Problems after de-clamping with caustic soda 2

[Nicksun]

Dear All,

I got a UPW plant:MMF->ACF->cat-anion->RO->MB->Polishing.

Now the MB resin can not separate well, becuase the inlet water is 1Mohm.cm and the setpoint of MB exhaust is according to 50pppt boron. so the H and OH resin still alive and absorb each other.

After discuss, we use NaOH to make a de-clamping precedure.
and after that, we did a double-regeneration.

After 10 hours rinsing, the resisitivity is fluctuating between 17.5Mohm to 13Mohm, and boron is fluctuating also from 100ppt to 0ppt. I am think the mixing was not done well. So I made another mixing. But still not satisfactory. The resistivity is still fluctuating between 17.5Mohm.cm to 16Mohm.cm.

Please help to figure out the causes.

Thanks very much.
Have a nice weekend!

Nixon
Pl

 

[sbush]

I've run into this separation problem on start-up of new system. I haven't had this problem on systems with longer operating experience, probably because non-regenerable polishing resin systems or "packed beds" were used.

I would "neutralize" the H-OH charges on the cation and anion resins using 4 - 5 pounds of high-purity sodium chloride per cubic foot of resin at a diluted concentration of about 10 percent (when passing through the resins) and a displacement rate of 0.5 to 0.75 gallons per minute per cubic foot of resin. If you are using warm water to regenerate the anion, this is okay to use with the brine solution too. After displacing all of the brine solution, perform a resin separation using a constant rate of backwash to expand the bed at least 75 percent for at least 15 minutes duration. Backwash flow rate should be carefully controlled to ramp up or ramp down very slowly. Abrupt changes in flow can cause a greater degree of interfacial mixing of cation/anion resins.

When a good separation occurs, continue regenerating the resins using acid and caustic per your usual recipe. You may choose to "double regenerate" to ensure maximum water quality output. Double regeneration simply means to extend the acid and caustic injection times to twice the normal duration which corresponds to using twice as much chemical volume. It does not mean to backwash and separate twice.

S. Bush

http://www.water-eg.com

 

[Nicksun]

Thanks S.Bush for the insights.

Becuase the high-purity NaCl not readily available.
Now I have killed the mixed bed resin with 55g 100%NaOH/L (anion resin).After this, displace and rinse the bed.
And the separation was very successful. After double regen, the rinsing water resisitivity is always fluctuating.

Now the question is: what leads to "The resistivity is still fluctuating between 17.5Mohm.cm to 16Mohm.cm."?

Could be:
1) Mixing was not done well.
2) Seperation was good, but the surface line was not even.(This line was very hard to judge by check the narrow sight glass), so the cross-contamination of anion resin was worse than normal.(There is no inert resin between the anion and cation resin, so there is alwasy a portion of anion resin was contaminated.)

Regards,
Nicksun

 

[sbush]

Is this a recent developement where you were accustomed to always seeing a relatively constant 17+ megohm quality but now the quality is fluctuating for some unknown reason?--or-- Is this a new system that has been fluctuating in this manner since start-up?

Either way, here are some suggestions that might help:

If you want to get the straightest line of separation, then turning off the backwash flow very slowly and incrementally (imagine stopping a car with a glass of water on the dashboard)will help.

When you begin air mixing, you need enough water above the bed to allow for good fluidization of the resins. After approximately 5 - 7 minutes, begin draining off the excess water freeboard volume while continuing to add air. Stop draining when the movement of resin in the bed is no longer fluid. This will inhibit the separation of the mixed bed resins due to their different specific gravities and physical size. When refilling the vessel with water after air mixing, be sure to control the flow rate to minimize disturbances to the top of the bed.

Another factor that effects output quality is starting and stopping the process flow or abrupt changes to the process flow during operation. If the demand for water downstream of the deionizer is intermittent or variable, then you should consider stabilizing the flow through the deionizer by adding a pump and a recirculation loop.

If it is possible for regenerant chemicals to leak into the vessel during service mode, you should consider using a block and bleed type valving arrangement (removeable spool piece or something similar) on the acid/caustic inlet lines.

If the vessel has places where exhausted resins and/or regenerant chemicals can "hide out" (i.e., avoiding rinsing) such as sight windows where the glass is not flush with or protruding slighly past the vessels interior wall or perhaps a resin clean-out nozzle with a blind flange on the exterior of the vessel; these can be sources of contamination that need to be fixed. Look for hide outs. These can sometime be found below the underdrain collector if there is too much space below the collectors. Sometimes collector can become clogged in localized areas that create dead-spots. Sometimes it can be a design defect by the manufacturer. I rehab a lot of brands, and frequently I'm surprised by what I find. Unfortunately, this industry has are a lot of "manufacturers" that know enough to be dangerous!

S. Bush

http://www.water-eg.com

 

[bimr]

Please provide additional information.

How old is the system?

Has the system ever worked properly?

How old is the resin? Resin older than 2-3 years will most likely not be able to meet your water quality requirements.

Is the resin that you are using designed for mixed bed use?

You probably have a resin problem or a separation problem. Exhausting the resin will not fix the problem.

 

[Nicksun]

Thanks for all the inputs.

To bimr:
The UPW system is new, started running in 2005 and has run well.

the MB resin can not separate well, becuase the inlet water is 1Mohm.cm and the setpoint of MB exhaust is according to 50pppt boron. so the H and OH resin still alive and attract each other.

Then we dosing NaOH(55g/L resin) to make good separation.
and it worked well.
and then we did 1.5times regeneration, then we have the problem I described here.

Now the latest information is:
we made double regeneration and the trend the much better - outlet resistivity is slowly climbing to 18Mohm.cm after 8 hrs rinsing.

So we think the root cause for the above problem is that we have not done a thorough regeneration after we killed the cation resin so severely.

 

[bimr]

Clumping of resins of resins should only occur with new resin. You have been operating for a while so it does not make any sense.

Resin manufacturers have addressed the clumping problem with new resins by treating the anion resin with a proprietary chemical to partially foul the surface of the resin to prevent clumping. A number of remedies have proven helpful in this regard, amongst which are the use of Triton X-100 in a 1/1000 dilution, or a complete exhaustion of the mixed bed resins.

However, I suspect that you are not finished with this problem. You indicate that you have ionic leakage.

Because of the intimate mixture of cation and anion resin in mixed bed units, there are multiple cation and anion stages and one would predict little to no significant ionic leakages when both resins are functioning properly.

You should not have to brine exhaust the resin to achieve good separation. There should be a definite density difference between the anion and cation resin that allows for easy separation even when the resins are just slightly exhausted.