Manufacture of chloroform
This application is a continuation of application Ser. No. 07/119,397 filed Nov. 10, 1987. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the manufacture of chloroform by partial chlorination of methyl chloridemethylene chloride mixtures. 
     2. Prior Art 
     The chlorinated methanes (methyl chloride, methylene chloride, chloroform and carbon tetrachloride) are well-known articles of commerce and are generally prepared by chlorination of an underchlorinated raw material. As the demand for the individual members of the series varies from time to time, it is desirable to be able to control their production accordingly. 
     At present the demand for chloroform is high, that for methylene chloride and carbon tetrachloride is low. The existing chlorination processes, however, are not entirely satisfactory for producing chloroform to the substantial exclusion of the other polychloromethanes. For example, the direct chlorination of methane for this purpose (described in SRI #126, C 1  Chlorinated Hydrocarbons, published August 1978, pages 49 to 96) is difficult to control and expensive to operate because of the necessity of recycling large amounts of methane, methyl chloride and methylene chloride. It also tends to produce undesirably high ratios of carbon tetrachloride to chloroform, which constitutes a cost penalty in view of the relatively low demand for the tetrachloride. 
     The production of methyl chloride is readily controlled by utilizing the reaction of hydrogen chloride with methanol, and for this reason it is an attractive starting material for the higher chlorinated methanes. 
     Baird, Baumgarten and Gentilucci, U.S. Pat. No. 3,502,734, describe a process of chlorinating methyl chloride and/or methylene chloride for the production of methylene chloride and/or chloroform with minimum formation of by-product carbon tetrachloride. The process involves (a) operating at pressures above the critical pressure of the reaction mixture, (b) pre-heating the ingredients, in specified proportions, to a specified temperature range to initiate the reaction, and (c) allowing this exothermic chlorination reaction to proceed adiabatically to a higher temperature close to the critical temperature of the reaction mixture, so as to capitalize on the high heat capacity of the mixture at such higher temperatures, until the chlorine reactant is substantially completely consumed. The mol ratio of chlorine to the chlorinated methane feedstock is in the range of 1:3 to 1:12. The mol ratio of methyl chloride to methylene chloride can also vary widely, ranging from 1.2:1 to 1.8:1 in the working examples. 
     Although the patent shows that more chloroform than methylene chloride can be produced under the disclosed conditions and that both can be obtained to the substantial exclusion of carbon tetrachloride, it is also apparent that the amount of methylene chloride produced is a substantial proportion of the total chlorinated methane production and corresponds to a significant net production of methylene chloride, which is disadvantaeous in times of low methylene chloride demand. 
     SUMMARY OF THE INVENTION 
     What has been discovered is a process for the manufacture of chloroform by the partial chlorination of methyl chloride and methylene chloride mixtures, which process comprises 
     providing an initial feed mixture consisting essentially of chlorine, methyl chloride, methylene chloride and optionally chloroform and/or hydrogen chloride in amounts such that the mol ratio of chlorine to said chlorinated methanes is in the range of from about 0.15:1 to about 0.21:1, the mol ratio of methyl chloride to methylene chloride is about 1.5:1 and the mol ratio of methylene chloride to chloroform is at least about 6:1; 
     feeding said mixture at a pressure of about 13 to 130 atmospheres to a reactor that minimizes backmixing of the mixture components and heating said mixture to a temperature in the range of from about 220° to about 500° C. at which reaction of the chlorine with the chloromethanes in the feed mixture is initiated; 
     maintaining said reaction under said conditions until substantially all the chlorine has reacted; 
     recovering a chlorinated product mixture containing chloroform, methyl chloride, methylene chloride and little or no carbon tetrachloride, the amount of methylene chloride being substantially equal to the amount employed in the initial feed mixture and the amount of carbon tetrachloride corresponding to less than 0.1 mol of chloroform in the product; and 
     separating chloroform from the product mixture. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reaction temperatures and pressures can vary widely as is well-known in the chlorination art. The temperature should be sufficiently high to initiate the chlorination reaction and allow it to proceed adiabatically. For example, the temperature can range from about 220° to 500° C. and preferably will be at least 250° C. The pressure can range from about 13 to 130 atmospheres and preferably will be at least about 17 atmospheres. More preferably, the pressure will be sufficiently high to maintain the reaction mixture in a super-critical fluidized state, thereby to minimize volumetric expansion of the reaction mixture when the temperature increases adiabatically as the exothermic chlorination proceeds. 
     Thus, a preferred embodiment of the invention is a continuous process which comprises 
     (1) providing a feed mixture consisting essentially of Cl 2 , CH 3  Cl, CH 2  Cl 2  and optionally CHCl 3  and/or HCl in amounts such that the mole ratio of Cl 2  to the chlorinatable chlorinated methanes is about 0.21:1, the mol ratio of CH 3  Cl to CH 2  Cl 2  is about 1.5:1 and the mol ratio of CH 2  Cl 2  to CHCl 3  is at least 6:1, 
     (2) continuously feeding the mixture at a pressure of at least 65 atmospheres to a heatable reaction zone, 
     (3) heating the mixture in said zone to a temperature at which the reaction of Cl 2  with said chloromethanes is initiated, 
     (4) continuously feeding the heated mixture to an adiabatic pipeline reaction zone under flow conditions that minimize backmixing of the mixture, 
     (5) allowing the chlorination reaction to proceed adiabatically under said flow conditions until substantially all the Cl 2  has reacted, then 
     (6) recovering a chlorinated product mixture containing CHCl 3 , CH 3  Cl and little or no CCl 4 , the amount of CH 2  Cl 2  being substantially equal to the amount employed in the feed mixture and the amount of CCl 4  corresponding to less than 0.1 mol per mol of the CHCl 3  product. 
     The reaction product mixture is separable by conventional methods, conveniently by distillation. The CH 3  Cl and CH 2  Cl 2  fractions, including any unseparated CHCl 3 , can be recycled along with make-up CH 3  Cl to provide a CH 3  Cl to CH 2  Cl 2  mol ratio of about 1.5:1 for further conversion to CHCl 3  in accordance with the process of the invention. 
     Except as modified by the disclosure that follows, the invention process will preferably be conducted as described in U.S. Pat. No. 3,502,734, which description is incorporated herein by reference. As disclosed in the referenced patent, the pressure on the reactants during the course of the chlorination is more preferably in the range of about 85 to 130 atmospheres, which pressures are above the critical pressure of the reaction mass. The reaction temperature will preferably also be above the critical temperature of the feed mixture or at least about 220° C. In a preferred embodiment the initial reaction mixture is fed to a heatable reaction zone, which may be a pipeline reactor, at a pressure above its critical pressure and heated to at least 220° C., preferably to at least 250° C., to initiate the reaction. The temperature during this stage of the reaction can be still higher, for example, around 300° C. The heated reaction mass then passes to an adiabatic reaction zone where chlorination continues with liberation of heat, resulting in a further increase in temperature. The temperature in this stage of the reaction can range from about 300° to about 450° C., and even as high as 500° C. Reaction is continued adiabatically until the chlorine is consumed. 
     The invention process is designed to provide net production of CHCl 3  with substantially no net production of CH 2  Cl 2  and little or no production of CCl 4 . The feed mixture mol ratios are critical, particularly the CH 3  Cl to CH 2  Cl 2  mol ratio. It is important to hold this ratio as close to 1.50:1 as is practicable to avoid net production of CH 2  Cl 2 . That the Cl 2  to chlorinatable chloromethanes mol ratio is less than stoichiometric means that unreacted CH 3  Cl and CH 2  Cl 2  will appear in the reaction product mixture along with product CHCl 3  and by-product HCl. For practical, i.e., economic, operation, it is desirable not only to recycle the unreacted chloromethanes but to minimize the total amount of such recycle material. Thus, although the Cl 2  to chloromethanes ratio can be as low as 0.15:1, it is preferred that it not be less than about 0.17:1 and highly preferred that it be held as close to 0.21:1 as is practicable in the initial feed mixture since the smaller this ratio the larger the amount of unreacted CH 3  Cl and CH 2  Cl 2  in the final product mixture. Ratios higher than about 0.21:1 tend to produce excessively high adiabatic reaction temperatures and are to be avoided. On the other hand, the CH 2  Cl 2  to CHCl 3  ratio, which is designed to minimize CCl 4  production, should be as high as possible. Although it can be as low as 6:1, it is preferred the CH 2  Cl 2  to CHCl 3  ratio be at least 10:1 and more highly preferred that the CHCl 3  be absent altogether from the initial feed mixture. Although HCl can be present in the initial feed mixture to serve as as a heat sink, it may sometimes be preferred that the HCl be substantially absent from the initial mixture. Thus, the ideal initial feed mixture will consist of Cl 2 , CH 3  Cl and CH 2  Cl 2  in the stated 0.21:1 and 1.50:1 mol ratios. 
     The term &#34;about&#34; as used herein to define the above mol ratios is meant to include variation of ±15 percent, preferably not more than ±10 percent, from the stated values, provided the sum of the deviation from the Cl 2  to chlorinatable chloromethanes ratio and the deviation from the CH 3  Cl to CH 2  Cl 2  ratio is not more than 15%, preferably not more than 10%, regardless of the sign (positive or negative) of the deviation. 
     By substantially no net production of CH 2  Cl 2  and CCl 4  is meant not more than 0.1 mol of each of these substances per mol of CHCl 3  produced. The feed stream mol ratios can be controlled within the defined limits in order to avoid substantial net production of CH 2  Cl 2  and CCl 4 . 
     It will be noted the production of CHCl 3  with substantially no net production of CH 2  Cl 2  and CCl 4  depends upon both CH 3  Cl and CH 2  Cl 2  as well as Cl 2  in the initial feed mixture. The chlorination process can be visualized as involving the utilization of one molar proportion of Cl 2  to produce CH 2  Cl 2  from CH 3  Cl and a second molar proportion to produce CHCl 3  from CH 2  Cl 2 . Overall, two mols of Cl 2  are required to produce one mol of CHCl 3 . Therefore, the maximum theoretical efficiency of the process for producing CHCl 3  with no net production of CH 2  Cl 2  and CCl 4  is the ratio of the mols of CHCl 3  produced to the mols of chlorine consumed in the reaction, or 0.50. Efficiencies less than 0.50 reflect net production of CH 2  Cl 2  and/or CCl 4 . The process of the invention can readily provide CHCl 3  in high production efficiencies as shown in the Examples. 
     It is also important for practical operation that the reactor be of the kind that minimizes backflow of the reactants during the chlorination reaction, since the greater the degree of backmixing that occurs the greater will be the amount of unwanted CCl 4  produced. 
     A preferred reactor for carrying out the continuous process of the invention is an unpacked pipeline reactor having a heatable zone and an adiabatic zone and whose overall length is at least 125 times its inside diameter. Rate of flow of the reaction mixture through the line should be high enough, for example, at Reynolds numbers of at least 5000, to minimize backmixing of the reactants. The total reaction time from when the reactants leave the heatable zone of the pipeline reactor to when they leave the adiabatic zone is generally around 0.5 to 3 minutes, most usually 1 to 1.5 minutes depending upon the temperature. The higher the temperature, the shorter the time required. 
     The reaction is further illustrated in the accompanying Examples. The reaction system involves two pumps--one for Cl 2 , another for the chloromethanes--a high pressure mixer for mixing the reactants and a pipeline reactor having a separate heatable zone and an adiabatic zone. The pumps are designed and operated to provide the reactants at pressures of at least about 65 atmospheres to the mixing device where the reactants are mixed and fed to the pipeline reactor made of high nickel alloy. The heatable section of the reactor is surrounded by a heating jacket, which serves as the means for heating the reactants to a chlorination temperature. The heatable zone leads to the adiabatic zone of the reactor, which is covered with pipe insulation. At the end of the pipeline reactor is a pressure let-down valve through which the reaction mixture passes for recovery and workup. The workup system includes means for venting and recovering by-product HCl gas and means for the distillation, separation and recovery of CH 3  Cl, CH 2  Cl 2 , CHCl 3  and CCl 4 . 
    
    
     EXAMPLES 1-3 
     A reactant feed composition as detailed below is fed at a pressure of 100 atmospheres to the heatable section of the pipeline reactor described above where it is heated to 250° C. to initiate the reaction. The reaction mass reaches a maximum temperature of about 450°-500° C. in the adiabatic section of the reactor. The residence time in the adiabatic section of the reactor is about 1 minute, which is sufficient for the initial charge of Cl 2  to be consumed. 
     The reaction is repeated two times with the feed mixture being varied as summarized in Table A below. Table A also includes the effluent composition and the distribution of products for each run. The sensitivity of the process to slight variations in the Cl 2  content and the presence of CHCl 3  in the initial feed mixture is shown in Table B. 
     Tables A and B show that the invention process is capable of producing CHCl 3  from controlled CH 3  Cl-CH 2  Cl 2  -CHCl 3  feed compositions to the substantial exclusion of CH 2  Cl 2  and CCl 4  on a net production basis. The data also show that the product distribution is sensitive to the Cl 2  /chloromethanes ratio and that it is beneficial to exclude CHCl 3  altogether from the initial feed composition. 
     
                       TABLE A                                                     
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Examples 1 to 3 of the Invention                                          
               Moles   Wt      Wt %                                       
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Example 1                                                                 
Reactor Feed Compositon                                                   
Cl.sub.2         2.24      158.8   18.7                                   
CH.sub.3 Cl      6.00      302.9   35.7                                   
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.5   40.0                                   
CHCl.sub.3       0.40      47.8    5.6                                    
CCl.sub.4        --        --      --                                     
                 12.64     849.0   100.0                                  
Reactor Effluent Composition                                              
CH.sub.3 Cl      4.92      248.4   29.3                                   
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6   40.0                                   
CHCl.sub.3       1.40      167.1   19.7                                   
CCl.sub.4        0.08      12.3    1.4                                    
HCl              2.24      81.6    9.6                                    
                 12.64     849.0   100.0                                  
Product                                                                   
CH.sub.2 Cl.sub.2                                                         
                 --        --      --                                     
CHCl.sub.3       1.00      119.4   90.7                                   
CCl.sub.4        0.08      12.3    9.3                                    
                 1.08      131.7   100.0                                  
CHCl.sub.3 /CCl.sub.4 Wt. Ratio                                           
                           9.7                                            
Recycle                                                                   
CH.sub.3 Cl      4.92      248.4                                          
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6                                          
CHCl.sub.3       0.40      47.6                                           
                 9.32      635.6                                          
By-Product HCl   2.24      81.6                                           
Example 2                                                                 
Reactor Feed Compositon                                                   
Cl.sub.2         2.12      150.3   19.0                                   
CH.sub.3 Cl      6.00      303.0   38.2                                   
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6   42.8                                   
CHCl.sub.3       --        --      --                                     
CCl.sub.4        --        --      --                                     
                 12.12     792.9   100.0                                  
Reactor Effluent Composition                                              
CH.sub.3 Cl      4.96      250.4   31.6                                   
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6   42.8                                   
CHCl.sub.3       1.00      119.4   15.1                                   
CCl.sub.4        0.04      6.2     0.8                                    
HCl              2.12      77.3    9.7                                    
                 12.12     792.9   100.0                                  
Product                                                                   
CH.sub.2 Cl.sub.2                                                         
                 --        --      --                                     
CHCl.sub.3       1.00      119.4   95.1                                   
CCl.sub.4        0.04      6.2     4.9                                    
                 1.04      125.6   100.0                                  
CHCl.sub.3 /CCl.sub.4 Wt. Ratio                                           
                           19.3                                           
Recycle                                                                   
CH.sub.3 Cl      4.96      250.4                                          
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6                                          
CHCl.sub.3       --         --                                            
                 8.96      590.0                                          
By-Product HCl   2.12      77.3                                           
Example 3                                                                 
Reactor Feed Compositon                                                   
Cl.sub.2         2.01      142.5   17.9                                   
CH.sub.3 Cl      6.00      302.9   38.0                                   
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6   42.6                                   
CHCl.sub.3       0.10      11.9    1.5                                    
CCl.sub.4        --        --      --                                     
                 12.11     796.9   100.0                                  
Reactor Effluent Composition                                              
CH.sub.3 Cl      4.99      251.9   31.6                                   
CH.sub.2 Cl.sub.2                                                         
                 4.07      345.5   43.3                                   
CHCl.sub.3       0.98      117.0   14.7                                   
CCl.sub.4        0.06      9.2     1.2                                    
HCl              2.01      73.3    9.2                                    
                 12.11     796.9   100.0                                  
Product                                                                   
CH.sub.2 Cl.sub.2                                                         
                 0.07      5.9     4.9                                    
CHCl.sub.3       0.88      105.0   87.4                                   
CCl.sub.4        0.06      9.2      7.7                                   
                 1.01      120.1   100.0                                  
CHCl.sub.3 /CCl.sub.4 Wt. Ratio                                           
                           11.4                                           
Recycle                                                                   
CH.sub.3 Cl      4.99      251.9                                          
CH.sub.2 Cl.sub.2                                                         
                 4.00      339.6                                          
CHCl.sub.3       0.10      12.0                                           
                 9.09      603.5                                          
By-Product HCl   2.01      73.3                                           
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                       TABLE B                                                     
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Selective Preparation of Chloroform                                       
                Example                                                   
Initial Feed Composition                                                  
                  1         2      3                                      
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Mol Ratio                                                                 
Cl.sub.2 /CH.sub.3 Cl, CH.sub.2 Cl.sub.2,CHCl.sub.3                       
                  0.215     0.21   0.199                                  
CH.sub.3 Cl/CH.sub.2 Cl.sub.2                                             
                  1.5       1.5    1.5                                    
CH.sub.2 Cl.sub.2 /CHCl.sub.3                                             
                  10.0      *      40.0                                   
Net Production, Mols                                                      
CHCl.sub.3        1.0       1.0    0.88                                   
CH.sub.2 Cl.sub.2 0         0      0.07                                   
CCl.sub.4         0.08      0.04   0.06                                   
Product Mol Ratio                                                         
CHCl.sub.3 /CH.sub.2 Cl.sub.2                                             
                  *         *      12.6                                   
CHCl.sub.3 /CCl.sub.4                                                     
                  12.5      25.0   14.7                                   
Chlorination Efficiency                                                   
Mols CHCl.sub.3 produced per                                              
                  0.45      0.47   0.44                                   
Mol Cl.sub.2 consumed                                                     
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 *infinite