Abstract:
Disclosed is a method for an energy-efficient improvement in the production of sulfur hexafluoride, and eliminates the generation of other byproducts. The process is an oxidative fluorination of sulfur tetrafluoride by CoF 3 /F 2 , where CoF 3  is solid stationary phase that can be regenerated.

Description:
TECHNICAL FIELD 
     The present invention generally relates to methods for production of sulfur hexafluoride by reacting sulfur tetrafluoride with fluorine with cobalt trifluoride as catalyst/fluorinating agent. The sulfur tetrafluoride is prepared by two step reactions from uranium tetrafluoride, or uranium hexafluoride in a process that was described in a previous U.S. patent. 
     BACKGROUND 
     Sulfur hexafluoride (SF 6 ) is a hypervalent, inorganic, colorless, odorless, non-toxic and non-flammable gas compound that is poorly soluble in water. It has a density of 6.1 g/L. Some 75% of the 8,000 tons of SF 6  produced per year is used as a gaseous dielectric medium in the electrical industry; as an inert gas for the casting of magnesium; and as an inert filling for insulated glazing windows. SF 6  plasma is also used in the semiconductor industry as an etchant. Because SF 6  is relatively slowly absorbed by the bloodstream, it is used to provide a long-term plug of a retinal hole in retinal detachment repair operations. SF 6  is employed as a contrast agent for ultrasound imaging of peripheral vein to enhance the visibility of blood vessels to ultrasound. This application has been utilized to examine the vascularity of tumors. Sulfur hexafluoride is also used as a fluorophilic gaseous diluent for ammonia in the efficient reaction with fluorine to produce nitrogen trifluoride. 
     New fluoride recovery methods for management of uranium fluorides in the nuclear industry could produce large quantities of uranium-free sulfur tetrafluoride. An efficient process for production of sulfur hexafluoride from sulfur tetrafluoride is required to further increase the commercial value of this approach to nuclear waste management. There is incentive to develop lower cost of producing sulfur hexafluoride. 
     Meanwhile, currently patented processes involve either direct fluorination of sulfur in an electrolytic reactor with byproducts including SF 4 , and S 2 F 10 ; or use of oxygen to oxidize sulfur in sulfur tetrafluoride with production of sulfur dioxide as byproduct. Also, the reaction of xenon tetrafluoride with sulfur tetrafluoride to produce sulfur hexafluoride has been reported in the literature. However, this process is cost prohibitive as an industrial process. Evaluation of the feasibility of efficient alternative process for production of sulfur hexafluoride from sulfur tetrafluoride has involved a review of the thermodynamic data, such as the enthalpy (ΔH), Gibbs free energy (ΔG), and equilibrium constant (log K) of the theoretical process by using the HSC Chemistry 7.0 software. The comparative data, (ΔH) and log k, for oxidation of S(IV) to S(VI) by oxygen (O 2 ), fluorine (F 2 ), xenon tetrafluoride (XeF 4 ), and cobalt trifluoride (CoF 3 ) are shown in Tables 1 and 2. 
     The results suggest that cobalt trifluoride, a strong fluorinating agent, could produce a basis for new reactor concept for the efficient oxidation of sulfur tetrafluoride. Indeed, CoF 3  fluorinates sulfur tetrafluoride in a gas-solid heterogonous reaction to produce sulfur hexafluoride. This process does not produce undesired side products like S 2 F 10  or SO 2 , but result in the desired sulfur hexafluoride. The simple reactor for this process avoids the cost of energy for the electrolytic method. 
     Cobalt trifluoride is a common fluorinating agent for commercial production of many perfluorinated organic compounds form saturated and unsaturated hydrocarbons. As shown in Equation 1, in those reactions cobalt trifluoride is reduced to difluoride:
 
CoF 3 +R—H→CoF 2 +R—F+HF  Eqn. 1
 
     Passing a steady flow of elemental fluorine through a CoF 3  bed produces an efficient CoF 2 /CoF 3  oxidation-reduction system. Therefore, a process for CoF 3  catalyzed fluorination of sulfur tetrafluoride; to produce sulfur hexafluoride is described in this patent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of the laboratory setup to perform the method, with the reactor in vertical mode. 
         FIG. 2  is a view of the laboratory setup to perform the method, with the reactor in horizontal mode. 
         FIG. 3  is an overlay of three FTIR scans from online monitoring the production of SF 6  from SF 4 . 
     
    
    
     SUMMARY OF THE INVENTION 
     The invention of this patent provides an energy-efficient improvement in the production of sulfur hexafluoride, and eliminates the generation of other byproducts. The process is an oxidative fluorination of sulfur tetrafluoride by CoF 3 /F 2 , where CoF 3  is solid stationary phase that can be regenerated. The overall process can be summarized as a combination of the following reactions in Equations 2-4: 
                     SF   4     +         F   2     ⁡     (   g   )       ⁢     ⟶     CoF   3       ⁢       SF   6     ⁡     (   g   )                 Eqn   .           ⁢   2               CoF 3 +SF 4 ( g )→CoF 2 +SF 6 ( g )  Eqn. 3
 
CoF 2 +F 2 ( g )→CoF 3   Eqn. 4
 
     Each of these independent reactions can proceed with varying degrees success at 25-450° C. However, 40-100% conversion of sulfur tetrafluoride to sulfur hexafluoride in continuous operation is achieved above 100° C., with flow regulation such that reagent-CoF 3  contact time is 1-10 minutes. The yield is also dependent on the gaseous reagent composition, which can be 1:1 to 1:1.5 SF 4 /F 2 . 
     The reactor can be made from a nickel or Teflon material, and must support the weight of the CoF 3  bed, as well as hold 20 PSIG pressure higher than the line pressure. The inlet reagent or sweep gases have to be at a minimum pressure that ensures flow through the reactor bed, generally at 1-50 PSIG. With a density of 3.88 Kg/L, the packing of the CoF 3  stationary bed is a significant factor in controlling the required pressure to drive the process flow through the reactor. The bed can be arranged as layered compartments, or filled around nickel or Teflon balls to engineer excellent diffusion of the reagents and/or process gases in a large scale reactor. To this end, the reactor can be placed in a vertical or a horizontal position ( FIGS. 1 and 2 ), to ensure a process flow that is sufficient for the preferred time of 2-5 minutes. 
     In actual practice, there can be a consideration for the rate of reaction versus the cost of energy for a high temperature process to determine the minimum cost of production of sulfur hexafluoride by this approach. Suitable reactors can be arranged in a series in order to increase contact time at a lower temperature where the reaction rate does not produce high yields at the process conditions. The reactor may be designed to any suitable configuration, such as cylindrical, tubular, cubic, etc. However, the appropriate reactor should produce excellent diffusion of process gas for maximum contact with the CoF 3  stationary bed. 
     Process yield of the method is a function of temperature, contact time, diffusion, and pressure. However, diffusion and contact time are also functions of pressure and the ratio of flow (slpm)/mass (Kg) of CoF 3  bed. During scale up, longer contact time is required for flow process at higher than 10 PSIG pressure in order to achieve the efficiency of lower pressure processes. 
     Stirring and significant agitation is very useful for improvement of the rates of process reactions. Mechanical agitation can be achieved by introduction of an auger in the horizontal reactor orientation, or a stirrer in the vertical orientation. Mechanical agitation improves diffusion, and potential for large gas-solid contact that could exponentially improve the reaction rates at lower temperatures. In a large scale process, the yields will be determined as a function of rate of agitation, temperature, and pressure. The yields will be a function of diffusion and the reaction kinetics. 
     The dilution of elemental fluorine feedstock is an important determinant of the flow efficiency. However, while low flow of pure fluorine feedstock will increase contact efficiency, it has higher handling risks, and produce significant corrosion of nickel reactor material, as well as costly loss of unreacted fluorine molecules. A compromise is the design of the reactor around a 20% F 2 /N 2  mixture. A compromise is the design of the reactor around a 20% F 2 /N 2  mixture, with 20% pure fluorine and 80% pure nitrogen. 
     The purity of the SF 4  feedstock is also an important consideration. High quality SF 4  reagent will produce high quality SF 6 , with minimum byproducts that require further purification. Typical gas phase impurities of SF 4  include thionyl fluoride (SOF 2 ), sulfur monofluoride (S 2 F 2 ), and sulfur difluoride (SF 2 ) and hydrogen fluoride (HF). It is necessary to ensure that sample delivery lines to the reactor are moisture free. Otherwise, the process will contain significant amounts of HF, which may generate a different type of reaction in the system. Total impurity content of less than 10,000 ppm (or 1%) has little to no effect on SF 6  production in a continuous flow process. Further, sulfur compounds in S(2+) and S(4+) oxidation states are reactive and get scrubbed when the process flow is channeled through a post-reactor alkaline scrubber. 
     The process flow stream includes N 2 , SF 6 , F 2 , and SF 4 . An online 5-L alkaline scrubber (1-5 M solution of potassium hydroxide) removed reactive F 2  (2-3% excess), and SF 4  from the flow stream; followed by a trap kept at −78° C. to remove SF 6 . The untrapped N 2  from the process is vented after trap  3 . This approach has produced crude SF 6  product with purity at 99.9%. Further cryogenic purification of this product has produced 99.995+% commercial quality. The total impurity content, including oxygen (O 2 ), water (H 2 O), nitrogen (N 2 ), total hydrocarbon content, carbon tetrafluoride (CF 4 ), and hydrogen fluoride (HF) is less than 50 ppm. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Fluorination of sulfur tetrafluoride with elemental fluorine in the presence of CoF 3  proceeds under external-kinetic control since the CoF 3  concentration in the surface layer is constant owing to timely or even advance supply of fluorine (Eqn. 4). The method of fluorination of SF 4  is designated  10  in the figures and is shown in  FIG. 1  in a vertical reactor column  12  made from nickel, with an internal diameter (i. d.)=1½″, height=16″. The reactor was packed with CoF 3  with a bed height of 12″, for a space volume of 21 in 3  or 348 cc. A hinged three-heater Lindberg Blue furnace  56  with thermostat controller was used to ensure that temperature variation throughout the bed did not exceed ±4° C. throughout the process. Independent flows to ensure stoichiometric reagents, i.e. SF 4  and F 2 , as well as the sweep gas (where necessary), i.e. N 2 , argon, or helium, were allowed to mix in the reactor, just before the introduction to the CoF 3  bed. The mixture was supplied at constant rates to the process. The experiments were performed at 100, 150, 200, and 250° C.; to determine the temperature dependent SF 6 :SF 4  yield in the effluent for total flow of 210 sccm (standard cubic centimeter). The yields are shown in Table 3, and an overlay of selected FTIR absorbance spectra of the process effluent at different temperatures is shown in  FIG. 3 . 
     Shown in  FIG. 1  is a laboratory scale version of the process  10  of the invention. Shown is a nickel reaction vessel  12  which can be oriented vertically as shown in  FIG. 1 , or which can be oriented horizontally as shown in  FIG. 2 . The reactor can be made from a nickel or Teflon material, and must support a weight of the CoF 3  bed and hold 20 PSIG of pressure. The inlet pressure is 1-50 PSIG, which is sufficient to ensure flow through the reactor bed. The reactor bed has a density of 3.88 kilograms per liter, with the packing of the solid CoF 3  in a stationary bed a significant factor of controlling the required pressure to drive the process flow through the reactor. The bed can be arranged as layered compartments, or filled around nickel or Teflon balls to engineer excellent diffusion of the reagents and/or process gasses in a large scale reactor. Show in  FIG. 1  is a source  19  of SF 4 , and a source  16  of 20% F 2 N 2 . Also present is a source  17  of N 2 . The N 2  source  17  and the SF 4    19  are on one line feeding the reaction vessel  12  and pass through a mass flow controller  30  and a first valve  26 . The transfer lines through which these two feed streams flow would typically be made of fluorine passivated stainless steel, with Nickel, or Monel, with Teflon also being suitable to 200 degree centigrade. 
     The other feed material is 20% F 2 N 2  from a tank  16 . This gas is 20% F 2  and 80% pure nitrogen. This gas flows through a pressure regulator  20 , through a mass flow controller  30 , through a valve  34  and into the reaction vessel  12 . The reaction vessel  12  is surrounded by a thermostat controlled heater  56 , which keeps the reaction in an optimal temperature range, which is typically 25-450 degrees centigrade, in which range 100% conversion of sulfur tetrafluoride to sulfur hexafluoride is achieved above 100 degrees centigrade. The flow is regulated so that the contact time with the CoF 3  reagent is 1-10 minutes. In the laboratory scale version, the reaction vessel  12  is approximately 3 inches in diameter, 18 inches in height, and is packed with approximately 1 Kg grams of CoF 3 , which serves as a stationary catalyst. The lines for conducting 20% F 2 N 2  from tank  16  into the reaction vessel  12  are typically made of fluorine passivated stainless steel, with Nickel, Monel, or Teflon, and in the laboratory scale version are approximately ¼ inches in diameter. 
     An agitator of some kind, such as auger  68 , helps to improve the reaction in the reaction vessel  12 . A valve  38  is shown in the figures and may be used to relieve pressure in the reaction vessel. 
     From the reaction vessel  12  a steady flow of SF 6  gas is routed to a KOH scrubber  42  designated trap  1 , via line  58 . Another line, designated  62 , exits the reaction vessel  12 , and may be used as a bypass of trap  42  and may route SF 6  directly to an analytical instrument  46  by opening valve  32 . Lines  62  and  58  in the laboratory scale version can be made of fluorine passivated stainless steel, with Nickel, Monel, or Teflon, with an ID of ¼ being suitable. In the laboratory scale version of the invention the KOH scrubber  42  is approximately 18 inches in diameter and 24 inches tall and holds approximately 5 L of 1-5 molar KOH. A vent line  64  exits the trap  42 , and allows venting of the trap  42  if need be, such as to a vacuum. 
     The KOH in the trap  42  absorbs unreacted fluorine and sulfur tetrafluoride, and the purified SF 6  product passes from the scrubber  42  through an inline dryer  44 . From the online dryer  44  the SF 6  gas is routed to analytical instrumentation  46 , which preferably is FTIR. From the FTIR device SF 6  gas passes into a trap  2 , designated  50 , which is kept at −78 degrees centigrade by a cryogenic cooling unit  22 . Trap  2  is designated  50  and serves as SF 6  storage. From trap  2  purified SF 6  may be removed for further purification to vent or storage. Associated with trap  50  is a pressure gauge  60 . A bypass line  66  is available to bypass the analytical instrumentation  46 . 
     The trap  2  designated  50  in the laboratory scale version is approximately 2 inches in diameter, 18 inches tall, and is made of fluorine passivated stainless steel. Other materials could also be used such as Nickel, Monel, or Teflon. 
       FIG. 2  shows the same laboratory set up with the reaction vessel  12  oriented vertically. 
       FIG. 3  shows an overlay of three Fourier transform infrared (FTIR) spectrometer scans from online monitoring of the production of SF 6  from SF 4 , using the method described above at different temperatures. These curves compare pure SF 4  diluted in N 2  with the gases produced in the method, with the ratio of SF 6  to SF 4  being optimized at higher temperatures. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Enthalpies (ΔH&#39;s) of the oxidation of SF 4  to produce SF 6   
               
             
          
           
               
                 Temperature  
                   
                   
                   
                   
               
               
                 (° C.) 
                 O 2   
                 F 2   
                 XeF 4   
                 CoF 3   
               
               
                   
               
             
          
           
               
                 100 
                 −107.516 
                 −109.441  
                 −169.747 
                 −166.552 
               
               
                 150 
                 −107.693 
                 −109.487  
                 −169.953 
                 −166.167 
               
               
                 200 
                 −107.818 
                 −109.503  
                 −170.107 
                 −165.758 
               
               
                 250 
                 −107.901 
                 −109.493  
                 −170.217 
                 −165.332 
               
               
                 300 
                 −107.947 
                 −109.463  
                 −170.290 
                 −164.896 
               
               
                 350 
                 −107.960 
                 −109.415  
                 −170.333 
                 −164.453 
               
               
                 400 
                 −107.947 
                 −109.354  
                 −170.349 
                 −164.005 
               
               
                 450 
                 −107.915 
                 −109.282  
                 −170.346 
                 −163.553 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Log K of the equilibrium constants for reagents that react  
               
               
                 with SF 4  to produce SF 6   
               
             
          
           
               
                 Temperature 
                   
                   
                   
                   
               
             
          
           
               
                 (° C.) 
                 O 2   
                 F 2   
                 XeF 4   
                 CoF 3   
               
               
                   
               
             
          
           
               
                 100 
                 48.970 
                 53.159 
                 90.565 
                 73.715 
               
               
                 150 
                 41.522 
                 45.583 
                 78.809 
                 62.200 
               
               
                 200 
                 35.640 
                 39.606 
                 69.528 
                 53.141 
               
               
                 250 
                 30.878 
                 34.772 
                 62.015 
                 45.832 
               
               
                 300 
                 26.945 
                 30.782 
                 55.810 
                 39.813 
               
               
                 350 
                 23.641 
                 27.433 
                 50.599 
                 34.774 
               
               
                 400 
                 20.829 
                 24.583 
                 46.161 
                 30.496 
               
               
                 450 
                 18.406 
                 22.129 
                 42.337 
                 26.819 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Temperature dependent SF 6 :SF 4  yield for total flow of  
               
               
                 210 sccm at 20 PSIG 
               
             
          
           
               
                 Process condition 
                 FTIR Yield (SF 6 :SF 4 ) at 10 minutes into process 
               
               
                   
               
             
          
           
               
                 Temperature 
                 100° C. 
                 150° C. 
                 200° C. 
                 250° C. 
               
               
                 Flow* @ 30 sccm SF 4 , 
                   
                   
                   
                   
               
               
                 30 sccm F 2 , and 
                 40:60 
                 60:40 
                 70:30 
                 90:10 
               
               
                 150 sccm N 2   
               
               
                   
               
               
                 *Total flow = 210 sccm. This implies that the contact time of SF 4 /F 2  with CoF 3  is 1 min 40 sec. 
               
             
          
         
       
     
     While certain exemplary embodiments are shown in the Figures and described in this disclosure, it is to be distinctly understood that the presently disclosed inventive concept(s) is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the disclosure as defined by the following claims. 
     While the presently disclosed inventive concept(s) is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the inventive concept(s) to the specific form disclosed, but, on the contrary, the presently disclosed and claimed inventive concept(s) is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the inventive concept(s) as defined in the claims.