Blow down PH and alkalinity

[mohtogh]

Hi guys,
I have a hot water stream, with PH=9.5, that goes to a steam separator.
My question is this:
After flashing,how much alkalinity goes to steam stream(as CO2)?
How much is blow down's alkalinity and PH?
I have heard different answers:
"...PH increased"
"...PH decreased"
"All carbonate species go to flashed steam"
"All carbonate species go to blow down"
What is your idea?

Thanks

 

[25362]

My thoughts: if the minute amount of CO[sub]2[/sub] remaining in the boiler (blowdown) water flashes away, the alkalinity of the remaining water would have to increase.

Approximately 0.034 mol of CO[sub]2[/sub] dissolves per liter water at 25[sup]o[/sup]C at atmospheric pressure. Most of the dissolved CO[sub]2[/sub] appears as CO[sub]2[/sub](aq) and only a small fraction actually reacts with water to give H[sub]2[/sub]CO[sub]3[/sub](aq).

Although describing carbonic acid as being solvated CO[sub]2[/sub] is an oversimplification, for the following discussion we shall assume carbonic acid (a weak diprotic acid) forms from solvated CO[sub]2[/sub].

See the example below to show the relative proportions of the carbonated ion species in equilibrium at differing pH values.

Carbonic acid can give up one hydrogen ion to form HCO[sub]3[/sub][sup]-[/sup], or two hydrogen ions to form CO[sub]3[/sub][sup]=[/sup].

H[sub]2[/sub]O(l) + CO[sub]3[/sub]H[sub]2[/sub](aq) <=> HCO[sub]3[/sub][sup]-[/sup](aq) + H[sub]3[/sub]O[sup]+[/sup](aq)​

where:

[H[sub]3[/sub]O[sup]+[/sup]][HCO[sub]3[/sub][sup]-[/sup]][&divide;][H[sub]2[/sub]CO[sub]3[/sub]] = 4.3x10[sup]-7[/sup]​

and

HCO[sub]3[/sub][sup]-[/sup](aq) + H[sub]2[/sub]O(l)<=> H[sub]3[/sub]O[sup]+[/sup](aq) + CO[sub]3[/sub][sup]=[/sup](aq)​

where:

[H[sub]3[/sub]O[sup]+[/sup]][CO[sub]3[/sub][sup]=[/sup]][&divide;][HCO[sub]3[/sub][sup]-[/sup]] = 4.8x10[sup]-11[/sup]​

At ambient temperature the equilibrium distribution of H[sub]2[/sub]CO[sub]3[/sub], HCO[sub]3[/sub]H[sup]-[/sup] and CO[sub]3[/sub][sup]=[/sup], in water at pH=9.5 (ie, [H[sub]3[/sub]O[sup]+[/sup]]=3.16x10[sup]-10[/sup] M), from the above formulas, is ~0.00064 : 0.85 : 0.15, respectively.

Measure the pH after flashing, upon cooling the sample to ambient temperature. Then estimate the relative amounts from the above formulas.

For example, if the pH rises to pH=10 (ie, [H[sub]3[/sub]O[sup]+[/sup]]=1.0x10[sup]-10[/sup] M) the respective presence of the mentioned species would be ~0.00016 : 0.68 : 0.32.

Is this the answer you were interested in ? I wonder.

 

[mohtogh]

Thank you for your help.