Correct. the surface area of the packing itself (scuffed or shiny) serves to hold the liquid layer to form evenly and stay stable. A scuffed surface may indeed help to set and maintain that liquid layer under some conditions. It is the surface area of the outside of that liquid layer which is the important measure, but only if that is considered along with the available void-space which is also necessary for fractionation.Bagasso wrote:I thought the point to be taken away from this thread was that surface area isn't an indicator of how well a packing material performs.
Will scuffed marbles work better than smooth? Could be, but the point is that smooth seems to have worked surprisingly well despite their low surface area, when compared to SPP or SS scrubbies.
My layman's guess would be that it has to do with the materials mass and thermal conductivity.
Working example:
Finely divided charcoal has a HUGE surface area/weight ratio. Think of it as infinitely small "marbles".
And it is useless as a fractionator packing material because the void-space is so tiny between all grains that capillary and viscosity effects cause a flow log-jam for both liquid AND vapour, because the liquid coating applied to the grains effectively closes all available void-spaces.
Exactly as MichiganCornhusker's sketches imply.
The bigger the marbles, the lower the ratio of surface area to void-space and the less you need worry about surface tension and viscosity effects clogging up that "breathing room".
Rad's contributions:
Rad's marbles experiments are beginning to give us a new, practical and comparative baseline for marbles and then onwards to all packing material studies, in HD-er terms too. He's already said that he intends to use identical conditions and compare scuffed marbles vs his shiny ones. If he sees any differences (and my bet is that he won't....) which become pronounced towards the "choking point" of operating range, the science-inclined amongst us will deduce that surface tension effects are an important consideration, and possibly viscosity too. I suspect that he is well away from closed voids (where such effects would be crucial).
Rad: will you give us detail on marble diameter and how many (or total Mass and individual mass) and the exact length and diameter of your column please? All of the previous work I've done along these lines was left behind as the property of my former employers....... my own current distilling prototyping doesn't involve a fractionating column.
The material's mass and thermal conductivity?
In a continuous distillation (don't bite me just yet!), all flow rates and temperatures are constant and no heat energy at all flows in or out of the packing material once it's "up to temperature", so both of these parameters are only important during the warm-up (stabilisation) phase.
In a more conventional (domestic) pot still, there is a very gradual change in these parameters, but I'd think that the change is so slow that, compared to the huge amounts of heat and mass transfer through the fractionator made between liquid and vapour across their contact surface area, it is unlikely to be too important.
It is the thermal and mass transfer across the contact surface area between the vapour and the liquid which determines all heat and mass exchange within the fractionator. The packing material will merely follow the gradual temperature changes which occur as the pot distillation progresses. Metal balls would change faster than glass balls, but once up to temperature, their contained heat is going nowhere, fast......
Reminds me of school! I am with your line of thinking though, I don't have any practical input and all this scientific mumbo jumbo gets nowhere until I taste the liquor coming out the pipe and compare it to what I have been brewing.
. It may well consummately astonish you to find that in fact most members don't in fact miscomprehend the metage that is "hight equivalent theoretical plates" and be further astounded to learn that most peoples stills in fact ain't crap and don't run poorly