Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
1999-05-13
2001-01-30
Knode, Marian C. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C194S220000, C194S215000, C194S293000
Reexamination Certificate
active
06180068
ABSTRACT:
FIELD OF THE INVENTION
This invention is directed to interbed quench and mixing of process gases and liquids in cocurrent downflow reactors using fixed hardware.
BACKGROUND
In fixed-bed fuels and lube hydroprocessing units, gas and liquid flow downward through multiple beds of solid catalyst. Heat is released from the catalytic reactions causing temperature to increase with distance down the bed. Cool hydrogen-rich gas is introduced between the beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. Three requirements of an effective quench zone are transverse gas mixing, transverse liquid mixing, and quench gas mixing. The introduction and mixing of quench into the process gas and liquid must be carried out in the interbed space which spans the full vessel diameter, but is often shorter than one vessel radius. Support beams, piping and other obstructions also occupy the interbed region so that unique hardware is required to perform efficient two-phase mixing in what amounts to limited volume.
Poor quench zone performance manifests itself in two ways. First, the quench zone fails to erase lateral temperature differences at the outlet of the preceding bed or, in the worst cases, amplifies them. An effective quench zone should be able to accept process fluids with 50 to 75° F. lateral temperature differences or higher and homogenize them sufficiently that differences do not exceed 5° F. at the following bed inlet. The second sign of poor performance is that inlet temperature differences following the quench zone increase as the rate of quench gas is raised. This indicates inadequate mixing of cooler gas with the hot process fluids.
Inadequate quench zone performance limits reactor operation in various ways. When interbed mixing in unable to erase temperature differences, these persist or grow as the process fluids move down the reactor. Hot spots in any bed lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at high temperatures; hot regions can cause color, viscosity and other qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850° F.), the exothermic reactions may become self-accelerating leading to a runaway which can damage the catalyst, the vessel, or downstream equipment. Cognizant of these hazards, refiners operating with poor internal hardware must sacrifice yield or throughput to avoid these temperature limitations. With present day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum quench zone design is a valuable low-cost debottleneck.
In U.S. Pat. No. 4,836,989 is described a method for quench zone design. The essential feature of this design is the rotational flow created in the mixing volume which increases fluid residence time and provides repeated contacting of liquid and gas from different sides of the reactor. This design is keyed to liquid mixing. More recent studies have shown it to be only a fair gas mixer. The trend to higher conversion and higher hydrogen circulation in fuels refining translates to gas/liquid ratios for this design is not well suited. Height constrained units cannot be fitted with mixing chambers of the type described in this patent that are deep enough to effectively mix both the gas and liquid phases.
A new interbed mixing system described in U.S. Pat. No. 5,462,719 offers some improvements over the design described above when gas mixing is paramount. This hardware is based again on a swirl chamber, but also includes at least three highly restrictive flow elements to enhance mixing, which necessarily increase pressure drop. Like the previously described system, this quench zone mixes the gas and liquid at once in a single chamber.
SUMMARY
The present invention provides a novel means to provide more effective mixing of quench gas and process fluids in a very-height constrained interbed space while not increasing pressure drop. The present invention embodies one or more of seven features, as follows.
1. Two separate swirl chambers are used to mix gas and liquid. Effective swirl-flow gas mixing requires a confining diameter smaller than that for liquid due to the much lower density of the gas relative to the liquid. In addition, the typically higher gas velocity is wasted accelerating the liquid, with no added mixing benefit, if the gas and liquid phases enter any confinement concurrently. Mixing is more effective if each phase is internally homogenized first, particularly since, despite lateral temperature differences, the average temperatures of the gas and liquid phases are usually very close. Separate gas and liquid chambers effectively partition the total mixing requirement into two disproportionately smaller tasks. The most space-efficient arrangement is for the gas and liquid swirl chambers to be concentric with the liquid chamber surrounding the gas chamber, although other constructions are possible.
2. Entrance to the gas swirl chamber is substantially tangential. Rotational flow in the gas is induced by forcing the flow through ducts or baffles which enter the swirl chamber at an angle less than 60 degrees from the tangential direction. The entrances are typically rectangular openings rather than narrow slots, and extend minimally into the swirl volume, although they may have significant approach length (possibly as much as one (1) swirl chamber diameter). The entering flow is substantially horizontal as this results in the maximum number of rotations in the swirl chamber. The gas entrances may take the form of many evenly spaced baffles or as few as two (2) opposing ducts. Fewer entrances provide more pre-mixing of gas approaching the chamber, but increase pressure drop.
3. Entrance to the liquid swirl chamber is substantially tangential. Rotational flow in the liquid is induced by forcing the flow through ducts or baffles, preferably only two (2) to four (4). Whereas the gas entrances create rotational flow by their orientation, the liquid entrances do so by their location close to the outer wall of the swirl chamber. Liquid entrances are oriented less than forty-five (45) degrees from horizontal, and preferably do not project into the swirl chamber at all, thus minimizing any disturbance to the rotation in the chamber and maximizing liquid swirling time.
4. Quench fluid is injected into or upstream of the swirl chamber of like phase. Gas quench is injected directly into the gas swirl chamber or into the process gas before entering. Liquid quench may be injected into the liquid swirl chamber, but is preferably mixed with the process liquid before entering. In either case, where quench enters the chamber directly, it is injected in a way which minimally disturbs the rotational flow, that is along the swirl axis or into the top or bottom of the chamber with a compatible swirl already established. Mixed-phase quench is preferably introduced upstream of both swirl chambers.
5. Exit from the gas chamber is against a stagnation-point surface. The outlet of the gas swirl chamber may feature various minor constrictions or steps to enhance swirling, but the final discharge is perpendicular against a flat surface which forces the gas into a thin outward-flowing sheet. The surface may be roughened or fitted with small objects to promote turbulence in the gas.
6. Exit from the liquid chamber is over a central circular weir and in the form of a thin sheet. A weir having a height between five (5) and ninety-five (95) percent of the liquid swirl chamber height is necessary to provide liquid holdup in the chamber. The weir is preferably concentric with a cylindrical surface of slightly smaller diameter extending downward from the ceiling of the chamber, such that the liquid exits through a narrow annular gap. This geometry meters flow out of the chamber without retarding the rotational flow, and forms a thin sheet to promote the subsequent interphase mixing step.
7. Gas and liquid exiting their re
Boyd Sherri L.
Muldowney Gregory P.
Keen Malcolm D.
Knode Marian C.
Mobil Oil Corporation
Ohorodnik Susan
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