Abstract:
A method and apparatus for filtering gas ( 58 ) with a moving granular filter bed ( 48 ) involves moving a mass of particulate filter material ( 48 ) downwardly through a filter compartment ( 35 ); tangentially introducing gas into the compartment ( 54 ) to move in a cyclonic path downwardly around the moving filter material ( 48 ); diverting the cyclonic path ( 58 ) to a vertical path ( 62 ) to cause the gas to directly interface with the particulate filter material ( 48 ); thence causing the gas to move upwardly through the filter material ( 48 ) through a screened partition ( 24, 32 ) into a static upper compartment ( 22 ) of a filter compartment for exodus ( 56 ) of the gas which has passed through the particulate filter material ( 48 ).

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
   This application is based upon Provisional Patent Application Ser. No. 60/314,103 filed Aug. 22, 2001 and Provisional Patent Application Ser. No. 60/388,201 filed Jun. 12, 2002. 

   GRANT REFERENCE 
   Work for this invention was funded in part by a grant from the United States Department of Energy, Contract No. DE-FG26-99FT40588. The government may have certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   Counterflow of gas and particles is highly desirable in moving bed granular filters to achieve high dust removal efficiency from contaminated gas streams. In counterflows the dirty entering gas is scrubbed by the dirtiest particles and the clean exiting gas is scrubbed by the cleanest particles. However, moving bed granular filters that utilize counterflow typically have a low gas throughput to prevent bed material from leaving the filter when the minimum fluidization velocity of the granules is exceeded. 
   A disadvantage of the cyclonic flow of the gas is a disturbance of granules and collected dust at the interface between the gas and granular bed. Momentum transfer from the cyclonic gas flow imparts a swirling flow to the granules and collected dust, which makes the flow through the filter non-uniform and adversely imparts filtration efficiency. 
   The lack of durable, low-cost filters to clean high temperature gas streams is one of the primary obstacles to commercial introduction of advanced power systems based on coal and biomass. The two most promising filtration systems being investigated by government and industry are ceramic barrier filters and moving bed granular filters. 
   Ceramic barrier filters have several disadvantages: they must be periodically regenerated (blow back) to remove accumulated dust; they are fragile; and they are expensive. 
   Moving bed granular filters have been developed in several different geometries. These can be roughly characterized as parallel flow, counterflow, and crossflow filters. Parallel flow of gas and particles results in clean gas disengaging from dust-laden granules. Under these circumstances, dust can be entrained with the gas, which reduces the dust collection efficiency of the filter. Crossflow filters require very complicated tuyeres to inject dirty gas into the moving bed of granules. These are expensive and do not fully solve the dust carryover problem of gas disengagement. 
   Moving bed granular filters operate on the principle that a flowing bed of particles can effectively scrub particulate contaminant from a gas stream. Although very promising for achieving high filtration efficiencies, the relatively large footprint of the equipment and high throughputs of granular material as filter media are cited as drawbacks to moving bed granular filters. 
   It is therefore a principal object of this invention to incorporate these novel design features: A tangential gas inlet, a flow straightening section at the interface between entering gas and the granular bed, a screened gas disengagement section, and a diamond shaped insert. 
   These and other objects will be apparent to those skilled in the art. 
   SUMMARY OF THE INVENTION 
   The design of this invention consists of four main features: a tangential gas inlet, a flow straightening section; a gas disengagement section as shown in  FIG. 1 , and a diamond shaped insert to enhance filter material flow. The gas enters the filter through a tangential gas inlet, which imparts a cyclonic motion to the gas flow. 
   The filter of this invention employs a counterflow of gas and particles, which substantially eliminates the problem of dust carryover in the gas disengagement region. Upward flowing gas can fluidize the granular bed at sufficiently high flow velocities, which greatly reduces filter efficiency and must be avoided. In the instant design, this problem is eliminated by use of a screen at the disengagement interface between the exiting gas and the granular bed. The screen prevents the bed from expanding; thus, it is not able to fluidize. With the screen in place, the gas flow rate through the filter is increased by as much as 67%, which translates to smaller, more economical filters. 
   Another advantage of the filter of this invention compared to other moving bed filter designs is the use of a tangential gas inlet, which reduces pressure drop through the filter. Traditional gas inlets inject gas perpendicularly through the side of the filter housing, which results in a large irreversible loss of gas momentum. The tangential inlet converts linear momentum of the gas flow into a cyclonic flow. The preserved momentum of the gas reduces pressure drop by 28-45% compared to a conventional perpendicular gas inlet. 
   In an alternate design, a diamond-shaped insert is in the lower region of the housing to change the media flow pattern, which enhances the performance of the filter. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic vertical sectional view through the filter system of this invention; 
       FIG. 2  is an enlarged scale sectional view taken on line  2 - 2  of  FIG. 1 ; 
       FIG. 3  is an enlarged scale sectional view taken on line  3 - 3  of  FIG. 1 ; and 
       FIG. 4  is a view similar to that of  FIG. 1 , but shows an alternate embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1  the filter assembly  10  has a conventional supporting frame  12  which supports a filter compartment  14  which is cylindrical in shape. The filter compartment has a top  16  with a center opening  17 , a bottom  18 , and cylindrical side walls  20 . An upper compartment  22  serves as a gas disengagement region and has a bottom partition  24  with a center opening  26  directly below center opening  17  in top  16 . A vertically disposed hollow particle inlet conduit  28  extends downwardly through the center opening  17  of top  16  as well as through the opening  26  in bottom partition  24  ( FIG. 2 ). The opening  26  in bottom partition  24  is sufficiently large to provide a circular space  30  around inlet conduit  28  ( FIG. 2 ). A screen mesh  32  extends over space  30  and is connected to bottom partition  24  in any convenient manner. 
   A plurality of radially disposed horizontal fins  34  are located in the gas engagement section  35  of filter compartment  14  at a level below the lower end of the inlet conduit  28 . The bottom  18  of filter compartment  14  has tapered bottom conical side walls  36  which terminate at the lower end thereof in exit port  38 . Port  38  is sealed to auger boot  40 . A sealed auger assembly  42  is conventionally connected to auger boot  40  in a sealed manner, and conventionally extends upwardly and outwardly therefrom. The upper end of the auger assembly  40  is in sealing engagement with a conventional residue container  44  by means of a sealed but detachable connection  46 . 
   In  FIG. 1 , a filter material  48  is placed in supply bin  49  which is in connection with the upper end of inlet conduit  28 . The numeral  48 A ( FIG. 1 ) designates an inverted conically-shaped pile or mass of the filter material which has entered the gas engagement section  35  through the inlet conduit  28 . While the shape and size of the mass  48 A may vary in size and shape during the operation of the filter assembly  10 , it will always be sufficiently broad at its upper end to cover the complete area of the screen mesh  32 . It typically will extend downwardly below the radial fins  34  to create a gas engagement section  50  where the gas interfaces with the surface of the filter material  48 . It should also be noted that a sufficient quantity of the filter material  48  completely fills the bottom of a filter compartment  14 . The material in exit port  38 , boot  40 , and in auger assembly  42  effectively seals the bottom of the filter compartment to prevent the escape of any gas material through those components. A valve  52  in the bottom of the filter compartment is available for selective access into the filter compartment from the bottom as maintenance or the like may require. 
   A horizontally disposed gas inlet  54  is tangentially secured to the filter compartment  14  at a level below the bottom partition  24  of the upper compartment  22 . A horizontal gas inlet pipe  56  extends outwardly from upper compartment  22  and serves to exit filtered gas from the upper compartment  22  as will be described hereafter. 
   It should be understood that the filter material  48 , which is granular in nature and which will be described hereafter is continuously flowing downwardly through the filter compartment  14 . The material is not free falling through the filter, for its downward movement is controlled and is restricted by the auger assembly  42  which determines and regulates the discharge of the filter material from the filter compartment  14 . 
   In operation, gas under pressure is introduced into the gas engagement section  35  through inlet  54  after the filter material  48  has sufficiently filled the filter compartment  14  to the extend completely under screen  32  as shown in  FIG. 1 . Again, it is important to note that the filter material at that point in time covers the lower surface of screen  32  to prevent any gas material from flowing from the inlet  54  directly into the upper compartment  22  through the screen  32 . The arrows  58  designate the general cyclonic path of the injected gas from inlet  54  around the surface of the conical shaped pile  48 . This cyclonic path is created by virtue of the tangential location and direction of the gas inlet  54 . As the gas approaches the fins  34 , the cyclonic path of the gas is interrupted and is changed to a vertical direction as indicated by the arrows  60 . It is within the engagement section  50  that the gas interfaces with the filter material  48  and begins to migrate upwardly through the conical shaped pile or mass of filter material  48 A as indicated by the arrows  62 . The upwardly moving gas through the material  48 A will not penetrate the inlet conduit  28  because of the weight and downward movement of the filter material moving downwardly therethrough. Rather, the gas will proceed to the area of least resistance which causes it to move upwardly through the screen  32  into the upper compartment  22 . The gas is cleansed by the filter material as it moves upwardly against the downwardly flow of the filter material. Thus, the gas entering the upper compartment  22  and exiting through the conduit  56  is substantially cleaner than the gas that was entering the gas inlet  54 . 
   The three main features of the preferred embodiment of this invention, i.e., a tangential gas inlet, a flow straightening section, and a gas disengagement section are shown in  FIG. 1 . 
   Inside the filter compartment  14 , the gas flows swirls downward towards the interface between the gas and granular bed (see arrows  58  in  FIG. 1 ). By imparting cyclonic flow, the momentum of the gas is preserved, reducing pressure drop normally associated with sudden expansion into a filter. However, bed granules and accumulated dust cake on the surface of the bed would be disturbed unless the radial component of the gas flow is redirected axially before it reaches the bed surface. The gas is redirected by a flow straightening fins  34  above surface of the bed, which consists of sixteen evenly spaced fins distributed circumferentially in the annular space. The fins  34  effectively redirect the radial momentum of the flow axially downward (arrow  60 ) onto the gas-solid contacting or engagement region  50 . The flow-straightening region  50  effectively distributes the gas flow over the surface of the bed, which is important to the efficient utilization of the filter media. It is at this gas/solid interface that much of the gas cleaning takes place: dust particles entrained in the gas flow impact on granules, where they are effectively captured. The accumulation of dust particles on the granules and in the voids between granules forms a thin dust cake, which further aids in the capture of dust particles in the gas flow. 
   The gas flows upward (arrows  62 ) through the granular material, which is moving downward as a moving bed. This counterflow of gas and granules means that the dirty gas engages the bed where the granules are the dirtiest while clean gas disengages the bed where the granules are the cleanest. This counterflow results in very efficient gas cleaning. 
   The gas disengagement region (upper compartment  22 ) consists of a central pipe  28  that feeds granular material to the top of the moving bed and an annular region  35  where clean gas exits the bed through a screen. As previously indicated, gas does not flow into the inlet conduit  28  by virtue of the large gas flow resistance in that direction. Gas flowing upward in the upper compartment  22  around the feed pipe tends to expand the bed against the screen, which prevents the moving bed from transitioning to a fluidized bed even at very high gas flow rates. This constraint is important because a fluidized bed would not be as efficient as a moving bed for dust collection. 
   As gas flows through the filter, the granular bed is continuously flowing downward under the force of gravity. The granular bed flow rate is controlled using either the slide gate (valve  52 ) positioned at the bottom of the unit, or the auger boot  40  and auger assembly  42  as described above. 
   Certain tests were conducted to determine the feasibility of the invention. For the tests described here, all parts were constructed from ¼″ thickness polycarbonate plexiglass to give a clear view of bed fluid dynamics. The part dimensions and shapes are listed in Table 1 below: 
   
     
       
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Size and Shape of Design Parts 
             
           
        
         
             
                 
               Part 
               Size and Shape 
             
             
                 
                 
             
             
                 
               Gas Inlet 54 
                4″ × 4″ OD tube 
             
             
                 
               Filter Compartment 14 
               12″ × 4″ OD tube 
             
             
                 
               Upper Compartment 22 
                8″ × 4″ OD tube 
             
             
                 
               Fins 34 
                ¾″ × 3″ rectangle 
             
             
                 
               Inlet Conduit 28 
                4″ × 4″ OD tube 
             
             
                 
               Gas Exit 56 
                4″ × 4″ OD tube 
             
             
                 
                 
             
           
        
       
     
   
   Sixteen fins were evenly spaced around the gas disengagement section. The granular bed consisted of washed 4-mm diameter soda-lime glass beads assumed to be perfectly spherical. Household window screen was stretched between the inlet conduit  28  and gas disengagement section (upper compartment  22 ) to prevent fluidization of the granular bed. Air at 70° F. simulated the gas to be cleaned in the model filter. All parts were welded together using standard plexiglass solvent. 
   Other parts used in these tests include a sealed funnel, slide gate, and a 5-gallon container. The granular bed material was stored above the filter compartment in the funnel where it flowed downwardly into the granular inlet conduit by gravity. It then flowed into the gas disengagement section and through the slide gate valve into the 5-gallon container. 
   The main purpose of the above tests was to establish gas and granular flow characteristics of the filter assembly. In addition to visual observations of the granular flow, pressure drop in the gas flow between the gas inlet and gas exit was measured as a function of incremented superficial gas velocities. For the purposes of these tests, superficial velocity was defined as the volumetric flow rate divided by the cross sectional area of the annular interfacial area where inlet gas engages the moving bed. Bed depth, cross-sectional flow area, granular size and density, and gas travel distance were kept constant in these tests. 
   The pressure drop was recorded at each superficial gas velocity using three Magnahelix differential pressure gauges (2, 5, and 10 in. H 2 O). Three differential pressure gauges were used to increase recording accuracy at different gas flow rates. A vertical, in-line 0-606 ft/min (0-100 SCFM) rotameter, corrected for temperature and pressure, was used to measure volumetric flow rate of air. Prior to each test, the differential pressure gauges and rotameter were calibrated at zero to ensure accurate measurements. 
   Triplicate measurements were performed for superficial gas velocities between 0 to 606 ft/min in increments of 30 ft/min. At each gas flow rate, the granular bed material was allowed to flow for five minutes to reach steady state before the gas flow rate, differential pressure, and visual observations of moving bed behavior were recorded. 
   It was hypothesized that the formation of dust cake to be important for efficient dust collection, the entering gas flow must not disrupt the interfacial area between bed and gas. This was accomplished by letting granules flow out of a solids downcomer (inlet conduit) to spread out into a large, conical interface. Dirty gas flows downward through this interface, depositing dust, and then turns to flow upward through the downward flowing granular bed. 
   The gas disengagement region (upper compartment) also required a special configuration to allow high gas flows through the filter. The upward flowing gas induced a drag on the granules that causes the bed to expand and eventually fluidize, an undesirable behavior that limits gas throughput through the filter. A gas disengagement section (upper compartment) consisting of a small diameter feeder tube conveying granular material to a larger diameter was used. At low gas velocities, the granules from the feeder tube spread out into a conical pile. However, at high gas velocities, these particles expanded upward against an annular porous plate or screen that prevented their continued expansion. The screen allowed gas to exit the filter while retaining granular material. Gas did not enter the feeder tube by virtue of the large gas flow resistance in that direction. 
   The filter was constructed from 6.35 mm thick polycarbonate plexiglass to give a clear view of bed fluid dynamics. The body of the filter was 19.8 cm dia., the downcomer (inlet conduit) was 14.2 cm dia., and the feeder tube was 10.2 cm dia. The feed hopper ( 49 ) provided fresh granular material to the top of the filter. A discharge barrel, which was sealed against the atmosphere, accepted dust-laden granular material from the bottom of the filter. Air at 20° C. simulated the gas to be cleaned in the filter. The granular bed is comprised of washed 4-mm diameter soda-lime glass beads assumed to be perfectly spherical. The granular bed flow rate was controlled with a slide gate at the bottom of the filter. Bed depth, cross-sectional flow area, granular size and density, and gas travel distance were kept constant in these tests. 
   As previously indicated, a rotameter, corrected for temperature and pressure, was used to measure volumetric flow rate of air up to 2.8 m3/min. Pressure drop was recorded at each superficial gas velocity using three Magnahelic differential pressure gauges (5, 12.7, and 25.4 mm H 2 O). Three differential pressure gauges were used to increase recording accuracy at different gas flow rates. Triplicate measurements were performed for superficial gas velocities between 0-3.1 mls in increments of 0.15 m/s. At each gas flow rate, the granular bed material was allowed to flow for five minutes to reach steady state before the gas flow rate, differential pressure, and visual observations of the moving bed were recorded. In some experiments, titanium oxide smoke was injected into the air flow to visualize gas entering the filter and obtain a qualitative indication of filtration efficiency. 
   Qualitative observations were made on the behavior of granular material moving through the downcomer as a function of superficial gas velocity, which is defined as the volumetric flow rate divided by the cross sectional area of the annular region formed between the filter body and the inlet conduit. The filter was tested both with and without the retaining screen installed above the disengagement section. Observations on the behavior of the granular bed in the presence and absence of the screen are recorded in Tables A and B. Without the screen, granules at the surface of the bed in the disengagement section become agitated with increasing superficial velocity and eventually elutriate from the filter. It was estimated that the maximum operating velocity to be 0.9 mls without a retaining screen. When the screen was installed above the disengagement section, the granules expand against the screen but were not able to fluidize. It was estimated that the maximum operating velocity to be 1.5 m/s. The screen increased gas throughput by 67% compared to the filter operated without the screen. 
   
     
       
             
           
             
             
           
         
             
               TABLE A 
             
           
           
             
                 
             
             
               Performance of downcomer with retaining screen 
             
             
               in disengagement section 
             
           
        
         
             
               Superficial gas 
                 
             
             
               velocity (m/s) 
               Observations of downcomer flow 
             
             
                 
             
             
                0-0.3 
               Smooth downward granular bed flow with 
             
             
                 
               no fluidization. 
             
             
               0.3-0.6 
               Smooth downward granular bed flow. Bed material 
             
             
                 
               begins to expand upward. 
             
             
               0.6-0.9 
               Smooth downward granular bed flow. Full expansion 
             
             
                 
               of the bed with the upper region becoming fully 
             
             
                 
               fluidized. 
             
             
               &gt;0.9 
               Material in top half of downcomer fully fluidized; 
             
             
                 
               granular material entrained elutriated through the 
             
             
                 
               gas exit. 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
         
             
               TABLE B 
             
           
           
             
                 
             
             
               Performance of downcomer without retaining screen 
             
             
               in disengagement section 
             
           
        
         
             
               Superficial gas 
                 
             
             
               velocity (m/s) 
               Observations of downcomer flow 
             
             
                 
             
             
                0-0.5 
               Smooth downward granular bed flow with 
             
             
                 
               no fluidization. 
             
             
               0.5-0.8 
               Smooth downward granular flow. Bed beginning to 
             
             
                 
               expand against screen with slight fluidization on top. 
             
             
               0.8-1.5 
               Smooth downward granular flow. Full expansion 
             
             
                 
               against the screen with no fluidization present. 
             
             
               &gt;1.5 
               Top half of granular bed in downcomer is stationary 
             
             
                 
               and fully expanded against screen while bottom half 
             
             
                 
               becomes fluidized. 
             
             
                 
             
           
        
       
     
   
   By increasing gas throughput, the overall size of the filter is decreased resulting in reduced costs associated with manufacturing the unit. At the same time, the low pressure drop decreases the operating costs by reducing the energy required by a blower to move gas through the filter. Furthermore, even distribution of gas at the interface between entering gas and the granular bed assures that granules of bed material and dust collected on the granules is not disturbed, which would adversely impact filtration efficiency. 
   The gas throughput with and without the screen installed in the disengagement section of the filter was measured. Without the screen, the maximum gas flow occurred when the moving bed began to fluidize. With the screen installed, the maximum gas flow occurred when gas flow in the lower region of the bed became turbulent and disrupted the smooth downward flow of granules. As indicated, the screen increased gas throughput by 67% compared to the filter operated without the screen. Observations on the behavior of the granular bed in the presence and absence of the screen are recorded in Tables 2 and 3. 
   Without the screen, granules at the surface of the bed in the disengagement section become agitated with increasing fluidization velocity and eventually blowout of the filter. It is estimated that the maximum operating velocity to be 182 ft/min with a screen. When a screen was installed in the disengagement section, the granules expanded against the screen but were not able to fluidize. The maximum operating velocity was estimated to be 303 ft/min. 
   
     
       
             
           
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Granular Bed Effects in Gas Disengagement 
             
             
               Section Without Screen 
             
           
        
         
             
               Superficial Gas Velocity (ft/min) 
               Granular Bed Effects 
             
             
                 
             
             
               0-61 
               Smooth downward granular bed flow 
             
             
                 
               with no fluidization. 
             
             
               91-121 
               Smooth downward granular bed flow. 
             
             
                 
               Beginning to expand to top of gas 
             
             
                 
               disengagement flow. 
             
             
               152-182  
               Smooth downward granular bed flow. 
             
             
                 
               Full expansion of the bed with the 
             
             
                 
               uppermost regions becoming fully 
             
             
                 
               fluidized. 
             
             
               &gt;182 
               Top half of bed fully fluidized; 
             
             
                 
               granular material entrained and 
             
             
                 
               blown out of the filter. 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Granular Bed Effects in Gas Disengagement Section 
             
             
               with Screen 
             
           
        
         
             
               Superficial Gas Velocity (ft/min) 
               Granular Bed Effects 
             
             
                 
             
             
                0-91 
               Smooth downward granular bed flow 
             
             
                 
               with no fluidization. 
             
             
               121-152 
               Smooth downward granular bed flow. 
             
             
                 
               Bed beginning to expand against 
             
             
                 
               screen with slight fluidization on top. 
             
             
               182-303 
               Smooth downward granular bed flow. 
             
             
                 
               Full expansion against the screen 
             
             
                 
               with no fluidization present. 
             
             
               &gt;303 
               Top half of granular bed is stationary 
             
             
                 
               and fully expanded against screen 
             
             
                 
               with bottom half becoming fluidized. 
             
             
                 
             
           
        
       
     
   
   The two types of inlets investigated were a conventional gas inlet entering perpendicularly to the body of the filter and a gas inlet entering tangentially to the body of the filter with flow straightening fins. Both configurations employed screens in the disengagement sections. Pressure loss was significantly less for the tangential inlet than for the perpendicular inlet. At the maximum operable superficial gas velocity of 303 ft/min the pressure loss for the tangential inlet was 44% less than for the perpendicular inlet. 
   The momentum of the gas was better conserved using the tangential gas inlet. When the gas entered through the tangential gas inlet, the momentum of the horizontally flowing gas was efficiently converted to cyclonic flow in the body of the filter. On the other hand, when the gas was injected perpendicularly into the filter, the gas lost momentum as the flow is rearranged. 
   As previously indicated, in tests with the tangential inlet, flow-straightening fins were employed just above the gas engagement section to prevent scouring of the interfacial region of the bed, as subsequently described. Pressure loss was significantly less for the tangential inlet than for the perpendicular inlet. At the maximum operable superficial gas velocity of 1.5 m/s the pressure loss for the tangential inlet was 44% less than for the perpendicular inlet. 
   The behavior of granules at the surface of the moving bed where gas entered was observed as a function of superficial velocity. Observations for-the perpendicular and tangential gas inlets are recorded in Tables 4 and 5, respectively. 
   
     
       
             
           
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               Gas-Solid Contacting Region Effects Using 
             
             
               Perpendicular Gas Inlet and Screen 
             
           
        
         
             
               Superficial Gas Velocity (ft/min) 
               Gas-Solid Contacting Region Effects 
             
             
                 
             
             
                0-182 
               Interface is smooth with no scouring 
             
             
                 
               present. 
             
             
               212-303 
               Interface becomes scoured on side 
             
             
                 
               opposite of gas inlet indicating uneven 
             
             
                 
               gas distribution. 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
         
             
               TABLE 5 
             
           
           
             
                 
             
             
               Gas-Solid Contacting Region Effects using Tangential 
             
             
               Gas Inlet, Screen, and Fins 
             
           
        
         
             
               Superficial Gas Velocity (ft/min) 
               Gas-Solid Contacting Region Effects 
             
             
                 
             
             
               0-303 
               Interface is smooth with no scouring 
             
             
                 
               present. 
             
             
               &gt;303 
               Gas is evenly distributed as granular 
             
             
                 
               interface becomes slightly scoured 
             
             
                 
               completely around gas disengagement 
             
             
                 
               section. 
             
             
                 
             
           
        
       
     
   
   For the perpendicular inlet, the gas flow appeared to be uniformly distributed over the granular bed interface up to superficial velocity of 182 ft/min. However, above this velocity, the interface of the bed on the side opposite of the inlet was strongly scoured, with granules violently tossed about. On the other hand, the tangential inlet showed uniform gas distribution at velocities as high as 303 ft/min. 
   High momentum gas entering the perpendicular inlet was not able to smoothly negotiate the downward turn and becomes concentrated at a point on the surface of the bed immediately opposite the inlet, the scouring effect showed how the gas was unevenly distributed over the gas-solid contact region. On the other hand, the tangential inlet combined with flow straightening fins, efficiently turns the flow and distributes it over the granular bed interface. 
   Although the tangential inlet greatly reduces pressure drop in the filter, the cyclonic flow scours the surface of the granular bed, disrupting the formation of dust cake deposited by entering gas. The fins which were installed in the annular space formed below the inlet conduit did straighten gas streamlines flowing downward toward the engagement section of the granular bed. Qualitative observations were made of the bed surface as a function of superficial gas velocity for perpendicular and tangential gas inlets and recorded in Tables C and D, respectively. 
   
     
       
             
           
             
             
           
         
             
               TABLE C 
             
           
           
             
                 
             
             
               Behavior of granules at bed interface 
             
             
               with no fins in gas engagement region 
             
           
        
         
             
               Superficial gas 
                 
             
             
               Velocity (m/s) 
               Gas-solid contacting region observations 
             
             
                 
             
             
                 0-0.9 
               Gas engagement interface is smooth with 
             
             
                 
               no scouring. 
             
             
               0.9-1.5 
               Interface becomes scoured on side 
             
             
                 
               opposite of gas inlet. 
             
             
                 
             
           
        
       
     
   
   
     
       
             
           
             
             
           
         
             
               TABLE D 
             
           
           
             
                 
             
             
               Behavior of granules at bed interface 
             
             
               with fins in gas engagement region 
             
           
        
         
             
               Superficial gas 
                 
             
             
               Velocity (m/d) 
               Gas-solid contacting region observations 
             
             
                 
             
             
               0-1.5 
               Gas engagement interface is smooth with 
             
             
                 
               no scouring. 
             
             
               &gt;1.5 
               Gas is evenly distributed with little scouring. 
             
             
                 
             
           
        
       
     
   
   The counterflow moving bed granular filter design handles a high gas throughput and produces a low-pressure drop while evenly distributing the gas over the gas-solid contacting region. These effects are achieved by using three main design features: a tangential gas inlet, a flow straightening section, and a screened gas disengagement section. The cold flow tests indicate that these design features can improve the performance of moving bed granular filters. 
   DESCRIPTION OF ALTERNATE EMBODIMENT 
     FIG. 4  shows an alternate form of the invention which includes essentially the same structure contemplated by  FIGS. 1-3 , except that a diamond shaped insert  64  supported generally on fins  34  in the configuration of a plumb bob has an inverted conical shaped top  66  and a conical shaped bottom  68  with side walls  70 . The side walls  70  are spaced from the sidewalls  36  of bottom  18  to provide a conical shaped material passageway  72 . The shape of the insert  64  creating the passageway  72  beneficially changes the media flow pattern, which enhances the flow and performance of the filter material  48 . It also serves to reduce the change of the media in the filter which reduces start up time. More specifically, the presence of the diamond-shaped insert  64  serves to encourage mass flow of the filter media, thereby directing the filter media to the gas/media interfacial region  50 . Exchange of clean filter media for dirty filter media in the interfacial region  50  is critical for high efficiency filtration. 
   Among the advantages of the embodiments of the present invention are the following: 
   1. The disengagement section  22  employs a screen  32  to prevent the free expansion of the granular bed  48 A. This constraint prevents the bed  48 A from fluidizing, which would otherwise limit the maximum gas flow rate through the filter. 
   2. With use of flow-straightening fins  34 , the tangential gas inlet  54  provides a low-pressure drop as the gas enters a moving bed granular filter. This reduces pumping costs and provides a more energy efficient filter. 
   3. With use of flow-straightening fins  34  in combination with the tangential gas inlet  54 , the gas is evenly distributed over the gas-solid contacting region. This maximizes gas-granule contact time and provides even utilization of the granular bed. 
   4. The use of the diamond shaped insert  64  serves to beneficially change the media flow pattern, which enhances the flow of the filter material  48 . 
   It is therefore seen that this invention will accomplish at least all of its stated objectives.