NaOH (Caustic) dilution system

[OneManWolfpack]

I'm designing bulk caustic storage facilities for an existing 2 MGD water treatment plant. The plant currently receives bulk caustic shipments in 50 gallon drums and dilutes it to 25% in a day tank prior to feeding. Client would like to switch to tank truck delivery, so the obvious solution is to put in a new bulk tank to hold the 50% caustic from the tank truck and keep the day tank setup how it is.

Besides safety concerns, the main problem with storing 50% caustic is the low freezing point (~58 degrees). So any tank holding 50% will require heating or be located indoors.

It makes sense to me to go ahead and dilute it down to 25% right off the truck. This eliminates the freezing/safety concerns with storing 50% on site and also would not require daily batch mixing in the day tank. My idea wasn't received too well initially, but after some internet research I see that there are systems out there that can do this:

http://www.powellfab.com/products/dilution/causticdilution.aspx

http://www.noritake.co.jp/eng/products/eeg/mixing/engineering/mixing_sys/soda_dilution.html

Now that the idea has caught on at the office, I've been given the go-ahead to investigate this option further and cost it out against the 50% storage solution. I would appreciate any input on the advantages/drawbacks of this type of system as well as information on other companies that may offer something that would work.

Thanks in advance.

 

[OneManWolfpack]

Another idea that's been suggested is to pre-fill the tank with enough water so that when the truck pumps 50% into the tank, the resulting solution in the tank is 25%. This seems simple, but I know you have to make sure the solution is constantly mixed or it will blow up and kill everybody or something like that...

 

[djs]

I built and installed a control system for a 250 MGD surface water treatment plant where the caustic soda was diluted on delivery. A magnetic flow meter was installed on the delivery line, and controlled valve with another magnetic flow meter was placed on a fresh water supply line. Both feeds were then feed into the storage tank. Mixing ratio was controlled by the control system with several cascading flow control loops. Recirculating pumps were also installed on the storage tanks. The only down side was that if the tanks were left setting for any amount of time without recirculation, the caustic soda would settle out to the bottom of the tank. Big problem as the pump motors were not sized to handle the density of the settled caustic soda.

 

[bimr]

One would expect that you will not use enough caustic to justify the expense of the dilution system and storage tank. In addition, with a small plant, one would not expect the operators to be qualified to operate the dilution system.

You should investigate other options. You can buy caustic in larger tote containers. Caustic is also available at lower concentrations than 50%, like 25% for example. A rough comparison of shipping costs should be made. How far away are you located from the producer?

Local chemical suppliers are generally happy to dilute the chemicals prior to delivery to the site. Of course it will cost more, but the operators will not be upset at you for giving them additional work unrelated to water treatment.

I presently work at a site now where the chemical delivery is pumped off a tote within the delivery truck into a tote on ground by the delivery person. My operators do not have to touch the chemicals during the delivery process.

Note, caustic is typically diluted to 20% because that level has the lowest freezing point.

 

[bimr]

http://treatmenttech.net/caustic-soda-50-sodium-hydroxide-solution/