Continuous Still Design

I've finally got around to making a continuous still. Its very easy to convert your existing column - all you need is a small (1-2L) boiler - eg an old electric kettle.

This would be a great tool to use for stripping down large quantities of wash, to say 85-90% purity, for redistilling the regular way. Eg 200 L of wash at 12%, if stripped at 85% purity, would give 28 L. Redistilling this would then give you the confidence that any heads and tails have been completely removed.

Concept

The design is quite straight forward. Use your existing column as the rectifying portion. Introduce the wash above a stripping column. The purpose of the stripping portion is to ensure that there is no alcohol left in the liquid by the time its dropped down to the lower boiler. This means that there wont be a loss of alcohol out the overflow. The boiler can be quite small in volume. It has a simple overflow so that the excess water is removed from the system automatically without need of control.

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Rectifying Section

The rectifying portion of the column is just your standard reflux column - a packed pipe, with an overhead condensor & reflux control. Use either a Nixon-Stone layout or whatever your choice is (modified "gattling gun" condenser ?). As usuall, the taller this portion the greater the final purity will be - eg if using stainless steel scouring pads for packing, then 330mm = 87%, 660mm = 92%, 990mm = 94% etc. At least 1m would be required, but I'd suggest using 1.5m - this will allow a lower reflux ratio for the same purity (eg much faster offtake available). Fully insulated etc. Use a thermometer in the head of the column to monitor the vapour temperature and hence purity - adjust the reflux ratio to inc/dec as required. For a 800mm column I was getting a very consistant 84 %. Its very easy to control - first set it to a very high reflux ratio, then slowly open up the collection valve until the temperature begins to climb a little. Back off a bit, and theres your collection rate. Approx 5-7 mL/min (fast dripping) for a 1500W heat source. If the reflux ratio is kept too high for too long, you'll eventually notice it coming out via the boiler - you'll see the temperature there decrease as the water begins to contain some alcohol. So dont push your column too much - learn what sort of % you can expect for its height, and dont ask for too much more.

You could also simply convert your existing column, by tapping a feed point into it 40cm up from the bottom. It would then mean that the rectifying section was only around 80cm tall, but thats ok for just doing stripping runs.

Feed Point

The feed point only needs to be a short collared section that joins the rectifying & stripping columns, with a inlet that will introduce the wash to the centre of the column. Have the tube from the feed point loop downwards into a small vapour lock, to prevent and steam escaping out there. Open it up into a small funnel, so that its easy to drip the wash into it. You need to have the wash freely dripping, so that you can judge how fast it is flowing. If you were to plumb it direct from the fermentor to the column, you would have no idea how fast the liquid is flowing.

It would probably pay to have a simple paper filter in the feed line, to ensure that any yeast cells etc are removed from the feed, so that the column does clog up with gunge during the run.

In a commercial column, the feed would generally be preheated before it is introduced to the column. I found that that was not necessary - this set up works well without preheating the feed.


Stripping Section

Approx 3 HETP (eg 36cm of scrubber packing) should do the job to strip the feed to less than 0.5% alcohol. If not, or if you want to reduce the % even smaller, then simply make it taller. Fully insulate this portion, just like the rectifying section. 40 cm height works well - stripped all ? the alcohol out - the boiler was always at > 99.8C provided I kept the feedrate low enough (around 30-60 mL/min).

Boiler and Overflow

1-2L capacity should be suitable for the boiler. For a 38mm column, around 1500-1800 W should be suitable. A bit of this will depend on just how fast you run the feed into the column. The purpose of the boiler is to provide the steam that will strip the feed of alcohol. I used an old electric kettle from a garage sale : $1. Bent the spout flat and soldered it watertight. Made the lid have a fitting that would mate with the existing one on the bottom of my column, and stuck an overflow though the side of it. Thermometer in the top and its ready.

The overflow is necessary to discard the excess water - eg if stripping 24L of wash in 4 hours (typical hobby setup), this is a feedrate of 100 mL/min. If its 10% alcohol, then you've got 90mL/min of water overflow (and around 10 mL/min distillate collection). As to whether or not 1800W is suitable for 100 mL/min feed rate is open to argument, but given thats the energy input for the same setup but batchwise, then it should still hold for the continuous case.

Use a thermometer in the boiler headspace to watch the purity there. If the temperature starts dropping below say 99.8C , then slow the feedrate. If this cannot be sustained, then look at increasing the height of the stripping section.

The overflow requires its inlet to be below the liquid level, so that the steam doesn't exit it too, but positioned higher than the element, so that there is no chance of emptying the liquid level below the elements & burning them out. You will also require a vent to atmosphere off the overflow too, to prevent it from acting like a syphon. Mount the overflow through the side of the boiler, as the overflow level needs to be lower than the bottom of the column, so that the bottom of the column does not become fully blocked off by liquid. Because there is always a steady dribble of liquid coming out the overflow, you're always certain that the boiler is full, and not boiling dry.

Running it

The still is very easy to run. Set up your fermentor so that is sitting well above the feedpoint (eg on a ladder or a couple of boxes). Place a bucket beneath the overflow tube.

Fill the boiler with water, and turn on the power. Wait for this to start boiling, and all the column preheated. At that stage, start the condenser water flowing.

Start the feedrate slowly feeding in via the funnel. Keep the head at total reflux, until the temperature there has reduced down to 78-80 C, then slowly open it up. Slowly increase the feedrate to the column, until it causes the boiler to start dropping in temperature below 99.9C - at that point slow it down a touch. Open up the head, until its purity likewise starts to suffer - eg it rises above 80C, then just back off a touch too. Don't try and run the head too pure, or else you'll overload the column with alcohol and simply push it into the boiler from where it will be lost.

Once its balanced & running, it wont need further adjusting, unless the feedrate slows as the liquid level in the fermentor drops. This can be minimised by mounting the fermentor as high as is practical.

The thermometer in the boiler shows you how to adjust the feedrate, and the one in the head of the column shows how to adjust the collection rate. Very simple. Do the maths - if feeding at 12%, and collecting at 85%, then you'd expect to be collection one drip at the head for every seven into the funnel.

Methanol & other Heads

The question remains as to what happens to the heads & tails that you'd normally take the effort to discard. It is not practical (in my opinion) to try and set this up as a multi-stage column to try and collect the heads at the column top, and the ethanol a little lower down - the heads collection rate would just be too small to be practical, and you'd need a fair bit of height between the two locations. To minimise the amount of heads, only use this setup for sugar-wash runs, which are said not to produce any methanol.

All going well, the tails should be exiting out the water from from the boiler. If you suspect that instead some are still turning up in the product, maybe it would be of some benefit to reduce the boiler temperature (eg to 99.0 C ?) This way you'd be losing some valuable ethanol out there too, but also any of the other impurities. It would be an economic choice for you. Personally, I don't think its warranted.

The easiest solution is just to treat the continuous column as a stripping device, to produce a product that you are going to redistill later, and remove any heads or tails should they be present. When I've redistilled the product, I've found it to have little/none of them present.

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