Process for producing middle distillates by hydroismerizing and hydrocracking a heavy fraction from a fischer-tropsch effluent

A process for producing middle distillates from a paraffinic feed produced by Fischer-Tropsch synthesis unit, comprising the following steps in succession:

• a) separating at least one gaseous C4− fraction, termed the light fraction, with an end boiling point of less than 20° C., from an effluent derived from the Fischer-Tropsch synthesis unit to obtain a single C5+ liquid fraction, termed the heavy fraction, with an initial boiling point in the range of 20° C. to 40° C.;
• b) hydrogenating the unsaturated olefinic type compounds of at least a portion of said heavy liquid fraction C5+, in the presence of hydrogen and a hydrogenation catalyst at a temperature in the range of 100° C. to 180° C., at a total pressure in the range of 0.5 to 6 MPa, at an hourly space velocity in the range of 1 to 10 h−1, and at a hydrogen flow rate corresponding to a hydrogen/hydrocarbons volume ratio in the range of 5 to 80 Nl/l/h to obtain a liquid hydrogenated effluent;
• c) hydroisomerizing/hydrocracking the entire liquid hydrogenated effluent from step b) with no prior separation step in the presence of hydrogen and a hydroisomerization/hydrocracking catalyst;
• d) distilling resultant hydrocracked/hydroisomerized effluent from step (c)
• and wherein said hydroisomerization/hydrocracking catalyst comprises up to 3% by weight of metal of at least one hydro-dehydrogenating element selected from noble metals from group VIII of the periodic table of the elements, 0.01% to 5.5% by weight of oxide of a doping element selected from phosphorus, boron and silicon, and a non-zeolitic support based on silica-alumina containing a quantity of more than 15% by weight and 95% or less by weight of silica (SiO2), said silica-alumina having the following characteristics:
• a mean pore diameter, measured by mercury porosimetry, in the range of 20 to 140 Å;
• a total pore volume, measured by mercury porosimetry, in the range of 0.1 ml/g to 0.5 ml/g;
• a total pore volume, measured by nitrogen porosimetry, in the range of 0.1 ml/g to 0.6 ml/g;
• a BET specific surface area in the range of 100 to 550 m2/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 140 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 160 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 200 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 500 Å, of less than 0.1 ml/g;
• an X ray diffraction diagram which contains at least the principal characteristic peaks of at least one transition alumina included in the group composed of alpha, rho, chi, eta, gamma, kappa, theta and delta aluminas;
• a settled catalyst packing density of more than 0.55 g/cm3.

A process for producing middle distillates from a paraffinic feed produced by Fischer-Tropsch synthesis unit, comprising the following steps in succession:

• a) separating at least one gaseous C4− fraction, termed the light fraction, with an end boiling point of less than 20° C., from an effluent derived from the Fischer-Tropsch synthesis unit to obtain a single C5+ liquid fraction, termed the heavy fraction, with an initial boiling point in the range of 20° C. to 40° C.;
• b) hydrogenating the unsaturated olefinic type compounds of at least a portion of said heavy liquid fraction C5+, in the presence of hydrogen and a hydrogenation catalyst at a temperature in the range of 100° C. to 180° C., at a total pressure in the range of 0.5 to 6 MPa, at an hourly space velocity in the range of 1 to 10 h−1, and at a hydrogen flow rate corresponding to a hydrogen/hydrocarbons volume ratio in the range of 5 to 80 Nl/l/h to obtain a liquid hydrogenated effluent;
• c) hydroisomerizing/hydrocracking the entire liquid hydrogenated effluent from step b) with no prior separation step in the presence of hydrogen and a hydroisomerization/hydrocracking catalyst;
• d) distilling resultant hydrocracked/hydroisomerized effluent from step (c)
• and wherein said hydroisomerization/hydrocracking catalyst comprises between 2.5% and 5% by weight of oxide of an element from group VIII and between 20% and 35% by weight of oxide of a group VIB element with respect to the weight of the final catalyst, optionally 0.01% to 5.5% by weight of oxide of a doping element selected from phosphorus, boron and a non-zeolitic support based on silica-alumina containing a quantity of more than 15% by weight and 95% by weight or less of silica (SiO2), said silica-alumina having the following characteristics:
• a mean pore diameter, measured by mercury porosimetry, in the range of 20 to 140 Å;
• a total pore volume, measured by mercury porosimetry, in the range of 0.1 ml/g to 0.5 ml/g;
• a total pore volume, measured by nitrogen porosimetry, in the range of 0.1 ml/g to 0.6 ml/g;
• a BET specific surface area in the range of 100 to 550 m2/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 140 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 160 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 200 Å, of less than 0.1 ml/g;
• a pore volume, measured by mercury porosimetry, included in pores with a diameter of more than 500 Å, of less than 0.1 ml/g;
• an X ray diffraction diagram which contains at least the principal characteristic peaks of at least one transition alumina included in the group composed of alpha, rho, chi, eta, gamma, kappa, theta and delta aluminas;
• a settled catalyst packing density of more than 0.55 g/cm3.