Gas separation: processes – Sound waves used
Reexamination Certificate
1998-12-31
2001-08-28
Lithgow, Thomas M. (Department: 1724)
Gas separation: processes
Sound waves used
C095S032000, C095S034000, C095S269000, C096S389000, C055S396000, C055S448000, C055S449000, C055S456000
Reexamination Certificate
active
06280502
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to removing solids from a fluid.
BACKGROUND TO THE INVENTION
Centrifugal force is useful for separation of solids from streams of fluids. Such separation can find application in various industrial processes, for example in removing of catalyst particles in fluidized bed catalytic cracking operations, and separation of ash from flue gases.
Fluids, when rotating around a central axis, will be accelerating toward the central axis, and inertia of the solids will force the solids outward away from the central axis. Outward flow of particles will be against the flow of fluids. Fluids containing fewer solids are then typically withdrawn from the center axis of rotation, and the solids are removed from the radially outer surface of the separator. The rate of flow of solids outward is limited by the resistance of the fluid. This rate is dictated by Stokes Law. Practically, only about five micron sized particles can be separated by conventional cyclone separators.
U.S. Pat. No. 5,073,177 suggests an apparatus for cyclonic separation of solids from a gas stream. This apparatus includes a rotating element of a large number of small conduits so that the particles in each small conduit only travel a short distance before contacting a surface, on which the particles will agglomerate. The element is said to rotate at 3000 rpm, to maximize the centrifugal force acting upon the particles being separated. The speed of rotation of the particles is limited to the speed at which the element can be rotated. This high rotational speed results in a relatively high maintenance apparatus. It is desirable to have a greater rotational speed imparted to the fluids without a rotating element that rotates at a high speed.
Dutch patent application No. 8901841 discloses a method of removing a selected gaseous component from a stream of fluid containing a plurality of gaseous components, wherein the stream is induced to flow at a supersonic velocity through a conduit so as to decrease the temperature of the fluid in the conduit to below a temperature at which the selected component condenses. The conduit is provided with swirl imparting means to impart a swirling motion to the stream of fluid flowing at supersonic velocity. The condensed particles are extracted in a first outlet stream from a radially outer section of the stream and the remaining fluid is collected in a second outlet stream from a central part of the stream. The velocity in the radially outer section and in the central part of the stream is supersonic.
In an embodiment of the device for separating a gas from a gas mixture as disclosed in NL-8901841, separate shock waves occur in the first and second outlet streams, leading to a relatively large flow resistance of the fluid. Furthermore, the separation efficiency is relatively low so that substantial amounts of the condensed particles are still present in the second outlet stream. This reference does not suggest utilizing such an apparatus for separation of solids from fluids.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a method and apparatus for removing particles from a particle containing fluid, the method including the steps of: inducing the stream to flow at supersonic velocity through a conduit; inducing a swirling motion to the supersonic stream of fluid thereby causing the particles to flow to a radially outer section of a collecting zone in the stream; and extracting the particles into an outlet stream from the radial outer section of the collecting zone. The apparatus is an apparatus effective for performance of this method.
Creation of the swirl in a fluid stream of sonic velocity results in considerably greater G-forces than are typically created in cyclone separtors.
In a preferred embodiment of the present invention, a shock wave caused by transition from supersonic to subsonic flow occurs upstream of the separation of the solids from the collecting zone. It was found that the separation efficiency is significantly improved if collection of the particles in the collecting zone takes place after the shock wave, i.e. in subsonic flow rather than in supersonic flow. This is because the shock wave dissipates a substantial amount of kinetic energy of the stream and thereby strongly reduces the axial component of the fluid velocity while the tangential component (caused by the swirl imparting means) remains substantially unchanged. As a result the density of the particles in the radial outer section of the collecting zone is significantly higher than elsewhere in the conduit where the flow is supersonic. It is believed that this effect is caused by the strongly reduced axial fluid velocity and thereby a reduced tendency of the particles to be entrained by a central “core” of the stream where the fluid flows at a higher axial velocity than nearer the wall of the conduit. Thus, in the subsonic flow regime the centrifugal forces acting on the condensed particles are not to a great extent counteracted by the entraining action of the central “core” of the stream, so that the particles are allowed to agglomerate in the radially outer section of the collecting zone from which they are extracted.
Preferably the shock wave is created by inducing the stream of fluid to flow through a diffuser. A suitable diffuser is a supersonic diffuser. A diffuser may be, for example, a diverging volume, or a converging and then diverging volume.
In an advantageous embodiment, the collecting zone is located adjacent the outlet end of the diffuser.
The present invention may be practiced in combination with other operations to effect drying of the fluid stream, or may be practiced in front of conventional separators in order to reduce the capacity and size required of those separators. Also, either of the stream containing the particles from the collecting zone or the stream from which the particles have been separated could be subjected to an additional separation step, for example, a dryer or separator.
The supersonic flow of the present invention also causes a rapid expansion, resulting in cooling of a compressible fluid stream. This cooling results in condensation of vapors to the extent that such cooling brings the temperature of the stream to a temperature below a dew point of the fluid stream. Creation of this liquid can improve separation of solids from the fluids by providing surfaces to which the solids can agglomerate, resulting in larger particles that are more easily separated.
Advantageously, any gaseous fraction separated from the radial outer section of the collecting zone can be recycled back to the inlet, preferably using an inductor to increase the pressure back to the pressure of the inlet stream.
Suitably the means for inducing the stream to flow at supersonic velocity comprises a Laval-type inlet of the conduit, wherein the smallest cross-sectional flow area of the diffuser is larger than the smallest cross-sectional flow area of the Laval-type inlet.
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C. A. Tjeenk Willink.Extraction of Condensables From Gases in a Wellbore, U.S. Pattent application Ser. No. 09/223,885,
Betting Marco
van Veen J. W. H. M.
Lithgow Thomas M.
Shell Oil Company
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