Efficiency of radiant section of steam reformer

[ibrahim]

what i quoted above was a thumb rule or rough calculation for reformers. you have to sum three heat values, first one is for reforming reaction (exothermic), second one if for shift reaction (endothermic) and third one the delta of inlet T and out T, taking average Cp values per unit mass. values of heat of reactions was taken such that if inlet temperatures are close to 900F (+ - 50F), then you can roughly evaluate heat of reaction directly using these values, rather than cooling it to 25C and then heating it back.
don't confuse yourself. here it is in simple form
Total heat absorbed by Radiant tubes = (delta lb-mol CH4) x 88700 + (delta lb-mol CO2) x 17700 + (Cp x delta T x mass flow)

[dhproeng]

My simple question is that , why do we need to evaluate heat of reaction at 900 F rather we should evaluate it at 1445 F at which reforming reaction takes place in reformer tubes.
I have sent data, please calculate efficiency of radiant section for me by spreadsheet u have.
Cp x delta T x mass flow)??? hee which mass will be considered, higher hydrocarbons , methabe, steam or reforming products because hydrocarbons don't exist at 900 F but not at 1445 F.

[ibrahim]

waooo, just hold on. dhproeng, i would suggest you to to check your peers avaliable at your workplace. Its a common sence of engineering, mass flow will not change due to any chemical reaction. so you can take either of these. in general heat of reaction is available at 25C, so called standard heat of reaction. just ignore what i said before about 900F. now proceed with heat of reactions of both reforming + shift and add sensible heat. that would be total heat intake of radiant tube...... erase any thing related to 900F from your memory.
heat of reforming reaction ~ 88700 BTU/lb-mol
heat of shift reaction ~ -17700 BTU/lb-mol

[whaider]

I just got your data, will reply shortly

[whaider]

well do let me know of my mistakes.

[dhproeng]

I am thankful to u for ur cooperation but I have some foolowing concers for my understanding hope u will clear them for me.
I am surprised, what is the need of calculating the heat of reaction for N2, argon while they are not taking part in the reaction.
You have calculated heat of reaction for combustion reaction while it should be calcultaed as (lhv*N.G. flow- Flue gas flow*enthalpy of flue gases at exit temperature.) for the fuel side calculations.
While in calculating heat of reaction u have latent heat from 77-900 while this is not provided in radiant section instead it should be from 900-77 as reactants will cool down to 77 for reaction as illustrated in David himublaue.
U have also not cosidered sensble heat from 900-1465 F.

[whaider]

Buddy the point is change in enthalpy across radiant section.

So 1st you find enthalpy of entering stream no matter wheather a component is taking part in reaction or not, it is carrying certain enthalpy and that is = Hf + CpdT

Similarly you find for outlet stream and the subtraction of two gives change in enthalpy please note that it is change in enthalpy we are interested in and not heat of reaction as heat of reaction is automaticaly taken care of.

Another thing, The concept is that enthalpy of any substance is equal to its heat of formation at that temperature. So at an elevated temperature enthalpy increases by CpdT.

LHV is a sort of heat of combustion, now it can not be used together with heat of formation. Means that for given stream at both states (inlet/outlet) you have to use same method.

I hope this clarifies, i suggest you read the chapter of book you mentioned in totality including the text not just the problems.

-----------------------------------END OF DISCUSSION------------------------------------

[dhproeng]

I think it will work. Thanks