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Carbon Deposition In A Natural Gas Steam Reforming Plant.

carbon deposition steamreforming

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#1 A_D_M_MII

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Posted 19 August 2021 - 12:41 PM

Hi everyone,

 

I am doing my final project for my master degree, it is about a steam reforming plant that uses natural gas as a feedstock and i have a doubt.

 

I have read that the fact of preheating the feed before the inlet of the reformer tube would improve the gas synthesis, furthermore, the furnace would need less fuel in order to achieve the final temperature.

 

Let's suppose that the feed current has high content in methane because it is natural gas and there is a prerreforming unit before the reformer.

My question is, has the increase of the inlet temperature any high limit?

 

I have read about the carbon deposition but i am not able to understand the phenomenon, so, if the feed currently enters at 650 ºC, would it be a problem if we heated the feed upper to 650ºC ? Would it appear carbon deposition if we did it?

 

Thank you so much!

 

ADM  :D

 

 

 



#2 Pilesar

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Posted 19 August 2021 - 02:59 PM

At high temperatures, hydrocarbons 'crack' and the hydrogen atoms dissociate leaving the carbon atoms in the solid state (coke.) Consider that natural gas has more components than just methane. The heavier components are more at risk of cracking. In many steam reforming units, the hydrocarbon feed is heated inside serpentine finned tubes in the flue gas convection section. Once these tubes are fouled with coke, there is no easy way to clean them in place and they must be cut out at great expense. So the preheat conditions are kept at a good margin below the coking temperature. There are also several other good uses for the flue gas heat. It can be used to heat boiler feed water, superheat steam, heat the fuel, heat the combustion air. The flue gas temperature is lower after each heat transfer coil, and the goal is to transfer the heat at the proper temperatures to achieve the best overall energy efficiency. The coil metallurgy requirements also change depending on the heat transfer temperature.



#3 A_D_M_MII

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Posted 20 August 2021 - 04:16 AM

Thank you Pilesar, i know what you mean, but i have the next question:
What is the coking temperature in this case?

So, inside the reformer tube is the carbon deposition inevitable? Because it usually reaches temperatures like 850 ºC to 900 ºC.

Is there any fact associated with the catalyst that allows less carbon depostion?

In my steam reforming plant there is a preheating convection section before the reformer, like the one you have described and the temperature at the outlet of this coil (Inlet of reformer) is about 650ºC, let's imagine that i have found a usable hot current that could increase the feed even more than 650 ºC, could be any problem?

At high temperatures, hydrocarbons 'crack' and the hydrogen atoms dissociate leaving the carbon atoms in the solid state (coke.) Consider that natural gas has more components than just methane. The heavier components are more at risk of cracking. In many steam reforming units, the hydrocarbon feed is heated inside serpentine finned tubes in the flue gas convection section. Once these tubes are fouled with coke, there is no easy way to clean them in place and they must be cut out at great expense. So the preheat conditions are kept at a good margin below the coking temperature. There are also several other good uses for the flue gas heat. It can be used to heat boiler feed water, superheat steam, heat the fuel, heat the combustion air. The flue gas temperature is lower after each heat transfer coil, and the goal is to transfer the heat at the proper temperatures to achieve the best overall energy efficiency. The coil metallurgy requirements also change depending on the heat transfer temperature.



#4 PingPong

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Posted 20 August 2021 - 09:08 AM

You said that you are using a prereformer, so all C2 and heavier will be converted into methane before the feed gas is further preheated in the convection bank to 650 oC (or whatever).

 

Methane will not coke in the convection bank, even well above 650 oC, because it is mixed with steam in a molar ratio of typically 2.5 to 3.

 

See for example:

 

Thermodynamically-predicted-coke-formati

Source: https://www.research..._fig2_254259386

 

Without a prereformer you would have to be more careful how far you can preheat the feed gas in the convection bank, depending on the actual composition of the natural gas.


Edited by PingPong, 20 August 2021 - 09:14 AM.


#5 Pilesar

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Posted 20 August 2021 - 09:57 AM

My work in design and operation of steam reformers was over 30 years ago and I had not heard of prereformers before this thread. I had interpreted 'prereformer' as a desulfurization bed and the preheating under discussion was for natural gas prior to steam mixing.
The coke formation chart for methane-steam mixtures is interesting! I am not sure what the y-axis means. Is that supposed to reflect the completion percentage of the methane cracking reaction or something else?
 
Carbon formation equilibrium and the risk areas in steam reformers are discussed in the following references:
 
J. R. Rostrup-Nielsen, “Catalytic steam reforming” in J. R. Anderson, M. Boudart (Eds.) Catalysis, Science and technology, Springer, Berlin (1984), pp. 1-130.
 
L. Altrup, Journal of Catalysis, 109, 241-251 (1988).
 
J. R. Rostrup-Nielsen, Catalysts Today, 19, 305-324 (1993).
 
B. J. Cromarty, “Carbon formation and removal in the primary reforming process”, lCI/Katalco Technical papers, ICI Group 113 W/075/2 CATPREF.
 
H. J. Renner, F. Marschner, G. Hochgesandt, “Gas Production” in Ullmann’s Encyclopedia of Industrial Chemistry, VCH, Weinheim, vol. A 12, p. 190.


#6 PingPong

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Posted 20 August 2021 - 02:10 PM

 

The coke formation chart for methane-steam mixtures is interesting! I am not sure what the y-axis means. Is that supposed to reflect the completion percentage of the methane cracking reaction or something else?

It is the molar percentage of CH4 that is converted into C.

 

In the complete article that is clear as there are also other graphs that show what happens with 1 kmol of CH4 under different S/C ratios and temperatures.

 

You can download the article as a pdf at the webpage that I mentioned before.

 

For convenience and future reference (in case the url does not work anymore some years from now) I attach the relevant part of the article:

 

Attached File  Methane Coke Formation.jpg   190.05KB   0 downloads






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