Gas separation: processes – Filtering – Moving filter
Reexamination Certificate
2001-10-29
2003-02-11
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Filtering
Moving filter
C095S270000, C095S282000, C055S304000, C055S400000, C055S521000, C055S528000
Reexamination Certificate
active
06517612
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for separating particulates from a fluid using a centrifugal filtration device. More specifically, this invention relates to a method and apparatus for removing liquid or solid particles from a fluid, which can be a liquid or a gas, using a continuously cleanable high efficiency rotating filtration device utilizing microporous filtration material.
BACKGROUND
The removal of particulates from a fluid stream has long been a practice in a variety of industrial fields. Systems for filtering particulates from fluid streams include barrier and non-barrier inertial filtration devices. Barrier filtration devices can include porous media in the forms of filter bags, filter tubes, filter cartridges, and filter panels, while non-barrier devices can include electrostatic precipitators and inertial filtration devices such as rotating disk separators, cyclones, and venturi scrubbers.
Non-barrier, inertial devices offer several advantages over barrier filtration devices by avoiding reliance on barrier layers to stop and trap particles in fluids as the fluids pass through the layers. In one type of non-barrier, inertial device, the cyclone separator, contaminated fluid, such as a dust laden gas or particle-laden liquid tangentially enters near the top of an inverted cylindrical chamber. The curved tapering walls of a lower conical section of the cyclone impose a vortex motion on the incoming fluid, and the denser particles in the fluid are displaced towards the cyclone walls under the influence of centrifugal forces. The particles follow a downward spiral towards the cyclone exit near the apex of the cone, while separated fluid is drawn off from a central location in the upper portion of the cyclone. Cyclones are generally suitable for removing particles from gases when the particles are over about 5 &mgr;m in diameter, and contain no filter elements that need periodic cleaning or replacement. Also, in multiple cyclone systems, 80% to 85% efficiency can be attained for removing particles of about 3 &mgr;m diameter and above.
Another non-barrier, inertial device is a rotating disk separator, such as described in U.S. Pat. No. 2,569,567. Further refinement of rotating disk separators is embodied in U.S. Pat. No. 5,746,789. This type of separator comprises a plurality of annular disks rotatably mounted in a housing and concentrically stacked so that their hollow centers define a central plenum. Spacers closely regulate the gaps between the disks. One end of the plenum is sealed off with a cap, and the other end is left open and serves as a filtered fluid exit. As the disks rotate, particle-laden fluid enters the housing through an inlet and flows through the gaps between the rotating disks towards the plenum. Disk rotation creates boundary layers by imparting a rotational velocity component to particle laden fluid layers adjacent the disks, as the particle-laden fluid flows inwards and towards the plenum. The boundary layer fluid also imparts rotational velocity to particles entrained therein, which thereby experience a centrifugal force. Under appropriate conditions, the centrifugal force experienced by some particles can be greater than the drag forces on the particles caused by the fluid flow into the plenum. These particles are outwardly ejected from the rotating disk device. The fluid, now free of the ejected particles flows into the plenum and out the plenum exit. Rotating disk separators contain no filter elements that need periodic cleaning or replacement. Although these centrifugal filters have demonstrated submicron particle removal capabilities on a small scale, in practice, with higher volumetric flow rates common in many industrial applications, it is typically more challenging to remove particles smaller than about 2 &mgr;m due to the higher rotational speeds and pressure drops required. Thus, particle filtration for fluids containing a high percentage of particles of less than about 2 &mgr;m diameter using non-barrier, inertial devices remains impractical.
High efficiency barrier type filtration devices suitable for removing particles of less than about 2 &mgr;m in diameter are known. In barrier layer devices, barrier layers comprise filtration media formed into filter elements through which the flow of particle-laden fluid is directed. Over time, filter element performance can deteriorate as filtered particles accumulate on the surface or through the depth of the filter element. The flow of fluids, whether liquid or gas, produces a pressure differential, or pressure drop, across the element. Preferably, the pressure differential is as small as possible for a given fluid flow rate in order to minimize the power required to filter the fluid. In many of these conventional techniques, filtration efficiency increases as filtered particulates accumulate on the filter element. However, as particles accumulate in or on the element, the pressure differential may increase, or the flow rate of fluid through the element may be reduced, or both. Therefore, after an amount of particulate material has accumulated or when limits of acceptable pressure differential or flow rate reduction have been reached, the filter element is either removed or cleaned.
Periodic element replacement and cleaning is generally needed to minimize filtration performance degradation in these systems. Filter element replacement and cleaning can be inconvenient and costly especially when considering the cost of shutting down industrial processes to allow this maintenance to be completed. In addition, the filter element can fail mechanically as a result of the stresses caused by cleaning the filter, thus resulting in loss of filtration performance.
Filter elements are typically constructed from filtration materials, or media such as, for example, felts and fabrics made from a variety of materials, including polyesters, polypropylenes, aramids, glasses and fluoropolymers. Selection of the type of material used is typically based on the fluid stream with which the filter element comes in contact, the operating conditions of the system and the type of particulate being filtered.
Polytetrafluoroethylene (PTFE) has demonstrated utility in many areas. As an industrial material, such as a filtration material, for example, PTFE has exhibited excellent utility in harsh chemical environments, which normally degrade many conventional metals and polymeric materials. A significant development in the area of particle filtration was achieved when expanded PTFE (“ePTFE”) membrane filtration media were incorporated as surface laminates on conventional filter elements. One example is taught in U.S. Pat. No. 4,878,930, directed to a filter cartridge for removing particles of dust from a stream of moving gas or air. Preferred filter materials for the cartridge are felt or fabric composites containing a layer of porous expanded polytetrafluoroethylene membrane.
Use of the ePTFE membrane greatly enhanced the performance of filter elements because the particles collected on the surface of the ePTFE, rather than in the depth of the elements as was occurring in the absence of the ePTFE layer. Several significant advantages were observed with these filter elements. For example, the filtration efficiency of the elements was high immediately from the outset of the filtration process, and it was not necessary to build up a cake of particles to achieve high efficiency.
Despite the superior performance and high filtration efficiency of cleanable ePTFE filtration media, filter element cleaning remains a problem, and filtration systems must frequently be shut down to remove and maintain filters, although cleaning methods have been developed to minimize these maintenance shut downs. For example, pulse jet cleaning, where the flow of the filtered fluid is temporarily reversed to dislodge accumulated material from the filter element surface, has been used for in situ filter element cleaning without shutting down the filtration system. However, in certain equipment where reverse fluid f
Crouch Steven
Poon Wai S.
Stark Stephen K.
Storm Jeffrey
Thorpe Daniel W.
Gore Enterprise Holdings Inc.
Lewis White Carol A.
Spitzer Robert H.
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