I have been contemplating the sizing of dephlegmators for a while, and testing water flows and observing temperature changes. I have read what others say about configuring dephlegmator and condenser as well, and most folk want to run water through the condenser and directly into the dephlegmator, which would be the most efficient way to do it if you can deal with the necessary balancing act.
Let’s look at the objectives of a dephlegmator:
- to knock back relux to load the plates with liquid initially,
- control the temperature of vapour passing the dephlegmator after the plates are loaded.
Each of these objectives requires a different water temperature; initially you want high flow water and a low temperature to knock back most, if not all, the vapour. Thereafter, you want vapour to pass the dephlegmator and into the final condenser where it will be cooled to a liquid. Pure ethanol will vapourise at 78°C so we know that if our dephlegmator is too cold no vapour will pass through, so we need it to be above 78°C during distillation. In fact, you don’t even need it to during distillation, only if you want to knock back liquid. The plates will do all the purifying you need, but to get additional purity you will need to knock back some vapour, or add more plates.
Let’s look at the objective of the final condnser:
- to cause the all vapour to liquify.
The only requirement for the final condenser is that it is not running too hot else you will have hot liquid and some vapour coming out. This is easy to arrange, just increase the flow of water and you don’t to worry if it is too much because you will still get all the vapour liquidised. Not entirely true, you don’t want to waste water, so you try to run it just right or you have a recycle tank.
So if you are running your dephlegmator and final condenser independently you don’t have to worry about matching sizes and flows, just make adjustments to each one as required and that is that. QED – quite easily done.
But what if you want to run water through your condenser and into the dephlegmator? I have figured that there is a balancing act going on here and it can be quite tricky. What if you have a small condenser and a big dephlegmator? You’d have to run more water through the condenser and your outlet temperature would be low and this high flow, low temperature water would enter the dephlegmator and knock back way too much vapour that you’d hardly get anything out. You can remedy this by reducing your power input with a power controller such that your condenser can cool the liquidise the vapour and have an outlet temperature of 75°C or above. Or increase the length of your condenser to match the power of your boiler.
Generally you can get dephlegmators on the market that are at least 170mm high. And condenser come standard at about 400mm high. The respective volumes of these items are 1.34 and 0.79 litres. The dephlegmator has a much higher cooling capacity which makes no sense at all. It only needs a high cooling capacity in the first 5 minutes of operation while you load the plates up and this is easily achieved with a high water flow, not a high capacity dephlegmator.
My experiements have shown that the volume of the condenser ought to be twice that of the dephlegmator in order to have maximum control of both simultaneously. So if the dephlegmator is 100mmØ and 100mm high you would need a condenser that is 50mmØ and 800mm high. I have used this combination very successfully and maximised my alcohol output while maintaining a high purity with 7kW of power in the 120L boiler and copper condensers.
And to confuse the matter even more, here is a little thought – how does the material affect performance of the condensers? I said above that I used copper. Copper has a thermal conductivity of around 400 W/m.°K. This is second only to gold. Stainless steel has a thermal conductivity of around 16 W/m.°K!!! You will get your best cooling, and therefore your smallest size, when using copper.
To take account of this I have designed what I call a Top Hat Dephlegmator which is a 170mm high item, with 100mm of heat exchanger inside, and 70mm of void with a 50mm outlet from the void at 90°. It is literally like putting a top hat on a dephlegmator and it works a treat. Instead of having a dephlegmator + reducer + bend, which traditionally add 350mm of height, you can do it all in copper for 170mm. Based on my prices do the traditional setup for $690 excl. GST, or the top hat dephlegmator + elbow for $530 and have enough space for an additional bubble tee.
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