【Q&A Question 129】2017.05.09
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This post was last edited by 955559 on 2017-7-21 22:56. [Q&A Question 129] 2017.05.09: Why does carbon buildup occur on catalysts? How to determine catalyst deactivation? The reference answers will be visible after responding; points are awarded by identifying the key points. (Unless otherwise specified, all questions and answers are based on hydrogenation units.) ) Causes of carbon deposition: (1) Significant fluctuations in reaction temperature or a drop in system pressure; (2) A decrease in the hydrogen-to-oil ratio; (3) Excessively high dry point of the feedstock or presence of water in it; (4) High levels of impurities in the feedstock; (5) Overheating of the catalyst bed. Under unchanged operating conditions, if the space velocity, reaction temperature, and pressure remain constant but hydrogen consumption decreases and the temperature rise of the catalyst bed becomes smaller, as well as if product quality parameters such as desulfurization efficiency decline and the bromine value increases, this indicates that the catalyst has become inactive. Scoring criteria: 2 Wealth points for incorrect responses; *incomplete* answers earn 3–8 points. 10 Wealth points are given for correct answers accompanied by detailed explanations; responses that are correct but lack sufficient details earn 15–20 Wealth points. Please score according to these criteria. 2017 Q&A Summary Thread (updating now) http://bbs.hcbbs.com/thread-1657793-1-1.html 2017 Daily Question Summary Thread (updating now) http://bbs.hcbbs.com/thread-1657794-1-1.html 2016 Q&A Summary Thread (already updated) http://bbs.hcbbs.com/thread-1597792-1-1.html
Catalyst carbon deposition is a type of catalyst poisoning. Carbon buildup, also known as carbon deposition, refers to the formation of carbon on the surface of catalysts. It refers to the gradual deposition of a layer of carbon-containing compounds on the surface of the catalyst during its use, which reduces the available surface area and thus leads to a decline in the catalyst’s activity. Carbon deposition can be regarded as a reflection of the toxic effect of by-products. The mechanism of carbon deposition is due to the dehydrogenation and polymerization of substrate molecules, resulting in non-volatile polymers. These polymers can undergo further dehydrogenation to form pyrolysis-like substances with very low hydrogen content; alternatively, polymerization at low temperatures may lead to the formation of dendritic structures that cover the active centers and block the catalyst pores, thereby causing the loss of the active surface
The temperature is extremely high, causing hydrocarbon molecules to decompose at high temperatures and resulting in carbon deposition. The reaction temperature is low, the product quality is unsatisfactory, and the pressure difference increases
Coking is the formation of large molecular compounds such as CH (and possibly O as well) that accumulate on the surface of the catalyst or within its pores, while carbon deposition refers to the formation of carbon-based substances such as graphite carbon that cover the catalyst’s surface or accumulate in its pores, thereby having an adverse effect on the reaction. It is generally believed that further loss of H in coking leads to the formation of carbon deposits. In organic catalytic reactions, sintering and carbon deposition occur simultaneously, both of which have varying degrees of adverse effects on the performance of the catalyst. Personally, I believe that by applying different calcination treatments to fresh catalysts, or by subjecting them to thermal aging at 500–600 degrees in an inert atmosphere, catalysts with varying degrees of sintering can be obtained. These catalysts can then be used to evaluate catalytic performance, thereby estimating the impact of sintering on catalytic efficiency ； By comparing the performance degradation caused by carbon deposition and sintering both present, it should be possible to determine which of the two plays a major role in the catalyst deactivation process.
Due to the presence of acidic sites, carbon deposits gradually form on the surface of the catalyst. The main phenomenon is an increase in the load on the reactor and compressor (primarily an increase in gas yield and a decrease in liquid yield). ), an increase in the regenerator bed temperature (an increase in coking amount), etc
The reaction takes place on the surface of the catalyst; if the reaction is exothermic, excessive temperatures can cause the organic material to overheat and decompose, resulting in the formation of carbon. As the amount of carbon deposition increases, the specific surface area, pore volume, surface acidity, and number of active centers of the catalyst all decrease accordingly; when the carbon deposition reaches a certain level, it leads to the deactivation of the catalyst. The faster carbon buildup occurs, the shorter the service life of the catalyst. Compared to catalyst poisoning, the amount of carbon deposit that causes catalyst deactivation is much greater than that of poisons. Carbon deposits can, to a certain extent, slow down catalyst poisoning, but catalyst poisoning accelerates the formation of carbon deposits. Compared to catalyst deactivation caused solely by physical blockage, carbon deposition-induced deactivation also involves a series of chemical reactions of reactant molecules in the gas phase and on the catalyst surface.
Causes of carbon deposition: (1) Significant fluctuations in reaction temperature or a drop in system pressure; (2) A decrease in the hydrogen-to-oil ratio; (3) Excessively high dry point of the feedstock or presence of water in it; (4) High levels of impurities in the feedstock; (5) Overheating of the catalyst bed. Under unchanged operating conditions, with constant space velocity, reaction temperature, and pressure, a decrease in hydrogen consumption and a reduced increase in bed temperature, along with declines in product quality parameters such as a decrease in desulfurization efficiency and an increase in bromine value, all indicate catalyst deactivation.
Due to the presence of acidic sites, carbon deposits gradually form on the surface of the catalyst. Carbon deposition reduces the catalyst’s activity, as carbon-forming compounds strongly adsorb onto the acidic sites, covering those active sites. Moreover, the accumulation of coke blocks the pores, preventing reactants from reaching the active sites for strong adsorption; this in turn covers the active sites and **reduces the surface utilization efficiency of the catalyst. At the beginning of the reaction, the amount of carbon deposition starts to increase rapidly, and the catalyst’s activity also declines quickly. As time passes, the amount of carbon deposit approaches stability, and the catalyst activity also enters a stable phase. In the later stages of the reaction, excessive carbon deposits clog the pores of the catalyst, resulting in a significant drop in its activity; therefore, raising the temperature is necessary to compensate for this.
The formation of carbon deposits is generally caused by factors such as uneven catalyst packing, an imbalance in the water-to-carbon ratio, increased load, heavier feed oil, catalyst poisoning or deactivation, a decline in activity or anti-deposit properties, and significant fluctuations in conversion temperature and pressure. 1. Decline in production ； 2. The temperature difference decreases ； 3. Bed temperature drops ； 4. Reduction in by-product steam ； 5. The bed temperature at the hot spot decreases.
Reasons: (1) Significant fluctuations in reaction temperature or a drop in system pressure; (2) A decrease in the hydrogen-to-oil ratio; (3) Excessively high dry point of the feedstock or presence of water in it; (4) High levels of impurities in the feedstock; (5) Overheating of the catalyst bed. Under unchanged operating conditions, if the space velocity, reaction temperature, and pressure remain constant but hydrogen consumption decreases and the temperature rise of the catalyst bed becomes smaller, as well as if product quality parameters such as desulfurization efficiency decline and the bromine value increases, this indicates that the catalyst has become inactive.