Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Fluidized bed
Reexamination Certificate
1998-11-20
2002-02-12
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Fluidized bed
C422S139000, C422S140000, C422S144000, C422S147000, C137S512000, C137S533000, C222S531000, C222S532000, C222S537000
Reexamination Certificate
active
06346219
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 catalyst 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 The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.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.
U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for a fluid catalytic cracking unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles.
U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles.
The injection devices of the '540 and '049 patents rely on relatively high fluid velocities and pressure drops to achieve atomization of the oil into fine droplets. Providing this higher pressure drop burdens the design and increases the cost of equipment such as pumps and exchangers that are typically used to supply liquid and gas to the feed injection device. The need to replace such equipment may greatly increase the cost of retrofitting an existing liquid-solid contacting installation with such an injection apparatus.
U.S. Pat. No. 4,717,467 shows a method for injecting an FCC feed into an FCC riser from a plurality of discharge points. The discharge points in the '467 patent do not radially discharge the feed mixture into the riser.
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 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.
Feed atomization, lift-gas and radial injection of feed have been used to more uniformly disperse feed over the cross-section of a riser reaction zone. As feed contacts the hot catalyst, cracking and volumetric expansion of the hydrocarbons causes an increase in the volumetric rate of fluids passing up the riser. A large portion of this volumetric increase occurs immediately downstream of the feed injection point. U.S. Pat. No. 5,562,818 controls the volumetric expansion occurring simultaneously with mixing of catalyst and hydrocarbon feed to avoid the mal-distribution that can adversely affect the quantity and quality of the products obtained from the cracking reaction by eliminating turbulent back mixing as well as quiescent zones in the riser section immediately downstream of the feed injection point.
Further control of feed atomization has been added by U.S. Pat. No. 5,298,155 and U.S. Pat. No. 5,188,805 that show a movable head in nozzle tip to adjust the flow opening and flow characteristics of multiple orifice nozzles. A movable shaft positions the head at variable locations within the orifice throat of the nozzle as the unit operates.
Feed contacting methods have also focused attention on the distribution of catalyst before it contacts the feed. U.S. Pat. No. 5,318,691 provides an extended contacting chamber for adjustment of the catalyst flow regime before a distributor radially injects multiple streams of hydrocarbon feed to form a catalyst and feed mixture that passes through a restricted opening and then upwardly in a ris
Doroshenk Alexa A.
Paschall James C.
Tolomei John G.
Tran Hien
UOP LLC
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