Abstract:
A method for liquefying gas involving pre-treating the gas stream in a pre-treater to remove impurities, and then passing the gas stream through a first flow path of a first heat exchanger to lower a temperature of the gas stream. The gas stream is then passed through the gas expansion turbine to lower a pressure of the gas stream and further decrease the temperature of the gas stream. The gas stream is then passed into a primary separator to separate the gas stream into a liquid stream and a cold gas stream. The liquid stream is collected. Selected quantities of the cold gas stream are passed through a second flow path of the first heat exchanger whereby a heat exchange takes place to cool the gas stream flowing through the first flow path to maintain the temperature of the gas stream entering the gas expansion turbine at a temperature which promotes the production of liquids.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method to liquefy natural gas from a transmission gas pipeline. The described process was developed to efficiently produce liquid natural gas (LNG). 
       BACKGROUND OF THE INVENTION 
       [0002]    LNG is a natural gas that has been cooled to a cryogenic condition to condense methane, the natural gas main component. A temperature of approximately −161 C is required to produce and keep natural gas in a liquid state at standard atmospheric pressure. Liquefaction reduces the volume by approximately 600 times thus making it more economical to transport over great distances versus traditional pipelines. At present, LNG is primarily transported across continents thus making it available throughout the world. LNG is also produced in small scale liquefaction plants to supply peak saving demands, as well as to make available natural gas to regions that need it but where it is not economical or technically feasible to build pipelines. 
         [0003]    There are differences in liquefaction selection processes for large versus small LNG plants. For large plants, the main criteria is minimization of capital cost whereas the minimization of energy consumption is left as a second objective. These two objectives can also go together; thus an optimization of the efficiency of the plant may involve a reduction in the investment of the equipment. On the other hand, a higher efficiency can result in an increase in LNG production, so the efficiency factor has a significant impact on the plant economics. In small to medium LNG plants, it is not the efficiency, but other factors such as simplicity, modularization, ease of maintenance, operation and installation that have an higher criteria when selecting a liquefaction technology. The direct consequence of these different selection criteria is that liquefaction technologies for small to medium scale applications are not the same as the ones that are used in large LNG plants. 
         [0004]    The two main groups of liquefaction technologies are the mixed refrigerant technologies and expansion based technologies. The mixed refrigerant technologies are “condensing type” processes, where the refrigerant used for the liquefaction makes use of its latent heat of vaporization to cool the natural gas. The expansion based technologies are processes where the refrigerant is always in gas phase and only makes use of its sensible heat to cool the natural gas. 
         [0005]    The following mixed refrigerant technologies are the most representative processes in the industry: PRICO (Poly Refrigerated Integrated Cycle Operation) is licensed by Black and Veatch and it consists of one cycle of mixed refrigerant (a mixture of methane, ethane, propane, butane, nitrogen and sometimes isopentane), the advantages claimed by the licensor are operating flexibility, modular design and reduced refrigerant inventory. The AP-M (Air Products) is licensed by APCI, is a single mixed refrigerant that is vaporized at two different levels of pressure. The dual pressure cycle is more efficient than the single pressure cycle, resulting in smaller heat exchangers and compressor. The LiMuM (Linde Multistage Mixed Refrigerant) is licensed by Linde and consists of a spiral wound heat exchanger and one 3-stage single mixed refrigeration loop for the pre-cooling, liquefaction and sub-cooling of the natural gas. This process allows for high capacity throughput. PCMR (Pre-cooled Mixed Refrigerant) is licensed by Kryopak and consists of a pre-cooling stage (ammonia or propane cycle) followed by a single mixed refrigerant cycle, where the mixed refrigerant is a mixture of nitrogen, methane, ethane, propane and butanes, this process is used primarily in small plants. OSMR (Optimized Single Mixed Refrigerant) is licensed by LNG Limited, the process is a single mixed refrigerant process complemented with a standard package ammonia absorption process. The utilization of an ammonia process improves the efficiency of the process and an increase in LNG output compared to traditional single mixed refrigerant processes. In all of the above mixed refrigerant technologies, the main differences between them are the composition of the mixed refrigerant (although the refrigerants are the same i.e.; nitrogen, methane, ethane, etc.), the metallurgy of the heat exchangers, the orientation of the equipment and the operations set points. In all the mixed refrigerants processes, the objective of innovation is to increase efficiency, reducing capital and operating costs. 
         [0006]    The expansion based technologies have various processes based on the use of nitrogen as a refrigerant to liquefy natural gas, the N 2  expansion cycle. Some of these processes use a single cycle, others use a dual expansion cycle and in other cases a pre-cooling cycle is added to improve efficiency. Several licensors i.e., APCI, Hamworthy, BHP Petroleum Pty, Mustang Engineering and Kanfa Oregon offer the N 2  expansion cycles processes, and they differ by proprietary process arrangement. In all these processes, the cooling is provided by an external refrigeration plant using nitrogen expanders. The Niche LNG process is licensed by CB&amp;I Lummus, consists of two cycles: one cycle uses methane as a refrigerant and the other uses nitrogen. The methane provides cooling at moderate and warm levels while the nitrogen cycle provides refrigeration at the lowest temperature level. The OCX process is licensed by Mustang Engineering and is based on the use of the inlet gas as a refrigerant in an open refrigerant cycle with turbo-expanders, there are variations such as OCX-R which adds a closed loop propane refrigerant to the OCX process and OCX-Angle which incorporates LPG recovery. 
         [0007]    As demonstrated, presently there are many variations and processes to liquefy LNG. All of the processes operate based on the expansion of low boiling fluids be it through expanders or JT valves, be it closed or open cycle, the difference between them is in the process efficiencies which result in lower capital and operating costs per unit of LNG produced. 
         [0008]    What is required is an alternative method to liquefy gas, such as LNG. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect, there is provided a method for liquefying gas where a gas stream is passed through a gas expansion turbine. The method involves pre-treating the gas stream in a pre-treater to remove impurities, and then passing the gas stream through a first flow path of a first heat exchanger to lower a temperature of the gas stream. The gas stream is then passed through the gas expansion turbine to lower a pressure of the gas stream and further decrease the temperature of the gas stream. The gas stream is then passed into a primary separator to separate the gas stream into a liquid stream and a cold gas stream. The liquid stream is collected. Selected quantities of the cold gas stream are passed through a second flow path of the first heat exchanger whereby a heat exchange takes place to cool the gas stream flowing through the first flow path to maintain the temperature of the gas stream entering the gas expansion turbine at a temperature which promotes the production of liquids. 
         [0010]    The method will hereinafter, as applied to the natural gas. The impurities removed are carbon dioxide and water. The liquids collected are natural gas liquids. 
         [0011]    Although beneficial results may be obtained through the use of the method, as described above, greater efficiencies can be achieved through the use of a recycle stream. The recycle stream already has impurities removed. This involves a step of compressing the cold gas stream in a compressor after the cold gas stream has passed through the first heat exchanger to create a recycled gas stream and directing the recycled gas stream into the gas stream downstream of the pre-treater and upstream of the first heat exchanger. 
         [0012]    Passing the recycled gas stream through the compressor will unavoidably raise the temperature of the recycled gas stream. It is, therefore, preferred that a step be included of passing the recycled gas stream through a first flow path of a second heat exchanger downstream of the compressor to lower the temperature of the recycled gas stream prior to the recycled gas stream being directed into the gas stream. 
         [0013]    In accordance with the teachings of this method, a steady state will be reached in which a ratio of the recycled gas stream entering the gas stream is maintained constant. 
         [0014]    In a variation of the method, where the liquids one wishes to collect are Liquid Natural Gas (LNG), a further step is included of mixing a slip stream of liquid natural gas (LNG) drawn from the primary separator into the gas stream via a mixer positioned downstream of the first heat exchanger and upstream of the gas expansion turbine. 
         [0015]    In another variation of the method, a further step may be taken of passing the gas stream through a preliminary separator positioned downstream of the mixer and upstream of the gas expansion turbine to separate natural gas liquids (NGLs) from the gas stream, collecting the NGLs and directing the gas stream to the gas expansion turbine. 
         [0016]    An advantage of the above method is that it can operate without external power inputs, resulting in substantial savings in both capital and operating costs. The above described method was developed with a view to collecting natural gas liquids and liquefying natural gas to form Liquid Natural Gas (LNG). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein: 
           [0018]      FIG. 1  is a schematic diagram of a facility equipped with a gas pre-treatment, an heat exchanger, an expander and a compressor to produce LNG. 
           [0019]      FIG. 2  is a schematic diagram of a facility equipped with an alternate cooling medium for the compression of the recycled vapour fraction. 
           [0020]      FIG. 3  is a schematic diagram of a facility equipped with the ability to recover natural gas liquids (NGLs). 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    The method will now be described with reference to  FIG. 1 . 
         [0022]    As set forth above, this method was developed with a view to liquefying natural gas to form Liquid Natural Gas (LNG). The description of application of the method to LNG should, therefore, be considered as an example. 
         [0023]    Referring to  FIG. 1 , a pressurized pipeline natural gas stream  1  provides natural gas to users through line  29 , valve  30  to flow distribution  37 . A natural gas stream  2  is routed through flow control valve  3 . The controlled flow enters the gas pre-treatment unit  5  through line  4 . Pre-treatment is to remove contaminants and may not be required if the gas used is of sufficient quality. The pre-treated gas exits through line  6  and is mixed with recycled gas stream  25  through valve  26 . The mixed gas stream  7  enters heat exchanger  8  where it is pre-cooled. The pressurized pre-cooled gas stream  9  enters expander  10  where the pressure is dropped resulting in a substantial temperature drop. The nearly isentropic expansion also produces torque and therefore shaft power that is converted into electricity through generator  11 . The expanded gas stream  12  enters LNG receiver  13  where the liquid and vapour fractions are separated. The vapour stream  17  is routed through heat exchanger  8  to pre-cool inlet gas stream  7 . The now warmed gas stream  18  enters compressor  20  through line  19  for re-compression. The compressor  20  shaft power is provided by a gas engine  22  which receives its fuel from gas line  21 . The compressed recycled gas stream  23  is cooled in heat exchanger  24  before mixing it with inlet feed gas stream  6  through line  25 . To prevent a buildup of nitrogen in the recycle gas stream  25 , a bleeding gas stream  27  is routed to gas transmission line  29  through valve  28 . The cooling of compressed recycled gas stream  23  is provided by a once through heat exchange from gas transmission line  29 . The required gas coolant is routed through valve  31  and line  32  into heat exchanger  24  and the once through flow is returned to gas transmission line  29  through line  34  and valve  33 . The LNG receiver  13  accumulates the LNG produced. LNG exits receiver  13  through stream  14  to supply LNG product pump  15 , where it is pumped to storage through line  16 . 
         [0024]    A main feature of this invention is the simplicity of the process which eliminates the use of external refrigeration systems. Another feature of the invention is the flexibility of the process to meet various operating conditions since the ratio of LNG production is proportional to the cold vapour stream generated and recycled. The invention also provides for a significant savings in energy when compared to other processes since it uses its recycled vapour stream as the coolant medium, the process produces its own refrigeration stream. The proposed invention can be used in any LNG production plant size. 
         [0025]    Referring to  FIG. 2 , the main difference from  FIG. 1  is in the heat exchanger to cool recycle stream  23 . In  FIG. 2 , the heat exchanger  50  is an air cooling heat exchanger where ambient air is used to cool stream  23 . This process orientation provides an alternative method to produce LNG at albeit less efficient than when using heat exchanger  24  as shown in  FIG. 1 . A pressurized pipeline natural gas stream  1  provides natural gas to users through line  29 , valve  30  to flow distribution  37 . A natural gas stream  2  is routed through flow control valve  3 , and enters the gas pre-treatment unit  5  through line  4 . The pre-treated gas exits through line  6  and is mixed with recycle gas stream  25  through valve  26 . The mixed gas stream  7  enters heat exchanger  8  where it is pre-cooled. The pressurized pre-cooled gas stream  9  enters expander  10  where the pressure is dropped resulting in a substantial temperature drop. The nearly isentropic expansion also produces torque and therefore shaft power that is converted into electricity through generator  11 . The expanded gas stream  12  enters LNG receiver  13  where the liquid and vapour fractions are separated. The vapour stream  17  is routed through heat exchanger  8  to pre-cool inlet gas stream  7 . The now warmed gas stream  18  enters compressor  20  through line  19  for re-compression. The compressor  20  shaft power is provided by a gas engine  22  which receives its fuel from gas line  21 . The compressed recycled gas stream  23  is cooled in heat exchanger  51  before mixing it with inlet feed gas stream  6  through line  25 . To prevent a buildup of nitrogen in the recycle gas stream  25 , a bleeding gas stream  27  is routed to gas transmission line  29  through valve  28 . The cooling of compressed recycled gas stream  23  is provided by an air cooling heat exchanger  51 . The LNG receiver  13  accumulates the LNG produced. LNG exits receiver  13  through stream  14  to supply LNG product pump  15 , where it is pumped to storage through line  16 . 
         [0026]    Referring to  FIG. 3 , the main difference from  FIGS. 1 and 2  is the recovery of natural gas liquids before expansion. This is achieved by circulating a portion of the generated liquid natural gas (LNG), stream  42  and mixing it in  43  with the pre-cooled gas stream  51  to meet the temperature required to condense the heavier fractions present in the natural gas stream such as; butane, propane and ethane. This process orientation provides an alternative method to produce both LNG and NGLs. A pressurized pipeline natural gas stream  1  provides natural gas to users through line  29 , valve  30  to gas flow transmission line  37 . A natural gas stream  2  is routed through flow control valve  3 , and enters the gas pre-treatment unit  5  through line  4 . The pre-treated gas exits through line  6  and is mixed with recycle gas stream  25  through valve  26 , the mixed gas stream  7  enters heat exchanger  8  where it is pre-cooled. The pressurized pre-cooled gas stream  43  enters mixer  44 , a LNG stream  42  is also added to mixer  44 . The addition of LNG stream to mixer  44  is controlled by temperature control valve  41 . The mixed stream  45 , enters separator  46  where the NGLs are separated and accumulated. The NGLs exit separator  46  through line  47  to NGL pump  49  and pumped to storage through line  50 . The pressurized, pre-cooled and de-liquified gas stream  9  enters expander  10  where the pressure is dropped resulting in a substantial temperature drop. The nearly isentropic expansion also produces torque and therefore shaft power that is converted into electricity through generator  11 . The expanded gas stream  12  enters LNG receiver  13  where the liquid and vapour fractions are separated. The vapour stream  17  is routed through heat exchanger  8  to pre-cool inlet gas stream  7 . The now warmed gas stream  18  enters compressor  20  through line  19  for re-compression. The compressor  20  shaft power is provided by a gas engine  22  which receives its fuel from gas line  21 . The compressed recycled gas stream  23  is cooled in heat exchanger  24  before mixing it with inlet feed gas stream  6  through line  25  and valve  26 . To prevent a buildup of nitrogen in the recycle gas stream  25 , a bleeding gas stream  27  is routed to gas transmission line  29  through valve  28 . 
         [0000]    The cooling of compressed recycled gas stream  23  is provided by a once through heat exchange from gas transmission line  29 . The required gas coolant is routed through valve  31  and line  32  into heat exchanger  24  and the once through flow is returned to gas transmission line  29  through line  34  and valve  33 .
 
The LNG receiver  13  accumulates the LNG produced. LNG exits receiver  13  through stream  14  to supply LNG product pump  15 , where it is pumped to storage through line  16 . A portion of the produced LNG is routed through line  38  to high pressure LNG pump  39 . The pressurized LNG liquid stream is controlled by temperature valve  41  to a pre-set temperature through temperature transmitter  47 . The controlled LNG stream  42  enters mixer  44  to cool and condense the desired natural gas liquids.
 
The proposed invention addresses both large and small plants in which process simplicity and ease of operation are the main components. The invention eliminates the need for refrigeration cycle plants and the use of proprietary mixed refrigerants. By simplifying the process, it reduces capital, maintenance, and operations costs.
 
In the preferred method, natural gas is first pre-cooled with produced cold vapor then expanded through a gas expander. The gas expander produces electricity. The expanded gas produces a vapour and a liquid stream. The vapour stream is recycled by first pre-cooling the feed gas to the expander and then recompressed, cooled and recycled. A portion of the produced LNG provides the cold energy required as a recycle stream to cool and liquefy the pre-treated natural gas stream to recover desired natural gas liquids. The proposed invention eliminates the practice and use of mixed refrigerant cycles resulting in lower capital and operating costs. The process is applicable to any LNG plant size.
 
       Variations  
       [0027]    It should be noted that the motive force for the compressor can be provided by an electric motor versus a gas driven engine as proposed. Moreover, the compressed vapour stream can be discharged into gas transmission line  29  rather than recycled as proposed. 
         [0028]    In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0029]    The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given a broad purposive interpretation consistent with the description as a whole.