Integrated method for producing middle distillate with a recycling loop in hydrotreatment

A hydrocracking method for treating hydrocarbon feedstocks containing at least 20% by volume and preferably at least 80% by volume of compounds boiling above 340° C., said method comprising:

• a) hydrotreatment of said feedstocks in the presence of hydrogen and at least one hydrotreatment catalyst, at a temperature of between 200 and 450° C., under a pressure of between 2 and 18 MPa, at a volumetric flow rate of between 0.1 and 6 h−1, and with an amount of hydrogen introduced such that the volumetric ratio of liter of hydrogen/liter of hydrocarbon is between 100 and 2,000 L/L,
• b) hydrocracking of at least one part of the effluent obtained from step a), with the hydrocracking step b) being carried out, in the presence of hydrogen and at least one hydrocracking catalyst, at a temperature of between 250 and 480° C., under a pressure of between 2 and 25 MPa, at a volumetric flow rate of between 0.1 and 6 h−1, and with an amount of hydrogen introduced such that the volumetric ratio of liter of hydrogen/liter of hydrocarbon is between 100 and 2,000 L/L,
• c) passing into at least one heat exchanger of at least said effluent obtained from step b) in which said effluent is cooled by exchanging the liquid hydrocarbon feedstock entering into step f) in at least one exchanger,
• d) gas/liquid separation of the cooled effluent obtained from step c) to produce at least one hydrogen-rich gas efflux and at least one liquid efflux,
• e) fractionation of said liquid effluent obtained from step d) into at least one fraction comprising the converted hydrocarbon products having boiling points lower than 340° C. and an unconverted liquid fraction having a boiling point higher than 340° C.,
• f) hydrotreatment of a liquid hydrocarbon feedstock comprising at least 95% by weight of compounds boiling at a boiling point of between 150 and 400° C., preheated in advance in step c), with said step f) being carried out in the presence of hydrogen and at least one hydrotreatment catalyst, at a temperature of between 200 and 390° C., under a pressure of between 2 and 16 MPa, at a volumetric flow rate of between 0.2 and 5 h−1, and with an amount of hydrogen introduced such that the volumetric ratio of liter of hydrogen/liter of hydrocarbon is between 100 and 2,000 L/L,
• g) gas/liquid separation of the effluent obtained from step f) for producing at least one hydrogen-rich gas efflux and at least one liquid efflux,
• h) fractionation of the liquid effluent obtained from step g) making possible the separation of at least one light gas fraction, a naphtha fraction, and a middle distillate fraction having a boiling point higher than 150° C.,
• i) compression of the hydrogen-rich gas effluent obtained from step g) in a hydrogen makeup compressor supplying steps a), b) and f) and comprising n stages, with n being an integer that is greater than or equal to 2, with said hydrogen-rich gas effluent being injected and compressed in an intermediate stage of said compressor before being recycled upstream from step f),
• j) hydrocracking of at least one part of the liquid fraction having a boiling point higher than 340° C. that is unconverted during the first hydrocracking step b) and obtained from the fractionation step e), with said second hydrocracking step j) being carried out in the presence of hydrogen and at least one hydrocracking catalyst, at a temperature of between 250 and 480° C., under a pressure of between 2 and 25 MPa, at a volumetric flow rate of between 0.1 and 6 h-1, and with an amount of hydrogen introduced such that the volumetric ratio of liter of hydrogen/liter of hydrocarbon is between 100 and 2,000 L/L.