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#1 Vakhit

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Posted 18 April 2016 - 07:33 AM

Hello everyone,
I would like to ask You to share Your experience (if any) for the following issue:
We have fixed bed reactors for H2S removal (iron based scavengers). Taking the laboratory analysis and gas flow rates we calculated that full cycle (H2 inlet = H2S outlet) of one absorber is about 65-75 % comparing with contract guarantees.
If worse quality of adsorbent is not considered, what else can be the reason for such poor unit performance? All other parameters are in design envelope (T, P, H2S concentration, gas composition). Only gas flow rate is about 20 % of design one. But I do not think this could be the answer. We suppose it can be channeling. Do you have any experience with this issue? What measures can be done: may be periodic purging (in reverse direction to gas) or "pressurizing / depressurizing" or something else? The goal is to break the channels.
Thank You very much in advance! It will be helpful to hear your opinion and advises.

#2 Art Montemayor

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Posted 18 April 2016 - 01:42 PM

Vakhit:

I am having trouble understanding what you wrote.  Perhaps you can help clear up the following points in your post:

  • You state you "have fixed bed reactors for H2S removal".  However, further down in your post you state " If ... quality of adsorbent is not considered, what else can be the reason for such poor unit performance?"  This confuses me.  Are you using reactors or adsorbers?  In other words, are you employing a chemical reaction (hot zinc oxide) to remove the H2S OR are you adsorbing the H2S impurity with an adsorbent that is regenerated?
  • If you are really using an adsorption process to remove the H2S, what adsorbent are you employing, what is your adsorption cycle, what is your regeneration cycle, and how do you regenerate the adsorption beds?  Show a sketch of how your beds/vessels are piped and valved.
  • What are the flow rates of your process stream and regeneration stream - as well as the compositions, temperatures, & pressures?
  • You state "... we calculated that full cycle (H2 inlet = H2S outlet) of one absorber".  Do you really mean the hydrogen content in the inlet stream is equal to the hydrogen sulfide content in the outlet stream?  Or is this a "typo" error?  Please be specific and cite your units.
  • How is your reactor/adsorber vessel constructed and compartmentalized?  Please furnish a detailed sketch or drawing of your vessel and show its internal construction and how the catalyst/adsorbent is installed.  How do you suspect channeling is taking place?
  • What is the temperature of any regeneration gas you are using (if you are adsorbing) and what is the make and type of valves you have installed between your adsorber vessels?


#3 Vakhit

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Posted 18 April 2016 - 03:16 PM

Art,
Thank You for reply.
I am sorry if You were confused by my statement. May be I wrongly mixed some different terms. Actually, we are using iron based scavengers - non-regenerated adsorbent. From this point I can call it "chemosorbtion". That is why even in our project documentation, contractor uses two terms "catalyst" and "adsorbent" equally. But it is not the main issue.
Let's call it aDsorbtion. It is non-regenerated type. According to contract specifications this adsorbent should provide "aDsorbtion capacity" around 290 kg H2S / m3 of adsorbent.
We have already carried out two "change-outs". Analysis of laboratory data showed that full cycle (H2S inlet equals H2S outlet, full saturation) is around 70 % of specified one in the contract.
All parameters are in the project envelope (T - 55 C, P - 12 bar, H2S inlet - 1000 ppm) and only gas flow rate is about 20 % (10000 Nm3/he vs 50000 Nm3/hr), but it will increase in the future. Approximate gas composition: C1 - 70 %, C2 - 15 %, C3 - 10 %, C4+ - 3 %, CO2 - 2 % (rough numbers).
Please, find attached the sketch of the adsorber. We suspect that the channeling can occur, because we do not have any ceramic balls in the middle layer of the bed. It would help to distribute the gas stream more smoothly.
But do You have any other thoughts? And may be remedies that can be helpful in the operation stage. I would like to try to perform periodic purging (in reverse direction to the gas) and pressurization / depressurazation and hope this can break or close the existing (possible) channels and help us to extend the life-cycle.
Thank You! Appreciate Your help!

Attached Files



#4 Bobby Strain

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Posted 18 April 2016 - 11:00 PM

Probably you must wait until you achieve design capacity. Your guarantee probably contains a required flow for which the capacity is guaranteed. And, yes, there is likely maldistribution along with reduced mass transfer due to the low velocity. Let us know how it works out for you.

 

Bobby



#5 Vakhit

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Posted 19 April 2016 - 12:58 AM

Thanks for Your reply,
But as I understand from theory, for the chemosorbtion one of the main parameter is contact time, which is much lower currently and it should give only benefits. If this is right, we can expect only deterioration when the gas flow rate will be higher.




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