Diesel fuel or diesel fuel base stock and production method thereof

1. A method for producing a diesel fuel or a diesel fuel base stock, comprising:

a hydrotreating step (A) comprising

(i) a hydroisomerization step (A1) of contacting a hydroisomerization feedstock comprising a Fischer-Tropsch (FT) synthesis oil, which contains a middle distillate and/or a wax fraction that is heavier than the middle distillate, with a hydroisomerization catalyst at a temperature of 320 to 350° C. to produce a hydroisomerized oil (a1), and

(ii) a hydrocracking step (A2) of contacting a hydrocracking feedstock comprising the wax fraction with a hydrocracking catalyst at a temperature of 300 to 320° C. to produce a hydrocracked oil (a2); and

a fractionation step (B) comprising

(i) transferring at least a portion of a hydrotreated oil (a) comprising the hydroisomerized oil (a1) and the hydrocracked oil (a2) to a fractionator and,

(ii) obtaining a middle distillate (b1) with a 5% distillation point of 130 to 170° C. and a 95% distillation point of 240 to 300° C. and a heavy oil (b2),

wherein with respect to the hydroisomerized oil (a1), a proportion of branched paraffins with 18 carbon atoms accounts for 85 to 98 mass % of the hydrocarbons with 18 carbon atoms,

wherein the middle distillate (b1) has a flash point of 30 to 40° C. a pour point of −45° C. or lower, a kinematic viscosity at 30° C. of 1.3 mm 2 /s or more, and a proportion of branched paraffins of 60 mass % or more with respect to the total amount of paraffins, and

wherein the middle distillate (b1) is used as a diesel fuel or a diesel fuel base stock.

2. The method according to claim 1 , wherein

the hydrotreated oil (a) is a mixture of a portion of the hydroisomerized oil (a1) and a portion of the hydrocracked oil (a2).

3. The method according to claim 1 , wherein

the hydroisomerization feedstock is a FT synthesis middle distillate (F1) with a 10% distillation point of 85 to 180° C. and a 90% distillation point of 325 to 355° C. and the hydrocracking feedstock is a wax fraction (F2) that is heavier than the FT synthesis middle distillate (F1).

4. The method according to claim 1 , further comprising a recycling step (C) in which at least a portion of the heavy oil (b2) is mixed with the hydroisomerization feedstock and/or the hydrocracking feedstock to produce a recycled heavy oil.

5. The method according to claim 4 , wherein

with respect to the recycling step (C), the at least a portion of the heavy oil (b2) is a fractionator bottom fraction containing hydrocarbons with 15 carbon atoms and higher, and the fractionator bottom fraction is mixed with the hydrocracking feedstock.

6. The method according to claim 4 , wherein

with respect to the hydrocracking step (A2), a single-pass decomposition yield of the recycled heavy oil is 75 to 90 volume %.

7. The method according to claim 1 , wherein

with respect to the middle distillate (b1), a proportion of branched paraffins with 14 to 16 carbon atoms accounts for 75 mass % or more of the hydrocarbons with 14 to 16 carbon atoms.

8. The method according to claim 1 , wherein

with respect to the middle distillate (b1), a proportion of hydrocarbons with 9 carbon atoms is 5 to 30 mass percent of the middle distillate (b1), a proportion of hydrocarbons with 16 carbon atoms is 0.5 to 10 mass percent of the middle distillate (b1), and a proportion of branched paraffins with 9 carbon atoms accounts for 45 to 75 mass % of the hydrocarbons with 9 carbon atoms.

9. The method according to claim 1 , wherein

with respect to the middle distillate (b1), a proportion of hydrocarbons with 17 carbon atoms is 10 mass % or less of the middle distillate (b1).

10. A diesel fuel or a diesel fuel base stock produced by the method according to claim 1 .