Martin & Gaw & Peter ... Some more comments on how to design a larger still. Its a process where you need to ask the right questions as you go through it. The answers then change how you proceed from there, or send you back to the start ...
1) What purity and flowrate do you want to produce at ? So, for example, wanting 5 gal/day (19 L/day), do you want this in 12 hours or 4 hours ? Say you chose that you want it in 4 hours. This means that you want to collect your distillate at around 80 mL/min. But you'll need a certain amount of reflux action happening, to get the purity you require. So we'll need a heat source which can actually generate quite a bit more vapour than that. But how much reflux ? That going to depend on the column design. Normally I'd suggest that you'd use stainless steel wool/scrubbers for the packing of the column. These will give you about 1 HETP (theoretical stage = equivalent to a single pot distillation) for each 13-15cm of packing. Fine for small stills. But, if you're into these high flowrates, and likely to be using it quite frequently, if not daily, then may want something less likely to block up/foul and is easier to clean. Its also harder to pack a wider column such that it will get a nice even flow over all the packing - structured packing will do this easier for you if you're into large diameters. So then you're talking about using small marbles or rachig rings or very short lengths of copper tubing (=homemade rachig tubes). These have greater HETP's and you may need 20-40cm of each to get the same purification happening. 2) Depending on cleaning needs, etc decide on the type of packing This will now make the column quite a bit taller (and a touch wider), if you decided to use something less than stainless steel scrubbers. But there's a trick here.. extra height isn't really going to cost too much more in the long run. So why not go silly and make it quite a bit taller. Say you've worked out, from http://www.geocities.com/kiwi_distiller/refluxdesign.htm that you need to do the equivalent of 8 stages/HETP's to get the purity you want. These HETP's are assuming a particular reflux ratio. If you make the column taller (guess, lets say 10-15 stages), then you can afford to run less reflux (and downgrade how good the HETP's are), as you have surplice capacity to get the purity you want. E.g. so rather than needing to run at a reflux ratio of say 4 (return 40 mL for every 10 mL you keep), you may be able to deliver the same purity with a taller column running at a reflux ratio of 2. This will speed up your collection rate heaps. It just means that you'll need a wee ladder to read the top thermometer (why not use a thermocouple instead ?), and that your condenser will be up in the roof too ... (fun if you have low water pressure) Then you can use the calculator in http://www.geocities.com/kiwi_distiller/dtw.htm#use_fract backwards ... e.g. use it to estimate the heater size you need. E.g. if you want 80 mL/min at a reflux ratio of 4, then you'll need approx. 6.1 kW heat, but only 3 kW if you only need a reflux ratio of 2. So, even though its cost a little more for the taller column, you'll save when trying to heat it. A smaller element also means that the column can be skinnier (thus not quite so much packing etc required) See how it sorta goes in circles ? 3) So you've picked your element size, This then lets you estimate the column diameter from http://www.geocities.com/kiwi_distiller/reflux_calc.htm (remember just to beef it up slightly from that predicted). Note that this calculator also estimates how long the pot will take to heat up. Consider using a second "boost" element in there if this time is too long for you. 4) Size your condenser. Once you know how much heat you're putting into the system, you then know (exactly !) how much heat you have to take out of it (clue - its the same amount). Size your condenser to handle this .. see http://www.geocities.com/kiwi_distiller/cond_calc.htm 5) Pick the pot. This won't really affect any of the above. Just make sure you can fill/empty/clean it ok. In a nutshell, what I'm suggesting, is that if you want to make something bigger than a usual hobby still, possibly down-grade the type of packing you're going to use, but definitely make it heaps taller than what you would usually. This will allow you to run at a lower reflux ratio, and hence get the through-put you're after, without needing a huge heat input. Tony Biofuel at Journey to Forever: http://journeytoforever.org/biofuel.html Please do NOT send "unsubscribe" messages to the list address. To unsubscribe, send an email to: [EMAIL PROTECTED] Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
