[tripton]

Hi everybody, I have a very basic question, but I can´t understand the answer. I will be very gratefull if anyone helpme.
 
I must calculate the residence time in a fixed bed reactor (Time=Volume/Flow rate) and I don´t know which Volume I must use. Must I use the "total Volume" of the reactor or just the volume Available for liquid in the bed "Total Volume x Porosity"


Complete description of the problem
Introduction (you could skip it) :
I´m trying to know the residence time distribution in a fixed reactor (because isn´t an ideal plug flow). I made some experiments by a tracer inyection in the reactor feed and then I have measured the tracer concetration in the output. The theory associated is described in "Chemical Reaction Engineering" writen by O.Levenspiel (Non ideal flow chapter).
I get very nice results and I constructed the Residence Time Distribution curve (E(t)). I calculate the experimental average time with numerical methods using RTD curve. My problem is when I try to calculate the theoretical Residence Time in order to compare it with the experimental data.
 
Question :
All of us knows the ecuation : Residence Time = Volume / Flow rate. My question is about how to calculate the volume for this ecuation in the case of fixed bed reactor.
I thought that the volume is the volume available for the liquid flow in the reactor (Total Volume x Porosity), but some collegue told me "that´s wrong" the volume is just the TOTAL Volume of the bed. The experimental data fix well if I use the total volume. I don´t understand clearly why because I guest each particle will stay in the reactor : Volume for liquid particles flow / liquid flow rate.
 
Numerical Example :
Porosity (P) = holes volume / total volume of the bed
Total Volume = 1 L
Porosity = 0,4
Liquid Available Volume = 1 x 0,4 = 0,4 L
Residence Time = Volume (total or liquid available? ) / Flow rate
 
Thank you very much.

Tripton

[tripton]

I found that the correct answer is just to consider the liquid available volume.

Tripton

[eugenedakin]

Hi tripton,

My answer is 'it depends'.  I have written many simulation programs and sometimes you need to determine the liquid portion only, and sometimes you need to consider the gas and liquid, and sometimes you need to consider solids, liquids and gas. 

There are many examples of this in the oilfield.  One possible example with a heater-treater (separates water from oil) - if the gas fraction is low, then the residence time is usually calculated with the liquid portion only.  Really, it depends on your specific reaction and application.  This is a quick example only.

I wish you the best.

Sincerely,

Eugene