Homegrown Oxidizers Part Six
WSM

The Continuous System in Practice

The potassium chlorate batch systems previously discussed are simple compared to a continuous system. To run a chlorate system on a continuous basis involves a great deal of attention to myriad details (many of which can be ignored in batch systems), and to omit any one of them invites headaches and an inefficient system.

Where, in batch systems it’s nice to track various conditions in the cell, in continuous systems, it’s required.

This is a broad subject and we’ll cover various cell configurations in showing and describing different continuous systems (keeping in mind that they share many common features, despite their fundamental differences).

Building the System

We’ve chosen to use PVC (polyvinyl chloride) for our continuous system tanks. The reaction chamber (RC) is a piece of pipe set vertically and cemented to a square piece of PVC plate for the bottom. The best practice is to apply cement to both pieces till they soften slightly and then put them together with pressure and leave the assembled combination alone for 24 hours, to make the strongest, permanent bond.

Thick-walled PVC pipe glued to a thick PVC plate to fabricate a reaction chamber (RC). The seam is sealed with extra cement filler, as described in the text.

It’s best to fit the two parts before cementing, so there are no gaps between them. Some amount of filing or sanding may be required to get the ideal fit. If some gaps appear between them after the cement has fully dried, apply medium to heavy bodied cement with disposable, one-piece polyethylene droppers to the gaps till they’re completely filled. Repeat the process with a fresh dropper each time, till no gaps exist, and be sure to let the cement dry for 24 hours between applications for the best results.

For bigger and/or thicker PVC pipes and plate, the slower and heavier bodied cements should be used. Before mating the parts together, use several applications of cement to both parts, with one minute intervals between applications, to assure proper softening of the hard PVC before they’re put together with pressure, so a better final bond will result. To fill any gaps found, use the same techniques mentioned in the paragraph above.

To put a threaded fitting in PVC involves drilling and tapping. The problem with drilling hard PVC with standard machine drills is the drill flutes are too aggressive and tend to break, crack and chip the hard plastic materials. The authors have had better results using specialized Forstner and step-less taper drills and then tapping the hole with NPT (National Pipe Taper) pipe taps to create the needed threads for standard sized fittings made of materials compatible with the cell liquor.

Once the proper sized hole is drilled in the PVC tank or lid, the pipe tap easily cuts the threads required to hold the fitting, connecting pipe or tubing to the cell.


This photo shows a Forstner drill bit in the upper left corner, “step-less” taper drills on the right and NPT pipe taps in the lower left corner.


A clear PVC cell lid with Kynar (PVDF) fittings mounted in NPT threaded holes.

We typically use Teflon tape on the threads of the fittings when assembling the components of our systems. Using three wraps (without allowing any to cover the openings) is a good practice and assures a leak-tight seal when properly installed.

The RC (and EC, if used) is vented to atmosphere, and it’s helpful if the vent tubing is large enough to prevent pressure buildup in the tanks. It’s important to keep the vent tubing from curling, where low spots can collect moisture and block the exit of gasses generated (primarily hydrogen). Using PVC pipe will also work, and has the advantage of being rigid so it tends to stay where it’s put (unlike flexible tubing).

Because of the explosive nature of hydrogen, for safety, be sure to vent it upward and away from any source of spark or ignition.

To mitigate any corrosive exhaust vapors, some have routed the vent through a glass or plastic jar to condense and collect any warm salt spray. If free chlorine gas is generated, it can be neutralized by reacting in the jar with lye and rendered harmless.


Setting the System up to Run


A partially completed continuous system assembly with an external electrode chamber (EC, in dark grey on the right) plumbed to the reaction chamber (RC, pale green in the center) and then gravity fed to a crystallizing chamber (CC, white with a dark grey lid on the left).

latest incarnation.pdf
A fellow enthusiast’s continuous potassium chlorate system, set up to run. Note the foam insulation and myriad tubes, wires for sensors and other connections. This setup produces over a pound of potassium chlorate a day.

Even though the photos above show external electrodes and chambers, much of our current thinking is to place the electrodes inside the reaction chamber (RC) to simplify the continuous setup. Also, placing the electrodes lower in the cell encourages electrolyte circulation by an effect called (by us), “hydrogen lift” due to the upward flow of hydrogen gas from the cathodes.

The fundamental idea of our system is to separate the RC, which runs at higher temperatures and creates the chlorate that stays in solution, from the CC (downstream), which at lower temperatures allows the chlorate to drop out of solution as crystals where we will collect them. The depleted liquor is then pumped from the CC through the heat well back to the RC, where it’s recharged by addition of new concentrated salt solution.

The schematic drawing below shows the concept with the electrodes run from the lid of the RC and positioned deep in the cell. The design is such that the fluid level in the RC is constant and overflows to the crystallization chamber (CC), as replacement salt solution is added to the RC. This scheme allows limited fluid level variation in the CC, controlled by float switches and the diaphragm pump in the heat well, placed in the CC.

The heat well in the CC is necessary to prevent crystal fouling of the pump and float switches by keeping the chlorate dissolved and from dropping out of solution there.

Internal Electrode RC Concept.doc
By putting the electrodes inside the reaction chamber we simplify the entire system.

This concept depends on careful control of various conditions in the cell and keeping them in balance. For example, it’s important to keep the temperature high enough in the RC that crystals don’t form there. Also, it’s important to maintain a low enough temperature in the CC (whether by passive or active means) that the crystals do form. For energy savings and simplicity, passive cooling is preferred.
The temperature maintained in the RC is largely a function of electrode size plus properly matching the power supply to the system. Insulating the external parts of the RC can be instrumental in maintaining higher internal temperatures, if necessary. Venting and positive airflow around the CC can help reduce the internal temperature and help increase the yield. With a wide and shallow CC, we’re striving for greater natural cooling, because of the large surface area of the tank.

With an optimal design, hopefully we can eliminate extra measures and keep the total system simple.


Fine Tuning the System

Once our continuous system is set up, filled and running, we need to “fine tune” it to make it as efficient as possible. By monitoring all the parameters of the system as it runs, we can track its progress and determine where improvements can be made. Some suggested things to follow are:

• Temperature of the RC and CC
• Voltage feeding the cell
• Current feeding the cell (more important than the voltage)
• The time the cell runs (keep a logbook)
• The pH (while aiming for the ideal of 6.8)
• The specific gravity of the cell liquor, to determine dissolved salt or oxidizer content
  

A few enthusiasts have worked on data acquisition systems to track parts of this information. The notion of using a watt/hour controller to actively control the pH has also been considered. There are many ways to approach these parts of the system and overcome challenges that appear in the various setups.

We are limited primarily by our imaginations, budgets and time constraints. Truly, “it’s amazing how complicated a simple concept can get”. Once the bugs are worked out of our system, we can strive to simplify the designs and operations of them; otherwise we can satisfy ourselves with “good enough” and move on to other challenges (such as entertainment and artistic uses for the products we make).


Improved Product Refinement


A bench top Buchner funnel large enough to process several kilos of oxidizer crystals at a time.

With larger scale production of oxidizer salts appears the need for upgrading our processing system to increase the efficiency and speed of preparing the product for use in pyrotechnics.

By the use of a large bench top Buchner funnel and properly sized laboratory grade filter papers, this step is greatly enhanced. Heavy walled vacuum tubing connected to the lower side port of the funnel is run to a large, thick-walled glass bottle used to collect the extracted fluids. A vacuum connection on the bottle activates the filtering by creating negative pressure, drawing the liquids out of the crystal mass, through the filter paper, and collecting in the vacuum bottle.

Rinsing the residual liquor off the crystals is accomplished by simply washing them with cold distilled water while the vacuum continues to run. The depleted liquor and wash water are taken, recharged with more chloride salt and used to replenish the running cell.


Next, the “Holy Grail”, making perchlorates…