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[SP500]

I am a student doing some research outside the scope of my course to help me understand reactor design better.

 

I have designed a PFR for a 2nd order homogeneous reaction assuming ideal flow.

I am now trying to modify my design to take into account the deviation from ideal plug flow.

 

I have found a tracer study for the particular reaction I am looking at and know how to use  the segregated flow model to estimate conversion but have read that this is not suitable or at least sufficient for reactions other than 1st order, is this true? and if so,why ?

 

Besides finding the actual conversion, what else can I do to account for non-ideal flow? I know that this alone is not sufficient. 

 

Also, I would like to ask another question irrelevant to the above.I know how mass transfer can affect heterogeneous reactions but how do I incorporate mass transfer in my design if i am looking at a homogeneous reaction?

[DanBR]

Degree of segregation model is going to tell you the maximum conversion of a plug flow, depending on how your plug flow mixes in the radial direction (molecules of same age E), either early or late in the reactor. It is still a plug flow, nevertheless, so there is no longitudinal mixing. Imagine if the molecules react rapidly in the beginning of the reactor, and the reactant concentrations go to very low, or if it happens along the reactor or in the end of the reactor. But always as a plug. The model can be applied for any order of reactions if the expansion factor is zero. I'm not sure about it if the expansion factor is different than zero. (pg 356, Levenspiel)

 

To take into account the "non-ideality" for a tubular reactor (not plug flow anymore), the Dispersion model is usually used, especially for gas mixture, but can be also used for liquids and solid particles, depending on the type of transport (homogeneus flow of particles or packed bed reactors).

 

In your case, I would start with Dispersion model. For gases and liquids, the coefficient can be directly estimated with a graph or empirical formulas found in the books of Levenspiel (Pg 310), Fogler, etc.. If the model is too dispersed, a tubular reactor behaves similar to a mixed reactor, which drops the conversion.

 

I advise reading the chapters 11 to 16 of the Chemical Reactions Engineering, Levenspiel and also look for Fogler book. Both are really good.

 

Besides the conversion, you can look for Dead Zones and by-passes in real reactors. The are well modeled by the Compartment models, which are sequences of plug-flows and CSTR reactors to represent a single reactor and its non-idealities.

 

Homogeneous reactions consider only 1 phase (ideal solution), so there is no mass transfer between phases. Consider it is fully mixed.

 

Any corrections on my answers are appreciated.