Inactivation means that some substances with biological activity (such as protein, amino acid, gene, etc.) are affected by physical or chemical factors, resulting inbiological activityThe phenomenon of loss also means that the catalyst used in industry loses its catalytic effect.
The industrial catalyst needs to operate in the reactor for a long time. Although the catalyst is not consumed from the perspective of reaction formula, there are always some reasons in the process of long-term operation that reduce the catalyst activity to the predetermined conversion rate or selectivity.The main reason is the poisoning caused by the interaction between the impurities in the reaction gas stream and the active center;The carbon on the surface of the catalyst covers the active center so that it cannot play its role;Small metal ion sintering at high temperature reduces the active surface area and the amount of active components due to loss in the reaction process.This phenomenon is called catalyst deactivation.
The catalyst has a certain service life.Generally, the activity of catalyst is very high when it is first used. After a short time, the activity will drop to a relatively stable value, which can last for a period of time and is the time range of normal use of catalyst.The catalyst and operating conditions are different, and the stability period is different, some months, some up to 3-5 years.After a certain period of time, the catalyst gradually loses its activity due to changes in composition and structure.If part or all of the activity is lost in a short time, it belongs to abnormal inactivation, which should be prevented.[1]
Type and cause of inactivation
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The types and causes of inactivation are complex and can be roughly divided into three categories:
(1) Structure change The physical structure of the catalyst changes during the reaction process, such as the change of crystal form, the growth of fine dispersed grains, the sintering of particles, and the pulverization of the carrier, thus losing its activity.This inactivation is irreversible.The main reason for this is that the temperature is too high, which may also be caused by the impurities brought into the gas.assilicon dioxideUnder the action of HF, vanadium catalyst as carrier will cause catalyst pulverization and decrease activity.
(2) Physically toxic solid impurities such as dust, carbon, etc. are deposited on the surface of the catalyst to cover the active center, which increases the internal diffusion resistance and can lead to a decline in activity.This deactivation is caused by the physical action of impurities. After impurities are removed, most of the activity can be recovered.The phenomenon of "carbon deposition" in petrochemical catalytic reaction belongs to physical poisoning.In addition, some inert gas components may be strongly adsorbed on the catalyst surface, occupying part of the active center and reducing the catalyst activity.When this part of the component in the reaction gas decreases, it can also be desorbed to restore the catalyst activity.
(3) Chemical poisoning Some adsorbedGaseous impurityOccurrence of molecules (poisons) and catalyst active substancesIrreversible reaction, generating inactive substances, such as sulfur, phosphorus and other compounds that are toxic to copper, zinc, nickel and other catalysts;Or generate volatile substances to escape into the gas phase;Or fixed on the surface of the catalyst to catalyze side reactions and reduce the selectivity of the target product.For example, when a small amount of nickel, vanadium, iron and other poisons in petroleum raw materials are deposited on the cracking catalyst, the yield of hydrogen and coke will increase and the yield of gasoline will decrease.The catalyst deactivation caused by chemical poisoning is difficult to recover, which is called permanent deactivation.[2]
Methods of dealing with inactivation
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The deactivation of catalyst has a significant impact on the industrial production process. Although a lot of research work has been carried out, it is still insufficient.The following measures are adopted in industry to prevent and compensate for catalyst deactivation, ensure the normal operation of the process and improve the economic benefits of the process.
(1) Improved catalyst
In order to reduce the possibility of catalyst deactivation and extend the period of catalyst replacement, catalyst formulations and processes are constantly improved at home and abroad to provide catalysts with high low-temperature activity, high mechanical strength, high temperature resistance and poison resistance.These improvements have played an important role in maintaining stable activity of the catalyst and achieving long-term operation.
(2) Reactor design with "intermediate activity" or safety factor
The production capacity must be guaranteed in the reactor design.That is, it is required to reach the specified output and quality stably within a certain period.Therefore, the amount of catalyst must be reasonably determined.However, it is difficult to determine the deactivation rule of the catalyst.For the sake of reliability, the catalyst with "medium-term activity" used in industry is used to study the catalytic kinetics, and the obtained kinetic data is taken as the design basis.This design is much more reliable.However, the inactivation phenomenon is different in different factories;Even in the same factory, the situation is different at different times;At the same time, catalysts at different stages are different.In order to ensure that the catalyst bed has a stable production capacity in a long period of time, someone has made probability statistics on the deactivation of the industrial bed. On this basis, the design institute has a safety factor for each section of the catalyst (that is, increase the factor several times on the basis of the calculated amount of catalyst, called the safety factor).Although this method is not scientific and reasonable, it is effective.
(3) Strictly control operating conditions
In most cases, catalyst deactivation is caused by improper control of operating conditions, such as over temperature, excessive toxic substances in intake air, etc.To ensure normal operation, the following points must be achieved:
Fully purify the feed gas to avoid toxic substances exceeding the indicators entering the catalyst bed, such as Cu, Zn-N: O, and the catalyst. It must ensure that the sulfide in the feed gas is less than the specified indicators to ensure the life of the catalyst.
Strictly control the operating temperature to prevent overheating, reduce the structural change of active components, and avoid permanent deactivation caused by sintering, decomposition, etc.
It is necessary to reactivate the catalyst that causes temporary poisoning due to physical changes. In order to ensure the activity of the catalyst, it must be reactivated.For example, the catalyst deactivated by adsorption of water vapor can be regenerated with dry air under high temperature.
(4) Optimize operation to make up for the decline in production capacity caused by deactivation
Due to the slow deactivation of the catalyst, if the operating conditions remain unchanged, the product output or quality will not meet the requirements of the original design.In order to ensure the production capacity, with the gradual deactivation of the catalyst, the operating conditions must be optimized.The most effective method is to gradually increase the reaction temperature according to the inactivation to make up for the lost activity.For a single reversible exothermic reaction, when using a multi-stage adiabatic catalytic reactor, the method of maximum conversion section by section (also known as maximum temperature difference section by section) can be used to make each section of the catalytic bed obtain the maximum conversion under the condition of obtaining the maximum conversion in the front section, and the entire catalytic bed is in the best state under certain conditions to achieve the maximum production capacity.The catalyst in the front section is first fully utilized by adjusting the sequence from front to back.[2]
