85
small amounts to satisfy these constraints by improving a specific yield (gasoline,
LCO, propylene), reducing emissions (SO x , NO x , H 2 S) and producing cleaner products. These additives become a part of the unit’s catalyst inventory, increase the
FCC unit’s operating window, and make it more flexible to process heavier, contaminated, and hence cheaper hydrocarbon feeds and thereby increasing the profitability and flexibility of the FCC unit.
5.4.1 CO Combustion Promoter
While cracking hydrocarbons, coke gets deposited onto the catalyst pores in the
reactor section after which the catalyst enters the regenerator. In the regenerator, the
coke deposits are burned off using heated air along with some additional oxygen
added optionally. Based on this coke combustion process, the FCC units can be
categorized as partial combustion or full combustion mode of operation. In full
combustion mode of operation, excess hot air is supplied to the regenerator to completely convert some amount of the deposited coke into carbon dioxide. Since coke
has a complex molecular structure, the mechanism of its combustion is quite complicated. However, all of the carbon atoms participating in the combustion are converted to carbon dioxide. In partial combustion as the name suggests, limited air is
provided for combustion of deposited coke. Hence, some of the carbon atoms participating in the combustion remain as coke and carbon monoxide. The partial combustion units are accompanied by a CO boiler, which converts the carbon monoxide
in the flue gas coming from the regenerator into carbon dioxide. The heat from this
reaction is utilized in steam generation. Partial combustion units accompanied with
a catalyst cooler are very flexible in terms of maintaining the desired severity of
operation.
The heat associated with CO combustion is quite high (10,100 kJ/kg std) [48].
For high-severity operations, it was deemed profitable to be incorporated this heat
into the dense phase of the regenerator [51]. However, in the case of incomplete
combustion, some CO combustion can occur in the dilute phase where it may affect
the metallurgy of the regenerator. This phenomenon is called afterburning.
CO combustion promoters are additives that restrict the combustion of coke in
the dense phase of the regenerator only. This helps in optimizing yields by a flexible
switch of operation between partial and full combustion modes. The key components of any CO promoter are transition metals that catalyze the CO combustion but
at the same time remain stable in the hydrothermal conditions of the regenerator
[49, 50]. The metals in the CO combustion promoter should not add to the existing
metal contamination of the base catalyst.
The initial CO promoters were chromium-based and used in TCC units, and they
showed satisfactory combustion promotion but affected overall activity. Around
1972, it was first proposed to use minute quantities of platinum (1–10 ppm) to manufacture CO promoter additive. The first CO promoter was prepared in early 1973
by R. C. Wilson, Jr. by impregnating Re-Y zeolite with H 2 PtCl 6 . During trials, the
regeneration gas contained only CO 2 , no CO; cracking yields were acceptable.
Recent Developments in FCC Process and Catalysts
small amounts to satisfy these constraints by improving a specific yield (gasoline,
LCO, propylene), reducing emissions (SO x , NO x , H 2 S) and producing cleaner products. These additives become a part of the unit’s catalyst inventory, increase the
FCC unit’s operating window, and make it more flexible to process heavier, contaminated, and hence cheaper hydrocarbon feeds and thereby increasing the profitability and flexibility of the FCC unit.
5.4.1 CO Combustion Promoter
While cracking hydrocarbons, coke gets deposited onto the catalyst pores in the
reactor section after which the catalyst enters the regenerator. In the regenerator, the
coke deposits are burned off using heated air along with some additional oxygen
added optionally. Based on this coke combustion process, the FCC units can be
categorized as partial combustion or full combustion mode of operation. In full
combustion mode of operation, excess hot air is supplied to the regenerator to completely convert some amount of the deposited coke into carbon dioxide. Since coke
has a complex molecular structure, the mechanism of its combustion is quite complicated. However, all of the carbon atoms participating in the combustion are converted to carbon dioxide. In partial combustion as the name suggests, limited air is
provided for combustion of deposited coke. Hence, some of the carbon atoms participating in the combustion remain as coke and carbon monoxide. The partial combustion units are accompanied by a CO boiler, which converts the carbon monoxide
in the flue gas coming from the regenerator into carbon dioxide. The heat from this
reaction is utilized in steam generation. Partial combustion units accompanied with
a catalyst cooler are very flexible in terms of maintaining the desired severity of
operation.
The heat associated with CO combustion is quite high (10,100 kJ/kg std) [48].
For high-severity operations, it was deemed profitable to be incorporated this heat
into the dense phase of the regenerator [51]. However, in the case of incomplete
combustion, some CO combustion can occur in the dilute phase where it may affect
the metallurgy of the regenerator. This phenomenon is called afterburning.
CO combustion promoters are additives that restrict the combustion of coke in
the dense phase of the regenerator only. This helps in optimizing yields by a flexible
switch of operation between partial and full combustion modes. The key components of any CO promoter are transition metals that catalyze the CO combustion but
at the same time remain stable in the hydrothermal conditions of the regenerator
[49, 50]. The metals in the CO combustion promoter should not add to the existing
metal contamination of the base catalyst.
The initial CO promoters were chromium-based and used in TCC units, and they
showed satisfactory combustion promotion but affected overall activity. Around
1972, it was first proposed to use minute quantities of platinum (1–10 ppm) to manufacture CO promoter additive. The first CO promoter was prepared in early 1973
by R. C. Wilson, Jr. by impregnating Re-Y zeolite with H 2 PtCl 6 . During trials, the
regeneration gas contained only CO 2 , no CO; cracking yields were acceptable.
Recent Developments in FCC Process and Catalysts