Patent Publication Number: US-6220052-B1

Title: Apparatus and method for liquefying natural gas for vehicular use

Description:
This invention relates to an apparatus and method for liquefying natural gas for vehicular use. 
     A method and apparatus for liquefying natural gas for a fuel for vehicles and a fuel tank for use therewith is disclosed in U.S. Pat. No. 5,327,730 issued on Jul. 12, 1994. In connection with the method and apparatus therein disclosed, difficulties have been encountered in reducing the pressure of natural gas being supplied through a fixed orifice because of changes in temperature of the natural gas. Additional difficulties have been encountered because of freezing of carbon dioxide in the natural gas. There is therefore a need for a new and improved apparatus and method for liquefying natural gas, particularly for vehicular use. 
     In general, it is an object of the present invention to provide an apparatus and method for liquefying natural gas for vehicular use which substantially increases the proportion of natural gas becoming a liquid during each cycle. 
     Another object of the invention is to provide an apparatus and method of the above character in which carbon dioxide in the natural gas is removed before liquefaction of the natural gas. 
     Another object of the invention is to provide an apparatus and method of the above character in which an adjustable orifice is provided in the Joule-Thompson valve to accommodate different temperatures of the incoming natural gas by maintaining a constant inlet pressure. 
     Another object of the invention is to provide an apparatus and method of the above character which by controlling the pressure of the compressed gas makes it possible to operate at very high liquefaction efficiencies. 
     Another object of the invention is to provide an apparatus and method of the above character in which the Joule-Thompson valve utilized is mounted in an assembly directly mounted on the dewar which can accommodate expansion and contraction in the dewar on which it is mounted. 
     Another object of the invention is to provide an apparatus and method of the above character in which all of the piping for the dewar is provided through the Joule-Thompson valve assembly for reducing the cost of the dewar. 
     Another object of the invention is to provide an apparatus and method of the above character which does not require the use of a cryogenic pump to transfer liquefied natural gas from the dispenser. 
     Another object of the invention is to provide an apparatus and method of the above character which does not require the use of a cryogenic pump to transfer liquefied natural gas from the dispenser. 
    
    
     Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings. 
     FIG. 1 is a schematic representation of the apparatus of the present invention and the flow diagram for use therewith. 
     FIG. 2 is a partial cross-sectional view of the cryogenic liquid methane storage vessel shown in FIG. 1 with the Joule-Thompson valve of the present invention mounted thereon. 
     FIG. 3 is a top plan view of the Joule-Thompson valve shown in FIG.  2 . 
     FIG. 4 is side elevational view of the tri-tower regenerating molecular sieve bed shown in FIG.  1 . 
     FIG. 5 is a top plan view of the molecular sieve bed shown in FIG.  4 . 
     FIG. 6 is a cross-sectional view of one of the desiccant vessels shown in FIG.  4  and taken along the line  6 — 6  of FIG.  4 . 
     FIG. 7 is an exploded view of the desiccant vessel as shown in FIG.  6 . 
     FIG. 8 is a simplified flow diagram of the present invention showing the manner in which the tri-tower regenerating molecular sieve bed is operated to perform the method of the present invention. 
    
    
     In general, the apparatus for liquefying natural gas includes means for removing the carbon dioxide from the natural gas. A compressor is provided for compressing the natural gas. A chiller is provided for reducing the temperature of the compressed gas. A heat exchanger is provided for further cooling of the compressed gas. A dewar is provided. A Joule-Thompson valve assembly is provided which is mounted on the dewar and has an orifice with an adjustable needle valve for controlling the size of the orifice for maintaining a constant pressure of the natural gas ahead of the Joule-Thompson valve to provide a controlled expansion of the gas from a high pressure to a lower pressure in the dewar to thereby cause liquefaction of a substantial portion of the gas. 
     More in particular as shown in the drawings the apparatus  21  for liquefying natural gas is for on-site natural gas liquefaction for dispensing compressed natural gas or liquid natural gas at a site accessible by the vehicles. The apparatus  21  is to be used with a source  22  of natural gas available at the site. The natural gas typically available at such a site has components which include methane and the heavier hydrocarbons. The heavier hydrocarbons are ethane, propane, butane, pentane, etc. Also included are inerts such as nitrogen, carbon dioxide and water. Methane which is the principal component of natural gas is only a liquid at an extremely cold temperature and within a certain pressure range. At atmospheric pressure, methane is a liquid at −260° F. (−160° C.) 
     The apparatus  21  uses such natural gas from the source  22  which supplies the gas to piping  23  connected to the first stage  26  of a four stage compressor  27  (see FIGS. 1 and  8 ) through a pneumatic control valve  24  and a check valve  25 . The gas after passing to the first stage  26  passes through piping  28  through a multi-tower regenerating molecular sieve bed  31  of the type hereinafter described in more detail consisting of three desiccant vessels or towers  32 ,  33  and  34  also respectively identified as DF 1 , DF 2  and DF 3  which are disposed in close proximity to each other and interconnected by valving hereinafter described. Briefly, one tower is used for absorbing the contaminants while the other two towers are being regenerated with the second tower being in a heating cycle and the third tower being in a cool-down mode. As hereinafter explained, the molecular sieve bed  31  is utilized for removing water and carbon dioxide. The water and the carbon dioxide are removed to prevent clogging of the processing equipment utilized in the apparatus  21  since water and the carbon dioxide solidify at the low temperature encountered during processing of the natural gas in the apparatus  21 . 
     After the undesired substances such as water and carbon dioxide have been removed from the natural gas, the natural gas is connected by piping  36  to the second stage  37  of the compressor  27 . Piping  38  (FIG. 8) connects the second stage  37  to the third stage  39  and piping  41  connects the third stage  39  to the fourth stage  42 . After passing through these four stages of compression, the natural gas has been compressed to a suitable pressure as for example approximately 2200 to 3000 psi and preferably 2700 to 2800 psi and supplied to piping  46  which is connected to a pressure reducing regulator  47  to reduce the pressure to approximately 150 psi. This compressed gas from regulator  47  is delivered by piping  48  to the desiccant tower  33 . 
     The four-stage compressor  27  is driven in a suitable matter as for example by a natural gas internal combustion engine  51  which drives hydraulic fluid pumps  52 . The hydraulic fluid from one of the pumps  52  is supplied through piping  53  to a hydraulic motor  54  that drives the four-stage compressor  27 . The hydraulic pumps  52  also include two additional hydraulic pumps (not shown) that drive other accessories including fans (not shown). 
     The compressed gas from the compressor  27  through the piping  46  is supplied to an industrial type gas chiller  56  using a mechanical refrigerant. As is well-known to those skilled in the art, the chiller  56  includes a compressor  57  which is driven by hydraulic fluid supplied on piping  58  from one of the hydraulic pumps  52 . The chiller  56  also includes an evaporator heat exchanger  59 . The gas in the chiller  56  is cooled to a suitable temperature as for example −60° F. and is supplied through outlet piping  66  at approximately this temperature to a pneumatic control valve  67  which is connected to piping  68  to a dispenser hereinafter described and is supplied by piping  68  through desiccant vessel  33  to the fuel intake of the internal combustion engine  51  as hereinafter described. The remaining compressed gas is then supplied through piping  69  to a main methane-to-methane countercurrent heat exchanger  71  which reduces the temperature of the compressed gas to approximately −100° F. 
     The cooled compressed gas after being cooled to −100° F. is supplied through tri-axial piping  76  to a Joule-Thompson (JT) valve assembly  77  mounted on top of a dewar or cryogenic liquid methane storage vessel  78 . As hereinafter explained, the JT valve assembly  77  is computer controlled to provide relatively high liquefaction efficiencies over a fluctuating range of temperatures and pressures. The gas in passing through the JT valve assembly  77  is expanded to a pressure of 90-125 psig under a method which uses a closed loop system identifying temperatures and pressures and properly controlling the orifice in the JT valve assembly as hereinafter described. Typically approximately 50% of the flow across the orifice of the JT valve assembly  77  is liquefied with the remaining 50%, still a gas being very cold in the range of −180° F. is withdrawn from the dewar  78  and is withdrawn through the tri-axial piping  76  and supplied to the return cooled gas countercurrent heat exchanger  71 . This cold countercurrent gas reduces the temperature of the feed stock natural gas from −60° F. −100° F. 
     Although liquefaction of natural gas can be achieved at pressures as low as 681 psig, the most effective pressure to liquefy natural gas for small scale on-site liquefaction as in the present apparatus  21  appears to be between 2700 psig and 3000 psig. There is a lower efficiency in the apparatus beyond 3100 psig which means that the energy spent for compression over 3000 psig yields very little if any increase to the liquefaction rate as can be ascertained from the entropy chart for natural gas. 
     As shown in FIG. 1, the apparatus  21  includes a compressed natural gas dispenser  86  and a liquid natural gas dispenser  87  under the control of a card lock apparatus  88  for use in dispensing the desired fuel to a vehicle  89  (see FIG. 8) having access to the apparatus  21  at the site. Piping  91  is provided for connecting the liquid natural gas in the dewar  78  to the liquid natural gas dispenser  87 . Compressed gas which has not been liquefied is returned from the countercurrent heat exchanger  71  through piping  92  through a pressure reducing regulator  93  and then through piping  94  through a check valve  95  (FIG. 8) to the piping  22  for reprocessing in the apparatus  21 . 
     A fuel nozzle  101  of the type disclosed in co-pending application Ser. No. 09/375,662 filed Aug. 17, 1999 (A-68329) is provided for supplying liquefied natural gas to a fuel tank  102  on the vehicle  89 . The nozzle  101  includes a liquefied gas line  103  and a vent return line  104  which are connected through a tri-axial line  106  to the dispenser  87 . Since the vent return line  104  is included in the nozzle  101 , the vent return line is coupled to the piping  23  through a check valve  105  when the vale  24  is closed under the control of the nozzle  101  when operated to cause gas vapors to be removed from the tank  102 . Removal of gas vapor from the tank  102  causes a reduction in pressure in tank  102  which causes LNG to flow from the dewar  78  through the nozzle  101  into the tank  102  until delivery is terminated or when the tank is full. Such a method eliminates the need for an expensive cryogenic pump. 
     In connection with the apparatus  21 , a data acquisition, communication, computer management system  108  (FIG. 1) is provided which is connected to various sensors (not shown) and controls (not shown) for controlling the operation of the apparatus  21  as hereinafter described in more detail. 
     The construction of the JT valve assembly  77  and its mounting on the dewar  78  may now be described more in detail. The dewar  78  is comprised of an inner stainless steel tank  111  and an outer carbon steel tank  112  with a space  113  therebetween which is provided with superinsulation (not shown) and a vacuum. The inner stainless steel tank  111  when it gets colder will shrink with respect to the carbon steel tank  112  which contraction must be accommodated by a weld-neck flange assembly  116  mounted on the dewar  78 . The weld-neck flange assembly  116  consists of weld-neck flange  117  which is mounted in an opening  118  in the outer tank  112  and an opening  119  in general registration with the opening  118  in the inner tank  111 . A cylindrical pipe  121  has its lower extremity welded to the inner tank  111  in the opening  119  and extends upwardly through the opening  118  in the outer tank  112  and is welded to the lower extremity of the weld-neck flange  117 . A bellows  122  is provided which has its upper extremity welded to the weld-neck flange  117  and has its lower extremity welded to the outer tank  112  at the opening  118 . Thus, the bellows  122  serves to permit expansion and contraction of the inner tank  111  with respect to the outer tank  112  and to maintain an air-tight and liquid-tight seal between the flange  117  and the outer tank  112  and the inner tank  111 . 
     A cylindrical sleeve  126  of a suitable material such as stainless steel is welded to the pipe  121  and extends upwardly through the weld-neck flange  117  as shown in FIG.  2  and forms a slip fit with respect to a slip-on flange assembly  127 . 
     A slip-on flange assembly  127  is provided consists of a slip-on flange  128  which is removably secured to the weld-neck flange  117  by circumferentially spaced-apart threaded rods  129  with nuts  131  secured to opposite ends thereof. A pipe  132  is welded to the slip-on flange  128  and extends upwardly therefrom and forms a part of the JT valve assembly  77 . 
     The JT valve assembly  77  also includes an inner cylindrical member  136 , the lower extremity of which is welded to an annulus  137  which is welded to the lower extremity of the sleeve  126 . The inner cylindrical member  136  extends upwardly in the pipe  132  and is provided with a top cover plate  138  which is welded to the top of the inner cylindrical member  136 . A dip slide tube  139  is mounted on the top cover plate  138  and extends upwardly therefrom and has a support plate  140  mounted thereon. The tube  139  houses an electronic dipstick (not shown). A bellows  141  is connected between the support plate  140  and the upward extremity of the pipe  132  by an annulus  142 . The bellows  141  serves to permit contraction and expansion of the inner tank  111  with respect to the outer tank  112  and provides a liquid-tight connection between the plate  142  and the pipe  132 . The JT valve assembly  77  thus provides a manway  143  in the form of a cylindrical passage into the inner tank  111 . 
     The JT valve assembly  77  includes a JT valve  144  that has a body  146  mounted within the manway  143  in the inner cylindrical member  136  and is supported by the top cover plate  138 . The valve body  146  is provided with a flow passage  147  therein which opens into an orifice  148 . The flow passage  147  is also in communication with an inlet flow passage  151  extending at right angles to the flow passage  147 . A needle valve  152  extends into the orifice  148  for adjusting the size of the orifice  148  as hereinafter described. The needle valve  152  passes through a packing nut  153  provided on the valve body  146  and extends upwardly through the top cover  138  through a needle valve enclosure  156  that also extends through the support plate  142 . The needle valve  152  is adjustable axially by threads  157  in the valve body  146  engaging mating threads  158  on the stem of the needle valve  152 . A shroud  161  is provided at the upper extremity of the needle valve  152  and accommodates movement of the needle valve between open and closed positions with respect to the orifice  148 . 
     Needle valve drive means  164  is provided for the needle valve  152  and includes a spur gear  166  mounted on the upper end of the needle valve  152  and which moves with the needle valve  152  as it is moved between open and closed positions with respect to the orifice  148 . The spur gear  166  is provided with a pin  168  which extends therethrough and which is adapted to pass through slotted infrared sensor housings  171  and  172  mounted in fixed positions on opposite sides of the gear. The pin  168  actuates the infrared sensor in the sensor housing  171  when the needle valve  152  is in a fully open position with respect to the orifice  148  and conversely the pin  168  actuates the infrared sensor in the sensor housing  172  when the needle valve  153  is in a fully closed position with respect to the orifice  148 . The needle valve drive means  164  also includes a spur gear  176  that drives spur gear  166 . Spur gear  176  is mounted on the output shaft  177  of a stepper motor  178  carried by a bracket  179  on the mounting plate  140 . The stepper motor  178  is a high resolution stepper motor as for example one having 12,800 steps per revolution to make it possible to precisely control the movement of the needle valve  152  with respect to the orifice  148 . 
     The needle valve  152  and the orifice  148  have been selected so that the JT valve  144  is an eleven-turn valve. Thus, when the pin  168  interrupts the infrared beam in the sensor housing  172 , the JT valve  144  is in a closed or home position. After eleven turns the JT valve  144  is moved from the closed position to an open position. 
     As hereinbefore explained, the cold compressed gas is supplied to the JT valve assembly  77  through tri-axial piping  76 . As shown in FIG. 2, this tri-axial piping  76  includes an inner pipe  181  which supplies this cooled and compressed gas to the inlet flow passage  151  and into the orifice  148 . A pressure sensor  182  is provided in the inner pipe  181  and is connected to the computer  106 . 
     With the cooled compressed gas being delivered to the inlet flow passage at a pressure of typically between 2700 and 2800 psi as it expands through the orifice  148 , a large proportion of the gas as for example 50% or greater is liquefied and passes through a pipe  186  welded to the valve body  146  and extending down into the upper portion of the inner tank  111  of the dewar  78  that contains the liquefied natural gas. At the same time the portion of the cooled compressed gas which is not liquefied passes down through the pipe  186  into the upper part of the inner tank  111  and is returned from through a pipe  187 , also a part of the JT valve assembly  77 . The pipe  187  is connected by a 90° elbow  188  to an outer pipe  189  that is concentric with the inner pipe  181  and which forms a part of the tri-axial piping  76  hereinbefore described. Thus this cold returned gas is returned to the countercurrent heat exchanger  71  to aid in cooling of the incoming natural gas being supplied to the heat exchanger  71 . An outer annulus  191  is provided as a part of the tri-axial piping  76  and typically is under a vacuum to provide the desired insulation for the cold gas passing through the outer pipe  189 . The outer pipe  189  also serves to insulate the pipe  181 . 
     A temperature sensor  196  is provided in the pipe  186  for sensing the temperature of the liquefied natural gas passing through the pipe  186  down into the dewar  78 . Conductive wires (not shown) are connected to the computer  106  through a tube  197  forming a part of the JT valve assembly  77 . A fill pipe  199  is provided as a part of the JT valve assembly  77  and extends upwardly through the support plates  138  and  140  and is connected to an elbow  201  to which a connection can be made from the exterior of the JT valve assembly  77  for supplying liquefied natural gas through the pipe  191  to the top of the dewar  78 . In addition as shown in FIG. 3 there is provided a vent pipe  202  and a pressure relief vent  203 . There is also provided a radio frequency level sensor  206 . A fitting  207  is provided for the temperature sensor  196  and a fitting  208  for the pressure sensor  182 . A housing  204  is mounted on the support plate  140  and encloses the drive means  164 . The operation of this JT valve assembly in conjunction with the apparatus  21  will hereinafter be described more in detail. 
     The molecular sieve bed  31  hereinbefore identified and which is more particularly shown in FIGS. 4,  5 ,  6 ,  7  and  8  consists of the three tanks, towers or filters  32 ,  33  and  34  and also identified respectively as DF 1 , DF 2  and DF 3 . As shown in FIGS. 4,  5  and  6 , these filters  32 ,  33  and  34  are interconnected by piping  211  which has provided therein a plurality of air actuated valves  212  bearing an AV designation as hereinafter set forth supplied with air from a conventional electric motor-driven air compressor (not shown). The physical arrangement of this piping  211  with respect to the three vessels or filters  32 ,  33  and  34  is shown in FIGS. 4,  5  and  8  in a physical format and in FIG. 8 in a diagrammatic format. As shown in FIG. 4, each desiccant filter of the vessels or filters  32 ,  33  and  34  consists of an outer pressure vessel  221  formed of steel and having a suitable size as for example a diameter of 24″ and a height of approximately 8′6″. This outer vessel  221  is provided with a cylindrical wall  222  with its open ends being enclosed by a top dome  223  and a bottom dome  224 . The outer vessel  221  is supported in a vertical position by a circular support  226  welded to the lower extremity of the cylindrical wall  222 . The outer vessel  221  is designed to withstand 150 psi and a temperature of 650° F. with a designed operating range of 0° F. to 550° F. 
     An inner vessel or liner  231  is disposed within the outer vessel  221  and is formed of a suitable thin-wall material such as 10 gauge stainless steel and has a suitable diameter as for example 16″. The inner vessel or liner  231  is provided with a cylindrical wall  232  with a bottom plate  234  enclosing the bottom open end. The top is open to outer pressure vessel  221  so that there is no pressure differential between the anterior of the inner vessel  231  and the interior of the outer vessel  221 . Thus the vessel  231  has the thin wall which accelerates heating and cooling of the vessel  231  during operation as hereinafter described. A support  236  is welded between the cylindrical wall  232  and the bottom  224  so that the inner vessel or liner  231  is supported in an upwardly spaced position with respect to the bottom dome  224  and in such a manner so that there is an annular space  241  which is filled with insulation which surrounds the cylindrical wall  232  and the bottom plate  234 . Circumferentially spaced-apart liner spacers  242  are only welded to the inner vessel or liner  231 . This permits the liner to expand and contract with respect to the outer vessel during operating cycles. 
     A gas inlet pipe  246  of a suitable diameter such as 1″ and forming a part of the piping  211  is mounted in the top dome  223  of the outer vessel  221  for supplying gas to the inner vessel or liner  231 . Similarly a gas outlet pipe  247  also of a suitable size such as 1″ and forming a part of the piping  211  is connected into the bottom plate  234  of the inner vessel or liner  231 . 
     A plurality of circumferentially spaced-apart grate supports  251  are welded to the interior of the inner vessel  231 . A circular grate  252  approximately 15¾″ in diameter rests upon the grate supports  251 . The circular grate has circular openings  253  of a suitable size of ¼″ in diameter with spaced apart centers of ⅜″. A plurality of dispersing elements in the form of ceramic balls  256  having various sizes ranging from ⅛″ to ½″ at a depth of approximately 6″ overlie the grate  252 . A circular mesh  258  of a suitable diameter such as 16″ with the mesh being formed of 20 wires per inch in two orthogonal directions to provide openings  259  of a size of approximately 0.036″ square. The space in the inner vessel or liner  231  above the mesh  258  is filled with a suitable desiccant material  261  of a suitable type such as a synthetic sodium potassium compound that absorbs carbon dioxide and water as for example Z402 supplied by Zeochem Corporation of Louisville, Ky. The desiccant material can be identified as a 4A material having a very small particle size similar to that of sand. This desiccant material has a relatively long lifetime as for example 2 to 3 years after which it can be vacuumed out and replaced. A mesh  263  similar to the mesh  258  overlies the top of the desiccant material  261 . The mesh  263  is overlaid with ceramic balls  264  similar to the ceramic balls  256  and having a depth of approximately 6″. 
     The piping  211  hereinbefore described in connection with the desiccant towers or filters  32 ,  33  and  34  and as shown in FIGS. 4 and 8 have relative positions in two stacks as indicated by the two rows of numbers set forth below from 1 to 9 and 10 to 18. 
     
       
         
           
               
             
               
                   
               
               
                 Chart I 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 AV8 
                 10 
                 AV2 
               
               
                 2 
                 AV9 
                 11 
                 AV3 
               
               
                 3 
                 AV10 
                 12 
                 AV4 
               
               
                 4 
                 AV11 
                 13 
                 AV5 
               
               
                 5 
                 AV12 
                 14 
                 AV6 
               
               
                 6 
                 AV13 
                 15 
                 AV7 
               
               
                 7 
                 AV17 
                 16 
                 AV14 
               
               
                 8 
                 AV18 
                 17 
                 AV15 
               
               
                 9 
                 AV19 
                 18 
                 AV16 
               
               
                   
               
            
           
         
       
     
     These valves  212  are operated in various sequences in three cases in which in each case has one of the desiccant towers performing filtering, one of them regenerating and the third cooling. These three cases are set forth below: 
     
       
         
           
               
               
               
               
               
             
               
                 CHART II 
               
               
                   
               
             
            
               
                 Case 1 
                 DF1 Filtering 
                 DF2 Regenerating 
                 DF3 Cooling 
                 SEQ 3 
               
            
           
           
               
               
            
               
                   
                 Open valves: AV2, AV5, AV10, AV12, AV16, AV18 
               
            
           
           
               
               
               
               
               
            
               
                 Case 2 
                 DF1 Regenerating 
                 DF2 Cooling 
                 DF3 Filtering 
                 SEQ 1 
               
            
           
           
               
               
            
               
                   
                 Open valves: AV4, AV7, AV9, AV11, AV15, AV17 
               
            
           
           
               
               
               
               
               
            
               
                 Case 3 
                 DF1 Cooling 
                 DF2 Filtering 
                 DF3 Regenerating 
                 SEQ 2 
               
            
           
           
               
               
            
               
                   
                 Open valves: AV3, AV6, AV8, AV13, AV14, AV19 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen from above, the valves  212  are operated in predetermined sequences as set forth in Sequence 1, Sequence 2 and Sequence 3. The condition of the air valves  212  in each sequence is set forth below: 
     
       
         
           
               
               
               
               
             
               
                   
                 CHART III 
               
               
                   
                   
               
               
                   
                 Valve Sequence 1 
                 Valve Sequence 2 
                 Valve Sequence 3 
               
               
                   
                 changes from: 
                 changes from: 
                 changes from: 
               
               
                   
                 Case 1 to Case 2: 
                 Case 2 to Case 3: 
                 Case 3 to Case 1 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 15 
                 O AV19 
                  2 
                 O AV18 
                  6 
                 O AV17 
               
               
                   
                 10 
                 C AV12 
                  7 
                 C AV11 
                 16 
                 C AV13 
               
               
                   
                 14 
                 C AV10 
                  1 
                 C AV9 
                  5 
                 C AV8 
               
               
                   
                 11 
                 O AV15 
                  8 
                 O AV14 
                 17 
                 O AV16 
               
               
                   
                 15 
                 C AV19 
                  2 
                 C AV18 
                  6 
                 C AV17 
               
               
                   
                 17 
                 C AV16 
                 11 
                 C AV15 
                  8 
                 C AV14 
               
               
                   
                 13 
                 O AV4 
                  3 
                 O AV3 
                  4 
                 O AV2 
               
               
                   
                 18 
                 O AV7 
                 12 
                 O AV6 
                  9 
                 O AV5 
               
               
                   
                  9 
                 C AV5 
                 18 
                 C AV7 
                 12 
                 C AV6 
               
               
                   
                  4 
                 C AV2 
                 13 
                 C AV4 
                  3 
                 C AV3 
               
               
                   
                  6 
                 O AV17 
                 15 
                 O AV19 
                  2 
                 O AV18 
               
               
                   
                  7 
                 O AV11 
                 16 
                 O AV13 
                 10 
                 O AV12 
               
               
                   
                  1 
                 O AV9 
                  5 
                 O AV8 
                 14 
                 O AV10 
               
               
                   
                  2 
                 C AV18 
                  6 
                 C AV17 
                 15 
                 C AV19 
               
               
                   
                   
               
               
                   
                 O = Open  
               
               
                   
                 C = Close  
               
               
                   
                 At the end of SEQ 1 valves are left in Case 2  
               
               
                   
                 At the end of SEQ 2 valves are left in Case 3  
               
               
                   
                 At the end of SEQ 3 valves are left in Case 1  
               
               
                   
                 Sequences are initiated when the SEQ buttons are turned from OFF to ON.  
               
            
           
         
       
     
     The above-identified sequences are initiated under the control of the computer  106 . However, sequence buttons (not shown) are provided which can be turned from OFF to ON to manually initiate a sequence. 
     In connection with the piping  211  there is provided a coil  271  which is wrapped around a muffler  272  provided on the internal combustion engine  51 . (See FIG. 8.) 
     Operation and use of the apparatus  21  for liquefying natural gas and utilizing the method of the present invention may now be briefly described as follows. The overall operation of the apparatus in performing the method has already been set forth in conjunction with the description of the apparatus shown in FIG.  1 . 
     The JT valve assembly  77  which is used in connection with the method of the present invention creates the cryogenic liquid natural gas. It creates it on the top of the dewar  78  and introduces it directly into the top of the inner tank  111  through the pipe  186  while at the same time permitting an expansion and contraction of the inner cryogenic tank  111  with respect to the outer tank  112 . 
     It is the function of the JT valve assembly  77  of the present invention to maintain a constant pressure immediately before the JT valve  144  regardless of the temperature of the gas supplied to the JT valve  144  whereby there is provided a controlled expansion of the gas from the high pressure in the inlet pipe  181  to the lower pressure in the tank  111  of the dewar  78 . The lower pressure in the dewar  78  is controlled by an adjustable back pressure regulator  183  (FIG. 1) in piping to provide a running pressure in the dewar ranging from 70 to 125 psi. In connection with the present invention, it is the purpose of the JT valve assembly  77  to optimize the pressure difference across the JT valve  144  to provide the final cooling of the gas which forces it to liquefy. In connection with the present invention it has been found that the optimum results in liquefication have been obtained by utilizing a pressure in the inlet gas to the JT valve  144  at a pressure ranging from 2200 to 3000 psi and preferably from 2700 to 2800 psi. Utilizing such pressures, it has been found that it is possible using the method of the present invention to liquefy approximately 50% or more of the gas stream in each pass through the JT valve  144 . 
     In placing the apparatus  21  of the present invention in operation, it has been found that until the heat exchanger  82  (FIG. 8) is very cold which only occurs after operation for a substantial period of time, the gas being supplied to the inlet  181  is not very cold and therefore the gas is very expansive creating higher pressures in the inlet flow passage  151 . It is therefore necessary that the computer  106  programs opening of the JT valve  144  to let more gas pass through the orifice  148  to maintain a constant pressure in the inlet  151  and to prevent the pressure from going too high. As the heat exchanger  71  becomes colder, the gas being supplied to the inlet  151  becomes more dense and the pressure tends to drop. Since a pressure drop is undesirable, the JT valve  144  under the control of the computer is moved to begin closing down of the JT valve  144  by moving the needle valve  152  downwardly to reduce the size of the orifice  148 . By controlling this pressure in the inlet  151  it is also possible to control the differential between the inlet pressure and the dewar pressure to thereby maximize the liquefication of the gas passing through the JT valve  144 . 
     It has been found in connection with the present invention that pressures above 3000 psi in the inlet  181  are undesirable because the pressure lines on the methane entropy chart at higher pressures are almost vertical so that there is very little increase in liquefaction with the increase in pressure above 3000 psi. However, with a decrease in pressure, the liquefaction rate drops rather rapidly. Thus in accordance with the present invention it is undesirable to perform liquefaction at pressures below 2200 psi and above 3000 psi with the optimum pressure being 2700 to 2800 psi. 
     As well known to those skilled in the art, the amount of liquid in the dewar can be readily ascertained by measuring the differential pressure in the liquid from the top of the tank and at the bottom of the tank. 
     In connection with the present invention it has been found that because the apparatus cannot run continuously it is necessary to ensure that substantially all the carbon dioxide and water have been removed in the early stages of processing of the natural gas in order to prevent freezing in the event of a shutdown of the apparatus which can occur when demand for fuel does not match the rate of production of fuel by the apparatus. 
     In connection with the operation of the molecular sieve bed  31  as a part of the apparatus  21  it can be seen from FIG. 8 that the gas stream from the first stage  26  of the compressor  27  is supplied to the piping manifold  211  which under the control of the valves  212  can be passed through any one of the three desiccant filters  32 ,  33  and  34  also identified as DF 1 , DF 2  and DF 3 . The gas after passing through one of these filters is returned to the input of the second stage of the compressor  27 . At the same time, a gas stream from a higher pressure point in the piping is used to cool one of the desiccant filters selected through the valving  212 . Thereafter this gas passing from this desiccant filter being cooled is supplied to the coil  271  that is wrapped about the engine muffler  272 . This heated gas is then returned to heat a selected desiccant filter for regeneration. 
     In connection with the present invention it has been found that a single desiccant filter can act as a filter for absorbing carbon dioxide and water for a period of approximately four hours, after which carbon dioxide can be detected as passing from the gas outlet pipe  247  indicating that the desiccant filter is saturated. This condition is sensed by the computer  108  which operates the valves  212  through a sequence to change the order in which the filter is being used and for what. For example, when a desiccant filter has become saturated, the gas which has been heated up to 600° F. by the muffler  272  passes from the bottom of the desiccant filter up towards the top for a period of approximately four hours. During this four-hour period of time most of the carbon dioxide has been removed and loosened from the desiccant filter. That filter with appropriate control of the valving  212  is then supplied with a cooling stream of gas. Within approximately 2½ to 3 hours it is found that the gas coming out of the top of the desiccant filter no longer contains any carbon dioxide. After that has occurred, the desiccant filter is ready to be put back into use for performing another cycle of removing carbon dioxide and water from the natural gas. 
     The sequencing for operating the valves has been hereinbefore set forth in connection with Charts II and III. When it is found that it is desired to shut the system down either for lack of demand for fuel or for example for overnight when there may be no demand, the desiccant filter which is in a cycle of being heated is typically very rich in carbon dioxide that is still present even though it is not contained in the desiccant within a desiccant filter. Upon cooling, this carbon dioxide which is present within the desiccant filter is reabsorbed back into the desiccant in the desiccant filter making it ineffective when placed back into service. In connection with the apparatus and the method of the present invention, this problem is overcome by running the desiccant stacks at a higher pressure, as, for example, 135 to 145 psi, which is the pressure available after the first stage  26  of the compressor  27 . In addition, the desiccant filters that were being regenerated by cooling and heating are emptied of gas by continuing running of the natural gas engine  51  until the pressure in these desiccant filters has dropped to 20 psi or less. By doing so it has been found that it is possible to clear substantially all of the carbon dioxide out of both of the regenerating desiccant filters so that the apparatus can be restarted successfully with all of the desiccant towers functioning in the appropriate manner. 
     In connection with the present invention it is desirable to control the shutting down of the apparatus to a selected time at which one of the desiccant filter has just been heated. 
     In connection with the desiccant filters forming the molecular sieve bed  31  it has been found that natural gas flowing at about approximately 250 cubic feet per minute can be accommodated. Typically approximately 0.7% carbon dioxide is in the gas which content can be removed by one of the desiccant filters becoming saturated in approximately four hours of operation. This flow of gas corresponds to the flow of gas supplied to the internal combustion engine  51  which consumes approximately 30 cubic feet per minute representing the heavy hydrocarbons in the natural gas. 
     The use of three desiccant filters is necessary because it takes two full cycles to regenerate a desiccant filter as by first heating and then cooling, with the heating and cooling taking approximately 5½ to approximately 6½ hours to completely regenerate. This makes it possible to utilize three desiccant filters in three cycles to achieve continuous operation in four hour increments. Another constraint on the apparatus is that the regenerative flow is the only flow that the internal combustion engine can consume. Thus the nitrogen, the carbon dioxide, the water and the oil from the compressor which are all unwanted elements embedded in the natural gas stream are supplied to the internal combustion engine and burned therein and then exhausted to the atmosphere. 
     It has been found in conjunction with operation of the apparatus it has been possible to cycle the desiccant filters without monitoring the carbon dioxide by conducting the cycling at timed intervals. 
     With the valve sequencing disclosed herein, the entire apparatus can continue working without stopping the flow of gas to the engine  51  or stopping flow between the first stage and the second stage of the compressor  27  all under the control of the programmed computer  108 . Thus in connection with the valving utilized, it is important to appreciate that fuel must be continuously supplied to the internal combustion engine  51  during operation and that there must be a continuous gas path from the first stage of the compressor to the second stage of the compressor. In the valving sequence, it is necessary to take one stack out of the service that it was in, for example a cooling stack can have the gas passing therethrough supplied to the engine. Another stack is brought into parallel and put it in the filtering cycle and then taking the stack that was in a filtering cycle out of service and place it into the heating regeneration cycle. Thus in the valve sequencing, it is always desirable to feed gas to the engine and to safely put a second stack on line into the compressor and then to take the first stack off line from the compressor. Thereafter the stack that was filtering is placed in the heating cycle to complete a sequence. 
     From the foregoing it can be seen that there has been provided an apparatus and method for liquefying natural gas for vehicular use. The apparatus is an on-site semi-portable liquefier which enables liquid natural gas to become a viable, economical, environmentally clean transportation fuel. Utilizing such fuel it has been found that current design liquid methane gas powered vehicles achieve reduction of 87% of reactive hydrocarbons and 82% of carbon monoxide and virtually eliminate particulate pollution over comparable gasoline and diesel powered vehicles. The method of liquefaction incurs no boil-off or atmospheric increases to the greenhouse effect. Because natural gas has the highest hydrogen-to-carbon ratio of all fuels, other than hydrogen itself, natural gas should remain the dominant alternative transportation fuel until the use of pure hydrogen occurs. The tank of a vehicle can be filled from the apparatus without the use of a cryogenic pump because vapor from the tank is withdrawn by the compressor.