Recovery of low pressure steam during production of EVA and reuse device and method

Abstract

The present application relates to the technical field of EVA production waste heat recovery, and is a device and method for recycling low-pressure steam by-produced during EVA production, which comprises the following steps: when EVA products with VA content are produced, a screw type steam compressor is used to compress low-pressure steam with a pressure lower than 0.45 MPag to 0.45 MPag; when EVA products with high VA content are produced, a screw type steam compressor is used to compress mixed low-pressure steam to 0.45 MPag. When the steam by-produced from the hot water system of the EVA production device is compressed by using a compression device, the present application can maximize the volumetric efficiency of the screw type steam compressor by mixing the two by-produced streams with a large flow difference before compression, so as to achieve the purpose of energy saving; and compared with the comparative example, the present application has better energy saving benefits, investment recovery rate and standard coal saving rate.

Description

Technical Field

[0001] This invention relates to the field of waste heat recovery technology in EVA production, and is a device for recovering and reusing low-pressure steam produced as a byproduct during EVA production, as well as a method for recovering and reusing low-pressure steam produced as a byproduct during EVA production. Background Technology

[0002] Currently, the EVA production unit adopts the Lupotech@T-tube process from LyondellBasell (LYB) in Germany. This process absorbs the heat of polymerization reaction through low-pressure hot water and medium-pressure hot water in the hot water system, and then flashes the low-pressure steam in the low-pressure hot water tank through a flash valve, producing low-pressure steam as a byproduct, and medium-pressure steam in the medium-pressure hot water tank.

[0003] For the production process of EVA products with low VA content (≤5wt%), 0.6 MPa of steam (approximately 64.5 t / h) is produced as a byproduct in the low-pressure hot water tank, and 0.9 MPa of steam (approximately 12.7 t / h) is produced as a byproduct in the medium-pressure hot water tank. Since the EVA production unit consumes 0.6 MPa of steam (10.8 t / h) and 0.9 MPa of steam (11.9 t / h), the remaining majority of the 0.6 MPa and 0.9 MPa steam is depressurized to 0.45 MPa of low-pressure steam (approximately 54.5 t / h), which can be transported outside the production area. Therefore, in the production process of EVA products with low VA content (≤5wt%), the pressure of the byproduct steam is relatively high, eliminating the need for compression equipment.

[0004] For the production process of EVA products with medium VA content (5wt% < VA content ≤ 18wt%), 0.26 MPa of low-pressure steam (approximately 68.1 t / h) is produced as a byproduct in the low-pressure hot water tank, and 0.52 MPa of medium-pressure steam (approximately 14.4 t / h) is produced as a byproduct in the medium-pressure hot water tank. This low-pressure steam cannot be used by the plant. The medium-pressure steam can be reduced to 0.45 MPa for plant use. To ensure normal plant operation, a low-pressure steam condenser (design load 53500 kW, motor load 370 kW) is required to condense all of this low-pressure steam into condensate. Under this condition, the plant consumes only approximately 12.9 t / h of low-pressure steam, and the remaining 1.5 t / h of low-pressure steam can be transported outside the plant boundary for use. However, the plant needs to replenish 8.7 t / h of medium-pressure steam from outside the plant boundary.

[0005] For the production process of EVA products with high VA content (>18wt%, the mainstream EVA product), 0.1 MPa of low-pressure steam (approximately 67.9 t / h) is produced as a byproduct in the low-pressure hot water tank, and 0.26 MPa of medium-pressure steam (approximately 16.6 t / h) is produced as a byproduct in the medium-pressure hot water tank. Because the pressure of this steam is too low to meet the needs of the unit, 15.5 t / h of low-pressure steam and 9.8 t / h of medium-pressure steam need to be supplied from outside the plant. Similarly, to ensure normal operation, low-pressure steam condensers and medium-pressure steam condensers (design load 12400 kW, motor load 60 kW) are required to condense this steam into condensate. During EVA production, the overall energy consumption of EVA products is as high as 322 kg of standard oil per t of product. It is estimated that the inability to recover and utilize the steam produced as a byproduct of the hot water system will directly increase the overall energy consumption of EVA products by approximately 116 kg of standard oil per t of product.

[0006] Currently, LyondellBasell (LYB) has not yet solved the problem of recovering low-pressure and medium-pressure steam byproducts from the hot water system during EVA production. There is no good steam recovery solution for LDPE (EVA) plants in China. Some plants use this low-pressure steam to heat heating water / traffic hot water, but the utilization rate is low (less than 10%). Some plants use high-pressure or medium-pressure steam as a power source and use ejectors to recover low-pressure steam, which is inefficient (ejection coefficient of about 0.8) and has a short actual commissioning time (high noise, steam leakage and pipeline corrosion).

[0007] A patent search was conducted on this technical solution, and two patent documents with high relevance to this technology were found, namely:

[0008] 1) Patent application CN117065377A 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 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. In this document, before mechanically compressing the low-pressure by-product steam, the impact of merging steam with large flow differences on pipelines and compression equipment is not considered. Over time, this will shorten the service life of pipelines and compression equipment and increase their failure rate.

[0009] 2) Patent application CN118681523A discloses a method and apparatus for preparing polyethylene and ethylene-vinyl acetate copolymer in a tubular reactor. The method includes providing a jacketed tubular reactor, introducing an initiator, ethylene, a polymerization regulator, and comonomers into the tubular reactor to prepare polyethylene or ethylene-vinyl acetate copolymer. The apparatus includes at least a tubular reactor, a heat recovery system, a product cooler, a high-pressure separator, a low-pressure separator, a high-pressure circulation system, and a low-pressure circulation system. It uses a heat exchange medium to remove the heat of polymerization from the reaction zone of the tubular reactor and produce by-product steam. This document only mentions connecting the by-product steam to the reactor jacket via pipelines, without specifying how to recover the by-product steam. Summary of the Invention

[0010] This invention provides a device and method for recycling low-pressure steam produced as a byproduct during EVA production, which can effectively recover all steam produced as a byproduct of the hot water system during EVA production, especially byproduct steam with large flow differences.

[0011] One of the technical solutions of this invention is achieved through the following measures: a method for recycling and reusing low-pressure steam, a byproduct of EVA production, comprising:

[0012] When producing EVA products with a VA content, the low-pressure hot water tank of the hot water system in the EVA production unit produces low-pressure steam with a pressure lower than 0.45 MPa. The flow rate of the low-pressure steam with a pressure lower than 0.45 MPa is greater than the flow load of the screw steam compressor. The low-pressure steam with a pressure lower than 0.45 MPa is sent to the steam buffer mixing tank for buffering. The flow rate of the low-pressure steam is adjusted by the valve at the inlet end of the screw steam compressor. After the flow rate of the low-pressure steam with a pressure lower than 0.45 MPa is reduced to match the flow load of the screw steam compressor, the screw steam compressor compresses the low-pressure steam with a pressure lower than 0.45 MPa to 0.45 MPa. The 0.45 MPa steam is sent to the low-pressure steam system for use by the EVA production unit.

[0013] When producing high-VA-content EVA products, both the low-pressure and medium-pressure hot water tanks in the hot water system of the EVA production unit produce low-pressure steam with a pressure lower than 0.45 MPa. The flow rates of the low-pressure steam produced by the low-pressure and medium-pressure hot water tanks differ significantly. The low-pressure steam produced by the low-pressure and medium-pressure hot water tanks is fed into a steam buffer mixing tank for mixing. Then, a screw steam compressor compresses the mixed low-pressure steam to 0.45 MPa and sends the 0.45 MPa steam into the low-pressure steam system for use by the EVA production unit.

[0014] The following are further optimizations and / or improvements to one of the above-mentioned technical solutions:

[0015] Furthermore, when producing EVA products with high VA content, the flow rate of low-pressure steam produced by the low-pressure hot water tank and the medium-pressure hot water tank differs by more than 10 t / h.

[0016] Furthermore, during the production of EVA products with VA content, the low-pressure hot water tank of the hot water system of the EVA production unit produces 0.26 MPa low-pressure steam with a flow rate of 68.1 t / h. The 0.26 MPa low-pressure steam enters the steam buffer mixing tank for buffering. The flow rate is adjusted by the valve at the inlet of the screw steam compressor to reduce the flow rate of the 0.26 MPa low-pressure steam to match the flow load of the screw steam compressor. The screw steam compressor then compresses the 0.26 MPa steam produced by the low-pressure hot water tank to 0.45 MPa. The 0.45 MPa steam is then sent to the low-pressure steam system for use by the EVA production unit.

[0017] When producing high-VA-content EVA products, the low-pressure hot water tank of the hot water system of the EVA production unit produces 0.1MPag low-pressure steam with a flow rate of 67.9t / h, and the medium-pressure hot water tank of the hot water system of the EVA production unit produces 0.26MPag low-pressure steam with a flow rate of 16.6t / h. After the 0.1MPag and 0.26MPag low-pressure steam are mixed in a steam buffer mixing tank, the screw steam compressor compresses the mixed low-pressure steam to 0.45MPag. The 0.45MPag steam is then sent to the low-pressure steam system for use by the EVA production unit.

[0018] One of the technical solutions of this invention is achieved through the following measures: a device for recycling low-pressure steam generated during EVA production, comprising: a hot water system and a steam boosting system; the hot water system includes a low-pressure hot water tank, a medium-pressure hot water tank, a low-pressure steam main pipe, and a medium-pressure steam main pipe; the steam boosting system includes a steam compressor unit and a steam buffer mixing tank; the exhaust end of the low-pressure hot water tank is connected to a low-pressure exhaust pipe, which is connected to the low-pressure steam main pipe; the exhaust end of the medium-pressure hot water tank is connected to a medium-pressure exhaust pipe, which is connected to the medium-pressure steam main pipe; the medium-pressure steam main pipe and the low-pressure steam main pipe are connected through a connecting pipe; the steam compressor unit includes at least one screw-type steam compressor; the outlet end of the steam buffer mixing tank is connected to the inlet end of the steam compressor unit via an inlet pipe for compression; the outlet end of the steam compressor unit is connected to a booster pipe; the low-pressure exhaust pipe and the medium-pressure exhaust pipe are both connected to the inlet end of the steam buffer mixing tank; and the booster pipe is connected to the low-pressure steam main pipe.

[0019] The following are further optimizations and / or improvements to one of the above-mentioned technical solutions:

[0020] Furthermore, the aforementioned steam compressor unit includes two screw steam compressors with the same load, which are connected in parallel. The outlet end of the steam buffer mixing tank is connected to the inlet end of each screw steam compressor via a steam inlet pipe, and the outlet end of each screw steam compressor is connected to a pressure boosting pipe.

[0021] Furthermore, the aforementioned low-pressure exhaust pipe is connected to the inlet end of the steam buffer mixing tank via a first low-pressure delivery pipe.

[0022] Furthermore, the aforementioned medium-pressure exhaust pipe is connected to the inlet end of the steam buffer mixing tank via a second low-pressure delivery pipe.

[0023] Furthermore, valves are fixedly installed on the aforementioned low-pressure exhaust pipe, medium-pressure exhaust pipe, first low-pressure conveying pipe, second low-pressure conveying pipe, steam inlet compression pipe, and booster pipe, and valves are fixedly installed on the low-pressure steam main pipe between the first low-pressure conveying pipe and the connecting pipe.

[0024] The beneficial effects of this invention are:

[0025] This invention only adds an item to the original process flow of the hot water system of the EVA production unit. The addition is a steam pressurization system, and the original low-pressure and medium-pressure steam condensers can be used as backup systems, making the process stable and reliable.

[0026] This invention can simultaneously compress two by-product steam streams with different pressures and large flow rates, making it applicable not only to the recovery of by-product steam during the production of medium-VA-content EVA products, but also to the recovery of by-product steam during the production of high-VA-content EVA products.

[0027] This invention achieves good economic benefits and significantly reduces the energy consumption of the equipment by recovering the by-product steam from the hot water system during EVA product production. Attached Figure Description

[0028] Appendix Figure 1 This is a process flow diagram of the low-pressure steam reuse device, a byproduct of EVA production.

[0029] Appendix Figure 2 This is a comparative application process flow diagram.

[0030] The codes in the attached diagram are as follows: 1 is a low-pressure hot water tank, 2 is a medium-pressure hot water tank, 3 is a low-pressure steam main, 4 is a medium-pressure steam main, 5 is a steam buffer mixing tank, 6 is a low-pressure exhaust pipe, 7 is a medium-pressure exhaust pipe, 8 is a connecting pipe, 9 is a screw steam compressor, 10 is a steam inlet pipe to be compressed, 11 is a pressure boosting pipe, 12 is the first low-pressure delivery pipe, 13 is the second low-pressure delivery pipe, 14 is a valve, 15 is a tubular reactor, 16 is a low-pressure steam condenser, 17 is a medium-pressure steam condenser, 18 is the first screw steam compressor, and 19 is the second screw steam compressor. Detailed Implementation

[0031] The present invention is not limited to the following embodiments, and the specific implementation can be determined according to the technical solution of the present invention and the actual situation.

[0032] In this invention, it should be noted that the terms "first," "second," etc., are used only for the convenience of describing the invention and simplifying the description, and are not intended to indicate or imply that the pipes or compressors referred to must have a specific order and operation, and therefore should not be construed as limiting the invention.

[0033] For ease of description, the relative positions of the components are described based on the appendix to the instruction manual. Figure 1 The layout is described using a diagrammatic method, such as the positional relationships of front, back, top, bottom, left, and right, which are based on the instructions attached. Figure 1 The orientation of the pattern is determined by the direction of the pattern. Unless otherwise specified, all percentages in this invention refer to mass percentages.

[0034] When the existing EVA production unit is in operation, the low-pressure hot water tank 1 provides low-pressure hot water to its corresponding tubular reactor 15, and the medium-pressure hot water tank 2 also provides medium-pressure hot water to its corresponding tubular reactor 15. The steam produced by the low-pressure hot water tank 1 is condensed into liquid by the low-pressure steam condenser 16 and returned to the low-pressure hot water tank 1 for use. Similarly, the steam produced by the medium-pressure hot water tank 2 is condensed into liquid by the medium-pressure steam condenser 17.

[0035] The use of this low-pressure steam recycling device, which is a byproduct of EVA production, can effectively recover and utilize the steam produced by low-pressure hot water tank 1 and medium-pressure hot water tank 2. As a result, low-pressure steam condenser 16 and medium-pressure steam condenser 17 can be shut down, avoiding the energy consumption generated by the operation of low-pressure steam condenser 16 and medium-pressure steam condenser 17, and using low-pressure steam condenser 16 and medium-pressure steam condenser 17 as a backup system.

[0036] Compared to existing technologies (Publication No.: CN117065377A; Title: A Method and Apparatus for Improving Waste Heat Utilization Rate in LDPE and EVA Production Systems), which require controlling the pressure difference between the low-pressure hot water tank and the medium-pressure condensate tank to not exceed 0.01 MPa, this method achieves a temperature difference of 10°C to 20°C and a pressure difference of 0.1 MPa to 0.2 MPa during implementation. Therefore, it eliminates the need to adjust the pressure difference between the low-pressure hot water tank and the medium-pressure condensate tank. The medium-pressure condensate tank is also part of the hot water system in the EVA production unit.

[0037] Furthermore, in the existing technology (publication number: CN117065377A; title: A method and apparatus for improving the waste heat utilization rate of LDPE and EVA production system), part of the by-product steam is sent to a steam compressor for compression and pressurization, and the other part is used as the power source of the condensing saturated steam turbine unit that drives the steam compressor. On the surface, the by-product steam can be used as both the power source of the steam compressor and the steam to be compressed. However, when the by-product steam is used as the power source of the steam compressor, since this part (about 43%) of the by-product steam cannot be recovered, its overall energy consumption is significantly higher than that of an electrically driven steam compressor.

[0038] The present invention will be further described below with reference to embodiments:

[0039] Example 1: As Figure 1 As shown, a method for recycling low-pressure steam, a byproduct of EVA production, includes:

[0040] When producing EVA products with a VA content of 5wt% < VA content ≤ 18wt%, the low-pressure hot water tank 1 of the hot water system of the EVA production unit produces low-pressure steam with a pressure lower than 0.45MPag. The flow rate of the low-pressure steam produced by the low-pressure hot water tank 1 with a pressure lower than 0.45MPag is greater than the flow load of the screw steam compressor 9. The low-pressure steam with a pressure lower than 0.45MPag is sent to the steam buffer mixing tank 5 for buffering. The flow rate of the low-pressure steam is adjusted by the valve 14 at the inlet end of the screw steam compressor 9. After the flow rate of the low-pressure steam with a pressure lower than 0.45MPag is adjusted to match the flow load of the screw steam compressor 9, the screw steam compressor 9 compresses the low-pressure steam with a pressure lower than 0.45MPag to 0.45MPag. The 0.45MPag steam is sent to the low-pressure steam system through the low-pressure steam main pipe 3 for use by the EVA production unit, thereby realizing the recovery of the steam produced by the low-pressure hot water tank 1.

[0041] When producing EVA products with high VA content (>18wt%, mainstream EVA products), both the low-pressure hot water tank 1 and the medium-pressure hot water tank 2 of the hot water system of the EVA production unit produce low-pressure steam with a pressure lower than 0.45MPag. Because the pressure of this steam is too low, it cannot meet the needs of the unit. Furthermore, the flow rates of the low-pressure steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2 differ significantly. The low-pressure steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2 is fed into the steam buffer mixing tank 5 for mixing. Then, the screw steam compressor 9 compresses the mixed low-pressure steam to 0.45MPag. The 0.45MPag steam is then sent into the low-pressure steam system through the low-pressure steam main pipe 3 for use by the EVA production unit, thereby realizing the recovery of the steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2.

[0042] When producing EVA products with low to medium VA content, the medium-pressure hot water tank 2 of the hot water system of the EVA production unit produces low-pressure steam with a pressure of not less than 0.45 MPa. The low-pressure steam with a pressure of not less than 0.45 MPa is sent into the low-pressure steam system through the low-pressure steam main pipe 3 for use by the EVA production unit.

[0043] When recovering steam byproducts from the hot water system using the method described in this embodiment, two streams of steam with significantly different flow rates are mixed in a steam buffer mixing tank 5. The flow rate of the mixed steam is matched with the flow load of the screw steam compressor 9, which maximizes the volumetric efficiency of the screw steam compressor 9.

[0044] If the flow rate of the steam to be compressed is greater than the flow load of the screw steam compressor 9, the excess steam needs to be diverted to prevent the screw steam compressor 9 from operating under overload. If the flow rate of the steam to be compressed is less than the flow load of the screw steam compressor 9, the volumetric efficiency of the screw steam compressor 9 will be lower. Generally, the higher the flow load of the screw steam compressor 9, the higher the equipment cost and the greater the energy consumption. Therefore, when processing steam with a flow rate less than the flow load of the screw steam compressor 9, the unit energy consumption is higher due to the low volumetric efficiency. For example, if the flow load of a screw steam compressor is 10t / h, when using this screw steam compressor to process steam with a flow rate of 5t / h, its energy consumption is the same as that of processing steam with a flow rate of 10t / h, but its energy consumption per ton of compressed steam (i.e., unit energy consumption) is obviously higher than that of processing steam with a flow rate of 10t / h.

[0045] Therefore, mixing two steam streams with significantly different flow rates through the steam buffer mixing tank 5 can maximize the volumetric efficiency of the screw steam compressor 9 on the one hand, and avoid the impact on the pipeline or screw steam compressor 9 when the two steam streams with significantly different flow rates merge at the inlet end of the pipeline or screw steam compressor 9.

[0046] Example 2: As Figure 1 As shown, as an optimization of the above embodiment 1, when producing EVA products with high VA content, the flow rate of low-pressure steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2 differs by more than 10 t / h.

[0047] Example 3: As Figure 1 As shown, a device for recycling low-pressure steam, a byproduct of EVA production, includes a hot water system and a steam booster system. The hot water system includes a low-pressure hot water tank 1, a medium-pressure hot water tank 2, a low-pressure steam main 3, and a medium-pressure steam main 4. The steam booster system includes a steam compressor unit and a steam buffer mixing tank 5. The exhaust end of the low-pressure hot water tank 1 is connected to a low-pressure exhaust pipe 6, which is connected to the low-pressure steam main 3. The exhaust end of the medium-pressure hot water tank 2 is connected to a medium-pressure exhaust pipe 7, which is connected to the medium-pressure steam main 4. The medium-pressure steam main 4 and the low-pressure steam main 3 are connected through a connecting pipe 8. The steam compressor unit includes at least one screw steam compressor 9. The outlet end of the steam buffer mixing tank 5 is connected to the inlet end of the steam compressor unit via a steam inlet pipe 10. The outlet end of the steam compressor unit is connected to a booster pipe 11. The low-pressure exhaust pipe 6 and the medium-pressure exhaust pipe 7 are both connected to the inlet end of the steam buffer mixing tank 5. The booster pipe 11 is connected to the low-pressure steam main 3.

[0048] Example 4: Figure 1 As shown, as an optimization of the above embodiment 3, the steam compressor unit includes two screw steam compressors 9 with the same load. The two screw steam compressors 9 with the same load are connected in parallel. The outlet end of the steam buffer mixing tank 5 is connected to the inlet end of each screw steam compressor 9 by a steam inlet pipe 10. The outlet end of each screw steam compressor 9 is connected to a pressure boosting pipe 11.

[0049] Using two identical screw steam compressors 9 allows for a one-for-one standby configuration, enabling the other to be activated if one fails. Furthermore, when replacing parts, only identical parts need to be prepared, as both screw steam compressors 9 can use each other's spare parts. Compared to the purchase price of different parts for different screw steam compressors 9, the purchase price of identical spare parts is lower.

[0050] Example 5: Figure 1 As shown, as an optimization of the above embodiment 3, the low-pressure exhaust pipe 6 and the inlet end of the steam buffer mixing tank 5 are connected through the first low-pressure delivery pipe 12.

[0051] Example 6: As Figure 1 As shown, as an optimization of the above embodiment 3, the medium-pressure exhaust pipe 7 and the inlet end of the steam buffer mixing tank 5 are connected through the second low-pressure delivery pipe 13.

[0052] Example 7: As Figure 1As shown, as an optimization of the above embodiment 3, valves 14 are fixedly installed on the low-pressure exhaust pipe 6, the medium-pressure exhaust pipe 7, the first low-pressure conveying pipe 12, the second low-pressure conveying pipe 13, the steam inlet compression pipe 10, and the pressure boosting pipe 11. Valve 14 is also fixedly installed on the low-pressure steam main pipe 3 between the first low-pressure conveying pipe 12 and the connecting pipe 8.

[0053] The opening and closing of valve 14 on the corresponding pipeline depends on whether there is a medium flowing through the corresponding pipeline. For example, when the steam produced by the low-pressure hot water tank 1 needs to be compressed and pressurized by the screw steam compressor 9, the steam produced by the tank passes through the low-pressure exhaust pipe 6 and the first low-pressure delivery pipe 12 in sequence into the steam buffer mixing tank 5, and then passes through the steam inlet pipe 10 to enter the screw steam compressor 9. In this direction of medium flow, valve 14 on the low-pressure exhaust pipe 6, the first low-pressure delivery pipe 12, and the steam inlet pipe 10 needs to be opened.

[0054] Example 8: As Figure 1 As shown, a method for recycling low-pressure steam, a byproduct of EVA production, includes:

[0055] When producing EVA products with a VA content of 5wt% < VA content ≤ 18wt%, the low-pressure hot water tank 1 of the hot water system of the EVA production unit produces 0.26MPag low-pressure steam with a flow rate of 68.1t / h. The medium-pressure hot water tank 2 of the hot water system of the EVA production unit produces 0.52MPag medium-pressure steam, which is sent to the low-pressure steam system via the low-pressure steam main pipe 3 for use by the EVA production unit. The 0.26MPag low-pressure steam enters the steam buffer mixing tank 5 for buffering. The flow rate is adjusted by the regulating valve at the inlet end of the screw steam compressor 9, reducing the flow rate of the 0.26MPag low-pressure steam to the flow load of the screw steam compressor 9. The screw steam compressor 9 then compresses the 0.26MPag steam produced by the low-pressure hot water tank 1 to 0.45MPag. The 0.45MPag steam is sent to the low-pressure steam system via the low-pressure steam main pipe 3 for use by the EVA production unit, thereby realizing the recovery of the steam produced by the low-pressure hot water tank 1.

[0056] When producing EVA products with high VA content (>18wt%, the mainstream EVA product), the low-pressure hot water tank 1 of the hot water system of the EVA production unit produces 0.1MPag low-pressure steam with a flow rate of 67.9t / h, and the medium-pressure hot water tank 2 of the hot water system of the EVA production unit produces 0.26MPag low-pressure steam with a flow rate of 16.6t / h. Because the pressure of these steams is too low to meet the needs of the unit, the 0.1MPag low-pressure steam and the 0.26MPag low-pressure steam are mixed in the steam buffer mixing tank 5, and then the screw steam compressor 9 compresses the mixed low-pressure steam to 0.45MPag. The 0.45MPag steam is sent into the low-pressure steam system through the low-pressure steam main pipe 3 for use by the EVA production unit, thereby realizing the recovery of the steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2.

[0057] The operating parameters of the screw steam compressor 9 used in Example 8 are shown in Table 1.

[0058] Comparative Example: This comparative example also uses a screw steam compressor to recover the steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2. The difference from Example 8 is that the steam buffer mixing tank 5 is not used, and the steam produced by the low-pressure hot water tank 1 and the medium-pressure hot water tank 2 are each processed separately using screw steam compressors with corresponding flow loads (including the first screw steam compressor 18 and the first screw steam compressor 19). The process flow diagram is shown below. Figure 2 As shown in Table 2, the operating parameters of the screw steam compressor used are as follows.

[0059] The method for recovering steam byproducts from the hot water system of the EVA production unit is as follows:

[0060] When producing EVA products with a VA content of 5wt% < VA content ≤ 18wt%, the low-pressure hot water tank 1 produces 0.26MPag of low-pressure steam (approximately 68.1t / h), and the medium-pressure hot water tank 2 produces 0.52MPag of medium-pressure steam. The 0.26MPag steam produced by the low-pressure hot water tank 1 is compressed to 0.45MPag using the first screw steam compressor 18. The 0.45MPag steam is then sent into the low-pressure steam system through the low-pressure steam main pipe 3. The first screw steam compressor 19 is then shut down.

[0061] When producing EVA products with high VA content (>18wt%, the mainstream EVA product), low-pressure hot water tank 1 produces 0.1MPag of low-pressure steam (approximately 67.9t / h), and medium-pressure hot water tank 2 produces 0.26MPag of medium-pressure steam (approximately 16.6t / h). At this time, the steam produced by low-pressure hot water tank 1 is compressed to 0.45MPag using the first screw steam compressor 18, and the 0.45MPag low-pressure steam is connected to the low-pressure steam network via the low-pressure steam main 3. The steam produced by medium-pressure hot water tank 2 is compressed to 1.0MPag of medium-pressure steam using the first screw steam compressor 19, and the 1.0MPag medium-pressure steam is connected to the medium-pressure steam network via the medium-pressure steam main 4.

[0062] When using a comparative method to recover by-product steam, the 0.1 MPa steam produced by low-pressure hot water tank 1 can be compressed into 0.45 MPa low-pressure steam, and the 0.26 MPa steam produced by medium-pressure hot water tank 2 can be compressed into 1.0 MPa medium-pressure steam. In other words, the comparative method can produce 0.45 MPa low-pressure steam and 1.0 MPa medium-pressure steam through steam recovery.

[0063] The economic benefits of Example 8 and the comparative example are shown in Table 3. In Table 3, the calculation is based on the following: electricity price 0.44 yuan / kWh, circulating water 0.4 yuan / ton, instrument air 0.27 yuan / standard cubic meter, nitrogen 0.48 yuan / standard cubic meter, steam 119 yuan / ton, and annual operating time of 8000 hours.

[0064] As can be seen from Table 3, the annual energy-saving benefits, investment recovery rate, and standard coal saving rate of Example 8 and the comparative scheme are considerable, and the carbon dioxide emission reduction is significant. However, the energy-saving benefits, investment recovery rate, and standard coal saving rate of Example 8 are obviously better than those of the comparative scheme.

[0065] Compared to Example 8, the comparative example has a larger difference in the flow rate of the by-product steam from low-pressure hot water tank 1 and medium-pressure hot water tank 2, and the compressors of different models cannot be used as backups for each other. Therefore, the actual comprehensive benefits of the comparative example project may be reduced (during the single unit failure maintenance period), and the project modification cost of the comparative example project is slightly higher by about 10%.

[0066] In summary, when the hot water system of the EVA production unit produces steam, and the same compression equipment is used to compress and pressurize the steam, the present invention, by mixing and then compressing two streams of steam with significantly different flow rates, can not only maximize the volumetric efficiency of the screw steam compressor, thereby achieving energy saving, but also achieve better energy-saving benefits, investment recovery rate, and standard coal saving rate compared to the comparative example.

[0067] Table 1

[0068] .

[0069] Table 2

[0070] .

[0071] Table 3 Economic Benefit Calculation

[0072] .

[0073] The above technical features constitute various embodiments of the present invention, which have strong adaptability and implementation effect. Unnecessary technical features can be added or removed according to actual needs to meet the needs of different situations.

Claims

1. A method for recycling low pressure steam, which is a by-product during EVA production, for reuse, characterized in that, The device for recycling low-pressure steam by-product during the production of recycled EVA, which implements the method for recycling low-pressure steam by-product during the production of recycled EVA, comprises a hot water system and a steam pressurization system. The hot water system comprises a low-pressure hot water tank, a medium-pressure hot water tank, a low-pressure steam main pipe and a medium-pressure steam main pipe. The steam pressurization system comprises a steam compressor set and a steam buffer mixing tank. The low-pressure hot water tank is connected with a low-pressure steam discharge pipe at the steam discharge end. The low-pressure steam discharge pipe is connected with the low-pressure steam main pipe. The medium-pressure hot water tank is connected with a medium-pressure steam discharge pipe at the steam discharge end. The medium-pressure steam discharge pipe is connected with the medium-pressure steam main pipe. The medium-pressure steam main pipe is connected with the low-pressure steam main pipe through a communication pipe. The steam compressor set comprises at least one screw steam compressor. The steam buffer mixing tank is connected with the steam compressor set through a steam compression pipe at the inlet end. The steam compressor set is connected with a pressure increasing pipe at the outlet end. The low-pressure steam discharge pipe and the medium-pressure steam discharge pipe are connected with the steam buffer mixing tank at the inlet end. The pressure increasing pipe is connected with the low-pressure steam main pipe. The method for recycling low-pressure steam by-product during the production of recycled EVA comprises the following steps: When the EVA product with medium VA content is produced, the low-pressure hot water tank of the hot water system of the EVA production device produces low-pressure steam with a pressure lower than 0.45 MPag. The flow rate of the low-pressure steam with a pressure lower than 0.45 MPag is greater than the flow load of the screw steam compressor. The low-pressure steam with a pressure lower than 0.45 MPag is sent to the steam buffer mixing tank for buffering. The flow rate of the low-pressure steam is adjusted through the valve at the inlet end of the screw steam compressor. After the flow rate of the low-pressure steam with a pressure lower than 0.45 MPag is adjusted to match the flow load of the screw steam compressor, the screw steam compressor compresses the low-pressure steam with a pressure lower than 0.45 MPag to 0.45 MPag. The steam with a pressure of 0.45 MPag is sent to the low-pressure steam system for use by the EVA production device. When the EVA product with high VA content is produced, the low-pressure hot water tank and the medium-pressure hot water tank of the hot water system of the EVA production device both produce low-pressure steam with a pressure lower than 0.45 MPag. The flow rates of the low-pressure steam produced by the low-pressure hot water tank and the medium-pressure hot water tank are quite different. The low-pressure steam produced by the low-pressure hot water tank and the medium-pressure hot water tank is mixed in the steam buffer mixing tank. The screw steam compressor compresses the mixed low-pressure steam to 0.45 MPag. The steam with a pressure of 0.45 MPag is sent to the low-pressure steam system for use by the EVA production device. The EVA product with medium VA content refers to an EVA product with a VA content of 5wt% to 18wt%. The EVA product with high VA content refers to an EVA product with a VA content greater than 18wt%.

2. The method according to claim 1, wherein, When the EVA product with high VA content is produced, the flow rates of the low-pressure steam produced by the low-pressure hot water tank and the medium-pressure hot water tank are different by more than 10t / h. When the EVA product with high VA content is produced, the flow rates of the low-pressure steam produced by the low-pressure hot water tank and the medium-pressure hot water tank are different by more than 10t / h.

3. The method according to claim 1 or 2, wherein, When producing EVA products with a VA content, the low-pressure steam tank of the hot water system of the EVA production device produces 68.1t / h of 0.26MPag low-pressure steam, which is buffered in the steam buffer mixing tank, and the flow is adjusted by the valve at the inlet end of the screw steam compressor, and the flow of the 0.26MPag low-pressure steam is adjusted to match the flow load of the screw steam compressor, and then the screw steam compressor compresses the 0.26MPag steam produced by the low-pressure steam tank to 0.45MPag, and the 0.45MPag steam is sent to the low-pressure steam system for use by the EVA production device; When producing EVA products with a high VA content, the low-pressure steam tank of the hot water system of the EVA production device produces 67.9t / h of 0.1MPag low-pressure steam, and the medium-pressure steam tank of the hot water system of the EVA production device produces 16.6t / h of 0.26MPag low-pressure steam, and the 0.1MPag low-pressure steam and the 0.26MPag low-pressure steam are mixed in the steam buffer mixing tank, and then the screw steam compressor compresses the mixed low-pressure steam to 0.45MPag, and the 0.45MPag steam is sent to the low-pressure steam system for use by the EVA production device.

4. The method according to claim 1 or 2, wherein, The steam compressor set includes two screw steam compressors with the same load, the two screw steam compressors with the same load are connected in parallel, the outlet end of the steam buffer mixing tank is in communication with the inlet end of each screw steam compressor through a steam inlet pipe for compression, and the outlet end of each screw steam compressor is in communication with a pressure boosting pipe.

5. The method according to claim 3, wherein the low-pressure steam is used for the production of steam. The steam compressor set includes two screw steam compressors with the same load, the two screw steam compressors with the same load are connected in parallel, the outlet end of the steam buffer mixing tank is in communication with the inlet end of each screw steam compressor through a steam inlet pipe for compression, and the outlet end of each screw steam compressor is in communication with a pressure boosting pipe.

6. The method according to claim 1 or 2 or 5, wherein, The low-pressure exhaust pipe is in communication with the inlet end of the steam buffer mixing tank through a first low-pressure conveying pipe.

7. The method according to claim 3, wherein the low-pressure steam is used for the production of steam. The low-pressure exhaust pipe is in communication with the inlet end of the steam buffer mixing tank through a first low-pressure conveying pipe.

8. The method according to claim 4, wherein the low-pressure steam is used for the production of steam. The low-pressure exhaust pipe is in communication with the inlet end of the steam buffer mixing tank through a first low-pressure conveying pipe.

9. The method according to claim 1 or 2 or 5 or 7, wherein, The medium-pressure exhaust pipe is in communication with the inlet end of the steam buffer mixing tank through a second low-pressure conveying pipe.

10. The method of claim 8, wherein the low pressure steam is produced during the production of EVA. The medium-pressure exhaust pipe is in communication with the inlet end of the steam buffer mixing tank through a second low-pressure conveying pipe.

11. The method according to claim 10, wherein the low-pressure steam is used for the production of steam in the EVA production process. The low-pressure exhaust pipe, the medium-pressure exhaust pipe, the first low-pressure conveying pipe, the second low-pressure conveying pipe, the steam inlet pipe for compression, and the pressure boosting pipe are fixedly installed with valves, and the low-pressure steam pipe between the first low-pressure conveying pipe and the communication pipe is fixedly installed with a valve.

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