APPARATUS FOR CONTINUOUSLY FILTERING A SLUDGE SUSPENSION

Abstract

Anapparatus for continuously filtering a sludge suspension, having a hollow shaft (2) that is rotatably mounted in a housing (1) and which that is fluidically connected to an inner chamber (6), which is surrounded by a filter membrane (5), of a discoid filter element (3) that radially protrudes from the hollow shaft (2), for removing a filtrate. A higher throughput of the filtrate, independently of the solids content of the sludge suspension to be filtered, is provided by the filter cake thickness and the filtration duration. The the radius (r) of the filter element (3) increases in the circumferential direction from a low pressure radius r.sub.i to a high pressure radius r.sub.h,reducing the free housing cross section.

An apparatus for continuously filtering a sludge suspension, having a hollow shaft (2) that is rotatably mounted in a housing (1) and that is fluidically connected to an inner chamber (6), which is surrounded by a filter membrane (5) of a discoid filter element (3) that radially protrudes from the hollow shaft (2), for removing a filtrate. A higher throughput of the filtrate, independently of the solids content of the sludge suspension to be filtered, is provided by the filter cake thickness and the filtration duration. The radius (r) of the filter element (3) increases in the circumferential direction from a low pressure radius r.sub.i to a high pressure radius r.sub.h, reducing the free housing cross section.

Claims

1. An apparatus for continuous filtration of a sludge suspension, said apparatus comprising: a hollow shaft rotatably mounted in a housing and flow-connected to the an inner chamber, said inner chamber being surrounded by a filter membrane having a discoid filter element that radially projects from the hollow shaft; wherein the discoid filter element has a radius that increases in the a circumferential direction from a low-pressure radius to a high-pressure radius so as to reduce the a free housing cross-sections; and wherein the filter element has a compression section defined by an increase of the radius of the filter element from the low-pressure radius to the high-pressure radius, and an expansion section defined by a decrease of the radius of the filter element from the high-pressure radius to the low-pressure radius; and wherein the decrease of the radius in the expansion section is faster than the increase of the radius in the compression section.

2. The apparatus according to claim 1, wherein the filter element is one of a plurality of filter elements arranged on the hollow shaft in an axial direction, the filter elements having respective high-pressure radii that are offset from one another in a circumferential direction.

3. The apparatus according to claim 2, wherein the the high-pressure radii between two of the filter elements following one another in the axial direction is are offset between 1° and 45°.

4. The apparatus according to claim 1, wherein the hollow shaft is one of at least two mutually parallel hollow shafts provided in the housing, each of the hollow shafts having the filter elements that are staggered with respect to one another in the axial direction.

5. The apparatus according to claim 4, wherein the filter elements of the mutually parallel hollow shafts at least partially overlap in the axial direction.

6. The apparatus according to claim 4, wherein the hollow shafts with the filter elements arranged thereon are arranged mirrored relative to one another about a common plane of symmetry.

7. The apparatus according to claim 1, wherein a pelletizing device driven by the hollow shaft is supported downstream of the filter elements.

8. The apparatus according to claim 1, wherein the filter element comprises a plurality of filter element segments detachably connected to one another.

9. The apparatus according to claim 8, wherein the filter element segments are of porous plastic.

10. The apparatus according to claim 1, wherein the filter membrane has two filter membrane disks arranged parallel to one another at a distance.

11. The apparatus according to claim 10, wherein the filter membrane disks are inserted into a fluid-impermeable base body of the filter element segment.

12. The apparatus according to claim 10, wherein the filter membrane disks are of porous plastic.

13. The apparatus of claim 10 wherein the filter membrane disks each comprise a ceramic core .

14. The apparatus according to claim 2, wherein the hollow shaft is one of at least two mutually parallel hollow shafts provided in the housing, each of the hollow shafts having filter elements that are staggered with respect to one another in the axial direction.

15. The apparatus according to claim 3, wherein the hollow shaft is one of at least two mutually parallel hollow shafts provided in the housing, each of the hollow shafts having filter elements that are staggered with respect to one another in the axial direction.

16. The apparatus according to claim 15, wherein the filter elements of the mutually parallel hollow shafts at least partially overlap in the axial direction; and wherein the hollow shafts with the filter elements arranged thereon are arranged mirrored relative to one another about a common plane of symmetry.

Description

BRIEF DESCRIPTION OF THE INVENTION

[0019] In the drawing, the subject matter of the invention is shown by way of example, wherein:

[0020] FIG. 1 shows an exposed top view of the apparatus according to the invention,

[0021] FIG. 2 shows a detailed view of the partially exposed apparatus in perspective view on an enlarged scale,

[0022] FIG. 3 shows a section extending along line III-III of FIG. 1 in enlarged scale,

[0023] FIG. 4 shows a section along line IV-IV of FIG. 3,

[0024] FIG. 5 shows a diagram of the pressure curve in the suspension along an axial flow line and in the filtered-out filtrate in the axial direction of the apparatus,

[0025] FIG. 6 shows a sectional view, corresponding to FIG. 3, of a second embodiment of the apparatus according to the invention,

[0026] FIG. 7 shows a vertical section through the filter elements of the second embodiment of the apparatus according to the invention, and

[0027] FIG. 8 shows a section along line VIII-VIII of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] As can be seen from FIG. 1, an apparatus for continuous filtration of a suspension has a housing 1 in which a hollow shaft 2 is rotatably mounted. As is disclosed in particular in FIG. 4, the hollow shaft 2 is flow-connected to a plurality of discoid filter elements 3 projecting radially from the hollow shaft 2. This can be achieved by the hollow shaft 2 being flow-connected via apertures 4 to an inner chamber 6 surrounded by a filter membrane 5. If a suspension flows against the filter element 3, the solid particles are retained by the filter membrane 5, while the liquid component penetrates the filter membrane 5, flows into the inner chamber 6 and leaves the apparatus as filtrate via the hollow shaft 2. The driving force of filtration is the pressure difference between the suspension side and the filtrate side. The higher this pressure difference, the faster the filtration. To further increase the pressure difference, an additional pump can be provided on the suspension side and/or a vacuum pump on the filtrate side. In the course of filtration, a filter cake forms on the filter membrane 5 due to the retained solid particles, which forms a flow resistance and therefore reduces the filtration performance or filtration efficiency with increasing thickness. Therefore, the filter cake must be cleaned off regularly. However, increasing the pressure on the suspension side, which is desirable in order to generate a large driving force for filtration, also results in compaction of the filter cake that has formed, making it more difficult to clean it off. Therefore, to enable a large driving force of filtration and thus a high filtration efficiency, the radius r of the filter element 3 increases in the circumferential direction from a low-pressure radius r.sub.l to a high-pressure radius r.sub.h to reduce the free housing cross-section, as disclosed in FIG. 3. As a result, the free housing cross-section changes as the filter elements rotate, causing the suspension at a space-fixed reference point to be subjected to a pressure profile that changes over time. As the free cross-section narrows, the pressure differential between the suspension side and the filtrate side increases, favoring rapid filtration. As the free cross-section increases, the pressure difference between the suspension side and the filtrate side decreases. Due to these constant pressure fluctuations, the filter cake is loosened and can detach from the filter membrane. The centrifugal force generated by the rotation of the filter elements 3 can further improve the cleaning effect. A rapid pressure drop can also promote cleaning. This can be achieved if the filter element 3 has a compression section 7 formed starting from the low-pressure radius r.sub.i by increasing the radius r of the filter element 3 to the high-pressure radius r.sub.h, and an expansion section 8 formed starting from the high-pressure radius r.sub.h by decreasing the radius r of the filter element 3 to the low pressure r.sub.i, wherein the decrease of the radius r in the expansion section 8 is faster than the increase in the compression section 7. Accordingly, the expansion section 8 occupies a smaller sector of the disk-shaped filter element 3 than the compression section 7.

[0029] In order to achieve a varying pressure distribution in the axial direction as well, several filter elements 3 can be arranged on the hollow shaft 2, the high-pressure radii r.sub.h of which are offset from one another in a circumferential direction. As can be seen from FIG. 3, a filter element 3 is always offset in the same circumferential direction to the previous filter element 3, so that the filter elements 3 form a spiral running around the hollow shaft 2. Particularly favorable conditions result when the offset of the high-pressure radii r.sub.h between two filter elements 3 following one another in the axial direction is between 1° and 45°. The offset can be 10°, for example, as shown in the exemplary embodiment. FIG. 5 shows the schematic pressure curve generated by the offset of the filter elements 3 along an axial exemplary flow line (not drawn) extending along the length of the apparatus at a specific point in time. While the pressure 9 on the filtrate side remains constant, the pressure 10 on the suspension side exhibits an undulating course due to the offset of the filter elements 3 in the axial direction and thus due to the different sizes of the free cross-sectional areas. The pressure increasing over the length is accompanied by the increasing solids content of the suspension. The areas with large pressure difference δp.sub.h favor a high filtration rate. The areas with small pressure difference δp.sub.l favor effective cleaning of the filter cake. In addition, the regular change in pressure differences creates turbulence, which further enhances the cleaning of the filter cake, resulting in an overall increase in filtration efficiency. It should be noted that the pressure curve shown in FIG. 5 corresponds to the pressure curve along a flow line at a fixed point in time. Due to the constant rotation of the hollow shaft 2, there is naturally a phase shift in the pressure curve over time.

[0030] As shown in particular in FIG. 2, at least two hollow shafts 2 extending parallel to each other can be provided in the housing 1, the filter elements 3 of which are offset in the axial direction towards each other to form a gap. As a result, particularly large housing cross-sections can also be used for high throughput without requiring special fabrications with regard to the size of the filter elements 3. This also allows geometrically more complex housing cross-sections to be used and fitted with filter elements.

[0031] The filter elements 3, which are staggered with respect to each other, can also overlap at least partially, forming an overlap area 11 which is variable in time by the rotation. As a result, a corresponding relative movement of the hollow shafts 2 can lead both to mutual shearing of the filter cakes of adjacent filter elements 3 located on the filter membranes 5 and to an increase in turbulence in the suspension. The filter elements 3 arranged according to the invention also act as a crushing or mixing unit for any inhomogeneities in the suspension.

[0032] In order that the suspension can also be actively and uniformly conveyed by the apparatus according to the invention, the hollow shafts 2, as disclosed for example in FIGS. 1 to 3, with the filter elements 3 arranged thereon, can be arranged relative to one another about a common plane of symmetry. With the hollow shafts moving accordingly at the same speed but in opposite directions, the suspension is actively pressed in the axial direction, which means that no further conveying devices are required.

[0033] For making up the filtered suspension, a pelletizing plate 12 can be used, which is arranged downstream of the filter elements 3 and is also driven by the hollow shaft 2.

[0034] The suspension can be fed in or discharged via connection pipes 13.

[0035] As can be seen from FIG. 6, a filter element 3 can comprise several filter element segments 14 in each case. The filter element segments 14 can be detachably connected to one another via a form fit to form the filter element 3. For example, the filter element segments 14 can be releasably connected to one another via a tongue-and-groove connection 15, 16 extending in the radial direction.

[0036] As can be seen from FIGS. 6 to 8, in particular from FIGS. 7 and 8, a filter element segment 14 can have a fluid-impermeable base body 17 into which two filter membrane disks 20 acting as cover or bottom surfaces 18, 19 are inserted. Together with the filter membrane disks 20, the base body 17 spans the cavity 6 of the filter element segment 14. The filter element segments 14 can be detachably flow-connected to the hollow shaft 2 via connecting nipples 21. The filter membrane disks 20 can in turn be detachably connected to the base body 17 via a tongue-and-groove connection 22, 23 (FIG. 8).

[0037] The filter membrane disks 20 may be made of porous plastic. In addition, the filter membrane disks 20 may comprise a ceramic core 24.