8.1. Basic properties and characteristics of ammonia

When developing security measures for handling ammonia, it is fundamental to have an understating of its main properties and chemical characteristics. Ammonia is transported in the liquid state; therefore, it must either be compressed or refrigerated, or some combination of the two. Fully refrigerated ammonia storage tanks contain liquid at -33 °C at atmospheric pressure, while fully pressurised tanks are designed for 18 bar which corresponds to the ammonia vapour pressure at 45 °C. Under atmospheric temperature and pressure, ammonia is a colourless, toxic gas with a strong and pungent odour. Ammonia is hygroscopic, which means that it has a high affinity for water, and dissolves easily in water to form ammonium hydroxide (NH4OH), a caustic solution and weak base. In its pure form, ammonia is referred to as anhydrous (“without water”) ammonia.

In gaseous form ammonia is lighter than air. However, due to its hygroscopic properties, relase of anhydrous ammonia will create a visible white vapour cloud due to absorption of, and reaction with, the moisture in the air. This cloud may have a higher density than air. Upon catastrophic rupture of a tank and release, the anhydrous ammonia will be spilled partly as vapor and partly as a boiling liquid, which will spread over the ground whilst evaporating. Initially the cloud is cold and heavy, but gradually, while dispersing and drifting with the wind, it heats up and becomes buoyant [32].

The most critical property to control is the toxicity. Since ammonia vapors are lighter than air, and a certain level of concentration must be reached for it to be harmful for people, the greatest risk of harm is to employees and workers exposed to high concentrations and standing closest to the release.