Chapter 1 9
In the cross-section average the local concentration is weighted by the local
area and integrated over the cross-section. One then divides this by the total
area to get the cross-sectional average.
In the cup mixing average, the local concentration is weighted by the local
volumetric flow rate and integrated over the cross-section. One the divides
this by the total volumetric flow rate to get the cup mixing concentration.
18. What assumptions are involved in plug flow model?
The cup mixing concentration is assumed to be the same as the cross-sectional
average concentration for a plug flow idealization.
19. What assumptions are involved in a completely backmixed model?
The average concentration in the reactor and the exit concentration are
assumed to be the same in a completely backmixed reactor.
20. What additional closure is needed for mass transport in turbulent flow? Why?
The contribution of the turbulent diffusivity (eddy diffusivity) should be
added in addition to molecular diffusivity to calculate the flux across a con-
trol surface. This extra term arises due to the fluctuations in velocity causing
additional transport.
21. What is an ideal stage contactor? How do you correct if the stage is not
ideal? The exit streams leaving a two phase contactor are assumed to be in
equilibrium in an ideal stage contactor. If the stage is not ideal, it is corrected
by using a stage efficiency factor.
22. What is the dispersion coefficient and where is it needed?
Dispersion coefficient connects the cup mixing and cross-sectional averages by
using a Fick’s type of relation. It is needed to close the mesoscopic models in
systems where a chemical reaction is taking place, e. g., tubular flow reactors.Companion to Mass Transfer Processes, P. A. Ramachandran.
