Failing T-15 SWG Cell Experiment

[JamesW]

There’s a pretty big change in solubility of chlorine gas in water with temperature -

View attachment 605557

Almost 50% less as you go from 20°C to 40°C. I know in commercial chlorine production, a lot of energy is expended keeping the electrochemical cells cooled.

Higher water temperatures improve production, so I would think that you would not want to cool the solution too much.

 

[mas985]

While the production of Cl2 goes up with temperature, the production of side products also increases by even more. From some measurements I did a while back, the cell produces 1.64 lbs/24hrs @ 7.24 amps & 100F (0.23 lbs/24hr/amp) while it produced 1.46 lbs/24hr @ 5.5 amps & 70F (0.27 lbs/24hr/amp). So the efficiency of the cell is actually better at lower temperatures which may be more important for commercial production.

 

[JamesW]

the efficiency of the cell is actually better at lower temperatures which may be more important for commercial production.

Commercial production uses a much higher salinity, so the efficiency is probably not affected as much.

If the process produces heat, then you would need to remove heat to prevent boiling.

They probably have ideal conditions like salinity, water temperature, plate spacing and voltage for commercial production.

 

[mas985]

Reynolds is proportional to the flow velocity and the hydraulic diameter. The hydraulic diameter for one pair of cells (4mmx62mm) is 7.5mm so with a velocity of 0.28 m/sec, the Reynolds number for the plate pair is only 1572. So yes, it is near the top of the laminar region.

I started with the low flow rate case because many people run at lower flow rates and to allow more time for the H2 to rise higher in the cell before being expelled out of the end of the cell. Plus, the lower velocity decreases run time and increases the model stability so was a good starting point for this type of analysis. To get into turbulent region, a velocity of 0.72 m/sec would be required but then there is less time in the cell as well so mixing may not be that much better but I can try it next.

The turbulence length for fully developed unobstructed pipe flow is approximately 3.8% of hydraulic diameter which for this case is 7.5 mm (4mm x 62mm duct dimensions) or turbulence length of 0.28 mm and is about 7% of the 4mm gap distance. So turbulence is fairly localized unless there is an obstruction but then, the turbulence tends to be more down stream than cross stream. So I think this is why you don't see much cross mixing.

Turbulence length scale -- CFD-Wiki, the free CFD reference

www.cfd-online.com

That was added to the simulations done in post #73 to represent scale in the cell in order see if it would increase turbulence and mixing. Plus conveniently, It is also used as an inlet for the H2/Cl2+O2 gas streams.


True but that is not captured in this analysis and doesn't really affect the outcome in terms of mixing since the two gas streams never really mix anyway.

Also, even at 0.28 m/sec, an CL2 molecule at the beginning of the cell would only spend less than a second in the cell and there is some evidence that the dissolution rate for Cl2 may be longer than that depending on conditions so Cl2 may not dissolve until it leaves the cell.

One thing I forgot to mention in this post is that the flow is laminar near a smooth tangential wall and along the plates is where the gases and ions are generated so the turbulence along the plates is minimal at best even when the central flow is fully turbulent.

This can best be illustrated with the following steady state analysis which represents ion concentrations. These are perfectly smooth plates. I am using the same injection points I did earlier but I added a separate injection point dead center in the middle of the plates where turbulence is the highest. For this case, I am also increasing velocity to 1 m/sec for a Reynolds number of 5655 so well within the turbulent region.

The first two plot are the concentrations for the injection points along the plate surfaces which represent ion flow from the anode and cathode. The third plot is the concentration for the injection point between the plates. You will notice that when there is an injection near the center of the plates, the concentration spreads out fairly rapidly due to the turbulence while the injection near the plates does not diffuse as quickly.

This is the primary reason that there is very little mixing in the scenarios that I showed earlier.