As you pointed out, mixing enhances mass transfer which in turn makes the mean residence time to approach the ideal time. However, mixing alone does to affect mass transfer, there is also the effect due to the species mass transfer diffusion coefficient. So, the best mixing cannot give a mean residence time better than the ideal case, even in the absences of dead zones, this is determined by the rate limiting step. Yes better mixing results in lower variance of RTD which translate to lower mean time

Generally speaking, the real mean residence time is lower than in the ideal case. Stagnancy causes the mean time to be lower than expected. Therefore, in a CSTR, better mixing will most likely increase the mean residence time. Not beyond the ideal case, though.

Variance should not be too high, but I would say, don't focus on variance. Dimensionless variance should always be lower than 1, other than that, estimate dead volume, short-circuiting, and look at the shape of the RTD curve and the dimensionless mean time tmean/tideal.

Generally speaking, the real mean residence time is lower than in the ideal case. Stagnancy causes the mean time to be lower than expected. Therefore, in a CSTR, better mixing will most likely increase the mean residence time. Not beyond the ideal case, though.

Variance should not be too high, but I would say, don't focus on variance. Dimensionless variance should always be lower than 1, other than that, estimate dead volume, short-circuiting, and look at the shape of the RTD curve and the dimensionless mean time tmean/tideal.

No what are you saying?

Always real residence time(due to the presence of only bypassing and channeling) is higher than the ideal case(well mixed case).

Always real residence time(due to the presence of only dead zone) is lower than the ideal case(well mixed case).

Vivek Arora  you are wrong in saying that : "Always real residence time(due to the presence of only bypassing and channeling) is higher than the ideal case(well mixed case)."

Think about that.

The second of your sentences says just opposite to what the first sentence means.

Dead zone is the part of the reactor which is bypassed  or channeled out

What i am saying that take a  CSTR which is well mixed but due to the presence of outlet nearby of inlet some fluid channels out- this is the bypassing 

If the CSTR is such that inlet-outlets are far apart then this bypassing is not there. But suppose in this case, there is no well mixing and at the bottom corners of the reactor, dead liquid zone will be created- this is what i meant by dead zone. In this case effective volume will be less compared to the total reactor volume, causing the lower residence time than ideal one.

Yes real reactor contains both dead volume as well as channeling.

What i am getting that, in the case of  a stirred tank reactor, ideal residence time is 50 sec.

While i am studying the mixing using a stirrer rotating at 3000 rpm in a CSTR using some software, i am getting a residence time of 80 sec. 

Is this correct? How to explain it?

Thanks

Vivek

Vivik,   blame the software :-)

But seriously speaking, we do not know what experiment you are doing.  Is it adding an impulse signal of the tracer on the input and you measure tracer concentration o the outlet.

Another methos is step wise changing of the concentration on the inlet.

In both method there is a need to integration of concentration in time.

The mean residence time can be calculated after finishing experiment.  The total time should be at least 3 times longer than mean residence time - or even more.

The question is how you calculate this 50 sec time.

What is the time you notice peak concentration on the outlet?

PS Do not use "ideal residence time"  instead of  "residence time in ideally mixed reactor"

What Roberto  said above, residence time in real reactor cant be greater than residence time in ideal reactor.

How is it possible?

just we take integration of tE(t) to calculate the residence time, where E(t) is the residence time distribution. We are using step input method of tracer study. We follow the outlet mass fraction of tracer until it reaches the inlet mass fraction.

Thanks

Vivek

Vivek

Can you show data about your reactor and concentration in time.?

And the name of that software you use?

As I am chemical engineer and computer science is my second nature :) I believe in software bugs.

My advice is use another software OR  find  number "2" missing in your calculation.

PS  Is stepwise change of inlet concentration from zero  or from other NON ZERO value.  In the latter case you should integrate in time The Difference between final and initial concentrations.

Initially there was a liquid and we know the velocity profile and then start the tracer injection and maintain a tracer mass fraction of 1 at the inlet.

Now E(t)=d/dt of (C(t)/C0) c(t) outlet concentration and co is the inlet concentration.

My question is why the real residence time cant be greater than residence time in ideal reactor? Can you please explain?

I believe the residence time can be greater than the residence time in ideal reactor

thanks

Vivek

I do not follow your last explanations, maybe because there is night here but...

The only reason to such behaviour asthe real residence time is greater than theoretical calculated for ideal-mixed reactor residence time is that your injection tracer goes to the dead zone and is slowly mixed out of this place.

Sorry fir typos :-)

If you go through the Fogler, elements of chemical reaction engineering book, you will find that, in the presence of dead zone, residence time decreases. In the presence of bypassing, residence time increases.

How that happens?

Please explain.

Thanks

Vivek

Bypassing means that apart from the stream flowing through reactor there is kind of external flow which is not flowing through reactor. However one measure sum of both streams and use that measurement to calculation of mean residence time. From the equation lower value of MRT is calculated.

But the real stream flowing through the reactor is lower than measured and the real MRT is thus greater than calculated.

That is simple theory but I can advice you that you should check the equipment like rhotameter as this can be calibrated for different medium and can give systematic error.  You should also check the volume of liquid in the reactor.  Is it glass or full metal?  If you not see the level of liquid in the metal reactor it can happen that during filling reator with liquid the air is trapped in the top part of the reactor and cannot be evacuated.

THere are many errors possible in the experiment.

Thanks for explaining this nicely.

In the same way one can say that if the reactor is having a dead zone, then the reactants coming with the inlet flow may fall into that zone. That may lead to higher residence time than the ideal well mixed case.

So essentially in real reactors residence time is higher than the ideal reactors.

Is it correct?

Thanks

Vivek

Vivek,

Your final conclusion may be correct if and only if the reactor is badly designed.

Real reactors if they are used for experiments and further calculations of reaction kinetics should be as close to ideal (means reactor with ideal mixing) reactor as possible.

1) Existence of bypassing means bad design or bad assembly of the test rig.

2) Existence of dead zones means bad design or bad geometry or bad positioning of mixing device

3) Injection of traces should be placed in the tube which delivers main stream. And after the injection tube there should be kenics static mixer or similar device to evenly distribute tracer in the whole volume of the stream.

4) inlet and outlet tube in the reactor should be placed as far as possible apart

5) Stream Flow should be visually examined with transparent fluid like water or air and small amount of color tracer.

sorry for typos

hope attached file helps

Dear  Dr. Bharatam,

I have gone through your notes.This is quite helpful.

For ideal CSTR reactor, there should not have any RTD. 

If we see the case of well mixed CSTR (there is no dead zone as well as channeling), then we will always have RTD. Is not it?

Thanks

Vivek

yes very well mixed cstr

inlet out let nozzles location and special fixtures on to the shell might cause some problems.

computational fluid dynamics are being used by people to understand issues in depth and find solutions.

yes very well mixed cstr

inlet out let nozzles location and special fixtures on to the shell might cause some problems.

computational fluid dynamics are being used by people to understand issues in depth and find solutions.

Vivek,

show  RDT curve and your calculation.  Without that all our answers are repating text books.

It is necessary to calculate the Peclet number. The Peclet number gathers the variance and tmean.