Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
2001-05-07
2004-06-01
Elve, M. Alexandra (Department: 1725)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S141000
Reexamination Certificate
active
06743401
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a closed cyclone system having a flow rate distributor for the flow rate equalization, especially directed to closed cyclone systems of fluid catalytic cracking units (FCC). More specifically, the present invention relates to a distributor placed in the interconnecting section of the cyclones of the disengager (separator) vessel of the gas-solid suspension in fluid catalytic cracking processes, which distributes the gases from the cyclones of the reaction zone and the disengager vessel, so as to promote a desirable by and uniform flow rate of said gases throughout said area. The distributor further absorbs the relative movements among the cyclones caused by the differential thermal expansion.
BACKGROUND INFORMATION
The fluid cracking catalytic process (FCC) is the backbone of many refineries, since most of the products are highly valuable.
Therefore, for the convenient working of a refinery, it is highly desirable that the FCC units operate without interruption during long periods of time and process various kinds of feeds, including heavy feeds having boiling points above 570° C.
Broadly, the fluid cracking catalytic process (FCC) aims at converting high boiling point hydrocarbons into light hydrocarbon fractions such as gasoline and liquid petroleum gas (LPG). Most of the gasoline included in the gasoline pool of a refinery originates generally from the FCC unit.
As a catalytic process, FCC uses catalysts that are responsible for the optimized yields in valuable products of high commercial value, such as gasoline, diesel oil and LPG, while the yields in less desirable products such as coke and fuel gas are minimized.
The friction strength features of such catalysts are as important as their chemical features. The reason of the concern with the friction strength of the FCC catalysts is easily understood in view of the high cost of the catalyst as well as of the increasing requirements in terms of the control of the particulate emissions by the refineries.
In spite of all the concern as well as the high technology involved, even so the FCC catalyst undergoes abrasion due to the high circulation velocities of the catalyst in the units. The abrasion of the FCC catalyst results in the reduction of the catalyst particle size—initially microspheres having a diameter around 70 micra, towards smaller size ranges, below 20 micra.
The state-of-the-art system usually employed for the separation (disengagement) of the catalyst from the gaseous products of the FCC unit is that of cyclone disengagers, or simply cyclones.
The separation of gas-solids mixtures using cyclones is one of the oldest industrial systems and is based on the common principle of the use of centrifuge force to separate solid particles from gases. In spite of the apparent simplicity, much is still studied and discussed in the search of a better cyclone performance.
Being at first apparatuses used aiming solely to separate and recover the catalyst from the products of the unit, with time and due to the various feeds and process modifications, the FCC cyclones acquired a new perspective, intervening even in the conversion results of the unit
In order to better evaluate the above statement, it is necessary to understand the basics of the FCC process.
In the fluid catalytic cracking unit the pre-heated catalyst is thoroughly admixed to the atomized feed stock of the unit and the so-formed mixture is conveyed to the reaction zone of the unit where occur the cracking reactions. The reaction zone is generally an elongated vertical tube, which may have the mixture flow in the upward direction (riser), or downward (downer).
The cracking products, hydrocarbon gases and vapors form a gas-solid suspension with the catalyst. Such suspension should be quickly and efficiently separated on exiting the reaction zone in order to minimize the overcracking reactions, which lead to the formation of less desirable products (fuel gas and coke). Such separation is achieved through the use of cyclones placed within the disengager vessel.
In order to improve the separation of the solid catalyst particles sets of cyclones (first and second stages) are installed.
The first cyclone stage carries most of the separation job and usually recovers around 99% of the total solids fed. The first cyclone stage has the easiest job since the catalyst particles have an average particle size between 60 and 70 micra, there is a huge amount of available gas to provide the centrifugal forces and there are no problems for discharging significant amounts of gas through the cyclone leg.
The second cyclone stage receives approximately the same amount of gas as the first cyclone stage, but the solids content is several orders of magnitude less. Due to such adverse conditions as well as to the fact that the particles that reach the second stage are of a smaller average size, the typical-recovery of a second stage cyclone is of from 95 to 98% of the total solids fed.
Once the catalyst has been separated from the gases and cracking reaction products, the stripping is started, whereby the hydrocarbons adsorbed on the catalyst or entrained by the flow are removed by entrainment with stripping steam.
After the stripping section the catalyst is conveyed to the regenerator, where the combustion reactions for the removal of the coke deposited on the catalyst occur. In the interior of the regenerator, the flow rates of combustion gases may also be considerable so in order to avoid loss of catalyst entrained by the gas produced in the burning reactions, cyclones are also installed for the separation of the gas-solid mixture.
Normally in the regenerators are also installed sets of cyclones of different and consecutive stages, as in the case of the disengager vessel.
Catalyst make-up also occurs in the regenerator through the addition of amounts of virgin catalyst calculated to keep the catalyst inventory as well as the FCC unit conversion.
In the disengager as well as in the regenerator, the adequate working of the cyclones is paramount to secure a suitable performance of the unit, with at the same time minimum catalyst loss and thus minimum particulate emission.
With passing of time as well as in view of the modifications introduced in the FCC units as a function mainly of the composition of the feed stocks, it was found that there was a requirement of an ever smaller residence time for the feed and of the reaction products with the catalyst in order to minimize the overcracking reactions.
Various methods and procedures have been proposed aiming at reaching these objectives.
One of the well-known and utilized modes aiming at this objective is the so-called “closed cyclones”, which is based on the concept of the reaction zone directly connected to the cyclone disengager.
According to the concept of “closed cyclones”, the cyclones installed in is the interior of the disengager vessel are directly linked to the reaction zone of the unit; a second cyclone stage is linked to the cyclones of the reaction zone, in series, by means of a linking pipe made up of cylinders of different diameters and mounted in telescopic form to absorb the movements due to differential thermal expansions between these cyclone stages.
This concept allows to minimize the presence of particulate matter in the exiting gases from the disengager vessel and the reaction products are more rapidly separated from the catalyst so that the overcracking reactions are significantly reduced by the reduction of the contact time between the catalyst and the cracking products.
U.S. Pat. No. 4,502,947 corresponding to Brazilian patent PI 8404451 teaches the efficient and quicker separation of the products obtained by the use of cyclone disengagers directly connected to the bottom of the riser, those being linked to cyclone sets of first and second stages. Concentric pipes, mounted in telescopic form to absorb movements due to the differential thermal expansions between the riser cyclones and these cyclone stages make the link between the exit of the riser cyclone and t
Elve M. Alexandra
Petroleo Brasileiro S.A.
Sughrue & Mion, PLLC
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