Polycrystalline silicon tail gas recovery system and method based on graded absorption and membrane separation coupling

Abstract

The invention discloses a polycrystalline silicon tail gas recovery system and method based on graded absorption and membrane separation coupling. The system comprises a pretreatment unit, a first absorption tower, a second absorption tower, a membrane separation and purification unit, an analysis and regeneration unit and a light component membrane removal unit. The pretreatment unit performs condensation, compression and gas-liquid separation on the reduction tail gas. The first absorption tower adopts high-purity silicon tetrachloride to remove hydrogen chloride; and the second absorption tower adopts an absorbent rich in trichlorosilane to deeply purify hydrogen. And refining the purified hydrogen through a membrane separator to obtain the electronic-grade high-purity hydrogen. Hydrogen chloride is desorbed from rich liquor from the two absorption towers in the desorption tower; and feeding the light-component-rich liquid extracted from the refluxing tank of the desorption tower into a pervaporation membrane assembly, selectively removing light components such as dichlorosilane and the like, and returning to the system. A traditional adsorption tower is omitted, the cryogenic energy consumption is reduced through graded absorption, and the industrial problem of light component circulation accumulation is solved while the high purity of hydrogen and hydrogen chloride products is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of polycrystalline silicon production technology, specifically a polycrystalline silicon tail gas recovery system and method based on the coupling of graded absorption and membrane separation. Background Technology

[0002] Polysilicon is a core material for the semiconductor and photovoltaic industries, and its mainstream production process is currently the modified Siemens process. During the reduction step of this process, a large amount of waste gas rich in hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and dichlorosilane is generated. To achieve resource recycling and reduce production costs, these waste gases must be efficiently separated and recovered.

[0003] Currently, the industrial process commonly employs a dry recovery method combining compression, cryogenics, absorption, and adsorption. A typical process, as described in Chinese patent CN112520697A, uses high-purity silicon tetrachloride (STC) as the absorbent to absorb HCl from the tail gas at a low temperature of approximately -45°C to -55°C. The hydrogen is then removed via an adsorption tower to remove trace amounts of chlorosilanes, ultimately yielding recovered hydrogen. While this process achieves material recovery, it suffers from the following significant drawbacks: (1) To meet the hydrogen quality requirements, the absorption process needs to maintain an extremely low temperature (below -55℃), resulting in huge energy consumption of the refrigeration unit; (2) Relying on the activated carbon adsorption tower for final purification, the adsorbent has a saturation problem and needs to be frequently regenerated, which not only increases energy consumption and operational complexity, but also causes fluctuations in hydrogen purity when the regeneration is incomplete, making it difficult to stably meet the production requirements of electronic-grade polysilicon; (3) During system operation, light components such as dichlorosilane (DCS) are prone to accumulate in the absorption-desorption cycle, affecting the operational stability and absorption efficiency of the absorption tower. Usually, some materials need to be discharged irregularly to control the concentration of light components, resulting in material loss and increasing the burden of subsequent processing.

[0004] Therefore, existing technologies suffer from problems such as high energy consumption, poor product purity and stability, and unstable system operation. There is an urgent need to develop a new exhaust gas recovery technology that can achieve higher product purity and lower overall energy consumption under milder conditions and can automatically solve the problem of light component accumulation. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a lactic acid cooling crystallization system and method, which aims to reduce the energy consumption of cryogenic treatment while eliminating easily deactivated adsorption units and actively and continuously removing light components in the system through technological integration and process innovation, thereby stably producing electronic-grade high-purity hydrogen and high-purity hydrogen chloride, and achieving the comprehensive goals of improving quality, increasing efficiency and reducing consumption.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation.

[0007] The system includes a pretreatment unit, a first absorption tower, a second absorption tower, a membrane separation and purification unit, a desorption and regeneration unit, a first absorbent circulation loop, a second absorbent circulation loop, and a light component membrane removal unit.

[0008] The pretreatment unit includes a multi-stage condenser, a compressor, and a gas-liquid separator connected in sequence, used for staged cooling, pressurization, and preliminary gas-liquid separation of the high-temperature reduction exhaust gas.

[0009] The first absorption tower has a first air inlet at its lower part, connected to the gas phase outlet of the gas-liquid separator; a first air outlet at its top; and a first rich liquid outlet at its bottom. A first spray device is located in the upper part of the tower, below which is a first packing layer. The first absorption tower is connected to a first absorbent circulation loop, which is used to send the rich liquid after absorbing hydrogen chloride to the desorption tower for regeneration, and to return the regenerated and cooled lean liquid to the tower for recycling. Specifically, the loop includes a first circulation pump, a first lean-rich liquid heat exchanger, and a first cooler arranged in sequence, and is connected to a first absorbent storage tank. The high-temperature lean liquid from the desorption tower is pre-cooled by heat exchange with the low-temperature rich liquid from the first absorption tower in the first lean-rich liquid heat exchanger, and then cooled to the target temperature (-40℃ to -30℃) by the first cooler before being stored in the first absorbent storage tank. Finally, it is sent into the tower through the first spray device. The first absorption tower uses high-purity silicon tetrachloride (STC) as the absorbent, mainly responsible for removing most of the hydrogen chloride from the tail gas.

[0010] The second absorption tower has a second air inlet at the bottom, connected to the first air outlet, a second air outlet at the top, and a second rich liquid outlet at the bottom. A second spray device is located in the upper part of the tower, below which is a second packing layer. The second absorption tower is connected to a second absorbent circulation loop, similar in structure to the first loop, including a second circulation pump, a second lean-rich liquid heat exchanger, and a cryogenic unit, and is connected to a second absorbent storage tank. This loop allows the absorbent to regenerate and cool to a lower temperature (-70°C to -60°C). The second absorption tower uses a chlorosilane mixture rich in trichlorosilane (TCS) as the absorbent, responsible for deep purification of the hydrogen after the first-stage dechlorination, removing residual trace amounts of chlorosilane.

[0011] The membrane separation and purification unit includes a membrane separator, which has a feed gas inlet connected to the second gas outlet, a first permeate gas outlet for outputting high-purity hydrogen, and a residual gas outlet for outputting impurity-enriched gases. The first permeate gas outlet is connected to a high-purity hydrogen buffer tank. This unit utilizes the significant difference in permeation rates between hydrogen and other impurity gases in the membrane to achieve terminal ultrapurification of hydrogen without the use of adsorbents, directly obtaining high-purity hydrogen that meets the requirements for electronic-grade polycrystalline silicon preparation.

[0012] The desorption and regeneration unit includes a desorption column, a reboiler, a condenser, and a reflux tank. The upper part of the desorption column has an inlet for receiving rich solutions from the two absorption columns, a gas phase outlet at the top, and a lean solution outlet at the bottom. The reboiler provides a heat source for the desorption column, causing the hydrogen chloride in the rich solution to desorb. The desorbed hydrogen chloride gas is condensed into liquid by the condenser and then enters the reflux tank. The lower part of the reflux tank has a reflux port, which returns most of the liquid to the top of the desorption column as reflux; its sidewall has a conventional sampling outlet and a higher-positioned light component sampling outlet.

[0013] The light component membrane removal unit is the core innovation of this invention in solving the problem of light component accumulation. It includes a pervaporation membrane module, a condenser, and a light component collection tank. The inlet of the pervaporation membrane module is connected to the light component outlet of the reflux tank. The liquid rich in light components such as dichlorosilane (DCS) drawn from the top of the reflux tank enters the membrane module. Under the selective action of the membrane, the light components preferentially vaporize and permeate through the membrane. After condensation, it is collected in the light component collection tank and can be sent to the distillation system for processing. The residual liquid after the light components are removed is returned to the absorbent storage tank. This process achieves active, continuous, and selective removal of light components from the circulating system, fundamentally avoiding their accumulation.

[0014] In addition, the present invention also provides a method for recovering polycrystalline silicon tail gas using the above system, which specifically includes the following steps: S1. Preprocessing: After the reduction tail gas is cooled by a multi-stage condenser and compressed by a compressor, it enters a gas-liquid separator to separate most of the chlorosilane condensate, and obtains non-condensable gas containing mainly hydrogen, hydrogen chloride and a small amount of chlorosilane. S2, First-stage absorption: The non-condensable gas obtained in step S1 is sent into the first absorption tower through the first air inlet and comes into countercurrent contact with high-purity silicon tetrachloride absorbent from the first absorbent storage tank at a temperature of -40°C to -30°C in the first packing layer to remove most of the hydrogen chloride and form the first rich liquid. S3, Second-level absorption: The gas processed in step S2 is sent into the second absorption tower through the second inlet. It comes into countercurrent contact with the trichlorosilane-rich absorbent from the second absorbent storage tank at a temperature of -70°C to -60°C in the second packing layer to deeply remove chlorosilane impurities and form the second rich liquid. S4. Membrane separation and purification: The hydrogen gas processed in step S3 is fed into the membrane separator from the feed gas inlet. Driven by the pressure difference, the hydrogen gas permeates through the membrane and is discharged from the first permeate gas outlet to obtain high-purity hydrogen gas. S5. Absorbent regeneration and light component removal: The first rich liquid formed in step S2 and the second rich liquid formed in step S3 are fed into the desorption tower and desorbed under the heating of the reboiler. The desorbed hydrogen chloride gas is condensed by the second condenser and then enters the reflux tank. At the same time, a portion of the liquid is taken out from the light component outlet of the return tank and sent to the pervaporation membrane module, so that the light component selectively permeates through the membrane and is condensed and collected by the first condenser. The residual liquid after the light component is removed is returned to the first absorbent storage tank or the second absorbent storage tank. S6. Recycling: The lean liquid regenerated at the bottom of the analytical column is discharged from the lean liquid outlet. Part of it exchanges heat with the first rich liquid in the first lean-rich liquid heat exchanger and is cooled by the first cooler before entering the first absorbent storage tank. The other part exchanges heat with the second rich liquid in the second lean-rich liquid heat exchanger and is cooled by the cryogenic unit before entering the second absorbent storage tank.

[0015] The polycrystalline silicon tail gas recovery system and method based on the coupling of graded absorption and membrane separation provided by this invention have the following beneficial effects by adopting the above structure and method: (1) Through the dual guarantee of graded deep absorption and membrane terminal purification, the adsorption tower that is easy to fail and needs to be regenerated in the traditional process is completely eliminated, and electronic grade high-purity hydrogen with a total impurity content of less than 1 ppm can be stably and continuously produced, while obtaining high-purity hydrogen chloride products. (2) By adopting a staged absorption strategy, the operating temperature of the main dechlorination process is increased from below -55℃ in the traditional process to -40℃ to -30℃, which greatly reduces the cryogenic load. At the same time, the membrane separation process operates at near room temperature with extremely low energy consumption. In addition, the lean and rich liquid heat exchange design fully recovers the cold and heat in the system. (3) A pervaporation membrane removal unit was introduced and coupled with the light component outlet of the reflux tank. Light components such as dichlorosilane can be separated from the circulating liquid of the system online, continuously and selectively, ensuring the stability of the absorbent composition in the absorption tower, thereby maintaining efficient and stable absorption conditions and avoiding periodic operation adjustments or material discharge losses caused by the accumulation of light components. Attached Figure Description

[0016] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the overall system structure of the present invention.

[0017] In the diagram: multi-stage condenser 101, compressor 102, gas-liquid separator 103, first absorption tower 200, first air inlet 201, first air outlet 202, first rich liquid outlet 203, first spray device 204, first packing layer 205, first absorbent storage tank 206, first circulating pump 207, first lean-rich liquid heat exchanger 208, cooler 209, second absorption tower 300, second air inlet 301, second air outlet 302, second rich liquid outlet 303, second spray device 304, second packing layer 305, second absorbent storage tank 306, second circulating pump 307, and so on. 308 Lean and rich liquid heat exchanger, 309 cryogenic unit, 401 membrane separator, 402 feed gas inlet, 403 first permeate outlet, 404 high-purity hydrogen buffer tank, 405 residual gas outlet, 501 pervaporation membrane module, 502 feed inlet, 503 residual liquid outlet, 504 second permeate outlet, 505 first condenser, 506 light component collection tank, 601 stripping tower, 602 liquid inlet, 603 gas phase outlet, 604 reflux tank, 605 lean liquid outlet, 606 reflux port, 607 sampling outlet, 608 light component sampling outlet, 609 reboiler, 610 second condenser. Detailed Implementation

[0018] Example 1: The overall process flow diagram of the system of this invention is as follows: Figure 1 As shown, its core lies in the efficient and low-consumption recovery of polycrystalline silicon tail gas through the synergistic coupling of units such as pretreatment, two-stage absorption, membrane separation, desorption regeneration and light component removal.

[0019] Example 1: Standard Production Mode This embodiment adopts Figure 1 The standard configuration system shown is described in detail, along with its process flow and operating conditions.

[0020] Step 1: Tail Gas Pretreatment. Tail gas from the modified Siemens reduction furnace first enters a multi-stage condenser 101 consisting of three heat exchangers connected in series. The first stage uses circulating cooling water to cool the gas to approximately 70°C; the second stage uses chilled water at approximately 7°C to cool it to approximately 15°C; and the third stage uses an ethylene glycol solution at approximately -20°C for deep cooling to approximately -5°C. During this stepped condensation process, most of the trichlorosilane and silicon tetrachloride components in the tail gas are condensed into liquids. The gas-liquid mixture then enters a gas-liquid separator 103, where the separated chlorosilane condensate is sent as an intermediate product to the distillation system. The separated non-condensable gas, mainly composed of hydrogen, hydrogen chloride, and trace amounts of gaseous chlorosilanes, then enters a compressor 102 and is pressurized to approximately 1.3 MPa.

[0021] Step 2: First-stage absorption (hydrogen chloride removal). The pressurized gas enters the first absorption tower 200 through the first inlet 201 at the bottom. Simultaneously, high-purity silicon tetrachloride liquid (purity ≥99.9%) stored in the first absorbent storage tank 206 is pumped out, first flowing through the shell side of the first lean-rich liquid heat exchanger 208 for preliminary cooling, and then entering the first cooler 209. In the first cooler 209, chilled brine at approximately -35°C is used as a refrigerant to precisely cool the silicon tetrachloride absorbent to the target temperature of -35°C. The cooled, low-temperature absorbent is evenly sprayed downwards through the first spray device 204 at the top of the tower, making full counter-current contact with the upward-flowing gas in the first packing layer 205 within the tower. Under these conditions, most of the hydrogen chloride in the gas is selectively absorbed by silicon tetrachloride. The hydrogen-rich gas (hydrogen chloride content less than 100 ppm) after hydrogen chloride removal is discharged from the first outlet 202 at the top of the tower. A silicon tetrachloride-rich liquid rich in hydrogen chloride is formed at the bottom of the tower, with a temperature still around -35°C, and is pumped out by the first circulation pump 207.

[0022] Step 3: Second-stage absorption (deep purification). Hydrogen-rich gas from the first absorption tower 200 enters the second inlet 301 at the bottom of the second absorption tower 300. The absorbent from the second absorbent storage tank 306 is a chlorosilane mixture rich in trichlorosilane (trichlorosilane mass fraction ≥90%). This absorbent first flows through the shell side of the second lean-rich liquid heat exchanger 308 for pre-cooling, and then enters the cryogenic unit 309. The cryogenic unit 309 provides cooling below -70°C, deeply cooling the absorbent to the target temperature of -65°C. The cooled absorbent is sprayed down through the second spray device 304 at the top of the tower, making deep counter-current contact with the gas within the second packing layer 305. This process mainly utilizes the strong solubility of trichlorosilane for similar chlorosilane impurities to further remove trace amounts of dichlorosilane and other impurities from the hydrogen. The high-purity hydrogen after this deep purification is discharged from the second outlet 302 at the top of the tower.

[0023] Step 4: Membrane separation terminal purification. Deeply purified hydrogen enters the feed gas inlet 402 of the membrane separator 401. In this embodiment, the membrane separator 401 uses a built-in polyimide hollow fiber membrane module. Operating conditions are: feed gas side pressure approximately 1.8 MPa, permeate gas side pressure approximately 0.3 MPa, and temperature approximately 40°C. Driven by this pressure difference, hydrogen preferentially permeates through the membrane, obtaining highly pure product hydrogen on the permeate side. This hydrogen is drawn out from the first permeate gas outlet 403 and enters the high-purity hydrogen buffer tank 404, directly meeting the requirements for electronic-grade polysilicon production. Unpermeated residual gas (rich in impurities) is discharged from the residual gas outlet 405. In this embodiment, this gas is recycled back to the second inlet 301 of the second absorption tower 300.

[0024] Step 5: Regeneration and Online Removal of Light Components. The rich solutions from the first absorption tower 200 and the second absorption tower 300 are preheated after flowing through the tubes of the first lean-rich solution heat exchanger 208 and the second lean-rich solution heat exchanger 308, respectively, and then mixed and enter the inlet 602 in the upper part (approximately 40% of the tower height) of the stripping tower 601. The bottom of the stripping tower 601 is heated by a reboiler 609 (e.g., 0.3 MPa saturated steam), controlling the bottom temperature at approximately 125°C and the operating pressure at approximately 0.5 MPa. Inside the tower, hydrogen chloride in the rich solution is desorbed by heating. The desorbed high-purity hydrogen chloride gas is drawn from the top gas phase outlet 603, condensed into liquid by the second condenser 610, and then enters the reflux tank 604. A portion of the condensate is pumped back to the top of the stripping tower 601 as reflux liquid from the reflux port 606 at the bottom of the reflux tank 604, while the other portion can be collected as product. The key innovation of this invention lies in the continuous extraction of a liquid rich in dichlorosilane, accounting for approximately 10% of the total liquid volume in the reflux tank 604, from the light component extraction outlet 608 on the upper side wall of the reflux tank 604, and feeding it into the inlet 502 of the pervaporation membrane module 501. This membrane module uses a polydimethylsiloxane composite membrane and operates at an operating temperature of approximately 60°C and a low permeate-side pressure (approximately 5 kPa). Light components such as dichlorosilane preferentially vaporize and permeate through the membrane. The permeate gas is drawn out from the second permeate gas outlet 504, condensed by the first condenser 505, and collected in the light component collection tank 506, where the concentration of light components is significantly enriched, facilitating subsequent processing. The residual liquid after the removal of light components is returned to the second absorbent storage tank 306 from the residual liquid outlet 503, thereby achieving active, continuous, and selective removal of light components from the circulating system.

[0025] Step 6: Absorbent Cooling and Circulation. The high-temperature lean liquid (approximately 130°C) regenerated at the bottom of the stripping tower 601 is discharged from the lean liquid outlet 605 and splits into two streams. One stream enters the shell side of the first lean-rich liquid heat exchanger 208, where it undergoes countercurrent heat exchange with the cold rich liquid (-35°C, tube side) from the first absorption tower 200. After the temperature drops to approximately 10°C, it is further cooled to -35°C by the first cooler 209 and finally returns to the first absorbent storage tank 206, completing the first absorbent circulation. The other stream enters the shell side of the second lean-rich liquid heat exchanger 308, where it exchanges heat with the cold rich liquid (-65°C, tube side) from the second absorption tower 300. After the temperature drops to approximately -20°C, it is further cooled to -65°C by the cryogenic unit 309 and finally returns to the second absorbent storage tank 306, completing the second absorbent circulation.

[0026] Example 2: Energy Saving Optimization and Configuration Adjustment This embodiment, based on the system architecture of Embodiment 1, adjusts some operating parameters and internal logistics paths to demonstrate the system's flexibility and energy-saving potential.

[0027] The main adjustments are as follows: Operating parameters optimization: To reduce energy consumption, the operating temperature of the first absorption tower 200 was appropriately increased from -35℃ to -30℃; the operating pressure difference of the membrane separator 401 was appropriately reduced from 1.5MPa; at the same time, the extraction ratio of the light component membrane removal unit was reduced from 10% to 8%.

[0028] Logistics path adjustment: The permeate outlet 405 of the membrane separator 401 is connected to the inlet pipe of the compressor 102, so that this part of the gas is returned to the front end of the system for reprocessing, aiming to improve the overall material recovery rate. At the same time, the permeate produced by the pervaporation membrane module 501 is diverted from the permeate outlet 503 to the first absorbent storage tank 206 as a supplement to the absorbent in this loop.

[0029] Operation and Results: The system operates according to the adjusted parameters and path. Although the first-stage absorption temperature is increased, the purity of the final hydrogen produced still meets stringent electronic-grade standards due to the dual protection of staged absorption and membrane separation. The adjusted operating conditions result in significant energy savings. The light component membrane removal unit continues to operate at a lower recovery rate, effectively controlling the concentration of light components within the system and maintaining the high efficiency and stability of the absorption tower. Different material flow configurations demonstrate that the system of this invention can be flexibly adapted to actual plant layouts and material balance requirements, exhibiting good engineering practicality.

Claims

1. A polycrystalline silicon tail gas recovery system based on the coupling of staged absorption and membrane separation, characterized in that, include: The pretreatment unit includes a multi-stage condenser (101), a compressor (102), and a gas-liquid separator (103) connected in sequence. The first absorption tower (200) has a first air inlet (201) at the bottom for receiving the gas output from the gas-liquid separator (103), a first air outlet (202) at the top, a first rich liquid outlet (203) at the bottom, a first spray device (204) at the upper part of the tower, and a first packing layer (205) below the first spray device (204). The second absorption tower (300) has a second air inlet (301) at the bottom that communicates with the first air outlet (202), a second air outlet (302) at the top, a second rich liquid outlet (303) at the bottom, a second spray device (304) at the upper part of the tower, and a second packing layer (305) below the second spray device (304). The membrane separation purification unit includes a membrane separator (401), which is provided with a raw material gas inlet (402) connected to a second gas outlet (302), a first permeate gas outlet (403) for outputting high-purity hydrogen gas, and a permeate gas outlet (405) for outputting impurity-rich gas. The analysis and regeneration unit includes an analysis tower (601) and a reflux tank (604). The analysis tower (601) has an inlet (602) for receiving the rich absorbent liquid in the upper part of the tower body, a gas phase outlet (603) at the top of the tower, and a lean liquid outlet (605) at the bottom of the tower. The gas phase outlet (603) is connected to the top inlet of the reflux tank (604) through a second condenser (610). The reflux tank (604) has a reflux port (606) at the bottom and returns to the top of the analysis tower (601) through a pipeline. Its side wall has a sampling outlet (607) and a light component sampling outlet (608). The first absorbent circulation loop is used to regenerate and recycle the absorbent of the first absorption tower (200); The second absorbent circulation loop is used to regenerate and recycle the absorbent in the second absorption tower (300); The light component membrane removal unit includes a pervaporation membrane assembly (501), which is provided with an inlet (502) connected to the light component collection outlet (608), a residual liquid outlet (503), and a second permeate outlet (504) connected to the light component collection tank (506) through a first condenser (505).

2. The polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation according to claim 1, characterized in that, The path of the first absorbent circulation loop is as follows: The first rich liquid outlet (203) at the bottom of the first absorption tower (200) is connected to the liquid inlet (602) of the desorption tower (601) through the first circulation pump (207) and the first channel of the first lean and rich liquid heat exchanger (208); The lean liquid outlet (605) of the analytical tower (601) is connected to the second channel inlet of the first lean and rich liquid heat exchanger (208), and the second channel outlet of the first lean and rich liquid heat exchanger (208) is connected to the first spray device (204) of the first absorption tower (200) through the first cooler (209) and the first absorbent storage tank (206) in sequence.

3. The polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation according to claim 2, characterized in that: In the first absorbent circulation loop, the rich liquid from the first absorption tower (200) flows through the first channel of the first lean-rich liquid heat exchanger (208), and the lean liquid from the desorption tower (601) flows through the second channel of the first lean-rich liquid heat exchanger (208). The two fluids exchange heat in countercurrent in the first lean-rich liquid heat exchanger (208).

4. The polycrystalline silicon tail gas recovery system based on graded absorption and membrane separation coupling according to claim 1, characterized in that, The path of the second absorbent circulation loop is as follows: The second rich liquid outlet (303) at the bottom of the second absorption tower (300) is connected to the liquid inlet (602) of the desorption tower (601) through the first channel of the second circulating pump (307) and the second lean and rich liquid heat exchanger (308); The lean liquid outlet (605) of the analytical tower (601) is connected to the second channel inlet of the second lean and rich liquid heat exchanger (308), and the second channel outlet of the second lean and rich liquid heat exchanger (308) is connected to the second spray device (304) of the second absorption tower (300) in sequence through the cryogenic unit (309) and the second absorbent storage tank (306).

5. A polycrystalline silicon tail gas recovery system based on graded absorption and membrane separation coupling according to claim 4, characterized in that: In the second absorbent circulation loop, the rich liquid from the second absorption tower (300) flows through the first channel of the second lean-rich liquid heat exchanger (308), and the lean liquid from the desorption tower (601) flows through the second channel of the second lean-rich liquid heat exchanger (308). The two fluids exchange heat in countercurrent in the second lean-rich liquid heat exchanger (308).

6. The polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation according to claim 1, characterized in that: The permeate outlet (405) of the membrane separator (401) is connected to the second air inlet (301) of the second absorption tower (300) and the inlet of the compressor (102).

7. The polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation according to claim 1, characterized in that: The residual liquid outlet (503) of the light component membrane removal unit is connected to the first absorbent storage tank (206) and the second absorbent storage tank (306).

8. The polycrystalline silicon tail gas recovery system based on the coupling of graded absorption and membrane separation according to claim 1, characterized in that: The liquid inlet (602) of the analytical column (601) is located at 1 / 3 to 1 / 2 of the column height. A rectification section is formed above the liquid inlet (602), and a stripping section is formed below it.

9. The method for recovering polycrystalline silicon tail gas according to any one of claims 1-8, characterized in that... Includes the following steps: S1. Preprocessing: After the reduction tail gas is cooled by a multi-stage condenser (101) and compressed by a compressor (102), it enters a gas-liquid separator (103) to separate most of the chlorosilane condensate, and obtains a non-condensable gas containing mainly hydrogen, hydrogen chloride and a small amount of chlorosilane. S2, First-stage absorption: The non-condensable gas obtained in step S1 is sent from the first air inlet (201) into the first absorption tower (200), and comes into countercurrent contact with the high-purity silicon tetrachloride absorbent from the first absorbent storage tank (206) at a temperature of -40°C to -30°C in the first packing layer (205) to remove most of the hydrogen chloride and form the first rich liquid. S3, Second-level absorption: The gas processed in step S2 is sent from the second inlet (301) into the second absorption tower (300), and comes into countercurrent contact with the trichlorosilane-rich absorbent from the second absorbent storage tank (306) at a temperature of -70°C to -60°C in the second packing layer (305) to deeply remove chlorosilane impurities and form a second rich liquid. S4. Membrane separation and purification: The hydrogen gas processed in step S3 is fed into the membrane separator (401) from the raw gas inlet (402). Driven by the pressure difference, the hydrogen gas permeates through the membrane and is discharged from the first permeate outlet (403) to obtain high-purity hydrogen gas. S5. Absorbent regeneration and light component removal: The first rich liquid formed in step S2 and the second rich liquid formed in step S3 are fed into the desorption tower (601) and desorbed under the heating of the reboiler (609). The desorbed hydrogen chloride gas is condensed by the second condenser (610) and then enters the reflux tank (604). Meanwhile, a portion of liquid is taken out from the light component outlet (608) of the return tank (604) and sent to the pervaporation membrane module (501) so that the light component selectively permeates through the membrane and is condensed and collected by the first condenser (505). The residual liquid after the light component is removed is returned to the first absorbent storage tank (206) or the second absorbent storage tank (306). S6. Recycling: The lean liquid regenerated at the bottom of the analytical column (601) is discharged from the lean liquid outlet (605). Part of it exchanges heat with the first rich liquid in the first lean-rich liquid heat exchanger (208) and is cooled by the first cooler (209) before entering the first absorbent storage tank (206). The other part exchanges heat with the second rich liquid in the second lean-rich liquid heat exchanger (308) and is cooled by the cryogenic unit (309) before entering the second absorbent storage tank (306).

10. The method for recovering polycrystalline silicon tail gas based on the coupling of graded absorption and membrane separation according to claim 9, characterized in that: In step S2, the operating pressure of the first absorption tower (200) is 1.2-1.4 MPa; In step S5, the proportion of liquid drawn from the reflux tank (604) to the pervaporation membrane module (501) is 5%-20% of the total liquid in the reflux tank (604), and the operating temperature of the pervaporation membrane module (501) is 40-80℃.

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