Method for analyzing desalination performance of seawater desalination system based on engine waste heat recovery

Abstract

This invention relates to the field of DCMD module desalination performance testing, specifically a method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery. This invention constructs a closed-loop thermal-mass-phase change model driven by engine waste heat and centered on the membrane interface for steady-state analysis of the seawater desalination process; it also comprehensively considers the coupling relationships between multiple parameters. These parameters collectively affect vapor pressure and heat transfer processes, thereby enabling the testing and analysis of the desalination performance of the seawater desalination system. By introducing the Knudsen number of the permeate membrane as a criterion, the microscopic mechanism of water vapor diffusion during membrane distillation is correlated with macroscopic performance testing. Based on the identification of diffusion types, the corresponding physical model is selected to calculate the mass transfer coefficient, improving the accuracy of mass flux and subsequent desalination rate calculations. This allows for real-time testing of the desalination performance of the DCMD module in the seawater desalination system with high accuracy.

Description

Technical Field

[0001] This invention relates to the field of DCMD module desalination performance testing, specifically a method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery. Background Technology

[0002] Currently, the large amount of waste heat generated by the high power, high exhaust flow, and high temperature of various types of marine diesel engines remains an underutilized resource. Only about one-third of the energy released from engine fuel combustion is effectively utilized, and effectively recovering and utilizing the waste heat from gas turbine exhaust is the most effective way to promote and expand the development of gas turbines. While introducing intercoolers and regenerators into traditional engine structures can effectively utilize engine exhaust waste heat to improve performance, it leads to a more complex engine structure and significantly increases manufacturing difficulty and development costs. To better meet the needs of ships in open-sea environments, refrigeration and seawater desalination systems utilizing engine waste heat, as described in publication number "CN118149496A," are being introduced into ships. This system's seawater desalination module uses direct contact membrane distillation (DCMD). After heat exchange between the engine waste heat and seawater, the high-temperature seawater is used as feed liquid into the DCMD. The DCMD module is divided into several segments, each corresponding to a membrane unit, as shown in the attached diagram. Figure 1 As shown, the permeate membrane divides each membrane unit into two channels. One set of channels is for the feed liquid (i.e., high-temperature seawater), and the other set is for cooling water. Due to the vapor pressure difference across the permeate membrane, the pressure difference acts as the driving force across the membrane, causing vapor on the feed side to pass through the membrane pores and condense in the cooling water on the cooling side, thus achieving seawater desalination. During seawater desalination, desalination performance is one of the key indicators for evaluating the efficiency of the DCMD module. Desalination performance can assess the desalination effect of the DCMD module under specific conditions, thereby determining whether it meets application requirements. However, the desalination performance of the DCMD module changes dynamically with variations in temperature and salinity. Therefore, how to dynamically measure the desalination performance of the DCMD module has become an urgent technical problem to be solved. Summary of the Invention

[0003] To avoid and overcome the technical problems existing in the prior art, this invention provides a method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery. This invention can perform real-time testing of the desalination performance of the DCMD module in a seawater desalination system with high testing accuracy.

[0004] To achieve the above objectives, the present invention provides the following technical solution: A method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery includes the following steps: S1. When the system is in a stable state, measure the waste heat of flue gas, the temperature of the main fluid, the salinity of the main fluid, and the operating parameters of the permeation membrane in the DCMD module of the seawater desalination system. S2. Determine the diffusion type of the permeation membrane based on the Knudsen number. The diffusion types of the permeation membrane include molecular diffusion, Knudsen diffusion, and transition diffusion. S3. After determining the diffusion type of the permeation membrane, calculate the mass transfer coefficient of the permeation membrane. S4. Using the temperature difference between the hot and cold sides of the permeate membrane as the driving force, establish the phase change equivalent heat transfer coefficient and energy balance equation, and obtain the overall heat transfer equation and the hot and cold side temperature of the permeate membrane surface through the overall thermal resistance of multi-segment heat transfer. S5. Calculate the salt concentration on the surface of the permeation membrane based on the salt mass conservation principle; S6. Using the vapor pressure difference between the hot and cold sides of the permeate membrane as the driving force and the membrane surface temperature and the salt concentration of the main fluid as constraints, the membrane interface mass transfer equation is established in combination with the mass transfer coefficient to calculate the water vapor mass flux. S7. Using the effective area of ​​each stage of the permeate membrane and the mass flux of water vapor passing through the permeate membrane, determine the total mass flow rate of the water produced by the DCMD module to obtain the desalination rate of the DCMD module.

[0005] As a further aspect of the present invention: the Knudsen number of the permeation membrane is Kn :

[0006] in, K b Boltzmann's constant; T Absolute temperature; The diameter of the pores in the permeate membrane structure; The pressure of the gas inside the membrane pores; d e The effective diameter of gas molecules in the permeation membrane; when Kn When <0.1, the diffusion type of the permeation membrane is molecular diffusion; when Kn When the value is greater than 10, the diffusion type of the permeable membrane is Knudsen diffusion; When 0.1≤ Kn When the value is ≤10, the diffusion type of the permeable membrane is transition diffusion.

[0007] As a further aspect of the present invention: when the diffusion type of the permeation membrane is molecular diffusion, the mass transfer coefficient of the permeation membrane... C m for:

[0008] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; T This refers to absolute temperature.

[0009] As a further aspect of the present invention: when the diffusion type of the permeation membrane is Knudsen diffusion, the mass transfer coefficient of the permeation membrane... C m for:

[0010] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

[0011] As a further aspect of the present invention: when the diffusion type of the permeation membrane is transition diffusion, the mass transfer coefficient of the permeation membrane... C m for:

[0012] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

[0013] As a further aspect of the present invention: in S4, an equation relating the phase change equivalent heat transfer coefficient to the energy balance is established. And the overall heat transfer equation is derived. ,in: ; ; ;

[0014] ; in, For heat transfer on the feed side of the permeation membrane; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. This refers to the temperature of the main fluid on the feed side. h f The convective heat transfer coefficient of the fluid on the hot side of the permeation membrane; For heat transfer on the permeate side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. The temperature of the main fluid on the product water side; h p The convective heat transfer coefficient of the fluid on the cold side of the permeation membrane; J w The mass flux of water vapor passing through the permeation membrane; The latent heat of vaporization on the surface of the permeate membrane; h m The thermal conductivity and heat transfer coefficient of the permeate membrane bulk; This refers to the heat flux through the permeation membrane; The thermal conductivity of water vapor; The thermal conductivity of the membrane material itself; The porosity of the permeable membrane; The thickness of the permeation membrane; Total heat transfer; The total heat transfer coefficient from the feed-side hot fluid to the product water-side cold fluid; It represents the equivalent phase change heat transfer coefficient of the water vapor phase change process.

[0015] As a further aspect of the present invention: the hot and cold side temperatures of the permeate membrane surface are: ; .

[0016] As a further aspect of the present invention: in S5, the salt concentration on the surface of the permeation membrane is... :

[0017] ; in, This refers to the salt concentration of the mainstream on the hot side of the permeate membrane. The density of the mainstream on the hot side of the permeation membrane; K The mass transfer coefficient for salt content; It is the Reynolds number; D h The hydraulic diameter; The diffusion coefficient of water vapor in air; The dynamic viscosity of the fluid; The density of the fluid.

[0018] As a further aspect of the present invention: in S6, the mass flux of water vapor permeating through the permeation membrane... J w for: ; in, The mass transfer coefficient of the permeation membrane; This is the saturated vapor pressure of the brine; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. The salinity of the membrane surface on the hot side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. This refers to the surface salinity of the membrane on the cold side of the permeate membrane.

[0019] As a further aspect of the present invention: the DCMD module is uniformly divided into multiple membrane segments, and the total mass flow rate of the water produced by the DCMD module is... for:

[0020] in, For DCMD module number The mass flux of water vapor permeating through a permeable membrane; For DCMD module number The effective membrane area of ​​the graded permeable membrane; j The total number of levels for the DCMD module.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention constructs a closed-loop thermal-mass-phase change model driven by engine waste heat and centered on the membrane interface for steady-state analysis of the seawater desalination process. It comprehensively considers the coupling relationships between multiple parameters, including waste heat input, fluid temperature change, membrane interface temperature, vapor pressure difference, water vapor flux, mass change, and concentration change. These parameters collectively affect the vapor pressure and heat transfer process, thereby enabling the testing and analysis of the desalination performance of the seawater desalination system. By introducing the Knudsen number of the permeate membrane as a criterion, the microscopic mechanism of water vapor diffusion during membrane distillation is correlated with macroscopic performance testing. Based on the identification of diffusion types, the corresponding physical model is selected to calculate the mass transfer coefficient, improving the accuracy of mass flux and subsequent desalination rate calculations. This allows for real-time testing of the desalination performance of the DCMD module in the seawater desalination system with high accuracy.

[0022] 2. This invention establishes a complete testing process from the determination of microscopic membrane parameters, the judgment of diffusion mechanism, the calculation of macroscopic mass and heat transfer, and the output of system performance. It also establishes an iterative calculation method for key boundary parameters such as membrane surface temperature and salinity, making the entire testing process highly reliable. By establishing and solving the calculation, the true temperature of the cold and hot sides of the permeate membrane surface and the surface salinity under concentration polarization are obtained, which effectively solves the calculation error caused by directly using the main fluid parameters and improves the calculation accuracy of permeate flux.

[0023] 3. Based on the possible temperature and salinity variations along the DCMD module, this invention calculates the membrane module in segments, calculates the permeate flux of each segment and sums them up to obtain the total permeate flow, which more accurately reflects the overall desalination performance of the system under non-uniform operating conditions and makes the test closer to the actual operating conditions. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the working state of a DCMD membrane unit.

[0025] Figure 2 This is a schematic diagram of the refrigeration and seawater desalination system in this invention.

[0026] In the picture: 1. Absorber; 11. First heat exchanger; 12. Second heat exchanger; 121. First electric regulating valve; 13. Solution pump; 2. High-voltage generator; 3. First low-voltage generator; 31. First variable frequency pump; 4. Second low-pressure generator; 41. First throttle valve; 5. Condenser; 6. Evaporator; 61. Second expansion valve; 62. Chilled water inlet; 63. Chilled water outlet; 64. Refrigerant variable frequency pump; 7. Diesel engine; 71. Third heat exchanger; 8. Seawater desalination module; 81. Flue gas heat exchanger; 82. Direct contact membrane still; 821. Hydrophobic membrane; 83. Shell and tube heat exchanger; 831. Circulating pump; 832. Second variable frequency pump; 84. Seawater heat exchanger; 841. Third variable frequency pump; 85. Water storage unit; 86. Seawater inlet; 87. Concentrated brine outlet; 871. Second electric regulating valve; 88. Fresh water outlet; 9. Latent heat storage unit; 91. Fourth variable frequency pump. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] Please see Figures 1 - 2 In this embodiment of the invention, the desalination performance analysis method of a seawater desalination system based on engine waste heat recovery utilizes the engine waste heat through methods such as...Figure 2 The refrigeration and seawater desalination system shown is used for recycling. The refrigeration and seawater desalination system is described below: I. Refrigeration System: It includes a refrigerant circulation pipeline, which is equipped with a high-pressure generator 2, a first low-pressure generator 3 and a second low-pressure generator 4, and the refrigerant is preferably a lithium bromide solution.

[0029] The low-temperature refrigerant is first discharged from the absorber 1 and then enters the cold end of the first heat exchanger 11. After heat exchange, it becomes a medium-temperature state and then flows through the cold end of the first heat exchanger 11 before being split.

[0030] After being diverted, a portion of the medium-temperature refrigerant is directly fed into the first low-pressure generator 3 and sprayed onto the finned heat exchanger of the first low-pressure generator 3. During the spraying process, the cylinder liner coolant of the diesel engine 7 passing through the finned heat exchanger of the first low-pressure generator 3 heats the medium-temperature refrigerant sprayed onto the finned heat exchanger and evaporates it into coolant vapor. The partially evaporated medium-temperature refrigerant changes from a dilute solution to a semi-dilute solution and flows to the second low-pressure generator 4. After being sprayed onto the finned heat exchanger of the second low-pressure generator 4, it is heated and evaporated again to produce coolant vapor. At this time, the semi-dilute medium-temperature refrigerant solution becomes a concentrated low-temperature refrigerant solution.

[0031] The diverted medium-temperature refrigerant enters the cold end of the second heat exchanger 12, where it becomes a high-temperature refrigerant. This high-temperature refrigerant is then introduced into the high-pressure generator 2 and sprayed onto the finned heat exchanger of the high-pressure generator 2. During the spraying process, the high-temperature flue gas passing through the finned heat exchanger of the high-pressure generator 2 exchanges heat with the high-temperature refrigerant. At this time, the high-temperature refrigerant is heated and boiled under low pressure, evaporating into refrigerant vapor. After evaporation, the high-temperature refrigerant changes from a dilute solution to a semi-dilute solution, flows into the hot end of the second heat exchanger 12, and after heat exchange, becomes a medium-temperature refrigerant. This vapor mixes with the medium-temperature refrigerant leaving the first low-pressure generator 3 and is sprayed onto the finned heat exchanger of the second low-pressure generator 4.

[0032] The refrigerant vapor evaporated from the finned heat exchanger of the high-pressure generator 2 is fed into the finned heat exchanger of the second low-pressure generator 4. After condensation and heat release, it becomes a high-temperature refrigerant solution. After throttling, it is sprayed onto the finned heat exchanger of the condenser 5, where it exchanges heat with the fresh water passing through the finned heat exchanger of the condenser 5. After condensation and cooling, it becomes a low-temperature refrigerant solution. At the same time, the refrigerant vapor evaporated from the first low-pressure generator 3 and the second low-pressure generator 4 exchanges heat with the finned heat exchanger in the condenser 5, and after condensation and cooling, it becomes a low-temperature refrigerant solution.

[0033] A temperature sensor and a pressure sensor are installed at the steam outlet of the high-pressure generator 2, and a temperature sensor is installed at the outlet of the finned heat exchanger of the high-pressure generator 2; a first electric regulating valve 121, a flow meter, and a temperature sensor are installed at the cold end outlet of the second heat exchanger 12; a first throttle valve 41 is installed at the outlet of the finned heat exchanger of the second low-pressure generator 4; and a solution pump 13, a flow meter, a temperature sensor, and a pressure sensor are installed at the solution outlet of the absorber 1.

[0034] The refrigerant solution discharged from condenser 5 is fed into evaporator 6 and sprayed onto the finned heat exchanger of evaporator 6, where a portion evaporates to form refrigerant vapor. The low-temperature concentrated refrigerant solution discharged from the second low-pressure generator 4 passes through the hot end of the first heat exchanger 11 and enters the absorber 1, where it mixes with the refrigerant vapor discharged from evaporator 6, diluting the concentrated solution into a dilute solution. The heat generated during the dilution process is recovered through the low-temperature fresh water in absorber 1. The unevaporated portion of the refrigerant solution sprayed onto evaporator 6 is transported by the third variable frequency pump 841 and re-sprayed onto the finned heat exchanger of evaporator 6. High-temperature chilled water from the ship passes through the finned heat exchanger of evaporator 6 from chilled water inlet 62, absorbing heat and cooling to become low-temperature chilled water, which can be used for cooling the ship's living environment and electronic equipment. A resistivity meter and a refrigerant variable frequency pump 64 are installed on the refrigerant solution collection pipeline of evaporator 6, a temperature sensor is installed at chilled water outlet 63, and a second throttle valve 61 is installed at the spray nozzle of evaporator 6.

[0035] The low-temperature fresh water in the latent heat storage unit 9 flows back into the latent heat storage unit 9 after passing through the finned heat exchanger of the absorber 1 and the finned heat exchanger of the condenser 5. During this process, the low-temperature fresh water absorbs heat and becomes high-temperature fresh water. After returning to the latent heat storage unit 9, it transfers heat to the phase change material of the latent heat storage unit 9, thereby enabling the charging process of the battery. Simultaneously, the high-temperature fresh water becomes low-temperature fresh water again, and this cycle continues. A fourth variable frequency pump 91 for water delivery is installed upstream of the inlet of the absorber 1 finned heat exchanger. Temperature sensors are arranged on the water supply pipeline between the condenser 5 and the latent heat storage unit 9.

[0036] Each of the high-pressure generator 2, the first low-pressure generator 3, the second low-pressure generator 4, the condenser 5, the evaporator 6, and the absorber 1 is independently equipped with a vacuum pump to maintain the internal vacuum level. The vacuum level inside the two low-pressure generators is much higher than that inside the high-pressure generator 2.

[0037] The cylinder liner coolant of the diesel engine 7 enters the finned heat exchanger of the first low-pressure generator 3 after passing through the hot end of the third heat exchanger 71 for heat exchange. After being transported by the first variable frequency pump 31, it flows back into the diesel engine 7 through the cold end of the third heat exchanger 71. Temperature sensors are installed at both the hot and cold end outlets of the third heat exchanger 71; temperature and pressure sensors are installed at the exhaust gas outlet of the diesel engine 7.

[0038] II. Seawater desalination system: The high-temperature flue gas from the diesel engine 7 enters the seawater desalination module 8 after passing through the finned heat exchanger of the high-pressure generator 2, where it exchanges heat with seawater. Seawater enters the flue gas heat exchanger 81 through the cold end of the seawater heat exchanger 84 via the coastal water inlet 86, where it exchanges heat with the high-temperature flue gas to form water vapor. This water vapor is then separated and purified by the hydrophobic membrane 821 of the direct contact membrane still 82 before passing through the hot end of the seawater heat exchanger 84. The hot end of the seawater heat exchanger 84 splits the flow: one part flows directly into the water storage unit 85, while the other part passes through the cold fluid side of the shell-and-tube heat exchanger 83 to form a freshwater circulation. After the seawater at the seawater inlet 86 is split, one part passes through the cold end of the seawater heat exchanger 84, and the other part exchanges heat with freshwater through the shell-and-tube heat exchanger 83 before being discharged into the water storage unit 85. The concentrated brine separated by the direct contact membrane still 82 is partially discharged through the concentrated brine outlet 87, and the other part is recirculated back into the cold end of the seawater heat exchanger 84 for a secondary circulation.

[0039] A temperature sensor is installed at the seawater inlet 86. A conductivity meter, flow meter, pressure sensor, and third variable frequency pump 841 are installed at the cold end inlet of the seawater heat exchanger 84. A second variable frequency pump 832 is installed at the freshwater inlet of the shell-and-tube heat exchanger 83. A temperature sensor and a pressure sensor are installed at the freshwater outlet 88 of the shell-and-tube heat exchanger 83. A circulation pump 831 is installed at the seawater inlet of the shell-and-tube heat exchanger 83.

[0040] The water storage unit 85 delivers stored fresh water to the outside through the fresh water outlet 88. A conductivity meter and a flow meter are installed at the water inlet of the water storage unit 85. The fresh water circulates between the water storage unit 85 and the latent heat storage unit 9 to heat the fresh water. A conductivity meter and a second electric regulating valve 871 are installed at the concentrated brine outlet 87 of the direct contact membrane distiller 82.

[0041] The flue gas discharged from the high-pressure generator 2 enters the flue gas heat exchanger 81 to heat the seawater. After the flue gas temperature decreases, it is directly discharged outside the ship. The seawater is heated and boiled under a certain vacuum to produce water vapor, which flows into the direct contact membrane still 82. Under the pressure difference across the hydrophobic microporous membrane, the hot water vapor permeates through the membrane pores to the cold side of the membrane, where it is cooled by the circulating distilled low-temperature fresh water, thus achieving purification. After continuous circulation, the increased distilled fresh water flows into the water storage unit 85. The water storage unit 85 provides fresh water for the ship's domestic use and also provides water for charging and discharging the batteries in the latent heat storage unit 9. During the seawater desalination process, the system can be adjusted in real time according to the data from the conductivity meter, so that a portion of the high-temperature concentrated brine mixes with the low-temperature seawater and enters the membrane distillation cycle, thereby improving the utilization rate of seawater and waste heat from the flue gas.

[0042] The direct contact membrane still 82, also known as the DCMD module, is divided into several sections, each of which is equipped with a membrane unit. The arrangement of the membrane units in the DCMD module is not limited; they can be arranged in a coaxial sleeve or a linear interval.

[0043] The specific steps for analyzing the desalination performance of a seawater desalination system include: S1. Determine the steady-state assumption, that is, the seawater desalination system satisfies the conservation of energy and mass in any computing unit; at this time, measure the waste heat of flue gas, the temperature of the main fluid, the salinity of the main fluid, and the operating parameters of the permeation membrane in the DCMD module of the seawater desalination system. S2. Determine the diffusion type of the permeation membrane based on the Knudsen number. The diffusion types of the permeation membrane include molecular diffusion, Knudsen diffusion, and transition diffusion. The Knudsen number of the permeable membrane is Kn :

[0044] in, K b Boltzmann's constant; T Absolute temperature; The diameter of the pores in the permeate membrane structure; The pressure of the gas inside the membrane pores; d e The effective diameter of gas molecules in the permeation membrane; when Kn When <0.1, the diffusion type of the permeation membrane is molecular diffusion; when Kn When the value is greater than 10, the diffusion type of the permeable membrane is Knudsen diffusion; When 0.1≤Kn When the value is ≤10, the diffusion type of the permeable membrane is transition diffusion. S3. After determining the diffusion type of the permeation membrane, calculate the mass transfer coefficient of the permeation membrane. S31. When the diffusion type of the permeation membrane is molecular diffusion, the mass transfer coefficient of the permeation membrane is... C m for:

[0045] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; T This refers to absolute temperature.

[0046] S32. When the diffusion type of the permeation membrane is Knudsen diffusion, the mass transfer coefficient of the permeation membrane is... C m for:

[0047] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

[0048] S33. When the diffusion type of the permeation membrane is transition diffusion, the mass transfer coefficient of the permeation membrane... C m for:

[0049] in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

[0050] S4. Using the fluid temperature difference between the hot and cold sides of the permeate membrane as the driving force for seawater desalination, determine the phase change equivalent heat transfer coefficient and energy balance equation in the permeate membrane desalination process, and convert the multi-stage heat transfer process into an overall thermal resistance to obtain the overall heat transfer equation of the system and the hot and cold side temperatures of the permeate membrane surface. ; ; ;

[0051] ; ; ; in, For heat transfer on the feed side of the permeation membrane; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. This refers to the temperature of the main fluid on the feed side. h f The convective heat transfer coefficient of the fluid on the hot side of the permeation membrane; For heat transfer on the permeate side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. The temperature of the main fluid on the product water side; h p The convective heat transfer coefficient of the fluid on the cold side of the permeation membrane; J wThe mass flux of water vapor passing through the permeation membrane; This refers to the latent heat of vaporization of the permeation membrane surface at the corresponding temperature and salinity. h m The thermal conductivity and heat transfer coefficient of the permeate membrane bulk; The heat flux through the permeation membrane includes heat conduction through the membrane material itself and latent heat transfer from water evaporation / condensation.

[0052] The thermal conductivity of water vapor; The thermal conductivity of the membrane material itself; The porosity of the permeable membrane; The thickness of the permeation membrane; For the total heat transfer, under steady state, it equals , and under steady state, it equals . .

[0053] The total heat transfer coefficient from the feed-side hot fluid to the product water-side cold fluid; The equivalent phase change heat transfer coefficient represents the phase change process of water vapor; During the calculation, the mass flux of water vapor permeating through the permeation membrane... J w Depends on the hot and cold side temperatures of the permeate membrane surface as well as At the start of the calculation, for J w Assign a hypothetical value and substitute it into the hypothetical value. J w Value calculated as well as ; calculate the as well as Substitute into S6 for calculation J w Comparing the new and old J w Check if the values ​​are consistent; if not, use the newly obtained value. J w The value was recalculated. as well as Repeat this process until... J w as well as The value no longer changes, indicating convergence.

[0054] S5. Based on the salt concentration change of the mainstream on the hot and cold sides of the permeate membrane, determine the salt mass conservation equation on the nth stage feed side of the permeate membrane, that is, the salt mass flow rate entering the nth stage per unit time is equal to the salt mass flow rate leaving the nth stage (entering the n+1th stage) per unit time.

[0055] ; ; in, This represents the salt concentration on the surface of the permeate membrane. This refers to the salt concentration of the mainstream on the hot side of the permeate membrane. The density of the mainstream on the hot side of the permeation membrane; K Salt transfer coefficient represents the ability of salt to return from the membrane surface to the bulk feed solution; It is the Reynolds number; D h The hydraulic diameter; The diffusion coefficient of water vapor in air; The dynamic viscosity of the fluid; The density of the fluid; Let be the mass flow rate of the fluid at the feed inlet of the nth stage permeation membrane; Let be the density of the fluid at the feed inlet of the nth stage permeate membrane; Salt concentration of the main current on the hot side of the nth stage permeation membrane; The mass flow rate of the fluid at the feed inlet of the (n+1)th stage permeation membrane; The density of the fluid at the feed inlet of the (n+1)th stage permeate membrane; Salt concentration of the main current on the hot side of the (n+1)th stage permeation membrane; S6. Using the fluid vapor pressure difference between the hot and cold sides of the permeate membrane as the driving force for seawater desalination, and taking the temperature of the hot and cold sides of the permeate membrane surface and the salt concentration change of the mainstream on the hot and cold sides of the permeate membrane as constraints, the mass transfer equation of the permeate membrane interface is determined. Combined with the mass transfer coefficient of the permeate membrane, the mass flux of water vapor passing through the permeate membrane can be calculated. Mass flux of water vapor passing through the permeation membrane J w for: ; in, The mass transfer coefficient of the permeation membrane; This is the saturated vapor pressure of the brine; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. The salinity of the membrane surface on the hot side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. This refers to the surface salinity of the membrane on the cold side of the permeate membrane.

[0056] S7. Based on the effective area of ​​each stage of the permeation membrane and the mass flux of water vapor passing through the permeation membrane, the mass of water distilled by the DCMD module is determined and calculated, and the desalination rate of the DCMD module can be obtained.

[0057] The DCMD module is evenly divided into multiple membrane segments, and the total mass flow rate of the water produced by the DCMD module is... for:

[0058] in, For DCMD module number The mass flux of water vapor permeating through a permeable membrane; For DCMD module number The effective membrane area of ​​the graded permeable membrane; j The total number of levels for the DCMD module.

[0059] The basic principles of this application have been described above with reference to specific embodiments. However, it should be noted that the advantages, benefits, and effects mentioned in this application are merely examples and not limitations, and should not be considered as essential features of each embodiment of this application. Furthermore, the specific details disclosed above are for illustrative and facilitative purposes only, and are not limitations. These details do not limit the application to the necessity of employing the aforementioned specific details for implementation.

[0060] The block diagrams of devices, apparatuses, devices, and systems involved in this application are merely illustrative examples and are not intended to require or imply that they must be connected, arranged, or configured in the manner shown in the block diagrams. As those skilled in the art will recognize, these devices, apparatuses, devices, and systems can be connected, arranged, and configured in any manner. Words such as “comprising,” “including,” “having,” etc., are open-ended terms meaning “including but not limited to,” and are used interchangeably with them. The terms “or” and “and” as used herein refer to the terms “and / or,” and are used interchangeably with them unless the context clearly indicates otherwise. The term “such as” as used herein refers to the phrase “such as but not limited to,” and is used interchangeably with it.

Claims

1. A method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery, characterized in that, Includes the following steps: S1. When the system is in a stable state, measure the waste heat of flue gas, the temperature of the main fluid, the salinity of the main fluid, and the operating parameters of the permeation membrane in the DCMD module of the seawater desalination system. S2. Determine the diffusion type of the permeation membrane based on the Knudsen number. The diffusion types of the permeation membrane include molecular diffusion, Knudsen diffusion, and transition diffusion. S3. After determining the diffusion type of the permeation membrane, calculate the mass transfer coefficient of the permeation membrane. S4. Using the temperature difference between the hot and cold sides of the permeate membrane as the driving force, establish the phase change equivalent heat transfer coefficient and energy balance equation, and obtain the overall heat transfer equation and the hot and cold side temperature of the permeate membrane surface through the overall thermal resistance of multi-segment heat transfer. S5. Calculate the salt concentration on the surface of the permeation membrane based on the salt mass conservation principle; S6. Using the vapor pressure difference between the hot and cold sides of the permeate membrane as the driving force and the membrane surface temperature and the salt concentration of the main fluid as constraints, the membrane interface mass transfer equation is established in combination with the mass transfer coefficient to calculate the water vapor mass flux. S7. Using the effective area of ​​each stage of the permeate membrane and the mass flux of water vapor passing through the permeate membrane, determine the total mass flow rate of the water produced by the DCMD module to obtain the desalination rate of the DCMD module.

2. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 1, characterized in that, The Knudsen number of the permeable membrane is Kn : in, K b Boltzmann's constant; T Absolute temperature; The diameter of the pores in the permeate membrane structure; The pressure of the gas inside the membrane pores; d e The effective diameter of gas molecules in the permeation membrane; when Kn When <0.1, the diffusion type of the permeation membrane is molecular diffusion; when Kn When the value is greater than 10, the diffusion type of the permeable membrane is Knudsen diffusion; When 0.1≤ Kn When the value is ≤10, the diffusion type of the permeable membrane is transition diffusion.

3. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 2, characterized in that, When the diffusion type of the permeation membrane is molecular diffusion, the mass transfer coefficient of the permeation membrane is... C m for: in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; T This refers to absolute temperature.

4. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 2, characterized in that, When the diffusion type of the permeation membrane is Knudsen diffusion, the mass transfer coefficient of the permeation membrane is... C m for: in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

5. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 2, characterized in that, When the diffusion type of the permeation membrane is transition diffusion, the mass transfer coefficient of the permeation membrane... C m for: in, The porosity of the permeable membrane; The radius of the membrane pores; The tortuosity of the pores in the permeable membrane; The thickness of the permeation membrane; M The molar mass of water; R This is the universal gas constant; The diffusion pressure of water vapor in the membrane pores; The average pressure of the system; T This refers to absolute temperature.

6. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to any one of claims 3 to 5, characterized in that, In S4, the phase change equivalent heat transfer coefficient and energy balance equation are established. And the overall heat transfer equation is derived. ,in: ； ； ； ； in, For heat transfer on the feed side of the permeation membrane; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. This refers to the temperature of the main fluid on the feed side. h f The convective heat transfer coefficient of the fluid on the hot side of the permeation membrane; For heat transfer on the permeate side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. The temperature of the main fluid on the product water side; h p The convective heat transfer coefficient of the fluid on the cold side of the permeation membrane; J w The mass flux of water vapor passing through the permeation membrane; The latent heat of vaporization on the surface of the permeate membrane; h m The thermal conductivity and heat transfer coefficient of the permeate membrane bulk; This refers to the heat flux through the permeation membrane; The thermal conductivity of water vapor; The thermal conductivity of the membrane material itself; The porosity of the permeable membrane; The thickness of the permeation membrane; Total heat transfer; The total heat transfer coefficient from the feed-side hot fluid to the product water-side cold fluid; It represents the equivalent phase change heat transfer coefficient of the water vapor phase change process.

7. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 6, characterized in that, The hot and cold side temperatures of the permeate membrane surface are: ； 。 8. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 6, characterized in that, In S5, the salt concentration on the surface of the permeate membrane is : ； in, This refers to the salt concentration of the mainstream on the hot side of the permeate membrane. The density of the mainstream on the hot side of the permeation membrane; K The mass transfer coefficient for salt content; It is the Reynolds number; D h The hydraulic diameter; The diffusion coefficient of water vapor in air; The dynamic viscosity of the fluid; The density of the fluid.

9. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to claim 8, characterized in that, In S6, the mass flux of water vapor permeating through the permeation membrane J w for: ； in, The mass transfer coefficient of the permeation membrane; This is the saturated vapor pressure of the brine; This refers to the surface temperature of the membrane on the hot side of the permeate membrane. The salinity of the membrane surface on the hot side of the permeate membrane; This refers to the surface temperature of the membrane on the cold side of the permeate membrane. This refers to the surface salinity of the membrane on the cold side of the permeate membrane.

10. The method for analyzing the desalination performance of a seawater desalination system based on engine waste heat recovery according to any one of claims 1 to 9, characterized in that, The DCMD module is evenly divided into multiple membrane segments, and the total mass flow rate of the water produced by the DCMD module is... for: in, For DCMD module number The mass flux of water vapor permeating through a permeable membrane; For DCMD module number The effective membrane area of ​​the graded permeable membrane; j The total number of levels for the DCMD module.

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