Abstract:
A system and method for liquefying gas are provided. The method includes combining an input gas and a first recycle stream to form a first blended stream, and producing a cooled first blended stream and a heated second blended stream by passing the first blended stream and a second blended stream through a heat exchanger. The method also includes producing a mixture of gas and liquefied gas from the cooled first blended stream using a first expander, and producing the liquefied gas at an outlet. The method further includes producing a second recycle stream from the gas from the output of the first expander using a first compressor, and combining the second recycle stream and a third recycle stream to form the second blended stream. The method still further includes producing the first recycle stream from the heated second blended stream using a second compressor, and producing the third recycle stream from the first recycle stream using a second expander.

Description:
[0001]    The present application is related to U.S. Provisional Patent Application No. 61/932,596, which was filed on Jan. 28, 2014, and is entitled “Modified Claude Process for Producing Liquefied Natural Gas (LNG).” Provisional Patent No. 61/932,596 is hereby incorporated by reference into the present application as if fully set forth herein. The present application hereby claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent No. 61/932,596. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure is directed, in general, to energy technologies, and more specifically, to a modified Claude process for producing liquefied gas. 
       BACKGROUND 
       [0003]    Requirements often arise for liquefying gas. A gaseous substance such as natural gas may be easier or more economical to transport as liquefied natural gas (“LNG”). Liquefied air, nitrogen, oxygen, hydrogen, helium, and other liquefied gases are widely used in industry. 
         [0004]    Producing liquefied gas requires at least the following steps: removing sensible heat and removing latent heat. 
         [0005]    Numerous technologies have been developed to produce liquefied gas. Classical processes employ a cascade of single-component refrigerators, typically using propane, ethylene, and methane as the working fluids. More recently, mixed-refrigerant systems have been developed that use a blend of nitrogen, methane, ethylene, propane, and butane. One of the oldest processes for producing liquefied gas is based on the Claude process, which employs a combination of compressors and expanders to chill the gas. In the final step, the high-pressure chilled gas passes through a Joule-Thomson throttling valve where a portion of the gas liquefies. An exemplary system employing the Claude process is shown in  FIG. 1 . Pretreated natural gas passes through a heat exchanger to reduce its temperature, and then the cooled natural gas passes through a Joule-Thomson valve to convert some of the cooled natural gas to liquefied natural gas. Other elements of the system of  FIG. 1  operate to cool a working fluid to a temperature below the temperature of the pretreated natural gas and to pass the cooled working fluid through the heat exchanger, which transfers heat from the natural gas to the working fluid to cool the natural gas. Although the system of  FIG. 1  is shown liquefying natural gas, it will be understood that the system could be used to liquefy other gases. Further, it is understood that the working fluid being compressed and expanded can be the same as the pretreated feed gas, or it can be different. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    According to a first embodiment of the present disclosure, a system for liquefying gas includes a first combiner that forms a first blended stream from a combination of an input gas and a first portion of a first recycle stream. The system also includes a heat exchanger that transfers heat from the first blended stream to a second blended stream, producing a cooled first blended stream and a heated second blended stream. The system further includes a first expander that produces a mixture of gas and liquefied gas from the cooled first blended stream, and an outlet to produce the liquefied gas. The system still further includes a first compressor that produces a second recycle stream from the gas from the mixture of gas and liquefied gas. The system also includes a second combiner that forms the second blended stream from the second recycle stream and a third recycle stream and provides the second blended stream to the heat exchanger. The system further includes a second compressor that receives the heated second blended stream from the heat exchanger and produces the first recycle stream and provides the first portion of the first recycle stream to the first combiner. The system still further includes a second expander that receives a second portion of the first recycle stream and produces the third recycle stream and provides the third recycle stream to the second combiner. 
         [0007]    According to a second embodiment of the present disclosure, a method of liquefying gas includes combining an input gas and a first portion of a first recycle stream to form a first blended stream. The method also includes producing a cooled first blended stream and a heated second blended stream by passing the first blended stream and a second blended stream through a heat exchanger adapted to transfer heat from the first blended stream to the second blended stream. The method further includes producing a mixture of gas and liquefied gas from the cooled first blended stream using a first expander, and producing the liquefied gas at an outlet. The method still further includes producing a second recycle stream from the gas from the mixture of gas and liquefied gas using a first compressor, and combining the second recycle stream and a third recycle stream to form the second blended stream. The method also includes producing the first portion and a second portion of the first recycle stream from the heated second blended stream using a second compressor, and producing the third recycle stream from the second portion of the first recycle stream using a second expander. 
         [0008]    According to a third embodiment of the present disclosure, a gas liquefying system includes a heat exchanger and a gerotor expander. The heat exchanger receives an input gas and produces a cooled gas. The gerotor expander receives the cooled gas and to produce a mixture of gas and liquefied gas. 
         [0009]    Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0011]      FIG. 1  shows an exemplary system employing the Claude process to liquefy natural gas; 
           [0012]      FIG. 2  shows a first system for liquefying gas according to this disclosure; 
           [0013]      FIG. 3  shows a second system for liquefying gas according to this disclosure; 
           [0014]      FIG. 4  shows appropriate gas pressure as a function of gas temperature in a system for liquefying methane according to this disclosure; 
           [0015]      FIG. 5  shows energy consumption for a system for liquefying methane according to this disclosure; 
           [0016]      FIG. 6  shows a calculation of a theoretical minimum energy consumption for producing LNG assuming the gas is pure methane; and 
           [0017]      FIG. 7  shows typical energy consumption for various processes that produce LNG. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    It should be understood at the outset that, although example embodiments are illustrated below, the present invention may be implemented using any number of techniques, whether currently known or not. The present invention should in no way be limited to the example implementations, drawings, and techniques illustrated below. Additionally, the drawings are not necessarily drawn to scale. 
         [0019]      FIG. 2  shows a system  200  for liquefying natural gas according to this disclosure. The system  200  is similar to the Claude process, however an expander  202  replaces the Joule-Thomson valve. In both  FIG. 1  and  FIG. 2 , the liquefaction process is incomplete so the LNG discharge contains gas that must be recycled. The expander  202  allows a greater portion of the gas to liquefy and thereby improves efficiency of the system  200  over the system shown in  FIG. 1 . However, the expander  202  must be able to tolerate the presence of liquid. 
         [0020]    Dynamic expanders (e.g., centrifugal, axial) must operate at high tip speeds, which makes the equipment susceptible to erosion by liquids. In contrast, positive displacement expanders (e.g., reciprocating, screw, sliding vane, and gerotors) operate at lower speeds and can more readily tolerate liquids. However, reciprocating expanders have valves that can interfere with the discharge of liquids, which risks damaging the machine if the reciprocating piston attempts to compress retained liquid. Both screw and sliding-vane expanders require a lubricant fluid for lubricating and sealing purposes, which becomes mixed with the liquefied gas produced by the expander and must be removed in a subsequent, additional step. Because the expander is very cold, it is difficult to find a suitable lubricant that remains liquid at cryogenic temperatures. In contrast, gerotors do not have these operational shortcomings. Generally, the competing expanders are approximately 50 to 80% efficient, depending on scale. In contrast, gerotor expanders are 80 to 90% efficient, which is required to make gas liquefying systems according to this disclosure economically attractive. 
         [0021]    StarRotor Corporation of College Station, Tex., has developed gerotor expanders that may be used in gas liquefying systems according to this disclosure. Certain embodiments of this disclosure may avail from other StarRotor technologies disclosed, for example, in U.S. Pat. No. 7,186,101, U.S. Pat. No. 7,695,260, and United States Printed Publication No. 2011/0200476—the contents of all three disclosures are incorporated herein by reference. 
         [0022]    By optimizing the process configuration, the modified Claude process is extremely efficient and approaches the theoretical minimum energy requirement to produce LNG. By inference, the process should be similarly efficient for other gases. Further advantages of the process are its simplicity and the fact that it requires no additional working fluids other than the gas being liquefied. 
         [0023]      FIG. 3  shows a system  300  for liquefying gas according to this disclosure. The system  300  illustrates the process of liquefying natural gas, however the system  300  may be engineered to liquefy any gas. In  FIG. 3 , input stream of natural gas  302  is supplied at pipeline pressure (approx. 5.3 MPa, or 780 psia) and a temperature of approx. 35° C. (or 95° F.). If the gas is not available at high pressures, an additional compression step is required to feed the gas at high pressure. 
         [0024]    The pipeline gas  302  is blended at combiner  304  with a first recycle stream  306  from a compressor section  308 . First blended stream  309  flows through a sensible heat exchanger  310  where it is pre-cooled to temperature T 2 . Then, the first blended stream  309  flows through expander E 1  where the pressure is lowered to approx. 0.1 MPa (1 atmosphere). Work is extracted in the expander E 1 , which rotates a shaft  312 . 
         [0025]    Expander E 1  cools the first blended stream  309  and liquefies a portion of it. The resulting LNG  315  is knocked out of the expander discharge in a container  316  and is produced at an outlet  314 . The natural gas that remains as vapor  318  is recycled and is fed to compressor C 1  where it is pressurized to pressure P and temperature T 1  to form second recycle stream  322 . As will be discussed in more detail below, compressor C 1  is powered by shaft  312 . 
         [0026]    The second recycle stream  322  is blended at combiner  324  with a third recycle stream  326  from the compressor section  308  to form a second blended stream  328 , which is passed through the sensible heat exchanger  310 . The temperature of the second blended stream  328  is colder than the first blended stream  309 , so the second blended stream  328  extracts heat from the first blended stream  309  and becomes warmer. The second blended stream  328  emerges from the sensible heat exchanger  310  at approx. 25° C. and is sent to the compressor section  308 . 
         [0027]    The heat exchanger of  FIG. 1  and  FIG. 2  includes the compressed working fluid from the booster compressor along part of the heat exchanger, but not the entire length. As a result, the heat duty varies in different sections of the heat exchanger, which makes it impossible to maintain a substantially uniform temperature difference along the length. In some portions of the heat exchanger, the temperature difference will be large, which results in thermodynamic irreversibilities that reduce energy efficiency. 
         [0028]    In contrast, the heat exchanger  310  of the system  300  includes only the first blended stream  309  and the second blended stream  328 , both of which flow along the entire length. Further, the mass flow rate of the two streams is substantially the same. This allows the heat exchanger  310  to maintain a substantially constant temperature differential along its length between the first blended stream  309  and the second blended stream  328 . This uniform and small temperature difference reduces thermodynamic irreversibilities, which improves energy efficiency. 
         [0029]    In the compressor section  308 , the second blended stream  328  is compressed in a series of compressors with inter-stage cooling. As will be discussed in more detail below, the compressors of the compressor section  308  are powered by shaft  312 . In  FIG. 3 , five compression stages are shown; however, more or fewer stages can be used. More stages will improve efficiency because the compression approaches isothermal conditions. Fewer stages will reduce capital costs. 
         [0030]    Compressed gas  330  exiting the compressor section  308  is split into two portions. A first portion, the first recycle stream  306 , is blended with the incoming pipeline gas  302 , as described above. In some configurations, the pipeline gas  302  has substantially the same temperature and pressure as the compressed gas  330  (and the first recycle stream  306 ). However, in other configurations, the temperature and/or pressure may be different. The second portion of the compressed gas  330 , stream  332 , is coupled to expander E 2 . Work is extracted in the expander E 2 , which rotates the shaft  312 . Discharge from the expander E 2 , the third recycle stream  326 , is blended at the combiner  324  with the second recycle stream  322  from the compressor C 1 . 
         [0031]    As mentioned above, a single drive shaft  312  may be common to all expanders and compressors of the system  300 . Work extracted from the expanders E 1  and E 2  contributes to rotating the shaft  312 . In turn, the shaft  312  operates to rotate the compressors C 1  through C 6 . A motor  334  is also coupled to the shaft  312  to compensate for inefficiencies in components of the system  300  and to provide the work done by the system  300  in liquefying the pipeline gas  302 . 
         [0032]    The following equations further describe the design of the system  300 , 
         [0000]    
       
         
           
             Q 
             = 
             
               
                 
                   ( 
                   
                     1 
                     + 
                     m 
                   
                   ) 
                 
                  
                 
                   ( 
                   
                     
                       H 
                       
                         h 
                         , 
                         o 
                       
                     
                     - 
                     
                       H 
                       
                         h 
                         , 
                         j 
                       
                     
                   
                   ) 
                 
               
               = 
               
                 - 
                 
                   n 
                    
                   
                     ( 
                     
                       
                         H 
                         
                           l 
                           , 
                           o 
                         
                       
                       - 
                       
                         H 
                         
                           l 
                           , 
                           i 
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
       
         
           
             n 
             = 
             
               
                 ( 
                 
                   1 
                   + 
                   m 
                 
                 ) 
               
                
               
                 
                   ( 
                   
                     
                       H 
                       
                         h 
                         , 
                         o 
                       
                     
                     - 
                     
                       H 
                       
                         h 
                         , 
                         i 
                       
                     
                   
                   ) 
                 
                 
                   ( 
                   
                     
                       H 
                       
                         l 
                         , 
                         i 
                       
                     
                     - 
                     
                       H 
                       
                         l 
                         , 
                         o 
                       
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where Q=is the heat duty of heat exchanger  310 , m=mass of the compressed gas in the recycle stream  306 , n=mass of the gas in the second blended stream  328 , H h,i =enthalpy of the first blended stream  309  entering the heat exchanger  310 , H h,o =enthalpy of the first blended stream  309  exiting the heat exchanger  310 , H l,i =enthalpy of the second blended stream  328  entering the heat exchanger  310 , and H l,o =enthalpy of the second blended stream  328  exiting the heat exchanger  310 . Further, 
         [0000]    
       
         
           
             ϕ 
             = 
             
               
                 Vapor 
                  
                 
                     
                 
                  
                 fraction 
               
               = 
               
                 m 
                 
                   1 
                   + 
                   m 
                 
               
             
           
         
       
       
         
           
             m 
             = 
             
               
                 ϕ 
                 
                   1 
                   - 
                   ϕ 
                 
               
               . 
             
           
         
       
     
         [0033]    To increase efficiency, the pressure of the compressed gas  330  exiting the compressor section  308  is selected so that its entropy is substantially the same as the entropy of the vapors  318  in equilibrium with the LNG  315 . By following this rule and assuming isoentropic compression/expansion, the gas temperatures discharged from the compressor C 1  and the expander E 2  will be identical, which eliminates inefficiencies in the combiner  324  associated with mixing two gases of differing temperatures.  FIG. 4  shows the appropriate theoretical gas pressure discharged from the compressor section  308  as a function of gas temperature discharged from the compressor section  308 . 
         [0034]    It should be emphasized that the pressures and temperatures used in the above illustration are examples only. The LNG  315  may be stored at pressures other than 0.1 MPa and the pipeline gas  302  may be supplied at pressures other than 5.3 MPa and temperatures other than 35° C. To achieve optimal performance, it may be necessary to pre-compress or pre-expand the pipeline gas  302  to match the pressure exiting the compressor section  308  in the system  300 . 
         [0035]      FIG. 5  shows the energy consumption for the system  300  as a function of the heat exchanger inlet temperature T 1  and the efficiency of the all compressors and expanders in the system, which are assumed to be the same for simplicity. Energy consumption is minimal at the “sweet spot” (T 1 =−90 to −95° C.; P=0.7 MPa.) At the sweet spot, the energy consumption is 0.18 to 0.30 kWh/kg at compressor/expander efficiencies of 95 to 80%, respectively. It should be noted that the energy consumption reported in  FIG. 5  is idealized and does not include the impact of compressor and expander inefficiencies on mass flows. 
         [0036]    The theoretical minimum energy consumption (0.147 kWh/kg) for any system producing LNG is calculated in  FIG. 6 . This value is similar to a literature value (0.133 kWh/kg), which used a slightly different reference condition. See C. W. Remeljej, A. F. A. Hoadley, An exergy analysis of small-scale liquefied natural gas (LNG) liquefaction processes,  Energy,  31, 2005-2019 (2006) (“Remeljej”). 
         [0037]    Assuming the compressors and expanders have efficiencies of about 90%, the energy consumption is 0.22 kWh/kg LNG. Assuming electricity sells for $0.036/kWh, this is equivalent to $0.0203/gal diesel equivalent—roughly two pennies per gallon of diesel equivalent. A world-scale LNG plant produces about 5 million tonne per year. See Remeljej. Reducing energy consumption from 0.35 to 0.22 kWh/kg saves $23.4 million per year at $0.036/kWh. 
         [0038]    The above discussion emphasizes the energy efficiency at large scale (90% compressor efficiency). Even at small scale, gerotor compressors are very efficient (˜85% efficiency); thus the system  300  will scale down for regional LNG plants that can service trucking routes without transporting large quantities of LNG on the highways. The above discussion is repeated for a regional plant. 
         [0039]    A regional LNG plant will have compressors and expanders with efficiencies of about 85%. At this efficiency level, the energy consumption is 0.26 kWh/kg LNG. This energy consumption can be expressed as a percentage of the energy content of the natural gas 
         [0000]    
       
         
           
             
               Energy 
                
               
                   
               
                
               Fraction 
             
             = 
             
               
                 0.26 
                  
                 
                   
                     
                       kW 
                       · 
                       h 
                     
                      
                     
                         
                     
                      
                     work 
                   
                   
                     kg 
                      
                     
                         
                     
                      
                     LNG 
                   
                 
                 × 
                 
                   
                     3600 
                      
                     
                         
                     
                      
                     s 
                   
                   h 
                 
                 × 
                 
                   
                     kJ 
                      
                     
                         
                     
                      
                     work 
                   
                   
                     kW 
                     · 
                     s 
                   
                 
                 × 
                 
                   kg 
                   
                     55.5 
                      
                     
                         
                     
                      
                     MJ 
                      
                     
                         
                     
                      
                     heat 
                   
                 
                 × 
                 
                   
                     MJ 
                      
                     
                         
                     
                      
                     heat 
                   
                   
                     1000 
                      
                     
                         
                     
                      
                     kJ 
                      
                     
                         
                     
                      
                     heat 
                   
                 
               
               = 
               
                 0.0169 
                  
                 
                   
                     kJ 
                      
                     
                         
                     
                      
                     work 
                   
                   
                     kJ 
                      
                     
                         
                     
                      
                     heat 
                   
                 
               
             
           
         
       
     
         [0040]    If an engine is 40% efficient, 4.2% of the natural gas must be burned to liquefy the remaining 95.8%. 
         [0041]    When expressed on a volume basis in traditional units, the energy consumption is 0.417 kWh/gal LNG. 
         [0000]    
       
         
           
             Work 
             = 
             
               
                 0.26 
                  
                 
                   kWh 
                   
                     kg 
                      
                     
                         
                     
                      
                     LNG 
                   
                 
                 × 
                 
                   
                     423.97 
                      
                     
                         
                     
                      
                     kg 
                   
                   
                     m 
                     3 
                   
                 
                 × 
                 
                   
                     m 
                     3 
                   
                   
                     1000 
                      
                     
                         
                     
                      
                     L 
                   
                 
                 × 
                 
                   
                     3.78 
                      
                     
                         
                     
                      
                     L 
                   
                   gal 
                 
               
               = 
               
                 0.417 
                  
                 
                   kWh 
                   
                     gal 
                      
                     
                         
                     
                      
                     LNG 
                   
                 
               
             
           
         
       
     
         [0042]    Retail electricity sells for about $0.12/kWh [5]; thus, it takes $0.05 to liquefy a gallon of natural gas—roughly one nickel per gallon of LNG. Expressed per gallon of diesel equivalent, the energy cost of liquefying LNG is 
         [0000]    
       
         
           
             Work 
             = 
             
               
                 0.26 
                  
                 
                   kWh 
                   
                     kg 
                      
                     
                         
                     
                      
                     LNG 
                   
                 
                 × 
                 
                   
                     kg 
                      
                     
                         
                     
                      
                     LNG 
                   
                   
                     55.5 
                      
                     
                         
                     
                      
                     MJ 
                   
                 
                 × 
                 
                   
                     44.8 
                      
                     
                         
                     
                      
                     MJ 
                   
                   
                     kg 
                      
                     
                         
                     
                      
                     diesel 
                   
                 
                 × 
                 
                   
                     0.840 
                      
                     
                         
                     
                      
                     kg 
                   
                   L 
                 
                 × 
                 
                   
                     3.78 
                      
                     
                         
                     
                      
                     L 
                   
                   gal 
                 
               
               = 
               
                 0.666 
                  
                 
                   kWh 
                   
                     gal 
                      
                     
                         
                     
                      
                     LNG 
                   
                 
               
             
           
         
       
     
         [0043]    Again, assuming electricity sells for $0.12/kWh, this is equivalent to $0.08/gal diesel equivalent—roughly a dime per gallon of diesel equivalent. 
         [0044]    As shown in  FIG. 7 , small simple LNG systems require 2 to 3 times more energy than the system  300 . On a diesel equivalent basis, the energy cost for their system is about $0.16 to $0.24/gallon of diesel equivalent. 
         [0045]    Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
         [0046]    To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.