Swede
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Bucket cell - interesting observations
My bucket cell adapter is running with a full data stream to a DATAQ electronic chart recorder, measuring voltage, current, and temperature. pH will
not be possible due to the harsh environment, but I may execute a data run in the future using a cheap, disposable probe.
The original plan was to hopefully correlate current and voltage so as to determine EOR (End of Run) conditions without having to measure chloride ion
concentration. It is also the first use of an upgraded HCl delivery scheme using an Auber Instruments inexpensive timer.
The cell is a 2.5 gallon HDPE bucket with 8.7 liters of pure KCl at 17.9% chloride, MMO anode, Ti cathode, and a good CC supply. The liquor was
saturated at 50 degrees C, quickly added to the cell, and the current cranked quickly to keep the liquor hot. Here is where it gets interesting...
With no cooling, the temperature at 40 amps CC climbed to as high as 76 degrees C. At this point, I pointed a small fan at the bucket, and this
proved dramatic in lowering the temperature. Fan on, the temp drops to 55 degrees C. from nothing more than blowing air over the bucket.
Between 55 and 75 degrees C, I noticed a very distinct correlation between temperature and voltage to maintain 40 amps. As the temperature climbed,
the voltage dropped, and as the fan cooled the bucket, the voltage climbed once more. Several data points confirmed that higher temps require less
power overall. Over approximately 18 hours, while the cell chemistry is still fairly young, 40.00 amps:
V------------T
5.05----60.49
4.80----71.02
4.76----76.32
4.84----70.86
4.99----61.74
5.05----59.71
5.16----55.81
5.26----54.17
The fan was turned off, the temp allowed to climb again:
4.96----70.63
A solid correlation that should have been obvious from other runs. It pretty much kills my notion that the voltage and current data can be used to
determine EOR, unless a cell's temperature remains constant, which it rarely does.
I'm certain that the literature on this process describes the reduced power requirement at higher temps, but I had not run into it before, or if I
did, it went over my head.
Temperature is your friend for the bulk process, just watch out for the materials! 75 degrees C is pushing it on a small HDPE bucket. And again, it
shows that heating is always a problem when attempting a higher-current run measured in days rather than weeks.
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Swede
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Oh yes, I forgot to mention - the bucket cell hardware is performing admirably. As usual, I made the vent too small, and the liquor level was too
high. I noticed some seepage, drew off 300 ml of liquor to lower the level of electrolyte, and since then, there has been nothing more than a slight
salt creep. I am using a bubbler for agitation rather than a mechanical stirrer, which I do have fitted to my large cell.
Each electrode shank has a small heat sink on it, and at 40 amps, they are not too hot to touch. I'll get some pics up soon.
I also wanted to re-arrange the data, from cooler to hotter:
V------------T
5.26----54.17
5.16----55.81
5.05----59.71
5.05----60.49
4.99----61.74
4.96----70.63
4.84----70.86
4.80----71.02
4.76----76.32
[Edited on 19-11-2009 by Swede]
[Edited on 19-11-2009 by Swede]
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12AX7
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I would be interested to see the V-I curve (for instance, at constant V instead of constant I). That would have to be plotted for different
temperatures, which would take a long time per datapoint and an external heater/chiller.
Offhand, your datapoints line up fairly linearly as T(V) = -44.7*V + 287.38. Evidently, if you put it in an autoclave it'll proceed all its own
around 287C. 5.87V would seem to be the initial voltage (i.e., room temperature).
Tim
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tentacles
Hazard to Others
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Tim, that's all we need, the pressure cooker chlorate cell!
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12AX7
Post Harlot
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Location: in my cozy, warm furnace
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Mood: lazy
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What's the steam pressure at that temperature, 50 atm? Not bad... Too bad the majority reaction would run in reverse; I can't see hydrogen
seperating from the steam, anyway.  Good way to dispose of chlorate though I
suppose? Nahh... there are far more entertaining reactions. 
Tim
[Edited on 11-19-2009 by 12AX7]
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watson.fawkes
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Quote: Originally posted by Swede  | | Between 55 and 75 degrees C, I noticed a very distinct correlation between temperature and voltage to maintain 40 amps. As the temperature climbed,
the voltage dropped, and as the fan cooled the bucket, the voltage climbed once more. |
Ion mobility is higher
at higher temperatures, which leads to a negative temperature coefficient. This is because current flow is the electrolyte is a diffusion-drift
process.
As temperature increases, a second process, inelastic collisions, start becoming significant. This is the driving process behind positive coefficients
of solid conductors, since their majority charge carriers (electrons) are much more mobile that ions are in liquid. I am doubtful that you'd see much
attenuation in conductivity at atmospheric pressures and sub-boiling temperatures.
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Swede
National Hazard
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I have allowed the temperature to stabilize, and at about 2.5 days, I am starting to see the behavior I noticed earlier... at a given temperature, as
the chloride concentration decreases, the voltage required to maintain CC goes up. Right now the bucket is at 55 degrees, and at 14.2% chloride, vs a
starting concentration of 17.9%, 5.55 V is needed to maintain 40 amps, compared to 5.16V when the chemistry was young.
Initial hacks at efficiency show 87% based upon chloride reduction over the first 24 hours. This will probably drop a bit as the run continues. The
pH is remarkably stable at 6.8, give or take a few hundredths. This is based upon the acid delivery rule of thumb:
0.057 ml of concentrated (32%) HCl per ampere, per hour.
I have the pump scheduled to deliver an appropriate mass of acid, which I diluted to 16%, every two hours. After a few runs now with a dosing pump, I
have come to the conclusion that a superior method is simply an appropriate solenoid valve (probably PTFE, available on eBay cheaply), a decent timer,
and a simple gravity system.
Anyway, this has been a data-rich run, and it will be interesting to create and compare some of the curves generated by the data-collection scheme.
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Contrabasso
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Can you get a driver to adjust the fan speed to keep the temp at 50c then you can fix the temp and do V/I plots at that temp,
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