Pressure filter with a flow distributor and a method for washing the pressure filter

Abstract

A pressure filter for treating liquor suspensions of a cellulose pulp mill, the pressure filter including a container having tubular filtering elements suspended in a horizontal plate, through which elements filtrate flows and the interior of which communicates with a filtrate chamber located above. The filtering elements collect on their outer surface precipitate that is arranged to be periodically removed by making the filtrate in the filtrate chamber travel counter-currently through the filtration surface of the filtering elements. The filtering elements are cleaned by wash liquid passing from the filtrate chamber into the filtering elements and through their filtration surface. The interior of the filtering elements is provided with a flow distributor above the middle of a filtering element for distributing the flow from the filtrate chamber onto the filtration surface of the filtering element.

Claims

1. A pressure filter comprising: a container connected to an inlet conduit including an inlet configured to be coupled to a source of pressurized feeding liquor in a chemical pulp mill, and connected to an outlet configured to discharge a thickened suspension of the container; an impervious plate separating an upper chamber of the container and a lower chamber of the container, wherein the inlet and the outlet are aligned with the lower chamber; an opening in the impervious plate; a filter tube extending through and filling the opening, the filter tube including a sidewall, bottom and an upper region, wherein the opening is aligned with the upper chamber and at least a portion of the sidewall and bottom extending into the lower chamber is porous so as to be impervious to at least some solids in the feeding liquor and allow liquids in the lower chamber to flow into the upper chamber, and a flow distributor within an interior filter chamber defined by the sidewall of the filter tube, wherein the flow distributor has a cross-sectional area configured to create a backpressure in the interior filter chamber, wherein the flow distributor includes a circular plate having a diameter smaller than an inner diameter of the interior filter chamber such that an opening is formed between a perimeter of the circular plate and the interior filter chamber, and the circular plate abuts an inner wall of the interior filter chamber, and the flow distributor includes an elongated leg located in the interior filter chamber, the elongated leg includes an upper end section supporting the circular plate and a lower end section seated at the bottom of the filter tube such that the elongated leg stands on the bottom to support the circular plate, wherein the circular plate and the upper end section of the elongated leg are below an upper region of the filter tube and at an elevation above a middle elevation of the filter tube, and the upper end section of the elongated leg is below and not directly connected to the impervious plate.

2. The pressure filter according to claim 1 wherein the flow distributor includes protrusions extending outward from a perimeter of the circular plate, wherein the protrusions are arranged symmetrically around the perimeter.

Description

SUMMARY OF DRAWINGS

(1) The present invention is described in more detail by means of an embodiment of the filter according to the invention and with reference to the accompanying figures, in which

(2) FIG. 1 illustrates a pressure filter provided with tubular filtering elements.

(3) FIG. 2 illustrates a top view of an exemplary flow distributor.

(4) FIG. 3 illustrates a side view of the flow distributor shown in FIG. 2.

(5) FIG. 4 is a cross-section view of a flow distributor mounted in a filtering element.

DETAILED DESCRIPTION OF INVENTION

(6) FIG. 1 shows a filter container 2 having a horizontal plate 3 from which hangs tubular filtering elements 4. For illustrative purposes, only one filter element 4 is shown in FIG. 1 and this element is shown in a significantly enlarged view. A large number of filter elements 4, such as 200, may hang from the horizontal plate. The filter elements many be arranged in rows or another array arrangement.

(7) The fluid suspension to be filtered, e.g. such as lime milk, flows into the filter container 2 from a lime milk container (not shown) along inlet pipes 11, 13. A feed pump 15 applies pressure in the inlet pipe 11 to move the suspension to the filter container. While the suspension flows through the filter container, valve 19 is closed and valve 18 open.

(8) The lime milk enters an interior chamber of the filter container from the inlet pipe 13. The lime milk flows over the outer surface of each of the filtering elements 4. The outer surface is of a sock 5, e.g., a porous tube having an open end and a closed end. The sock 5 is fitted over a metallic tube 7 included in the filtering element 4. The sock 5 may be formed of a filtering cloth. The tube 7 may be perforated, mesh or otherwise porous. The tube provides a structural support for the sock and prevents the sock from collapsing. The metallic tube 7 is welded at its upper edge to a flange 8 which is mounted on the horizontal plate 3.

(9) White liquor from the lime milk passes through the porous surfaces of the socks and the metal tubes, and enters an interior region of each of the elements. By flowing through the sock and metal tubes, the while liquor is filtered from the lime milk. The white liquor flows through the socks and metal tubes 7 under prevailing pressure in the filter container 2. Solids, e.g., lime mud from the lime milk, are deposited as a layer 6 on the outer surface of the filtering sock 5. The filtrate, e.g., white liquor, flows up through the interior of each of the filtering elements 4, an open outlet of each element adjacent the flange 8 and into a filtrate chamber 12. The chamber is above the horizontal plate 3. White liquor flows from the filtrate chamber over an overflow edge 9 or other opening at the top of the filtrate chamber 12. From the edge 9 or other opening, the filtrate enters a chute 14 flows via pipe 17 for further treatment.

(10) Periodically the precipitate 6 on the outer surface of the filter sock is removed. The removal of precipitate 6 from the outer surface of the filter sock 5 may be accomplished as follows: The valve 19 is opened to divert flow from the pipe 11 and reduce pressure in the filter container 2. The hydrostatic pressure of liquid in the filtrate chamber 12 forces the filtrate, e.g., white liquor, to flow down into the interior of the filter elements and create a back pressure against the precipitate layer 6 on the socks. The back pressure releases the precipitate from the sock. The releases of the precipitate, also referred to as a cake-removal stage, may last for 2-10 seconds.

(11) After the cake removal stage, a settling stage is performed, such as for 30 seconds. In the settling stage, the valve 18 is closed and valve 19 remains open. The precipitate released from the socks 5 settles at the bottom the filtering container 2. The settled precipitate is removed via outlet conduit 20. A rotating blade 21 prevents the precipitate from sticking on the bottom of the container 2 and assists in moving the precipitate to the outlet conduit. After the settling stage and removal of the precipitate from the filtering container, the filter stage may be resumed by opening valve 18 and closing valve 19.

(12) The feed pump 15 may continually pump lime milk and need not be stopped while the lime mud precipitate is removed from the filtering elements. During the cake-release stage, the pump 15 speed may be slowed and the speed may be slowed further during the settling stage. Just prior to turning the valves 19, 18 for the filtering stage, the rotational speed of the pump 15 is increased to avoid no back flow of lime mud.

(13) The filter is periodically washed. To wash the filter, the filtering container 2 is emptied. Washing is accomplished using a similar process as the cake-release process in that fluid from the filtrate chamber 12 creates a back pressure inside each of the filtering elements 4. The washing liquid may be an acid or water which flows into the filter chamber via filtrate discharge conduit 17. Valves or pipe connections (not shown) direct the washing liquid to the filtrate discharge conduit 17. An acid wash may be repeatedly circulated through the filtrate chamber 12, filtering container 2, outlet 20, an container (not shown) and back to the filtrate discharge conduit 17. In a water-only wash, the water may flow once through the filter and need not be repeatedly circulated through the filtering elements.

(14) FIGS. 2 to 4 illustrate an exemplary flow distributor 22 to be located in a filtering element 4. In FIG. 2, the flow distributor is shown from the above. In FIG. 3 the flow distributor is shown from the side. FIG. 4 shows the flow distributor in a filtering element.

(15) The flow distributor 22 comprises a circular plate 23, the outer circumference of which is provided with protrusions 24, e.g. tabs that may be symmetrically arranged around the perimeter of the circular plate 23. The number of protrusions 24 may be three.

(16) The flow distributor 22 includes a rod 25 that serves as a support member. The middle point of the plate has an opening 26, through which the rod 25 is connected to the plate. The end of the rod has a protrusion 27 that keeps the rod in place over the plate and prevents the rod from sliding down over the rod.

(17) A flow distributor 22 is placed in each of the filtering elements 4 during the washing stage. The flow distributor is supported in a downward direction via the rod 25 extending down into the filter element and in a lateral direction via the protrusions 24 that seat on an inner wall of the filter element. The distal end of the rod seats on the bottom of the sock portion 5 of the filter element. The length of the rod 25 is such that the plate is positioned a desired elevation in the filtering element. The desired elevation is selected to position the plate immediately below an upper region of the sock portion of the filter element and above a mid-elevation (M) of the filtering element.

(18) A flow distributor may be inserting into each of the filtering elements during the washing stage or any other stage. The flow distributor 22 rests loosely in the filtering element. Gravity holds the flow distributor 22 in the filtering element. The flow distributor remains in the filtering element under the force of gravity, since during the filtering stage the flow velocity up through the interior of each of the filtering elements is slow.

(19) The diameter 28 of the plate is smaller than the inner diameter 10 (FIG. 1) of the filtering element. The diameter of the plate is 50-80%, preferably 60-75% of the inner diameter 10 of the filtering element. The diameter of the plate is such that the filtrate liquid passes through a generally annular opening between the plate and the inner surface of the interior of the filtering element. As the wash liquid flows into the interior of each filter element, the plate 23 creates a backpressure in the upper region of the interior of the filtering element. This back pressure assists in causing the solid precipitate on the upper outside surface of the filter elements to release during the wash stage.

(20) The flow distributor plate is mounted inside the filtering element distributes the flow during the cleaning stage of the filtering element. The flow distribution plate better to the upper part of the filtering element and as a result the filtering element is cleaned along its whole length more efficiently,

(21) Although the above description relates to an embodiment of the invention that in the light of the present knowledge is considered the most preferred embodiment, it is clear for a person skilled in the art that the invention can be modified in many different ways within the broadest possible scope defined by the appended claims only.

(22) While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.