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13290Re: [Distillers] Question for Mike Nixon

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  • Mike Nixon
    Aug 4, 2003
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      timmur64 wrote:
      Subject: [Distillers] Question for Mike Nixon

      Mike,    Hi Tim
      I have taken the liberty of putting in the metric values for your measurements ... just makes it a bit easier for me to get a picture in my mind as it's what I'm used to ... and advancing senility needs all the crutches it can get  :-)

      In "The Compleat Distiller" chapter 8 there's a section titled "Supercooling and Column Stability". If I understand it
      correctly, you seem to be saying that you don't need to worry about "excessive" cooling water flow to the condenser. If the
      condenser happens to cool the distllate, the column can accomdate this via a small buffer zone right at the top of the packing. My still is as outlined in the book "THE DISTILLATION OF ALCOHOL A Professional Guide for Amateur Distillers" except that the column is 1.875" [4.8cm] and the condenser is made of 21' [6.4m] 1/4" [6.4mm] copper tubing.
      Spot on!  Just think of it as the distillate falling onto a hotplate which has the full power of the boiler heating it.  Sizzle!
      The column has done all it can to separate by internal reflux on the packing, and has passed on the vapor that reaches the top to be recycled repeatedly in the head and so achieve greater separation.

      Yesterday was the first run. While the run went well, there was one anomoly. After the boil started, the temp shot up to 178 [81C] then went down over the next 20 minutes and settled at 168 [75.6C]. I kept waiting for it to drop lower, but it never did (I only waited about 45 minutes after the initial temp drop). Since the temp stopped at 168 [75.6C], I figured that the column had equalibriated, even though it didn't seem to take very long. The first stuff coming off (1 drop/sec) smelled like lighter fluid and was discarded (125 ml). I kept expecting the temp to go up at some point, but it never did. The only time the temp ever went up was when I cranked up the output to find the upper limit. It went to 170 [76.7C]and I turned it back down so
      that I was collecting 7 ml/min. This kept the temp from climbing. I'm wondering if the condenser was cooling the distillate so much that the temp was lower than expected (because of the "buffering" area at the top of the column?). The condenser has tons of extra capacity as it only gets warm about 1/4-1/3 of its length (depending on how much heat is being applied). What do you make of this? Thanks.
      81C is a bit on the low side, but in the the ballpark area for what you might expect for the vapor coming out the top of the column at the beginning, reflecting the separation that the column on its own can achieve.  Ditto with all the other temperatures once it has settled down.  This 'low temperature' is probably due to your thermometer, either its accuracy or its placement.  It's often surprising how the temperature reading can vary if you move the temp probe around even a few millimeters, or even between two different runs.  Just comes down to how the vapor flow changes from time to time.  There is always turbulence, and the vapor flow can settle into first one configuration and then suddenly switch to another.  A bit like balancing a stick on the end of your finger ... you can get it to stay upright, but the top will wobble around a bit, even if you are an expert juggler in the Cirque du Soleil.  Used to worry me too before the penny dropped and I realised what was happening and checked with two thermometers in slightly different places.  They both showed slightly different temperatures (taking their calibration into account), and they both showed small erratic jumps from time to time.  Taking 75.7C as your steady state before you cranked up the power, the mix you had in the head area was probably actually around 78C (the bp of both pure eth and the 96% azeotrope are within a couple of decimal places off 78C, so your thermometer was probably reading a consistent couple or three degrees low.  This would make your actual first temp 83-84C at the beginning, which is what you might expect with a well-built rig like yours.  Good news is ... don't bother changing the thermometer ... just use it as a guide to when you reach steady states and for when it starts to shift from them :-)
      With 6.4m of tubing in your condenser you do indeed have heaps of capacity!  Way to go!!  Far better to have excess capacity for all occasions than too little.  The vapor only rises as far as it needs to up that coil before it is all condensed, so the top part doesn't contribute any more cooling.  It is only when you increase the power so the vapor is pushed up faster and in greater volume that it rises higher up that coil and the condensate is in contact longer as it trickles back down.  So you get the apparent anomaly that the more power you pump into the boiler, the colder the condensate is when it finally gets back to the packing!  Mind you, this does not mean very much colder, as the condensate is also being subjected to heating from the hot vapor rising past it.  So on balance it will be cooler at high power (physical contact with the metal of the coil beats vapor every time with heat transfer), but not as much as you might expect.
      So no worries!  Sounds like you have a rig that performs as well as it looks (very pretty!).  Rising just one degree when you pushed the power up shows that the mix you were getting was slightly less "pure", but by only a small amount.  In fact, it sounds like you not only have a very fine still Tim, but a very stable one too  :-)
      Nice job!
      Mike N
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