The coke generated during the cracking reaction is deposited on the surface of the catalyst and can reduce the activity and selectivity of the catalyst. Therefore, when a certain amount of carbon has accumulated on the catalyst it has to be burnt off to restore the catalyst activity. Usually the catalyst (to be regenerated) leaves the reactor with a carbon content of about 1% and after burning, for molecular sieve catalysts (regenerating agents), the carbon content is generally required to be reduced to 0. 1% or even 0. 05% or less.
1. Causes of catalyst deactivation
1) Hydrothermal deactivation
At high temperatures, especially in the presence of water vapour, the surface structure of the cracking catalyst changes, the specific surface area decreases, the pore volume decreases and the crystal structure of the molecular sieve is destroyed, resulting in a decrease in the activity and selectivity of the catalyst. Once this deactivation occurs it is irreversible and usually only operational conditions can be controlled to minimise hydrothermal deactivation, e.g. avoiding repeated contact with water vapour at superheat.
2) Coking deactivation
Coke generated from the catalytic cracking reaction is deposited on the surface of the catalyst, covering the active centre on the catalyst and reducing the activity and selectivity of the catalyst. This deactivation is the most severe and fastest, generally causing the catalyst to lose most of its activity within 1s, although it is a "temporary deactivation" and can be recovered after regeneration.
3) Deactivation caused by toxins
Under catalytic cracking reaction conditions, these metallic elements can cause catalyst poisoning or contamination, resulting in a reduction in catalyst activity, known as "toxic deactivation", and high levels of alkaline nitrides in some raw materials can also cause catalyst poisoning and deactivation. High levels of alkaline nitrides in some feedstocks can also poison the catalyst.
2. Catalyst regeneration
Catalyst deactivation can be restored by regeneration due to coking, but not due to structural changes or metal contamination. In order to restore the activity of the catalyst for reuse, the deposited coke must be burned off with air at a high temperature, a process known as catalyst regeneration. The catalyst regeneration reaction, in which oxygen from the air is used to burn away the coke deposited on the catalyst, can be expressed as:
In addition to CO, CO2 and H2O, the products of the regeneration reaction will contain SOx and NOx if the raw material contains both S and N elements.
In practice, the catalyst leaving the reactor contains approximately 1% carbon and is known as the catalyst to be regenerated (referred to as the regenerant) and the regenerated catalyst is known as the regeneration catalyst (referred to as the regenerant). The carbon content of the regenerant is subject to certain requirements: for silica-aluminium catalysts it is less than 0.5%; for zeolite catalysts it is less than 0.2%. The regeneration process of the catalyst determines the heat balance and the production capacity of the whole plant.
If the heat generated is not sufficient to supply the heat required for the reaction, additional heat is required (by spraying combustion oil into the regenerator) and if there is a surplus of heat generated, the excess heat is removed from the regenerator for other use to maintain the heat balance of the whole system.