Abstract:
A process to convert light hydrocarbons such as natural gas to a liquid or liquids. Vacuum pressure swing adsorption (VPSA) is used to produce a stream of relatively high purity oxygen. The relatively high purity oxygen is reacted with light hydrocarbons and steam in an autothermal reformer in order to produce synthesis gas. The synthesis gas is thereafter converted to a hydrocarbon liquid or liquids via a Fischer Tropsch or related reaction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/441,789, filed Feb. 11, 2011, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to a process to convert natural gas into liquid fuels and chemicals. In particular, the present invention relates to a process utilizing vacuum pressure swing adsorption to produce high purity oxygen to react in an autothermal reformer to produce synthesis gas. 
     2. Prior Art 
     The Fischer-Tropsch (“FT”) Synthesis has been used to convert synthesis gas (carbon monoxide and hydrogen) into hydrocarbon products. These resulting hydrocarbon products can be useful as a synthetic crude oil or further refined into various fuels, chemicals and chemical intermediate products. The FT feedstock synthesis gas can be produced using a wide range of raw materials including, for example, coal, biomass and natural gas. The method of process design and integration is, essential to any effort to convert a raw material into useful hydrocarbon products by FT chemistry. Conversion of the raw material into synthesis gas is often the critical step as it is capital intensive. 
     Production of synthesis gas requires the introduction of oxygen and/or heat. The source of oxygen can be atmospheric air, enriched air, substantially pure oxygen (90%+O 2 ), steam, or a combination of these. 
     When the feedstock is natural gas, historically steam has been used as the source to obtain oxygen. This process is known as steam methane reforming. An example of steam methane reforming may be seen in Tio (U.S. Pat. No. 7,550,635). Methane is partially oxidized in a non-catalyzed reaction followed by steam reforming of methane. 
     Prasad et al. (U.S. Pat. No. 6,695,983) discloses another example of steam methane reforming. 
     In recent years however, autothermal reforming has gained acceptance for syngas production. Autothermal reforming uses a small amount of steam and substantially pure oxygen to produce synthesis gas by a combination of combustion and reforming reactions. The exothermic partial oxidation reaction provides the necessary heat for endothermic steam methane reforming reactions that occur over a single catalyst bed. 
     The commercial development of these processes has been driven to larger scale in order to gain advantage from economy of scale. Since steam methane reforming requires a large number of small catalyst filled tubes in a large fired heater, the autothermal reforming process with a single catalyst bed has become a preferred method for many processes requiring synthesis gas. The autothermal reformer is particularly preferred for use with a FT process because it produces synthesis gas at or near the ideal 2/1 hydrogen to carbon monoxide H 2 /CO molar ratio. 
     While the technology development is being driven by economy of scale, there are commercial reasons to consider smaller scale development. Natural gas fields vary significantly in size and location, and a substantial portion of the natural gas discovered is stranded and, therefore, has no ready access to a market. This lack of market access is a result of the difficulty to transport the natural gas to a market. The smaller gas fields are disadvantaged compared to larger fields due to limited pipeline access. If the natural gas could be converted to liquid fuels or chemical intermediates it can easily be transported to a market. There are also many more small fields than large fields. Therefore, there is a need to develop a process that can convert natural gas into fuels and/or chemicals efficiently and economically at a relatively small scale. 
     Beer (U.S. Pat. No. 5,755,840) suggests using an oxygen-sorbent material to add oxygen to a feed stream where the feed stream can be natural gas. The combined feed stream and oxygen can be passed to a reactor for conversion to syngas. In this process, the feed gas is used to desorb the oxygen off of the sorbent. The feed gas/oxygen mixture is a combustible mixture and therefore the time it can be retained prior to reaction is very short. Also, the ability to preheat the mixture is very limited and the process may well be found to be hazardous to practice. 
     Accordingly, there remains a need for a safe effective method to produce syngas from natural gas at a relatively small scale. 
     Other processes to obtain enriched air or substantially pure oxygen have been proposed. Baksh et al. (U.S. Pat. No. 7,867,320) discloses a specific mechanical design using a vacuum pressure swing adsorption process. 
     There remains a need to adapt vacuum pressure swing adsorption technology to conversion of natural gas into liquid fuels and chemicals. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process for converting natural gas into liquid fuels and/or liquid chemicals. The process uses an adsorption method to separate oxygen from atmospheric air known as Vacuum Pressure Swing Adsorption (VPSA). 
     The oxygen rich stream of 80% plus, and preferably 90% plus, oxygen then is compressed to a pressure necessary to make synthesis gas for the process, at preferably between 20-40 Bar. 
     The oxygen rich stream is reacted with a combination of natural gas and steam in an autothermal reformer to produce synthesis gas. 
     The synthesis gas is then converted to useful fuels and/or chemicals in one or more synthesis conversion reactors. The synthesis gas can be converted to methanol, higher alcohols or higher hydrocarbons and further refined to marketable products. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram of a process to convert natural gas into liquid fuels and chemicals in accordance with the present invention; and 
         FIG. 2  is a table with examples of components from streams in the process of the present invention shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments discussed herein are merely illustrative of specific manners in which to make and use the invention and are not to be interpreted as limiting the scope of the instant invention. 
     While the invention has been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the invention&#39;s construction and the arrangement of its components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification. 
     The present invention can be better understood by referring to the non-limiting example shown in a simplified schematic diagram in  FIG. 1  with stream example in properties shown in a table in  FIG. 2  on a dry basis.  FIG. 1  gives an example of a process  10  capable of being used to convert 10.4 MMSCFD of natural gas into 1000 Bbl/d of liquid hydrocarbon product. 
     The process uses a Vacuum Pressure Swing Adsorption (VPSA) unit to produce a stream (Stream 1) 6.53 MMSCFD of a 90% pure oxygen stream from atmospheric air. As seen in  FIG. 1 , atmospheric air  12  is delivered to a Vacuum Pressure Swing Adsorption Unit  14 . Blowers, compressors or other mechanisms may be utilized to deliver the atmospheric air  12  under pressure to the Vacuum Pressure Swing Adsorption Unit  14 . 
     In one non-limiting embodiment, solid or liquid absorbents are utilized in the VPSA unit which absorb nitrogen more strongly than oxygen. When the pressurized air is introduced to the absorbents, an oxygen-rich gas is left. The oxygen-rich gas is then compressed in a compressor as shown at reference number  16 . The oxygen-rich gas may be pressurized from between 20 to 40 Bar. In the example herein, the relatively pure oxygen is collected at low pressure and compressed to approximately 30 Bar. 
     A stream of natural gas  20  is combined with steam  22  and preheated. The relatively pure oxygen stream 1 is preheated and mixed with Stream 2 comprising 10.4 MMSCFD of natural gas which contains steam in a 0.6 Steam/Carbon molar ratio and is also preheated. 
     The two streams mix in an autothermal reformer or ATR  30  where they are partially combusted. The partially combusted stream is then reacted over a reforming catalyst adiabatically to produce a syngas stream. The exothermic reaction typically takes place in a chamber  28  where the methane is partially oxidized. 
     The syngas stream 32 is then cooled in a heat exchanger  34 , dried and subjected to a syngas cleanup process to reduce the level of trace components that have an adverse effect on the FT catalyst. This gas as shown in Stream 3, is at approximately 25 Bar and is approximately 30.5 MMSCFD with a H2/CO ratio of approximately 2.2. 
     The synthesis gas is then subjected to a hydrogen membrane unit  36 . A small amount of hydrogen as shown in Stream 4 is removed by a membrane that is hydrogen (H 2 ) permeable. The H 2  is alternately cleaned up and optionally purified for use in product upgrading or for export. This stream contains approximately 0.44 MMSCFD of hydrogen. Alternately, more hydrogen could be removed to adjust the H 2 /CO ratio feed to the FT reactor. 
     A feed ratio that is less than the stoichiometric requirement (greater than 2.1) will result in an even lower H 2 /CO ratio at the exit of the FT reactor. The excess hydrogen can be compressed and added to the tail gas or waste gas from the first stage FT reactor to replenish the H2/CO ratio for the second stage FT reactor. Such manipulation of the H2/CO ratio resulting in a ratio in the FT reactors that is below the stoichiometric requirement will result in reduced methane selectivity in the FT reactors and increased yield of heavy hydrocarbon products. 
     The adjusted synthesis gas as shown in Stream 5 is at approximately 23 Bar and has a volume of 30.06 MMSCFD. This gas is then reacted over a Fischer Tropsch catalyst producing heavy hydrocarbon products as shown by box FT1 at reference numeral  40 . A small amount of light hydrocarbons are produced and exit the reactor with unreacted hydrogen and carbon monoxide. 
     After removal of water, the residue stream from the first reactor is fed to the second FT reactor. This gas as shown in Stream 6 is at approximately 21 Bar and consists of 13.82 MMSCFD of hydrogen, carbon monoxide, carbon dioxide and light hydrocarbons. The reactive components react over the FT catalyst in the second stage FT reactor as shown by box FT2 at reference number  42  where they are converted to additional hydrocarbon products. 
     The tail gas or waste gas shown in Stream 7 from the second stage FT reactor is significantly reduced in volume from the feed gas with 5.17 MMSCFD. This gas has higher levels of inerts as CO 2  and N2/Ar which have built up to about 40% of the stream due to the conversion of H 2  and CO into hydrocarbon products. This gas can be used as fuel to preheat feed gases and/or recycled to the ATR. The FT reactor section can also be configured to include a recycle stream around one or both reactors, or configured with a single FT reactor with a recycle stream. 
     The result of the above process is the production of approximately 1,000 BPD of synthetic liquid crude oil as shown at box  50 . This syncrude can optionally be upgraded to fuels, chemicals and/or chemical intermediate products such as diesel, jet fuel, solvents, waxes, lubricants and gasoline. 
     Whereas, the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention.