Diesel oil is a product obtained by fractional distillation of petroleum under normal pressure. If it is directly obtained from petroleum refining, the content of sulfur, nitrogen and oxygen in diesel oil is relatively high, and the content of olefin is also relatively high. The high content of olefins in the product is prone to discoloration, and it also affects the service life of the motor vehicle, and the degree of atmospheric pollution is relatively large.
In order to overcome the above problems and improve the performance of diesel oil, in the processing of diesel oil, a hydrogenation process is generally introduced: on the one hand, the content of sulfur, nitrogen, oxygen and olefin in diesel oil can be reduced, and the stability of the product is improved; On the other hand, it can also reduce the impact of the product on the atmosphere.
Why choose the hydrotreating process?
This has to start with the cetane number of diesel.
The cetane number is the self-ignitability index of diesel when it is burned in a diesel engine. The higher the cetane number, the better the combustion performance of diesel, and the less likely it is to cause abnormal combustion such as knocking. It differs from the diesel label: the diesel label only indicates the anti-coagulation properties of the diesel, and the cetane number is a true indicator of the combustion performance.
The cetane number is related to the nature of the diesel component. In general, alkanes have the largest cetane number, the smallest aromatic hydrocarbons, and the naphthenes and olefins are somewhere in between. N-hexadecane (CN is 100), 1-methylnaphthalene (CN is 0) diesel standard fuel is from n-hexadecane and 2,2,4,4,6,8,8-heptamethyl The decane (CN specification 15) is mixed in a certain ratio. The test oil is compared with the standard fuel on a standard engine. If the test oil has the same degree of knocking as a standard fuel, then it is the same as the standard fuel. The cetane number, the cetane number of the standard fuel, can be calculated from the formula: cetane number = n-hexadecane volume fraction + 0.15 x heptamethyl decane volume percent. Generally, the cetane number of coking diesel is generally around 50, the cetane number of Ciba is generally around 45, and the cetane number of Chai is generally around 35.
The same petroleum, the different process of adding diesel oil, the diesel has the same cetane number: straight-run diesel with high alkane content, high cetane number; catalytic diesel isoparaffin and olefin higher than straight-run diesel, Therefore, its cetane number is lower than that of straight-run diesel; the content of olefins and aromatics in coking diesel is higher than that of catalytic diesel, so its cetane number is obviously lower. The diesel fuel subjected to catalytic hydrorefining is converted into a naphthene by conversion of an olefin into an alkane, so that the cetane number is improved and the performance is improved.
In general, the light diesel fuel used in cars should have a cetane number of no less than 45. If the cetane number of diesel is lower than the working conditions, combustion delay and incomplete combustion may occur, and even knocking may occur, reducing engine power and increasing diesel consumption. However, if the cetane number is too high, it will also cause incomplete combustion and smoke, and increase diesel consumption. The high-speed diesel fuel has a cetane number of about 40-56. Adding a small amount of additives (such as amyl nitrate) can also increase the cetane number of diesel.
