Abstract:
A method of producing decabromodiphenyl alkanes includes the steps of charging a reaction vessel with bromine and a bromination catalyst and introducing a diphenyl alkane into the vessel at a location above the level of the charge bromine and catalyst. A dip tube apparatus for introducing the diphenyl alkane includes an inner tube and an outer tube, each of which are disposed above the surface of the bromine reaction vessel. The inner tube is fitted with a plug having an opening. Diphenyl alkane flows through the inner tube, out the opening in the plug, and into the reactor. The outer tube is disposed around and along the inner tube. Reaction mass from the vessel is recirculated from the vessel, through the outer tube and back to the vessel so as to form a curtain of reaction mass around the stream of diphenyl alkane being simultaneously fed into the reaction vessel.

Description:
This application is a divisional application of and claims the benefit of U.S. patent application Ser. No. 10/323,012 filed on Dec. 18, 2002, now U.S. Pat. No. 6,841,707, and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/345,063 filed Dec. 21, 2001, the complete disclosures of which are hereby expressly incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and an apparatus for producing decabromodiphenyl alkanes. More specifically, the field of the invention is that of producing decabromodiphenyl ethane. 
     2. Description of the Related Art 
     Halogenated aromatic compounds are often employed as flame retardant agents. Flame retardants are substances applied to or incorporated into a combustible material to reduce or eliminate its tendency to ignite when exposed to a low-energy flame, e.g., a match or a cigarette. The incorporation of flame retardants into the manufacture of electronic equipment, upholstered furniture, construction materials, textiles and numerous other products is well known. 
     Brominated aromatic compounds are often utilized as flame retardant agents in polymer compositions such as the outer housing of computers, television sets, and other electronic appliances. One group of halogenated flame retardants are decabromodiphenyl alkanes. The manufacture of decabromodiphenyl alkanes is known. Conventionally, decabromodiphenyl alkanes are prepared by reacting a diphenyl alkane with bromine in the presence of a bromination catalyst, such as AlCl 3  or FeCl 3 . 
     For example, U.S. Pat. No. 5,030,778 to Ransford discloses a process for producing decabromodiphenyl alkanes in which bromine and a bromination catalyst are charged to a reaction vessel. Liquid diphenyl alkane is fed by a dip tube into the reaction vessel at a point which is beneath the level of the charged liquid bromine and catalyst. The stated advantages of this sub-surface addition method are that (1) a product with a high average bromine number is obtained faster when the diphenyl alkane is fed below the surface of the charged liquid bromine and catalyst; and (2) splattering of the reaction mass associated with the addition of the diphenyl alkane into the vessel is reduced. 
     One disadvantage to adding the diphenyl alkane to the vessel at a location below the surface of the charged bromine and catalyst is that the dip tubes used for adding the diphenyl alkane to the vessel are prone to plugging. It is believed that the sub-surface addition dip tubes become plugged when a small amount of diphenyl alkane remains at the tip of the tube and reacts in place, thereby forming insoluble, high melting point material. It is believed that this is more likely to occur at the end of the addition or if the diphenyl alkane addition is interrupted. This susceptibility to plugging prevents the manufacturer from being able to stop and start the diphenyl alkane addition, which is sometimes desirable for controlling the evolution HBr gas. 
     It is also believed that the agitation of the reaction mass may create a vortex within the tip of the sub-surface addition dip tube. This vortex may pull solids from the reaction mass into the tube, thereby creating a blockage. Additionally, because some diphenyl alkanes, such diphenylethane (“DPE”), are solids at room temperature and are fed to the reaction vessel as liquids, they may begin to crystallize in an unheated dip tube if the feed is interrupted for any reason. In the event that the sub-surface dip tube does become plugged, regardless of the reason, the diphenyl alkane feed must be stopped and the tube must be pulled out of the reactor in order to remove the blockage. It is desirable to avoid the need to remove the dip tube, as the vapor space of the reaction vessel is filled with toxic and corrosive bromine vapors which may escape during removal. 
     The above-surface diphenyl alkane addition technique of the present invention reduces dip tube plugging, thereby providing a more efficient method of adding diphenyl alkane to a reactor charged with bromine and catalyst. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for producing decabromodiphenyl alkanes, particularly decabromodiphenylethane. In one embodiment of the invention, a diphenyl alkane, such as diphenylethane, is fed into a reaction vessel containing liquid bromine and a bromination catalyst via a dip tube located above the surface level of the charged liquid bromine and catalyst. The DPE may be fed to the reaction vessel under pressure and at a relatively high velocity from a point above the level of the bromine and catalyst in the reaction vessel. 
     Another method for preparing decabromodiphenyl alkanes according to the present invention includes the steps of charging a reaction vessel with bromine and a bromination catalyst, providing a dip tube apparatus having a first end and a second end located in the reaction vessel above the surface level of the bromine and the bromination catalyst, introducing diphenyl alkane through the dip tube apparatus such that the diphenyl alkane flows from the first end, to the second end and enters the reaction vessel at a point above the surface level of the bromine and bromination catalyst in the reaction vessel, reacting the diphenyl alkane with the bromine and the bromination catalyst thereby forming a reaction mass, and recirculating the reaction mass through the dip tube apparatus so as to form a curtain of recirculated reaction mass around the diphenyl alkane being introduced into the reaction vessel. An excess of bromine above that needed to brominate the diphenyl alkane to the desired degree is utilized. 
     In accordance with the present invention, the crude decabromodiphenyl alkane, such as decabromodiphenylethane, obtained by the aforementioned process is isolated and purified. Any one of numerous known isolation and purification methods may be utilized. For example, the solid may be isolated through direct removal from the slurry by filtration or centrifugation. The solid decabromodiphenyl alkane could as be removed by combining the reaction mass with water and striping the bromine through the use of heat and/or a vacuum. In one method of the invention, the crude decabromodiphenyl alkane is first made into a water slurry, and is transferred to a another reactor. The slurry is washed with an alkaline solution and the solid decabromodiphenyl alkane is separated and washed with water. The solid decabromodiphenyl alkane is then in the form of a filter cake. The filter cake may be treated in any one of a number of known ways. For example, the cake may first be dried and then fractured and/or heat treated in a number of ways. The wet decabromodiphenyl alkane product may also be treated without first drying, as disclosed in U.S. Pat. No. 4,659,021 to Bark, et al. In one embodiment of the present invention, the filter cake is dried and then ground twice in an air mill using air which is heated to an inlet temperature of approximately 260° C. The ground decabromodiphenyl alkane is then heat-treated at approximately 240° C. for three to four hours in order to release any free bromine which may be present in the product. The release of free bromine improves the purity and the color of the end product. Other known isolation and purification methods may also be utilized, such as those disclosed in U.S. Pat. No. 4,327,227 to Ayres, et al. and U.S. Pat. No. 5,030,778 to Ransford. 
     An apparatus for preparing decabromodiphenyl alkanes according to the present invention includes a dip tube apparatus disposed above the surface level of the charged liquid bromine in the reaction vessel and includes an inner tube and an outer tube. The inner tube has a first end and a second end. The first end is adapted to receive a flow of diphenyl alkane. The second end of the inner tube is adapted to receive a plug. The plug includes a hollow portion and an opening. When the inner tube and the plug are engaged, the plug and the inner tube are in fluid communication with one another. The diphenyl alkane flows through the inner tube, through an opening in the plug, and into the reaction vessel at a location which is above the surface level of the charged liquid bromine and the bromination catalyst. Plugs having a variety of opening sizes and configurations may be chosen in order to achieve the desired flow rate diphenyl alkane. Alternatively, the plug can be eliminated and the opening through which the diphenyl alkane flows can be formed in one end of the inner tube. 
     The outer tube of the dip tube apparatus extends around the inner tube. The reaction mass within the reaction vessel is pumped out of the reaction vessel, through the outer tube, and then back into the reaction vessel. The outer tube is spaced from the inner tube, such that the recirculated reaction mass re-enters the reaction vessel in the form of a curtain surrounding the diphenyl alkane stream. The curtain of recirculated reaction mass does not come into contact with the stream of diphenyl alkane being simultaneously added to the reaction vessel through the inner tube. 
     Other features of the present invention will be apparent to those of skill in the art from the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating the dip tube apparatus according to one embodiment of the present invention disposed in a reaction vessel; 
         FIG. 2  is a perspective views of a plug that is a component of the dip tube apparatus of  FIG. 1 ; and 
         FIG. 3  is an end view of the dip tube apparatus taken along line  3 - 3  of  FIG. 1 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The examples set out herein illustrate a preferred embodiment of the invention, in one form, and are not to be construed as limiting the scope of the invention is any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , dip tube apparatus  10  is disposed in reaction vessel  11 . Reaction vessel  11  may be any size desired, but for commercial applications, a reactor of at least 3,000 gallons is typically preferred. 
     Dip tube apparatus  10  includes inner tube  12  and outer tube  14 . Inner tube  12  may be manufactured out of Teflon® (polytetrafluoroethylene) or any other suitable material having similar properties, and outer tube  14  may be manufactured of Kynar® (polyvinylidene fluoride) or any other suitable material having similar properties. Inner tube  12  includes first end  13   a  and second end  13   b . First end  13   a  is adapted for receiving a flow of a diphenyl alkane, such as, for example, DPE, as indicated by arrow A. Second end  13   b  is adapted for receiving plug  18 , as described below. 
     Outer tube  14  extends along and surrounds inner tube  12  as shown. Outer tube  14  includes first end  15   a  and second end  15   b .  FIG. 3  illustrates an end view of dip tube apparatus  10  taken along line  3 - 3  of  FIG. 1 . As shown in  FIG. 3 , spacers  25  serve to separate inner tube  12  from outer tube  14 . Spacers  25  are may be manufactured from a 15% glass Teflon® material or any other suitable material. 
     With reference to  FIG. 2 , plug  18 , in the embodiment shown, includes a hollow portion  20  and a generally disc-shaped portion  24 . Disc-shaped portion  24  includes a first surface  26   a  having an opening  26   b  therein. Note that in the embodiment shown, the side of disc-shaped portion  24  is flared. Hollow portion  20  and opening  26   b  are in fluid communication such that DPE flowing through hollow portion  20  will exit plug  18  through opening  26   b . The size and shape of opening  26   b  may be varied depending upon the desired flow rate of DPE into reaction vessel  11 . Typically, opening  26   b  will have a diameter of between about 0.0625″ and about 0.0937″. In the embodiment shown, second end  13   b  of inner tube  12  includes a threaded portion  16  which is adapted to receive corresponding threads  16  of plug  18  such that plug  18  is secured within second end  13   b  of inner tube  12 . This arrangement allows the size and shape of opening  26   b  to be changed by changing plug  18 . Other mechanisms for securing plug  18  to inner tube  12  may also be utilized. When plug  18  is secured within second end  13   b  of inner tube  12 , inner tube  12  and plug  18  are in fluid communication such DPE flowing through inner tube  12  will flow into hollow portion  20  and out opening  26   b . Plug  18  may be manufactured from Teflon® or any other suitable material. 
     Note that plug  18  can be completely eliminated by constructing second end  13   b  of inner tube  12  with an appropriately sized and shaped opening for introducing DPE into reaction vessel  11 . Of course, this would mean that the entire inner tube  12  would have to be changed to change the size and shape of the opening. 
     In the embodiment shown in  FIG. 1 , dip tube apparatus  10  is disposed in reaction vessel  11  such that second end  15   b  of outer tube  14  is located at the same level as second end  13  of inner tube  12 . Both second end  15   b  of outer tube  14  and second end  13   b  of inner tube  12  are located a distance D above surface  26   a  of plug  18 . In the embodiment shown, distance D corresponds to the thickness of portion  24  of plug  18 . In one embodiment of the invention, distance D is about 0.5 inches. Note also that it is not necessary that surface  26   a  of plug  18  be located below second end  15   b  of outer tube  14 . That is, inner tube  12  and plug  18  may be positioned such that surface  26   a  is recessed within outer tube  14  and is located further from level L than is second end  15   b  of outer tube  14 . As further shown in  FIG. 1 , surface  26   a  of plug  18  is located a distance X above surface level L of the charged bromine and catalyst. In one embodiment of the invention, distance X is about 4 inches. Typically, distance X will be between about 1 inch and about 12 inches. 
     If an inner tube  12  is utilized without a plug  18 , as described above, inner tube  12  and outer tube  14  would be positioned such that second end  13   b  of inner tube  12  is located a distance D below second end  15   b  of outer tube  14 . Similarly, inner tube  12  would be positioned such that second end  13   b  is located a distance X above level L. 
     In use, DPE is fed, under pressure, through inner tube  12  and emitted as a stream  36  via opening  26   a  into reaction vessel  11 . The DPE preferably has a purity level of about 99.7% or greater, however, DPE of different purity levels can be used depending on the desired characteristics of the final product. The pressure at which the DPE is fed is preferably at least about 20 psig and preferably between about 20 psig and about 60 psig. The DPE stream  36  preferably has a velocity within the range of about 9 meters per second to about 25 meters per second. In one embodiment of the invention, the DPE is fed at about 17.5 meters per second under a pressure of about 30 psig. An agitator (not shown) operating at approximately 47-88 rpm and disposed within reaction vessel  11  mixes the DPE and the bromine reaction medium, thereby facilitating the reaction. 
     As the DPE is added to reaction vessel  11 , the reaction mass  38  is recirculated to first end  15   a  of outer tube  14  and allowed to flow through outer tube  14  (shown in  FIG. 1  by arrows B), along inner tube  12  and back into reaction vessel  11 . Preferably, reaction mass  38  is recirculated at a rate of between about 45 gallons per minute to 250 gallons per minute. In this manner, a curtain  34  of reaction mass  38  is formed around DPE stream  36 . The distance Y between curtain  34  and DPE stream  26  is preferably at least about 0.5 inches. In one embodiment of the invention, distance Y is about 0.867″. Note that DPE stream  26  does not come into contact with curtain  34 . The flared sides of portion  24  of plug  18  assists in keeping the DPE stream  26  and the curtain  34  separated. Note that any portion of reaction mass  38  that splashes as a result of the DPE feed will be contained by curtain  34  and carried back into the remainder of the reaction mass  38 . 
     Decabromodiphenylethane may be obtained according to the present invention by reacting DPE in the presence of an excess of bromine and a bromination catalyst. The molar ratio of bromine to DPE is between about 18:1 and about 50:1. Preferably, the ratio is between about 18:1 to about 39:1. Appropriate bromination catalysts include aluminum halides, such as aluminum chloride and aluminum bromide, as well as iron powder. Other catalysts may also be used. 
     During the DPE addition, the temperature of the reaction mass is preferably in the range of about 50° C. to about 60° C. Preferably the temperature of the reaction mass is maintained at 55° C. until the DPE feed is complete. The temperature is then increased to approximately 60° C. and held constant for the duration of the reaction time. The reaction time will vary depending upon the amount of DPE being added, and upon the rate at which the DPE is added. For commercial production, reaction times will likely be between three and six hours. 
     As noted above, isolation and purification of the resulting decabromodiphenylethane may be carried out in a variety of ways. In one method, the decabromodiphenylethane slurry is placed in a pressurized vessel charged with water which has been heated to approximately 70° C. Once the slurry addition is complete, the temperature within the vessel is increased to approximately 100° C. to facilitate the removal of any remaining free bromine. The water slurry is transferred to a tank which is charged with a solution which is 25% alkaline. The resulting slurry is then fed to a centrifuge where the solid decabromodiphenylethane product is separated and washed with water. The solid is in the form of a filter cake. The filter cake of decabromodiphenylethane is then dried and ground twice in an air mill using air heated to an inlet temperature of approximately 260° C. The resulting material is heat-treated at approximately 240° C. for 3-4 hours. The final product preferably has a yellowness index of below about 10, and more preferably below about 9. 
     EXAMPLE 
     A 3000 gallon glass lined Pfaudler reactor was equipped with a vertical H-type baffle, condenser system, 41″ Pfaudler Cryo-lock reverse curve agitator, temperature sensor, and the dip tube apparatus described above. 21,210 kg (132,278.41 moles) of liquid bromine and 63.5 kg (476.26 moles) of aluminum chloride were charged to the reaction vessel. The agitator was then turned on and ranged in speed from 47 to 88 rpm. The reactor was heated to 55° C. 618 kg (3395.6 moles) of DPE was then charged through the dip tube, as described above, at a rate of 4-8.5 lbs./min. The DPE addition ranged from 3.5 to 5 hours. Once the feed was completed, the reaction temperature was increase to 60° C. and held at that temperature for thirty minutes. 
     A 4000 gallon glass lined Pfaudler reactor was equipped with a vertical H-type baffle, condenser system, 41″ Pfaudler Cryo-lock reverse curve agitator, and temperature sensor. The vessel was charged with 1850 gallons of water. The water was then heated to 70° C. The bromine slurry from the reactor was fed into the vessel over a 2.5 to 3 hour period, while maintaining 5 to 5.5 lbs. of pressure within the vessel. Once the slurry addition was complete, the temperature within the vessel was raised to 100° C. to complete the bromine removal. 
     The water slurry was transferred to a 5000 gallon tank equipped with an agitator. The tank was charged with 475 gallons of a 25% caustic (alkaline) solution. The resulting slurry was then fed to a centrifuge where the solid was separated and washed with water. The filter cake was then dried. The resulting material was then ground twice by passing it through a Fluid Energy Aljet air mill using air heated to 260° C. The product was then heat treated in a Wyssmont drier at 240° C. for 3 to 4 hours. 
     The conditions and results of the reactions run are illustrated in the following table. Note that some of the reactions were run without utilizing the curtain of recirculated reaction mass. In these runs, the DPE was simply fed under pressure to the reaction vessel at a point above the level of the bromine and bromination catalyst. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Bromine 
                 Bromine 
                 Bromine 
                 Bromine 
                   
                   
                 DPE 
                 DPE 
                   
               
               
                   
                 Agitation 
                 Hole 
                 Weight 
                 Weight 
                 temp 
                 temp 
                   
                 DPE 
                 Weight 
                 Linear 
               
               
                   
                 Rate 
                 Diameter 
                 (rcyl) 
                 (vir) 
                 initial 
                 final 
                 DPE 
                 Rate 
                 total 
                 Feed Rate 
                 Wet cake 
               
               
                 Run # 
                 RPM 
                 (inches) 
                 lbs. 
                 lbs. 
                 ° C. 
                 ° C. 
                 Assay % 
                 lbs/min 
                 lbs. 
                 m/sec 
                 GC Assay % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Above Surface 
               
             
          
           
               
                 20 
                 88 
                 0.0625 
                 23360 
                 23360 
                 56.7 
                 54.2 
                 98.7 
                 4.62 
                 1363 
                 17.7 
                 99.07 
               
               
                 21 
                 88 
                 0.0937 
                 47510 
                 0 
                 53.3 
                 53.3 
                 98.7 
                 5.4 
                 1363 
                 9.2 
                 98.93 
               
               
                 22 
                 88 
                 0.07813 
                 37360 
                 9340 
                 58.3 
                 51.7 
                 98.7 
                 4.4 
                 767 
                 10.8 
                 98.82 
               
               
                 23 
                 88 
                 0.07813 
                 46700 
                 0 
                 57.8 
                 53.3 
                 98.7 
                 10 
                 640 
                 24.5 
                 98.7 
               
               
                 24 
                 88 
                 0.07813 
                 46710 
                 0 
                 57.2 
                 57.2 
                 98.7 
                 4.3 
                 741 
                 10.5 
               
               
                 25 
                   
                 0.07813 
                 46710 
                 0 
                 56.7 
                   
                 98.7 
                   
                 463 
                 0.0 
               
             
          
           
               
                 Above surface curtain 
               
             
          
           
               
                 26 
                 75 
                 0.07813 
                 46710 
                 0 
                 56.7 
                 55.6 
                 98.7 
                 5 
                 1363 
                 12.2 
                 99.102 
               
               
                 27 
                 75 
                 0.07813 
                 46720 
                 0 
                 55.0 
                 56.1 
                 98.7 
                 5.05 
                 1363 
                 12.4 
                 99.05 
               
               
                 28 
                 75 
                 0.07813 
                 43280 
                 0 
                 56.7 
                 58.3 
                 98.7 
                 4.99 
                 1263 
                 12.2 
                 99.24 
               
               
                 29 
                 46 
                 0.07813 
                 43340 
                 0 
                 59.2 
                 57.3 
                 98.7 
                 5.3 
                 1263 
                 13.0 
                 99.14 
               
               
                 30 
                 46 
                 0.07813 
                 43321 
                 0 
                   
                   
                 98.7 
                   
                 1263 
                 0.0 
                 98.83 
               
               
                 31 
                 47 
                 0.07813 
                 46710 
                 0 
                   
                   
                 98.7 
                   
                 1338 
                 0.0 
                 99.18 
               
               
                 32 
                 47 
                 0.07813 
                 45110 
                 0 
                 59.2 
                 56.4 
                 98.7 
                 5.4 
                 1315 
                 13.2 
                 98.98 
               
               
                 33 
                 47 
                 0.07813 
                 45100 
                 0 
                 56.1 
                 56.7 
                 98.7 
                 4.7 
                 326 
                 11.5 
                 98.8 
               
               
                 34 
                 47 
                 0.07813 
                 45100 
                 0 
                 56.1 
                 57.8 
                 98.7 
                 5.89 
                 1315 
                 14.4 
                 99.01 
               
               
                 35 
                 47 
                 0.0625 
                 45120 
                 0 
                 55.9 
                 56.6 
                 98.7 
                 5 
                 1315 
                 19.1 
                 99.08 
               
               
                 36 
                 47 
                 0.07813 
                 46660 
                 0 
                 58.9 
                 57.8 
                 98.7 
                 5.37 
                 1315 
                 13.1 
                 99.2 
               
               
                 37 
                 47 
                 0.07813 
                 46730 
                 0 
                 56.5 
                   
                 98.7 
                   
                 1773 
                 0.0 
                 98.6 
               
               
                 38 
                 47 
                 0.07813 
                   
                   
                   
                   
                 98.7 
                   
                   
                 0.0 
                 99.27 
               
               
                   
               
             
          
         
       
     
     While this invention has been described with reference to specific embodiments, the present invention may be further modified within the spirit and scope of the disclosure. This application covers such departures from the present disclosure as come within the known or customary practice in the art to which the invention pertains and which fall within the limits of the appended claims.