【Q&A Question 009】April 29, 2016

A senior in the community once said that, in the presence of hydrogen, carbon monoxide can undergo methanation reactions at the active sites on the surface of a catalyst. The methanation reaction competes with other normal reactions for the catalyst. Therefore, if the accumulation of carbon monoxide is allowed to continue, it is necessary to increase the catalyst temperature. Under extreme conditions, the entry of a large amount of carbon monoxide into the device can cause a highly exothermic reaction, and even lead to a surge in temperature. Another senior once said that carbon monoxide and carbon dioxide have the following effects on the system: ① Carbon dioxide is converted into carbon monoxide through hydrogenation. 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 between 200°C and 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 heat up excessively, resulting in an uneven temperature distribution and compromising the operation of the facility. ③Carbon monoxide, carbon dioxide, and hydrogen compete for adsorption at the catalyst’s 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. To be honest, I’m not a process engineer and really don’t understand processes; I can only reply by searching for answers from previous colleagues. I hope this can solve your problem.

To prevent methanation reactions.

This is because CO and CO2 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, leading to uneven temperature distribution, which deteriorates the operation of the plant. Moreover, carbon monoxide, carbon dioxide, and hydrogen compete for adsorption at the active sites of the catalyst, affecting the utilization of those 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

Methanation reaction and formation of **nickel carbonyl

Controlling alkylation reactions and catalyst poisoning

CO and CO2 can be converted into methane and water at active sites on the catalyst surface in the presence of hydrogen. This is called the methanation reaction. The methanation of CO and CO2 competes with the reaction of ordinary hydrocarbon reactants for the catalyst. Therefore, if the accumulation of CO and CO2 is allowed to continue, the temperature of the catalyst needs to be increased. In extreme cases, if a large amount of CO and CO2 enters the hydrocracking unit in a very short period of time, and since the methanation reaction is highly exothermic, it is theoretically possible for a spike in temperature to occur. In practical operation, it is required that if the concentration of CO + CO2 exceeds the maximum design limit, the temperature of the catalyst shall not be increased to compensate for the resulting decrease in conversion rate. The temperature of the catalyst should be maintained or reduced until the problem of increased CO + CO2 levels is resolved. Only in this way can the catalyst’s activity be prevented from being impaired by rising temperatures, and accidents caused by excessive temperature rise due to the methanation reaction can also be avoided.

The effects of carbon monoxide and carbon dioxide on hydrogenation reactions: 1. Carbon dioxide is hydrogenated to form carbon monoxide; this is an endothermic reaction. The hydrogenation conditions favor the forward reaction, resulting in a higher concentration of carbon monoxide than that of carbon dioxide in the recycled hydrogen； 2. Under the action of nickel- or cobalt-containing catalysts, carbon monoxide and carbon dioxide react with hydrogen at temperatures of 200–350°C to produce methane, releasing a large amount of heat in the process. The heat generated by this methanation reaction causes the temperature of the catalyst bed in the reactor to rise excessively, leading to uneven temperature distribution and compromising the operation of the facility ； 3. Carbon monoxide, carbon dioxide, and hydrogen compete for adsorption at the active sites of the catalyst, affecting the utilization of the hydrogenation active sites. Carbon monoxide reacts with the metal components on the catalyst to form toxic, volatile carbonyl compounds, thereby causing corrosion of the catalyst and reducing its activity. 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 can easily lead to coking and deactivation.

Ensure hydrogen purity and maintain a satisfactory hydrogen partial pressure

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 between 200°C and 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 heat up excessively, resulting in an uneven temperature distribution and compromising the operation of the facility. ③Carbon monoxide, carbon dioxide, and hydrogen compete for adsorption at the catalyst’s 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.