This invention relates to filters for filtering undesirable materials or impurities from liquids, such as used cooking oils, for example.  More particularly, this invention relates to filters which include a filter envelope, wherein the filter envelope is formed from filter panels formed from materials having different thicknesses, structural strengths, and/or permeabilities, whereby flow of the liquid to be filtered is directed through the panel having the greater permeability. 
The term xe2x80x9cpermeability,xe2x80x9d as used herein, means the ability of a material to permit the flow of a liquid therethrough.  Thus, materials which have a low resistance to the flow of liquids therethrough have greater permeabilities, and materials which have a high resistance to the flow of liquids therethrough have lesser permeabilities. 
In a typical frying operation, large quantities of edible cooking oils or fats are heated in pans or vats to temperatures of from about 315xc2x0 F. to about 400xc2x0 F. or more, and the food is immersed in the oil or fat for cooking.  During repeated use of the cooking oil or fat, the high cooking temperatures, in combination with water from the food being fried, cause the formation of free fatty acids (or FFA).  An increase in the FFA decreases the oil""s smoke point and results in increasing smoke as the oil ages.  In addition, the cooking oil may include solid impurities, such as, for example, small pieces of the food being cooked. 
In addition to hydrolysis, which forms free fatty acids, there occurs oxidative degeneration of fats which results from contact of air with hot oil, thereby producing oxidized fatty acids (or OFA).  Heating transforms the oxidized fatty acids into secondary and tertiary by-products which may cause off-flavors and off-odors in the oil and fried food. 
Caramelization also occurs during the use of oil over a period of time, resulting in a very dark color of the oil, which, combined with other by-products, produces dark and unappealing fried foods. 
Such solid and dissolved impurities in general are removed from such used cooking oils by passing such oils through a filter.  Such a filter includes one or more of the following components including, but not limited to, screens, metal spacer grids, filter papers, filter pads, or combinations thereof.  In connection with the filter, the oil may be contacted with a variety of materials or filter aids, adsorbents, or neutralizing agents, generally in the form of powders or particles, to remove free fatty acids, oxidized fatty acids and other by-products from the oil.  Such materials include, but are not limited to, magnesium silicate, diatomite, calcium silicate, and alkali materials such as alkaline earth metal hydroxides, alkaline earth metal oxides, alkali metal carbonates, alkali metal bicarbonates, alkaline earth metal carbonates, and alkali metal silicates.  Such materials may be placed upon the filter or contained within the filter.  For example, when a filter pad or filter paper is employed, such materials may be impregnated in the filter pad or filter paper. 
One example of a filter includes a metal spacer grid which is contained within a filter paper envelope.  The paper envelope may be impregnated with a filter aid, such as those hereinabove described.  Such a filter in general is disposed at the bottom of a pan of used cooking oil.  The metal spacer grid has a fitting which extends from the grid through a panel or side of the filter envelope.  The fitting may be connected to a pipe which is connected to a pump.  When the pump is activated, the oil flows through the paper envelope on both sides of the filter, through the interstices of the metal spacer grid, and through the metal fitting and pipe.  The filtered oil then may be recycled for further cooking. 
When a filter paper envelope is employed, however, in general, a limited quantity of filter aid may be impregnated in the filter paper.  Thus, the treated oil may retain an unacceptable amount of undesirable materials such as free fatty acids and oxidized fatty acids, as well as other by-products.  Furthermore, it is well known that xe2x80x9cdepth filtrationxe2x80x9d is the most effective means of removing particulate materials without xe2x80x9cblindingxe2x80x9d or plugging the filter.  Depth filtration is achieved by forming a thick filter cake on a filter septum such as a filter paper or screen or by the use of a filter pad made of fibrous material such as cellulose.  The filter cake or pad creates a tortuous path of increasingly smaller capillaries with a sizable surface area which trap fine particles.  Although a filter pad can be impregnated with a sufficient quantity of filter aid, such pads in general are less stable structurally than filter papers for use as an envelope and, therefore, have not been used to form filter envelopes surrounding spacer grids.  Filter paper, however, has sufficient structural strength to be fused easily around its peripheral edges to form a filter envelope. 
It is an object of the present invention to provide a filter for filtering undesired materials from a fluid, such as cooking oil, for example, which can be impregnated with a desired amount of filter aid yet remain stable structurally.  Such object is accomplished when a filter is formed from first and second filter panels having different permeabilities, such as, for example, wherein the first panel is a filter pad and the second panel is formed from at least one filter paper.  The filter paper, which has a greater structural strength than the filter pad, gives support to the filter pad when the peripheral edges are fused. 
Furthermore, in order to obtain the full effect of the ingredient(s) impregnated into the filter pad, it is necessary to direct the liquid flow through the filter pad and not through the filter paper.  This is accomplished by employing a filter paper that has a lower permeability than the filter pad. 
In accordance with an aspect of the present invention, there is provided a filter for removing undesired materials from a fluid.  The filter comprises a filter envelope including a first panel and a second panel.  The first panel has a first permeability, and the second panel has a second permeability.  The first permeability is greater than the second permeability.  Each of the first and second panels includes a peripheral region, wherein a portion but not all of the peripheral regions are fused to each other.  The non-fused portions of the peripheral regions of the first panel and the second panel provide a first opening in the envelope.  In one embodiment, one of the first panel and the second panel has a second opening for receiving a conduit means for conducting a liquid from the filter. 
In general, the permeabilities of the first and second panels may be determined from the amount of vacuum drawn through the first and second panels from an inlet side to an outlet side.  The vacuum drawn through the panel is related inversely to the permeability.  Thus, the first panel has a lower amount of vacuum drawn therethrough than the amount of vacuum drawn through second panel. 
In one embodiment, the permeabilities of the materials which form the first and second panels are measured in accordance with a filter medium permeability test, the procedures for which are given hereinbelow.  In this test, a filter apparatus is employed.  The filter apparatus includes an outer cylindrical container which contains a circular and planar screen upon which is placed the filter medium to be tested.  The container is attached to a conduit which is connected to a positive displacement pump.  The apparatus also includes a vacuum gauge for measuring the vacuum drawn through the filter.  In general, in each test of a filter medium, a disc of the filter medium is placed on top of the screen.  In general, the diameter of the disc is greater than the diameter of the screen.  An inner cylinder having an inside diameter which is less than that of the disc of the filter medium then is placed on top of the filter medium.  Thus, the filter medium is prepared for the permeability testing. 
For each filter medium tested, a sample of 150 g of corn oil is placed in a 250 ml beaker.  A Whatman 41 filter paper is placed upon the screen in the outer cylindrical container.  The inner cylinder then is placed inside the outer cylinder and upon the filter paper.  The apparatus is flushed for 5 minutes with at least 150 g of hot oil at 150xc2x0 C. The Whatman 41 filter paper is removed and replaced with a sample of the filter medium, such as, for example, a filter pad or filter paper.  A sample of 150 g of oil is heated to 150xc2x0 C.xc2x12xc2x0 C. The oil then is poured into the inner cylinder and contacts the filter medium.  The positive displacement pump then is activated such that the oil is circulated and recycled through the filter medium.  The oil is circulated at a flow rate or positive displacement, of 52 ml/min.  per square inch of surface area of the filter medium.  After the oil has been circulated for five minutes, and the amount of vacuum being drawn in inches of mercury is measured. 
The oil then is redirected to a collection beaker.  The first panel, which has a greater permeability than the second panel, will have a lower amount of vacuum drawn therethrough than the second panel.  In general, when the first panel is a filter pad and the second panel is a filter paper, the filter pad has a vacuum drawn therethrough, according to the procedure hereinabove described, of from about 1.0 in.  to about 12.0 in.,  preferably from about 2.5 in.  to about 8.0 in.  of mercury at a displacement of 52 ml/min.  per square inch of surface area of filter pad, and the filter paper has a vacuum drawn therethrough of from about 5.0 in.  to about 30.0 in.,  preferably from about 19.0 in.  to about 28.0 in.  of mercury at a displacement of 52 ml/min.  per square inch of surface area of filter paper, provided that the amount of vacuum drawn through the filter pad is less than that drawn through the filter paper. 
In another aspect, the first and second panels of the filter envelope may be fused to each other by a variety of means, such as by stitching or by an adhesive.  In a further aspect, when the first panel is a filer pad and the second panel is a filter paper a small strip of the same material and permeability as the filter paper may be placed upon the peripheral region of the filter pad.  The strip, filter pad, and filter paper then are stitched to each other to form the filter envelope. 
The filter envelope may be of any shape, including square, rectangular, trapezoidal, and circular.  The shape of the filter envelope may be in conformity with the shape of the pan or vat of used cooking oil into which the filter envelope is placed. 
In a further aspect, the opening for receiving a conduit means is in the first panel.  In another embodiment, the opening for receiving a conduit means is in the second panel. 
In a preferred aspect, the filter further comprises a spacer grid contained within the filter envelope.  In general, the spacer grid is the same shape as that of the filter envelope, and is inserted into the filter envelope through the opening formed by the non-fused portions of the peripheral regions of the first and second panels of the filter envelope. 
In general, the spacer grid may be a metal grid formed from a material including, but not limited to, perforated metal, expanded metal, or wire cloth containing a plurality of openings or interstices through which liquid may pass.  In one embodiment, the grid is expanded metal formed by a multiplicity of intersecting undulating metal strips which form a plurality of interstices through which the fluid may pass.  An example of a metal grid which may be employed is sold by Semrow Perforated and Expanded Metal Corporation, of Des Plaines, Ill. In another preferred embodiment, the filter further comprises a conduit fitting extending from the spacer grid through the opening for receiving a conduit means in one of the first panel or the second panel.  In one embodiment, the conduit fitting means is a threaded nipple attached, such as by welds, for example, to the spacer grid. 
In another embodiment, the filter further comprises a clip means, which is fitted over the opening provided by the non-fused portions of the peripheral regions of the first panel and the second panel.  In one embodiment, the clip may include an opening for receiving a conduit means or a conduit fitting means, whereby liquid is conducted from the filter by passage through the clip.  In such an embodiment, neither the first panel nor the second panel includes an opening for receiving a conduit means. 
In a most preferred embodiment, the filter envelope includes a first panel, which is a filter pad, and a second panel, which is a filter paper.  The filter pad and filter paper may be formed from materials such as those herein described.  An opening for receiving a conduit means is provided in the filter pad.  The filter pad has a permeability which is greater than that of the filter paper.  The filter pad is impregnated with a filter aid as herein described. 
In a further aspect, the filter envelope has a square or rectangular shape, and a strip of filter paper is placed on top of the filter pad at three sides of the filter pad.  The strip, filter pad, and filter paper are stitched together at the three sides.  The fourth side is not stitched together, but is left open, forming an envelope with a transverse opening between the pad and paper.  A spacer grid then is inserted through the opening into the filter envelope.  The spacer grid may be formed from intersecting and undulating metal strips that are attached to each other by spot welds.  A conduit fitting is welded to the spacer grid and projects through the opening in the filter pad.  Once the spacer grid is inserted into the filter envelope, a metal clip is fitted over the opening formed by the non-fused sides of the filter pad and the filter paper.  Thus, the filter now is sealed on all sides.