The main influencing factors of the hydrogenation process

Reaction pressure:

The influence of reaction pressure is often reflected by the hydrogen partial pressure of the system, which depends on the operating pressure, the hydrogen to oil ratio, the purity of the circulating hydrogen and the vapourisation rate of the feedstock.

1)Gasoline hydrorefining

After a hydrogen partial pressure of 2.5 MPa to 3.5 PMa, the depth of the gasoline hydrorefining reaction is not thermodynamically controlled, but depends on the reaction rate and reaction time. Carried out under gas phase conditions, increasing the reaction pressure makes the reaction time of gasoline longer, the pressure has little effect on its reaction rate and therefore the depth of hydrofinishing increases. If the pressure remains the same and the hydrogen partial pressure is increased by the hydrogen to oil ratio, the refining depth decreases.

2)Diesel hydrorefining

Under refining conditions, it can be either in the gas phase or in the mixed gas-liquid phase. In the gas phase, the reaction time of the gasoline is increased by increasing the reaction pressure and therefore the hydrogen refining depth is increased. However, in the presence of a liquid phase, increasing the pressure will make refining less effective. The rate of hydrogen diffusion through the liquid film to the catalyst surface is often the controlling factor in the rate of reaction, and increasing the reaction pressure will thicken the liquid layer on the catalyst surface, thus reducing the rate of reaction. The best refinement results can be obtained if the total pressure is kept constant and the hydrogen partial pressure is increased. Generally, at a pressure of 4.5~5.0MPa, a hydrogen to oil ratio of 150~600Nm3/m3 can be used to obtain the most suitable hydrogen partial pressure.

3)Heavy distillate hydrorefining and hydrocracking

Heavy distillates above 350°C are often in the gas-liquid mixed phase under hydrofinishing conditions, so increasing the hydrogen partial pressure can significantly increase the reaction rate and improve the refining effect. The conversion of aromatic hydrogenation reactions increases significantly with higher reaction pressure, which also speeds up the reaction rate. At low pressures, it is not possible to have the high conversions at high pressures, even if the reaction rate is fast. Hydrocracking feedstocks are generally heavier distillate oils, which contain more polycyclic aromatic hydrocarbons. The cracking of PAHs is carried out by hydrogenation of the aromatic rings and therefore the pressure is chosen to ensure a sufficient equilibrium conversion of the aromatics with the highest number of rings for a given catalyst and reaction temperature.

Reaction temperature:

Increasing the reaction temperature results in a faster reaction rate for both hydrorefining and hydrocracking. In the commonly used pressure range, the reaction temperature for hydrorefining is generally in the range of 250 to 420°C. More cracking and dehydrogenation reactions will occur above 420°C.

1)Reforming feedstock refining: 400~420°C. The higher temperature and the reactions of dehydrogenation are not relevant.

2)Hydrorefining of jet coal: 350~360°C. The equilibrium conversion of tetrahydronaphthalene dehydrogenation to naphthalene rises sharply at temperatures above 370°C (at 5.0 MPa).

3)Diesel hydrogenation: 400~420°C. When the reaction temperature is too high, the dehydrogenation of monocyclic and bicyclic alkanes will occur, resulting in a lower cetane number, while the hydrocracking reaction will intensify and increase the hydrogen consumption.

4)Hydrocracking: 260~400℃. The increase in temperature accelerates the cracking reaction, resulting in an increase in the number of low boiling components in the reaction products, an increase in the alkane content and a decrease in the cycloalkane content, and a decrease in the iso-alkane/n-alkane ratio. Depending on the performance of the catalyst, the nature of the raw material and the requirements of the product, it is generally not more than 400~420°C.

Airspeed:

The air velocity reflects the processing capacity of the unit. Depending on the catalyst activity, the nature of the raw material and the required reaction depth, it is generally in the range of 0.5 to 10 h-1. A lower air speed is usually used for the hydrogenation of heavy raw materials and raw materials obtained from secondary processing.

Hydrogen to oil ratio:

Increasing the hydrogen-to-oil ratio can be hydrogen partial pressure, which is generally beneficial to the hydrogenation reaction. Hydrogen refining can have a lower hydrogen-to-oil ratio, while hydrocracking has to be higher.