Abstract:
Unexpected corrosion of downstream sections of a dialkyl carbonate manufacturing apparatus has been traced to alkyl chloroformate impurities, which slowly decompose to yield hydrochloric acid. An improved process and apparatus for dialkyl carbonate synthesis reduce corrosion by physically removing or chemically decomposing the alkyl chloroformate impurities within the corrosion-resistant upstream sections of the apparatus.

Description:
BACKGROUND OF INVENTION  
         [0001]    Polycarbonate resins are useful materials valued for their physical and optical properties. Methods for the preparation of polycarbonate resins include interfacial processes and melt processes. In interfacial processes, as described, for example, in U.S. Pat. No. 4,360,659 to Sikdar, a bisphenol is reacted with phosgene in the presence of solvents. In melt processes, as described, for example, in U.S. Pat. No. 3,153,008 to Fox, a bisphenol is reacted with a diaryl carbonate. Melt processes are presently preferred because they avoid the use of phosgene and solvents.  
           [0002]    Use of a melt process for polycarbonate synthesis requires an industrially efficient process for producing diaryl carbonates. There are several known processes for producing diary carbonates. One example of such a process is described by U.S. Pat. No. 4,182,726 to Illuminati et al. In this process, diaryl carbonates are produced by reacting dialkyl carbonates with aryl hydroxides (see Scheme I, below).  
                         
 
           [0003]    U.S. Pat. No. 4,182,726 also demonstrates that diaryl carbonates can be reacted together with dihydric phenols to produce polycarbonates (see Scheme II, below).  
                         
 
           [0004]    A preferred process for making dialkyl carbonates is illustrated in Scheme III, below, and described, for example, in U.S. Pat. Nos. 5,527,943 to Rivetti et al.; and 4,218,391 and 4,318,862 to Romano et al.  
                         
 
           [0005]    U.S. Pat. No. 5,527,943 (the &#39;943 Patent) also describes a known drawback of the dialkyl carbonate process according to Scheme (III): it produces water as a by-product. Also, hydrochloric acid (HCl) may be continuously added to the reaction mixture to maintain a desired molar ratio of chloride to copper. Therefore, HCl, CuCl catalyst, and water are typically found in the stream exiting the reactor vessel. Hydrochloric acid and copper chlorides are very corrosive in the presence of water, so equipment made from corrosion-resistant materials, such as glass-lined vessels, must be used in the reaction section of a chemical plant making dialkyl carbonates by this process. As corrosion-resistant equipment is expensive, there is a desire to use it in as little of the plant as possible.  
           [0006]    A typical plant for performing preparing dialkyl carbonates according to Scheme III may contain three sections: a reaction section for converting raw materials to dialkyl carbonate, a separation section for isolating the dialkyl carbonate from unreacted monomers and by-products, and a purification section for removing water and further isolating the dialkyl carbonate. The &#39;943 Patent teaches that one can minimize the amount of corrosion-resistant equipment required by removing the HCl from the process stream immediately after the reaction section. This eliminates the necessity of using expensive corrosion-resistant materials in the separation and purification sections of the plant. The &#39;943 Patent further suggests that removal of HCl and possible copper halide salts from the stream immediately after the reaction section can be accomplished by exposing the gas-liquid mixture produced by the reaction to a liquid stream consisting of one of the process fluids. The &#39;943 Patent also states that the operating conditions employed are preferably adjusted such that the gaseous mixture from the reactor does not condense, or condenses only to a negligible extent, before the acid removal section in order to avoid the necessity of having to reheat the mixture before removing the HCl (col. 3, lines 17-30).  
           [0007]    In view of the above, it was desirable to construct a plant wherein the HCl and any copper halide salts would be removed from the stream after the reaction section to avoid corrosion in the separation and purification sections. However, a technique similar to that described by the &#39;943 Patent—removing HCl and copper salts by treatment of a vaporized feed in a column using a counterflowing azeotrope fluid from the reaction mixture—failed to prevent corrosion in the downstream separation and purification sections.  
           [0008]    There is therefore a need for a dialkyl carbonate process that recognizes and eliminates additional sources of corrosion.  
         SUMMARY OF INVENTION  
         [0009]    The above-described and other drawbacks and disadvantages of the prior art are alleviated by a method of preparing a dialkyl carbonate, comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalyst to form a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, and carbon monoxide; and removing alkyl chloroformate from said mixture.  
           [0010]    After considerable effort, the present inventors have discovered that dialkyl carbonate synthesis can form alkyl chloroformate by-products that lead to problematic corrosion. For example, in the reaction of methanol, carbon monoxide, and oxygen to form dimethyl carbonate (hereinafter “DMC”), methyl chloroformate (hereinafter “MCF”) may be formed as a by-product. The MCF may pass through the HCl removal column into the separator and purification sections, where it reacts slowly with methanol and/or water to form corrosive HCl. Therefore, it was determined that steps were needed to remove MCF prior to the separation and purification sections.  
           [0011]    Other embodiments, including an apparatus for preparing dialkyl carbonates, are described below. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 is a diagrammatic view of a first embodiment of the apparatus.  
         [0013]    [0013]FIG. 2 is a simplified diagrammatic view of a comparative apparatus that is susceptible to corrosion.  
         [0014]    [0014]FIG. 3 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  comprises two holding vessels  120 .  
         [0015]    [0015]FIG. 4 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  comprises four holding vessels  120 .  
         [0016]    [0016]FIG. 5 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  comprises a tubular section  130 .  
         [0017]    [0017]FIG. 6 is a simplified diagrammatic view of an embodiment of the apparatus comprising an ion exchange resin bed  190 .  
         [0018]    [0018]FIG. 7 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  comprises a first gas-liquid separator  90  and a second gas-liquid separator  100 .  
         [0019]    [0019]FIG. 8 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  precedes the first gas-liquid separator  90 .  
         [0020]    [0020]FIG. 9 is a simplified diagrammatic view of an embodiment of the apparatus in which the fluid passageway  110  follows the azeotrope column  180 .  
         [0021]    [0021]FIG. 10 is a plot of chloride concentrations at the bottom of an azeotrope column  180  as a function of apparatus type (FIG. 2 and FIG. 3) and time.  
         [0022]    [0022]FIG. 11 is a plot of methyl chloroformate concentrations entering and exiting the fluid passageway  110  as a function of time for an apparatus corresponding to FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0023]    One embodiment is a method, comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalyst to form a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, and carbon monoxide; and removing alkyl chloroformate from said mixture.  
         [0024]    There is no particular limitation on the alkanol used in the method. Suitable alkanols include primary, secondary, and tertiary C 1 -C 12  alkanols, with primary C 1  -C 6  alkanols being preferred. Highly preferred alkanols include methanol.  
         [0025]    Oxygen may be provided in any form, with gaseous forms being preferred. Suitable oxygen sources include, for example, air, and oxygen-containing gases having at least about 95 weight percent molecular oxygen, preferably at least about 99 weight percent molecular oxygen. Suitable oxygen-containing gases are commercially available from, for example, Air Products.  
         [0026]    Carbon monoxide is preferably supplied as a gas having at least about 90 weight percent, preferably at least about 95 weight percent, more preferably at least about 99 weight percent, carbon monoxide. Suitable carbon monoxide-containing gases are commercially available from, for example, Air Products.  
         [0027]    Suitable catalyst include those comprising iron, copper, nickel, cobalt, zinc, ruthenium, rhodium, palladium, silver, cadmium, rhenium, osmium, iridium, platinum, gold, mercury, and the like, and combinations comprising at least one of the foregoing metals. Preferred catalysts may comprise copper. A highly preferred catalyst comprises copper and chloride ion in a molar ratio of about 0.5 to about 1.5. Within this range, a molar ratio of at least about 0.8 may be preferred. Also within this range, a molar ratio of up to about 1.2 may be preferred. Highly preferred catalysts include cuprous chloride (CuCl) and cupric chloride (CuCl 2 ), with cuprous chloride being more highly preferred. During operation of the process, a suitable chloride ion concentration may be maintained by the addition of hydrochloric acid (HCl).  
         [0028]    [0028]FIG. 1 illustrates a dialkyl carbonate plant  10  having linked reaction section  20 , separation section  30 , and purification section  40 . With reference to FIG. 1, the catalyzed reaction of alkanol, oxygen, and carbon monoxide may be performed in a single reactor  50 , or in two or more reactors  50 . The conditions for performing this step should be selected to maximize the yield of dialkyl carbonate while minimizing the degradation of dialkyl carbonate. Preferably, the reaction is performed in a single reactor  50 , at a temperature of about 50° C. to about 250° C. Within this range, the temperature may preferably be at least about 100° C. Also within this range, the temperature may preferably be up to about 150° C. The reactor  50  is preferably kept at a pressure of about 15 to about 35 bar gauge (barg). Within this range, a pressure of at least about 20 barg may be preferred. Also within this range, a pressure up to about 28 barg may be preferred. In the case of dual reactor systems, the catalyst may be recycled between tanks. The catalyst concentration should be sufficiently high to produce an acceptable yield, but should be kept below a concentration that would cause solid setting of the catalyst in the reactor  50  or clogging of the equipment. The reactants alkanol, oxygen, and carbon monoxide are preferably added in a molar ratio of (about 0.5 to about 0.7):(about 0.04 to about 0.06):(about 0.8 to about 1.2), respectively. A highly preferred molar ratio of alkanol:oxygen:carbon monoxide is (about 0.6):(about 0.05):(about 1).  
         [0029]    The amount of catalyst used relative to the reactants will depend on the identity of the catalyst. For example, when the catalyst comprises CuCl, a highly preferred catalyst concentration is about 140 to about 180 grams per liter of reaction mixture. During operation, the catalyst may initially be added from a catalyst tank  60 . Sufficient HCl is preferably added to reactor  50  from a hydrochloric acid tank  70  during the course of the reaction to maintain a molar ratio of Cu:Cl close to 1.0. The concentration of HCl is preferably continuously determined and controlled by the addition of HCl. A typical mass ratio for HCl feed to total liquid feed is about 6×10 −4  to about 8×10 −4 .  
         [0030]    The reaction produces a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, and carbon monoxide. The mixture may further comprise residual methanol and oxygen, as well as side-products such as alkyl chlorides and dialkyl ethers. The mixture is typically withdrawn from the reactor  50  in a gas/vapor form. The term “vapor” is meant to refer to gaseous organic components of the mixture such as, for example, evaporated dialkyl carbonates, alcohols, alkyl chloroformates, etc., and to water vapor. That is, the term “vapor” refers to fluids having a boiling point of at least −50° C. at one atmosphere. In contrast, the term “gas” is meant to refer to the gaseous oxygen, carbon dioxide, carbon monoxide, and optional nitrogen. That is, the term “gas” refers to fluids having a boiling point less than −50° C. at one atmosphere. The vapor may be at least partially condensed in condenser  80 , and fed to a first gas-liquid separator  90 . The apparatus may optionally employ a single gas-liquid separator, or a plurality of (i.e., at least 2; preferably up to about 5) gas-liquid separators. The first gas-liquid separator  90  may be kept at a pressure within about 10%, more preferably within about 1%, of the pressure of the reactor  50 . The gas effluent from the first gas-liquid separator  90  may be recycled, for example to reuse excess carbon monoxide. The mixture may be sent to a second gas-liquid separator  100 , which preferably has a pressure less than about 20% of the pressure of the reactor  50  (e.g., preferably less than 3 bar gauge, more preferably about 0.2 bar gauge) to preferably achieve separation of at least about 90%, more preferably at least 95%, by weight of the remaining gas in the mixture. In a highly preferred embodiment, substantially all of the gas is removed from the mixture. The gas effluent removed from the second gas-liquid separator  100  can also be recycled. It is preferred that the vapor in the mixture be in a partially condensed form (i.e., at least about 10% condensed), more preferably a fully condensed form (i.e., at least about 90% condensed), before entering the first gas-liquid separator  90 , and between the first gas-liquid separator  90  and the second gas-liquid separator  100 .  
         [0031]    In the embodiment shown in FIG. 1, the mixture exiting the second gas-liquid separator  100  may be in a single liquid phase. After the second gas-liquid separator  100 , the mixture may proceed through a fluid passageway  110  that removes alkyl chloroformate from the mixture. It will be understood that the terms “remove” and “removal” in reference to a particular chemical species encompass any chemical or physical process that reduces the concentration of the species in the mixture. The alkyl chloroformate may be removed from the condensate by any method. Some preferred methods include heating, increasing pressure, increasing residence time, adding a polar solvent, adsorbing, separating with a membrane (including gas and liquid membrane separation), pervaporating, passing through an ion exchange resin, exposing to a stoichiometric reagent, exposing to a catalytic reagent, and the like, and combinations comprising at least one of the foregoing techniques. In a preferred embodiment, the alkyl chloroformate is removed from mixture by reaction with water (see Scheme IV) or alkanol (see Scheme V).  
                         
 
         [0032]    It may also be preferred to remove the alkyl chloroformate without passing the mixture through an ion exchange resin, because such resins are expensive to install and operate. It may be preferred to remove at least about 50 percent, more preferably at least about 90 percent, yet more preferably at least about 95 percent, even more preferably at least about 99 percent, of the alkyl chloroformate from the mixture. In one embodiment, it may be preferred to reduce the alkyl chloroformate concentration in the mixture to less than about 500 ppm, more preferably less than about 100 ppm, yet more preferably less than about 30 ppm. In any of these embodiments, it may be preferred to remove less than about 10%, more preferably less than about 5%, yet more preferably less than about 1%, of the dialkyl carbonate. Although the method may be described as “removing less than about 10% of said dialkyl carbonate”, it will be understood that the concentration of dialkyl carbonate need not be reduced and may even increase. For example, the concentration of dialkyl carbonate may increase if the Scheme V reaction of alkyl chloroformate with methanol forms dialkyl carbonate faster than dialkyl carbonate decomposes due to other reactions.  
         [0033]    Through extensive kinetic studies of the dimethyl carbonate process utilizing variations in factors including temperature, residence time, water concentration, methanol concentration, and hydrochloric acid concentration, the present inventors have found that the rate of methyl chloroformate decomposition may be given by the equation (1)  
           −r   MCF =( k   1 [H 2 O]+ k   2 [MeOH)][MCF]  (1)  
         [0034]    where r MCF  is the rate of change of the moles of methyl chloroformate (MCF) per unit volume, [H 2 O], [MeOH], and [MCF] are the instantaneous concentrations of water, methanol, and methyl chloroformate, respectively, in moles per unit volume, and k 1  and k 2  are rate constants that vary with temperature according to equations (2) and (3), respectively  
           k   1   =k   1   0   e   −6381/T   (2)  
           k   2   =k   2   0   e   −7673/T   (3)  
         [0035]    where k 1   0 =2.09×10 9  mL/mol-min, k 2   0 =4.14×10 10  mL/mol-min, and T is the temperature in degrees kelvin.  
         [0036]    In many cases, it is valid to assume that the concentrations of water and methanol, and the density of the solution are essentially constant. Within these general kinetic constraints, different kinetic expressions may be used for different process and apparatus types. With knowledge of the relevant chemical reactions and rate constants provided in this application, these expressions may be readily derived by those of ordinary skill in the art. For example, in a batch process, the rate of methyl chloroformate decomposition may be expressed as a function of residence time, as shown in equation (4):  
         − d [MCF]/ dt =( k   1 [H 2 O]+ k   2 [MeOH])[MCF]  (4) | 
         [0037]    where t is residence time in minutes. The residence time t may be defined as the total time spent by an average molecule in the fluid passageway  110 . In a batch process, at least about 50% of the methyl chloroformate may be removed by maintaining the mixture under conditions comprising a water concentration ([H 2 O]), a methanol concentration ([MeOH]), a temperature ( T), and a residence time ( t), such that a parameter X according to the equation (5)  
           X=exp {−[(2.09×10 9 ) e   (−6381/T) [H 2 O]+(4.14×10 10 ) e   −7673/ T )[MeO]] t}   (5)  
         [0038]    has a value less than about 0.9, wherein the water concentration and the methanol concentration are expressed in moles per milliliter, the temperature is expressed in degrees Kelvin, and the residence time is expressed in minutes. The value of X may preferably be less than about 0.5, more preferably less than about 0.2, yet more preferably be less than about 0.1, even more preferably less than about 0.05, still more preferably less than about 0.01. The water concentration may be about 0.1 to about 50 moles per liter (mol/L). Within this range, the water concentration may preferably be at least about 0.5 mol/L, more preferably at least about 1 mol/L. Also within this range, the water concentration may preferably be up to about 30 mol/L, more preferably up to about 20 mol/L, yet more preferably up to about 10 mol/L, even more preferably up to about 5 mol/L. The methanol concentration may be about 1 to about 25 mol/L. Within this range, the methanol concentration may preferably be at least about 5 mol/L, more preferably at least about 10 mol/L. Also within this range, the methanol concentration may preferably be up to about 20 mol/L, more preferably up to about 18 mol/L. The residence time may be about 0.5 hour to about 10 hours. Within this range, the residence time may preferably be at least about 1 hours, more preferably at least about 2 hours. Also within this range, the residence time may preferably be up to about 8 hours, more preferably up to about 6 hours. The temperature may be about 30 to about 130° C. Within this range, the temperature may preferably be at least about 40° C., more preferably at least about 50° C., yet more preferably at least about 60° C. Also within this range, the temperature may preferably be up to about 110° C., more preferably up to about 100° C., yet more preferably up to about 90° C.  
         [0039]    In the limit of an ideal steady state plug flow reactor, and assuming constant density of the mixture, the rate of methyl chloroformate decomposition may be expressed according to equation (3), with t representing residence time in minutes.  
         [0040]    For an ideal steady state continuous stirred tank reactor (CSTR), the concentration of methyl chloroformate is given by equation (6)  
         [MCF]=[MCF] t=0 (1/(1 +kt ))  (6)| 
         [0041]    where [MCF] t=0  is the initial concentration of methyl chloroformate in moles per milliliter, t is the residence time in minutes, and k is given by equation (7)  
           k=k   1 [H 2 O]+ k   2   [MeOH]   (7)| 
         [0042]    where k 1 , k 2 , [H 2 O], and [MeOH] are as defined above.  
         [0043]    In another embodiment that relates to a batch reactor, removing alkyl chloroformate from the mixture comprises maintaining the mixture under conditions comprising an initial concentration of methyl chloroformate ([MCF] t=0 ), a water concentration ([H 2 O]), a methanol concentration ([MeOH]), a temperature ( T), and a residence time ( t), such that a parameter Z calculated according to the equation (8)  
           Z =[MCF] t=0   exp{−[( 2.09×10 9 ) e   (−6381/T) [H 2 O]+(4.14×10 10 ) e   (−7673/T) [MeOH] t}   (8)| 
         [0044]    has a value less than about 5×10 −6 , preferably less than about 1×10 −6 , more preferably less than about 5×10 −7 , even more preferably less than about 5×10 −8 , wherein the initial concentration of methyl chloroformate, the water concentration, and the methanol concentration are expressed in moles per milliliter, the temperature is expressed in degrees Kelvin, and the residence time is expressed in minutes. The temperature, residence time, methanol concentration, and water concentration in this expression are as described above. The initial concentration of methyl chloroformate will depend on the reactor conditions, but it is typically about 5×10 −3  moles per liter to about 5×10 −1  moles per liter. Within this range, the initial concentration of methyl chloroformate may be at least about 1×10 −2  moles per liter. Also within this range, the initial concentration of methyl chloroformate may be up to about 1×10 −1  moles per liter.  
         [0045]    In a preferred embodiment that relates to a batch reactor, removing alkyl chloroformate comprises subjecting the mixture to conditions comprising an initial dimethyl carbonate concentration ([DMC]t=0), an initial water concentration ([H2O]t=0), an initial methanol concentration ([MeOH]t=0), an initial hydrochloric acid concentration ([HCl]t=0), a temperature (T), and a residence time (t), such that a parameter X calculated according to the equation (9)  
           X=exp {−[(2.09×10 9 ) e   (−6381/T) H 2 O] t=0 +(4.14×10 10 ) e   (−7673/T) [MeOH] t=0   ]t}   (9)  
         [0046]    has a value less than about 0.9, and a parameter Y calculated according to the equation (10)  
             Y   =       (     1   -         [       H   2        O     ]       t   =   0           [     D                 M                 C     ]       t   =   0           )       (     1   -       (         [       H   2        O     ]       t   =   0           [     D                 M                 C     ]       t   =   0         )          (     exp        (       (     6.6   ×     10   10       )                  (     exp        (       -   6636     /   T     )       )          [     H                 C                 l     ]         t   =   0            [     D                 M                 C     ]         t   =   0            (           [       H   2        O     ]       t   =   0           [     D                 M                 C     ]       t   =   0         -   1     )        t     )       )         )               (   10   )                               
 
         [0047]    has a value of at least about 0.9, wherein the initial dimethyl carbonate concentration, the initial water concentration, the initial methanol concentration, and the initial hydrochloric acid concentration are expressed in moles per milliliter, the temperature is expressed in degrees Kelvin, and the residence time is expressed in minutes. The value of Y may preferably be at least about 0.95, more preferably at least about 0.99. Suitable analytical techniques to determine initial concentrations of water, methanol, hydrochloric acid, and dimethyl carbonate in reaction mixtures are well known in the art. The term “initial concentration” refers to the concentration of a species before intentional removal of alkyl chloroformate. The initial water and methanol concentrations are the same as the water and methanol concentrations described above (under typical reaction conditions, the water and methanol concentrations are large are essentially constant during alkyl chloroformate removal). The initial dimethyl carbonate concentration may be about 0.5 to about 10 mol/L. Within this range, the initial dimethyl carbonate concentration may preferably be at least about 1 mol/L, more preferably at least about 2 mol/L. Also within this range, the initial dimethyl carbonate concentration may preferably be up to about 8 mol/L, more preferably up to about 6 mol/L. The concentration of HCl in the mixture will depend on the type and concentration of catalyst employed. The initial hydrochloric acid concentration will depend on the type and amount of catalyst, but it is typically about 1×10 −3  to about 2×10 −1  moles per liter. Within this range, the initial hydrochloric acid concentration may preferably be at least about 5×10 −3 , more preferably at least about 1×10 −2  mol/L. Also within this range, the initial hydrochloric acid concentration may preferably be up to about 1×10 −1  more preferably up to about 7×10 −2  mol/L.  
         [0048]    The method may be operated, for example, in a batch, semi-batch, or continuous manner.  
         [0049]    In the particular embodiment shown in FIG. 1, the mixture passes through a first heat exchanger  140  to adjust the temperature of the mixture about 30° C. to about 130° C. Within this range, the temperature may preferably be at least about 40° C., more preferably at least about 50° C. Also within this range, the temperature may preferably be up to about 80° C., more preferably up to about 70° C. The term “heat exchanger” describes a well-known device for heating chemical reaction streams, typically by exchanging heat between a thermal energy source (e.g., steam) and a cooler chemical reaction stream, but it is understood that other types of equivalent heaters (e.g., electrical heaters) are also included. The condensate may proceed into a fluid passageway  110 , which serves to increase the residence time of the mixture under conditions to maximize decomposition of alkyl chloroformate while minimizing decomposition of dialkyl carbonate. The condensate may preferably remain fully condensed within the fluid passageway  110 . It is desirable to keep the condensate fully condensed because at least some alkyl chloroformates (e.g., methyl chloroformate) are more stable in the vapor phase than the liquid phase under conditions used for this process.  
         [0050]    The residence time and temperature in the fluid passageway  110  are preferably sufficient to remove enough alkyl chloroformate to prevent unacceptable downstream corrosion, but they should not be so excessive as to cause unnecessary reductions in the productivity and yield of the desired dialkyl carbonate product. FIG. 2 shows a simplified process diagram representative of a comparison process. In this process, the mixture flows directly from a first gas-liquid separator  90  to a first heat exchanger  140 , then to an acid removal column  160 . Three specific embodiments of the fluid passageway  110  are shown in FIGS. 3, 4, and  5 . In a preferred embodiment, at least about 50% of the alkyl chloroformate is removed, more preferably at least 80% is removed. In a highly preferred embodiment, the alkyl chloroformate concentration is reduced to less than about 500 parts per million (ppm) by weight, more preferably less than about 100 ppm by weight, yet more preferably less than about 30 ppm by weight, based on the total weight of the mixture after alkyl chloroformate removal.  
         [0051]    The fluid passageway  110  is preferably selected such that the total residence time between the reactor  50  and the acid removal column  160  is about 0.5 hour to about 10 hours. Within this range, the residence time may preferably be at least about 1 hour, more preferably at least about 2 hours. Also within this range, the residence time may preferably be up to about 8 hours, more preferably up to about 7 hours.  
         [0052]    In one embodiment, illustrated in FIG. 3, the fluid passageway  110  comprises 2 holding vessels  120 . These holding vessels  120  may, for example, maintain the mixture at a temperature of about 55° C. for about 2 hours. Each holding vessel  120  may preferably have a length to volume ratio (L/V) less than 5, preferably less than about 2. While two holding vessels  120  are illustrated in this figure, there is no particular limitation on the number of holding vessels  120  in the fluid passageway  110 . It may be preferred to use at least 2 holding vessels  120 , and configurations comprising 3, 4, 5, 6, or more holding vessels  120  may also be preferred.  
         [0053]    In another embodiment, illustrated in FIG. 4, the fluid passageway  110  comprises 4 holding vessels  120 . These holding vessels  120  may, for example, maintain the mixture at a temperature of about 70° C. for about 4 hours. Each holding vessel  120  may preferably have a length to volume ratio (L/V) less than 5, preferably less than about 2.  
         [0054]    In yet another embodiment, illustrated in FIG. 5, the fluid passageway  110  may comprise a section having L/V of at least 5, preferably at least about 10. For brevity, this section may be referred to as a tubular section  130 . Such a tubular section  130  having L/V&gt;5 may promote plug flow of the mixture through the fluid passageway  110 , thereby efficiently utilizing the residence time for removal of the alkyl chloroformate. In this embodiment, it may be preferred that the mixture reside in one or more narrow sections having L/V&gt;5 for at least about 50% of the total residence time spent in the fluid passageway  110 , more preferably at least about 80% of the total residence time spent in the fluid passageway  110 .  
         [0055]    Referring again to FIG. 1, after exiting the fluid passageway  110 , the mixture may, optionally, pass through a second heat exchanger  150  to at least partially vaporize the mixture. This second heat exchanger  150  may have a residence time of less than 10 minutes. This vaporization step may also be accomplished without a heat exchanger by lowering the pressure applied to the condensed mixture (e.g., by passing the condensate into an acid removal column  160  that is kept at a relatively lower pressure). The vaporized mixture may then, optionally, be treated to remove HCl, preferably by injecting it into an acid removal column  160 . The acid removal column  160  may also help remove any entrained catalyst (e.g., CuCl) that could otherwise contribute to downstream corrosion. In the acid removal column  160 , the vaporized condensate may preferably encounter a counter-flowing liquid supplied by counter-flowing liquid line  170  to a higher point in the column (e.g., the upper third). The counter-flowing liquid may trap the remaining HCl and other reactants, which may be removed from the bottom of the acid removal column  160  and recycled to the reactor  50 . The dialkyl carbonate mixture may be removed from the top of the acid column  160 , and, optionally, passed into an azeotrope column  180 . As shown in FIG. 6, an optional ion exchange resin bed  190  may be included after the acid removal column  160 , or at any other position downstream with respect to the acid removal column  160 . It may be advantageous to include an optional ion exchange resin bed  190  after water is removed from the product dialkyl carbonate stream in the purification section  40 . In a preferred embodiment, the apparatus does not include an ion exchange resin bed  190 .  
         [0056]    In a preferred, embodiment, the method comprises reducing the concentration of hydrochloric acid in the mixture to less than about 1×10 −3  mol/L, more preferably less than about 5×10 −4  mol/L, even more preferably less than about 1×10 −4  mol/L, based on the total composition after removing hydrochloric acid.  
         [0057]    In a preferred embodiment, the portions of the separation section  30  downstream from the azeotrope column  180 , and the purification subsection  40  are not required to be corrosion-resistant. Equipment upstream of the azeotrope column  180  is preferably corrosion-resistant; for example, it may be glass lined. The term “corrosion-resistant” is meant to describe a material capable of withstanding an HCl content of 500 ppm at a temperature of about 50° C. to about 135° C. in the reaction mixture without substantial corrosion in a relatively brief time period (e.g., six months). Glass lined vessels, precious metal (e.g., tantalum) lined vessels and special steels such as HASTELLOY® and CHROMALLOY® would be considered corrosion-resistant materials, while ordinary stainless steels not modified to enhance corrosion resistance would not be considered corrosion-resistant. The azeotrope column  180  can be made at least in part from corrosion-resistant metals. In a preferred embodiment, the bottom of the azeotrope column  180  may be made from a corrosion-resistant steel, whereas the top of the column can be ordinary stainless steel.  
         [0058]    In one embodiment of the apparatus, illustrated in FIGS. 1 and 3- 6 , alkyl chloroformate is removed in a fluid passageway  110 .  
         [0059]    In another embodiment of the apparatus, illustrated in FIG. 7, the mixture is present in the gas-liquid separation vessels  90  and  100  for sufficient residence time and at sufficient temperature to remove alkyl chloroformate. In other words, the fluid passageway  110  comprises the gas-liquid separation vessels  90  and  100 . For example, the mixture may remain in the condense phase in the gas-liquid separation vessels to be substantially decomposed by reactions with water and methanol. In this embodiment, the first heat exchanger  140  and the holding vessels  120  may be unnecessary.  
         [0060]    In another embodiment of the apparatus, illustrated in FIG. 8, the alkyl chloroformate may be removed in a fluid passageway  110  that precedes the gas-liquid separation vessels  90  and  100 . In this embodiment, one of the above-mentioned techniques for removing alkyl chloroformate may be employed upstream of the gas-liquid separation vessels  90  and  100 .  
         [0061]    In another embodiment of the apparatus, illustrated in FIG. 9, the hydrochloric acid may be removed from the mixture before removing the alkyl chloroformate. In this embodiment, the alkyl chloroformate may be removed in the vapor, rather than the liquid phase. For example, referring to FIG. 9, the fluid passageway  110  may follow the azeotrope column  180 ; for example, it may be inserted into the azeotrope column vapor exit line  210 . In this embodiment, the first heat exchanger  140  and the holding vessels  120  illustrated in FIG. 3 may be omitted. In this embodiment, the fluid passageway  110  may preferably comprise an apparatus suitable for removing alkyl chloroformate from the vapor phase (e.g., ion exchange resins, absorption beds, vapor phase membranes, etc.), and the alkyl chloroformate need not be condensed.  
         [0062]    A preferred embodiment is a method of preparing a dialkyl carbonate, comprising:  
         [0063]    reacting an alkanol, oxygen, carbon monoxide, and a catalyst to form a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, and carbon monoxide; and passing said mixture through a fluid passageway  110  at a temperature of about 50° C. to about 80° C. and for a residence time of about 1 hour to about 10 hours.  
         [0064]    Another preferred embodiment is an apparatus for preparing a dialkyl carbonate, comprising: means for reacting an alkanol, oxygen, carbon monoxide, and a catalyst to form a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, carbon dioxide, and carbon monoxide; and means for removing alkyl chloroformate from said mixture.  
         [0065]    Another preferred embodiment is an apparatus for preparing a dialkyl carbonate, comprising: a reactor for reacting an alkanol, oxygen, carbon monoxide, and a catalyst to a produce a mixture comprising a dialkyl carbonate, an alkyl chloroformate, hydrochloric acid, water, and carbon dioxide; and a fluid passageway  110  for removing alkyl chloroformate.  
         [0066]    Dialkyl carbonates prepared according to the method are useful for the preparation of diaryl carbonates. For example, diaryl carbonates may be generated by the reaction of a dialkyl carbonate with an aryl hydroxide (see Scheme I, above). The diaryl carbonate may in turn be reacted with a dihydric phenol to form a polycarbonate (see Scheme II, above). For example, dimethyl carbonate prepared according to the method may be reacted with phenoxide to form diphenyl carbonate, which in turn may be reacted with bisphenol A to form a polycarbonate.  
         [0067]    The invention is further illustrated by the following non-limiting examples.  
       EXAMPLE 1  
       [0068]    A plant according to simplified FIG. 2 was built and operated to produce dimethyl carbonate. Corrosion damage was observed in and downstream of the azeotropic column  180 . After extensive experimentation, it was determined that the corrosion damage was caused by methyl chloroformate passing through the acid separation column. Specifically, methyl chloroformate was found to be present in the azeotrope column  180  at a concentration of 300 parts per million (ppm) by weight.  
       EXAMPLES 2-5  
       [0069]    The decomposition kinetics of methyl chloroformate were studied under four different conditions. A procedure for determining methyl chloroformate in a sample was as follows. For Example 2, 32 milliliters (mL) of dimethyl carbonate, 10 mL of dimethyl carbonate containing 50 mg of a biphenyl internal standard 63 mL of methanol, and 5 ml of water were added to a 250 mL flask equipped with a thermometer, a condenser, and a port for sampling. (Toluene may be used instead of the methanol/water solution.) The resultant homogeneous solution was placed in an oil bath and the temperature of the solution was held constant at 50° C. At time zero, 81.7 microliters of pure methyl chloroformate were added to the solution (1,000 ppm on a weight basis). Samples were withdrawn at various time intervals and were quenched by reacting the methyl chloroformate in the sample with diisobutyl amine to convert the methyl chloroformate to N,N′-diisobutyl methyl carbamate. The amount of N,N′-diisobutyl methyl carbamate was then analyzed via titration with a standard silver nitrate solution to quantify the amount of ionic chloride present. The amount of methyl chloroformate could then be inferred by analyzing the original sample for ionic chloride. The difference in chloride concentration is equal to the methyl chloroformate concentration because each equivalent of methyl chloroformate liberates one equivalent of ionic chloride upon derivatization. Alternatively, gas chromatography can be used for direct analysis of the N,N′-diisobutyl methyl carbamate using an internal standard.  
         [0070]    Table I below show the observed decomposition rate constants (k) at 50° C. for various conditions. Example 2 corresponds to the case described above. Example 3 has added hydrochloric acid that is generally present in the authentic reaction mixture. In Example 4, the effect of a small amount of sodium bicarbonate was tested. In Example 5, the ratio of dimethyl carbonate to methanol was held constant, but the amount of water was increased from 5% to 10%. The results are summarized below in Table 1.  
         [0071]    [t4] 
                                                                 TABLE I                                   DMC   MeOH                       (wt %)   (wt %)   H 2 O (wt %)   Temp (° C.)   k (min −1 )                                    Ex. 2   45   50   5   50   0.043       Ex. 3*   45   50   5   50   0.043       Ex. 4**   45   50   5   50   0.480       Ex. 5***   43   47   10   50   0.055                                          
 
         [0072]    Plots of the logarithm of methyl chloroformate concentration versus time were linear, fitting a pseudo-first-order kinetic model. This behavior was observed even in the presence of hydrochloric acid, and therefore this method can be used to determine the concentration of methyl chloroformate in a particular sample. Comparison of Examples 2 and 5 indicates that only minor variations in the rate coefficient, k, are observed when analyzing samples having water contents varying by a factor of two. Comparison of Examples 2 and 3 shows, surprisingly, that added HCl did not affect the observed rate of methyl chloroformate decomposition. Comparison of Examples 2 and 4 shows that even a small amount of base increased the reaction rate by more than ten-fold. As a practical matter, however, it may be desirable to avoid strongly basic conditions because they also may increase the decomposition rate of dimethyl carbonate.  
       Example 6, Comparative Example 1  
       [0073]    These experiments show that the fluid passageway  110  is effective to reduce the concentration of methyl chloroformate that can react to form HCl in downstream sections of the plant. With reference to FIG. 1, two samples were obtained by sampling the process fluid at different points in a dimethyl carbonate plant having a configuration with a first heat exchanger  140  and two holding vessels  120  (i.e., a configuration corresponding to FIG. 3). The first sample (Comparative Example 1) was taken immediately before the first heat exchanger  140 . The second sample (Example 6) was taken after the second holding vessel  120  (i.e., after the fluid passageway  110 ). Each same was taken to the lab, and its chloride content was determined as a function of time elapsed from sampling. The results are presented in Table II. The Ex. 6, data show essentially constant levels of chloride ion, indicating that labile, chloride-generating species such as methyl chloroformate are not present in the sample. In contrast, the data for Comp. Ex. 1 show an increase with chloride level over time, consistent with presence of methyl chloroformate in the initial sample and its decomposition over time to form additional chloride ion. Thus, the data collectively show that in the absence of the fluid passageway  110 , substantial chloride formation may take place in downstream (post-acid removal column  160 ) sections of the plant, causing corrosion, but the presence of the fluid passageway  110  is effective to decompose alkyl chloroformate to chloride ion before the acid removal column  160 , thereby preventing downstream corrosion.  
         [0074]    [t5] 
                                     TABLE II                                       Chloride Concentration (ppm)            Time (h)   Ex. 6   Comp. Ex. 1               0   374   189       2   408   312       4   374   339       8   372   368       10    372   357       25    381   368                  
 
       Example 7, Comparative Example 2  
       [0075]    For Comparative Example 2, a dimethyl carbonate plant according to simplified FIG. 2 was operated according to the conditions described in Table III , below. This plant was similar to that shown in more detail in FIG. 1, with the exception that the first heat exchanger  140  and the fluid passageway  110  were absent. Corrosion was observed in and downstream of the azeotrope column  180 . Next, this plant was modified to include the first heat exchanger  140  and two holding vessels  120  were added to increase residence time (i.e., FIG. 3 configuration). FIG. 10 presents measurements of residual ionic chlorides found in samples taken from the bottom of the azeotrope column  180 , comparing the FIG. 2 and FIG. 3 configurations, each over time. Residual chlorides were determined by titration using a silver nitrate solution, as described above. The data for the FIG. 2 configuration have an average of 671 ppm chloride with a standard deviation of 370 ppm chloride, whereas the data for the FIG. 3 configuration have an average of 35 ppm chloride and a standard deviation of 25 ppm chloride. The data thus show a dramatic reduction in chloride levels for the FIG. 3 configuration vs. the FIG. 2 configuration. It is predicted this reduction would be even greater for the configurations according to FIGS. 4 and 6, in which four holding vessels  120  are used to provide a residence time of four hours at 70° C. FIG. 11 presents measurements of methyl chloroformate concentration entering and exiting the fluid passageway  110  of the FIG. 3 concentration. In other words, the points signified by “+” and labeled “MCF feed to Phase 0” in FIG. 11 correspond to measurements on the mixture as it was entering the fluid passageway  110 ; these points have an average value of 930 parts per million by weight (ppmw) and a standard deviation of 412 ppmw. And the points signified by “▪” and labeled “MCF from Phase 0” correspond to measurements on the mixture as it exits the fluid passageway  110 ; these points have an average value of 45 ppmw and a standard deviation of 77 ppmw. These data clearly show that an apparatus according to FIG. 3 is effective to dramatically reduce the concentration of methyl chloroformate in the process stream.  
         [0076]    [t3] 
                       TABLE III                               Control Ex. 2           Ex. 7 (FIG. 2   (FIG. 3       Conditions   Configuration)   Configuration)                   Mass Ratio MeOH/O 2 /CO   0.7/0.06/1   0.7/0.06/1       Catalyst Content   Fixed   Fixed       Reaction Temperature (° C.)   133    133        Reaction Pressure (barg)   23   23       Temp. of Pre-Residence Time Heater   60   —       (° C.)       Temp. of Acid Column Feed Vaporizer   90   90       (° C.)       Residence Time between flash vessel    2      0.03       and acid column, excluding both       (hours)                  
 
         [0077]    [t1] 
                                     TABLE IV                                       average chloride               concentration ± standard           Configuration   deviation (ppm)                                    FIG. 3 (comparison)   671 ± 370       FIG. 2 (invention)    35 ± 25                  
 
         [0078]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.  
         [0079]    Where not specifically defined herein, technical terms in this specification may be interpreted according to Grant and Hach&#39;s Chemical Dictionary, 5 th  ed., McGraw-Hill, Inc.  
         [0080]    All cited patents and other references are incorporated herein by reference in their entirety.