4 replies to this topic

[gunley]

Hi

I can't figure out how to convert reaction rate for methane - steam reforming. In various articles it is given as kmol*Pa/s*kg_cat. For reaction rate calculation partial pressure of components are used. I want to change it to molar concentration. For components it's easy, I'm just dividing by RT. But what about reaction rate constant ? Should I use temperature of the process or temperature of the relevance state ? (298 K) 

[thorium90]

so you want to convert from kmol.Pa/(kg.s) to?

 

Anyway, are you sure you got the units right? Do check because some of the exponents may be fractional and thus the conversion would be different, although that depends on the form of the reaction equation. You will need to check by balancing the units. You are using the kinetics by Xu and Froment?

 

Pressure=(Concentration)*(Gas Constant)*(Temperature)

 

PV=nRT

P=(n/V)*RT

P=CRT

 

Rate constant is not a real constant. It depends on temperature.

Edited by thorium90, 21 February 2013 - 02:25 AM.

[gunley]

Yeah, I'm using the kinetics proposed by Xu and Froment. To simulate reaction I'm using COCO CAPE-OPEN simulator. 

I want to convert kmol*Pa/kg*s to kmol^2/m^3*kg*s where kg is the amount of catalyst.

Anyway I figured it out, if could do double check I would be grateful.

Let's assume that reaction A -> B with rate r = k1*pa, where k1 is the reaction constant and pa is the partial pressure of component A.

For T = 273.15, k10 = 1 mol/s*Pa, pa = 100 Pa, E = 1 J/mol, reaction rate r = 1*exp(-1/R*273.15)*100 = 99 mol/s. 

When I convert partial pressure to concentration I got ca = pa/RT = 100/8.314*273.15 = 0.044 mol/m3. k1' = 1*8.134*273.15 = 2271 m3/s. Thus reaction rate r = 2271*exp(-1/8.314*273.15)*0.044 = 99 mol/s.

It is clear that reaction rate constant must be divided by current temperature.

[thorium90]

I want to convert kmol*Pa/kg*s to kmol^2/m^3*kg*s where kg is the amount of catalyst.

 

These two dont match. Your form of the reaction equation is not the LH kinetics that Xu and Froment published. Yes, you did the conversion correctly. Based on the two sets of units you said, its definitely from his LH kinetics. Conversion in the typical reaction from r=kC is simple. The fact that you said you wanted to convert kmol*Pa/kg*s to kmol^2/m^3*kg*s implies you may not have understood the kinetics correctly.

 

Another question, why is it necessary for you to convert to concentration units? Industrial gas phase reactors calculate stuff in partial pressures. Industrial liquid phase reactors like CSTR calculate stuff in concentration. Doing it in partial pressure makes it easier as your equations of state will be done in pressure units too. Things like pressure drop also use pressure units...

Edited by thorium90, 21 February 2013 - 04:04 AM.

[gunley]

I think I haven't make myself clear and I typed the wrong reaction rate constant unit. Obviously I want to convert all of the constants in the methane steam reforming reaction, so I can use the concentrations instead of partial pressures. 

I want to do this, because for reason unknown to me when I'm trying to simulate PFR reactor in COCO COPE-OPEN, with Peng Robinson as my EOS, it can't use fugacity of the components for reaction rate equation. So, I'm forced to use concentrations of the components instead of fugacities. As far as I know syntax only supports activity, concentration and mole fraction