3 replies to this topic

[kikisapsal]

Greetings,
 
I want to model a nonisotermal, nonisobaric multitube furnace to be used as primary methane-steam reformer. I calculated the final composition of first iteration of methane steam reformer with Runge-Kutta Fourth Order method. 
 
a) I begin the calculation by using the following formulas for determining kinetic constants, equilibrium constants, and adsorption constant:
 1. Formulas for Calculating Kn, kn, and Keqn.png   45.52KB   0 downloads
and I compared the results from my calculation with this source (Silva and de Abreu, 2016, p. 4)

 2. Results and Comparations of Kn, kn, and Keqn.png   20.35KB   0 downloads

which is weird. There is no difference in the input. I model my reactor with design temperature of 500 deg C and use similar constant R = 8,314 kPa m³/ kmol K, exactly what the journal models their FBR (fixed-bed reactors).

 

Then I calculate the reaction rate formula with partial pressure of each composition that evaluated from molar flow per hour specified as follows:

 3. Reaction rate formulas.png   10.12KB   0 downloads

 4. Partial pressure calculation.png   15.08KB   0 downloads

 5. Molar flows.png   14.07KB   0 downloads

 6. Reaction rate results.png   16.57KB   0 downloads

 

c) Lastly, using the RK 4th Order and the following formulas, I present my results and my calcuation methods to Silva and de Abreu (2016), obviously wrong.

 7. Conversion formulas.png   13.38KB   0 downloads

 8. Final composition formula.png   9KB   0 downloads

 9. Final composition results.png   18.73KB   0 downloads

 

What I want to ask is:

1. What is wrong with this calculation? I got very unrealistic, too large values. I search for number of reasons.
2. Maybe the universal gas constant is wrong? I search for 9 different configuration of formulas and try three of them. The universal gas constant usually different in units in the whole formula of, for example, in kinetic constant, R is in unit of J/kmol K. (that in Table 5 Silva and de Abreu, 2016, it uses kPa m3/kmol-K, 1000 times than J/kmol-K.) and when calculated, the unit usually don't match (for example, in Xu and Froment (1989) they use kJ/mol for energy activation, the value on nominator part of the kinetic equation inside exponential of natural number e, but the R uses kJ/kmol, which needs to be converted. I found that I can't do it because it will give different result). Even if we calculated them in exponential, the exponent in e will have different value if we divide the value with different values of R, which in some journals, don't state.
3. There is no explicit formula for final composition and conversion that I can use directly. Furthermore, there are no guidance of how to use the rate formula (in some journals, they use net rates, and some uses the value of the rate and incorporate them into final calculation of molar flow rate) and have no luck. However, the biggest problem may be the result of kinetic constant values, I think.

 The calculation is in the Excel document

 (R-03) Neraca Massa, Konversi, dan Panjang Tube.xlsx   193.72KB   1 downloads

in sheet 4) TLength 9 for my calculations and 4) TLength 17 for the calculations from specification stated in Silva and de Abreu (2016)

 

Formulas of kinetic, adsorption, and equilibrium constants is in page 4-6 in this Word document.

 (R-03) Iterasi Awal Konversi.docx   96.25KB   0 downloads

 

The question and my short design temperature and pressure is in the last page of

 Final Composition Calculation of First Iteration Step of Methane Steam Reformer With Runge.docx   27.74KB   3 downloads

 

 

Source:

1) Silva, J. D., de Abreu, C. A. M., Modelling and simulation in conventional fixed-bed and fixed bed membrane reactors for the steam reforming of methane, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10...ene.2016.01.023

2) Xu, Jianguo, Froment, Gilbert F. Methane Steam Reforming, Methanation and Water-Gas Shift: I. Intrinsic Kinetics. AIChE Journals, Jan 1989, Vol 35, No. 1, p. 90-96

[breizh]

Hi,

My advice, drop a message to PingPong  using (envelope), top right corner. He has a lot of knowledge about this process.

BTW, consider the search engine in this forum, you will find his replies to students to similar questions.

https://terpconnect....SteamReform.pdf

 

EDIT :

R =8314.47 J/kmol K

R=8.31447 Kpa m3/kmol K 

Some relations are using Celsius and other kelvin!

Good luck,

Breizh

[kikisapsal]

Crazy, right? How we suppose to know what values to use if the journal itself doesn't give any number to begin with.

[breizh]

Hi,

Did you check your model using the data published in the literature, this will a good to validate your work?

Start with the constants and then the reactions rate.

My 2 cents

Breizh