Abstract:
A method and apparatus for converting natural gas from a source, such as a wellhead, pipeline, or a storage facility, into hydrocarbon liquid stable at room temperature, comprising a skid or trailer mounted portable gas to liquids reactor. The reactor includes a preprocessor which desulfurizes and dehydrates the natural gas, a first stage reactor which transforms the preprocessed natural gas into synthesis gas, and a liquid production unit using a Fischer-Tropsch or similar polymerization process. The hydrocarbon liquid may be stored in a portable tank for later transportation or further processed on site.

Description:
RELATED APPLICATIONS 
     This application is a divisional of pending U.S. patent application Ser. No. 13/769,079, filed Feb. 15, 2013, and titled “Natural Gas to Liquid Fuels,” which is a continuation-in-part of pending U.S. patent application Ser. No. 13/472,793, filed May 16, 2012, and titled “Gas to Liquid Fuels,” the disclosures of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The oil and gas industry is faced with the need to produce more fossil energy or to prove that the production of such energy is possible. Efforts expended heretofore in this regard have, among other things, revealed that a considerable amount of fossil energy may be obtained from shale deposits, which were previously thought to be only barriers to the migration of subterranean hydrocarbons. Rather recently it has been learned that many of the same shales, whose only function was thought to serve as a caprock or impermeable barrier to subterranean hydrocarbon migration, in fact, have served as massive agents to absorb natural gas. This natural gas can be converted to commercial production. Following drilling and fracturing the shale, so much of this type of unconventional gas production has been proven up at this point in time that the supply has exceeded the demand and prices of natural gas have diminished significantly. 
     Many operators who have paid the cost to drill, such as operators in the Barnett Shale in the area of Fort Worth, North Texas, have shut in their successful shale gas wells because the market price of the gas has fallen below acceptable economic levels for the operation of such wells. Therefore, there is a need to enhance the natural gas market and thereby encourage added drilling aimed at improving the natural gas reserves. 
     Gas to Liquid (GTL) technology for converting natural gas, which consists primarily of methane, to a liquid fuel has existed for nearly a century. A recent resurgence of interest is providing significant advancements in the rapidly growing art. Prior art teaches that natural gas may be converted to higher molecular weight hydrocarbons by generally two techniques, either a direct transformation with an intermittent step of creating a synthesis gas (syngas) or a gas composed generally of hydrogen and carbon monoxide. 
     Direct transformation into higher molecular weight hydrocarbons may occur through pyrolysis, during which methane generally at 250° C. to 100° C. is passed through a catalyst in the absence of substantial amounts of oxygen. Processes and catalysts are described in U.S. Pat. Nos. 4,199,533; 4,547,607; 4,704,496; 4,801,762; 5,093,542; 5,157,189; and 5,245,124. These processes require high activation energy and can be difficult to control. As a result, there is minimal commercial use of direct GTL processes. 
     Two or three GTL processes where the natural gas is first converted to syngas have more prevalent commercial use than the direct processes. For example, Mobil has developed M-gasoline which is created by a three stage process. Natural gas is converted to syngas which is transformed into methanol which is finally made into M-gasoline. However, the most common GTL process is a two stage process in which the natural gas is first converted to syngas which is then changed into liquid hydrocarbons via the Fischer-Tropsch process. 
     SUMMARY 
     The embodiments described herein are based upon a process that first removes sulfur compounds from natural gas, and then converts the processed gas using a catalyst-aided process to a liquid that is useable for transportation or other fuel. This process may be performed in a relatively small unit that could be portable, skid mounted, and/or located adjacent to a source of the natural gas. The liquid to which the natural gas is expected to be converted is anticipated to be a sulfur free mixture of various fuels: for example, gasoline, diesel fuel, jet fuel, and light bunker fuel. This mixture of fuels may then be separated to render them commercially saleable. Therefore, a third process involves the use of a small skid-mounted fractionation tower to separate and stabilize the various fuel products. 
     The skid-mounted conversion unit and processing equipment may be readily moved to any location where fuel is needed and where the gas can be piped to the skid. In an alternative embodiment, the skid may be placed at natural gas supply location, such as at a gas wellhead, pipeline, storage facility, or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a top view of one embodiment of an apparatus comprising a heavy duty truck trailer with a gas to liquids (GTL) conversion unit for transforming natural gas into a liquid phase at ambient temperature and pressure. 
         FIG. 2  is a side view of one embodiment of the apparatus of  FIG. 1 . 
         FIG. 3  is a cross-section of one embodiment of a Fischer-Tropsch reactor. 
         FIG. 4  illustrates an alternative embodiment of a portable GTL apparatus. 
         FIG. 5  illustrates a hydrogenation unit and Fischer-Tropsch reactors according to one embodiment 
         FIG. 6  illustrates an alternative embodiment of a hydrogenation unit and Fischer-Tropsch reactors 
         FIG. 7  illustrates an apparatus for holding catalyst within a hydrogenation unit and/or Fischer-Tropsch reactor pipe or tube. 
     
    
    
     DETAILED DESCRIPTION 
     The invention now will be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. One skilled in the art may be able to use the various embodiments of the invention. 
     The embodiments disclosed herein relate generally to a method and apparatus for converting sulfur-free natural gas to a non-cryogenic liquid for storage and/or transport by land vehicle to another location or for conversion to a motor fuel on-site. A large number of stranded gas fields exist, meaning that they are not close enough to a pipeline to be economically feasible for production. As a consequence, such fields may not be economically developed and the economic value of the gas remains trapped in the earth&#39;s crust. Oil wells on the other hand can be developed even if such wells are in a remote location because the liquid oil may be accumulated in a designated tank placed at the well location and then transferred to a refinery by a tanker truck. In some cases, natural gas may be available at a remote location, for example, in a pipeline. However, such natural gas has greater utility if converted in situ to a liquid motor fuel. 
     The apparatus may comprise at least one truck trailer, a desulfurizing unit, a gas to liquid conversion unit for transforming natural gas into hydrocarbon, characterized by having a liquid phase at atmospheric pressure and ambient temperature, said gas to liquid conversion unit on top of said truck trailer and fastened thereto. A natural-gas-driven electric generator sufficient to provide electricity for all requirements of the truck trailer and the equipment mounted on the truck trailer may also be provided on the truck trailer. 
       FIG. 1  is a top-view illustration of an apparatus comprising at least one heavy-duty truck trailer  100  with a gas to liquids (GTL) conversion unit for transforming natural gas into a liquid phase at ambient temperature and pressure according to one embodiment.  FIG. 2  is a side view of the apparatus. The GTL conversion unit is designed and outfitted for filtering, desulfurizing, dehydrating, and regulating the pressure of the gas and liquid processed by the apparatus. The gas to liquid conversion unit comprises a first stage reactor  103  for converting the effluent of pipeline quality gas into synthesis gas by a hydrogenation source, such as a steam methane reformer, hydrogen generator, hydrogen tank, or auto thermal reformer, and a Fischer-Tropsch reactor  104  for polymerizing said synthesis gas to produce said liquid hydrocarbon. 
     The gas to liquid conversion unit of the apparatus is characterized by having catalyst sites that are designed and arranged with high surface-area to volume ratios. 
     The gas to liquid conversion unit comprises a gas preprocessing section for filtering, desulfurizing, dehydrating, and regulating the pressure of said unit. A first stage converts the effluent of said gas preprocessing unit into synthesis gas by a hydrogen generation process or source, such as a steam methane reforming reaction. Thereafter, the gas is conducted through a Fischer-Tropsch reactor to polymerize said synthesis gas to produce liquid. 
     The apparatus further comprises one or more storage tanks into which the liquid fuels emanating from a fractionation tower may be temporarily stored until they are delivered to a collection tank truck. 
     In one embodiment, a method for converting natural gas at remote terrestrial sources to hydrocarbon is characterized by having a liquid phase at atmospheric pressure and ambient temperature, comprising the steps of moving the trailer mounted GTL equipment in close proximity to the gas source, coupling the equipment to the gas source, and conducting natural gas through the gas to liquid conversion unit while located near said source. 
     Referring to  FIG. 1 , the apparatus comprises a natural gas inlet  101  that may be coupled to a natural gas source, such as a natural gas well or a pipe line. Natural gas flows through the inlet  101  to a sulfur gas removal unit  102 , which is loaded with iron shavings in one embodiment. The natural gas flows over the iron filings in desulfurization unit  102 , which removes the sulfur content from the natural gas. The removal of sulfur from the natural gas improves the lifetime of the catalysts, such as that used in the Fischer-Tropsch reactions. For sulfur in the form H 2 S, the iron shavings react with the sulfur in the natural gas to form FeS 2 . Depending on the content of the sulfur in the natural gas source any other suitable desulfurization unit  102  that is adapted to remove sulfur from natural gas may be used. 
     The output of sulfur gas removal unit  102  is provided to a hydrogenation unit  103 , such as a steam methane reformer that is loaded with a Nickel catalyst. In other embodiments, any hydrogen source generator may be used in place of a steam methane reformer, such as a hydrogen generator, an auto thermal reformer, or a tank of commercially available hydrogen gas. 
     The output of hydrogenation unit  103  is input to Fischer-Tropsch reactors  104 . In the case where a steam methane reformer is used as hydrogenation unit  103 , the Fischer-Tropsch reactors  104  will cause the methane molecule to act as follows: CH 4 +H 2 O→CO+3H 2 . The reactant products are converted from natural gas to a liquid as the substance passes through the Fischer-Tropsch reactors  104 . The long chain hydrocarbon molecules structure produced by the Fischer-Tropsch reactors may vary depending on the hydrogen source selected. 
     A cross section of one embodiment of Fischer-Tropsch reactor  104  is shown in  FIG. 3 . Fischer-Tropsch reactors  104  comprise sections of 1″ extra heavy (XH) line pipe  301  wrapped around a 16″ lightweight center pipe  302 . The 1″ XH line pipe may be loaded with Ruthenium catalyst to effect a Fischer-Tropsch reaction. In other embodiments, a Cobalt catalyst or any other suitable catalyst that will effect a Fischer-Tropsch reaction may be used. Ambient air may be circulated through the 16″ pipe  302  to control the system temperature. In other embodiments, the pipes  301  in the Fischer-Tropsch reactor  104  may be arranged in other configurations that allow air to circulate for cooling. The Fischer-Tropsch reactors  104  may be air cooled using any method of air generation including, for example, high velocity fans. Additionally, water may be used as a coolant and may enhance the reactant process. 
     The Fischer-Tropsch reactors  104  may be positioned at an angle (e.g., at a 3° slope) to cause the liquid to flow through the reactors toward a fractionation tower  105 . The output of Fischer-Tropsch reactors  104  passes through a gas trap (not shown). Any gases trapped in the gas trap are recirculated to the front of the Fischer-Tropsch reactors  104  for further GTL processing. A back-pressure control valve  106  is positioned on the pipe linking the Fischer-Tropsch reactors  104  to the fractionation tower  105  in order to control the pressure, flow rate and temperature into fractionation tower  105 . In one embodiment, fractionation tower  105  may be hinged or otherwise adapted to be rotated from a vertical position for transport of the apparatus in order to avoid low hanging structures or wires. 
     By the time the reactant products reach the base of the fractionation tower  105 , the reactant products are hot liquids capable of boiling and, as such, the products will be segregated as they cool in the fractionation tower  105 . Fractionation tower  105  may have a number of outlets, such as an outlet  107  to a gasoline storage tank, an outlet  108  to a diesel storage tank, an outlet  109  to an aviation fuel storage tank, and an outlet  110  to a heavier fuel storage tank. 
       FIG. 4  illustrates an alternative embodiment of a portable GTL apparatus  400 . The GTL equipment is mounted on a truck trailer, skid, pallet, or other portable or mobile platform  401  that allows it to be moved and deployed near natural gas sources, such as gas wellhead, pipeline, storage tank, or other location or facility. The portable platform  401  may be driven, dragged, pushed, airlifted, floated, or otherwise moved to a natural gas source in any location whether easily accessible or remote. A desulfurization unit  402  has an inlet  403  that may be coupled to the natural gas source. Desulfurization unit  402  may use iron filings, for example, to react with and remove sulfur from the natural gas. The natural gas output from desulfurization unit  402  is fed by pipe  403  to a hydrogenation unit  404 . 
     The desulfurized natural gas passes through a hydrogenation unit  404  from input  405  to output  406 . In one embodiment, hydrogenation unit  404  comprises twenty-nine 1″ pipes  407  surrounding a 16″ central pipe (not shown). The twenty-nine 1″ pipes contain a Nickel catalyst that reacts with the natural gas to generate hydrogen (H 2 ) molecules. The natural gas and hydrogen is provided to two Fischer-Tropsch reactors  408  by pipe  409 . In other embodiments, hydrogenation unit  404  may be, for example, a steam methane reformer, an auto thermal reformer, or a hydrogen tank that generates or provides hydrogen to mix with the natural gas. 
     The natural gas and hydrogen enter the Fischer-Tropsch reactors  408  at inputs  410 . In one embodiment, Fischer-Tropsch reactors  408  each comprise twenty-nine 1″ pipes  411  surrounding a 16″ central pipe (not shown). The twenty-nine 1″ pipes contain a catalyst, such as Ruthenium or Cobalt, that effect a Fischer-Tropsch process so that the natural gas and hydrogen is converted to liquid at exits  412 . The Fischer-Tropsch reactors  408  are constructed at an angle, such as a 3° angle, so that the liquid will flow from input  410  toward output  412 . 
     The liquid is then provided by pipe  413  to a fractionation tower  414 . The liquid rises in fractionation tower  414  and is output at a port  415  selected depending upon the desired liquid fuel type. A valve  416  on pipe  413  may be used to regulate the pressure output from Fischer-Tropsch reactors  408  and input to fractionation tower  414 . 
     It is expected that the temperatures created in hydrogenation unit  404  and the Fischer-Tropsch reactors  408  will be very high. For example, a hydrogenation unit  404  using a Nickel catalyst as described above may operate at approximately 800° C. and the Fischer-Tropsch reactors  408  may operate at approximately 300° C. Therefore, hydrogenation unit  404  and the Fischer-Tropsch reactors  408  will likely require cooling. In one embodiment, hydrogenation unit  404  and the Fischer-Tropsch reactors  408  are air-cooled by an air source  417 , such as a fan, blower, or turbine that provides air to the center 16″ pipe of the hydrogenation unit  404  and the Fischer-Tropsch reactors  408  via air pipes  418 . The air flows though the center pipe to cool the respective hydrogenation unit  404  and/or the Fischer-Tropsch reactor  408 . The cooling air exits the hydrogenation unit  404  and the Fischer-Tropsch reactors  408  via exhaust  419 . 
       FIG. 5  illustrates the hydrogenation unit  404  and Fischer-Tropsch reactors  408  according to one embodiment. A central 16″ pipe receives air at input  502  from an air source. Twenty-nine 1″ pipes  503  are wrapped around the center pipe  501 . Catalyst, such as Nickel in a hydrogenation unit  404  or Ruthenium or Cobalt in a Fischer-Tropsch reactor  408 , is placed in the twenty-nine pipes  503 . Any other catalyst proven to affect the desired reaction(s) may be used in the alternative. An input manifold  504  distributes the incoming natural gas (hydrogenation unit  404 ) or natural gas and hydrogen (Fischer-Tropsch reactors  408 ) to the twenty-nine pipes  503 . An output manifold  505  collects the output natural gas and hydrogen (hydrogenation unit  404 ) or liquid (Fischer-Tropsch reactors  408 ) and provides the output to pipes  409  or  413 , respectively, at outlet  506 . Cooling air exits the assembly via outlet  507 . 
       FIG. 6  illustrates an alternative embodiment of the hydrogenation unit  601  and Fischer-Tropsch reactors  602 . Instead of wrapping the 1″ pipes around a center pipe for cooling as illustrated in  FIGS. 1 ,  4 , and  5 , the 1″ pipes  604  are held in bracket-like structure  600  that provides spacing between the 1″ pipes  604  so that air can flow between the pipes  604  for cooling. An air source  605 , such as fans, blowers, or turbines, is used to provide ventilation through the bracket structure  600  and across pipes  604  for cooling. 
     It will be understood that other cooling arrangements, such as water cooling, may be used for the hydrogenation unit and Fischer-Tropsch reactors depending upon the configuration of the system. Additionally, it will be understood that the embodiments illustrated herein are merely examples and that the number of pipes, other pipe sizes and other configurations may be used for the hydrogenation unit and Fischer-Tropsch reactors. 
       FIG. 7  illustrates an apparatus for holding catalyst within a hydrogenation unit and/or Fischer-Tropsch reactor pipe or tube. Each device, as illustrated above, comprises a plurality of pipes filled with a catalyst. Over time, the catalyst will become “poisoned” as other compounds bond to its active surface sites, which reduces the usefulness of the catalyst. When that occurs, the catalyst in the hydrogenation unit and/or Fischer-Tropsch reactor will need to be replaced. In order to simplify the replacement of the catalyst within the pipes  701  of these devices, a tray  702  is adapted to carry the catalyst  703 . This makes it easier to load, unload, and replace the catalyst. The operator simply has to load the catalyst  703  on the tray  702  and inserts the tray  702  into a pipe  701  of the hydrogenation unit or Fischer-Tropsch reactor (e.g., pipes  301  ( FIG. 3 ) or  503  ( FIG. 5 )). 
     The catalyst  703  may have any appropriate form that is required by the process or available from a manufacturer, such as pellets, disks, rings, or other shapes. Tray  702  may be adapted to hold a particular form of the catalyst in a desired position, for example, to maximize an available surface area exposure or to generate turbulence or to otherwise improve the desired reaction. Tray  702  may also be adapted to evenly distribute the catalyst in pipe  701  and to prevent unwanted shifting of the catalyst during movement of the GTL apparatus. In one embodiment, trays  702  extend the entire length of the pipe  701  and provide for easy loading and uniform flow of hydrogen over the catalyst  703  in order to maximize contact with the catalyst and provide uniformity of reaction. 
     In other embodiments a series of trays  702  may be used in a pipe  701  instead of a single long tray. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. While preferred embodiments of the invention have been described, modifications and adaptations of the preferred embodiments will occur to those skilled in the art. It is to be expressly understood that modifications and adaptations are in the spirit and scope of inventions set forth in the following claims.