Calculations for a Reflux Still

Heres the start of this. Still HEAPs to go ... I've so far totally ignored the cooling water & heat loss calculations (will include them later (much))

The calculations for sizing column diameter (at 65% of flooding) are based on Sherwoods equation (Perry 18-47), whereas the performance of the packing, and the resulting number of theoretical plates is based on Onda's method (Coulson 11-113).

If you don't know the power input to your still (eg you use gas), adjust the "power" values used until the "time to heat up" match with what you expect from experience.

This page is still very much work in progress. It may well all be totally wrong, and may very much change from one day to the next. Thats my perogative. I make absolutely no claims about its accuracy at this stage, but I'm trying to do my best.

Known errors at present : innaccurate for small number of plates, as result is rounded to closest plate, eg 1.45 plates rounded to Plate 1,but 1.55 plates rounded to Plate 2
There is still a small discrepancy vs Excel version of this - suspect rounding errors are at fault

Input your Reflux Still characteristics
Initial Volume of Wash L Timestep for Calculation minutes
Alcohol Content % by volume Initial Temperature oC
Power during Heat-up W Height of Packing m
Power during Distillation W Diameter of Column m
Reflux Ratio mL Distillate refluxed / mL Distillate kept

Select Packing Used
(note: the values these selections generate are only approximate guesses)
Marbles 6 mm Raschig rings (ceramic)
Stainless Steel Wool 13 mm Raschig rings (ceramic)
or guess your own values ...
% Fractional Voids in Dry Packing m-1 Diameter of Packing m
Surface Area of Packing m2/m3 Critical Surface Tension N/m

Estimate of Reflux Still Performance
Packing Guesses Used :
  Packing Voidage   Diameter : m   Surface Area : m2/m3   Critical Surface Tension: N/m
Height Equivalent of Packing (HETP): m  
(depends on Reflux ratio and type & size of packing)
Number of Theoretical Plates :   (height/HETP)
Time to heat still contents up to Boiling : minutes    ( oC)
Minimum Internal Diameter of Column : mm.
(Smaller than this will cause the vapour to blow any refluxing distillate out of the column)
Recommended Internal Diameter (65% Flooding Diameter) : mm

Time StepPotVapourDistillateCollected
  VolumeBoil TempPurityCond Temp.VolumePurityTotalPurity
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %
min L C % C mL % mL %

So the Purity of the distillate is determined mostly by the height of the column, and the type of packing (via the HETP), whereas the rate of product removal is dominated by the Reflux ratio. For purity say 80-90 %, the amount of reflux doesn't become too important, the column height is what delivers for you. However, to really push the limit towards 95% however, this is when you need to increase the reflux ratio, and wait a while longer to obtain the product.

To use this calculator to help you with your still design or operation, spend some time once to rigourously understand its operation. Treat this one run as an experiment, and spend the time on it. Measure & write down the temperatures every 5 or 10 minutes, and graph them as you go. Collect the distillate into 200 mL or so containers, and note how long it took to fill them (eg the flowrate). Once they're each cool, measure their %. Measure the cooling water flowrate & temperature, and work out how much it needs to be increased near the end of the run to keep it cool.

This will allow you to get an accurate idea of the heat input to the still, so that you can then optimise its diameter (you may need to widen it if too small, or decrease it (by stuffing it with some stainless steel rods or suchlike) if too large) if too far from that desired.

By understanding the flowrate of distillate you are collecting, you can determine the reflux ratio that you are getting from your cooling coils etc. Once you know this, you can work out the HETP for your packing, and thus the number of "theoretical plates" your still has. This will determine the maximum purity that you can achieve - if close to it, you're doing well, if miles away from it then something aint working right. If the maximum is less than what you want, then you'll need to make the column taller or change the packing (or both !).

The cooling water temperature will tell you if you're sub-cooling the reflux distillate. If this is happening, then you're wasting the top portion of your column as all its doing is trying to heat it back up to equilibrium tempeature again, before it can start stripping the vapour of its water content. If the cooling water flowrate is really large (eg more than about 2/3L per minute) then your cooling isn't as efficient as it could be - see if it needs more surface area to work over.

Once you understand how your temperatures & distillate flowrate changes at the end of a run, you can predict more accurately when you're starting to receive the tails. Compare the total distillate collected, and its purity, against what is theoretically possible. There's no point is trying to wring too much out of the still, or else you'll just be into collecting those tails and ruining your whole batch. (This is where the table of "amount collected over time" might be usefull to you - as a guide to when to stop)

These calcs aren't thecomplete picture, but should be usefull as a guide along with reflux design in terms of understanding and getting the best from your still.
Also read all of the other submenu's UNDER the reflux design. They have a LOT of information.     This page last modified Tue, 20 Jan 2015 20:51:05 -0800