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Dephlegmator Vs. Reflux Condenser

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I see a lot of people referring to their reflux condensers as dephlegmators, which is inaccurate considering the way they're using them.

While the dephlegmators provide reflux, not all reflux condensers are dephlegmators. Dephlegmators use warmer/hot water (and generally more surface area) to selectively condense higher weight compounds, and causing more specific separation of components. While a typical reflux condenser does allow you to control the reflux ratio and keep your plates/packing filled, it isn't as finely tuned.

It's why a lot of German stills use hot condenser water to feed their dephlegmators, rather than a separate cold water feed.

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Really? Can you cite literature on this? Somewhat surprised, because based on my very limited understanding of the scientific principals involved, you can not selective condense substances by varying the cooling temperature. No different from looking at it from the opposite phase change direction, you can not selectively evaporate them by varying heating temperatures. If you could, you wouldn't need additional plates or distillation stages to increase purity. Isn't the chemical make up of the condensate always be based on the chemical make up of the vapor that preceded condensation?

I would think the reason why warm water condensers have larger surface areas, is because they are less efficient than a condenser designed for cooler temps, in some cases significantly so.

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Found an interesting PDF that seems to go into some of the detail around this topic (from an industrial perspective).



www.nt.ntnu.no/users/skoge/.../Trial%20_lecture_presentation.ppt



Looks like in the partial condenser, there is some amount of additional mass transfer/rectification taking place (vapor moving upward against condensate moving downward).



Pretty neat.

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Good question. I can't cite any literature, as I'm relying on how it was explained to me by a German engineer. You might be right regarding a single phase change of a vapor back to a liquid. However a real world condenser is much more dynamic with the vapor stream and condensate interacting over a range of surface areas and temperatures. It doesn't seem illogical to me that you could have two different condensers that provide the same amount of latent cooling but could have different reflux compositions. For instance, a lower volume of reflux at a cooler temperature compared to a higher volume of reflux at a higher temperature. A different reflux composition would mean a different vapor composition going to the product condenser.

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I missed your last reply before responding. Glad you found some info. Would you mind pasting the full link? That shortened one isn't working for me.

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Here is another one:

http://www.chemicalprocessing.com/articles/2005/614/

Question is, how important is mass transfer within the dephlegmator? Based on the first piece it looks like the additional rectification is minimal, likely less than adding an additional plate. Since we're not selectively removing components of the vapor stream, it wouldn't really be any different from enabling an additional plate (if you have that capability) or slightly increasing reflux ratio (by reducing the dephlegmator temperature) to yield the same output abv.

I still think the most effective approach is to separate the dephlegmator and product condenser temperature controls and run them independently. Gives you additional flexibility that you wouldn't otherwise have, and is likely more energy efficient.

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