50% Caustic Piping in Cold Climate

Caustic is difficult stuff, you need to keep it above the freezing point and below a max value. The band is not that big.

EHT with line sensing is the best option. The insulation integrity is very important, don't try to save money on that part.

Try to go for self regulating, I know that it is not that popular in Canada, but in Europe we only use self regulating heaters on Caustic lines.

There is a good supplier of EHT in Edmonton, they will solve your problem with a smile.

Quite. Self-regulating electric trace heating tape, covered with a substantial quantity of pipe heat insulation and protected from water ingress, will do the job.

Steam has also been used to trace-heat caustic lines in heritage installations, 1/4" copper pipework for the steam being secured to the process pipe before the insulation goes on.

Where I am at is not nearly that cold but from experience, we used 50% solution and it will start to gel at around 45 degrees farenheit. Not the freezing point but a little above it, it will start to gel up and give you problems if you're trying to pump it somwhere. Plenty of heat trace and insulation unless you have another heat source (assuming it's in a pipe). Diluting it won't help much as it didn't for us, just postpones the inevitable slightly and you're considerably colder than where I am. Good luck with it. It was a PITA while we were using it.

Typically we use Potasssion Hydroxide around here rather than Sodium hydroxide for the very reason you are talking about. The freezing point for 50% KOH is around -11 C, much easier to work with than regular 50% NaOH from a freezing point of view.

I would encourage a system design which would ensure that the caustic would not freeze. You are able to assess the flowrate, the cooling conditions etc in your piece of pipe, the lowest outside temperature etc. From all this data you will be able to work out whether the system will be sufficient to prevent a caustic plug forming.

This would be my approach. If you are wanting to use a 50% solution of caustic then EHT would be a requirement as my humble engineering opinion in a 1500m pipe length in a location close to Fort McMurray.

Kind regards

Alex Chatwin

Thank-you all for your input.

I think we are going to settle on a dilution skid to bring the concentration from 50 to 20 wt%. The dilution skid will be monitored with mass flow meters so we can keep an eye on the density of the product to ensure that we are not producing offspec caustic which could freeze at a different point than expected. We will use redundant, monitored heat tracing (probably self regulating) and insulation, taking care to do an excellent job on the insulation. We will also have a flushing procedure in place to evacuate the lines immediately should we have a failure in heat tracing or should the product go off spec (pure water for instance which will freeze at 0 deg C vs -27 deg C for 20 wt% caustic). After that, we'll hope for the best.

You may want to try a different approach - magnesium hydroxide may work for your application. It freezes at 0C. A circulating loop maintaining pipe velocity would be recommended. See magspecialties.com

Thanks,

We are using caustic to neutralize heat stable salts in amine in an electrodialysis membrane stack. There is no tolerance for solids. We filter all fluids entering the stack down to 1 micron. I don't think that a magnesium hydroxide suspension with 3 micron particles is going to work very well. None-the-less, thanks for the input. It is appreciated.