Dry emergency gas scrubbers: the dream machine for removing chlorine

Karen Gailey
Communications Specialist
Purafil, Inc.
Doraville, Georgia

The general duty clause of the Environmental Protection Agency's Clean Air Act requires owners and operators of "stationary sources [of extremely harmful substances like chlorine]...to design and maintain a safe facility and to minimize the consequences of releases when they occur." Wet and dry scrubbers both prevent accidental chlorine (Cl2) releases, but only dry scrubbers provide a virtually maintenance-free solution.

Wet scrubbers follow a simple operation process. When a wet scrubber senses Cl2, the gas is drawn from the storage area to the packed bed tower; it then flows countercurrent to the liquid caustic solution, which is sodium hydroxide (NaOH). The caustic neutralizes the gas, and the discharge concentration of Cl2 is approximately 0.5 parts per million (ppm).

Though they function easily, wet scrubbers require time-consuming maintenance on a regular basis. They use moving parts such as pumps, valves, spray nozzles, and pH controls, and they need heaters for outdoor applications. Moreover, liquid caustic is toxic, and because elements in the air gradually neutralize NaOH, the substance must be replaced every two or three years regardless of an accidental release. Sensors that detect Cl2 also have limited shelf life; if the Cl2 concentration is especially high, sensors might give false results.

For owners and operators looking for a hassle-free way to prevent accidental Cl2 releases, they should look no further than dry scrubbers. Dry scrubbers remove the entire contents of a fully-loaded one-ton cylinder in a worst-case release scenario without the time and money it takes to maintain wet scrubbers.

In the event of an unintentional Cl2 emission, dry scrubbers follow these steps to eliminate the gas from the air:

  1. Released Cl2 is drawn into the scrubber through a blower and contacts dry-scrubbing media.

  2. Media react with Cl2 and permanently convert the gas to nontoxic solids.

  3. Clean air is discharged to the outdoors with a Cl2 concentration of less than 5 parts per billion (ppb).
Dry scrubbers require significantly less maintenance than wet scrubbers. They do not need heaters for outdoor applications, and they have one moving part—a blower. Instead of using caustic, dry scrubbers neutralize Cl2 with dry-scrubbing media, which are highly porous, spherical pellets that permanently transform the gas into harmless solids.
Installed at the Silvan Disinfection Plant in Melbourne, Victoria, Australia, this dry scrubber uses 25,000 lbs of dry-scrubbing media to remove one ton of chlorine in the event of an accidental release.

As long as media do not react with Cl2, they do not need to be replaced; they only require occasional testing to determine remaining life and to project change-out dates. Nontoxic and non-hazardous, media do not require special handling and can be disposed in landfills.

Despite the benefits of dry-scrubbing technology, dry scrubbers have not been used as widely as wet scrubbers in emergency gas applications. However, they have begun to receive more attention domestically and internationally. For example, Melbourne Water in Australia installed a dry scrubber at a disinfection plant to protect human and environmental health from an accidental Cl2 release. The scrubber is 10 feet in diameter and 15 feet tall and holds 25,000 lbs of dry-scrubbing media.

"The Silvan Disinfection Plant is classified as a Major Hazard Facility under state government legislation," said project manager Stephen Answerth. "Under this legislation, Melbourne Water is required to reduce the risk of a chlorine release 'so far as practicable.' The dry scrubber was seen as part of this effort; [it] was selected because it is a simpler system and does not involve having liquid caustic soda onsite."

With the rapid development of residential communities near water and wastewater treatment plants, preventing accidental Cl2 releases has become increasingly important. Owners and operators can save lives and the environment using wet scrubbers, but with dry scrubbers, they also save valuable time and money.

To reach Karen Gailey at Purafil, Inc., call (770) 662-8545 or send e-mail to karen_gailey@purafil.com.