Sable Chemicals Process Description
Ammonia Storage Plant
Imported ammonia is received via rail tank cars (RTCs) from South Africa and decanted into ammonia storage vessels, alternatively called spheres at the Ammonia Storage plant. Each sphere nominally carries approximately 1,000 tonnes of liquid ammonia. The main purpose of the ammonia storage plant is to store liquid ammonia for use in downstream plants ie. the nitric acid and ammonium nitrate plants.
The spheres are heavily insulated to minimize the effect of variations in ambient conditions, which would otherwise result in vaporization of the liquid ammonia. Ammonia is stored as a liquid at a temperature of 2ºC inside the two spherical vessels while its vapors are monitored under pressure control at a pressure of 52psi (3.6 kg/cm2). If the pressure exceeds 52psi the vapors are withdrawn from the sphere, compressed and condensed in the refrigeration circuit before being channeled back to spheres in liquid form.
- Inerts content below 1%
- Moisture content below 0.02%
- Oxygen content below 0.01ppm
- Oil content below 10 ppm
Liquid ammonia is pumped at 14.5 bars by ammonia storage pumps from the spheres to the Burn tank where it is vaporized to a pressure of 12.5 bars in preparation for use in the Nitric acid and Ammonium Nitrate (AN) processes.
Nitric Acid Plant
The main purpose of the Nitric acid plants is to produce 57% nitric acid for use in the production of Ammonium Nitrate. This is achieved through the oxidation of anhydrous ammonia vapour by oxygen in compressed atmospheric air. The plant comprises of two Acid plants, each with a capacity of 300 tons per day (TPD), equivalent to 10,000 TPM of AN per plant.
Atmospheric air is drawn through filters and is compressed using a Brown Boveri Compressor. Gaseous Ammonia and compressed air are intimately mixed in a gas mixer before entering the reactor or converter. In the converter, the reaction occurs as the gas mixture passes over a platinum rhodium catalyst at a temperature of 930oC and pressure of 110psi. The process gas resulting from the reaction, mainly consisting of nitrogen oxides, undergoes cooling through a series of heat exchangers till it gets into an absorption column, where the gas is absorbed in high purity water to obtain 57% Nitric acid. The acid is stored in a stainless steel storage tank pending use at the Ammonium nitrate plant.
Ammonium Nitrate Plant
Nitric Acid, at approximately 57% concentration, is supplied to the neutralizer from the Acid Storage tank where it is mixed with vaporized ammonia in a neutralization vessel. An exothermic neutralization reaction takes place between the nitric acid and ammonia forming an 83% ammonium nitrate solution. The latter is subsequently concentrated in a series of evaporators until it becomes 99.8%. A stabilizer (Magnesium Oxide) is added to the melt to improve the stability of the product at approximately 0.55 – 0.65%. After prilling of the AN melt, it is cooled, screened and coated with dolomite at a concentration of between 0.1 – 0.25% before sending it for bagging and dispatch.
To augment the key plants and processes outlined above, other ancillary plants are the water treatment plant and the steam raising plant
Key support services to the process are maintenance and laboratory services.