Main influencing factors of catalytic hydrogenation processes

The main operational factors affecting the catalytic hydrogenation process in practice are the reaction temperature, reaction pressure, hydrogen to oil ratio, reaction air velocity, catalyst activity, recycled hydrogen purity and the composition and nature of the raw material.

1. Reaction temperature
The reaction temperature is the main means to control the rate of desulphurisation, denitrogenation and conversion of the resulting oil. Increasing the reaction temperature can improve the desulphurisation and denitrogenation rates and create conditions for the cracking reaction. Increasing the reaction temperature has a significant effect on the chemical composition of the product, with the n-alkane content increasing, the iso-alkane content decreasing, the iso-alkane/n-alkane ratio decreasing and the degree of olefin saturation increasing, resulting in a better product stability. The hydrogenation process is an exothermic process and the increase in temperature is not conducive to the process. Too high a reaction temperature will lead to a reduction in conversion, so the appropriate reaction temperature should be chosen in practice.
An increase in reaction temperature will result in faster coking of the catalyst surface and affect its lifetime. Therefore, the choice of temperature conditions is generally influenced by the catalyst activity, the operating conditions and the catalytic performance.
The choice of temperature conditions is generally influenced by a number of factors such as catalyst activity, operating temperature limits and product distribution. As a general rule, the lowest possible temperature is used when the catalyst activity allows. The lowest possible reaction temperature is used as long as the catalyst activity allows.

2. Reaction pressure
The actual factor in reaction pressure is the partial pressure of hydrogen. Increasing the hydrogen partial pressure of the system promotes the hydrogenation reaction, accelerates the hydrogenation of olefins and aromatics, increases the rate of desulphurisation and denitrogenation, is beneficial for the removal of gum and asphaltenes, so the resulting product has a low bromine price, contains less sulphur and nitrogen compounds, has a good oil stability and can increase the smoke point of jet fuel and the sixteen burn value of diesel. At the same time, increasing the hydrogen partial pressure can also prevent or reduce coking, which is conducive to maintaining the catalyst activity and improving the stability of the catalyst. The choice of reaction pressure is related to the nature of the raw material to be treated, the more polycyclic aromatic hydrocarbons and impurities are contained in the raw material. The more PAHs and impurities are present in the feedstock, the higher the required reaction pressure. Excessive hydrogen partial pressures lead to increased investment in the plant and correspondingly increased operating costs. Therefore, the reaction pressure should be determined according to the requirements of the intended product and the nature of the raw material to be processed.

3. Hydrogen to oil ratio
The size of the hydrogen-to-oil ratio or the amount of circulating hydrogen is directly related to the hydrogen partial pressure and the residence time of the oil and also affects the vapourisation rate of the oil. An increase in circulating hydrogen ensures that the system has sufficient hydrogen partial pressure for the hydrogenation reaction to take place. In addition, the excess hydrogen
gas serves to protect the catalyst surface and within certain limits prevents the oil from condensing and coking on the catalyst surface. Also
The increased hydrogen-to-oil ratio can bring the reaction heat out of the system in time, which is conducive to the thermal balance of the reaction bed and thus makes the reactor. The temperature inside the reactor can be controlled smoothly.
However, an excessive hydrogen to oil ratio will increase the pressure drop in the system and shorten the contact time between oil and catalyst, which will lead to a decrease in reaction depth, an increase in circulator load and an increase in power consumption.
                                  Hydrogen to oil ratio = circulating hydrogen volume (Nm³ /h) / feed volume (m³ /h)
Normally the circulating hydrogen flow rate should remain constant throughout the catalyst's operating cycle, as frequent changes in compressor operation are not possible.

4. Airspeed
The airspeed is the ratio of the feed volume to the catalyst charge and is divided into two types: volumetric airspeed and mass airspeed.
                                   Airspeed (h^-1) = reactor feed volume (m³/h) / reactor catalyst charge volume (m³)
By reducing the airspeed, the reaction time of the feedstock is extended, the depth is increased and the conversion rate is increased. However, if the airspeed is too low, the secondary cracking reaction intensifies and although the total conversion can then be increased, there is a corresponding increase in the gaseous hydrocarbons produced. At the same time, the chance of condensation coking increases at certain temperatures due to the extended residence time of the oil molecules in the catalyst. Therefore, a low airspeed for a long period of time is detrimental to the catalyst activity. The choice of airspeed varies with the nature of the feedstock and the catalyst. An increase in airspeed means an increase in processing capacity, so the airspeed should be increased as much as possible without affecting the depth of conversion of the feedstock. However, the increase in air speed is limited by the design load of the equipment and the corresponding temperature limits.

5. Catalyst activity
Catalyst activity has a significant impact on hydrogenation operating conditions, product yield and product properties, increasing catalyst activity can reduce reactor temperature and pressure, increase air velocity or reduce the hydrogen to oil ratio. Increasing catalyst selectivity results in the production of more of the intended product, reduces unnecessary side reactions and increases the catalyst's resistance to toxicity. As the start-up cycle increases, the catalyst activity gradually decreases, at which point the reaction temperature must be increased accordingly to maintain the designed conversion rate. It should be noted that the level of operation and various incorrect operating methods have a significant impact on catalyst activity during production and must be brought to the attention of those involved.

6. Circulating hydrogen purity
The purity of circulating hydrogen is directly related to the partial pressure of hydrogen in the catalyst bed, and maintaining a high purity of circulating hydrogen maintains a high partial pressure of hydrogen, which is beneficial to the hydrogenation reaction and is a key part of improving product quality. At the same time, maintaining a high purity of circulating hydrogen also reduces oil condensation and coking on the catalyst surface, which protects the catalyst and helps to improve its activity and stability and extend its service life. However, if too high a purity of circulating hydrogen is required, a large amount of waste hydrogen has to be discharged. This results in increased hydrogen consumption and higher costs. Generally, the purity of circulating hydrogen is controlled to be no less than 85%. If the hydrogen purity is below 85%, some of the recycled hydrogen will have to be discharged from the plant and new hydrogen will have to be added.

7. Nature of the raw material
The relative constancy of the nature of the feedstock is an important factor for smooth operation. If the feedstock becomes heavy, the bed temperature needs to be increased to maintain a certain conversion rate. In addition, changes in the content of feedstock impurities (e.g. sulphur, nitrogen) have a large impact on the hydrorefining and hydrocracking reactions. From the point of view that both desulphurisation and denitration reactions are exothermic, an increase in both sulphur and nitrogen content will affect the rise in reaction temperature. However, an increase in sulphur content will produce large amounts of H2S, which will form salts with the NH3 produced by denitrogenation and block the system. At the same time, high concentrations of H2S can also cause corrosion to equipment and the concentration of H2S in the system should not normally be higher than 2%. When the concentration of hydrogen sulphide in circulating hydrogen is higher than 2%, additional circulating hydrogen desulphurisation devices should be installed. The nitrogen from hydrogenolysis produces NH3, which will reduce the catalyst activity. Therefore, the nature of the feedstock must be strictly controlled.
Where a catalyst and feedstock oil have been selected, the effect of temperature is most important. Under normal production conditions, system pressure and new hydrogen purity do not vary greatly and the hydrogen-to-oil ratio is essentially constant, so temperature becomes the most effective control.
Impurities in the feedstock (e.g. sulphur), especially nitrogen, affect both the hydrorefining and hydrocracking reactions. For this reason, the reactor bed temperature needs to be adjusted according to the amount of sulphur and nitrogen contained in the feedstock.