h2s scrubbing from a mixture of h2s and nh3

[ReemaK]

Hi
I am tasked to design a process to remove h2s from a mixture of H2S and NH3 using scrubbing . As of now, we have decided to use caustic, though by simulation results there is some ammonia also being absorbed by the NaOH. The composition of H2S in the inlet is 4 wt%. The objective of the process is to produce ammonia free of h2s.
Can some other solvent be used in this case ?
Can amines be used in the presence of ammonia ?

Thank you in advance !!

 

[georgeverghese]

H2S is easily absorbed into liquid propane while NH3 is not. See if this works for you.

 

[ReemaK]

thank you for your prompt response @georgeverghese...
but first we want to try out commonly used solvents like amines and caustic available in refinery...

does ammonia have any degrading effect on amine solvents ? amines are used for h2s removal but for mixtures of h2s and co2 mostly..can it be used for h2s and nh3 mixture ?
It is okay if a some amount nh3 is lost in the solvent..

 

[georgeverghese]

You havent said much
a) do you need the solvent to be regenerated and recycled back to contacting tower, or is this feed gas stream flow so low that you can use a non regenerable contacting agent?
b) What is feed stream press, temp and flowrate? Can you compress it to a higher pressure?

Alkanol amine solvents ( MEA, DEA, MDEA, activated MDEA, Selexol, Sulfinol, n-methyl pyrolidone)are all 10-20% by wt ( or thereabouts) in water, so co absorption of NH3 into aqueous amine solution will be high. The only other solvents I can see commonly used are the Fluor solvent - propylene carbonate and the methanol process from Lurgi. The Lurgi process is refrigeration intensive, so may be not for you. And obviously, weak caustic solution also is not suitable for the same reason.

You've got a tough problem on your hands.

 

[ReemaK]

the feed gas stream is around 4100 kg/hr and has 4 wt% h2s, 60wt% nh3 and rest water vapor...at a temperature of 90C and 1 barg
we are not specific about solvent regeneration
But with caustic the h2s cant be removed to <10 ppm level in a single step...to go for adsorption we dont have sufficient pressure in the system unless we put a compressor
so we were thinking of initially treating with some solvent to reduce h2s and then go for caustic...so that even spent caustic will be less
we are looking at different combinations of solvents for absorption or if we cant avoid adsorption..we have to put a compressor anyhow then which seems unavoidable now..as the system pressure is very less

Thank you !!

 

[georgeverghese]

Suggest trying this liquid propane (or butane/isobutane may be better) on a reliable simulator and see what you get. Maybe a split feed of lean propane into a 2 section column may do it. Agreed, 1 barg is too low, try 700kpag at an operating t=50degC. And superheat the feedgas by 10degC or so to the column so that water vapor does not condense. <10ppmv H2S in exit gas may be difficult, even with 2 split feeds of lean butane.

Ammonia converts to ammonium sulfide / bisulfide with H2S, and will solidify, blocking gas lines / instrumentation impulse lines. So there is a solid phase to this stream also. And the water vapor phase, when it condenses will be amm. sulfide / bisulfide saturated. So tell us how /why H2S gets into this NH3 stream. Can you remove the sulfur from further upstream before it causes all these problems?

 

[ReemaK]

this gas is basically sour water stripper overhead gas...(from 2nd stripper...i.e ammonia stripper)..
We want to remove h2s from this stream
yes...when temperature is low...ammonium bisulfide forms and plugs the lines..
<10ppm is difficult in single step...we need to incorporate a 2nd polishing step to remove h2s further...
ZnO is reported in literature for h2s adsorption..

Thanks a lot for your prompt response !!!

 

[georgeverghese]

Non regenerable Girdler ZnO works with dry gas for trace H2S in gas streams.
Suggest heat tracing SWS ovhd gas line to SRU and also insulate to prevent water condensation which then leads to supersaturation of liquid water with amm sulfides. Heat tracing should cover all pressure instruments on this line also. Steam tracing would be okay, but electric tracing would be better. Pipe wall temp controller setpoint may be say 90-92degC.
Why has this become a problem now - is the SRU offline ?

 

[ReemaK]

yes sir

 

[pierreick]

Hi,
You may find pointers in the document attached.
Pierre

 

[ReemaK]

thank you @pierreick