Question

Which model is commonly used to describe non-ideal flow with backmixing?

• A. Ideal PFR model
• B. Ideal CSTR model
• C. Axial Dispersion Model
• D. Batch reactor model

Hint:

Whenever you encounter a question on non-ideal flow with backmixing, look for models that incorporate dispersion or deviation from plug flow — the Axial Dispersion Model is your go-to.

Answer 1

The Correct Option is C

In chemical reactor engineering, real flow often deviates from ideal models due to effects such as channeling, stagnation, or backmixing.
Backmixing refers to the phenomenon where the flow reverses partially, causing mixing between the inlet and outlet regions, which is not captured by the Ideal Plug Flow Reactor (PFR) model.
The Axial Dispersion Model is used to simulate this type of non-ideal flow.
It combines aspects of plug flow with an added term to represent diffusion (dispersion) along the flow direction. This dispersion accounts for the spreading of concentration profiles and represents backmixing.
Mathematically, the model incorporates a dispersion coefficient ($D_{ax}$), and the governing differential equation modifies the ideal PFR equation to include axial diffusion.
The other options do not fit the purpose:
- The Ideal PFR assumes no backmixing — flow is perfectly ordered.
- The Ideal CSTR assumes complete mixing, not partial or axial dispersion.
- The Batch reactor has no flow at all, so the concept of backmixing does not apply.
Therefore, the Axial Dispersion Model is the most suitable and widely used model to describe non-ideal flow with backmixing.