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| coppercone2:
i found usually even if you leave something out in a open dish you get a decent amount of purification but it depends on how close the boiling points are. its the effect of vapor pressure/ake of stuff at the surface. the coefficents are different across temperature though I think. i think the effect of backflow purification is more related to vapor/liquid infusion/solubility then condensation, you actually don't want a temperature gradient on your fractioning column. This is why you want very high surface area in a fractioning column. Good ones are actually vacuum insulated and silvered. I think this is why a spinning band column works, because you are increasing gas pressure on the film surface, but I need to study that more. I think if the gas molecule has velocity from the spin drag then you have more diffusion of molecules into the fluid film with bigger momentum. |
| IanB:
--- Quote from: coppercone2 on November 17, 2018, 09:16:17 pm ---i think the effect of backflow purification is more related to vapor/liquid infusion/solubility then condensation, you actually don't want a temperature gradient on your fractioning column. This is why you want very high surface area in a fractioning column. Good ones are actually vacuum insulated and silvered. I think this is why a spinning band column works, because you are increasing gas pressure on the film surface, but I need to study that more. I think if the gas molecule has velocity from the spin drag then you have more diffusion of molecules into the fluid film with bigger momentum. --- End quote --- FYI, I am a chemical engineer and I know quite a lot about how distillation works. We ChemE's design and operate distillation columns for a living. A temperature gradient is exactly the thing that makes a fractionating column work. The temperature gradient up the column is the driving force that allows the separation to be achieved. Even though the column may be insulated there is still a temperature gradient due to the varying composition of the material inside the column. Industrial scale fractionating columns work by putting heat in at the bottom in a reboiler and taking heat out at the top through a condenser. Some of the condensate is refluxed back into the column so that there is vapor flowing up the column and liquid flowing down the column. Putting heat in at a high temperature and taking heat out at a lower temperature means that thermodynamic work is being performed, and this work is used to separate the different molecules apart that are mixed together in the feed. When the different molecules got mixed together there was an increase in entropy, and to undo that mixing you have to put energy into the system to reverse the process. Laboratory scale distillation apparatus is not necessarily as efficient as industrial apparatus, but it works using the same principles. |
| coppercone2:
then why do hempel beads and other fillers like specially woven stainless steel braid that increase surface area increase theoretical plates without increasing length? They are used to the point where they become clog dangers (i.e. virguex column vs proper column)? It's known that a proper column filled with hempel beads, stainless steel shavings, etc, does a better job but is much harder to use then a simple low surface area column of the same length that is used unmonitored. I am not trying to be condescending it was my understanding of it that its diffusion into a large surface area of liquid flow. I thought it would work just the same if its isothermal so long you have flow along the interior surface area of the column and vapor crashing into it? I thought the function was much different then a condenser, with the rate of condensation being important only enough to wet the walls. I was under the impression that not having a temperature gradient in the column, only down flow, results in easier control. I believe but now am unsure of it being mainly related on path length of the gas travel in regions covered by downwards moving film and that it was primarily related to absorption into the film and that the different absorption rates of the gas mixture into the film result in purification. Do you understand it having to do with gas turbulence instead (what else does a filler in a silvered vacuum insulated column do? In that case why does it need to be wetted? I mean this for a single stage seperator not something advanced like in a petroleum plant. |
| IanB:
There are two factors at play in a fractionation column. The first factor is equilibrium between vapor and liquid, and the second factor is mass (and heat) transfer across the vapor liquid phase boundary. The simplest model of fractionation assumes that equilibrium is reached between vapor and liquid in each theoretical stage. (The reason this might be called a theoretical plate is that industrial columns commonly use actual metal plates with holes in to bring the vapor and liquid into contact.) As with electronics you have models with varying degrees of rigor, some models making simplifying assumptions to reduce the complexity. One of the assumptions commonly made is that fractionation columns can be modeled as a series of equilibrium stages. More rigorous models relax this assumption and consider transfer rate limitations. A halfway model assumes some kind of efficiency for each stage. Therefore if a column contains packing instead of plates then a certain height of packing will correspond to an equilibrium stage. And better packing achieves more efficient contact between vapor and liquid, which results in a shorter height of packing to reach equilibrium. In industry, packing vendors put a lot of effort into creating more efficient packing designs. But at the end of the day, it is equilibrium that governs what kind of ultimate separation can be achieved. Improved mass and heat transfer with better packing just allows that separation to be achieved more efficiently (i.e. in a smaller column). What it comes down to is that the limit to the theoretically achievable separation is modeled as a sequence of equilibrium stages, and each stage will have a different temperature from the ones above and below it. You get a temperature profile because the things to be separated must have different boiling points, and as you go up the column you get more of the lower boiling substances and less of the higher boiling substances. If two substances have very similar boiling points it becomes very difficult to separate them by distillation. You get bigger and bigger columns with more and more stages. |
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