Method of manufacturing diesel fuel

1. A method for producing diesel fuel, comprising:

fractionating a synthetic oil obtained by Fisher-Tropsch synthesis into a plurality of fractions including a middle fraction, and a wax fraction in a first fractionator, wherein the wax fraction contains a wax component heavier than the middle fraction;

hydroisomerizing the middle fraction with a hydroisomerizing catalyst, thereby producing a hydroisomerized middle fraction;

hydrocracking the wax fraction with a hydrocracking catalyst, thereby producing a hydrocracked wax decomposition compound;

separating a mixture of the hydroisomerized middle fraction and the hydrocracked wax decomposition compound into a plurality of fractions including a kerosene fraction and a gas oil fraction by fractionation in a second fractionator;

analyzing the composition of each of the plurality of fractions including the kerosene fraction and the gas oil fraction;

predicting one or more properties of a diesel fuel formed from the fractions based on the composition of each of the fractions;

determining a mixing ratio of the fractions so as to have the values of the properties within respective predetermined ranges; and

mixing the fractions including the kerosene fraction and the gas oil fraction at the mixing ratio, thereby producing the diesel fuel.

2. The method for producing diesel fuel according to claim 1 , wherein

the kerosene fraction contains 80% by volume or more of a component having a boiling point of 150° C. to 250° C., and the gas oil fraction contains 80% by volume or more of a component having a boiling point of 250° C. to 360° C.

3. The method for producing diesel fuel according to claim 1 , wherein,

when bringing the middle fraction into contact with the hydroisomerizing catalyst, the reaction temperature is 180° C. to 400° C., the hydrogen partial pressure is 0.5 MPa to 12 MPa, and the liquid hourly space velocity is 0.1 h −1 to 10.0 h −1 , and

when bringing the wax fraction into contact with the hydrocracking catalyst, the reaction temperature is 180° C. to 400° C., the hydrogen partial pressure is 0.5 MPa to 12 MPa, and the liquid hourly space velocity is 0.1 h −1 to 10.0 h −1 .

4. The method for producing diesel fuel according to claim 1 ,

wherein the one or more properties are kinematic viscosity at 30° C. and pour point of a diesel fuel.

5. A method for producing diesel fuel, comprising:

fractionating a synthetic oil obtained by Fisher-Tropsch synthesis into a plurality of fractions including a middle fraction, and a wax fraction in a first fractionator, wherein the wax fraction contains a wax component heavier than the middle fraction;

hydroisomerizing the middle fraction with a hydroisomerizing catalyst, thereby producing a hydroisomerized middle fraction;

hydrocracking the wax fraction with a hydrocracking catalyst, thereby producing a hydrocracked wax decomposition compound;

separating a mixture of the hydroisomerized middle fraction and the hydrocracked wax decomposition compound into a plurality of fractions including a kerosene fraction and a gas oil fraction by fractionation in a second fractionator; and mixing the fractions including the kerosene fraction and the gas oil fraction, thereby producing the diesel fuel, wherein,

an appropriate mixing ratio of the kerosene fraction and the gas oil fraction is obtained by the following steps (1) to (3):

(1) analyzing the composition of the kerosene fraction and the composition of the gas oil fraction based on an all-component analysis by gas chromatography in advance, and predicting the composition of the produced diesel fuel with respect to x [M.W.] and [nC19 + ] when assuming that the kerosene fraction and the gas oil fraction are mixed at a specific ratio;

(2) calculating the kinematic viscosity [Vis.] at 30° C. of the diesel fuel by Equation 1 and calculating the pour point of the diesel fuel by Equation 2 based on the composition of the produced diesel fuel predicted in step (1); and

(3) when the kinematic viscosity at 30° C. of the diesel fuel calculated in step (2) is 2.5 mm 2 /s or more and the pour point of the diesel fuel calculated in step (2) is −7.5° C. or less, completing the steps by determining that the specific ratio of the kerosene fraction and the gas oil fraction assumed in step (1) is the appropriate mixing ratio of the kerosene fraction and the gas oil fraction, and

when the kinematic viscosity at 30° C. of the diesel fuel calculated in step (2) is less than 2.5 mm 2 /s and the pour point of the diesel fuel calculated in step (2) is more than −7.5° C., repeating the steps (1) to (3) until the appropriate mixing ratio of the kerosene fraction and the gas oil fraction where the kinematic viscosity of the diesel fuel at 30° C. is 2.5 mm 2 /s or more and the pour point of the diesel fuel is −7.5° C. or less is obtained,

where

[Vis.]=0.1309× e (0.0144× x [M.W.] )  Equation 1,

and

[PP]=46.37×log([ nC 19 + ]+1.149)−45  Equation 2

wherein, [Vis.] represents the kinematic viscosity at 30° C.; [PP] represents the pour point;

x [M.W.] represents the average molecular weight of the diesel fuel; and [nC19 30 ] represents the content of normal paraffins having 19 or more carbon atoms in the diesel fuel by mass percentage.