Abstract:
An apparatus and method to perform multiple filtration steps with a modular filtration apparatus and single cycle. A stackable modular filter cup has features to allow stacked cups to be separated without the need for substantial effort yet without diminishing the effect of vacuum assisted filtration. Each cup is formed with a slip seal segment and/or a series of substantially vertical channels or ridges on an inner or outer wall to reduce surface contact between nested filter cups.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to filter cups used to separate solids from liquids. More particularly, the invention concerns apparatus and methods to mechanically separate solids dissolved in liquids through multiple filtration steps with a single pour of a fluid into a stacked column of stackable filter cups, each having a dedicated filter for filtering solids from liquids. 
       BACKGROUND OF THE INVENTION 
       [0002]    To perform filtrations of solutions (fluids or gases containing suspended solids) bearing multiple forms of solids having different particle sizes and solubility constants, and/or complex solids, filtration systems, particularly those that employ vacuum assisted filtration, use a rather laborious and time consuming manual procedure of introducing a solution into a filter cup having a porous barrier, i.e., a filter, to retain particles of a desired size (solute) and allow passage of a solvent (filtrate) component. Often times, the desired solid cannot be separated and isolated without the use of multiple filtration operations that may require a multitude of reagents to break down the solution components and multiple filter media to obtain the desired solid. 
         [0003]    With conventional gravimetric, or vacuum-assisted filtration systems, a series of filters have to be used to achieve the desired result. For example, as shown in U.S. Pat. No. 5,695,639, a modular filter system is shown that includes a spout member designed to direct a solvent into a container. A top portion of the spout is configured to receive in releasable engagement, a reservoir element, or modular filter cup. 
         [0004]    To secure a filter to the apparatus, the filter is sandwiched between the tight fitting, but releasable reservoir and spout. To perform a multi-step filtration process with this apparatus, each filtration operation requires the reservoir and spout to be disassembled and the filter carefully removed and replaced with a different filter for the next step. If sterility of the solute is important to the procedure, a sterile implement, e.g., forceps, may have to be used to remove the filter. 
         [0005]    The fluid portion that flows via the spout into a container, e.g., a flask, is reintroduced into the reservoir to begin another filtration cycle. The steps are repeated serially until the desired solid(s) has been separated out of the solution. The process is slow and cumbersome due to the structure of the filter cup/filter/stem assembly. 
         [0006]    In U.S. Pat. No. 4,301,010, a multi-component vacuum filter funnel is disclosed that solves the problem of solids migrating around the perimeter of filters placed in filter cups. The apparatus includes a vacuum intake member (similar to a filter cup stem) that creates a seal between the filter cup assembly and a container, such as a flask. An optional funnel having a spout is placed within the intake member and forms an airtight seal with the intake member. The spout extends beyond a bottom outlet of the intake member. An upper funnel member (filter cup) having a perforated plate formed on a bottom has threading on an outer lower end that engages threading on an upper inner perimeter edge of the intake member. 
         [0007]    A filter is placed on the perforated plate to perform the filtration function. A threaded cylindrical sleeve member has threading formed around an outside bottom edge of the member that engages corresponding threading on a lower end of an inner wall of the upper funnel member. The engaged features of the sleeve member and upper funnel member secure the filter firmly against the perforated plate, particularly about the filter&#39;s perimeter edge. This configuration ensures a tight seal of the filter&#39;s edge to prohibit the migration of solids around the filter&#39;s edge. 
         [0008]    While solving the problem of solute migration around the filter, this configuration is enormously cumbersome when a multi-step filtration process is required. Each change of a filter requires the sleeve member to be rotated out of the upper funnel member and the upper funnel member to be rotated out of the intake member to enable retrieval of the filter. This assembly and disassembly process has to be repeated for each successive filter. 
         [0009]    As gravimetric and vacuum-assisted filtration processes often require multiple filtration steps, achieving this by multiple exchanges of filter cups with changed filters may cause additional problems with respect to solute loss and material contamination. Each problem can adversely affect quantitative as well as qualitative analyses. 
         [0010]    The process of stacking conventional filter cups is equally problematic as the registered walls of stacked cups result in too much surface area contact. With the addition of vacuum assisted filtration, the binding of nested cups is increased making separation potentially very difficult due to the negative pressure coupled with likely capillary action of fluid migrating into the interface of adjacent cups. 
         [0011]    What is needed and desired is a filter cup apparatus that simplifies and streamlines multi-step filtration processes including pre-filtration and final filtration steps by providing a filter train to prevent or eliminate solute loss and contamination, and that provides a means to facilitate separation of nested cups to retrieve desired solutes. These and other objects of the invention will become apparent from a reading of the following summary and detailed description of the invention. 
       SUMMARY OF THE INVENTION 
       [0012]    In one aspect of the invention, an apparatus for performing multiple filtration steps in a single pour includes two or more stacked filter cups having features for facilitating the releasable engagement of nested cups. An inner wall, and/or an outer wall, of each cup is formed with one or more substantially vertical or helical grooves, or, alternatively, ribs to prevent unwanted adhesion of the interfacing side walls. 
         [0013]    A substantially annular lower portion of the inner wall of each cup is formed with a frustum angle different than the angle of an upper portion of the wall. This allows the lower portion to register against the outer wall of an adjacent second filter cup placed nested within the first filter cup to form a luer lock type slip seal without engaging the upper portion. This reduces the overall contact of the nested walls to reduce overall adhesion between the nested cups. This dramatically reduces the amount of energy needed to separate the cups and retrieve the desired solute(s) when the filtration process is complete. 
         [0014]    A bottom surface of each cup has portions defining an annular shoulder to receive and secure an o-ring seal between the cup and a stem. The stem has portions defining an annular chase into which the o-ring is secured. This creates an air-tight and fluid-tight seal between the filter cup and stem. The bottom edge of the annular shoulder acts as a stop when the cup is nested inside another filter cup. 
         [0015]    In another aspect of the invention, an annular ledge is formed on the inner wall of each cup that acts as a stop when a second filter cup is inserted into the filter cup. The annular shoulder of the second filter cup registers against the ledge so as to define a space between the filters of the two cups. This ensures to formation of an adequate space to contain the desired precipitate without the precipitate coming into contact with, and potentially clinging to, a bottom surface of the second nested filter cup. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a sectional view of a stackable filter cup and filter assembly according to one embodiment of the disclosure. 
           [0017]      FIG. 2  is a bottom perspective view of a stackable filter cup assembly according to another embodiment of the disclosure. 
           [0018]      FIG. 3  is a top perspective view of a stackable filter cup assembly according to another embodiment of the disclosure. 
           [0019]      FIG. 4  is a sectional view of a stackable filter cup assembly according to another embodiment of the disclosure. 
           [0020]      FIG. 5  is a top perspective view of a stackable filter cup assembly according to another embodiment of the disclosure. 
           [0021]      FIG. 6  is a bottom perspective view of a stackable filter cup assembly according to another embodiment of the disclosure. 
           [0022]      FIG. 7  is a sectional view of a stackable filter cup assembly with an additional stacked up according to one embodiment of the disclosure. 
           [0023]      FIG. 8  is a partial sectional view taken at I in  FIG. 7  showing the registration surfaces of nested filter cups according to one embodiment of the disclosure. 
           [0024]      FIG. 9  is an exploded view of a stackable filter cup assembly according to one embodiment of the disclosure. 
           [0025]      FIG. 10  is a bottom perspective view of a stackable filter cup according to one embodiment of the disclosure. 
           [0026]      FIG. 11  is a top perspective view of a stackable filter cup stem according to one embodiment of the disclosure. 
           [0027]      FIG. 12  is a top perspective view of a stackable filter cup stem according to another embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Referring to  FIGS. 1-3 ,  7  and  10 , in one aspect of the invention, a stackable filter cup assembly, shown generally as  10 , includes a stackable filter cup subassembly  12  configured substantially as an inverted frustum of a cone with a top end having a diameter generally greater than a bottom end to which a filter material is attached. Filter cup subassembly  12  is secured to a filter stem  14  that has features for securing the filter cup assembly to a container, such as a flask. Filter cup subassembly  12  may be constructed from materials (disclosed below) suitable for disposal after one or more uses. 
         [0029]    Filter cup  12  has a substantially circular bottom edge. A lower portion  21  of a sidewall of the cup forms an annular shoulder  16  for receiving a top end of filter stem  14 . Formed on an inner wall of cup  12  is an annular flange  18  that provides a binding/connection point for a filter membrane  19 . The combination of flange  18  and shoulder  16  form a cavity into which filter stem  14  is seated. 
         [0030]    Formed on an inner wall of cup sub-assembly  12  are a plurality of release channels  20 . In one embodiment, channels  20  run substantially vertically from a top end of the cup to a termination point  22  set above the plane occupied by the filter membrane disclosed below. The distance between termination point  22  and the filter membrane provides an area  23  for solutes to accumulate during a filtration process. This distance can be increased or decreased to alter the available volume for solute precipitation. 
         [0031]    Alternatively, channels  20  may extend to a point proximate to, but not congruent with, the plane occupied by the top end of the cup. In a further alternative embodiment, channels  20  may define a spiral pattern along the cup inner wall. The starting point of each spiral channel  20  may coincide with the plane occupied by termination point  22 . Channels  20  may conform to other configurations as long as the channels are not oriented substantially horizontally within cup  12 . This ensures any trapped air and/or fluid will be channeled away from the adjoining surfaces of two cups when urged into a nested configuration. 
         [0032]    In a further aspect of the disclosure, channels  20  may be formed on an outer wall of cup subassembly  12 . The orientation and length of channels  20  formed on the outer wall may be the same as disclosed for channels  20  formed on the inner wall. In a yet further aspect of the disclosure, channels  20  may be formed on both the inner wall and outer wall of cup subassembly  12  to maximize friction and/or capillary action reduction when two or more cups are nested together. 
         [0033]    In an alternate embodiment, a plurality of ribs  20 ′ may be formed on the inner wall, or outer wall of cup sub-assembly  12  to perform the same function as release channels  20 . For purposes of this disclosure, a rib is defined as a segment of inner or outer cup wall, either integral or attached, extending proud of the general inner or outer wall surface and with a width less than the width of an inner or outer wall segment extending between adjacent ribs. 
         [0034]    Like channels  20 , ribs  20 ′ limit surface area contact between nested filter cups so as to reduce any adhesion caused by friction and/or capillary action of fluids introduced into the filter cups. Ribs  20 ′ may be oriented substantially vertically within or without cup  12 , may define a helical pattern, or may conform to other configurations so long as the ribs are not oriented substantially horizontally within cup  12 . The gap formed between nested cups can be altered by altering the thickness of the ribs. The thicker the ribs, the shorter the distance a cup can be nested into a lower cup and the larger the gap between nested cup filters. 
         [0035]    With this embodiment, it should be understood that the number and width of the ribs will impact the effectiveness of vacuum assist. If a significantly large area of the walls of nested cups is maintained separate so as to form gaps, a substantial portion of the negative vacuum pressure may be lost between the gaps between the nested walls without the presence of the slip seal disclosed and described below. 
         [0036]    Secured to annular flange  18  is filter membrane  19 . Membrane  19  may be made of any material suitable for use as a filtering medium including, but not limited to, woven screens made from nylon, polyester, polypropylene and stainless steel, cellulose fiber, activated carbon fibers, melt-blown micro-fiber and nano-fiber non-woven filter media made from polypropylene, nylon, polyvinylidene fluoride (PVDF), glass-fibers, nanoporous or microporous polymer-based membranes made from polytetrafluoro-ethylene (Teflon® PTFE), polyvinylidene fluoride (PVDF), perfluoroalkoxy (PFA), Ethylene-clorotrifluoroethylene copolymer (ECTFE), polyolefins like polypropylene (PP), high density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE or UPE), polyethersulfone (PES), polysulfone (PS), Nylon 6, Nylon 66, regenerated cellulose, mixed esters of cellulose, polycarbonate, polyester, and mixtures thereof. 
         [0037]    The filtration media&#39;s retention feature and pore size can be altered over a wide range of values to accommodate different filtration applications. In addition, the membranes or filtration media can be made with functional surface properties to have absorption, adsorption, and/or ion exchange sites for specific filtration and purification requirements. The pore sizes on the filter media can range from about 0.001 micron up to about 100 micron, depending on the target application and/or the size of the particle or molecule to be removed. 
         [0038]    The functional surfaces may be discrete portions of the filter, or the entire filter having anionic or cationic sites to adsorb charged molecules in the passing solution. For example, adsorbents for heavy metals are available in powder form, or porous disc form. In porous disc form, the specialized discs may be used in place of, or in combination with, resident filters in the filter cups. If used as a supplemental filter means, the specialized discs may be placed on, or adhered to, the resident filter. 
         [0039]    Membrane  19  may be bonded to flange  18  via sonic bonding, thermal lamination, chemical bonding, adhesive, press-fit, and the like. The membrane or filter media can also be secured to flange  18  by insert molding. The type of bonding may be varied to accommodate any possible reaction to the contents of a fluid to be filtrated. 
         [0040]    In a further aspect of the disclosure, a slip seal segment  26  (shown in  FIG. 8 ) formed on a lower end of the cup inner wall is set at an angle relative to the plane occupied by filter  19  and flange  18  different than the angle formed by the remainder of the cup wall with the same plane. The angle of segment  26  is less than the angle of the remainder of the cup wall when measured in terms of degrees of angle. 
         [0041]    By making the angle of segment  26  less than the angle of the upper portion of the cup wall, the amount of cup wall registering with an adjacent nested cup can be controlled and limited by the length of segment  26 . 
         [0042]    With respect to the embodiment utilizing ribs  20 ′, a top annular end of segment  26  should align with the inward faces of ribs  20 ′ to ensure contact with the corresponding outer slip seal surface segment  21  of another cup nested therein. Segment  21  does not require modification when ribs  20 ′ are used. 
         [0043]    To create a uniform slip seal, an outer surface of corresponding wall segment  21  is formed at an angle relative to the plane occupied by flange  18  and filter  19  substantially similar to the angle formed by segment  26 . In one embodiment, the difference between the angle of segment  26  and the remainder of the cup wall can be from an angle greater than about 0° to an angle of about 15°. An angle from about 4° to about 8° provides a superior slip seal surface suitable for securely nesting cups and releasing nested cups. It should be understood that angles outside this stated range may also provide an adequate slip seal surface. 
         [0044]    In another aspect of the disclosure, referring now to  FIGS. 1 ,  2 ,  9  and  11 , filter stem  14  includes a main body  30  defining a hollow interior for receiving fluid. Main body  30  may be formed with one or more trusses  31  to add structural rigidity and to support filter  19 . Main body  30  is connected to, and in fluid communication with, either a modular or integral spout  32  extending downwardly from main body  30 . Spout  32  may be substantially uniformly cylindrical along its length, or may be formed with a tapered wall with the lower end of the spout having a diameter smaller than the diameter of the spout portion proximate main body  30 . Spout  32  is substantially hollow throughout its length to allow fluid to flow therethrough from main body  30 . 
         [0045]    In one embodiment, a bottom end of main body  30  has portions defining an annular channel  34  configured to receive an open top end of a fluid receiving vessel, e.g., a flask. The spout end proximal to main body  30  tapers outwardly towards body  30  and defines an inner wall of channel  34 . Due to the taper of the upper spout end, the diameter of channel  34  becomes smaller as the channel walls progress to an upper terminus. This configuration ensures the engagement with numerous fluid receiving vessels having variable neck sizes and results in a seal sufficiently tight to support the introduction of vacuum assistance to facilitate fluid flow through the filter system. 
         [0046]    In an alternate embodiment, main body  30  does not have portions defining channel  34 . Instead, a stopper  35  having a through bore configured to receive spout  32  is placed onto spout  32  to create a seal with the spout. 
         [0047]    An outer wall of stopper  35  may have an upwardly extending taper to create a seal with a fluid receiving vessel when the filter apparatus is placed onto the vessel. The outer wall may be varied in both diameter and taper to accommodate a wide range of receiving vessel necks. Stopper  35  may be integral to spout  32 , or may be a separate component as disclosed and described above. 
         [0048]    Extending from a top end of main body  30  is an annular filter cup receiving segment  36 . Receiving segment  36  is dimensioned to fit within circular shoulder  16  so as to provide a substantially air-tight connection with filter cup  12 . Formed on a perimeter of receiving segment  36  is o-ring channel  38 . Channel  38  is configured to receive an o-ring  40 . O-ring  40  interfaces with channel  38  and an inside wall of shoulder  16  to provide an air-tight seal between filter cup  12  and stem  14 . This configuration also allows ease of disassembly of the cup and stem due to the lubricious nature of the materials used to construct o-rings. 
         [0049]    O-ring  40  may be constructed from a variety of materials including, but not limited to silicone, nitrile, neoprene, ethylene propylene (EPM or EPDM), fluorocarbon, perfluorelastomers, and combinations thereof. Regardless of the material used for o-ring  40 , it should be understood that channel  38  may alternatively be formed on an inner wall of shoulder  16 . In a yet further alternative, o-ring channels may be formed on both the inner wall of shoulder  16  and the perimeter of receiving segment  36  to receive o-ring  40 . 
         [0050]    In a further aspect of the invention as shown in  FIGS. 4-6  and  12 , stem  14  may be formed with a vacuum stem  42  to receive a hose to supply vacuum pressure to the filtration apparatus in the event the fluid receiving vessel is not provided with a vacuum assist nozzle. Vacuum stem  42  may further be formed with integral hose barbs  44  to secure an attached vacuum hose (not shown). Barbs  44  are preferably configured to prevent unintended migration of the hose off the vacuum stem. Other fittings can also be used, such as male/female quick couplings that provide a quick release feature for easy attachment and separation of the filtration system from the vacuum hose. 
         [0051]    To use the novel filter apparatus, a stem/filter cup assembly (as shown in  FIG. 1 , is assembled and placed on a fluid receiving vessel so that either channel  34  or stopper  35  register against the receiving vessel to form a substantially airtight seal. Next, one or more additional filter cups are placed onto the initial assembly to form a series of nested filter cups wherein a space is formed between the filters of each cup as shown in  FIG. 7 . 
         [0052]    Additional filtration media, such as activated carbon fiber media, or molecular selective resin or chromatographic media can be placed in the space between the nested cups to facilitate and achieve highly selective separation and/or purification processes. The added filtration medium may be adhered to the structures of the nested cups, or may be free floating and contained by the defined inter-cup space. If free floating, the added filtration media is inserted onto the filter surface of a first filter cup and secured in place when a second filter cup is inserted into and nested with the first filter cup. As with filter  19 , the additional filtration medium may be customized with respect to retention features and pore sizes, and optionally formed with functional surface properties as disclosed with respect to the description of filter  19 . 
         [0053]    Each cup should have a different filter (different media, filter pore size), to capture different precipitates, although one or more cups may have the same filter characteristics in terms of retention features and pore size, among others. If vacuum assistance is desired or required, vacuum pressure is introduced via the vacuum stem or via a vacuum stem formed or placed on the receiving vessel. The user then pours the subject liquid into the uppermost filter cup along with any reagents and allows the gravimetric and/or vacuum assisted filtration process to run its course through the successive filter cups. 
         [0054]    When the fluid has completed its passage through each filter cup with the filtrate being deposited in the receiving vessel, the cups are removed one from another by applying opposing forces to adjacent nested cups with hands or grasping implements, e.g., forceps. Whether ridges or channels are used to construct the cups, the cups should disengage with relative ease as there will be insufficient surface area contact to impede cup separation. Once the desired precipitate has been removed from filter cup  14  (and any additional filter cups in the filter cup train), using methods commonly known in the art, e.g. scraping, back washing, flushing and/or dissolutions, the filter cup may be disposed or cleaned for subsequent use. 
         [0055]    Material selection for the filter cups and stem and filter type and pore size selection is determined by the acceptable or required bulk flow of fluid and retention of precipitate through the apparatus. Ideally, the material selected should be able to withstand emersion into strong solvents, acids, bases, and potentially oxidizing fluids, and should be able to withstand the high temperatures of autoclave (135° C.) and other sterilizing equipment, e.g., gamma e-beam. Polysulphones, polyethylene, nylon, PVDF (polyvinylidene fluoride (e.g., Kynar Flex®)), gamma stabilized PP (polypropylene), and/or combinations thereof are well suited for these functions. 
         [0056]    While the present invention has been described in connection with several embodiments thereof, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. Accordingly, it is intended by the appended claims to cover all such changes and modifications as come within the true spirit and scope of the invention. What I claim as new and desire to secure by United States Letters Patent is