50% Caustic Soda Feed - Storage Temperature and Line Material (CPVC?)

[RVAChemE]

Hello folks,

I've done a bit of searching and wasn't able to find any good answer to this question.

We're designing a 50% caustic feed system for a water treatment plant in north Texas. Design flow is 50 GPH. The chemical skid will be outdoors and I believe completely exposed to the elements.

Questions are:
1) What temperature should we maintain in the chemical storage tank? I've read numerous sources that say to target 80*F but our lead engineer wants to go with 65*F. I've looked at the viscosity difference between the two temperatures which shows 80 cP vs. ~50 cP for 80*F and 65*F. Is there any additional rationale for the higher temperature? We're above the freezing point (54*F).

2) We are planning on using CPVC piping for the feed lines on this skid, 1" and 3/4". Is this a wise choice for a skid that is outdoors and unprotected? I know CPVC has a higher chemical resistance but can be rather brittle from my experience with continued UV exposure.

Thanks in advance for the help guys!

 

[moltenmetal]

I would not use CPVC outdoors without at least white mastic to protect it from sunlight- unless it is a short term project. If lines will be insulated, that will protect them from UV.

Targeting 65 F is probably going to let you sag below the freeze point somewhere in the line. Unless you can scrupulously avoid it, target a higher temperature.

I suspect the non-chlorinated polymers (PP, PE) are going to have better long-durability resistance to caustic, but a quick check of my references had CPVC listed as acceptable (some degradation) as long as temps are below about 100 F.

 

[chemEcaleb]

The temperature you're targeting should depend on the volume of the storage tank, and the recirculation flow rate on your heating system. If your tank is 100 gallons, and you're recirculating 10 gpm, and there's only 20 feet of piping on your skid, 65F is probably fine. But if it's a substantially sized tank/skid I agree with moltenmetal that 80F is safer. But just design in some overcapacity in your heater and you can change your mind to heat the tank up to 80F after you freeze a line!

 

[bimr]

Is there any reason that you are not following the caustic soda manufacturers recommendations on storage and handling?

CPVC is a safety hazard as it may break. You will also have to have pipe supports every 4 feet or so.

Seamless carbon steel, butt-welded Schedule 40 pipe is recommended. Lined, flanged pipe has also been used successfully. All piping should be installed above ground.

http://msdssearch.dow.com/Published...soda/pdfs/noreg/102-00011.pdf&fromPage=GetDoc

 

[itdepends]

How big is the storage tank? Would it be economical to eliminate the heating system by diluting the caustic in situ (assuming you don't precipitate any solids in the water you're using to dilute it).

A relatively small change in tank cost might save you considerable sums in electric heat tracing on your pipes (which you'll need if you use steel and it gets cold enough).

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"