Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1999-05-24
2003-01-28
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S144000, C422S214000, C422S215000, C208S113000, C208S146000, C208S153000, C208S156000
Reexamination Certificate
active
06511635
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the catalytic cracking of hydrocarbons in the presence of a fluidized phase catalyst. More particularly its objectives are a method and device that make it possible to introduce a homogenous flow of catalyst grains in the reaction section, at the injection level of the charge to be cracked.
As known, the oil industry uses heavy hydrocarbon charge conversion methods in which hydrocarbon molecules with a high molecular weight and high boiling point are split into smaller molecules that can boil in lower temperature ranges that befit the sought use.
In this field, the method most used is currently the method called fluid catalytic cracking (or FCC method). In this type of method, the hydrocarbon charge, pulverized into fine droplets, is put in contact with high temperature cracking catalyst grains that circulate in the reactor in the form of a fluidized bed, meaning in a more or less heavy medium within a gaseous fluid that ensures or helps their transport. On contact with the hot catalyst, the charge vaporizes, this is followed by the cracking of the hydrocarbon molecules on the active sites of the catalyst. Once the desired molecular weight range has been reached in this manner, accompanied by a depression of the corresponding boiling points, the effluents of the reaction are separated from the catalyst grains. These catalyst grains, deactivated as a result of the coke that has deposited on their surface, are then stripped in order to recuperate the hydrocarbons that have been swept away, then are regenerated by combustion of the coke, and lastly, are put back in contact with the charge to be cracked.
The reactors used are generally vertical reactors of the tubular type, in which the catalyst moves following a flow that is mostly upward (the reactor is then called “riser”) or following a flow that is mostly downward (the reactor is then called “dropper” or “downer”).
We know that one of the key factors of the catalytic cracking process lies in the quality of the mixture of the charge that is injected in liquid state in the form of fine droplets, with the flow of hot catalyst grains resulting from the regeneration. Indeed, it is essential to ensure that the hydrocarbons are quickly, closely and evenly put in contact with the catalyst flow, as this determines the efficiency of the thermal transfer from the hot catalyst grains to the droplets of the charge. The speed and evenness of the vaporization of the charge depend on the efficiency of this transfer, and therefore, so does the quality of the conversion of the charge since the catalytic cracking reaction takes place in the gaseous state.
However, the studies completed in this field by the applicant have revealed that the yields obtained with the highest performing cracking units remain below what was predicted by the theoretical studies and that this difference is due among other things to the fact that the droplets of charge were not put in contact with the catalyst particles in an adequate fashion. We assumed that it was in part due to an inhomogeneity of the density of the fluidized bed of the catalyst that arrives in the injection area of the charge, or in other words to a signification segregation within the mixture consisting of the catalyst grains and the gaseous fluid that ensures their transportation.
In particular, we have illustrated two main factors of segregation:
On the one hand, in conventional devices, the circulation pattern of the catalyst grains often lacks stability. In particular, the catalyst grains emanating from the regenerator tend to show up “in bunches” and a phenomenon called pulsation phenomenon is then noticed: the feeding of catalyst grains into the reactor is not continuous, and the density of the catalyst flow arriving in the cracking zone may then fluctuate considerably in time around an average value. This pulsating pattern shows a fluctuation in time of the actual C/O ratio of the quantity of catalyst C introduced in the reaction zone to quantity O of the injected charge to be cracked.
On the other hand, in particular for units equipped with a upward flow reactor (riser), in the slanted conduit that ensures the transfer of the regenerator's catalyst grains toward the reactor, these grains tend to gather on the bottom, while the conveyor vapor creates “pockets” in the upper part of this transfer conduit. The elbow that is present at the point of connection between this slanted conduit and the reactor only accentuates the segregation. As the catalyst's fluidization device that is present at the entry of the reactor does not allow for the re-balancing of the distribution of catalyst grains on the section of the reactor, we notice, for one same section, an inhomogeneity of the density of the catalyst. The results in an inhomogeneity of the actual C/O ratio and therefore of the temperature profile for one same section of the reactor.
At the injection area of the charge, these spatial and temporal variations of the actual C/O ratio have proved to be particularly problematic, since they lead to an inhomogeneity of the vaporization and the cracking of the injected charge. In areas where the density of the catalyst is too high, the charge runs the risk of overcracking, which generates dry gases and coke at the expense of the sought intermediate hydrocarbons. In return, in area where the density of the catalyst is insufficient, the charge is only partially vaporized, which leads to an increased deposit of hydrocarbons on the surface of the catalyst, by collision of the catalyst grains with the non vaporized droplets of the charge, from which results a greater coking of the catalyst. In other respects, the deficit in catalytic sites favors the thermal cracking reactions, which are not very selective, at the expense of the catalytic cracking reactions.
In the end, all these phenomena translate into a significant penalty in terms of conversion yields and selectivity, and lead to a significant coking of the separation and stripping chamber and inside the reactor
In order to remedy the problems described above, the applicant has already proposed a certain number of solutions.
In patent EP-0 326 478, we proposed a new form for the connection that ties the regenerator to the reactor of a catalytic cracking unit that operates in the upward mode. In particular, this connection consists of tubing connected by incurved elbows that dictate neither an upturning point on the particles' path, nor sudden modifications of the tubing diameter. Injections of a make up carrier gas are also planned in order to accelerate the catalyst particles in a controlled manner at the level of the reactor's connection elbow. By using this process we can connect the catalyst transfer line and the upward reactor following a curvilinear profile, which makes it possible to limit the dehomogenization of the catalyst and of the fluid that ensures its transport, but it does not however, make it possible to completely eliminate the inevitable segregation that takes place in a solid/gas biphasic mixture, nor the pulsating pattern of the circulation of the catalyst. Furthermore, it corresponds to an optimization of the configuration of a unit that contains a reactor that operates in an upward mode, and therefore in no way affects the unit where the reactor operates in a downward mode.
In patent EP-0 191 695, the applicant has described an advantageous fluidization system in two steps at the base of an upward reactor. The proposed solution consists in slowly injecting a first fluid into the reactor, below the level of introduction of the catalyst grains emanating from the regeneration area, in order to maintain a dense and homogenous catalyst fluidized bed at the bottom of the “riser”, and in simultaneously injecting a second fluid below the upper part of the dense catalyst bed, in order to obtain a more diluted and homogenous fluidized phase with a constant flow of catalyst grains, upstream from the injection area of the charge. Such a procedure, although efficient, doe
Barthod Daniel
Del Pozo Mariano
Mauleon Jean-Louis
Sughrue & Mion, PLLC
Total Raffinage Distribution , S.A.
Tran Hien
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