Caustic injection into tank for pH control 1

[AJAvery]

We are having issues with caustic injection into 30,000 gallon pH adjust waste tanks. The "injection" is done with a control valve feeding a 1" PVC line into the top of the tank. The issue we are experiencing is crytallization of the casutic in and around the 1" pipe. The pipe ends ~5 feet above water line. The tank is closed but does have exhaust.

1. What causes the crystallization?

2. What is the best method to inject cuastic soda into a water solution?

Thanks in advance,

Andy

 

[ash9144]

Caustic alone will crystallize. More readily the higher the concentration. It is usually easily flushed away with hot water. If your crystals are outside the line, I'd say you have a small leak. If the crystals are forming on your feed pipe and restricting flow, I'd guess your flows are fairly low and are getting crystal build up.

A better injection point would be on a circulating pumps suction line. Assuming you have enough caustic pressure to overcome the head pressure.

 

[JimCasey]

You did not say, but I suspect you are using 50% (Commercial) caustic. It's pretty concentrated, so any caustic that wicks off the flow stream or spatters will dry and crystalize wherever it lands. Another thing: 50% caustic freezes at 52F if I recall, so be aware of the potential for a clogged line/valve this winter. The slushy semi-frozen mixture is abrasive to the valve as well.

Pouring it in from 5 feet above is, as you have found, a little messy, and caustic poses a personnel hazard, too.
If you have a continuous flow into the tank you could tee into that line and the flow would help mix the caustic, or dip your pipe in below the normal waterline and squirt the additive in there. Look for a spot with turbulence and do not inject it where the unmixed plume of caustic will drift over the outlet of the vessel. Frequently it is appropriate to use a mechanical (Lightnin') mixer to homogenize the solution.

 

[JStephen]

If you're intentionally trying to grow crystals, you can take a saturated solution and cool it, or let water evaporate out of it- and either one could explain the crystallization going on here.

 

[hacksaw]

glad you qualified the term "injection"

why dribble caustic into a tank, you have little or no control of the mixing, dilution, and or contact time.

there are common injection methods that involve circulation and mixing tees.

of greater concern is that by injecting in the top of the tank how are you planning to control pH? where is your analyzer point located?

are you batching or continually treating your waste water?

 

[MedicineEng]

Is dilution an option? Here we use 20% soda for our neutralization of the waste water. As far as I remember, only once we had a problem with crystals. I believe that the guy adding water didn't know how much needed to add and made a more concentrated solution.

 

[bimr]

Caustic begins to crystallize and become very thick at temperatures as high as 55 Degrees F making it difficult to feed. Diluting chemicals allows them to be sucessfully fed at ambient temperatures down to freezing and reduces the need for heat tracing on piping, valves, etc.

 

[Shimasu]

If your soda is being cristallized the easiest way out I see without making major changes is to dilute it, however you need to consider if you can afford a more dilluted solution (because of the increased flow, the mixing conditions etc)

if it is still crystallizing see if you can somehow heat your solution besides diluting like some hot stream you have nearby.

Now if you say you have crystallization around the pipe and that the tank is closed I would think of leaks, give it a look since Soda is a very dangerous chemical, and all the time wear googles, the soda diffuses very fast in the retine.

 

[controlnovice]

If you can't dilute it, heat trace it to 60-70F.

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This is normally the space where people post something insightful.

 

[pumpking]

Why not submerge the end of the pipe into the tank to which you are dosing ? Caustic crystalises when coming into contact with air, so submerge it to eliminate the opportunity - it will also remove potential to splash and risk personell safety.



Ash Fenn

http://www.cdrpumps.co.uk