【Q&A Question 021】2017.01.21

The pressure difference increases, the temperature rise is minimal, and the product quality is unsatisfactory!

The inlet temperature of each section increases gradually, while the temperature of the reaction furnace decreases

The inactivation process usually consists of three stages: ① Initial inactivation. This phase lasts for about a few days. During this stage, intense carbon deposition occurs, and the activity declines rapidly, until a dynamic equilibrium of coking is reached and the activity stabilizes. Initial inactivation requires increasing the temperature to compensate for the loss of activity. ②Intermediate inactivation. Catalyst deactivation at this stage is caused by the deposition of metal sulfides. Due to the stability of heavy metals and carbon deposits, the activity decreases slowly. ③Terminal inactivation. At the end of operation, the high operating temperature exacerbates carbon and metal deposition, causing the catalyst to become rapidly deactivated.

The reasons for catalyst deactivation are generally classified into three categories: poisoning, sintering and thermal deactivation, and coking and blockage. 1. Inactivation caused by poisoning (1) Temporary poisoning (reversible poisoning): When a toxin adsorbs onto or reacts with the active site, the bonds formed are relatively weak; it is possible to use appropriate methods to remove the toxin, thereby restoring the catalyst’s activity without affecting its properties. This type of poisoning is known as reversible poisoning or temporary poisoning. (2) Permanent poisoning (irreversible poisoning): When a toxin interacts with the active components of a catalyst, strong chemical bonds are formed, making it difficult to remove the toxin using conventional methods in order to restore the catalyst’s activity. This type of poisoning is known as irreversible or permanent poisoning. (3) Selective poisoning: After catalyst poisoning, it may lose its catalytic ability for a certain reaction, but remain catalytically active for other reactions; this phenomenon is known as selective poisoning. In a chain reaction, if the poison only poisons the active sites that initiate subsequent reactions, it can halt the reaction at an intermediate stage, yielding intermediates in high yield. 2. Deactivation caused by coking and blockage: Carbonaceous deposits on the surface of the catalyst are known as coking. Coking almost always occurs in heterogeneous catalytic reaction processes that use organic substances as reactants and solids as catalysts. The phenomenon in which the pore size decreases (or the pore openings shrink) due to the deposition of carbonaceous substances and/or other materials within the catalyst pores, preventing reactant molecules from diffusing into those pores, is known as clogging. Therefore, clogging is often classified under coking, and the overall decline in activity is referred to as coking deactivation; it is the most widespread and common form of catalyst deactivation. Typically, carbonaceous deposits can be removed by gasification in reaction with water vapor or hydrogen, so coking and deactivation are reversible processes. Compared to catalyst poisoning, there are many more substances that cause catalyst coking and blockage than catalyst poisons. In actual coking studies, it has been found that catalyst coking involves a rapid initial deactivation, followed by a quasi-steady state in terms of activity. It has been reported that coking deposition occurs mainly during the initial stage (within 0.15 seconds), and it has also been observed that approximately 50% of the carbon formed gets deposited within the first 20 seconds. Coking deactivation is also reversible; by controlling coking during the early stages of the reaction, the activity of the catalyst can be significantly improved. This is precisely why research on coking deactivation is becoming increasingly active. 3. Sintering and thermal deactivation (solid-state transformation): The sintering and thermal deactivation of catalysts refer to the changes in the structure and properties of the catalysts caused by high temperatures. In addition to causing sintering of the catalyst, high temperatures can also lead to other changes, including alterations in chemical and phase composition, semi-melting, grain growth, the encapsulation of active components by the carrier, and the loss of active components due to the formation of volatile or sublimable substances. In fact, at high temperatures, all catalysts will gradually undergo irreversible structural changes; the rate of these changes varies depending on the catalyst. Sintering and thermal deactivation are related to various factors, such as the pretreatment, reduction, and regeneration processes of the catalyst, as well as the promoters and carriers added. Of course, the reasons for catalyst deactivation are complex; the deactivation of each catalyst does not occur based on just one of the categories mentioned above, but is rather caused by two or more factors

Early stage: Low conversion rate and low temperature rise in the bed; under the same temperature, the load cannot be increased. Late stage: The bed resistance gradually increases, and the conversion efficiency becomes worse. It is forced to raise the catalyst temperature, or even produce using a electric furnace.

The inactivation process usually consists of three stages: ① Initial inactivation. This phase lasts for about a few days. During this stage, intense carbon deposition occurs, and the activity declines rapidly, until a dynamic equilibrium of coking is reached and the activity stabilizes. Initial inactivation requires increasing the temperature to compensate for the loss of activity. ②Intermediate inactivation. Catalyst deactivation at this stage is caused by the deposition of metal sulfides. Due to the stability of heavy metals and carbon deposits, the activity decreases slowly. ③Terminal inactivation. At the end of operation, the high operating temperature exacerbates carbon and metal deposition, causing the catalyst to become rapidly deactivated.

The inactivation process is usually divided into three stages: (1) initial inactivation. This phase lasts for about a few days. During this stage, intense carbon deposition occurs, and the activity declines rapidly, until a dynamic equilibrium of coking is reached and the activity stabilizes. Initial inactivation requires increasing the temperature to compensate for the loss of activity. (2) Intermediate inactivation. In one stage, catalyst deactivation is caused by the deposition of metal sulfides. Due to the stability of heavy metals and carbon deposits, the activity decreases slowly. (3) Terminal inactivation. High temperatures at the end of operation exacerbate carbon and metal deposition, causing the catalyst to become rapidly deactivated.

The inactivation process usually consists of three stages: ① Initial inactivation. This phase lasts for about a few days. During this stage, intense carbon deposition occurs, and the activity declines rapidly, until a dynamic equilibrium of coking is reached and the activity stabilizes. Initial inactivation requires increasing the temperature to compensate for the loss of activity. ②Intermediate inactivation. Catalyst deactivation at this stage is caused by the deposition of metal sulfides. Due to the stability of heavy metals and carbon deposits, the activity decreases slowly. ③Terminal inactivation. At the end of operation, the high operating temperature exacerbates carbon and metal deposition, causing the catalyst to become rapidly deactivated.

Temperature rise decreases； The product quality is substandard ；

The inactivation process usually consists of three stages: ① Initial inactivation. This phase lasts for about a few days. During this stage, intense carbon deposition occurs, and the activity declines rapidly, until a dynamic equilibrium of coking is reached and the activity stabilizes. Initial inactivation requires increasing the temperature to compensate for the loss of activity. ②Intermediate inactivation. Catalyst deactivation at this stage is caused by the deposition of metal sulfides. Due to the stability of heavy metals and carbon deposits, the activity decreases slowly. ③Terminal inactivation. At the end of operation, the high operating temperature exacerbates carbon and metal deposition, causing the catalyst to become rapidly deactivated.