[mirandamix]

hey everyone,

i have this question:

What are some causes for the residence time to be equal to 0 ??
or when does that happen??

thanks in advance...

[RealEngineer]

The residence time in a tank approaches zero as the volume of the tank becomes very small in comparison to the flowrate through it.  When the residence time becomes zero, it can be considered to be plug flow - see http://en.wikipedia.org/wiki/Plug_flow.  Plug Flow is a subject discussed at length in many chemical engineering textbooks.

Regards

RealEngineer

[virenchocha]

Residence time for plug flow is can not be zero otherwise how conversion will take place inside the plug flow reactor ?
Residence time is associated with every single pipe and piece of vessel.

[RealEngineer]

Yes, I stand corrected.  Plug flow does not have a zero residence time. 

However, as my previous post says, residence time will approach zero as the flowrate is increased and / or the volume of the reactor is reduced.

[Siddarth]

Residence time is essentially the time spent by the particles(either the reactants or products) in a reactor. A plot of the time spent by each particle is called the residence time distribution curve. We may plot this curve experimentally by using a non reactive tracer(sometimes even a reactive one).

The spreading of this tracer in the reactor is assumed to be a diffusion like process, and is called "dispersion" or longitudinal diffusion. The measure of spreading of this RTD curve is indicated by the term "variance".   

In ideal plug flow, there is no dispersion. As a result, the tracer does not spread, and the variance is zero.

"Variance" and "RTD" are not one and the same. How can Residence time turn to zero? If a particle spends "zero" time in a reactor, can you get what that translates to? The particles do not enter or leave the reactor at all. The reactor has nothing in it. You do not even carry out the reaction.