Hydrogen Chloride (HCl) is a colorless, corrosive, pungent gas used in a variety of industries including pharmaceutical, chemical, metal finishing, and semiconductor. It is sometimes stored and used as a gas, but typically its source is aqueous Hydrochloric Acid solutions. These solutions tend to have a significant HCl vapor pressure. Hydrochloric Acid is formed when HCl gas is dissolved in water. Hydrogen Chloride is not flammable but is fairly toxic. It has an Immediately Dangerous to Life and Health (IDLH) concentration of 50 ppmv with OSHA Permissible Exposure Limit (PEL) of 5 ppmv. The odor threshold for HCl is 0.26 ppmv, giving ample olfactory warning of potential danger. For the sake of comparison, Ammonia has an odor threshold of 5 ppmv, and Hydrogen Sulfide (rotten egg smell) has odor threshold of 0.5 ppbv. Hydrogen Chloride is a Hazardous Air Pollutant (HAP), and its emissions are highly regulated.

Hydrochloric acid solutions have low to moderate HCl vapor pressure, which means plain recycled water may be used in HCl scrubbers. The wastewater stream purged from such air scrubbers will contain in many instances as much as 20% by weight Hydrochloric Acid. This may be acceptable or even desirable if the plant has a use for this acid solution. Due to the acidity of HCl, it is somewhat more effectively scrubbed using an alkaline solution. This allows absolute minimum usage of water due to the HCl reacting to form a non-volatile Chloride Salt. The most common base used in scrubbers is Sodium Hydroxide, due to its availability, affordability, solubility and non-volatility. The product formed when HCl is scrubbed with Sodium Hydroxide is Sodium Chloride, otherwise known as “table salt.”

Hydrogen Chloride scrubbers incorporate a variety of gas-liquid contacting techniques, including packed beds, spray chambers and water jets. Depending on concentration, special mist elimination techniques are required to prevent acid “fog” from exiting the air scrubber. This “fog” or aerosol forms spontaneously when high concentrations of HCl, or moderate concentrations at elevated temperature, are introduced into a humid environment. In every application, AAT, Inc. considers all factors in selecting the optimum design. We offer complete packages, including instrumentation and controls, skid-mounting of equipment, caustic storage tank and metering pump, and installation.