Gradient utilization system based on low-grade heat energy and LNG cold energy

Abstract

The invention discloses a gradient utilization system based on low-grade heat energy and LNG cold energy, the gradient utilization system based on low-grade heat energy and LNG cold energy comprises a cold energy power generation device and a light hydrocarbon separation system which are communicated with each other, the light hydrocarbon separation system is used for recycling and separating C2 + light hydrocarbon in LNG rich liquor to obtain ethane, LPG and LNG barren liquor respectively, and the LNG barren liquor is used for storing the ethane, the LPG and the LNG barren liquor. The cold energy power generation device carries out cold energy power generation based on ethane and LNG barren liquor obtained by the light hydrocarbon separation system as cold sources. The cold energy power generation device comprises a first circulation pipeline, a second circulation pipeline, a third circulation pipeline and an intermediate heat exchanger. And low-grade heat energy circulates in the intermediate heat exchanger and the third circulating pipeline. The low-grade heat energy is utilized, coupling of the light hydrocarbon separation system and the cold energy power generation device is achieved, the cooling capacity of LNG barren liquor and ethane is fully utilized, external cooling capacity supply is not needed, comprehensive utilization of the low-grade heat energy and LNG cold energy is achieved, and the cold energy utilization rate is effectively increased.

Description

Technical Field

[0001] The present invention relates to the field of petrochemical technologies, and particularly to a cascaded utilization system based on low-grade thermal energy and LNG cold energy. Background Art

[0002] During the regasification process of LNG, a huge amount of cold energy is contained, enabling its application in aspects such as air separation, light hydrocarbon separation, and low-temperature pulverization of waste rubber in the low-temperature region, cold energy power generation, dry ice production, and seawater desalination in the medium and low-temperature regions, or cold storage, refrigeration air conditioners, etc. in the normal-temperature region.

[0003] Due to the high light hydrocarbon content in liquefied natural gas rich liquid, light hydrocarbons can be separated at LNG receiving terminals to obtain high-value-added products such as ethane and liquefied petroleum gas (LPG).

[0004] In the prior art, the ethane recovery rate of the LNG light hydrocarbon separation process is usually between 90% and 95%. The operating pressure of the demethanizer is mostly in the range of 2 - 3.5 MPag, and the energy consumption of the rectification tower is significant. In addition, there are generally defects such as a complex heat exchange network, low cold energy utilization rate, and weak adjustment performance of the process system to raw material fluctuations.

[0005] Cold energy power generation has characteristics such as a short industrial chain, mature technology, strong industrial independence, high utilization efficiency, and strong feasibility, and thus has become the main development direction of cold energy utilization at home and abroad.

[0006] The prior art CN103075250A discloses a method for cascaded utilization of LNG cold energy for power generation, which includes two parts: a natural gas medium Rankine cycle and a refrigerant medium Rankine cycle. It uses vaporized natural gas and another refrigerant as power generation working fluids, and realizes multiple cascaded heat exchanges with LNG by controlling the pressures of the natural gas medium and the refrigerant medium. This system can be understood as five cold energy recovery cycles. Although the cold energy recovery efficiency is relatively high, the process is too complex and is only applicable to occasions with high requirements for cold energy utilization efficiency. The process of this system is relatively complex, and the acquisition of heat sources is often restricted by geography and cannot be applied to typical LNG receiving terminals.

[0007] The existing LNG cold energy utilization technologies are mainly single utilization methods, with insufficient LNG cold energy utilization, low cold energy utilization rate, generally about 25%. The process is too complex and the operation difficulty in engineering practice is relatively large. In view of this, it is urgently necessary to carry out reasonable design for cascaded utilization of cold energy for the rich liquid of LNG receiving terminals. By analyzing the characteristics of different cold energy utilization methods and the cold energy demand range, grasping the principle of "temperature matching and cascaded utilization", and combining with the actual operation of engineering projects, a reasonable integrated scheme for efficient cascaded utilization of LNG cold energy at LNG receiving terminals is established.

[0008] Therefore, there is an urgent need to develop a system that can comprehensively and hierarchically utilize the cold energy of LNG, effectively improve the cold energy utilization rate, reduce engineering costs, and simultaneously produce high-value-added products. Summary of the Invention

[0009] In view of the above problems, the present invention is proposed to provide a hierarchical utilization system based on low-grade heat energy and LNG cold energy that can overcome or at least partially solve the above problems.

[0010] The present invention provides a hierarchical utilization system based on low-grade heat energy and LNG cold energy. The hierarchical utilization system based on low-grade heat energy and LNG cold energy includes: a cold energy power generation device and a light hydrocarbon separation system that are interconnected. The light hydrocarbon separation system is used to recover and separate C 2 + light hydrocarbons in the LNG rich liquid to obtain ethane, LPG, and LNG lean liquid respectively. The cold energy power generation device uses the ethane and LNG lean liquid obtained from the light hydrocarbon separation system as cold sources respectively to generate cold energy power.

[0011] Among them, the cold energy power generation device includes a first circulation pipeline, a second circulation pipeline, a third circulation pipeline, and an intermediate heat exchanger. The first circulation pipeline includes a first power generation circuit and a first heat exchanger that are interconnected, and a first circulation working fluid flows in the first circulation pipeline. The second circulation pipeline includes a second power generation circuit and a second heat exchanger that are interconnected, and a second circulation working fluid flows in the second circulation pipeline. The second circulation working fluid is LPG obtained from the light hydrocarbon separation system. The third circulation pipeline includes a third power generation circuit and a third heat exchanger that are interconnected, and a third circulation working fluid flows in the third circulation pipeline.

[0012] The first power generation circuit and the second power generation circuit are respectively connected to the intermediate heat exchanger.

[0013] The light hydrocarbon separation system is sequentially connected to the first heat exchanger, the second heat exchanger, and the intermediate heat exchanger, and the light hydrocarbon separation system is connected to the third heat exchanger.

[0014] The LNG lean liquid obtained from the light hydrocarbon separation system is used as a cold source and is sequentially introduced into the first heat exchanger, the second heat exchanger, and the intermediate heat exchanger for heat exchange. The LNG lean liquid is used as a cold source and exchanges heat with the first circulation working fluid in the first heat exchanger and then is introduced into the second heat exchanger. The first circulation working fluid is introduced into the first power generation circuit for power generation after heat exchange. The LNG lean liquid exchanges heat with LPG in the second heat exchanger and then is introduced into the intermediate heat exchanger. The LPG is introduced into the second power generation circuit for power generation after heat exchange. The LNG lean liquid is introduced into the pipeline network after heat exchange in the intermediate heat exchanger.

[0015] The ethane obtained by the light hydrocarbon separation system is introduced into the third heat exchanger. The ethane acts as a cold source and exchanges heat with the third circulating working fluid in the third heat exchanger. The third circulating working fluid is introduced into the third power generation loop for power generation after heat exchange;

[0016] Low-grade heat energy circulates in the intermediate heat exchanger and the third circulating pipeline.

[0017] Optionally, the first power generation loop includes a first expander, a first liquid separation tank, and a first booster pump. The first expander, the first heat exchanger, the first booster pump, the third heat exchanger, and the first liquid separation tank are sequentially connected in a loop.

[0018] Optionally, the second power generation loop includes a second expander, a second liquid separation tank, and a second booster pump. The second expander, the second heat exchanger, the second booster pump, the third heat exchanger, and the second liquid separation tank are sequentially connected in a loop. The LPG input port of the second booster pump is also connected to the light hydrocarbon separation system for receiving the LPG obtained by the light hydrocarbon separation system.

[0019] Optionally, an electric heater is also provided on the connecting pipeline between the second expander and the second liquid separation tank.

[0020] Optionally, the third power generation loop includes a third expander, a fifth heat exchanger, and a third booster pump. The third expander, the third heat exchanger, the third booster pump, and the fifth heat exchanger are sequentially connected in a loop.

[0021] Optionally, the first expander, the second expander, and the third expander are centrifugal, reciprocating, or screw expanders.

[0022] Optionally, the temperature of the lean LNG increases sequentially after passing through the first heat exchanger, the second heat exchanger, and the intermediate heat exchanger.

[0023] Optionally, the first heat exchanger, the second heat exchanger, and the third heat exchanger are plate-fin heat exchangers, shell-and-tube heat exchangers, or printed circuit board heat exchangers.

[0024] Optionally, low-grade heat energy circulates in the intermediate heat exchanger and the fifth heat exchanger. The heat source of the low-grade heat energy is seawater, and the temperature difference of the seawater at the inlet and outlet of the intermediate heat exchanger and the fifth heat exchanger is not greater than 5°C.

[0025] Optionally, a fifth booster pump is provided on the connecting pipeline between the ethane input port of the third heat exchanger and the light hydrocarbon separation system, and an air-cooled vaporizer is connected to the ethane output port of the third heat exchanger.

[0026] Optionally, the light hydrocarbon separation system includes a condensation assembly, a flash absorption tower, a demethanizer, a deethanizer, a fourth heat exchanger, a fourth booster pump, and a fifth booster pump. The condensation assembly, the flash absorption tower, the demethanizer, the fifth booster pump, the deethanizer, and the fourth heat exchanger are connected in sequence. The top of the flash absorption tower and the top of the demethanizer are respectively connected to the condensation assembly. The condensation assembly is connected to the LPG input port of the fourth heat exchanger through the fourth booster pump. The LPG output port of the fourth heat exchanger is connected to the first heat exchanger, and the ethane output port of the fourth heat exchanger is connected to the third heat exchanger.

[0027] Optionally, the condensation assembly includes a plurality of condensers connected in sequence.

[0028] Optionally, the first circulating working fluid is one or a mixture of methane, ethane, propane, and butane, or is a refrigerant.

[0029] Optionally, the third circulating working fluid is a single or mixed working fluid, including butane, pentane, or R152.

[0030] The technical solution provided in the embodiments of the present invention has at least the following technical effects or advantages:

[0031] The cascade utilization system based on low-grade thermal energy and LNG cold energy provided in the embodiments of the present invention consists of a light hydrocarbon separation system and a cold energy power generation device. The cold energy power generation device includes three independent cold energy power generation cycle systems, namely a first circulation pipeline, a second circulation pipeline, and a third circulation pipeline, all of which are Rankine cycle power generation. The light hydrocarbon separation system recovers and separates the LNG rich liquid to obtain ethane, LPG, and LNG lean liquid respectively. The LNG lean liquid and ethane are respectively used as cold sources for cold energy power generation in the first circulation pipeline and the third circulation pipeline. The LPG is used as the working medium of the second circulation pipeline to assist power generation, and at the same time, low-grade thermal energy is utilized, thereby realizing the coupling of the light hydrocarbon separation system and the cold energy power generation device, fully utilizing the cold energy of the LNG lean liquid and ethane, without the need for external supply of cold energy, realizing the comprehensive utilization of low-grade thermal energy and LNG cold energy, and effectively improving the cold energy utilization rate.

[0032] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features, and advantages of the present invention more obvious and understandable, the specific embodiments of the present invention are given below. Description of the Drawings

[0033] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the following detailed description of the preferred embodiments. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Also, throughout the drawings, the same reference numerals are used to represent the same components.

[0034] In the drawings:

[0035] Figure 1 is a schematic diagram of the principle structure of a cascaded utilization system based on low-grade thermal energy and LNG cold energy provided by an embodiment of the present invention;

[0036] Figure 2 is another schematic diagram of the principle structure of a cascaded utilization system based on low-grade thermal energy and LNG cold energy provided by an embodiment of the present invention.

[0037] Description of the reference numerals in the drawings:

[0038] 100, cold energy power generation device;

[0039] 110, first circulation pipeline; 111, first heat exchanger; 112, first expander; 113, first liquid separation tank; 114, first booster pump; 120, second circulation pipeline; 121, second heat exchanger; 122, second expander; 123, second liquid separation tank; 124, second booster pump; 125, electric heater; 130, third circulation pipeline; 131, third heat exchanger; 132, third expander; 133, fifth heat exchanger; 134, third booster pump; 135, sixth booster pump; 136, air temperature vaporizer; 140, intermediate heat exchanger;

[0040] 200, light hydrocarbon separation system;

[0041] 201, condensation component; 202, flash absorption tower; 203, demethanizer; 204, deethanizer; 205, fourth heat exchanger; 206, fourth booster pump; 207, fifth booster pump. Detailed embodiments

[0042] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the drawings.

[0043] Various schematic diagrams according to the embodiments of the present disclosure are shown in the drawings. These figures are not drawn to scale, where for the purpose of clear expression, some details are enlarged and some details may be omitted. The shapes of various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are only exemplary. In practice, there may be deviations due to manufacturing tolerances or technical limitations, and those skilled in the art can design regions / layers with different shapes, sizes, and relative positions according to actual needs.

[0044] To better understand the above technical solution, the above technical solution will be described in detail below in combination with specific implementation manners. It should be understood that the embodiments of the present disclosure and the specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. Without conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0045] Figure 1 is a schematic diagram of the principle structure of a cascaded utilization system based on low-grade thermal energy and LNG cold energy provided by an embodiment of the present invention. As Figure 1 shown, the cascaded utilization system based on low-grade thermal energy and LNG cold energy includes: a cold energy power generation device 100 and a light hydrocarbon separation system 200 that are interconnected. The light hydrocarbon separation system 200 is used to recover and separate C 2 + light hydrocarbons, and respectively obtain ethane, LPG, and LNG lean liquid. The cold energy power generation device 100 uses the ethane and LNG lean liquid obtained by the light hydrocarbon separation system 200 as cold sources respectively to generate cold energy power.

[0046] Combined with Figure 2 shown, wherein, the cold energy power generation device 100 includes a first circulation pipeline 110, a second circulation pipeline 120, a third circulation pipeline 130, and an intermediate heat exchanger 140; the first circulation pipeline 110 includes a first power generation loop and a first heat exchanger 111 that are interconnected, and a first circulation working fluid flows in the first circulation pipeline 110; the second circulation pipeline 120 includes a second power generation loop and a second heat exchanger 121 that are interconnected, and a second circulation working fluid flows in the second circulation pipeline 120, and the second circulation working fluid is LPG obtained by the light hydrocarbon separation system 200; the third circulation pipeline 130 includes a third power generation loop and a third heat exchanger 131 that are interconnected, and a third circulation working fluid flows in the third circulation pipeline 130.

[0047] The first power generation circuit and the second power generation circuit are respectively connected to the intermediate heat exchanger 140; the light hydrocarbon separation system 200 is sequentially connected to the first heat exchanger 111, the second heat exchanger 121 and the intermediate heat exchanger 140, and the light hydrocarbon separation system 200 is connected to the third heat exchanger 131; the LNG lean liquid obtained by the light hydrocarbon separation system 200 is used as a cold source and is respectively passed into the first heat exchanger 111, the second heat exchanger 121 and the intermediate heat exchanger 140 in sequence for heat exchange. The LNG lean liquid, as a cold source, exchanges heat with the first circulating working medium in the first heat exchanger 111 and then enters the second heat exchanger 121. The first circulating working medium generates electricity in the first power generation circuit after heat exchange. The LNG lean liquid exchanges heat with LPG in the second heat exchanger 121 and then enters the intermediate heat exchanger 140. The LPG generates electricity in the second power generation circuit after heat exchange. The LNG lean liquid enters the pipeline network after heat exchange in the intermediate heat exchanger 140; the ethane obtained by the light hydrocarbon separation system 200 enters the third heat exchanger 131. The ethane, as a cold source, exchanges heat with the third circulating working medium in the third heat exchanger 131. The third circulating working medium generates electricity in the third power generation circuit after heat exchange.

[0048] Low-grade thermal energy flows in the intermediate heat exchanger 140 and the third circulation pipeline 130 (the fifth heat exchanger 133). In the embodiment of the present invention, the heat source of the low-grade thermal energy is seawater, and the temperature difference of the seawater at the inlet and outlet of the intermediate heat exchanger 140 and the fifth heat exchanger 133 is not greater than 5°C.

[0049] The cascade utilization system based on low-grade thermal energy and LNG cold energy described in the embodiment of the present invention is composed of a light hydrocarbon separation system 200 and a cold energy power generation device 100. The cold energy power generation device 100 includes three independent cold energy power generation cycle systems, namely the first circulation pipeline 110, the second circulation pipeline 120 and the third circulation pipeline 130, all of which are Rankine cycle power generation. The light hydrocarbon separation system 200 recovers and separates the LNG rich liquid, and respectively obtains ethane, LPG and LNG lean liquid. The LNG lean liquid and ethane are respectively used as cold sources of the first circulation pipeline 110 and the third circulation pipeline 130 for cold energy power generation. The LPG is used as the working medium of the second circulation pipeline 120 to assist in power generation, and at the same time, low-grade thermal energy is utilized, so as to realize the coupling of the light hydrocarbon separation system 200 and the cold energy power generation device 100, make full use of the cold energy of the LNG lean liquid and ethane, do not require external supply of cold energy, realize the comprehensive utilization of low-grade thermal energy and LNG cold energy, and effectively improve the cold energy utilization rate.

[0050] Optionally, in the embodiment of the present invention, the light hydrocarbon separation system 200 includes a condensation assembly 201, a flash absorption tower 202, a demethanizer 203, a deethanizer 204, a fourth heat exchanger 205, a fourth booster pump 206, and a fifth booster pump 207. The condensation assembly 201, the flash absorption tower 202, the demethanizer 203, the fifth booster pump 207, the deethanizer 204, and the fourth heat exchanger 205 are connected in sequence. The top of the flash absorption tower 202 and the top of the demethanizer 203 are respectively connected to the condensation assembly 201. The condensation assembly 201 is connected to the LPG input port of the fourth heat exchanger 205 through the fourth booster pump 206. The LPG output port of the fourth heat exchanger 205 is connected to the first heat exchanger 111, and the ethane output port of the fourth heat exchanger 205 is connected to the third heat exchanger 131.

[0051] Specifically, the LNG-rich feed liquid pumped out from the storage tank of the LNG receiving terminal enters the light hydrocarbon separation system 200. The LNG-rich feed from the LNG receiving terminal releases cold energy through the condensation assembly 201, and then enters the flash absorption tower 202 and the demethanizer 203 in sequence. The bottom product of the flash absorption tower 202 is pressurized by the demethanizer feed pump and then enters the demethanizer 203. The demethanizer 203 operates at a pressure lower than 1.5 MPag, and methane at -116 °C is produced at the top. The bottom of the demethanizer 203 is C2+ light hydrocarbons, which are sent to the deethanizer 204 as feed. The deethanizer 204 operates at 0.2 MPag. The gaseous ethane product at the top exchanges heat with the LNG lean liquid and then becomes a liquid ethane product. After being pressurized by a pump, it exchanges heat with the third circulating working medium in the third circulation pipeline 130 and provides cold energy for it. Subsequently, the temperature of the ethane rises and it is sent out of the battery limit for storage or outside. The ethane is in a pressurized normal temperature state. The methane gas produced at the tops of the flash absorption tower 202 and the demethanizer 203 is introduced into the condensation assembly 201 and reasonably distributed therein. The cold energy carried by itself is re-liquefied into LNG lean liquid by the LNG-rich feed, which is convenient for returning to the inlet of the vaporizer of the receiving terminal in an energy-saving way by pumping. The LNG lean liquid is introduced into the upper parts of the flash absorption tower 202 and the demethanizer 203 respectively as the absorption liquid and the cold reflux, so as to achieve a high recovery rate of light hydrocarbons. Especially for light hydrocarbons such as ethane, the recovery rate is high, and the purity and recovery rate can reach more than 98%. The cold energy of the ethane product is recovered, so that the ethane product is in a pressurized normal temperature state, and the investment cost of the subsequent vaporization device is also saved.

[0052] The liquid LNG lean liquid from the light hydrocarbon separation system 200 exchanges heat with ethane in the fourth heat exchanger 205, with a pressure of 60 - 90 barg and a temperature of about -110 to -100 °C, and then enters the cold energy power generation device 100 to provide the cold energy required for power generation for the first circulation pipeline 110 and the second circulation pipeline 120.

[0053] In the embodiments of the present invention, the demethanizer 203 and the deethanizer 204 adopted have relatively low operating pressures, and the energy consumption is significantly reduced compared with similar processes. Moreover, the bottom reboilers of both the demethanizer 203 and the deethanizer 204 can use steam, circulating hot water, and the surrounding ambient heat source (such as seawater) as the heat source.

[0054] The temperature of the lean LNG increases sequentially after passing through the first heat exchanger 111, the second heat exchanger 121, and the intermediate heat exchanger 140 in sequence. After being heated to a certain temperature, it is output to the pipe network.

[0055] Optionally, the condensing assembly 201 includes a plurality of condensers connected in sequence, which can be two groups or multiple groups of condensers connected in series in sequence. The appropriate number can be selected according to actual needs, which can achieve the cascaded release of cold energy and improve the cooling effect.

[0056] In the embodiments of the present invention, the first power generation loop includes a first expander 112, a first liquid separation tank 113, and a first booster pump 114. The first expander 112, the first heat exchanger 111, the first booster pump 114, the third heat exchanger 131, and the first liquid separation tank 113 are connected in a circulating manner in sequence. The first circulating working fluid enters the first heat exchanger 111 to exchange heat with the lean LNG and becomes a liquid working fluid. Subsequently, it is pressurized by the first booster pump 114 and then enters the intermediate heat exchanger 140 to be heated and vaporized by seawater. The gaseous working fluid enters the first expander 112 for power generation by turbine (the turbine outlet is gaseous) and then enters the first heat exchanger 111 again for circulation.

[0057] Specifically, the liquid lean LNG enters the first circulation pipeline 110 and exchanges heat with the first circulating working fluid. The temperature is about -80 to -70 °C. After the first circulating working fluid at the outlet of the first expander 112 exchanges heat with the lean LNG, the temperature is -70 °C and the pressure is 5 barg, becoming a liquid product. After being pressurized by the first booster pump 114, the pressure is 23 barg. It exchanges heat with seawater through the intermediate heat exchanger 140, and the first circulating working fluid is vaporized and enters the first expander 112 to drive the first expander 112 to generate electricity.

[0058] Optionally, the first power generation loop utilizes the cold energy in the low-temperature section of the lean LNG, and the temperature range is -160 to -80 °C. The first circulating working fluid is a single working fluid or a mixed working fluid, which can be alkanes or freons, etc. The alkanes include one or more mixtures of methane, ethane, propane, butane, etc. In the embodiments of the present invention, the first circulating working fluid is preferably freons, such as R01. Compared with propane, etc., this product can effectively increase the power generation per unit of LNG by 10% under the same seawater temperature conditions, and solves the problem of the discharge of the alkane (such as propane) circulating power generation system. Moreover, it can be directly purchased externally, reducing additional equipment investment and lowering costs.

[0059] In an embodiment of the present invention, the second power generation loop includes a second expander 122, a second liquid separation tank 123, and a second booster pump 124. The second expander 122, the second heat exchanger 121, the second booster pump 124, the third heat exchanger 131, and the second liquid separation tank 123 are sequentially connected in a loop. The LPG input port of the second booster pump 124 is also connected to the light hydrocarbon separation system 200 for receiving the LPG obtained by the light hydrocarbon separation system 200. The second power generation loop utilizes the cold energy in the medium-temperature section of the lean LNG, and the temperature range is -80 to -40 °C. The LPG (mixed hydrocarbon working medium) from the bottom of the deethanizer 204 enters the second heat exchanger 121 to exchange heat with the lean LNG in the medium-temperature section, is cooled into a liquid working medium, is pressurized by the second booster pump 124, enters the intermediate heat exchanger 140 to be gasified into a gas state, and then enters the second expander 122 to expand and generate electricity.

[0060] Specifically, the temperature of the lean LNG after passing through the first circulation pipeline 110 is -80 to -70 °C, providing cold energy for the circulation of the second circulation pipeline 120. After passing through the second circulation pipeline 120, the temperature rises to -40 °C and enters the intermediate heat exchanger 140, where seawater heats it to the network export temperature. The low-pressure and low-temperature working medium (i.e., LPG) in the second circulation pipeline 120 is cooled to about -40 °C by the lean LNG in the second heat exchanger 121, is pressurized to 6 - 7 barg by the second booster pump 124, enters the intermediate heat exchanger 140 where seawater heats it, is gasified and then enters the second expander 122 to expand and generate electricity.

[0061] Optionally, an electric heater 125 may be provided on the connecting pipeline between the second expander 122 and the second liquid separation tank 123, which can be adapted to different low-grade heat sources in cooperation with the liquid separation tank, breaking through the limitations of the heat sources used in similar technologies.

[0062] In an embodiment of the present invention, the third power generation loop includes a third expander 132, a fifth heat exchanger 133, and a third booster pump 134. The third expander 132, the third heat exchanger 131, the third booster pump 134, and the fifth heat exchanger 133 are sequentially connected in a loop. The third circulation working medium exchanges heat with the liquid ethane in the third heat exchanger 131, and the temperature of the ethane further rises. The third circulation working medium in the third circulation pipeline 130 is cooled to -20 °C in the third heat exchanger 131, is pressurized to 4 barg after passing through the third booster pump 134, enters the fifth heat exchanger 133, is heated and gasified by seawater, enters the third expander 132, expands and generates electricity, completing the Rankine cycle.

[0063] The present invention optimizes the division of the heat exchange temperature section based on three independent circulation pipelines, effectively reducing the exergy loss of the system, improving the heat exchange efficiency between LNG and the working medium, and the power generation per unit of LNG.

[0064] Optionally, the third power generation loop utilizes the cold energy of the overhead product ethane of the deethanizer 204, with a temperature range of -40 to 0 °C. The third circulating working fluid is a single or mixed working fluid, including butane, pentane, R152, etc.

[0065] In the embodiment of the present invention, a sixth booster pump 135 is provided on the pipeline connecting the ethane input port of the third heat exchanger 131 to the light hydrocarbon separation system 200. The ethane output port of the third heat exchanger 131 is connected to an air-cooled vaporizer 136. The third circulating working fluid exchanges heat with liquid ethane in the third heat exchanger 131, and the temperature of ethane further rises. Then it enters the air-cooled vaporizer 136 and is sent out of the battery limit for storage after becoming a pressurized normal temperature state.

[0066] Optionally, the first expander 112, the second expander 122, and the third expander 132 can be centrifugal, reciprocating, or screw expanders, and can be selected according to the actual application scenario. The present invention has no limitation in this regard.

[0067] In the embodiment of the present invention, the first heat exchanger 111, the second heat exchanger 121, and the third heat exchanger 131 can be plate-fin heat exchangers, shell-and-tube heat exchangers, or printed circuit board heat exchangers.

[0068] The cascade utilization system based on low-grade thermal energy and LNG cold energy according to the embodiment of the present invention has the following advantages compared with the prior art:

[0069] 1. It couples the light hydrocarbon separation system 200 and the cold energy power generation device 100. The ethane, LPG, and LNG lean liquid obtained by the light hydrocarbon separation system 200 have a high recovery rate of light hydrocarbons such as ethane, and the purity and yield can reach more than 98%. Among them, by recovering the cold energy of the ethane product, the ethane product is in a pressurized normal temperature state, saving the investment cost of subsequent gasification devices. The LNG lean liquid sent to the cold energy power generation device 100 can be used as a multi-stage cold source, and LPG is used as the working fluid in the second circulation pipeline 120. Through the three independent circulation pipelines of the cold energy power generation device 100, the cascade utilization of LNG cold energy is realized based on the organic Rankine cycle, making full use of the cold energy of LNG lean liquid and ethane, without the need for additional external cold supply, and at the same time can be applicable to different low-grade heat sources. The comprehensive cold energy utilization rate of this system can reach 50%;

[0070] 2. It can use low-grade ambient thermal energy, such as using seawater as a heat source, improving the electrification rate of the system, reducing carbon emissions, and having good environmental protection effects.

[0071] In the specification provided herein, numerous specific details are set forth. It will be understood, however, that embodiments of the invention may be practiced without these specific details. In some instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0072] Similarly, it should be understood that in order to streamline this disclosure and help understand one or more of the various inventive aspects, in the foregoing description of the exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof. However, the disclosed method should not be construed as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, the inventive aspects lie in less than all the features of the single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of the invention.

[0073] It should be noted that the above embodiments are illustrative of the invention and not restrictive thereof, and that alternative embodiments may be devised by those skilled in the art without departing from the scope of the appended claims.

Claims

1. A cascaded utilization system based on low-grade thermal energy and LNG cold energy, characterized in that, The cascaded utilization system based on low-grade thermal energy and LNG cold energy includes: a cold energy power generation device and a light hydrocarbon separation system that are interconnected. The light hydrocarbon separation system is used to recover and separate C 2 + light hydrocarbons, and respectively obtain ethane, LPG, and LNG lean liquid. The cold energy power generation device uses the ethane and LNG lean liquid obtained by the light hydrocarbon separation system as cold sources respectively for cold energy power generation; wherein, the cold energy power generation device includes a first circulation pipeline, a second circulation pipeline, a third circulation pipeline and an intermediate heat exchanger; the first circulation pipeline includes a first power generation loop and a first heat exchanger that are connected to each other, and a first circulation working fluid flows in the first circulation pipeline; the second circulation pipeline includes a second power generation loop and a second heat exchanger that are connected to each other, and a second circulation working fluid flows in the second circulation pipeline, and the second circulation working fluid is LPG obtained by the light hydrocarbon separation system; the third circulation pipeline includes a third power generation loop and a third heat exchanger that are connected to each other, and a third circulation working fluid flows in the third circulation pipeline; the first power generation loop and the second power generation loop are respectively connected to the intermediate heat exchanger; the light hydrocarbon separation system is sequentially connected to the first heat exchanger, the second heat exchanger and the intermediate heat exchanger, and the light hydrocarbon separation system is connected to the third heat exchanger; the LNG lean liquid obtained by the light hydrocarbon separation system is used as a cold source and is sequentially introduced into the first heat exchanger, the second heat exchanger and the intermediate heat exchanger for heat exchange. The LNG lean liquid is used as a cold source and exchanges heat with the first circulation working fluid in the first heat exchanger and then is introduced into the second heat exchanger. The first circulation working fluid is introduced into the first power generation loop for power generation after heat exchange. The LNG lean liquid exchanges heat with LPG in the second heat exchanger and then is introduced into the intermediate heat exchanger. The LPG is introduced into the second power generation loop for power generation after heat exchange. The LNG lean liquid is introduced into the pipeline network after heat exchange in the intermediate heat exchanger; the ethane obtained by the light hydrocarbon separation system is introduced into the third heat exchanger. The ethane is used as a cold source and exchanges heat with the third circulation working fluid in the third heat exchanger. The third circulation working fluid is introduced into the third power generation loop for power generation after heat exchange; low-grade thermal energy flows in the intermediate heat exchanger and the third circulation pipeline.

2. The cascaded utilization system based on low-grade thermal energy and LNG cold energy according to claim 1, characterized in that, the first power generation loop includes a first expander, a first liquid separation tank and a first booster pump, and the first expander, the first heat exchanger, the first booster pump, the third heat exchanger and the first liquid separation tank are sequentially connected in a loop.

3. The cascaded utilization system based on low-grade thermal energy and LNG cold energy according to claim 2, characterized in that, the second power generation loop includes a second expander, a second liquid separation tank and a second booster pump, and the second expander, the second heat exchanger, the second booster pump, the third heat exchanger and the second liquid separation tank are sequentially connected in a loop. The LPG input port of the second booster pump is also connected to the light hydrocarbon separation system for receiving the LPG obtained by the light hydrocarbon separation system.

4. The cascaded utilization system based on low-grade thermal energy and LNG cold energy according to claim 3, characterized in that, an electric heater is further provided on the connection pipeline between the second expander and the second liquid separation tank.

5. The cascaded utilization system based on low-grade thermal energy and LNG cold energy according to claim 3, characterized in that, The third power generation loop includes a third expander, a fifth heat exchanger, and a third booster pump, and the third expander, the third heat exchanger, the third booster pump, and the fifth heat exchanger are sequentially connected in a cycle.

6. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 5, wherein, the first expander, the second expander, and the third expander are centrifugal, reciprocating, or screw expanders.

7. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 5, wherein, the temperature of the lean LNG increases sequentially after it is respectively introduced into the first heat exchanger, the second heat exchanger, and the intermediate heat exchanger in sequence.

8. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 5, wherein, the first heat exchanger, the second heat exchanger, and the third heat exchanger are plate-fin heat exchangers, shell-and-tube heat exchangers, or printed circuit board heat exchangers.

9. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 5, wherein, low-grade heat energy flows through the intermediate heat exchanger and the fifth heat exchanger, the heat source of the low-grade heat energy is seawater, and when the seawater enters and exits the intermediate heat exchanger and the fifth heat exchanger, the temperature difference is not greater than 5°C.

10. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 5, wherein, a fifth booster pump is provided on the pipeline connecting the ethane input port of the third heat exchanger to the light hydrocarbon separation system, and an air-cooled vaporizer is connected to the ethane output port of the third heat exchanger.

11. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 1, wherein, the light hydrocarbon separation system includes a condensation component, a flash absorption tower, a demethanizer, a deethanizer, a fourth heat exchanger, a fourth booster pump, and a fifth booster pump. The condensation component, the flash absorption tower, the demethanizer, the fifth booster pump, the deethanizer, and the fourth heat exchanger are sequentially connected. The top of the flash absorption tower and the top of the demethanizer are respectively connected to the condensation component. The condensation component is connected to the LPG input port of the fourth heat exchanger through the fourth booster pump. The LPG output port of the fourth heat exchanger is connected to the first heat exchanger, and the ethane output port of the fourth heat exchanger is connected to the third heat exchanger.

12. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 11, wherein, the condensation component includes a plurality of condensers connected in sequence.

13. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 1, wherein, the first circulating working fluid is one or a mixture of methane, ethane, propane, and butane, or is Freon.

14. The cascade utilization system based on low-grade heat energy and LNG cold energy according to claim 1, wherein, the third circulating working fluid is a single or mixed working fluid, including butane, pentane, or R152.

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