WILSON, Kei th (125 Leland Avenue South, Thunder Bay, Ontario P7E 2N3, CA)
CLAIMS:
1. An anaerobic digester comprising: an elongate vessel having opposite first and second ends; an inlet defined at the first end of the vessel; an outlet defined at the second end of the vessel opposite the inlet; a shaft extending a distance along a longitudinal axis of the vessel between the opposite first and second ends thereof and being supported for driven rotation about said longitudinal axis; a plurality of mixing arms connected to the shaft at spaced positions therealong and extending outward therefrom toward peripheral walls of the vessel defined between the opposite ends thereof; and a plurality of discs supported within the vessel at spaced positions along the longitudinal axis thereof in orientations transverse thereto, the discs alternating between small diameter discs and large diameter discs along the longitudinal axis of the vessel, an outer diameter of each large diameter disc being greater than an outer diameter of an adjacent small diameter disc, the large diameter discs being annular to each define a central opening therein through which the shaft passes, the central opening of each large diameter disc being greater in diameter than the shaft passing therethrough, and each small diameter disc being fixed to the shaft for rotation therewith; the discs being arranged to force waste introduced into the vessel through the inlet to flow through the central openings in the large diameter discs and around the small diameter discs to the outlet.
2. The anaerobic digester according to claim 1 wherein the discs are parallel.
3. The anaerobic digester according to either one of claims 1 and 2 wherein the discs are perpendicular to the longitudinal axis of the vessel.
4. The anaerobic digester according to any one of claims 1 to 3 wherein the large diameter discs are fixed to the shaft for rotation therewith. 5. The anaerobic digester according to any one of claims 1 to 4 wherein each small diameter disc is sealingly fixed to the shaft to prevent any flow of waste therebetween.
6. The anaerobic digester according to any one of claims 1 to 3 wherein the inner diameter of each large diameter disc is less than the outer diameter of the adjacent small diameter disc.
7. The anaerobic digester according to any one of claims 1 to 6 wherein the mixing arms are grouped into sets, the mixing arms of each set being angularly spaced about the shaft from an adjacent set.
8. The anaerobic digester according to claim 7 wherein the sets of mixing arms are arranged with equal spacing about the shaft from one set to a next set along the shaft.. 9. The anaerobic digester according to any one of claims 1 to 8 wherein each mixing arm extends radially from the shaft.
10. The anaerobic digester according to any one of claims 1 to 9 wherein end discs of the plurality of discs adjacent the opposite ends of the vessel are defined by small diameter discs. 11. The anaerobic digester according to any one of claims 1 to
10 wherein bearings installed on the shaft for rotational support thereof are mounted only proximate opposite ends of the shaft.
12. The anaerobic digester according to claim 11 wherein the bearings are mounted outside the vessel. 13. The anaerobic digester according to either one of claims 11 and 12 wherein none of the bearings are mounted on the shaft between any two of the mixing arms.
14. The anaerobic digester according to any one of claims 1 to 13 wherein the vessel is mounted on a slope to situate the inlet at a greater elevation than the outlet with the inlet and outlet horizontally spaced apart, the anaerobic digester further comprising a crust breaker is carried on the shaft radially outward therefrom for rotation thereabout, the crust breaker extending along the shaft with a first end of the crust breaker adjacent the inlet being radially inward of a second end of the crust breaker adjacent the outlet relative to the shaft.
15. The anaerobic digester according to claim 14 wherein the crust breaker comprises a plurality of longitudinal members supported parallel to the shaft, the longitudinal members being spaced radially further from the shaft from the first end of the crust breaker to the second end thereof.
16. An anaerobic digester comprising: an elongate vessel having opposite first and second ends horizontally spaced apart, the vessel being and supported on a slope to situate the first end at a greater elevation than the second end; an inlet defined at the first end of the vessel; an outlet defined at the second end of the vessel opposite the inlet; a shaft extending a distance along a longitudinal axis of the vessel between the opposite first and second ends thereof and being supported for driven rotation about said longitudinal axis; a plurality of mixing arms connected to the shaft at spaced positions therealong and extending outward therefrom toward peripheral walls of the vessel defined between the opposite ends thereof; and a crust breaker carried on the shaft radially outward therefrom for rotation thereabout, the crust breaker extending along the shaft with a first end of the crust breaker adjacent the inlet being radially inward of a second end of the crust breaker adjacent the outlet relative to the shaft.
17. The anaerobic digester according to claim 16 wherein the crust breaker comprises a plurality of longitudinal members supported generally parallel to the shaft, the longitudinal members being spaced progressively radially further from the shaft from the first end of the crust breaker to the second end thereof.
18. The anaerobic digester according to claim 17 further comprising baffles installed at spaced positions along the shaft within the vessel to force waste introduced into the vessel through the inlet to deviate from a linear path through the vessel, each longitudinal member being positioned between a respective pair of the baffles along the shaft.
19. The anaerobic digester according to claim 18 the baffles comprise large diameter annular discs and small diameter discs carried on and alternately arranged along the shaft, which passes through the discs, the longitudinal members of the foam breaker each being positioned between a respective pair of adjacent large diameter discs outward of a respective small diameter disc positioned between the respective pair of adjacent large diameter discs relative to the shaft. |
ANAEROBIC DIGESTER
This invention relates generally to anaerobic digesters and more particularly to anaerobic digesters having baffles or bulkheads mounted therein to extend the time it takes waste to pass through the digester by diverting its flow from a linear path between the inlet and outlet and to anaerobic digesters installed at a slope to horizontally space apart the inlet and outlet with the inlet at a greater elevation than the outlet. BACKGROUND OF THE INVENTION
In the filed of anaerobic digesters, it is known to use an agitator within a horizontally oriented cylindrical vessel to ensure a complete mixing of the digester contents as they flow from the infeed end to the outfeed end of the vessel. The agitation process is used to move the food sources to the methane producing bacteria and vice versa. The agitator also promotes the movement of heavy indigestible solids to the outfeed end for extraction and breaks up the natural formation of crust at the liquid/gas interface. The agitator also prevents thermal and material stratification, thereby minimizing temperature differentials throughout the vessel. The substrate enters the digester vessel at the infeed end and is pushed through the digester by subsequent feedings. This is known as a "plug flow" operation. However, the "plug" must be broken up and thoroughly mixed with the existing contents of the vessel. This enables the methane producing bacteria within the vessel to access this new food source. The infeed "plug" must also be prevented from bypassing any portion of the vessel and shorten its journey. This would create inactive bacterial concentrations or "dead zones", thereby lowering the overall efficiency of the digester. There is also the possibility of an accelerated journey through the digester, which would result in a lower residence time. This shortened residence time within the vessel may consequently result in a higher pathogen content in the outfeed.
It is known in the art to install baffles or bulkheads within the interior of the anaerobic digester to force the material passing therethrough to take a winding or non-linear path through the digester to extend the time it takes for the waste to reach the digester outlet from the inlet at the opposite end of the digester vessel. It has been proven that the addition of bulkheads impedes the progress of the "plug" down the length of the vessel, thereby preventing "short-
circuiting" of the process. This ensures sufficient residence time for pathogen destruction. In an agitator or mixer equipped digester having a rotatable shaft running centrally along the vessel interior to rotably support a plurality of mixing utensils projecting outward from the shaft, it is known to form the baffles or bulkheads from flat plates fixed to the interior peripheral wall of the cylindrical vessel with the shaft passing through bearings mounted in the plates, the installation of the bearings within the baffles or bulkheads adding to the cost and complexity of the digester construction.
It is also known in the art to support an elongate member parallel to the agitator shaft over the length of a horizontal digester vessel at a radial distance outward from the shaft corresponding to the height of the waste passing through the vessel such that revolution of the elongate member about the shaft under driven rotation thereof will break up crust or foam that may form at the surface of the waste. However, tilting of a horizontal digester to create a downward slope from its inlet end to outlet end horizontally spaced therefrom to encourage solids toward the outlet, as also known in the art, will cause sloping of the waste level within the vessel relative to the bottom thereof along its length. This will reduce the effectiveness of the foam breaker structure described above, as the position of the shaft-parallel elongate member relative to the surface of the waste will vary over the length of the vessel. SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided an anaerobic digester comprising: an elongate vessel having opposite first and second ends; an inlet defined at the first end of the vessel; an outlet defined at the second end of the vessel opposite the inlet; a shaft extending a distance along a longitudinal axis of the vessel between the opposite first and second ends thereof and being supported for driven rotation about said longitudinal axis; a plurality of mixing arms connected to the shaft at spaced positions therealong and extending outward therefrom toward peripheral walls of the vessel defined between the opposite ends thereof; and
a plurality of discs supported within the vessel at spaced positions along the longitudinal axis thereof in orientations transverse thereto, the discs alternating between small diameter discs and large diameter discs along the longitudinal axis of the vessel, an outer diameter of each large diameter disc being greater than an outer diameter of an adjacent small diameter disc, the large diameter discs being annular to each define a central opening therein through which the shaft passes, the central opening of each large diameter disc being greater in diameter than the shaft passing therethrough, and each small diameter disc being fixed to the shaft for rotation therewith; the discs being arranged to force waste introduced into the vessel through the inlet to flow through the central openings in the large diameter discs and around the small diameter discs to the outlet
Having bulkheads or baffles of alternating disc and annular shapes promotes radial mixing and minimizes "dead zones" significantly improving the digester efficiency. Fixing the bulkheads to the central rotating agitator eliminates the need for bulkhead penetrations and internal shaft bearings. Preferably the discs are parallel.
Preferably the discs are perpendicuiar to the longitudinal axis of the vessel, Preferably wherein the large diameter discs are fixed to the shaft for rotation therewith.
Preferably each small diameter disc is sealiπgly fixed to the shaft to prevent any flow of waste therebetween.
Preferably the inner diameter of each large diameter disc is less than the outer diameter of the adjacent small diameter disc.
Preferably the mixing arms are grouped into sets, the mixing arms of each set being angularly spaced about the shaft from an adjacent set.
Preferably the sets of mixing arms are arranged with equal spacing about the shaft from one set to a next set along the shaft. Preferably each mixing arm extends radially from the shaft.
Preferably end discs of the plurality of discs adjacent the opposite ends of the vessel are defined by small diameter discs.
Preferably bearings installed on the shaft for rotational support thereof are mounted only proximate opposite ends of the shaft.
Preferably the bearings are mounted outside the vessel. Preferably none of the bearings are mounted on the shaft between any two of the mixing arms.
The vessel may be mounted on a slope to situate the inlet at a greater elevation than the outlet with the inlet and outlet horizontally spaced apart, the anaerobic digester further comprising a crust breaker is carried on the shaft radially outward therefrom for rotation thereabout, the crust breaker extending along the shaft with a first end of the crust breaker adjacent the inlet being radially inward of a second end of the crust breaker adjacent the outlet relative to the shaft. The crust breaker may comprise a plurality of longitudinal members supported parallel to the shaft, the longitudinal members being spaced radially further from the shaft from the first end of the crust breaker to the second end thereof.
According to a second aspect of the invention there is provided an anaerobic digester comprising: an elongate vessel having opposite first and second ends horizontally spaced apart, the vessel being and supported on a slope to situate the first end at a greater elevation than the second end; an inlet defined at the first end of the vessel; an outlet defined at the second end of the vessel opposite the inlet; a shaft extending a distance along a longitudinal axis of the vessel between the opposite first and second ends thereof and being supported for driven rotation about said longitudinal axis; a plurality of mixing arms connected to the shaft at spaced positions therealong and extending outward therefrom toward peripheral walls of the vessel defined between the opposite ends thereof; and a crust breaker carried on the shaft radially outward therefrom for rotation thereabout, the crust breaker extending along the shaft with a first end of the crust breaker adjacent the inlet being radially inward of a second end of the crust breaker adjacent the outlet relative to the shaft.
The crust breaker may comprise a plurality of longitudinal members supported generally parallel to the shaft, the longitudinal members being spaced progressively radially further from the shaft from the first end of the crust breaker to the second end thereof. There may be provided baffles instailed at spaced positions along the shaft within the vessel to force waste introduced into the vessel through the inlet to deviate from a linear path through the vessel, each longitudinal member being positioned between a respective pair of the baffles along the shaft.
The baffles may comprise large diameter annular discs and small diameter discs carried on and alternately arranged along the shaft, which passes through the discs, the longitudinal members of the foam breaker each being positioned between a respective pair of adjacent large diameter discs outward of a respective small diameter disc positioned between the respective pair of adjacent large diameter discs relative to the shaft. BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, which illustrate an exemplary embodiment of the present invention:
Figure 1 is a perspective view of an agitator assembly of an anaerobic digester according to a first embodiment of the present invention. Figure 2 is a side elevational view of the agitator assembly of Figure
1.
Figure 3 is an end elevational view of the agitator assembly of Figure 1.
Figure 4 is a schematic side elevational view of an anaerobic digester according to a second embodiment of the present invention. DETAILED DESCRIPTION
Figure 1 shows a first embodiment agitator assembly 10 for installation within an elongate cylindrical vessel of an anaerobic digester oriented to have its central longitudinal axis about which the cylindrical periphery of the hollow vessel closes extending generally horizontally, or at least in a more horizontal than vertical direction. Figure 4 shows a similar second embodiment anaerobic digester 100 in which the elongate cylindrical vessel 102 is oriented to
have a slight downward slope from its inlet end 104 to its outlet end 106 to encourage flow of material through the vessel.
The first embodiment agitator 10 and the second embodiment agitator 10' each feature solid small diameter discs 12 and annular large diameter discs 14 alternately arranged at spaced positions along a central shaft 16 supporting the rest of the agitator assembly's components for rotation about an axis along which the shaft extends, the discs being perpendicular to the shaft and coaxial therewith such that their peripheries revolve in a circular path about the shaft under rotation thereof. As shown in Figure 4 for the second embodiment, and similarly applying to the first embodiment, the round large diameter discs 14 have an outer diameter slightly smaller than the interior diameter of the hollow cylindrical vessel 102 and an inner diameter not exceeding the outer diameter of the small discs, so that material passing through the vessel 102 is forced to take a non-linear path through the vessel, needing to instead adopt inward and outward radial components of motion toward and away from the shaft to pass through the openings in the annular large diameter discs 12 and pass around the solid small diameter discs 14. Fixed to the shaft 16, the bulkheads or baffles formed by the discs rotate therewith, allowing the use of a shaft of continuous length without the bulkhead-carried bearing assemblies needed to support agitator shafts in fixed bulkhead constructions.
In the second embodiment, the agitator assembly 10' includes radially-offset foam-breaking longitudinal members 108 extending parallel to the shaft 16 at increasing distances therefrom moving toward the outlet end 106, so that each foam-breaking member 108 is situated proximate the generally horizontal upper level of the material flowing through the vessel 102, which is not parallel to the shaft 16 due to the sloped orientation of the vessel to encourage migration of solids toward the outfeed end. The offsetting of these foam-breaking members 108 forms a foam-breaker that extends substantially the full length of the vessel's interior from a position nearer the shaft 16 at the foam breaker's end adjacent the inlet end 104 than its position proximate the peripheral wall of the vessel at its end adjacent the outlet end 106 of the vessel 102, thereby acting under rotation of the shaft on which the foam-breaking members 108 are carried
to break up crust tending to form at the surface of the material passing through the vessel 102.
With reference to Figure 1 , at five equally spaced locations along the shaft 16, four radial arms 18 project radially from shaft, perpendicularly from four respective sides of the shaft's square cross section to form a cross shape having the four arms equally spaced around the shaft with right angles between the adjacent arms at each of these five locations. These five sets of arms 18 divide the agitator assembly into four sections 20 of equal length along the central shaft 16. Four outer longitudinal members 22 extend parallel to the shaft over nearly the fuil length thereof, specifically spanning the full distance between the two end sets of radiai arms 18 nearest the ends of the shaft 16, each of these outer longitudinal members 22 being fixed to a respective one of the four radial arms 18 at each of the five positions along the assembly at a distal end of this respective radial arm. At a central position along each of these sections 20, a respective one of the small diameter discs 12 is mounted to the shaft 16 in a manner generally sealed thereto prevent material from passing by the smaller disc 12 by flowing through any opening or gap that otherwise may be left between the disc 12 and the shaft 16. In the illustrate embodiments, where the central shaft 16 is continuous over the full length of the agitator, the reference to the disc 12 as being solid thus refers to all portions of the disc outward of its sealed connection to the shaft 16, unlike the annular larger diameter discs 14 each having a central opening significantly larger than the shaft's cross section 16 that is left open to define a flow passage between the shaft 16 and the large diameter disc 14. Except for the two outermost sets of four radial arms 18 adjacent to, but inward from, opposite ends of the shaft 16, each set of four radial arms has a respective one of the large diameter discs 14 adjacent to it, one planar face of the disc 14 being fixed to the set of radial arms and the outer periphery of the disc 14 being fixed at nintety degree intervals thereabout to the four outer longitudinal members 22. The positioning of the four small diameter discs 12 centrally along the four sections 20 of the assembly 10 and the positioning of the three large diameter discs 14 at the ends of the two central sections creates generally equal spacing between adjacent discs along the shaft 16. In the illustrated embodiments, the
iarge diameter discs share a common shape and dimension, as do the small diameter discs.
With reference to Figures 1 and 2, in each of the four sections 20 of the agitator assembly 10, there are provided a plurality of mixing arms 24 projecting radially outward from the shaft 16 on a common side thereof to a respective one of the longitudinal members 22 situated outward from that same side of the shaft 16. The mixing arms 24 of each section are parallel, coplanar and equally spaced from one another along the shaft 16. In each section, spacing along the shaft from each of the two sets of radial arms 18 at opposite ends of the section 20 to the respective plurality of mixing arms 24 is equal to the spacing between adjacent mixing arms 24 of that section, which also equals the spacing between adjacent mixing arms in each of the other sections. The mixing arms 24 of each section are angularly offset about the shaft 16 by ninety degrees from the mixing arms 24 of the adjacent section(s), the mixing arms of two sections on opposite sides of a section between them projecting in opposite directions along the same plane. Moving from one end of the shaft 16 to the other, the angular offsetting from each set of mixing arms 24 to the next is in the same direction about the shaft 16.
As the assembled agitator 10 is a single unit supportable for rotation at only two bearing supports disposed at opposite ends of the shaft 16 outward from the outermost sets of radial arms 18, stiffening members 26 are used to stiffen and strengthen the agitator assembly. Each stiffening member 26 extends from the distal end of one of the four radial arms 18 at one of the five positions along the shaft 16, at the connection of this radial arm to the respective one of the outer longitudinal members 22, to the opposite end of the corresponding one of the four radial arms 18 at an adjacent one of the five positions along the shaft. A pair of such stiffening members 26 is connected in this manner between two parallel radial arms at opposite ends of each section 20. These two stiffening members 26 are fixed to opposite sides of the two radial arms they extend between so as not to interfere with the installation of one another and cross one another approximately half way along the section 20, just radially outward from the respective small diameter disc 12, allowing the
peripehery of this small diameter disc 12 to be fixed to these crossing stiffening members 26 proximate their crossing.
With reference to Figure 2, any two of the agitator's stiffening members 26 connected adjacent the same end of a single radial arm 18 form part of a generally triangular arrangement completed by a base defined by either the shaft 16 or the respective outer longitudinal member 22 to which this radial arm is fixed, depending on whether these end-to-end stiffening members connect to the radial arm 18 near its inner or distal end. For example, with reference to Figure 2, members 26a and 26b are connected to the distal end of radial arm 18a. Member 26a defines a first side of the triangle extending obliquely outward from the shaft 16 along a radial plane with member 26b defining a second side extending obliquely along the same plane back toward the shaft 16 from its connection with the member 26a outward from the shaft at the distal end of radial arm 18a opposite the shaft. The triangle is closed by a third side or base defined by the shaft 16 along two sections 20a and 20b of the agitator spanned respectively by members 26a and 26b. Similarly, members 26b and 26c are connected to the inner end of radial arm 18a. Member 26c defines a first side of a second triangle extending obliquely inward toward the shaft 16 along the same radial plane with member 26d defining a second side extending obliquely back outward away from the shaft in the same plane from its connection with member 26d adjacent the shaft at the inner end of radial arm 18a. The second triangle is closed by a third side or base defined by the respective outer longitudinal member 22a along two sections 20a and 20b of the agitator spanned respectively by members 26c and 26d. These two triangular stiffening arrangements span the same sections of the agitator assembly between the shaft 16 and the same longitudinal member 22a, thus aligning along the shaft 16 to provide increased stiffness or strength over that which would be provided by one such arrangement alone. As the stiffening members 26 are arranged the same in all of the agitators sections 20, each shaft-based triangular arrangement overlaps with an adjacent shaft-based arrangement on the same side of the shaft by one section, just as each outer member based triangular arrangement overlaps with an adjacent outer member based arrangement relying on the same outer longitudinal member by one section, and non-overlapping triangular stiffening arrangements sharing
the same base-defining component (either the shaft or one of the outer longitudinal members) are arranged end-to-end along the shaft. The described use of stiffeners 26 also results in like-based stiffening arrangements being provided in sets of four that are aligned with one another along the shaft, sharing the same section 20, but offset from one to the next by ninety degrees about the shaft 16 with pairs of these like-based stiffening arrangements projecting to opposite sides of the shaft 16.
At each of the two end sets of radial arms 18 where no disc is installed, as these sets will be positioned proximate the inlet and outlet ends of the digester vessel as illustrated for the second embodiment in Figure 4, stiffening between the radial arms 18 is provided reinforcement members 28, each fixed to one radial arm proximate the distal end thereof and extending to linearly to similarly fasten to an adjacent radial arm, offset ninety degrees about the shaft 16 from the first as described herein above, proximate its distal end. No disc is provided at these end sets of radial arms due to their close proximity to the ends 104, 106 of the vessel 102 when installed, so as thereby not to obstruct the infeed and outfeed of material to and from the vessel interior 102. The endmost discs closest these opposite ends of the vessel interior are small diameter discs 12 in the illustrated embodiments. Figure 4 schematically illustrates the common way in which the first and second embodiment agitator assemblies are installed within a cylindrical vessel 102 that is circular in cross-section and elongated in the dimension corresponding to the central axis about which its periphery closes. The shaft 16 is supported at only two points adjacent its opposite ends positioned outward past the opposite ends 104, 106 of the cylindrical vessel 102. Bearing supports 110 house the shaft 16 at these points outside the vessel 102 with the shaft 16 extending along the central axis of the vessel to support all the small diameter discs 12, large diameter discs 14, radial arms 18, outer longitudinal members 22, mixing arms 24, stiffening members 26 and reinforcement members 28 inside the vessel 102 between its opposite ends 104, 106.
The second embodiment agitator assembly 10' illustrated schematically in Figure 4 is of substantially the same structure of the first embodiment agitator assembly 10 of Figures 1 to 3, except that the stiffener
members 26 are not provided in crossing pairs in the two end sections adjacent the opposite ends 104, 106 of the anaerobic digester vessel 102. More specifically, at the eπdmost sets of radial arms, only a single stiffener member 26e is fixed to each radial arm, specifically at its connection to the shaft 16 so as to extend obliquely outward therefrom to connect to the distal end of a corresponding radial arm at the adjacent set of radial arms at the opposite end of the same section. Also, the second embodiment differs in that the small diameter discs each have an outer diameter larger than the inner diameter of the annular large diameter discs, which may force a greater radial component of motion during passage of material around the small diameter discs 12 and through the annular large diameter discs 14 than the first illustrated embodiment, where the outer diameter of the small solid discs is generally equal to the inner diameter of the larger annular discs. The second embodiment agitator 10' features the addition of the longitudinal foam breaking members 108 extending parallel to the shaft 16 radially outward therefrom.
In the second embodiment, the cylindrical digester vessel 102 is not oriented completely horizontally along its longitudinal axis, but rather is sloped somewhat to situate its inlet end 104 at a slightly higher elevation than its outlet end 106 to encourage migration of solids toward the outfeed end 106. The generally horizontal upper surface of material passing through the digester is represented by iine 300 in Figure 4, illustrating that, due to the sloped orientation of the digester vessel 102, a greater amount of space exists between the material and the uppermost region of the vessel's cross section proximate the inlet end 104 than proximate the outlet end 106. The outer longitudinal members 22 act as foam breakers in the section of the agitator assembly adjacent the outlet end 106 of the vessel 102, where the upper surface 300 of the material in the digester is at its closest the to upper region of the vessel's interior periphery, as the outer longitudinal members 22 define the radially outermost parts of the agitator assembly and thus revolve about the shaft's axis in closest proximity to the digester's peripheral wall. As each outer longitudinal member 22 is driven through an uppermost arcuate portion of its revolving path about the shaft axis just inward from the peripheral vessel wall, it is forced to sweep through and generally along the upper surface
300 of the material in the digester, breaking up any crust or solidification tending to form or occur there. However, moving from this section toward the inlet end 104 of the vessel, the upper surface 300 of the material slopes generally linearly downward relative to the top of the vessel 102, and so the outer longitudinal members 22 overshoot the upper surface 300 of the material during the upper portion of their revolution about the rotating shaft 16 at these other three sections of the agitator, traveling instead in the empty space between the materia! and the top of the vessel interior. These other three sections are thus each equipped with four foam breaking longitudinal members 108, each extending parallel to the shaft 16 from a radial arm at one end of the section to the parallel corresponding radial arm at the other end of the section at a position radially inward from the outer longitudinal members 22. In each of these three sections, the four foam breaking members 108 are generally equidistant from the shaft 16 to share a common circular path about the shaft axis during rotation of the agitator assembly. Moving from the inlet end 104 of the vessel toward the outlet end 106, the foam breaking members 108 are situated further radially outward from the shaft 16 from section to section. It will be appreciated that the first embodiment agitator may be similarly equipped with a foam breaking arrangement when intended for installation in a sloped digester vessel. The agitators described herein operate in a horizontal or gently downward sloping tank with an integral, fuil length, continuous centre shaft supported by external bearings at each end. The agitator may be driven at both ends, for example by pneumatic actuators through a ratchet and pawl system. The agitator may also have outboard thrust bearings at both ends of the centre shaft. The radial fingers or mixing arms evenly spaced along the shaft length ensure every area of the vessel is swept by them. The outer longitudinal members at the extremities of these fingers or arms move the solids along the vessel bottom, with the second embodiment also featuring additional longitudinal members to break up the crust formation. The rotating bulkheads fixed to the agitator are of disc and annular configurations alternately and evenly spaced along the length of the shaft, ensuring a radial flow component and mixing of the substrate from the circumference of the vessel interior toward the vessel centerlϊne and back again.
In an alternate embodiment in which the shaft is still only supported outward from the mixing sections 20 of the agitator, the large diameter discs 14 may instead be stationary and fixed to the vessel 102, in which case the installed shaft 16 would extend through the openings in these annular discs 14 while the smaller diameter discs 12 would again rotate with the shaft as a result of being fixed thereto. Although the illustrated embodiments show four sections 20 each having nine mixing arms 24 between each pair of parallel radial arms at opposite ends of the section, it will be appreciated that the numbers and inter-member spacing of various components of the agitator assemblies of the illustrated embodiments may be varied while still featuring rotating baffles or bulkheads fixed to the shaft or foam breaking members spaced increasingly further from the shaft 16 moving from the inlet end of the digester to the outlet end.
The agitator may be made of welded together metal components, but it will be appreciated that other materials and fastening methods suitable to achieve an assembly of suitable strength may be known to those of skill in the art and readily applied to the agitators described herein.
Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.