Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1998-11-16
2003-11-25
Johnson, Jerry D. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S139000, C422S140000, C422S144000
Reexamination Certificate
active
06652815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a stream of fluidized particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the catalyst can dramatically affect the performance of the reaction zone. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Ideally the feed as it enters the riser is instantaneously dispersed throughout a stream of catalyst that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of catalyst particles. However, the dispersing of the feed throughout the catalyst particles takes some time, so that there is some non-uniform contact between the feed and catalyst as previously described. Non-uniform contacting of the feed and the catalyst exposes portions of the feed to the catalyst for longer periods of time which can in turn produce overcracking and reduce the quality of reaction products.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solids. Preferably, the droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods are known for shearing such liquid streams into fine droplets.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Catalyst particles first enter the riser with zero velocity in the ultimate direction of catalyst flow through the riser. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser the turbulence can cause mal-distribution and variations in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and back mixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 issued to Hammershaimb and Lomas.
There are additional references which show the use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 to Bartholic, a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom part of a vertical confined conduit and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 to Busch et al., a C
4
minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil
U.S. Pat. Nos. 5,554,341; 5,173,175; 4,832,825 and 3,654,140 all shows the use of radially directed feed injection nozzles to introduce feed into an FCC riser. The nozzles are arranged in a circumferential band about the riser and inject feed toward the center of the riser. The nozzle arrangement and geometry of the riser maintain a substantially open riser cross-section over the feed injection area and downstream riser sections. The angled feed nozzles are typical of those used to inject feed or other fluids at an intermediate portion in the riser conduit. The angled feed injectors present a number of problems for the operation of the risers. The nozzles typically extend away from the wall of the riser and into the flow path of the catalyst. Passing particles over the nozzles at high velocity can result in erosion. The nozzle protrusion can also result in quiescent zones that promote backmixing and provide sites for coke build-up to begin. The protrusion of the feed injectors can provide such zones by protecting coke from the natural erosion action of the flowing catalyst which would otherwise eliminate the coke from these sites. Excessive coke buildup can upset the hydraulic balance in a unit to the point where it is eventually forced to shut down. The processing of heavier feeds such as residual hydrocarbons can exacerbate coke production problem due to their higher coking tendencies.
An obvious solution to the problem of nozzle protrusion would be to recess the nozzles completely into the wall of the riser and thereby remove them from the catalyst flow path. This solution is not satisfactory since the feed injector tips are specifically designed to provide a relatively uniform coverage of the hydrocarbon feed over the cross-section of the riser by expanding the pattern of feed injection as it exits from the nozzle. Completely recessing the tips of the injector nozzles within the wall of the riser disrupts the ability to obtain a spray pattern over the majority of the riser cross-sectional area
It is an object of this invention to more uniformly distribute catalyst and oil over the cross-section of the riser.
It is another object of the invention to reduce areas of local variation in particle density to improve oil penetration into the particles.
It is a further object of the invention to minimize areas of backmixing and quiescence around the feed injectors that can lead to coke formation.
BRIEF SUMMARY OF THE INVENTION
These objects are achieved by providing a hydrodynamic mixing zone where a plurality of feed injectors circle an intermediate portion of a contacting conduit to inject a feed into a flowing stream of particulate material. The hydrodynamic zone is also referred to as the injector zone. The invention locates the outlets of the feed injector nozzles in a shelf from which the tips of the nozzles protrude. The shelf is formed by an abrupt change in the diameter of the conduit relative to the adjacent upstream portion of the conduit. This divergence in the diameter of the conduit locates the protruding tips of the feed injectors outside of the direct flow path of the passing particulate material and maintains active and flowing particles in the regions immediately upstream and downstream of the injector tips. The shelf thereby improves the hydrodynamics in the co
Doroshenk Alexa Ann
Johnson Jerry D.
UOP LLC
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