Patent Publication Number: US-7592416-B2

Title: Polystyrene processing apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 60/773,664, filed Feb. 16, 2006, the entire contents of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention is an apparatus and method for efficient processing of polystyrene (e.g., discarded expanded or foam polystyrene). 
     BACKGROUND OF THE INVENTION 
     Waste polystyrene, such as expanded polystyrene, presents particular economic and environmental problems. The density of expanded polystyrene is such that carrying waste expanded polystyrene on trucks is very wasteful. More specifically, expanded polystyrene has such a low density, and hence high relative volume per unit mass, that trucks used to transport waste expanded polystyrene often “bulk out”. In other words, trucks, semi-trucks, tractor-trailers, large rigs, 18-wheeler rigs, etc., can each be filled with expanded polystyrene without any reaching their load carry capacity. 
     In addition, the environmental impact of waste expanded polystyrene is a serious matter. Waste expanded polystyrene takes up a disproportionate amount of space at landfill sites. Recycling expanded polystyrene is economically wasteful in light of the “top out” problem. 
     There are several issued U.S. Patents devoted to dealing with the economic and environmental issues surrounding the burying or recycling of waste expanded polystyrene, such as U.S. Pat. No. 6,169,121, issued Jan. 2, 2001 to Noguchi et al. The &#39;121 patent describes a processing method for effectively and promptly removing insoluble components contained in a solution of a styrene resin waste material to enable regeneration of high-quality regenerated styrene. A solution of the styrene resin waste material in an organic solvent is processed with, for example, a dehydrating agent, to remove insoluble components. The resulting clarified solution is degasified on heating in vacuum to remove the organic solvent for recycling the resulting mass as a regenerated styrene resin. 
     U.S. Pat. No. 6,098,649, issued Aug. 8, 2000 to Aug. 8, 2000, describes a waste recovery system using this apparatus and a liquid container ideal for use with the waste processing apparatus and waste recovery system. The waste processing apparatus comprises a processing part for processing supplied waste (for example foam polystyrene) with a processing liquid (for example limonene) and a storing part for storing produced liquid (for example limonene containing dissolved foam polystyrene) produced in this processing part and provided with a removable liquid container constructed to both supply processing liquid to the processing part and receive produced liquid produced by the waste processing apparatus. In the waste recovery system, the liquid container is removed from the waste processing apparatus after receiving the produced liquid and carried to a waste recycling apparatus, the produced liquid is transferred from the liquid container into the waste recycling apparatus, waste or waste and processing liquid is recovered from the produced liquid by the waste recycling apparatus and the empty liquid container is filled with new or recycled processing liquid and returned to the waste processing apparatus again. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for processing polystyrene. The method includes the steps of: receiving a solution in the form of polystyrene polymer dissolved in an organic solvent; and separating the polystyrene from the solvent by applying heat to the solution to provide a heated solution and then spraying the heated solution to separate the polystyrene from the solvent. The apparatus includes: a holding tank at least partly filled with a solution of polystyrene dissolved in an organic solvent; at least one heat exchanger ( 170 ); and a separator in the form of a sprayer unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a processing flow diagram of a polystyrene processing apparatus according to an embodiment of the present invention. 
         FIG. 2  shows a further process layout according to the invention. 
         FIG. 3  shows a schematic diagram of how polystyrene and a solvent such as d-limonene is mixed and filtered prior to heating by a heat exchanger. 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is directed to an apparatus and method for efficient processing of polystyrene (e.g., expanded or foam polystyrene). The apparatus of the present invention is denoted generally by the numeric label  100 . 
       FIG. 1  is a processing flow diagram of a polystyrene processing apparatus  100  according to an embodiment of the present invention. Polystyrene, such as expanded polystyrene, dissolved in solvent is held in resin/solvent tank  120  from which it is directed through a particle remover  140  to remove solids to provide a substantially solid free solution of polystyrene and solvent. The substantially solid free solution of polystyrene and solvent is thence directed to at least one heat exchanger  170  by operation of a pump  180 . The heat exchanger  170  heats up the substantially solid free solution of polystyrene, which is then directed to sprayer  200 . The sprayer  200  is located inside a chamber  220 . The chamber  220  includes a heating jacket  240 . At least one heater  260  is used to provide heated liquid or steam to heating jacket  240  under the operation of at least one pump  270 . 
     The amount of heat delivered to the bottom  240   b  of the heating jacket  240  should be adjusted to prevent the separated polystyrene polymer from becoming too viscous for efficient plant operation in the bottom part  220   b  of chamber  220 . For example, the Applicant has noted that a temperature of about 370° F. maintained in the bottom part  240   b  of the heating jacket  240  might be sufficient, but it will be understood by a person or ordinary skill in the art that the temperature of the bottom  240   b  of heating jacket  240  (and, therefore  220   b ) may vary according to the particular set-up and configuration of the chamber  220  and a person of ordinary skill in the art could establish by routine operation of the plant what temperature should be applied to the bottom  240   b  of the heating jacket  240 . Likewise, the temperature of in the vicinity of an at least one spray nozzle  205  can be adjusted to ensure that the solution (polystyrene/solvent) is atomized to a sufficient degree to allow for efficient separation of the polymer from the solvent through evaporation of droplets  210 . A filter, such as glass wool filter  305 , can be located at the top of chamber  220  near an output port  310  to prevent strands of styrene carrying over from the styrene-polymer/solvent into the heat exchanger  340 . 
     The chamber  220  is used to separate the dissolved polystyrene from the solvent. A vacuum pump  320  is used to lower the pressure inside chamber  220 . The vacuum pump  320  can be a liquid ring vacuum pump to allow the solvent to be moved while maintaining a vacuum. The sprayer  200  generates droplets  210  of polystyrene dissolved in solvent, which are directed downward. The solvent evaporates from the droplets  210  and is drawn upwards by operation of vacuum pump  320 . The solvent is then cooled by heat exchanger  340 , then by heat exchanger  350  (with the vacuum pump  320  located between heat exchangers  340  and  350 ). Solvent is collected in a solvent recovery tank  360 . 
     The chamber  220  is under a vacuum during operation. The vacuum pressure and nozzle pressure can vary and still provide satisfactory operation. More particularly, the vacuum pressure in chamber  220  and nozzle  205  pressure may vary depending based on such factors as heat in the chamber  220  (residual or maintained), GPM or nozzle volume throughput, pressure of produce and laminar flow at nozzles  205 , vacuum separation inherent of atomization based on Newton fall. During optimum operation a vacuum of 18″ Hg (eighteen inches Mercury pressure) in chamber  220  should be sufficient to maintain satisfactory operation, but the vacuum pressure in chamber  220  can vary between about 10″ Hg and about 29″ Hg vacuum pressure. Nozzle  205  pressure of about 77 PSI (77 pounds pressure per square inch) should provide satisfactory operation, but the nozzle pressure can vary between about 12 PSI and about 85 PSI. 
     Recovered solvent held in tank  360  can be reused to dissolve unprocessed polystyrene. As the droplets  210  lose solvent, the previously dissolved polystyrene now exists in the form of dense polystyrene polymer. The polymer is optionally agitated by an agitation section  380  prior to exiting at  400 . A sufficient amount of heat is delivered to the heating jacket  240  to maintain a temperature of about 370° F. at the bottom of the chamber  220  to prevent the separated styrene polymer from becoming too viscous and difficult to handle. The separated styrene polymer can be converted into pellets by a pelletizer (not shown) as described in U.S. Pat. No. 6,169,121, issued Jan. 2, 2001 to Noguchi et al. 
     It should be understood that members, such as member  170  (i.e., heat exchanger  170 ), might comprise a single unit (i.e., one heat exchanger  170 ) or operate in parallel to increase or meet throughput requirements. Thus, it will be understood by a person of ordinary skill in the art that the process and apparatus shown in  FIG. 1  can vary in number, rated throughput or rearranged (e.g., in units operating in parallel) and still remain within the scope of the claimed invention. 
     Still referring to  FIG. 1 , a heater  280  is used to provide heat transfer medium to the heat exchanger  170  under the operation of pump  300 . The heat transfer medium can take any suitable form such as liquid or gas (e.g., steam). It will be understood by persons of ordinary skill in the art that the various elements shown in  FIG. 1  can be arranged differently without detracting from the spirit of the invention. For example, heaters  260  and  280  can be replaced with a single heater without detracting from the spirit of the invention. 
     The Applicant has devoted a considerable amount of effort into solving the problem of separating polystyrene polymer from resin-solvent solution by means of vacuum spray drying. The resin-solvent solution is extremely viscous and difficult to heat uniformly in a heat exchanger. Polystyrene-solvent ratios of 30/70 (i.e., 30% polystyrene resin concentration) present serious problems in terms of throughput and uniform heating. The resin tends to stick to heat transfer surfaces thereby deleteriously impacting on heat transfer. Resin-solvent ratios of 30/70 are difficult to heat up using, for example, steam heated tube or multiple tube heat exchangers. The resin sticks to the interior surfaces of the tubes thus creating a low heat conducting barrier and thence causing lower throughput due to compromised heat transfer. Resin-solvent ratios of 30/70 will gum up the sprayer  200  if the resin-solvent is not sufficiently heated. Heating reduces viscosity such that under temperature 30/70 ratio will have too high a viscosity thereby causing gumming up of sprayer  200 . After a considerable amount of effort, the Applicant evolved a solution by proposing spray drying of polystyrene resin/solvent in combination with a heat exchanger upstream of the sprayer  200  and a heating jacket  240  in combination with vacuum pump  320  as shown in  FIG. 1 . 
     Instead of using spray drying under vacuum to separate the solvent from the dissolved polystyrene as taught in the present invention, the prior art teaches applying boiling to separate the solvent from the polystyrene. For example, U.S. Pat. No. 6,169,121 teaches clarifying the solution of polystyrene and solvent by a separator ( 8 ) into polystyrene and the solvent. This separator ( 8 ) is heated by the thermal medium of a boiler ( 9 ) to approximately 240° C. Boiling at such a high temperature is at least partly necessary because the rate of separation of the solvent from the polystyrene is a function of surface area through which the solvent can escape from the polystyrene. 
     In the present invention, the Applicant makes use of a heat exchanger  170  in combination with sprayer  200  and vacuum pump  320 , wherein the heat exchanger  170  enables rapid uniform heating of the polystyrene/solvent liquid and the sprayer  200  produces droplets with surface area favorable for more rapid conversion of the solvent into gas and hence speaks to a more rapid separation of polystyrene from the solvent leading to increased throughput of polystyrene/solvent. The combination of sprayer  200  (especially with twister type nozzles, see later) in combination with the heat exchanger  170  enables separation of the polystyrene/solvent mix without boiling the solvent off as required in the prior art &#39;121 patent. Thus, the process of the present invention is far more efficient than that of the prior art as disclosed, for example, in the prior art &#39;121 patent. 
     It should be understood that the solvent (i.e., processing liquid for dissolving polystyrene such as expanded polystyrene dissolved in a solution of solvent such as d-limonene at, for example, about 30% concentration, i.e., a 30/70 polystyrene/solvent solution, respectively) is not limited to one particular solvent, though limonene and more specifically d-limonene is particularly suited as a solvent for dissolving expanded polystyrene to provide a polystyrene/solvent solution. The solvent can be at least one solvent chosen from a group consisting of an aromatic organic solvent, a hydrocarbon organic solvent, an ether organic solvent, an ester organic solvent, a ketone organic solvent and a monoterpene organic solvent. Also, the solvent can be at least one solvent chosen from the group consisting of limonene, isoamyl acetate, benzyl propionate and ethyl butyrate. In addition, the solvent can be at least one solvent chosen from the group consisting of d-limonene, isoamyl acetate, benzyl propionate and ethyl butyrate. The solvent can be d-limonene. 
     The temperature range of operation for the heat exchanger  170  is typically chosen to deliver the solution (i.e., polymer/solvent) at the sprayer  200  in the range between about 235° F. and 255° F. A person of ordinary skill in the art would monitor the temperature of the solution entering the sprayer and adjust the heater  280  to ensure the temperature of the solution leaving the heat exchanger  170  is high enough to ensure that the temperature of the fluid entering the sprayer  200  is in the range between about 235° F. and 255° F. It is preferred that the solution is not heated above about 350° F. Expanded polystyrene is sometimes pre-treated with a bromine-based fire retardant, which can cause problems if the solution is heated above about 350° F. 
     The sprayer  200  should be capable of atomizing the solution to ensure rapid evaporation of the solvent and hence separation of the polystyrene from the solvent. The sprayer  200  makes use of at least one nozzle  205 . Any suitable nozzle can be used so long as the nozzle atomizes the solution, i.e., produces a stream of droplets  210  sufficiently small to allow rapid evaporation of the solvent. The at least one nozzle  205  can produce, for example, a hollow coned spray, a full coned spray or a flat spray. The at least one nozzle  205  can be at least one nozzle chosen from a group consisting of a hollow coned spray nozzle, a full coned spray nozzle, a flat spray nozzle, and a twister nozzle. An example of a twister nozzle is the YS series twister nozzle supplied by BEX Incorporated (located at various sites including: 37709 Schoolcraft Road, Livonia, Mich., 48150-1009 USA). BEX Inc. also provides a wide selection of hollow coned, full coned and flat spray nozzles. 
     Spraying Systems Company also provide a range of spraying system nozzles such as, but not limited to, a full jet, ⅜″, 316SS with a 22 drill (Part # 3/8HH-316SS22). Spraying Systems Company can be contacted at: PO Box 7900, Wheaton Ill. 60189-7900, Phone 630-665-5000. A suitable nozzle type is described in U.S. Pat. No. 3,104,829 issued Sep. 24, 1963 to Wahlin. In addition, Spray Systems Company provides a new range of stainless steel IceJet™ inlet cooling and evaporation nozzles. The IceJet™ inlet cooling and evaporation nozzle utilizes a whirl chamber to produce a fine mist of droplets. 
       FIG. 2  shows a further process layout according to the invention. 
       FIG. 3  shows a schematic diagram of how polystyrene and a solvent such as d-limonene is mixed and filtered prior to heating by heat exchanger  170 . Example ranges of polystyrene concentration (as volume %) with respect to d-limonene are provided. Specifically, the example range is 1/99 (where the d-limonene solvent contains 1 volume % polystyrene) to 60/40 (where d-limonene solvent contains 60 volume % polystyrene). It will be understood that the exact way of mixing can vary without detracting from the spirit of the claimed invention. 
     In one non-limiting example (referred to here as “Example #1”), the bottom  240   b  of the heating jacket  240  is configured to function, at least partly, as a porous heated plate. In this example, the heating jacket  240  is a basic round vessel that is approximately ½ inch thick (variable based on throughput) and has oil circulating in between 2-10 gauge plates at the bottom end  240   b , which is also angled inwards and has, for example, multiple ½ inch openings with oil caused to flow around them. The angle is in relation so that molten polystyrene will flow through into the bottom of the vessel without allowing the mist (originating from droplets  210 ) to go straight through. The intent of this heated and angled bottom part  240   b  with holes is to allow the polystyrene to flatten and thereby increase surface area then flow into the bottom  220   b  of vessel  220 . The temperature at the bottom  240   b  can be in the range 350° F. to 375° F. The upper part of the chamber  220  is maintained at about 390° F. 
     Still referring to Example #1, the solution (of polystyrene/solvent) enters #22 nozzle at 60-80 psi in (larger than needed 3.4 GPM @1800cP) but there is a possibility of a stringing effect (due to the larger orifice) up to 165° F. in a 70/30 (solvent/polystyrene) ratio and stringing or sporadic effect up to 180° F. due to possible laminar flow and centipoises value of the mass. Hence a spiral incorporated into the at least one nozzle  205  to control momentum diffusion at a specific point and keeping the product temp around 235° F. at the least one spray nozzle  205  (if d-limonene solvent is used, 235° F. is about 15° F. degrees above boiling point of the solvent). Some atomizing effect might be lost because of substance consistency but the height of the vessel and substance velocity should be large enough that any un-flashed solvent will loosen from entrainment through the product/chamber vessel  220  temperature being high enough to create vapor pressure greater than the polymer entrainment ability. Also the Newton force is pulling on the mass (polystyrene) not on the vapor (unless entrained), hence the use of the porous heated section  240   b  (any entrained vapor will be released upon impact with  240   b ). It is preferred, but not totally necessary, that no more than about 5% of vapors are recovered at blast plate  240   b . It should be understood that the terms “blast base plate  240   b ” and “bottom  240   b  of the heating jacket  240 ” are regarded in this embodiment as equivalent terms. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the spirit of the invention.