Liquid purification or separation – Particulate material type separator – e.g. – ion exchange or...
Reexamination Certificate
1999-05-12
2001-10-09
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Particulate material type separator, e.g., ion exchange or...
C210S502100, C210S265000, C210S505000, C210S508000
Reexamination Certificate
active
06299771
ABSTRACT:
The present invention relates to water purification, and, more particularly, to the use of a composite activated carbon element that exhibits reduced flow resistance and improved adsorption properties to improve the quality of potable water.
BACKGROUND OF THE INVENTION
The need to purify water as, for example, at home (point-of-use water purification systems) is increasing because of the increased incidence of man-made contaminants, the growth of population world wide, and the concomitant increasing burden on the water resources. As more medical and environmental information becomes available about the ability of contaminants in water to cause illnesses, the need to use pure water becomes increasingly evident.
Typical point-of-use water purification systems comprise a bed made of activated carbon that is used to remove chlorine and organic and other impurities. Most often the activated carbon is in granular form. Advantages of the use of an adsorption bed made of granular activated carbon are that granular activated carbon is inexpensive, granular activated carbon in the bed does not compact under flow, and granular activated carbon has relatively little resistance to the flow of water therethrough. A disadvantage of using a granular activated carbon bed is its slow adsorption rate. In industrial applications, it is recommended to have at least 15 minutes contact time between the fluid and the activated granular carbon adsorbent bed. Another disadvantage of a granular activated carbon bed is that, due to the relatively large size of the granules and the void spaces therebetween, it does not remove smaller particulate contaminants.
Powdered activated carbon is known to adsorb impurities faster than granular activated carbon, but the inclusion of carbon fines provides high flow resistance and leads to undesirable compaction under flow. Attempts to avoid compaction commonly involve the incorporation of a binder. While filtration elements comprising activated carbon particles bound into a rigid structure using a polymeric thermoplastic material will not compact under flow, a disadvantage of these so called “carbon-block” structures is that a part of the adsorption capacity of activated carbon is lost due to contact with the immobilizing binding material. Another disadvantage is that, in such filters, the activated carbon occupies only a portion of the adsorption bed volume. The rest of the adsorption bed volume is taken by the immobilizing polymeric binding material. The binder is not active in adsorbing impurities, and its incorporation leads to increased bed sizes as compared to an adsorption bed that contains only activated carbon. Yet another disadvantage of carbon-block material is that its ability to be regenerated or sterilized with steam is hindered due to the presence of the thermoplastic binding material. In addition, making carbon-block material is costly, requiring exact temperatures and other precisely controlled processing conditions.
Activated carbon fiber has been used, but it is expensive and it does compact under flow, leading to moderate flow resistance, although compaction of activated carbon fibers is considerably less than that experienced when employing powdered activated carbon.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a composite filter element that exhibits reduced flow resistance and improved adsorption properties; including high adsorption activity, high adsorption capacity, and which causes less pressure drop than corresponding conventionally available filters, and which novel composite element of the invention is easy to prepare.
A further object of the invention is to provide a method for purifying water using a composite activated carbon filter formed from a combination of GAC and ACF in which the activated carbon fibers are positioned between the activated carbon granules in a manner to eliminate all of the larger voids therebetween.
A still further object is to provide adsorption materials which comprise a mixture of ACF and GAC.
Yet another object is to provide a process for producing the above described adsorption material, mixing milled activated carbon fiber and granular activated carbon in water and removing the excess water from the mixture.
The above and further objects and novel features of the invention will appear more fully from the following detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The adsorption material of the present invention comprises a mixture of granules and fibers, and particularly a mixture of granular activated carbon (GAC) and activated carbon fiber (AGF). In an alternate embodiment of the invention, we employ a mixture of ion exchange resin granules mixed with ion exchange fibers.
The term “GAC” as used herein means granular activated carbon in its art recognized sense, i.e., porous carbon particles which range in size from 50 to 3000 microns. Preferably, the granules used when practicing this invention will have diameters from about 100 to 2,000 microns, and most preferably about 200 to 1,000 microns. Granules useful in accordance with this invention will also have a specific surface area of at least 300 m
2
/g, and more preferably at least 500 m
2
/g. They will also have an adsorption capacity of methylene blue of at least 50 mg/g, and more preferably at least 100 mg/g.
The term “ACF” as used herein means activated carbon fiber in its art recognized sense; that is, porous carbon fibers obtained from organic fibers through a carbonization treatment and an activation treatment. ACF is classified by the starting material used to prepare the same. Common starting materials include rayon, polyacrylonitrile, pitch, and the like. Any of these types of ACF is suitable for use in the present invention. Of the various, commercially available ACF materials, rayon type ACF is particularly preferred in view of its high adsorption capacity and its ability to rapidly adsorb impurities. Rayon type ACF can be prepared, for example, by the process described in U.S. Pat. No. 5,521,008, the disclosure of which is incorporated herein by reference..
Activated carbon fiber has the ability to adsorb impurities faster than powdered activated carbon. In addition, while it does compact under flow leading to moderate flow resistance, compaction of activated carbon fibers is considerably less than that experienced when employing powdered activated carbon. One of the advantages of activated carbon fiber materials is that they have smaller quantities of carbon fines than powder materials. In addition, carbon fines that are generated when ACF is used do not impede the flow nearly as much as the carbon fines generated by powdered activated carbon. Another advantage of using activated carbon fibers is that due to their small diameter, and the proximity of the fibers to each other, suspended solids and silt can readily be removed from incoming aqueous feed. Working as a depth filter, it also removes bacteria from water.
ACF that is useful for the present invention will have a specific surface area of at least about 400 m
2
/g, and preferably at least 750 m
2
/g. The individual fibers will also have an average diameter which is significantly smaller that the average diameter of the GAC which is employed. Desirably, the average diameter of the GAC is an order of magnitude larger than the average diameter of the ACF, and the maximum diameter of the ACF will be significantly less than about half of the smallest diameter of the granules, i.e., the diameter of the ACFs will be on the order of from 1 to 30 microns, and preferably of from 4 to 20 microns. They will also have an adsorption capacity of at least about 200 mg/g of methylene blue, preferably at least 350 mg/g. As the average diameter of the fibers falls below 4 microns the activated carbon fiber tends to create excessive amount of carbon fines and the packing density tends to become excessively large. Below 1 micron average diameter, the packing density becomes so large that effective filtration becomes impractical. The length distri
Lieberman Alexander I.
Pimenov Alexander V.
Shmidt Joseph L.
Cornell Ronald S.
Electrophor, Inc.
Fortuna Ana
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