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Ammonia Synthesis Loop Purge Gas Flow


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#1 Riaz01

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Posted 10 August 2017 - 10:58 AM

In Ammonia synthesis plant, in Synthesis Loop under what criteria purge gas flow  is increased or decreased??



#2 Saml

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Posted 10 August 2017 - 07:04 PM

Basically it is optimization.

Purge less: higher pressure for same production, accumulation of methane and argon, more power to compress syn-gas and recirculate the loop.

 

Purge more: Lydrogen loses if your recovery unit efficiency is not 100%. Higher production for the same pressure (albeit more inefficiently because of the hydrogen loss)



#3 Riaz01

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Posted 13 August 2017 - 01:39 AM

Thanks Sami,

 

Can you kindly guide me If Purge gas flow is increased, Hydrogen is more reduced than Nitrogen in loop, and H/N will reduces. Why Hydrogen is escaped more than Nitrogen??

I this is due to partial pressure?? 



#4 Saml

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Posted 13 August 2017 - 09:10 AM

Purge will tend to increase whatever imbalance from 3:1 ratio you have.

I assume you are purging after ammonia separation and before syngas make up point and that "more reduced than nitrogen" you mean "the concentration tends to decrease faster than nitrogen concentration does".

If you are operating above 3:1 (say 3.05). Then the reaction consumes exactly in the 3:1 ratio, and you wil have a higher ratio in the purge. It is just a simple mass balance. Check the two simple examples below, where you can see that the ratio at reactor outlet moves away from 3:1.

Inlet	Outlet	
10.0	11.6	Inerts (Methane + Argon)
2.0	18.6	Ammonia
21.8	17.2	Nitrogen
66.2	52.6	Hydrogen	
3.04	3.06	H2/N2 Ratio


Inlet	Outlet	
10.0	11.6	Inerts (Methane + Argon)
2.0	18.6	Ammonia
22.2	17.7	Nitrogen
65.8	52.1	Hydrogen
2.96	2.94	H2/N2 Ratio


#5 Riaz01

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Posted 13 August 2017 - 09:50 PM

Thanks Sami,

 

It means if we increase purge gas flow H/N ratio will reduce ....

 

And second if CH4 increased or decreased in Natural gas at Reformer inlet what are effects on Synthesis loop and H/N ratio and what to do to maintain H/N ratio in that case?? 



#6 Saml

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Posted 15 August 2017 - 01:33 PM

"It means if we increase purge gas flow H/N ratio will reduce ...."

 

No, the purge H:N ratio is the same as the ration in the reactor outlet. Since the purge is further from the 3:1 ratio that the inlet to the reactor, usually higher purge tends to bring the ratio closer to 3:1

 

"And second if CH4 increased or decreased in Natural gas at Reformer inlet what are effects on Synthesis loop and H/N ratio and what to do to maintain H/N ratio in that case??"

 

From this question, it seems that you have not fully investigated the process and the reactions that take place. Please post what you have investigated so far, and the doubts you have on specific issues.



#7 astro

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Posted 22 August 2017 - 09:32 AM

You could take a look at this paper:

https://www.scribd.c...tate-Simulation

paying particular regard to Table 6 and the row "Inerts Level".

 

Hopefully it will assist with the understanding of the sound advice from Saml.






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