Abstract:
The present invention is a system and method for improving the useful life of sorbent beds, such as activated carbon filter beds. It is known that breakthrough in carbon filter beds first occurs at the low gas velocity regions of the filter bed, i.e., at the filter&#39;s edges. A recirculation jacket filter system diverts already filtered air from the downstream side of the filter bed toward the peripheral region of the upstream side of the filter bed so that contaminants near the filter&#39;s edges are reduced and filter life is improved.

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used and/or licensed by or for the Government of the United States of America. 
   BACKGROUND OF THE INVENTION 
   This invention relates generally to fluid separation, and more particularly to fluid separation with recycle means. 
   Sorbent filters, such as carbon filters typically composed of one or more carbon beds, have been used for many years to remove offensive or harmful organic compounds entrained in air. Although the compounds are retained in all parts of the sorbent beds, studies have shown that a sorbable gas normally demonstrates earliest breakthrough (i.e., complete forward progression through the sorbent) first in the low velocity regions. In a sorbent bed of uniform dimensions and distribution of sorbent, such breakthrough occurs near the bed&#39;s edge zones or near a protrusion through the bed. 
   The finding of an early breakthrough pattern near boundaries, such as edge regions of a sorbent bed, may be explained based on the principles of shearing forces, kinetic energy, and surface area of sorbent pores and research related thereto. A carbon bed, for example, is composed of numerous carbon particles, typically activated carbon particles, confined within a defined volume. The carbon particles forming the bed have numerous tortuous pores of various diameters, generally defined as macropores (&gt;50 nanometers in diameter), mesopores (˜2 to 50 nanometers in diameter) and micropores (&lt;2 nanometers in diameter). Macropores and mesopores serve as primary transport channels. If a sorbent bed is considered having substantially planar upstream and downstream surfaces that are also substantially parallel to one another and designed to have a gas flow through the bed in a direction normal to the bed&#39;s surfaces, the sorbent bed may be characterized as having a central region or midsection in which most of the efficient sorption takes place and a surrounding peripheral region at the edge of the filter bed, proximate the outer wall of the bed or sorption chamber in which the sorbent is disposed. Studies have shown that most separation of gaseous components takes place in the micropores where shearing forces created by rapidly moving gases are not sufficient to cleave such compounds, in contrast to the mesopores and macropores, the primary transport channels, particularly in the midsection of a filter bed, where such shearing forces are sufficient to cleave sorbable compounds from carbon. Likewise, it is believed that a minimum threshold level of forced flow kinetic energy (KE, expressed mathematically as KE=½ mV 2 , where m is mass and V is velocity in consistent units) is required to open the micropore channels, form streamlines through them, and cause sorbable molecules to penetrate the pores deeply before their movement is stopped. Below such a level, sorbable molecules will instead simply continue to travel in forward directed macropores and mesopores. Thus, at low velocities and corresponding weak shearing forces near the edge regions of the sorbent bed, sorption takes place primarily in macropores and mesopores. 
   The rate of forward progression of the penetration front is determined by the relative rates of filling of the micropores in the midsection and the macropores near the boundary. Sufficient kinetic energy in the midsection enables sorbable compounds to deeply penetrate the micropores. In the midsection of the sorbent bed, the penetration front will advance at a given rate. Simultaneously, near the edge portions of the sorbent bed, where kinetic energy and shearing forces fall off markedly, sorbable compounds bind to macropore wall surfaces at a different rate than to micropore wall surfaces. Due to the much larger (orders of magnitude) surface area of micropores compared to macropores, the penetration front of the gas can advance more rapidly in edge zone macropores than in midsection micropores. Accordingly a penetration front bulge, and concomitant breakthrough, can occur near the edge of the sorbent bed. As a result, the edges or peripheral region demonstrates less capture efficiency (i.e. weight of contaminant captured by the sorbent/weight of sorbent) than in the midsection. Hence, earliest breakthrough generally occurs at the edges, or peripheral regions, of the bed. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of this invention to minimize or eliminate contaminant breakthrough at edge or peripheral regions of sorbent beds. 
   It is another object to trap larger amounts of sorbable contaminant fluids, particularly sorbable gases, than in similar conventional sorbent bed designs and methods used for such purposes. 
   These and other objects of the invention are achieved in one aspect by a filter system for removing adsorbable compounds from a contaminant-containing fluid. The filter system includes a sorbing chamber having an inlet and an outlet, the inlet, sorbing chamber and outlet defining a primary fluid flow path through the sorbing chamber; an inlet conduit in fluid communication with the inlet of the sorbing chamber; and an effluent conduit in fluid communication with the outlet of the sorbing chamber. Additionally, the filter system includes a sorbent bed disposed within the sorbing chamber transverse to the primary fluid flow path, means for diverting effluent fluid passing from a downstream surface of the sorbent bed at its peripheral region and directing the diverted effluent fluid toward a peripheral region of an upstream surface of the sorbent bed thereby defining a secondary fluid flow path; and a blower disposed downstream of the sorbent bed to draw fluid through the sorbent bed. The edge or peripheral region of the adsorbent bed receives fluid only from the secondary (recirculating) flow path. 
   Another aspect of the invention involves a method for removing adsorbable compounds from a contaminant-containing fluid, comprising the steps of introducing a contaminant-containing fluid to a sorbing chamber containing a sorbent bed having an upstream surface and a downstream surface; actuating a blower disposed downstream of the sorbent bed; drawing the contaminant-containing fluid through the sorbent bed with the blower such that substantially all of the contaminant is retained by the sorbent bed while effluent fluid, substantially free of contaminant, emerges from the downstream surface of a central region of the sorbent bed; diverting effluent fluid passing through the sorbent bed at its peripheral region; conducting the diverted effluent fluid passing through a peripheral region of the sorbent bed toward the upstream surface of the sorbent bed; and directing the conducted effluent fluid toward a peripheral region of the upstream surface of the sorbent bed. 
   One of the main advantages of the filter system of the present invention is that for a given volume of sorbent an increased sorption efficiency can be realized. Alternatively, for a given entrapment design criterion, the sorbent bed may be made physically smaller than conventional sorbent beds using the same type of sorption agent. In situations where reduced size sorbent beds may be employed, several cost benefits may be realized. These include reduced acquisition costs, reduced energy costs, reduced hazardous materials costs, and reduced structural support costs. 
   The filter system may be used in free standing units or incorporated into other devices in which air is forced into or drawn through such other devices. It may be used in any commercial, industrial, or military application where gaseous contaminants are to be removed from another predominant gas, such as air. Examples of situations where the filter system might be used include such diverse applications as scrubbing effluents, filtering air in HVAC and air conditioning systems, filtering air at sites where harmful gases or vapors may be encountered, filtering air at potential terrorist sites, and filtering air in vehicles. 
   Additional advantages and features will become more apparent as the subject invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a filter system constructed in accordance with the present invention. 
       FIGS. 2–7  show modifications of the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a filter system  10  is provided with a sorbing chamber  12  having a fluid inlet  14  at the upstream region of the chamber and a fluid outlet  16  at the downstream region of the chamber. A fluid inlet conduit  18  is provided in fluid communication with the fluid inlet  14  and a fluid effluent conduit  20  is provided in fluid communication with the fluid outlet  16 . A sorbent bed  22  is provided within the sorbing chamber  12 . Typically, the sorbent bed  22  is disposed transverse to the width or diameter of the sorbing chamber  12  and substantially perpendicular to the direction of fluid flow; however, other orientations may be used for a particular purpose or situation. 
   A motor driven blower  24  is disposed within the sorbing chamber  12 , downstream of the sorbent bed  22 . The blower  24  is of sufficient dimensions and power, when actuated, and the fan blades, or similar mechanism, are of such a pitch as to draw fluid in through the inlet conduit  18  and expel it through outlet conduit  20 . Fluid passing through the filter system  10  defines a primary fluid flow path composed of the fluid inlet  14 , sorbing chamber  12 , and fluid outlet  16 . Small arrows in  FIGS. 1 and 2  indicate the direction of primary fluid flow through the filter system  10 . 
   The filter system  10  is further provided with a recirculation jacket  26  for diverting effluent fluid passing from the downstream surface of the sorbent bed  22  at its peripheral region and directing the diverted effluent fluid toward a peripheral region of the upstream surface of the sorbent bed, thereby defining a secondary fluid path. The peripheral region of sorbent bed  22  is exposed substantially exclusively to the secondary fluid path, which only carries previously filtered fluid. Thus, the peripheral region of the bed  22  sees few, if any, contaminants. 
   In operation, a contaminant-containing fluid, such as air, is fed to the fluid inlet conduit  18 . As a result of the negative pressure upstream of the sorbent bed  22  and the positive pressure downstream of the sorbent bed, caused by the blower  24 , the contaminant-containing fluid is drawn into the fluid inlet  14  and the sorbing chamber  12  and through the sorbent bed  22 . Contaminants present in the contaminant-containing fluid passing through the central region of the sorbent bed  22  and any small amounts of contaminants that may reach the peripheral region of the sorbent bed adjacent to the wall of the sorbing chamber  12 , are sorbed, or retained, by the sorbent bed. Under typical operating conditions, the effluent fluid that passes through the sorbent bed  22  is contaminant-free and continues to flow past the blower  24  and out of the sorbing chamber  12  through the fluid outlet  16  and the fluid outlet conduit  20 . The portion of the effluent fluid that passes through the peripheral region of the sorbent bed  22  is essentially contaminant free but may have small amounts of contaminant present, particularly when the system is initially activated. The effluent fluid emanating from the peripheral region of the downstream surface of the sorbent bed  22  enters the recirculation jacket  26  at its inlet  28  and is drawn through the recirculation jacket by the negative pressure created at its outlet  30 , located upstream of the sorbent bed. As the previously filtered effluent fluid flows from the recirculation conduit outlet  30  it is directed to the peripheral region of the upstream surface of the sorbent bed. Thus, the peripheral region of the sorbent bed sees, at most, only a minuscule amount of contaminants. In comparison to traditional filter bed arrangements in which the peripheral region is the first region to experience breakthrough, the peripheral region of the filtration system of the invention is the last region to show breakthrough. 
   While the filter system  10  may be used to separate one or more sorbable fluids from other fluids, either liquids or gases, its greatest use may be to separate gaseous or volatile components from one another. Most typically, the filter system  10  is expected be used to separate one or more contaminant gases from another gas that is a major component of a gaseous mixture. Most applications are expected to be directed to removal of offensive gases, such as noxious or toxic gases, from air. Accordingly, the particular sorbent employed depends on the nature of the fluids being separated. When those fluids are gases, most typically, the sorbent is carbon; however, other sorbents that employ the same or similar sorbing mechanisms as carbon, or a molecular sieve, may be used. When the fluid being treated is air, carbon is generally the preferred sorbent, with some form of finely divided or activated carbon being most preferred. In some instances, to efficiently remove specific contaminants, the sorbent may be treated with a particular reagent reactive with the contaminant. The filter system  10  illustrated in  FIGS. 1 and 2  is exemplary of a system to purify gases such as air. 
   MODIFICATIONS 
   Reference is now made to the systems shown in  FIGS. 2–7 , which are similar to the system described above. 
   The difference in the system of  FIG. 2  is the addition of a high efficiency particulate air (HEPA) filter  32 , for retaining fine particulate matter, disposed upstream of the sorbent bed  22 , within and transverse to the inlet duct  18 . 
   The difference in the system of  FIG. 3  is the addition of a HEPA filter  32  and a pre-filter  34  placed upstream of the HEPA filter  32 , also within and transverse to the inlet duct  18  in case the presence of larger particulate matter is suspected in the contaminated fluid. 
   The difference in the system of  FIG. 4  is the addition of a diffuser  36  disposed within the sorbing chamber  12  intermediate the sorbent bed  22  and the blower  24  and transverse to the primary fluid flow path. The diffuser is used to thoroughly mix and distribute the effluent fluid after passing through the sorbent bed  22 . 
   The difference in the system of  FIG. 5  is the addition of a monitoring system  38  located within and transverse to the sorbing chamber  12  to determine whether breakthrough has occurred. The exact nature of the monitoring system may vary depending on the exact nature of the contaminant(s) present in the fluid being treated by the filtration system. Typically, a pipe tap could be located in the wall of the apparatus housing or sorbing chamber, fitted with an appropriate plug. When the system is monitored, the plug can be removed and a small amount of effluent fluid bled off and analyzed by appropriate electroanalytic means, such as a gas chromatograph. 
   The difference in the system of  FIG. 6  is the addition of a second sorbent bed  40  disposed within the sorbing chamber  12  downstream from the primary sorbent bed  22 . Typically, the orientation of the second sorbent bed  40  is the same as that of the primary sorbent bed  22 , transverse to the direction of fluid flow; however, in particular situations it may be desirable to employ other orientations, such as V-bed configurations. Although the primary sorbent bed  22 , in combination with the recirculation jacket  29 , is quite efficient in preventing breakthrough of contaminant during operation of the filtering system of the invention, the secondary sorbent bed  40  may be used to remove small traces of contaminants that might be found in effluent during the first few seconds of operation of the system or that might occur should the contaminants be present in extremely high concentrations upstream of the primary sorbent bed  22 . Either the primary sorbent bed  22 , when used alone, or when used in combination with the second sorbent bed  40 , may be impregnated with a reagent (e.g. triethylenediamine) known to react with a specific contaminant or class of contaminants, for example, blood agents. 
   The difference in the system of  FIG. 7  is the addition of a fluid shutoff mechanism  42  disposed transverse to the direction of fluid flow in the inlet conduit  18 , upstream of the sorbent bed  22 . This mechanism, taking the form of a valve or series of gates or locks, is provided to terminate all flow of contaminant-containing fluids in the event of a power disruption or blower malfunction so as to prevent any flow of unfiltered fluids through the recirculation jacket. 
   Obviously, numerous other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as described.