Preparation and purification of N2 O4 containing rocket propellant oxidizer
BACKGROUND OF THE INVENTION 
     It is apparent that a need exists for supply of improved grades of nitrogen tetroxide propellants for use as oxidizers for rocket engines during system design and development tests and at the launching sites of vehicles to be projected into space, such as satellites, space shuttles, or the like. Propellant supply/oxidizer flow rate decay problems such as are prone to interfere with design/testing programs and launch scheduling of the vehicle, as well as to its space flight life duration prospects have plagued the industry. For example, the following publications report on current state of the art: 
     &#34;Flow Decay in Nitrogen Tetroxide Systems&#34;, a report published at pages 7-14 of Chemical Propulsion Information Agency Publication No. 171, July 1968. 
     &#34;Operational Nitrogen Tetroxide Handling&#34;, a report published at pages 41-45 of Chemical Propulsion Information Agency Publication No. 171, July 1968. 
     &#34;Nitric Acid/Nitrogen Tetroxide Oxidizers&#34;, pages 2.3-1, 2.3-15, 3-26, 3-27, 3-28, 3-32, 3-33, 3-40, 3-41 and 3-45 of USAF Propellant Handbooks, Vol. II, AFRPL-TR-76-76, February 1977. 
     &#34;Flow Decay&#34;, pages 101, 103, 104, 115, 117, 122, 124, 126, 132, 133, 135, 136 and 139 of Final Scientific Report, June 30, 1972, AFRPL-TR-72-84. 
     As a result of a professional search conducted with respect to this invention, Applicant is aware of the following U.S. Pat. Nos.: 2,298,255; 2,355,817, 2,403,932; 2,759,418; 3,095,693; 3,146,139; 3,310,444; 3,345,821; 3,383,859; 3,534,554; 3,536,543; 3,562,035; and 3,582,412. None of these references is considered relevant to the subject matter claimed herein. 
     Such problems are now of major concern due to the recent emergence of nitrogen tetroxide propellants as major oxidizers for liquid propellant rocket engines of such vehicles, and occur in connection with the storing, transferring, testing, launching and maneuvering operations of satellites; MX missiles; space shuttles; and the like. This is because of the tendency of such oxidizers to corrode their storage/transport/transfer containers; as well as other steel parts of the engine oxidizer supply system. Commercially available nitrogen tetroxide propellants for such purposes are typically supplied to the test site or launch-pad-mounted vehicle, either into the vehicle inboard supply tanks or into close-by &#34;ready storage&#34; ground-based intermediary supply relay tanks. 
     In either case, the supplied nitrogen tetroxide tends to react with the metal of the container and of the system components so as to cause solid precipitates of iron salts to form therefrom. Such solids in the supply stream, along with other &#34;dirt&#34; particulates such as are typically present in commercially supplied nitrogen tetroxide propellants, tend to plug the filters and other components of the propulsion system such as valves and injector orifices; thereby seriously interfering with the propellant transfer operations as well as the test or vehicle launching operations. Furthermore, and perhaps more importantly, the incidence thereof seriously reduces the reliable duration prospect of the rocket engine performance. Typical pre-storage and transfer/operational ambient temperature conditions contribute to these problems. 
     SUMMARY OF THE INVENTION 
     I have observed that a supply of relatively warm/hot nitrogen tetroxide propellant such as is typically furnished commercially to a test site or to a space vehicle at the launch site, precipitates solid particulates out of solution at progressive rates as it cools down throughout the supply transfer process. This problem is exaggerated whenever the supply is furnished from a ground-based stock after long term storage such as in stockpiling operations; and in any case operates to plague the engine test and/or vehicle launch control operations, as well as to limit the duration of the ensuing mission in space. This invention is concerned with the problem of more precisely furnishing to a propulsion system test facility and/or to a facility for launching a vehicle such as a satellite or the like into space in accordance with a prescribed test or launching program a more reliable propellant supply. Thus, the end-user vehicle may be maintained in space for a longer period of time than has heretofore been possible when using present day commercial grades of &#34;Red&#34; or &#34;Green&#34; nitrogen tetroxide (MON). The terms &#34;Red&#34; and &#34;Green &#34; MON as used herein refer to propellants as set forth in Military Procurement Specification MIL-P-26539C. 
     Table I from such Mil. Spec. and which specifies chemical composition and physical properties is reproduced as follows: 
     
         ______________________________________                                    
                       Limits          Test                               
            NTO        MON-1    MON-3  Para-                              
Composition (Red-Brown)                                                   
                       (Green)  (Green)                                   
                                       graphs                             
______________________________________                                    
Nitrogen tetroxide                                                        
            99.5                       4.5.3                              
assay (N.sub.2 O.sub.4)-per-                                              
cent by weight                                                            
Nitric oxide (NO)      1.0      3.0    4.5.2                              
content-max                                                               
percent by weight -                                                       
            .sup.1     0.6      1.5                                       
min                                                                       
N.sub.2 O.sub.4 + NO - per-                                               
                       99.5     99.5   4.5.3                              
cent by weight                                                            
minimum                                                                   
Water equivalent -                                                        
            0.17       0.17     0.17   4.5.4                              
percent by weight                                                         
maximum                                                                   
Chloride content -                                                        
            0.040.sup.2                                                   
                       0.040    0.040  4.5.5                              
percent by weight                                                         
maximum                                                                   
Particulate -                                                             
            10         10       10     4.5.6                              
mg/liter -                                                                
max                                                                       
______________________________________                                    
 .sup.1 The NO content shall be limited to that which does not change the 
 specified RedBrown color of the propellant (3.4).                        
 .sup.2 This test need not be performed on NTO propellant if the material 
 was manufactured by the ammoniaoxidation process.                        
 
    
     More specifically, in accordance with the present invention, the type of propellant being commercially furnished, whether it is &#34;Red&#34; or &#34;Green&#34;, is first determined by appropriate analysis. If it is of the &#34;Green&#34; type, an excess of oxygen is added to it in order to convert it to the &#34;Red&#34; type for introduction into the process of the present invention. Then, in any case, the dissolved iron salts included in the propellant supply resulting from previous contact of the propellant with the steel components of the manufacturing, storing and/or manufacturing equipment are precipitated out of solution by reducing the temperature of the solution to within the range of +20° F. to +35° F. The referred to precipitates (as well as other &#34;dirt&#34; particulates such as typically occur in commercially furnished grades of nitrogen tetroxide supplies) are then filtered out so as to thereby provide to the test or launch site a more pure form of &#34;Red&#34; nitrogen tetroxide propellant. This improved propellant eliminates at minimum expense the currently frustrating problems of design modifications and of scheduling blast-off procedures. This product may be used &#34;as is&#34; for fueling engines of the type such as are used for example in the TITAN II missiles; or it may be blended to contain for example from 0.6 to 1.0 wt% nitric oxide (NO) to produce a purer &#34;MON-1&#34; type propellant; such as is used in MX type engines. Or, it may be blended with 1.5 to 3.0 wt% nitric oxide to produce a purer &#34;MON-3&#34; type propellant such as is used in the space shuttle engines, as well as in various bi-propellant satellite engines. Various amounts of nitric oxide may be added to the base material in similar manner in accordance with this invention in order to produce propellants such as may be required for other applications. 
     Thus, the invention provides an improved base material from which may be blended any desired formulations of nitrogen tetroxide rocket engine oxidizers, which are in any case of improved purity when supplied to the propulsion system; thereby substantially increasing the operational duration functioning of such engines by reason that the propellant flow rate decay problems are reduced. This advance in the art is acquired without adding foreign matter &#34;inhibitors&#34; or the like, such as are suggested for example in U.S. Pat. Nos. 2,403,932; 3,534,554 and 3,536,543. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow diagram illustrating a first stage of the invention, wherein a &#34;raw&#34; (commercial grade) of MON is beneficiated before proceeding further in accordance with the process of the invention; 
     FIG. 2 is a flow diagram showing optional treatments of the beneficiated material to be processed, as well as placements of typical flow control valves and filters in the system; 
     FIG. 3 is a flow diagram illustrating flow control valves and filter placements in that portion of the system which is located in a test facility or inboard of a space vehicle; 
     FIG. 4 graphically illustrates relative changes in solubility of corrosion products in nitrogen tetroxide propellants at constant temperature as a function of the nitric oxide content of various propellants; and 
     FIG. 5 similarly illustrates changes in solubility of corrosion products at constant nitric oxide content as a function of propellant temperature. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with one preferred version of this invention as shown at FIG. 1, a typical commercially supplied raw MON, such as may be in the form of either the so-called &#34;Red&#34; or &#34;Green&#34; type, may first be heated as is indicated at 10 if necessary to bring it into liquid state. For example, the MON may be supplied in containers under lower than 12° F. ambient temperature conditions, thereby being in frozen solid condition. The liquid MON is then transferred into a reactor 12 and in event the raw MON material if of the &#34;Green&#34; type, an excess of oxygen gas is bled into the MON, such as from a supply tank 14 under control of valve 32. Accordingly, in any event the output from the vessel 12 is assured to be MON of the &#34;Red&#34; type. This is then beneficiated by cooling it down to within the range of +20° F. to +35° F. such as by passing it through a cooler as is shown at 16. Then it is filtered through a suitable device as is indicated at 18. This filter should be at least of a rating such as is known in the trade as &#34;6 microns absolute&#34;; meaning that the largest particle passing through the filter will be of the order of 6 microns. If possible, a still finer filter will be employed. Thus, the solid precipitates which occur in the &#34;Red&#34; MON in the form of iron salts or the like resulting from previous contact with the walls of its containers are removed, as well as any other so-called &#34;dirt&#34; particles such as may have accumulated therein throughout its manufacture, transfer/storage procedures. 
     A purer form of &#34;Red&#34; nitrogen tetroxide is thereby provided under delivery control by a valve 32 1 , from which supplies of MON-1; MON-3, etc. may be formulated by by-passing it under control of a valve 32 2  for addition of appropriate amounts of NO gas under control of a valve 32 3  such as from a supply source indicated at 20 leading into a reactor as shown at 22 (FIG. 2). Provision is made for cooling the reactor prior to admission of NO gas in order to obtain an efficient reaction between the NO and dissolved oxygen (O 2 ) and nitrogen dioxide (NO 2 ) present in the purified &#34;Red&#34; MON; and a filter such as shown at 34 is provided downstream of the reactor to remove solid precipitates from the stream. Valves 32 4  and 32 5  are included in the delivery lines to cooperate with the valve 32 2  to control the by-pass operation. Thus, the products delivered through valve 32 5  are therefore initially substantially free of particulate inclusions such as would plug filters and/or feed system components in the rocket test or propulsion system; and therefore will exhibit longer shelf life and provide for longer duration trouble-free rocket engine test or engine performance. 
     In any case, in accordance with this invention, in order to obtain desirably extended shelf life and trouble-free rocket engine performance, nitric oxide is added to the product at this stage of the process, as allowed by procurement specifications. However, to be assured of maximum shelf life and trouble-free rocket engine performance, nitric oxide would be added at this stage up to the maximum amount permitted by procurement specifications. It will be recognized that this same process may be employed to rectify existing stock-piled supplies of nitrogen tetroxide propellants before they are released for use. 
     FIG. 2 also shows how the purified MON supply of the present invention may be furnished under control of a strategically placed conduit and valve system as designated generally at 31; whereby the purified propellant may be optionally transported either into a plant storage facility 24; a shipping container 26; a ready storage facility 28; or directly into the rocket engine test or propulsion system propellant feed tank 30 under control of a manifold system including valves 32 6  ; 32 7  ; 32 8  ; 32 9  ; 32 10  ; 32 11  ; 32 12  ; 32 13  and 32 14 . It is an important feature of this invention that appropriate filters are installed in this manifold circuitry such as are illustrated at 34a, 34b and 34c. Thus, it will be noted that the product is filtered after each cooling operation and after each transfer and storage step, as well as just prior to introduction into the test facility or propulsion system tank. FIG. 3 illustrates how the material is preferably filtered again at 36 just prior to its entry into the rocket engine flow control valve system as shown at 38 leading into the engine thrust chamber injector. It is to be noted that the filter 36 may preferably be of slightly larger pore size than the filter 18 and the 34 series of filters which are ground-based, and are therefore readily replaceable when becoming clogged. The rocket engine in toto is designated generally by the numeral 40. 
     FIGS. 4 and 5 illustrate graphically how changes in solubility of corrosion products in nitrogen tetroxide propellants occur as functions of NO content and temperature. For example, FIG. 4 graphs typical solubility changes in relation to nitric oxide content, whereas FIG. 5 illustrates solubility changes in relation to temperature changes. Incidentally, just prior to initial loading an/or reloading of the tank and plumbing components of the propellant handling system, all parts thereof are preferably cleansed of any metal corrosion/oxidation by-products such as may exist on their propellant contacting surfaces. This may be done by flushing them with an appropriate cleaning solution. For example, components made of stainless steel may be &#34;pickled&#34; with an aqueous solution of nitric and hydrofluoric acids. Then, the cleansed surfaces are passivated such as with an aqueous nitric acid solution. 
     EXAMPLE #1 
     Approximately 200 grams of typically commercially furnished &#34;Green&#34; MON (mixed oxides of nitrogen containing an equilibrium amount of nitrogen dioxide and nitric oxide) were cooled down to a temperature of approximately 32° F. for beneficiation. A small quantity (approximately 50 grams) of the chilled material was then analyzed in order to determine its nitric oxide content. The remainder of the material was then weighed into a tared glass pressure vessel. The vessel and contents were then cooled to approximately 32° F. and contents bleached by bubbling an excess of commercially pure oxygen gas into the liquid, thus converting the nitric oxide present in the liquid to nitrogen tetroxide, according to the equation: 
     
         2NO+O.sub.2 →N.sub.2 O.sub.4 
    
     Addition of more than the stoichiometric amount of oxygen was then confirmed by analysis. The cooled product (impure &#34;Red&#34; MON) was then transferred by nitrogen gas pressure through a chilled plastic filter assembly rated at 6 microns absolute, into a clean, chilled glass receiver. This product was pure &#34;Red&#34; MON, which by analysis substantially exceeded the requirements for &#34;Red-Brown&#34; propellant as set forth in Military Procurement Specification, MIL-P-26539C. 
     EXAMPLE #2 
     100 grams of &#34;Red&#34; MON (mixed oxides of nitrogen) as received from a plant which commercially produces a mixture of nitrogen tetroxide containing an equilibrium amount of nitrogen dioxide and dissolved oxygen, was cooled down to 32° F. and then passed by nitrogen gas pressure through a chilled plastic filter assembly rated at 6 microns absolute into a clean, chilled glass receiver. The product was pure &#34;Red&#34; MON, which by analysis exceeded the requirements for &#34;Red-Brown&#34; nitrogen tetroxide type propellant as set forth in Military Procurement Specification MIL-P-26539C. 
     EXAMPLE #3 
     A quantity of pure &#34;Red&#34; MON, as produced in Example 1 or Example 2, was cooled down to 32° F. and then transferred into a glass pressure vessel. The vessel was weighed before and after charging. The vessel and contents were then again cooled down to 32° F. Commercially pure nitric oxide was then bubbled into the liquid to convert any excess oxygen into nitrogen tetroxide, and to convert some of the nitrogen dioxide present to dinitrogen trioxide according to the equation: 
     
         NO+NO.sub.2 →N.sub.2 O.sub.3 
    
     Addition of the nitric oxide was monitored by weight change and terminated when analysis confirmed the desired degree of dinitrogen trioxide formation. The method of analysis employed is provided in Military Procurement Specification MIL-P-26539C. The product was pure &#34;Green&#34; MON containing the equivalent of 0.6-1.0 wt% nitric oxide, therefore exceeding the purity of the MON-1 required in the aforementioned Military Procurement Specification. 
     EXAMPLE #4 
     Approximately 100 grams of the product obtained in Example 3 was furnished additional nitric oxide as in Example 3, so that sufficient nitric oxide was added so as to produce a final value equivalent to 1.5-3.0 wt% nitric oxide. The final product was a pure grade of MON-3 propellant exceeding the requirements of Military Procurement Specification MIL-P-26539C.