WO1994022553A1

WO1994022553A1 - Membrane vent filter cartridge

Info

Publication number: WO1994022553A1
Application number: PCT/US1993/004161
Authority: WO
Inventors: Stephen Stark
Original assignee: WL Gore and Associates Inc
Current assignee: WL Gore and Associates Inc
Priority date: 1993-03-29
Filing date: 1993-05-04
Publication date: 1994-10-13
Anticipated expiration: 1995-09-29
Also published as: AU4035193A

Abstract

A membrane vent filter cartridge combined with a pneumatic particle conveying system for keeping dust of noxious or dirty particles inside the system while air flow into and out of the system is filtered. A porous polymer membrane (3), especially of expanded polytetrafluoroethylene, backed by non-woven textile fibers (4) and an apertured support screen (5) effectively filter out dust of toners, pigments, noxious chemicals and the like.

Description

TITLE OF THE INVENTION

MEMBRANE VENT FILTER CARTRIDGE

FIELD OF THE INVENTION

The invention relates to vent filters for pneumatic conveying systems which transfer particulate materials, such as grains, powders, or dusts through lines between supply tanks and silos to other tanks, reactors, and the like, by air flow under positive or negative pressure.

BACKGROUND OF THE INVENTION

Pneumatic conveying systems are well known in the art and involve the movement of particulate materials of quite variable sizes such as small pieces of minerals, beads, grains, powder, and dust. The particles are usually contained in tanks or silos and are moved through conduit or pipe transfer lines to other locations, such as smaller tanks, feed chambers, bins, holding receptacles for package loading lines, and many other forms. Both low pressure, high pressure, low flow rate, high flow rate, intermittent flow, and continuous flow are utilized. The particulate material is allowed to flow into a transfer line where it is picked up and pushed or pulled by a stream of air through the line to another receptacle.

Many of the materials being transferred by these systems are minerals, chemical compounds, pharmaceuticals, or pigments, for example, and have an element of toxicity, staining, or danger associated with ingestion by people operating the systems or present nearby. Pigments, dyes, and toxic materials can cause particular problems if they leak from the system, even in

small amounts. The vent filter of the invention is designed to control this leakage while at the same time allowing the air carrying the particles being transported to escape through the filter. SUMMARY OF THE INVENTION

The invention comprises a membrane vent filter cartridge constructed of a top cap and a bottom cap between which are fixed into place a cylindrical pleated filter medium inside a support screen to hold the filter medium in place inside it under the pressure of air flowing through it from inside the tank or vessel to which it is attached. Both the top cap and bottom cap may have valved attachment and access means to control air flow and access for cleaning purposes or for aiding the operation of the transport systems.

The pleated filter membrane is formed by adhering together a porous polymer membrane and a fibrous textile fiber non-woven backing sheet or felt, the porous polymer membrane being on the inside of the medium facing the interior of the vent filter and the material being filtered. The pores of the membrane are of a size to completely block the passage of the particles being filtered. Surface filtration, rather than in-depth filtration, is important to the function of the invention, in that surface filtration allows easy removal of collected dust from the inside of the vent filter and reuse of the filter, rather than the necessary replacement and discard of an in-depth type filter.

A preferred membrane material is expanded polytetrafluoroethylene, which has a hydrophobic surface. Particles tend not to adhere to it. It is generally resistant to abrasion, heat, and chemical attack. The top and bottom caps may be of metal or plastic, the potting material may be any customarily used in the art for such purposes, and the support screen may be made from plastic or metal, apertured sheet, woven strands, or slit and expanded sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a partially broken perspective view of the vent filter cartridge of the invention to show interior layers.

Figure 2 is a broken perspective view of the vent filter cartridge of the invention with an attachment tube held into the throat of the top cap of the cartridge by a jam nut.

Figure 3 is a broken perspective view of the cartridge with a valved inlet-outlet tube for access to the cartridge for cleaning it. Figure 4 is a schematic diagram of an assembly of two vent filter cartridges of the invention fitted in place on a pneumatic particle conveying system.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the drawings, the details of the cartridge and its assembly with a particle conveying system are described to more fully delineate the articles of the invention, the materials useful in manufacturing it, and how it operates to solve a problem associated with particles and dust of noxious character or having inconvenient properties during handling and storage. Figure 1 describes the basic membrane vent filter cartridge of the invention which has a pleated filter medium (made up of 3 and 4) housed between a top cap 2 and a bottom cap I . The medium is inside support screen 5. Support screen 5_ and the filter medium are potted into top 2 and bottom caps I by a standard method with materials known in the art. Figure 1 shows the various layers broken away to show the layers of support screen 5 and the pleated filter medium. The pleated filter medium has an inside layer of porous polymer membrane 3. Adhered to the membrane is a fibrous textile backing layer 4. Membrane 3 and backing layer 4 together make up a sheet of the filter medium. The sheets are pleated and folded into a generally cylindrical shape, which has the edges adhered together with an adhesive or heat bonding to seal the edges together into a cylinder, porous membrane 3 facing into the cylinder, support backing layer 4 facing out. The cylinder of filter medium is inserted inside a cylindrical support screen 5, and the medium and screen 5 potted as a unit into the top 2 and bottom caps I.

The top cap 2 and bottom cap \ may be either metal or plastic as needed to meet the conditions under which the cartridge will be used. The filter medium layer of porous membrane 3 is preferably of a low surface energy material, and often is hydrophobic as well. A preferred material for the membrane is expanded polytetrafluoroethylene (ePTFE), which is described in U.S. Patents 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, and 4,478,665, assigned to W. L. Gore & Associates, Inc. Other equivalent materials may be used.

The backing layer 4 adhered to the porous membrane layer 3 can be a non-woven sheet of polymer fibers, such as polyester, polyamide, polypropylene, polysulfone, polyether ether ketone, polycarbonate, fluorinated hydrocarbon, or other suitable fiber.

The support screen 5, with the cylinder of filter medium inside it, is potted as a unit in the top cap 2 and bottom cap I with one of the usual materials for that purpose, such as rubber, urethane rubber, silicone rubber, epoxy resin, or a plastisol, for example.

Figure 2 shows in a broken perspective view a top portion of the vent filter of the invention fitted with a molded plastic screw fitting JJ. which fits inside the center aperture of the top cap 2 and is held in place by a jam nut J_0 which is screwed over fitting JJ until it rests against cap 2 to clamp cap 2 and fitting JJ. together. An expanded metal support screen 5 is shown on this cartridge.

Figure 3 describes in a broken perspective view a bottom cap 1 of a cartridge fitted with a screw-threaded fitting 1 which embodies a valve 13 for quick opening and closing the cartridge by throwing lever J4-

Figure 4 is a schematic drawing of a pneumatic particle conveying system with two vent filter cartridges 20 fixed in place to vent air or other gases from silo 21 and bin 26 and to block any particles or dust from leaking out of the system at the same time. Dusty particles 23 are stored in silo 2_1 with filter cartridge 2J) in place to let air in and out, but filtering out any dust 27. When it is desired to refill bin 26 with particles 23, a source of air 24 pushes air into the conveying tube 22, as central valve 2j> is opened and the requisite amount of particles 23 blown over into bin 26 via tube £2. In bin 26, the dust particles 27 cannot escape through vent filter cartridge 20 as excess air flows out of the cartridge as bin 26 fills with particles 2_. to displace the air therein.

EXAMPLE 1

Construction of a Vent Filter Cartridge

A cartridge was constructed from a sheet of a 6 oz./yd. spun bonded polyester backing laminated with a layer of expanded polytetrafluoroethylene (ePTFE). The measured permeability of this filtration laminate was 6.0 cubic feet per minute per square foot at a pressure drop of 0.5 inches w.g. It had pleats 1 inch deep and the filter contained a total of 51 pleats. The total filter area was 8.3 square feet.

This flat pleated sheet was cut and formed into a cylindrical shape and the ends sealed such that the ePTFE membrane was facing inward toward the center of the cylinder. A core was constructed with 24 gauge, expanded 304 stainless steel, by cutting a rectangular sheet 12 inches wide by 19 inches long. A cylinder was formed of this sheet by rolling the sheet and overlapping the ends by 1-3/4 inches. The overlap was spot welded to form a core 12 inches long with an outside diameter of 5.5 inches. The vent filter cartridge was assembled by pouring 1/2 pound of polyvinyl chloride primerless plastisol potting compound into a galvanized steel end cap with an outside diameter of 5-9/16 inches and an inside diameter of 3-3/16 inches. The metal core was then inserted into the potting compound just inside the outer lip of the end cap. The filter medium was then inserted into the inside of the metal core. The potting compound was cured on a hot plate at 385°F. for 25 minutes. After the top of the filter was assembled, the potting process was repeated on the bottom end using a closed galvanized steel end cap with an outside diameter of 5-9/16 inches. After both ends of the filter were potted and cured, a 3/16 inch thick neoprene foam gasket was attached to the top end cap using pressure sensitive adhesive. EXAMPLE 2

Installation and Testing of a Vent Filter Cartridge

The vent filter cartridge was bolted to a metal flange with two 1/4 inch diameter x 1 inch long bolts attached to the top of the cartridge. The metal flange was constructed with a 1/4 inch thick carbon steel disc with an outside diameter of 5-9/16 inch. A piece of standard 2 inch I.D. pipe was press fit into a hole cut in the center of the disc.

The cartridge with the flange assembly attached was then screwed into a pneumatic conveying pressure vessel built by Macawber Engineering, Inc., Marysville, TN.

This 40-gallon pressure vessel was charged with 15 pounds of xerographic toner dust, then pressurized to 15 pounds per square inch. The exit valve was opened, allowing the toner dust to flow into the pipe exiting at the bottom of the vessel. After the toner dust was transferred through the exit line, a pressure of 2 pounds per square inch remained in the vessel. This pressure was reduced to ambient pressure by bleeding air through an exhaust valve diaphragm into the cartridge vent filter. The toner dust entrained in the air during the exhaust procedure was captured on the inside of the cartridge vent filter. The air passed through the filter media into the area surrounding the equipment. The filling, pressuring, transfer, and exhaust cycle occurred every 20 minutes while the equipment was operating. The cartridge vent filter was tested over a four-week period during which no toner dust was detected leaking through the media or around the seals. The filter was then removed to determine the amount of toner dust collected inside the filter. A total of 350g of toner dust was removed from inside the filter by turning it upside down and tapping the bottom end cap. After cleaning out the toner dust, the filter was re-attached to the pressure vessel and continued to perform as before.

Thus, particles of a material, such as a xerographic toner or a noxious chemical powder, can be easily moved from receptacle to receptacle in a sealed conveyor system without leakage to harm operators and bystanders if the air conveying medium is filtered of dust of these materials before it exits the system. Troublesome materials are completely isolated inside the systems thereby ensuring safety and cleanliness.

Claims

I CLAIM :

1. A membrane vent filter cartridge comprising:

(a) a top cap and a bottom cap between which are potted into sealing engagement with (b) a cylindrical pleated filter medium comprising adhered ■ sheets of porous polymer membrane and textile fiber backing sheet, said membrane facing the interior of said cartridge; and (c) a support screen surrounding said fflter medium.

2. A filter cartridge of Claim 1 wherein said top cap, said bottom cap, and said support screen are selected from the group consisting of metal and plastic.

3. A filter cartridge of Claim 1 wherein said porous polymer membrane comprises expanded polytetrafluoroethylene.

4. A filter cartridge of Claim 1 wherein said textile fiber backing sheet is non-woven.

5. A filter cartridge of Claim 1 wherein said textile fibers of said backing sheet are selected from the group consisting of polyester, polyamide, polypropylene, polysulfone, polyether ether ketone, polycarbonate, and fluorinated hydrocarbon polymers.

6. An assembly of a filter cartridge of Claim 1 with a particulate-conveying pneumatic materials transport system.

7. A process of filtering particles from air being vented from a component of a particulate-conveying pneumatic materials transport system by a vent filter cartridge comprising a top cap and a bottom cap between which are potted into sealing engagement with a cylindrical pleated filter medium comprising adhered sheets of porous polymer membrane and textile fiber backing sheet, said membrane facing the inside of said cartridge, and a support screen surrounding said filter medium, which process comprises:

(a) attaching said cartridge to a pneumatic system; and

(b) passing air that contains a particulate from said pneumatic system through the cylindrical interior of the cartridge and through the filter medium.