A method and apparatus for improving waste heat utilization in LDPE and EVA production systems

Abstract

This invention discloses a method and apparatus for improving the waste heat utilization rate of LDPE and EVA production systems, including flash pressure control of hot water tanks and condensate tanks, by-product steam recovery and utilization, and equipment start-up and shutdown control. The flash pressure control of the hot water tanks and condensate tanks is based on the saturation pressure corresponding to the highest circulating hot water inlet temperature required by the high-temperature cooling section of the reactor, thereby controlling the pressure of the high-pressure hot water tank and sequentially controlling the pressure difference between the subsequent low-pressure hot water tank and medium-pressure condensate tank to not exceed 0.01 MPa. The by-product steam recovery and utilization involves mechanically compressing the by-product steam to increase its pressure to the same pressure level as the lowest pressure level steam supplied from outside the boundary area, for use by low-pressure steam users, with the surplus supplied outside the boundary area. The steam source for mechanical compression comes from the by-product steam venting main. This invention can effectively improve the by-product steam recovery rate and waste heat utilization rate during EVA product production in LDPE and EVA production systems, achieving the goals of energy conservation and emission reduction.

Description

Technical Field

[0001] The present invention relates to the technical field of LDPE / EVA production, and particularly relates to a method and device for improving the waste heat utilization rate of an LDPE / EVA production system. Background Art

[0002] LDPE (low density polyethylene) and EVA (ethylene-vinyl acetate copolymer resin) have good market prospects. Currently, there are multiple LDPE and EVA production systems that have been put into operation or are under construction in the market, and can be switched between different product categories and different product grades of LDPE and EVA.

[0003] In the production process, high-pressure steam (HS: 2.5 MPa), medium-pressure steam (MS: 0.8 MPa), and low-pressure steam (LS: 0.5 MPa) supplied outside the device boundary are used to preheat and heat-insulate the materials corresponding to the requirements. The production of LDPE / EVA belongs to a polymerization exothermic reaction, and a large amount of heat is released during the polymerization reaction. In order to maintain continuous production and improve production efficiency, the heat of the reaction needs to be removed through the jacket of the reaction kettle by circulating hot water. After the hot water enters the reaction kettle, the temperature of the circulating hot water rises, and then it returns to the hot water tank. The flashing pressure is controlled according to the inlet water temperature required for the specific product in production. After flashing, the temperature of the hot water decreases, and so on in a cycle. Among them, the water loss caused by flashing is supplemented by external water.

[0004] The LDPE and EVA production systems have specific requirements for the temperature of the circulating hot water entering the reaction kettle for different products. When producing LDPE products, the temperature of the hot water entering the jacket of the reaction kettle is required to be relatively high, controlled at about 170 °C; when producing EVA products, the temperature of the hot water entering the jacket of the reaction kettle is required to be relatively low, controlled at about 135 °C. Due to the different temperature control requirements, according to the flashing principle, the corresponding flashing steam pressures are also different. When producing LDPE products, the flashing steam pressure can reach above 0.8 MPa, which can meet the pressure grade requirements of the low-pressure steam (LS: 0.5 MPa) users in the device area. When producing EVA products, the flashing steam pressure can only reach about 0.3 MPa. This part of the flashing steam cannot be incorporated into the low-pressure steam pipe network. Usually, it is cooled and condensed through the by-product steam condenser supporting the high- and low-pressure hot water tanks, and the condensed water in the recycled steam is reused. The uncondensed by-product steam is transported to the centralized venting tank for gas-liquid separation, and then the water vapor is discharged into the atmosphere and the liquid water is discharged into the sewage treatment system. In this process, a large amount of heat energy is wasted and the electric energy consumption of the by-product steam condensation system is increased. Moreover, since the hot water is discharged into the sewage treatment system, the treatment cost of the sewage system is increased.

[0005] After the steam of different pressure levels supplied outside the plant boundary for process preheating and insulation releases heat, the high-pressure condensate and medium-pressure condensate return to the medium-pressure condensate tank, and the low-pressure condensate returns to the low-pressure condensate tank. Because the medium-pressure condensate tank recovers high-pressure steam and medium-pressure steam condensate, it has the capability to flash-evaporate low-pressure steam for reuse. However, due to the need for level balance among the high-pressure hot water tank, low-pressure hot water tank, medium-pressure condensate tank, and low-pressure condensate tank, a hot water / condensate drain line is installed. The drain sequence is high-pressure hot water tank - low-pressure hot water tank - medium-pressure condensate tank - low-pressure condensate tank, and then the condensate is pumped to hot water users. Excess condensate is discharged outside the plant boundary. Because the unit's condensate return water requires a pressure difference between the tanks, the control pressure of each tank is from high to low as follows: high-pressure hot water tank - low-pressure hot water tank - medium-pressure condensate tank - low-pressure condensate tank. This causes a problem, especially when producing EVA products, where the high-pressure hot water tank controls the flash evaporation pressure too low, preventing the medium-pressure condensate tank from being pressurized. This reduces the flash steam pressure in the medium-pressure condensate tank, making it impossible to integrate it into the low-pressure steam network for reuse. Instead, it can only be discharged to a centralized vent tank, resulting in a waste of thermal energy and demineralized water resources.

[0006] The water missing from the system is supplied from outside the boundary area. Since the water contains dissolved oxygen, it needs to be thermally deoxygenated before entering the low-pressure condensate tank. The deoxygenated exhaust gas in the low-pressure condensate tank is condensed and vented.

[0007] Since the polymerization process of LDPE / EVA is an exothermic reaction, each kg of product releases 3000-3600 kJ of heat. After deducting heat loss, assuming an average heat production of 3250 kJ per kg of product and an hourly production capacity of 36 t / h (designed annual capacity of 300,000 tons), including the heat from polymerization reaction and steam condensation, 134 GJ / h of heat needs to be removed from the main production unit. Apart from about 27 GJ / h used by some equipment in the production unit (such as the hot water type lithium bromide chiller), the remaining 107 GJ / h (equivalent to 35 t / h of 0.31 MPa saturated steam) needs to be carried away through cooling, condensation, and venting, resulting in a large waste of heat, electricity, and water resources. At the same time, the vented steam has an adverse effect on the equipment and structures in the plant area.

[0008] When the plant produces EVA, there are no heat users in the plant area with corresponding by-product steam pressure levels. To recover and utilize the flash steam, from the perspective of improving steam recovery rate, the quality of the by-product steam needs to be improved to meet the requirements of the lowest pressure level heat users, or the by-product steam needs to be converted into electrical or mechanical energy. Steam quality improvement typically involves technical routes such as thermal compression (TVR), conventional mechanical compression (MVR), power generation via turbine-driven generators, and turbine-driven equipment (such as fans and pumps). The analysis of each technical route is as follows:

[0009] 1. Thermal compression

[0010] Thermal compression refers to the use of a steam ejector to eject flash steam with high-pressure steam. After mixing, low-pressure steam is discharged for use by heat users at the corresponding pressure level. Although this technology has no auxiliary energy consumption, it presents a steam balance problem in LDPE / EVA plants. Taking a mainstream LDPE / EVA plant with a capacity of 300,000 tons / year as an example, the average flow rates of high-pressure steam, medium-pressure steam, and low-pressure steam supplied to the plant from outside the boundary are approximately 10 t / h, 2 t / h, and 10 t / h, respectively. Using high-pressure steam (2.5 MPa) to eject flash steam (0.31 MPa), the mixed pressure needs to reach the low-pressure steam pressure (0.5 MPa). The ejection coefficient is too low, requiring a large amount of high-pressure steam to eject flash steam, resulting in a significant increase in high-pressure steam. The discharged low-pressure steam cannot be absorbed by the plant area, making this route unfeasible in LDPE and EVA production systems.

[0011] Conventional mechanical compression

[0012] Conventional mechanical compression refers to using an electric motor to drive a steam compressor to increase the pressure of by-product steam for use by the plant's heat users, with the surplus being sent outside the plant area. Since the amount of by-product steam fluctuates within a certain range depending on process conditions, the electric motor needs to be equipped with a frequency converter to adapt to these fluctuations. Taking a mainstream LDPE and EVA production system with a capacity of 300,000 tons / year as an example, the by-product steam at 0.31 MPa is approximately 33 t / h. Increasing it to 0.6 MPa through mechanical compression would require 1980 kW of power. The plant area lacks such a large available power capacity, and the required high-voltage frequency converter would occupy a large area in the power distribution room, which is already limited by space constraints. Therefore, this technical approach faces significant implementation difficulties.

[0013] Generating electricity by driving a generator through a turbine.

[0014] Because the flash steam pressure is low, the installed capacity of the generator is not large, and power generation requires various administrative procedures and communication with the power grid dispatch, resulting in huge costs per unit installed capacity. Therefore, this power generation technology route is generally not adopted.

[0015] Equipment directly driven by a turbine (such as fans, water pumps, etc.)

[0016] Because the by-product steam has a low pressure, it is not suitable for long-distance transportation outside the plant area and must be utilized locally within the plant. Due to the fluctuating nature of the by-product steam, it cannot meet the requirements for stable operation of fans, pumps, and other equipment within the plant; therefore, this technical approach is not suitable for implementation.

[0017] Given the conventional waste heat recovery system design process of LDPE and EVA production systems, especially the drawback of not being able to recover and utilize a large amount of heat during EVA product production, and the analysis of conventional heat recovery technology routes, it is particularly important to invent a design method to improve the waste heat utilization rate of LDPE and EVA production systems. Summary of the Invention

[0018] The purpose of this invention is to provide a method and apparatus for improving the waste heat utilization rate of LDPE and EVA production systems. This invention can effectively improve the by-product steam recovery rate during EVA product production in LDPE and EVA production systems, thereby increasing waste heat utilization and achieving energy conservation and emission reduction.

[0019] The technical solution of the present invention is a method for improving the waste heat utilization rate of LDPE and EVA production systems, including flash pressure control of hot water tanks and condensate tanks, recovery and utilization of by-product steam, and equipment start-up and shutdown control.

[0020] The flash pressure control of the hot water tank and condensate tank is based on the saturation pressure corresponding to the highest circulating hot water inlet temperature required by the high-temperature heat removal section of the reactor. This controls the pressure of the high-pressure hot water tank and sequentially controls the pressure difference between the subsequent low-pressure hot water tank and medium-pressure condensate tank to not exceed 0.01 MPa.

[0021] The by-product steam recovery and utilization adopts the method of mechanically compressing the by-product steam to increase its pressure to the same pressure level as the lowest pressure level steam supplied from outside the boundary to this boundary, for use by low-pressure steam users, with the surplus supplied to outside the boundary, and the steam source for mechanical compression comes from the by-product steam venting main.

[0022] The equipment start-up and shutdown control determines the start-up and shutdown of the mechanical compression equipment based on the by-product steam pressure generated during the production of different products.

[0023] In the above-mentioned method for improving the waste heat utilization rate of the LDPE and EVA production system, in the flash evaporation pressure control of the hot water tank and condensate tank, if the resistance of the drainage pipeline between adjacent upstream and downstream tanks is greater than 0.01 MPa during drainage, the diameter of the drainage pipeline is increased to make the pressure difference less than 0.01 MPa.

[0024] In the aforementioned method for improving the waste heat utilization rate of LDPE and EVA production systems, the by-product steam recovery and utilization process uses a steam compressor for mechanical compression, and the power source for driving the steam compressor that compresses the by-product steam is a condensing saturated steam turbine unit. The steam source of the condensing saturated steam turbine unit and the steam source of the steam compressor are both the same steam source of the by-product steam, and the inlet steam pressure is determined based on the lowest by-product steam pressure.

[0025] The aforementioned method for improving the waste heat utilization rate of LDPE and EVA production systems involves calculating the rated steam intake of the steam compressor and condensing saturated turbine unit as follows: based on the balance between the input power required by the steam compressor to compress 1 ton of steam to the required pressure and the output power of the condensing saturated turbine unit through the work done by 1 ton of steam, the steam intake allocated to the steam compressor and condensing saturated turbine unit is calculated.

[0026] The aforementioned method for improving the waste heat utilization rate of LDPE and EVA production systems involves adjusting the steam intake of the steam compressor and condensing saturated steam turbine unit proportionally according to the calculated rated steam intake ratio when the by-product steam volume fluctuates.

[0027] The aforementioned method for improving the waste heat utilization rate of the LDPE and EVA production system involves the following equipment start-up and shutdown control: when the production of LDPE products generates by-product steam at a pressure greater than 0.5 MPa, the shut-off valve connecting the discharge port in the LDPE and EVA production system is cut off, the shut-off valves connecting the steam pipelines of various pressure levels in the LDPE and EVA production system are opened, the main shut-off valve connecting the mechanical compression equipment and the LDPE and EVA production system is cut off, and the mechanical compression equipment is shut down.

[0028] The aforementioned method for improving the waste heat utilization rate of the LDPE and EVA production system involves the following equipment start-up and shutdown control: when the production of EVA products is insufficient to generate by-product steam greater than 0.5 MPa, the shut-off valve of the discharge port in the LDPE and EVA production system is opened, the shut-off valves of the steam pipelines connecting various pressure levels in the LDPE and EVA production system are shut off, and the main shut-off valve connecting the mechanical compression equipment and the LDPE and EVA production system is opened, allowing the mechanical compression equipment to operate.

[0029] The aforementioned apparatus for improving the waste heat utilization rate of an LDPE and EVA production system includes an LDPE and EVA production system, wherein the LDPE and EVA production system includes a high-pressure hot water tank, a low-pressure hot water tank, a medium-pressure condensate tank, a low-pressure condensate tank, and a by-product steam venting tank; the high-pressure hot water tank is connected to a high-pressure hot water by-product steam cooling condenser, a first discharge valve, and a first steam valve; the low-pressure hot water tank is connected to a low-pressure hot water by-product steam cooling condenser, a second discharge valve, and a second steam valve; the medium-pressure condensate tank is connected to a third discharge valve and a third steam valve; the low-pressure condensate tank is connected to a low-pressure condensate by-product steam cooling condenser and a fourth discharge valve; the high-pressure hot water tank is connected to a high-pressure hot water by-product steam cooling condenser, a fourth discharge valve; the high-pressure hot water tank is connected to a high-pressure hot water by-product steam cooling condenser, a second discharge valve, and a third steam valve; the high-pressure hot water tank is connected to a high-pressure hot water by-product steam cooling condenser, a second discharge valve, and a third steam valve; the medium-pressure condensate tank is connected to a high-pressure hot water by-product steam cooling condenser, a third discharge valve, and a fourth steam valve; the high-pressure hot water tank is connected to a high-pressure hot water by-product steam cooling condenser, a fourth discharge valve, and a third ... The discharge pipes of the high-pressure hot water tank, low-pressure hot water tank, medium-pressure condensate tank, and low-pressure condensate tank are connected together to the vent main of the by-product steam vent tank; it also includes mechanical compression equipment, which includes a steam compressor and a condensing saturated steam turbine unit. The steam compressor and the steam inlet pipe of the condensing saturated steam turbine unit are connected to the vent main of the by-product steam vent tank, and the vent main is equipped with a main shut-off valve; the steam compressor and the steam inlet pipe of the condensing saturated steam turbine unit are equipped with inlet shut-off valves; the steam outlet pipe of the steam compressor is connected to the lowest pressure level steam main pipe of the LDPE and EVA production system; the steam outlet pipe of the steam compressor is equipped with an outlet shut-off valve.

[0030] Compared with existing technologies, this invention utilizes mechanical compression to compress and pressurize flash steam. Through mechanical compression, a portion of the flash steam is used to power a saturated steam turbine. The turbine output power drives a steam compressor to pressurize the portion of steam other than that consumed by the turbine, and then returns it to the steam pipeline network. This invention can effectively improve the by-product steam recovery rate in LDPE and EVA production systems during EVA product production, increase waste heat utilization, and achieve the goals of energy conservation and emission reduction. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the device of the present invention. Detailed Implementation

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0033] Example: A method for improving waste heat utilization in LDPE and EVA production systems, applied to LDPE / EVA production systems, such as... Figure 1 As shown, the device includes an LDPE and EVA production system, which includes a high-pressure hot water tank 1, a low-pressure hot water tank 2, a medium-pressure condensate tank 3, a low-pressure condensate tank 4, a by-product steam venting tank 5, and hot water users, water replenishment, and condensate return systems. The high-pressure hot water tank 1 is connected to a high-pressure hot water by-product steam cooling condenser 101, a first discharge valve 102, and a first steam valve 103. The low-pressure hot water tank 2 is connected to a low-pressure hot water by-product steam cooling condenser 201, a second discharge valve 202, and a second steam valve 203. The medium-pressure condensate tank 3 is connected to a third discharge valve 302 and a third steam valve 303. The low-pressure condensate tank 4 is connected to a low-pressure condensate by-product steam cooling condenser 401 and a fourth discharge valve 402. The discharge pipes of the high-pressure hot water tank 1, low-pressure hot water tank 2, medium-pressure condensate tank 3, and low-pressure condensate tank 4 are connected together to the venting main pipe of the by-product steam venting tank 5.

[0034] It also includes mechanical compression equipment, which includes a steam compressor 6 and a condensing saturated steam turbine unit 7. The steam compressor 6 and the steam turbine unit 7 are connected to the venting main pipe of the by-product steam venting tank 5, and the venting main pipe is equipped with a main shut-off valve 8. The steam compressor 6 and the steam turbine unit 7 are equipped with a steam inlet shut-off valve 9. The steam compressor 6 is connected to the lowest pressure level steam main pipe of the LDPE and EVA production system. The steam compressor 6 is equipped with a steam outlet shut-off valve 10.

[0035] The method of the present invention will be further described based on the above-described LDPE / EVA production apparatus. The method of the present invention includes flash pressure control of the hot water tank and condensate tank, recovery and utilization of by-product steam, and equipment start-up and shutdown control, as detailed below:

[0036] I. Flash pressure control of hot water tank and condensate tank;

[0037] The flash pressure control of the hot water tank and condensate tank is based on the saturation pressure corresponding to the highest circulating hot water inlet temperature required by the high-temperature heat removal section of the reactor. This controls the pressure of the high-pressure hot water tank and sequentially controls the pressure difference between the subsequent low-pressure hot water tank and medium-pressure condensate tank to not exceed 0.01 MPa. If the resistance of the drainage pipeline between adjacent upstream and downstream tanks is greater than 0.01 MPa during drainage, the drainage pipeline is modified by increasing the pipe diameter to make the pressure difference less than 0.01 MPa.

[0038] II. Recovery and utilization of by-product steam;

[0039] 2. By mechanically compressing the by-product steam to increase its pressure to the same level as the lowest pressure steam supplied from outside the boundary to this boundary, the steam will be used by low-pressure steam users, with the surplus supplied to users outside the boundary.

[0040] 2.1 The power source for the steam compressor that drives the compression of by-product steam is a condensing saturated steam turbine unit. The steam source of the condensing saturated steam turbine unit and the steam source of the steam compressor are both by-product steam from the same source, and the inlet steam pressure is determined based on the lowest by-product steam pressure.

[0041] 2.2 Calculation method for the rated steam intake allocation of steam compressor and condensing saturated steam turbine unit: Based on the balance between the input power required by the steam compressor to compress 1 ton of steam to the required pressure and the output power of the condensing saturated steam turbine unit through the work done by 1 ton of steam, the steam intake allocated to the steam compressor and condensing saturated steam turbine unit is calculated.

[0042] 2.3 When the amount of by-product steam fluctuates, the steam volume of the steam compressor and the condensing saturated steam turbine unit shall be adjusted proportionally according to the steam volume calculated in 2.2.

[0043] 2.4 The steam source for condensing saturated steam turbine units and steam mechanical compressors both comes from the by-product steam venting main;

[0044] 2.5 The by-product steam introduction point is set on the vent main, and the principle is to prevent the introduction of flash steam from the low-pressure condensate tank and the flash steam from the high-pressure hot water tank / low-pressure hot water tank / medium-pressure condensate tank from entering the centralized vent tank.

[0045] 2.6 The steam compressor outlet is connected to the main steam pipe of the lowest pressure level required by each heat-using device in the plant area.

[0046] III. Equipment Start-up and Shutdown Control

[0047] 3.1 When the production of LDPE products generates by-product steam with a pressure greater than 0.5MPa, shut off the shut-off valves (first discharge valve 102, second discharge valve 202, third discharge valve 302) connecting the discharge ports of the LDPE and EVA production systems, open the shut-off valves (first steam valve 103, second steam valve 203, third steam valve 303) connecting the steam pipelines of various pressure levels in the LDPE and EVA production systems, shut off the main shut-off valves (inlet steam shut-off valve 9, outlet steam shut-off valve 10) connecting the mechanical compression equipment and the LDPE and EVA production systems, and stop the mechanical compression equipment.

[0048] The equipment start-stop control is as follows: when the production of EVA products is insufficient to generate by-product steam greater than 0.5MPa, the shut-off valves connecting the discharge ports of the LDPE and EVA production systems (first discharge valve 102, second discharge valve 202, and third discharge valve 302) are opened, the shut-off valves connecting the steam pipelines of various pressure levels in the LDPE and EVA production systems (first steam valve 103, second steam valve 203, and third steam valve 303) are shut off, and the main shut-off valves connecting the mechanical compression equipment and the LDPE and EVA production systems (inlet steam shut-off valve 9 and outlet steam shut-off valve 10) are opened, and the mechanical compression equipment is put into operation.

[0049] This example uses a 300,000-ton / year LDPE and EVA production system built in China in 2010 as an example. The system has an operational flexibility of 65%-115%. Since its completion and operation, it has mainly produced EVA products, with an actual annual operating time of about 8,000 hours.

[0050] The power distribution system of the plant is supplied with a voltage level of 110kV from the power station to the boundary area, and then stepped down to 10kV and 0.4kV by step-down transformers. The actual power distribution system has a spare capacity of about 1000kW, and the plant area is relatively compact.

[0051] According to 2022 production data, the unit produces approximately 36 tons / hour of EVA, generating 3250 kJ of heat per kg of product polymerized, totaling 117 GJ / hour of heat. This heat is removed from the reactor via a hot water circulation system, flash-cooled in a hot water tank, and reused. The lost water is replenished externally. As per the reactor design requirements, the cooled hot water temperature must be maintained at 130℃-140℃ before entering the reactor for heat removal. The average pressure of the by-product steam is 0.31 MPa saturated steam (specific enthalpy 2727 kJ / kg).

[0052] The system consumes an average of 12.5 t / h of steam at a pressure of 2.5 MPa, 1.0 t / h of steam at a pressure of 0.8 MPa, and 10.8 t / h of steam at a pressure of 0.5 MPa from outside the consumption area. The system is equipped with a hot water type lithium bromide chiller unit that supplies water at a rated temperature of 7℃ for process use. The unit consumes 27 GJ / h of heat under rated operating conditions.

[0053] The by-product steam pressure generated during EVA production is lower than the minimum steam consumption pressure level of the plant area, and therefore cannot be integrated into the low-pressure steam network. During production, approximately 27 GJ / h of hot water returning from the reactor is supplied to the hot water-type lithium bromide via a hot water circulation pump. The remaining heat is flashed out in a hot water tank and then cooled and condensed by a cooling condenser to recover the demineralized water. Flash steam exceeding the condenser's load capacity is centrally discharged into the air. This means that 117-27=90 GJ / h of reaction heat is lost per hour through condensation and centralized discharge, equivalent to 33 t / h of 0.31 MPa saturated steam = 90 × 1000000 ÷ 2727 ÷ 1000, which is not recovered. The total power consumption of the air cooler used for condensing the exhaust steam is 312 kW, with an actual average operating power of 187 kW. In addition, according to the original system design requirements for condensate drainage, approximately 1.9 t / h of high and medium pressure steam condensate in the unit is also discharged into the air at a centralized venting point to avoid being recycled.

[0054] The steam pressure levels in the plant area are divided into three categories, with the lowest pressure level being 0.5 MPa and an average hourly steam consumption of 10.8 t / h. The by-product steam pressure is 0.31 MPa, which is lower than the minimum steam pressure level of the plant area. The by-product steam cannot be connected to the low-pressure steam network, and there are no other heat users with lower requirements than the low-pressure steam level. Therefore, the pressure of the by-product steam needs to be increased to 0.5 MPa, with a portion supplied to the low-pressure steam users in this plant area and the surplus returned to the low-pressure steam network outside the plant area.

[0055] Based on the thermodynamic calculations of steam compression and the technical parameters of the steam compressor manufacturer, compressing 0.31MPa steam to 0.6MPa (considering the steam transportation after compression, the compressor outlet pressure is slightly higher than the pipeline pressure) consumes 60kW.h / t of power per ton of steam. The total amount of by-product steam in this system is 34.9t / h on average. The power required for compression by electric motor is 60×34.9=2094kW. This plant area does not have such a large power load, so it is not possible to use an electric motor to drive the compressor to pressurize the flash steam.

[0056] This invention utilizes a combination of a saturated steam turbine and a steam compressor to compress and pressurize flash steam. A portion of the flash steam is used to power the saturated steam turbine, and the turbine's output power drives the steam compressor to pressurize the remaining steam (excluding the steam consumed by the turbine) and return it to the low-pressure steam network.

[0057] Determination of main equipment parameters

[0058] The by-product steam pressure rating is 0.3 MPa, and the saturation temperature is [not specified]. Based on calculations and the equipment manufacturer's product series, the saturated steam turbine steam consumption rate is 12.5 kg / kW·h, meaning that each ton of by-product saturated steam can output 80 kW of power through the turbine. The steam compressor pressurizes each ton of steam from 0.3 MPa to 0.6 MPa, requiring an input of 60 kW to compress each ton of steam.

[0059] Since the steam turbine directly drives the steam compressor, the total output power of the steam turbine equals the total input power of the steam compressor, i.e., 80 × steam turbine input steam capacity = 60 × steam compressor input steam capacity. Also, the steam turbine input steam capacity + steam compressor input steam capacity = 34.9 t / h. Therefore, the steam capacity entering the steam turbine is 15 t / h, and the steam capacity entering the steam compressor is 19.9 t / h. The main equipment parameters are as follows:

[0060]

[0061] Table 1

[0062] After modification by this invention, an average of 19.9t / h of flash steam is recovered and pressurized, saving 187kW of power consumption in the original flash steam condenser. The power consumption of the supporting equipment in this project is 93kW, resulting in a total saving of 19.9t / h of 0.5MPa steam and 94kW of electricity.

[0063] Based on a steam cost (0.5MPa) of 108 yuan / ton, an electricity price of 0.6 yuan / kW.h, and an annual operating time of 8000 hours, the annual benefits generated through this project are as follows:

[0064] (108×19.9+0.6×94)×8000=17,644,800 yuan / year.

[0065] Based on a standard coal equivalent conversion factor of 0.094 tce / t for saturated steam at 0.5 MPa and a standard coal equivalent conversion factor of 0.299 kgce / kW·h, the annual savings are equivalent to:

[0066] 19.9×8000×0.094+94×8000×0.299÷1000=15190 tons / year.

[0067] It is evident that this invention can effectively improve the by-product steam recovery rate and waste heat utilization rate of the LDPE and EVA production system during EVA product production, thereby achieving the goal of energy conservation and emission reduction.

Claims

1. A method for improving the waste heat utilization rate of an LDPE and EVA production system, characterized in that: This includes flash pressure control for hot water tanks and condensate tanks, recovery and utilization of by-product steam, and equipment start-up and shutdown control. The flash pressure control of the hot water tank and condensate tank is based on the saturation pressure corresponding to the highest circulating hot water inlet temperature required by the high-temperature heat removal section of the reactor. This controls the pressure of the high-pressure hot water tank and sequentially controls the pressure difference between the subsequent low-pressure hot water tank and medium-pressure condensate tank to not exceed 0.01 MPa. The by-product steam recovery and utilization adopts the method of mechanically compressing the by-product steam to increase its pressure to the same pressure level as the lowest pressure level steam supplied from outside the boundary to this boundary, for use by low-pressure steam users, with the surplus supplied to outside the boundary, and the steam source for mechanical compression comes from the by-product steam venting main. The equipment start-up and shutdown control determines the start-up and shutdown of the mechanical compression equipment based on the by-product steam pressure generated during the production of different products.

2. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 1, characterized in that: In the flash evaporation pressure control of the hot water tank and condensate tank, if the resistance of the drainage pipeline between adjacent upstream and downstream tanks is greater than 0.01 MPa during drainage, the diameter of the drainage pipeline is increased to make the pressure difference less than 0.01 MPa.

3. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 1, characterized in that: In the recovery and utilization of by-product steam, mechanical compression is performed using a steam compressor, and the power source for driving the steam compressor that compresses the by-product steam is a condensing saturated steam turbine unit. The steam source of the condensing saturated steam turbine unit and the steam source of the steam compressor are both the same steam source of the by-product steam, and the inlet steam pressure is determined based on the lowest by-product steam pressure.

4. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 3, characterized in that: The calculation method for the rated steam intake allocation of the steam compressor and the condensing saturated steam turbine unit is as follows: based on the balance between the input power required by the steam compressor to compress 1 ton of steam to the required pressure and the output power of the condensing saturated steam turbine unit through the work done by 1 ton of steam, the steam intake allocated to the steam compressor and the condensing saturated steam turbine unit is calculated.

5. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 4, characterized in that: When the amount of by-product steam fluctuates, the steam intake of the steam compressor and the condensing saturated steam turbine unit is adjusted proportionally according to the calculated rated steam intake ratio.

6. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 1, characterized in that: The equipment start-stop control is as follows: when the production of LDPE products generates by-product steam with a pressure greater than 0.5MPa, the shut-off valve of the discharge port connecting the LDPE and EVA production systems is cut off, the shut-off valves of the steam pipelines connecting the LDPE and EVA production systems at various pressure levels are opened, the main shut-off valve connecting the mechanical compression equipment and the LDPE and EVA production systems is cut off, and the mechanical compression equipment is shut down.

7. The method for improving the waste heat utilization rate of the LDPE and EVA production system according to claim 1, characterized in that: The equipment start-stop control is as follows: when the production of EVA products is insufficient to generate by-product steam greater than 0.5MPa, the shut-off valve of the discharge port connecting the LDPE and EVA production system is opened, the shut-off valves of the steam pipelines connecting the LDPE and EVA production system at various pressure levels are shut off, and the main shut-off valve connecting the mechanical compression equipment and the LDPE and EVA production system is opened, and the mechanical compression equipment is started.

8. An apparatus for improving the waste heat utilization rate of an LDPE and EVA production system according to any one of claims 1-7, comprising an LDPE and EVA production system, the LDPE and EVA production system comprising a high-pressure hot water tank (1), a low-pressure hot water tank (2), a medium-pressure condensate tank (3), a low-pressure condensate tank (4), and a by-product steam venting tank (5); the high-pressure hot water tank (1) is connected to a high-pressure hot water by-product steam cooling condenser (101), a first discharge valve (102), and a first steam valve (103); the low-pressure hot water tank (2) is connected to... It has a low-pressure hot water by-product steam cooling condenser (201), a second discharge valve (202), and a second steam valve (203); the medium-pressure condensate tank (3) is connected to a third discharge valve (302) and a third steam valve (303); the low-pressure condensate tank (4) is connected to a low-pressure condensate by-product steam cooling condenser (401) and a fourth discharge valve (402); the discharge pipes of the high-pressure hot water tank (1), the low-pressure hot water tank (2), the medium-pressure condensate tank (3), and the low-pressure condensate tank (4) are connected together to the venting main pipe of the by-product steam venting tank (5); its characteristic is that: It also includes mechanical compression equipment, which includes a steam compressor (6) and a condensing saturated steam turbine unit (7). The steam compressor (6) and the steam inlet pipe of the condensing saturated steam turbine unit (7) are connected to the venting main pipe of the by-product steam venting tank (5). The venting main pipe is equipped with a main shut-off valve (8). The steam compressor (6) and the steam inlet pipe of the condensing saturated steam turbine unit (7) are equipped with a steam inlet shut-off valve (9). The steam outlet pipe of the steam compressor (6) is connected to the lowest pressure level steam main pipe of the LDPE and EVA production system. The steam outlet pipe of the steam compressor (6) is equipped with a steam outlet shut-off valve (10).

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