Fluidized catalytic cracking process

Details Fluidized catalytic cracking process Fluidized catalytic cracking process

1998-11-30

2001-07-31

Griffin, Walter D. (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Cracking

C208S155000, C208S080000, C502S044000, C585S910000

Reexamination Certificate

active

06267873

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a fluidized catalytic cracking process and apparatus for resid in general and heat integration of reaction-regeneration sections of the resid FCC unit in particular.
2. Description of the Related Art
Fluidized catalytic cracking (FCC) is one of the most important conversion processes in the refining industry. FCC was initially designed for a silica-alumina matrix type catalyst with a dense bed reactor-regenerator system. However, since the introduction of zeolite type catalysts, the FCC reactor has been converted to all riser cracking with significant reduction in riser residence time and catalyst inventory.
With further improvement in the catalyst composition, FCC could be run at higher metal level (5000-7000 ppm nickel and vanadium) on an equilibrium catalyst. Simultaneously, for reduction of bottom of the barrel, conventional FCC units were modified for handling heavy residue, e.g., atmospheric and vacuum resid, etc. The modification involved the improvement in feed atomization, quick riser termination and quench, better catalyst striping, two stage catalyst regeneration, external catalyst cooler, catalyst and air distributors, etc.
In a resid FCC unit, the feed is preheated to 150-250° C. and injected radially at the bottom of the riser with steam as a dispersing medium. The contact time of the riser is kept in the range of 2-6 secs and the temperature in the riser bottom and top normally remains around 540-580° C., respectively. Suitable riser terminator devices are attached at the end of the riser to quickly disengage the catalyst from the product vapor. The catalyst is guided to a bubbling bed stripper where steam at the rate of 2-5 kg/1000 kg of catalyst is injected at the bottom of the stripper to remove the entrapped hydrocarbon vapor from the catalyst. The product vapor after the riser terminator is quenched or guided to the second stage cyclone and finally to the main column fractionator. The stripper catalyst is fed to the 1st stage of regenerator which works in the temperature range of 650-690° C. The carbon on catalyst is significantly reduced (70-80%) in this stage which then is pneumatically conveyed to the second stage regenerator where the temperature is kept much higher (710-740° C.) with sufficiently excess oxygen for near complete removal of carbon (<0.05%) on catalyst. The regenerator catalyst from the second stage of the regenerator is fed to the riser bottom through regenerated catalyst slide valve where the catalyst circulation rate is controlled to maintain the riser top temperature. Typically, the resid FCC unit operates at a 5-8 cat/oil ratio. In some resid FCC units where the quality of resid (Conradson cokes 3-4%), the catalyst in the regenerator is cooled in an external catalyst cooler to maintain over all heat balance of the unit.
Although many modifications in the original FCC unit have been made earlier to process residues, such resid FCC units can not handle very heavy residues where the Conradson carbon is more than 30-50 ppm. Several problems are associated in the known resid FCC units to economically process resid. These problems are as follows:
i) Excessive coke with the resid produces a large amount of excess heat and therefore the heat balance of the reactor regenerator is disturbed.
ii) Higher metal level on the resid leads to significant deactivation of the catalyst and requires a very large catalyst addition rate to keep the metal level or equilibrium catalyst in an acceptable range.
iii) Crackability of some of the residue, in particular, aromatic residue, are not quire good. Sufficient residence time for such residues are required in the riser and the extra coke generated from such aromatic residue cracking is required to be handled.
iv) Poor strippability of the catalyst: Strippability of the heavier unconverted residue inside the catalyst pores is not al all efficient.
v) SO
2
emissions from present resid FCC units are very high and present resid FCC conditions are not very conducive for efficient functioning of SO
x
removal additives.
vi) NO
x
generation in the present resid FCC unit is quite high due to high temperature regeneration.
These problems are further discussed in the following sections.
Excess Coke Formation Associated Higher Regeneration Temperature
Coke make in FCC is the most critical parameter to maintain the heat balance. Coke produced in the riser is burnt in the presence of air in the regenerator. The heat produced through exothermic coke burning reactions supplies the heat demand of the reactor, i.e., heat of vaporization, and associated sensible heat of the feedstock, endothermic heat of cracking etc. Typically, the coke yield in a conventional FCC unit with vacuum Gas Oil remains in the range of 4.5-5.5 wt %. The heat produced from burning (complete combustion) is sufficient to supply the reactor heat load. However, in a resid FCC unit, since the feedstock contains large amounts of coke precursors with higher amounts of Conradson coke and aromatic rings, the coke make is significantly increased which in turn increases regenerator temperature from 650-860° C. in conventional FCCUs to 720-250° C. in residue crackers.
The higher regenerator temperature has multiple deleterious effects in resids FCC units. However, the following are the major issues involved in high temperature regeneration:
i) High regenerator temperature reduces the catalyst circulation rate for a given riser top temperature to maintain in the reactor heat balance. Thus, the effective cat/oil ratio drops significantly resulting in reduced conversion.
ii) Higher regenerator temperature significantly increases catalyst deactivation both due to the metal, as well as hydrothermal factors. In fact, a regenerator temperature beyond 700° C. exponentially increases the zeolite crystallinity loss which is further aggravated in the presence of vanadium impurities on the catalyst. The maximum vanadium level which can be tolerated in FCC depends on the regenerator temperature. The tolerable vanadium level can be significantly improved by 4-5 times if regenerator temperature is reduced from say 730° C. to 680° C. Similarly, hydrothermal deactivation of catalyst also drops significantly with regenerator temperature reduction.
iii) High regenerator temperature is not conducive for SO
x
additive which works better at moderate regenerator temperatures (680-700° C.). Similarly, NO
x
emission is significantly increased beyond regenerator temperature of 720° C.
iv) Higher regenerator temperature requires better lining and metallurgy of the regenerator which increases the capital expenditure.
Therefore, it is essential to keep the regenerator temperature within limits below 700° C. and preferably within 680-690° C. to minimize the above damaging effects but at the same time without reducing the coke burning rate to less than an acceptable level. Unfortunately, most of the present resid FCC regenerators operate at high temperatures in complete combustion mode. Regenerator temperature to some extent can be reduced by partial combustion of the coke and by installing a CO boiler to take care of the unconverted carbon monoxide. However, with partial combustion regenerators, particularly at very high coke on catalyst as in resid FCC, it is difficult to maintain uniformity of bed temperature and after burning. Also, the catalyst inventory required to maintain sufficient residence time of the catalyst in the regenerator goes up with a reduced coke burning rate at lower temperatures. Therefore, running a partial combustion regenerator with resid spent catalyst is not commonly observed now a days.
Another way to control the regenerator temperature in resid FCC, is use of an external catalyst cooler with a suitable cooling capacity; regenerator temperature may be reduced by 20-30° C. However, catalyst cooler is not desirable normally from the heat efficiency point of view since steam is generated in the cooler essentially at the cost of feedstock.
An important point to note is that the

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