12 replies to this topic

[Fardu]

Hi all,

I am new here and wanted help with a project. I was given a project to design a plant based on partial combustion of a given hydrocarbon to produce synthesis gas. I have carried out some material and energy balance based on some data collected online and elsewhere and assuming the shell gasification technology. Now I am stuck with designing the reactor - the process design. I am planning to do it as a MFR but I don't have a clue of the rate equation, reaction mechanism or the kinetic data.

A part of the hydrocarbon undergoes combustion first that'll produce enough heat for the later endothermic reforming reactions.

CxHy + O2 = CO2 + H2O
CxHy + CO2 = CO + H2
CxHy + H2O = CO + H2

I don't have rate equation of any of the reactions. Moreover on what basis the design be based, i.e. the slowest reaction step (which one?) or the multiple reaction mechanism (simplified to series and parallel steps).

Thanks...

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[kkala]

Your project, interesting and challenging, needs specific knowledge that I do not have. Some useful (or not so) remarks, having visited a coal gasification plant (1969).
1. Shell Gasification Process (http://www.escet.urj...propuesta18.pdf), gives mainly CO + H2 as product syn gas, whose cooler is the waste heat boiler reported in Statement of the Problem (SoP).
2. SoP expressions (feeds mixed in a combustion chamber before entering the reaction zone; the heat produced pyrolyzes the remeining hydrocarbons into gas) give the impression of thermal cracking (kinetics rather well known). Burning a small part of feed can raise temperature to 1350 oC; yet subsequent endothermic cracking shall require further combustion. I do not know whether this concept of thermal cracking, along with several simplifications, could be applicable here. Proprietary Shell's models promote CHx + 0.5O2 --> CO + 0.5xH2 (not hinted by SoP), results will be different at any case.
3. Above is based on the impression that kinetics specific for the case will not be "public domain" yet. An expert's opinion on this would be helpful. Conversion of heavy (black) petroleum products to lighter ones have much commercial interest nowadays.
4. In case of no data, an option is to address to Shell Global Solutions, Uhde, etc. Kinetics will not be disclosed, but you could obtain illustrative material, so that you can assume something about reactor size and product composition. In my student years this was common practice, but instructor was commercially oriented. SoP does not seem to have this spirit, but you still have to do a lot technical work downstream (waste heat boiler, cleaning systems, etc). The systems would not be so "compact" as in Shell's design.
5. Requirement No 3 of the design report seems to miss something. Operating factor 95% , then assumed feed composition is not complete.
6. Hopefully your supervisor is willing to help in this project, good luck!
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Edited by kkala, 10 December 2011 - 02:28 PM.

[ankur2061]

Fardu,

The link provided below may help you in getting started where a whole section is devoted to partial oxidation of hydrocarbon including flow diagrams:

http://kimia.um.edu....esis_041218.pdf

Hope this helps.

Regards,
Ankur.

[Fardu]

Your project, interesting and challenging, needs specific knowledge that I do not have. Some useful (or not so) remarks, having visited a coal gasification plant (1969).
1. Shell Gasification Process (http://www.escet.urj...propuesta18.pdf), gives mainly CO + H2 as product syn gas, whose cooler is the waste heat boiler reported in Statement of the Problem (SoP).
2. SoP expressions (feeds mixed in a combustion chamber before entering the reaction zone; the heat produced pyrolyzes the remeining hydrocarbons into gas) give the impression of thermal cracking (kinetics rather well known). Burning a small part of feed can raise temperature to 1350 oC; yet subsequent endothermic cracking shall require further combustion. I do not know whether this concept of thermal cracking, along with several simplifications, could be applicable here. Proprietary Shell's models promote CHx + 0.5O2 --> CO + 0.5xH2 (not hinted by SoP), results will be different at any case.
3. Above is based on the impression that kinetics specific for the case will not be "public domain" yet. An expert's opinion on this would be helpful. Conversion of heavy (black) petroleum products to lighter ones have much commercial interest nowadays.
4. In case of no data, an option is to address to Shell Global Solutions, Uhde, etc. Kinetics will not be disclosed, but you could obtain illustrative material, so that you can assume something about reactor size and product composition. In my student years this was common practice, but instructor was commercially oriented. SoP does not seem to have this spirit, but you still have to do a lot technical work downstream (waste heat boiler, cleaning systems, etc). The systems would not be so "compact" as in Shell's design.
5. Requirement No 3 of the design report seems to miss something. Operating factor 95% , then assumed feed composition is not complete.
6. Hopefully your supervisor is willing to help in this project, good luck!
-

Dear Mr. kkala,

Thanks for your help and guidance. Actually, the model I am assuming use the process of the shell which is suitably fit the SoP. The problem with the kinetics can be tackled by assuming the thermal cracking as rightly suggested by you rather than going with the properitery aspects of the partial combustion of Shell's. You told that the kinetics of the thermal cracking is well known, would you please suggest some references or some rate equations for the problem. Moreover I didn't get what you exactly meant by the feed composition is not complete by operating factor of 95%. What I thought of the operating factor was that the plant operates for 95% of the capacity (production rate).

Thanks and Regards,

[Fardu]

Dear Mr. Ankur,

The link gives only a preliminary idea of the overall POX process which I already have got. Please suggest some links or references linked specifically with the reactor design aspects.

Thanks and Regards,

[Fardu]

Dear Mr. kkala,

Are you sure that the Shell uses the partial combustion model suggested by you? Actually I have got some material related with the process (from NFL Panipat, one that uses Shell technology) that also describes the mechanism I wrote in the original post.

Regards,

Edited by Fardu, 15 December 2011 - 10:25 AM.

[ankur2061]

Dear Mr. Ankur,

The link gives only a preliminary idea of the overall POX process which I already have got. Please suggest some links or references linked specifically with the reactor design aspects.

Thanks and Regards,

Fardu,

Design Details for Partial Oxidation Reactors would be diifficult to find as open information on the internet. You need to find some book on reactor design and try to develop a theoretical design. I am posting some links, however, I am not sure whether they would be of help:

http://alexandria.tu...2/200011459.pdf

http://www.sciencedi...009250905800028

Regards,
Ankur.

[Fardu]

Dear Mr. Ankur,

Thanks for the support but the links are really not useful for me as I am supposed to design a non-catalytic reactor. You said it correctly the design aspects are not available in the public domain as I've searched the net thoroughly. What I exactly wanted was a reaction mechanism for the above mentioned reactions or rather more specifically the rate equations based on a global one step model (if possible) so that based on the MFR perfomance equation I would be able to calculate the volume and other kinetic aspects of the reactor.

Thanks and Regards,

[kkala]

...You told that the kinetics of the thermal cracking is well known, would you please suggest some references or some rate equations for the problem.
Moreover I didn't get what you exactly meant by the feed composition is not complete by operating factor of 95%. What I thought of the operating factor was that the plant operates for 95% of the capacity (production rate). Thanks and Regards,

1. Judging from the exercises of my instructor on reactors (1971), kinetics of thermal cracking in furnace tubes had been well developed at that time. Googling "thermal cracking kinetics" reveals a lot of links, a few free like the http://www.sid.ir/en...84320110414.pdf.
"Petroleum Refinery Engineering" by W. L. Nelson, Mc-Graw Hill, 1958, 4rth edition seems good enough for someone interested not only in kinetics, but also in other data related to thermal cracking (decomposition heat, yields, descriptions). Reaction velocity constants from asphalt to ethane are presented in a diagram (Fig 19-5). Chapter 19 is devoted to "Thermal cracking and decomposition processes" (68 pages).
"Reaction kinetics for chemical engineers" by S. W. Walas, McGraw-Hill, 1959, reports mentioned diagram by Nelson in Chapter 9 (Fig 9-12).
Probably Nelson is a good source for familiarization with the subject.
2. It seems that some phrase is missing from para 3 of Statement of the Problem, after "assume an operating factor of 95 percent". Probably feed composition follows, which is not complete.

[kkala]

Dear Mr. kkala, Are you sure that the Shell uses the partial combustion model suggested by you? Actually I have got some material related with the process (from NFL Panipat, one that uses Shell technology) that also describes the mechanism I wrote in the original post. Regards,

Not at all, as pointed out in the post of 10 Dec, para 2. Shell's proprietary model is not known, but something (concept - subsequent rates) has to be "invented" instead of it, based on assumptions. Results of the latter cannot be same as those of Shell.
One could think of partial combustion looking at Shell's gasifier in the web reference of mentioned post by kkala (long reactor, short flame, refractory lining) or at the statement of the problem; however any model based on this concept has to pass from numerous compromises to be meaningful. Another basic concept could probably provide background for model development too.
This specific topic of kinetics had better be discussed with your instructor in advance; so that you will not have to revise it after few months.

[Fardu]

Mr. kkala Sir,

Thank you so much for your guidance. I am presently going through the topic you suggested and try to find out if something could be worked out.

Regards,

[kkala]

Just one note, based on kkala's post of 10 Dec 11, para 4. Is it necessary to have some picture of the kinetics for an "elementary" sizing of the gasification reactor? SoP seems to require it, but there is much other technical work, then cost estimating. I wonder if you can "bypass" reactor sizing, accepting a size and "conversions" without much justification, just supposing something from an article. It depends on the purpose of this student project.
As students we did so in a polyethylene plant, but instructor was paying attention rather to cost estimate than to reaction kinetics.

[Fardu]

Dear Mr. kkala,

That was exactly I was thinking when I started to lose hopes of finding any kinetics of the gasification. Since I didn't mention the weightage of marks in the SoP, the cost estimation carries 30 marks (out of 200) and the reactor design a mere 20 which includes the mechanical design aspects as well (rightly anticipated by you).
I have gone through the points suggested by you regarding thermat cracking but couldn't satisfy myself with it. Wrote to Shell also but no response so far, now I am positively thinking about assuming a size in absence of any practical data.
The best I could find from the net was that the partial oxidation (of methane) catalytically follows two mechanisms, one by direct partial combustion reaction (DPO) and the other by the combination of the combustion and reforming reactions (CRR) (both by steam and CO2) suggested by different researches (from the net) but these process are all catalytic with different temperature and pressure conditions and do not serve my purpose.

Thanks for your guidance.

Regards,