In organic chemistry an alkene or olefin is an unsaturated chemical compound containing one or more carbon-to-carbon double bond. The simplest alkene is ethylene, C2H4, which, according to the International Pure & Applied Chemistry (IUPAC) standards, is called ‘ethene’. Alkenes or olefins, is an archaic synonym widely used in the petrochemical industry to describe these compounds.
The ethylene unit is the core of RLOC plant. It consists of furnaces, compressors, separators, heat exchangers, and reactors. The unit receives rich-ethane feedstock from suppliers and thermally cracks the gas in nine ethylene furnaces. During cracking, the furnaces also produce high pressure superheated steam from recovered furnace heat. The steam is collected and used to drive the turbines for compressing cracked gas, the propane refrigeration compressor, and the ethylene refrigeration compressor.
In simple language the ethane gas is processed through three important stages; Crack, Pack, and Frac.
Crack: The ethane gas is heated in nine furnaces to near 850°C and it thermally reacts (cracks) to form ethylene and other byproducts. It takes just 0.3 seconds to travel through the furnaces. The gas is cooled by the primary and secondary Transfer Line Exchangers (TLE) steam generators. It then passes through a third TLE and a Quench Tower where the gas is further cooled in preparation for compression. This cooling is essential to stop secondary reactions from happening which may adversely affect the target product slate.
Pack: Once cooled, the gas undergoes through five stages of compression in a steam driven centrifugal compressor called the Cracked Gas Machine. The high pressure cracked gas, containing Hydrogen, Methane, Ethylene, Ethane, and other molecules and is now ready for distillation.
Frac: This is the “fractionation” or distillation process. An ultimate refrigeration temperature of -135°C, along with finely tuned heat integration, is utilized to separate by-products from ethylene. Hundreds of individual processing stages take place as the gas travels through multiple distillation towers and heat exchangers. The fractionation of components in the process gas yields 99.9% pure ethylene product. The resultant ethylene product is then transported in the form of compressed fluid at a pressure over 100 kg/cm² through a 133-km pipeline from Ras Laffan to Qatofin which receives 46.15% (600 kmtpa) and the other portion to the Q-Chem II plant which receives 53.85% (700 kmtpa).