Using CO2 for pH control in cooling water systems 1

CO2 can cause corrosion in pipes (CO2 cracking). This must be a negative factor

Morten

Hello.

Substitution of acidic CO2 for mineral acid feed in the reduction and control of cooling water pH is an interesting thought.

The CO2 will not be corrosive, unlike in high purity systems such as steam condensate, where CO2 will form carbonic acid and reduce pH to corrosive values (like <6 pH or so). You are likely attemptimg to control pH at 7 or 8.5, or so, depending upon the cooling water treatment program in use. So, you will not get significant carbonic acid concentrations.

As cooling water evaporates from the tower, the tower make-up water alkalinity is concentrated, causing it's pH to rise, along with the water's scaling tendancy. Part of this pH rise is due to CO2 flashing to atmosphere. Hence, of course, the need to feed mineral acid to lower and control cooling water pH.

The fact that CO2 (carbonic acid or H2CO3 in water) is in equilibrium with bicarbonate (HCO3) and carbonate (CO3) in the water, and CO2 in the air, as you pointed out, is the problem with this process. The purpose is to lower pH to get away from high pH, high carbonate alkalinity related scaling problems (particularly CaCO3 scale). The lower the pH and/or (carbonate) alkalinity that your cooling water treatment program requires, the higher the tendancy for CO2 to flash to atmosphere from the tower.

Detailed equilibrium calculations or even a trial or pilot run will of course show how CO2 feed affects mineral acid feed requirement. However, you should find that the cooling water carbonate alkalinity is ALREADY IN EQUILIBRIUM WITH THE CO2 in the atmosphere. Years ago we tried waste CO2 feed at one small cooling tower with alkaline make-up water, and even though it was an alkaline water treatment program (pH approx 8), mineral acid feed reduction was small. The benefit of eliminating mineral acid feed, not surprisingly, was not achieved. My bet is that CO2 will be ineffective at mineral acid feed elimination at your facility.

An exception is if you had make-up water with negligible alkalinity (eg: Canadian west coast). However, in such a case, you may need to feed alkalinity (eg: soda ash, Na2CO3, or caustic if you had money to burn) as acid feed may not be required in the first place.

Hope this helps.

Wayne Gilraine at

Thanks for your comments Delta Cascade. What I am not sure of is the equilibrium balance will be between the CO2, carbonate, bicarbonate, and carbonic acid. I was hoping that carbonic acid would react with the carbonate to form bicarbonate and the reaction would be final. There (hopefully) would not be enough heat in the cooling water system to reverse the reaction and result in significant loss of CO2 in the tower. However, I am not sure if this will be the case. I had hoped to hear that you would have had better success in your trial operation. I am thinking of doing a pilot test to determine how well it works (or doesn't work).

Regards,
Grant

Hello

Wayne's reply is thorough. The issue is that the CO2, bicarb and carbonate are in equilibrium, and as long as the system is open, the CO2 will blow off, and the equilibrium will be re-established. The temperature affects the equilibrium, but since the rate of alkalinity accumulation is related to the make-up and blowdown, and those are dependant upon the heat load to the system, it doesn't really affect the outcome.

What are the driving economics?

Keith

Tx, folks. As &quot;Feather&quot; added, it is an open system, and as such subject to an equilibrium state with the environment. Hence, reactions are not &quot;final&quot;.

CO2 <==> H2CO3 <==> NaHCO3 <==> Na2CO3

(Na+ could be replaced by 1/2 Ca++, 1/2 Mg++, etc)

Estimated pH ranges at which each forms of carbonate alkalinity are present are:

CO2 + H2O <--> H2CO3 pH: < 6.4
H2CO3 <--> (H+) + HCO3- pH: 6.4 - 10.33
HCO3- <--> (H+) + CO32- pH: > 10.33


One would expect an open cooling water system to already be at, or very very close to, equilibrium with CO2 in the atmosphere. The &quot;energy&quot; that &quot;pushes&quot; the CO2 to atmosphere is the very same &quot;energy&quot; used to &quot;push&quot; the CO2 into the cooling water ... the CO2 &quot;doesn't water to be there&quot; in the first place as it was already in equilibrium.

Adding CO2 shifts the equilibrium to the right ... but as the cooling water is in equilibrium with the atmosphere, if you wanted to reduce cooling water pH, increasing CO2 concentration of the air would be the answer!

Exception may be if the cooling water was significantly supersaturated with CO2. Since cooling tower's purpose is to bring air and cooling water into thermal equilibrium (much air-water contact), chemical equilibrium with air is also expected to result. Hence, loss of CO2 fed to cooling water, to atmosphere.

If you had enough CO2 to dissolve into the cooling water returning from the cooling tower to the heat exchange part of the system, making up for what is lost to atmosphere in each cooling water pass through the cooling tower, you could reduce pH that way, but only in the heat exchange part of the whole system. You lose pH control at the cooling tower and introduce dynamics to control of pH in the whole system.

Dissolving the CO2 into the water in the first place, would require a contacting system, as well, and high CO2 concentrations or pressures (which are assumed available). With the introduction of dynamics of CO2 dissolution, would pH be well controlled (eg: 8.3-8.7 pH units) throughout the cooling water system? pH too high at one location may cause scaling, pH too low at another location may cause corrosion. What is the cost of the contacting system?

&quot;Feather&quot;'s comment that the typical system is open, brings in the idea of closing the system. Perhaps, then, if one has enough waste CO2, vent it through cooling tower to increase the AIR's effective CO2 concentation (starting to &quot;close&quot; the system). This could save some sulfuric acid feed and sequester some greenhouse gas to the water (it may also increase evaporation by reducing the &quot;air&quot;'s relative humidity). However, this is likely very uneconomic. Sulpuric acid and its handling costs are generally quite low. Greenhouse gas sequestration is likely only temporary, as well (ie: when blowdown water returns from the cooling system to the environment, and comes into equilibrium with the environment).

If it was to be made to work, the new cooling water alakalinity equilibrium, and control of it, could adversely affect your cooling water treatment program. Even if CO2 control was good, CaCO3 concentration may rise, moreso than expected at observed pH. Water treatment chemical vendors may not find it economical to design treatment programs for such unusual conditions. Increased scale inhibiting polymer dosage, if required, could negate benefits of closing the sytem. Undiscovered increase in system scaling may further reduce ecomonics.

One may also need to consider the effect on cooling water system of any impurities in the CO2 stream.

Altering system temperature may also improve CO2 dissolution, but the tower's primary purpose is to provide economical cooling, so the practical benefit of temperature change is expected to be negligible, or negative.

In summary, feeding CO2 to reduce mineral acid feed to open cooling tower water systems appears to have some limited technical, and little, if any, economic, feasability. Indeed, it is likely very unecomonic.

You may find better uses for your CO2, such as neutralizing alkaline streams that are not already in equilibrium with CO2 in the atmosphere (eg:
If you decide to trial your process, it would be interesting to see results. For example, ability to economically maintain supersaturated CO2 concentrations in water would have interesting and widespread application. If it was possible, you'd think we would have heard of it already?

PS: Once through cooling water systems using alkaline water (eg: seawater) may be used to sequester CO2, but i think that this is a separate issue...

Wayne at

Thanks again for additional comments Wayne and &quot;Feather&quot;.

A question that remains is why is the system necessarily in equilibrium. To have the reaction reverse (bicarbonate to go back to carbonate and CO2) requires heat (at a temperature of about 50C at low or atmospheric pressures and a pH of around 8.3 - I believe - correct me, as I may be wrong!). It this is true, the loss of CO2 may be insignificant. I am also assuming that bicarbonates have a lower scaling tendency than carbonates (again this maybe incorrect). The desired outcome would be eliminating sulfuric acid, but not causing a scaling problem (either in the exchangers or cooling tower fill).

My main interest in doing this would be to eliminate the sulfuric acid systems we have for pH control on our cooling water systems. If the CO2 losses to atmosphere were minimal, the operating costs for using CO2 would be about the same as sulfuric acid. However, there would be a number of other significant benefits. CO2 would be much safer to handle, less maintenance (less corrosion of acid lines and vessels, less operation attention (maintaining proper nitrogen purge on acid storage tanks, etc. etc.)).

It may well prove to be an idea that simply will not work in practice. That is what I am trying to determine. Sometimes things work well enough and don't have that high of cost that we never have reason to challenge the status quo and investigate alternatives. Therefore, things always get done the same. I was hoping that this was the case with sulfuric acid and pH control.

Your situation is very common. pH control, with the elimination of chrome and chlorine gas, has been an ongoing concern. In the bad old days, small levels of chrome could be used and bio and corrosion were eliminated, and since the system could run acidic, scaling was negligable. But, times change.

Chlorine gas was another source of acid for alkalinity reduction, as half of the cl- went to acid. The replacements, such as hypo, add alkalinity, and therefore compound the problem. pH neutral compounds require tighter pH ranges to be effective, alkaline treatment programs are tricky to control, and other alternatives scale the tower. Chlorine gas has its own set of environmental and operator concerns, and unless one is ready to tackle those significant issues, that option is out.

The best advice is to figure out why the sulfuric, (or sulfamic, or others) addition system is a problem. Ones we use are trouble free, no operator attention (or at least minimal).

Keith

This is getting a little wordy, and pretty much beaten up, but:

Carbonates are the scale forming species, but note they are in equilibrium with bicarbonates, etc too. Calcium carbonate displays retrograde solubility, that is, the warmer the water, the less its solubility and thus it tends to precipitate and scale at warm spots .. that is, in heat exchangers, etc. Yes, perhaps in cooling tower fill too, as concentrations increase there due to evaporation (particularly at spots with poor flow distribution, spots that evaporate to dryness!!). In any case, when CaCO3 drops from solution, HCO3 will shift to CaCO3 to maintain equilibrium, etc. Cooling water treatment program dispersants help keep the precipitated CaCO3 in suspension.

Loosely stated:
CO2 <-> H2CO3 <-> HCO3 <-> CaCO3 <-> CaCO3 precipitate

You might want to review equilibrium reactions in general .. reactions are &quot;constantly reversing and going forward and backward&quot;, maintaining an equilibrium. Having a clear understanding will be useful in many fields of water treatment.

Due to continuous mineral acid feed, your cooling water is probably already ever so slightly supersaturated with CO2, at least before it goes over the cooling tower... like opening the lid on a soda pop bottle .. CO2 fizzes out as the bottle's contents comes into equilibrium with the new pressure ...

One time we desludged a large cooling water system by tempporarily increasing mineral acid feed to operate a stabilized phosphate treatment program at a pH of approx 6.5 .. CO2 appeared to be fizzing out of the water at sidestream filter flow splitter boxes... as (some) CaCO3 was being redissolved as equilibrium was shifted down to low pH range...indeed, equilibrium was being approached as CaCO3 redissolved.

OK, here's a final word: Check the Nalco Water Handbook .. there are several sections dealing with carbonate alkalinity. A cooling tower is an excellent water aerator. The cooling water section says: atmospheric CO2 concentration is about 0.03% by volume (depending upon location near flue gas stacks, etc)... if atmospheric CO2 concentration doubles, and cooling water alkalinity is held constant, water pH will decrease by log 2, or, by 0.3 pH units. Conversely, if water alkalinity doubles and CO2 is constant, pH will rise log 2 or 0.3 pH units.

As you will likely need to maintain water alkalinity control as a first line of defence against CaCO3 scaling, keeping alkalinity constant with make-up quality and blowdown rate control, and increasing carbon dioxide concentration of the air ten-fold will reduce pH by 1 unit. Figure out the water pH without mineral acid feed to see what CO2 concentration in the air you would need. Considering air flow rate, you would need one heck of a lot of CO2...

As Keith commented, acid feed shouldn't be a significant problem. I wonder if nitrogen purge on H2SO4 tank is necessary, as concentrated H2SO4 should not be corrosive to mild steel if fluid velocities are not high (dilute acid is a different story), just keep it dry and free of condensation (dessicant on tank vent?)... etc .. or use fibreglass piping, etc .. but that is a different topic...

What about CO2 leaks causing lack of breathable air in confined spaces, particularly re any maintenance or inspection work in or near the cooling tower .. that could be a safety concern (it sure burns your nose, judging from my industrial fermentation experiences)... and CO2 is heavier than air, yes?

Go ahead, trial or pilot the CO2 feed for cooling tower water pH control and see what happens. I still wonder how you will ge the CO2 into solution in the first place. Looking at it as increasing the air's CO2 content is the way to go.

I agree that it is good to question the status quo! But, if it worked, you'd think that it would be done more often and we'd hear about it... it seems that folks seldom publish articles about their &quot;failures&quot;, particularly in industry, as they want their competitors to learn the hard way, too (as in my experience with ozone treatment enabling zero blowdown and other amazing &quot;perpetual motion machine type&quot; claims (where would the CaCO3 go without blowdown, anyhow)). Your results may ge a good paper to publish ... again, maybe a detailed literature search could find something already published ... (eg: engineering index search)... a literature search would be my recommendation.

Quite the complicated topic, really. I have heard of one cooling tower that had ammonia contamination and bacteria converted it to nitrates and nitric acid i believe); result was acid feed was turned off. A peculiar example of bacterial contamination causing a benefit? ... watch out for background noise in your testing.

I really don't think the process of adding CO2 to the water will work ... you would need to increase the concentration of CO2 in the air .. look at it that way.

Nuff said. Good luck!

Wayne at

Hi Grant
I am also looking into using CO2 for cooling water pH control. I have done some short tests (2 hours) feeding CO2 to our cooling tower through a sparger in the bottom of the tower. pH dropped very fast to 7.4 from 8.5, when the CO2 supply was turned off the pH rose back to 8.5 with in 10 minutes. Based on the rapid rise in pH I think the reaction reverses back to CO2 very quickly and then vents off. Our tower is operating at 32 degrees C. Tower circulation 80,000 LPM.

Todd

Todd's results are most interesting. Thanks for sharing.

Questions would be: was the mineral acid feed shut off (I would presume so)? Is there normaly a high acid feed requirement (which provides insight into the make-up water quality and/or % blowdown operated at). What is the % blowdown (cycles of concentration). Wonder what the CO2 flowrate was. Was the pH sample point in the cold supply water just after the cooling water sump and pumps? 32C cooling water is return temperature? What is supply temperature? (in winter, here in Canada at least, it can be quite low).

At this pH sample point, I wonder what happened to the alkalinity during the CO2 sparging and lowrered pH (M-alk and P-alk), and subsequently, the LSI (Langelier stability index, a measure of scaling and corrosion tendancies, calculated from temperature, calcium, alkalinity and pH values). M-Alk likely rose considerably? This would have big implications in cooling water scale and corrosion inhibition program. With normal operation at 8.5 pH the high LSI provides bulk of corrosion protection, and one feeds dispersants to prevent scaling, in TRADITIONAL treatment programs, at least! Also, traditionally at least, at pH <7.5 you would have near zero LSI and likely need a stabilized PO4 program (high PO4 corrosion inhibitor concentration, 10-15 ppm PO4) for corrosion inhibition ... and less dispersant .. depending largely upon LSI.
(cooling water treatment programs are typically alkaline (high pH) with low corrosion inhibitor and high dispersant concentration, or, neutral pH, with high corrosion inhibitor concentration)

The dynamics of the system, as demonstrated, are dramatic.
Estimating that it may take about 10 minutes for the water to cycle once around the cooling sytem, it appears that all CO2 is lost as soon as the water comes back to the tower and is exposed to &quot;normal atmosphere&quot;, when CO2 is shut off. When CO2 was fed and the low pH was reached and presumably stabilized there, I wonder how high the pH (and LSI) was at the water return just before re-entering to the cooling tower, in the hot wells exposed to atmosphere at the top of the tower (if any), and at the top of the cooling tower fill (if any). If CO2 flashed off quickly, and pH was high, scaling tendancy there would be high, too.


Wayne at DeltaCascade

Additional information and another opinion has been provided by a well respected technical professional at BetzDearborn, Mr Gary Geiger:

&quot;Carbon dioxide has been successfully used to control pH in the 8-8.5 range with relatively cold systems (return water < 100 F). It has found some utility in small cooling systems where mineral acid is not used because of safety concerns. However, it has not proven to be cost-effective compared to sulfuric acid for large systems (>5,000 gpm recirculating rate). Unlike mineral acid, CO2 does not destroy alkalinity, so loss of CO2 feed can
result in rapid calcium carbonate scaling.&quot;

It appears that a consensus is developing that CO2 feed can be made to replace sulfuric acid feed, although for a limited number of cases. However, there is significant increased risk of calcium carbonate scaling.

A risk-benefit analysis, I suppose, after all, could possibly justify CO2 use in a limited number of cases. Awareness of the pitfalls may help one to avoid them: if it works very well, watch out for premature evaluation as the scaling caused by one pH upset perhaps several years down the road could more than eliminate the benefits achieved. With excellent long term operational control, there may be potiential for success.

Hopefully, someone actually having experience with the process will share their sucesses or failures with us.