As a background I've got a boka slant plate 2 1/2" stainless still with a fairly tall column with a lot of copper packing and a condenser that works well enough to not let any vapour out. For all intents and purposes it appears to work very efficiently. I use gas and it has a fast start-up and no problems maintaining heat. The boiler is an old autoclave which I think was about 40 litres. I've been making 25 litre washes so it has a lot of breathing space. Product I am looking for is high purity neutral. So that's the background, now I have a couple of questions about operation which I can't find any concrete information on.
Firstly, how does one determine the optimum take-off rate? While there are probably equations that would answer this, I reckon calculating this would be very difficult outside of controlled environmental conditions (like a lab). I have done enough research to know that the 'rule of thumb' is to run the take-off of the hearts at a thin, barely broken stream. I also know that there is much talk about larger diameter columns being capable of producing product more efficiently and faster. The 2 1/2 inch column is likely moving into this category (of being able to shorten run times), so how do I work out the optimum? Do I run it very slow and work out the maximum purity (currently at about 93% when run at a dribble), then run it faster until the purity declines and wind it back? Or can it be wound up until the temperature rises, then wound back? Basically I don't want to be sitting there for hours with a dribble of a take-off if it is capable of running twice as efficiently as I'm operating it. I am aware that with smaller columns and/or less packing, you have to keep it slow to get higher purity neutral. If the dribble is the optimum I'm of course happy to stick with that.
Secondly, the time to get to equilibrium in the column. The Compleat Distiller suggests 2 hours to let the column equilibrate. I see people in the forum mentioning such things as 'at least one hour to an hour and a half'. How does column size and packing height measure into this? Are there longer or shorter times for larger or smaller columns/packing heights? Is there some measurement that helps to determine equilibrium?
Thirdly, going from foreshots to heads I'm not seeing any temperature differential and the smell is a little hard to differentiate. I'm taking maybe 50ml of foreshots, then once again as per The Compleat Distiller, taking 1cup of heads (because once again there isn't really any telltale with separating them). Trying that today, the first take of hearts seems pretty good - I don't detect anything untoward in the mix. I'm not trying to squeeze less than 1cup of heads out of the equation - quite the opposite, I'll take more if there is a better way to differentiate. In contrast, with this still design, tails have all kinds of telltale signs - the ethanol purity drops, the smell comes into effect, and the temperature begins to rise - temperature is the last thing to change at the tails in my rig.
Whilst I can probably do comparitive tests to work some of this stuff out, I know that much of that has already been nutted out (as have the still designs). I do like to work things out for myself with the help of some research, but on these topics I'm not getting definitive results from searching. I also severely lack time so I can't be tinkering with too many tests for the sake of reinventing the wheel. Even given the answers, there's still a hell of a lot that is going to simply come down to experience, really taking notice of what's happening, and most of all recognising subtleties that are going to affect the brew for good or for bad (and in the process, developing the taste and smell for it). So I would really appreciate any experience on these questions. Thanks in advance.
A few questions - reflux still
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Re: A few questions - reflux still
ebswift...I'll try and at least shed some general light on things, maybe other mentors can pitch in and fill any holes. When you stabilize/equalize the column...the lower boiling point volitiles will move to the top and the higher temp ones stratify down through the column such that they are stacked. This takes some time to happen. I think using a longer amount of time here isn't going to hurt anything (particularly if you are after neutral). You want to give it plenty of time to separate and form a "pocket" if you will at the top of the lowest temp gradient.
Now, the trick in drawing it off..is to draw it off at such a speed that you can "maintain" that temp gradient in the column with it's sweet spot at the top. The best description/visual I ever heard for that was it's like a guy walking on a tight rope with a pole that has a sack of potatoes on each side. If you suddenly take away a lot out of one side or the other...he can't react fast enough to maintain balance and will have to shift or could fall. But, if you just take one or two potatoes at a time...he can make the small adjustments along the way such as it does not upset his balance. So, the ideal take off rate is one such that it maintains the purity level and temp. And this is inherently going to be "slow"(er) if you are after the highest purity through the run. What helps here is height. If your column is higher...(up to a point) It gives you a bit more room to draw things off the top faster...and not destabilize the column. Take that same stack gradient and put it in a much shorter tube...and you see how you have to compress it to get it to fit. The slightest change can throw it out of whack (another reason to insulate well). So, for take off speed, making sure your boka's packed column is proper height/length is important. Diameter is more about "volume"...for a given inch of column height..a wider diameter has more volume. So, this can improve your take as well, but not for the same reasons.
Next up would be do you have the volume of alc in the pot necessary to support and feed such a column? I mean, if you put like 5 gals of 10% wash in...by the time you get the vapor stacked in a wide/tall column...you are about done. So, you'll find you can stabilize it up front, but the min you start pulling anything off it starts to destabilize. Using low-wines will really help in this regard.
If you draw it off too fast...you'll collapse that temp gradient, and pull hotter vapor higher up into the column, ie., the temp goes up...(ie., smearing your vapor as well). If that happens, you just close the valve (on a boka) and let it re-stack itself under full reflux. Ideally, you have to balance how much you are taking off vs how much you are refluxing..such that it maintains everything. A common mistake here is in not using "enough" heat and cooling to produce enough reflux. More vapor that goes up...the more reflux action coming back down you gonna have. Too much heat, and you'll get smeared vapor. But, the trick to this kind of compound reflux is to finding a balance..and it's usually a bit more heat than you think.
As to operation...you've got it. If the temp starts to rise...you can a) reduce the output so there is more reflux heading back down to restabilize/stack the column or b) increase the reflux ratio by turning the heat up slightly (watch your condenser at top and make sure you've got enough cooling to knock it all down). Sounds counter intuitive but you'll just have to experiment with it to see how it works. IF neither one of those works...to bring it back in line...then shut the output completely and give it time to restack/stabilize inside the column...and start drawing it back off....only slower so it can maintain balance. The other thing working against you here is as you extract the alc from the pot...the vapor temp going up the bottom is going to be continually rising...pushing more heat. It may require some fiddling during the run in other words...particularly towards the end of a run where...it starts not wanting to hold temp. When it does that...don't let the temp rise. Shut the output, let it squeeze/bring the temp back down. Then drip off slower. Keep doing that till you can't hold the temp anymore.
So, long as your seeing the temp stable at the top...you are running it right speed. If the purity is not what you want...then close the valve and let it equalize itself a bit more to see if the temp drops. If the temp doesn't drop...increase the heat slightly to force more reflux (again watch your condenser to make sure it's taking care of everything). Once you get it back down to temp at highest purity...open the take off really slow (this is why you have a needle valve). You "can" increase speed..up and too you see the temp start to bump up. Then..back it back down. If you are too late and it just keeps ticking up...then close the valve..but remember the previous rate. Once you get it stable again, open the valve to the rate it was stable and leave it. It is what it is at that point in so far as output rate.
When you can no longer maintain that even after equalizing..etc..you are at tails, you can flush them out.
Now, the trick in drawing it off..is to draw it off at such a speed that you can "maintain" that temp gradient in the column with it's sweet spot at the top. The best description/visual I ever heard for that was it's like a guy walking on a tight rope with a pole that has a sack of potatoes on each side. If you suddenly take away a lot out of one side or the other...he can't react fast enough to maintain balance and will have to shift or could fall. But, if you just take one or two potatoes at a time...he can make the small adjustments along the way such as it does not upset his balance. So, the ideal take off rate is one such that it maintains the purity level and temp. And this is inherently going to be "slow"(er) if you are after the highest purity through the run. What helps here is height. If your column is higher...(up to a point) It gives you a bit more room to draw things off the top faster...and not destabilize the column. Take that same stack gradient and put it in a much shorter tube...and you see how you have to compress it to get it to fit. The slightest change can throw it out of whack (another reason to insulate well). So, for take off speed, making sure your boka's packed column is proper height/length is important. Diameter is more about "volume"...for a given inch of column height..a wider diameter has more volume. So, this can improve your take as well, but not for the same reasons.
Next up would be do you have the volume of alc in the pot necessary to support and feed such a column? I mean, if you put like 5 gals of 10% wash in...by the time you get the vapor stacked in a wide/tall column...you are about done. So, you'll find you can stabilize it up front, but the min you start pulling anything off it starts to destabilize. Using low-wines will really help in this regard.
If you draw it off too fast...you'll collapse that temp gradient, and pull hotter vapor higher up into the column, ie., the temp goes up...(ie., smearing your vapor as well). If that happens, you just close the valve (on a boka) and let it re-stack itself under full reflux. Ideally, you have to balance how much you are taking off vs how much you are refluxing..such that it maintains everything. A common mistake here is in not using "enough" heat and cooling to produce enough reflux. More vapor that goes up...the more reflux action coming back down you gonna have. Too much heat, and you'll get smeared vapor. But, the trick to this kind of compound reflux is to finding a balance..and it's usually a bit more heat than you think.
As to operation...you've got it. If the temp starts to rise...you can a) reduce the output so there is more reflux heading back down to restabilize/stack the column or b) increase the reflux ratio by turning the heat up slightly (watch your condenser at top and make sure you've got enough cooling to knock it all down). Sounds counter intuitive but you'll just have to experiment with it to see how it works. IF neither one of those works...to bring it back in line...then shut the output completely and give it time to restack/stabilize inside the column...and start drawing it back off....only slower so it can maintain balance. The other thing working against you here is as you extract the alc from the pot...the vapor temp going up the bottom is going to be continually rising...pushing more heat. It may require some fiddling during the run in other words...particularly towards the end of a run where...it starts not wanting to hold temp. When it does that...don't let the temp rise. Shut the output, let it squeeze/bring the temp back down. Then drip off slower. Keep doing that till you can't hold the temp anymore.
So, long as your seeing the temp stable at the top...you are running it right speed. If the purity is not what you want...then close the valve and let it equalize itself a bit more to see if the temp drops. If the temp doesn't drop...increase the heat slightly to force more reflux (again watch your condenser to make sure it's taking care of everything). Once you get it back down to temp at highest purity...open the take off really slow (this is why you have a needle valve). You "can" increase speed..up and too you see the temp start to bump up. Then..back it back down. If you are too late and it just keeps ticking up...then close the valve..but remember the previous rate. Once you get it stable again, open the valve to the rate it was stable and leave it. It is what it is at that point in so far as output rate.
When you can no longer maintain that even after equalizing..etc..you are at tails, you can flush them out.
Re: A few questions - reflux still
@Usge, this turned out to be awesome advice. I went from drawing off 2.8L in the last run to drawing off 4.7L at ~93% out of 25L of wash. Silly me originally thought once the temp rose and alc went down, the show was over, it was all tails. Little did I know you could shut off the tap for a minute or two to bring the temp back down and the alc back up! I kept the temp fairly high throughout the run and it has the same smell as the last run that was done at a lower temp. I'm starting to understand my build and why the boka dual slant is so great a design for simplicity of build.
It seems I was drawing a little bit fast which was drawing warmer vapour up. Though this isn't a really big deal whilst temp and alc content is being monitored, you can catch the vapour pretty quick. I see some talk around the traps about a temperature gauge lower in the column, perhaps this is the best way to 'tune' the takeoff by way of stabilising the vapour pocket? Having it in the right spot would be a bit of a trick though, I'm not sure how much of the top of the column sits at 78 degrees celcius during normal takeoff.
Interestingly, I really screwed up the wash on this run. I got the bentonite wrong and ended up with a wash that was almost uniformly cloudy with some sludge at the bottom. Time didn't permit me to try and clear it more. I think it goes to show the efficiency of the boka when you have a good wide, tall column - it makes neutral like it should be - neutral, and all the crap you don't want stays at the bottom of the vapour pocket.
It seems I was drawing a little bit fast which was drawing warmer vapour up. Though this isn't a really big deal whilst temp and alc content is being monitored, you can catch the vapour pretty quick. I see some talk around the traps about a temperature gauge lower in the column, perhaps this is the best way to 'tune' the takeoff by way of stabilising the vapour pocket? Having it in the right spot would be a bit of a trick though, I'm not sure how much of the top of the column sits at 78 degrees celcius during normal takeoff.
Interestingly, I really screwed up the wash on this run. I got the bentonite wrong and ended up with a wash that was almost uniformly cloudy with some sludge at the bottom. Time didn't permit me to try and clear it more. I think it goes to show the efficiency of the boka when you have a good wide, tall column - it makes neutral like it should be - neutral, and all the crap you don't want stays at the bottom of the vapour pocket.
Re: A few questions - reflux still
Glad it worked out eb. 