,

Thank you for your prompt response..
This was not the compressor I asked about..in the PFD and P&ID's ,confirmed by line designation tables ..The unit I am looking at was supplied be LINDE,it shows 3 compressors:

1. Hydrogen recirculation for desulfurisation
2. Nitrogen (suction from impure hydrogen line) going to PSA unit,and discharge to the feed preheat exchange train ,
It's funny it says Nitrogen,suction from impure hydrgen after the condensate drum,discharge pressure (6 atm-PSV set pressure at 6.85atm)..and the system looks as if it is normal operation(suction valve white -normally open)..I thought this was a purge compressor -for shutdown-because how do discharge N2 (6 atm) into a hydrocarbon line (25 atm)
It does not make sense..
3. Pure Hydrogen product compressor for bottling section(discharge 220 atm)

I hope my question is clear,somthing illogical in my documents..

Could you pass me your contact info..e-mail,so I can give more details.

Regards,

Whylie
wayil@hotmail.com

 

[dimitris]

Dear Whylie,
I am afraid that I am not aware of the system your are talking about, but one thing that crosses my mind is that this may be a strat up unit only.
As you may already know, when you start a reforming hydrogen production unit, to heat up the reformer you pass nnitrogen through it first which you gradually switch with steam and when you acheive temperatures at which th refornming reaction can occur ( above 700-750 C) you enter the HC feed. The compressor you are talking about may be a strat up N2 unit, that recirculates N2 while heating up the reformer.
What plant are we talking about, an ammonia plant and if so where is it located,
If you want to further contact me my hotmail account is
d_orfanidis@hotmail.com

Regards,
Dimitris....

 

[pkv61]

yes, ammonia will kill the zeolite sieve immediatly and is irreversible