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Continuous still ?

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  • Tony & Elle Ackland
    I ve been toying with the idea of a continuous still. Applicable to either drinking alcohol or for fuel. A bit of fun for a 20L hobbiest, but probably worth
    Message 1 of 4 , Feb 17, 2001
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      I've been toying with the idea of a continuous still. Applicable to either
      drinking alcohol or for fuel. A bit of fun for a 20L hobbiest, but
      probably worth the effort for those doing bigger washes. Once you knew
      what the "balanced" flowrates were for your particular setup, it wouldn't
      take too much stuffing around - simply turn on, get it up to temperature in
      5-10 minutes, then you'd be in business. Don't know if I'd get around to
      making it, but something to ponder on a Sunday afternoon.

      Here's my thoughts so far ...any suggestions/comments/questions would be
      helpful. There's a poorly drawn diagram attached to assist with the
      description.

      Basically, provide a rectifying section to attach below the usual 1.0 -
      1.5m packed reflux column (eg Nixon-Stone design.).

      It would comprise a small "boiler", and a 0.5m packed rectifying column,
      preheating the feed by passing it through the boiler. (Go have a look at
      the drawing now)

      For it to work, it would need to have about 40-50cm of packing (use
      stainless steel scrubbers), in say a 1.5 to 2 inch column. This should be
      enough room to fully strip the ethanol from the incoming wash, and end up
      with a bottoms product very low in ethanol - low enough to discard on a
      continuous basis. Just need to double-check the maths sometime, but should
      be ball-park for now.

      Probably need a bit of mesh/perforated plate or something to keep the
      packing in place.

      To preheat the incoming wash, pass it through some 3/16 or 1/4 inch tubing
      doing a few coils in the "boiler" section at the bottom. Need enough
      length (and slow enough flow rate inside it) so that the wash would be
      superheated to over 95C. Would want a fair length of tube here (5-10 feet
      ?), but without creating too much pressure restriction to cause the feed to
      not feed (haven't worked out the head required to overcome this (yet))

      Introduce the wash into/through this coil by siting the wash barrel higher
      than the feed location - gravity to do the pushing. Given that the feed
      location (top of the packing in the rectifying section) is only about 0.5m
      off the ground, this only means sitting the still on the floor, and the
      wash barrel on a bench or higher.

      After pasing through the heating zone, the tubing goes up the center of the
      packed rectifying section, and discharges onto the top of the packing in
      the bottom section.

      The "boiler" section would be as small as possible. Just enough space to
      fit a heating element and the preheating coil. Less than 1 litre ?

      Maintain the bottoms liquid level by having an overflow tube. Immerse the
      inside end under the liquid level, so that the vapour doesn't try to escape
      out it. Have a vent on the outside of it so that it doesn't try to siphon
      out all the contents.

      To avoid boiling it dry, have the simple precaution of always ensure that
      there is overflow occuring (there should be anyhow due to mass balance).
      Maybe get an element with some sort of automatic cut-out on it that
      protects against boiling dry. If you wanted, you could mount a simple
      sight-glass for the liquid level, by having a clear tube from the base of
      the boiler up the outside, open at the top - the liquid level in the tube
      would be the liquid level in the boiler

      The top of the rectifying section would just screw into the bottom of your
      regular column. If making it from scratch, maybe just do in one single
      column, say 1.75m+ tall.

      Using the bottom element to do the preheating takes a bit of the energy,
      and changes the collection rate a little. eg usually an 1800W element
      would produce around 127 mL/minute at 95% purity from the top of the column
      (once the pot contents were up to distilling temperature). But preheating
      a continuous feed of wash at 10% purity would take about 1436W, leaving
      only 364W to make vapour from (eg 26 mL/min of vapour). For mass balance,
      you'd need to limit the wash flowrate to around 245 mL/minute, from which
      you'd collect a proportion of the 26 mL/min depending on what reflux ratio
      your column needed to make the 95% purity.

      How would you control the column ? Only a valve on the wash feed line, and
      the one at the top of the column that controls the reflux ratio/collection
      rate. Might even pay to have a simple rotameter in the wash feed line to
      see what the flowrate there was (its like a wee cone/float in a transparent
      bit of tube - the faster the flow the further up it gets pushed). Or just
      use a measuring cup and a stopwatch, and make sure that your collection
      rate of distillate was less than feed rate * (feed purity/distillate
      purity). The joy of the continous set up would be that while you're doing
      this balancing, any stuff-ups just get recycled back into the wash barrel -
      no loss other than your time.

      It would probably pay to have a thermometer also in the boiler section, to
      check on its temperature/purity. If the temperature started slipping down
      below 100.0C too much, it could be an indication that there was a bit of
      alcohol getting down there, possible due to too high a feed rate and/or
      insufficient preheating of the feed.

      This sorta setup would probably only be suited to sugar/water washes if
      used for drinking alcohol. This is because there is no way to collect the
      methanol, and I don't quite know if the tails are likely to exit with the
      bottoms water. So what you'd want is to minimise the heads & tails before
      you start - eg a clean & healthy wash with happy fermentation. I
      personally doubt if there's more than a couple of mL's methanol in a 20L
      wash, and similar of the fusel oils. It would be nice to get a wash
      analysed for content. It wouldn't be such an issue for fuel production,
      but you'd want to still keep the wash well filtered, so that you didn't
      block the narrow coil nor clog the packing in the rectifying section with
      bits of grain, dead yeast etc.

      Well - any comments ?

      Tony
    • Ted Palmer
      Tony, what about all the heat in the condenser? you could get 75C by using the wash to cool the condenser before it gets to the reboiler. Instead of going
      Message 2 of 4 , Feb 18, 2001
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        Tony, what about all the heat in the condenser? you could get 75C by using the wash to cool the condenser before it gets to the reboiler. Instead of going right into the reboiler how about going through an instant hot water heater with a backflow preventer. This type of setup should give you steam right out of the instant hot water heater letting you keep the reboiler set to a very low wattage or you could direct the flow of steam down into the bottom of the still like a thumper and get rid of the reboiler heater altogether.
        _____________
        Ted Palmer
        tpalmer@...
      • Ackland, Tony (CALNZAS)
        First point to note ...I ve mixed up my terms ... the new section below the feed point is actually a stripping section; the standard column above this is the
        Message 3 of 4 , Feb 18, 2001
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          First point to note ...I've mixed up my terms ... the new section below the
          feed point is actually a "stripping" section; the standard column above this
          is the "rectifying" section (not the other way round!) - thanks Mike

          Tim,

          I'd considered some sort of bleed-off for the other fractions, but the
          sticking point was knowing what sort of flowrate you'd get of each of them.
          e.g. off a 20L wash, I'd be surprised if there were more than a few mL of
          the tails present (in concentrated form). So, even if they were being
          carried in a "mostly ethanol" stream at the different heights, you'd still
          only want to limit their collection to a VERY slow drip. But I couldn't see
          how to do this accurately. Then I got to thinking, that if the total volume
          of wash was still say less than 100-200L (plenty for personal consumption
          !), then there would still only be a small total amount of tails, and it
          might be likely that they could either be fully retained within the column
          (at their own level), or likely to find their way out with the continuously
          discarded bottoms. I'm figuring that even as a "continuous" still, it is
          limited to batch running, and doesn't have quite the same concerns as
          something designed to run continuously for months at a time ? Its hard to
          know, given we don't know the relevant concentrations of them in the
          incoming wash. But what you suggest is an easy enough modification to make,
          should my guessing be wrong.

          Ted,

          Good thinking... my plan was to try and keep it as simple as possible,
          without using anything fancy (not even a pump) or extra - see if I could do
          it as a simple add-on to a normal column...but you got me thinking more
          about trying to use the condenser, rather than the boiler.

          I originally had problems with seeing how to pass the wash through the
          condenser coil (as my present one has quite a large pressure drop - doubtful
          that a simple gravity feed system would push through it). But - how about a
          "cold finger" design ? It doesn't have to be perfect - just as long as the
          feed does a fair chunk of its heating up there, it is freeing up the boiler
          to make the vapour. And keep the water cooled coil above it, for backup in
          times of low wash feedrate. The coldfinger could be a large diameter, with
          little pressure drop. The way I have my condenser at present, this could
          easily be added as a separate unit between the condenser and the metering
          valve - no modifications required. The only downside of not perfectly
          heating (or superheating) the feed is that it will use a bit more of the
          column to heat up - so get a small dead zone not doing much below the feed
          point, other than transferring heat - no dramas - just make the stripping
          section a little bit longer.

          I've included a second diagram - not to scale, but to show the layout

          By taking the coil out of the boiler, the boiler could be made even smaller.
          The new problem that this introduces is that the wash barrel now has to be
          higher than the top of everything - probably a couple of meters up. Need
          someway of getting it on top of a ladder, or using a header tank (and pump,
          damm !)

          I'll do the maths this evening and see how using the condenser for some
          preheating changes the flowrates etc.

          thanks for the input.

          Tony
        • Ted Palmer
          I feel that to get any real return from such a setup you have to be heating the wash to the temperature of the steam that is condensing, in this case the
          Message 4 of 4 , Feb 18, 2001
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            I feel that to get any real return from such a setup you have to be heating the wash to the temperature of the steam that is condensing, in this case the ethanol. That means a large surface area(best) and/or long contact time with the steam(not good for this usage). I don't think a "cold finger" will work. Do the math using a larger id tubing for pressure drop and flow rate. You'll probably find that 20 feet of 1/2" refrigeration copper is the best solution, which means a redesign of the condenser to accommodate such a large coil. Also solves the low feedrate problem.
            What to do when you run out of wash? just put water in the feed tank to keep the condenser cold till the run is done.
            A pump is the only way to go. You'll need a preheating system to get up to speed and a pump with a few valves to recirculate the wash from the thumper section back into the instant hot water heater is an easy solution. The pump can then be used to move the feed into the system with a few valve turns. My grandfather abandoned a gravity system after the still kept running dry when the pressure dropped from having less weight of wash to push down with.
            So why even go through with all this? 30% energy savings is nice but the amount of time that you can save is worth even more!
            _____________
            Ted Palmer
            tpalmer@...
            Ted,

            Good thinking... my plan was to try and keep it as simple as possible,
            without using anything fancy (not even a pump) or extra - see if I could do
            it as a simple add-on to a normal column...but you got me thinking more
            about trying to use the condenser, rather than the boiler. 

            I originally had problems with seeing how to pass the wash through the
            condenser coil (as my present one has quite a large pressure drop - doubtful
            that a simple gravity feed system would push through it).  But - how about a
            "cold finger" design ?  It doesn't have to be perfect - just as long as the
            feed does a fair chunk of its heating up there, it is freeing up the boiler
            to make the vapour.  And keep the water cooled coil above it, for backup in
            times of low wash feedrate.  The coldfinger could be a large diameter, with
            little pressure drop.  The way I have my condenser at present, this could
            easily be added as a separate unit between the condenser and the metering
            valve - no modifications required.  The only downside of not perfectly
            heating (or superheating) the feed is that it will use a bit more of the
            column to heat up - so get a small dead zone not doing much below the feed
            point, other than transferring heat - no dramas - just make the stripping
            section a little bit longer.

            I've included a second diagram - not to scale, but to show the layout

            By taking the coil out of the boiler, the boiler could be made even smaller.
            The new problem that this introduces is that the wash barrel now has to be
            higher than the top of everything - probably a couple of meters up.  Need
            someway of getting it on top of a ladder, or using a header tank (and pump,
            damm !)


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