Patent Publication Number: US-2022236004-A1

Title: Systems and Methods for Improving the Efficiency of Combined Cascade and Multicomponent Refrigeration Systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/872,317, filed Jul. 10, 2019, which is incorporated herein by reference. This application, U.S. Provisional Application No. 62/872,318, and U.S. Provisional Application No. 62/885,958, which are incorporated herein by reference, are commonly assigned to Bechtel Oil, Gas and Chemicals, Inc. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to systems and methods for improving the efficiency of combined cascade and multicomponent refrigeration systems. More particularly, the systems and methods improve the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate compression stages. 
     BACKGROUND 
     The natural gas liquefaction process takes natural gas, primarily comprised of methane at high pressure and passes it through consecutive refrigeration cycles. These refrigeration cycles can be single or multi-component. The present disclosure relates to multi-component refrigeration cycles. Two examples of such processes are presented in  FIGS. 1 and 2 . 
     In  FIG. 1 , a schematic diagram illustrates a conventional propane, pre-cooling system with mixed refrigerant liquefaction (hereinafter collectively the “C3MR system”). Feed Gas  102  enters the system and is mixed with boil-off gas from line  104  that is recompressed. The feed gas  102  is chilled by means of three consecutive heat exchangers ( 106 ,  108 ,  110 ). The heat exchangers chill the feed gas to a temperature of approximately 23° F. in line  111 . The chilled feed gas in line  111  is then distributed through two zones  112 ,  114  of a spiral-wound design heat exchanger, reaching a temperature of—about 262° F. in line  116 . 
     The propane pre-cooling system is comprised of a three-stage compressor  118 , three flash drums  120 ,  122 ,  124  and a chiller  126  to reject heat. Vapor propane is introduced into the first compression stage of compressor  118  and is compressed through three successive stages before being transferred through line  128  to the chiller  126 . The chiller  126 , typically an air cooler or cooling water exchanger, chills the compressed propane to a temperature of about 100° F. The outlet pressure in line  130  is equivalent to the pressure at which the refrigerant is liquid, which for propane is approximately 190 psia. The liquid propane in line  130  is flashed through an expansion valve  132  to a pressure of approximately 80 psia and a temperature of about 41° F. It is then introduced into the first heat exchanger  106  to pre-chill the feed gas  102 . The outlet of the first heat exchanger  106  consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the high stage of flash drum  124 . The vapor is recompressed in the compressor  118 . The liquid is flashed to a lower pressure through a second expansion valve  134  to a temperature of approximately 25° F. and 61 psia and is introduced into the second heat exchanger  108 . The outlet of the first heat exchanger  108  consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the middle stage flash drum  122 . The vapor is recompressed in the compressor  118 . The liquid is flashed to a lower pressure through a third expansion valve  136  to a temperature of approximately −35° F. and 18 psia and is introduced into the third heat exchanger  110 . The propane is completely vaporized in the third exchanger  110  and is distributed into a compressor suction drum  120 . 
     The mixed refrigerant system consists of a two-stage compressor  138 , a refrigerant chiller  140  for heat rejection, a flash drum  142  for separating the mixed refrigerant into liquid and vapor phases and the spiral wound heat exchanger with two zones,  112  and  114 . Vapor mixed refrigerant is introduced to the two-stage compressor  138 . The mixed refrigerant composition is variable and is designed to closely match the cooling curve of the feed gas  102 . In this example, the mixed refrigerant composition is described in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 Mole Fraction 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 H 2 O 
                 0.000 
               
               
                   
                 CO 2   
                 0.000 
               
               
                   
                 Nitrogen 
                 0.100 
               
               
                   
                 Methane 
                 0.400 
               
               
                   
                 Ethane 
                 0.350 
               
               
                   
                 Propane 
                 0.150 
               
               
                   
                 i-Butane 
                 0.000 
               
               
                   
                 n-Butane 
                 0.000 
               
               
                   
                 i-Pentane 
                 0.000 
               
               
                   
                 n-Pentane 
                 0.000 
               
               
                   
                 n-Hexane 
                 0.000 
               
               
                   
                 Ethylene 
                 0.000 
               
               
                   
                 Propene 
                 0.000 
               
               
                   
                   
               
            
           
         
       
     
     The mixed refrigerant is compressed to a pressure of approximately 601 psia and is then chilled to a temperature of 118° F. by the mixed refrigerant chiller  140 . The mixed refrigerant chiller  140  can typically be an air cooler or cooling water exchanger. The chilled mixed refrigerant in line  144  is introduced into the second and third heat exchangers  108 ,  110  and is subsequently chilled to about −30° F. At this state, the refrigerant consists of a liquid and vapor mixture in line  146 . In this example, the vapor molar fraction is approximately 43%. The refrigerant is separated in flash drum  142  into liquid and vapor refrigerants and is then inserted into the top and bottom sections of the spiral wound heat exchanger. The refrigerant is then collected at the bottom of the spiral wound exchanger  112  and sent back to the suction of the two-stage compressor  138 . 
     Liquefied gas in line  116  is flashed to atmospheric pressure via an expansion valve  148  as well as via the line losses in the transfer pipe and stored in a cryogenic liquefied natural gas (LNG) tank. Because the liquefied gas is subcooled, no vapor is generated. Boil-off gas from the LNG tank is recompressed to pipeline pressure via a boil-off gas compressor  150  and chiller  152 . The chiller is typically an air cooler or cooling water exchanger. 
     In  FIG. 2 , a schematic diagram illustrates a conventional multi-component, integrated, propane pre-cooling system with single mixed refrigerant liquefaction (hereinafter collectively the “IPSMR system”). Feed Gas  202  enters the system and is mixed with boil-off gas from line  204  that is recompressed. The feed gas  204  is chilled by means of three consecutive heat exchangers ( 206 ,  208 ,  210 ). The heat exchangers chill the feed gas to a temperature of approximately −35° F. The chilled feed gas is then distributed through a brazed aluminum heat exchanger  212 , reaching a temperature of about −265° F. at the outlet in line  216 . 
     The propane pre-cooling system is comprised of a three-stage compressor  218 , three flash drums  220 ,  222 ,  224  and a chiller  226  to reject heat. Propane in the vapor phase is introduced into the first compression stage of compressor  218  and is compressed through three successive stages before being transferred through line  228  to the chiller  226 . The chiller  226 , typically an air cooler or cooling water exchanger, chills the compressed propane to a temperature of about 100° F. The outlet pressure in line  230  is equivalent to the pressure at which the refrigerant is liquid, which for propane is approximately 190 psia. The liquid propane in line  230  is flashed through an expansion valve  232  to a pressure of approximately 80 psia and a temperature of about 41° F. It is then introduced into the first heat exchanger  206  to pre-cool the feed gas  202 . The outlet of the first heat exchanger  206  consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the high stage flash drum  224 . The vapor is recompressed in the compressor  218 . The liquid is flashed to a lower pressure through a second expansion valve  234  to a temperature of approximately 25° F. and 61 psia and is introduced into the second heat exchanger  208 . The outlet of the first heat exchanger  208  consists of a two-phase mixture of liquid and vapor propane. The mixture is flashed in the middle stage flash drum  222 . The vapor is recompressed in the compressor  218 . The liquid is flashed to a lower pressure through a third expansion valve  236  to a temperature of approximately −35° F. and 18 psia and is introduced into the third heat exchanger  210 . The propane is completely vaporized in the third exchanger  210  and is distributed into a compressor suction drum  220 . 
     The mixed refrigeration system consists of a brazed aluminum heat exchanger  212 , three flash drums  242 ,  244 ,  246 , a two-stage mixed refrigerant compressor  238 , a mixed refrigerant chiller  240 , and a mixed refrigerant pump  248 . Vapor mixed refrigerant  250  is introduced into the mixed refrigerant compressor  238 . The mixed refrigerant composition is variable and is designed to closely match the cooling curve of the feed gas  202 . In this example, the mixed refrigerant composition is described in Table 2 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Component 
                 Mole Fraction 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 H 2 O 
                 0.0000 
               
               
                   
                 CO 2   
                 0.0000 
               
               
                   
                 Nitrogen 
                 0.1115 
               
               
                   
                 Methane 
                 0.2903 
               
               
                   
                 Ethane 
                 0.4008 
               
               
                   
                 Propane 
                 0.0000 
               
               
                   
                 i-Butane 
                 0.0000 
               
               
                   
                 n-Butane 
                 0.1520 
               
               
                   
                 i-Pentane 
                 0.0453 
               
               
                   
                 n-Pentane 
                 0.0000 
               
               
                   
                 n-Hexane 
                 0.0000 
               
               
                   
                 Ethylene 
                 0.0000 
               
               
                   
                 Propene 
                 0.0000 
               
               
                   
                   
               
            
           
         
       
     
     The mixed refrigerant is compressed to a pressure of approximately 718 psia and is then chilled via the mixed refrigerant chiller  240  to a temperature of about 95° F. At this state, the mixed refrigerant is approximately 70% vapor in line  252 . The vapor and liquid mixed refrigerant is then transferred to a flash drum  246 . The vapor and liquid are transferred through various sections of the brazed aluminum heat exchanger  212  and flashed through three separate let-down valves  254 ,  256 ,  258 . The refrigerant is partially condensed in the brazed aluminum heat exchanger  212  and is then returned to flash drum  242 . Liquid from the flash drum  242  is transferred to the middle stage flash drum  244  via a pump  248 . The middle stage flash drum  244  operates at a pressure of approximately 196 psia and a temperature of about 95° F. Liquid from the flash drum  244  is transferred to the brazed aluminum heat exchanger  212  and recycled back to flash drum  242 . Vapor from the middle stage flash drum  244  is compressed in the mixed refrigerant compressor  238 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying drawings, in which like elements are referenced with like reference numbers, in which: 
         FIG. 1  is a schematic diagram illustrating a conventional C3MR system. 
         FIG. 2  is a schematic diagram illustrating a conventional IPSMR system. 
         FIG. 3  is a schematic diagram illustrating one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process. 
         FIG. 4  is a schematic diagram illustrating one embodiment of the present disclosure applied to a C3MR liquefied natural gas process. 
         FIG. 5  is a schematic diagram illustrating another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     The subject matter of the present disclosure is described with specificity, however, the description itself is not intended to limit the scope of the disclosure. The subject matter thus, might also be embodied in other ways, to include different structures, steps and/or combinations similar to and/or fewer than those described herein, in conjunction with other present or future technologies. Although the term “step” may be used herein to describe different elements of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless otherwise expressly limited by the description to a particular order. Other features and advantages of the disclosed embodiments will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within the scope of the disclosed embodiments. Further, the illustrated figures and dimensions described herein are only exemplary and are not intended to assert or imply any limitation with regard to the environment, architecture, design, or process in which different embodiments may be implemented. The pressures and temperatures described herein thus, illustrate exemplary advantages and/or parameters of the various embodiments. 
     In one embodiment, the present disclosure includes a system for chilling a feed gas, which comprises: i) a first heat exchanger enclosing a first portion of a feed gas line and a portion of a first chilled refrigerant line; ii) a first flash drum in fluid communication with the first chilled refrigerant line for receiving a two-phase refrigerant from the first heat exchanger, the first flash drum having a first vapor outlet line and a first liquid outlet line; iii) a second heat exchanger enclosing a second portion of the feed gas line and a portion of a second chilled refrigerant line; iv) a second flash drum in fluid communication with the second chilled refrigerant line for receiving a two-phase refrigerant from the second heat exchanger, the second flash drum having a second vapor outlet line and a second liquid outlet line; v) a third heat exchanger enclosing a third portion of the feed gas line and a portion of a third child refrigerant line; vi) a drum in fluid communication with the third chilled refrigerant line for receiving a vaporized refrigerant from the third heat exchanger, the drum having a drum vapor outlet line; vii) an ejector in fluid communication with the drum vapor outlet line, the first chilled refrigerant line, and a compressed refrigerant line; and vii) a compressor in fluid communication with the first vapor outlet line, the second vapor outlet line, and the compressed refrigerant line connected to a chiller for chilling a compressed refrigerant in the compressed refrigerant line 
     In another embodiment, the present disclosure includes a method for chilling a feed gas, which comprises: i) introducing a feed gas stream through a first heat exchanger, a second heat exchanger and a third heat exchanger; ii) chilling the feed gas stream in the first heat exchanger by circulating a first chilled refrigerant stream adjacent the feed gas stream in the first heat exchanger using a compressor and a chiller to convert a first vapor refrigerant stream from a first flash drum into a liquid refrigerant stream and an ejector to convert the liquid refrigerant stream into the first chilled refrigerant stream; iii) chilling the feed gas stream in the second heat exchanger by circulating a second chilled refrigerant stream adjacent the feed gas stream in the second heat exchanger using a first liquid refrigerant stream from the first flash drum; iv) chilling the feed gas stream in the third heat exchanger by circulating a third chilled refrigerant stream adjacent the feed gas stream in the third heat exchanger using a second liquid refrigerant stream from the second flash drum; v) transferring a vapor refrigerant stream from the third heat exchanger to a drum; and vi) returning at least a portion of the vapor refrigerant stream in the drum to the ejector for lowering the temperature of the first chilled refrigerant stream. 
     Referring now to  FIG. 3 , a schematic diagram illustrates one embodiment of the present disclosure retrofitted in a pre-existing liquefied natural gas process. A vaporized refrigerant from the lowest stage drum  120  is taken through line  302  to an ejector  304  that is preferably a liquid motive ejector. The motive for the ejector  304  is supplied via line  130  and is passed at saturated liquid conditions through a high-efficiency pump  306 . The propane chilling compressor  118  can comprise three stages or can employ two stages of compression and instead redirect the total flow of vaporized refrigerant from the lowest stage drum  120  through line  302 . This facilitates a significant decrease in mass flow to the compressor  118 , as depicted in Table 3 below, based on a HYSYS™ simulation. As a result, capacity in the propane chilling system is increased, facilitating the change in temperature profile. The adjustment of the temperature profile differs by implementation, but generally is a reduction of about 2° F. to about 4° F. in the feed gas stream  102  and about 5° F. to about 100° F. in the supplemental refrigeration system  312 . The supplemental refrigeration system  312  produces one of a chilled feed gas stream and a liquified feed gas stream in line  116 . The supplemental refrigeration system  312  may be a mixed refrigeration system that includes a mixed refrigerant. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                   
                 Prior Art 
                 FIG. 4 
               
               
                   
                 Refrigeration 
                 Equipment 
                 Mass Flow 
                 Mass Flow 
               
               
                   
                 Loop 
                 Tag 
                 % Difference 
                 % Difference 
               
               
                   
                   
               
             
            
               
                   
                 Mixed 
                 K-6000.I 
                 Base 
                 −11.66% 
               
               
                   
                 Refrigerant 138 
                 K-6000.II 
                 Base 
                 −11.66% 
               
               
                   
                 Propane 118 
                 K-3001.I 
                 Base 
                 — 
               
               
                   
                   
                 K-3001.II 
                 Base 
                 −52.48% 
               
               
                   
                   
                 K-3001.III 
                 Base 
                 −10.86% 
               
               
                   
                   
               
            
           
         
       
     
     Due to the fact that the chilled feed gas stream or the liquified feed gas stream in line  116  is subcooled, the boil-off gas recompression system can be eliminated in favor of another liquid motive ejector  310  that is controlled by means of the letdown valve  148 . Pressure in the form of vapor suction through line  308  to the ejector  310  is monitored to ensure that the LNG tank does not reach vacuum pressure. A small temperature increase of approximately 3-5° F. is noted from HYSYS simulation models, but due to the significant subcooling of the chilled feed gas stream or the liquified feed gas stream at the letdown valve  148 , no vapor generation occurs. 
     Referring now to  FIG. 4 , a schematic diagram illustrates one embodiment of the present disclosure applied to a C3MR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum  120  to the compressor  118  is eliminated. The entire vaporized refrigerant from drum  120  is thus, diverted through line  302  to the liquid motive ejector  304 . The resultant effect is a reduction in the temperature in line  111  from about 23° F. to about 19° F. The temperature of the mixed refrigerant in line  146  is also reduced from about −30° F. to about −34° F. As a result, the vapor fraction in flash drum  142  is adjusted from about 43% to about 41%. The inter-stage flashes conditions in the supplemental refrigeration system improve from about −162° F. in the conventional C3MR liquefied natural gas process to about −190° F. The temperature remains the same. Table 4 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case maintains the natural gas feed rate to the natural gas liquefaction terminal. The second case increases the feed rate to maintain the compressor  118  at a capacity like a conventional C3MR liquefied natural gas process. An observed feed rate increase of nearly 17% is depicted when the terminal is revamped with the present embodiment. Additionally, a brake power reduction of nearly 22% is observed. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                 FIG. 4 (w/increased 
               
               
                   
                 Prior Art 
                 FIG. 4 
                 feed rate and revamp) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Brake Power 
                 hp 
                 % Difference 
                 Base 
                 −21.96% 
                 −11.98% 
               
               
                 Feed Rate 
                 MMtpa 
                 % Difference 
                 Base 
                 0.00% 
                 16.57% 
               
               
                 Feed Temperature 
                 ° F. 
                 Value 
                 60.00 
                 60.00 
                 60.00 
               
               
                 Product Rate 
                 MMBtu/hr 
                 % Difference 
                 Base 
                 1.82% 
                 18.82% 
               
               
                   
                 MMtpa 
                 % Difference 
                 Base 
                 0.33% 
                 18.42% 
               
               
                 Thermal Efficiency 
                 % 
                 % 
                 92.75 
                 94.34 
                 94.52 
               
               
                 UA 
                 Btu/hr-° F. 
                 % Difference 
                 0.00% 
                 48.13% 
                 564.08% 
               
               
                   
               
            
           
         
       
     
     Referring now to  FIG. 5 , a schematic diagram illustrates another embodiment of the present disclosure applied to an IPSMR liquefied natural gas process. In this embodiment, the lowest compression stage from the drum  220  to the compressor  218  is eliminated. The entire vaporized refrigerant from drum  220  is thus, diverted through line  302  to the liquid motive ejector  304 . Because of the relatively small size of the propane chilling system in this embodiment, compared to the embodiment in  FIG. 4 , the increased capacity in the propane chilling system is used to cool the mixed refrigerant in line  501 . The mixed refrigerant passed through heat exchangers  206 ,  208  and  210  is chilled to a temperature of about −13° F. in line  502 . The mixed refrigerant is then re-introduced into the brazed aluminum heat exchanger  212 . Table 5 (below) illustrates the impact of this embodiment applied to a natural gas liquefaction process for two cases, modeled using HYSYS™. One case only modifies the propane chilling system. The second case modifies both the propane chilling system and the mixed refrigeration system depicted in  FIG. 5 . Additionally, a brake power reduction of nearly 12% is observed. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 FIG. 5 (w/MR 
               
               
                   
                 Prior Art 
                 FIG. 5 
                 integration) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Brake Power 
                 hp 
                 % Difference 
                 Base 
                 −1.52% 
                 −11.54% 
               
               
                 Feed Rate 
                 MMtpa 
                 % Difference 
                 Base 
                 0.00% 
                 0.00% 
               
               
                 Feed Temperature 
                 ° F. 
                 Value 
                 95 
                 95 
                 95 
               
               
                 Product Rate 
                 MMBtu/hr 
                 % Difference 
                 Base 
                 0.11% 
                 0.81% 
               
               
                   
                 MMtpa 
                 % Difference 
                 Base 
                 0.11% 
                 0.81% 
               
               
                 Thermal Efficiency 
                 % 
                 % 
                 93.35 
                 93.45 
                 94.10 
               
               
                 UA 
                 Btu/hr-° F. 
                 % Difference 
                 0.00% 
                 1.68% 
                 221.98% 
               
               
                   
               
            
           
         
       
     
     The systems and methods disclosed herein thus, improve the efficiency of combined cascade and multicomponent refrigeration systems by utilizing one or more ejectors to reduce and/or eliminate conventional compression stages. The systems and methods change the temperature profile, which reduces the energy consumption of both the mixed refrigeration system and the pre-cooling system. 
     While the present disclosure has been described in connection with presently preferred embodiments, it will be understood by those skilled in the art that it is not intended to limit the disclosure to those embodiments. For example, the present disclosure may be implemented in the mixed refrigeration systems described herein and other multi-stage refrigeration processes for chilling a feed gas, such as other cascade refrigeration cycles and mixed refrigerant cycles, to achieve similar results. Although propane is used as an exemplary refrigerant for the pre-cooling system, it is not intended to preclude other refrigerants from being used instead of propane. It is therefore, contemplated that various alternative embodiments and modifications may be made to the disclosed embodiments without departing from the spirit and scope of the disclosure defined by the appended claims and equivalents thereof.