2 replies to this topic

[Miccon]

After doing some literatures study, i found that nomarally pressure drop for wet packed is higher than dry packed pressure drop.One from famous chemical handbook-Perry state that. However, when i study a book name distilatiaon design by Kister. The autor said that dry packed bed pressure drop is higher than wet packed pressure drop (pages 477 from distillation design). It make me really confuse. Thus, in random packing column, dry packed pressure drop higher than wet packed pressure drop? Why??
Hope can get someone reply me soon.
Thank you..

Edited by Miccon, 06 October 2009 - 07:31 AM.

[katmar]

I have not checked your references, but I suspect that the reason for the apparent contradiction is that the different authors are using the same words to describe different situations.

When Perry (and others) say that the wet packing pressure drop is higher than the dry packing pressure drop what they mean is that if you had an existing packed column and you measured the pressure drop for a given gas flow, you would get a lower pressure drop if the packing was bone dry than if you had first flooded the column with liquid and then drained it, leaving the packing wet. And of course if you have liquid flowing down at the same time as the gas flows up, the pressure drop will increase as the liquid flow increases.

On the other hand, I suspect that Kister is talking about the way the column was originally packed. If the packing is added to the column with everything dry (i.e. you just open the bags of packing and dump the contents into the empty column) the packing pieces pack more tightly together than if the column is first filled with water (or other liquid) and then the packing is added. This second method of filling the column with packing is called "wet packing" and because the individual pieces fall more slowly into place because of the bouyancy in the liquid they are not so tightly packed. The "dry packing" method with the more tightly packed bed results in a higher pressure drop, because there is more packing per unit volume. Also, the dry packing method can result in ceramic packing fracturing and making smaller pieces which raise the packing density and metal or plastic packings can be deformed.

So the difference is that Perry is using "dry packing" to describe the condition of the packing, while Kister used "dry packing" to describe the way the column was packed.

[siretb]

I fully agree with Katmar.