A compound absorbent for polyolefin high-pressure flare gas light hydrocarbon recovery, a preparation method, and a recovery system and recovery method

Abstract

The application discloses a kind of for polyolefin high-pressure flare gas light hydrocarbon recovery complex absorbent, preparation method and recovery system and recovery method, it is related to petrochemical tail gas resource utilization technical field.Complex absorbent includes: main absorption component, auxiliary absorption component, heat stabilizer and viscosity modifier by mass fraction.The system includes compression impurity removal unit, absorption desorption purification unit and catalytic oxidation tail gas treatment unit, and forms integrated complete equipment.The method includes compression impurity removal, low-temperature oil absorption, flash evaporation, desorption purification, product separation and catalytic oxidation tail gas purification process.The application solves the technical problems of poor C2 selectivity, high nitrogen co-absorption rate, high temperature easy deterioration, high viscosity of existing absorbent, low separation precision of existing device, high energy consumption and tail gas not up to standard through the synergistic effect of four-function complex absorbent and double-tower coupled integrated device, realizes the efficient recovery and resource utilization of C2+ Light hydrocarbon in high-nitrogen-content polyolefin high-pressure flare gas.

Description

Technical Field

[0001] This invention relates to the field of petrochemical tail gas resource utilization technology, specifically to a compound absorbent, preparation method, recovery system, and recovery method for recovering light hydrocarbons from polyolefin high-pressure flare gas. Background Technology

[0002] During continuous operation, low-density polyethylene (LDPE) and high-density polyethylene (HDPE) production units require the polymerization reactor to periodically discharge a mixed gas containing unreacted monomers, oligomers, and inert gases to ensure stable system pressure and product quality. This discharged gas is collected by a high-pressure flare system to form "high-pressure flare gas." High-pressure flare gas is characterized by its large volume and high pressure (single unit discharges can reach several tons to tens of tons per hour, with discharge pressure typically between 0.5 and 2.0 MPa), extremely high nitrogen content (nitrogen fraction as high as 80% to 90%, far exceeding that of conventional refinery dry gas), dispersed light hydrocarbon components (rich in high-value light hydrocarbon components such as ethylene, ethane, propylene / propane, butene / butane, C5 and C6, but with a total concentration of only 10% to 20%), and high impurity content (containing trace amounts of oxygen, hydrogen, methane, liquid oligomers, and moisture).

[0003] The high nitrogen content, high impurity content, and dispersed light hydrocarbon components of polyolefin high-pressure flare gas present significant technical challenges for resource recovery. On the one hand, direct incineration of large quantities of high-value light hydrocarbons not only results in substantial economic losses (up to tens of millions of yuan annually for a single ethylene unit), but also increases carbon emissions and environmental pollution. On the other hand, the high nitrogen and impurity content imposes stringent requirements on traditional separation technologies, making it difficult for conventional techniques to achieve efficient, stable, and low-energy-consumption recovery of light hydrocarbons.

[0004] Currently, the industry's technologies for treating high-pressure flare gas from polyolefins mainly fall into two categories: The first is direct incineration, where the flare gas is directly sent to the flare system for venting and combustion, achieving only safe disposal without resource recovery. The second is simplified recycling, where the flare gas is simply compressed and then fed into the fuel gas pipeline network for use as low-calorific-value fuel, without separating and purifying high-value light hydrocarbons. The common drawback of both methods is that a large amount of high-value C2+ light hydrocarbon resources are wasted through incineration, causing significant economic losses and increasing carbon emissions and environmental pollution, which is seriously inconsistent with the current requirements for green and low-carbon development in the petrochemical industry.

[0005] For the separation and recovery of light hydrocarbon components in flare gas, existing technologies mainly employ three routes: simple condensation, membrane separation, and oil absorption. Simple condensation, through cooling and condensation, can only separate a portion of heavy light hydrocarbons, but its recovery effect on light components such as ethylene, ethane, propylene, and propane is extremely poor, with a large amount of light hydrocarbons being lost with the non-condensable gas. Membrane separation utilizes the permeability selectivity of polymer membranes to separate different components, but the membrane materials have limited selectivity for ethylene, ethane, and nitrogen, and high nitrogen content leads to decreased permeate flux, increased membrane fouling, and poor industrial operational stability. Oil absorption selectively dissolves light hydrocarbon components with liquid absorbents and recovers them through subsequent desorption; it is currently the most widely used industrial technology for light hydrocarbon recovery from flare gas. Among these three technical routes, oil absorption is considered the most feasible for light hydrocarbon recovery from polyolefin high-pressure flare gas due to its high separation accuracy, high operational flexibility, and high technological maturity.

[0006] The absorbent is the core of the oil absorption process, and its performance directly determines the light hydrocarbon recovery efficiency, product quality, and energy consumption of the unit. The absorbents used in existing oil absorption technologies are mainly of three types: C4 / C5 fractions, crude diesel oil, and heavy aromatics. Among them, C4 / C5 fractions are widely available and have low cost, but they have weak selective solubility for ethylene and ethane, and the nitrogen co-absorption rate is as high as 3% to 5%, which leads to a large amount of nitrogen being entrained into the rich liquid, increasing the load on the desorption tower and the overall energy consumption. Crude diesel oil has a high boiling point and a large absorption capacity for heavy light hydrocarbons, but it has poor selectivity for ethylene and ethane, and its kinematic viscosity is high, usually greater than 5 square millimeters per second at room temperature, resulting in low gas-liquid mass transfer efficiency, large pressure drop in the tower, and high pumping energy consumption. Heavy aromatics have a certain solubility for light hydrocarbons, but they are prone to oxidation and deterioration at high temperatures, coking and carbon deposition, resulting in a short absorbent life, frequent replacement, and high operating costs. The common drawback of the aforementioned traditional absorbents is that they are all single-component or simple mixtures, and are not functionally designed for the specific operating conditions of high-nitrogen-content polyolefin flare gas. Specifically, their insufficient selectivity for dissolving ethylene and ethane results in recovery rates typically below 90%, leading to the loss of a large amount of high-value ethylene resources. Under conditions with high nitrogen content of 80%–90%, the nitrogen co-absorption rate is as high as 3%–5%, and a large amount of ineffective nitrogen enters the rich liquid, which not only increases the steam consumption of the desorption tower but also reduces the purity of the product. This causes the separated light hydrocarbon components to be mixed with non-condensable gases, making them unusable as direct polyolefin feedstocks. At desorption tower operating temperatures typically of 120–150 °C, traditional absorbents are prone to oxidation and deterioration, increased viscosity, and darkening of color. After long-term cyclic use, their absorption performance significantly declines, and their kinematic viscosity at room temperature is typically greater than 5 mm. 2 / s, under low-temperature absorption conditions, the viscosity further increases, leading to a decrease in gas-liquid mass transfer efficiency, an increase in pressure drop inside the tower, and an increase in pumping energy consumption; the above-mentioned performance defects of the absorbent are the fundamental reason why traditional oil absorption technology is difficult to adapt to the high nitrogen content polyolefin flare gas conditions, directly resulting in a series of technical problems such as low light hydrocarbon recovery rate, poor product purity, high operating energy consumption, and large absorbent loss.

[0007] Furthermore, existing oil absorption methods for light hydrocarbon recovery have the following drawbacks: Impurities such as liquid oligomers, free water, and solid particles in the polyolefin flare gas directly entering the absorption tower can clog the packing or trays, contaminate the absorbent, and reduce absorption efficiency. Existing technologies typically only include simple gas-liquid separators without advanced impurity removal equipment. Single-tower absorption and separation precision is low; absorption and desorption are completed in the same tower or only a single absorption tower is used, making it impossible to simultaneously achieve high recovery rates and high purity, resulting in poor recovery of ethylene and ethane. The rich solution (approximately 5°C) needs to be heated to 130°C, and the lean solution (approximately 130°C) needs to be cooled to 5°C. The rich solution is directly heated by steam, and the lean solution is directly cooled by cooling water, resulting in a lack of cascaded heat utilization and high consumption of steam and cooling water. Residual light hydrocarbons, carbon monoxide, and hydrogen in the non-condensable gas at the top of the absorption tower cause volatile organic compound concentrations to typically exceed 100 mg / m³. 3 The levels of non-condensable gases far exceed the limits specified in GB31571-2015, posing a risk of environmental non-compliance if directly discharged. Furthermore, the equipment is dispersed and lacks integration; compression, absorption, desorption, and tail gas treatment equipment are scattered and not integrated into a single structure, resulting in large footprints, cumbersome operation and maintenance, and poor industrial adaptability. In addition, during the operation of polyolefin production units, the volume and composition of flare gas fluctuate with production load and product grade switching, with load fluctuations reaching 70% to 110%. Existing oil absorption units are typically designed for fixed operating conditions, lacking frequency conversion regulation and load linkage control functions. When operating conditions fluctuate, problems such as decreased absorption efficiency, product purity fluctuations, and equipment instability easily occur, resulting in insufficient process adaptability and resistance to fluctuations.

[0008] In summary, existing technologies suffer from significant technical deficiencies in both absorbent performance and process structure. Particularly under high nitrogen content conditions, there is a lack of a dedicated absorbent that combines high C2+ selectivity, low nitrogen co-absorption rate, good high-temperature stability, and suitable viscosity. Furthermore, there is a lack of integrated recovery systems and methods that are compatible with and work in conjunction with such an absorbent. These problems severely restrict the efficient recovery and resource utilization of high-value light hydrocarbons from polyolefin high-pressure flare gas. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a compound absorbent, its preparation method, and a recovery system and method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. This solves the problems of existing absorbents, such as poor C2 selectivity, high nitrogen co-absorption rate, easy deterioration at high temperatures, high viscosity, and inability to adapt to high-nitrogen operating conditions. It also addresses the problems of existing recovery systems, such as low recovery rate, poor product purity, high operating energy consumption, substandard exhaust emissions, and low system integration.

[0010] The specific technical solution of this invention is as follows: In a first aspect, the present invention provides a compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, comprising, by mass parts: The main absorber component consists of 65-75 parts, which is a complex of C6-C8 cycloalkanes and C6-C8 n-alkanes. The absorption aid component consists of 20-25 parts, which are C9-C10 isoalkanes; 0.3~0.8 parts of heat stabilizer, which is a hindered phenolic heat stabilizer; 3-5 parts of viscosity modifier, selected from at least one of polyα-olefin, polyisobutylene and ethylene-propylene copolymer.

[0011] Furthermore, in the main absorbent component, the mass ratio of the C6-C8 cycloalkanes to the C6-C8 n-alkanes is 1:(0.8-1.5).

[0012] Furthermore, the hindered phenolic heat stabilizer is selected from at least one of 2,6-di-tert-butyl-p-cresol, β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene and 2,4,6-tri-tert-butylphenol.

[0013] Further, the molecular weight of the polyα-olefin is 300-500, the molecular weight of the polyisobutylene is 400-1000, and the molecular weight of the ethylene-propylene copolymer is 5000-50000.

[0014] Furthermore, the mass fractions of each component in the compound absorbent are as follows: 70 parts of a compound of C6-C8 cycloalkanes and C6-C8 n-alkanes, wherein the mass ratio of the C6-C8 cycloalkanes to the C6-C8 n-alkanes is 1:1.2; 23 portions of C9~C10 isoalkanes; 0.5 parts of 2,6-di-tert-butyl-p-cresol; 6.5 parts of polyα-olefin, wherein the molecular weight of the polyα-olefin is 300-500.

[0015] Secondly, the present invention provides a method for preparing the above-mentioned compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, comprising the following steps: mixing the main absorbent component and the auxiliary absorbent component uniformly at a temperature of 15~30 ℃ and a pressure of 0.1~0.15 MPa, adding the heat stabilizer and the viscosity modifier and continuing to mix until completely dissolved, and filtering to obtain the compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins.

[0016] Thirdly, the present invention provides a system for recovering light hydrocarbons from high-pressure flare gas of polyolefins, comprising a compression and impurity removal unit, an absorption and desorption purification unit, and a catalytic oxidation tail gas treatment unit connected in sequence by pipelines. The compression and impurity removal unit includes a dry gas compressor unit, a cooler, a gas-liquid separator, and a dehydration bed, which are connected in series. The absorption and desorption purification unit includes an absorption tower, a desorption tower, a flash tank, a lean-rich liquid heat exchanger, a lean liquid cooler, and a product separator. The bottom outlet of the absorption tower, the flash tank, the lean-rich liquid heat exchanger, and the middle inlet of the desorption tower are connected in series. The bottom outlet of the desorption tower, the lean liquid cooler, and the top inlet of the absorption tower are connected in series to form an absorbent circulation loop. The top outlet of the desorption tower is connected to the product separator. The catalytic oxidation tail gas treatment unit includes a tail gas heat exchanger, a heater, a catalytic oxidation reactor, and a chimney. The top outlet of the absorption tower, the cold medium inlet of the tail gas heat exchanger, the heater, the catalytic oxidation reactor, and the chimney are connected in series. The outlet of the catalytic oxidation reactor is connected to the hot medium inlet of the tail gas heat exchanger, and the hot medium outlet of the tail gas heat exchanger is connected to the chimney. The outlet of the dewatering bed is connected to the bottom inlet of the absorption tower.

[0017] Furthermore, the absorption-desorption-purification unit also includes a product gas compressor and a product gas cooler. The product gas compressor is located between the top outlet of the desorption tower and the product separator, and the product gas cooler is located between the product gas compressor and the product separator.

[0018] Furthermore, the lean liquid cooler includes a lean liquid water cooler and a lean liquid intercooler connected in sequence.

[0019] Furthermore, the catalytic oxidation tail gas treatment unit also includes a continuous emission monitoring system (CEMS) and an online volatile organic compound (VOCs) analyzer, both of which are located at the chimney outlet.

[0020] Fourthly, the present invention provides a method for recovering light hydrocarbons from polyolefin high-pressure flare gas, employing the aforementioned compound absorbent for recovering light hydrocarbons from polyolefin high-pressure flare gas, comprising the following steps: S1. Compression and impurity removal: The polyolefin high-pressure flare gas is sequentially subjected to pressurization, cooling, gas-liquid separation and dehydration to obtain pretreated gas; S2. Absorption: After cooling the pretreated gas, it is contacted counter-currently with a lean absorbent containing the compound absorbent to absorb C2+ light hydrocarbons, resulting in non-condensable gas and a rich absorbent. S3. Flash evaporation: Flash evaporation of the rich absorbent liquid separates some C2+ light hydrocarbons, resulting in a flash-evaporated rich absorbent liquid. S4. Desorption: After preheating the flash-evaporated rich absorbent, desorption is performed to obtain carbon dioxide-rich and lean absorbent. The lean absorbent is cooled and returned to step S2 for recycling. S5. Product separation: The carbon-rich gas is compressed, cooled and then separated into liquids to obtain C2+-rich product gas and C2+-rich product liquid. S6. Catalytic oxidation: The non-condensable gas is preheated and then subjected to catalytic oxidation. The flue gas after the reaction is discharged after the waste heat is recovered.

[0021] Furthermore, in step S1, the pressurization pressure is 1.5~1.6 MPa.

[0022] Furthermore, in step S2, the temperature of the pretreated gas after cooling is 4~6 ℃.

[0023] Furthermore, in step S4, the temperature of the preheated absorbent is 128~132 ℃.

[0024] Furthermore, in step S6, the temperature of the preheated non-condensable gas is 295~305 ℃.

[0025] Beneficial effects This invention provides a compound absorbent, its preparation method, and a recovery system and method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. Compared with the prior art, it has the following significant advantages: (1) Significantly improved light hydrocarbon recovery rate: Existing technologies use single-component absorbents such as C4 / C5 fractions, crude diesel oil, or heavy aromatics, which have insufficient selectivity for dissolving C2 light hydrocarbons such as ethylene and ethane, with C2 recovery rates typically below 90%. This invention uses a quaternary compound system with C6-C8 cycloalkanes and C6-C8 n-alkanes as the main absorbent component and C9-C10 isoalkanes as the auxiliary absorbent component. Utilizing the principle of solubility parameter matching and the directional inclusion effect of branched structures, the selective dissolution capacity for C2 light hydrocarbons is significantly enhanced. The recovery rate of C2 light hydrocarbons is significantly improved compared to existing technologies, fundamentally preventing the loss of high-value ethylene resources.

[0026] (2) Significantly reduced nitrogen co-absorption rate, resulting in a substantial decrease in desorption load and energy consumption: High nitrogen content is a core characteristic of high-pressure flare gas for polyolefins. Existing absorbents exhibit a high nitrogen co-absorption rate, leading to a large amount of ineffective nitrogen entering the rich liquid, which not only increases steam consumption in the desorption tower but also reduces product purity. This invention utilizes the molecular sieving effect of C6-C8 cycloalkanes / n-alkanes and the steric hindrance of C9-C10 isoalkanes to reduce the nitrogen co-absorption rate by an order of magnitude compared to existing technologies. The reduction in nitrogen co-absorption rate directly brings the following chain benefits: significantly reduced steam consumption in the desorption tower, significantly reduced nitrogen content in the product gas, and product purity meeting the requirements for feedstock recycling in polyolefin plants.

[0027] (3) Excellent high-temperature stability and significantly reduced operating costs: Existing absorbents are prone to oxidation and deterioration at desorption temperatures, resulting in increased viscosity and darker color. After long-term cyclic use, their absorption performance significantly declines, requiring frequent replenishment or replacement. This invention, by adding hindered phenolic heat stabilizers, utilizes their free radical capture mechanism to interrupt the chain reaction of high-temperature alkane oxidation, enabling the compound absorbent to maintain stable performance after long-term cyclic use at desorption temperatures. This significantly extends the oxidation induction period, significantly reduces the amount of absorbent to be replenished, and substantially lowers operating costs.

[0028] (4) Significantly reduced kinematic viscosity and greatly improved gas-liquid mass transfer efficiency: Existing crude diesel absorbents have a relatively high kinematic viscosity at room temperature, which further increases under low-temperature absorption conditions, leading to reduced gas-liquid mass transfer efficiency, increased pressure drop in the tower, and increased pumping energy consumption. This invention adds a viscosity modifier to reduce the frictional resistance between the molecules of each component, thereby controlling the kinematic viscosity of the compound absorbent within a suitable range at low temperatures. This significantly improves gas-liquid mass transfer efficiency, significantly reduces pressure drop in the tower, and effectively reduces pumping energy consumption.

[0029] (5) Significantly improved separation precision, product purity meets reuse requirements: Existing technologies mostly adopt a single-tower absorption structure, where absorption and desorption are completed in the same tower or only a single absorption tower is used. The separation precision is limited, and it is impossible to achieve high recovery rate and high purity at the same time, and it is impossible to produce high-purity C2-rich products. This invention adopts a dual-tower coupled structure of absorption tower and desorption tower, combined with flash tank for deep purification, which significantly improves the separation precision and can simultaneously produce two products: C2-rich gas and C2-rich liquid. The product has a high content of light hydrocarbons and can be directly reused as raw material for polyolefin plants or sold externally without additional refining processes.

[0030] (6) Cascaded utilization of waste heat significantly reduces energy consumption: In existing technologies, the rich liquor is directly heated by steam, the lean liquor is directly cooled by cooling water, and the high-temperature flue gas is directly discharged, resulting in no cascaded utilization of heat and high consumption of steam and cooling water. This invention utilizes the waste heat of the high-temperature lean liquor to preheat the rich liquor by setting up a lean-rich liquor heat exchanger, reducing steam consumption in the desorption tower reboiler; and utilizes the waste heat of the high-temperature flue gas after the reaction to preheat the inlet non-condensable gas, reducing heater power consumption. The comprehensive waste heat recovery measures significantly reduce the overall energy consumption of the device compared to traditional processes.

[0031] (7) Emissions meet emission standards and environmental risks are effectively controlled: In existing technologies, the non-condensable gas at the top of the absorption tower is directly discharged or used as low-calorific-value fuel, and the VOCs concentration far exceeds the emission standards. This invention adds a catalytic oxidation tail gas treatment unit to completely oxidize and decompose the residual hydrocarbons, carbon monoxide, hydrogen and other combustible pollutants in the non-condensable gas under the action of a catalyst. The VOCs emission concentration is stably controlled within the emission standards, achieving zero emissions and effectively controlling environmental risks.

[0032] (8) Integrated system design significantly reduces floor space and operating costs: In existing technologies, compression, absorption, desorption, and tail gas treatment equipment are scattered and not integrated into a single structure, resulting in a large floor space and cumbersome operation and maintenance. This invention connects the compression and impurity removal unit, the absorption and desorption purification unit, and the catalytic oxidation tail gas treatment unit sequentially through pipelines to form an integrated complete device. The equipment layout is compact, the floor space is greatly reduced, the operation and maintenance process is simplified, the number of operators is significantly reduced, and the industrial adaptability is significantly improved. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the polyolefin high-pressure flare gas light hydrocarbon recovery system of the present invention.

[0034] Figure label.

[0035] 1-Dry gas compressor unit, 2-Cooler, 3-Gas-liquid separator, 4-Dehydration bed, 5-Absorption tower, 6-Desorption tower, 7-Cold box, 8-Absorption tower reboiler, 9-Flash tank, 10-Lean and rich liquid heat exchanger, 11-Desorption tower reboiler, 12-Desorbed gas cooler, 13-Return liquid tank, 14-Lean liquid water cooler, 15-Lean liquid intercooler, 16-Product gas compressor, 17-Product gas cooler, 18-Product liquid separator, 19-Heater, 20-Catalytic oxidation reactor, 21-Tail gas heat exchanger, 22-Chimney. Detailed Implementation

[0036] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described in detail below. It should be noted that the following embodiments are only used to illustrate the implementation methods and typical parameters of the present invention, and are not intended to limit the parameter range described in the present invention. Reasonable variations derived therefrom are still within the protection scope of the present invention.

[0037] It should be noted that the endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0038] Unless otherwise defined, all terms, symbols, and other scientific terms used herein are intended to have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In some instances, terms having a conventional meaning are defined herein for clarification or ease of reference, and such definitions should not be construed as indicating a significant difference from conventional understanding in the art. The technical methods described or referenced herein are generally well understood by those skilled in the art and employed by conventional methods. Unless otherwise stated, the use of commercially available kits, reagents, and instruments shall be performed according to the manufacturer's instructions and parameters.

[0039] In a first aspect, the present invention provides a compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, comprising, by mass fraction: The main absorber component consists of 65-75 parts, which is a complex of C6-C8 cycloalkanes and C6-C8 n-alkanes. The absorption aid component consists of 20-30 parts, which are C9-C10 isoalkanes; 0.3~0.8 parts of heat stabilizer, which is a hindered phenolic heat stabilizer; 3-5 parts of viscosity modifier, selected from at least one of polyα-olefin, polyisobutylene and ethylene-propylene copolymer.

[0040] The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins provided by this invention constructs a synergistic and complementary absorbent system at the molecular scale by limiting the ratio of the main absorbent component, the auxiliary absorbent component, and the thermal stability and viscosity modifier quaternary functional components: The main absorber is a combination of C6-C8 cycloalkanes and C6-C8 n-alkanes, whose molecular size matches that of C3-C6+ light hydrocarbons. Based on the molecular sieving effect, it provides a suitable solubilization cavity: the solubility is insufficient if the number of carbons is lower than C6, and desorption is difficult if the number of carbons is higher than C8. At the same time, the cyclic structure of cycloalkanes enhances the solubility of alkenes by inducing dipole-dipole interactions, while the straight-chain structure of n-alkanes enhances the solubilization of alkanes by dispersion forces. The combination of the two makes the solvent have both polar and non-polar solubility sites, achieving broad-spectrum selective absorption of the C3-C6+ mixed components.

[0041] The auxiliary absorbent is selected from C9-C10 isoalkanes. Utilizing the characteristic of high-carbon-number absorbents that abnormally increase the solubility of low-carbon-number solutes, it directionally enhances the absorption driving force of C2 components. Simultaneously, the branched structure of C9-C10 isoalkanes generates a steric hindrance effect, increasing the diffusion barrier of N2 molecules. Furthermore, the inter-branch gap sites form a more suitable force field with C2 molecules. Through a synergistic mechanism of molecular size exclusion and force interaction, it suppresses N2 co-absorption while directionally solubilizing C2. A mass ratio of 20-30 parts, together with 65-75 parts of the main absorbent, constitutes a bifunctional gradient absorption architecture: the main absorbent is responsible for the broad-spectrum absorption of C3-C6+, while the auxiliary absorbent is responsible for the directional solubilization and nitrogen resistance of C2. Their complementary ratio ensures continuous selective absorption of light hydrocarbons across the entire C2-C6+ spectrum.

[0042] During high-temperature desorption and long-term cyclic operation, absorbents are prone to free radical chain oxidation degradation. The molecular structure of hindered phenolic heat stabilizers contains active hydrogen atoms, which can provide hydrogen atoms to react with alkoxy radicals or peroxy radicals, interrupting the oxidation chain reaction and protecting the main / co-absorbent components from oxidative damage. At the same time, its tert-butyl steric hindrance group can protect the phenolic hydroxyl group and prevent it from being rapidly consumed, thus achieving long-term thermal stability. An addition amount of 0.3~0.8 parts ensures sufficient free radical scavenging ability without affecting absorption selectivity.

[0043] The long-chain alkyl side chains of the viscosity modifier extend in hydrocarbon solvents, reducing the frictional resistance between component molecules through intermolecular lubrication, thereby significantly reducing low-temperature viscosity. An addition of 3-5 parts can effectively reduce viscosity while improving gas-liquid contact efficiency, thus increasing the probability of collisions between C2+ molecules and active sites, while the intrinsic solubility of N2 remains unchanged. This kinetically prioritizes the absorption of C2+ components, relatively suppressing the co-absorption of N2.

[0044] The main absorbent component, the auxiliary absorbent component, the thermal stability and viscosity modifier work together to solve the problems of poor C2 selectivity, high nitrogen co-absorption rate, high temperature deterioration and high viscosity of traditional single-component absorbents.

[0045] Furthermore, in the main absorbent component, the mass ratio of C6-C8 cycloalkanes to C6-C8 n-alkanes is 1:(0.8-1.5). Limiting the mass ratio of C6-C8 cycloalkanes to C6-C8 n-alkanes to 1:(0.8-1.5) ensures that the compound absorbent balances C2 selectivity and C4+ solubility over a wide temperature range, adapting to the operating conditions of light hydrocarbon dispersion in high-nitrogen flare gas.

[0046] Furthermore, the hindered phenolic heat stabilizer is selected from at least one of 2,6-di-tert-butyl-p-cresol, β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, and 2,4,6-tri-tert-butylphenol. All four of these hindered phenolic antioxidants possess active hydrogen atoms and sterically hindered protecting groups, enabling them to capture alkoxy radicals and peroxy radicals by providing hydrogen atoms, thereby interrupting the chain reaction of high-temperature alkane oxidation and preventing the absorbent from oxidizing, deteriorating, coking, and increasing viscosity at the desorption temperature.

[0047] Furthermore, the molecular weight of polyalphaolefin is 300-500, that of polyisobutylene is 400-1000, and that of ethylene-propylene copolymer is 5000-50000. All three viscosity modifiers are polyolefin polymers. In hydrocarbon solvents, they can reduce the internal friction between absorbent molecules by extending the molecular chains, thereby reducing the overall kinematic viscosity and improving gas-liquid mass transfer efficiency. Specifically: polyalphaolefin (MW 300-500) has a comb-like alkyl side chain structure, which reduces intermolecular frictional resistance through intermolecular lubrication; its molecular weight of 300-500 ensures that it reduces viscosity in a colloidal dispersion state without forming a three-dimensional network entanglement; polyisobutylene (MW 400-1000) is a saturated linear polymer, which reduces the cohesive force of the system through the interaction of long-chain alkyl groups with solvent molecules; and ethylene-propylene copolymer (MW 5000-50000) has a relatively long molecular chain, which, in addition to reducing viscosity, can also improve the viscosity-temperature properties of the system.

[0048] Furthermore, the mass fractions of each component in the compound absorbent are as follows: 70 parts of a compound of C6-C8 cycloalkanes and C6-C8 n-alkanes, wherein the mass ratio of C6-C8 cycloalkanes to C6-C8 n-alkanes is 1:1.2; 23 portions of C9~C10 isoalkanes; 0.5 parts of 2,6-di-tert-butyl-p-cresol; 6.5 parts of polyalphaolefin, the molecular weight of which is 300-500.

[0049] The above absorbent formulation is the optimal embodiment determined through optimization, achieving optimal synergy among the components: 70 parts of the main absorbent component provide sufficient solubility and basic selectivity, with a 1:1.2 mass ratio of cycloalkanes to n-alkanes balancing C2 mass transfer rate and C4+ solubility; 23% of the co-absorbent component ensures C2 directional selectivity without significantly increasing system viscosity; and 0.5 parts of the heat stabilizer achieve a balance between antioxidant effect and cost. Under this formulation, the nitrogen co-absorption rate of the compound absorbent can be reduced to below 0.45%, and the C2 recovery rate reaches over 98.39%, resulting in optimal overall technical performance.

[0050] Secondly, the present invention provides a method for preparing the above-mentioned compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, comprising the following steps: mixing the main absorbent component and the auxiliary absorbent component uniformly at a temperature of 15~30 ℃ and a pressure of 0.1~0.15 MPa, adding a heat stabilizer and a viscosity modifier and continuing to mix until completely dissolved, and filtering to obtain the compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins.

[0051] The method for preparing a compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins provided by this invention ensures the full dissolution and stable performance of each component by employing stepwise feeding and mild conditions. The first step involves mixing the main absorbent component and the auxiliary absorbent component, taking advantage of their good compatibility as both are alkanes, to rapidly form a homogeneous system under mild conditions of 15–30 °C and 0.1–0.15 MPa. The second step involves adding a heat stabilizer and a viscosity modifier, which further disperse and dissolve the components within this homogeneous system. The ambient temperature and pressure conditions of 15–30 °C and 0.1–0.15 MPa allow the preparation process to be completed without special equipment, making the process simple, easy to implement industrially, and avoiding the risk of component volatilization or degradation under high temperature and high pressure conditions.

[0052] Thirdly, the present invention provides a system for recovering light hydrocarbons from high-pressure flare gas of polyolefins, comprising a compression and impurity removal unit, an absorption and desorption purification unit, and a catalytic oxidation tail gas treatment unit connected in sequence by pipelines. The compression and impurity removal unit includes a dry gas compressor unit 1, a cooler 2, a gas-liquid separator 3, and a dehydration bed 4, which are connected in series. The absorption and desorption purification unit includes an absorption tower 5, a desorption tower 6, a flash tank 9, a lean and rich liquid heat exchanger 10, a lean liquid cooler, and a product separator 18. The bottom outlet of the absorption tower 5, the flash tank 9, the lean and rich liquid heat exchanger 10, and the middle inlet of the desorption tower 6 are connected in series. The bottom outlet of the desorption tower 6, the lean liquid cooler, and the top inlet of the absorption tower 5 are connected in series to form an absorbent circulation loop. The top outlet of the desorption tower 6 is connected to the product separator 18. The catalytic oxidation tail gas treatment unit includes a tail gas heat exchanger 21, a heater 19, a catalytic oxidation reactor 20, and a chimney 22. The top outlet of the absorption tower 5, the cold medium inlet of the tail gas heat exchanger 21, the heater 19, and the catalytic oxidation reactor 20 are connected in sequence. The outlet of the catalytic oxidation reactor 20 is also connected to the hot medium inlet of the tail gas heat exchanger 21, and the hot medium outlet of the tail gas heat exchanger 21 is connected to the chimney 22. The outlet of dewatering bed 4 is connected to the bottom inlet of absorption tower 5.

[0053] The system for recovering light hydrocarbons from high-pressure flare gas of polyolefins provided by this invention has an integrated structure of "compression and impurity removal - oil absorption - desorption and purification - tail gas catalytic oxidation", with each unit function connected and synergistically enhanced.

[0054] The compression and impurity removal unit addresses the issues of low recovery rate and poor product purity through pretreatment: the dry gas compressor unit 1 pressurizes the flare gas to increase gas density and gas-liquid mass transfer driving force; the cooler 2 cools down the gas to condense some condensable components and reduce the volumetric flow rate; the gas-liquid separator 3 removes condensate droplets to prevent flooding; and the dehydration bed 4 deeply removes free water to prevent emulsification and deactivation of the absorbent and low-temperature freezing blockage. By removing impurities, the cleanliness of the feed to the absorption tower 5 is ensured, maintaining stable and efficient gas-liquid mass transfer, while preventing moisture and impurities from entering the product, thereby improving the recovery rate of C2+ light hydrocarbons and the purity of the product.

[0055] The absorption-desorption purification unit, through the synergistic effect of a dual-tower coupling structure, flash tank 9, lean and rich liquid heat exchanger 10, lean liquid cooler, and product separator 18, solves the problems of low recovery rate, poor product purity, and high operating energy consumption. Absorption tower 5 and desorption tower 6 each perform their respective functions, independently carrying out absorption and desorption, avoiding the contradictions between absorption and desorption conditions found in a single-tower structure, thus improving the recovery rate of C2+ light hydrocarbons. Simultaneously, the high-concentration C2+-rich gas produced at the top of desorption tower 6 can be directly reused after subsequent separation. Flash tank 9 is used to separate the rich liquid before it enters desorption tower 6. The pressure is reduced before the desorption tower 5 is activated, allowing some light hydrocarbons and non-condensable gases to flash out, recovering some products in advance and reducing the load and steam consumption of the desorption tower. The lean and rich liquid heat exchanger 10 allows the high-temperature lean liquid and the low-temperature rich liquid to exchange heat in a countercurrent manner, realizing the cascade utilization of heat and reducing the steam consumption and cooling water consumption of the reboiler. The lean liquid cooler cools the regenerated lean absorbent to the optimal absorption temperature, ensuring the efficient operation of the absorber tower 5. The product separator 18 separates the C2+-rich gas into C2+-rich product gas and product liquid, avoiding purity fluctuations caused by gas-liquid miscibility.

[0056] The catalytic oxidation tail gas treatment unit solves the problem of substandard tail gas emissions through the synergistic effect of tail gas heat exchanger 21, heater 19, catalytic oxidation reactor 20, and chimney 22: tail gas heat exchanger 21 uses the waste heat of high-temperature flue gas after reaction to preheat inlet low-temperature non-condensable gas, reducing the load on heater 19; heater 19 further heats the preheated non-condensable gas to the catalytic oxidation reaction temperature; under the action of catalyst, catalytic oxidation reactor 20 causes residual hydrocarbons, carbon monoxide, hydrogen, and other combustible pollutants to undergo oxidation reactions with oxygen to generate carbon dioxide and water, completely removing combustible pollutants; through catalytic oxidation, residual combustible pollutants in the tail gas are completely decomposed, so that the emission concentration of volatile organic compounds is stably controlled within the emission standards, achieving zero emissions exceeding the standards.

[0057] The three-unit integrated structure connects the compression and impurity removal unit, the absorption and desorption purification unit, and the catalytic oxidation tail gas treatment unit sequentially through pipelines to form a complete closed-loop process, overcoming the problem of low system integration: the unit equipment is connected in series, and the material flows automatically within the system from the raw gas inlet to the product outlet and tail gas emission. The absorbent is automatically recycled back to the absorption tower 5 after being regenerated at the bottom of the desorption tower 6, achieving closed-loop utilization. This integrated design makes the equipment layout compact, reduces the floor space, simplifies the operation and maintenance process, reduces the number of operators, and significantly improves industrial adaptability.

[0058] Furthermore, the absorption-desorption purification unit also includes a product gas compressor 16 and a product gas cooler 17. The product gas compressor 16 is located between the top outlet of the desorption tower 6 and the product separator 18, and the product gas cooler 17 is located between the product gas compressor 16 and the product separator 18. The carbon-rich gas produced at the top of the desorption tower 6 is pressurized by the product compressor 16 and cooled by the product cooler 17 before entering the product separator 18. This improves the separation efficiency and accuracy, further increasing the concentration of light hydrocarbons in the gas phase product and increasing the content of heavy components in the liquid phase product, thus achieving high-quality production of both gas and liquid products.

[0059] Furthermore, the lean solution cooler includes a lean solution water cooler 14 and a lean solution intercooler 15 connected in sequence. The temperature of the lean absorbent produced at the bottom of the desorption tower 6 is as high as 120~135 ℃. If it is directly returned to the top of the absorption tower 5 for spraying, the high-temperature lean solution will destroy the low-temperature absorption environment of the absorption tower 5, resulting in a significant decrease in C2+ absorption efficiency. Through the two-stage gradient cooling of the lean solution water cooler 14 and the lean solution intercooler 15, the lean solution is reduced to the optimal absorption temperature before returning to the absorption tower 5, which not only ensures the absorption efficiency, but also reduces the load and energy consumption of a single-stage cooling through two-stage cooling.

[0060] Furthermore, the catalytic oxidation tail gas treatment unit also includes a continuous emission monitoring system (CEMS) and an online volatile organic compound (VOCs) analyzer, both located at the chimney outlet 22. The CEMS and VOCs analyzer at the chimney outlet 22 allow for real-time continuous monitoring of the VOCs concentration in the purified flue gas, ensuring traceable and verifiable emission data. When the VOCs concentration abnormally increases, the system can promptly issue an early warning and adjust the operating parameters of the catalytic oxidation unit accordingly, ensuring long-term stable emissions that meet the requirements of GB31571-2015 standards, while also providing data support for the unit's operational adjustments.

[0061] Fourthly, the present invention provides a method for recovering light hydrocarbons from polyolefin high-pressure flare gas, employing the aforementioned compound absorbent for recovering light hydrocarbons from polyolefin high-pressure flare gas, comprising the following steps: S1. Compression and impurity removal: The polyolefin high-pressure flare gas is sequentially subjected to pressurization, cooling, gas-liquid separation and dehydration to obtain pretreated gas; In step S1, free water and liquid impurities in the flare gas are removed by pressurization, cooling, gas-liquid separation and dehydration to ensure the cleanliness of the feed to the subsequent absorption tower and avoid equipment blockage and reduced absorption efficiency.

[0062] S2. Absorption: After cooling the pretreated gas, it is contacted counterclockwise with a lean absorbent containing a compound absorbent to absorb C2+ light hydrocarbons, resulting in non-condensable gas and a rich absorbent. In step S2, the selective solubility characteristics of the special compound absorbent are utilized to efficiently enrich C2+ light hydrocarbons from the high-nitrogen flare gas to the liquid phase under low-temperature conditions, and the non-condensable gas at the top of the tower is discharged and sent to the downstream purification.

[0063] S3, flash evaporation: Flash evaporation of the rich absorbent liquid separates some C2+ light hydrocarbons, resulting in a flash-evaporated rich absorbent liquid; In step S3, the rich absorbent is subjected to reduced pressure flash evaporation to separate some C2+ light hydrocarbons in advance, thereby reducing the processing load of the subsequent desorption tower and the heat consumption of the reboiler.

[0064] S4. Desorption: After flash evaporation, the rich absorbent is preheated and then desorbed by heating to obtain carbon dioxide-rich and lean absorbent. The lean absorbent is cooled and returned to step S2 for recycling. In step S4, the C2+ light hydrocarbons dissolved in the rich solution are released in gaseous form by heating, achieving the dual purpose of absorbent regeneration and light hydrocarbon purification. The regenerated lean solution is cooled and returned to the absorption tower for recycling, realizing the closed-loop utilization of the absorbent.

[0065] S5. Product separation: The carbon-rich gas is compressed, cooled and separated into liquids to obtain C2+-rich product gas and C2+-rich product liquid. In step S5, the carbon-rich gas is further separated into two forms, C2+-rich product gas and C2+-rich product liquid, through compression, cooling and separation, to suit different reuse scenarios.

[0066] S6. Catalytic oxidation: The non-condensable gas is preheated and then subjected to catalytic oxidation. The flue gas after the reaction is discharged after the waste heat is recovered.

[0067] In step S6, combustible pollutants such as hydrocarbons, CO, and H2 in the non-condensable gas at the top of the absorption tower are completely oxidized into CO2 and H2O under the action of a catalyst. The purified flue gas is discharged after waste heat recovery, thus achieving both environmental compliance and energy recovery.

[0068] The polyolefin high-pressure flare gas light hydrocarbon recovery method provided by this invention integrates six steps—compression and impurity removal, absorption, flash evaporation, desorption, product separation, and catalytic oxidation—to achieve a balance between high recovery rate, high purity, and low energy consumption.

[0069] Furthermore, in step S1, the pressurization pressure is 1.5~1.6 MPa. The pressure range of 1.5~1.6 MPa satisfies the requirements of the subsequent absorption tower operating pressure on the feed pressure, while also taking into account compression energy consumption and absorption efficiency.

[0070] Furthermore, in step S2, the temperature of the pretreated gas after cooling is 4~6 ℃. Cooling the pretreated gas to 4~6 ℃ can significantly improve the solubility and absorption rate of C2+ light hydrocarbons in the compound absorbent. At the same time, the lower temperature can reduce the physical dissolution and entrainment of non-condensable gases such as N2 in the absorbent, which helps to reduce the nitrogen co-absorption rate.

[0071] Furthermore, in step S4, the temperature of the preheated absorbent is 128~132 ℃. The temperature range of 128~132 ℃ provides sufficient thermodynamic driving force for the desorption of C2+ light hydrocarbons in the compound absorbent, allowing the dissolved gas in the rich liquid to be fully released into the gas phase; at the same time, this temperature is lower than the decomposition temperature of the heat stabilizer in the absorbent, ensuring the stability of the absorbent during long-term cyclic operation.

[0072] Furthermore, in step S6, the temperature of the preheated non-condensable gas is 295~305 ℃. The temperature range of 295~305 ℃ is the optimal reaction temperature window for the catalytic oxidation reaction, ensuring that the catalytic oxidation unit operates under optimal reaction kinetics conditions, which is conducive to achieving stable VOCs emissions that meet standards.

[0073] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. The raw materials used in the embodiments are all industrial-grade products, wherein the C6-C8 cycloalkanes are a mixture of industrial cyclohexane and methylcyclohexane, the C6-C8 n-alkanes are a mixture of n-hexane and n-heptane, the C9-C10 isoalkanes are industrial isoalkan solvent oils, and the polyα-olefins are commercially available low molecular weight synthetic base oils.

[0074] Example 1 This embodiment provides a compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, which is prepared by the following steps: 700 kg of a compound of C6-C8 cycloalkanes and C6-C8 n-alkanes (mass ratio 1:1.2) and 230 kg of C9-C10 isoalkanes are added to a stirred tank and mixed evenly at a temperature of 25 ℃ and a pressure of 0.12 MPa; 5 kg of 2,6-di-tert-butyl-p-cresol and 65 kg of poly-α-olefin (molecular weight 400) are added and mixed until completely dissolved; after 5 μm precision filtration, 1000 kg of the finished compound absorbent is obtained.

[0075] Example 2 This embodiment provides a compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, which is prepared by the following steps: 650 kg of a compound of C6-C8 cycloalkanes and C6-C8 n-alkanes (mass ratio 1:0.8) and 250 kg of C9-C10 isoalkanes are added to a stirred tank and mixed evenly at a temperature of 20 ℃ and a pressure of 0.1 MPa; 8 kg of 2,6-di-tert-butyl-p-cresol and 50 kg of poly-α-olefin (molecular weight 300) are added and mixed until completely dissolved. After 5 μm precision filtration, 958 kg of the finished compound absorbent is obtained, and C9-C10 isoalkanes are added to bring the total mass to 1000 kg.

[0076] Example 3 This embodiment provides a compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins, which is prepared by the following steps: 750 kg of a compound of C6-C8 cycloalkanes and n-alkanes (cycloalkanes to n-alkanes mass ratio 1:1.5) and 200 kg of C9-C10 isoalkanes are added to a stirred tank and mixed evenly at a temperature of 28 ℃ and a pressure of 0.15 MPa; 3 kg of octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (antioxidant 1076) and 30 kg of polyisobutylene (molecular weight 600) are added, and mixing is continued until dissolved. The mixture is filtered to obtain 983 kg of the finished compound absorbent, and C9-C10 isoalkanes are added to bring the total mass to 1000 kg.

[0077] Example 4 This embodiment provides a system for recovering light hydrocarbons from high-pressure flare gas of polyolefins, such as... Figure 1 As shown, it includes a compression and impurity removal unit, an absorption and desorption purification unit, and a catalytic oxidation tail gas treatment unit connected in sequence through pipelines; The compression and impurity removal unit includes a dry gas compressor unit 1, a cooler 2, a gas-liquid separator 3, and a dehydration bed 4, which are used for pressurizing, cooling, dehydrating, and removing impurities from polyolefin high-pressure flare gas. The absorption-desorption purification unit adopts a dual-tower absorption-desorption coupling structure, including absorption tower 5, desorption tower 6, cold box 7, absorption tower reboiler 8, flash tank 9, lean and rich liquid heat exchanger 10, desorption tower reboiler 11, desorbed gas cooler 12, return liquid tank 13, lean liquid water cooler 14, lean liquid intercooler 15, product gas compressor 16, product gas cooler 17, and product separator 18. It is used to selectively absorb C2+ light hydrocarbons in the raw gas with a compound absorbent. After dual-tower coupled desorption and deep purification, C2-rich gas and C2-rich liquid products are produced simultaneously, and the absorbent is recycled. The catalytic oxidation tail gas treatment unit includes a heater 19, a catalytic oxidation reactor 20, a tail gas heat exchanger 21, and a chimney 22, and is used for the harmless treatment of non-condensable gases. Among them, the outlet of dry gas compressor unit 1 is connected to the inlet of cooler 2, the outlet of cooler 2 is connected to the inlet of gas-liquid separator 3, the gas phase outlet of gas-liquid separator 3 is connected to the inlet of dehydration bed 4, the outlet of dehydration bed 4 is connected to the inlet of cold box 7, and the outlet of cold box 7 is connected to the bottom inlet of absorption tower 5. The top outlet of the absorption tower 5 is connected to the middle inlet of the cold box 7, the middle outlet of the cold box 7 is connected to the first inlet of the tail gas heat exchanger 21, the bottom outlet of the absorption tower 5 is connected to the inlet of the flash tank 9, the bottom outlet of the flash tank 9 is connected to the first inlet of the lean and rich liquid heat exchanger 10, and the first outlet of the lean and rich liquid heat exchanger 10 is connected to the middle inlet of the desorption tower 6; the top outlet of the flash tank 9 is connected to the upper inlet of the cold box 7, and the upper outlet of the cold box 7 is connected to the inlet of the product gas compressor 16.

[0078] The top outlet of desorption tower 6 is connected to the inlet of desorbed gas cooler 12, the outlet of desorbed gas cooler 12 is connected to the inlet of return liquid tank 13, the gas phase outlet of return liquid tank 13 is connected to the inlet of product gas compressor 16, the outlet of product gas compressor 16 is connected to the inlet of product gas cooler 17, and the outlet of product gas cooler 17 is connected to the inlet of product separator 18. The bottom outlet of desorption tower 6 is sequentially connected to lean and rich liquid heat exchanger 10, lean liquid water cooler 14, and lean liquid intercooler 15. The outlet of lean liquid intercooler 15 is connected to the top inlet of absorption tower 5. The bottom outlet of desorption tower 6 is also connected to the inlet of desorption tower reboiler 11, and the outlet of desorption tower reboiler 11 is connected to the bottom inlet of desorption tower 6. The first outlet of the exhaust gas heat exchanger 21 is connected to the inlet of the heater 19, the outlet of the heater 19 is connected to the inlet of the catalytic oxidation reactor 20, the outlet of the catalytic oxidation reactor 20 is connected to the second inlet of the exhaust gas heat exchanger 21, and the second outlet of the exhaust gas heat exchanger 21 is connected to the chimney 22.

[0079] In this embodiment, when using the system, first check the equipment sealing, absorbent reserves, and the status of the online monitoring device. Prepare the compound absorbent according to the mass fraction and fill the circulation pipeline. After startup, perform six steps in sequence: compression and impurity removal, low-temperature oil absorption, flash evaporation pretreatment, desorption purification and absorbent regeneration, product separation, and catalytic oxidation tail gas purification. The raw gas is pressurized by the dry gas compressor unit 1 and cooled and impurity removed. It then selectively absorbs C2+ light hydrocarbons in the absorption tower by counter-current contact with the lean absorbent. The rich liquid enters the desorption tower for desorption after flash evaporation and heat exchange between the lean and rich liquids. The carbon-rich gas produced at the top of the desorption tower is compressed and separated to obtain product gas and product liquid. The lean liquid at the bottom of the tower is recycled after multi-stage cooling. The non-condensable gas at the top of the absorption tower is discharged after catalytic oxidation. During operation, it is necessary to monitor key parameters such as pressure, temperature, recovery rate, and tail gas VOCs concentration. When shutting down, gradually reduce the load first and then shut down the equipment in sequence.

[0080] Example 5 This embodiment provides a method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. It uses the compound absorbent for recovering light hydrocarbons from high-pressure flare gas of polyolefins as described in Example 1 and the system for treating high-pressure flare gas byproducts of LDPE as described in Example 4. The total raw material quantity is 7.85 t / h. The main components and their contents are shown in Table 1. Table 1. Composition of the high-pressure flare gas produced as a byproduct of LDPE in Example 5 Specifically, the following steps are included: S1. Compression and impurity removal: The high-pressure flare gas produced by LDPE is pressurized to 1.6 MPa by a dry compressor unit, cooled to 40 ℃ by a cooler, and sent to a gas-liquid separator to remove residual droplets. Then, it is removed by a dehydration bed to remove free water and liquid impurities, resulting in pretreated gas. S2. Absorption: The pretreated gas is cooled to 5°C by heat exchange in a cold box and then sent to the bottom of the absorption tower, where it comes into countercurrent contact with the lean absorbent (compound absorbent) sprayed from the top of the absorption tower. The mass ratio of the compound absorbent circulation rate to the high-pressure flare gas produced by LDPE is 10:1. C2+ light hydrocarbons such as ethylene, ethane, butene, butane, C5, and C6 are selectively absorbed. Non-condensable gases containing nitrogen, hydrogen, methane, and oxygen are discharged from the top of the absorption tower, while a rich absorbent is produced at the bottom of the absorption tower. S3, Flash Evaporation: The rich absorbent enters the flash tank for flash evaporation. The C2+ light hydrocarbons flashed out enter the recovery unit after heat exchange in the cold box. The rich absorbent after flash evaporation enters the desorption process. S4. Desorption: After flash evaporation, the rich absorbent is preheated to about 90°C by a lean-rich liquid heat exchanger and then sent to the middle of the desorption tower. The desorption tower is heated to 130°C by a reboiler to desorb and purify the C2+ light hydrocarbons in the rich liquid. The carbon-rich gas produced at the top of the desorption tower is cooled by a desorbed gas cooler and then enters a return liquid tank for gas-liquid separation. The lean absorbent produced at the bottom of the desorption tower is cooled in two stages by a lean liquid water cooler and a lean liquid intercooler and then returned to the absorption tower for recycling. S5. Product separation: After being pressurized by the product gas compressor and cooled by the product gas cooler, the carbon-rich gas enters the product separation tank. The top of the product separation tank gets C2+-rich product gas, and the bottom of the separation tank gets C2+-rich product liquid. S6. Catalytic Oxidation: The non-condensable gas at the top of the absorption tower is preheated by the tail gas heat exchanger and then heated to 300 ℃ by the heater before being sent to the catalytic oxidation reactor. Under the action of the catalyst, a catalytic oxidation reaction occurs to remove residual hydrocarbons, carbon monoxide, and hydrogen. After the reaction, the high-temperature flue gas is cooled by recovering waste heat through the tail gas heat exchanger and then discharged through the chimney. A continuous emission monitoring system and an online volatile organic compound analyzer are installed at the chimney outlet for real-time monitoring.

[0081] Table 2 shows the method for recovering light hydrocarbons from polyolefin high-pressure flare gas in this embodiment and the various indicators of the recovered products. Table 2. Data on various indicators of the polyolefin high-pressure flare gas light hydrocarbon recovery method and the recovered products in Example 5. As shown in Table 2, in this embodiment, the C2 recovery rate reached 98.39%, the C4+ recovery rate reached 98.53%, the nitrogen co-absorption rate was only 0.45%, and the total flue gas flow rate after catalytic oxidation purification was approximately 5900 Nm³. 3 / h, the composition meets the following requirements: nitrogen 94.9 mol%, carbon dioxide 0.8 mol%, oxygen 3.0 mol%, water 1.4 mol%, VOCs emission concentration 18 mg / m³ 3 It fully complies with the requirements of GB31571-2015 standard.

[0082] Example 6 This embodiment provides a method for recovering light hydrocarbons from polyolefin high-pressure flare gas. It uses the compound absorbent for recovering light hydrocarbons from polyolefin high-pressure flare gas from Example 1 and the system for treating LDPE by-product high-pressure flare gas from Example 4. The total feed rate is 5.495 t / h, and the main components and their contents are the same as in Example 5. Specifically, the following steps are included: S1. Compression and impurity removal: Same as in Example 5, except that the high-pressure flare gas produced by LDPE is pressurized to 1.5 MPa by a dry gas compressor unit; S2, Absorption: Same as in Example 5, except that the pretreated gas is cooled to 5°C by heat exchange in a cold box; S3, flash evaporation: Same as in Example 5; S4. Desorption: Same as in Example 5, except that the absorbent liquid after flash evaporation is heated to 128°C by a reboiler; S5. Product separation: Same as Example 5; S6, Catalytic oxidation: Same as in Example 5, except that the non-condensable gas is heated to 295°C by a heater; Table 3 shows the method for recovering light hydrocarbons from polyolefin high-pressure flare gas in this embodiment and the various indicators of the recovered products. Table 3. Data on various indicators of the polyolefin high-pressure flare gas light hydrocarbon recovery method and the recovered products in Example 6. As shown in Table 3, the environmental protection index in this embodiment is: 4130 Nm³ of purified flue gas. 3 / h, VOCs emission concentration 17mg / m³ 3 It still meets the national emission standards.

[0083] Example 7 This embodiment provides a method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. It uses the compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins as described in Example 1 and the system for processing the feed gas as described in Example 4. The total feed gas volume is 7.85 t / h. The main components and their contents are shown in Table 4. Table 4 Composition of the raw gas in Example 7 Specifically, the following steps are included: S1. Compression and impurity removal: Same as in Example 5; S2. Absorption: Same as in Example 5, except that the pretreated gas is cooled to 4°C by heat exchange in a cold box, and the mass ratio of the compound absorbent circulation rate to the LDPE by-product high-pressure flare gas is 11:1. S3, flash evaporation: Same as in Example 5; S4. Desorption: Same as in Example 5, except that the absorbent liquid after flash evaporation is heated to 132°C by a reboiler; S5. Product separation: Same as Example 5; S6, Catalytic oxidation: Same as in Example 5, except that the non-condensable gas is heated to 305°C by a heater; Table 5 shows the method for recovering light hydrocarbons from polyolefin high-pressure flare gas in this embodiment and the various indicators of the recovered products. Table 5. Data on various indicators of the polyolefin high-pressure flare gas light hydrocarbon recovery method and the recovered products in Example 7. As can be seen from the data in Table 5, this embodiment still maintains efficient and stable operation under high nitrogen conditions.

[0084] Comparative Example 1 This comparative example provides a method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. It employs a traditional single-tower oil absorption unit and an industrial C5 fraction absorbent to treat the high-pressure flare gas produced as a byproduct of LDPE. The total feed rate is 7.85 t / h. The main components and their contents are shown in Table 1. Specifically, the following steps are included: The high-pressure flare gas produced by LDPE directly enters the bottom of the absorption tower of a traditional single-tower oil absorption unit and comes into countercurrent contact with the C5 absorbent sprayed at the top of the tower; the non-condensable gas at the top of the absorption tower is directly discharged; the rich liquid at the bottom of the tower is heated and desorbed, and the desorbed gas is cooled and directly output as a mixed product. There is no pre-compression to remove impurities, no catalytic oxidation tail gas treatment, no lean and rich liquid heat exchange, and no separation and purification.

[0085] Table 6 shows the various indicators of the polyolefin high-pressure flare gas light hydrocarbon recovery method and the recovered products in this comparative example. Table 6. Data on various indicators of the light hydrocarbon recovery method and recovered products from polyolefin high-pressure flare gas in Comparative Example 1. The data in Table 6 show that, with the traditional single-tower unit and C5 absorbent, the C2 recovery rate is only 86.72%; no high-purity product is produced, and it can only be used as a low-calorific-value fuel; the VOCs emission concentration is 124 mg / m³. 3 The levels of nitrogen were severely exceeded, with a co-absorption rate as high as 4.3%, resulting in high energy consumption and poor operational stability of the equipment.

[0086] Comparative Example 2 This comparative example provides a method for recovering light hydrocarbons from high-pressure flare gas of polyolefins. The system and industrial crude diesel absorbent in Example 4 are used to treat the high-pressure flare gas by-product of LDPE. The total feed amount is 7.85 t / h, and the main components and contents are the same as in Example 5.

[0087] Specific steps: Same as in Example 5, except that the absorbent is replaced with crude diesel oil.

[0088] Table 7 shows the various indicators of the light hydrocarbon recovery method and the recovered products in this comparative example of polyolefin high-pressure flare gas: Table 7. Data on various indicators of the polyolefin high-pressure flare gas light hydrocarbon recovery method and the recovered products in Comparative Example 2. The data in Table 7 show that even with the complete system of this invention, simply replacing the absorbent with traditional crude diesel reduces the C2 recovery rate to 90.45%, the nitrogen co-absorption rate to 3.2%, and the VOCs emission concentration to 28 mg / m³. 3 The slightly excessive levels of the compound absorbent, indicating insufficient product purity for direct reuse, demonstrate the crucial role of the compound absorbent provided by this invention in achieving superior technical results.

[0089] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas in polyolefins, characterized in that, Included by weight parts: The main absorber component consists of 65-75 parts, which is a complex of C6-C8 cycloalkanes and C6-C8 n-alkanes. The absorption aid component consists of 20-25 parts, which are C9-C10 isoalkanes; 0.3~0.8 parts of heat stabilizer, which is a hindered phenolic heat stabilizer; 3-5 parts of viscosity modifier, selected from at least one of polyα-olefin, polyisobutylene and ethylene-propylene copolymer.

2. The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins according to claim 1, characterized in that, In the main absorbent component, the mass ratio of C6-C8 cycloalkanes to C6-C8 n-alkanes is 1:(0.8-1.5).

3. The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins according to claim 1, characterized in that, The hindered phenolic heat stabilizer is selected from at least one of 2,6-di-tert-butyl-p-cresol, β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene and 2,4,6-tri-tert-butylphenol.

4. The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins according to claim 1, characterized in that, The molecular weight of the polyα-olefin is 300-500, the molecular weight of the polyisobutylene is 400-1000, and the molecular weight of the ethylene-propylene copolymer is 5000-50000.

5. The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins according to claim 1 or 2, characterized in that, The mass fractions of each component in the compound absorbent are: 70 parts of a compound of C6-C8 cycloalkanes and C6-C8 n-alkanes, wherein the mass ratio of the C6-C8 cycloalkanes to the C6-C8 n-alkanes is 1:1.2; 23 portions of C9~C10 isoalkanes; 0.5 parts of 2,6-di-tert-butyl-p-cresol; 6.5 parts of polyα-olefin, wherein the molecular weight of the polyα-olefin is 300-500.

6. A method for preparing a compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins as described in any one of claims 1-5, characterized in that, The process includes the following steps: mixing the main absorbent component and the auxiliary absorbent component evenly at a temperature of 15~30 ℃ and a pressure of 0.1~0.15 MPa, then adding the heat stabilizer and the viscosity modifier and continuing to mix until completely dissolved, followed by filtration to obtain the compound absorbent for the recovery of light hydrocarbons from high-pressure flare gas of polyolefins.

7. A system for recovering light hydrocarbons from high-pressure flare gas of polyolefins, characterized in that, It includes a compression and impurity removal unit, an absorption and desorption purification unit, and a catalytic oxidation tail gas treatment unit connected in sequence through pipelines; The compression and impurity removal unit includes a dry gas compressor unit (1), a cooler (2), a gas-liquid separator (3), and a dehydration bed (4), which are connected in series. The absorption and desorption purification unit includes an absorption tower (5), a desorption tower (6), a flash tank (9), a lean and rich liquid heat exchanger (10), a lean liquid cooler, and a product separator (18). The bottom outlet of the absorption tower (5), the flash tank (9), the lean and rich liquid heat exchanger (10), and the middle inlet of the desorption tower (6) are connected in series. The bottom outlet of the desorption tower (6), the lean liquid cooler, and the top inlet of the absorption tower (5) are connected in series to form an absorbent circulation loop. The top outlet of the desorption tower (6) is connected to the product separator (18). The catalytic oxidation tail gas treatment unit includes a tail gas heat exchanger (21), a heater (19), a catalytic oxidation reactor (20), and a chimney (22). The top outlet of the absorption tower (5), the cold medium inlet of the tail gas heat exchanger (21), the heater (19), and the catalytic oxidation reactor (20) are connected in sequence. The outlet of the catalytic oxidation reactor (20) is also connected to the hot medium inlet of the tail gas heat exchanger (21), and the hot medium outlet of the tail gas heat exchanger (21) is connected to the chimney (22). The outlet of the dewatering bed (4) is connected to the bottom inlet of the absorption tower (5).

8. The system for recovering light hydrocarbons from high-pressure flare gas of polyolefins according to claim 7, characterized in that, The absorption, desorption, and purification unit further includes a product gas compressor (16) and a product gas cooler (17). The product gas compressor (16) is located between the top outlet of the desorption tower (6) and the product separator (18), and the product gas cooler (17) is located between the product gas compressor (16) and the product separator (18). And / or, the lean solution cooler includes a lean solution water cooler (14) and a lean solution intercooler (15) connected in sequence; And / or, the catalytic oxidation tail gas treatment unit further includes a continuous emission monitoring system for flue gas and an online volatile organic compound (VOC) analysis device, both of which are located at the outlet of the chimney (22).

9. A method for recovering light hydrocarbons from high-pressure flare gas of polyolefins, characterized in that, The compound absorbent for light hydrocarbon recovery from high-pressure flare gas of polyolefins, as described in any one of claims 1-5, comprises the following steps: S1. Compression and impurity removal: The polyolefin high-pressure flare gas is sequentially subjected to pressurization, cooling, gas-liquid separation and dehydration to obtain pretreated gas; S2. Absorption: After cooling the pretreated gas, it is contacted counter-currently with a lean absorbent containing the compound absorbent to absorb C2+ light hydrocarbons, resulting in non-condensable gas and a rich absorbent. S3. Flash evaporation: Flash evaporation of the rich absorbent liquid separates some C2+ light hydrocarbons, resulting in a flash-evaporated rich absorbent liquid. S4. Desorption: After preheating the flash-evaporated rich absorbent, desorption is performed to obtain carbon dioxide-rich and lean absorbent. The lean absorbent is cooled and returned to step S2 for recycling. S5. Product separation: The carbon-rich gas is compressed, cooled and then separated into liquids to obtain C2+-rich product gas and C2+-rich product liquid. S6. Catalytic oxidation: The non-condensable gas obtained in step S2 is preheated and then subjected to catalytic oxidation. The flue gas after the reaction is discharged after the waste heat is recovered.

10. The method for recovering light hydrocarbons from high-pressure flare gas of polyolefins according to claim 9, characterized in that, In step S1, the pressurization pressure is 1.5~1.6 MPa; And / or, in step S2, the temperature of the pretreated gas after cooling is 4~6 ℃; And / or, in step S4, the temperature of the preheated absorbent liquid is 128~132 ℃; And / or, in step S6, the temperature of the preheated non-condensable gas is 295~305 ℃.

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