Recovery of Olefins in Liquid Feedstock Cracking Plants
The raw gas from the furnaces contains a large spectrum of hydrocarbons (hydrogen to oil) and is separated into the main products (C2H4 and C3H6) 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 oil fractionation and the water quench sections. Heat is removed from the raw gas by direct contact with circulating pyrolysis oil and water streams. The recovered heat is utilised in the plant for reboiling and process steam generation. Due to the reduction in temperature of the raw gas, pyrolysis oil and pyrolysis gasoline are condensed as products.
Basic Flow Diagram: Liquid Feedstock Cracking Process Sequence
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, 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+ stream in the C2/C3 separation column. All free water is removed from the gas stream in a dryer after the initial cooling step. The partial condensation of the cracked gas is achieved by vapourizing C3H6 refrigerant and recycle C2H6. Superheating of H2 and CH4 streams from the low temperature section is an additional source of cooling medium.
The C2H2 in the C2- stream is selectively hydrogenated to C2H4 in a hydrogenation reactor.
In the following low temperature section, the acetylene free C2- gas stream is partially condensed to form a H2 rich vapour fraction , a CH4 rich liquid fraction and a C2 rich liquid feed for the C1/C2 separation. The partial condensation of the C2- gas stream in the low temperature section is done by vapourizing and superheating of C2H4 refrigerant. H2 and CH4 streams are also utilized as cooling medium in counter current heat exchangers.
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 and low temperature sections, it is used as a cooling medium for the cracked gas and C2- fraction.
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 C4+ fraction is separated into a C4 and C5+ fraction. The C5+ fraction is joined with the pyrolysis gasoline fraction from the Water Quench system and the raw gas compression section.
Before the final step in the C3 processing path where the C3H6 and the C3H8 are separated, the raw C3 fraction is hydrogenated to convert all C3H4 to C3H6 and C3H8. The C3H8 is recycled back to the furnaces as feed stock.
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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