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flyhigher87

Pulsing Still

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Hey guys hope all is well.

So I'm in the middle of a vodka run and my distillate is pulsing.  By this I mean, the distillate will completely stop for like 10-15 seconds and then all of a sudden open the flood gates for like 5 seconds.  This has only happened once before on a previous run.  Normally my still is pretty steady stream a little variation in flow rate but nothing this drastic.  My still is a small 50 gallon it has four plates and a large random packed section with copper scrubbers and raschig rings.  I am running a dephlag with little trickle of water through it to maintain 95% abv.  

So Why is my distillate flow rate so crazy today??

 

thank you.

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Sounds like a back pressure issue. Did this just start ot has it always done that?

Does it only do it when you're running at 95%if you lower it to 90 do you still get the pulsation?

Might be useful to share a pic of the still.

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Is the spirit outlet open to the air or submerged as in a parrot?

If open then it is possible for cold air to get sucked into the condenser if the still slows for a moment.

If the condenser is short the cold air gets into the still and cools the vapor in there causing a slight negative pressure which sucks more cold air in. Then as that air heats up it rushes out again, when the air has finished expanding the output stops for a moment and more cold air gets drawn in. Huffing.

If that is what is happening then a submerged outlet will stop it.

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Google "Parrot Surge Breaker".  Most of the vendors here offer them. It will reduce/eliminate the backpressure/surge cycle. 

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So I have a parrot but before the parrot it has holes on the side below the condenser, so it is essentially open to the air.  And @Silk City Distillers You may be spot on about the cold condenser water, I converted an old Air conditioner into our water chiller and the water will get down to almost 0 C, I'm going to install a temp probe and controller soon, but I'll be sure to watch the condenser water temperature.

Thanks for the replies guys

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So I’m not sure anyone is following this or cares. But if you are and you do I fixed the problem. I was reading a home distillers forum and saw someone talking about boiling chips. So I threw in some ceramic rings and problem solved. So I guess the gas had no nucleation point and the liquid was super heating and then flash boiling. 

 

So this worked but I never in a million years would have thought that a still with an agitator would have needed boiling chips. 

Anyone else need them in a commercial still?

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I get this a little bit when I’m running to much heat to the kettle and also real cold water to the condenser.  

 

who made your still. Do they have any recommended fixes?

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Sorry i posted my last comment before i saw your response.   Boiling chips.. Hm i agree never would have thought about needing them.  Are you thinking that it was boiling then stoping then boiling then stoping?  Causing the pulsing? 

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@whiskeytango absolutely.  Now that I have the "boiling chips" in the still it still pulses a little but not as bad.  And I can actually here the rings rattling in the still(boiling) and then go dead silent(not boiling).

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If you use a standard parrot, make sure it's vented.  This stops it immediately.

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I have been thinking about the physics of why a still would pulse. Interest for physics nerds only.

The boiling  chips comment gave me an idea, the boiling appears to stop and start.

A plausible explanation is when the pot boiling vigorously there is a very slight increase in pressure in the still because the rapidly evolving vapour

This slight pressure increase will increase the boiling point of the still contents and boiling stops, (but temperature now increases because no heat lost as Latent)

Now not boiling or producing vapour and pressure drops, boiling point drops and the contents will start boiling again, even more vigorously because the liquid is now slightly superheated.

Pressure builds again and boiling stops........................etc.....

Someone mentioned very cold condenser water could cause pulsing. The above theory still applies, the really cold water creates a slight negative pressure allowing rapid boiling then pressure builds .... etc......

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From one physics nerd to another....

I believe you are on the right track with your interpretation that it is the boiling that stops and starts.  I have seen a similar phenomenon in continuous columns that have significantly oversized external reboilers.  These get into a similar cycle of pulsing and the accepted interpretation was always that boiling was stopping and starting.

With the very oversized reboiler, heat can be passed through the tubes into the liquid so fast that a vapor blanket is formed on the inside of the tubes.  The heat transfer through vapor is much slower than through liquid and suddenly the heat transfer stops.  The bouyancy of the vapor gradually causes the vapor to rise out of the reboiler and colder liquid flows in from the bottom of the column.  Heat transfer picks up because of the higher thermal conductivity of the liquid and the cycle repeats.

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makes sense pete and meerkat , so with this in mind what would you guys say the optimum temp for the preheated feed to be when entering a continuous column , not factoring in the pressure change from the feed line to atmospheric in the column . 

tim

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On the last continuous I ran we fed bourbon mash beer to the column at 130F. The purpose of heating the feed is mainly to ensure all alcohol is flashed out before the mash reaches the base of the column

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@Hudson bay distillers If there is such a thing as a universal optimum temperature for the feed to a continuous column then I would say it is the boiling point corresponding to the composition of the feed.  If it is colder than this it causes internal reflux in the bottom part of the column which does not do much towards providing a high strength top product.  On the other hand if it is hotter than the boiling point you will find that you need to provide a higher reflux ratio from the condenser which increases the water requirements on the condenser but saves a bit of heat on the boiler.

But I have seen feed temperatures from substantially below the boiling point all the way through to feeding the column with 100% vapour.  If the feed is not a liquid at its boiling point then  a change in diameter of the column at the feed point may be required.

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thanks captn and meerkat that confirms what i was thinking . i would think that a feed temp and pressure change that would flash off the alcohol vapors but keep heavier components at a liquid would be optimal temp . ...sorryfor getting off topic

tim  

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From some of the design threads around the interwebs, running the feed on the bleeding edge of boiling is problematic as you risk boiling in the feed lines from the last heat exchanger to the input, causing all sorts of sputtering and inconsistent feed rates.

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true but upping  the pressure in the feed line drops the boiling point  keeping  it at a stable nonboiling point while its in the feed line  , dropping it back to atmospheric pressure when released into the column drops the boiling point causing it to boil and vaporise  instantly  without taxing any energy from the column . 

tim 

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I'm currently wrastling with this same issue as the O.P.. 50 gallon pot, steam heated, 6" diameter column still has a three foot packed section below six bubble plates. It will chug along all happy (but very slowly - about 1.25 gallons an hour) for about 5 hours or so and them out of no where start a massive surge cycle. Parrot has surge breaker holes, I also read about the flash boiling a while ago so I cut up several sections of copper pipe and put them in the pot. Didn't help. I have been somewhat successful by turning down the dephlag and running real low heat with the thinking that my surging was coming from too much condensate return 'chocking' the packed section and creating a sort of liquid blockage for the rising vapors. My packed section is pretty fine copper mesh (used to keep mice and rats from chewing through holes). No sure if this is the issue or not but this is driving me fairly crazy as every time it surges I have to crank up my dephlag, turn down my heat, and re-equalize my column. 

Any thoughts?

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