8 replies to this topic

[Srikrishna Chaitanya]

Dear All,
I am doing a kind of research on Natural gas steam reforming for syngas production , my basic objective is to find out methane slip ( Amount of methane i.e. mole percent that can be allowed in the reformer outlet ) of a reformer with respect to its application like Ammonia production , Methanol production , hydrogen production , Carbon monoxide production etc . I am trying to understand whether methane slip depends on factors like
1. Reformer outlet temperature: At high temperatures methane slip will be low (change in temperature will affect energy optimization and change the steam balance in the reforming section)
If this is the only reason then methane slip is fixed by energy optimization
2. Catalyst: If methane slip depends on the catalyst then catalyst selection is the guiding factor for fixing the methane slip , I doubt this because equilibrium composition at reformer outlet is limited by thermodynamics , which may not depend on catalyst .
Actually I am not clear how to approach this
Any experts having worked on reforming can throw some light on this , you can suggest some papers or books or your views
I appreciate your help
thank you

Regards ,

Satish

[astro]

Satish, reviewing the chemical reactions (the 1st is largely endothermic, the 2nd is slightly exothermic) involved is useful to support discussion:

CnHm + n H2O → n CO + (m/2 + n) H2

CO + H2O → CO2 + H2

Given the net endothermic nature of the reactions, you are quite correct in identifying that increasing temperature will generally drive the reaction forward. However as Chemical Engineers we need to be aware of issues that extend beyond chemistry and one of those areas is Materials Engineering. The high alloy steels that are used in steam reformer tubes are susceptible to creep rupture at the elevated temperatures that drive the reforming reactions forward. So much so, that a small increase in tube wall temperature can have a marked reduction in the service life of the tube metal.

Reverting to chemistry and applying Le Chatlier's principle of mass action, the reactions imply that an increase in the proportion of steam will also drive the reaction forward. However this comes at an energy cost to heat the steam and increases the fluid flow through the system, increasing pressure drop (and hence energy costs) and increasing the size of the process plant.

Catalyst activity is very important, which is why it is sensible to design for "end of run" catalyst conditions to give a worst credible case scenario for design and sizing. Catalyst suppliers go to great pains to show that their catalyst provides the best activity while offering low pressure drop and robustness (low propensity to breakage and dusting) over time.

It is difficult to be specific because the conditions and constraints around an individual application will influence the design (which is a balance of compromises). Typically the design process will involve the catalyst suppliers, as they have substantial experience that is supported with process guarantees.

To my knowledge, the cornerstone reference for this area of study is the Catalyst Handbook:
Catalyst Handbook
If you can source a copy, you would be well advised to hold on to it and treat it with care.

[iplan]

Satish,

In the steam methane reforming, following factors can contribute to a reduction in methane slip.

1. Increased reformer outlet temperature .

2. Increased steam / carbon ratio.
There is a term called "Approach to equilibrium" in steam methane reforming reactions( CH4 + H20 -> CO + 3H2). It is the difference between the actual reformer outlet temperature at which you have a particular methane slip and the theoretical temperaure at which the same methane slip would have been obtained had there been a perfect equilibrium. Normally designers design around 20 deg F as Approach. So if you increase the temperature your methane slip will decrease but you will also move away from equilibrium.

As Astro points out, the reformer outlet temperature is limited by the mechanical design of the tube material. They are subjected to creep and are normally designed to have a life of 100000 hours of operation.

Lastly,by Le Chatlier's principle number of moles increase in the products and therefore decreased pressure drives the reaction to forward direction and decreases methane slip. But again as Astro says, if the pressure is reduced, the size of equipment / piping go up which may not be economical.


Thanks
iplan

[Srikrishna Chaitanya]

Dear Astro ,
thank you very much for the reply and sharing your knowledge , this going to help me a lot

[Srikrishna Chaitanya]

Dear Iplan ,

thank you very much for the reply
this approach, has it got some thing to do with Application like Reformer in Ammonia plant , Reformer in Hydrogen plant etc. ?? Can you please throw some more light on approach

[iplan]

Satish,

This Approach to equilibrium is applicable for all steam methane reforming processes. Hence it is applicable in Ammonia plant steam refomers as well as hydrogen plant steam reformers. All equilibrium driven reactions have this Approach to equilibrium concept. To know more on this , You can refer to the techology supplier like Haldor Topsoe manuals or ICI katalco manual.

I am not sure what is the objective of your research , but it will be greatly helpful if you refer these manuals.


Many thanks,
iplan

[Srikrishna Chaitanya]

Dear ,
Can you please tell me manual means thats the operating manual given by TOPSOE or broachers we get in TOPSOE website , how do I get that , I appreciate if you provide me some links

[Ahmed Mohamed Khater]

The steam reforming of saturated hydrocarbons can be represented by the three reactions: C
n
H
m
+ nH
2
O → nCO + (n+m/2) H
CO + 3H
2
→ CH
4
+ H
O CO + H
2
O → CO
2
+ H
2
2
2
An excess of steam is required to suppress carbon formation and promote the reforming reaction, so that the steam ratio is normally in the range 2.5 – 3.5 for ammonia and methanol plants, 2.5-5.0 for hydrogen plants, and 1.6-2.5 for H
/CO plants. Also despite the thermodynamics of the process, reformers operate at high pressure to optimize the overall

[S.AHMAD]

Attached graphs may be helpful

Attached Files

•  SteamReformingGraphs.pdf   47.59KB   380 downloads