Objective The aim is to solve the problem that the diesel discoloration for two sets of vacuum gas oil high pressure hydrocracking units of a refinery brought adverse effects on the products transportation.

Method The causes of diesel discoloration were analyzed from the following three aspects: the conventional properties and stability of diesel products, the influence of different storage environments on product properties, and the effect of additives on the appearance color of diesel. Meanwhile, the chromogenic substances in the discolored diesel were enriched, and molecular characterization was carried out.

Result The colors of the two diesel products were sensitive to ultraviolet light, especially for the vacuum gas oil hydrocracking unit processing naphthenic distillate oil, which produced diesel with a high final boiling point (FBP) and a slightly higher aromatics content. These aromatics were extremely easy to generate trace amount of chromogenic substances under the catalysis of ultraviolet light, thus affecting the appearance of diesel appearance. In view of the above reasons, some suggestions were put forward to improve the color of diesel products. Firstly, reduce the FBP of the distillation range of diesel or increase the hydrogenation depth to reduce aromatic content. Secondly, completely avoid light in the whole process of transportation and storage of diesel products. Thirdly, moderately add trace amount of photothermal stabilizer to improve the photothermal stability of diesel.

Conclusion By taking the above measures, the problem of diesel discoloration can be solved, which can provide a reference for other hydrocracking units.