Process for deeply desulfurizing catalytic cracking gasoline

A process for desulfurizing a catalytic cracking gasoline fraction by solvent extraction, comprising steps of:

• introducing the gasoline fraction into an extraction tower at an about middle part thereof, introducing a solvent into the extraction tower at the top thereof, and injecting saturated C5 hydrocarbon into a reflux device at the bottom of the extraction tower, wherein the temperature at the top of the extraction tower is controlled between 65˜80° C., the temperature at the bottom of the extraction tower is controlled between 50˜60° C., and the pressure (absolute) at the top of the extraction tower is controlled between 0.5˜0.6 MPa, a feeding ratio by volume of the solvent to the gasoline fraction is controlled between 2.0˜3.0, and a feeding ratio by volume of the saturated C5 hydrocarbon to the gasoline fraction is controlled between 0.2˜0.3, and wherein the gasoline fraction and the solvent are contacted at an upper section of the extraction tower via a multi-stage countercurrent, meanwhile the saturated C5 hydrocarbon and the solvent are contacted at a lower section of the extraction tower, the gasoline fraction which is desulfurized by extraction flows out from the top of the extraction tower, as a material A, and the solvent that has extracted sulfide, aromatics and the C5 hydrocarbon flows out from the bottom of the extraction tower, as a material B;
• washing the material A for removing solvent therein, to obtain a desulfurized gasoline fraction;
• further treating the material B to separate a C5 hydrocarbon-containing light component, a sulfur-rich component, water and the solvent, wherein the C5 hydrocarbon-containing light component contains the saturated C5 hydrocarbon and a C5 olefin, wherein the further treating the material B specifically comprises steps of:
• {circle around (1)} introducing the material B into an extraction distillation tower at the top thereof, wherein the pressure (absolute) of the extraction distillation tower is controlled between 0.15˜0.3 MPa, and the temperature at the bottom of the extraction distillation tower is controlled between 150˜180° C., the C5 hydrocarbon-containing light component is distilled out as material C from the top of the extraction distillation tower, and a sulfur-rich solvent is obtained as material D at the bottom of the extraction distillation tower;
• {circle around (2)} after being condensed, returning the material C obtained in step {circle around (1)} to the reflux device at the bottom of the extraction tower;
• introducing the material D into a recycling tower from a middle part thereof, wherein the pressure (absolute) of the recycling tower is controlled between 0.015˜0.05 MPa, and the temperature at the bottom of the recycling tower is controlled between 130˜180° C.;
• a material E, i.e, a sulfur-rich oil containing sulfide, aromatics and cycloolefin, is obtained at the top of the recycling tower;
• a material F that mainly contains solvent is obtained at the bottom of the recycling tower;
• and
• {circle around (3)} after being condensed, conducting a water oil separation on the material E obtained in step {circle around (2)}, to obtain water and a sulfur-rich component G;
• returning a portion of the water to the top of the recycling tower in step {circle around (2)}, and returning the rest as washing water to the step of washing the material A for removing solvent;
• returning the material F obtained in step {circle around (2)}, after heat-exchange, to the top of the extraction tower for recycling;
• wherein the solvent is a mixed solvent containing one or two of diethylene glycol, triethylene glycol, tetraethylene glycol, dimethyl sulfoxide, sulfolane, N-formyl morpholine, N-methyl pyrrolidone, polyethylene glycol, and propylene carbonate;
• and the water content of the solvent is 0.6˜0.8% by weight.