Objective The aim is to solve the issues of insufficient production capacity of low pour point diesel in the refinery, high C₃⁺ content in coking dry gas, and an increase in operation costs after adding a circulating hydrogen desulfurization unit.

Method Based on the concept of molecular refining, the unit models for diesel blending, absorption and stabilization, and circulating hydrogen desulfurization were established using simulation software to provide references for optimizing the process scheme.

Result The first cut of atmospheric gas oil and the second cut of atmospheric gas oil were mixed at a mass ratio of 1∶2. The pour point of the blended diesel was −35 ℃ and the cetane index was 52.6, which could meet the requirements for blending low pour point diesel. The volume fraction of C₃⁺ components in the coking dry gas was reduced from 10.0% to 3.5%, the compressor load of cracking gas was reduced by 18.7 kmol/h, and 4 776 t/a more naphtha could be processed. The rich amine solution from the sulfur recovery unit was used as the lean amine solution for the circulating hydrogen desulfurization of the hydrocracking unit. After the desulfurization unit was put into operation, the volume fraction of hydrogen sulfide (H₂S) in the circulating hydrogen reached 0.11%-0.22%, and the stripping steam consumption of the unit decreased by 1.37 t/h, while the consumption of lean amine solution decreased by 27 t/h.

Conclusion The optimization of the molecular refining process flow can effectively enhance the economic benefits of the facility.