Methanator

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Ramprasad
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Methanator

Post by Ramprasad »

I have a doubt on methanator section .
Why are you not using the adsorbers in the place of methanator for absorbing the methane carbon monoxide ,crbondioxide and argon .
It will helps the decrease the consumption of the hydrogen and reduce the load on synthesis gas compressor.
PKS1964
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Re: Methanator

Post by PKS1964 »

Hi,
I think that reaction help to convert CO and CO2 to CH4 which we removed as inert and use as fuel.
For adsorption there will be some issue in ammonia plant
1- Operation energy of adsorption tower for handling of big quantity of gas for CH4 adsorption.
2- Where we use this adsorbed CH4. because for adsorption there will be some adsorbing media.

regards
Zubairtalha
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Re: Methanator

Post by Zubairtalha »

Hi,
Following could be some Reds,

1- There will be no savings on Capitals. You need Two or more vessels for Dryers in comparison to Methanator. Size of the vessels will be huge as Stream is saturated with water ( may require chilling to remove free water),Loading of CH4, CO2 and CO is much higher. And further we are trying to completely remove CO (difficult).
2- For regeneration, probably treated outlet stream will be utilized which will contain Hydrogen far more than Methanator consumption?
3- Increased number of Instruments and Control valves. A single passing valve can ruin the complete regeneration cycle of dryers.
4- Requirement of filtration at main stream to avoid dust carry over to Synthesis section. Similarly burners chocking issue for usage of Waste gas as fuel if no filtration.
5- requirement of chillers (Cooling step of adsorbers), putting load on refrigeration circuit and Heaters (for heating cycle), ie more Steam demand.
6- Replacement frequency of Molecular sieve is high while Methanator catalyst lasts for Years.
etc. etc. etc.

Regards;
Zubair Talha
naseemce
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Re: Methanator

Post by naseemce »

It will be very expensive and methanator catalyst life is more than 15 years and generated CH4 will work as fuel instead of wasting .
astromo
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Re: Methanator

Post by astromo »

Assuming that this question is reviewing the trade off between a pressure swing adsorber as the final step of syngas production versus a methanator, then the answer lies in the process advantages and compromises that the original designer drives for.

I'd say that once that decision has been made, the magnitude of the sunk capital (especially when you consider the intricacy of an ammonia plant's energy recovery and exchange systems) is such that a retrofit of one technology for the other is unlikely. Maybe there's a project report out there that can be shared?

A PSA requires multiple vessels plus a buffer drum to smooth the pressure cycles so that the off gas can be supplied to the plant's fuel gas system. Intuitively, one of the key advantages that a PSA provides in terms of the process energy balance is that it reduces the inert loading in the make up gas to the syngas compressor and avoids the need for additional treatment and heat exchange to deal with the moisture and temperature rise that a methanator reintroduces to the syngas.

How that sits with the other trade offs at play, will come down to economics. For some, such as Linde, it works. For others, methanator technology wins.

If an in-depth study write up is available that reviews the CAPEX/OPEX trade-offs, then it would make good reading provided that the author(s) were not commercially motivated. Every process licensor will go to all sorts of lengths to convince you why their process is a winner and there are all sorts of reporting tricks that can be used to sway the argument.

Process licensors (generally) have a slate of technology in which they're strong and they will favour those technologies, rather than necessarily developing a process that is the ultimate best of the best. This latter hypothetical approach would invariably require using someone else's unit operation that would need to be licenced (assuming that it can be released because the owner doesn't see a competitive threat). Practically, this is not seen very often.
khaneteknon
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Re: Methanator

Post by khaneteknon »

Hello Ramprasad,
First it is key to note that that methanation step is critically key to the process for two major reasons. Absorption would not 'completely' remove the oxides of carbon from the gas stream. But the question is why do you need to remove those oxides so badly? It is because oxides of carbon are catalyst poisons to the iron catalyst in the synthesis converter and if allowed, they would greatly reduce the activity of the catalyst thereby decreasing the yield of ammonia.

The second reason is as earlier mentioned by someone, which is to convert it to combustible methane which can then be purged out (as an inert) and be used in the steam reforming section to provide the heat necessary for the endothermic reforming process.


I hope you found this helpful. Feel free to contact me at anytime. Have a great day ahead.
naseemce
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Re: Methanator

Post by naseemce »

what is the inlet /outlet limit of oxides and what will b the minimum and maximum operational temperature of methanation.
bedmonds
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Re: Methanator

Post by bedmonds »

Hi. Good questions-most methanators are of 1.25 Cr 0.25 Mo and H2 partial pressure is around 330 PSI so H2 embrittlement is an issue at much over 550 deg C BUT the vessel itself is limited by allowable stress constraints as temperature rises and trips are typically at 400-430 deg C. Rise is 74 deg C/% for CO and 60 deg/% for CO2. For maximum temperature, to me, normal operation should not be much over 350 deg C to have margin from trip for upsets. Another factor, for plants with feed/effluent exchangers, is ability to control temperature with the bypass that is normally supplied. With good catalyst an inlet tmperature of 260 deg C is feasible and with special catalysts for lower temperature, that option is possible. The outlet allowable is a delicate question and depends on a few factors. CO2 is more difficult to methanate.
a. if syngas driers are used they can remove CO2
b. if gas is washed with ammonia CO2 is removed
If neither of these are present CO2>5 ppm will take a toll on the converter catalyst over an extended period. Also, carbamate can be made and plug chillers, strainers etc. In the worst case, for older compressors using "rivetted" attachments peened to assemble the front and back of the impeller can be attacked by SCC in carbamate forms. In the HP compressor, in some configurations, the balance connector goes from recycle to HP stage suction which can be cooled enough, if a chiller is used, to allow carbamate formation. To avoid that risk CO2 should be less than 10 ppm. Interestingly, high oxides at the inlet does not lead to high oxides at the outlet. The exotherm is bigger and kinetics favor lower outlet oxides. Finally, when a new charge of methanator is brought online to maximize activity, operation at higher than normal temperature for a few hours is helpful. I hope others offer additional thoughts on this matter.
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