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[Ebrahim]

Dear all
What is "mixing effect"?
how can it show effect of muldistribution on efficiency?

Ebrahim

[Ebrahim]

I asked these questions also in student forum.
Following are answers



hey!
pinching effect is an interesting property. lets look at this very simply for a binary system. we have a y-x diagram. now, on this diagram, using the various formulae that we have studied, we draw the operating lines of the rectification and stripping sections.
now, the slope of both of these lines depend on the reflux ratio. if we reduce the reflux ratio, at one point, we will reach a state such that the two lines hit the equlibrium curve simultaneously. this reflux ratio is called the pinch reflux and the point where these curves meet is called the pinch point.
the 'mal-distribuion' of efficiency can be found out by studying the lost work in the column. this is done very easily by using a software called Design II. { this is true iff i have understood your question properly.}
about mixing effect, you boggle me. please explain in a little bit more detail the exact question.
Regards,
Ankur
P>S: we are still talking about distillation towers right?? the pinch concept in heat exchnager network design is totally a different issue.
if u need more clarifications, do mail back...


Hi
I will like to add a bit.
At the Pinch point the vapor and liquid concentration do not change and even with an infinitely high column there will be no change in the vapor and liquid concentration.



Dear Ankur
Thank you your response helped me. But according to following paragraphs maybe my question becomes clearer:
1. Pinching effect: mal-distribution delivers less liquid to some areas l/v ratio is relatively low causing a composition pinch. the pinched areas contribute little to mass transfer.(as you said they contact by e.q.curve in pinched area).vapor leaving this areas contribute is rich with the less volatile components, witch contaminate the vapor rising from the rest of the bed (why contaminates? and what is the problem ?) similarly, lights-rich liquid leaving this areas contaminates the liquid descending from rest of the bed.(why contaminates? and what is the problem ?).The pinches also create non uniform liquid and vapor composition along the cross section of the column.(why contaminates? and what is the problem ?).
2. Latral mixing effect: packing particles deflect posh liquid and vapor laterally. This promotes mixing of vapor and liquids,(why?) and contracts the pinching effect. This is referred to as the lateral mixing effect.
Any explanation with any detail is more useful.
Regards



I think I only agree to some extent with some comments above.
Pinching, which is not releted only to distillation and column, occurs when an operating line gets close to an equilibrium line. One may have a pinch with heat exchange as well as with mass exchange is a column.
I think that maldistribution is not directly related to pinching. Of course, it is possible that a severe maldistribution generates a pinch somewhere, but this is a different issue.
In terms of fractionation, I think is is more appropriate to see the effect of maldistribution as the results of several sections of a column operating at different reflux ratios, or different L/V if you prefer.
It is true that packings will allow for some lateral remixing, and that this mitigates to some extent the effect of maldistribution. But not much. Why do you think that major packing suppliers will ask for an even, uniform wetting, of the packing, sometimes with as much as 120 drip points per square meter. This is becaus the axial redistribution of liquid is limited.
How much does mal distribution will affect a column. It depends. If you ak for extreme purities at any end, a lot. For moderate separation, not much.
I dug out a simulation done years ago for a fractionation column (case study) the fractionnation is similar to the xexample of Perry's handbook 13.37, I did it to convince my management that maldistribution could be a problem. The approach was to have the column split in two, each column handling one half of the vapor and one half on the feed, by with one column receiving more than its share of reflux. Its a C3-C4-C5 splitter. The graph shows the bottoms purity (C5) versus the un-even-ess of reflux.
You see that "best" performances are obtained with even split of reflux.
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