Patent Publication Number: US-4257781-A

Title: Process for enhancing the fuel value of low BTU gas

Description:
DESCRIPTION 
     This application is a continuation-in-part of my copending application, U.S. Ser. No. 817,576, filed July 21, 1977 now U.S. Pat. No. 4,134,907. 
    
    
     TECHNICAL FIELD 
     The need to use the extensive coal resources in this country as a source of fuel gas is now quite evident in view of the rapid depletion of other sources. Accordingly, it has become essential to develop processes for the economic production of fuel gas for industrial uses from coal. 
     Atmospheric coal gasification processes are well known and well developed. Typical of these proven processes are the Koppers-Totzek, Winkler, Wellman-Galusha, Woodall-Duckman, and others. The gas produced from these gasification processes is a low BTU gas comprising a mixture of carbon monoxide and hydrogen. This gas mixture has a low fuel value of about 300 Btu/ft 3  or less, on the average, which is too low for most industrial uses. 
     The fuel value of the gas produced by the atmospheric coal gasification processes can be enhanced with the use of high temperatures and pressures, sometimes accompanied by the use of oxygen and/or catalysts, to make the hydrogen and carbon monoxide present react to produce methane. Methane has a heat of combustion of 1013 Btu/ft 3 , whereas carbon monoxide and hydrogen has Btu&#39;s of about 322 and 325, respectively. The chief disadvantage, of course, of these procedures for enhancing the fuel value of the low Btu gas is the expense involved. The expense is so great that low Btu gas enhanced in this manner is not competitive with other fuels available for industrial uses. 
     So-called intermediate Btu gas is suitable for industrial uses, this gas having a Btu value of 450 Btu/ft 3  or more. It will burn well in existing gas burner equipment in power plants and other industrial applications with only minor modification in the burner head. The Btu value is high enough so that its use does not result in loss of boiler efficiency and, further, this gas can be economically piped moderate distances, which is not true for low Btu gas. 
     Accordingly, it is an object of this invention to provide a relatively inexpensive process for enhancing the fuel value of the low Btu gas produced by coal gasification processes. 
     DISCLOSURE OF THE INVENTION 
     A process for increasing the fuel value of a gas mixture of carbon monoxide and hydrogen by converting part of the hydrogen, and part of the carbon in the carbon monoxide of the gas mixture to methane, which comprises continuously introducing the gas mixture into a fluid bed in the presence of a mixture of iron and iron carbide under conditions of pressure and temperature which promote the reduction of carbon monoxide to carbon along with the formation of iron carbide by the reaction of iron and carbon followed by the formation of methane and iron by the reaction of iron carbide with hydrogen, while continuously removing from the fluid bed a gas mixture including methane, carbon monoxide and hydrogen having a substantially increased fuel value over the gas mixture introduced into the fluid bed. The gas mixture removed has a Btu value of about 600 on the average and is a suitable industrial or utility fuel. If methane alone is required it can be recovered from the gas mixture removed from the fluid bed by conventional procedures. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIGS. 1-3 are stability diagrams indicating the gas phase relationships between iron carbide and the hydrogen-carbon-oxygen system. The symbol αC refers to the activity of carbon in the system. The symbol &#34;P&#34; represents partial pressure. The amounts of gases are essentially directly related to the partial pressures. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The invention is based on establishing and maintaining conditions in a fluid bed which promote the following three reactions: 
     (1) CO+H 2  →C+H 2  O 
     (2) C+3Fe→Fe 3  C 
     (3) Fe 3  C+2H 2  →3Fe+CH 4   
     These reactions will proceed under atmospheric pressures, although slightly elevated pressures may be preferred. 
     In the fluid bed reaction, the iron acts as an acceptor of carbon in reaction (2) and as a donor of carbon in reaction (3). It will be noted that iron is reformed or regenerated in reaction (3) and that the iron carbide is reformed or regenerated in reaction (2) so that after the first addition of iron and iron carbide they are always present in the reaction zone without further additions. 
     Reaction (3) can be made to proceed to the right either by the addition of hydrogen or the removal of methane. Hydrogen and carbon monoxide are being continuously added in reaction (1) and methane, along with the carbon monoxide and hydrogen not converted, is being continuously removed as part of the enriched fuel gas. 
     The reactions can be made to proceed and controlled by controlling the ratio of the various gases present, that is, the ratio of methane to hydrogen, water to hydrogen, carbon dioxide to carbon monoxide, etc. Charts will be described hereinafter illustrating how control of these ratios results in the reactions proceeding in the required manner. 
     The fluidized bed reactor referred to herein is of the conventional type in which finely divided feed material on a grate or perforate support is fluidized by upwardly flowing gases which may include or entirely comprise the reactant gases. Auxiliary equipment includes heating and temperature control and monitoring equipment, heat exchangers, scrubbers, cyclones, gas cycling equipment and other conventional equipment which is used to remove solids from the off-gas stream and to remove water and CO 2  from a recycle gas loop to shift the gas system equilibrium. 
     The reactants introduced into the reactor after the initial charge of iron carbide and iron are the low Btu coal gasification gases containing carbon monoxide and hydrogen. 
     By proper balancing of the ratios of the hydrogen and carbon bearing materials in accordance with the stability diagrams, it is possible to make the hydrogen serve a reducing function to reduce the carbon monoxide to carbon, and the carbon serve a carburizing function as iron carbide is formed. As stated previously, conditions are established and maintained so that iron serves both a carbon acceptor function and a carbon donor function. Additionally, reaction conditions are adjusted so that hydrogen performs an additional reducing function in reducing iron carbide to iron and forming methane with the released carbon. 
     Because of the equilibrium conditions involved in hydrogen-carbon-oxygen gas systems, the required hydrogen-carbon ratios will automatically require that methane be present in the gas system. The quantity of methane present or produced will be a function of carbon to hydrogen ratios, as well as temperature and pressure conditions, and all of these can be controlled. 
     FIGS. 1, 2 and 3 are stability diagrams indicating the gas phase relationships between iron carbide and the hydrogen-carbon-oxygen system at temperatures of 1160°, 1070° and 1250° F., respectively. The stability diagrams indicate the relationship between log plots of partial pressure ratios of the various gas components which are in equilibrium with iron carbide in the present process. These illustrate that definite amounts of methane will exist in the system in the presence of the iron carbide, and that the amount of methane present or produced can be controlled by controlling the other variables in the system. These stability diagrams show that for any given temperature and pressure there is a fixed relationship between the gas components and the solid reactants iron and iron carbide thus indicating that fixed gas compositions will be obtained if mixed gases are contacted with iron carbide. Furthermore, the equilibrium gas composition can be altered at any given temperature and pressure by removal of one or more reaction products from the system. For example, if water vapor and/or carbon dioxide are continuously removed by scrubbing in the recycle gas loop, then the quantity of methane will continue to increase in the reaction zone. The charts indicate the operative range of variables at specified temperatures for insuring that Fe 3  C is present in the fluid bed. They also show the effect of temperature on the production of methane and Fe 3  C when the other variables for insuring the presence of Fe 3  C in the fluid bed are maintained substantially constant. 
     A feasible temperature range for the process is about 600° F. to about 1200° F., preferably about 600° F. to about 950° F. Temperatures outside these ranges are not economically feasible. Atmospheric pressures can be used and are preferred, although slightly elevated pressures of up to about 10 atmospheres are also suitable. Higher pressures are uneconomical. 
     The iron to iron carbide ratio in the reaction area can vary between about 10 percent iron carbide to 96 percent or more iron carbide. Iron may be added in metallic form or supplied from various sources, including iron oxide. Some carbon dioxide can be used in the feed gas as a source of carbon. It is an advantage of the process that oxygen is removed from the process in the form of water which is easily recovered. If any methane is fed into the reactor, it is unreacted and recovered with the product gas. 
     A 50 percent mixture of methane with carbon monoxide and hydrogen gives a gas mixture of 600 Btu. As can be seen from the examples below, this intermediate fuel gas is easily produced by the process of the invention. 
     EXAMPLE 1 
     Using the stability diagrams, a computer program was constructed which gives the equilibrium gas composition expected for the process when various hydrogen and carbon bearing gases are contacted with iron-iron carbide mixtures at various temperatures. Table 1 below shows examples of results obtained from this computer program under varying conditions of inlet gas composition, temperature and pressure under which the process is performed within the favorable methane production gas ratios illustrated in FIGS. 1-3. 
     
                                           TABLE 1                                 
__________________________________________________________________________
 Equilibrium Shift Calculations for Fe.sub.3 C System                     
Pres-                                    Btu/scf                          
Temp                                                                      
    sure                                                                  
       Inlet Gas, Volume Percent                                          
                        Off-gas, Volume Percent                           
                                         Inlet                            
                                            Off                           
°F.                                                                
    Atm                                                                   
       H.sub.2                                                            
          H.sub.2 O                                                       
             CO CO.sub.2                                                  
                   CH.sub.4                                               
                      N.sub.2                                             
                        H.sub.2                                           
                           H.sub.2 O                                      
                              CO CO.sub.2                                 
                                    CH.sub.4                              
                                       N.sub.2                            
                                         Gas                              
                                            Gas                           
__________________________________________________________________________
Section 1                                                                 
750  1 48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        4.6                                               
                           9.7                                            
                              1.7                                         
                                 38.0                                     
                                    37.5                                  
                                       8.6                                
                                         292                              
                                            434                           
840 1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        8.0                                               
                           9.0                                            
                              4.5                                         
                                 35.6                                     
                                    34.7                                  
                                       8.3                                
                                         292                              
                                            422                           
930 1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        13.0                                              
                           8.5                                            
                              9.7                                         
                                 30.9                                     
                                    30.0                                  
                                       7.8                                
                                         292                              
                                            405                           
1020                                                                      
    1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        19.8                                              
                           8.0                                            
                              17.1                                        
                                 24.2                                     
                                    23.6                                  
                                       7.2                                
                                         292                              
                                            382                           
1110                                                                      
    1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        27.9                                              
                           7.1                                            
                              25.2                                        
                                 17.0                                     
                                    16.3                                  
                                       6.5                                
                                         292                              
                                            356                           
1200                                                                      
    1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        35.6                                              
                           5.9                                            
                              32.1                                        
                                 10.8                                     
                                    9.7                                   
                                       5.8                                
                                         292                              
                                            331                           
1290                                                                      
    1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        41.6                                              
                           4.6                                            
                              36.8                                        
                                 6.7                                      
                                    4.9                                   
                                       5.4                                
                                         292                              
                                            312                           
Section 2                                                                 
750 1  53.0                                                               
          1.0                                                             
             31.0                                                         
                1.0                                                       
                   13.0                                                   
                      1.0                                                 
                        7.7                                               
                           16.4                                           
                              0.9                                         
                                 19.4                                     
                                    54.0                                  
                                       1.7                                
                                         400                              
                                            674                           
840 1  53.0                                                               
          1.0                                                             
             31.0                                                         
                1.0                                                       
                   13.0                                                   
                      1.0                                                 
                        12.7                                              
                           14.0                                           
                              2.5                                         
                                 18.9                                     
                                    50.0                                  
                                       1.6                                
                                         400                              
                                            638                           
930 1  53.0                                                               
          1.0                                                             
             31.0                                                         
                1.0                                                       
                   13.0                                                   
                      1.0                                                 
                        19.3                                              
                           11.4                                           
                              5.9                                         
                                 17.0                                     
                                    44.9                                  
                                       1.5                                
                                         400                              
                                            595                           
Section 3                                                                 
930 1  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        13.0                                              
                           8.5                                            
                              9.7                                         
                                 30.9                                     
                                    30.0                                  
                                       7.8                                
                                         292                              
                                            405                           
930 5  48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        6.3                                               
                           10.0                                           
                              4.6                                         
                                 35.4                                     
                                    35.3                                  
                                       8.4                                
                                         292                              
                                            429                           
930 10 48.0                                                               
          2.0                                                             
             39.0                                                         
                5.0                                                       
                   1.0                                                    
                      5.0                                                 
                        4.6                                               
                           10.4                                           
                              3.3                                         
                                 36.5                                     
                                    36.7                                  
                                       8.5                                
                                         292                              
                                            435                           
__________________________________________________________________________
 
    
     The results recorded in section 1 of Table 1 show the theoretical change in composition resulting when a gas having a composition similar to commercially produced &#34;blue water gas&#34; is subjected to the computerized program. 
     The results is section 2 of the Table show the theoretical change in composition obtained when a gas having a composition similar to gas produced by the Lurgi oxygen-pressure gasification is subjected to the computerized process. The large increase in yields of methane within a well defined temperature range graphically illustrates the critical effect of temperature on the yield of methane. 
     The results in section 3 of the Table show the theoretical effect of pressure on the yield of methane when the computerized process is applied to the same gas used for the section 1 tests. Methane yield is increased from 30 volume percent to 36.7 volume percent by increasing the pressure from one to ten atmospheres. Increased pressures would probably show slight increase in methane production but such pressures become uneconomic. 
     EXAMPLE 2 
     In order to further illustrate the operativeness of the invention and to illustrate the correlation between the results obtained by the computer application of the process and actual operation of the process, bench scale tests were made of the process. The tests were run in accordance with previously described procedure. Adequate iron and iron carbide were present in the fluid bed to start the reaction. No further addition of these components was necessary. Results from actual tests are recorded in each section with results from the computerized test under identical conditions. The results are recorded in Table 2. 
     
                                           TABLE 2                                 
__________________________________________________________________________
 Experimental Shift Data for Fe.sub.3 C System                            
                                                    Btu/scf               
Temp       Pressure                                                       
                Inlet Gas, Volume Percent                                 
                                  Off-gas, Volume Percent                 
                                                    Inlet                 
                                                       Off-               
°F. Atm  H.sub.2                                                   
                   H.sub.2 O                                              
                      CO CO.sub.2                                         
                            CH.sub.4                                      
                               N.sub.2                                    
                                  H.sub.2                                 
                                     H.sub.2 O                            
                                        Co CO.sub.2                       
                                              CH.sub.4                    
                                                 N.sub.2                  
                                                    Gas                   
                                                       Gas                
__________________________________________________________________________
Section 1                                                                 
Actual                                                                    
      1020 1    65.0                                                      
                   2.0                                                    
                      33.0                                                
                         0  0  0  60.7                                    
                                     2.5                                  
                                        12.6                              
                                           2.4                            
                                              21.8                        
                                                 0  317                   
                                                       461                
Computer                                                                  
      1020 1    65.0                                                      
                   2.0                                                    
                      33.0                                                
                         0  0  0  38.8                                    
                                     1.7                                  
                                        10.5                              
                                           17.2                           
                                              31.8                        
                                                 0  317                   
                                                       481                
Section 2                                                                 
Actual                                                                    
      1020 1    22.0                                                      
                   1.7                                                    
                      17.7                                                
                         13.3                                             
                            7.8                                           
                               37.5                                       
                                  30.2                                    
                                     1.1                                  
                                        13.0                              
                                           4.8                            
                                              12.5                        
                                                 38.4                     
                                                    207                   
                                                       264                
Computer                                                                  
      1020 1    22.0                                                      
                   1.7                                                    
                      17.7                                                
                         13.3                                             
                            7.8                                           
                               37.5                                       
                                  15.9                                    
                                     1.9                                  
                                        16.2                              
                                           5.7                            
                                              13.3                        
                                                 47.0                     
                                                    207                   
                                                       239                
__________________________________________________________________________
 
    
     The results recorded in section 1 of Table 2 are from a test program using a 3:1 mixture of hydrogen to carbon monoxide as the inlet gas, this gas representing a gasification process working with oxygen. At 1020° F. the actual test produced a gas with a 21.8 percent methane and a Btu value of 461 as compared to the predicted values of 31.8 percent methane and 481 Btu&#39;s. 
     The results recorded in section 2 of Table 2 show the change in composition obtained by the process in a representative gas containing relatively large amounts of inert nitrogen, this gas representing a gasification process working with air. The actual test produced a gas with 12.5 percent methane and a Btu value of 264 as compared to a predicted methane content of 13.3 percent and a Btu value of 239. An increase in Btu value of over 30 percent was obtained in both instances. 
     The test results established the operativeness of the process for producing methane, and prove the validity of the stability diagrams of FIGS. 1-3 for use in selecting conditions for operative and feasible production of methane. 
     EXAMPLE 3 
     Various gases were fed at a rate of 200 cubic feet per minute to a two foot diameter fluidized-bed reactor containing sufficient iron and iron carbide to start the reaction. No further addition of these materials was necessary. The inlet gases consisted of hydrogen, carbon monoxide and carbon dioxide introduced in amounts conforming to favorable methane production ratios illustrated in FIGS. 1-3. A temperature of 930° F. and atmospheric pressure were used for all the tests. The inlet gas had a composition of approximately 82 percent hydrogen, 8 percent carbon dioxide and 10 percent methane with a Btu value of about 370. The ratio of iron carbide to iron varied from a ratio of about 73/27 percent to 96/4 percent. 
     Analyses were made of the off-gas taken at half-hour intervals for a 12 hour period, the results of which are presented in Table 3. 
     
                       TABLe 3                                                     
______________________________________                                    
 Pilot Plant Gas Composition Data                                         
Reactor Products-Solid, Gas                                               
                      Ratio                                               
Off Gas                 CO/    H.sub.2 /                                  
                                      H.sub.2 /                           
Time H.sub.2 O                                                            
            CO.sub.2                                                      
                   CO   N.sub.2                                           
                             H.sub.2                                      
                                  CH.sub.4                                
                                        CO.sub.2                          
                                             H.sub.2 O                    
                                                  CH.sub.3                
______________________________________                                    
2400 1.2    4.5    3.9  8    35   44    0.9  29.2 0.8                     
0030 1.2    4.5    3.9  8    33   44    0.9  27.5 0.8                     
0100 1.0    4.5    3.9  8    35   44    0.9  35.0 0.8                     
0130 1.0    4.8    4.2  8    35   43    0.9  35.0 0.8                     
0200 1.0    4.8    4.2  8    35   44    0.9  35.0 0.8                     
0230 1.0    4.8    4.2  8    35   44    0.9  35.0 0.8                     
0300 1.0    4.8    4.0  8    34   42    0.8  34.0 0.8                     
0330 1.0    4.8    4.2  8    34   43    0.9  34.0 0.8                     
0400 1.0    4.8    4.2  8    34   43    0.9  34.0 0.8                     
0430 1.0    5.5    4.2  8    35   43    0.8  35.0 0.8                     
0500 1.0    5.5    4.0  8    35   43    0.7  35.0 0.8                     
0530 1.0    6.7    4.8  8    35   40    0.7  35.0 0.9                     
0600 1.0    6.2    4.8  8    35   40    0.8  35.0 0.9                     
0630 1.0    6.2    4.8  8    35   41    0.8  35.0 0.9                     
0700 1.0    6.2    5.0  8    35   40    0.8  35.0 0.9                     
0730 1.0    6.7    5.1  8    35   40    0.8  35.0 0.9                     
0800 2.4    7.5    7.9  7    35   40    1.1  14.6 0.9                     
0830 2.4    7.75   8.25 6.5  35   39    1.1  14.6 0.9                     
0900 2.4    8.6    8.9  7    34   38.3  1.0  14.2 0.9                     
0930 2.4    5.3    6.6  7    38   40    1.3  15.8 1.0                     
1000 2.3    4.4    4.5  5.5  41   33.5  1.0  17.8 1.2                     
1030 2.3    3.6    4.5  5.5  40   40    1.3  17.4 1.0                     
1100 2.4    4.5    5.2  7    39   41.5  1.2  16.3 0.9                     
1130 2.3    4.8    6.5  7    37   41.5  1.4  16.1 0.9                     
______________________________________                                    
 
    
     The average methane content of the off-gas during the 12-hour period exceeded 40 percent and the off-gas had a Btu average value of about 560 as compared to the Btu value of only 370 for the inlet gas. 
     Again, the results of the table show the feasibility of the process for strongly enhancing the Btu value of a gas, including one containing methane. The results illustrate the feasible time period for the enhancement. Further, the results show that large amounts of methane are produced with large percentages of iron carbide to iron present in the fluid bed. For example, at 1000 the percentage of iron carbide to iron in the bed was about 96 percent. The results further establish the validity of the stability diagrams of FIGS. 1-3 for use in selecting favorable operating conditions for the process. 
     EXAMPLE 4 
     Gases containing hydrogen and carbon monoxide in a ratio of three parts of hydrogen to one part of carbon monoxide were fed at a rate of 200 cubic feet to a two-foot diameter fluidized-bed reactor containing sufficient iron and iron carbide to start the reaction. No further addition of these materials was necessary. A temperature of 840° F. and atmospheric pressure were used for all tests. After the system reached equilibrium with respect to off-gas composition, a portion of the off-gas was taken through a water scrubbing step to remove the water formed in the reaction and this scrubbed gas was substituted for part of the incoming hydrogen and carbon monoxide feed gas. As predicted from the stability diagrams, this removal of water shifted the equilibrium and permitted additional quantities of hydrogen and carbon monoxide to react and form methane. Analysis of the off-gas taken at half-hour intervals for a period of twelve hours are presented in Table 4. 
     
                                           TABLE 4                                 
__________________________________________________________________________
Pilot Plant Gas Composition Data                                          
for Reactor Products with Water Scrub Circuit                             
Off-gas             Recycle Gas                                           
                           Ratio                                          
Time                                                                      
   H.sub.2 O                                                              
      CO.sub.2                                                            
         CO N.sub.2                                                       
               H.sub.2                                                    
                 CH.sub.4                                                 
                    H.sub.2 O                                             
                           CO/CO.sub.2                                    
                                H.sub.2 /H.sub.2 O                        
                                     H.sub.2 /CH.sub.4                    
__________________________________________________________________________
1800                                                                      
   8  6  4  10 32                                                         
                 40 3      0.7  4.0  0.8                                  
1900                                                                      
   8  6  4  8  31                                                         
                 42 3      0.7  3.9  0.7                                  
2000                                                                      
   8  6  5  7  29                                                         
                 44 3      0.9  3.6  0.7                                  
2100                                                                      
   8  6  5  5  26                                                         
                 48 3      0.9  3.3  0.5                                  
2200                                                                      
   8  6  5  3  24                                                         
                 52 3      0.9  3.0  0.5                                  
2300                                                                      
   8  6  5  4  24                                                         
                 52 3      0.9  3.0  0.5                                  
2400                                                                      
   7  4  5  4  22                                                         
                 56 2      1.3  3.1  0.4                                  
0100                                                                      
   7  4  4  4  20                                                         
                 60 2      1.0  2.9  0.3                                  
0200                                                                      
   6  5  5  4  22                                                         
                 58 2      1.0  3.7  0.4                                  
0300                                                                      
   7  4  4  4  20                                                         
                 60 2      1.0  2.9  0.3                                  
0400                                                                      
   6  4  3  4  20                                                         
                 62 2      0.8  3.3  0.3                                  
0500                                                                      
   6  5  3  3  20                                                         
                 62 2      0.6  3.3  0.3                                  
0600                                                                      
   6  5  3  3  20                                                         
                 62 2      0.6  3.3  0.3                                  
__________________________________________________________________________
 
    
     The average methane content of the gas increased from 42 percent at the beginning of the run to a value of 62 percent at the end of the run to give an increased Btu value of 690 as compared to the 300 Btu value of the inlet gas. By shifting the equilibrium through the removal of water vapor, the methane content was increased as indicated by the stability diagrams. 
     Similar results were obtained when the equilibrium was shifted by removing CO 2  from the recycle gas through the addition of caustic to the water scrub recycle loop. These results further establish the validity of the stability diagrams of FIGS. 1-3 for use in selecting favorable operating conditions for the process.