Demetallization of petroleum feedstocks with zinc chloride and titanium tetrachloride catalysts
BACKGROUND OF THE INVENTION 
     This invention relates to the demetallization of hydrocarbon charge stocks. More particularly, it relates to the removal of metallic contaminants, especially those of nickel and vanadium, from residual oils. 
     Various petroleum feedstocks, such as crude petroleum oils, heavy vacuum gas oils, shale oils, oils from bituminous sands, topped crudes, and atmospheric or vacuum residual fractions contain varying amounts of non-metallic and metallic impurities. The non-metallic impurities usually include nitrogen, sulfur, and oxygen; the metallic impurities usually include nickel, vanadium, iron, sodium, copper, zinc, and arsenic. In addition, it is known that most the the metallic contaminants are present as inorganic sulfides, oxides, and water soluble constituents, while the remainder is usually in the form of relatively thermally stable organometallic complexes such as metal porphyrins and derivatives thereof. It is generally conceded by the art that for purposes of demetallization the removal of the organometallics is more difficult than that for the inorganics. 
     The presence of these contaminants in petroleum feedstocks presents assorted difficulties in the refining process. In most petroleum refineries today, however, the problems associated with the inorganic metallics and non-metallic impurities have largely been resolved. Catalytic hydrotreating or hydrofining operations effectively remove sulfur, nitrogen, and oxygen from petroleum stocks by hydrodecomposition. Inorganic metallics, on the other hand, are generally removed at least in part, by filtration, water washing, or electric desalting operations performed prior to hydrofining. But removal of the organometallics, especially those of vanadium and nickel, is especially troublesome. These components are readily adsorbed by conventional hydrofining catalysts and other catalysts used in cracking and reforming operations, resulting in rapid deactivation thereof. 
     A general method by which these organometallics can be at least partially removed prior to the above catalytic contacting processes is to pass the desalted feedstock (especially those with metals content in excess of 150 ppm) over catalytic adsorptive materials. Many such materials are known in the art. For example, spent cobalt-molybdate hydrotreating catalysts have been used for this purpose, but generally they are too badly coked to provide effective service. Bauxite has also been used for this purpose, as reported in U.S. Pat. No. 2,687,985. And manganese nodules removed from the sea floor have been reported in a series of patents (see U.S. Pat. Nos. 3,772,185, 3,716,479, 3,766,054 and 3,813,331) as being effective catalytic adsorbents for demetallizing petroleum feedstocks. 
     Although the aforementioned catalytic adsorbents are effective for demetallization purposes, their use usually results in one or more disadvantages. For example, because these catalysts are usually of low capacity, frequent shutdowns for replacement of catalyst are necessitated. Also, to decompose the metal porphyrins into forms adsorbable in the catalyst, high operating temperatures, i.e., above 900° F., are required. This, of course, not only results in high heat energy requirements for the process, but also causes deactivation of the catalysts by coking, a problem which is usually avoided by using hydrogen and elevated pressures. But this in turn simply increases the overall cost of operation, primarily because the amount of hydrogen consumed to prevent coking is substantial. The net effect, therefore, of demetallizing with catalytic adsorbents is that high costs are incurred in a process designed to remove an extremely small proportion of contaminants from a feedstock. 
     Of course, for feedstocks which are destined to be hydrocracked, demetallizing under severe hydrocracking conditions may prove economical despite the cost of demetallizing with catalytic adsorbents. But for feedstocks which would ordinarily be sent directly to a catalytic cracking unit, or for heavy residua and the like, which would ordinarily be sold as a fuel for power plants and the like, the operating costs of using hydrocracking conditions is uneconomical, primarily because of the costs involved in consuming hydrogen. Thus, for such feedstocks, a process is required for removing nickel and vanadium contaminants that would cause deactivation of cracking catalysts, or would cause corrosion on the external side of power plant boiler tubing, but at the same time not require the undesirable and severe hydrocracking conditions necessary in prior art processes. 
     SUMMARY OF THE INVENTION 
     According to the present invention, liquid hydrocarbon feedstocks, such as heavy hydrocarbon fractions and the like are demetallized by blending therewith zinc chloride and/or titanium tetrachloride and charging the resulting mixture to a demetallization reactor, wherein under conditions of elevated temperature and pressure, more than about 70% of the vanadium and nickel contaminants are converted to insoluble components removable by filtration or sedimentation. The products removed from the demetallization reactor include liquid product oil, coke containing insoluble vanadium and nickel components, and either or both of the catalysts zinc chloride and titanium tetrachloride. Titanium tetrachloride is recovered in vapor form for recycle to the demetallization reactor by means of a conventional liquid-gas separator. The zinc chloride and coke are removed from the liquid product oil in a liquid-solid separation zone, wherefrom a liquid product oil is obtained containing nickel plus vanadium in a concentration less than 30%, preferably less than 10%, of the concentration of nickel plus vanadium in the feedstock. The separated solid zinc chloride and coke are fed to a combustion zone wherein the coke is burned so as to produce a vanadium and nickel-rich ash marketable to steel producers and the like. However, the primary purpose of the combustion zone is to vaporize zinc chloride, which may then be separated from the other products of combustion by condensation so that the zinc chloride may then be recycled to the demetallization reactor. 
     As calculated herein, the concentrations of nickel and vanadium are based on the weight of the metals themselves in the feedstock, product oil, and coke, regardless of the forms (elemental or combined) in which the nickel and vanadium may happen to be present. Likewise, as determined herein, the removal percentages of nickel and vanadium contaminants is based on the weights of nickel and vanadium, as the metals themselves, in the feedstock and product oil. Lastly, as used herein, the term &#34;coke&#34; refers to all solids produced in the demetallization reactor, less any added metal chloride also present therein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Feedstocks contemplated to be treated by the process described herein include broadly all liquid hydrocarbon mixtures, including whole crudes, which contain nickel plus vanadium contaminants in a total concentration of at least about 50 ppmw. Especially contemplated feedstocks are those which, but for the high nickel and vanadium concentrations therein, would be marketable. Thus, the process will be most commonly applicable to heavy fuel oils and other high boiling fractions which would be ideal fuels for power plants and the like except for the corrosion problems associated with the high metals content of such high boiling fractions. Especially contemplated feedstocks, therefore, are atmospheric and vacuum distillation residua containing more than 50 ppmw of nickel plus vanadium components. Typical of such charge stocks include those having an initial boiling point below 900° F. with a substantial proportion, generally at least 35% by volume, boiling above 900° F. Other contemplated feeds are those which would normally be further refined by methods other than hydrocracking, such as by catalytic cracking. 
     Referring now to the drawing, the feedstock to be treated in line 1 is combined with hydrogen fed at a rate of between 1000 and 5000 ft 3  /bbl via line 2 and with zinc chloride and/or titanium tetrachloride injected through line 3 at a total metal chloride catalyst feed rate between about 0.5 and 10.0 lb/bbl, preferably 2.0 and 4.5 lb/bbl, with the higher metal chloride feed rates in these ranges being preferred for hydrocarbon feedstocks containing in excess of 500 ppmw of nickel plus vanadium contaminants. The mixture so produced is passed through line 4 into demetallization reactor 5 wherein the nickel and vanadium components are converted under conditions of elevated temperatures and pressures to insoluble forms. 
     Conditions within reactor 5 are preferably chosen to effect substantial demetallization of the feedstock with a minimum of coking and hydrocracking. For this purpose conditions in the following ranges are required: 500-2000 psig, 550°-900° F., and 0.5-10.0 hours residence time. Under such conditions, and with sufficient catalyst present in the mixture entering reactor 5, at least 70% of the nickel plus vanadium contaminants are converted to oil-insoluble forms with concomitant coke formation being less than 3% by weight. Also preferably, the ratio of percent nickel plus vanadium contaminants converted to insoluble forms to the percent coke formed is above about 45, preferably above 80. 
     Of course, it must be realized that to effect a desired percentage of nickel plus vanadium removal requires proper correlation of the operating conditions within the demetallization reactor 5 and the rate at which the zinc chloride and/or titanium tetrachloride catalyst is added via line 3. To varying extents, increasing catalyst feed rate, operating pressure, operating temperature, or residence time also results in increasing the conversion of nickel and vanadium contaminants to insoluble forms. Therefore, although the use of operating conditions and catalyst feed rates specified hereinbefore are effective for removing at least 70% of the nickel plus vanadium contaminants, such conditions and feed rates may also be adjusted so as to increase the removal percentage to at least 80%, and usually to at least 90%. However, if such increased demetallization is desired, it is preferred that the catalyst feed rate, rather than the temperature, pressure, or residence time, be increased until such higher demetallization efficiency is obtained. Operating in this manner insures that the maximum proportion of nickel plus vanadium is removed without substantially increasing the proportion of coke formed. 
     A mixture of product oil, catalyst, gas, and coke containing insoluble nickel and vanadium components is directed from reactor 5 via line 6 into liquid-gas separator 7 of conventional design wherefrom essentially all the hydrogen and titanium tetrachloride added in lines 2 and 3, respectively, are recovered in line 8. In condenser-separator 9 titanium tetrachloride is condensed at an appropriate temperature and pressure and then recycled via line 10 to reactor 5. The uncondensable off-gases, consisting essentially of hydrogen and an extremely minor proportion of hydrocarbonaceous vapors generated in reactor 5, are also recycled via lines 11 and 12, with the excess make gas being removed via line 13 at a rate sufficient to compensate for the production of hydrocarbonaceous vapors in reactor 5. 
     From liquid-gas separator 7 a mixture of product oil, zinc chloride catalyst, and coke containing insoluble nickel and vanadium components is passed through line 14 into solids separator 15, wherein, by filtration, settling, or centrifuging, the product oil is separated at a temperature in the 100°-500° F. range from essentially all undissolved solids, which product oil is then recovered in line 16. This product oil, being of essentially the same composition as the feedstock except for metals content, may either be further refined, as by hydrocracking, catalytic cracking, or hydrotreating, or sold as a metals-free fuel for use in power plants and the like. 
     The solids-containing sludge removed from solids separator 15 is preferably directed by line 17 to a combustion zone 18 wherein the sludge containing coke and zinc chloride is completely burned in the presence of air added via line 19 so as to vaporize essentially all the zinc chloride in said sludge and to recover an ash rich in nickel and vanadium compounds via line 20. The combustion zone 18 is preferably of the fluidized combustion type, similar to those designed to recover zinc chloride from spent melts obtained from hydrocracking coal extract with a catalyst consisting of molten zinc chloride. A more thorough discussion of fluidized combustion processes for recovering zinc chloride is found in an article in I &amp; E.C. PROCESS DESIGN AND DEVELOPMENT, Vol. 8, No. 4, (Oct. 1969) entitled &#34;Fluidized Combustion Process for Regeneration of Spent Zinc Chloride Catalysts&#34; by Clyde W. Zielke et al, herein incorporated by reference. 
     The gases in line 21, in addition to containing vaporized zinc chloride, contain products of combustion from the burning of coke in combustion zone 18. To remove these undesired combustion products, the gases are passed to condenser-separator 22 wherein, at a suitable temperature and pressure, preferably at about 550°-1300° F. and 1 atm, the zinc chloride is condensed as a liquid and the combustion products discharged as an off-gas via line 23. The molten zinc chloride is then recycled to reactor 5 via line 24. 
     When steady state conditions are reached, the rate at which hydrogen is fed via line 2 and catalyst is fed via line 3 will be substantially reduced. Because of the recycling features of the preferred embodiment of the invention, hydrogen, zinc chloride, and titanium tetrachloride need only be added at rates sufficient to make-up for whatever losses of these components are incurred in the overall process. In general, it can be expected that no more than about a 5% loss of catalyst and a 15% loss of hydrogen, the latter being recovered via line 13, will be incurred in the process. 
     Finally, it is noted that, in non-preferred embodiments of the invention, hydrogen is not added via line 2. Although this results in more coking in demetallization reactor 5, the demetallization efficiency, i.e., the proportion of nickel plus vanadium removed, remains about the same as that obtained when hydrogen is used in the process. Thus, it should be apparent that the conversion of vanadium and nickel contaminants to insoluble components in demetallization reactor 5 does not involve the conversion of the metal-containing contaminants to light hydrocarbon fragments, as would occur if the feedstock were hydrocracked in demetallization reactor 5. In fact, even when hydrogen is added in line 2 and recycled through line 12, conditions are preferably chosen to prevent hydrocracking the feedstock, thereby avoiding the unnecessary consumption of hydrogen. Hence, in the preferred embodiment, the product oil obtained in line 16 will, except for the vanadium and nickel content, have substantially the same composition and properties as the original feedstock. 
     The following example demonstrates the superiority of zinc chloride and titanium tetrachloride for demetallizing a heavy residuum in comparison to ferric chloride and aluminum chloride. 
     EXAMPLE 
     A heavy Iranian residuum of composition and properties as shown in Table I was divided into eight 400-gram samples, which were individually introduced into a stirrer-equipped autoclave fitted with a glass liner. Also introduced into the autoclave were the various metal chloride catalysts in the amounts shown in Table II. The pressure within the autoclave was then raised to 1500 psig with hydrogen and the temperature to 650° or 700° F. as shown in Table II. The mixture within the autoclave was stirred at the rate shown in Table II, and all conditions were maintained for the residence time also indicated in Table II. 
     At the conclusion of each experiment, analyses were run to determine the nickel and vanadium content of the product oil, the gravity of the product oil, the fraction of the product oil boiling above 500° F., the amount of coke, and the concentration of nickel and vanadium contained in said coke. These data are also recorded in Table II. 
     
                       TABLE I                                                     
______________________________________                                    
IRANIAN RESIDUUM FEEDSTOCK                                                
COMPOSITION AND PROPERTIES                                                
Properties of Feed                                                        
Gravity, ° API  16.9                                               
Pour Point, ° F.                                                   
                       75                                                 
Carbon Residue, wt. %  9.0                                                
Asphaltenes            17.1                                               
Hydrogen 11.52                                                            
Sulfur                 2.62                                               
Nitrogen               0.356                                              
Metals Content, ppmw                                                      
Nickel,                41                                                 
Vanadium               126                                                
Iron                   4                                                  
D-1160 Engler IBP ° F.                                             
                       503                                                
              5%       614                                                
             10        677                                                
             20        746                                                
             30        808                                                
             40        868                                                
             50        936                                                
             Max       1036                                               
             Rec. %    68.0                                               
______________________________________                                    
 
    
     
                                           TABLE II                                
__________________________________________________________________________
Test No.:     69  74  76  80  81  82  83  88                              
__________________________________________________________________________
Conditions                                                                
Residence Time, hours                                                     
              2   2   2   2   6   6   6   6                               
Stirrer speed, rpm                                                        
              400 400 700 700 700 700 700 700                             
Temperature, ° F.                                                  
              650 650 650 650 650 650 650 700                             
Catalyst                                                                  
Compound      None                                                        
                  ZnCl.sub.2                                              
                      FeCl.sub.3                                          
                          ZnCl.sub.2                                      
                              ZnCl.sub.2                                  
                                  TiCl.sub.4                              
                                      AlCl.sub.3                          
                                          ZnCl.sub.2                      
Weight, lb/bbl                                                            
              --  4.2 3.3 4.2 2.1 4.2 4.2 4.2                             
lb mole/bbl   --  0.031                                                   
                      0.020                                               
                          0.031                                           
                              0.015                                       
                                  0.022                                   
                                      0.031                               
                                          0.031                           
Products                                                                  
Dissolv. Ni, ppmw                                                         
              42  13  36  11  20 4.6  9.8 2.6                             
Dissolv. V, ppmw                                                          
              134 30  118 20  48 22   62  3.4                             
500+° F., vol %                                                    
              96.9                                                        
                  96.5                                                    
                      --  96.9                                            
                              -- --   --  --                              
Gravity, ° API                                                     
              15.9                                                        
                  16.7                                                    
                      --  16.9                                            
                              -- --   --  --                              
Coke, wt %.sup.1                                                          
              0.03                                                        
                  0.80                                                    
                      0.49                                                
                          0.80                                            
                              0.07                                        
                                 1.80 2.88                                
                                          2.08                            
 Ni, ppmw     --  3500                                                    
                      1000                                                
                          3800                                            
                              30000                                       
                                 2000 110 190                             
 V, ppmw      --  12000                                                   
                      1600                                                
                          13000                                           
                              110000                                      
                                 5800 22000                               
                                          5900                            
% Demetallization.sup.2                                                   
              0   74  7.8 81  59  83  55  94                              
% Demetallization to                                                      
              0   93  16  100 840 46  19  45                              
% Coke ratio                                                              
__________________________________________________________________________
 .sup.1 The weight of coke obtained, for purposes of calculating the      
 percent coke formed, was determined by weighing all solids obtained from 
 centrifuging the product recovered in each experiment after such solids  
 had been dried at 160° F. and corrected for metal chloride content
 i.e., by subtracting the weight of metal chloride from the total weight o
 solids.                                                                  
 .sup.2 % Demetallization = (100-wt. % coke) (Ni + V in feed - Ni + V in  
 product) ÷ (Ni + V in feed)                                          
 
    
     As the data in Table II make clear, titanium tetrachloride and zinc chloride are superior to aluminum and ferric chloride for removing nickel and vanadium from the liquid feed and converting these metals into oil-insoluble constituents within the coke. Other conclusions from these results are as follows: 
     (1) Increasing residence time and slightly increasing temperature substantially increases demetallization and coking. Compare Test Nos. 80 and 88. 
     (2) Increasing the residence time from 2 to 6 hours while decreasing the amount of zinc chloride by one-half decreased coke formation at the expense of demetallization. Compare Test Nos. 80 and 81. 
     (3) Essentially no hydrocracking occurred in Test Nos. 74 and 80 because the 500+° F. fraction of the product oil was essentially the same as that of the original Iranian feedstock and the same as that of the sample tested with no additive (Test No. 69). 
     (4) The low demetallization to coke selectivity ratios in Test Nos. 76 and 83 indicate aluminum and ferric chloride promote indiscriminate coking compared to zinc chloride, i.e., the amount of coke formed is highly disproportionate to the vanadium and nickel contained therein, at least when compared to the cokes obtained with zinc chloride or titanium tetrachloride. 
     Although the invention has been described in conjunction with specific embodiments and examples thereof, it is evident that many alterations, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. For example, the embodiment of the invention shown in the drawing involves the use of both zinc chloride and titanium tetrachloride. It is, of course, not necessary to use both of these catalysts, and if only one is used, then it is clearly apparent that the equipment related to recovering and recycling the other catalyst would not be needed. Accordingly, it is intended to embrace these, and all such alternatives, modifications, and variations, that fall within the spirit and scope of the appended claims.