Azeotropic working medium power circulation system regulated and controlled by variable components of membrane separator

Abstract

The invention belongs to the field of liquefied natural gas cold energy power generation, and discloses an azeotropic working medium power circulation system regulated and controlled by variable components of a membrane separator, which comprises an expansion machine, the membrane separator, an evaporator and a heater. Azeotropic components are heated by seawater in the evaporator to be in a steam state and enter the expansion machine to do work through expansion, dead steam of the expansion machine enters the membrane separator to be separated, under the action of pressure difference, working media of the azeotropic components are divided into two non-azeotropic component flows with different pressures, and the two non-azeotropic component flows enter the two condensers to be subjected to heat exchange with LNG and are completely condensed to be in a liquid state; after being pressurized by the two working medium pumps, the mixture is mixed to become a mixture of azeotropic components again for the next circulation. According to the system, LNG cold energy is utilized step by step, components of the azeotropic working medium are regulated and controlled through the membrane separator, cooperative matching of the working medium, the heat source side and the cold source side is achieved, irreversible loss of the system is reduced, meanwhile, components and components of the mixed working medium are designed as required according to the characteristics of the cold source and the heat source, and high flexibility and controllability are achieved.

Description

Technical Field

[0001] The invention belongs to the field of liquefied natural gas (LNG) cold energy power generation, and relates to an azeotropic working fluid power circulation system regulated by a membrane separator with variable components. Background Art

[0002] Due to the development of the global economy and the growth of population, energy demand has been rising at an annual rate of 2.4% over the past few decades. Natural gas is one of the cleanest fossil fuels that power modern society. Due to its advantages such as transportability, high combustion efficiency and low contribution to greenhouse gas emissions, it has become the fastest growing primary energy source. These ideal characteristics make natural gas an excellent choice to bridge the gap between traditional energy and future renewable energy. Natural gas is extracted in a gaseous state. In order to facilitate storage and improve transportation efficiency, natural gas is liquefied into liquefied natural gas (LNG) at low temperatures. 1 cubic meter of LNG contains about 625 cubic meters of natural gas, making the energy density of LNG significantly higher than that of natural gas. LNG needs to be re-vaporized before use. Due to the large temperature difference between LNG and the environment, a large amount of cold energy will be released during the vaporization process. In the traditional LNG vaporization process, the cold energy of LNG is usually transferred to seawater or air, and is not utilized, resulting in extreme waste of energy. At present, the methods of utilizing LNG cold energy include air liquefaction separation, light hydrocarbon separation, seawater desalination, refined ethylene, dry ice manufacturing, cold energy power generation, food freezing and rubber crushing. Among them, the LNG cold energy power generation system has a short industrial chain, is not affected by external factors, and can recover cold energy in most temperature ranges, so it has received widespread attention.

[0003] At present, the main practical application of LNG cold energy power generation method is the Rankine cycle method. A large number of scholars at home and abroad mainly focus on improving the performance of the Rankine cycle to improve the utilization efficiency of LNG cold energy, including the improvement of system structure and the design of circulating working fluids. The system structure improvement is mainly divided into the series, parallel and mixed connection of the single loop Rankine cycle; the CO2 cycle and Kalina cycle that draw on the experience of waste heat recovery and renewable energy utilization; the cascade Rankine cycle that draws on the experience of natural gas liquefaction system, etc. Since these improvement methods are mainly borrowed from other applications, they have not deeply explored the thermodynamic essence of the LNG cold energy utilization process, so there is a lack of construction of a circulation system suitable for LNG cold energy power generation.

[0004] Different from waste heat recovery and renewable energy utilization systems, the main characteristics of the thermodynamic cycle for power generation using liquefied natural gas cold energy are: within the entire operating temperature range, the temperature change of the heat source is extremely small and can be approximately considered isothermal, while the temperature change of the cold source is huge. If a pure working fluid is used, due to its constant temperature phase change characteristics, it cannot form a good match with the temperature curve of the cold source side of the LNG vaporization process; while a non-azeotropic working fluid with variable temperature phase change characteristics can be well matched with the LNG cold source side, but cannot be well matched with the seawater heat source side. Therefore, whether it is a pure working fluid or a non-azeotropic working fluid, there is a problem that the temperature matching on one side is good, but it cannot meet the problem of coordinated matching of the temperature curve on the heat absorption side and the heat release side.

[0005] In summary, it is necessary to find a suitable working fluid and design a power cycle that meets the temperature characteristics of the cold and hot sources of the LNG cold energy power generation system. Summary of the invention

[0006] In order to solve the above problems, the present invention proposes an azeotropic working fluid power cycle regulated by changing components with a membrane separator. Through the characteristics of the azeotropic working fluid, it is found that the azeotropic working fluid will not experience temperature slip during the phase change process under the azeotropic component, but temperature slip will occur under other components. This characteristic enables the azeotropic working fluid to form a good temperature match with both the heat source and the cold source, but the components of the azeotropic working fluid need to be changed in this process. In the chemical industry, the commonly used methods for separating azeotropes are constant boiling distillation, extractive distillation and membrane separation. Among them, the membrane separation method has the advantages of low energy consumption, single-phase separation, and no addition of other components compared to the other two methods. Therefore, the membrane separation method is selected to regulate the azeotropic working fluid components.

[0007] The technical solution of the present invention:

[0008] An azeotropic working fluid power cycle system controlled by a membrane separator with variable components, comprising a working fluid evaporator 1, a first expander 2, a working fluid heater 3, a membrane separator 4, a second expander 5, a first-stage condenser 6, a first working fluid pump 7, a second-stage condenser 8, a second working fluid pump 9, a mixer 10 and a reheater 11;

[0009] The azeotropic working fluid component of the present invention is a mixture of alkanes, olefins (HC) and hydrofluorocarbons (HFC), and the azeotropic components at 10° C. enter the membrane separator. According to the prediction of the RefProp state equation, the available azeotropic working fluids are as follows: the binary azeotrope of pentafluoroethane (R125) and propane (R290), with a molar composition of 64.3% R125, 35.7% R290, and an azeotropic pressure of 1102 kPa; the binary azeotrope of pentafluoroethane (R125) and propylene (R1270), with a molar composition of 61.6% R125, 39.4% R1270, and an azeotropic pressure of 1032 kPa; the binary azeotrope of tetrafluoroethane (R134a) and propane (R290), with a molar composition of 37.9% R134a, 62.1% R290, and an azeotropic pressure of 775 kPa; the binary azeotrope of trifluoroethane (R143a) and propane (R290), with a molar composition of 62.4% R125, 39.4% R1270, and an azeotropic pressure of 1032 kPa. R143a, 37.6% R290, azeotropic pressure of 1004kPa; binary azeotrope of trifluoroethane (R143a) and propylene (R1270), its molar composition is 57.3% R143a, 42.7% R1270, azeotropic pressure of 939kPa; binary azeotrope of difluoromethane (R32) and propane (R290), its molar composition is 65.9% R32, 34.1% R290, azeotropic pressure of 1340kPa; binary azeotrope of difluoromethane (R32) and propylene (R1270), its molar composition is 60.9% R32, 39.1% R1270, the azeotropic pressure is 1503kPa; according to the physical properties of different azeotropic working fluids, the operating range of the parameters of the input system is as follows: the outlet pressure range of the first expander 2 is 1 / 5 of the azeotropic pressure to 1 / 2 of the azeotropic pressure, the outlet pressure range of the second expander 5 is 5 to 20kPa, and the pressure range of the membrane permeation side is 100-300kPa.

[0010] The azeotropic component is heated by seawater in the evaporator 1 to become a steam state and enters the first expander 2 for expansion and work. Then, the exhaust steam of the first expander 2 passes through the working fluid heater 3 to exchange heat with the seawater. The heated working fluid enters the membrane separator 4 for separation. Under the action of the pressure difference, the working fluid of the azeotropic component is divided into two non-azeotropic working fluids with different pressures. Among them, the working fluid on the retentate side of the membrane separator 4 enters the second expander 5 for expansion and work. The exhaust gas at the outlet of the second expander 5 is condensed by LNG in the first-stage condenser 6 and then pressurized and heated by the first working fluid pump 7; the working fluid on the permeate side of the membrane separator 4 enters the second-stage condenser 8, is completely condensed into a liquid state by LNG, and then enters the second working fluid pump 9 for pressurization; the two non-azeotropic component flows pressurized by the first working fluid pump 7 and the second working fluid pump 9 are mixed in the mixer 10, and then returned to the evaporator 1 to form an azeotropic component for the next cycle.

[0011] The LNG gasification process is as follows: an LNG storage tank filled with raw liquid LNG is connected to a first-stage condenser 6, a second-stage condenser 8 and a reheater 11 in sequence through an LNG pipeline; the LNG in the LNG storage tank first condenses the non-azeotropic components at the outlet of the second expander 5, and then the cold side outlet of the first-stage condenser 6 is connected to the cold side inlet of the second-stage condenser 8, so that the LNG continues to condense the non-azeotropic components entering the second-stage condenser 8, and then the LNG at the outlet of the second-stage condenser 8 is heat exchanged with seawater through the reheater 11, and finally the natural gas heated to the pipeline transportation temperature is sent to the pipeline network, thereby realizing the multi-stage utilization of LNG cold energy.

[0012] Beneficial effects of the present invention:

[0013] (1) Using an azeotropic mixture as the working fluid, the temperature curves of the endothermic and exothermic processes can be switched between isothermal and variable temperature, achieving efficient temperature matching of the entire cycle. There is no component migration under the azeotropic composition, and the heat transfer coefficient is higher than that of the non-azeotropic working fluid, which is easy to improve the economy and operability of the system.

[0014] (2) The components are regulated by homogeneous separation, which is independent of the phase equilibrium characteristics of the working fluid and can be used for the separation of azeotropic systems. The feed does not require phase change, and the energy consumption is lower than that of two-phase separation. The heat capacity can be changed by the components and flow rate, thereby regulating the heat transfer curve of the endothermic or exothermic process. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a flow chart of the azeotropic working fluid power cycle regulated by changing components of a membrane separator in the present invention.

[0016] In the figure: 1 evaporator; 2 first expander; 3 working fluid heater; 4 membrane separator; 5 second expander; 6 first stage condenser; 7 first working fluid pump; 8 second stage condenser; 9 second working fluid pump; 10 mixer; 11 reheater. DETAILED DESCRIPTION

[0017] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0018] Example 1

[0019] In this embodiment, the initial pressure of liquefied natural gas is 7000 kPa, the temperature is -162 ° C, the processing capacity is 3600 kg / h, and its molar composition is methane 91.33%, ethane 5.36%, propane 2.14%, n-butane 0.47%, isobutane 0.46%, n-pentane 0.01%, isopentane 0.01%, and nitrogen 0.22%. The working fluid required for the circulation process is a binary azeotrope of pentafluoroethane (R125) and propane (R290), and the molar composition of the azeotropic component at 10 ° C is 64.3% R125 and 35.7% R290. The heat source is seawater, the pressure is 101.325 kPa, and the temperature is 15 ° C. The membrane in the membrane separation device is polydimethylsiloxane (PDMS), and the selectivity to R290 / R125 is about 10.

[0020] The azeotropic components of the binary mixture of R125 and R290 (temperature 10°C, pressure 1102kPa, molar composition 64.3% R125, 35.7% R290) after heat exchange with seawater 1 in the evaporator 1 enter the first expander 2, the temperature drops to -30.99°C, the pressure drops to 274.21kPa, and the work done by the first expander 2 in this process is 56.02kW; the outlet exhaust gas of the first expander 2 exchanges heat with the seawater 2 in the working fluid heater 3 to increase the temperature of the azeotropic working fluid entering the membrane separator 4, the temperature rises to 10°C, and the pressure remains unchanged (274.21kPa); then the azeotropic working fluid is converted into two non-azeotropic working fluids by the pressure difference in the membrane separator, the retentate side working fluid (temperature 10°C, pressure 274.21kPa, molar composition 76.64% R125, 23.36% R290) enters the second expander 5, the temperature drops to -58.68 ° C, the pressure drops to 10.10 kPa, the second expander does 55.80 kW of work in this process, the exhaust gas at the outlet of the second expander is condensed in the first condenser 6, the temperature drops to -93.53 ° C, the pressure remains unchanged (10.10 kPa), and then is pressurized to 1102 kPa by the first working fluid pump 7, the temperature becomes -93.12 ° C, the first working fluid pump consumes 1.072 kW of work, and then the working fluid enters the mixer 10; the working fluid on the permeate side (temperature 10 ° C, pressure 202.84 kPa, molar composition 56.19% R125, 43.81% R290) enters the second-stage condenser 8 and is condensed to -40°C by LNG; it is pressurized to 1102kPa by the second working fluid pump 9, and the temperature becomes -39.49°C. The second working fluid pump consumes 1.507kW of power, and the pressurized working fluid also enters the mixer 10; after the two working fluids are mixed in the mixer, the temperature becomes -61.45°C, the pressure is 1102kPa, and the molar composition is 64.30% R125, 35.70% R290, and then returns to the evaporator 1 for the next cycle.

[0021] LNG gasification process: LNG with a temperature of -162°C and a pressure of 7000kPa absorbs the heat of the working fluid in the first-stage condenser 6, and the temperature rises to -88.34°C, and the pressure remains unchanged (7000kPa). The heated LNG enters the second-stage condenser 8, absorbs the heat of the working fluid again, and the temperature rises to -32.01°C, and the pressure remains unchanged (7000kPa). Then, LNG is heated by seawater 3 in the reheater 11 to become natural gas (NG) at 10°C, and finally input into the pipeline network.

[0022] According to the calculation of Aspen Hysys simulation software, the net output power per unit mass of LNG in this process is 108.9kW, the thermal efficiency is 15.21%, and the available energy efficiency is 29.09%. Compared with the traditional Rankine cycle, the net output power under the same working conditions is increased by about 53%, the thermal efficiency is increased by about 58.97%, and the available energy efficiency is increased by about 49.84%, realizing the efficient utilization of liquefied natural gas cold energy.

[0023] The above-described embodiments merely express the implementation methods of the present invention, but they cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. An azeotropic working fluid power circulation system controlled by changing components of a membrane separator, characterized in that: The azeotropic working fluid power cycle system comprises a working fluid evaporator (1), a first expander (2), a working fluid heater (3), a membrane separator (4), a second expander (5), a first-stage condenser (6), a first working fluid pump (7), a second-stage condenser (8), a second working fluid pump (9), a mixer (10) and a reheater (11); The azeotropic component is heated by seawater in the evaporator 1 to a steam state and enters the first expander (2) to expand and perform work. Then, the exhaust steam of the first expander (2) passes through the working fluid heater (3) to exchange heat with the seawater. The heated working fluid enters the membrane separator (4) for separation. Under the action of the pressure difference, the working fluid of the azeotropic component is divided into two non-azeotropic working fluids with different pressures. The working fluid on the retentate side of the membrane separator (4) enters the second expander (5) to expand and perform work. The exhaust gas at the outlet of the second expander (5) is condensed by LNG in the first-stage condenser (6) and then pressurized and heated by the first working fluid pump (7). The working fluid on the permeate side of the membrane separator (4) enters the second-stage condenser (8) and is completely condensed into a liquid state by LNG. Then, it enters the second working fluid pump (9) for pressurization. The two non-azeotropic component flows pressurized by the first working fluid pump (7) and the second working fluid pump (9) are mixed in the mixer (10) and then returned to the evaporator 1 to form an azeotropic component for the next cycle. The LNG gasification process is as follows: an LNG storage tank filled with raw liquid LNG is connected to a first-stage condenser (6), a second-stage condenser (8) and a reheater (11) in sequence through an LNG pipeline; the LNG in the LNG storage tank first condenses the non-azeotropic components at the outlet of the second expander (5); then the cold side outlet of the first-stage condenser (6) is connected to the cold side inlet of the second-stage condenser (8), so that the LNG continues to condense the non-azeotropic components entering the second-stage condenser (8); then, the LNG at the outlet of the second-stage condenser (8) is heat exchanged with seawater through the reheater (11); finally, the natural gas heated to the pipeline network transmission temperature is sent to the pipeline network, thereby realizing multi-stage utilization of LNG cold energy.

2. The azeotropic working fluid power cycle system according to claim 1, characterized in that: The azeotropic working fluid is selected as follows: the binary azeotrope of pentafluoroethane R125 and propane R290, whose molar composition is 64.3% R125, 35.7% R290, and the azeotropic pressure is 1102kPa; the binary azeotrope of pentafluoroethane R125 and propylene R1270, whose molar composition is 61.6% R125, 39.4% R1270, and the azeotropic pressure is 1032kPa; the binary azeotrope of tetrafluoroethane R134a and propane R290, whose molar composition is 37.9% R134a, 62.1% R290, and the azeotropic pressure is 775kPa; the binary azeotrope of trifluoroethane R143a and propane R290, whose molar composition is 64.3% R125, 35.7% R290, and the azeotropic pressure is 1102kPa; the binary azeotrope of pentafluoroethane R125 and propylene R1270, whose molar composition is 61.6% R125, 39.4% R1270, and the azeotropic pressure is 1032kPa; The molar composition of the binary azeotrope of trifluoroethane R143a and propylene R1270 is 62.4% R143a, 37.6% R290, and the azeotropic pressure is 1004kPa; the molar composition of the binary azeotrope of trifluoroethane R143a and propylene R1270 is 57.3% R143a, 42.7% R1270, and the azeotropic pressure is 939kPa; the molar composition of the binary azeotrope of difluoromethane R32 and propane R290 is 65.9% R32, 34.1% R290, and the azeotropic pressure is 1340kPa; the molar composition of the binary azeotrope of difluoromethane R32 and propylene R1270 is 60.9% R32, 39.1% R1270, and the azeotropic pressure is 1503kPa.

3. The azeotropic working fluid power cycle system according to claim 1, characterized in that: According to the physical properties of different azeotropic working fluids, the operating range of the parameters of the input system is as follows: the outlet pressure of the first expander (2) is 1 / 5 of the azeotropic pressure to 1 / 2 of the azeotropic pressure, the outlet pressure of the second expander (5) is 5 to 20 kPa, and the pressure on the membrane permeate side is 100-300 kPa.

Images