How do I analyse for, or what do I need to consider, when comparing gravity-opposed vs gravity assisted flow in a tube containing a porous bed that partially shrinks (through partial mass loss) over a fixed time period?
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$\begingroup$ What is the function of the porous media? $\endgroup$– J. AriMar 11, 2018 at 21:10
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$\begingroup$ It's plant matter, being extracted $\endgroup$– BeerhunterMar 11, 2018 at 21:13
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$\begingroup$ How large is the volume change? $\endgroup$– J. AriMar 11, 2018 at 22:12
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$\begingroup$ Approximately 10-15% drop $\endgroup$– BeerhunterMar 11, 2018 at 23:48
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With gravity assisted flow, you have a higher potential of channel flow through the media, so sections of your plant packing won't be wetted by the solvent. Making sure you have the optimal flow pattern is one of the primary factors in packed column performance. You indicated a 10 to 15% volume drop in the plant matter, so this poses an issue that over time the solvent flow in will squish the plant matter against one end of the container; the magnitude of this problem will depend on when the extraction of the molecules you want takes place. It's also possible that you could fluidize this plant material after the volume starts to decrease. Fluidizing could be beneficial to get high surface area but you have to control it to avoid the bed compaction effect mentioned above.
The main thing to measure is the pressure drop through the column. Too low a pressure drop means you have channel flow and too high a pressure drop can indicate plant matter compaction. If you can use a clear container to hold the plant matter, you can tune your setup to keep the flow pattern you want - provided you can control inlet pressure of your solvent.
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$\begingroup$ Thanks for the answer; I left out some detail that the flow is under high pressure (>60 bar), which I hypothesise, would give compaction irrespective of the flow direction. I also rationalised that channel flow would be more likely in the upward flow, but that this is contingent on the effect of gravity. Modified Reynolds number calculation doesn't take into account flow direction. I didn't know if Sherwood number was relevant, but it's difficult to measure Scmidt number. I don't know if I'm right in thinking this pressure would negate a fluidised bed effect taking place. $\endgroup$ Mar 12, 2018 at 0:20
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$\begingroup$ 60 bar is very high, I don't think you even need to worry about the contribution of gravity to the flow unless your system is very large. If you have bed compaction, you won't have sustained fluidization. $\endgroup$– J. AriMar 12, 2018 at 0:31
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$\begingroup$ By "very large", I would assume diameter would have more of an effect. I don't think 200L is very large but towards 1000L might be. It seems you suggest compaction would be more problematic in downward flow. I read there could be more axial dispersion in upward flow systems but I have no measure to determine this. $\endgroup$ Mar 12, 2018 at 0:46
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$\begingroup$ Compaction can occur in either direction, it is a function of the liquid's energy and how it's allowed to move through the plant matter. If there isn't enough unfilled volume, the liquid will compact the bed and then take any available flow path - hence the large dP, a lot of energy was expended in compacting the bed. If you have a column-like vessel, then yes, diameter will have a large influence on the intrinsic dP of the system (meaning necessary dP to flow through the column with no plant matter). $\endgroup$– J. AriMar 12, 2018 at 1:29