13 replies to this topic

[T'ien]

Hello,

 

I have a basic question regarding the PSA tail gas from a steam reforming process. It is my understanding that this tail gas is 100% recycled back to the reformer furnace as there is a significant amount of unreacted methane in this gas. However, I was under the impression that there is also a significant amount of carbon dioxide in this gas stream. My question is, do reforming processes usually have a unit to separate the methane in this tail gas from CO2 and other gases? If not, why not? If the methane is separated, is this usually done with an amine scrub? Is the CO2 and other waste gases simply vented off? Or is this CO2 commonly used for some othe process? Thank you.

 

T'ien

[PingPong]

You misunderstood the "recycling" of the PSA tailgas to the furnace.

 

The tailgas is used as fuel gas for the burners of the furnace, to supply the heat required for the reforming reaction.

[T'ien]

Hi pingpong. Thank you for your reply.

 

I apologize if I didn't word my questions clearly. But yes, that is exactly what I meant... recycling the tail gas to use the remaining methane as fuel gas for the burners. My question still remains though... do we just recycle the stream as it is (with CO2, CO, traces of H2O, etc.) back to the furnace or do we separate the methane from these other gases before it is sent back to the burners?

 

The reason I am asking is because my group and I have modelled the whole process using VMGSim and we found that about 50% of the tail gas is CO2, 37% methane and the rest is CO and H2O. We are not sure how you decide whether it's worth recycling the remaining methane back to the furnace burners or simply send the entire stream to a flare stack. All of this of course depends on whether some energy input is needed to separate the remaining methane from the other gases (and that's what we are unsure of).

[PingPong]

do we just recycle the stream as it is (with CO2, CO, traces of H2O, etc.) back to the furnace or do we separate the methane from these other gases before it is sent back to the burners?

The tailgas is sent to the burners as is, without any treatment, or separation, or whatever. For the burners it does not matter that it contains CO₂.

 

 

50% of the tail gas is CO2, 37% methane and the rest is CO and H2O.

37 % methane in the tailgas is extremely high, so I fear your model is not correct.

Tailgas usually contains less than 10 % methane. It contains however also H₂ (because H₂ product recovery in PSA unit is less than 90 %) and CO that together provide more than half of the tailgas heating value.

Edited by PingPong, 27 February 2014 - 04:22 PM.

[rychurek]

Hi
Tail gas is main fuel for methane steam reformer when used PSA process for hydrogen purification. For 12 bed mode PSA hydrogen recovery is 91%, so tail gas concentration is approx. 20% hydrogen, 60% CO2, 10% CO, and about 10% methane.(as PingPong mentioned)

[rychurek]

... so tail gas concentration is approx...[/quote]
I meant to write: tail gas composition is approx...

[BJD]

pl explain how 91% H2 recovery in 12 bed PSA is possible ? what are the PSA inlet , Product H2 outlet & PSA off gas pressure?How much Kf is maintained? What is your adsrobtion subcycle time?/

[rychurek]

BJD
91% hydrogen recovery is for PSA 12 beds design condition( 89% for 10 beds and 87% for 8 beds). It could be different for various feeds and hydrogen purity expectation. PSA hydrogen recovery depends on: adsorbent activity, valve tightness and process conditiions : syngas hydrogen concentration, adsorption and desorption pressure and temperature and hydrogen purity. PSA recovery should be in your PSA manual guide. If I remember well in our 77000 m3/h hydrogen methane staem reformer PSA working condition was: PSA upstream condittion:hydrogen concentration 75%, P=24,5 barg, temp. 30 C. PSA downstream: hydrogen purity ( 5 ppm CO), pressure 24.2 barg. Tail gas pressure 0.2 barg. Adsorption subcycle time 30 sec. with KF 850

[BJD]

rychurek

 

Is it possible to recycle back PSA tail gas in to the feed to steam reformer or somewhere else to recover 30% H2 & 15 % CH4 contained in 35000 Nm3/hr PSA tail gas? Otherwise the PSA tail gas (almost 80%) is being used as fuel to the reformer and the rest 20% is going to flare vent due to limitation in reformer tube skin temperature. What do you suggest for better hydrogen and CH4 recovery from PSA tail gas?PSA tail gas pressure is 0.2 barg and contains 55% CO2, 0.8% CO & less than 0.5 % C2+ component.In our 12 BED UOP PSA( Capacity 67412 Nm3/hr H2 by steam reforming of Natural Gas or naphtha), the PSA upstream condittion:hydrogen concentration 75%, P=20.0 barg, temp. 40 C. Is there any scope to improve H2 recovery by increasing PSA inlet pressure to say 22.0 barg?

[rychurek]

BJD
1. What's tail gas composition/concentration in heat and material balance.
2. What's your design PSA hydrogen recovery?
3. When Did you begin to flare tail gas?

[BJD]

rychurek

 

 1. PSA offgas contains 30% H2, 15 % CH4 , 55% CO2, 0.8% CO & less than 0.5 % C2+ component . Off gas LHV=1700 Kcal/Nm3 .Total flow=35000 Nm3/hr.

 

2. PSA H2 Recovery is =85%

 

3. Few months back( say 6 Months)

 

What do you say regarding Off gas recycle & Improvement scope of H2 recovery by increasing inlet pressure? 

[rychurek]

BJD
I assume that 85% is actual hydrogen recovery, not design. You asked about KF factor and subcycle time, that I can presume that you reached min subcycle time to keep hydrogen purity as low as possible, am I right? Hydrogen that should be send to consumer is now send with tail gas to reformer furnace. Did you try to find root cause this situation. C2+ present in Tail gas is not typical for Methane steam reformers. Did you compare your reformer outlet gas analysis with PSA feed gas? C2+ in PSA feed gas can indicate heat exchanger leakage

[BJD]

85% Recovery is design. I dont understand why design recovery in PSA can not be achieved atleast 90%. Actually In our PSA feed we put other 2 offgas stream such as hydrocracker low pressure section offgas & Motor spirit plant reforming unit off gas which contains C2+ components less than 1% and to take care of this components we have adsorbents in PSA from which C2+ are released in PSA tail gas.

PSA outlet product H2  is 67412 Nm3/hr with 99.99 % purity. Presently PSA tail gas is sent to reformer furnace as fuel and part of that is going to flare due to limitation in Reformer tube skin temperature.Is it feasible to incorporate one compressor to recycle back part of the tail gas (30% H2 & 15% CH4 and balance are CO, CO2 & C2+) into reformer feed?Is there any scope for increasing H2 recovery by raising PSA inlet pressure by around 2 kg/cm2 from present 20 Kg/cm2g ? You can reach me by e-mail: bubuljyoti@gmail.com      

[rychurek]

85% Recovery is design. I dont understand why design recovery in PSA can not be achieved atleast 90%.

I think you can increase your recovery to 90% but it's involve with PSA revamps by increase adsorber
beds, replace adsorbent etc. Contact with UOP, for sure they'll answer your questions.

I don't want to advice you to increase PSA feed pressure or decrease feed temperature, because all of these changes should be evaluated with UOP specialists.