Patent Publication Number: US-2010115992-A1

Title: Methods for recovering argon

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from International Application Serial No. PCT/US2007/087341 filed 13 Dec. 2007 (published as WO 2008/076782 A2, with publication date 26 Jun. 2008), which claims priority from U.S. Provisional Patent Application Ser. No. 60/875,509, filed Dec. 18, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     Current cosmological models predict the existence of “dark matter” to explain the observed expansion of the universe. Dark matter is so called because it hypothetically lacks the property of emissivity of electromagnetic radiation while still having substantial mass. Consequently, one shouldn&#39;t be able to “see” it as one can see normal matter reacting to electromagnetic radiation, but one should be able to detect it by its interactions with normal matter. 
     Many cosmological models predict that dark matter should be prevalent in a form called WIMPs, or “Weakly Interacting Massive Particles”. The large mass of these particles should make collisions with similar large atoms of normal matter observable, and the physics have been derived to quantify what these interactions should look like. As a result, it appears that argon forms the perfect collision target for WIMPs. Several investigators have designed a series of WIMP detectors using liquid Ar to capture the collisions and distinguish them from other radioactive phenomena. See “Measurement of the Specific Activity of  39 Ar in Natural Argon”, http://www.arxiv.org/pdf/astro-ph/0603131, 6 Mar. 2006. 
     A problem exists with the argon source, however, which has led some to seek new sources. The collision between a WIMP and the normal Ar isotope,  40 Ar, can yield  39 Ar as an artifact. Normal atmospheric argon contains a low level of  39 Ar (1 part in 1 million billion) due to the effects of solar radiation, so this background level creates “noise” in the detector signal. Argon from underground sources should in principle be free from  39 Ar and thus would constitute a more sensitive detector for WIMP. See “A Dating Method with  39 Ar”, H. H. Loosli,  Earth and Planetary Science Letters,  63 (1983) 51-62. 
     Argon is known to exist in natural gases around the world. See “Helium—Its Relationship to Geologic Systems and its Occurrence with the Natural Gases, Nitrogen, Carbon Dioxide, and Argon”, Claude A. Tongish, United States, Bureau of Mines Report of Investigations 8444 (1980). 
     Some analyses show argon at average levels around 0.05-0.1%. This level is quite low and would not be economically recoverable by conventional means, especially without concomitant recovery and purification of any contained hydrocarbons and helium. The present invention is to directed to a means to concentrate and recover argon economically from natural gas. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for recovering argon from natural gas. The proposed method improves on earlier methods of trying to separate the argon from the bulk of the natural gas stream, thereby reducing the size of the stream to be treated, and allowing for recovery of the valuable components (hydrocarbons and helium) that result from the separation. The liquid off the primary feed separator is currently used as part of liquid nitrogen circuit in the helium plant, but can be replaced by imported liquid nitrogen. 
     Accordingly, there is disclosed a method for recovering argon from bottoms liquid from the flash separation of a helium gas mixture containing argon comprising subjecting the bottoms liquid to a rectification process. 
     Further, the present invention provides for a method for recovering argon present in a natural gas stream during the purification of a helium gas mixture withdrawn from the natural gas stream comprising recovering the liquid bottoms from the helium purification and subjecting the liquid bottoms to a rectification process specific for argon. 
     Accordingly, in one embodiment of the invention, there is disclosed a method for recovering argon from a natural gas stream comprising the steps: 
     a) directing a natural gas stream containing natural gas, hydrocarbons, crude helium and argon to a helium recovery unit;
 
b) separating a gas stream containing helium and argon from the natural gas stream and directing the helium and argon to a helium purifier;
 
c) purifying the helium; and
 
d) recovering argon from the purified helium.
 
     In a further embodiment of the present invention there is disclosed a method for recovering argon from a natural gas stream comprising the steps; 
     a) separating crude helium and argon from the natural gas stream;
 
b) directing the separated gas stream to a first helium separation unit, wherein argon is separated from the gas stream and directed to an argon concentration unit;
 
c) directing the helium from the first helium separation unit to an enriched helium separation unit;
 
d) concentrating argon by removing nitrogen from the argon concentration unit; and
 
e) recovering concentrated argon from the argon concentration unit.
 
     In a further embodiment of the present invention, there is disclosed a method for recovering argon during the purification of a helium gas stream mixture separated from a natural gas stream comprising recovering the liquid bottoms from the purification of the helium and subjecting the liquid bottoms to a rectification process to recover argon. 
     The purification of helium is typically performed in a helium recovery unit which can be a cryogenic rectifier. Once purification is achieved the helium is directed to either a helium liquefier or to storage. 
     Argon will typically be present in the natural gas stream in an amount ranging from about 10 ppm to about 10000 ppm. 
     The rectification that is performed on the argon is designed to separate the argon from any further impurities originating in the natural gas stream and passing through to the helium separation processes. This impurity typically is nitrogen and the rectification processes are typically cryogenic rectification and selective adsorption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGURE is a schematic representation of the process whereby argon is recovered from a helium purification process. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The FIGURE is a schematic representation of a crude helium stream  1  which has been recovered from a natural gas prior to the natural gas being added to a pipeline. The crude helium stream contains impurities such as nitrogen, methane, argon and other noble gases (see Table 1). 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Typical composition of crude helium extracted 
               
               
                 from the National Helium Reserve. 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Concentration by Volume 
               
               
                   
                   
               
               
                   
                 He 
                 77.3% 
               
               
                   
                 N 2   
                 20.3% 
               
               
                   
                 CH 4   
                  1.6% 
               
               
                   
                 H 2   
                 8000 ppm  
               
               
                   
                 Ar 
                 680 ppm 
               
               
                   
                 CO 2   
                 110 ppm 
               
               
                   
                   
               
            
           
         
       
     
     This stream is pre-treated to remove hydrogen and carbon dioxide, and then fed into the primary feed separator A where the crude helium is flash separated by being cooled to below about −300° F. The flashed helium which is about 95 to 98% by volume helium leaves the primary feed separator A through line 2 and enters the enriched helium separator B. 
     The enriched helium separator B is also a flash separator where the further purified helium is further purified and is at a lower temperature below about −315° F. than the primary feed separator A. This further purified helium can be recovered through line 6 for storage purposes. The liquids leaving the enriched helium separator B are primarily nitrogen and small amounts of other gases (Table 2) and exit through line 7 to connect with line 4 nitrogen discharge from concentrator C where the nitrogen can be stored for other uses or discharged to the atmosphere. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Typical composition of rejected liquid from helium purification. 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Concentration by Volume 
               
               
                   
                   
               
               
                   
                 He 
                 0.5% 
               
               
                   
                 N 2   
                 96.0%  
               
               
                   
                 CH 4   
                 3.5% 
               
               
                   
                 H 2   
                 0 ppm 
               
               
                   
                 Ar 
                 2800 ppm   
               
               
                   
                 CO 2   
                 0 ppm 
               
               
                   
                   
               
            
           
         
       
     
     The bottoms from the primary feed separator A leave through line 3 and are directed to the  40 Ar concentrator C. The concentrator is designed to rectify the liquids mixture from the bottoms liquid from the primary feed separator A which comprise nitrogen, methane, argon and other noble gases. The rectification can be performed by a variety of techniques, including cryogenic rectification or adsorptive separation depending upon the concentration of the argon and other constituents in the bottoms liquid. 
     The rectified argon will leave the concentrator C through line 5 where it can be directed to storage or to an on-site usage as necessary. The nitrogen present in the bottoms liquid is also recovered through line 4 and can be directed to storage or discharged into the atmosphere. 
     While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.