Cascade refrigeration natural gas liquefaction system and process

Abstract

A cascade refrigeration natural gas liquefaction system and process, which relate to the technical field of refrigeration and cryogenic engineering. The process may include an ethane refrigeration cycle and a mixed refrigerant refrigeration cycle. Refrigerants according to the present disclosure may include propane, ethane and a mixed refrigerant that may be composed of nitrogen, methane and ethane. Except for nitrogen supplied from public utilities, the refrigerants may be sourced from the natural gas itself. The comprehensive energy consumption of liquefaction per unit according to the present disclosure may be lower than the propane precooling mixed refrigerant process and the conventional cascade process. Although it may be slightly higher than that of the dual mixed refrigerant process in winter working condition, the energy consumption of the dual mixed refrigerant process is exceedingly high in summer, and the total energy consumption of the process of the present disclosure is still low comprehensively.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of refrigeration and cryogenic engineering, and more specifically, to a cascade refrigeration natural gas liquefaction system and process.BACKGROUND

[0002] Liquefied Natural Gas (LNG) is currently the fastest growing energy source in the world. Vigorously developing LNG plays an important role in optimizing the energy structure, effectively addressing the dual issues of energy supply security and ecological environment protection, and achieving sustainable economic and social development.

[0003] LNG, as a vehicle fuel, has various advantages. First of all, LNG can save about 25-35% of costs than the fuel oil, i.e., substituting gas for oil can bring about considerable economic benefits. Secondly, natural gas is a fuel with a high octane number, where the octane number is an important indicator for evaluating the fuel performance. If a fuel with a high octane number is used in a vehicle, the engine is less likely to experience knocking combustion, which can prolong the service life of the engine. Thirdly, as compared to gasoline, diesel and LPG, LNG is cheaper, safer and more environment-friendly, has a higher storage efficiency, a longer vehicle driving range and a longer engine service life, and can use the cold energy released during operation of the engine for air conditioning. This can not only save energy but also enable comprehensive utilization of energy.

[0004] Small and medium-sized natural gas liquefaction plants are mainly built near the natural gas main network to serve the surrounding markets. Basically, a single mixed refrigerant (SMR) refrigeration process is used. Although the process incurs higher energy consumption than the propane-precooled mixed-refrigerant (C3-MR) or Cascade process, it requires simple devices and a small investment, which is the optimal process from the perspective of comprehensive investment return cycle.

[0005] However, for multi-base type large-scale LNG plants, the SMR process is not the optimal solution, because the power of the refrigeration compressor is increased as the liquefaction scale is increased. According to the current motor or gas-turbine manufacturing capabilities, if the power is too great, it is required to connect multiple compressors in parallel since only one compressor cannot meet the requirement. In the case that the scale of a single production line is large, if the SMR process is still used, it is required to connect multiple compressors in parallel, which will greatly increase the fixed investment cost and bring about high energy consumption. In the circumstance, the propane-precooled mixed-refrigerant (C3-MR) or Cascade process, or other cascade refrigeration process has obvious advantages. As compared with the SMR, the cascade refrigeration process needs a similar, or even lower, fixed investment and brings about significantly reduced energy consumption. Therefore, most of the multi-base type large-scale LNG plants adopt the cascade refrigeration process, particularly the C3-MR or Cascade process, but a few of them utilize the dual mixed refrigerant process or the multi-stage mixed refrigerant process.SUMMARY

[0006] The objectives of the present disclosure are to solve the problems existing in the prior art and provide a cascade refrigeration large-scale natural gas liquefaction system and process.

[0007] The technical solution adopted in the present disclosure specifically includes:

[0008] In a first aspect, the present disclosure provides a cascade refrigeration natural gas liquefaction system, which comprises an ethane refrigeration cycle system and a mixed refrigerant refrigeration cycle system;

[0009] the ethane refrigeration cycle system comprises a multi-stage ethane compressor, an ethane air cooler, an ethane buffer tank, a multi-stage throttling refrigeration device, a multi-stage natural gas precooler and a multi-stage mixed refrigerant precooler; the multi-stage ethane compressor, the ethane air cooler, the ethane buffer tank and the multi-stage throttling refrigeration device are sequentially connected via an ethane transmission pipeline and form an ethane circulation loop such that the ethane can be buffered and stored in the ethane buffer tank after compressed by the multi-stage ethane compressor and cooled by the ethane air cooler, then enter the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage, and flow back to the multi-stage ethane compressor after providing cold energy to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler; an input end of the multi-stage natural gas precooler is connected to a natural gas inlet I such that the input natural gas to be liquefied can be transmitted via a natural gas transmission pipeline to the mixed refrigerant refrigeration cycle system after precooled stage by stage using the cold energy of the ethane in the multi-stage natural gas precooler;

[0010] the mixed refrigerant refrigeration cycle system comprises a mixed refrigerant compressor, a mixed refrigerant air cooler, a mixed refrigerant gas-liquid separation tank and a main heat exchanger cold box; a mixed refrigerant outlet of the mixed refrigerant compressor is connected sequentially to the mixed refrigerant air cooler, the multi-stage mixed refrigerant precooler and the mixed refrigerant gas-liquid separation tank via a mixed refrigerant transmission pipeline, such that, after compressed by the mixed refrigerant compressor and air-cooled by the mixed refrigerant air cooler, the mixed refrigerant can be precooled stage by stage using the cold energy of the ethane in the multi-stage mixed refrigerant precooler and then enter the mixed refrigerant air-liquid separation tank for gas-liquid separation; a gas phase outlet and a liquid phase outlet of the mixed refrigerant gas-liquid separation tank are respectively connected to inlet ends of two throttling refrigeration units in the main heat exchanger cold box such that the two throttling refrigeration units can provide the main heat exchanger cold box with cold energy required for supercooling the natural gas through throttling refrigeration of the mixed refrigerant; outlet ends of the two throttling refrigeration units are connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; the main heat exchanger cold box is provided therein with a heat exchange pipe for supercooling the natural gas, wherein the heat exchange pipe is connected at an input end to the natural gas transmission pipeline and connected at an output end to a liquefied natural gas outlet O.

[0011] As a preferred option of the first aspect described above, the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler are the same and correspond one-to-one; each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttle valve installed on an inlet pipe of the gas-liquid separation tank, wherein a liquid phase outlet of the gas-liquid separation tank is divided into three branches that respectively lead to refrigerant inlets of the corresponding stage of the natural gas precooler and the mixed refrigerant precooler, and refrigerant outlets of the natural gas precooler and the mixed refrigerant precooler and a gas phase outlet of the gas-liquid separation tank are connected to an inlet of the corresponding stage of the ethane compressor.

[0012] As a preferred option of the first aspect described above, the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler are from 3 to 5.

[0013] As a preferred option of the first aspect described above, each throttling refrigeration unit comprises an inflow heat exchange pipe, a throttle valve, a two-phase flow uniform distribution tank and an outflow heat exchange pipe, wherein the inflow heat exchange pipe, the two-phase flow uniform distribution tank and the outflow heat exchange pipe are all disposed in the main heat exchanger cold box; an inlet of the inflow heat exchange pipe is used for inputting the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank, an outlet of the inflow heat exchange pipe is sequentially connected to inlets of the throttling valve and the two-phase flow uniform distribution tank, a gas phase outlet and a liquid phase outlet of the two-phase flow uniform distribution tank are respectively connected to an inlet of the outflow heat exchange pipe, and an outlet of the outflow heat exchange pipe is connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank further undergoes throttling refrigeration via the throttle valve after absorbing cold energy in the inflow heat exchange pipe, then enters the two-phase flow uniform distribution tank for gas-liquid uniform distribution, and is input to the outflow heat exchange pipe to provide the cold energy for the natural gas and the mixed refrigerant.

[0014] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed between the ethane air cooler and the ethane buffer tank.

[0015] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed between the mixed refrigerant air cooler and the multi-stage mixed refrigerant precooler.

[0016] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed respectively between the natural gas inlet I and the multi-stage natural gas precooler.

[0017] In a second aspect, the present disclosure provides a cascade refrigeration natural gas liquefaction process, which is implemented using the cascade refrigeration natural gas liquefaction system of any of the embodiments of the first aspect described above, the cascade refrigeration natural gas liquefaction process comprising:

[0018] S1: gas ethane is compressed in multiple stages by the multi-stage ethane compressor, the compressed ethane is air-cooled by the ethane air cooler into liquid ethane, and the liquid ethane is then stored in the ethane buffer tank; the liquid ethane in the ethane buffer tank is stably delivered to the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage; each stage of throttling refrigeration device throttles and depressurizes the input liquid ethane and then further performs gas-liquid separation; the gas ethane obtained by separation is reflowed back and delivered to the multi-stage ethane compressor to be recompressed; the liquid ethane obtained by separation is partly delivered to a next stage of throttling refrigeration device, and partly delivered to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler to provide cold energy by means of heat exchange, wherein the gas ethane after heat exchange is reflowed back and delivered to the multi-stage ethane compressor to be recompressed;

[0019] S2: the mixed refrigerant is compressed by the mixed refrigerant compressor, the compressed mixed refrigerant is air-cooled by the compressor air cooler into a liquid mixed refrigerant, the liquid mixed refrigerant passes through the multi-stage mixed refrigerant precooler and then exchanges heat with the liquid ethane delivered in the multi-stage throttling refrigeration device to thus lower the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling, and the precooled liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank to implement gas-liquid separation; the gas mixed refrigerant and the liquid mixed refrigerant obtained through gas-liquid separation enter the main heat exchange cold box, respectively, to further absorb cold energy to lower the temperature, and then after throttled and depressurized via the respective throttling refrigeration units, are returned to the main heat exchanger cold box for providing cold energy for lowering the temperature of the natural gas and the mixed refrigerant, which in turn causes the temperature thereof to increase; the mixed refrigerant with the increased temperature flows back to the mixed refrigerant compressor to be recompressed; and

[0020] S3: the purified natural gas to be liquefied is input to the multi-stage natural gas precooler via a natural gas inlet I to exchange heat with the liquid ethane delivered in the multi-stage throttling refrigeration device, thus lowering the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling; the precooled natural gas further enters the main heat exchanger cold box, and exchanges heat with the mixed refrigerant returned to the main heat exchanger cold box after throttled and depressurized to complete liquefaction supercooling; the liquefied and supercooled natural gas is output via a liquefied natural gas outlet O.

[0021] As a preferred option of the second aspect described above, a temperature of the precooled liquid mixed refrigerant and the precooled natural gas is from −60° C. to −80° C.

[0022] As a preferred option of the second aspect described above, the mixed refrigerant is composed of nitrogen, methane and ethane.

[0023] As compared with the prior art, the present disclosure has the following advantageous effects:

[0024] 1. For the conventional cascade process, ethene is used in the second-stage refrigeration phase, which needs to be purchased externally. However, a cascade refrigeration cycle natural gas liquefaction process is adopted in the present disclosure, and refrigerants used therein include propane, ethane and a mixed refrigerant, wherein the mixed refrigerant is composed of nitrogen, methane and ethane. Except for nitrogen that needs to be supplied externally (e.g., from public utilities), the rest refrigerants are sourced from the natural gas per se, for which additional purchase is not required.

[0025] 2. Nowadays, the most commonly used process for natural gas liquefaction is C3-MR, namely a propane-precooled mixed refrigerant process, which is most advantageous as having low energy consumption and being convenient in operation. However, the most prominent problem of the process is a great discrepancy between the power of the propane compressor and the power of the mixed refrigerant compressor, wherein the power of the mixed refrigerant compressor is limited by the maximum power of the gas turbine or the motor, thus restricting the single-line production capacity. In order to solve the problem, the American Pharmacy Cooperative, Inc. (APCI) employs the AP-Split MR® process wherein a part of power of the mixed refrigerant compressor is distributed to the gas turbine or motor that drives the propane compressor. In the way, this process can reduce the power of the gas turbine or motor driving the mixed refrigerant compressor while maximizing the single-line production capacity. Such control system is relatively complex, since a gas turbine or motor needs to drive two compressor systems, and startup and shutdown control of the compressors is complicated.

[0026] The mixed refrigerant precooled using ethane adopted herein can cool the natural gas and the mixed refrigerant to −80° C., which can only be cooled to −40° C. if precooled by propane. Accordingly, the present disclosure can achieve a lower precooling temperature, and can enable the power of the ethane compressor and the power of the mixed refrigerant compressor to be close to each other by adjusting the temperature after precooling, to thus maximize the single-line production capacity. Moreover, this can avoid a complex solution wherein a gas turbine or motor controls two compressors.

[0027] 3. For the current C 3-MR process and the Cascade process, generally three-stage or four-stage propane precooling is adopted to precool the raw gas and the refrigerant to about −35° C. In contrast, according to the present disclosure, a propane refrigeration or seawater precooling step may be added before precooling using ethane, and the propane refrigeration may adopt a one-stage or two-stage refrigeration method according to the needs, to further improve the efficiency of the entire device. The process according to the present disclosure has the following advantage: propane one-stage or two-stage precooling or seawater auxiliary precooling is used therein, which can cool the raw gas and the refrigerant to about 12° C. The process is especially suitable for regions with a great temperature difference between winter and summer, in particular frigid or polar zones. If the ambient temperature is low, the propane compressor refrigeration cycle or seawater precooling can be stopped, since the raw gas and the refrigerant are cooled to 12° C. or even lower by the ambient temperature. In the way, the present disclosure can greatly reduce the energy consumption for operation. The present disclosure is especially suitable for the frigid or polar zones. In these zones, since the ambient temperature above 0° C. lasts only one or two months per year, the propane precooling compressor or seawater precooling only needs to run for a short period. However, for the conventional three-stage or four-stage propane precooling process with the requirement for cooling the raw gas and the refrigerant to −35° C., it is hard to cool the two to such temperature only by means of the ambient temperature, even in winter in the frigid zones. Therefore, the propane compressor needs to keep running, which wastes lots of energy.

[0028] 4. The comprehensive liquefaction energy consumption per unit in the present disclosure is lower than that in the propane precooling mixed refrigerant process and the conventional cascade process. Although it is slightly higher than that of the dual mixed refrigerant process in the winter working condition, the energy consumption of the dual mixed refrigerant process is exceedingly high in summer. To sum up, the present disclosure is still advantageous in terms of energy consumption.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a schematic diagram of a structure of a cascade refrigeration natural gas liquefaction system;

[0030] FIG. 2 is an enlarged schematic diagram of the ethane refrigeration cycle system in FIG. 1;

[0031] FIG. 3 is an enlarged schematic diagram of the mixed refrigerant refrigeration cycle system in FIG. 2;

[0032] FIG. 4 is a schematic diagram of a structure of a further cascade refrigeration natural gas liquefaction system;

[0033] FIG. 5 is an enlarged schematic diagram of the ethane refrigeration cycle system in FIG. 4; and

[0034] FIG. 6 is an enlarged schematic diagram of the mixed refrigerant refrigeration cycle system in FIG. 4.

[0035] Reference signs in the drawings are described below: an ethane refrigeration cycle system A, and a mixed refrigerant refrigeration cycle system B, wherein: the ethane refrigeration cycle system A includes: a multi-stage ethane compressor C-201, an ethane air cooler AC-201, an ethane buffer tank V-201, a first ethane gas-liquid separation tank V-202, a second ethane gas-liquid separation tank V-203, a third ethane gas-liquid separation tank V-204, a fourth ethane gas-liquid separation tank V-205, a natural gas-ethane first-stage precooler E-301, a natural gas-ethane second-stage precooler E-302, a natural gas-ethane third-stage precooler E-303, a natural gas-ethane fourth-stage precooler E-304, a mixed refrigerant-ethane first-stage precooler E-101, a mixed refrigerant-ethane second-stage precooler E-102, a mixed refrigerant-ethane third-stage precooler E-103, a mixed refrigerant-ethane fourth-stage precooler E-104, a first valve Val-201, a second valve Val-202, a third valve Val-203, a fourth valve Val-204, a fifth valve Val-205, a sixth valve Val-206, a seventh valve Val-207, an eighth valve Val-208, a ninth valve Val-209, a tenth valve Val-210, an eleventh valve Val-211, and a twelfth valve Val-212; the mixed refrigerant refrigeration cycle system B includes: a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation tank V-101, a first two-phase flow uniform distribution tank V-401, a second two-phase flow uniform distribution tank V-402, a main heat exchanger cold box CB-401, a first throttle valve Val-401, and a second throttle valve Val-402; and connecting pipelines in the two systems include: an ethane transmission pipeline P-1, a natural gas transmission pipeline P-2, a mixed refrigerant transmission pipeline P-3, a first inflow heat exchange pipe P-4, a second inflow heat exchange pipe P-5, an outflow heat exchange pipe P-6, and a heat exchange pipe P-7.DETAILED DESCRIPTION OF EMBODIMENTS

[0036] In order to make the above objective, features and advantages of the present disclosure much clearer, detailed description about the specific implementations of the present disclosure will be provided below with reference to the drawings. In the following description, lots of details will be illustrated to enable full understanding of the present disclosure. However, the present disclosure could be implemented in other manners than the one described herein, and allows those skilled in the art to make similar improvements without departing from the spirit of the present disclosure. The present disclosure is therefore not limited to the specific embodiments described below. The technical features in the various embodiments of the present disclosure could be combined correspondingly, without conflicting with each other.

[0037] In the description of the present disclosure, it would be appreciated that, if an element is considered to be “connected” to a further element, the element may be directly connected to the further element, or indirectly connected to the latter (i.e., there is an intermediate element). However, if the element is described as being “directly” connected to the further element, no intermediate element is included.

[0038] As used herein, the terms “first,”“second,” and the like, are used only for differentiation and should not be construed as indicating or implying relative importance, or implicitly indicating a number of technical features involved.

[0039] The present disclosure provides a cascade refrigeration natural gas liquefaction system, in which a purified natural gas and a compressed and cooled mixed refrigerant are respectively precooled by ethane in multiple stages, the precooled natural gas and mixed refrigerant are then cooled, liquefied and supercooled by a throttled, depressurized and reversely-flowing mixed refrigerant, and the liquefied and supercooled natural gas is delivered to a downstream LNG storage tank for storage while the liquefied and supercooled mixed refrigerant is throttled and returned to the main heat exchanger to provide cold energy for the system.

[0040] Referring to FIG. 1, in a preferred embodiment of the present disclosure, a specific implementation of the cascade refrigeration natural gas liquefaction system is shown, which includes an ethane refrigeration cycle system A and a mixed refrigerant cycle system B.

[0041] Referring to FIG. 2, the ethane refrigeration cycle system A in the embodiments of the present disclosure includes a multi-stage ethane compressor C-201, an ethane air cooler AC-201, an ethane buffer tank V-201, a multi-stage throttling refrigeration device V-2, a multi-stage natural gas precooler E-3, and a multi-stage mixed refrigerant precooler E-1; the multi-stage ethane compressor C-201, the ethane air cooler AC-201, the ethane buffer tank V-201 and the multi-stage throttling refrigeration device V-2 are sequentially connected via an ethane transmission pipeline P-1 and form an ethane circulation loop such that the ethane can be buffered and stored in the ethane buffer tank V-201 after compressed by the multi-stage ethane compressor C-201 and cooled by the ethane air cooler AC-201, then enter the multi-stage throttling refrigeration device V-2 to be throttled, depressurized and refrigerated stage by stage, and flow back to the multi-stage ethane compressor C-201 after providing cold energy to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1; an input end of the multi-stage natural gas precooler E-3 is connected to the natural gas inlet I such that the input natural gas to be liquefied can be transmitted via the natural gas transmission pipeline P-2 to the mixed refrigerant refrigeration cycle system B after precooled stage by stage using the cold energy of the ethane in the multi-stage natural gas precooler E-3.

[0042] It is worth noting that the multi-stage ethane compressor C-201 is a compressor having a multi-stage compression function and formed by a plurality of compression units in cascade, wherein each stage of compression unit can receive the ethane output by the previous stage of compression unit and further compress the same, and wherein each stage of compression unit may have an air inlet, and the ethane input via the air inlet of the compression unit, if any, can be mixed with the ethane output from the previous stage of compression unit and then compressed. In this way, the ethane can be compressed stage by stage to the required pressure to facilitate subsequent cooling and liquefaction. The multi-stage throttling refrigeration device V-2 is formed by a plurality of throttling refrigeration devices in cascade, which can throttle and depressurize the cooled and liquefied liquid ethane stage by stage, thus generating cold energy to further cool the liquid ethane. A part of the liquid ethane cooled by each stage of throttling refrigeration device enters a next stage of throttling refrigeration device, and the rest thereof can supply cold energy required for precooling the natural gas and the mixed refrigerant to the outside, thus evaporating and forming gas ethane. The gas ethane resulting from throttling and the gas ethane resulting from precooling and evaporation can both return to the multi-stage ethane compressor C-201 for compression. The multi-stage natural gas precooler E-3 is formed by a plurality of precoolers in cascade, wherein each stage of precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cold energy, so as to precool the natural gas, after heat exchange with the natural gas, to a certain low temperature before the natural gas enters the mixed refrigerant refrigeration cycle system B. The multi-stage mixed refrigerant precooler E-1 is formed by a plurality of precoolers in cascade, wherein each stage of precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cold energy, so as to precool the mixed refrigerant, after heat exchange with the mixed refrigerant, to a certain low temperature before the mixed refrigerant enters the mixed refrigerant refrigeration cycle system B. The specific temperature at which the mixed refrigerant and the natural gas are precooled before entering the mixed refrigerant refrigeration cycle system B could be adjusted as actually required. In the embodiments of the present disclosure, the temperature of the precooled liquid mixed refrigerant and the precooled natural gas is preferably from −60° C. to −80° C.

[0043] The mixed refrigerant according to the present disclosure could be selected as actually required, which is preferably a mixed refrigerant composed of nitrogen, methane and ethane.

[0044] In addition, the number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 can be adjusted as actually required, which is generally set to 3 to 5 stages. The number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 may be the same, or may be different. In the embodiments of the present disclosure, the number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 are the same and correspond one-to-one. Each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttle valve installed on an inlet pipe of the gas-liquid separation tank, wherein a liquid phase outlet of the gas-liquid separation tank is divided into three branches which respectively lead to refrigerant inlets of the corresponding stage of the natural gas precoolers and the mixed refrigerant precoolers, and a refrigerant outlet of the natural gas precooler, a refrigerant outlet of the mixed refrigerant precoolers and a gas phase outlet of the gas-liquid separation tank are connected to the inlet of the corresponding stage of the ethane compressor.

[0045] In the embodiments of the present disclosure, the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 have four stages, respectively. The multi-stage ethane compressor C-201 includes 4 stages of compressors, namely a fourth-stage compressor, a third-stage compressor, a second-stage compressor and a first-stage compressor according to the compression order, wherein each stage of compressor has an inlet for inputting the gas ethane and is simultaneously configured to receive the ethane compressed by the previous stage of the compressor. The multi-stage throttling refrigeration device V-2 includes 4 gas-liquid separation tanks, namely a first ethane gas-liquid separation tank V-202, a second ethane gas-liquid separation tank V-203, a third ethane gas-liquid separation tank V-204 and a fourth ethane gas-liquid separation tank V-205. On the inlet and outlet pipelines of the four gas-liquid separation tanks, 12 valves are installed, namely a first valve Val-201, a second valve Val-202, a third valve Val-203, a fourth valve Val-204, a fifth valve Val-205, a sixth valve Val-206, a seventh valve Val-207, an eighth valve Val-208, a ninth valve Val-209, a tenth valve Val-210, a eleventh valve Val-211 and a twelfth valve Val-212, wherein the first valve Val-201, the third valve Val-203, the sixth valve Val-206 and the ninth valve Val-209 are all throttle valves, and the remaining valves are used to control connection or disconnection of the pipelines. The multi-stage natural gas precooler E-3 includes a natural gas-ethane first-stage precooler E-301, a natural gas-ethane second-stage precooler E-302, a natural gas-ethane third-stage precooler E-303 and a natural gas-ethane fourth-stage precooler E-304, and the multi-stage mixed refrigerant precooler E-1 includes a mixed refrigerant-ethane first-stage precooler E-101, a mixed refrigerant-ethane second-stage precooler E-102, a mixed refrigerant-ethane third-stage precooler E-103 and a mixed refrigerant-ethane fourth-stage precooler E-104.

[0046] The first valve Val-201 is installed on a connecting pipeline between the ethane buffer tank V-201 and the first ethane gas-liquid separation tank V-202; the second valve Val-202 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the mixed refrigerant-ethane first-stage precooler E-101; the third valve Val-203 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the second ethane gas-liquid separation tank V-203; the fourth valve Val-204 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the natural gas-ethane first-stage precooler E-301; the fifth valve Val-205 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the mixed refrigerant-ethane second-stage precooler E-102; the sixth valve Val-206 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the third ethane gas-liquid separation tank V-204; the seventh valve Val-207 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the natural gas-ethane second-stage precooler E-302; the eighth valve Val-208 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the mixed refrigerant-ethane third-stage precooler E-103; the ninth valve Val-209 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the fourth ethane gas-liquid separation tank V-205; the tenth valve Val-210 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the natural gas-ethane third-stage precooler 303; the eleventh valve Val-211 is installed on a connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the mixed refrigerant-ethane fourth-stage precooler E-104; the twelfth valve Val-212 is installed on a connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the natural gas-ethane fourth-stage precooler E-304.

[0047] It is worth noting that the respective ethane working medium pressures in the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204 and the fourth ethane gas-liquid separation tank V-205 are reduced gradually, and the four ethane gas-liquid separation tanks respectively correspond to an ethane ultra-high pressure gas-liquid separation tank, an ethane high pressure gas-liquid separation tank, an ethane low pressure gas-liquid separation tank and an ethane ultra-low pressure gas-liquid separation tank. However, the ultra-high pressure, the high pressure, the low pressure and the ultra-low pressure only indicate relative levels of the pressures, but are not limitations to absolute values of the pressures.

[0048] In the four-stage ethane refrigeration cycle system A, the cyclic precooling process of the ethane is presented below:

[0049] After undergoing multi-stage compression by the multi-stage ethane compressor C-201 and undergoing cooling by the ethane air cooler AC-201 into the liquid, the ethane is delivered to the ethane buffer tank V-201. Subsequently, after throttled by the first valve Val-201, the liquid ethane enters the first ethane gas-liquid separation tank V-202 for gas-liquid separation. The gas generated from the gas-liquid separation enters the inlet of the four-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation within the first ethane gas-liquid separation tank V-202, a part thereof enters the natural gas-ethane first-stage precooler E-301 through the fourth valve Val-204 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane first-stage precooler E-101 through the second valve Val-202 as a pre-refrigerant for precooling mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane first-stage precooler E-301 and the mixed refrigerant-ethane first-stage precooler E-101, respectively, also enters the inlet of the fourth-stage compressor of the multi-stage ethane compressor C-201.

[0050] A further part of liquid ethane generated from the gas-liquid separation within the first ethane gas-liquid separation tank V-202 continues to be throttled by the third valve Val-203 and then enters the ethane high pressure gas-liquid separator V-203 for gas-liquid separation. The gas ethane generated from the gas-liquid separation enters the inlet of the third-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation within the ethane high pressure gas-liquid separator 203, a part thereof enters the natural gas-ethane second-stage precooler E-302 through the seventh valve Val-207 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane second-stage precooler E-102 through the fifth valve Val-205 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane second-stage precooler E-302 and the mixed refrigerant-ethane second-stage precooler E-102, respectively, enters the inlet of the third-stage compressor of the multi-stage ethane compressor C-201 together.

[0051] A further part of the liquid generated from the gas-liquid separation within the second ethane gas-liquid separation tank V-203 continues to be throttled by the sixth valve Val-206 and then enters the ethane low-pressure gas-liquid separator V-204 for gas-liquid separation. The gas ethane generated from gas-liquid separation enters the inlet of the second-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation in the ethane low pressure gas-liquid separator V-204, a part thereof enters the natural gas-ethane third-stage precooler E-303 through the tenth valve Val-210 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane third-stage precooler E-103 via the eighth valve Val-208 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane third-stage precooler E-303 and the mixed refrigerant-ethane third-stage precooler E-103 enters the inlet of the second-stage compressor of the multi-stage ethane compressor C-201.

[0052] A further part of the liquid generated from gas-liquid separation within the third ethane gas-liquid separation tank V-204 continues to be throttled by the ninth valve Val-209 and then enters the ethane ultra-low pressure gas-liquid separator V-205 for gas-liquid separation. The gas ethane generated from the gas-liquid separation enters the inlet of the first-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation in the ethane ultra-low pressure gas-liquid separator V-205, a part thereof enters the natural gas-ethane first-stage precooler E-304 via the twelfth valve Val-212 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane first-stage precooler E-104 via the eleventh valve Val-211 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane first-stage precooler E-304 and the mixed refrigerant-ethane first-stage precooler E-104 enters the inlet of the first-stage compressor of the multi-stage ethane compressor C-201 together.

[0053] The respective multi-stage devices in the ethane refrigeration cycle system A all have four stages and correspond to a precooling method including four stages of refrigeration. In practice, each of the multi-stage devices may have three or five stages and correspond to a precooling method including three or five stages of refrigeration. In the three-stage refrigeration mode, three precooling heat exchangers are selected, which reduces the costs of the ethane compressor but increases the power of the ethane compressor and the energy consumption of the device; in the five-stage refrigeration mode, five precooling heat exchangers are selected, which increases the costs of the ethane compressor but reduces the power of the ethane compressor and the energy consumption of the devices. Therefore, the specific number of stages could be set as actually required.

[0054] Referring to FIG. 3, the mixed refrigerant refrigeration cycle system B in the embodiments of the present disclosure includes a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation tank V-101 and a main heat exchanger cold box CB-401. The mixed refrigerant outlet of the mixed refrigerant compressor C-101 is connected sequentially to the mixed refrigerant air cooler AC-101, the multi-stage mixed refrigerant precooler E-1 and the mixed refrigerant gas-liquid separation tank V-101 via the mixed refrigerant transmission pipeline P-3. After compressed by the mixed refrigerant compressor C-101 and air-cooled by the mixed refrigerant air cooler AC-101, the mixed refrigerant is precooled stage by stage using the cold energy of the ethane in the multi-stage mixed refrigerant precooler E-1 and then enters the mixed refrigerant air-liquid separation tank V-101 for gas-liquid separation. A gas phase outlet and a liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to inlets of two throttling refrigeration units in the main heat exchanger cold box CB-401, wherein the two throttling refrigeration units provide cold energy required for supercooling the natural gas for the main heat exchanger cold box CB-401 by means of throttling refrigeration of the mixed refrigerant; outlets of the two throttling refrigeration units are connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101; the main heat exchanger cold box CB-401 is provided therein with a heat exchange pipe P-7 for supercooling the natural gas, wherein the heat exchange pipe is connected at an input end to the natural gas transmission pipeline P-2 and connected at an output end to the liquefied natural gas outlet O.

[0055] The two throttling refrigeration units disposed within the main heat exchanger cold box CB-401 are mainly used for throttling and depressurizing the mixed refrigerant, to further lower the temperature of the mixed refrigerant. In theory, any device that can implement mixed refrigerant throttling refrigeration can be used as the throttling refrigeration unit. The two throttling refrigeration units operate independently, and the gas phase outlet and the liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to different throttling refrigeration units. The outlets of the two throttling refrigeration units may be merged and connected to the mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101, or may be connected respectively to the mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101.

[0056] In the embodiments of the present disclosure, continuing with FIG. 3, each throttling refrigeration unit includes an inflow heat exchange pipe, a throttling valve, a two-phase flow uniform distribution tank and an outflow heat exchange pipe, wherein the inflow heat exchange pipe, the two-phase flow distribution tank and the outflow heat exchange pipe are all disposed in the main heat exchanger cold box CB-401. An inlet of the inflow heat exchange pipe is used for inputting the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank V-101, an outlet of the inflow heat exchange pipe is sequentially connected to inlets of the throttling valve and the two-phase flow uniform distribution tank, the gas phase outlet and the liquid phase outlet of the two-phase flow uniform distribution tank are respectively connected to an inlet of the outflow heat exchange pipe, and the outlet of the outflow heat exchange tube is connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101. The mixed refrigerant output from the mixed refrigerant gas-liquid separation tank V-101 further undergoes throttling refrigeration via the throttle valve after absorbing cold energy in the inflow heat exchange pipe. Thereafter, the mixed refrigerant enters the two-phase flow uniform distribution tank for gas-liquid uniform distribution and is then input to the outflow heat exchange pipe to provide the natural gas and the mixed refrigerant with cold energy. In the embodiments of the present disclosure, the two throttling refrigeration units share the same outflow heat exchange pipe, but since the inflow heat exchange pipes in the two throttling refrigeration units are independent of each other, the throttling refrigeration unit connected to the liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes a first inflow heat exchange pipe P-4, a first throttle valve Val-401, a first two-phase flow uniform distribution tank V-401 and an outflow heat exchange pipe P-6, and the throttling refrigeration unit connected to the gas phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes a second inflow heat exchange pipe P-5, a second throttle valve Val-402, a second two-phase flow uniform distribution tank V-402 and an outflow heat exchange pipe P-6. The mixed refrigerants input respectively from the first inflow heat exchange pipe P-4 and the second inflow heat exchange pipe P-5 finally converge into the outflow heat exchange pipe P-6. Moreover, the natural gas delivered via the natural gas transmission pipe P-2 can further absorb the cold energy of the mixed refrigerant within the outflow heat exchange pipe P-6 upon entering the heat exchange pipe P-7 in the main heat exchanger cold box CB-401, thus making it possible to supercool the natural gas to a required output temperature.

[0057] In the embodiments of the present disclosure, the mixed refrigerant compressor C-101 is preferably a two-stage compressor. In the case, in the mixed refrigerant refrigeration cycle system B, the overall processing flow is presented below: two-stage compression is utilized for the mixed refrigerant compressor C-101; the mixed refrigerant is compressed in two stages and then output to the mixed refrigerant air cooler AC-101 from the second-stage compression outlet for air cooling, and thereafter sequentially passes through the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane three-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104 such that the mixed refrigerant can absorb the cold energy of the liquid ethane to be cooled gradually. After ethane four-stage precooling, the mixed refrigerant is cooled to a preset precooling temperature (which may be set from −60° C. to −80° C.). Subsequently, the precooled mixed refrigerant is delivered into the gas-liquid separation tank V-101. The gas phase refrigerant exiting from the top of the gas-liquid separation tank V-101 enters into the second inflow heat exchange pipe P-5 in the main heat exchanger cold box CB-401 and absorbs cold energy therein; thereafter, the gas phase refrigerant further undergoes throttling refrigeration via the second throttle valve Val-402, enters the two-phase flow uniform distribution tank V-402 for gas-liquid uniform distribution, and is then delivered to the bottom of the outflow heat exchange pipe P-6. The liquid phase refrigerant exiting from the bottom of the separation tank V-101 enters the first inflow heat exchange pipe P-4 of the main heat exchanger cold box CB-401 and absorbs cold energy therein; the liquid phase refrigerant further undergoes throttling refrigeration via the first throttle valve Val-401 and enters the first two-phase flow uniform distribution tank V-401 for gas-liquid uniform distribution; then, the liquid phase refrigerant is delivered to the middle of the outflow heat exchange pipe P-6 to be mixed with the gas phase refrigerant flowing upwardly from the bottom, exits from the main heat exchanger cold box CB-401 after releasing cold energy to the natural gas in the heat exchange pipe P-7 and being reheated, and subsequently enters the inlet of the first-stage compressor of the mixed refrigerant compressor C-101 to continue the cycle refrigeration.

[0058] It is worth noting that, in the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, the control temperature of the natural gas in each stage can be reasonably adjusted and controlled as actually required. In the natural gas liquefaction system, the cooling temperature directly affects the flash evaporation amount, specifically: the higher the cooling temperature, the greater the flash evaporation amount, and the lower the cooling temperature, the smaller the flash evaporation amount. Therefore, the final specific cooling temperature of the natural gas can be determined according to the actual requirement of the system for the flash evaporation amount. In the embodiments of the present disclosure, the temperature of the natural gas in each stage can be controlled according to the following temperature interval: after being purified and cooled by the ambient air, the natural gas enters the multi-stage natural gas precooler E-3 to gradually lower the temperature of the natural gas. Having been precooled by the ethane, the natural gas is cooled to −60 to −80° C. Subsequently, the precooled natural gas enters the main heat transfer cold box CB-401, and is then output from the liquefied natural gas outlet O until cooled to −150 to −162° C.

[0059] In addition, according to the present disclosure, in the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, for the natural gas, the ethane and the mixed refrigerant, an air cooler is utilized for first precooling. However, if the ambient temperature is high, it is hard to cool the natural gas and the mixed refrigerant through air cooling, or there will be high energy consumption in the subsequent procedure even though the natural gas and the mixed refrigerant can be cooled. Accordingly, if the present disclosure is applied to the scenario of a high ambient temperature, a one-stage or two-stage propane or seawater precooling system is added before or behind the air cooler, to improve the efficiency of the system. In a further embodiment of the present disclosure, three water coolers are added to cooperate with the original air cooler, as shown in FIGS. 4, 5 and 6. Specifically, a first water cooler E-100 is added between the mixed refrigerant air cooler AC-101 and the multi-stage mixed refrigerant precooler E-1, a second water cooler E-200 is added between the ethane air cooler AC-201 and the ethane buffer tank V-201, and a third water cooler E-300 is added between the natural gas inlet I and the multi-stage natural gas precooler E-3. With the addition of the three water coolers, it can be guaranteed that the mixed refrigerant, the ethane and the raw natural gas can be cooled to 13° C. by the combination of air cooling and water cooling, to facilitate subsequent cooling. For the three additional water coolers, seawater is preferably used as the cooling water, i.e., seawater precoolers are preferably used, to reduce the energy consumption costs for water cooling. Of course, the three additional water coolers can be replaced by propane precoolers, which could be selected as actually required.

[0060] In the embodiments of the present disclosure, there may also be provided a cascade refrigeration natural gas liquefaction process according to the cascade refrigeration natural gas liquefaction system, which includes steps S1 through S3.

[0061] S1: gas ethane is compressed in multiple stages by the multi-stage ethane compressor C-201, the compressed ethane is air-cooled by the ethane air cooler AC-201 into liquid ethane, and the liquid ethane is then stored in the ethane buffer tank V-201; the liquid ethane in the ethane buffer tank V-201 is stably delivered to the multi-stage throttling refrigeration device V-2 to be throttled, depressurized and refrigerated stage by stage; each stage of throttling refrigeration device throttles and depressurizes the input liquid ethane and then further performs gas-liquid separation; the gas ethane obtained by separation is reflowed back and delivered to the multi-stage ethane compressor C-201 to be recompressed; the liquid ethane obtained by separation is partly delivered to a next stage of throttling refrigeration device, and partly delivered to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 to provide cold energy by means of heat exchange, wherein the gas ethane after heat exchange is reflowed back and delivered to the multi-stage ethane compressor C-201 to be recompressed.

[0062] S2: the mixed refrigerant is compressed by the mixed refrigerant compressor C-101, the compressed mixed refrigerant is air-cooled by the compressor air cooler AC-101 into a liquid mixed refrigerant, the liquid mixed refrigerant passes through the multi-stage mixed refrigerant precooler E-1 and then exchanges heat with the liquid ethane delivered in the multi-stage throttling refrigeration device V-2 to thus lower the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling, and the precooled liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank V-101 to implement gas-liquid separation; the gas mixed refrigerant and the liquid mixed refrigerant obtained through gas-liquid separation enter the main heat exchange cold box CB-401, respectively, to further absorb cold energy to lower the temperature, and then after throttled and depressurized via the respective throttling refrigeration units, are returned to the main heat exchanger cold box CB-401 to provide cold energy for lowering the temperature of the natural gas and the mixed refrigerant, which in turn causes the temperature thereof to increase; the mixed refrigerant with the increased temperature flows back to the mixed refrigerant compressor C-101 to be recompressed.

[0063] S3: the purified natural gas to be liquefied is input to the multi-stage natural gas precooler E-3 via a natural gas inlet I to exchange heat with the liquid ethane delivered in the multi-stage throttling refrigeration device V-2, thus lowering the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling; the precooled natural gas further enters the main heat exchanger cold box CB-401, and exchanges heat with the mixed refrigerant that is returned to the main heat exchanger cold box CB-401 after throttled and depressurized to complete liquefaction supercooling; the liquefied and supercooled natural gas is output via a liquefied natural gas outlet O.

[0064] In the natural gas liquefaction process described above, the temperature of the precooled liquid mixed refrigerant and the precooled natural gas is controlled preferably between −60 and −80° C.

[0065] Hereinafter, a specific embodiment is provided to describe the specific implementations and technical effects of the cascade natural gas liquefaction system and process.Embodiment

[0066] The cascade natural gas liquefaction system according to this embodiment is shown in FIG. 1. The details about the structures of the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B are omitted herein for brevity, and the focus below will be given to respective fluid circulation processes in the two systems and respective process parameters.

[0067] In the embodiment, the purified natural gas with a raw gas pressure of 55 kg / cm2 sequentially passes through the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303 and the natural gas-ethane fourth-stage precooler E-304 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., then enters the main heat exchanger cold box CB-401 to be further cooled to −162° C., outputs from the cold box via the liquefied natural gas outlet O, and is finally delivered as the liquefied and supercooled natural gas to the LNG storage tank for storage.

[0068] The mixed refrigerant according to the embodiment is comprised of nitrogen, methane and ethane. After pressurized to 55 kg / cm2 by the mixed refrigerant compressor C-101, the mixed refrigerant enters the mixed refrigerant air cooler AC-101 to be cooled to 13° C., and then sequentially passes through the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane third-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., and thereafter enters the mixed refrigerant gas-liquid separation tank V-101. The liquid-phase refrigerant exiting from the bottom of the mixed refrigerant gas-liquid separation tank V-101 enters the main heat exchanger cold box CB-401 to be further cooled, and is drawn out and enters the first two-phase flow uniform distribution tank V-401 after passing through the first throttle valve Val-401, and then injected into the middle of the outflow heat exchange pipe P-6. The gas-phase refrigerant exiting from the top of the mixed refrigerant gas-liquid separation tank V-101 enters the main heat exchanger cold box CB-401 to be further cooled to −162° C. ; and is drawn out and enters the second two-phase flow uniform distribution tank V-402 after passing through the second throttle valve V-402, and then injected into the bottom of the outflow heat exchange pipe P-6, further discharged out of the cold box until reheated to −73° C. by absorbing, in the heat exchanger, the heat of the natural gas in the heat exchange pipe P-7, and finally returned to the mixed refrigerant compressor C-101.

[0069] After pressurized by the multi-stage ethane compressor C-201 wherein the multiple stages are specifically four stages, the ethane enters the ethane air cooler AC-201, and is then condensed therein into a liquid and stored in the ethane buffer tank V-201. Subsequently, the ethane is throttled, depressurized and refrigerated stage by stage in the four-stage throttling refrigeration device with a body formed by the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204, the fourth ethane gas-liquid separation tank V-205, the first valve Val-201, the third valve Val-203, the sixth valve Val-206 and the ninth valve Val-209, then provides cold energy to the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303, the natural gas-ethane fourth-stage preccoler E-304, the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane third-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104, and finally evaporates and flows back to the multi-stage ethane compressor C-201 to be recompressed stage by stage. See above for the detailed process of the ethane flow path, which is omitted herein for brevity.

[0070] It is worth noting that the specific form of the main heat exchanger cold box CB-401 is not limited, and the four heat exchange pipes therein, namely the first inflow heat exchange pipe P-4, the second inflow heat exchange pipe P-5, the outflow heat exchange pipe P-6 and the heat exchange pipe P-7, should be designed according to the type of the heat exchanger. In the embodiment of the present disclosure, the main heat exchanger cold box CB-401 can be implemented by a plate-fin heat exchanger including heat exchange channels therein, wherein the form of the heat exchange pipes may be directly the heat exchange channels inside the heat exchanger. Moreover, if the main heat exchanger cold box CB-401 adopts a coil-wound heat exchanger, the form of the heat exchange pipes may be built-in heat exchange pipes.

[0071] Hereinafter, comparative analysis of energy consumption benefits of the present disclosure and other prior art process will be made according to the cascade natural gas liquefaction system and process provided in the embodiment.

[0072] The analysis of energy consumption benefits of the present disclosure: in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the multi-stage ethane compressor in the present disclosure is about 49,800 KW, and the shaft power of the mixed refrigerant compressor is about 49,600 KW. The shaft powers of ethane compressor and the mixed refrigerant compressor are substantially the same, which can avoid the limitation of the maximum power of the gas turbine and the motor and maximize the single-line production capacity. In addition, the present disclosure can also avoid complex control. The system and the process according to the present disclosure are suitable for a high-latitude cold region, in particular the Arctic region with a low ambient temperature all year round, where this process can be carried out only through air cooling. For the process according to the present disclosure, the energy consumption of natural gas liquefaction per unit is as low as about 0.18 KW / Nm3.

[0073] The analysis of energy consumption benefits of the first type of comparative process: if a traditional propane pre-cooling mixed refrigerant is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the propane compressor is 30,300 KW, and the shaft power of the mixed refrigerant compressor is about 75,300 KW. The two compressors have a great discrepancy in shaft power, wherein the shaft power of the mixed refrigerant compressor is particularly large. For this process, the energy consumption of liquefaction per unit is about 0.21 KW / Nm3.

[0074] The analysis of energy consumption benefits of the second type of comparative process: if a dual mixed refrigerant process is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, when in the summer working condition, the power of the mixed refrigerant in the precooling stage is 79,000 KW, and the power of the mixed refrigerant in the deep cooling stage is 53,000 KW; when in the winter working condition, the power of the mixed refrigerant in the precooling stage is 35,000 KW, and the power of the mixed refrigerant in the deep cooling stage is 53,000 KW. That is, the power of the mixed refrigerant in the precooling stage is large in summer while being small in winter. In theory, the dual mixed refrigerant process is an LNG process suitable for a region with a low ambient temperature. However, the process has the following disadvantages of: complex working conditions of the two mixed refrigerants, difficult on-site operation, and frequent adjustments of the refrigerant ratio in the precooling stage according to the ambient temperature (otherwise, the adjustment can only be implemented through the backflow of the compressor, which will result in exceedingly high power consumption). For this process, the energy consumption for liquefaction per unit in summer is 0.256 KW / Nm3, and the energy consumption for liquefaction per unit in winter is 0.175 KW / Nm3.

[0075] The analysis of energy consumption benefits of the third type of comparative process: if a conventional cascade process is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the propane compressor is 53,000 KW, the shaft power of the ethane compressor is 41,300 KW, and the shaft power of the methane compressor is 14,200 KW. For this process, three compressors are required, where the propane compressor has the greatest power, the total power is large, and the energy consumption is high. The energy consumption for liquefaction per unit is about 0.234 KW / m3.

[0076] As seen above, the comprehensive energy consumption for liquefaction per unit in the present disclosure is lower than that in the propane-precooling mixed refrigerant process and the conventional cascade process. Although it is slightly higher than that of the dual mixed refrigerant process in the winter working condition, the energy consumption of the dual mixed refrigerant process is exceedingly high in summer. In a word, the present disclosure is advantageous over other processes in comprehensive energy consumption.

[0077] The embodiments described above are only a part of the preferred implementations of the present disclosure, without suggesting any limitation to the present disclosure. Those skilled in the related technical field are allowed to make various changes and variations thereto, without departing from the spirit and scope of the present disclosure. Therefore, any technical solution obtained by equivalent replacement or variation should fall within the scope of protection of the present disclosure.

Examples

embodiment

[0066]The cascade natural gas liquefaction system according to this embodiment is shown in FIG. 1. The details about the structures of the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B are omitted herein for brevity, and the focus below will be given to respective fluid circulation processes in the two systems and respective process parameters.

[0067]In the embodiment, the purified natural gas with a raw gas pressure of 55 kg / cm2 sequentially passes through the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303 and the natural gas-ethane fourth-stage precooler E-304 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., then enters the main heat exchanger cold box CB-401 to be further cooled to −162° C., outputs from the cold box via the liquefied natural gas outlet O, and is finally delivered as the liquefied and supercooled n...

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of refrigeration and cryogenic engineering, and more specifically, to a cascade refrigeration natural gas liquefaction system and process.BACKGROUND

[0002] Liquefied Natural Gas (LNG) is currently the fastest growing energy source in the world. Vigorously developing LNG plays an important role in optimizing the energy structure, effectively addressing the dual issues of energy supply security and ecological environment protection, and achieving sustainable economic and social development.

[0003] LNG, as a vehicle fuel, has various advantages. First of all, LNG can save about 25-35% of costs than the fuel oil, i.e., substituting gas for oil can bring about considerable economic benefits. Secondly, natural gas is a fuel with a high octane number, where the octane number is an important indicator for evaluating the fuel performance. If a fuel with a high octane number is used in a vehicle, the engine is less likely to experience knocking combustion, which can prolong the service life of the engine. Thirdly, as compared to gasoline, diesel and LPG, LNG is cheaper, safer and more environment-friendly, has a higher storage efficiency, a longer vehicle driving range and a longer engine service life, and can use the cold energy released during operation of the engine for air conditioning. This can not only save energy but also enable comprehensive utilization of energy.

[0004] Small and medium-sized natural gas liquefaction plants are mainly built near the natural gas main network to serve the surrounding markets. Basically, a single mixed refrigerant (SMR) refrigeration process is used. Although the process incurs higher energy consumption than the propane-precooled mixed-refrigerant (C3-MR) or Cascade process, it requires simple devices and a small investment, which is the optimal process from the perspective of comprehensive investment return cycle.

[0005] However, for multi-base type large-scale LNG plants, the SMR process is not the optimal solution, because the power of the refrigeration compressor is increased as the liquefaction scale is increased. According to the current motor or gas-turbine manufacturing capabilities, if the power is too great, it is required to connect multiple compressors in parallel since only one compressor cannot meet the requirement. In the case that the scale of a single production line is large, if the SMR process is still used, it is required to connect multiple compressors in parallel, which will greatly increase the fixed investment cost and bring about high energy consumption. In the circumstance, the propane-precooled mixed-refrigerant (C3-MR) or Cascade process, or other cascade refrigeration process has obvious advantages. As compared with the SMR, the cascade refrigeration process needs a similar, or even lower, fixed investment and brings about significantly reduced energy consumption. Therefore, most of the multi-base type large-scale LNG plants adopt the cascade refrigeration process, particularly the C3-MR or Cascade process, but a few of them utilize the dual mixed refrigerant process or the multi-stage mixed refrigerant process.SUMMARY

[0006] The objectives of the present disclosure are to solve the problems existing in the prior art and provide a cascade refrigeration large-scale natural gas liquefaction system and process.

[0007] The technical solution adopted in the present disclosure specifically includes:

[0008] In a first aspect, the present disclosure provides a cascade refrigeration natural gas liquefaction system, which comprises an ethane refrigeration cycle system and a mixed refrigerant refrigeration cycle system;

[0009] the ethane refrigeration cycle system comprises a multi-stage ethane compressor, an ethane air cooler, an ethane buffer tank, a multi-stage throttling refrigeration device, a multi-stage natural gas precooler and a multi-stage mixed refrigerant precooler; the multi-stage ethane compressor, the ethane air cooler, the ethane buffer tank and the multi-stage throttling refrigeration device are sequentially connected via an ethane transmission pipeline and form an ethane circulation loop such that the ethane can be buffered and stored in the ethane buffer tank after compressed by the multi-stage ethane compressor and cooled by the ethane air cooler, then enter the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage, and flow back to the multi-stage ethane compressor after providing cold energy to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler; an input end of the multi-stage natural gas precooler is connected to a natural gas inlet I such that the input natural gas to be liquefied can be transmitted via a natural gas transmission pipeline to the mixed refrigerant refrigeration cycle system after precooled stage by stage using the cold energy of the ethane in the multi-stage natural gas precooler;

[0010] the mixed refrigerant refrigeration cycle system comprises a mixed refrigerant compressor, a mixed refrigerant air cooler, a mixed refrigerant gas-liquid separation tank and a main heat exchanger cold box; a mixed refrigerant outlet of the mixed refrigerant compressor is connected sequentially to the mixed refrigerant air cooler, the multi-stage mixed refrigerant precooler and the mixed refrigerant gas-liquid separation tank via a mixed refrigerant transmission pipeline, such that, after compressed by the mixed refrigerant compressor and air-cooled by the mixed refrigerant air cooler, the mixed refrigerant can be precooled stage by stage using the cold energy of the ethane in the multi-stage mixed refrigerant precooler and then enter the mixed refrigerant air-liquid separation tank for gas-liquid separation; a gas phase outlet and a liquid phase outlet of the mixed refrigerant gas-liquid separation tank are respectively connected to inlet ends of two throttling refrigeration units in the main heat exchanger cold box such that the two throttling refrigeration units can provide the main heat exchanger cold box with cold energy required for supercooling the natural gas through throttling refrigeration of the mixed refrigerant; outlet ends of the two throttling refrigeration units are connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; the main heat exchanger cold box is provided therein with a heat exchange pipe for supercooling the natural gas, wherein the heat exchange pipe is connected at an input end to the natural gas transmission pipeline and connected at an output end to a liquefied natural gas outlet O.

[0011] As a preferred option of the first aspect described above, the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler are the same and correspond one-to-one; each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttle valve installed on an inlet pipe of the gas-liquid separation tank, wherein a liquid phase outlet of the gas-liquid separation tank is divided into three branches that respectively lead to refrigerant inlets of the corresponding stage of the natural gas precooler and the mixed refrigerant precooler, and refrigerant outlets of the natural gas precooler and the mixed refrigerant precooler and a gas phase outlet of the gas-liquid separation tank are connected to an inlet of the corresponding stage of the ethane compressor.

[0012] As a preferred option of the first aspect described above, the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler are from 3 to 5.

[0013] As a preferred option of the first aspect described above, each throttling refrigeration unit comprises an inflow heat exchange pipe, a throttle valve, a two-phase flow uniform distribution tank and an outflow heat exchange pipe, wherein the inflow heat exchange pipe, the two-phase flow uniform distribution tank and the outflow heat exchange pipe are all disposed in the main heat exchanger cold box; an inlet of the inflow heat exchange pipe is used for inputting the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank, an outlet of the inflow heat exchange pipe is sequentially connected to inlets of the throttling valve and the two-phase flow uniform distribution tank, a gas phase outlet and a liquid phase outlet of the two-phase flow uniform distribution tank are respectively connected to an inlet of the outflow heat exchange pipe, and an outlet of the outflow heat exchange pipe is connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank further undergoes throttling refrigeration via the throttle valve after absorbing cold energy in the inflow heat exchange pipe, then enters the two-phase flow uniform distribution tank for gas-liquid uniform distribution, and is input to the outflow heat exchange pipe to provide the cold energy for the natural gas and the mixed refrigerant.

[0014] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed between the ethane air cooler and the ethane buffer tank.

[0015] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed between the mixed refrigerant air cooler and the multi-stage mixed refrigerant precooler.

[0016] As a preferred option of the first aspect described above, a propane precooler or a water cooler is disposed respectively between the natural gas inlet I and the multi-stage natural gas precooler.

[0017] In a second aspect, the present disclosure provides a cascade refrigeration natural gas liquefaction process, which is implemented using the cascade refrigeration natural gas liquefaction system of any of the embodiments of the first aspect described above, the cascade refrigeration natural gas liquefaction process comprising:

[0018] S1: gas ethane is compressed in multiple stages by the multi-stage ethane compressor, the compressed ethane is air-cooled by the ethane air cooler into liquid ethane, and the liquid ethane is then stored in the ethane buffer tank; the liquid ethane in the ethane buffer tank is stably delivered to the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage; each stage of throttling refrigeration device throttles and depressurizes the input liquid ethane and then further performs gas-liquid separation; the gas ethane obtained by separation is reflowed back and delivered to the multi-stage ethane compressor to be recompressed; the liquid ethane obtained by separation is partly delivered to a next stage of throttling refrigeration device, and partly delivered to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler to provide cold energy by means of heat exchange, wherein the gas ethane after heat exchange is reflowed back and delivered to the multi-stage ethane compressor to be recompressed;

[0019] S2: the mixed refrigerant is compressed by the mixed refrigerant compressor, the compressed mixed refrigerant is air-cooled by the compressor air cooler into a liquid mixed refrigerant, the liquid mixed refrigerant passes through the multi-stage mixed refrigerant precooler and then exchanges heat with the liquid ethane delivered in the multi-stage throttling refrigeration device to thus lower the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling, and the precooled liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank to implement gas-liquid separation; the gas mixed refrigerant and the liquid mixed refrigerant obtained through gas-liquid separation enter the main heat exchange cold box, respectively, to further absorb cold energy to lower the temperature, and then after throttled and depressurized via the respective throttling refrigeration units, are returned to the main heat exchanger cold box for providing cold energy for lowering the temperature of the natural gas and the mixed refrigerant, which in turn causes the temperature thereof to increase; the mixed refrigerant with the increased temperature flows back to the mixed refrigerant compressor to be recompressed; and

[0020] S3: the purified natural gas to be liquefied is input to the multi-stage natural gas precooler via a natural gas inlet I to exchange heat with the liquid ethane delivered in the multi-stage throttling refrigeration device, thus lowering the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling; the precooled natural gas further enters the main heat exchanger cold box, and exchanges heat with the mixed refrigerant returned to the main heat exchanger cold box after throttled and depressurized to complete liquefaction supercooling; the liquefied and supercooled natural gas is output via a liquefied natural gas outlet O.

[0021] As a preferred option of the second aspect described above, a temperature of the precooled liquid mixed refrigerant and the precooled natural gas is from −60° C. to −80° C.

[0022] As a preferred option of the second aspect described above, the mixed refrigerant is composed of nitrogen, methane and ethane.

[0023] As compared with the prior art, the present disclosure has the following advantageous effects:

[0024] 1. For the conventional cascade process, ethene is used in the second-stage refrigeration phase, which needs to be purchased externally. However, a cascade refrigeration cycle natural gas liquefaction process is adopted in the present disclosure, and refrigerants used therein include propane, ethane and a mixed refrigerant, wherein the mixed refrigerant is composed of nitrogen, methane and ethane. Except for nitrogen that needs to be supplied externally (e.g., from public utilities), the rest refrigerants are sourced from the natural gas per se, for which additional purchase is not required.

[0025] 2. Nowadays, the most commonly used process for natural gas liquefaction is C3-MR, namely a propane-precooled mixed refrigerant process, which is most advantageous as having low energy consumption and being convenient in operation. However, the most prominent problem of the process is a great discrepancy between the power of the propane compressor and the power of the mixed refrigerant compressor, wherein the power of the mixed refrigerant compressor is limited by the maximum power of the gas turbine or the motor, thus restricting the single-line production capacity. In order to solve the problem, the American Pharmacy Cooperative, Inc. (APCI) employs the AP-Split MR® process wherein a part of power of the mixed refrigerant compressor is distributed to the gas turbine or motor that drives the propane compressor. In the way, this process can reduce the power of the gas turbine or motor driving the mixed refrigerant compressor while maximizing the single-line production capacity. Such control system is relatively complex, since a gas turbine or motor needs to drive two compressor systems, and startup and shutdown control of the compressors is complicated.

[0026] The mixed refrigerant precooled using ethane adopted herein can cool the natural gas and the mixed refrigerant to −80° C., which can only be cooled to −40° C. if precooled by propane. Accordingly, the present disclosure can achieve a lower precooling temperature, and can enable the power of the ethane compressor and the power of the mixed refrigerant compressor to be close to each other by adjusting the temperature after precooling, to thus maximize the single-line production capacity. Moreover, this can avoid a complex solution wherein a gas turbine or motor controls two compressors.

[0027] 3. For the current C 3-MR process and the Cascade process, generally three-stage or four-stage propane precooling is adopted to precool the raw gas and the refrigerant to about −35° C. In contrast, according to the present disclosure, a propane refrigeration or seawater precooling step may be added before precooling using ethane, and the propane refrigeration may adopt a one-stage or two-stage refrigeration method according to the needs, to further improve the efficiency of the entire device. The process according to the present disclosure has the following advantage: propane one-stage or two-stage precooling or seawater auxiliary precooling is used therein, which can cool the raw gas and the refrigerant to about 12° C. The process is especially suitable for regions with a great temperature difference between winter and summer, in particular frigid or polar zones. If the ambient temperature is low, the propane compressor refrigeration cycle or seawater precooling can be stopped, since the raw gas and the refrigerant are cooled to 12° C. or even lower by the ambient temperature. In the way, the present disclosure can greatly reduce the energy consumption for operation. The present disclosure is especially suitable for the frigid or polar zones. In these zones, since the ambient temperature above 0° C. lasts only one or two months per year, the propane precooling compressor or seawater precooling only needs to run for a short period. However, for the conventional three-stage or four-stage propane precooling process with the requirement for cooling the raw gas and the refrigerant to −35° C., it is hard to cool the two to such temperature only by means of the ambient temperature, even in winter in the frigid zones. Therefore, the propane compressor needs to keep running, which wastes lots of energy.

[0028] 4. The comprehensive liquefaction energy consumption per unit in the present disclosure is lower than that in the propane precooling mixed refrigerant process and the conventional cascade process. Although it is slightly higher than that of the dual mixed refrigerant process in the winter working condition, the energy consumption of the dual mixed refrigerant process is exceedingly high in summer. To sum up, the present disclosure is still advantageous in terms of energy consumption.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a schematic diagram of a structure of a cascade refrigeration natural gas liquefaction system;

[0030] FIG. 2 is an enlarged schematic diagram of the ethane refrigeration cycle system in FIG. 1;

[0031] FIG. 3 is an enlarged schematic diagram of the mixed refrigerant refrigeration cycle system in FIG. 2;

[0032] FIG. 4 is a schematic diagram of a structure of a further cascade refrigeration natural gas liquefaction system;

[0033] FIG. 5 is an enlarged schematic diagram of the ethane refrigeration cycle system in FIG. 4; and

[0034] FIG. 6 is an enlarged schematic diagram of the mixed refrigerant refrigeration cycle system in FIG. 4.

[0035] Reference signs in the drawings are described below: an ethane refrigeration cycle system A, and a mixed refrigerant refrigeration cycle system B, wherein: the ethane refrigeration cycle system A includes: a multi-stage ethane compressor C-201, an ethane air cooler AC-201, an ethane buffer tank V-201, a first ethane gas-liquid separation tank V-202, a second ethane gas-liquid separation tank V-203, a third ethane gas-liquid separation tank V-204, a fourth ethane gas-liquid separation tank V-205, a natural gas-ethane first-stage precooler E-301, a natural gas-ethane second-stage precooler E-302, a natural gas-ethane third-stage precooler E-303, a natural gas-ethane fourth-stage precooler E-304, a mixed refrigerant-ethane first-stage precooler E-101, a mixed refrigerant-ethane second-stage precooler E-102, a mixed refrigerant-ethane third-stage precooler E-103, a mixed refrigerant-ethane fourth-stage precooler E-104, a first valve Val-201, a second valve Val-202, a third valve Val-203, a fourth valve Val-204, a fifth valve Val-205, a sixth valve Val-206, a seventh valve Val-207, an eighth valve Val-208, a ninth valve Val-209, a tenth valve Val-210, an eleventh valve Val-211, and a twelfth valve Val-212; the mixed refrigerant refrigeration cycle system B includes: a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation tank V-101, a first two-phase flow uniform distribution tank V-401, a second two-phase flow uniform distribution tank V-402, a main heat exchanger cold box CB-401, a first throttle valve Val-401, and a second throttle valve Val-402; and connecting pipelines in the two systems include: an ethane transmission pipeline P-1, a natural gas transmission pipeline P-2, a mixed refrigerant transmission pipeline P-3, a first inflow heat exchange pipe P-4, a second inflow heat exchange pipe P-5, an outflow heat exchange pipe P-6, and a heat exchange pipe P-7.DETAILED DESCRIPTION OF EMBODIMENTS

[0036] In order to make the above objective, features and advantages of the present disclosure much clearer, detailed description about the specific implementations of the present disclosure will be provided below with reference to the drawings. In the following description, lots of details will be illustrated to enable full understanding of the present disclosure. However, the present disclosure could be implemented in other manners than the one described herein, and allows those skilled in the art to make similar improvements without departing from the spirit of the present disclosure. The present disclosure is therefore not limited to the specific embodiments described below. The technical features in the various embodiments of the present disclosure could be combined correspondingly, without conflicting with each other.

[0037] In the description of the present disclosure, it would be appreciated that, if an element is considered to be “connected” to a further element, the element may be directly connected to the further element, or indirectly connected to the latter (i.e., there is an intermediate element). However, if the element is described as being “directly” connected to the further element, no intermediate element is included.

[0038] As used herein, the terms “first,”“second,” and the like, are used only for differentiation and should not be construed as indicating or implying relative importance, or implicitly indicating a number of technical features involved.

[0039] The present disclosure provides a cascade refrigeration natural gas liquefaction system, in which a purified natural gas and a compressed and cooled mixed refrigerant are respectively precooled by ethane in multiple stages, the precooled natural gas and mixed refrigerant are then cooled, liquefied and supercooled by a throttled, depressurized and reversely-flowing mixed refrigerant, and the liquefied and supercooled natural gas is delivered to a downstream LNG storage tank for storage while the liquefied and supercooled mixed refrigerant is throttled and returned to the main heat exchanger to provide cold energy for the system.

[0040] Referring to FIG. 1, in a preferred embodiment of the present disclosure, a specific implementation of the cascade refrigeration natural gas liquefaction system is shown, which includes an ethane refrigeration cycle system A and a mixed refrigerant cycle system B.

[0041] Referring to FIG. 2, the ethane refrigeration cycle system A in the embodiments of the present disclosure includes a multi-stage ethane compressor C-201, an ethane air cooler AC-201, an ethane buffer tank V-201, a multi-stage throttling refrigeration device V-2, a multi-stage natural gas precooler E-3, and a multi-stage mixed refrigerant precooler E-1; the multi-stage ethane compressor C-201, the ethane air cooler AC-201, the ethane buffer tank V-201 and the multi-stage throttling refrigeration device V-2 are sequentially connected via an ethane transmission pipeline P-1 and form an ethane circulation loop such that the ethane can be buffered and stored in the ethane buffer tank V-201 after compressed by the multi-stage ethane compressor C-201 and cooled by the ethane air cooler AC-201, then enter the multi-stage throttling refrigeration device V-2 to be throttled, depressurized and refrigerated stage by stage, and flow back to the multi-stage ethane compressor C-201 after providing cold energy to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1; an input end of the multi-stage natural gas precooler E-3 is connected to the natural gas inlet I such that the input natural gas to be liquefied can be transmitted via the natural gas transmission pipeline P-2 to the mixed refrigerant refrigeration cycle system B after precooled stage by stage using the cold energy of the ethane in the multi-stage natural gas precooler E-3.

[0042] It is worth noting that the multi-stage ethane compressor C-201 is a compressor having a multi-stage compression function and formed by a plurality of compression units in cascade, wherein each stage of compression unit can receive the ethane output by the previous stage of compression unit and further compress the same, and wherein each stage of compression unit may have an air inlet, and the ethane input via the air inlet of the compression unit, if any, can be mixed with the ethane output from the previous stage of compression unit and then compressed. In this way, the ethane can be compressed stage by stage to the required pressure to facilitate subsequent cooling and liquefaction. The multi-stage throttling refrigeration device V-2 is formed by a plurality of throttling refrigeration devices in cascade, which can throttle and depressurize the cooled and liquefied liquid ethane stage by stage, thus generating cold energy to further cool the liquid ethane. A part of the liquid ethane cooled by each stage of throttling refrigeration device enters a next stage of throttling refrigeration device, and the rest thereof can supply cold energy required for precooling the natural gas and the mixed refrigerant to the outside, thus evaporating and forming gas ethane. The gas ethane resulting from throttling and the gas ethane resulting from precooling and evaporation can both return to the multi-stage ethane compressor C-201 for compression. The multi-stage natural gas precooler E-3 is formed by a plurality of precoolers in cascade, wherein each stage of precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cold energy, so as to precool the natural gas, after heat exchange with the natural gas, to a certain low temperature before the natural gas enters the mixed refrigerant refrigeration cycle system B. The multi-stage mixed refrigerant precooler E-1 is formed by a plurality of precoolers in cascade, wherein each stage of precooler can use the liquid ethane provided by the throttling refrigeration device as a refrigerant to provide cold energy, so as to precool the mixed refrigerant, after heat exchange with the mixed refrigerant, to a certain low temperature before the mixed refrigerant enters the mixed refrigerant refrigeration cycle system B. The specific temperature at which the mixed refrigerant and the natural gas are precooled before entering the mixed refrigerant refrigeration cycle system B could be adjusted as actually required. In the embodiments of the present disclosure, the temperature of the precooled liquid mixed refrigerant and the precooled natural gas is preferably from −60° C. to −80° C.

[0043] The mixed refrigerant according to the present disclosure could be selected as actually required, which is preferably a mixed refrigerant composed of nitrogen, methane and ethane.

[0044] In addition, the number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 can be adjusted as actually required, which is generally set to 3 to 5 stages. The number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 may be the same, or may be different. In the embodiments of the present disclosure, the number of stages included in each of the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 are the same and correspond one-to-one. Each stage of the throttling refrigeration device is composed of a gas-liquid separation tank and a throttle valve installed on an inlet pipe of the gas-liquid separation tank, wherein a liquid phase outlet of the gas-liquid separation tank is divided into three branches which respectively lead to refrigerant inlets of the corresponding stage of the natural gas precoolers and the mixed refrigerant precoolers, and a refrigerant outlet of the natural gas precooler, a refrigerant outlet of the mixed refrigerant precoolers and a gas phase outlet of the gas-liquid separation tank are connected to the inlet of the corresponding stage of the ethane compressor.

[0045] In the embodiments of the present disclosure, the multi-stage ethane compressor C-201, the multi-stage throttling refrigeration device V-2, the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 have four stages, respectively. The multi-stage ethane compressor C-201 includes 4 stages of compressors, namely a fourth-stage compressor, a third-stage compressor, a second-stage compressor and a first-stage compressor according to the compression order, wherein each stage of compressor has an inlet for inputting the gas ethane and is simultaneously configured to receive the ethane compressed by the previous stage of the compressor. The multi-stage throttling refrigeration device V-2 includes 4 gas-liquid separation tanks, namely a first ethane gas-liquid separation tank V-202, a second ethane gas-liquid separation tank V-203, a third ethane gas-liquid separation tank V-204 and a fourth ethane gas-liquid separation tank V-205. On the inlet and outlet pipelines of the four gas-liquid separation tanks, 12 valves are installed, namely a first valve Val-201, a second valve Val-202, a third valve Val-203, a fourth valve Val-204, a fifth valve Val-205, a sixth valve Val-206, a seventh valve Val-207, an eighth valve Val-208, a ninth valve Val-209, a tenth valve Val-210, a eleventh valve Val-211 and a twelfth valve Val-212, wherein the first valve Val-201, the third valve Val-203, the sixth valve Val-206 and the ninth valve Val-209 are all throttle valves, and the remaining valves are used to control connection or disconnection of the pipelines. The multi-stage natural gas precooler E-3 includes a natural gas-ethane first-stage precooler E-301, a natural gas-ethane second-stage precooler E-302, a natural gas-ethane third-stage precooler E-303 and a natural gas-ethane fourth-stage precooler E-304, and the multi-stage mixed refrigerant precooler E-1 includes a mixed refrigerant-ethane first-stage precooler E-101, a mixed refrigerant-ethane second-stage precooler E-102, a mixed refrigerant-ethane third-stage precooler E-103 and a mixed refrigerant-ethane fourth-stage precooler E-104.

[0046] The first valve Val-201 is installed on a connecting pipeline between the ethane buffer tank V-201 and the first ethane gas-liquid separation tank V-202; the second valve Val-202 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the mixed refrigerant-ethane first-stage precooler E-101; the third valve Val-203 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the second ethane gas-liquid separation tank V-203; the fourth valve Val-204 is installed on a connecting pipeline between the first ethane gas-liquid separation tank V-202 and the natural gas-ethane first-stage precooler E-301; the fifth valve Val-205 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the mixed refrigerant-ethane second-stage precooler E-102; the sixth valve Val-206 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the third ethane gas-liquid separation tank V-204; the seventh valve Val-207 is installed on a connecting pipeline between the second ethane gas-liquid separation tank V-203 and the natural gas-ethane second-stage precooler E-302; the eighth valve Val-208 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the mixed refrigerant-ethane third-stage precooler E-103; the ninth valve Val-209 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the fourth ethane gas-liquid separation tank V-205; the tenth valve Val-210 is installed on a connecting pipeline between the third ethane gas-liquid separation tank V-204 and the natural gas-ethane third-stage precooler 303; the eleventh valve Val-211 is installed on a connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the mixed refrigerant-ethane fourth-stage precooler E-104; the twelfth valve Val-212 is installed on a connecting pipeline between the fourth ethane gas-liquid separation tank V-205 and the natural gas-ethane fourth-stage precooler E-304.

[0047] It is worth noting that the respective ethane working medium pressures in the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204 and the fourth ethane gas-liquid separation tank V-205 are reduced gradually, and the four ethane gas-liquid separation tanks respectively correspond to an ethane ultra-high pressure gas-liquid separation tank, an ethane high pressure gas-liquid separation tank, an ethane low pressure gas-liquid separation tank and an ethane ultra-low pressure gas-liquid separation tank. However, the ultra-high pressure, the high pressure, the low pressure and the ultra-low pressure only indicate relative levels of the pressures, but are not limitations to absolute values of the pressures.

[0048] In the four-stage ethane refrigeration cycle system A, the cyclic precooling process of the ethane is presented below:

[0049] After undergoing multi-stage compression by the multi-stage ethane compressor C-201 and undergoing cooling by the ethane air cooler AC-201 into the liquid, the ethane is delivered to the ethane buffer tank V-201. Subsequently, after throttled by the first valve Val-201, the liquid ethane enters the first ethane gas-liquid separation tank V-202 for gas-liquid separation. The gas generated from the gas-liquid separation enters the inlet of the four-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation within the first ethane gas-liquid separation tank V-202, a part thereof enters the natural gas-ethane first-stage precooler E-301 through the fourth valve Val-204 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane first-stage precooler E-101 through the second valve Val-202 as a pre-refrigerant for precooling mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane first-stage precooler E-301 and the mixed refrigerant-ethane first-stage precooler E-101, respectively, also enters the inlet of the fourth-stage compressor of the multi-stage ethane compressor C-201.

[0050] A further part of liquid ethane generated from the gas-liquid separation within the first ethane gas-liquid separation tank V-202 continues to be throttled by the third valve Val-203 and then enters the ethane high pressure gas-liquid separator V-203 for gas-liquid separation. The gas ethane generated from the gas-liquid separation enters the inlet of the third-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation within the ethane high pressure gas-liquid separator 203, a part thereof enters the natural gas-ethane second-stage precooler E-302 through the seventh valve Val-207 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane second-stage precooler E-102 through the fifth valve Val-205 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane second-stage precooler E-302 and the mixed refrigerant-ethane second-stage precooler E-102, respectively, enters the inlet of the third-stage compressor of the multi-stage ethane compressor C-201 together.

[0051] A further part of the liquid generated from the gas-liquid separation within the second ethane gas-liquid separation tank V-203 continues to be throttled by the sixth valve Val-206 and then enters the ethane low-pressure gas-liquid separator V-204 for gas-liquid separation. The gas ethane generated from gas-liquid separation enters the inlet of the second-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation in the ethane low pressure gas-liquid separator V-204, a part thereof enters the natural gas-ethane third-stage precooler E-303 through the tenth valve Val-210 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane third-stage precooler E-103 via the eighth valve Val-208 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane third-stage precooler E-303 and the mixed refrigerant-ethane third-stage precooler E-103 enters the inlet of the second-stage compressor of the multi-stage ethane compressor C-201.

[0052] A further part of the liquid generated from gas-liquid separation within the third ethane gas-liquid separation tank V-204 continues to be throttled by the ninth valve Val-209 and then enters the ethane ultra-low pressure gas-liquid separator V-205 for gas-liquid separation. The gas ethane generated from the gas-liquid separation enters the inlet of the first-stage compressor of the multi-stage ethane compressor C-201. For the liquid ethane generated from the gas-liquid separation in the ethane ultra-low pressure gas-liquid separator V-205, a part thereof enters the natural gas-ethane first-stage precooler E-304 via the twelfth valve Val-212 as a pre-refrigerant for precooling the natural gas, and a part thereof enters the mixed refrigerant-ethane first-stage precooler E-104 via the eleventh valve Val-211 as a pre-refrigerant for precooling the mixed refrigerant. The ethane evaporated after precooled within the natural gas-ethane first-stage precooler E-304 and the mixed refrigerant-ethane first-stage precooler E-104 enters the inlet of the first-stage compressor of the multi-stage ethane compressor C-201 together.

[0053] The respective multi-stage devices in the ethane refrigeration cycle system A all have four stages and correspond to a precooling method including four stages of refrigeration. In practice, each of the multi-stage devices may have three or five stages and correspond to a precooling method including three or five stages of refrigeration. In the three-stage refrigeration mode, three precooling heat exchangers are selected, which reduces the costs of the ethane compressor but increases the power of the ethane compressor and the energy consumption of the device; in the five-stage refrigeration mode, five precooling heat exchangers are selected, which increases the costs of the ethane compressor but reduces the power of the ethane compressor and the energy consumption of the devices. Therefore, the specific number of stages could be set as actually required.

[0054] Referring to FIG. 3, the mixed refrigerant refrigeration cycle system B in the embodiments of the present disclosure includes a mixed refrigerant compressor C-101, a mixed refrigerant air cooler AC-101, a mixed refrigerant gas-liquid separation tank V-101 and a main heat exchanger cold box CB-401. The mixed refrigerant outlet of the mixed refrigerant compressor C-101 is connected sequentially to the mixed refrigerant air cooler AC-101, the multi-stage mixed refrigerant precooler E-1 and the mixed refrigerant gas-liquid separation tank V-101 via the mixed refrigerant transmission pipeline P-3. After compressed by the mixed refrigerant compressor C-101 and air-cooled by the mixed refrigerant air cooler AC-101, the mixed refrigerant is precooled stage by stage using the cold energy of the ethane in the multi-stage mixed refrigerant precooler E-1 and then enters the mixed refrigerant air-liquid separation tank V-101 for gas-liquid separation. A gas phase outlet and a liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to inlets of two throttling refrigeration units in the main heat exchanger cold box CB-401, wherein the two throttling refrigeration units provide cold energy required for supercooling the natural gas for the main heat exchanger cold box CB-401 by means of throttling refrigeration of the mixed refrigerant; outlets of the two throttling refrigeration units are connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101; the main heat exchanger cold box CB-401 is provided therein with a heat exchange pipe P-7 for supercooling the natural gas, wherein the heat exchange pipe is connected at an input end to the natural gas transmission pipeline P-2 and connected at an output end to the liquefied natural gas outlet O.

[0055] The two throttling refrigeration units disposed within the main heat exchanger cold box CB-401 are mainly used for throttling and depressurizing the mixed refrigerant, to further lower the temperature of the mixed refrigerant. In theory, any device that can implement mixed refrigerant throttling refrigeration can be used as the throttling refrigeration unit. The two throttling refrigeration units operate independently, and the gas phase outlet and the liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 are respectively connected to different throttling refrigeration units. The outlets of the two throttling refrigeration units may be merged and connected to the mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101, or may be connected respectively to the mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101.

[0056] In the embodiments of the present disclosure, continuing with FIG. 3, each throttling refrigeration unit includes an inflow heat exchange pipe, a throttling valve, a two-phase flow uniform distribution tank and an outflow heat exchange pipe, wherein the inflow heat exchange pipe, the two-phase flow distribution tank and the outflow heat exchange pipe are all disposed in the main heat exchanger cold box CB-401. An inlet of the inflow heat exchange pipe is used for inputting the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank V-101, an outlet of the inflow heat exchange pipe is sequentially connected to inlets of the throttling valve and the two-phase flow uniform distribution tank, the gas phase outlet and the liquid phase outlet of the two-phase flow uniform distribution tank are respectively connected to an inlet of the outflow heat exchange pipe, and the outlet of the outflow heat exchange tube is connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor C-101. The mixed refrigerant output from the mixed refrigerant gas-liquid separation tank V-101 further undergoes throttling refrigeration via the throttle valve after absorbing cold energy in the inflow heat exchange pipe. Thereafter, the mixed refrigerant enters the two-phase flow uniform distribution tank for gas-liquid uniform distribution and is then input to the outflow heat exchange pipe to provide the natural gas and the mixed refrigerant with cold energy. In the embodiments of the present disclosure, the two throttling refrigeration units share the same outflow heat exchange pipe, but since the inflow heat exchange pipes in the two throttling refrigeration units are independent of each other, the throttling refrigeration unit connected to the liquid phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes a first inflow heat exchange pipe P-4, a first throttle valve Val-401, a first two-phase flow uniform distribution tank V-401 and an outflow heat exchange pipe P-6, and the throttling refrigeration unit connected to the gas phase outlet of the mixed refrigerant gas-liquid separation tank V-101 includes a second inflow heat exchange pipe P-5, a second throttle valve Val-402, a second two-phase flow uniform distribution tank V-402 and an outflow heat exchange pipe P-6. The mixed refrigerants input respectively from the first inflow heat exchange pipe P-4 and the second inflow heat exchange pipe P-5 finally converge into the outflow heat exchange pipe P-6. Moreover, the natural gas delivered via the natural gas transmission pipe P-2 can further absorb the cold energy of the mixed refrigerant within the outflow heat exchange pipe P-6 upon entering the heat exchange pipe P-7 in the main heat exchanger cold box CB-401, thus making it possible to supercool the natural gas to a required output temperature.

[0057] In the embodiments of the present disclosure, the mixed refrigerant compressor C-101 is preferably a two-stage compressor. In the case, in the mixed refrigerant refrigeration cycle system B, the overall processing flow is presented below: two-stage compression is utilized for the mixed refrigerant compressor C-101; the mixed refrigerant is compressed in two stages and then output to the mixed refrigerant air cooler AC-101 from the second-stage compression outlet for air cooling, and thereafter sequentially passes through the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane three-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104 such that the mixed refrigerant can absorb the cold energy of the liquid ethane to be cooled gradually. After ethane four-stage precooling, the mixed refrigerant is cooled to a preset precooling temperature (which may be set from −60° C. to −80° C.). Subsequently, the precooled mixed refrigerant is delivered into the gas-liquid separation tank V-101. The gas phase refrigerant exiting from the top of the gas-liquid separation tank V-101 enters into the second inflow heat exchange pipe P-5 in the main heat exchanger cold box CB-401 and absorbs cold energy therein; thereafter, the gas phase refrigerant further undergoes throttling refrigeration via the second throttle valve Val-402, enters the two-phase flow uniform distribution tank V-402 for gas-liquid uniform distribution, and is then delivered to the bottom of the outflow heat exchange pipe P-6. The liquid phase refrigerant exiting from the bottom of the separation tank V-101 enters the first inflow heat exchange pipe P-4 of the main heat exchanger cold box CB-401 and absorbs cold energy therein; the liquid phase refrigerant further undergoes throttling refrigeration via the first throttle valve Val-401 and enters the first two-phase flow uniform distribution tank V-401 for gas-liquid uniform distribution; then, the liquid phase refrigerant is delivered to the middle of the outflow heat exchange pipe P-6 to be mixed with the gas phase refrigerant flowing upwardly from the bottom, exits from the main heat exchanger cold box CB-401 after releasing cold energy to the natural gas in the heat exchange pipe P-7 and being reheated, and subsequently enters the inlet of the first-stage compressor of the mixed refrigerant compressor C-101 to continue the cycle refrigeration.

[0058] It is worth noting that, in the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, the control temperature of the natural gas in each stage can be reasonably adjusted and controlled as actually required. In the natural gas liquefaction system, the cooling temperature directly affects the flash evaporation amount, specifically: the higher the cooling temperature, the greater the flash evaporation amount, and the lower the cooling temperature, the smaller the flash evaporation amount. Therefore, the final specific cooling temperature of the natural gas can be determined according to the actual requirement of the system for the flash evaporation amount. In the embodiments of the present disclosure, the temperature of the natural gas in each stage can be controlled according to the following temperature interval: after being purified and cooled by the ambient air, the natural gas enters the multi-stage natural gas precooler E-3 to gradually lower the temperature of the natural gas. Having been precooled by the ethane, the natural gas is cooled to −60 to −80° C. Subsequently, the precooled natural gas enters the main heat transfer cold box CB-401, and is then output from the liquefied natural gas outlet O until cooled to −150 to −162° C.

[0059] In addition, according to the present disclosure, in the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B, for the natural gas, the ethane and the mixed refrigerant, an air cooler is utilized for first precooling. However, if the ambient temperature is high, it is hard to cool the natural gas and the mixed refrigerant through air cooling, or there will be high energy consumption in the subsequent procedure even though the natural gas and the mixed refrigerant can be cooled. Accordingly, if the present disclosure is applied to the scenario of a high ambient temperature, a one-stage or two-stage propane or seawater precooling system is added before or behind the air cooler, to improve the efficiency of the system. In a further embodiment of the present disclosure, three water coolers are added to cooperate with the original air cooler, as shown in FIGS. 4, 5 and 6. Specifically, a first water cooler E-100 is added between the mixed refrigerant air cooler AC-101 and the multi-stage mixed refrigerant precooler E-1, a second water cooler E-200 is added between the ethane air cooler AC-201 and the ethane buffer tank V-201, and a third water cooler E-300 is added between the natural gas inlet I and the multi-stage natural gas precooler E-3. With the addition of the three water coolers, it can be guaranteed that the mixed refrigerant, the ethane and the raw natural gas can be cooled to 13° C. by the combination of air cooling and water cooling, to facilitate subsequent cooling. For the three additional water coolers, seawater is preferably used as the cooling water, i.e., seawater precoolers are preferably used, to reduce the energy consumption costs for water cooling. Of course, the three additional water coolers can be replaced by propane precoolers, which could be selected as actually required.

[0060] In the embodiments of the present disclosure, there may also be provided a cascade refrigeration natural gas liquefaction process according to the cascade refrigeration natural gas liquefaction system, which includes steps S1 through S3.

[0061] S1: gas ethane is compressed in multiple stages by the multi-stage ethane compressor C-201, the compressed ethane is air-cooled by the ethane air cooler AC-201 into liquid ethane, and the liquid ethane is then stored in the ethane buffer tank V-201; the liquid ethane in the ethane buffer tank V-201 is stably delivered to the multi-stage throttling refrigeration device V-2 to be throttled, depressurized and refrigerated stage by stage; each stage of throttling refrigeration device throttles and depressurizes the input liquid ethane and then further performs gas-liquid separation; the gas ethane obtained by separation is reflowed back and delivered to the multi-stage ethane compressor C-201 to be recompressed; the liquid ethane obtained by separation is partly delivered to a next stage of throttling refrigeration device, and partly delivered to the multi-stage natural gas precooler E-3 and the multi-stage mixed refrigerant precooler E-1 to provide cold energy by means of heat exchange, wherein the gas ethane after heat exchange is reflowed back and delivered to the multi-stage ethane compressor C-201 to be recompressed.

[0062] S2: the mixed refrigerant is compressed by the mixed refrigerant compressor C-101, the compressed mixed refrigerant is air-cooled by the compressor air cooler AC-101 into a liquid mixed refrigerant, the liquid mixed refrigerant passes through the multi-stage mixed refrigerant precooler E-1 and then exchanges heat with the liquid ethane delivered in the multi-stage throttling refrigeration device V-2 to thus lower the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling, and the precooled liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank V-101 to implement gas-liquid separation; the gas mixed refrigerant and the liquid mixed refrigerant obtained through gas-liquid separation enter the main heat exchange cold box CB-401, respectively, to further absorb cold energy to lower the temperature, and then after throttled and depressurized via the respective throttling refrigeration units, are returned to the main heat exchanger cold box CB-401 to provide cold energy for lowering the temperature of the natural gas and the mixed refrigerant, which in turn causes the temperature thereof to increase; the mixed refrigerant with the increased temperature flows back to the mixed refrigerant compressor C-101 to be recompressed.

[0063] S3: the purified natural gas to be liquefied is input to the multi-stage natural gas precooler E-3 via a natural gas inlet I to exchange heat with the liquid ethane delivered in the multi-stage throttling refrigeration device V-2, thus lowering the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling; the precooled natural gas further enters the main heat exchanger cold box CB-401, and exchanges heat with the mixed refrigerant that is returned to the main heat exchanger cold box CB-401 after throttled and depressurized to complete liquefaction supercooling; the liquefied and supercooled natural gas is output via a liquefied natural gas outlet O.

[0064] In the natural gas liquefaction process described above, the temperature of the precooled liquid mixed refrigerant and the precooled natural gas is controlled preferably between −60 and −80° C.

[0065] Hereinafter, a specific embodiment is provided to describe the specific implementations and technical effects of the cascade natural gas liquefaction system and process.Embodiment

[0066] The cascade natural gas liquefaction system according to this embodiment is shown in FIG. 1. The details about the structures of the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B are omitted herein for brevity, and the focus below will be given to respective fluid circulation processes in the two systems and respective process parameters.

[0067] In the embodiment, the purified natural gas with a raw gas pressure of 55 kg / cm2 sequentially passes through the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303 and the natural gas-ethane fourth-stage precooler E-304 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., then enters the main heat exchanger cold box CB-401 to be further cooled to −162° C., outputs from the cold box via the liquefied natural gas outlet O, and is finally delivered as the liquefied and supercooled natural gas to the LNG storage tank for storage.

[0068] The mixed refrigerant according to the embodiment is comprised of nitrogen, methane and ethane. After pressurized to 55 kg / cm2 by the mixed refrigerant compressor C-101, the mixed refrigerant enters the mixed refrigerant air cooler AC-101 to be cooled to 13° C., and then sequentially passes through the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane third-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., and thereafter enters the mixed refrigerant gas-liquid separation tank V-101. The liquid-phase refrigerant exiting from the bottom of the mixed refrigerant gas-liquid separation tank V-101 enters the main heat exchanger cold box CB-401 to be further cooled, and is drawn out and enters the first two-phase flow uniform distribution tank V-401 after passing through the first throttle valve Val-401, and then injected into the middle of the outflow heat exchange pipe P-6. The gas-phase refrigerant exiting from the top of the mixed refrigerant gas-liquid separation tank V-101 enters the main heat exchanger cold box CB-401 to be further cooled to −162° C. ; and is drawn out and enters the second two-phase flow uniform distribution tank V-402 after passing through the second throttle valve V-402, and then injected into the bottom of the outflow heat exchange pipe P-6, further discharged out of the cold box until reheated to −73° C. by absorbing, in the heat exchanger, the heat of the natural gas in the heat exchange pipe P-7, and finally returned to the mixed refrigerant compressor C-101.

[0069] After pressurized by the multi-stage ethane compressor C-201 wherein the multiple stages are specifically four stages, the ethane enters the ethane air cooler AC-201, and is then condensed therein into a liquid and stored in the ethane buffer tank V-201. Subsequently, the ethane is throttled, depressurized and refrigerated stage by stage in the four-stage throttling refrigeration device with a body formed by the first ethane gas-liquid separation tank V-202, the second ethane gas-liquid separation tank V-203, the third ethane gas-liquid separation tank V-204, the fourth ethane gas-liquid separation tank V-205, the first valve Val-201, the third valve Val-203, the sixth valve Val-206 and the ninth valve Val-209, then provides cold energy to the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303, the natural gas-ethane fourth-stage preccoler E-304, the mixed refrigerant-ethane first-stage precooler E-101, the mixed refrigerant-ethane second-stage precooler E-102, the mixed refrigerant-ethane third-stage precooler E-103 and the mixed refrigerant-ethane fourth-stage precooler E-104, and finally evaporates and flows back to the multi-stage ethane compressor C-201 to be recompressed stage by stage. See above for the detailed process of the ethane flow path, which is omitted herein for brevity.

[0070] It is worth noting that the specific form of the main heat exchanger cold box CB-401 is not limited, and the four heat exchange pipes therein, namely the first inflow heat exchange pipe P-4, the second inflow heat exchange pipe P-5, the outflow heat exchange pipe P-6 and the heat exchange pipe P-7, should be designed according to the type of the heat exchanger. In the embodiment of the present disclosure, the main heat exchanger cold box CB-401 can be implemented by a plate-fin heat exchanger including heat exchange channels therein, wherein the form of the heat exchange pipes may be directly the heat exchange channels inside the heat exchanger. Moreover, if the main heat exchanger cold box CB-401 adopts a coil-wound heat exchanger, the form of the heat exchange pipes may be built-in heat exchange pipes.

[0071] Hereinafter, comparative analysis of energy consumption benefits of the present disclosure and other prior art process will be made according to the cascade natural gas liquefaction system and process provided in the embodiment.

[0072] The analysis of energy consumption benefits of the present disclosure: in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the multi-stage ethane compressor in the present disclosure is about 49,800 KW, and the shaft power of the mixed refrigerant compressor is about 49,600 KW. The shaft powers of ethane compressor and the mixed refrigerant compressor are substantially the same, which can avoid the limitation of the maximum power of the gas turbine and the motor and maximize the single-line production capacity. In addition, the present disclosure can also avoid complex control. The system and the process according to the present disclosure are suitable for a high-latitude cold region, in particular the Arctic region with a low ambient temperature all year round, where this process can be carried out only through air cooling. For the process according to the present disclosure, the energy consumption of natural gas liquefaction per unit is as low as about 0.18 KW / Nm3.

[0073] The analysis of energy consumption benefits of the first type of comparative process: if a traditional propane pre-cooling mixed refrigerant is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the propane compressor is 30,300 KW, and the shaft power of the mixed refrigerant compressor is about 75,300 KW. The two compressors have a great discrepancy in shaft power, wherein the shaft power of the mixed refrigerant compressor is particularly large. For this process, the energy consumption of liquefaction per unit is about 0.21 KW / Nm3.

[0074] The analysis of energy consumption benefits of the second type of comparative process: if a dual mixed refrigerant process is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, when in the summer working condition, the power of the mixed refrigerant in the precooling stage is 79,000 KW, and the power of the mixed refrigerant in the deep cooling stage is 53,000 KW; when in the winter working condition, the power of the mixed refrigerant in the precooling stage is 35,000 KW, and the power of the mixed refrigerant in the deep cooling stage is 53,000 KW. That is, the power of the mixed refrigerant in the precooling stage is large in summer while being small in winter. In theory, the dual mixed refrigerant process is an LNG process suitable for a region with a low ambient temperature. However, the process has the following disadvantages of: complex working conditions of the two mixed refrigerants, difficult on-site operation, and frequent adjustments of the refrigerant ratio in the precooling stage according to the ambient temperature (otherwise, the adjustment can only be implemented through the backflow of the compressor, which will result in exceedingly high power consumption). For this process, the energy consumption for liquefaction per unit in summer is 0.256 KW / Nm3, and the energy consumption for liquefaction per unit in winter is 0.175 KW / Nm3.

[0075] The analysis of energy consumption benefits of the third type of comparative process: if a conventional cascade process is adopted, still in the case of taking the purified natural gas flow rate of 3.3 million tons / year as an example, the shaft power of the propane compressor is 53,000 KW, the shaft power of the ethane compressor is 41,300 KW, and the shaft power of the methane compressor is 14,200 KW. For this process, three compressors are required, where the propane compressor has the greatest power, the total power is large, and the energy consumption is high. The energy consumption for liquefaction per unit is about 0.234 KW / m3.

[0076] As seen above, the comprehensive energy consumption for liquefaction per unit in the present disclosure is lower than that in the propane-precooling mixed refrigerant process and the conventional cascade process. Although it is slightly higher than that of the dual mixed refrigerant process in the winter working condition, the energy consumption of the dual mixed refrigerant process is exceedingly high in summer. In a word, the present disclosure is advantageous over other processes in comprehensive energy consumption.

[0077] The embodiments described above are only a part of the preferred implementations of the present disclosure, without suggesting any limitation to the present disclosure. Those skilled in the related technical field are allowed to make various changes and variations thereto, without departing from the spirit and scope of the present disclosure. Therefore, any technical solution obtained by equivalent replacement or variation should fall within the scope of protection of the present disclosure.

Examples

embodiment

[0066]The cascade natural gas liquefaction system according to this embodiment is shown in FIG. 1. The details about the structures of the ethane refrigeration cycle system A and the mixed refrigerant refrigeration cycle system B are omitted herein for brevity, and the focus below will be given to respective fluid circulation processes in the two systems and respective process parameters.

[0067]In the embodiment, the purified natural gas with a raw gas pressure of 55 kg / cm2 sequentially passes through the natural gas-ethane first-stage precooler E-301, the natural gas-ethane second-stage precooler E-302, the natural gas-ethane third-stage precooler E-303 and the natural gas-ethane fourth-stage precooler E-304 to be respectively cooled to −13° C., −33° C., −54° C. and −70° C., then enters the main heat exchanger cold box CB-401 to be further cooled to −162° C., outputs from the cold box via the liquefied natural gas outlet O, and is finally delivered as the liquefied and supercooled n...

Claims

1. A cascade refrigeration natural gas liquefaction system, comprising an ethane refrigeration cycle system and a mixed refrigerant refrigeration cycle system;wherein the ethane refrigeration cycle system comprises a multi-stage ethane compressor, an ethane air cooler, an ethane buffer tank, a multi-stage throttling refrigeration device, a multi-stage natural gas precooler, and a multi-stage mixed refrigerant precooler;wherein the multi-stage ethane compressor, the ethane air cooler, the ethane buffer tank, and the multi-stage throttling refrigeration device are sequentially connected via an ethane transmission pipeline and form an ethane circulation loop such that the ethane can be buffered and stored in the ethane buffer tank after being compressed by the multi-stage ethane compressor and cooled by the ethane air cooler, then enters the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage, and flows back to the multi-stage ethane compressor after providing cold energy to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler;wherein an input end of the multi-stage natural gas precooler is connected to a natural gas inlet such that the input natural gas to be liquefied can be transmitted via a natural gas transmission pipeline to the mixed refrigerant refrigeration cycle system after being precooled stage by stage using the cold energy of the ethane in the multi-stage natural gas precooler;wherein the mixed refrigerant refrigeration cycle system comprises a mixed refrigerant compressor, a mixed refrigerant air cooler, a mixed refrigerant gas-liquid separation tank, and a main heat exchanger cold box;wherein a mixed refrigerant outlet of the mixed refrigerant compressor is connected sequentially to the mixed refrigerant air cooler, the multi-stage mixed refrigerant precooler, and the mixed refrigerant gas-liquid separation tank via a mixed refrigerant transmission pipeline such that, after being compressed by the mixed refrigerant compressor and air-cooled by the mixed refrigerant air cooler, the mixed refrigerant can be precooled stage by stage using the cold energy of the ethane in the multi-stage mixed refrigerant precooler and then enters the mixed refrigerant air-liquid separation tank for gas-liquid separation;wherein a gas phase outlet and a liquid phase outlet of the mixed refrigerant gas-liquid separation tank are respectively connected to inlet ends of two throttling refrigeration units in the main heat exchanger cold box such that the two throttling refrigeration units can provide the main heat exchanger cold box with cold energy required for supercooling the natural gas through throttling refrigeration of the mixed refrigerant;wherein outlet ends of the two throttling refrigeration units are connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; andwherein the main heat exchanger cold box is provided therein with a heat exchange pipe for supercooling the natural gas, wherein the heat exchange pipe is connected at an input end to the natural gas transmission pipeline and connected at an output end to a liquefied natural gas outlet.

2. The cascade refrigeration natural gas liquefaction system of claim 1, wherein the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler, and the multi-stage mixed refrigerant precooler are the same and correspond one-to-one; andwherein each stage of the throttling refrigeration device comprises a gas-liquid separation tank and a throttle valve installed on an inlet pipe of the gas-liquid separation tank, wherein a liquid phase outlet of the gas-liquid separation tank is divided into three branches that respectively lead to refrigerant inlets of the corresponding stage of the natural gas precooler and the mixed refrigerant precooler, and refrigerant outlets of the natural gas precooler and the mixed refrigerant precooler and a gas phase outlet of the gas-liquid separation tank are connected to an inlet of the corresponding stage of the ethane compressor.

3. The cascade refrigeration natural gas liquefaction system of claim 1, wherein the number of stages comprised in the multi-stage ethane compressor, the multi-stage throttling refrigeration device, the multi-stage natural gas precooler, and the multi-stage mixed refrigerant precooler are in the range from 3 to 5.

4. The cascade refrigeration natural gas liquefaction system of claim 1, wherein each throttling refrigeration unit comprises an inflow heat exchange pipe, a throttle valve, a two-phase flow uniform distribution tank, and an outflow heat exchange pipe, and wherein the inflow heat exchange pipe, the two-phase flow uniform distribution tank, and the outflow heat exchange pipe are all disposed in the main heat exchanger cold box;wherein an inlet of the inflow heat exchange pipe is used for inputting the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank, an outlet of the inflow heat exchange pipe is sequentially connected to inlets of the throttling valve and the two-phase flow uniform distribution tank, a gas phase outlet and a liquid phase outlet of the two-phase flow uniform distribution tank are respectively connected to an inlet of the outflow heat exchange pipe, and an outlet of the outflow heat exchange pipe is connected to a mixed refrigerant backflow inlet of the mixed refrigerant compressor; andwherein the mixed refrigerant output by the mixed refrigerant gas-liquid separation tank further undergoes throttling refrigeration via the throttle valve after absorbing cold energy in the inflow heat exchange pipe, then enters the two-phase flow uniform distribution tank for gas-liquid uniform distribution, and is input to the outflow heat exchange pipe to provide the cold energy for the natural gas and the mixed refrigerant.

5. The cascade refrigeration natural gas liquefaction system of claim 1, wherein a propane precooler or a water cooler is disposed between the ethane air cooler and the ethane buffer tank.

6. The cascade refrigeration natural gas liquefaction system of claim 1, wherein a propane precooler or a water cooler is disposed between the mixed refrigerant air cooler and the multi-stage mixed refrigerant precooler.

7. The cascade refrigeration natural gas liquefaction system of claim 1, wherein a propane precooler or a water cooler is disposed respectively between the natural gas inlet and the multi-stage natural gas precooler.

8. A cascade refrigeration natural gas liquefaction process, wherein the process is implemented based on the cascade refrigeration natural gas liquefaction system of claim 1, the cascade refrigeration natural gas liquefaction process comprising:S1: gas ethane is compressed in multiple stages by the multi-stage ethane compressor, the compressed ethane is air-cooled by the ethane air cooler into liquid ethane, and the liquid ethane is then stored in the ethane buffer tank; the liquid ethane in the ethane buffer tank is stably delivered to the multi-stage throttling refrigeration device to be throttled, depressurized and refrigerated stage by stage; each stage of throttling refrigeration device throttles and depressurizes the input liquid ethane and then further performs gas-liquid separation; the gas ethane obtained by separation is reflowed back and delivered to the multi-stage ethane compressor to be recompressed; the liquid ethane obtained by separation is partly delivered to a next stage of throttling refrigeration device, and partly delivered to the multi-stage natural gas precooler and the multi-stage mixed refrigerant precooler to provide cold energy by means of heat exchange, wherein the gas ethane after heat exchange is reflowed back and delivered to the multi-stage ethane compressor to be recompressed;S2: the mixed refrigerant is compressed by the mixed refrigerant compressor, the compressed mixed refrigerant is air-cooled by the compressor air cooler into a liquid mixed refrigerant, the liquid mixed refrigerant passes through the multi-stage mixed refrigerant precooler and then exchanges heat with the liquid ethane delivered in the multi-stage throttling refrigeration device to thus lower the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling, and the precooled liquid mixed refrigerant enters the mixed refrigerant gas-liquid separation tank to implement gas-liquid separation; the gas mixed refrigerant and the liquid mixed refrigerant obtained through gas-liquid separation enter the main heat exchange cold box, respectively, to further absorb cold energy to lower the temperature, and then after throttled and depressurized via the respective throttling refrigeration units, are returned to the main heat exchanger cold box for providing cold energy for lowering the temperature of the natural gas and the mixed refrigerant, which in turn causes the temperature thereof to increase; the mixed refrigerant with the increased temperature flows back to the mixed refrigerant compressor to be recompressed; andS3: the purified natural gas to be liquefied is input to the multi-stage natural gas precooler via a natural gas inlet to exchange heat with the liquid ethane delivered in the multi-stage throttling refrigeration device, thus lowering the temperature stage by stage using the cold energy provided by the liquid ethane to complete precooling; the precooled natural gas further enters the main heat exchanger cold box, and exchanges heat with the mixed refrigerant returned to the main heat exchanger cold box after throttled and depressurized to complete liquefaction supercooling; the liquefied and supercooled natural gas is output via a liquefied natural gas outlet.

9. The cascade refrigeration natural gas liquefaction process of claim 8, wherein a temperature of the precooled liquid mixed refrigerant and the precooled natural gas is from −60° C. to −80° C.

10. The cascade refrigeration natural gas liquefaction process of claim 8, wherein the mixed refrigerant comprises nitrogen, methane, and ethane.

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