Catalytic stability of the performance parameter of the catalyst

Although according to the definition of a catalyst, the catalyst will not be consumed during the chemical reaction, the catalyst can still maintain its original chemical state before and after the reaction, and can continue to function to catalysis. However, in the actual process, it cannot be used unlimited times. The catalyst will be deactivated after multiple uses, because the catalyst will undergo a series of physical and chemical changes under certain temperature, pressure and chemical reactions, such as catalyst fragmentation and lattice deformation. , Changes in pore structure and specific surface area, loss of active components, coverage of active sites, and coking or carbon deposition of reaction products on the surface, etc. Catalytic stability refers to the change of catalyst activity and selectivity over time, which can be used to measure the catalyst’s ability to maintain activity and selectivity, such as service life, cycle times, thermal decomposition temperature, acid and alkali resistance, and mechanical strength. . Among them, catalyst life refers to the time for maintaining a certain activity level (one-way life) or the cumulative time for regenerating and returning to the required activity level after each activity decline (total life) under the conditions of use; life is the catalyst stability. summary description of . Testing the activity and selectivity of a catalyst takes little time, whereas knowing its stability takes a lot of time.

For industrial catalysis, stability is critical. The stability of the catalyst mainly includes:

①Chemical stability:The chemical composition and chemical state of the catalyst are stable during the catalytic process, and the active components and additives do not react Or loss, of course, for a specific catalytic environment, the catalyst is required to be resistant to alkali, acid or strong oxidation.

②Heat stability: The catalyst does not undergo changes such as sintering, crystallite growth and crystal phase transformation during the catalytic process; a A good catalyst should be able to have a certain level of activity for a long time under high temperature and harsh reaction conditions. Most catalysts have a service limit temperature, beyond a certain range, the activity will be reduced, or even completely deactivated. To measure the thermal stability of the catalyst, it is to gradually increase the operating temperature, and record how high the catalyst can withstand the temperature and how long it can last without changing the activity. The higher the heat-resistant temperature and the longer the time, the longer the life of the catalyst.

③Anti-toxicity stability:The ability of the catalyst to resist the deactivation of adsorption active poisons is called anti-toxicity stability. Compounds such as sulfur, phosphorus, halogen, and arsenic may be impurities in the raw materials, or by-products or intermediate compounds produced in the reaction; various catalysts have different anti-toxicity to various harmful impurities, and the same catalyst has different effects on the same Impurities also have different anti-toxin abilities under different reaction conditions. The criteria for measuring the anti-toxicity stability of catalysts are as follows: add a certain amount of related poisons to the reaction gas to poison the catalyst, and then test with pure raw material gas to observe the degree of activity and selective retention; The relevant poisons are gradually added to the gas until the activity and selectivity are maintained at a given level, and the high allowable concentration of the poisons is observed; the poisoned catalyst is regenerated to observe whether the activity and selectivity can be restored and the degree of recovery.

④Mechanical stability: The ability of the catalyst to resist friction, impact and gravity is called mechanical stability, which determines the catalyst’s use process Broken and worn in. Catalysts with high mechanical stability are able to withstand particle-to-particle, particle-to-fluid, and particle-to-wall friction.

During the use of the catalyst, the efficiency will gradually decrease, which will affect the catalytic process. For example, due to the decrease of catalytic activity or selectivity, and the increase of bed pressure drop due to catalyst crushing, etc., the economic benefit of the production process is reduced, and even the normal operation cannot be performed. There are many reasons for the attenuation of the efficiency of the catalyst and the shortening of its life. The main reasons are: the poisoning of impurities in the raw materials; the thermal effect at high temperature increases the grain size of the active components in the catalyst, resulting in a decrease in the specific surface area or deterioration of the catalyst. ; The dust in the reaction raw materials or the carbon deposits generated during the reaction cover the surface of the catalyst; the active ingredients in the catalyst are lost during the reaction; strong thermal shock or pressure fluctuations break the catalyst particles; the scouring of the reactant fluid makes the catalyst Pulverization and blow-off etc.

During the catalytic reaction process, the activity and selectivity of the catalyst may decrease due to various reasons described above. If appropriate measures are taken to maintain the stability of the catalyst, the It has a long enough life. For example, adding some additives to the catalyst can improve the stability of the active structure and the thermal conductivity of the catalyst; purify the reaction material to avoid catalyst poisoning; improve the mechanical strength of the catalyst to reduce the wear and tear of the catalyst, and reasonable regeneration and other measures.