Bok Mini Temp Problems

p_su · Post by **p_su** » Thu Dec 20, 2007 6:51 pm

I made the Bokakob mini still and am having some problems getting the temp to stay even as I try to get close to the recommended take off rate. I am taking off with a column temp of 174F - and if the rate goes too high (>3 drops/sec) the temp will go up to 175F. If it goes up to 175 - even if I immediately shut off the output (reducing at that point doesn't help)- it will spike to 185F or so and then take about 7-15 minutes to go back down to 174. I have to keep the valve closed for it to come back down. I am still in the middle of a run, so I'm not out of ethanol in the solution. I'll try and list as much of the setup as I can to help with troubleshooting.
This seems to be far short of the "steady dripping close to a weak stream" that is specified in the mini instructions. Is that temp spike normal, or should it equalize faster? Should I keep the hot plate at full blast- or reduce power?

1.5" dia column - insulated to take off point
Stainless scrubbers for packing
1/4" output tubing with needle valve
1/4" cooling coil - (sufficient to knock down water vapor)
20qt stock pot with ~2gal of sugar wash in it
1000w hot plate with 3/8" thick cast iron pan for heat spreader
the boiler and SS bowl are both insulated too

I am using a digital thermometer that as far as I can tell is accurate. It is about 1/3 down from the top of the column packing. The heat also seems even, despite the fact that the hot plate does cycle - if I am taking off at 1-2 drops a second or so - the temp does not waver from 174F.

I am running the cooling water so that the output is warm - not cold to the touch, as specified in the instructions. Should I keep this as it is?

Let me know if I've left out any details that would help.

CoopsOz · Post by **CoopsOz** » Fri Dec 21, 2007 1:15 am

I know you said the hot plate has a spreader but do you think the cycling might still be affecting it?

arkansas · Post by **arkansas** » Fri Dec 21, 2007 2:55 am

CoopsOz wrote:I know you said the hot plate has a spreader but do you think the cycling might still be affecting it?

I am with Coops on this one, a hotplate will surge on you unless modifed like pinto's electric controller is used.

p_su · Post by **p_su** » Fri Dec 21, 2007 11:53 am

So would the surging explain the temperamental nature of the column? It is collapsing the layers in it every time it cycles... which is why when I increase take off it totally freaks out?

I've read thru Pinto's temp sticky - and there is one thing that still isn't clear to me. If I build the controller and operate at 100% for boil up, won't the plate still cycle as it will be the equivalent of no controller at all? What causes the cycling in the plate anyway.. is it the temperature of the top of the plate, or determined by some sort of current draw to avoid stressing house wiring...?

I also realized something else... I am keeping my plate at full power during a run - should I reduce it back from that to a lower setting?

Post by **HookLine** » Fri Dec 21, 2007 5:01 pm

p_su wrote:I also realized something else... I am keeping my plate at full power during a run - should I reduce it back from that to a lower setting?

Yes, 1000 w is too much for a 1.5" column. Try 400-500 w.

tracker0945 · Post by **tracker0945** » Fri Dec 21, 2007 5:17 pm

I am no electrical engineer but cycling is a cheap way of controlling the hot plate temperature.
There is a thermostat switch built in to the stove which when the temperature reaches a set level, the current is switched off until the temp drops to another set level when the unit will receive full power again.
You will notice the same thing happening in most microwave ovens when you lower the power setting - on for 5 seconds, off for 10 seconds, on for 5 seconds etc, etc, etc.
What this all means with the hot plate is that you will be running flat out for 2 or 3 minutes, then have no power for the next 2 or 3 minutes while the plate is cooling down.
Removing the thermostat and fitting a heavy duty voltage regulator means that you can set the heat by means of adjusting the power supplied to the unit. This will provide an even heat supply continuously.
And - Yes, 1000 watts is too high to be running your Bok Mini on. 500 - 600 watts or even less is ample once it is up to temp.

p_su · Post by **p_su** » Fri Dec 21, 2007 8:16 pm

Thanks for the replies - I will try to reduce power and see where that gets me. It just seems somewhat counterintuitive that you'd have to lower power to increase output...

I was able to use a gas stove today - and solves my problem. I the heat is much more even - and my column temp is much more stable.

Post by **HookLine** » Fri Dec 21, 2007 8:35 pm

You are not just after raw output rate, you are also after purity, and it is a trade-off between the two.

You only get purity by getting good separation of the different components in the vapour within the column.

To get decent separation the vapour speed through the column needs to be below about 20 feet per second.

Vapour speed is controlled by heat input. Reduce heat input to slow vapour speed, and increase separation (purity).

The reason your output rate is lower at higher heat input, is because to get the same purity you have to use a higher reflux ratio. You have to return proportionally more of the distillate to the column for re-distillation. Which lowers the output rate of high purity distillate.

p_su · Post by **p_su** » Fri Dec 21, 2007 8:57 pm

I appreciate the clarification Hookline. I had assumed that the output rates quoted from the mini instructions meant those rates were achievable at a high purity - perhaps I shouldn't have. Guess I'll just chug along at a few drops a second and keep a high purity output. I think in the future I'll use a pot to strip my washes... otherwise I'll be at this for a looonnng time before I get much out of the still and into my glass.

Post by **HookLine** » Fri Dec 21, 2007 9:23 pm

You gotta be patient if you want quality. My 50 mm reflux column has 1500 mm of packing, and I take-off at 4 drops a second (about 500 ml an hour).

Definitely use a pot column for stripping, much easier.

p_su · Post by **p_su** » Sat Dec 22, 2007 2:41 am

I think that the stripping was the point that I was missing. I was not looking forward to running xxx liters of wash with a column that takes some sweet time to output. I don't mind waiting for quality - but in that case it wasn't going to be worth it.

I used my bok mini to strip a couple of gallons of wash earlier today. I removed the packing, and opened up the valve wide open. It only took 1.5 hours or so (including heat up time) at about 1/2 oz. per minute. Not as fast as a purpose built pot still, but beats the living crap out of running wash thru the column with packing and full reflux... which would have taken about 2-3 hours longer. Guess I'd better start thinking about making a pot attachment...

Not to mention making one of the VM stills from the compleat distiller... i am jealous hookline.

After I read that section I realized I should have made that instead of the mini... if only I'd have known.

Post by **HookLine** » Sat Dec 22, 2007 3:43 am

p_su wrote:Not to mention making one of the VM stills from the compleat distiller... i am jealous hookline. After I read that section I realized I should have made that instead of the mini... if only I'd have known.

Ahh, the benefits of reading lots before you make your still. I was originally going to build a Bokabob slanted plate (which is a very good design), but after reading more I decided on a VM, and I am very happy with that choice. Some say that they are not as simple as a Bokabob because they need two condensers, but the big Liebig product condenser on my VM column also doubles as the condenser on my pot column.

I also don't see the point of making a reflux column less than 2" diameter. WOFTAM.

tracker0945 · Post by **tracker0945** » Sat Dec 22, 2007 5:51 pm

What you can do is some stripping runs with packing removed etc. and collect everything (as in pot still mode) which will give you a 40 - 50%abv wash which you combine all collections together and re-run slowly with packing replaced. This should (does with my 2") be able to be collected at 94-5% @ 1.5 litres per hour, reducing to 400 mls/hr before it is finished.

p_su · Post by **p_su** » Sat Dec 22, 2007 10:21 pm

Hookline- You're right about the reading. I read fairly extensively on the homedistiller site as well as a few others... I just didn't get my hands on the compleat distiller until it was too late

I may have to borrow the liebig idea from you.. use the same part on both a pot and a VM - that is a fine idea.

Tracker - Thanks for the advice. That was exactly what I was planning on doing. I doubt that I'll be able to match your collection rates though - I have a 1.5" column, and as yet have only been able to hit something like 200mL / hr at 90%. It could be that I am not quite expert at balancing the heat input and takeoff rate yet - I'm sure that Bokakob's design is sound as many are happy with it.

Post by **HookLine** » Sat Dec 22, 2007 10:54 pm

tracker0945 wrote:This should (does with my 2") be able to be collected at 94-5% @ 1.5 litres per hour, reducing to 400 mls/hr before it is finished.

I followed the general advice on these forums and have only tried collecting at 500-600 ml an hour, though that is for the whole of the hearts run, I don't adjust the take-off rate at all during the hearts run. But I will try faster rate and see what happens.