Recovery of Olefins in Gas Feedstock Cracking Plants
The raw gas from the furnaces contains a large spectrum of hydrocarbons (hydrogen to oil) and is separated into the main product C2H4 and various by-products in the separation section.
In the first step of this separation sequence the raw gas from the furnaces is cooled down in the water wash section. Heat is removed from the raw gas by direct contact with a circulating water stream. The recovered heat is utilised in the plant for reboiling purposes. Due to the reduction in temperature of the raw gas, pyrolysis oil and pyrolysis gasoline are condensed as products.
In order to separate the remaining hydrocarbons, the pressure of the cracked gas is increased in the cracked gas compressor. A small amount of pyrolysis gasoline is condensed here. Acid gas (CO2 and H2S) is also removed by a caustic wash in this section.
Following the compression, water is removed from the cracked gas in a drier. In the hydrogenation section, all C2H2 contained in the cracked gas is selectively hydrogenated to Ethylene. Other multiple unsaturated hydrocarbons, MAPD and butadiene, are partially hydrogenated.
Basic Flow Diagram: Gas Feedstock Cracking Process Sequence
Following the hydrogenation, the cracked gas temperature is step by step reduced in a pre-cooling section. This enables the separation of the cracked gas into a C2- and C3+ product stream in the C2/C3 separation column. The partial condensation of the cracked gas is achieved by vapourizing C3H6 refrigerant and recycle C2H6. Superheating of H2-rich and CH4-rich streams from the low temperature section is an additional source of cooling medium.
In the C1/C2 separation column, CH4 is separated from the C2's. The methane from the C1/C2 separation joins the methane fraction from the low temperature section.
The final step in the C2 processing path to produce the ethylene product is the separation of C2H4 and C2H6. The C2H6 is recycled back to the furnaces as feed stock. By vapourizing and superheating the C2H6 in counter current heat exchangers in the pre-cooling section, it is used as a cooling medium for the cracked gas and C2- fraction.
In the case of ethane cracking the C3+ fraction is used as fuel as the small quantity does not justify further processing.
In the case of cracking larger amounts of propane and C4's, the C3+ fraction from the C2/C3 separation step is separated in additional processing steps.
In the first step, C3's are separated from the C4+ fraction and in the next step, the C3's are separated into polymer grade propylene product and a propane rich fraction. In an additional step the C4 fraction is separated in a C5+ and in a C4 stream. The C5+ fraction is joined with the pyrolysis gasoline fraction from the Water Quench system and the raw gas compression section.
The C4 fraction as well as the propane fraction is recycled to the cracking furnaces, contributing to a net saving in fresh feedstock.
The main energy carrier for the separation is a steam system that supplies steam to the various turbines in the plant, driving compressors and pumps and supplying steam to energy consumers. Steam is produced in the cracking furnaces by recovering waste heat from the cracking process.
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