Advanced desulfurization technology for cracked diesel

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Adopting a new mild hydrocracking route enables the conversion of vacuum gas oil (VGO) to increase diesel production. Meanwhile, the hydrocracking reaction improves the quality of the remaining VGO, making it an excellent feedstock for catalytic cracking (FCC), and it also increases gasoline yield and facilitates the production of low-sulfur diesel. Typically, to produce diesel with 10 μg/g of sulfur via hydrocracking, it is necessary to carry out the process under medium to high pressure (greater than 12 MPa), resulting in a relatively high conversion rate (greater than 50%). When mild hydrocracking (MHC) is used, although the pressure is low to medium (5 – 12 MPa) and the conversion rate ranges from 20% to 40%, it is difficult for the diesel fraction obtained through MHC to meet the requirement of a sulfur content of 10 μg/g. Additionally, when hydrogenated VGO is used as feed for FCC, the sulfur content in the diesel increases both at the beginning and at the end of operation. To address both of these issues simultaneously, Europe has developed a new patented process that allows for independent control of diesel quality. The new process takes MHC as the core and combines specialized purification stages to form an integrated process. Fresh VGO feed is mixed with recycled hydrogen and fed into the MHC reactor, while the hydrogenated VGO fraction is sent to the FCC unit. The diesel fraction is then mixed with fresh hydrogen and fed into a dedicated single-pass refining reactor. The refining reactor uses a high hydrogen partial pressure to ensure maximum hydrorefining of diesel that is difficult to desulfurize by hydrogenation. Compared to two separate units, this process saves on equipment investment and operating costs, reduces the need for compressors and air coolers, and enables flexible heat integration. It allows diesel from units such as distillation, FCC, viscosity reduction, and coking to also be fed into the refining reactor for refinement. A new unit using this process was put into operation in the second half of 2004 at the Puertillano refinery of Repsol-YPF in Spain, with an MHC conversion rate of 35%, enabling the production of diesel containing 10 μg/g of sulfur. By using SR (straight-run) VGO and coker heavy gas oil (high in nitrogen), diesel with 33.6% (by volume) sulfur content can be produced