Deactivation of catalytic reforming catalysts

There are many reasons for reforming catalyst deactivation (reduced activity) during the production process, such as catalyst carbon build-up, agglomeration of platinum grains and poisoning by impurities in the raw material. In normal production, the main cause of catalyst deactivation is the accumulation of carbon on the catalyst surface.

1. Catalyst deactivation due to carbon build-up
For general platinum catalysts, when the carbon build-up increases to 3% to 10%, most of the activity is lost; for platinum rhenium catalysts, when the carbon build-up reaches 20%, most of the activity is lost.
The reduction in catalyst activity due to carbon build-up can be compensated for by increasing the reaction temperature, but there are limitations to increasing the reaction temperature.
There are certain limitations. Domestic platinum reformers generally limit the reaction temperature to a maximum of 520°C, at which point the amount of carbon build-up on the catalyst is 8% to 10%. When the reaction temperature is raised to the limit and the activity still does not meet the requirement, the catalyst can only be regenerated by burning away the carbon. The catalyst is regenerated by burning off the carbon to restore its activity.
The rate of catalyst carbon build-up is related to the nature of the feedstock and operating conditions. The rate of carbon build-up is dependent on the nature of the feedstock and the operating conditions, with high final distillation points and high unsaturated hydrocarbon content. The rate of carbon build-up is fast. Severe reaction conditions (e.g. high temperatures, low air velocities, low hydrogen-to-oil ratios, etc.) also increase the rate of carbon build-up.

2. Agglomeration and deactivation of platinum grains
The dispersion of the platinum grains is closely related to their activity, which can be caused by the high temperature of the platinum grains on the catalyst during operation. The presence of impurities and water in the raw material will gradually agglomerate and grow, resulting in a reduction in activity.

3. Catalyst poisoning and deactivation
There are two types of catalyst poisoning: permanent poisoning, where the activity of the catalyst can never be restored; and temporary poisoning, where the activity of the catalyst can never be restored. The other type is temporary poisoning, where the activity of the catalyst can be restored by removing the poison.
(1) Permanent poisons
Arsenic (As) and platinum have a high affinity for each other and can form platinum-arsenic compounds with the platinum grains on the catalyst surface, resulting in permanent catalyst poisoning. Usually when the arsenic content on the platinum catalyst is >200X10-6", the activity of the catalyst can never be restored and this poisoning is called permanent poisoning. Therefore, the arsenic content of platinum reforming feedstock should be strictly controlled. The arsenic content of platinum reformers is usually limited to 1X10^-9 to 2X10^-9 or less.

Metals such as lead, copper, mercury, iron and sodium can also cause permanent catalyst poisoning, so care should be taken that the reforming feedstock is not contaminated with leaded petrol.
Therefore, care should be taken that the reforming oil is not contaminated with leaded gasoline, and that copper, iron and mercury chips are not allowed to enter the system during maintenance. The use of sodium compounds such as sodium hydroxide to treat the feedstock is prohibited.

(2) Non-permanent toxins
a. Sulphur
H2S can react with platinum to form metal sulphides, thus reducing the dehydrogenation-hydrogenation activity of the catalyst. However, rhenium is more sensitive to sulphur and does not recover easily once poisoned.
Studies have shown that permanent poisoning can also occur if the catalyst is exposed to sulphur for a long period of time. For fresh or recently regenerated
platinum-rhenium and platinum-iridium series catalysts, pre-sulphurisation with sulphur is required at the beginning of the process to suppress excessive hydrogenolysis activity, but should not be excessive.
but not excessively.
b. Nitrogen
Nitrogen compounds in the feedstock will produce NH3 under reforming conditions. NH3 can adsorb to the acidic centre of the catalyst or react with chlorine to form ammonium chloride, thus reducing the acidic function of the catalyst and the isomerisation activity. The catalyst activity can be restored as long as the feedstock no longer contains nitrogen and is also properly supplemented with chlorine.

c. Carbon monoxide and carbon dioxide
can form complexes with platinum, causing permanent poisoning. Carbon dioxide can be reduced to carbon monoxide, which is also toxic. Carbon monoxide is not normally present in the feedstock and is not present in the raw gas. Its main source is the industrial hydrogen or replacement nitrogen introduced into the system at start-up, which usually requires <0.1% carbon monoxide and <0.2% carbon dioxide in the gas used.