Thursday, February 23, 2012

Understanding Anhydrous Ammonia

This "Chemical of the Quarter" excerpt is from the U.S. Coast Guard “Proceedings of the Marine Safety & Security Council” magazine, by Ms. Sara S. Ju, senior chemical engineer, U.S. Coast Guard Hazardous Materials Standards Division.

What is it?
Ammonia (NH3), or anhydrous ammonia, is one of the most commonly produced industrial chemicals in the United States. About 80 percent of the ammonia produced by industry is used in agriculture as fertilizer.

It is a colorless gas at ambient conditions. It is poisonous, extremely irritating, corrosive, and pungent. It can also easily dissolve in water to form ammonium hydroxide, a caustic solution.

How is it shipped?
Bulk anhydrous ammonia is typically shipped as a liquefied compressed gas. In the U.S., ammonia is transported in pipelines, pressure tank cars, pressure tank trucks, pressure tanks, and refrigerated barges. For long-distance marine shipping, ammonia is usually carried in mid-size liquefied petroleum gas (LPG) ships.

Why should I care?
Shipping concerns.
Liquefied ammonia is usually shipped at a low temperature. LPG ships or barges carrying ammonia are either fully refrigerated (FR) or semi-refrigerated (SR).

FR LPG ships have a large cooling capacity and keep the ammonia fully refrigerated at -27 degrees Fahrenheit and a vapor pressure below the atmospheric pressure. SR LPG ships have a less powerful cooling capacity and can keep the ammonia at the liquefied condition with a temperature of -15 to 5 degrees F and a vapor pressure of 4 to 5 atmosphere pressure. Because ammonia is poisonous, it is very important to prevent it from leaking out of its cargo tanks.

Health concerns.
Anhydrous ammonia is extremely irritating and corrosive. It is classified by the Department of Transportation as a poisonous, non-flammable compressed gas and defined by the Coast Guard as a “toxic cargo” (46 CFR 154.7).

As a gas, it is an inhalation hazard and can cause breathing difficulty, coughing, lung injury, and a burning sensation and pain in the eyes and respiration system. As a liquid, it can cause burns and frostbite.

Fire or explosion concerns.
Ammonia is a fire hazard when in high concentrations and at high temperature. Presence of oil or other combustible vapors increase the fire hazard.

What is the Coast Guard doing about it?
Ships carrying liquefied compressed ammonia are regulated by the Coast Guard in 46 CFR Part 154—Safety Standards for Self-Propelled Vessels Carrying Bulk Liquefied Gases. Barges carrying liquefied compressed ammonia are regulated by 46 CFR Part 151—Barges Carrying Bulk Liquid Hazardous Material Cargoes.