IMPROVEMENTS IN OR RELATING TO FLUID FILTRATION

Abstract: Filtration apparatus has parallel filtration chambers (A, B, C, D) each of which has a filter screen and a cake of particulate material accumulated thereon for enhanced filtration. In normal use three chambers are in filtering operation, filtering fluid between an inflow conduit (100) and an outflow conduit (106); whilst one chamber, D as shown, is off-line. When another chamber, for example A, needs to have its filter screen de-caked it is taken out of operation and chamber D is brought into operation. After de-caking it is necessary to prepare the filter screen of chamber A for subsequent filtration and this is achieved by switching a fan (114) on, so that fluid filtered through chamber A is drawn through a recirculation conduit (108) and back into the inflow conduit (100). It cannot leave the apparatus until it has undergone effective filtration in one of the other filtration chambers. After an interval the filter screen in chamber A will have an adequate cake and is ready to be brought on-line.

Description: IMPROVEMENTS IN OR RELATING TO FLUID FILTRATION This invention relates to improvements in or relating to filtration of entrained particulate matter from a fluid. In EP-A-244938 there is described a multi-stage fluid filtration apparatus in which a fluid passes through a primary filtration stage, and then through secondary filtration stage, before passing to the outlet. There is described a filtration apparatus having a pair of parallel filtration chambers for the primary filtration stage, a pair of parallel filtration chambers for the secondary filtration stage, and one further filtration chamber, a standby chamber brought into operation when one of the other chambers is being cleaned. The filtration chambers are all identical. Thus if the filter screen area of one chamber is called A the total filter screen area of the apparatus is 5A whilst the filter screen area for the primary filtration stage is 2A, and the filter screen area for the secondary filtration stage, is also 2A. The filter screen ratio (which we define as the total filter screen area divided by the filter screen area used in the primary filtration stage, in normal operation) is 2.5. In EP-A-602100 there is described a multi-stage filtration apparatus in which there is a pair of identical filtration chambers. In normal use valves are arranged such that fluid passes through one filtration chamber, in which primary filtration occurs, then through the other filtration chamber, in which secondary filtration occurs, then to the outlet. Thus the filter screen ratio is 2.0. For cleaning, one of the filtration chambers is taken out of commission and all of the fluid passes through the other filtration chamber. In such prior art apparatus the normal operation is one in which the fluid undergoes primary and secondary filtration. In accordance with a first aspect of the present invention there is provided a method of filtering particulate material from a fluid using filtration apparatus having at least three filtration chambers, wherein fluid is divided into fluid streams passing through respective filtration chambers (hereinafter the "active chambers") arranged in parallel, the apparatus having a filtration chamber through which a fluid stream does not then pass (hereinafter the "standby chamber") , wherein in each chamber the filtration medium comprises a filter screen and a cake of said particulate material accumulated thereon, and wherein to clean one of the active chambers it is taken out of operation, and at substantially the same time the standby chamber is brought into operation. The term "parallel" as used herein, applied either to the filtration chambers or to the fluid streams, is not used in its mathematical sense, but in the sense that the fluid is divided into fluid streams. The filtration chambers need not be arranged side by side so that these fluid streams are geometrically parallel; although this will often be the most convenient installation arrangement. Preferred methods of the present invention includes the following procedures: - in normal filtration operation the fluid passing along the inflow conduit is split, to pass as parallel fluid streams through the active chambers, and thence into the outflow conduit. The fluid streams thereby undergo primary filtration. There is no secondary filtration. - when an active chamber is taken out of operation for cleaning a standby chamber having a cake of proven efficiency already on its filter screen is brought into operation. The fluid is split, to pass as parallel fluid streams through the remaining active chambers and the standby chamber, and thence to the outflow conduit. Again, there is no secondary filtration. - to bring the cleaned active chamber back into operation a fluid stream is passed through it as a primary filtration stage, but before this fluid stream is allowed to pass out of the apparatus it must undergo a secondary filtration stage. The entire fluid stream issuing from the active chamber being brought back into operation is passed through one or more of the other chambers. In certain embodiments the entire fluid stream issuing from the active chamber being brought back into operation is passed through one other chamber, preferably the said standby chamber. In other, preferred, embodiments the entire fluid stream from the active chamber being brought back into operation is passed back to the upstream region of the inflow conduit, where it mixes with the newly admitted fluid to be filtered. The latter embodiment preferably employs a said impeller means. Preferably the method employs apparatus which comprises a common inflow conduit feeding the respective upstream sides of the filtration chambers with parallel fluid streams, a common outflow conduit leading from the downstream sides of the chambers, a common recirculation conduit leading from the downstream sides of the chambers, the recirculation conduit leading towards the upstream sides of the chambers, impeller means to impel fluid through the recirculation conduit to the upstream sides of the chambers, and valve means downstream of each chamber, able to direct the fluid stream from the respective chamber into the outflow conduit, or into the recirculation conduit. Preferably the fluid passing through the recirculation conduit is fed into the inflow conduit rather than directly into one or more of the chambers. Preferably at the downstream sides of the chambers there are respective outlet branches which communicate selectively with the outflow conduit and with the recirculation conduit, the selection being under the control of the said valve means. In general terms if there are N chambers, with N being at least 3, one chamber will typically be a standby chamber, and N-l chambers will typically be active chambers, at any given time. N may, for example, be 3-7. Preferably N is at least 4. However, in filtration apparatus with a large number of filtration chambers, there may be more than one standby chamber at any one time. Suitably said active chambers are identical to each other. Said one or more standby chambers may differ from the active chambers, for example in terms of their cross- sectional area, and/or area of the filter screens. A standby chamber may be a dedicated standby chamber, in which case it may differ from the active chambers (but does not have to) , or it may be a designated standby chamber only for a particular time, at other times being designated as an active chamber, with another chamber taking over as a standby chamber. In such embodiments all of the filtration chambers are suitably identical. In relation to all aspects of the invention, the filter screen is rigid, and is of a material inert to the fluid undergoing filtration, and to the particulate material. The filter screen could be of plastics or ceramic material in certain environments but is preferably metallic. The filter screen could be a woven mesh or a perforated plate. A preferred material for the filter screen is stainless steel. The fluid, containing particulates to be removed, may be a liquid but is preferably a gas. Preferably it is a waste stream in which nitrogen gas is the primary carrier. For example, it may be a waste stream in which particulates are suspended in air, or it may be a combustion gas stream. In each case nitrogen gas is the primary carrier. The filtration chambers and the filter screens therein are suitably stationary, in use. In relation to all aspects of the present invention, cleaning can be carried out by taking a filtration chamber out of operation, by interrupting the fluid flow therethrough. The cleaning of the cake on the filter screen can in certain cases be carried out by mechanical means, for example by means of a mechanical shaker or a mechanical rub-down arrangement. In certain embodiments the cake may be removed from the filter screen by high- pressure water jets (this is not to be confused with the aspects expressed above relating to the increase in water content of the cake prior to cleaning) . In most embodiments, however, to achieve a partial cake removal the preferred way of cleaning filter screens is by use of compressed fluid, preferably compressed air. This may be delivered to the filter screen from the side which is downstream in normal use. Thus the method may include providing a reverse flow of fluid (i.e. from what is normally the downstream side, to the upstream side) , and there may be superimposed onto that reverse flow one or more fluid pressure pulses. Reverse flow may easily be provided for by means, for example a bypass conduit arrangement and valve means controlling the flow therein, whereby fluid already filtered by the apparatus can be received from the outflow conduit and delivered to the downstream side of the chamber to be cleaned, suitably via the recirculation conduit. It should be noted that references herein to "cleaning" and "removal" of a cake do not imply that there must be complete removal, to a bare screen. Indeed this is undesirable. In most cases, to provide a reasonable level of filtration when a filtration chamber is brought back into operation, a partial cake removal is preferred. An aim in such cases is to remove only a controlled amount of the cake. The apparatus may include means for increasing the water content of the cake, intermittently, continually or occasionally, in the method. Increasing the water content of the cake may have a number of benefits. Firstly, it may assist the build-up of the cake during the initial stages. Secondly, it may enhance the filtration efficiency of the cake. Thirdly, it may ease cake removal . Further aspects of the invention relate to filtration apparatus per se as defined and described herein. The invention will now be further described, by way of example, with reference to the accompanying drawings in which :- Figures 1 to 3 all relate to a first embodiment of filtration apparatus having three filtration chambers, the figures showing different flow arrangements through the apparatus; Figures 4 to 6 relate to a second embodiment having provision for reverse flow of fluid to aid cleaning; and Figures 7 to 10 relate to a third embodiment. The apparatus shown in Figures 1 to 3 comprises four filtration chambers A, B, C and D positioned side by side in a row. Within each chamber there is a stainless steel woven mesh as the filter screen (not shown) made up of tubular filter screen sections. Each filter screen is intended to have a cake of accumulated particulate material on it. An inflow conduit 100 has four inlet branches 102 leading respectively to the upstream side of each chamber, that is, upstream of the filter screen. On the downstream side of each chamber, that is, downstream of the filter screen, there is a corresponding outlet branch 104. Downstream of each outlet branch 104 there is a bifurcation, whereby fluid, typically combustion gases in the context of the particular apparatus described herein, flows either to an outflow conduit 106, or to a recirculation conduit 108. A flap valve 110 is provided at each bifurcation to determine whether fluid flows from a respective chamber into the outflow conduit 106 or the recirculation conduit 108. The recirculation conduit 108 is linked to the upstream end of the inflow conduit 100 by a return conduit 112. Located between the recirculation conduit 108 and the return conduit 112 is a fan 114. The operation of the apparatus will now be described, with reference to Figures 1 to 3. In Figure 1 the chambers A, B and C are shown in operation to filter the fluid passing through the apparatus. The respective valves 110 are set so that fluid flows through the outlet branches 104 of chambers A, B and C, and into the outflow conduit 106. The valve 110 for the chamber D is located in the alternative position and the fan is not running. This means that there is no flow of fluid through the chamber D. D is therefore designated as a standby chamber, but in this embodiment chambers A, B, C and D are identical and each chamber serves as a standby chamber at different times. Chambers A, B and C provide primary filtration. There is no secondary filtration. In Figure 2 chamber A is shown taken out of filtering operation, for cleaning. This is simply achieved by moving the valve 110 for chamber A to its alternative position to that shown in Figure 1. To maintain the filtering capacity of the apparatus chamber D is brought into filtering operation, this being achieved by the movement of the respective valve 110 for chamber D to its alternative position to that of Figure 1. The fan 114 is still not running. A fluid stream then passes through chamber D and into the outflow conduit 106. Chamber D already had on it a cake of proven efficiency from previous phases of the operation. Chambers B, C and D provide primary filtration. There is no secondary filtration. Once chamber A has been cleaned it is necessary to prepare its filter screen for the time when chamber B requires cleaning. It would be unacceptable simply to switch it back into operation then, and take chamber B out of operation. The filter screen within chamber A would not have a good cake of accumulated material. The filtration efficiency of chamber A would be inadequate and the overall filtration efficiency of the filtration apparatus would be reduced. Therefore, to prepare chamber A the apparatus is operated in a partial recirculation mode. Without altering the positions of the valves 110 from those shown in Figure 2 the fan 114 is operated. Chambers B, C and D remain in filtering operation. However a fluid stream passes through the filtration chamber A, and this fluid stream is then recirculated rather than expelled then from the apparatus. Thus, fluid to be filtered passes through the inflow conduit 100, and is split, to pass through each of the chambers A, B, C and D. The parallel fluid streams which pass through chambers B, C and D are diverted by the respective valves 110 into the outflow conduit 106. The fluid stream which passes through chamber A is diverted into the recirculation conduit 108 and then, via the fan 114 and return conduit 112, back into the inflow conduit 100. It then mixes at the upstream end of the inflow conduit 100 with more fluid to be filtered. Thus, the fluid stream which underwent primary filtration in chamber A, and only this fluid stream, must undergo secondary filtration. In this way an efficient cake may be built up on the filter screen in chamber A quickly and efficiently by a primary filtration stage, but with the fluid thereby filtered having to undergo a further filtration stage in chambers B, C or D before it can leave the apparatus. Once chamber A has a good cake the fan is switched off. The fan could be controlled by a time period, by a differential pressure switch judging the cake condition or preferably by use of an emission monitoring system (triboelectric or optical) . Chamber A can then be left until chamber B is to be cleaned. If wished a short further recirculation stage may be effected using chamber A, to check that its cake is still in good condition, immediately prior to taking chamber B out of operation. To take chamber B out of operation and bring chamber A into operation the valves 110 at the outlet sides of chambers A and B are switched to their respective alternative positions, from those shown in Figure 3, with the fan still off. Chamber A then provides primary filtration between the inflow conduit and the outflow conduit, as do chambers C and D. After cleaning, chamber B is operated in a recirculation mode, until it is ready, with a good cake on its filter screen. The fan is then switched off and chamber B is left out of operation until chamber C requires cleaning; then, chamber C is taken out of operation and chamber B is brought into operation; and so forth. The embodiment shown in Figures 4 to 6 is similar to that shown in Figures 1 to 3 but additionally has provision by means of a small amount of extra ductwork in the region of the fan, and two extra flap valves, for delivering fluid already filtered by the apparatus to the upstream side of any of the chambers, in order to assist cleaning. Instead of there being a single flow pathway between the recirculation conduit and the inflow conduit, in which pathway the fan is located (as in Figures 1 to 3), there is an additional bypass conduit 120, in which the fan 114 is located. This bypass conduit has a lower end connected to the return conduit 112, between the recirculation conduit 108 and the inflow conduit 100, and an upper end connected to a further conduit 122 colinear with the return conduit 112 and located between the return conduit 112 and the outflow conduit 106. Flap valves 124 are located at the upper and lower ends of the by-pass conduit . In Figure 4 the fan is off and the valve 124 at the upper end of the bypass conduit is in such a position that fluid cannot flow from the outflow conduit 106 to the inflow conduit 100. In Figure 5 the apparatus is shown configured for cleaning chamber A, this cleaning being assisted by reverse flow of fluid. The valves 124 are set such that fluid is drawn by the fan from the outflow conduit, through the bypass conduit 120, and into the recirculation conduit 108 in the reverse direction to that described above in relation to Figures 1 to 3. Valve 110 at the top end of chamber A is set such that a fluid stream is impelled through the chamber, in the reverse direction to normal . In Figure 6 the valves 124 are both in their alternative positions from those shown in Figure 5, and the fan is left running. Now, chamber A is simply being operated in its recirculation mode to build up its filter cake, in the same manner as was described with reference to Figure 3 of the first embodiment, but with fluid passing through the bypass conduit 120 and thence through the return conduit 112. By appropriate selections of the positions of valves 110 and 124, chambers B, C and D can be likewise subjected to reverse fluid streams. The third embodiment shown in Figures 7 to 10 does not have a fan and, unlike the embodiments described with reference to Figures 1 to 6 , has a dedicated standby chamber D, which may be different to the other chambers. Another difference from the embodiments of Figures 1 to 6 is that there are valves at the inlet sides of the respective chambers, butterfly valves 140 in the case of the active chambers A, B, C, and a flap valve 142 in the case of standby chamber D. A third difference is that there is a return conduit 144 leading from the end of the recirculation conduit 108 near the outlet branch from standby chamber D, to active chamber C, connectincj to a low position thereof. At the end of the conduit 144 adjacent to active chamber C there is a butterfly valve 146. In Figure 7 the normal operation of the apparatus is shown. The butterfly valves 140 are all open, to permit parallel fluid streams to pass through the active chambers A, B and C, from the inflow conduit 100 to the outflow conduit 106, the flap valves 110 on the downstream sides of the chambers A, B and C being configured for this. The position of valve 142 prevents fluid from entering standby chamber D. In Figure 8 standby chamber D has been brought into operation and active chamber A has been taken out of operation for cleaning, this being achieved by switching all four valves at the downstream and upstream sides of chambers A and D to their alternative positions, from those shown in Figure 7. The filter in standby chamber D already has on it a good cake from previous phases of the operation. The fluid stream passing through standby chamber D passes directly into the outlet pathway 106, because of the position of its downstream valve 110. After pulse cleaning and permitting the dust to settle for a delay period, active chamber A needs to be prepared to be brought back into operation, and this is achieved by setting the valves as shown in Figure 9, so that a fluid stream passes through chamber A in a primary filtration stage, the valves being set so that the fluid stream filtered in active chamber A is then delivered to standby chamber D, in which it undergoes a secondary filtration stage. Thus chamber D fulfils two functions, being a standby filtration chamber and a filtration chamber for use as a secondary filtration stage when any of the active chambers A, B and C need to be prepared for bringing back into operation, after cleaning. Once chamber A has a good cake on it, the valves are switched again so that chamber D is taken out of operation, and the filtration is achieved by means of primary filtration only in chambers A, B and C. That is to say, the operation is once again as shown in Figure 7. This operation is later repeated with chamber B, then with chamber C. During all of the operations described above with reference to Figures 7 to 9 , the butterfly valve 146 at the downstream end of return conduit 144 was closed. However from time to time standby chamber D needs to be cleaned, and after cleaning it needs to be prepared for later operations in which it must again filter a fluid stream, whilst one of the active chambers A, B or C is cleaned. To prepare its filter screen with a good cake in readiness for this the valves are set as shown in Fig. 10. Thus active chambers A and B filter as before; active chamber C does not receive a fluid stream for a primary filtration stage; standby chamber D does, but the fluid stream issuing from it passes, via the recirculation conduit 108 and return conduit 144, through the active chamber C, in which it undergoes a secondary filtration stage, before passing to the outflow conduit 106. In all of the embodiments of the present invention, the primary filtration is being carried out (sometimes with secondary filtration during cake build up; otherwise not) through three chambers, whilst altogether there are four chambers. Thus the filter screen ratios as defined above are 1.33. This is much more space-efficient than the earlier proposals described above, in which the respective filter screen ratios were 2.5 and 2.0. Yet, despite this, it is found that the embodiments of the present invention offer highly effective cleaning, even though secondary filtration is not being carried out as part of the normal filtering operation. Our improvements described herein in relation to cake building, namely use initially of a relatively low flow rate and then increasing this (e.g. by changing valves over slowly) , and/or the use of water to increase the rate of cake building, may alleviate the problem previously found, of low filtration efficiency in the early stages of cake building. However, by the methods described above we have determined that it is feasible to achieve a controlled, partial removal of a cake, from a filter screen. It does not have to be removed completely. In the past cakes have normally been removed substantially in their entirety, by application of a single pulse of fluid, for example air, at high pressure, from the downstream side of the filtration chamber. We have found that in many situations a number of fluid, preferably air, pulses of lower pressure may be useful in achieving a partial removal, optionally superimposed onto a reverse fluid flow, and that this may be done, in many installations, in a remarkably controlled fashion. For example if a cake is thicker than is optimal, it may be possible to determine a cleaning regime, of controlled pressure pulses, superimposed onto a reverse fluid flow, remove approximately three-quarters of the cake, leaving one quarter as a residual base layer. A similar effect may be achieved by intermittently resisting the outlet from the chamber. The use of water in the cleaning operation, as described herein, may assist in controlled, partial cake removal. This is significant because the filtration chamber can be brought back on line at, at the least, a reasonable level of filtration efficiency. The building up of the cake can then take place at a rapid rate, this being assisted because a base layer is already present, and by the delivery of water, and/or by the gradual increase of the flow rate through the filtration chamber, as described above.

Claims: CLAIMS 1. A method of filtering particulate material from a fluid using filtration apparatus having at least three filtration chambers, wherein fluid is divided into fluid streams passing through respective filtration chambers (hereinafter the "active chambers") arranged in parallel, the apparatus having a filtration chamber through which a fluid stream does not then pass (hereinafter the "standby chamber") , wherein in each chamber the filtration medium comprises a filter screen and a cake of said particulate material accumulated thereon, and wherein to clean one of the active chambers it is taken out of operation, and at substantially the same time the standby chamber is brought into operation. 2. A method as claimed in Claim 1, wherein the method includes the following procedures: - in normal filtration operation the fluid passing along the inflow conduit is split, to pass as parallel fluid streams through the active chambers, and thence into the outflow conduit, said fluid streams undergoing primary filtration and no secondary filtration; - when an active chamber is taken out of operation for cleaning and a standby chamber having a cake already on its filter screen is brought into operation, the fluid is split, to pass as parallel fluid streams through the remaining active chambers and the standby chamber, and thence to the outflow conduit, said fluid streams undergoing primary filtration and no secondary filtration; - to bring a cleaned active filtration chamber back into operation a fluid stream is passed through it as a primary filtration stage, but before this fluid stream is allowed to pass out of the apparatus it undergoes a secondary filtration stage, wherein the entire fluid stream issuing from the active chamber being brought back into operation is passed through one or more of the other filtration chambers. 3. A method as claimed in Claim 2, wherein the entire fluid stream issuing from the active chamber being brought back into operation is passed through the standby chamber. 4. A method as claimed in Claim 2, wherein the entire fluid stream from the active chamber being brought back into operation is passed back to the upstream region of the inflow conduit, where it mixes with the newly admitted fluid to be filtered. 5. A method as claimed in any preceding claim, wherein the method employs apparatus which comprises a common inflow conduit feeding the respective upstream sides of the filtration chambers with parallel fluid streams, a common outflow conduit leading from the respective downstream sides of the chambers, a common recirculation conduit leading from the respective downstream sides of the chambers, the recirculation conduit leading towards the upstream sides of the chambers, impeller means to impel fluid through the recirculation conduit to the upstream sides of the chambers, and valve means downstream of each chamber, able to direct the fluid stream from the respective chamber into the outflow conduit, or into the recirculation conduit. 6. A method as claimed in Claim 5, wherein a reverse flow of fluid may be impelled through a chamber to aid cleaning, the apparatus comprising a bypass conduit arrangement and valve means controlling the flow therein, whereby fluid can be received from the outflow conduit and delivered to the downstream side of the chamber to be cleaned, via the recirculation conduit. 7. A method as claimed in any preceding claim, wherein all of the filtration chambers are identical, and the standby chamber is a designated standby chamber only for a particular time, at other times being designated as an active chamber, with another chamber taking over as a standby chamber. 8. A method as claimed in any preceding claim, wherein the fluid is a gas. 9. Filtration apparatus per se for use in a method as claimed in any preceding claim. 10. Filtration apparatus having at least three parallel filtration chambers, in each of which chambers the filtration medium comprises a filter screen able to retain a cake of said particulate material thereon; a common inflow conduit leading to the upstream sides of the chambers; a common outflow conduit leading from the downstream sides of the chambers; a common recirculation conduit leading from the downstream sides of the chambers and leading towards the upstream sides of the chambers; impeller means to selectively impel fluid through the recirculation conduit to the upstream sides of the chambers; and valve means downstream of each chamber, able to direct the fluid stream from the respective chamber into the outflow conduit, or into the recirculation conduit.

Inventors: JONES CEDRIC RICHARD, NIEDERER KURT WILLY, JONES, CEDRIC, RICHARD, NIEDERER, KURT, WILLY
Applicants: JONES CEDRIC RICHARD, NIEDERER KURT WILLY, PARFTEC LIMITED, JONES, CEDRIC, RICHARD, NIEDERER, KURT, WILLY
Application year: 1997
Main classification: B01D35
IPC codes: B01D35, B01D46