The effect of CO/CO2 on diesel hydrogenation catalysts

The carbon monoxide and carbon dioxide in the recycle hydrogen of the hydrogenation unit originate mainly from the following sources: ① Carried in by the hydrogen used for hydrogen production into the reaction system； ②The water in the feedstock reacts with the carbon deposits on the catalyst surface to produce carbon monoxide and carbon dioxide (this reaction hardly occurs when the water content in the feedstock is very low) ； ③Carbon monoxide and carbon dioxide contained in the protective gas of the raw material tank dissolve in the oil and enter the reaction system ； ④Coking wax oil is blended into the crude oil; carbon monoxide and carbon dioxide generated during the coking process dissolve in the coking wax oil and enter the reaction system along with it. Carbon monoxide and carbon dioxide have the following effects on the system: ① Carbon dioxide is hydrogenated to form carbon monoxide. This reaction is endothermic, and it proceeds in the forward direction under hydrogenation conditions, resulting in a higher concentration of carbon monoxide than that of carbon dioxide in the recycle hydrogen. ②Under the action of nickel- or cobalt-containing catalysts, carbon monoxide and carbon dioxide react with hydrogen at temperatures of 200°C to 350°C to produce methane, releasing a large amount of heat in the process. The heat generated by the methanation reaction causes the catalyst bed in the reactor to overheat, resulting in an uneven temperature distribution and deteriorating the operation of the plant. ③Carbon monoxide, carbon dioxide, and hydrogen compete for adsorption at the catalyst active sites, affecting the utilization of the hydrogenation active sites. Carbon monoxide may form toxic, volatile carbonyl compounds with the metal components on the catalyst, thereby causing corrosion of the catalyst and reducing its activity. 50℃ 230℃ 4CO+Ni Ni(CO)4 Ni+4CO Since the above reactions occur more easily at low temperatures, the catalyst’s activity is reduced each time it is put into use when carbon monoxide is present in the circulating hydrogen. At low temperatures, carbon monoxide reacts with the nickel component on the catalyst to form nickel carbonyl; when the temperature rises, nickel carbonyl volatilizes and sublimates, exposing metallic nickel on the surface of the catalyst. Metallic nickel has a very high hydrogenolysis activity, which easily leads to coking and deactivation.

1. CO causes temporary poisoning of the catalyst, but this condition is reversible. CO2 acts as a diluent, reducing the hydrogen partial pressure. 2. When CO and CO2 react with hydrogen to produce CH4 and H2O, reaction heat is generated, which increases the temperature rise in the catalyst bed; the water produced can also increase the gelation rate of the catalyst. 3. CO and CO2 can affect the denitration activity and cracking activity of hydrogenation catalysts.

Under the action of a nickel catalyst, CO2 readily undergoes a methanation reaction with hydrogen at high temperatures >220 degrees, releasing heat that causes coking of the catalyst; CO, on the other hand, at lower temperatures

Among all metals, only Ni and Co readily react with CO to form carbonyl compounds. The active components of hydrogenation catalysts are mainly Ni, Mo, Co, and W; therefore, an excessive amount of CO can lead to the formation of carbonyl compounds, resulting in metal loss.