Patent Publication Number: US-2003233937-A1

Title: Apparatus and method for continuously removing air from a mixture of ground polyurethane particles and a polyol liquid

Description:
RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Application 60/372,270, filed Apr. 11, 2002. 
    
    
     
       BACKGROUND OF THE SYSTEM  
       [0002] 1. Field of the System  
       [0003] This system relates to devices and processes for continuously preparing mixtures of powdered solids in liquids, further relates to processes for continuous removal of entrained bubbles from such mixtures. The system is particularly useful for the preparation of bubble-free slurries of finely ground polyurethane-foam particles in polyol, and for the preparation of new polyurethane articles that contain the finely ground polyurethane-foam particles.  
       [0004] 2. Background  
       [0005] “Polyurethane” (PUR) describes a general class of polymers prepared by polyaddition polymerization of polyisocyanate molecules and one or more active-hydrogen compounds. “Active-hydrogen compounds” include water and polyfunctional hydroxyl-containing (or “polyhydroxyl”) compounds such as diols, polyester polyols, and polyether polyols. Active-hydrogen compounds also include polyfunctional amino-group-containing compounds such as polyamines and diamines. A common example of an active-hydrogen compound is a polyether polyol such as a glycerin-initiated polymer of ethylene oxide or propylene oxide. Polyether polyols useful for slabstock flexible polyurethane foams generally have a molecular weight in the range of 2000 to 6000 g/mol, a functionality in the range of 2 to 7 (preferably about 3), and a viscosity at 25° C. generally in the range of 100 to 10,000 mPa-s.  
       [0006] “PUR foams” are formed via a reaction between one or more active-hydrogen compounds and a polyfunctional isocyanate component, resulting in urethane linkages. PUR foams are widely used in a variety of products and applications. These foams may be formed in a wide range of densities and may be flexible, semi-flexible, semi-rigid, or rigid foam structures. Generally speaking, “flexible foams” are those that recover their shape after deformation. In addition to being reversibly deformable, flexible foams tend to have limited resistance to applied load and tend to have mostly open cells. “Rigid foams” are those that generally retain the deformed shape without significant recovery after deformation. Rigid foams tend to have mostly closed cells. “Semi-rigid” or “semi-flexible” foams are those that can be deformed, but may recover their original shape slowly, perhaps incompletely.  
       [0007] A foam structure is formed by use of so-called “blowing agents.” Blowing agents are introduced during foam formation through the volatilization of low-boiling liquids or through the formation of gas due to chemical reaction. For example, a reaction between water and isocyanate forms carbon dioxide (CO 2 ) gas bubbles in PUR foam. This reaction also generates heat and results in urea linkages in the polymer. Additionally, surfactants may be used to stabilize the polymer foam structure during polymerization. Catalysts are used to initiate the polymerization reactions forming the urethane linkages and to control the blowing reaction for forming gas. The balance of these two reactions, which is controlled by the types and amounts of catalysts, is also a function of the reaction temperature. A typical foam recipe includes at least one polyol, at least one isocyanate, and also typically includes water, surfactant, and catalysts, and also optionally includes additional blowing agent, fillers, and additives for color, fire performance, antimicrobial activity, etc.  
       [0008] Polyurethane foam can be ground into fine particles using, for example, cryogenic processes or roll mills (see for example, U.S. patent application Ser. No. 09/748,307 filed Dec. 21, 2000, incorporated herein by reference). These fine particles can then be used to replace chemicals in recipes for new foam; this provides an environmental benefit and often a cost savings.  
       [0009] In order to add polyurethane powder to the recipe, the powder must be mixed with liquid reactants. Further, the slurry must be free of entrained bubbles because they create an undesirable irregular cell structure in the foam, including holes and splits. Powder can be mixed with liquid reactants in a batch process by adding a known mass of powder to a known mass of polyol, mixing thoroughly, and allowing sufficient time (generally about 8 to 48 hours) for entrained bubbles to leave the slurry. Such a natural de-gassing process takes a long time because the slurry has a high viscosity, generally about 500 to 20,000 mPa-s. A continuous process for removal of entrained air is preferred over a batch process because the continuous process would not require waiting for entrained air to leave the slurry naturally, and the continuous processes would not require the large storage capacity necessary to hold the slurry needed for an entire day of foam production.  
       [0010] Methods are desirable for producing bubble-free slurries of fine particles of polyurethane in polyol, and for producing foam with these slurries. Particularly, a need exists for devices and processing methods to take finely ground polyurethane-foam particles, disperse them as a slurry in polyol, remove substantially all entrained bubbles from the slurry, and to use this slurry as a direct replacement for at least some of the polyol in the production of new polyurethane articles. It is further desirable for such a process to be able to run continuously, such that powder and polyol are mixed, de-gassed, and used to make new polyurethane articles without delay. The desired continuous process must be able to deliver bubble-free slurry with an accurately controlled solids concentration at an accurately controlled flow rate.  
       SUMMARY OF THE SYSTEM  
       [0011] The present system comprises devices, methods and an integrated process for the continuous production of substantially bubble-free slurries of solid powders in liquids and for the delivery of such slurries at an accurately controlled concentration and flow rate. The devices comprise a mixer and a surge tank.  
       [0012] Powder and liquid (for example, finely ground polyurethane powder and polyol liquid) are delivered continuously to a mixer where they are contacted intimately and a slurry is produced. The slurry, which may contain entrained air bubbles, is delivered from the mixer to a de-gassing step, where entrained bubbles are continuously removed. Substantially bubble-free slurry is continuously delivered from said de-gassing step to downstream uses.  
       [0013] The mixer is designed to mix into a vessel of liquid (for example, polyol) a light, fluffy, low-bulk-density solid that has a tendency to float (for example, finely ground polyurethane foam). The mixer drives the solids under the surface of the liquid in a tank using an auger, breaks up clumps of powder that otherwise tend to float rapidly because they hold interstitial air, and provides at least one mixing impeller to disperse and wet the powder. The mixer is integrated into a tank, which is optionally provided with internal baffles and an internal screen to retain any un-dispersed clumps of powder until they are fully wet out and dispersed. A slurry can be produced continuously with the mixer by delivering powder at a known, controlled rate to the mixer, delivering polyol at a known, controlled rate to the tank, and drawing the slurry away from the vessel.  
       [0014] The method for continuous removal of entrained bubbles from the slurry operates by means of a centrifuge operating in a vacuum environment. Slurry is delivered to the center of a bowl that is rotating with a high tangential velocity, for example about 10 to 100 m/s. The slurry spreads out thinly, rupturing any entrained bubbles. The entire rotating bowl is housed within a chamber in which the pressure is maintained at a vacuum. The rotating bowl throws the slurry along the wall of the bowl away from the center of the bowl. At some distance from the center of the bowl, the slurry is collected by a pickup tube. The energy imparted by the spinning bowl is sufficient to pump the de-gassed slurry out of the centrifuge, optionally through a one-way valve (check valve), into a surge tank.  
       [0015] The surge tank of the disclosed system provides a place for bubble-free slurry to accumulate without re-entrainment of bubbles. Bubble-free slurry can be continuously drawn from the bottom of the surge tank and continuously added from the centrifuge. The inlet connection where the bubble-free slurry flows into the surge tank has an internal weir that forces the slurry to flow down the wall of the tank. This keeps the slurry from re-entraining bubbles upon start-up and also allows the slurry to be added to the top of the tank. Adding slurry to the top of the tank and removing it from the bottom is important because it is preferable to operate the tank on a first-in-first-out basis. Additional inlets with weirs can be provided to accommodate slurry recirculating from downstream processes such as from the mix head of a polyurethane foam machine.  
       [0016] The method disclosed comprises delivering powder and liquid to the mixer to produce a slurry, and subsequently continuously removing entrained bubbles from the slurry. Powder is delivered to the mixer at a known, controlled rate by means known to those skilled in the art. For example, a loss-in-weight feeder may be used. The mixer drives the powder under the surface of the slurry in the mix tank. Polyol is delivered to the mix tank at a known, controlled rate by means known to those skilled in the art. For example, a non-cavitating positive-displacement pump may be used. The mixer disperses and wets out the powder. Mixed slurry is continuously drawn from the bottom of the mix tank at a rate substantially equivalent to the total rate of addition of powder and polyol. The slurry is transferred to the vacuum centrifuge, where entrained bubbles are removed. The bubble-free slurry is pumped by the centrifuge to the disclosed surge tank, where it runs down the wall of the tank to avoid re-entraining any bubbles. Bubble-free slurry is drawn from the bottom of the surge tank at a rate substantially equivalent to the rate at which slurry is added to the surge tank. The slurry drawn from the bottom of the surge tank is immediately ready for use in polyurethane foam manufacture. It can be pumped from the surge tank, optionally through a heat exchanger or a mass-flow meter directly to the mix head of a foam machine.  
       [0017] A better understanding of the features and advantages of the present system will be obtained by reference to the following detailed description of the system and accompanying drawings which set forth an illustrative embodiment in which the principles of the system are utilized. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0018]FIG. 1 is a process-flow diagram showing a process for the continuous production of substantially bubble-free slurry.  
     [0019]FIG. 2 is a simplified schematic diagram of a mixer according to the present system.  
     [0020]FIG. 3 is a simplified schematic diagram of the mixer of FIG. 2 integrated with a mix tank.  
     [0021]FIG. 4 is a simplified schematic diagram of a centrifuge within a vacuum chamber for removal of entrained air bubbles from a slurry.  
     [0022]FIG. 5 is a simplified schematic diagram of an embodiment surge tank of the present of system.  
     [0023]FIG. 6 is a flowchart illustrating an embodiment of a process for the continuous production of substantially bubble-free slurry. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0024] Those of ordinary skill in the art will realize that the following description of the present system is illustrative only and not in any way limiting. Other embodiments of the system will readily suggest themselves to such skilled persons.  
     [0025] A schematic diagram of a process of the present disclosure  100  is shown in FIG. 1. Solid powder  105 , such as polyurethane, melamine, calcium carbonate, barium sulfate, talc, pigment and fire retardant, (supplied for example in bulk bags) is conveyed by suitable means  110  (for example, a flexible screw conveyor) to the hopper of a powder feeder  120 . For instance, the powder feeder may be a loss-in-weight belt or screw feeder. The powder feeder  120  delivers powder at a known and controlled rate to the mixer  200  (for example the disclosed mixer described in detail below) in tank  250 . A liquid component such as liquid intermediate for manufacture of thermoset polymers (for example, polyol) is added to tank  250  through inlet  251 . This liquid component is added at a known and controlled rate, for example by means of a non-cavitating positive-displacement pump  170 , with a known and controlled speed of rotation, drawing from a tank (not shown) or other source of liquid  130 . Alternatively, the rate of liquid addition can be measured with a flow meter. The mixer  200  in tank  250 , as described in detail below, performs the function of thorough mixing of the powder into the liquid, with good dispersion and wet-out of the powder. The mixed slurry, which also generally contains entrained bubbles, exits tank  250  through an outlet  254 . Drain valves  260  are provided throughout the process for cleanup and shutdown. Tank  250  may have a vent  259 , a continuous-level sensor  258 , a high-level sensor  256 , and a low-level sensor  257  for measurement and control (see FIG. 6) of the liquid level in the tank. The ratio of powder addition rate to liquid addition rate to the mixer/tank is held constant.  
     [0026] At mix-tank outlet  254 , entrained bubbles may typically be present in the slurry at a concentration on the order of about 10% by volume. Mixed slurry is drawn from tank  250  through outlet  254  by any suitable means (gravity, pump, vacuum suction), for example a positive-displacement pump  270 , and transferred to inlet  320  of vacuum centrifuge  300 . Vacuum centrifuge  300  continuously removes entrained bubbles from the slurry as describe in connection with FIG. 4 below. Vacuum is drawn from vacuum centrifuge  300  to maintain within it a low absolute pressure, for example less than 1050 mbar, preferably between 1 and 300 mbar. Vacuum is drawn using suitable means, for example vacuum pump  353  with regulator  351 , condensation trap  352 , pressure measurement device  354 , and exhaust to the atmosphere  356 . Substantially bubble-free slurry exits the centrifuge  300  through outlet  335 . Check valve  350  allows the slurry to pass to an inlet  401  of at least one surge tank  400  without backflow of slurry or air into the centrifuge.  
     [0027] Surge tank  400 , or a plurality of such tanks, may be large enough to hold enough slurry for a full day of foam production. In such a case, the process is used to prepare a large batch of substantially bubble-free slurry. FIG. 1 depicts a process that is also suitable for producing slurry continuously and using that slurry for foam production without delay.  
     [0028] Surge tank  400 , described in detail below, may have a vent  459 , and continuous-level sensor  458 , a high-level sensor  456 , and a low-level sensor  457  for measurement and control (see FIG. 6) of the liquid level in the tank.  
     [0029] Substantially bubble-free slurry exits the surge tank through outlet  405 , drawn by any suitable means (gravity, pump, vacuum suction), for example a non-cavitating positive-displacement pump  470 . The mass flow rate is measured with meter  490 , for example  490  may be an in-line coriolis-effect mass-flow meter. By means of three-way diverter valve  494 , the slurry is transferred to downstream processes (for example, foam manufacture) via conduit  495 , or recirculated to inlet  404  of surge tank  400  via conduit  496 . The option for recirculation to the surge tank allows simple integration of the process to existing slabstock polyurethane-foam manufacturing plants. For instance, valve  494  may be located in close proximity to the mix head of a slabstock machine, mixing process  100  can be running prepared slurry via recirculation line  496 , and substantially bubble-free slurry can be available to the foam-manufacture process immediately with a turn of valve  494 . Similarly, valve  494  can be used to switch off downstream use of the slurry without interruption of mixing process  100 . This effectively decouples the downstream foam-manufacture process from any startup or shutdown transients of mixing process  100 .  
     [0030] Optionally, the prepared slurry may be heated or cooled using heat exchanger  480 . The temperature and pressure of the slurry may be measured using meters  492  and  460  respectively.  
     [0031] Referring now to FIGS. 2 and 3, the mixer assembly  200  comprises a barrel  201  within which a shaft  202  is rotated by a motor  203  through coupling  204 . An auger  205 , a lump-breaker  206 , and at least one impeller  207  are attached to and rotate with shaft  202 . Powder is added to the mixer at a known, controlled rate through port  208 . A flexible boot  209  seals the port to the perimeter of powder feeder  255  in such a way as to avoid blowing dust while maintaining mechanical isolation of the mixer from powder feeder. If the powder feeder is a loss-in-weight feeder, mechanical isolation of the feeder from the mix tank is important so that the weight measurement is not biased. Powder drops through port  208  onto auger  205 . The auger rapidly moves the powder through barrel  201  to the outlet  210 . A bearing plate  211  is positioned at the outlet  210  of the barrel  201  and supports a bearing  212  for shaft  202  by means of a spider (not shown).  
     [0032] The outlet of the barrel  210  is positioned below the working liquid level  214  in the tank  250 . Powder exits outlet  210  and is rapidly dispersed into the surrounding liquid by lump-breaker  206 . The lump-breaker comprises a radial arrangement of stiff pins affixed around a central hub. The pins are long enough to span the entire opening of outlet  210 . The lump-breaker is positioned very close to the outlet so that no large lumps of powder may pass.  
     [0033] Impellers  207  are positioned below the lump-breaker and are of suitable size and design to provide multiple turnovers of the tank volume within the mean residence time of the powder. Preferably, at least one impeller is a radial-flow high-shear design. Axial-flow impellers (for example, marine impellers) provide higher flow and more tank turnovers. Preferably, one impeller is placed near the lump-breaker for good mixing, and a second impeller is placed near the bottom of the tank to avoid settling of solids.  
     [0034] The liquid component of the slurry (for example, polyol) is added to tank  250 , preferably at a position near the working liquid level  214 , by means of at least one inlet  251 . The tank preferably has a plurality of baffles  252  to reduce the formation of a vortex. Near the bottom of the tank, a screen plate  253  is attached. The screen plate has a plurality of openings that allow mixed slurry to pass through, but that returns larger un-dispersed lumps of powder for additional mixing. Preferably the openings in the screen plate have a size of about 0.1 to 1 cm. Mixed slurry leaves the mix tank through an opening  254  in the bottom.  
     [0035]FIGS. 4 and 5 show schematic diagrams of a vacuum centrifuge  300  useful for removal of entrained bubbles from a liquid and a surge tank  400  useful for accumulating substantially bubble-free liquid without re-entrainment of bubbles. In the context of the vacuum centrifuge and surge tank, a “liquid” includes pumpable mixtures of solid particles in liquids, such as slurries of powders in liquids or slurries of solids such as polyurethane, melamine, calcium carbonate, barium sulfate, talc, pigment and fire retardant in a liquid intermediate for manufacture of thermoset polymers. In the present example, for illustrative purposes only, finely ground polyurethane powder is dispersed a polyol liquid.  
     [0036] Referring now to FIG. 4, liquid to be de-gassed is delivered to inlet  320  of the centrifuge and travels through conduit  321  to bowl  310 . Bowl  310  is attached to shaft  346  such that the bowl and shaft are rotated by suitable means, for example motor  345 . When bowl  310  is rotated such that the tangential velocity is high the liquid spreads out thinly along the inside surface of the bowl, forcing bubbles to the surface under centripetal force and rupturing them. In one embodiment, the tangential velocity may be between about 1 and 100 m/s. In another embodiment, the tangential velocity may be between about 5 and 75 m/s. Finally, in yet another embodiment, the tangential velocity may be between about 10 and 50 m/s. The liquid pools as far away from the axis of rotation as possible. A pickup tube  333  catches the liquid and directs it through conduit  334  to outlet  335 . The rotation of the bowl imparts sufficient energy to the liquid to pump it through conduit  334 .  
     [0037] A baffle  329  is affixed to conduit  321  and serves to keep liquid from splashing out of the bowl.  
     [0038] The spinning bowl  310  is disposed within a vacuum chamber  325 , which has front plate  326  and back plate  327  attached with gaskets to make the chamber vacuum tight. Plate  327  contains a bearing-and-seal assembly  340 , through which shaft  346  rotates. Plate  326  optionally has a sealed sight glass  328  to allow viewing of the spinning bowl. Vacuum is applied to the chamber, for example through fitting  330 . The low absolute pressure within the chamber prevents bubbles from being re-entrained in the liquid as it is picked up ( 333 ) and directed toward outlet  335 . In one embodiment, the absolute pressure may be less that 500 mbar. In another embodiment, the absolute pressure may be less than 300 mbar. Finally, in yet another embodiment, the absolute pressure may be between about 1 and 300 mbar. Substantially bubble-free liquid is available from outlet  335 . A drain  332  may be provided to facilitate cleanup and shutdown.  
     [0039] Referring now to FIG. 5, a liquid is delivered to inlet  401 . Inlet  401  is positioned near the top of the tank, while outlet  405  is at the bottom of the tank. This arrangement of inlet and outlet provides for desirable first-in-first-out operation of the tank, but presents the potential for entrainment of bubbles due to jetting or splashing of the liquid. Weir  402  prevents the liquid from jetting into the tank, rather the liquid is diverted such that it runs down the wall of tank  407 . Flexible weir extension  403  keeps the liquid running down the wall of the tank even at low flow rates. Liquid runs between the extension  403  and the wall of tank  407 , and sucks extension  403  against the wall. This avoids any liquid dripping, “raining”, or otherwise falling through an air space between inlet  401  and free surface  406 . Extension  403  may be made of flexible plastic sheet, for example.  
     [0040]FIG. 5 depicts the tank  407  as partially full, as it would be during startup from an initially empty condition. During initial filling, the liquid has the furthest distance to travel from inlet  401  to the free surface  406 , and there is potential for entrainment of bubbles. Forcing the liquid to run down the side of the tank with weir  402  and extension  403  greatly reduces the potential for bubble entrainment.  
     [0041] Surge tank  400  may have a plurality of inlets  401 ,  404  with their associated weirs  402  and extensions  403 . For example, inlet  401  may be used to receive liquid coming from outlet  335  of vacuum centrifuge  300 , while inlet  404  may be used to receive liquid recirculating from outlet  405  through downstream processes.  
     EXAMPLE 1  
     [0042] A mass of 83.5 kg of VORANOL 3010A polyether polyol from Dow Chemical Co. was initially charged to a 30-gallon mix tank. To this tank was added 16.7 kg of a powder of finely ground polyurethane foam with a maximum particle size of 250 microns. The initial batch was mixed thoroughly using the mixer shown in FIG. 2. The slurry obtained had a concentration of entrained air of about 10% by volume.  
     [0043] Using the process shown in FIG. 1, slurry was pumped from the mix tank, through the vacuum centrifuge and surge tank, and into a storage vessel. The concentration setpoint was 20 pphp (i.e., 20 parts of powder per 100 parts of liquid, or 16.7% by mass), and the flow rate setpoint was 20 kg/min of slurry. Samples of the slurry were taken as it left the process to go into the storage vessel. These samples were tested for slurry concentration by separating the powder from a known weight of slurry by means of filtering the powder from the slurry, washing away the polyol with methylene chloride solvent, drying and weighing the powder. These samples were also tested for entrained bubble volume by filling a 1000-mL graduated cylinder with slurry and observing the change in volume over a 24-hour period at constant temperature. Any volume change indicates the presence of entrained bubbles in the slurry product. For example, a 1-mL volume change would correspond to 0.1% entrained bubble concentration by volume.  
     [0044] The results shown in TABLE 1 were obtained, which show no entrained air bubbles in the product slurry, and powder concentrations that were accurately near the setpoint concentration of 20 pphp.  
                           TABLE 1                       sample   time   measured powder   measured volume % of       ID   (min:sec)   concentration (pphp)   bubbles in the slurry                                                A   2:30   19.9   0       B   2:45   20.2   not measured       C   3:00   20.1   0       D   3:15   19.8   0                  
 
     [0045] A flow chart of a process of the present disclosure  600  is shown in FIG. 6. Powder  602  (for example, finely ground polyurethane powder) and liquid  604  (for example, polyol) are delivered continuously to a mixing step  606  (for example, using disclosed mixer  200 ) where the powder and liquid are contacted intimately and a slurry is produced. The slurry, which may contain entrained air bubbles, is delivered from the mixing step to a de-gassing step  608 , where entrained bubbles are continuously removed. For example, the de-gassing step may use a vacuum centrifuge and may further use the disclosed surge tank described below. Next, substantially bubble-free slurry is continuously delivered to a downstream use  610 . For example, downstream use  610  may be the production of polyurethane foam. Optionally, some or all of the slurry may be recycled back to the de-gassing step via  612  to the mixing step via  614 .  
     [0046] It should be understood that various alternatives to the embodiments of the disclosed process and apparatus descried herein maybe employed in practicing the disclosed process and using the disclosed apparatus. It is intended that the following claims define the scope of the disclosed process and apparatus and that processes and structures within the scope of these claims and their equivalents be covered thereby.