Patent Publication Number: US-8974677-B2

Title: Fluid processing

Description:
CLAIM OF PRIORITY 
     This U.S. patent application is a continuation of and claims priority to U.S. patent application Ser. No. 12/702,602, filed Feb. 9, 2010, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application 61/151,053, filed on Feb. 9, 2009. The disclosures of each of the foregoing are considered part of the disclosure of this application and are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to fluid processing systems and related components and methods. 
     BACKGROUND 
     Mixing is a process in which two or more substances are combined while the chemical properties of each substance remain unchanged. The properties of the overall mixture, however, can differ from those of the component substances. Thus, mixing is often used to produce a medium with a desired set of physical and chemical properties. 
     For example, in semiconductor fabrication, chemical mechanical planarization (CMP) is used to make wafer surfaces flat. This process requires slurry of abrasive particles dispersed in a chemically corrosive agent. During CMP, movement of the abrasive particles on the wafer mechanically removes material from the wafer surface. The chemically corrosive agent of the slurry facilitates this mechanical removal of material by reacting with the material to be removed. To produce CMP slurry having desired properties, it can be useful to filter the CMP slurry to achieve the desired distribution of abrasive particles dispersed within the chemically corrosive agent. 
     SUMMARY 
     In one aspect of the invention, a fluid filtration apparatus includes a movable enclosure and a first plurality of filter housings disposed substantially within the movable enclosure. The first plurality of filter housings are in fluid communication with one another, and each filter housing is configured to support a filter therein to filter a substantially continuous flow of fluid. 
     In another aspect of the invention, a fluid filtration system includes a material feed, a flow controller, a pump, and a fluid filtration apparatus. The flow controller is in fluid communication with the material feed and a processing stream. The pump is in fluid communication with the material feed and the flow controller and is configured to move material from the material feed to the flow controller. The fluid filtration apparatus is in fluid communication with the pump. The fluid filtration apparatus includes a movable enclosure, a first plurality of filter housings, and a first plurality of filters. The first plurality of filter housings is disposed substantially within the movable enclosure and each filter housing of the first plurality of filter housings is in fluid communication with one another. Each filter of the first plurality of filters is supported in a respective filter housing to filter a substantially continuous flow of fluid. 
     Embodiments can include one or more of the following features. 
     In some embodiments, each filter housing is configured to be releasably detachable from the respective filter supported therein. 
     In certain embodiments, the filter housings are in fluid communication in series with one another to receive a substantially continuous flow of fluid. In some embodiments, the first plurality of filter housings is configured to support filters having different pore sizes. In certain embodiments, the plurality of filter housings are configured to support filters having progressively smaller pore sizes in the direction of the substantially continuous flow of fluid. 
     In some embodiments, each filter housing is configured to support a filter cartridge. In certain embodiments, each filter housing is configured to support an open-end cartridge. 
     In certain embodiments, the plurality of filter housings are arranged in parallel with one another such that each filter housing receives at least a portion of a substantially continuous flow of fluid. In some embodiments, the plurality of filter housings are configured to support filters having substantially the same pore sizes. 
     In some embodiments, the movable enclosure includes a plurality of wheels for moving the fluid filtration apparatus, the plurality of filter housings disposed substantially within the movable enclosure, above the plurality of wheels. In certain embodiments, at least one of the plurality of wheels is lockable in place. 
     In certain embodiments, the movable enclosure comprises at least one panel removable to provide access to at least one filter housing. 
     In some embodiments, the movable enclosure includes at least one door configured to provide access to at least one filter housing. In certain embodiments, the movable enclosure defines a substantially cuboid volume and the movable enclosure includes a door on more than one side (e.g., each side) of the cuboid. 
     In certain embodiments, the fluid filtration apparatus includes an inlet conduit passing through the movable enclosure for fluid communication with the first plurality of filter housings. In some embodiments, the fluid filtration apparatus includes a quick disconnect coupling disposed on the inlet conduit to establish fluid communication between the inlet conduit and a supply of a substantially continuous flow of fluid. 
     In some embodiments, the fluid filtration apparatus includes an outlet conduit passing through the movable enclosure for fluid communication with the first plurality of filter housings. In certain embodiments, the fluid filtration apparatus includes a quick disconnect coupling disposed on the outlet conduit to establish fluid communication between the outlet conduit and a processing stream for receiving a substantially continuous flow of fluid. 
     In certain embodiments, at least a portion of the movable enclosure is transparent. In some embodiments, at least a portion of the movable enclosure is poly(methyl methacrylate). 
     In some embodiments, the movable enclosure is dimensioned to fit within a size envelope with a height of about 50 inches, a width of about 40 inches, and a length of about 40 inches. 
     In certain embodiments, the fluid filtration apparatus includes a flexible tube extending between each of the filter housings to establish fluid communication between each of the filter housings. 
     In some embodiments, the fluid filtration apparatus includes a plurality of quick disconnect couplings disposed on each respective filter housing for connecting the filter housing to the respective filter tube. 
     In certain embodiments, each filter housing has an inlet portion and an outlet portion, each filter housing defines an elongate path between the inlet portion and the outlet portion, and each filter housing is configured to support the respective filter along the elongate flow path. In some embodiments, each filter housing is configured to receive a filter through at least one of the inlet portion and the outlet portion. In some embodiments, the fluid filtration apparatus, includes a stand disposed within the enclosure, the stand supporting each filter housing such that the elongate flow path of each housing is substantially parallel to a vertical axis. In certain embodiments, each filter housing is independently rotatable relative to the stand about a horizontal axis. In some embodiments, each filter housing is independently rotatable relative to the stand about a horizontal axis to form about a 45 degree angle between the elongate flow path of the respective filter housing and the vertical axis. In certain embodiments, the fluid filtration apparatus includes a plurality of sensors, each sensor disposed on a respective filter housing and in fluid communication with each respective filter housing. Each sensor extends away from the stand at an angle with respect to the respective elongate flow path. In some embodiments, the stand is rotatable about a vertical axis to move the first plurality of filter housings relative to the movable enclosure. In certain embodiments, the stand is lockable in place relative to the movable enclosure. 
     In some embodiments, each filter housing is polyvinyl chloride. 
     In certain embodiments, each filter housing is stainless steel. 
     In some embodiments, each filter housing defines a substantially cylindrical volume. In certain embodiments, the substantially cylindrical volume has an inner diameter of greater than about 2.5 inches and less than about 18 inches. In some embodiments, the substantially cylindrical volume has a length of greater than about 1 inch and less than about 50 inches. 
     In certain embodiments, each filter housing is configured to support a single open end filter cartridge. 
     In some embodiments, each filter housing is configured to support a double open end filter cartridge. 
     In certain embodiments, each filter housing is configured to support an O-ring type filter. 
     In some embodiments, the fluid filtration apparatus includes a second plurality of filter housings and a valve switch in fluid communication with each of the first plurality of filter housings and the second plurality of filter housings. The valve switch is configured to direct a substantially continuous flow of fluid between the first plurality of filter housings and the second plurality of filter housings. 
     In certain embodiments, the valve switch is a two-way valve. 
     In some embodiments, the fluid filtration apparatus includes a first plurality of sensors and a second plurality of sensors. The first plurality of sensors are each configured to measure a fluid parameter and each in fluid communication with a volume defined by a respective filter housing of the first plurality of filter housings. The second plurality of sensors are each configured to measure a fluid parameter and each in fluid communication with a volume defined by a respective filter housing of the second plurality of housings. 
     In some embodiments, the fluid filtration apparatus further includes a controller in electrical communication with the first and second plurality of sensors. The controller is configured to receive a measured fluid parameter from the sensors, and activate the valve switch based on a comparison between the measured fluid parameter and a threshold value. The activation of the valve switch redirects the substantially continuous flow from the first plurality of filter housings to the second plurality of filter housings. In certain embodiments, the controller is further configured to send an alarm signal to indicate the need for maintenance of the first plurality of filter housings. In some embodiments, the controller is further configured to indicate which filter housing of the first plurality of filter housings requires maintenance. In certain embodiments, the controller is further configured to compare the valve switch position to signals received from the plurality of sensors to determine an alarm condition. In some embodiments, each sensor is configured to measure pressure of fluid in the respective first and second plurality of filter housings. In certain embodiments, the controller is configured to determine the pressure drop across each filter supported in the respective filter housing. In some embodiments, each sensor is configured to measure large particle count of fluid in the respective first and second plurality of filter housings. In some embodiments, each sensor is configured to measure conductivity of fluid in the respective first and second plurality of filter housings. In certain embodiments, each sensor is configured to measure pH level of fluid in the respective first and second plurality of filter housings. 
     In certain embodiments, the fluid filtration apparatus further includes a plurality of filters, each filter supported in a respective filter housing to filter a substantially continuous flow of fluid. In some embodiments, each filter housing is configured to be releasably detachable from the respective filter supported therein. 
     In some embodiments, the first plurality of filter housings support filters having progressively smaller pore sizes in the direction of the substantially continuous flow of fluid. In certain embodiments, each filter has a pore size of greater than about 0.05 microns and less than about 200 microns. In some embodiments, each filter has a pore size of greater than about 0.1 microns and less than about 10 microns. In certain embodiments, each filter has a pore size of greater than about 0.2 microns and less than about 1 micron. 
     In some embodiments, the pump includes a diaphragm pump. In certain embodiments, the diaphragm pump is a bladder pump. In some embodiments, the diaphragm pump is an electro-mechanical diaphragm pump. 
     In certain embodiments, the flow controller is an adjustable orifice. 
     In some embodiments, the flow controller is a U-tube flow controller. 
     In certain embodiments, the fluid filtration system includes a plurality of flow controllers. Each flow controller is individually adjustable to achieve a target flow rate of the substantially continuous flow of fluid. 
     In some embodiments, the fluid filtration system further includes an in-line mixer in fluid communication with the pump and the fluid filtration apparatus. The in-line mixer is disposed between the pump and the fluid filtration apparatus. 
     Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view of a slurry blending plant with a filter station disposed between a product packaging station and a series of in-line mixers. 
         FIG. 2  is a schematic view of the filter station of  FIG. 1 . 
         FIG. 3  is a schematic of a filter housing of the filter station of  FIG. 1  tilting about a horizontal axis during filter replacement. 
         FIG. 4  is a schematic of a stand of the filter station of  FIG. 1  rotating about a vertical axis during repositioning of the filter station. 
         FIG. 5  is a schematic of inputs to and outputs from a controller of the plant of  FIG. 1 . 
         FIG. 6  is a flow chart of a process used to control the position of a valve switch of the filter station of  FIG. 1  based on pressure measurements received from pressure sensors in communication with a continuous flow of slurry through the filter station. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-3 , a continuous slurry blending plant  10  includes a mixing station  12 , a filter station  14 , and a product packaging station  31 . The filter station  14  is positioned along a processing stream  29 , between the mixing station  12  and the product packaging station  31 . As described below, the filter station  14  includes a first filter set  30 , a second filter set  32 , and a valve switch  34  in fluid communication with the first and second filter sets  30 ,  32  to direct a continuous flow of slurry to either the first filter set  30  or the second filter set  32 . 
     During use, the mixing station  12  mixes raw materials to form a substantially continuous flow of slurry that moves toward the filter station  14 , along the processing stream  29 . At the filter station  14 , cartridge filters  49  positioned in the first and second filter sets  30 ,  32  remove agglomerated particles and/or impurities from the slurry. The filtered slurry then flows to the product packaging station  31  where, for example, the slurry can be used in CMP of wafers or packaged for use at another location. As described below, the blending plant  10  controls the position of the valve switch  34  such that maintenance (e.g., filter changes) can be performed on one filter set (e.g., the first filter set  30 ) while slurry continues to flow through the other filter set (e.g., the second filter set  32 ). The on-line maintenance facilitated by filter station  14  can reduce the amount of operational downtime required for maintenance. The on-line maintenance facilitated by the filter station  14  can also improve the yield of wafer production, for example, by reducing the likelihood that agglomerated particles will reach and interfere with downstream processes performed at the product packaging station  31 . 
     The substantially continuous processing carried out by the blending plant  10  can have a higher volume throughput and/or require less space than a plant relying on batch processing. Additionally or alternatively, by reducing start and stop operation characteristic of batch processing, the blending plant  10  reduces slurry shearing and, thus, reduces the likelihood of particle agglomeration resulting from such shearing. As also described below, the filter station  14  is movable to facilitate assembly and/or reconfiguration of the blending plant  10 . 
     The mixing station  12  includes material feeds  13 ,  15 ,  16 ,  17 , pumps  18 ,  19 ,  20 ,  21 , flow controllers  22 ,  23 ,  24 ,  25 , and mixers  27 . Each pump  18 ,  19 ,  20 ,  21  is disposed between a respective material feed  13 ,  15 ,  16 ,  17  and a respective flow controller  22 ,  23 ,  24 ,  25 . The mixers  27  are in-line pipe mixers positioned in series along the processing stream  29 , and each mixer  27  includes baffles for inducing swirl (e.g., dual swirl) into the flow of materials introduced into the processing stream  29  through the flow controllers  22 ,  23 ,  24 ,  25 . Each mixer  27  is positioned downstream of a corresponding flow controller  22 ,  23 ,  24 ,  25  such that there is a respective mixer  27  downstream of each material feed  13 ,  15 ,  16 , and  17 . 
     Material feeds  13 ,  15 ,  16 ,  17  each provide an inlet for receiving raw material to be combined into slurry. Material feed  13  receives the chemically corrosive agent that acts as the dispersion medium of the slurry. Slurry components, such as abrasive particles, are introduced downstream, through material feeds  15 ,  16 ,  17 . A single raw material can be introduced through more than one of the material feeds  13 ,  15 ,  16 ,  17  to stratify the raw material along the processing stream  29  which can, for example, improve mixing quality. 
     Each pump  18 ,  19 ,  20 ,  21  moves a raw material from a respective material feed  13 ,  15 ,  16 ,  17  through a respective flow controller  22 ,  23 ,  24 ,  25 . Pumps  18 ,  19 ,  20 ,  21  can be electro-mechanical diaphragm pumps including a sealed diaphragm with one side in fluid communication with the working fluid and the other side in communication with a motor drive. Fluid is pumped as the motor drive flexes the diaphragm. 
     Each flow controller  22 ,  23 ,  24 ,  25  includes an adjustable orifice and an internal regulating valve that maintains a constant pressure drop across the orifice to achieve a constant volumetric flow rate. The volumetric flow rates through the flow controllers  22 ,  23 ,  24 ,  25  are independently adjustable such that the raw materials can be combined in desired proportions. For example, the volumetric flow rates through the flow controllers  22 ,  23 ,  24 ,  25  can be adjusted to achieve a target concentration of abrasive particles dispersed within the chemically corrosive agent. 
     Referring to  FIGS. 2-3 , the filter station  14  includes the first filter set  30 , the second filter set  32 , the valve switch  34 , a stand  36 , an enclosure  38 , an inlet conduit  42 , and an outlet conduit  44 . The filter sets  30 ,  32  are supported by the stand  36  within the enclosure  38 . The inlet conduit  42  and the outlet conduit  44  each pass through the enclosure  38  to allow the substantially continuous flow of slurry to pass through the filter station  14 . The inlet conduit  42  and the outlet conduit  44  each include a respective quick disconnect coupling  43  that can facilitate connection and disconnection of the filter station  14  to the processing stream  29 . Each filter set  30 ,  32  is in fluid communication with the valve switch  34  and the outlet conduit  44 . The valve switch  34  is in fluid communication with the inlet conduit  42  such that, during use, the substantially continuous flow of slurry flowing through the inlet conduit  42  can be directed through either filter set  30 ,  32  for filtering. 
     The enclosure  38  is substantially cuboid and includes wheels  58 . The cuboid shape can facilitate assembly, disassembly, and/or partial disassembly of the enclosure  38 . Of course, the enclosure  38  can be shapes other than cuboid. For example, the enclosure  38  can be cylindrical, pyramidal, conical, frusto-conical, and/or spherical. 
     The enclosure  38  has a size envelope of less than approximately 50″×40″×40″. Filter station  14  can be moved on wheels  58  as necessary to construct plant  10  within an available footprint. The enclosure  38  can protect filter sets  30 ,  32  from damage as the filter station  14  is moved. Additionally or alternatively, the enclosure  38  can protect nearby operators and equipment during operation of the plant  10 . The enclosure  38  is transparent plastic (e.g., poly(methyl methacrylate)) to facilitate visual inspection of filter station  14  during operation. The volume defined by the enclosure  38  is accessible from each side and from the top (e.g., through removable panels and/or doors) to facilitate access to components disposed within the enclosure  38  (e.g., filter sets  30 ,  32 ) during maintenance. 
     The filter sets  30 ,  32  are substantially identical. For the sake of clarity, components specific to filter set  30  are identified by reference numerals ending in “a,” and components specific to filter set  32  are identified by reference numerals ending in “b.” 
     Each filter set  30 ,  32  includes three elongate filter housings  46   a,b ,  47   a,b ,  48   a,b , three pressure sensors  50   a,b ,  51   a,b ,  52   a,b , and three cartridge filters  49 . Each filter housing  46   a,b ,  47   a,b ,  48   a,b  has an inlet portion  54  and an outlet portion  56  and defines an elongate flow path therebetween. For clarity of illustration, the inlet portion  54  and the outlet portion  56  is shown only for the filter housing  46   a . Filter housings  46   b ,  47   a,b ,  48   a,b  have analogous inlet and outlet portions which may be reversed with respect to the inlet portion  54  and the outlet portion  56  of the filter housing  46   a . For example, the inlet portion of filter housing  47   a  may be arranged proximate to the outlet portion  56  of filter housing  46   a.    
     A respective cartridge filter  49  is removably disposed along the elongate flow path defined by each filter housing  46   a,b ,  47   a,b ,  48   a,b . Each pressure sensor  50   a,b ,  51   a,b ,  52   a,b  is in fluid communication with a respective elongate flow path defined by a respective filter housing  46   a,b ,  47   a,b ,  48   a,b  to measure the pressure drop of the substantially continuous flow of slurry passing through the cartridge filter  49  disposed in the filter housing. For example, the pressure drop across filter housing  46   a  can be measured as the difference between the pressures measured at pressure sensors  50   a  and  51   a . Each measured pressure drop can indicate that the cartridge filter  49  disposed within the respective filter housing  46   a,b ,  47   a,b ,  48   a,b  is clogged and/or otherwise rendered ineffective for filtering the slurry. 
     The cartridge filters  49  are substantially cylindrical with a length of about 30 inches and an outer diameter of about 2.5 inches such that each cartridge filter  49  substantially fills the dimensions of the elongate flow path of each filter housing  46   a,b ,  47   a,b ,  48   a,b . The cartridge filters  49  can be single open end, double open end, and/or O-ring type filters. The pore size of the cartridge filters  49  can be greater than about 0.05 microns and/or less than about 200 microns (e.g., greater than about 0.1 microns and/or less than about 10 microns, greater than about 0.2 microns and/or less than about 1 micron). 
     Within each filter set  30 ,  32 , the respective filter housings  46   a,b ,  47   a,b ,  48   a,b  are in fluid communication in series. For example, when the valve switch  34  directs slurry through a given filter set  30 , the slurry enters filter housing  46   a , runs through filter housings  46   a ,  47   a ,  48   a  in succession, and then exits filter housing  48   a  into the outlet conduit  44 . In this successive arrangement, the cartridge filters  49  disposed in respective filter housings  46   a,b ,  47   a,b ,  48   a,b  have progressively smaller pore sizes such that the pore size of the cartridge filter  49  disposed in filter housing  47   a,b  is less than the pore size of the cartridge filter  49  disposed in the respective filter housing  46   a,b  and greater than the pore size of the cartridge filter  49  disposed in the respective filter housing  48   a,b . This arrangement of successively less porous cartridge filters  49  can allow the more porous cartridge filters  49  to remove particles that could otherwise damage and/or significantly clog the less porous downstream filter(s). Thus, this successive arrangement of cartridge filters  49  can increase the amount of time between filter changes, resulting in both time and cost savings. 
     In each filter set  30 ,  32 , the respective filter housing  46   a,b  is in fluid communication with the valve switch  34  through flexible tubing extending from an outlet of the valve switch  34  to a quick disconnect coupling such that filter housing  46   a,b  can be disconnected from the valve switch  34 . Similarly, in each filter set  30 ,  32 , the respective filter housing  48   a,b  is in fluid communication with the outlet conduit  44  through flexible tubing extending from a quick disconnect coupling at the outlet portion  56  of filter housing  48   a,b  to the outlet conduit  44 . Within each filter set  30 ,  32 , the filter housings  46   a,b ,  47   a,b ,  48   a,b  are connected in series using flexible tubing terminating at quick disconnect couplings at either end portion of the respective filter housing. The use of quick disconnect couplings to connect filter housings  46   a,b ,  47   a,b ,  48   a,b  in fluid communication with other parts of the plant  10  reduces maintenance time by improving access to cartridge filters in the filter housings  46   a,b ,  47   a,b ,  48   a,b . Additionally or alternatively, the flexibility of the tubing used to achieve fluid communication in the filter station  14  can facilitate moving the filter sets  30 ,  32 , for example, during initial configuration of the plant  10  or during maintenance. 
     The stand  36  supports the filter sets  30 ,  32  such that the respective elongate flow paths defined by the filter housings  46   a,b ,  47   a,b ,  48   a,b  are substantially vertical during operation of the plant  10 . Because off-the-shelf cartridge filters  49  are typically elongate, this substantially vertical orientation of the elongate flow paths of filter housings  46   a,b ,  47   a,b ,  48   a,b  can facilitate fitting the filter station  14  within a size envelope that is amenable to portability. For example, this substantially vertical orientation can facilitate fitting the filter station  14  within the envelope with dimensions of 50″×40″×40″. 
     Referring to  FIG. 3 , a portion (e.g., an inlet portion  54  and/or an outlet portion  56 ) of each filter housing  46   a ,  47   a ,  48   a  of filter set  30  is hinged to the stand  36  such that an unhinged portion of each filter housing  46   a ,  47   a ,  48   a  can be tilted (e.g., to about a 45 degree angle) about a horizontal axis. During use, the unhinged portion of each filter housing  46   a ,  47   a ,  48   a  can be disconnected, and each filter housing  46   a ,  47   a ,  48   a  can be independently tilted away from the stand  36  to improve, for example, accessibility of the cartridge filters  49  disposed in the filter housings  46   a ,  47   a ,  48   a . For example, the independent tilting of each filter housing  46   a ,  47   a ,  48   a  can facilitate replacement of a single cartridge filter  49  at a time. Thus, if two cartridge filters  49  of one of the filter sets  30 ,  32  are clogged, an operator can remove the clogged cartridge filters while leaving the third cartridge filter in place. As compared to systems that require replacement of all filters at once, the ability to replace cartridge filters  49  selectively can reduce the overall operating cost of the plant  10 . Additionally or alternatively, tilting the filter housings  46   a ,  47   a ,  48   a  away from the stand  36  can reduce the overall amount of space required to perform maintenance on the filter station  14  (e.g., by reducing the vertical head space required to pull or install the cartridge filters  49 ). 
     For the sake of clarity and ease of illustration, only the support of filter set  30  on stand  36  has been described. It should be noted, however, that filter set  32  is supported on stand  36  in an analogous manner. 
     Referring to  FIG. 4 , the stand  36  is rotatable about a vertical axis to facilitate positioning filter sets  30 ,  32  for ease of accessibility. For example, stand  36  can be rotated about a vertical axis during configuration or reconfiguration of the plant  10  such that filter station  14  can be moved to a corner while still allowing filter sets  30 ,  32  to be positioned for access during maintenance. Stand  36  can lock into place during operation of the plant  10  to reduce the likelihood that filter sets  30 ,  32  will move out of position. 
     Referring to  FIG. 5 , a controller  60  is in communication (e.g., electrical communication) with the pressure sensors  50   a,b ,  51   a,b ,  52   a,b , the valve switch  34 , and an input device  62 . During use, the controller  60  receives input signals from the pressure sensors  50   a,b ,  51   a,b ,  52   a,b  and the input device  62  (e.g., keyboard). The controller  60  sends output signals to the valve switch  34 . The controller  60  controls the position of the valve switch  34  based at least in part on the signals from the pressure sensors  50   a,b ,  51   a,b ,  52   a,b . Additionally or alternatively, the controller  60  can control the position of the valve switch  34  based on the signal from the input device  62 . The controller  60  receives pressure measurements from pressure sensors  50   a,b ,  51   a,b ,  52   a,b . Based on these pressure measurements, the controller  60  can send a command signal to the valve switch  34  to divert the continuous flow of slurry to another filter set as necessary to allow continuous operation of the plant  10 . 
       FIG. 6  shows an example of a controller process  63  used to control the position of the valve switch  34 . The controller process includes a setup stage  64  and a monitoring stage  66 . 
     In the setup stage  64 , the controller  60  receives  68  input parameters related to slurry processing (e.g., through input device  62 ). The input parameters include a first threshold pressure drop at which a warning signal will be generated and a second threshold pressure drop, greater than the first, at which the valve switch  34  will be activated to redirect slurry flow between filter sets  30 ,  32 . The input parameters also include an initial time delay, during which the valve switch  34  is maintained  70  in a fixed position to allow the plant  10  to reach steady state operation. 
     Once the controller  60  determines  72  that the initial time delay has lapsed, the controller  60  enters the monitoring stage  66  in which the controller receives  74  pressure measurements from each sensor  50   a,b ,  51   a,b ,  52   a,b  associated with filter sets  30 ,  32 . The controller  60  determines  76  which filter set  30 ,  32  is filtering slurry based on the magnitude of the pressure measurements of the pressure sensors  50   a,b ,  51   a,b ,  52   a,b  associated with each filter set  30 ,  32 . For example, the filter set  30 ,  32  associated with pressure measurements approximately equal to atmospheric pressure is the filter set that is not in use. The controller  60  checks  78  this determination with the last actual valve switch  34  position sent by the controller  60 . If the determined and actual valve switch  34  positions do not match, the controller  60  signals  80  an alarm and shuts down  82  the plant  10 . 
     For the filter set  30 ,  32  that is filtering slurry, the controller  60  determines  84  the pressure drop across each filter housing  46   a,b ,  47   a,b ,  48   a,b  (e.g., across each cartridge filter  49 ) based on the measurements received from pressure sensors  50   a,b ,  51   a,b ,  52   a,b . The controller  60  compares  86  the pressure drop across each filter housing  46   a,b ,  47   a,b ,  48   a,b  to the first and second threshold pressure drops entered during the setup stage  64 . For example, the pressure drop across filter housing  46   a  can be determined by subtracting the pressure reading from pressure sensor  51   a  from the upstream pressure reading from pressure sensor  50   a , and this pressure drop is compared to the first and second threshold pressure drops. If the pressure drop across all three cartridge filters  49  in a filter set  30 ,  32  is less than or equal to the first threshold pressure drop  88 , the controller  60  continues to monitor the pressure drop across the filter housings  46   a,b ,  47   a,b ,  48   a,b . If the pressure drop across one of the filter housings  46   a,b ,  47   a,b ,  48   a,b  is between the first and the second threshold pressure drops  90 , the controller  60  activates  92  a warning signal (e.g., a visual alert) indicating which filter housing  46   a,b ,  47   a,b ,  48   a,b  is associated with the warning signal. If the pressure drop across one of the filter housings  46   a,b ,  47   a,b ,  48   a,b  is greater than the second threshold pressure drop, the controller  60  activates  94  the valve switch  34  to direct the slurry flow to the other filter set  30 ,  32 . The controller  60  also indicates  96  which filter housing(s)  46   a,b ,  47   a,b ,  48   a,b  had the excessive pressure drop that caused the activation of the valve switch  34 . 
     In response to the indication  96  by the controller  60 , an operator can open one or more sides of the enclosure  38  to disconnect the flexible filter tubing connected to the appropriate filter housing(s)  46   a,b ,  47   a,b ,  48   a,b . The operator can tilt the filter housing(s)  46   a,b ,  47   a,b ,  48   a,b  away from the stand  36  to replace the filter in the filter housing(s)  46   a,b ,  47   a,b ,  48   a,b . While the operator is replacing these filters, a substantially continuous flow of slurry continues to flow through the filter set that is not being serviced. 
     Control process  63  can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof. Controller  60  can be implemented in a computer program product tangibly embodied or stored in a machine-readable storage device for execution by a programmable processor; and control process  63  can be performed by a programmable processor executing a program of instructions to perform functions of the invention by operating on input data and generating output. The control process  63  can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. 
     Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD_ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     While certain embodiments have been described, other embodiments are possible. 
     For example, while the controller  60  has been described as determining pressure drop across each filter housing  46   a,b ,  47   a,b ,  48   a,b , other embodiments are possible. For example, a controller can activate the valve switch  34  based on the pressure drop across the overall filter set  30 ,  32 . 
     As another example, while each filter set  30 ,  32  has been described as including filter housings  46   a,b ,  47   a,b ,  48   a,b  arranged in series, other embodiments are possible. The flexible tubing connected to the filter housings  46   a,b ,  47   a,b ,  48   a,b  can be reconnected and/or replaced such that slurry flows through filter housings  46   a,b ,  47   a,b ,  48   a,b  in parallel. 
     As yet another example, while the filter station  14  has been described as including two filter sets  30 ,  32 , other embodiments are possible. For example a filter station can include three or more filter sets and a valve switch having a respective number of positions (e.g., three positions if there are three filter sets). These additional filter sets can increase the amount of time that the plant  10  can be operated without maintenance, which can reduce the operating cost of a continuous slurry blending plant. 
     As still another example, while each filter set  30 ,  32  has been described as including three filter housings  46   a,b ,  47   a,b ,  48   a,b , other embodiments are possible. For example, each filter set can include two filter housings. As another example, each filter set can include four or more filter housings. Additionally or alternatively, the first filter set can include a different number (e.g., a greater number) of filter housings than the second filter set. 
     As another example, while filter housings  46   a,b ,  47   a,b ,  48   a,b  have been described as being connected in fluid communication with other components of the filter station  14 , other embodiments are possible. For example, substantially rigid pipes can connect filter housings to the other components of a filter station. These substantially rigid pipes can reduce the likelihood that leaks will develop over long periods of substantially continuous operation. 
     As yet another example, while the filter station  14  has been described as having an enclosure including wheels  58  and fitting within a 50″×40″×40″ envelope, other embodiments are possible. For example, in some embodiments, a filter station  14  can include filter housings  46   a,b ,  47   a,b ,  48   a,b  dimensioned to support cartridge filters  49  having an outer diameter of greater than about 2.5 inches and/or less than about 18 inches. In some embodiments, a filter station  14  can include filter housings dimensioned to support cartridge filters  49  having a length of greater than about 1 inch and/or less than about 50 inches (e.g., about 10 inches, about 20 inches). In certain embodiments, a filter station can be dimensioned for bench top filtration including cartridge filters  49  having a length of about 10 inches. Additionally or alternatively, a bench top filter station can include an enclosure that sits substantially flat (e.g., without wheels) on the bench top to reduce the likelihood that the bench top unit can be inadvertently moved. In some embodiments, a bench top filter station can include an enclosure having one or more handles to facilitate positioning the bench top filter station. 
     As another example, while the filter station  14  has been shown as having pressure sensors  50   a,b ,  51   a,b ,  52   a,b  extending substantially parallel to the elongate flow path defined by the respective filter housings  46   a,b ,  47   a,b ,  48   a,b , other embodiments are possible. For example, pressure sensors  50   a,b ,  51   a,b ,  52   a,b  can be mounted at an angle (e.g., about a 45 degree angle) to the elongate flow path such that each pressure sensor extends away from the stand  36 . Such angled pressure sensors can facilitate tilting filter housings  46   a,b ,  47   a,b ,  48   a,b  without interference from other components (e.g., other filter housings or the stand). 
     As still another example, while the filter station  14  has been described as having pressure sensors  50   a,b ,  51   a,b ,  52   a,b  in communication with filter housings  46   a,b ,  47   a,b ,  48   a,b , other embodiments are possible. For example, a filter station can include one or more of the following measurements downstream of each cartridge filter to indicate whether the respective filter is clogged: pH; large particle counting (LPC), and conductivity. 
     As yet another example, while pumps  18 ,  19 ,  20 ,  21  have been described as electro-mechanical diaphragm pumps, other embodiments are possible. For example, pumps  18 ,  19 ,  20 ,  21  can be bladder pumps. Each bladder pump includes a bladder disposed in a filter housing. During use, raw material is drawn into the bladder as it expands under vacuum pressure exerted outside of the bladder, between the bladder and the filter housing. With raw material in the bladder, pressurized fluid (e.g., air) is introduced outside of the bladder, between the bladder and the filter housing. The pressurized fluid collapses the bladder to force the raw material out of the bladder and into the processing stream. Without mechanical actuation, such bladder pumps are reliable over the long periods of substantially continuous operation that are possible with the filter station  14 . The bladder pump can be, for example, a NOWPAK® container assembly available from ATMI, Inc. of Danbury, Conn. 
     As still another example, while the mixing station  12  has been described as including flow controller  22 ,  23 ,  24 ,  25 , each including an adjustable orifice and an internal regulating valve, other embodiments are possible. For example, a mixing station can include a U-tube flow controller including a reservoir and an adjustable U-tube. The adjustable U-tube is positionable to set the maximum level of fluid in the reservoir. During use, fluid will continue to collect in the reservoir until the fluid level reaches the apex of the U-tube. Once the fluid in the reservoir reaches this level, additional fluid added to the reservoir will flow out of the reservoir through the U-tube (e.g., into an overflow volume to be recycled back into the system). Without mechanical actuation, U-tube flow controllers are reliable over the long periods of substantially continuous operation that are possible with the filter station  14 . An example of a U-tube flow controller is the U-Tube technology available from ChemFlow Systems, Inc. of San Jose, Calif. 
     While filter station  14  has been described as processing a CMP slurry used in semiconductor fabrication, a filter station can be used to filter any of various different types of fluids used in any of various different industries. For example, a filter station can include stainless steel filter housings and be used to filter milk during dairy processing.