Abstract:
A method and apparatus for carrying out a method for thermophoretically removing particles from a particulate containing liquid the method including providing a heated turbulent flowing particulate contain liquid through a first conduit; redirecting a portion of the particulate containing liquid through a second conduit to provide laminar flow having a flow direction substantially parallel to the first conduit; forming a thermal gradient in said second conduit substantially perpendicular to the flow direction; concentrating particles in the particulate containing liquid in a portion of the second conduit aided at least in part by thermophoretic forces; and, separating the particulate containing liquid into at least a relatively concentrated particle containing portion and a relatively unconcentrated particle containing portion.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to removal of particles from a liquid system and more particularly to a thermophoretic method and system for removing particles included in a particulate containing liquid.  
         BACKGROUND OF THE INVENTION  
         [0002]    The removal of small particles from chemical process fluids is a general problem to which a wide variety of techniques have been applied. Liquids are much more difficult to filter than gases for several physical reasons. For example, Brownian motion causes aerosol particles to travel much farther in gases than in liquids, providing an increased opportunity for the particles to collide with and stick to a membrane. In addition, viscous drag is much smaller in gases (for the same volume of material) so that smaller pore sized filters can be used in gases without unduly limiting pressure drops. Furthermore, the adhesion of particles to, for example, a membrane is less likely than in gases due to larger viscous drag forces present in liquids thereby making it more likely that particles are more likely to be re-entrained in the liquid.  
           [0003]    Most filters for liquids rely on sieving, using pores having dimensions smaller than the particles to be captures; this leads to more clogging problems than occur with filters for gases. Moreover, the strong forces exerted by liquid flow impingement on the filters can destroy the filters. In addition, poor initial wetting of filter membranes can reduce the sieving performance of liquid filters. Other shortcomings of membrane filters include the limited ability to recycle the membranes for use in different processes. In view of the several shortcomings of sieving (filtering) to remove particles from liquid processes, other techniques have been developed which have provided partial solutions.  
           [0004]    For example, electrophoretic techniques have been developed using dipole fields in molecular separations. Multipole fields are useful for filtering liquids having sufficiently low electrical conductivity. Uniform electric fields can be used to remove charged particles from high-resistivity (low conductivity) liquids. A general difficulty with using electric fields is that the magnitude and sign of the charge on a particle, if any, depends upon the pH of the solution, the type of particle and the type of solution. Further, any particles without an electrical charge will not be removed. These limitations restrict the applicability of filtering using electrophoresis techniques.  
           [0005]    Another example includes magnetophoretic techniques which apply inhomogeneous magnetic fields to exert variable forces on particles suspended in solution depending on a particle&#39;s magnetic susceptibility. Although magnetophoresis is a useful tool in separation of materials in the field of waste re-cycling, it appears inefficient for applications requiring ultra-small particle separation.  
           [0006]    Yet another technique that has been developed to remove particles from a liquid solution is a flotation mechanism or settling rate. Generally, depending on the particle size and particle charging characteristics determining particle adhesion characteristics, the particles including adhering groups of particles display certain settling behaviors. Generally, larger groups of particles settle faster than smaller groups. This method, however, is generally inefficient in terms of processing time and is limited to larger particles that settle within a reasonable time.  
           [0007]    One area where there has been limited application of the method to the problem of liquid filtering includes thermophoresis. Thermophoresis works by the presence of differential pressures exerted on particles due to the presence of a thermal gradient, causing the particles to migrate from an area of higher temperature to an area of lower temperature. The phenomenon of thermophoresis is well known and has been observed in gases, for example, aerosols, and liquids some time. For example, one application of thermophoresis relates to particles dispersed in a fluid trapped between two plates of differing temperatures will migrate towards and eventually deposit on the colder plate. This application, however, is not practical for use as a particle liquid since deposition on the surface of a flow cell or flow cavity is transitory, turbulence or pressure fluctuations being sufficient in many cases to agitate the particles back into the liquid flow stream.  
           [0008]    Therefore, there is a need to develop a thermophoretic system and method whereby a wide range of particle sizes present in a liquid may be effectively captured and removed from the liquid without creating an unacceptable pressure drop while providing for cost effective recycling of the particle filtering means.  
           [0009]    It is therefore an object of the invention to provide thermophoretic system and method whereby a wide range of particle sizes present in a liquid may be effectively captured and removed from the liquid without creating an unacceptable pressure drop while providing for cost effective recycling of the particle filtering means, while overcoming other shortcomings and deficiencies in the prior art.  
         SUMMARY OF THE INVENTION  
         [0010]    To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method and apparatus for thermophoretically removing particles from a particulate containing liquid.  
           [0011]    In a first embodiment according to the present invention, a method is provided for thermophoretically removing particles from a particulate containing liquid including providing a heated turbulent flowing particulate contain liquid through a first conduit; redirecting a portion of the particulate containing liquid through a second conduit to provide laminar flow having a flow direction substantially parallel to the first conduit; forming a thermal gradient in said second conduit substantially perpendicular to the flow direction; concentrating particles in the particulate containing liquid in a portion of the second conduit aided at least in part by thermophoretic forces; and, separating the particulate containing liquid into at least a relatively concentrated particle containing portion and a relatively unconcentrated particle containing portion.  
           [0012]    In a related embodiment the thermal gradient is formed by thermally conductively contacting said upper portion of the second conduit with the lower portion of said first conduit the thermal gradient having a decreasing temperature profile in a direction toward the lower portion of the second conduit.  
           [0013]    In another related embodiment, the step of separating the particulate containing liquid further comprises collecting the relatively concentrated particle containing portion in a lower portion of the second conduit such that the relatively unconcentrated particle containing portion is collected in an upper portion of the second conduit exiting through an upper exit portion of the second conduit to be returned to the first conduit the lower portion exiting through a lower exit portion of the second conduit.  
           [0014]    In another embodiment, thermal conductance in a direction other than substantially perpendicular to the flow direction is minimized.  
           [0015]    In yet another embodiment, the method further includes maintaining the temperature gradient by providing a heat sink at a relatively lower temperature than the first conduit and the second conduit said heat sink in thermally conductive communication with the second conduit.  
           [0016]    In another embodiment, the method further includes the step of removing particles from the particulate containing liquid included in the lower portion exiting through the lower exit portion of the second conduit.  
           [0017]    In a related embodiment, the step of removing particles includes passing the particulate containing liquid over at least one depositing surface to form a flow pathway such that particles included in the particulate containing liquid are captured and entrapped upon impacting said at least one depositing surface.  
           [0018]    In another related embodiment, the at least one depositing surface is in thermally conductive communication with a heat sink maintained at a temperature relatively lower than the at least one depositing surface temperature.  
           [0019]    In yet another related embodiment, the at least one depositing surface includes a plurality of protrusions extending into the flow pathway to form a plurality of particle capturing spaces.  
           [0020]    In a separate embodiment, the steps are performed in parallel according to a plurality of serially connected first and second conduits.  
           [0021]    In a separate embodiment according to the present invention, a thermophoretic system is provided for removing particles from a particulate containing liquid including a first conduit with means for generating a turbulent flow therein said first conduit having a second conduit formed substantially parallel to a flow direction thereto; said second conduit being in bypass flowable communication with the first conduit such that a portion of the turbulent flow is redirected to the second conduit to form a laminar flow in the second conduit; the second conduit being in thermal contact with the first conduit for forming a thermal gradient substantially perpendicular to the flow direction to exert a thermophoretic force on particles included in the laminar flow.  
           [0022]    In a related embodiment, an upper portion of the second conduit forms thermally conductive contact with the lower portion of said first conduit thereby forming the thermal gradient said thermal gradient having a decreasing temperature profile in a direction toward the lower portion of the second conduit.  
           [0023]    In another embodiment, the second conduit further includes an inlet flowable pathway and at least an upper outlet flowable pathway and a lower outlet flowable pathway said upper outlet flowable pathway in flowable communication with the first conduit to return a portion of the laminar flow to the first conduit. Further, the thermophoretic system further includes a means for selectively regulating a flow through the second conduit to maintain laminar flow conditions.  
           [0024]    In another embodiment, thermal conductance in a direction other than substantially perpendicular to the flow direction is minimized by providing thermally insulating plumbing connections.  
           [0025]    In another embodiment, the thermophoretic system further includes a heat sink in thermally conductive communication with the second conduit for maintaining the thermal gradient.  
           [0026]    In a separate embodiment, the thermophoretic system further includes a particle removing means in flowable communication with the lower outlet flowable pathway. Further, the particle removing means includes a plurality of depositing surfaces disposed along a flow pathway such that particles included in a particulate containing liquid are captured and entrapped upon impacting said depositing surfaces. Further, the plurality of depositing surfaces are in thermally conductive communication with a heat sink maintained at a temperature relatively lower than the depositing surfaces. Further yet, the plurality of depositing surfaces include a plurality of protrusions extending from the depositing surfaces into the flow pathway to form a plurality of particle capturing spaces.  
           [0027]    In a separate embodiment, a plurality of thermophoretic systems are disposed in serial flowable communication to increase a particle removing efficiency.  
           [0028]    These and other embodiments, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a schematic side view representation of one embodiment of the thermophoretic system according to the present invention.  
         [0030]    [0030]FIGS. 2A and 2B are a side view and a cross sectional view, respectively, of a thermophoretic tunnel included in the thermophoretic system according to the present invention.  
         [0031]    [0031]FIGS. 3A and 3B are conceptual schematic representations of the operation of the thermophoretic tunnel according to the present invention.  
         [0032]    [0032]FIGS. 4A through 4C are schematic representations of the particle collector and portions thereof included in the one embodiment of the thermophoretic system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Referring to FIG. 1 is shown a schematic side view representation of one embodiment of a thermophoretic particle removal system  10  for carrying out the method according to the present invention. A main liquid conduit  12 , for example a pipe for conveying a particulate containing liquid is preferably oriented with a flow direction  12 B substantially perpendicular to a gravitational force. It will be appreciated that other orientations may be used as well. In one aspect of the embodiment, particulate containing liquid flowing through main liquid conduit  12  acts as a heat source, being of a relatively higher temperature compared to the thermophoretic particle removal system so as to create a thermal gradient. In one exemplary operation, for example the particulate containing liquid temperature may be from about 100° C. to about 130° C. In thermal contact with the lower part of the main liquid conduit  12 , is thermal bridge  14 . The main liquid conduit  12  and the thermal bridge  14  are preferably made of material with good thermally conductivity such as metal, but with low chemical reactivity and a resistance to corrosion, for example stainless steel, anodized aluminum and alumina. A thermophoretic tunnel  16  is in thermal contact with the lower part of thermal bridge  14  arranged for optimized (high surface area) heat exchange. For example, the thermal bridge  14  is preferably a rectangular member with a length from about 50 percent to about 100 percent of the length of the thermophoretic tunnel and a thickness of about ¼ to about ½ of the diameter of the thermophoretic tunnel  16 . The thermal bridge  14  forms a thermal contact bridge for thermal conductance spanning a distance between the thermophoretic tunnel  16  and the main liquid conduit  12 .  
         [0034]    The thermophoretic tunnel  16  is preferably disposed below the main liquid conduit  12  and in thermally conductive contact with main liquid conduit  12  through thermal bridge  14  such that a thermal gradient originating from the main liquid conduit  12  heated by the particulate containing liquid acting is directed substantially perpendicular to a fluid flow direction e.g.,  16 B through thermophoretic tunnel  16 . Preferably, the thermal gradient through the thermophoretic tunnel is directed substantially parallel to a gravity force direction.  
         [0035]    The arrows, e.g.,  12 C and  16 C in main liquid conduit  12  and thermophoretic tunnel  16 , respectively, together schematically represent a temperature profile according to an exemplary operation of the thermophoretic system of the present invention. For example, the temperature profile in the main liquid conduit  12 , indicated by arrows e.g.,  12 C, the size of an arrow being representative of a relative temperature magnitude, gives a convex profile due in part to a turbulent flow in main liquid conduit  12 . Any known means for creating turbulent flow in main liquid conduit  12  may be used, for example, a converging diverging diffuser  13  disposed at an exit end of main liquid conduit  12  may be a suitable means to create an upstream turbulent flow in main liquid conduit  12 . The temperature profile in the thermophoretic tunnel  16 , indicated by arrows e.g.,  16 C, indicates a temperature magnitude decreasing in a direction toward a lower portion of thermophoretic tunnel  16  and substantially perpendicular to a flow direction  16 B.  
         [0036]    In one embodiment, thermophoretic tunnel  16  is in flowable communication with main liquid conduit  12 , for example an inlet pipe  18 A and an outlet pipe  18 B, each having a radius of curvature to maintain laminar flow through thermophoretic tunnel  16 . In exemplary operation about 5% of the fluid flowing in main liquid conduit  12  is redirected into inlet pipe  18 A. Preferably, a flow regulator  20 , including for example a flow sensor and a pressure regulator, is disposed in the flow path of outlet pipe  18 B between thermophoretic tunnel  16  and main liquid conduit  12  in order to selectively control a flow rate through thermophoretic tunnel  16  to ensure laminar flow through thermophoretic tunnel  16 . Preferably, the temperature gradient and the flow rate through thermophoretic tunnel  16  is sufficient to allow a thermophoretic force to separate particles contained in the particulate containing liquid to a lower part of the thermophoretic tunnel  16  prior to a flow exiting from thermophoretic tunnel  16  through, for example, outlet pipe  18 B. It will be appreciated that the efficiency of particle separation in thermophoretic tunnel  16  depends on a number of factors including the magnitude of the thermal gradient through thermophoretic tunnel  16  and the flow rate of the particulate containing liquid through thermophoretic tunnel  16 .  
         [0037]    Preferably, adiabatic connectors surround inlets  22 A and outlets  22 B, and  22 C, the connectors preferably made of a pliable, heat resistant, thermally insulating material, for example PTFE. The adiabatic connectors surrounding inlets  22 A and outlets  22 B, and  22 C, connect inlet pipe  18 A and outlet pipes  18 B and  18 C respectively to thermophoretic tunnel  16  to prevent lateral thermal conduction along those pathways thereby maintaining the magnitude of the temperature gradient substantially perpendicular to a flow direction  16 B in the thermophoretic tunnel  16 .  
         [0038]    In another embodiment, the thermophoretic system of the present invention includes flow channels passing through thermophoretic tunnel  16 . For example, FIG. 2A shows a side view of the thermophoretic tunnel  16  with inlet  22 A and outlets  22 B and  22 C. FIG. 2B shows a portion of a cross section of thermophoretic tunnel  16  taken along cross sectional cut a-a as shown in FIG. 2A. The flow channels e.g.,  24  are preferably rectangular with thermally insulating (adiabatic) walls e.g.,  24 A including walls  24 B making up the outer wall portion of peripheral flow channels. Heat conducting plates  26 A and  26 B, preferably having good thermal conductivity and chemically inertness, for example stainless steel, are disposed such that they form the top ( 26 A) and bottom ( 26 B) portions of the flow channels, e.g.,  24  to provide a thermally conductive pathway including a thermal gradient and a pressure gradient from an upper to a lower portion of thermophoretic tunnel  16  in a direction substantially perpendicular to a flow direction.  
         [0039]    Returning to FIG. 1, in operation, thermal bridge  14  is in thermal communication with main liquid conduit  12  and heat conducting plate  26 A (not shown in FIG. 1) disposed in an upper portion of thermophoretic tunnel  16  to conduct heat from a particulate containing liquid in main liquid conduit  12  through thermal bridge  14  to heat conducting plate  26 A said heat passing through the particulate containing liquid passing through flow channels, e.g.,  24  in thermophoretic tunnel  16  thereby heating conducting plate  26 B disposed in an lower portion of thermophoretic tunnel  16  to thereby creating a thermal gradient producing a thermophoretic force directed substantially perpendicular to a liquid flow direction  16 C in thermophoretic tunnel  16 . Heating plate  26 B disposed in a lower portion thermophoretic tunnel  16  forming a bottom portion of flow channels e.g.,  24  included in thermophoretic tunnel  16  is in thermal communication via an array of heat pipes e.g.,  28 A with heat sink  30  maintained at a constant lower temperature relative to the particulate containing liquid thereby conducting heat through thermophoretic tunnel  16  and maintaining a thermal gradient for exerting a thermophoretic force on particles included in the particulate containing liquid passing through a thermophoretic tunnel  16 .  
         [0040]    In operation, for example, turning to FIGS. 3A and 3B, upper portion  301  of thermophoretic tunnel  16  has a relatively higher temperature compared with a relatively lower temperature of lower portion  303  of thermophoretic tunnel  16 . As a result, an exemplary particle e.g.,  305  experiences a thermophoretic force vector  307  together with a flow force vector  309  to give a resultant force vector  311 . Preferably the magnitude of the resultant force vector  311  is sufficient to displace a particle to a lower portion of the thermophoretic tunnel prior to the particle exiting the thermophoretic tunnel. FIG. 3B is a schematic representation of the operation of the thermophoretic tunnel  16  displacing entrained particles, e.g.,  305 A, to a lower portion of the thermophoretic tunnel  16 , e.g.,  305 B, to exit through outlet  22 C, while a relatively particulate free liquid exits through outlet  22 B to return to the main liquid conduit  12 . Returning to FIG. 1, outlet pipe  18 C carries the separated entrained particles to a particle collector  32  for removing the entrained particles from the liquid.  
         [0041]    In another embodiment, heat pipe array e.g.,  34 A thermally connects particle collector  32  with heat sink  30  in order to maintain a thermal gradient to improve the thermophoretic performance of particle collector  32 . It will be appreciated that a separate heat sink may be optionally used to thermally communicate with particle collector  32 . The particle collector  32  preferably has a means to capture and entrap particles driven by, for example, a thermophoretic force into such means. In an exemplary embodiment, referring to FIG. 4A, outlet pipe  18 C from thermophoretic tunnel  16  forms inlet  18 C which feeds particle collector feed  34  centrally disposed in particle collector  32  to carry the particulate containing liquid into a bottom portion of particle collector  32  where the particulate containing liquid begins to fill the particle collector  32  following flow direction arrows e.g.,  32 A and  32 B. In an exemplary embodiment, disposed around particle collector feed  34  are particle depositors e.g.  36 , forming for example, a vertical spaced apart stack of disk shaped plates having a centrally located opening through which particle collector feed  34  passes, each plate having concentric protrusions or fins (see FIG. 4B) extending from an upper particle depositing surface (see FIG. 4B) for capturing and entrapping particles driven toward the depositing surface.  
         [0042]    For example, referring to FIG. 4B, is an expanded view of an exemplary embodiment of the particle collecting means including a portion of the particle depositors e.g.,  36 . The particle depositor e.g.,  36  is formed of, for example, a thermally conductive disk shaped plate  36 A having a depositing surface  36 B, a peripheral portion being thermally connected to the thermally conductive particle collector walls, e.g.  38  in FIG. 4A. The particle collector walls, e.g.  38  are further thermally connected via an array of heat pipes e.g.,  34 A (see FIG. 1) to a heat sink, for example heat sink  30 , for completing the thermal conduction pathway for forming a thermal gradient with a resulting thermophoretic force operating to displace entrained particles toward particle depositing surface  36 B. Each disk shaped plate  36 A is equipped with means for capturing and holding particles contacting depositing surface  36 A. For example, a series of concentric protrusions or fins e.g.,  36 C, also preferably thermally conductive and chemically resistant (e.g., stainless steel), are formed to protrude upward from particle depositing surface  36 B thereby capturing and entrapping particles e.g.,  36 D at the particle depositing surface  36 B. Referring to FIG. 4C, a top view of the depositors e.g.,  36  is shown with the concentrically formed fins, e.g.,  36 C for capturing and holding particles contacting depositing surface  36 B including by operation of a thermophoretic force. For example, in an exemplary embodiment the fins  36 C protrude above the surface  36 B from about 0.5 mm to about 3 mm, are about 0.5 mm to about 3 mm in width, and are concentrically spaced from about 0.5 mm to about 3 mm.  
         [0043]    In another embodiment, referring again to FIG. 4A, particle collector  32  includes a drain port  40  in the lower portion of particle collector  32  for capturing and removing particles from particle collector  32  thereby recycling the depositors e.g.,  36  for filtering particles from the particulate containing liquid. In operation, after the particulate containing liquid passes through particle collector  32 , a relatively, particulate free liquid is returned via outlet pipe  34  to main liquid conduit  12  preferably near the converging area in the diffuser  13  as shown in FIG. 1. The outlet pipe  34  preferably includes in a flow path disposed between particle collector  32  and main liquid conduit  12  a flow regulator  36  similar to flow regulator  20  to selectively control a flow rate.  
         [0044]    In related exemplary embodiments, the main liquid conduit  12  is cylindrical with a diameter of from about 1 cm to about 4 cm in diameter. Further, the inlet pipe  18 A and outlet pipe  18 B, creating a bypass flow path through thermophoretic tunnel  16 , are cylindrical with a diameter from about 0.25 to about 1 cm in diameter with a radius of curvature for example, extending from about 5 cm to about 20 cm in arc length such that laminar flow is maintained through the thermophoretic tunnel  16 . In exemplary operation, the inlet pipe  18 A is formed such that a volumetric flow from about 2.5 percent to about 10 percent, more preferably about 5 percent, is re-directed from main liquid conduit  12  through inlet pipe  18 A and thermophoretic tunnel  16  such that laminar flow is maintained through thermophoretic tunnel  16 . In addition, a flow regulator  20 , in flow pathway of outlet pipe  18 B is selectively controllable to alter a flow rate to maintain a laminar flow through thermophoretic tunnel  16 . A volumetric flow entering the thermophoretic tunnel  16 , for example, may range from about 250 CC/min to about 1000 cc/min, more preferably about 500 cc/min, depending on the Reynolds number of the system, the flow rate preferably remaining below the point for the onset of turbulence. In other exemplary embodiments the flow channels, e.g.,  24  in thermophoretic tunnel  16  have dimensions of, for example, a width of about 0.25 to about 1 cm, a height of about 0.025 to about 1 cm and a length of about 20 cm to about 30 cm.  
         [0045]    In yet other embodiments, a plurality of thermophoretic systems are connected in series to increase the efficiency of particle removal from the particulate containing liquid. Additionally, a thermophoretic system may include a plurality of particle collectors connected in series to increase the efficiency of particle removal in a single thermophoretic system. In operation, one or more thermophoretic systems are serially placed in a flow path of the particulate containing liquid.  
         [0046]    According to the apparatus and method of the present invention, a thermophoretic system and method has been presented whereby a wide range of particle sizes present in a liquid may be effectively captured and removed from the liquid without creating an unacceptable pressure drop while providing for cost effective recycling of the particle filtering means.  
         [0047]    The preferred embodiments, aspects, and features of the invention having been described, it will be apparent to those skilled in the art that numerous variations, modifications, and substitutions may be made without departing from the spirit of the invention as disclosed and further claimed below.