T-Cell Construction, III
Entry posted by Swede
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As usual, I underestimated the time and effort such a new system would entail. The good news - it is coming along very well, and I do enjoy this sort of hands-on work, so rather than being a chore, it is enjoyable.
While the T-Cell is underway, I decided to run a last chlorate batch in my old acrylic tower cell. Three distinct purposes:
- Acquire another kilo of stock
- Take detailed notes on voltage, time, and chloride ion concentration
- Test my roll of Tygon tubing for chemical compatibility with this process
First, the tubing. Saint Grobain plastics, the maker of this tube, suggests weighing of a tubing sample as a means to check for moisture uptake. The weight of the sample (before and after) and the physical characteristics (hazing, stiffening, surface finish) are used to determine compatibility.
A short section of the tubing was weighed, photographed, and examined. It weighed 7.39 grams before the test.
Into the electrolyte it went, and it was subjected fully to the heat, the chlorine, all the chemical hell that is a chlorate cell for the full 3-day run.
I lost track of it quickly in the growing pile of crystals. At the end of this run, I spotted it buried like a fossil.
I must say, the run was the best I've ever had. I feel like I have a good grasp on this particular process, and with the cell at 40 amps continuously, I had over a kilo of potassium chlorate in 2 days+ of running. As for the tubing, the hypochlorites stripped the red lettering from the outside of the tube section, and it absorbed 0.01g of water, but otherwise was fine. It looks like this tube will do OK for the T-Cell.
Back to the T-Cell construction... Sealing of the cells, with the exception of the vent, is critical, and difficult to do well. My tower cell uses a Viton O-ring, and from what I've seen, Viton passes the test with flying colors. It shrugs off the nasty brew with no ill effects. I decided, then to use a bead of Viton O-ring stock to seal the lid of the larger CC.
To do this, I needed to cut a continuous channel in the lid of the CC. The channel needed to grip the Viton stock, yet leave a percentage of it proud of the lid surface, so that when the bolts are tightened, a good seal will be made. I did some experiments on a scrap of PVC sheet, and found that a 3/16" cutter, 0.148" deep, worked perfectly. The 0.210" diameter Viton stock squeezes into the groove, and is held exceptionally well. One problem, the stock really cannot make a sharp 90 degree bend, so I relieved each corner with a 1/2" diameter plunging cut. The result:
Much time and angst was spent on the lid hold-down system. Tentacles sugested, and I agreed with him, the best way would be to install studs directly in the walls of the tank, but despite much fiddling, I could not figure out a way to do it well, as the Viton stock, plus a 1/4" diameter stud, together, just took up too much space on the 3/4" wall. I wanted the Viton to be close to the middle of the wall, so in the end, I elected to glue even more PVC onto the tank in the form of stud strips and lid attachments. This picture can do a better job explaining it than my text. The hardware is 316 stainless, the most corrosion-resistant stainless commonly available, outside of some exotic, NASA-like materials.
As far as I am concerned, the CC (Collection Chamber) is finished. It has two ports on the sidewalls, and 7 on the lid, an assortment of 1/4", 3/8" and 1/2" NPT threads, cut with pipe taps. No doubt some will be plugged and not see any use, but it is much easier to cut these ports now, and not need them, than to wish later on that I had done it earlier.
I moved on to the EC, the electrode chamber. The concept here was a small CPVC round section that will be run at a much hotter temperature than the CC. Hopefully the EC will NOT produce crystals; the CC, being cooled, should produce the bulk of the crystallization. Even more than the larger tank, this sucker needs to be very well sealed. I am hopeful of running upwards of 80 amps through this device, and with a bubbler creating agitation, this thing will produce a LOT of gasses.
There was no way in hell I was going to glue another box, so I decided to use a section of 4" schedule 80 CPVC pipe, and two end caps, to form the EC. The bottom end cap is mostly unmodified, other than to lathe-turn the bottom flat so the unit will stand up properly; it simply gets cemented in place with the chemically-resistant IPS CPVC glue.
Work begins using my old Hardinge lathe. I have been a hobby machinist for 20+ years, and it allows me to do a nice job, but if anyone decides to replicate this sort of system, rest assured it can be done without machine tools. Since I have them, I use them, to good effect.
One of the 4" CPVC end caps was mounted onto the lathe, and the bottom and sides were cleaned up a bit. The reason I am doing this will be evident shortly...
Removed from the lathe, the perimeter was spotted for eight 10-24, 316SS studs. While the end cap was still one piece (this will form the top of the unit), the holes were drilled and tapped 10-24. It is always better with this sort of exercise to drill all 8 holes (or however many you want) so that the pattern will match perfectly after it is cut into two pieces.
Back on the lathe, a tool called a parting tool quickly separates the end of the cap from the body. I now had a lid! The walls of this piece were a bit too small for 1/4" studs, so I did go with #10, but I have no doubt it will work well. I cut a 15" section of CPVC pipe, and slipped the (now-cut) end cap onto it, stopping short of flush. This makes a really nice channel for the O-ring! After checking the dimensions, it was cemented into place, permanently.
These O-rings compress significantly when the hold-down nuts are torqued a bit, so if you attempt something like this, be sure to allow for the compression. It does make for a very effective seal. I am very pleased with how this has turned out.
There's not too much more to do... the lid of the EC will need to be modified to accept electrodes, and I am still debating whether or not I want to install a permanent, fixed titanium cathode into it, as Ti is used for both chlorates and perchlorates. This will make swapping anodes a breeze. Also, I plan on boring a clean hole in the lid, and using a short section of CPVC round stock, with yet another O-ring, as a carrier for the anode. By releasing a clamp, or some other device, the round carrier can be removed, and the anode replaced. This would be much simpler (and more economical) than having to craft another lid when I want to swap anodes. Hopefully, future pictures will show this concept more clearly.
Additional work yet to be done to the EC:
- The lid (or perhaps the body) of the EC will need to be ported for a PTFE thermowell, for temperature
- Two ports for electrolyte inlet and outlet
- An agitation port for either a stirrer, or more likely, a PTFE tube bubbler
- A wide-bore vent tower
This latter device is important. It needs to have a wide bore so as to prevent electrolyte from being forced out due to gas pressure. In my older cells, I used 1/4" OD tube, and this was too small. There is so much misting, bubbling, and general agitation, that small amounts of liquid electrolyte are thrown into the tube, and the positive gas pressure then forces the liquid out the vent. I want to avoid that. Finally, since I will be using gravity to transfer liquid in at least one direction, a tall vent tower will act as a pressure reservoir. For example, let's say I'm pumping aggressively into the EC. The liquid level will rise, and when it reaches the outlet port, it will begin to flow out. If the flow is not the same as the pump input, the liquid level will continue to rise into the vent tower. At some point, the pressure from that column will contribute to the output flow, and the liquid will no longer rise. I hope this makes some sense. Since I have only one pump, gravity MUST be used to flow electrolyte, either to or from the EC.
Frank Rizzo said (and he is correct) that I'm making this too industrial... but I can't help it, I LOVE this shit! The challenge must be met for a true, miniature perc factory. At 80 amps, this thing should produce a kilo of chlorate a day, with a capacity I am estimating at 4 kilograms. Since it is plumbed, I can always daisy-chain additional volume, heat exchangers, etc, into the system. Modular is a good way to go.
One last note: Thanks to a very helpful and knowledgeable APC member, I may possibly have access to some modern MMO anode stock that we are theorizing MAY be able to produce perchlorate. That would be a huge boon to the hobby, being able to make perc with something other than a Pt or PbO2 anode. There is room enough in this process for years of experimentation! 
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