b_d_Dom
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Electrolysis of molten salt (current density and such)
hello
I am just finishing the plans for the Down's cell that I have been wanting to build for a while, but I am not very experienced in electrochemistry and
I need some help so I can determine the final measurements.
My design is a cylinder Dia. 28 cm hight 16 cm, so 5 liters of 40%NaCl + 60%CaCl. It has two internal ring heaters that can reach 650 C to supply
most of the heat, but some extra may need to come from the electrolysis to keep the entire 5 liters liquid.
I can probably supply 12 volts at a maximum of 100 amps to the electrodes, the heaters will use another source.
I need to know how:
Variables
- Distance between electrodes
- Surface area of electrodes
Affect
1. Heat created
2. The current drawn
3. And most importantly how much sodium will be produced!!
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Magpie
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Have you researched this site? See: "hot electrochemical sodium," 11 pages of information. If this question had been posted under that thread it
would be much more likely to receive response.
The single most important condition for a successful synthesis is good mixing - Nicodem
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b_d_Dom
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Yes I did read that thread
It's information is entirely useless unless you want to use NaOH and get a couple grams for hours of tedious work
My previous question still stands...
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b_d_Dom
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Is no one able to answer my question? I would greatly appreciate a helpful reply, just whatever you know concerning the question.
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12AX7
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I don't think that's a problem, salt is pretty conductive and current density should be "as high as you can get", right?
Tim
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b_d_Dom
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So the electrodes should have the least possible surface area as long as they still draw the maximum current?
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BromicAcid
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I lost this post once when I tried to submit it so I'm typing this reply up quickly, so it might ramble a bit and lack a cohesive point. Still...
your best bet is to find a book on the industrial process and go off their values regarding the size of your anode, cathode, etc. Like 12AX7 said
though, molten salts are fairly conductive. You'll loose some current efficiency toward the melt but you should get a respectable yield in terms of
current efficiency toward calculating how much sodium you should make. In the electrochemical sodium thread there is information on a Downs cell. I
can recall specifically some information regarding the calcium impurities (which are minimal) and some of the information regarding current though I
couldn't give you a page number on it.
A Downs cell is the way to go though. Eliminates a number of the complications as you know. Hence some of my more recent attempts at running a cell
for lithium electrolysis using a Lithium Bromide / Lithium Chloride melt rather then using LiOH. Just don't fix your electrodes in place if at all
possible. Make them adjustable as to how much goes into the melt that way you'll be prepared. Because the melt is fairly conductive you don't have
to position your electrodes that close to one another. Though you might loose some efficiency by the resistance of the cell, put them far enough
apart that you can comfortably separate the products formed (Chlorine and Sodium) because good separation should be fairly important. The design of
your cell, and the way you are going to separate and collect your products therefore accounts for the major determinate in distance between the
cathode and anode.
The heat created by all of this, the resistance of the electrolyte, current efficiency, those are all specific values that are usually more readily
garnered from literature though. It sounds like you have grand aspirations for this cell, and you may well be a capable engineer and I might be
preaching to the choir here, but start off small. My small scale Castner Cell worked okay, then my large scale.... it was a disaster because I
thought I corrected my mistakes without making yet another small scale test. Remember that molten salts are corrosive, you'll be generating chlorine
at high temps, reactivities are different, your sodium will be more volitile then in a Castner cell, and if something does go wrong you will have a
large amount of very very hot salt to deal with.
I wish you the best of luck though. I too had a jaunt where I was tempted to do pretty much the same thing. Nevertheless, do post pictures!
[Edited on 11/19/2006 by BromicAcid]
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b_d_Dom
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Ok great thanks for the tips, I was afraid everbody was too tired of hearing about sodium and I wasn't going to get any replies
I found several useful patents online that go into a very detailed description of their slightly newer versions of the original Downs cell, I think
these will answer many of my questions once I have enough time to sift through them. My new problem is power! I am removing the heating elements from
my design so I will need at least 600 Amps at 6 volts, I'm still not sure about those numbers however.
So far I think I have a workable design, the case is 3/8" steel so I am safe, it will have a cathode made of sheet metal giving it an adjustable
diameter, replacable graphite rod anodes, and a cooling riser to filter out calcium impurities. As well it will all be wrapped in a layer of Kaowool
to improve thermal efficiency.
Also BromicAcid, THIS is my small scale version ! I will attempt a few small
experiments first; but assuming I can find a source of power that supplies a few thousand amps, and this version works well enough, I will build a
second one with 3-4 times the capacity.
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b_d_Dom
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Does anyone know if I use a thin piece of sheet metal for the cathode and am determining current density, do I count both sides of it when adding up
the surface area?
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ciscosdad
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From my limited perspective, I suggest you use external heating for the pot. I believe the molten salts will be too conductive to give you sufficient
ohmic heating. Suspend the pot in an "oven" of firebrick and put a big propane burner underneath. You will need to be able to melt the charge intially
in any case, as there is zero current flow in the unmelted mix. You can get away with a loose wall of baked clay housebricks to surround the burner
and pot if you don't mind replacing some of the bricks after a few runs.
If you design your electrodes carefully, you should be able to adjust both area and spacing while operational to get the effect you need, although it
might be a bit complex.
It's my understanding the the effective area of the electrodes is that part that is in "line of sight" (for want of a better term) of the opposing
electrode. So, for example, if you had a hollow Cathode inside a larger cylindrical Anode, the outside of the Inner tube and the inside of the outer
cylinder would be the effective areas. I think the actual current flow per unit area also varies if one part is much further away from the opposing
electrode than the rest (carries less current).
Bromic's comments on disposal of chlorine are right on. If the cell works as you hope, there will be a vast volume of chlorine to get rid of. Are you
in a suburban neighborhood?
The authorities will take a dim view of a cloud of "war gas" and there will be endless neighbour suspicion if there is the slightest chemical smell.
(I'm sure you know all this!)
Do you have an effective method of sealing the chlorine take off path? I'm thinking of the junction between the high and low temp parts of the "tube"
(or whatever). It would not do to have a leak during full flow.
How do you intend to collect the sodium? Drain off the liquid while operational?
However the project is fascinating and I hope you succeed. I am an armchair chemist and lack the get up and go to try this stuff myself nowadays
although I did some minor dabbling as a youngster. I admire your ambition.
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b_d_Dom
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Thank you for rejuvinating my topic and for the advice
Since I have posted here I understand the Down's process and how a cell is constructed much better. I did realize the current alone would not be
sufficient to keep this volume of salt liquid, so I am preparing two electric heaters that will heat it from the inside out, that way I can completely
surround it in kaowool.
I believe that the electrode's surface area includes all of it that is submerged in the melt, whether facing the other electrode or not, is that
correct?
As for the chlorine, I plan to let it run free for the test runs (I doubt small amounts would be noticed where I am) then when my cell is more stable
I will probably cool it and bubble it up into large water jugs, to be safely vented away nearby. Eventually of course I'll try and build a condenser
to liquify it to be stored in propane tanks, I'm sure I can find some uses for it later.
The rest of the cell I have designed very well. The cathode ring will be fixed for simplification but I have researched the proper distance at my
current density well. The sodium collector however is somewhat complex, or at least original, but it has a simple job and shouldn't be a problem
either.
Once my cell is complete (2-3 months) I will come back and post many pictures and some detailed plans with all of the information I gathered in the
"hot electrochemical sodium" thread. That way none of you who aim to build your own cell will have to waste the many hours searching through Google
for old patents and forgotten chemistry pages!
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ciscosdad
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b_d_Dom
It's my understanding that you should count only that face of the electrode facing towards the opposite electrode.
There is current flowing through the opposite face(s) but as the electron flow is through a much longer electrolyte path to reach the
other eletrode, it is very small.
If you have ever electrolysed a solution that evolves gasses you will note that the vast majority of gas is generated on the "inner faces." For
practical purposes, count only those faces in direct line of sight to the opposing electrode.
Something else to consider in your design:
Direct gas burner heating is probably cheaper than electric heating (at least it is where I live). If you are fortunate enough to have very cheap
electric power, this does not apply of course. If you have limited current flow (as in a domestic power outlet) you may be better off going with the
gas heating so all the electrons available are making your sodium. 600A is a lot of juice, even at 6v.
Note that the gas heating is only supplemental. You will get a useful amount of ohmic heating though the elctrolysing current path, but probably not
enough to maintain temp.
Don't be blase about the vented chlorine. It will take very little to make a very big local problem. First for you and then for your neighbours
(unless you are kilometers away).
At least provide for a Sodium Hyroxide solution to capture it (with suckback protection). If you make 23 grams of sodium you will have made 35.5g of
Chlorine. Thats 22.4 litres at STP. Thats quite enough to make a house uninhabitable until ventilated.
Feel free to U2U if you want to discuss further. I haven't been checking this thread very frequently, and responses may be quicker.
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tumadre
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IIRC you need 9 volts, various authorities have used this value
the method I designed uses a vertical cathode entering the melt from the bottom, and steel/copper/stainless steel (whichever works the best &
lasts the longest) riser pipe over it.
same for the anode but the riser is ceramic with a port for air to enter and a copper rod makes the connection to the carbon anode through the ceramic
pipe, which dips into the salt an inch or so.
the Cl2 absorption is a water/ slacked lime mix where the cl2 enters from the bottom and an air pump pulls the cl2 and additional air through the
system.
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