Gas scrubbers, pH measurement, and you.

Let’s talk about acid rain and what we do about it. Yes, it still exists, but modern manufacturing practices have made it much rarer. And yes, this chat will involve pH. Be patient, young grasshopper.

In case you don’t know much about acid rain, once upon a time it polluted surface waters, killed wildlife, and damaged forests and buildings. It mostly comes from gases emitted as a byproduct of burning coal or oil-based fuel, which results in a high load of sulfur dioxide (SO2) compounds.

The SO2 reacts with the water in clouds to form sulfuric acid that returns to the earth as rain. This rain decreases the pH value in nearly everything it touches, causing monuments to crumble, fish and trees to die, and a number of other issues. To reduce SO2 in their gases, most plants use devices called gas scrubbers.

How do you scrub gas?

Don’t take the term too literally. Gas scrubbers neutralize SO2 in an industrial process. You can think of it like a big shower, where the polluted gas comes in contact with a scrubbing liquid. The chemicals in the liquid absorb the pollutants, and the resulting mess collects in a tank at the bottom of the scrubber. Sometimes that mess gets recycled by chemical companies, so it’s not always wasted.

Gas scrubbers
Courtesy of The Spray Nozzle People

In almost all gas scrubbers, you’ll find pH-measuring loops. They control the pH of the scrubbing liquid to improve scrubber efficiency. The fewer chemicals the scrubber uses and the more pollutants it absorbs, the more money the process saves.

How do you save money?

Consider the logarithmic pH scale – it takes 10 times the quantity of chemicals to adjust from pH 8 to 9 as it does from 7 to 8. Yikes! So an incorrect reading of only 0.5 pH above the true value leads to big waste.

What does this have to do with scrubbers? The removal of SO2 uses a simple chemical reaction. By adding lime, limestone, or hydrated lime and fresh air, sulfur dioxide turns into calcium sulfate, better known as plaster. The ideal pH for this process falls between 5.5 and 6.0.

So if the pH rises above 6, then it will waste chemicals and create clogs. Worst case scenario has your scrubber blocked and days of work to get it right again. How about a no on that? On the other hand, a pH below 5 risks corrosion and pollutants escaping into the environment. So yeah, spend a little on a pH loop and you’ll save in the long run.

Where do you put a pH sensor in a scrubber?

Generally, you have two places to install your pH electrode:

  • in the absorber sump at the bottom of the scrubber
  • in the pipe to the injectors past the pump

Most installers consider the first point the easy one, as you don’t have to deal with high pressure. But you might face scaling, where matter builds up on your sensor and blocks it. You’ll lose accuracy and make a lot of work if you let this happen. In this case, you should consider an automatic sensor cleaner. In any case, make sure you can remove your electrode without shutting down the process.

If you install the sensor in the pipeline after the pump, then the speed of the fluid creates a cleaning effect. Unfortunately, it also creates a wearing effect from undissolved particles in the scrubber solution. For easier maintenance, use a holder that can retract the electrode during the process.

Gas scrubbers
Courtesy of Endress+Hauser


Courtesy of Endress+Hauser

Endress+Hauser Liquiline M CM42 pH meter

The CM42 is a robust transmitter for pH/oxidation-reduction potential (ORP), conductivity, or oxygen measurement, making it a good choice for demanding environments. Its design simplifies commissioning, handling, and maintenance, saving you time. Easy parameter switching and seamless system integration lets you adapt it to your process.

Courtesy of Endress+Hauser

Endress+Hauser Orbisint CPS11D pH electrode

The CPS11D is a digital pH electrode for process and environmental technology. It measures even in highly alkaline media or hazardous areas. Designed for low maintenance and long life, this electrode offers you nifty value for your money. It resists corrosion and moisture, enables lab calibration, and supports predictive maintenance.

Second option

M400 2-wire
Courtesy of Mettler Toledo

Mettler Toledo M400 2-wire transmitter

The M400 2-wire is a single-channel multi-parameter device for pH/conductivity and dissolved oxygen digital sensors, wrapped in a IP66 enclosure. It sends data through analog output and can use HART, FOUNDATION Fieldbus, and PROFIBUS PA protocols. It also provides advanced predictive diagnostics that indicate the current status of the electrode.

inPro 4800
Courtesy of Mettler Toledo

Mettler Toledo InPro 4800(i) pH electrode

The 4800(i) is a digital pH electrode with a pressure-compensated double gel-electrolyte reference chamber system. It offers long life in aggressive applications, due to its prolonged diffusion path and dirt-repellent junction. Its digital technology provides a interference-free signal.


Even if neither of the above options work for you, they should give you some ideas about how to make your scrubbers more efficient. If you have other ideas, leave a comment and let us know!

Find and buy in our online shop: pH electrode and analytics.

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