The Good... and the Bad
Entry posted by Swede
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The last three days have been very, very busy for me with regards to this project. I had hoped to have the system running Monday. Instead, it was Wednesday before I finally had everything put together, and ready to plug in.
The odd thing is this - I don't have any pictures of the entire system once it was running! Just individual pictures, snippets here and there. The lead-up to this was pretty lengthy, work-intensive, and frankly, expensive.
One of the reasons I waited was the delivery of a Hanna Blackstone dosing pump, NIB from eBay. eBay is LOADED with dosing pumps. Some of them are no doubt real bargains, while others are disasters waiting to happen, mainly due to pump heads that may (or may not be) near the end of their useful lifetime. Dosing pumps are not all alike. They vary by capacity, and also by the materials. A chlorate system requires dosing of HCl, and the wetted materials need to be capable of pumping the concentrated HCl without harm. One of the reasons I picked the Hanna, along with the low cost, was the fact that wetted parts are either PTFE, PVDF, or glass - all of these are compatible with both concentrated HCl and chlorate cell liquor, if I ever need it to pump electrolyte.
For proper pump function, it is best to put the pump between HCl reservoir below, and the delivery point, above. The pump suction-feeds quite well. At the feed (input) end is a prefilter and a spring-loaded check valve, and the injection point also requires a check valve to keep the CC contents from back-siphoning.
I needed a good container for the HCl. In the end, despite the small, 1-liter size, I chose a Nalgene HDPE lab container. The main reason was the fact that the lid is threaded 3/4", making integration a snap.
I adapted some PVC fittings I had and made a nice connector for the Nalgene container. The tubing is Tygon "Lab" tubing that can handle the acid with ease. Tygon is interesting stuff. It comes in what must be dozens of formulations, each with particular characteristics that must be taken into account. The Saint Gobain Plastics website has all the info on Tygon you'll ever need.
The dosing pump was ready. Wheeling the cart outside, I added all of the used chlorate cell electrolyte that I had accumulated since I started this project, and all the remaining KCl feedstock I had on hand. I was still about a gallon short! Using a huge 4 liter borosilicate beaker on the kitchen stove, I boiled up another saturated solution of KCl, and added that to the batch. The lid was secured, the pump turned on, and the current was applied.
I had thermocouples on both the CC and EC thermowells. With the pump just ticking over, I soon had about a 25 degree C difference between the two. When I increased the pumping, the temperature differential dropped. Likewise, a slower pump created a higher temperature differential. It was working perfectly! The sight glass was a winner, showing the level properly. Most important of all, everything was nice and tight. Only the slightest bit of salt creep on a threaded fitting or two, easily corrected. I was ecstatic!
I went to bed Wed. night feeling confident.
Thursday morning, still in my bathrobe, I went out to check the system. Many readers are younger guys not into coffee, but older guys might understand... I was staggering about without my coffee, and was really out of it.
I turned on the thermometer, and pushed the "T2-T1" button which showed the temperature difference. It showed 60. I thought "That's weird, that's what the T2 should read, the EC." I cycled through the buttons, and 60 WAS the temp difference. T2 was at 90 degrees C!
The line leading to the pump was jammed with crystals; flow had stopped, and the EC was in a meltdown mode. Oh... Shit...
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It's beer-thirty, and time for my "King of the Hill" beer session with my neighbor friends. Check back later, I'll conclude the story by editing this blog. Here's a taste of what I saw when I tore it down...
There's good news too, not all is lost! 
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Back to the Good News / Bad News... with the EC (The Electrode Cell) near boiling at 90 degrees, I chopped the power, and started looking at some way to salvage this particular run. I tried massaging the crystal mass through the line and restore flow... I tried reversing the flow. No luck. With some reluctance, I decided to tear it down after only one day of running.
Outside, with a nice breeze carrying the chlorine and hypochlorite away, I took a look at the individual components.
The exterior of the EC looked god. I drained what I could, then popped the lid. The horrifying picture above is what I saw. The cathode plates had warped away like a banana peel from the anode! The side of the plates away from the anode had a chalky white substance, which I have seen before on previous runs... I'm not sure what it is. The inside of the cathode plates that faced the anode looked clean. The anode itself looked completely unaffected, which is a testament to the durability of this particular MMO formulation. On the bottom of the EC was a layer of chlorate crystals maybe 1" tall.
The CC (Collection Cell) had a thin, sparse layer of potassium chlorate crystals on the bottom. After only 24 hours of running, I did not expect much more than that. Physically, everything looked fine, except all the hose barb fittings were jammed with fine crystals. Here's what happened, my best guess... With the CC at maybe 30 degrees, and the EC at 60, the bulk of the crystallization was in fact taking place inside the CC, as designed. But last night was exceptionally cool, probably 8 to 12 C. As the 30 degree liquor hit the line to the pump, it cooled rapidly, forcing crystallization inside those portions of the system that were cooler than the heart of the cell, within the bulk of the liquor. So long as the system was not yet saturated with potassium chlorate, and all liquid, it was behaving perfectly. As soon as the solution WAS saturated with KClO3, it became susceptible to crystallization and clogging.
With the EC no longer recieving flow, it rapidly heated, and all the chloride in the EC was converted to chlorate. With no chloride ion left, "bad things" happened to the cathode plates. Remarkeably, the anode (MMO mesh) handled probably near zero chloride without any problems at all! That is a good sign - the MMO mesh is exceptionally durable, and can make chlorate right down to near zero chloride. Bye bye, 10% chloride rule. Your efficiency will drop with the chloride, but you're not going to hurt this MMO.
The lid looked good, and the viton seal performed perfectly. My choices of materials bore fruit in the sense that nothing was damaged beyond the cathode plates. It is remarkeable that the CPVC cell at 90 degrees did not suffer.
The problem was me, the engineer of this system. I should have anticipated this failure mode. I knew the crystals would form in the line, but I was counting on flow to carry the small crystal seeds into the cell(s), where they would sink, and grow.
I need to rethink the circulation method of (per)chlorate production. In the mantime, I still have a kick-ass monolithic cell in the form of the CC. Weeks ago, I had installed permanent cathodes in the CC lid so that it could, in fact, act as a stand-alone cell. I began to configure it in this direction.
One of the first things I did was to create PTFE supports for the cathode plates... no more warping for me! I had never seen such warpage before, but I had never run a cell at 90 degrees and zero chloride, either. The PTFE supports are nothing more than round bar, cut partway with a slitting saw, and installed on the cathodes. A Ti wire helps secure them.
I took some of the things that I had learned, and with a bit of pipe work, upgraded the cell a bit. One very useful feature is a sampling port. This can be used to sample, or drain, as needed. The nice 30 degree bend was done by applying a heat gun to the CPVC pipe and carefully manipulating it a bit. The CPVC ball valve was a common hardware store purchase.
I also decided to improve the HCl dosing pump integration. Specifically, I created a stand-pipe that would deliver the HCl in a bit more diffuse manner. This following picture is the bottom of the CC lid. The vertical pipe is what carries the HCl dosage. I drilled a few small holes in the pipe on the side away from the electrodes. When the HCl dose is introduced into the pipe, it seeps out the holes and mingles with the liquor in a hopefully gentle and gradual manner.
Another important improvement is an open pipe leading to the liquor, seen here:
This CPVC pipe bends at 90 degrees, and enters the cell at half-height. When the cell is full, the liquor travels up the pipe, and halts at the full level of the electrolyte, meaning it is about 3" below the funnel. Since bubbles really do not travel up that pipe, it does not emit noxious fumes, and does not need to be capped. I set a funnel into the pipe, and it is used for topoff, and for returning drawn samples back to the bulk of the liquor.
On the EC, the anode did get a bit hot, so I drilled and tapped the Al block connector and installed a big surplus heat sink:
The new lid carries two lines for air agitation, one which impacts the electrodes, and extends about 1/2 way down, and the other, at full depth. I have seen in the past that a full-depth PTFE line can become jammed with crystals, despite the air flow, so the mid-depth line is a backup as well. At least one of the air lines will operate throughout the run.
The Hanna HCl dosing pump was set on a timer. This pump is interesting in that it displaces the HCl with very audible "strokes," where one stroke = 1 ml. With the dial set at midpoint, it strokes about once very 2 seconds. At the low point (where I set it on the timer) it strokes once every 5 seconds. The Intermatic DT620CH timer can be set for up to 28 events. I programmed it to turn on for 1 minute, 4 times a day. Each ON cycle will deliver about 20ml of acid, which should keep it very close to 6.8 pH.
The previously modified Nalgene 1 liter container resides on the bottom shelf of the cart. I forgot to mention, it does have a miniscule hole drilled in the lid to provide a vent; otherwise, it might collapse (or the pump may lose prime) due to vacuum.
The T-Cell Junior is brewing right now, and the run is going very well indeed. It's hard to argue with the simplicity of a monolithic cell, but I have not given up on the circulation concept. Given the volume of this cell, I expect between 3 and 4 kilograms of potassium chlorate salt per run, and with pH now under control, I am hoping that the efficiency will rise, from 61% (previous cells) to hopefully 75 to 80%. So far, pH checks with my inexpensive Milwaukee brand pH probe show that the system is responding very nicely to HCl dosing. Whereas previous runs would take the pH to 9 or 10, this system has been closer to 7.2, with the goal being 6.6 to 6.8.
That's it for now. I'm a bit bummed, but no big deal. I expected setbacks, and I certainly learned a lot. That's what it's all about, isn't it? 
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