Most cost-effective way to lower process water alkalinity 3

[WillBryant]

I need to design a process that lowers the alkalinity of 50,000 L/day of water. The water in question required 34.2 mL of 0.02N H2SO4 to neutralize 100ml and I need to lower the alkalinity to where the same amount can be neutralized by 25 mL of 0.02N H2SO4. I am currently thinking to develop an acid-injection system, but I am not sure if this is the best route. I am new to this site and professional engineering, so feel free to give tips on both!

Let me know if this information is not sufficient. Thanks!

Will Bryant

 

[bimr]

Lowering the alkalinity with acid is possible and may be cost effective. However, the best alternative to accomplish this objective will depend on many factors including but not limited to:

[ul]
[li]The parameters in the water analysis.[/li]
[li]The use of the treated water.[/li]
[li]The type of facility and ability of the workers.[/li]
[li]Cost and availability of acid.[/li]
[/ul]

 

[WillBryant]

These are the parameters in the water analysis:

Alkalinity Test (Volume of 0.02N H2SO4 Required to Neutralize 100mL Sample Using Mixed Indicator (mL))
Results: 38.1
Limit: 25.0 Max

Acidity Test (Volume of 0.02N NaOH Required to Neutralize 100mL Sample Using Mixed Indicator (mL))
Results: NIL
Limit: 5.0 Max

Inorganic Solids (ppm)
Results: 497
Limit: 3000 Max

Sulphates As SO3 (ppm)
Results: 80
Limit: 400 Max

Chlorides Cl (ppm)
Results: 107
Limit: 2000 Max For Plain Concrete
500 Max For Reinforced Concrete

Suspended Matter (ppm)
Results: 15
Limit: 2000 Max

Organic Matter (ppm)
Results: 143
Limit: 200 Max

pH
Result: 8.0
Limit: Not Less than 6


The treated water will be used by a construction contractor, not sure on specifics but I can find out. The facility should be a long-term treatment plant, and I am not sure on the ability of the workers. This will be taking place in India. As far as the cost and availability of acid, I have someone looking for that information already and I will post results as I get them.

 

[QualityTime]

Using 93% sulphuric acid is a very cheap way to decrease alkalinity and pH. In the past I have used pH, Alkalinity, temperature tables to determine how much suphuric acid is requried to decrease alkalinity and what will happen to the pH at differnt water temperatures. Sulphuric acid is actually more corrosive the weaker it is. Whatever you do just make sure your materials are compatible. The design should be really simple

 

[QualityTime]

The book I used is "Alkalinity, pH changes with temperature for waters in industrial systems". It has 79 pages of almost all tables. It is pretty good and easy to use with examples. I scanned a copy from my local library years ago. I refer to it once in a while

 

[WillBryant]

That is very good to hear! Thanks a lot, I will definitely check out that reference.

Also, can anyone recommend a good reference manual for piping and control systems for this type of application?

I am thinking to use a system design similar to the setup recommended by Bimr for an RO acid-injection system here:

http://www.eng-tips.com/viewthread.cfm?qid=79099

I am glad that this is a relatively small application; I still can't believe I count as a licensed engineer, fresh out of school, in India!

 

[QualityTime]

I don't like to inject sulphuric acid into a pipeline. The backpressure valve on the sulphuric acid metering line will fail one day and or the main pipe/chemical metering line interface location may fail because of localized acid/water corrosion problems.


Look at introducing sulphuric acid via a metering pump into a closed top box from the above with an air break. Have the process water introduced submerged tangentially into the box so that you get a circular swirling pattern. Put vertical baffles on all four sides so that the water gets mixed with the acid. Use special carpenter stainless steel. Have another box in series to give the mixed water more mixing just to make sure. It is similar to designeing hydraulic flocculation tanks


No mechanical mixing requried....no issues with heat generated at the acid water interface .etc etc.


You have to understand the properties of the acids that you are contemplating to use. Acids are never easy to handle. Off gass from a HCL a day tank vent will offgases and will corrode any steel in a room. The room will look like hell in short order

 

[bimr]

It is a somewhat premature to design a system without knowing the specifics of your application. It is possible to do the pH injection inline or with a batch tank whatever is easier with your equipment and operating staff.

You should note the sodium concentration will increase as you lower the pH.

 

[QualityTime]

i can email a copy of the book "Alkalinity, pH changes with temperature for waters in industrial systems" if you want (rhum@rogers.com). You can do jar tests to confirm the calculations. You can even heat up the fluid or put it in the fridge to cool it down to confirm the temperature, ph, alkalinity changes predicted in the tables. It will give you a feel for the design boundary conditons. It was close enough for my purposes when I used it 15 years ago.

 

[bimr]

Regarding: 'The water in question required 34.2 mL of 0.02N H2SO4 to neutralize 100ml and I need to lower the alkalinity to where the same amount can be neutralized by 25 mL of 0.02N H2SO4."

It appears that very little acid will be required. What is the purpose of the neutralization and what will the water be used for.

 

[WillBryant]

The water will be used by a construction contractor. I am told that the water must meet certain specifications before it can be used for this purpose, as detailed by that assay listed above.

In regards to the mixing boxes, is there any sizing calculations for the size of the box, location of the feed, and width of the baffles? I am guessing that with such a low acid feed rate, I could just build it large and turbulent enough so as to guarantee proper mixing without them, but its probably best to double-check.

Thanks again for both of your extremely helpful responses!

 

[bimr]

Acid should readily mix into water. A rule of thumb is that the acid will be mixed after 4 pipe elbows or turns.

You should be aware that a process control system will be necessary to safely feed acid inline. Each flow should be monitored and controlled. If the water stops, then the acid feed must be shut off. A pH meter should also be used to shut the system down if the finished solution is out of specification.