RCO precious metal catalyst poisoning situation classification and response methods

Aug 17, 2022
Among the many VOCs organic waste gas treatment methods, Regenerative Catalytic Oxidation, referred to as RCO, is one of the most commonly used and effective technologies. Although regenerative catalytic oxidation treatment of organic waste gas is also a thermal oxidation technology, it differs from the RTO process in that its starting ignition temperature is usually 260-400°C, which is mainly due to the involvement of precious metal catalysts in the reaction. Therefore, precious metal catalyst is also the core unit of catalytic combustion method to treat organic waste gas. And catalyst poisoning is one of the most common problems.

1. Definition of catalyst poisoning

Catalyst poisoning is a phenomenon in which the activity and selectivity of a catalyst are significantly reduced or lost by trace impurities contained in the reaction material. The essence of the poisoning phenomenon is some kind of chemical interaction between the trace impurities and the active center of the catalyst, forming an inactive substance. In gas-solid multiphase catalytic reactions are formed by adsorption complexes. One type of poisoning is called reversible poisoning or temporary poisoning if the poison acts weakly with the active component and the activity can be restored by simple methods. The other category is irreversible poisoning, it is impossible to restore the activity by simple methods. In order to reduce the activity of the side reaction, it is sometimes necessary to selectively poison the catalyst.

2. RCO precious metal catalyst poisoning judgment method

(1) The degradation and removal rate decreases, catalytic combustion equipment has good removal rate for VOCs, if at a certain moment, the removal rate shows a large decrease, it is likely that there is poisoning of precious metal catalyst.

(2) The temperature inside the furnace needs to be increased to achieve the standard of the exhaust gas. Depending on the composition of the exhaust gas, the normal furnace temperature is 200-400°C. For example, the normal starting combustion temperature of toluene is 190°C, and 99% removal can be achieved at 230°C. Once its reaction temperature is too high, the catalyst needs to be checked.

3. Types of catalyst poisoning

(1) Recoverable poisoning: Toxic substances adsorb or chemically combine with active components such as platinum and palladium in the precious metal catalysts, thus creating chemical bonds with weak strength, which can be removed by physical or chemical means and regenerated in reverse to make the catalysts recover their activity. This is also the mildest form of poisoning.

(2) Selective poisoning: We call it selective poisoning of the catalyst when the toxicant causes the precious metal catalyst to have no catalytic effect on some or specific components.

(3) Permanent poisoning: The toxic substance interacts with the active components such as platinum and palladium in the precious metal catalyst and forms a strong and stable new chemical bond.

4. Catalyst poisoning response methods

(1) Recoverable poisoning, such as sulfate coverage, can be regenerated by water washing, acid washing and alkali washing, while chlorine poisoning can be restored by secondary calcination. The recoverable poisoning is simple in comparison with the performance recovery method and the regeneration cost is low.

(2) Selective poisoning, different noble metal bases have different reactivity or poignant temperature for different pollutants, and there is a match between different pollutants and noble metals, so when choosing catalyst types, the catalytic activity of the catalyst for the target pollutant and the poignant temperature should be fully verified

(3) Irreversible poisoning, such as alkali metal poisoning and heavy metal poisoning, cannot be regenerated by physicochemical methods when the precious metals form stable chemical bonds with toxic substances. To reduce losses, the precious metals in the catalyst can be recovered in pure form by classical separation through precipitation, filtration or washing.

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