Descaling of Plate & Frame Caustic Heat Exchanger

[Iomcube]

Hey there, In my chloralkali industry a P&F HXchanger (constructed out of Ni) is used to cool 32 w.t% NaOH from 87 to 75 Celsius using Cooling water (CW) available at 32C (in summers)

For previous x2 months the duty of HXchanger duty decreased such that outlet NaOH temperature cannot be decreased below 80C

Flowrates of NaOH, its entering temperature is same (thermocouples & flowmeter). CW flowrate is also same (indicated by pressure gauge at the inlet to HXchanger)

In an attempt to insitu descale HXchanger without disturbing running process I utilized an opening to inject HCl (34 w.t%) at the inlet of CW. Stopped CW flow completely for ~2mins & initiate HCl flow while keeping outlet temperature rise in check! Then again CW flow is resumed & HCl flow stopped after ~15min again the previous ~2min HCl cycle was repeated. In whole of this process outlet point of CW was open to drain (instead of being returned to Cooling Tower)

This HCl flush cycles were done ~6 times after the 4th cycle the outlet pH of HCl stopped increasing indicating no HCl consumption while descaling salt deposits.

The performance of HXchanger (after above activity) slightly increased but to no avail ..it now approached minimum temperature of 79C (a decrement of only 1-1.5C)

Now is there any other activity I should try, anything other than HCl! I believe that our Cooling Tower has deposited silica based impurities & what cleaning agent I should try to remove it.

Regarding Cooling Water specs: 6-6.5 pH; chromate inhibited; 2barg pressure

 

[ashtree]

What has lead you to the believe that it is a silica based impurity that is causing the problem>
Do you know exactly what this silica based impurity is?

Silica based scaling will be very difficult/im[possible to remove with standard CIP methods. Hydrochloric and similar acids will generally make little difference. You may have to resort to dismantling the HEX , plate by plate and physically chip out the silica scale.

The HEX supplier may be able to give you advice on what to do beyond what i have said above.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Compositepro]

Are you even sure that the fouling is on the cooling water side?

 

[Iomcube]

ashtree
Because the CW source specifications. We use water where you can remove the probability of algae growth & related stuff. However, mis-operations can be included where pH of CW remained high causing silica deposits

Compositepro
Because the caustic at the inner circuit is from Membrane electrolyzer where any impurities existence can easily be judged through extensive lab/quality checks round the clock.

...& the pH of HCl stayed constant AFTER 4th cycle ...how do you explain this observation?

 

[EdStainless]

You need to open it and retrieve a sample of the scale.
Send it to your water treatment people for analysis.
You need the correct chemicals, maybe a different acid, maybe a caustic, likely with additions to allow high solubility.

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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear Iomcube,

Can't agree more with EdStainless. Sometimes there is no alternative for old fashioned visual inspection.

Hydrojetting of the exchanger is the need of the hour.

Regards.

DHURJATI SEN