Chemical Dosing Rate Calculation - Acid Water Neutralisation 1

[jph90]

Hi All,

Looking for some assistance on a relationship/calculation to determine the caustic dosage rate required into a certain flow rate of acid water so as to neutralise it to a certain pH. If you refer to the attached sketch, it shows the parameters i am looking to get a relationship for. Hopefully i have the correct variables covered but let me know if not

Cheers

 

[moltenmetal]

The main neutralization (taking pH from 14 to about 9) requires no more information than what you have now, and a basic knowledge of chemistry.

But if you started with something other than ultrapure water and ultrapure sodium hydroxide, you will need more information about what else is in the water to accurately estimate how much acid is required to get you from pH 9 down to 7-8.

 

[jph90]

Molten - are you able to tell me the relationship?

Thanks

 

[moltenmetal]

Pretty basic question, no pun intended. Well within your Google skills I think. pH is the inverse log of the H+ concentration. You want to take it from pH 7 (i.e. [H+] = 10^-14 mol/L, i.e. basically no free protons) back up to pH 7 ( [H+] = 10-7 mol/L. Round numbers, you need to add a little less than 10^-7 mol/L H+ by adding a strong acid. However, by making that calc, you're asssuming that the water has just strong base in it, and contains mo other species that will absorb protons at a pH intermediate between 14 and 7. Since you don't actually know the full water chemistry, you need to do a titration and MEASURE how much acid it takes to actually bring the pH from 14 down to 7.

 

[moltenmetal]

My apology- the concentrations are in weirder units still- moles of H+ per mole of solution, not mol/L as I had previously stated. The rest should be correct, more or less.

 

[elridyeng]

i think you can control it manually from changing the dosing pump flow rate and and monitor it for awhile then you can fine the optimum flow rat to get your required PH.

 

[bimr]

If you know the fluid chemical properties, it is a simple matter to determine the dosing rate. It would WAG to size the dosing pump without the calculation.

 

[RuwanR]

In order to do the neutralization , you should know the following basic formulas beforehand.
pH = - log10 [ H⁺ ] …………………..①
pOH = - log10 [ OH⁻ ] ……………..②
pH + pOH = 14 ……………………..③
Your inlet water pH is 2. According to formula ① ,
H⁺= 10⁻² mol ̷ L
Suppose the inlet flow is 50 m3 ̷ h .
So the H⁺ flow rate , 10⁻²mol ̷ L X 50 m3 ̷ h X 1000 = 500 mol ̷ h
To bring the pH 2 to 7( neutral zone) , you need to dose NaOH in the rate of 500 mol ̷ h.
Molarity of NaOH is 40.01 mol ̷ L .When the Caustic strength diluted to 50 % ,
The molarity will be 40.01 mol ̷ L X 50 % = 20 mol ̷ L
The dosing rate of NaOH =( 500 mol ̷ h) ̷ (20 mol ̷ L )
= 25 L ̷ h
Like wise you can calculate the dosing rate for alkali water using ② and ③ formulas. But keep in mind that these are just theoretical , the actual could vary due to numerous reasons.