Abstract:
A process for producing polycarbonate is disclosed. Accordingly a pre-reaction mixture containing a bis(hydroxyaryl) compound and a diaryl carbonate is formed and introduced through the top of a reactive rectification column under condition calculated to bring about a reaction. The resulting hydroxyaryl is eliminated and diaryl carbonate in vapor form is introduced at the bottom of the reactive rectification column.

Description:
FIELD OF THE INVENTION  
         [0001]    This application relates to a process for the production of polycarbonate by the melt transesterification process using reactive rectification.  
         BACKGROUND OF THE INVENTION  
         [0002]    Production of polycarbonates by melt transesterification proceeds by reacting bisphenols [bis(hydroxyaryl) compounds], preferably bisphenol A, with diaryl carbonates, preferably diphenyl carbonate, with elimination of the hydroxyaryl component from the carbonic acid diester; when diphenyl carbonate is used, phenol is eliminated. By sustained continuous or discontinuous removal of the hydroxyaryl component, for example phenol, the reaction equilibrium is shifted, making the formation of high molecular weight polycarbonates possible. A distinction is generally drawn in this connection between the so-called low viscosity stages at the beginning of the reaction, during which polycarbonate oligomers are formed and a large proportion of the liberated hydroxyaryl is separated and the so-called high viscosity stages, during which highly viscous polycarbonates are obtained towards the end of the reaction using special surface-forming apparatus. Separation of the hydroxyaryl in the low viscosity stages generally proceeds by distillation. It is known that a stirred-tank reactor with an attached distillation column can be used. The disadvantage of this method is that the process is performed batchwise and not by distillation and that the long residence times in the stirred-tank reactor may result in damage to the product. Alternatively, the hydroxyaryl may be separated in a multistage evaporator cascade, for example of falling film evaporators. One disadvantage of this continuous process is that, on flash evaporation in such apparatus, relatively large quantities of diaryl carbonate are also evaporated and therefor lost from the reaction mixture. As a result an excess of diaryl carbonate has to be supplied to the process. In addition the evaporator cascade entails considerable plant and equipment costs and complexity.  
           [0003]    The object was to provide a simpler process for the production of polycarbonate using the melt transesterification process by reacting diaryl carbonates (DAC) with bisphenols [bis(hydroxyaryl) compounds]. Description of the figures 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a schematic description of the inventive process. Accordingly reaction unit (reactive rectification column) ( 1 ) has an upper inlet ( 2 ) a lower inlet ( 3 ) and a discharge location, or outlet unit ( 13 ). ( 4 ) indicates an evaporator for diaryl carbonate; ( 5 ) is the pre-reaction unit. A final falling film evaporator unit ( 9 ) connected downstream from ( 1 ) has a separate enrichment column ( 10 ) that includes a condenser ( 12 ) and enrichment column ( 11 ). Optional enrichment section ( 6 ) positioned at the top of reactive rectification column ( 1 ) includes a condensing unit ( 8 ) and an enrichment column ( 7 ). 
     
    
     SUMMARY OF THE INVENTION  
       [0005]    It has now surprisingly been found that the low viscosity stage may be performed with reactive rectification columns, in particular only one reactive rectification column. As a result it is possible to simplify the plant, to reduce residence times and consequently reduce product contamination by more effective removal of hydroxyaryl, to exercise greater control over product quality by means of the ratio of DAC introduced into a pre-reaction unit to DAC introduced in vapor form, and to reduce the required excess of DAC in comparison with the falling film evaporator cascade, with a consequent reduction in circulating volumes.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0006]    The present invention accordingly provides a process for the production of polycarbonate oligomers by reacting bis(hydroxyaryl) compounds (bisphenols) and diaryl carbonates (DAC) with elimination of the hydroxyaryl component from the DAC and with introduction of pure DAC vapor countercurrently to this pre-reaction mixture. Preferred bisphenol components which are used individually or as mixtures are 2,2-bis(4-hydroxyphenyl)propane (BPA), 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, 4,4′-dihydroxybiphenyl or 1,3-bis(1-(4-hydroxyphenyl)-1-methylethyl)benzene. BPA is particularly preferably used as the bisphenol component and diphenyl carbonate as the diaryl carbonate component.  
         [0007]    The pre-reaction mixture is preferably introduced into the upper part ( 2 ) of a reactive rectification column ( 1 ) and the DAC vapor into the lower part ( 3 ) of the reactive rectification column ( 1 ).  
         [0008]    The hydroxyaryl generated during the reaction of bis(hydroxyaryl) compounds with DAC is discharged from the reactive rectification column, preferably at the top ( 13 ) of the reactive rectification column. The partial pressure of the hydroxyaryl in the DAC introduction section ( 3 ) is zero. Since, in comparison with DAC, the corresponding hydroxyaryl is a low-boiling compound, it is displaced from the liquid phase by distillation and replaced by DAC from the vapor phase. As a result, in the preferred processing method, the vapor stream is continuously being concentrated from the bottom upward with hydroxyaryl and DAC increasingly passes into the liquid phase, where it is available for the reaction. In the preferred embodiment, all the energy required for vaporizing the hydroxyaryl formed in the reactive rectification column is introduced by the vapor-form DAC, i.e., thus by an evaporator ( 4 ), which does not come into contact with the polycarbonate. It is accordingly possible to dispense with any input of energy by hot evaporator surfaces which come directly into contact with the reaction product.  
         [0009]    Pre-condensation is preferably performed in a separate pre-reaction unit ( 5 ). In this case, the pre-reaction unit is operated with a large DAC deficit and DAC is constantly re-supplied by the vapor phase in the reactive rectification column. The hydroxyaryl formed in the pre-reaction unit may in part be vaporized by flashing off in the reactive rectification column or has partially been already vaporized in the pre-reaction unit ( 5 ).  
         [0010]    Alternatively, it is, of course, possible to introduce additional energy into the reactive rectification column to ensure improved vaporization of the hydroxyaryl or for control purposes by means of evaporator surfaces at the bottom of the reactive rectification column or heat exchangers in the reactive rectification column between the mass transfer elements.  
         [0011]    Instead of introducing the entire quantity of DAC into the reaction zone from beneath, this stream may also be split and introduced in part at one or more points in the central portion of the reaction zone.  
         [0012]    After this reactive rectification column, in order to take the reaction to completion, a final falling film evaporator unit ( 9 ) is preferably connected downstream. This unit is operated at reduced pressure in comparison with the reactive rectification column and has a separate condenser ( 12 ). It is, however, also possible to proceed without or with two or more evaporator units. In this case too, as described below for the reactive rectification column, an enrichment section ( 10 ), consisting of a condenser ( 12 ) and enrichment column ( 11 ) may optionally be operated in order to concentrate the secondary product vapor streams which are generated.  
         [0013]    The final falling film evaporator unit may, of course, also be operated at the same pressure as the reactive rectification column. In this case, it is possible to dispense with separate vapor condensation, as the vapors are introduced directly at the base of the reactive rectification column.  
         [0014]    It is entirely possible to utilize the enthalpy of the hydroxyaryl which leaves the top the reactive rectification column or the final falling film evaporator in vapor form for the purification thereof. In this case, savings are made with regard to additional energy and apparatus, for example, as described below:  
         [0015]    The reactive rectification column is optionally operated with an enrichment column ( 6 ) to concentrate the escaping hydroxyaryl. At the top of the enrichment column, the hydroxyaryl is condensed in a condensing unit ( 8 ) and, on the basis of a reflux ratio selectable as a function of the required purity, partially discharged. As a result, the energy introduced with the DAC may be utilized not only for separating the hydroxyaryl but also for purifying it. The reflux is returned to the enrichment column ( 7 ). The liquid stream leaving the bottom of the enrichment section is discharged entirely or in part and may, optionally together with the condensed vapors from the final falling film evaporator, be sent for working-up in a separate, distinctly smaller quantity.  
         [0016]    Alternatively, the vapors from the final falling film evaporator and the reactive rectification column may be combined at the lower pressure level, ie., in general at the pressure level of the final falling film evaporator. This combined vapor stream is introduced in vapor form into a separate enrichment column. Under certain circumstances, the feed points for the individual vapor streams may be located at differing points on this enrichment column.  
         [0017]    If homogeneous catalysts are used, they are introduced entirely or in part into the pre-reaction unit. The remaining proportion is apportioned directly above or also at a lower point into the reaction section of the reactive rectification column. High-boiling or non-volatile catalysts are preferred as any transition into the gas phase may give rise to problems in the optionally installed distillative top section. Homogeneously dissolved catalysts which may be used are suitable soluble basic compounds, such as for example alkali metal or alkaline earth metal hydroxides or carbonates or basic organic compounds containing N or P. Basic quaternary ammonium or phosphonium salts are preferably used, such as for example tetraalkylammonium hydroxides or tetraarylphosphonium phenolates. Conventional mass transfer elements may here be used as column internals, wherein the intention is to ensure not only intensive mass transfer between the vapor and liquid but also a sufficiently long residence time for the reaction to proceed. Various column plates or ordered or random packing known to the person skilled in the art are thus suitable. Different types of trays like bubble trays, sieve trays and valve trays and sheet metal packing may be used. If very long residence times are needed special tray-designs with great hold up (e.g., U.S. Pat. No. 5,026,549 incorporated herein by reference, Thormann-tray from Julius Montz, Germany) or special packing designs with great holdup (e.g., packing for heterogeneous catalytic reactions like DE19701045, U.S. Pat. No. 5,467,817, incorporated herein by reference) are preferred.  
         [0018]    If solid catalysts are used, they are introduced in a manner known to the person skilled in the art into the mass transfer elements, such as for example in ordered or random catalytic packing with a fabric structure to accommodate heterogeneous catalysts or alternatively in specific apparatus in distillation trays. Examples of such column internals are described in patents EP-A 670 178; EP-A 461 855; U.S. Pat. No. 5,026,549; U.S. Pat. No. 4,536,373; WO 94/08681; WO 94/08682; EP-A 470 655; WO 97/26971; U.S. Pat. No. 5,308,451; EP-A 755 706; EP-A 781 829; EP-A 428 265; EP-A 448 884; EP-A 640 385; EP-A 631 813; WO 90/02603; WO 97/24174; EP-A 665 041; EP-A 458 472; EP-A 476 938.  
         [0019]    Metal oxides or for example solid basic anion exchange resins may be used as the solid catalysts.  
         [0020]    The temperature of the DAC supplied in vapor form is between the boiling point of the DAC at the pressure prevailing in the reactive rectification column and 300° C., particularly preferably up to 270° C.  
         [0021]    The temperature of the mixture supplied in liquid form preferably from a pre-reaction unit is between 100 and 250° C., particularly preferably between 140 and 210° C.  
         [0022]    The temperature in the reactive rectification column is established on the basis of the selected pressure and the selected feed conditions and is between 130 and 230° C. in the area of the liquid feed and between 180 and 270° C. in the lower zone of the reactive rectification column.  
         [0023]    The temperature in the final falling film evaporator may be adjusted separately on the basis of the reduced pressure and is between 180° C. and 320° C., particularly preferably between 200 and 290° C.  
         [0024]    The pressure established in the reactive rectification column at the upper end of the reaction section is between 20 and 500 mbar, particularly preferably between 30 and 200 mbar. The selected pressure has a particular impact upon the temperatures and thus upon the rate of reaction and product quality and, via the temperature, upon the viscosity and thus the fluid dynamics in the reactive rectification column.  
         [0025]    The pressure established in the final falling film evaporator is determined in accordance with admissible temperatures for good product quality and is between 5 and 200 mbar, particularly preferably between 10 and 60 mbar.  
         [0026]    The molar ratio of the total quantity of DAC introduced into the pre-reaction unit and the reactive rectification column (i.e., in the pre-reaction unit and feed in vapor form) to the quantity of bis(hydroxyaryl) compounds is 0.95-1.5, particularly preferably 1.0-1.2.  
         [0027]    0% to 60%, particularly preferably 2% to 30% of the entire DAC feed are introduced into the pre-reaction unit. The remainder is introduced into the reactive rectification column by the feed in vapor form. The instance with 0% DAC corresponds to a reactive rectification column without a pre-reaction unit, into which the pre-heated bis(hydroxyaryl) compound is introduced.  
         [0028]    The greater part of the total amount of hydroxyaryl formed is flashed off in the reactive rectification column or stripped out by DAC in vapor form. The ratio of the hydroxyaryl stripped out in the final falling film evaporator to the total amount of hydroxyaryl formed is 0.1 %-20%, particularly preferably 0.1%-10%.  
         [0029]    Typical residence times in the pre-reaction unit are 1 min. to 60 min., particularly preferably 1 min. to 15 min.  
         [0030]    Average residence times of the reaction mixture in the reactive rectification column are 3 min. to 80 min., particularly preferably 5 min. to 30 min.  
         [0031]    Residence times in the final falling film evaporator including the associated equipment are 2 to 50 min., particularly preferably 5 to 25 min.  
         [0032]    In this manner, polycarbonate oligomers are obtained which exhibit a relative solvent viscosity eta rel  (measured on a solution in dichloromethane containing 5 g of polymer per liter at 25° C.) of 1.05 to 1.10, preferably of 1.06 to 1.08. The content by weight of the phenolic OH terminal groups x PhOH  in the resultant polycarbonate oligomers is 4000-10000 ppm, preferably 5000-7000 ppm. The products obtained in this manner may be used as prepolymers for the production of light-colored and solvent-free polycarbonate, as for example described in EPA 719 814 or EPA 694572. To this end, the prepolymers, optionally with the addition of a suitable catalyst, are condensation polymerized with continued elimination of the hydroxyaryl compound from the diaryl components to yield high molecular weight polycarbonate polymers.  
         [0033]    The following Example is intended to illustrate the present invention, but without restricting it.  
       EXAMPLE  
       [0034]    24.2 kg/h of mixture prepared from 94.4 wt. % of BPA, 5.6 wt. % of DPC and, relative to BPA, 1.5·10 −3  mol % of tetraphenylphosphonium phenolate as catalyst are introduced into a pre-reaction unit and heated and, at 190° C., reacted in a pre-reaction to yield polycarbonate (PC). The residence time in the pre-reaction unit was approx. 3 min. The resultant phenol/BPA/DPC/PC mixture is introduced into the top of a reactive rectification column. In so doing, a proportion of the hydroxyaryl formed in the pre-reaction unit flashes off. The temperature in the reactive rectification column around the input point is approx. 190° C. Pure DPC is evaporated in a lateral falling film evaporator and, at a temperature of 230° C at the base of the reactive rectification column, introduced at an input rate of 22.7 kg/h.  
         [0035]    The reactive rectification column has a diameter of 100 mm and is equipped with a conventional 350 m 2 /m 3  sheet metal packing. The total packing height is 13 m. The liquid is collected and redistributed at three points. The reactive rectification column is insulated by an adiabatic jacket. The reactive rectification column is operated at 100 mbar. Pressure is measured directly above the reaction zone. Temperature measurement points are provided in the reactive rectification column, measuring from the bottom upwards, after approx. 1.5 m and after 13 m of packing for monitoring the temperature profile. The temperature at the measuring point after 1.5 m is 232° C., that at the uppermost measuring point is 191° C. The residence time in the reactive rectification column is approx. 10 minutes.  
         [0036]    The reaction liquid is discharged from the reactive rectification column at the bottom and introduced into a 1.0 m 2  falling film evaporator to take conversion to completion. The falling film evaporator and its associated bottom pump receiver are operated at 25 mbar and 272° C. 27.1 kg/h of liquid product consisting of polycarbonate oligomers may be withdrawn from the falling film evaporator. The product is characterized by the values eta rel =1.063 and x PhOH =6030 ppm. A total of 3.3 kg/h of vapor from the falling film evaporator, primarily consisting of DPC and phenol, are condensed in a separate condenser.  
         [0037]    The vapor leaving the reactive rectification column is passed through an empty 1 m length of glass tube of a diameter of 100 mm to a condenser. The glass tube is also provided with an adiabatic jacket. 16.4 kg/h of vapor are condensed in the condenser installed thereover, which is operated at a coolant-side temperature of 50° C. The condensate contains approx. 98 wt. % phenol. In the laboratory, an enrichment section with mass transfer elements is not provided.  
         [0038]    Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.