Filtration Device Using Absorption for the Removal of Gas Phase Contaminants

The present invention relates generally to methods and apparatus for removing undesirable particles from air and more especially to such methods and apparatus for removing odors from air which is recirculated in an enclosed human environment such as an automobile, airplane or similar cabin.

The oil and intake air filters commonly encountered in conjunction with internal combustion engines are exemplary of the filtering art and its associated problems. These filters utilize a somewhat porous paper filter element or similar filter media on which any undesirable particles are retained as fluid is passed through such filter element or media. As filtering continues, the filter element or media becomes more and more blocked by the removed particles which results in a further reduction of the flow rate even though the particle removing efficiency increases. These filtering systems essentially rely on adsorption which is a surface phenomenon wherein the removed particles adhere to the surface of the filter element or media. Another example of an adsorptive particle removal technique, is the use activated charcoal or a zeolite material as an odor removing element or media to purify an air stream. However, such techniques or method of purifying an air stream are effective only for a relative short time period, are relatively inefficient at low odor concentration levels, and frequently result in a relatively high pressure differential across the element or media. As a result very large quantity of media is the only satisfactory way in which to use these materials however the relative large pressure differential problem would remain.

A costly and complex solution to the flow rate and efficiency problems can be provided by employing absorption techniques wherein a counterflowing liquid and gas are intermingled in a gas absorption tower with certain gas components being absorbed and removed by the liquid. Such "scrubbing" techniques are not suited to small solid filter applications since the interstices within the filter elements must be relatively large to avoid the liquid from clogging the filter elements and severely reducing air flow therethrough. These large openings significantly reduce the filtering efficiency.

An illustrative hybrid between the first two methods is shown in U.S. Pat. No. 4,323,373 which teaches an air cleaning system for a restaurant cooking grill wherein air is passed through filter elements which remove the undesirable materials and a scrubbing solution of water and detergent continuously circulates over the filter elements to remove congealed grease and other pollutants from those filter elements. Here, the filtering of particles from the air is an adsorption process, but the subsequent removal of the materials from the filter elements is an absorption process. This type system is relatively large and is adapted to high volume flow of relatively dirty air. While aptly suited to cooking grills, it is not well suited to more generalized odor removal functions. Also, as with the "scrubbers" mentioned above, the interstices within the filter elements must be relatively large to avoid the liquid from clogging the filter elements and severely reducing air flow therethrough.

It is desirable to provide a compact, economical absorptive air filter for the removal of odors which avoids the problems of reduced flow rates and reduced particle removing efficiency.

U.S. Pat. No. 5,057,368 entitled FILAMENTS HAVING TRILOBAL 0R QUADRILOBAL CROSS-SECTIONS discloses a trilobal or quadrilobal fiber formed from thermoplastic polymers wherein the fiber has a cross-section comprised of a central core and three or four T-shaped lobes. The legs of each of the lobes intersect at the center of the core so that the angle between the legs of adjacent lobes is from about 80 to 130 degrees. The thermoplastic polymer is typically a nylon, a polyester, a polyolefin or a combination thereof. For example, a combination of a polyester such as polyethylene terephthalate and a polyolefin such as polypropylene may be used.

The present invention provides solutions to the above problems by providing a cabin air filtration system having structure forming a cavity with an air flow path therethrough. The cavity which has a liquid inlet and a liquid outlet, retains an internally wicking fibrous material through which fluid is transferred from the inlet to the outlet. An air-borne material absorbing liquid is supplied to the inlet and that air-borne material absorbing liquid and associated absorbed materials exits at the liquid outlet. Typically air is continuously circulated through the cavity from a cabin or room along a closed flow path consisting of the cavity and the cabin or room. Through experimentation it has been determined that extruded fibers such as disclosed in U.S. Pat. No. 5,057,368 are well suited to the practice of the present invention.

In accordance with another form the invention, the cavity includes a liquid reservoir which contains the air-borne material absorbing liquid. In this embodiment, the internally wicking fibrous material within the cavity draws liquid through capillary attraction from the reservoir into the air flow path without significantly restricting the air flow path.

In accordance with another aspect of the invention, undesirable materials are removed from an air stream by interposing a plurality of at least partially hollow wicking fibers in the air stream and supplying a liquid including a component having an affinity for the undesirable material to those wicking fibers at a location outside the air stream. The wicking fibers convey the liquid into contact with the air stream while maintaining relatively open interstices between the fibers so that the air stream flow is substantially unimpeded by the liquid. A concentration factor induced molecular migration effectively conveys the undesirable material in solution within the liquid away from the air stream.

FIG. 1 is a view in cross-section of a cabin air filtration system wherein the filter cavity and associated liquid reservoir are common;

FIG. 2 is a view in cross-section of a cabin air filtration system wherein an air-borne material absorbing liquid is supplied to a filter cavity inlet and conveyed from a cavity outlet with cabin air circulated through the cavity;

FIG. 3 is a perspective view of an alternative filtration system cavity;

FIG. 4 is a view in cross-section of a modified cabin air filtration system wherein an absorbing liquid and associated absorbed materials are separated allowing the liquid to be returned for re-use; and

FIG. 5 is a partial perspective view illustrative of a hollow wicking fiber suitable for use as an internally wicking fibrous material for use in the filtration systems illustrated in FIGS. 1, 2, 3 and 4.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawing.

The present invention is particularly suited to freshening the air of a relatively small space such as the cockpit of an aircraft, interior of an automobile, spacecraft, building, or similar enclosed environment all of which are hereinafter generically identified as a cabin. Freshening of the air is achieved by removing particles or gas from an air stream flowing through the cabin.

In FIG. 1, a cabin air filtration system according to the present invention has a housing 21 with a filter cavity 19 defined therein having an air inlet port 11 for receiving cabin air and an air outlet port 13 for returning that air to the cabin. Air flows from the cabin along a laminar flow path 15, 16, and 17. Housing 21 includes a liquid reservoir 23 located in a lower liquid-tight portion of the cavity 19. The reservoir 23 contains an air-borne material absorbing liquid typically comprising an inert carrier with one or more chemically aggressive agents dissolved therein. The inert carrier preferably is a non-volatile liquid such as glycerol, polyethylene glycol, and polypropylene glycol although water may be used in some applications. Chemically aggressive agents which have an affinity for the undesired air-borne materials are typically selected. The undesirable materials may merely be taken into solution, or there may be a chemical reaction with the undesirable material. For example, sodium hydroxide effectively removes carbon dioxide by converting it to sodium carbonate. Sodium hydroxide is also effective to remove hydrogen sulfide or formaldehyde. Calcium chloride or common table salt may be used as a desiccant. Acetic or citric acids have been found suitable for some other odors. The possible carriers and aggressive agents are nearly unlimited.

The cavity 19 also contains an internally wicking fibrous material 25 which functions by capillary action to move liquid upwardly from the reservoir into the air flow path without significantly restricting the air flow path. Many common materials which are effective wicking agents also restrict any attempt to circulate air through the material. For example, wetting a common handkerchief with water essentially seals the material against any air flow through it. By employing internally wicking materials where the capillary force inside individual fibers is far greater than that outside the fiber, the unrestricted air flow path about the outsides of the individual wicking fibers is maintained. Such an internally wicking fibrous material is sometimes formed of extruded strands having hollow interior regions which communicate with the outer strand surface. Strands of such a wicking fibrous material are illustrated in FIG. 5 as being C-shaped in cross-section or multi-lobed. A fan may be used to draw air from the cabin through the cavity 19 and back into the cabin.

A system illustrated in FIG. 2 illustrates a closed loop cabin air filtration system to circulate air from the cabin, through the cavity 27 of a cylindrical housing 26 preferably along a laminar air flow path along arrows 29 and 31 from an inlet 48 and back into the cabin through an outlet 50. A liquid inlet port 33 receives an air-borne material absorbing liquid 35 from an external reservoir 37 as determined by flow control valve 39. The control of valve 39 may be timed to periodically release liquid, may be enabled only when an ignition switch is on, or otherwise controlled to limit liquid flow. Housing 26 has a liquid outlet port 41 for removing the air-borne material absorbing liquid and associated absorbed materials from the cavity 27 to, for example, a vehicle radiator 45 overflow tank 43. The internally wicking fibrous material 49 is generally annually disposed within the cavity 27 and transfers fluid from the inlet port 33 to a sump adjacent outlet port 41. Because the air flow path through the cavity is relatively long and narrow, turbulence may occur as illustrated by arrows 51 which can provide a mixing and may result in good exposure of the incoming air to the air-borne material absorbing liquid. An illustrative, fan 47 provides for circulation by drawing air through inlet 48 from a cabin, pushing this volume of air through the cavity 27 along the air flow path 29, 31 and back into the cabin through outlet 50.

In FIG. 3, the housing for the cavity of the air filtration system is not shown, but such housing would surround the parallelepiped-shaped mass of wicking material 53. In this illustration, the fibrous wicking material has a plurality of generally parallel elongated fibers oriented to extend generally from left to right as viewed and in the intended direction of the flow path for fluid transfer. The air flow path for air is shown by arrows 55 and 57 and lies generally perpendicular to flow path for the liquid. A liquid inlet from reservoir 65 is provided near an upper edge 59 and a liquid outlet is provided near a sump 60 on a lower edge 61. The sump 60 communicates the cavity to a reservoir 63 for holding the air-borne material absorbing liquid.

One of the reservoirs may be eliminated from FIG. 3 to more nearly mimic the closed loop system described in conjunction with FIG. 1. In such a closed loop system the undesirable materials absorbed in the mass 53 will be effectively conveyed to the used solvent reservoir 63 by migration due to a concentration factor difference. However, periodic replacement of the fluid would then be necessary.

The system in FIG. 4, which is modification of the system of FIG. 2, illustrates structure which eliminates the need for periodic replacement of the fluid in reservoir 37. In this system, the air-borne absorbing liquid is regenerated by heating to drive off the noxious materials which were absorbed in the filter chamber 27. Here, the airborne material absorbing liquid and associated absorbed materials are received in vessel 67 and heated or otherwise treated to separate the absorbed materials from the liquid. As the noxious materials are removed, the liquid may be returned by conduit 69 to the supply reservoir 37 and ultimately is provided to the liquid inlet port 33 for re-use in cavity 27 by the wicking fibrous material 49.

As in FIG. 3, the reservoir 37 of FIG. 4 could be eliminated and filter cavity or chamber 27 would function as a material pick-up zone while vessel 67 constitutes a restoration zone with a non-mechanical migration of undesirable particles from one zone to the other.

The wicking fibrous material 71, as illustrated in FIG. 5, is formed as an extruded strand having three hollow interior regions 73, 75 and 77 each of which communicates with the outer strand surface 79,79' by way of generally parallel slots 81, 83 and 85. The capillary forces within the individual inner grooves 73, 75, and 77 are so much greater than those external to the fiber that liquid is readily wicked up the interior of the fiber without appreciable wetting of the external surface 79,79'. Thus, a mesh of such fibers 71 would remain unclogged even in the presence of a liquid and as a result this function is generally referred to as internally wicking. Such a fibrous material may be made of one or more materials such as nylons, polyesters, and polyolefins. The three T-shaped cross-section segments may have the outer portion of the T curved as shown, or straight. While the wicking fibrous material is depicted as three-lobed, any other numbers of lobes are possible. In addition, a C-shaped or other cross-sectional configurations may also be suitable for an internally wicking fiber for various applications.

The method of operation of the invention should now be clear. Undesirable material is removed from an air stream by interposing a plurality of at least partially hollow wicking fibers in the air stream and a liquid including a component having an affinity for the undesirable material is communicated through the wicking fibers as the air stream from an external location flows along an air path. In this way, the wicking fibers can convey material absorbing liquid into contact with the air stream while maintaining relatively open interstices between the fibers so that the air stream flow is substantially unimpeded by the liquid. The undesirable material in solution within the liquid may then be conveyed the from the air stream by a concentration factor induced molecular migration, by gravity, or may be removed and the liquid recirculated for repeated use.