Patent Publication Number: US-2018038639-A1

Title: Robust recovery of natural gas letdown energy for small scale liquefied natural gas production

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application of U.S. Provisional Applicant No. 62/371,497, filed Aug. 5, 2016, which is herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for liquefaction of natural gas using available letdown energy of a high pressure natural gas pipeline. More specifically, embodiments of the present invention are related to liquefying a natural gas stream utilizing a combination of natural gas letdown and a nitrogen refrigeration cycle that provide for increased flexibility and efficiencies. 
     BACKGROUND OF THE INVENTION 
     Many locations utilize a high pressure (transmission) network and a lower pressure (distribution) network to supply natural gas through an area. The transmission network acts as a freeway to send the gas to the general area, while the distribution network acts as the roads to send the gas to the individual users within the area. Pressures of these networks vary by location, but typical values are 30-60 bara for transmission and 5-20 bara for distribution. 
     Some small scale LNG plants such as peak shaving plants are often located close to these letdown stations and are able to utilize this “free energy” to produce liquefied natural gas (LNG). The process scheme used is an “open natural gas cycle” where part of the gas is letdown through a turbine and used to provide the refrigeration necessary to liquefy the rest of the natural gas that makes LNG. This letdown energy can supplement an additional refrigeration cycle such as nitrogen expansion cycle or mixed refrigerant cycle, or provide all of the system&#39;s refrigeration requirements. In the latter case, the plant does not require an external source of energy, but its major drawback is that the ratio of expanded natural gas flow over the LNG production is very high (usually 6 to 10) leading to a limited LNG production capacity due to low thermodynamic efficiency. Moreover, the LNG production is totally dependent on the availability of letdown gas. Fluctuations in the feed pressure, return pressure, or flowrate yields significant fluctuations in the operation of the liquefier. 
     Therefore, there is a need for an improved process for using the letdown energy of the high pressure natural gas to produce LNG that is more efficient and is less sensitive to fluctuations of the process conditions of the letdown stream 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process that satisfies at least one of these needs. In certain embodiments, a process is provided that optimally utilizes the natural gas letdown energy while accommodating the system fluctuations. 
     A typical small scale LNG scheme utilizes a nitrogen cycle (N 2  recycle compressor and two turbine boosters) in a closed loop. However, certain embodiments of the present invention present flexible solutions that enable full utilization of the letdown energy by combining natural gas turbo-expansion for natural gas pre-cooling and a refrigeration cycle. Therefore, even when the letdown capacity varies, the production can be maintained while reducing costs. In certain embodiments of the invention, the refrigeration cycle used is either a nitrogen refrigeration cycle or a mixed refrigerant refrigeration cycle. 
     In one embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) withdrawing a pressurized natural gas stream from a natural gas pipeline; b) boosting a first portion of the pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream; c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; d) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream; e) expanding a second portion of the pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; f) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and g) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product. 
     In optional embodiments of the invention:
         the first natural gas turbine is configured to power the first natural gas booster;   the second natural gas turbine is configured to power a generator such that electricity is produced during step f);   the second natural gas turbine is configured to power a second natural gas booster;   the refrigeration cycle comprises a refrigerant by-pass configured to remove a portion of refrigerant coolant from an intermediate section of the heat exchanger thereby reducing the amount of cooling provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream;   the first warmed natural gas stream and the second warmed natural gas stream are combined within the heat exchanger or combined before entering the heat exchanger;   the method can further include the steps of: removing an impurity from the pressurized natural gas stream using an adsorption bed, wherein the impurity is selected from the group consisting of carbon dioxide, water, and combinations thereof; and regenerating the adsorption bed using a low pressure natural gas, wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof;   the first expanded natural gas stream and the second expanded natural gas stream are expanded to about the same pressure;   the method can further include the steps of: sending a flow of a low pressure natural gas to a user; adjusting the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine based on the flow of the low pressure natural gas sent to the user, wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof;   the amount of LNG product produced in step h) is unchanged by the amount of the second portion of the pressurized natural gas stream expanded in a second natural gas turbine in step f);   the pressure of the second portion of the boosted pressurized natural gas stream during liquefaction is unchanged by the combined flow rate of the first expanded natural gas stream and the second expanded natural gas stream;   the method can further include the steps of: modifying the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine in step f); and adjusting the refrigeration provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream in step h) in order to keep the amount of the LNG product produced within a targeted range;   the refrigeration cycle is selected from the group consisting of a nitrogen refrigeration cycle and a mixed refrigerant refrigeration cycle;   the refrigeration cycle has only one turbine-booster; and/or   the nitrogen cycle comprises two turbine-boosters.       

     In one embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) withdrawing a pressurized natural gas stream from a natural gas pipeline operating at a first pressure; b) boosting a first portion of the pressurized natural gas stream in a first natural gas booster to a second pressure to produce a boosted pressurized natural gas stream; c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas expansion turbine to a third pressure to produce a first expanded natural gas stream; d) liquefying a second portion of the boosted pressurized natural gas stream in a natural gas liquefier using refrigeration provided by a refrigeration cycle; e) expanding a second portion of the pressurized natural gas stream in a second expansion turbine to a fourth pressure to produce a second expanded natural gas stream; f) warming the first expanded natural gas stream and the second expanded natural gas stream by in-direct heat exchange against the second portion of the boosted natural gas stream to produce a first and second warmed expanded natural gas stream; g) sending the first and second warmed expanded natural gas stream to a downstream facility, wherein the downstream facility has a natural gas demand, wherein the first natural gas expansion turbine is configured to provide compression power for the first natural gas booster. 
     In optional embodiments of the invention:
         the flow rate of the second portion of the pressurized natural gas stream expanded in the second expansion turbine is adjusted based on changes in the natural gas demand of the downstream facility;   the flow rate of the second portion of the boosted pressurized natural gas stream is independent of the natural gas demand of the downstream facility; and/or   the method can further include the step of monitoring the first pressure; and adjusting the flow rates of the first portion of the boosted pressurized natural gas stream and the second portion of the pressurized natural gas stream based on the first pressure while maintaining the flow rate of the LNG stream produced in step d).       

     In another embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) boosting a first pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream; b) expanding a second pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; c) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream; d) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; e) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and f) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product, wherein the first pressurized natural gas stream, the second pressurized natural gas stream, and the third pressurized natural gas stream all originate from a common high pressure natural gas pipeline. 
     In another embodiment of the present invention, a method for the liquefaction of natural gas is provided. In one embodiment, the method can include the steps of: a) expanding a first pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream; b) warming the first expanded natural gas stream in a heat exchanger against a second pressurized natural gas stream to produce a first warmed natural gas stream; c) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream; d) warming the second expanded natural gas stream in the heat exchanger against the second pressurized natural gas stream to produce a second warmed natural gas stream; and e) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product, wherein the first natural gas turbine is configured to drive a first booster, wherein the first booster is configured to compress a stream selected from the group consisting of the first pressurized natural gas stream, the first warmed natural gas stream, the second pressurized natural gas stream 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
         FIG. 1  shows an embodiment of the prior art. 
         FIG. 2  shows an embodiment in accordance with the present invention. 
         FIG. 3  shows a second embodiment in accordance with the present invention. 
         FIG. 4  shows a third embodiment in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
       FIG. 1  is an example of a typical small LNG scheme that utilizes a nitrogen cycle (N 2  compressor and two turbine boosters) in a closed loop  10 . Natural gas NG is cooled and condensed into LNG in passages separate and adjacent to the N 2  in the heat exchanger  40 . In most small scale LNG plants, heavy hydrocarbons (HHC), which freeze at LNG temperatures, condense and are removed from the natural gas via a knock out drum  50 . The specific power of such plant is highly dependent on the natural gas feed pressure and usually varies between 450 and 550 kWh/ton of LNG produced. 
     In one embodiment of the present invention, the system presented in  FIG. 2  combines two natural gas turbines  15 ,  25  and a standard nitrogen liquefier  10 . In the embodiment shown, the first natural gas turbine  15  is driving a first natural gas booster  17  that is used to set the pressure of the natural gas to a specified optimum value at the inlet of the cold box  20 . This additional pressure boost to the natural gas stream is advantageous, since a high natural gas pressure (1) improves the heat exchange efficiency, (2) shrinks the size of the equipment, and (3) reduces overall costs. However, this pressure must also be maintained below the maximum allowable working pressure of the equipment design. 
     In certain embodiments, the natural gas operating pressure at cold box inlet can be limited by the critical pressure of the gas. This is because the HHC condensation requires the operating pressure to be less than the critical pressure for the separation of liquid and vapor to occur. Therefore, in certain embodiments, the limit to the natural gas critical pressure will set the maximum discharge pressure of the first natural gas booster  17  and thus the flow going to the first natural gas expander  15 . In certain embodiments, the letdown flow rate available is higher than the flow rate required to reach the booster maximum suction pressure. When this occurs, second natural gas turbine  25  can be utilized. 
     In one embodiment, second natural gas turbine  25  can be configured to drive a generator, thereby producing additional electricity. This turbine  25  is completely independent from the first turbine  15 , and uses the extra letdown flow available to produce electricity. In this way, the natural gas liquefaction stream can be maintained at its optimum pressure through a range of letdown flows and pressures. 
     Additionally, in certain embodiments, the nitrogen cycle flow may be adjusted such that the LNG production can be maintained independently from the letdown flow variation. 
       FIG. 3  presents an alternative embodiment in which a second booster  27  replaces the generator. The temperature of the booster aftercooler  30  may be adjusted such that no condensation appears at the discharge of second natural gas turbine  25 . Alternatively (in an embodiment not shown), components which condense may be removed prior to expansion. Depending on the pressure ratio and flows, the natural gas can also be expanded prior to boosting in second booster  27 . As this embodiment does not require a transformation of the mechanical energy into electrical energy, the embodiment presented in  FIG. 3  is generally more efficient and cost competitive compared to the embodiment presented in  FIG. 2 . 
     In another embodiment not shown, the energy of the second natural gas turbine  25  may drive a booster which is compressing expanded LNG flash after the letdown valve to the LNG tank. The advantage of such system is to provide free cold energy at both the warm end (thanks to the natural gas expansion) and the cold end (thanks to LNG flash) of the liquefier with no natural gas losses, as it is recompressed to the low pressure network. Therefore, this embodiment is particularly efficient and is especially suited when using a bullet tank type storage, which has sufficient pressure to send the flash gas back at the warm end of the heat exchanger. 
       FIG. 4  presents another embodiment that is particularly useful for varying demands for the low pressure natural gas. As the low pressure natural gas flow and pressures vary, the relative amount of natural gas letdown energy varies compared to the N 2  cycle energy. As a result, the warm end of the heat exchanger may receive more cold than is needed. In addition to the loss of thermal efficiency , the warm end of the heat exchanger could get to a temperature that is colder than it was designed for, which could lead to structural issues, as well as premature freezing of components within other parts of the heat exchanger, particularly the heavy hydrocarbons. To alleviate this issue, certain embodiments of the invention can include a by-pass  60  of cold nitrogen at the warm end of the heat exchanger. 
     This by-pass  60  advantageously (1) enables an increase of the heat exchange efficiency in the heat exchanger and (2) reduces the power consumption of the nitrogen cycle compressor by cooling down its suction temperature. 
     In summary, embodiments of the invention provide for many improvements over conventional liquefaction techniques. For example, by increasing the feed pressure of the natural gas using a combination turbine booster ( 15 ,  17 ), the heat exchange efficiency is greatly improved, which allows for either an increase in LNG production capacity by keeping the same equipment size or reducing the size of the equipment, and therefore the overall footprint of the plant while maintaining current production capacity. 
     Additionally, expansion of natural gas enables to pre-cool the warm end of the heat exchanger reducing the specific power of the nitrogen cycle. The embodiments of the invention are very robust as they can adapt to a wide range of natural gas flow rates. This is due to the decoupling of the natural gas turbines  15 ,  25  with the ability to maintain the natural gas liquefaction pressure  17  with the first natural gas turbine  15 . 
     In certain embodiments, the significant refrigeration brought to the warm end of the main exchanger by the natural gas letdown can allow for the removal of the warm nitrogen turbine and booster to reduce capital cost. 
     Moreover, the design of the main heat exchanger can optionally stay very similar to a standard nitrogen cycle plant, which means that no major changes in design are required. 
     In certain embodiments, all the expansion of the natural gas is carried out at ambient or warm temperatures, which results in limited risk of heavy hydrocarbon freezing at turbine outlets. 
     Additionally, for an incremental additional capital cost (natural gas turbine booster  15 ,  17  and turbine-generator  25 ), there can be a significant power savings as there is a corresponding reduction in nitrogen cycle size and power. This is shown in Table I below. 

 
     Table II below, provides data for an embodiment in which the flowrates and pressures of various streams could be adjusted based on the pressure of the natural gas coming from the pipeline in order to keep the LNG production at a constant pressure and flowrate. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Response of the system to a change of NG Feed pressure 
               
            
           
           
               
               
               
               
            
               
                   
                 Case 01 
                 Case 04 
                   
               
               
                   
                 (FIG. 2) 
                 (FIG. 2) 
                 Note 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 NG Feed Pressure 
                 bar abs 
                 31 
                 40 
                 +9 bar change in the feed gas pressure 
               
               
                 Liquefaction 
                 bar abs 
                 48 
                 48 
                 Kept Constant 
               
               
                 Pressure 
               
               
                 Letdown Pressure 
                 bar abs 
                 6.5 
                 6.5 
                 Kept Constant 
               
               
                 Letdown Flow 
                 MMSCFD 
                 29 
                 29 
                 Constant Demand 
               
               
                 LNG Production 
                 MMSCFD 
                 21 
                 21 
                 Constant Demand 
               
               
                 Flow to NG 
                 MMSCFD 
                 14 
                 5 
                 Adjusted to keep the discharge 
               
               
                 Turbine (15) 
                   
                   
                   
                 pressure constant 
               
               
                 Flow to NG 
                 MMSCFD 
                 15 
                 24 
                 Adjusted to deliver the rest of the 
               
               
                 Turbine (25) 
                   
                   
                   
                 letdown gas 
               
               
                 N 2  Cycle Power 
                 kW 
                 6,600 
                 6,260 
                 −5% 
               
               
                 (10) 
                   
                   
                   
                 Reduced power mainly due to the 
               
               
                   
                   
                   
                   
                 higher expansion power (higher 
               
               
                   
                   
                   
                   
                 pressure ratio and flow) of NG 
               
               
                   
                   
                   
                   
                 Turbine 25 
               
               
                 NG Turbine (15) 
                 kW 
                 750 
                 230 
                 −69% 
               
               
                 Power 
                   
                   
                   
                 Reduced Power due to the reduced 
               
               
                   
                   
                   
                   
                 pressure ratio, and therefore 
               
               
                   
                   
                   
                   
                 flowrate of Turbine 15 
               
               
                 NG Turbine (25) 
                 kW 
                 550 
                 1,008 
                 +83% 
               
               
                 Power 
                   
                   
                   
                 Higher power generation of 
               
               
                   
                   
                   
                   
                 Turbine 25 
               
               
                   
               
            
           
         
       
     
     The flows, pressure variations and impact on the machinery between Case 01 and Case 04 presented in Table II are merely one example, and are included herein for illustrative purposes. 
     While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device. 
     The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. The terms about/approximately a particular value include that particular value plus or minus 10%, unless the context clearly dictates otherwise. 
     Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.