Abstract:
Improved mist eliminator ( 50 ) for use in pulp washers, deinking cells and the like, and methods of use are disclosed. A cyclonic gas/liquid separator ( 50 ) is provided that includes an elongated housing ( 100 ) with a cyclonic flow inducing vane assembly ( 108 ) disposed at a medial location along the length of the housing ( 100 ) to divide the separator ( 50 ) into two tandem cyclonic action zones ( 150, 152 ) of approximate equal volume within the housing ( 100 ). An initial separation is effected in the first cyclonic action zone ( 150 ) followed by final separation in the downstream or second cyclonic action zone ( 152 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority filing benefit of (1) International PCT application PCT/US00/18964 filed Jul. 12, 2000, and published under PCT 21(2) in the English language and (2) U.S. Provisional Application Ser. No. 60/143,406 filed Jul. 12, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus for the separation of gas and liquid from a flowing mixture of gas and liquid. In particular, the invention relates to an improved gas/liquid separator of the cyclonic variety. 
     In various industrial processes, such as in washing wood pulp, it is advantageous to draw air from over a reservoir of liquid by means of a vacuum provided by a blower intake. The air is usually drawn through the blower and provided under pressure to a portion of an apparatus, such as a pulp washer, to create a region of positive pressure. However, the air drawn by the vacuum created by the blower may have liquid or solid particles entrained therein, particularly where the liquid has foam or froth covering its surface. For proper operation of the blower and the apparatus, it is necessary to separate the entrained particles from the air before the air is taken in by the blower. 
     Various means of separating a gas from a gas/liquid flow mixture are known in the art. In particular, separation of liquid and solid particles from gas streams by cyclonic action is known. For example, a contact-and-separating element of a vortex tray of a liquid-gas mass-transfer apparatus using cyclonic separation is disclosed in U.S. Pat. No. 4,838,906 to Kiselev. Similarly, a dust collection system using cyclonic separation is disclosed in U.S. Pat. No. 2,393,112 to Lincoln. 
     In a typical cyclonic separator, a gas mixture having particles entrained therein is drawn vertically upward. A cyclonic rotation is imparted to the flowing mixture, typically by means of helical or spiral vanes. Centrifugal force causes heavier particles to be forced radially outwardly toward the outer periphery of the flow mixture where the particles drop back downward under the force of gravity. The gas mixture without the heavier particles continues upwardly. 
     Cyclonic gas/liquid separators may be used in a flat bed wood pulp washer generally similar in construction and mode of operation to a Fourdrinier paper machine incorporating an endless foraminous belt (“wire”), a headbox which delivers the pulp suspension in a pulping liquor to one end of a horizontally traveling upper run of the wire, successive washing zones along the length of the run, and means at the downstream end of the run for receiving and removing the resulting washed pulp. Pulp washers of this type, manufactured by the assignee of the present invention in accordance with Ericsson U.S. Pat. No. 4,154,644 of 1979, have been notably successful, and the present invention was developed to improve the operation and results obtained by such pulp washers. 
     The operation of a pulp washer of this type may be described as being according to the displacement washing principle. That is, once the pulp mat has been formed, it is not rediluted, but simply is subjected to repeated washings by application on top of the mat of washing liquid with the liquid applied in each washing zone having a lower concentration of liquor than the filtrate from the preceding zone. The liquid applied in each zone enters the mat substantially en masse and thereby displaces the liquid which was carried into the zone in the mat and causes it to drain therefrom through the wire. 
     The mechanical elements of a washer according to the Ericsson patent include a hood which encloses the entire apparatus downstream from the headbox, and a series of receptacles below the operating run of the wire in sealed relation with the hood. In operation, vacuum is applied to the receptacles, and/or gas pressure is developed within the hood, to augment the action of gravity in forcing the washing liquid through the pulp mat on the wire. Gases and vapors drawn through the wire into the upper spaces in the receptacles are recycled back to the hood to increase the pressure differential above and below the wire. 
     In the pulp washer, a gas/liquid separator or mist eliminator is attached to each of the receptacles at a gas outlet on top of the receptacle to thereby permit the gases and vapors to be drawn from the flat top receptacles through the mist eliminator to the suction side of a pump or fan that recycles the gas, in this case air, to the hood. However, entrained within the gases and vapors drawn from the receptacle are particles of mist and foam from the space between the wire and the pulping liquor. 
     Prior art mist eliminators employing cyclonic separating means have been used to remove particles of mist and foam from the gas flow before the gas reaches the pump or blower. The typical mist eliminator has a cylindrical housing arranged vertically proximate the top of the receptacle with a vacuum line attached to the top end. The gas inlet is positioned above the level of the liquor with cyclonic-flow-inducing means, typically helical vanes or a swirler, positioned within the inlet. Droplets of mist and particles of foam are hurled radially outwardly by centrifugal force from the gas/liquid flow to drop back down into the liquor under the force of gravity. 
     However, in many cases the upward draft of gas through the mist eliminator impinges upon the falling, separated liquid droplets. This tends to retard the separation of undesired droplets of mist and particles of foam entrained within the gas flow. Thus, incomplete separation of foreign matter from the gas results. Mist and foam entrained within the gas flow result in a lower pressure differential being developed between the hood and the receptacle, thereby reducing the efficiency of the pulp washer. 
     In accordance with the disclosure of WO Publication 98/29179 (of common assignment herewith) an improved mist eliminator is provided wherein an annular zone is formed in the separator where falling droplets of separated water and liquid drain downwardly in the device substantially without resistance from countercurrent flow of gas and liquid. 
     Although the mist eliminator set forth in the aforementioned WO Publication has proven commercially successful, it was found that in some instances, where dense foams were encountered in the suction box, turbulence was actually increased in the liquid directly beneath the separator, leading to inefficient separation. Accordingly, there is a need in the art for a mist eliminator device of enhanced efficiency that is capable of separating foam components that may exist in the suction box. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are met by the provision of a cyclonic separation device wherein two distinct cyclonic action zones are provided to perform the desired separation of the liquid components from the gas component. The device includes a generally elongated cylindrical housing with inlet end and outlet end disposed at opposite axial ends of the cylinder. Within the cylinder and at an approximate medial position along the longitudinal axis of the housing, a swirl imparting vane structure is located. This vane structure forms a boundary defining a upstream cyclonic zone and downstream cyclonic action zone. The location of the swirl imparting vane structure at an approximate mid-point along the length of the cylinder contrasts with many prior art designs where the swirl vanes are located adjacent the inlet end of the separator. 
     The outlet of the housing is connected to a suitable suction source which draws the gas/liquid mixture to be separated into the inlet portion of the housing, passing generally upwardly through the housing in an upstream to downstream direction. The mixture is first separated in the upstream cyclonic zone with an additional separation performed in the downstream cyclonic action zone. 
    
    
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view in side elevation showing a horizontal Ericsson-type of washer that may incorporate, as a component thereof, an improved mist eliminator in accordance with the invention 
     FIG. 2 is a cross-sectional view taken along the lines and arrows  2 — 2  of FIG.  1  and detailing the system location of the improved mist eliminator of the invention; 
     FIG. 3 is a partially broken away side elevational view of a improved mist eliminator in accordance with the invention shown in its position in a suction box receptacle of the type provided in Ericsson washers 
     FIG. 4 is a partially broken away side elevational view of the improved mist eliminator of the invention. 
     FIG. 5 is a magnified cutaway side elevational view of the swirl imparting vane assembly of the mist eliminator shown in FIG. 4; and 
     FIG. 6 is a diagrammatic view illustrating use of the improved mist eliminator in a recycle pulp deinking cell. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1, there is shown diagrammatically a pulp washing apparatus and system in which the improved mist eliminator of the present invention may be advantageously employed. As shown, an endless foraminous belt  11 , usually a wire of woven plastic filaments, is trained around a breast roll  12  at the upstream wire run location, a couch roll  14  at the downstream run location and around drive rolls  16   a ,  16   b , and tensioning rolls  18   a ,  18   b  on the return run of the belt to the upstream location. A headbox deposits the pulp suspension to be washed onto the upstream end of the wire run. 
     A smooth, foraminous material  22  of low frictional coefficient such as a perforated polyethylene sheet is provided below and in supporting relation to the upper run of the wire  11 . Disposed directly underneath the sheet and mounted on a frame (not shown) are a plurality of receptacles  24   a - 24   f . Each of these is, in effect, a suction box, bounded at the top by the perforated sheet. As more fully explained in the Ericsson patent, the receptacles  24   a - 24   f  are interconnected and operated so that they form a series of successive zones along the path of the wire run comprising a formation zone  28  adjacent the headbox  20  and consecutive washing zones  30   a - 30   e , the last of which  30   e  is adjacent the downstream end of the wire run. 
     A hood  40  is supported by a frame (not shown) and is positioned in enclosing relationship to the zones  28  and  30   a - 30   e . As most clearly shown in FIG. 2, the hood provides a seal over the receptacles. 
     Turning back to FIG. 1, each of the receptacles is provided with a drain line  42   a-f . Drain lines  42   c-f  communicate with pumps  44   c  to  48   f  to return liquid to shower heads  46   c-f  through liquid return lines  48   c  to  48   f  to provide wash water to effect displacement washing of the mat  51  of fibers as it travels from the upstream to downstream direction along the upper run of the wire  11 . Clean water or white water is passed through line  252  so that the pulp mat adjacent the couch roll is washed with relatively clean water. The washed pulp after leaving washing zone  30   e  exits the device via the assistance of auger  254 . 
     Water drained from receptacle  24   b  through drain line  42   b  is forwarded to an evaporator tank, with water drained from upstream receptacle  42   a  forwarded to a blow tank. Lines  42   a  and  42   b  are connected via valve (not shown) so that water from line  42   a  may be directed to the evaporator if desired and, conversely water from line  42   b  could, if desired, be channeled to the blow tank. 
     In simplified form, the apparatus of FIG. 1 serves to evenly distribute pulp fed from the headbox onto the moving wire. In the formation zone  28 , pulp is dewatered from inlet consistency to displacement consistency, forming a pulp mat. Receptacles  24   a-e  serve as suction boxes under the wire to collect the liquid passing therethrough. 
     Displacement washing of the mat occurs when the mat goes under the shower where the filtrate from each succeeding washing stage flows onto and through the pulp. The device depicted in FIG. 1 employs multiple stages of displacement washing, the number of which will depend on the finish and the washing efficiency required. Dewatering and displacement of shower liquid is a function of the arrangement of the receptacle (suction box) pumps and a blower (explained thereinafter) and the hood. 
     Turning now to FIG. 2 it can be seen that gas (usually air) is provided to the hood by means of blower  256 . Air from each suction box is returned to the blower via line  258 . A pressure differential of between about 1-4″ Hg exists between the hood and the suction box atmosphere so that this pressure differential provides the driving force for the shower liquid to flow through the mat on the wire. 
     The hood maintains a seal between the atmosphere and the receptacle. The gauge value of the pressure in the hood may be positive, zero or negative depending on the desired washer operation. Filtrate from line  48   e  is used to provide wash water to shower head  46   e . Filtrate from receptacle  24   d  is drained through line  42   d  where it is pumped by pump  44   d  to upstream shower head  46   d  (See FIG.  1 ). Line  42   d  is provided with a liquid level control valve  260  operatively associated with pump  44   d  to control the liquid level inside of the receptacle  24 . 
     Disposed above the liquid level in receptacle  24   d  is improved liquid/gas separator  50  of the invention. Separator  50  is mounted in the receptacle  24   d . Drainage from the separator passes directly into the liquid in the receptacle  24   d  via drain tube  76 . A valve  78  controls the flow of gas back to the suction side of blower  256 . 
     Although separator  50  is shown mounted within suction box  24   d , it is to be appreciated that it could also be spaced closely adjacent to the box  24   d . The term proximate as used herein is intended to cover mounted arrangements where the separator is mounted in, contiguous to, or closely adjacent the receptacle or suction box provided that it is located upstream from a the control valve  78  that is used to regulate the fluid flow through the separator device. This disposition contrasts sharply with the disposition of the mist eliminators in many prior art Ericsson devices in which the mist eliminators were located close to the fan or blower. In these prior art devices a plurality of receptacles were connected to the mist eliminator via a manifold arrangement and lengthy inlet or feed line to the mist eliminator. 
     The separator  50  serves to separate foam and liquid from the gas that is to be recycled to the hood by blower  256 . Foam and liquid separation have become critically important as higher soap content pulps such as Southern Pine Kraft pulps are increasingly used and as washer throughput rates are increased. 
     Turning now to FIGS. 3 and 4, the separator  50  is shown in greater detail. As shown, the separator comprises an elongated cylindrical housing  100  having as upstream inlet  102  and downstream outlet  104  provided at opposite ends along the longitudinal axis of the housing. The separator of mist eliminator is oriented vertically in the receptacle  30  of the pulp washer with the inlet of the housing spaced above the pulping liquor level  32 . 
     At the upstream end of the housing, an air straightening baffle  106  may be securely disposed via welding, brazing, or other attachment means to the inside of the housing. Spaced above baffle  106  is a cyclonic flow inducing vane assembly  108 . The assembly  108  is housed in cylindrical shroud  110 , coaxial to the longitudinal axis of the housing. The shroud and vane assembly are secured in the housing by welding of the four spacer tabs  112   a,b,c,d  to the inside wall of the cylindrical housing. 
     The vane assembly comprises a central rod  114  coaxial with the housing axis and secured to the rod and shroud are a pair of swirl imparting vanes  116 , 118 . As shown, each of the vanes is in the form of a spiral flight with each flight spanning 270° of the circumference of the rod  114 . The pitch (length) of each flight is approximately 1.5 times the diameter of the rod. The flights  116 ,  118  are out of phase with each other at an angle of 180°. The configuration and spacing of the flights is not critical provided that they impart a cyclonic swirling motion to the liquid/gas mixture traveling through the separator in an upstream to downstream direction from the inlet to the outlet. 
     Fixed to the outside of the shroud and inside of the housing is an annular rim  120  which provides an air tight seal in the area between the shroud and inside wall of the housing. The rim  120  is inclined relative to the longitudinal axis of the housing at an acute angle of about 10-25°; preferable 15°. At the lowest point on the rim (or, stated differently, at the point on the rim closest to the liquor level  32 ) an opening  122  is provided to allow for drainage of liquid. A drain tube  124  may be provided in communication with the opening  122  to drain directly into the pulp liquor. As shown, the tube drain exit  126  is actually submersed in the liquor. Similar to the disclosure of WO Publication 98/29179, the rim and housing define a substantially cylindrical region wherein droplets of mist and foam separated from the gas flow travel downwardly toward the drain with reduced resistance from the counter flowing gas. This improves mist and foam separation from the mixture. 
     At the downstream, outlet end of the housing, a vortex finder tube  125  is secured to the frusto-conical end  128  of the outlet. The outlet  104  communicates with return line  258  to return gas (usually air) to the blower  256  (FIG.  2 ). Flow rate is controlled via valve  78 . 
     In accordance with the invention, a first cyclonic action zone  150  is provided in the cylindrical housing between the baffle  106  and the vane assembly  108 . In addition, a second cyclonic action zone  152  is provided in the housing between the assembly  108  and the outlet  104 . In practice, it has been found that the first and second cyclonic action zones should be approximate equal volume. 
     In one field application of the mist eliminator disposed as shown in the suction box receptacle of an Ericsson type washer, substantial amounts of the dense foam floating along the liquor level were desirably drawn into the zone  150  for preliminary separation therein. After this initial separation, the gas/liquid mixture was further separated in the second cyclonic action zone  152 . Location of the vane assembly  108  at a generally medial disposition within the housing provides a more elevated position that in some of the prior art devices and thereby increases head pressure over the opening  122  to improve liquid drainage thru tube  124 . 
     In another aspect of the invention, the improved separation device is used to separate ink particles from a recycled fiber slurry. In typical deinking applications recycled fibers including mixed office waste, and old newsprint etc., are commonly fiberized in a hydrapulper or the like in the presence of chemical additives that are adapted to facilitate separation of the ink particles from the pulp. The ink particles that are dislodged are released from the fiber surfaces due to mechanical and/or chemical reaction are separated from the slurry via dispersion, washing and flotation processes. 
     The separator  50  of the invention may be ideally used in deinking cells to aid in separation of ink particles from the recycle pulp. In such cells, air bubble generators or the like, with or without chemical flotation enhancement agents, are provided in a cell or series of cells in which the particles are released from the fibers and are carried to the slurry surface. The particles normally float atop the surface in a foamy mass. 
     Turning to FIG. 6 of the drawings, there is shown deinking cell  200  of the type normally encountered in deinking processes. As shown, the cell includes an agitator  202  to promote air bubble formation. The ink particles are floated in the form of a dense foamy mass  204  atop the recycle pulp slurry  206 . The separator  50  is spaced over the foam  204 . A suction line  210  is connected to the outlet  104  of the separator. In this embodiment, the air straightening baffle normally positioned adjacent the separator inlet is omitted. It is noted that although only one separator  50  is depicted in the drawings, a bank or plurality of the same may actually be employed in conjunction with a deinking cell. 
     In operation, the vacuum drawn through the line  210  picks the dense mass  204  off the top of the recycle pulp slurry. The foamy mass  204  enters the first cyclonic zone  150  where it is subjected to a preliminary separation, then moving downstream through the vanes and into the second cyclonic action zone  152 . The ink particles are lower in specific gravity than the liquid so the cyclonic action causes same to be separated and removed through line  210 . 
     While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.