Manufacturing olefins from gas oil
BACKGROUND OF THE INVENTION 
     This invention relates to the manufacture of C 2  and C 3  olefins from gas oil in a higher yield than was previously attained. In particular, it relates to manufacturing olefins from gas oil feedstocks that contain low concentrations of benzothiophenes, either because the feedstock has been selected for low benzothiophenes concentration or because benzothiophenes have been removed from the feedstock. 
     Gas oil is a crude oil distillation cut primarily consisting of a mixture of C 6  to C 16  hydrocarbons. Olefins can be manufactured from gas oil by cracking these hydrocarbons into smaller units. Cracking is accomplished by pyrolysis--heating the gas oil at high temperatures in the presence of steam and the absence of oxygen. Cracking produces a mixture of olefins of different chain lengths. Ethylene and propylene are the olefins which have the greatest commercial value. 
     SUMMARY OF THE INVENTION 
     We have discovered that certain components of some gas oils act as inhibitors in manufacturing olefins and reduce the yield of the most desirable olefins, ethylene and propylene. These inhibitors are benzothiophine and substituted benzothiophenes, particularly benzothiophenes that are unsubstituted in the three position. 
     As a result of this discovery, we have found that the yield of ethylene and propylene can be enhanced either by using gas oil that contains a low concentration of benzothiophenes or by reducing the concentration of benzothiophenes in the gas oil. The value of the increased yield of ethylene and propylene is very significant, worth tens of millions of dollars per year. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Gas oil is a cut from the distillation of crude oil that has a boiling point in the range of about 150° to about 350° C. The composition of gas oil varies with the location of the crude oil from which it was cut. Gas oil is principally a mixture of C 6  to C 16  hydrocarbons (e.g., paraffins, naphthenes, aromatics), but it also contains minor amounts of various other organic compounds. A typical gas oil feedstock may contain about 200 to about 5000 ppm (as sulfur) of various benzothiophenes. A benzothiophene (for the purpose of this invention) is a compound having a benzothiophene ring system ##STR1## where the seven positions indicated can be substituted with alkyl from C 1  to C 4  or alkylene from C 2  to C 4 . We have found that the presence of benzothiophenes in gas oil inhibits the production of C 2  and C 3  olefins, and that this is especially true when the 3-position of the benzothiophene ring is not substituted. 
     Thus, the essence of this invention is, in making olefins from gas oil, to use gas oil that is substantially free of benzothiophenes, especially benzothiophene and benzothiophenes that are unsubstituted in the 3-position. The feedstock used should contain less than 50 ppm (as sulfur) of benzothiophenes, and preferably should contain less than 10 ppm (as sulfur) of benzothiophenes. 
     Benzothiophenes-free gas oil feedstock can be obtained by selecting feedstock that already contains low concentrations of benzothiophenes. This can be accomplished by testing or analyzing the feedstock to determine the benzothiophene concentration therein and using only those feedstocks that contain low concentrations of benzothiophenes. The analysis of the feedstock for the presence of benzothiophenes is most advantageously accomplished using gas chromatography--mass spectroscopy (GC/MS) or a gas chromatograph equipped with a sulfur chemiluminescence detector. 
     Benzothiophenes-free gas oil can also be obtained by removing benzothiophenes from gas oil that contains high concentrations of benzothiophenes. That is, if a feedstock contains more than 10 or 50 ppm (as sulfur) benzothiophenes, a sufficient amount of the benzothiophenes is removed from the feedstock to lower their concentration below 50 ppm, and preferably below 10 ppm. This can be accomplished by distilling the feedstock and discarding distillate that boils between 45° and 60° C. under reduced pressure of about 0.05 to about 0.1 mmHg, as distillate boiling in that range contains benzothiophenes. The discarded distillate can be used for other purposes where the presence of benzothiophenes is not important. 
     Removal of benzothiophenes from the feedstock can also be accomplished by passing the feedstock through an adsorbent for benzothiophenes. We have found that activated alumina and activated carbon are good adsorbents for benzothiophenes. It is surprising that these substances are effective as adsorbents for benzothiophenes because we found that many other materials commonly used as adsorbents, such as Fuller&#39;s earth, silica gel, molecular sieves, sand, nickel oxide, copper oxide, sodium hydroxide beads, and manganese dioxide, were ineffective in adsorbing benzothiophenes from gas oil. At least about 1 bed volume of absorbent should be used for every 4 volumes of gas oil. When the bed has been exhausted, it can be reactivated with air heated to about 300° to about 400° C. 
     Once gas oil has been obtained with a low concentration of benzothiophenes it is pyrolyzed to form olefins. Pyrolysis is accomplished by heating the gas oil, typically at about 750° to about 850° C., in the presence of steam and the absence of oxygen. This cracks the gas oil into smaller carbon chains, and especially into valuable ethylene and propylene. The use of gas oil containing low concentrations of benzothiophenes can result in as much as a 5% increase in the yield of ethylene and propylene, as the examples which follow will demonstrate. 
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     Sulfur free gas oil was pyrolyzed in a 1-inch diameter quartz tube 12 inches long. For comparison some gas oil samples were doped with benzothiophene (BT) or dibenzothiophene (DBT). The gas oil flow rate was 0.203 g/min and the flow rate of the nitrogen diluent was 1.37 g/min. The following table gives the reaction temperature, the benzothiophenes added, and the yields of ethylene and propylene. 
     
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              Amount                                                      
         Amount                                                           
              of                                                          
         of   Additive                                                    
                   C.sub.2 H.sub.4 (wt %)                                 
                             C.sub.3 H.sub.6 (wt %)                       
Temp Addi-                                                                
         Additive                                                         
              as S Abso-     Abso-                                        
°C.                                                                
     tive                                                                 
         (ppm)                                                            
              (ppm)                                                       
                   lute                                                   
                       Difference*                                        
                             lute                                         
                                 Difference*                              
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775  --  --   --   17.8                                                   
                       --    10.9                                         
                                 --                                       
775  BT   100  24  17.0                                                   
                       -0.8  10.4                                         
                                 -0.5                                     
775  BT   500 119  16.7                                                   
                       -1.1  10.2                                         
                                 -0.7                                     
775  BT  1000 239  16.2                                                   
                       -1.6  9.9 -1.0                                     
775  BT  2000 478  15.7                                                   
                       -2.1  9.5 -1.4                                     
775  DBT 2900 504  17.9                                                   
                       0.1   10.9                                         
                                 0.0                                      
750  --  --   --   14.2                                                   
                       --    10.8                                         
                                 --                                       
750  BT  2000 478  12.2                                                   
                       -2.0  8.7 -2.1                                     
800  --  --   --   19.9                                                   
                       --    8.8 --                                       
800  BT  2000 478  18.0                                                   
                       -1.9  7.8 -1.0                                     
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 *With inhibitor minus without inhibitor.                                 
 
    
     The table shows that the benzothiophene reduced the yields of ethylene and propylene. The table also shows that dibenzothiophene did not reduce the yield of ethylene or propylene. 
     EXAMPLE 2 
     Example 1 was repeated using 2000 ppm (as sulfur) of 2 -methyl benzothiophene (2-MBT) or 2,5-dimethyl benzothiophene (DMBT). The following table gives the results. 
     
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         CH.sub.4                                                         
               C.sub.2 H.sub.4                                            
                         C.sub.3 H.sub.6                                  
                                 CH.sub.4 /C.sub.3 H.sub.6                
         (wt %)                                                           
               (wt %)    (wt %)  (wt %)                                   
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TEMPERATURE 775° C.                                                
No Additive                                                               
           6.0     13.4      8.4   0.71                                   
2-MBT      5.4     11.8      7.3   0.74                                   
DMBT       4.9     10.8      6.4   0.71                                   
TEMPERATURE 800° C.                                                
No Additive                                                               
           7.1     15.3      8.4   0.85                                   
2-MBT      6.6     14.2      7.6   0.87                                   
DMBT       6.0     13.0      7.4   0.81                                   
TEMPERATURE 825° C.                                                
No Additive                                                               
           8.9     18.4      8.8   1.01                                   
2-MBT      8.0     16.3      7.6   1.05                                   
DMBT       7.2     15.0      7.6   0.95                                   
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     The table shows that the benzothiophenes reduced the yield of ethylene and propylene. 
     EXAMPLE 3 
     Example 1 was repeated at 775° C. using 3-methylbenzothiophene (3-MBT). The following table gives the results: 
     
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               Yield (wt %)                                               
Product          Pure Gas Oil                                             
                            3-MBT                                         
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CH.sub.4         10.5       10.2                                          
C.sub.2 H.sub.6  2.0        2.0                                           
C.sub.2 H.sub.4  26.4       26.5                                          
C.sub.3 H.sub.8  0.3        0.3                                           
C.sub.3 H.sub.6  14.2       13.8                                          
C.sub.4 H.sub.10 0.3        0.3                                           
C.sub.4 H.sub.8  3.2        3.1                                           
C.sub.4 H.sub.6  6.2        6.0                                           
C.sub.4 &#39;s plus other minor                                               
                 36.9       37.8                                          
compounds                                                                 
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     The table shows that 3-MBT does not significantly reduce the yield of ethylene or propylene. 
     EXAMPLE 4 
     Gas oil was analyzed by GC/MS and was found to contain benzothiophenes. Using an Oldershaw 5 tray distillation column the gas oil was distilled under 0.05 to 0.1 mm Hg and cuts were collected at vapor temperatures of 49° C. (pot temperature=165° C.) and 57° C. (pot temperature=180° C.). Those cuts were analyzed by GC/MS and were found to contain benzothiophenes.