Patent ID: 8597502

Claim:
A method of manufacturing a diesel fuel base stock improved in low-temperature flowability, comprising: (i) fractionating in a first fractionator a synthetic oil obtained by Fisher-Tropsch synthesis into at least two fractions of a first middle fraction containing a component having a boiling range corresponding to diesel fuel oil and containing a light wax component of n-paraffins having 20 to 25 carbon atoms, and a wax fraction containing a wax component heavier than the first middle fraction; (ii) hydroisomerizing the first middle fraction by bringing the first middle fraction into contact with a hydroisomerizing catalyst to produce a hydroisomerized middle fraction; (iii) hydrocracking the wax fraction by bringing the wax fraction into contact with a hydrocracking catalyst to produce a wax-decomposition component; and (iv) fractionating in a second fractionator a mixture of the produced hydroisomerized middle fraction and the produced wax-decomposition component, wherein rectification conditions in the first fractionator and/or rectification conditions in the second fractionator are adjusted to reduce the content of n-paraffin having 19 or more carbon atoms in a heavy component contained in the diesel fuel base stock, and wherein the fractionation is conducted where the 90% by volume distillation temperature of the first middle fraction, which is a feedstock for the hydroisomerization, is higher than the 90% by volume distillation temperature of the diesel fuel base stock by 20° C. or more as one of the rectification conditions in the first fractionator.