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Re: Liebig Condenser. A puzzle.

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  • toddk63
    I re-visited this one again a few days ago. Here are my findings. In conventional Heat eXchangers (HX), a counter flow arrangement. is most effective.
    Message 1 of 11 , Mar 3, 2004
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      I re-visited this one again a few days ago. Here are my findings. In
      conventional Heat eXchangers (HX), a "counter flow" arrangement. is
      most effective. That is cold cooling water in, meets the the already
      cooled product out. This maximizes the Log Mean Temp Difference (LMTD)
      However in our application, with a BOKAKOB style column and
      condenser, I speculate that the distillate is a fairly constant temp
      across the HX, thus the direction of cooling water has little effect
      on LMTD. You can also minimize the temp difference of the cooling
      water in .vs. cooling water out by turning the flow way up, but this
      is wasteful.

      While testing my condenser, I made the following observations. With
      water, I measured a condenser load of around 6kW before the thing
      started spewing out the vent hole. With an ethanol run, I can't even
      make 2kW before it spews. After looking closer at the numbers, I
      discovered that the energy required to condense 1 ml of water was
      about 3 times more than 1 ml of ethanol (6:2). So I figured that the
      limiting factor of my condenser was volume condensed. It makes sense
      to me that the condensate falling back down the condenser tubes
      eventually builds up and "sheets" them with a heavy amount of liquid,
      thus reducing condensing effectivness.

      By having your cooling water enter at the bottom, you get the most
      condensing in the bottom section, where the condensate quickly exits
      the condenser and doesn't contribute to sheeting. If it where the
      other way around, a larger porportion of the condensate would be
      formed at the top and would run down the other tubes ,eventually
      building up and rendering them less effective.

      What do you think? Does this make sense? More testing required!

      Todd K. BSME P.E.

      --- In Distillers@yahoogroups.com, "jimpuchai" <puchai4@o...> wrote:
      >
      > Hello all,
      >
      > I was trying to work out why, with a Liebig condenser, we insert the
      > cold water at the low point and the waste water goes from the
      > highest point.
      > This arrangement would seem in direct variance with the accepted,
      > and clearly most efficient, method where the cold water is inserted
      > so as to meet the hottest vapour.
      >
      > It is obvious that if the cooling water was introduced at the
      > top/hot end, it would tend to trickle down the lower side of the
      > pipe and not fill it - very in-efficient. A simple piece of plumbing
      > would solve this, enabling the more efficient counter-flow
      > arrangement.
      >
      > Can anyone shine some light on this.
      >
      > If you prefer there are two pictures here
      > http://tinyurl.com/2akfd which show the ends only of the
      > (possibly) more efficient method. The short pipes are just
      > mechanical supports. You have to imagine the middle section. It is
      > just the pipes making the condenser a metre in length. I have not
      > tried this yet, but you will see that it is fitted with quick
      > release connectors so I can compare results for each direction of
      > flow. I shall try this in the near future, but can anyone suggest
      > what I may expect ?
      >
      > Cheers,
      >
      > Jim P.
    • jimpuchai
      ... findings. In ... already ... (LMTD) ... temp ... effect ... this ... With ... even ... the ... sense ... liquid, ... exits ... Hi Todd, I made my own
      Message 2 of 11 , Mar 4, 2004
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        --- In Distillers@yahoogroups.com, "toddk63" <toddk63@y...> wrote:
        > I re-visited this one again a few days ago. Here are my
        findings. In
        > conventional Heat eXchangers (HX), a "counter flow" arrangement. is
        > most effective. That is cold cooling water in, meets the the
        already
        > cooled product out. This maximizes the Log Mean Temp Difference
        (LMTD)
        > However in our application, with a BOKAKOB style column and
        > condenser, I speculate that the distillate is a fairly constant
        temp
        > across the HX, thus the direction of cooling water has little
        effect
        > on LMTD. You can also minimize the temp difference of the cooling
        > water in .vs. cooling water out by turning the flow way up, but
        this
        > is wasteful.
        >
        > While testing my condenser, I made the following observations.
        With
        > water, I measured a condenser load of around 6kW before the thing
        > started spewing out the vent hole. With an ethanol run, I can't
        even
        > make 2kW before it spews. After looking closer at the numbers, I
        > discovered that the energy required to condense 1 ml of water was
        > about 3 times more than 1 ml of ethanol (6:2). So I figured that
        the
        > limiting factor of my condenser was volume condensed. It makes
        sense
        > to me that the condensate falling back down the condenser tubes
        > eventually builds up and "sheets" them with a heavy amount of
        liquid,
        > thus reducing condensing effectivness.
        >
        > By having your cooling water enter at the bottom, you get the most
        > condensing in the bottom section, where the condensate quickly
        exits
        > the condenser and doesn't contribute to sheeting. If it where the
        > other way around, a larger porportion of the condensate would be
        > formed at the top and would run down the other tubes ,eventually
        > building up and rendering them less effective.
        >
        > What do you think? Does this make sense? More testing required!
        >
        > Todd K. BSME P.E.
        >

        Hi Todd,
        I made my own Liebig checks a few days ago. Nothing very elaborate,
        but enough to verify what my brain was rejecting. With a strip run,
        I adjusted the power input, and water feed rate to the bottom of the
        Liebig to produce a distillate output temperature of 60C. I then
        reversed the flow with suitable plumbing so that the Liebig jacket
        remained full for the test.
        There was an sustained rise of 8C in the distillate output. No
        doubt, as bubbles would start to collect at the high point, this
        would have pushed the temperature up progressively. QED for me, and
        I went back to the only sensible way, cold in at the low point.
        Some time later, it occurred to me that the descent angle of the
        Liebig may have some bearing on results! Somewhere between vertical
        and horizontal there may be a sweet spot. Any ideas? So far, I can't
        get my head around that.

        Your description of sheeting and its possible cause in the vertical
        compound type condenser makes perfect sense to me. I have to say
        that you should not hang your theory on my judgement. Experience has
        shown that they work better that way, but I never asked why.


        Many thanks for your input,

        Jim P.
      • toddk63
        Comparing distillate outlet temp is another good data point. A different approach that confirms my work. That s good! As far as setting the condenser
        Message 3 of 11 , Mar 4, 2004
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          Comparing distillate outlet temp is another good data point. A
          different approach that confirms my work. That's good! As far as
          setting the condenser off-vertical improving the effectiness. That
          makes sense to under the "sheeting" theory. On a vertical condenser,
          the condensate has only one path. Down along the other tubes. At an
          angle, some may drip off the tubes and ride the wall down.

          Todd K.

          --- In Distillers@yahoogroups.com, "jimpuchai" <puchai4@o...> wrote:

          > Hi Todd,
          > I made my own Liebig checks a few days ago. Nothing very elaborate,
          > but enough to verify what my brain was rejecting. With a strip run,
          > I adjusted the power input, and water feed rate to the bottom of the
          > Liebig to produce a distillate output temperature of 60C. I then
          > reversed the flow with suitable plumbing so that the Liebig jacket
          > remained full for the test.
          > There was an sustained rise of 8C in the distillate output. No
          > doubt, as bubbles would start to collect at the high point, this
          > would have pushed the temperature up progressively. QED for me, and
          > I went back to the only sensible way, cold in at the low point.
          > Some time later, it occurred to me that the descent angle of the
          > Liebig may have some bearing on results! Somewhere between vertical
          > and horizontal there may be a sweet spot. Any ideas? So far, I can't
          > get my head around that.
          >
          > Your description of sheeting and its possible cause in the vertical
          > compound type condenser makes perfect sense to me. I have to say
          > that you should not hang your theory on my judgement. Experience has
          > shown that they work better that way, but I never asked why.
          >
          >
          > Many thanks for your input,
          >
          > Jim P.
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