Thermal cracking is well known and widely accepted technology for olefin production. This technology is also called steam cracking, since steam is added to hydrocacbon before cracking to reduce the parial pressure of hydrocacbon and to produce a better yield performance. In the petrochemical industry, steam cracking is a core technology for producing olefins, although there are alternative routes from off - gas of fluid catalvtic cracking units in oil refineries or by dehydrogenation or prepane or outanes.
Thermal cracking reactions are bisically uses to break the C – C bonds of hydrocacbons non - catalytically at high temperature of around 800 – 9000C and at a low pressure of 0. 16 – 0. 2Mpa in the coils located in the radiant sections of furnaces. They finally produce lower molecular weight olefins. In including radical reactions, whereas gas oil cracking is more complex with more than 3000 reactions.
In industrial practie, non - reaced ethane or propane is separated in the cold separation sestion and recycled back to cracking furnaces where ethane and propane are cracked again. Therefore, the ultimate yields of olefins are much higher than those of once - through yields.
I.1.1. PETROCHEMICAL COMPLEX IN Japan.
Naphtha is used asa base feedstock for petrochemical complexes in Japan. LPG is also used as an alternative feedstock, but accounts for about 60% of such feedstock and the rest of the feedstock are LPG and gas oil. However, in the United States, ethane and LPG separated from natural gas are major feedstock, which are for 70 – 80% of total demand, while the rest are naphtha and gas oil.
Based on naphtha cracking, the product pattern consists of ethylene at 28%, propylene at 17%, butene and butadiene products at 11%, off - gas and pyrolysis heavy oil at 24% and pyrolysis gasoline at 20%.
A typical petrochemical complex consists of an ethylene plants such as polyolefin and aromatic plants using olefin products. The configuration of the complex depends on the final product types and the available feedstock. A complex based on naphtha - cracking ethylene plants is more elaborate than a complex based on cracking gases such as ethane or propane.
Some 60 – 70% of pyrolsis gasoline fractions consist of aromatic such as benzene, toluene and xylenes called a BTX fraction. These are recovered by extractive separations at 12 – 14% based on the feedstock rate. However, in the recent product pattern, the BTX extraction rate tends to decrease due to advances in cracking technology and a parafinic and lighter naphtha used as the feedstock. In the 1970s, about 50% of BTX demand was supplied from ethylene plants; however, in the 1990s, this has decreased to 40%. The rest of BTX demand is supplied from catalytic naphtha reforming plants.