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Distillation- Weeping Calculations


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#1 confusedstudent

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Posted 13 March 2011 - 04:09 PM

Hi guys i was hoping someone can help me out here

I'm calculating the weeping using the equation from coulson and richardson text book volume 6 4th ed. If you look at the attached excel file where an example is shown, how is the answer obtained in mm and m/s? because of these unit problems my answers are really big i would appreciate any help please

Thanks

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#2 golden_gate

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Posted 13 March 2011 - 10:57 PM

They're empirical correlations, usually valid for a certain range, for a specific set of units (SI in this case)

You should check if the units you're using is consistent with what is stated in the book.

#3 katmar

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Posted 14 March 2011 - 02:18 AM

The physical phenomena involved in weeping through a sieve tray are so complicated that nobody has been able to perform an analysis from first principles. The only way to get an answer, as is true of many operations in chemical engineering, is to resort to empirical correlations (as pointed out by golden_gate). This results in dimensional equations like this one from C&R.

A hole velocity of 14 m/s is not unreasonable. The hole area will typically be around 10% of the cross sectional area of the column. Even less sometimes. The superficial velocity is then about 1,4 m/s up the column. This is probably towards the upper end of the usual velocities, but is not obviously wrong.

I have seen other weeping correlations that are much more complicated than the C&R one, but they don't seem to offer any more accuracy. The most difficult part is to define the onset of weeping. Even in a tray operating well there will be some small leakage from the trays above. As the gas flowrate decreases the weeping will gradually increase. There is no magical point where it suddenly goes from 0% to 100%. So it is hard to say how accurate a weeping correlation really is.

#4 Art Montemayor

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Posted 14 March 2011 - 07:53 AM


Confused:

If what you copied and pasted into the workbook comes from your textbook, then Coulson & Richardson has an error in their published example numbers. Look at my modified copy of your workbook. Due to bad editing and checking, the constant 0.06 is shown instead of 0.6.

In today's world of down-sizing, this is happening more and more in technical textbooks.

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#5 golden_gate

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Posted 14 March 2011 - 01:47 PM

Hi Art,

That was really well spotted, I've just realised that mistake in my copy. Thanks for that!

#6 confusedstudent

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Posted 16 March 2011 - 03:02 PM

thankyou so much !




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