CN213708185U - Ethylene cryogenic recovery system - Google Patents

Abstract

The utility model relates to a chemical industry tail gas treatment field, concretely relates to ethylene cryrogenic recovery system. The ethylene cryogenic recovery system comprises a pretreatment unit and a cryogenic recovery unit, wherein the cryogenic recovery unit comprises a third heat exchanger and a third gas-liquid separation tank; the discharge end of a heating medium channel of the third heat exchanger is communicated with a third gas-liquid separation tank; a first liquid-state cryogen outlet is arranged on the third gas-liquid separation tank, the feed end of the cryogen passage of the third heat exchanger is communicated with the first liquid-state cryogen outlet, and a throttling expansion device is arranged between the cryogen passage of the third heat exchanger and the first liquid-state cryogen outlet. The technical problem of recycling the ethylene in the tail gas generated in the process of preparing the vinyl acetate-ethylene copolymer can be solved. The implementation of the scheme can effectively and reasonably utilize resources, reduce the production cost, reduce the emission of greenhouse gases and implement the development strategy of 'green and low carbon'.

Description

Ethylene cryogenic recovery system

Technical Field

The utility model relates to a chemical industry tail gas treatment field, concretely relates to ethylene cryrogenic recovery system.

Background

Vinyl acetate-ethylene copolymer (VAE) is an industrial material obtained by copolymerizing vinyl acetate and ethylene, and has a wide range of applications. In the preparation process of the vinyl acetate-ethylene copolymer, a large amount of tail gas is generated, the tail gas contains ethylene, vinyl acetate, the vinyl acetate-ethylene copolymer and other components, and the tail gas must be treated for achieving the purpose of green emission. In the prior art, the main treatment method of tail gas is to send the tail gas to a boiler or a flare for combustion through a compressor. However, ethylene and vinyl acetate contained in the tail gas are both raw material components for synthesizing vinyl acetate-ethylene copolymer, and the tail gas is directly combusted, so that the components with utilization values are not fully utilized, and resource waste is caused. Through calculation and industrial verification, 12 ten thousand tons of vinyl acetate-ethylene copolymer is produced, and the generated tail gas contains about 1100 tons of ethylene. So a large amount of ethylene is directly combusted and consumed, but not recycled and recycled, the direct economic loss brought by the method is nearly ten million yuan, and the combustion consumes materials with recycling value, so that the method does not accord with the concept of 'green and low carbon' of the modern society. It is urgently needed to design a system for recycling ethylene from tail gas generated in the process of preparing vinyl acetate-ethylene copolymer, so as to effectively and reasonably utilize resources, reduce production cost, reduce emission of greenhouse gases and practice the development strategy of 'green and low carbon'.

SUMMERY OF THE UTILITY MODEL

The utility model aims to provide an ethylene cryogenic recovery system to solve the technical problem of ethylene in the tail gas that recycle preparation vinyl acetate-ethylene copolymer in-process produced.

In order to achieve the above purpose, the utility model adopts the following technical scheme:

the ethylene cryogenic recovery system comprises a pretreatment unit and a cryogenic recovery unit, wherein the cryogenic recovery unit comprises a third heat exchanger and a third gas-liquid separation tank; the discharge end of a heating medium channel of the third heat exchanger is communicated with a third gas-liquid separation tank; a first liquid-state cryogen outlet is arranged on the third gas-liquid separation tank, the feed end of the cryogen passage of the third heat exchanger is communicated with the first liquid-state cryogen outlet, and a throttling expansion device is arranged between the cryogen passage of the third heat exchanger and the first liquid-state cryogen outlet.

The principle and the advantages of the scheme are as follows: by means of the pretreatment unit, part of impurities such as water, carbon dioxide and the like in the tail gas generated in the process of preparing the vinyl acetate-ethylene copolymer can be removed, and the substances are easy to freeze and block pipelines in the cryogenic recovery. After pretreatment, the gas material enters a cryogenic recovery unit, ethylene in the gas material is liquefied through a third heat exchanger, and the liquefied ethylene (liquid ethylene) and some non-condensable gas are separated in a third gas-liquid separation tank. And the liquid ethylene flows out of the third gas-liquid separation tank, the temperature of the liquid ethylene is further reduced through the throttling expansion device to form liquid refrigerant I (the liquid ethylene with lower temperature), and the liquid refrigerant I can be used as the refrigerant of the third heat exchanger to cool the gas material in the third heat exchanger. The liquid refrigerant I can also be heated by heat exchange to finally become the target product, namely the gaseous ethylene. Although liquid ethylene is already available in the third knock-out pot, liquid ethylene is generally not directly used for the synthesis of vinyl acetate-ethylene copolymer, and it is heated to be in a gaseous state before it can be used for the synthesis of vinyl acetate-ethylene copolymer. In the scheme, liquid ethylene is used as a refrigerant of the third heat exchanger, and the refrigerant is heated simultaneously to become gaseous ethylene, so that special heating equipment for the liquid ethylene is avoided. The third heat exchanger, the third gas-liquid separation tank and the throttling expansion device have two functions: condensing and liquefying ethylene to remove non-condensable gas impurities; and heating the liquid ethylene to obtain gaseous ethylene meeting the requirements of the vinyl acetate-ethylene copolymer synthesis process. The refrigerant is a cooling medium used for cooling the material in the heat exchanger.

The scheme has the advantages that through ingenious design, the recycled object (ethylene) is not only an object cooled by condensation, but also serves as a cold source. The purification of the target substance is realized through the process of condensation and temperature reduction, the purified substance is used as a cold source, the temperature reduction and liquefaction of the recovered object are separated from the non-condensable gas, and the cold source is simultaneously heated and gasified through heat exchange to form the required form of the raw material in the production of the vinyl acetate-ethylene copolymer. The circulation operation saves the cost of purchasing a cold source (for example, additionally purchasing nitrogen as the cold source) and the cost of additionally arranging a heating device to gasify the ethylene (the ethylene obtained by the cryogenic separation needs to be heated and gasified before being used for producing the vinyl acetate-ethylene copolymer). Most importantly, the scheme can recycle the tail gas generated in the production of the vinyl acetate-ethylene copolymer, thereby realizing energy conservation and emission reduction and reasonably utilizing the resource.

Further, the cryogenic recovery unit also comprises a second heat exchanger and a second gas-liquid separation tank; the discharge end of the heating medium channel of the second heat exchanger is communicated with the second gas-liquid separation tank, and the feed end of the heating medium channel of the third heat exchanger is communicated with the second gas-liquid separation tank; the feed end of the refrigerant channel of the second heat exchanger is communicated with the discharge end of the refrigerant channel of the third heat exchanger.

By adopting the technical scheme, the temperature of the gas material is reduced in the second heat exchanger, the liquefaction of the vinyl acetate in the gas material is realized through the second gas-liquid separation tank, the gaseous ethylene and the liquid vinyl acetate are separated, and the vinyl acetate impurities are removed. In addition, the feeding end of the refrigerant channel of the second heat exchanger is communicated with the discharging end of the refrigerant channel of the third heat exchanger, and the refrigerant in the heat exchange process of the second heat exchanger directly comes from the third heat exchanger, namely liquid ethylene. Thus, the low-temperature liquid ethylene refrigerant can be further heated, the temperature of the liquid ethylene is increased, and the liquid ethylene is gasified, because the gaseous ethylene can be directly used for synthesizing the vinyl acetate-ethylene copolymer. If liquid ethylene is not used as a refrigerant, other liquid ethylene heating devices are needed, and the cost is increased.

Further, the cryogenic recovery unit further comprises a first heat exchanger; the discharge end of the heat medium channel of the first heat exchanger is communicated with the feed end of the heat medium channel of the second heat exchanger; and a first gaseous refrigerant outlet is formed in the third gas-liquid separation tank, and the feed end of the refrigerant channel of the first heat exchanger is communicated with the first gaseous refrigerant outlet.

By adopting the technical scheme, the gaseous refrigerant is used for preliminarily cooling the gas material, and then the subsequent two-step heat exchange process is carried out, so that the gas material can be fully cooled, and the cryogenic separation of ethylene is realized. In addition, the gaseous refrigerant is actually non-condensable gas which is difficult to treat in the tail gas in the scheme, and the gaseous refrigerant needs to be combusted, so that the environment pollution is avoided. However, after cryogenic treatment, the temperature of the non-condensable gas (gaseous refrigerant) in the third gas-liquid separation tank is too low to be directly subjected to combustion treatment (the temperature is too low to be effectively combusted, and potential safety hazard exists), and the non-condensable gas needs to be properly heated to be subjected to subsequent combustion treatment. The heat exchange process carried out in the first heat exchanger therefore achieves two objectives: primarily cooling the gas material; avoid additionally setting up other noncondensable gas heating device, practiced thrift the cost.

Further, the discharge end of the refrigerant channel of the second heat exchanger is communicated with a first gaseous ethylene storage tank.

By adopting the technical scheme, the liquid ethylene becomes gaseous ethylene after passing through the double heat exchange of the first heat exchanger and the second heat exchanger, and is conveyed to the first gaseous ethylene storage tank for temporary storage. Gaseous ethylene will be used as a raw material for the synthesis of vinyl acetate-ethylene copolymer.

Further, the discharge end of the refrigerant channel of the first heat exchanger is communicated with a flare system.

By adopting the technical scheme, after the heat exchange of the first heat exchanger, the non-condensable gas serving as the refrigerant is heated, and the heated non-condensable gas is the waste gas to be treated and can be directly discharged into a torch system for combustion treatment.

Further, the third gas-liquid separation tank is also communicated with a discharge end of a liquid ethylene storage tank.

By adopting the technical scheme, the liquid ethylene is temporarily stored in the liquid ethylene storage tank and is used for providing low-temperature liquid ethylene for the third gas-liquid separation tank so as to start the whole circulation process. When the equipment is just started, no refrigerant exists in the whole system, the whole cryogenic recovery circulation process cannot be started, and a small amount of low-temperature liquid ethylene needs to be added from the outside as an initial refrigerant.

Further, the pretreatment unit comprises a first gas-liquid separation tank, a water washing tower, an alkaline washing tower and a drying tower which are sequentially arranged along the material flowing direction.

By adopting the technical scheme, the first gas-liquid separation tank settles the vinyl acetate-ethylene copolymer in the gas-liquid mixture (tail gas) to remove the impurities; the water washing tower mainly has the following functions: washing off substances such as methanol, acetaldehyde, ethyl acetate, tert-butyl alcohol, hydrogen peroxide, tert-butyl hydroperoxide and the like in the gas material; the main functions of the alkaline washing tower are as follows: and the carbon dioxide in the gas material is washed away, so that the carbon dioxide is prevented from easily forming dry ice to block the pipeline in the subsequent cryogenic recovery.

Further, the discharge end of the drying tower is communicated with the feed end of the heat medium channel of the first heat exchanger.

By adopting the technical scheme, the drying tower is used for removing moisture in the gas material, because water is easy to form ice to block a pipeline in subsequent cryogenic recovery, the drying tower is arranged to avoid the phenomenon.

Further, a water cooler is arranged between the alkaline washing tower and the drying tower.

By adopting the technical scheme, the water cooler can reduce the saturated water content of the material gas, and after the saturated water content is reduced, the drying tower is used for dewatering, so that dewatering is more sufficient, water molecules as few as possible are ensured to enter the copious cooling recovery unit, and the pipeline blockage caused by water freezing in the copious cooling recovery unit is avoided.

The water cooler and the alkaline washing tower can be omitted according to the actual condition of the device, and the effect of the scheme can still be obtained.

Further, a gas holder is arranged between the water washing tower and the alkaline washing tower.

Adopt above-mentioned technical scheme, the gas holder has the gaseous effect of keeping in, if when meeting the trouble shut down, gaseous material can be temporarily stored in the gas holder. And the emission of tail gas is intermittent type nature, sets up the gas holder after, can collect tail gas, in follow-up technology, can follow the gas holder in the gaseous material of continuous discharge, follow-up technology can go on in succession, convenient operation and management and control, also more economical and practical.

Drawings

FIG. 1 is a schematic of the cryogenic ethylene recovery system of example 1.

FIG. 2 is a schematic diagram of the cryogenic recovery unit of embodiment 1.

FIG. 3 is a schematic diagram of the cryogenic recovery unit of embodiment 2.

Detailed Description

The following is further detailed by way of specific embodiments:

reference numerals in the drawings of the specification include: the system comprises a cryogenic recovery unit 1, a first gas-liquid separation tank 2, a water scrubber 3, a gas holder 4, a liquid ring press 5, an alkaline washing tower 6, a water cooler 7, a drying tower 8, a first heat exchanger 9, a second heat exchanger 10, a third heat exchanger 11, a second gas-liquid separation tank 12, a third gas-liquid separation tank 13, a liquid ethylene storage tank 14, a throttling expansion device 15, a first gaseous ethylene storage tank 16, a torch system 17, a diaphragm compressor 18 and a second gaseous ethylene storage tank 19.

Example 1:

as shown in fig. 1, the cryogenic recovery system for ethylene includes a pretreatment unit and a cryogenic recovery unit 1, and the pretreatment unit and the cryogenic recovery unit 1 will now be described in detail, respectively.

1. Pre-processing unit

As shown in fig. 1, the pretreatment unit includes a first gas-liquid separation tank 2, a water scrubber 3, a gas holder 4, a liquid ring press 5, a caustic wash tower 6, a water cooler 7, and a drying tower 8, which are connected in sequence. A first material outlet is arranged above the first gas-liquid separation tank 2, and a first material inlet is arranged below the first gas-liquid separation tank; a second material inlet is arranged below the water washing tower 3, a second material outlet is arranged above the water washing tower, and the first material outlet is communicated with the second material inlet through a pipeline; a third material inlet and a third material outlet are arranged on the gas holder 4, and the third material inlet is communicated with the second material outlet through a pipeline; a fourth material inlet and a fourth material outlet are formed in the hydraulic ring press 5, and the fourth material inlet is communicated with the third material outlet through a pipeline; a fifth material inlet is arranged below the alkaline tower 6, a fifth material outlet is arranged above the alkaline tower, and the fifth material inlet is communicated with the fourth material outlet through a pipeline; the water cooler 7 is provided with a sixth material inlet and a sixth material outlet, and the sixth material inlet is communicated with the fifth material outlet through a pipeline; and a seventh material inlet and a seventh material outlet are formed in the drying tower 8, and the seventh material inlet is communicated with the sixth material outlet through a pipeline.

The specific parameters and use of the equipment in the pretreatment unit are as follows: the first gas-liquid separation tank 2 is a conventional gas-liquid separation tank in the chemical field in the prior art, and separates liquid and gas in a gas-liquid mixture by utilizing the difference of specific gravity of the gas and the liquid. The working pressure of 0.02-3 MPaG, the working temperature of 13-95 ℃ (in this embodiment, specifically 0.02MPaG, 65 ℃) water scrubber 3 and alkali scrubber 6 are common gas purification equipment in the prior art in the chemical field, and impurities are removed through spray water or alkali liquor to realize gas purification. The main functions of the water scrubber 3 (the amount of water sprayed is 1-5 tons/hour, in this example 4 tons/hour) are: and washing off substances such as methanol, acetaldehyde, ethyl acetate, tert-butyl alcohol, hydrogen peroxide, tert-butyl hydroperoxide and the like in the tail gas I to form a tail gas II. The main functions of the alkaline tower 6 (the alkaline tower 6 uses 2-30% sodium hydroxide solution by mass, the spraying amount is 1-5 tons/hour, in the present scheme, 20% sodium hydroxide solution, 4 tons/hour) are: and washing off carbon dioxide in the tail gas III to form a tail gas IV, wherein the carbon dioxide is easy to form dry ice to block a pipeline in the subsequent cryogenic recovery. The drying tower 8 (the filler uses a 3A molecular sieve) is a structure which is commonly used in the chemical field and used for drying industrial gas, and is used for removing moisture in tail gas V to form tail gas VI (the water content in the tail gas VI after water removal is reduced to 1-10ppm), and water is easy to form ice to block a pipeline in subsequent cryogenic recovery. The water cooler 7 is a conventional cooler using water as a cooling medium, and in this embodiment, the temperature of the water inlet end is 7 ℃ and the temperature of the water outlet end is 12 ℃. The water cooler 7 can reduce the water saturation degree of the tail gas IV by reducing the temperature of the tail gas IV (to 10-20 ℃), so that a tail gas V is formed, and the drying tower 8 is used for removing water after the water saturation degree is reduced. The water cooler 7 and the drying tower 8 are used together, so that the water removal efficiency is further improved. The use of the water cooler 7 is an optimum solution and the water cooler 7 may not be used. The gas holder 4 is a common industrial gas storage structure in the prior art, and the hydraulic ring press 5 is a common industrial gas pressurizing device in the prior art.

The pretreatment steps are as follows: tail gas gets into first gas-liquid separation jar 2 through first material entry, and gas-liquid separation takes place at first gas-liquid separation jar 2 for tail gas, and liquid phase part sinks to first gas-liquid separation jar 2 bottoms, and gas phase part is discharged from first material export, and this gas phase part is called tail gas I. And the tail gas I enters the water washing tower 3 through the second material inlet, the vinyl acetate-ethylene copolymer in the tail gas I is removed through the water washing effect of the water washing tower 3 to obtain a tail gas II, and the tail gas II is discharged from the second material outlet. Tail gas II gets into gas holder 4 through third material entry, again from third material export discharge gas holder 4, gas holder 4 has played the effect of keeping in and buffer gas. The arrangement of the gas holder 4 is set according to the requirements of upstream and downstream communication and long-period stable operation of the production process flow, in the process, because the gas led into the water washing tower 3 in the preorder process is intermittently led in, the gas holder 4 is arranged behind the water washing tower 3 to buffer the upstream incoming gas, and the continuous and stable operation of the subsequent procedures is ensured. The tail gas II flowing out of the gas holder 4 enters the liquid ring press 5 through the fourth material inlet (the gas material is pressurized so that the gas material has enough power to operate in the system), and the tail gas III is formed through pressurization of the liquid ring press 5 and flows out of the fourth material outlet. And the tail gas III enters the alkaline tower 6 from the fifth material inlet, the alkaline tower 6 can remove acid gases such as carbon dioxide and the like to obtain a tail gas IV, and the tail gas IV flows out from the fifth material outlet. The tail gas IV flows through the water cooler 7 through the sixth material inlet (in the embodiment, the temperature of the water inlet end is 7 ℃, and the temperature of the water outlet end is 12 ℃), the temperature is reduced, the tail gas V is obtained and flows out from the sixth material outlet, and the water saturation of the tail gas V is lower than that of the tail gas IV due to the reduction of the temperature. And tail gas V enters the drying tower 8 from the seventh material inlet, after water is fully removed, tail gas VI is obtained and flows out from the seventh material outlet, and finally enters the cryogenic recovery unit 1. The parameters of the tail gas at each stage are shown in Table 1.

2. Cryogenic recovery unit

The cryogenic recovery unit 1 includes a first heat exchanger 9, a second heat exchanger 10, a third heat exchanger 11, a second gas-liquid separation tank 12, a third gas-liquid separation tank 13, a liquid ethylene storage tank 14, a throttling expansion device 15, a first gaseous ethylene storage tank 16, and a flare system 17 as shown in fig. 2. The first heat exchanger 9 is provided with an eighth material inlet and an eighth material outlet which are communicated with each other, and is also provided with a second gaseous refrigerant inlet and a second gaseous refrigerant outlet which are communicated with each other. The seventh material outlet is communicated with the eighth material inlet through a pipeline, and the flare system 17 is communicated with the second gaseous refrigerant outlet through a pipeline. The second heat exchanger 10 is provided with a ninth material inlet and a ninth material outlet which are communicated with each other, and is also provided with a third liquid refrigerant inlet and a third liquid refrigerant outlet which are communicated with each other, and the ninth material inlet is communicated with the eighth material outlet. The first gaseous ethylene storage tank 16 is in communication with the third liquid refrigerant outlet via a conduit. The lower part of the second gas-liquid separation tank 12 is provided with a tenth material inlet, the upper part is provided with a tenth material outlet, and the tenth material inlet is communicated with the ninth material outlet through a pipeline. The third heat exchanger 11 is provided with an eleventh material inlet and an eleventh material outlet which are communicated with each other, and is also provided with a second liquid refrigerant inlet and a second liquid refrigerant outlet which are communicated with each other. The eleventh material inlet is communicated with the tenth material outlet through a pipeline. The lower part of the third gas-liquid separation tank 13 is provided with a twelfth material inlet, a first liquid refrigerant outlet and a first liquid refrigerant inlet, and the upper part of the third gas-liquid separation tank 13 is provided with a first gaseous refrigerant outlet. The first liquid cryogen outlet is communicated with the second liquid cryogen inlet through a pipeline, the pipeline is provided with a throttling expansion device 15, the throttling expansion device 15 comprises a compressor and a throttling valve which are sequentially arranged along the material flowing direction, the compressor and the throttling valve are conventional equipment in the prior art, and throttling expansion can be realized to cool the flowing liquid. The liquid ethylene storage tank 14 is in communication with the first liquid refrigerant inlet via a conduit. The second liquid refrigerant outlet is communicated with the third liquid refrigerant inlet through a pipeline.

The specific parameters and the use of the equipment in the cryogenic recovery unit 1 are as follows: the second gas-liquid separation tank 12 and the third gas-liquid separation tank 13 are conventional gas-liquid separation tanks in the chemical field in the prior art, and separate liquid and gas in a gas-liquid mixture by utilizing the difference of specific gravity between the gas and the liquid. The second gas-liquid separation tank 12 has a working pressure of 0.1 to 1MPaG and a working temperature of-75 to-65 ℃ (in this embodiment, 0.5MPaG and-70 ℃); the third gas-liquid separation tank 13 has a working pressure of 0.2 to 0.3MPaG and a working temperature of-125 to-80 ℃ (in this embodiment, 0.4MPaG and-105 ℃). The first heat exchanger 9, the second heat exchanger 10, and the third heat exchanger 11 are all conventional heat exchangers. In a conventional heat exchanger, a heat medium passage and a refrigerant passage are provided, and the heat medium and the refrigerant are separated by a solid partition wall and heat exchange is performed through the partition wall. And a heat medium channel and a refrigerant channel are arranged in the three heat exchangers. An eighth material inlet and an eighth material outlet are respectively arranged at two ends of a heat medium channel of the first heat exchanger 9; the two ends of the refrigerant channel of the first heat exchanger 9 are respectively a second gaseous refrigerant inlet and a second gaseous refrigerant outlet. A ninth material inlet and a ninth material outlet are respectively arranged at two ends of a heat medium channel of the second heat exchanger 10; the two ends of the refrigerant channel of the second heat exchanger 10 are respectively a third liquid refrigerant inlet and a third liquid refrigerant outlet. An eleventh material inlet and an eleventh material outlet are respectively arranged at two ends of a heat medium channel of the third heat exchanger 11; the two ends of the refrigerant channel of the third heat exchanger 11 are respectively a second liquid refrigerant inlet and a second liquid refrigerant outlet. The torch system 17 is a conventional waste gas treatment system in the chemical industry, and conventional treatment objects are combustible and combustible toxic gas and steam which cannot be recovered and reprocessed, and are used for realizing safe discharge. The first gaseous ethylene storage tank 16 and the liquid ethylene storage tank 14 are also conventional storage tanks in the chemical industry for storing industrial gases or liquids.

Cryogenic recovery process: the deep cooling recovery step mainly comprises three heat exchanges, namely preheating exchange, first separation heat exchange and second separation heat exchange.

The preheating exchange process comprises the following steps: and the tail gas VI enters the eighth material inlet, the temperature of the tail gas VI is reduced by heat exchange in the first heat exchanger 9 to form a tail gas VII, and the tail gas VII flows out of the eighth material outlet to the ninth material inlet. The preheating exchange realizes the primary cooling of the materials.

The first separation heat exchange process comprises the following steps: and the tail gas VII is subjected to heat exchange in the second heat exchanger 10, the temperature of the tail gas VII is reduced to form tail gas VIII, and the tail gas VIII flows out from the ninth material outlet to the tenth material inlet. And tail gas VIII enters the second gas-liquid separation tank 12 from a tenth material inlet, gas-liquid separation of the tail gas VIII is realized in the second gas-liquid separation tank 12, a liquid phase part is remained at the bottom of the second gas-liquid separation tank 12, a gas phase part is tail gas IX, and the tail gas IX flows out of the second gas-liquid separation tank 12 through a tenth material outlet and then enters an eleventh material inlet.

The process of the second separation heat exchange is as follows: the tail gas IX is subjected to heat exchange in the third heat exchanger 11, the temperature is reduced to form tail gas X, the tail gas X flows out of the third heat exchanger 11 through an eleventh material outlet and then enters the third gas-liquid separation tank 13 through a twelfth material inlet, and the gas-liquid separation of the tail gas X is realized in the third gas-liquid separation tank 13.

It is worth mentioning that the refrigerant used in the cryogenic recovery of the embodiment has the following relevant conditions: the liquid phase portion in the third knockout drum 13 sinks to the bottom of the third knockout drum 13 to form liquid ethylene, and flows out of the third knockout drum 13 from the first liquid refrigerant outlet. The gas phase part ascends and flows out of the third gas-liquid separation tank 13 through the first gaseous refrigerant outlet, and the gas phase part is the gaseous refrigerant. The liquid ethylene storage tank 14 temporarily stores liquid ethylene, and is used for supplying liquid ethylene (-90 ℃ to-80 ℃, 0.2 ℃ to 0.6MPaG, 99.5% of liquid ethylene, in this embodiment, 0.6MPaG, -85 ℃) to the third gas-liquid separation tank 13 when the equipment is just started, so as to start the whole circulation process and intermittently or continuously supplement refrigerant during operation. Liquid ethylene is used as a refrigerant, a pressurizing device is not needed to be arranged between the liquid ethylene storage tank 14 and the third gas-liquid separation tank 13, and the pressure of the gas moving in the pipeline can be provided only by slightly heating the liquid ethylene storage tank 14. Liquid ethylene is throttled and expanded to obtain low temperature under the action of a compressor and a throttle valve to obtain liquid refrigerant I, the liquid refrigerant I enters the third heat exchanger 11 through the second liquid refrigerant inlet, the liquid refrigerant I is used as refrigerant to cool materials (tail gas IX) in the third heat exchanger 11 through heat exchange (the flow direction of the liquid refrigerant I in the third heat exchanger 11 is opposite to that of the tail gas IX), and after cooling is completed, the liquid refrigerant I becomes liquid refrigerant II, flows out of the second liquid refrigerant outlet and then enters the third liquid refrigerant inlet. Liquid refrigerant II enters the second heat exchanger 10 through the third liquid refrigerant inlet, the liquid refrigerant II is used as refrigerant to cool the material (tail gas VII) in the second heat exchanger 10 through heat exchange (the flow direction of the liquid refrigerant II in the second heat exchanger 10 is opposite to that of the tail gas VII), the liquid refrigerant II becomes gaseous product (99.5% of gaseous ethylene, namely the target product for recovery) after the temperature reduction is finished, and the gaseous product flows out from the third liquid refrigerant outlet and then flows into the first gaseous ethylene storage tank 16 for storage.

Gaseous refrigerant (also called non-condensable gas) flows out of the third gas-liquid separation tank 13 through the first gaseous refrigerant outlet and enters the second gaseous refrigerant inlet, and the gaseous refrigerant is used as refrigerant to cool materials (tail gas VI) in the first heat exchanger 9 through heat exchange (the flow direction of the gaseous refrigerant in the first heat exchanger 9 is opposite to that of the tail gas VI). After the heat exchange is completed, the temperature of the gaseous refrigerant is raised, and waste gas to be treated is formed and enters the flare system 17 through the second gaseous refrigerant outlet to be combusted.

In summary, the refrigerant used in the preheating exchange is a gaseous refrigerant (non-condensable gas) generated by the third gas-liquid separation tank 13, and the liquid ethylene generated by the third gas-liquid separation tank 13 is used in both the second separation heat exchange and the first separation heat exchange. And after the liquid ethylene is subjected to the second separation heat exchange, carrying out the first separation heat exchange again. The refrigerant is a cooling medium used for cooling the material in the heat exchanger.

Parameters of tail gas, liquid ethylene, liquid refrigerant I, liquid refrigerant II, product (liquid ethylene), gaseous refrigerant (non-condensable gas) and waste gas to be treated at each stage are shown in Table 1. The solid line in FIG. 2 indicates the flow direction of the tail gas at each stage, the dotted line indicates the flow direction of the ethylene refrigerant (liquid ethylene, liquid refrigerant II, liquid refrigerant III and product), and the chain line indicates the flow direction of the gaseous refrigerant (gaseous refrigerant and exhaust gas to be treated). The initial tail gas is the product of the vinyl acetate-ethylene copolymer production process (the initial tail gas is the tail gas generated in the vinyl acetate-ethylene copolymer production process), and comes from a defoaming tank in the upstream process. In this embodiment, nitrogen is used for pressure-increasing and defoaming, and the obtained initial tail gas has a high nitrogen content, wherein the components and contents are as follows: 0.6% of oxygen, 0.12% of ethane, 17.33% of nitrogen, 1.44% of carbon dioxide, 0.60% of vinyl acetate, 0.23% of methane, 75.21% of ethylene and 0.18% of methanol, and the balance of a small amount of acetaldehyde, ethyl acetate, tert-butyl alcohol, hydrogen peroxide, tert-butyl hydroperoxide and vinyl acetate-ethylene copolymer (copolymer particles dispersed in the tail gas). After the cryogenic recovery of the process, the purity of the ethylene in the first gaseous ethylene storage tank 16 reaches 99.5 percent. The above percentages are all mole percentages.

Table 1: parameter setting

In table 1, the two data columns of "temperature range (c)", and "pressure range (MPaG)" indicate that the purification and recovery of gas can be achieved in both the above temperature and pressure units. The two columns of data, "temperature (. degree. C.)" and "pressure (MPaG)" represent the conditions specifically employed in this example.

3. The comprehensive effect of the scheme of the embodiment

Gaseous ethylene in the tail gas is cooled by the water cooler 7, the first heat exchanger 9, the second heat exchanger 10 and the third heat exchanger 11 to become liquid ethylene, the vinyl acetate is separated from the ethylene in the second gas-liquid separation tank 12, and noncondensable gases such as oxygen, ethane, nitrogen, methane and the like are separated from the ethylene in the third gas-liquid separation tank 13. In the process, liquid ethylene is not directly recovered, but is used as a liquid refrigerant in the second heat exchanger 10 and the third heat exchanger 11. In this way, the low temperature of the liquid ethylene is fully utilized to cool the materials in the second heat exchanger 10 and the third heat exchanger 11, and no additional refrigerant (such as liquid nitrogen) is purchased. In addition, liquid ethylene cannot be directly used for the synthesis of vinyl acetate-ethylene copolymer, and it is heated to be in a gaseous state so as to be used for the synthesis of vinyl acetate-ethylene copolymer. In the scheme, liquid ethylene is used as the liquid refrigerant of the second heat exchanger 10 and the third heat exchanger 11, and the liquid refrigerant is heated simultaneously to become gaseous ethylene, so that special heating equipment for the liquid ethylene is avoided.

The gaseous refrigerant is actually non-condensable gas which is difficult to treat in tail gas in the scheme, and the gaseous refrigerant needs to be combusted, so that the environment pollution is avoided. However, after the cryogenic treatment, the temperature of the noncondensable gas (gaseous refrigerant) in the third gas-liquid separation tank 13 is too low to be directly subjected to the combustion treatment (the temperature is too low to be efficiently combusted), and it is necessary to appropriately heat the gas to perform the subsequent combustion treatment. In this embodiment, the low temperature of the gaseous refrigerant is used as the refrigerant of the first heat exchanger 9. Therefore, the tail gas VI can be preliminarily cooled, the temperature of the non-condensable gas (gaseous refrigerant) can be increased, the waste gas to be treated is formed after the temperature is increased, and the waste gas can directly enter the torch system 17 for combustion treatment.

According to statistics, the ethylene recovery amount actually can be about 1000 tons/year by adopting the scheme. The recycled ethylene replaces part of purchased raw materials, and the production cost is reduced to 979.1 ten thousand yuan per year. And the cost required by the system for normal operation is 261.19 ten thousand yuan/year. Therefore, the direct economic benefit brought by the system can reach 580.43 ten thousand yuan per year. In addition, by using the system, the energy consumption can be reduced, so that the emission of greenhouse gases is reduced, the resources are effectively and reasonably utilized, and the aim of green and low-carbon development can be fulfilled.

Example 2

This embodiment is basically the same as embodiment 1, except that, as shown in fig. 3, the first gaseous ethylene storage tank 16 is further connected to a membrane compressor 18 through a pipe, and the membrane compressor 18 is connected to a second gaseous ethylene storage tank 19 through a pipe. The product is pressurized to 16MPaG by a diaphragm compressor 18, the pressurized product is conveyed into a second gaseous ethylene storage tank 19 by a pipeline, and in the process, the pressurized product is subjected to heat exchange with the outside and is gradually heated to normal temperature to obtain the final product. The final product obtained in this example can be used as it is for the reaction of synthesizing a vinyl acetate-ethylene copolymer using vinyl acetate and ethylene as raw materials. The diaphragm compressor 18 is a gas compression device conventional in the chemical field, and the second gaseous ethylene storage tank 19 is also a gas storage device conventional in the chemical field.

Example 3

This example is basically the same as example 1, except that in this example, the caustic washing tower 6 is not provided, and the hydraulic ring press 5 and the drying tower 8 are directly connected by a pipe. In this example, sodium hydroxide was used instead of sodium bicarbonate to adjust the pH in the synthesis of the vinyl acetate-ethylene copolymer in the upstream process, so that the initial off-gas contained no carbon dioxide and the caustic scrubber 6 was not required to remove carbon dioxide. After the recovery treatment of the process, gaseous ethylene with the purity of 99.5 percent can be obtained.

Example 4

This example 1 is basically the same as example 1, except that initial tail gas generated under different conditions is used, and the initial tail gas is obtained by pressure-increasing and defoaming ethylene in a defoaming tank, and in this example, the components of the initial tail gas are specifically (mole percentage): 0.2 percent of oxygen, 0.24 percent of ethane, 1.59 percent of nitrogen, 1.44 percent of carbon dioxide, 0.34 percent of vinyl acetate, 0.04 percent of methane, 85.84 percent of ethylene, 0.39 percent of acetaldehyde, 0.48 percent of methanol and 0.22 percent of water, and the balance of small amount of ethyl acetate, tertiary butyl alcohol, hydrogen peroxide, tertiary butyl hydroperoxide and vinyl acetate-ethylene copolymer (copolymer dispersed in tail gas). After the recovery treatment of the process, gaseous ethylene with the purity of more than 99.5 percent can be obtained.

While the embodiments of the present invention have been described in detail with reference to the accompanying drawings, it is to be understood that the present invention is not limited to the embodiments and that various changes and modifications may be made therein by those skilled in the art without departing from the spirit and scope of the invention. These should also be considered as the scope of protection of the present invention, and these do not affect the effect of the implementation of the present invention and the utility of the patent. The techniques, shapes, and structural parts, which are omitted from the description of the present invention, are all known techniques.

Claims (10)

1. The ethylene cryogenic recovery system is characterized by comprising a pretreatment unit and a cryogenic recovery unit, wherein the cryogenic recovery unit comprises a third heat exchanger and a third gas-liquid separation tank; the discharge end of a heating medium channel of the third heat exchanger is communicated with a third gas-liquid separation tank; a first liquid-state cryogen outlet is arranged on the third gas-liquid separation tank, the feed end of the cryogen passage of the third heat exchanger is communicated with the first liquid-state cryogen outlet, and a throttling expansion device is arranged between the cryogen passage of the third heat exchanger and the first liquid-state cryogen outlet.

2. The cryogenic ethylene recovery system of claim 1, wherein: the cryogenic recovery unit further comprises a second heat exchanger and a second gas-liquid separation tank; the discharge end of the heating medium channel of the second heat exchanger is communicated with the second gas-liquid separation tank, and the feed end of the heating medium channel of the third heat exchanger is communicated with the second gas-liquid separation tank; the feed end of the refrigerant channel of the second heat exchanger is communicated with the discharge end of the refrigerant channel of the third heat exchanger.

3. The cryogenic ethylene recovery system of claim 2, wherein: the cryogenic recovery unit further comprises a first heat exchanger; the discharge end of the heat medium channel of the first heat exchanger is communicated with the feed end of the heat medium channel of the second heat exchanger; and a first gaseous refrigerant outlet is formed in the third gas-liquid separation tank, and the feed end of the refrigerant channel of the first heat exchanger is communicated with the first gaseous refrigerant outlet.

4. The cryogenic ethylene recovery system of claim 3, wherein: and the discharge end of the refrigerant channel of the second heat exchanger is communicated with a first gaseous ethylene storage tank.

5. The cryogenic ethylene recovery system of claim 4, wherein: and the discharge end of the refrigerant channel of the first heat exchanger is communicated with a flare system.

6. The cryogenic ethylene recovery system of claim 5, wherein: and the third gas-liquid separation tank is also communicated with a discharge end of the liquid ethylene storage tank.

7. The cryogenic recovery system of ethylene according to any one of claims 1 to 6, characterized in that: the pretreatment unit comprises a first gas-liquid separation tank, a water washing tower, an alkaline washing tower and a drying tower which are sequentially arranged along the material flowing direction.

8. The cryogenic ethylene recovery system of claim 7, wherein: and the discharge end of the drying tower is communicated with the feed end of a heat medium channel of the first heat exchanger.

9. The cryogenic ethylene recovery system of claim 8, wherein: and a water cooler is arranged between the alkaline washing tower and the drying tower.

10. The cryogenic ethylene recovery system of claim 9, wherein: and a gas holder is arranged between the water washing tower and the alkaline washing tower.

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