On catalyst poisoning and dry gas composition analysis

This post was last edited by sdya123456 on 2019-9-2 at 14:56. Definition of catalyst poisoning: The phenomenon in which trace impurities present in the reaction materials cause a significant decrease or loss of the catalyst’s activity and selectivity. The essence of poisoning is a certain chemical reaction between trace impurities and the active centers of the catalyst, resulting in the formation of inactive species. Adsorption complexes are formed in gas-solid multiphase catalytic reactions. One type is when the toxin has a weak effect on the active component, and the activity can be restored using simple methods; this is known as reversible poisoning or temporary poisoning. Another type is irreversible poisoning, for which it is not possible to restore activity using simple methods. To reduce the activity of side reactions, it is sometimes necessary to selectively poison the catalyst. 　　 Catalysts can lose their activity during use due to various factors, one of the important ones being poisoning. There are several possible causes of catalyst poisoning: small amounts of impurities present in the raw material, strong adsorption (usually chemical adsorption) on the active sites, or chemical reactions with the active sites that result in the formation of other substances – all of these can poison the active sites. Additionally, such toxic substances may also be present in the reaction products ； During the preparation of the catalyst, impurities present in the support interact with the active components, and may also poison the active sites. Identification and effects of catalyst poisoning: There are various types of poisons, and for a given catalyst, it is only by considering the reaction it catalyzes that it can be determined which substances act as poisons. In other words, poisons affect not only the catalyst itself but also the reaction catalyzed by that catalyst. During the period of stable activity, catalysts often experience a significant decline in their activity due to contact with small amounts of impurities; this phenomenon is known as catalyst poisoning. Substances that cause a catalyst to lose its catalytic activity are known as catalyst poisons. If activity can be restored after the toxic factor is removed, it is called temporary poisoning; otherwise, it is called permanent poisoning. 　　 Some catalysts are toxic in certain reactions; some of these toxins have a temporary effect, while others are permanent. For example, in the synthesis of ammonia, iron-based catalysts are affected by water and oxygen as toxins. When such poisoning occurs, the catalyst can be reactivated through reduction or heating methods. This type of poisoning is temporary, or reversible ； Compounds of sulfur or phosphorus are also toxic to this catalyst and this reaction; when poisoning occurs as a result of them, it is very difficult to reactivate the catalyst, and this represents permanent poisoning, or irreversible poisoning. The latter type of poisoning can be prevented. 　　 Poisoning not only affects the activity of catalysts, leading to a decrease in their efficiency, but it also impacts their selectivity. Poisoning is a practical problem that often arises when using catalysts in chemical production, yet our understanding of it is not yet complete. As for how to prevent poisoning and remove toxic substances, this must be addressed through practical experience. Analyzing catalyst poisoning based on the components of dry gas: The hydrogen content in the components of dry gas plays a significant role; a high hydrogen level indicates a possible case of catalyst poisoning. For reference only.

This post was last edited by Refiner on 2020-1-30 08:54. Catalyst poisoning includes many types such as Fe, Ca, Vi, V, Na, etc., and is mainly manifested as changes in product distribution and fluidization. Different poisoning symptoms vary. You can search for posts on the forum; there are many detailed explanations there. Taking NI as an example, NI mainly involves dehydrogenation, which leads to an increase in the H2/CH4 ratio in the dry gas. V primarily causes damage to the catalyst’s structure, as evidenced by a decrease in pore volume and specific surface area in catalyst analysis. Similar to Ca, Fe causes the blockage of pores, leading to the formation of calcified hard shells or iron tumors. With the above criteria, electron microscope images can be accumulated, making it easy to understand at a glance. The above is mentioned in the yellow book as well as on the forum. Try to search more to improve yourself; don’t just keep asking questions, as the process is beneficial for your understanding and memory.

Is your catalyst often examined under an electron microscope? Do you do it yourselves or outsource it to another company?