Tail gas pressure swing adsorption system and method for vinyl chloride production

Abstract

The invention discloses a vinyl chloride production tail gas pressure swing adsorption system and method, the system comprises a PSA-1 process recovery unit, a PSA-2 process purification unit and a PSA-H2 process hydrogen extraction unit which are sequentially communicated along the gas flow direction, the PSA-1 unit adopts seven towers to recover vinyl chloride and part of acetylene, and product gas I and semi-purified gas are output; the PSA-2 unit adopts seven towers to deeply remove residual vinyl chloride and acetylene, and product gas II and purified gas are output; and the PSA-H2 unit adopts five towers to separate hydrogen and nitrogen, and outputs products of hydrogen and tail gas. The automatic control system integrates a fault automatic switching module and a self-adjusting control module, so that online fault-tolerant operation when a single valve breaks down is realized, and the adsorption time is dynamically optimized according to the working condition of feed gas. The total recovery rate of vinyl chloride and acetylene is larger than or equal to 99.9%, the product hydrogen purity is larger than or equal to 99.9%, the acetylene content is smaller than or equal to 10 ppm, and the device has the advantages of being high in separation efficiency, stable in operation and high in intelligent degree.

Description

Technical Field

[0001] This invention relates to the field of chemical gas separation and resource recovery technology, and in particular to a pressure swing adsorption system and method for the tail gas of vinyl chloride production. Background Technology

[0002] In the calcium carbide or ethylene process for producing polyvinyl chloride (PVC), the purification of vinyl chloride (VCM) monomer is a crucial step. The VCM distillation unit emits a tail gas containing a large amount of non-condensable gases. This tail gas has a complex composition, typically containing high concentrations of uncondensed vinyl chloride (VCM), acetylene (C2H2), nitrogen (N2), and valuable hydrogen (H2). Direct emission of this gas not only wastes valuable raw materials, but VCM, being a Group 1 carcinogen and VOC, also severely pollutes the environment.

[0003] Existing technologies for treating such exhaust gases mainly include cryogenic condensation, solvent absorption, and membrane separation, but all suffer from high energy consumption, complex processes, inability to recover hydrogen, or equipment susceptibility to damage. While traditional single-stage pressure swing adsorption (PSA) technology shows great promise, when treating multi-component gases, a single adsorbent cannot effectively address all separation targets, and it suffers from adsorbent poisoning, poor process stability, and inability to adapt to fluctuations in upstream operating conditions. Particularly in industrial PSA units, a malfunction of a single programmable valve often leads to unplanned system shutdowns, resulting in significant economic losses and environmental risks. Therefore, there is an urgent need to develop an exhaust gas treatment system that can efficiently recover vinyl chloride and acetylene and purify hydrogen, while also being stable and adaptable. Summary of the Invention

[0004] The purpose of this invention is to provide a pressure swing adsorption (PSA) system and method for the tail gas of vinyl chloride production, in order to solve the problems existing in the prior art that cannot simultaneously achieve high VCM recovery rate and high hydrogen purity extraction, that the entire unit is prone to shutdown due to single-point failure, and that it cannot adapt to fluctuations in upstream operating conditions.

[0005] To achieve the above objectives, the present invention provides a pressure swing adsorption (PSA) system for the tail gas of vinyl chloride production, comprising a PSA-1 recovery unit, a PSA-2 purification unit, and a PSA-H2 hydrogen extraction unit connected sequentially along the gas flow direction; the PSA-1 recovery unit includes a feed gas pretreatment device and at least six recovery towers connected in parallel, each recovery tower being filled with an adsorbent having selective adsorption properties for vinyl chloride (VCM) and acetylene, used to adsorb vinyl chloride and part of acetylene from the feed gas tail gas and output semi-purified gas and product gas; the PSA-2 purification unit includes at least six purification towers connected in parallel, each purification ... An adsorbent with high selectivity for low concentrations of vinyl chloride and acetylene is used to further remove residual vinyl chloride and acetylene from the semi-purified gas and output purified gas and product gas 2. The hydrogen extraction unit of the PSA-H2 process includes at least four purification towers arranged in parallel. Each purification tower is filled with an adsorbent with selective adsorption performance for nitrogen, used to separate hydrogen from the purified gas and output product hydrogen and tail gas. The system also includes a pressure equalization tank, a vacuum pump group, a product gas buffer tank, and an automatic control system. The automatic control system is used to control each adsorption tower to cycle through adsorption, pressure equalization, reverse pressure release, vacuum regeneration, pressure equalization rise, and final pressurization steps according to a preset time sequence to achieve continuous operation.

[0006] Furthermore, the PSA-1 process recovery unit and the PSA-2 process purification unit preferably adopt a seven-tower pressure swing adsorption cycle process, with at least two adsorption towers in the adsorption step at any given time; the single-tower cycle sequence includes: adsorption (A), first pressure equalization drop (E1D), second pressure equalization drop (E2D), third pressure equalization drop (E3D), reverse depressurization (D), evacuation and evacuation flushing (V&VP), third pressure equalization rise (E3R), second pressure equalization rise (E2R), first pressure equalization rise (E1R), and final pressurization (FR).

[0007] Furthermore, the hydrogen extraction unit of the PSA-H2 process preferably adopts a five-tower pressure swing adsorption cycle process (5 towers in parallel), with at least two purification towers in the adsorption step at any given time; the single-tower cycle sequence includes: adsorption (A), first pressure equalization drop (E1D), second pressure equalization drop (E2D), reverse depressurization (D), evacuation and evacuation flushing (V&VP), second pressure equalization rise (E2R), first pressure equalization rise (E1R), and final pressurization (FR).

[0008] Furthermore, the automatic control system includes an automatic fault switching module, which monitors the working status of key programmable valves associated with each tower, and controls the isolation of the corresponding adsorption tower when any key programmable valve is detected to allow the remaining towers to continue to perform the cycle according to the preset degradation sequence to maintain continuous operation.

[0009] Furthermore, the automatic control system also includes a self-adjusting control module, which is connected to an online monitoring instrument for detecting the flow rate, pressure, and composition of the raw material tail gas. The self-adjusting control module dynamically adjusts the adsorption step time of the recovery tower, purification tower, or refining tower based on the online monitoring signal, and adjusts the opening degree of the regulating valve or the pressurization rate of the final pressurization step in conjunction with it to adapt to fluctuations in upstream operating conditions.

[0010] Furthermore, the PSA-1 and PSA-2 single towers sequentially undergo adsorption, first pressure equalization drop, second pressure equalization drop, third pressure equalization drop, reverse depressurization, evacuation and evacuation flushing, third pressure equalization rise, second pressure equalization rise, first pressure equalization rise, and final pressurization steps during their operation cycle; the PSA-H2 single tower sequentially undergoes adsorption, first pressure equalization drop, second pressure equalization drop, reverse depressurization, evacuation and evacuation flushing, second pressure equalization rise, first pressure equalization rise, and final pressurization steps during its operation cycle.

[0011] Furthermore, the PSA-1 process recovery unit and the PSA-2 process purification unit maintain at least two towers operating in parallel during the adsorption step at any time during operation; the PSA-H2 process hydrogen extraction unit maintains at least two towers operating in parallel during the adsorption step at any time during operation.

[0012] On the other hand, the present invention provides a pressure swing adsorption method for the tail gas of vinyl chloride production using the above-mentioned system, comprising the following steps: (1) The vinyl chloride distillation tail gas is introduced as raw material gas into the PSA-1 process recovery unit for the first stage of pressure swing adsorption to obtain product gas rich in vinyl chloride and semi-purified gas. (2) The semi-purified gas is introduced into the PSA-2 process purification unit for the second stage of pressure swing adsorption to obtain product gas and purified gas rich in vinyl chloride and acetylene. (3) The purified gas is introduced into the hydrogen extraction unit of the PSA-H2 process for the third stage of pressure swing adsorption to obtain product hydrogen and tail gas.

[0013] Further, in step (1), the working pressure Pads of the raw gas entering the PSA-1 process recovery unit is controlled to be 0.50~0.54 MPaG and the temperature is −25±5℃; the product gas is pressurized and returned to the vinyl chloride monomer gas holder.

[0014] Further, in step (2), the product gas is pressurized by a booster and returned to the acetylene conversion system for recycling reaction; in step (3), the purity of the output product hydrogen is ≥99.9%, and the acetylene content in the product hydrogen is ≤10 ppm.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention utilizes a three-stage process—coarse separation and recovery, deep purification, and hydrogen purification—to achieve a total recovery rate of ≥99.9% for vinyl chloride and acetylene, thus meeting environmental standards and ensuring raw material recovery. Simultaneously, it provides clean raw materials for the hydrogen extraction unit, resulting in a product with hydrogen purity exceeding 99.9% and acetylene impurities ≤10ppm, yielding a high-value-added product.

[0016] 2. This invention achieves seamless online fault-tolerant operation of switching between seven and six towers or between five and four towers through an automatic fault-switching module built into the automatic control system. A single valve failure no longer leads to unplanned shutdowns of the entire line, greatly improving the online operating rate of the unit and avoiding the risk of exhaust gas venting.

[0017] 3. This invention, through a self-adjusting control module based on raw gas parameters, can respond in real time to fluctuations in upstream operating conditions, dynamically optimize adsorption time and pressurization rate, ensure stable product indicators, and achieve refined operation with energy saving and consumption reduction.

[0018] 4. This invention effectively recovers the effective gas and pressure energy in the dead space of the bed by using multi-stage pressure equalization (PSA-1 / 2 uses three pressure equalizations, and PSA-H2 uses two pressure equalizations) combined with vacuum regeneration technology, thereby reducing the load on the vacuum pump and the total energy consumption of the system. Attached Figure Description

[0019] To more clearly illustrate the technical solution of the present invention, the drawings used in the implementation examples will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0020] Figure 1 This is a schematic diagram of the overall process flow of a pressure swing adsorption system for the tail gas of vinyl chloride production according to the present invention. Figure 2 This is a schematic diagram comparing the traditional single-stage PSA with the cascaded PSA system of this invention.

[0021] In the picture: 100 - Raw material pretreatment; 101 - Vinyl chloride distillation tail gas; 102 - Heater; 103 - Raw material gas-liquid separator; 104 - Processed raw material gas; 200-PSA-1 process recovery unit; 201-7 parallel PSA-1 recovery towers; 202-First purified gas buffer tank; 203-First vacuum pump group; 204-Semi-purified gas; 205-Product gas one; 206-VCM gas holder; 207-First product gas buffer tank; 300-PSA-2 process purification unit; 301-7 parallel PSA-2 purification towers; 302-Second purified gas buffer tank; 303-Second vacuum pump group; 304-Second product gas buffer tank; 305-Product gas booster; 306-Purified gas; 307-Product gas II; 308-Conversion system; 400 - PSA-H2 process hydrogen extraction unit; 401 - Group of 5 parallel PSA-H2 purification towers; 402 - Hydrogen buffer tank; 403 - Third vacuum pump group; 404 - User or pipeline network; 405 - Standard emission; 406 - Product hydrogen; 407 - Tail gas; 500 - Automatic control system; 501 - Automatic fault switching module; 502 - Self-adjusting control module. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings.

[0023] Example 1: This embodiment provides a pressure swing adsorption (PSA) system for treating the tail gas from vinyl chloride production. The system is used to treat the transdermal tail gas from a 300,000-ton-per-year PVC plant. The designed processing capacity is 3200 Nm³ / h, with an operational flexibility of 60%~110%. The typical composition (volume fraction) of the feed gas is: vinyl chloride (VCM) 5~10%, acetylene (C₂H₂) 15~20%, hydrogen (H₂) 25~35%, with the remainder mainly consisting of nitrogen (N₂) and trace amounts of methane and carbon monoxide. Figure 1 As shown, this system mainly consists of three core parts connected sequentially along the gas flow direction: the PSA-1 process recovery unit 200, the PSA-2 process purification unit 300, and the PSA-H2 process hydrogen extraction unit 400, as well as an integrated automatic control system 500. The units are connected via buffer tanks to eliminate the impact of upstream switching fluctuations on the downstream.

[0024] In this embodiment, the PSA-1 process recovery unit 200 serves as the first stage, responsible for recovering the majority of VCM and acetylene from the raw material tail gas. The pretreated raw material gas is heated to −25±5℃ by heater 102 before entering the main inlet pipe. Each recovery tower is a parallel fixed-bed structure; in this embodiment, seven towers are preferably connected in parallel (this can be expanded or reduced depending on the scale of the unit). Each tower is designed to withstand a pressure ≥1.2 MPaG, with a typical adsorption working pressure of approximately 0.50–0.54 MPaG. Each tower bed is layered from bottom to top, consisting of an inert support layer, an activated alumina layer (adsorbing trace amounts of moisture and acidic impurities), and a main adsorption layer (layered or mixed packing of macroporous silica gel and special activated carbon). The layer height and packing ratio can be determined based on process optimization. After adsorption saturation, desorption and recovery of product gas 205 are achieved through pressure equalization, reverse pressure release, and vacuum evacuation (absolute pressure typically within the range of 5–20 kPaA).

[0025] In this embodiment, the feed gas tail gas 101 from the VCM distillation section first enters the feed gas pretreatment device 100. To ensure that the adsorption process takes place at the optimal temperature and to prevent the high-concentration VCM from liquefying under high pressure and damaging the adsorbent, the feed gas first enters a heater 102. By adjusting the heating medium of the heater 102, the temperature of the feed gas is precisely controlled and stabilized within the range of -25±5℃. The heated gas then enters a feed gas-liquid separator 103 to remove any droplets or solid particles that may be present, protecting the downstream programmable valves and the adsorbent bed. The separator is equipped with a level gauge and an automatic drain valve, ultimately yielding the treated feed gas 104.

[0026] In this embodiment, the pretreated feed gas 104 enters the main inlet pipe of the PSA-1 recovery unit 200. This unit has seven vertical fixed-bed adsorption towers with identical structure and dimensions, i.e., seven parallel PSA-1 recovery tower groups 201. Each tower is equipped with high-performance pneumatic programmable valves on its inlet and outlet pipelines, including feed gas inlet valves, semi-purified gas outlet valves, multi-stage pressure equalization valves (upper / lower), reverse pressure relief valves, evacuation valves, and final pressurization valves. All programmable valves are equipped with valve position feedback devices to provide real-time status feedback to the automatic control system 500. The system is equipped with a first purified gas buffer tank 202 to buffer pressure fluctuations during pressure equalization between multiple tower groups, making the pressure equalization process more stable.

[0027] In this embodiment, considering the high concentrations of VCM and acetylene in the feed gas, a composite bed is used in each recovery tower. The bottom of the tower is filled with 200mm high inert alumina ceramic balls to support and distribute the airflow. Above these, 300mm high activated alumina is packed to deeply remove trace amounts of moisture and acidic impurities that may be entrained in the feed gas, protecting the main adsorbent. The main body of the tower (approximately 7000mm high) is layered with macroporous silica gel adsorbent with high selective adsorption performance for vinyl chloride and acetylene, and special coal-based granular activated carbon. These two adsorbents are mixed in a certain proportion or layered to achieve high-capacity capture and rapid desorption of high-concentration organic matter.

[0028] In this embodiment, under the control of the automatic control system 500, the seven recovery towers operate in a cycle according to a specific pressure swing adsorption sequence (see Embodiment 2 for details). In the adsorption step, the treated feed gas 104 (approximately 0.5 MPaG) passes through the bed from bottom to top, and vinyl chloride and most of the acetylene are captured by the adsorbent. The semi-purified gas 204 flowing out from the top of the tower, whose main components are hydrogen, nitrogen, and a small amount of unadsorbed vinyl chloride (content reduced to <2000ppm) and acetylene, flows into the main pipe and enters the next unit. The adsorbed components (product gas 205) are desorbed in the reverse depressurization and vacuum evacuation steps (provided by the first vacuum pump group 203, with an absolute pressure of 5~10 kPaA). The first vacuum pump group 203 consists of two water ring vacuum pumps (one in use and one on standby), with a pumping rate of 1200 m³ / h. After the desorbed gas is pressure-stabilized by the first product gas buffer tank 207, it is continuously and stably sent to the VCM gas holder 206 for recovery at a slightly positive pressure (≥0.02 MPaG).

[0029] In this embodiment, the PSA-2 process purification unit 300 processes the semi-purified gas 204 from PSA-1, aiming to remove organic impurities to trace levels, provide clean raw materials for subsequent hydrogen extraction, and further improve the total recovery rate of organic matter.

[0030] In this embodiment, semi-purified gas 204 enters the second purified gas buffer tank 302 to smooth out pressure fluctuations caused by the PSA-1 cycle switching, and then enters the PSA-2 inlet manifold from the buffer tank. Preferably, seven PSA-2 units are connected in parallel, with an adsorption working pressure of approximately 0.48–0.50 MPaG. Microporous activated carbon or 5A molecular sieve, which has high selectivity for low concentrations of VCM and C2H2, is used as the main adsorbent. The product gas 207 desorbed from PSA-2 is pressure-stabilized by the second product gas buffer tank 304 and then pressurized by the product gas booster 305 before being returned to the acetylene conversion system 308 for recycling.

[0031] The purified gas 306 of the PSA-H2 unit enters directly from the PSA-2 outlet or through an independent buffer tank; preferably, 5 PSA-H2 units are connected in parallel. The tower is mainly filled with a selective adsorbent for nitrogen (such as X-type or 5A molecular sieve). The adsorption working pressure is about 0.45 MPaG. Desorption is carried out by vacuum evacuation (absolute pressure 5–20 kPaA) combined with pressure equalization regeneration. The product hydrogen 406 is stabilized and output through hydrogen buffer tank 402.

[0032] In this embodiment, in the PSA-2 process purification unit 300, semi-purified gas 204 passes through a group of seven parallel PSA-2 purification towers 301 under high pressure (approximately 0.48 MPaG), where residual trace amounts of vinyl chloride and acetylene are deeply captured by the adsorbent. The purified gas 306 flowing out from the top of the tower is extremely clean. Online gas chromatograph analysis shows that the vinyl chloride and acetylene content has decreased to ≤10 ppm (volume fraction), containing almost only hydrogen and nitrogen, and can be directly sent to the next stage PSA-H2 process hydrogen extraction unit 400. The gas desorbed during the regeneration process of the saturated purification tower (product gas 2 307) has a high concentration of vinyl chloride and acetylene. After being stabilized by the second product gas buffer tank 304, this gas is pressurized to approximately 0.08 MPaG by the product gas booster 305, and then continuously and stably sent back to the upstream conversion system 308 through a flow control valve. It then reacts with hydrogen chloride gas, achieving complete closed-loop recovery of organic matter. After two stages of treatment, PSA-1 and PSA-2, the total recovery rate of vinyl chloride and acetylene in the exhaust gas can theoretically reach over 99.9%, with a typical value of 99.95% in actual operation.

[0033] In this embodiment, the hydrogen extraction unit 400 (hydrogen purification section) of the PSA-H2 process is used to produce hydrogen, a high-value-added product. The clean purified gas 306 from PSA-2 first enters the second purified gas buffer tank 302 (Note: This second purified gas buffer tank has been described in the PSA-2 unit. In the actual process, the purified gas directly enters the PSA-H2 process hydrogen extraction unit after exiting the PSA-2 tower group 301. However, for pressure buffering, an independent buffer tank can be set between units. The purified gas 306 may pass through a buffer tank before entering the PSA-H2 process hydrogen extraction unit, but this is not listed separately in the numbering. In actual implementation, a tank can be set at the PSA-H2 inlet. However, based on the given numbering, we can understand the second purified gas buffer tank 302 as being used for semi-purified gas, while the purified gas 306 directly enters the PSA-H2 process hydrogen extraction unit, or an additional purified gas buffer tank can be added, but the number is not provided. We maintain the original numbering system and explain it reasonably in the description. Given the numbering...). 302 is already used in the second purified gas buffer tank (located before PSA-2), while purified gas 306 directly enters the hydrogen extraction unit of the PSA-H2 process. We assume that no additional buffer tank is needed here, or that the downstream hydrogen buffer tank 402 assumes part of the buffering function. Following the numbering, we will directly describe: Purified gas 306 enters the main inlet pipe of the hydrogen extraction unit 400 of the PSA-H2 process. This unit is equipped with 5 purification towers, namely 5 parallel PSA-H2 purification tower groups 401, with specifications of Φ2000mm×6000mm. The towers are filled with highly efficient adsorbents that selectively adsorb nitrogen but have virtually no adsorption of hydrogen, typically X-type molecular sieves or 5A molecular sieves, to maximize hydrogen recovery rate and purity. The unit is equipped with a hydrogen buffer system 402 consisting of two hydrogen buffer tanks connected in series and a third vacuum pump group 403.

[0034] In this embodiment, in the hydrogen extraction unit 400 of the PSA-H2 process, purified gas 306 passes under high pressure (approximately 0.45 MPaG) through a group of five parallel PSA-H2 purification towers 401 in the adsorption step. Since the molecular sieve has a stronger adsorption capacity for nitrogen than hydrogen, nitrogen is adsorbed onto the adsorbent, while hydrogen, as a weakly adsorbed component, flows out from the top of the tower, yielding high-purity product hydrogen 406. After being pressure-stabilized by a hydrogen buffer tank 402, the product hydrogen is continuously output to the user or pipeline network 404 via a flow meter and online analyzer. The purity of the final product hydrogen 406 can be stably maintained between 99.9% and 99.99%, with the acetylene content, which is harmful to downstream hydrogenation reactions, strictly controlled to ≤5 ppm, fully meeting the requirements for high-value-added utilization. The exhaust gas 407 (mainly nitrogen with a small amount of hydrogen) desorbed during adsorbent regeneration is extracted by the third vacuum pump group 403 and discharged through the high-point exhaust stack to meet emission standards 405, thus completely solving the exhaust gas pollution problem.

[0035] In this embodiment, the automatic control system 500 of this system achieves centralized control based on a DCS / PLC platform. The automatic fault switching module 501 monitors the valve position feedback of the listed key programmable valves (inlet valve, product gas valve, equalizing valve, evacuation valve, and final pressurization valve) in real time, and completes a safe switch according to a preset degradation sequence (e.g., downgrading a 7-tower system to 6-tower operation or a 5-tower system to 4-tower operation) when an abnormal state is detected, ensuring continuous operation of the device. In this embodiment, if the PLC does not receive normal valve position feedback within a timeout after issuing a valve action command, it triggers isolation logic (the preferred timeout time in this embodiment is 3 seconds, which can be determined by engineering debugging). The automatic control system also includes a self-adjusting control module 502, which reads signals such as online flow meters, pressure, and online gas chromatography (GC), and dynamically adjusts the adsorption step time and final pressurization rate based on a feedforward-feedback strategy to cope with upstream operating condition fluctuations and maintain stable product indicators.

[0036] Example 2: In this embodiment, in order to achieve efficient gas separation and pressure energy recovery, the adsorption towers of each unit operate in a time-sequence cycle.

[0037] PSA-1 and PSA-2 employ the exact same 7-1-3 / V (seven towers (preferred), one tower fed at any time, three pressure equalization cycles, with vacuum regeneration) process. The entire cycle is divided into multiple time steps. Taking tower A as an example, it sequentially undergoes the following 10 steps within a complete cycle. The time for each step can be adjusted according to the processing capacity. Typically, the adsorption step takes a longer time to ensure continuous feeding and simultaneous adsorption in both towers: Adsorption (A): The inlet valve and product gas valve of tower A are open. The feed gas flows from bottom to top through the bed, VCM and C2H2 are adsorbed, and the semi-purified gas / purified gas flows out from the top of the tower. In the seven-tower process, at any given time, two towers are in the adsorption state (e.g., tower A and tower B, but their adsorption start times are staggered by half a cycle) to ensure the continuity of the feed.

[0038] First Pressure Equalization Drop (E1D): Adsorption in column A ends, and the product gas valve is closed. Then, its equalization valve is opened, connecting it to another column (such as column E) which is in the "First Pressure Equalization Rise (E1R)" state. Hydrogen, nitrogen, and other gases in the high-pressure dead space of column A flow to column E, causing the pressure in column A to drop from the adsorption pressure Pads to P1, and the pressure in column E to rise from the low pressure PE1R to P1'. This step recovers approximately 20% of the pressure energy and usable gas.

[0039] Second pressure equalization drop (E2D): Tower A is disconnected from tower E and connected to a tower (such as tower F) that is in the "second pressure equalization rise (E2R)" state, to perform a second pressure equalization. The pressure of tower A drops to P2, and the pressure of tower F rises.

[0040] Third pressure equalization drop (E3D): Tower A is connected to a tower in the "third pressure equalization rise (E3R)" state (such as tower G) to perform the third pressure equalization. The pressure of tower A drops to P3 (close to atmospheric pressure), and the pressure of tower G rises.

[0041] Reverse pressure relief (D): Close all equalizing valves and open the reverse pressure relief valve at the bottom of column A. The pressure inside the column rapidly drops from P3 to near atmospheric pressure (or slightly above atmospheric pressure). At this time, a large amount of adsorbed VCM and C2H2 desorb, forming organic-rich product gas, which flows into the product gas buffer tank.

[0042] Vacuuming and Vulcanization Flushing (V&VP): Open the evacuation valve at the bottom of column A and connect the vacuum pump unit for deep evacuation. The pressure inside the column drops to negative pressure (absolute pressure 5~20 kPaA), and the strongly adsorbed components are completely desorbed. In the later stage of evacuation, open the flushing gas valve at the top of the column and introduce a small amount of product gas from other columns or from the product gas buffer tank to flush the bed from top to bottom in a countercurrent manner, further reducing the partial pressure of organic components in the micropores of the adsorbent and achieving bed regeneration.

[0043] Third Pressure Equalization Rise (E3R): Close the evacuation and flushing valves, open the equalization valve of column A, and accept the gas from a column (such as column C) that is in the "Third Pressure Equalization Drop (E3D)" state to perform the first pressure increase, and the pressure of column A rises to PE3R.

[0044] Second pressure equalization boost (E2R): Switch the equalization path to receive gas from the tower (such as tower D) which is in the E2D state, and perform a second pressure boost, so that the pressure in tower A rises to PE2R.

[0045] First pressure equalization boost (E1R): The equalization path is switched again to receive gas from the tower that is in the E1D state (such as tower E), and the pressure of tower A rises to PE1R, which is very close to the adsorption pressure.

[0046] Final pressurization (FR): Close all equalizing valves, open the final pressurization valve of column A, introduce some product gas (purified gas), and slowly and precisely pressurize column A to the final adsorption working pressure Pads through a high-precision regulating valve, in preparation for the next adsorption step.

[0047] PSA-H2 employs a 5-1-2 / V (five towers, one tower fed at any time, two pressure equalization cycles, with vacuum regeneration) process. Taking tower A as an example, its complete cycle involves the following eight steps: Adsorption (A), first pressure equalization drop (E1D), second pressure equalization drop (E2D), reverse depressurization (D), evacuation and evacuation flushing (V&VP), second pressure equalization rise (E2R), first pressure equalization rise (E1R), final pressurization (FR).

[0048] Since nitrogen adsorption capacity is relatively weak in hydrogen-nitrogen separation systems, and hydrogen recovery is the primary objective, two pressure equalization cycles are sufficient to efficiently recover hydrogen from the dead space in the bed. This sequence simplifies the process and valve configuration while ensuring a high hydrogen recovery rate (>85%).

[0049] Example 3: In this embodiment, the system's automatic control system 500 incorporates an automatic fault switching module 501, which is an online fault-tolerant mechanism that greatly improves the reliability of the device. The module monitors the status of all critical programmable valves in the adsorption towers in real time. Programmable valves refer to valves that directly affect tower isolation, such as inlet valves, product gas valves, and pressure equalization valves.

[0050] Each programmable valve is equipped with open and close limit switches. After issuing the switch command, the PLC starts a timer (e.g., set to 3 seconds) to wait for the feedback signal from the limit switch. During normal operation of the PSA-1 process recovery unit 200, the PLC commands the intake valve V103C of tower C to open. If no "valve fully open" feedback signal is received after 3 seconds, or if an error signal of "valve fully closed" is received, the PLC determines that V103C has malfunctioned (e.g., jamming, solenoid valve coil burnout, gas supply interruption, etc.). The automatic fault switching module 501 can immediately start the pre-programmed "7 to 6" degraded operation program, immediately canceling the next operation command for all programmable valves of tower C, and forcibly outputting a "close" signal to all connected valves (e.g., intake valve, equalizing valve, product gas valve, etc.), ensuring that the faulty tower is physically isolated to prevent internal leakage, cross-pressure, or pressure buildup. At the same time, a prominent audible and visual alarm window pops up on the HMI, highlighting tower C and its faulty valve in red, and recording the fault log. The state capture and calculation module needs to quickly capture the current position of the remaining 6 towers (A, B, D, E, F, G) in the cycle sequence (e.g., tower A is in the 20th second of adsorption, tower B is in the pressure drop equalization E1D, etc.).

[0051] The control program, based on a pre-calculated "7-to-6" jump logic table, finds the safest and smoothest entry point. It maps the current state of these six towers to a preset 6-tower process (e.g., 6-2-3 / V, i.e., 6 towers, 2 towers simultaneously adsorbing, 3 pressure equalization cycles) sequence. Within a very short time (usually <1 second) of the next step switch, the system completes the program jump, and the remaining 6 towers begin to run seamlessly and continuously according to the new 6-tower sequence. During this process, due to the upstream feed gas buffer tank and the downstream product gas buffer tank, the pressure and flow fluctuations in the system's main pipeline are controlled within a very small range allowed by the process, achieving a "seamless" transition between upstream and downstream.

[0052] For the PSA-H2 hydrogen extraction unit 400, a similar "5-to-4" logic is pre-programmed into the control system. When any of the five parallel PSA-H2 purification towers in group 401 fails, the system isolates it and switches the remaining four towers to a four-tower hydrogen extraction sequence (e.g., 4-1-2 / V). Process personnel can perform online maintenance on the isolated faulty tower without shutting down the unit, such as replacing the faulty programmable valve. After maintenance, the operator confirms the reset on the HMI. The PLC system will automatically execute the "6-to-7" or "4-to-5" recovery logic at the start of the next appropriate cycle, smoothly reintegrating the repaired tower back into the system and restoring the original optimal operating mode.

[0053] Example 4: In this embodiment, the system's automatic control system 500 also integrates a self-adjusting control module 502 to cope with unavoidable load fluctuations and component changes in the upstream distillation section, thereby achieving automatic adjustment of the unit's operation. Assume that due to adjustments in the operation of the upstream VCM distillation column, the flow rate of the discharged vinyl chloride distillation tail gas 101 suddenly increases from 3200 Nm³ / h to 3600 Nm³ / h (an increase of 12.5%), and simultaneously, the online gas chromatograph detects that the VCM concentration in the tail gas also increases from 8% to 9%. The self-adjusting control module 502 reads the 4-20mA signals from the vortex flow meter on the feed gas main and the online gas chromatograph in real time.

[0054] To prevent the adsorption front from moving too quickly due to increased load, causing premature penetration of the adsorption tower and resulting in product defects, the control module dynamically adjusts the adsorption sequence time of the PSA-1 process recovery unit 200. The original baseline adsorption time was 120 seconds, so the new target adsorption time is adjusted to approximately 100 seconds. The shorter adsorption time means a shorter overall cycle time. The time window allocated to the final pressurization (FR) step is also shortened. If pressurization continues at the original rate, the pressure inside the tower may not rise to the required adsorption pressure within the specified time, leading to poor adsorption performance in the next cycle. Therefore, the self-adjusting control module 502 simultaneously calculates the required new final pressurization rate. It sends a new set of settings to the PID controller of the final pressurization regulating valve in the PSA-1 process recovery unit 200, including either increasing the target value of the valve opening or directly setting a higher pressurization rate slope. This ensures that the tower pressure accurately and smoothly reaches the adsorption pressure setpoint within the shortened FR step time.

[0055] In this embodiment, similar self-adjusting logic is also applied to the PSA-2 process purification unit 300 and the PSA-H2 process hydrogen extraction unit 400. For example, the adsorption time of the PSA-2 process purification unit 300 may also need fine-tuning, and the final pressure valve of the PSA-H2 process hydrogen extraction unit 400 also needs to be adjusted accordingly. The system continuously monitors the quality of the outlet gases at each stage: the VCM content of the semi-purified gas 204 at the PSA-1 outlet, the trace organic matter content of the purified gas 306 at the PSA-2 outlet, and the purity of the final product hydrogen 406. If a certain indicator is found to still have a deviation trend (for example, the VCM content in the semi-purified gas is still slowly increasing), it indicates that the feedforward adjustment is insufficient. The self-adjusting module 502 will perform a new round of more refined fine-tuning based on these feedback signals until all product quality indicators are stabilized again within the target range. This composite control strategy of feedforward and feedback ensures that no matter how drastically the upstream operating conditions fluctuate, the system can always adaptively find the optimal operating point.

[0056] In this invention, all pressure values ​​are uniformly expressed as gauge pressure, with the unit being MPaG (gauge pressure); absolute pressure is expressed in kPaA or kPa(a). Design pressure refers to the highest working pressure that the equipment structure can withstand, while operating pressure or working pressure refers to the normal process pressure of the device during adsorption / desorption. Temperature is expressed in °C.

[0057] To explain the above embodiments in more detail, the start-stop control logic of the system in this embodiment is as follows: Start-up procedure: Confirm that all instruments, valves, and pumps are in normal standby condition. Purge the entire system with nitrogen until the oxygen content is below the safety setpoint (e.g., 0.5%). Start each stage of the vacuum pump group sequentially to achieve the standby vacuum level. With no feed gas available, start the PSA timing program, allowing all programmable valves to run idle for 1-2 cycles to verify the accuracy of valve action and logic. Slowly open the feed gas main valve while simultaneously gradually increasing the pressure through the automatic control system (500). The system will automatically switch from "shutdown mode" to "low load operation mode" based on the pressure increase, and ultimately adjust to "normal load operation mode" based on the intake gas volume. Before the product gas at each stage meets quality standards, temporarily release it through the vent valve.

[0058] Normal shutdown procedure: Close the main feed gas valve or notify the upstream supplier to stop gas supply. Continue to consume the gas in the tower using the product gas outlets at each stage until the system pressure drops to atmospheric pressure. After the pressure in the tower is completely released, stop the PSA sequence. Thoroughly purge the system with nitrogen to remove all residual flammable or toxic gases, and then reliably isolate the system from upstream and downstream process pipelines in preparation for maintenance.

[0059] Finally, by selecting four common processing technologies in the current industry—cryogenic condensation, solvent absorption, membrane separation, and traditional single-stage PSA—and comparing them with the "three-stage stepped PSA system" of this invention, a comparison was formed. Figure 2 Comparative data in the middle.

[0060] A comparison reveals that this invention employs a tiered pressure swing adsorption (PSA) structure. The PSA-1 process achieves efficient vinyl chloride recovery, the PSA-2 process achieves deep purification, and the PSA-H2 process extracts high-purity hydrogen. The overall recovery rate is significantly higher than that of cryogenic condensation, while energy consumption is significantly reduced. Using solid adsorbents instead of liquid absorbents eliminates solvent loss issues, resulting in a simpler system structure, higher operational stability, and the ability to achieve multi-component staged recovery, significantly improving environmental performance. The segmented PSA process uses specialized adsorbents in each stage, avoiding material swelling and reducing vinyl chloride and acetylene concentrations to the ppm level, achieving stable emissions compliance. By setting up a three-stage tiered structure consisting of a PSA-1 recovery unit, a PSA-2 purification unit, and a PSA-H2 hydrogen extraction unit, different separation targets are physically isolated, achieving an overall vinyl chloride recovery rate of ≥99.9%; deep acetylene removal; and hydrogen purity of ≥99.9%. The system has online fault-tolerant operation capabilities, allowing for switching from seven to six towers or from five to four towers. An adaptive control module automatically adjusts the adsorption cycle and pressurization rate, improving system stability. Therefore, the present invention is significantly superior to the prior art in terms of separation efficiency, operational stability, resource recovery rate, degree of automation, and overall economic benefits.

[0061] It should be understood that the present invention is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention.

Claims

1. A pressure swing adsorption system for the tail gas of vinyl chloride production, characterized in that, The system comprises a PSA-1 process recovery unit (200), a PSA-2 process purification unit (300), and a PSA-H2 process hydrogen extraction unit (400), which are connected sequentially along the gas flow direction. The PSA-1 process recovery unit (200) includes a raw material gas pretreatment device (100) and at least six recovery towers connected in parallel. Each recovery tower is filled with an adsorbent that selectively adsorbs vinyl chloride and acetylene, used to adsorb vinyl chloride and some acetylene from the raw material tail gas and output semi-purified gas (204) and product gas (205). The PSA-2 process purification unit (300) includes at least six purification towers connected in parallel, each filled with an adsorbent that has high selectivity for low concentrations of vinyl chloride and acetylene. The adsorbent is used to further remove residual vinyl chloride and acetylene from the semi-purified gas (204) and output purified gas (306) and product gas 2 (307); the PSA-H2 process hydrogen extraction unit (400) includes at least four purification towers connected in parallel, each purification tower is filled with an adsorbent that has selective adsorption performance for nitrogen, used to separate hydrogen from the purified gas (306) and output product hydrogen (406) and tail gas (407); the system also includes a pressure equalization tank, a vacuum pump group, a product gas buffer tank and an automatic control system (500); the automatic control system (500) is used to control each adsorption tower to cycle through adsorption, pressure equalization, reverse pressure release, vacuum regeneration, pressure equalization rise and final pressurization steps according to a preset time sequence.

2. The pressure swing adsorption system for vinyl chloride production tail gas as described in claim 1, characterized in that: The PSA-1 process recovery unit (200) and the PSA-2 process purification unit (300) preferably adopt a seven-tower pressure swing adsorption cycle process, with at least two adsorption towers in the adsorption step at any given time; the single-tower cycle sequence includes: adsorption (A), first pressure equalization drop (E1D), second pressure equalization drop (E2D), third pressure equalization drop (E3D), reverse depressurization (D), evacuation and evacuation flushing (V&VP), third pressure equalization rise (E3R), second pressure equalization rise (E2R), first pressure equalization rise (E1R), and final pressurization (FR).

3. A pressure swing adsorption system for the tail gas of vinyl chloride production as described in claim 1 or 2, characterized in that: The PSA-H2 process hydrogen extraction unit (400) preferably adopts a five-tower pressure swing adsorption cycle process, with at least two purification towers in the adsorption step at any given time; the single-tower cycle sequence includes: adsorption (A), first pressure equalization drop (E1D), second pressure equalization drop (E2D), reverse depressurization (D), evacuation and evacuation flushing (V&VP), second pressure equalization rise (E2R), first pressure equalization rise (E1R) and final pressurization (FR).

4. The pressure swing adsorption system for vinyl chloride production tail gas as described in claim 1, characterized in that: The automatic control system (500) includes an automatic fault switching module (501) for monitoring the working status of key programmable valves associated with each tower, and controlling the isolation of the corresponding adsorption tower and allowing the remaining towers to continue to run in a cycle according to a pre-set degradation sequence to maintain continuous operation when any key programmable valve is detected to be abnormal.

5. A pressure swing adsorption system for the tail gas of vinyl chloride production as described in any one of claims 1 to 4, characterized in that: The automatic control system (500) also includes a self-adjusting control module (502), which is connected to an online monitoring instrument for detecting the flow rate, pressure and composition of the raw material tail gas. The self-adjusting control module dynamically adjusts the adsorption step time of the recovery tower, purification tower or refining tower based on the online monitoring signal, and adjusts the opening degree of the regulating valve or the pressurization rate of the final pressurization step in conjunction with the adjustment to adapt to upstream operating condition fluctuations.

6. The pressure swing adsorption system for vinyl chloride production tail gas as described in claim 2, characterized in that: The PSA-1 and PSA-2 single towers sequentially undergo the following steps during their operation cycle: adsorption, first pressure equalization drop, second pressure equalization drop, third pressure equalization drop, reverse depressurization, evacuation and evacuation flushing, third pressure equalization rise, second pressure equalization rise, first pressure equalization rise, and final pressurization. The PSA-H2 single tower sequentially undergoes the following steps during its operation cycle: adsorption, first pressure equalization drop, second pressure equalization drop, reverse depressurization, evacuation and evacuation flushing, second pressure equalization rise, first pressure equalization rise, and final pressurization.

7. The pressure swing adsorption system for vinyl chloride production tail gas as described in claim 6, characterized in that: The PSA-1 process recovery unit and the PSA-2 process purification unit maintain at least two towers operating in parallel during the adsorption step at any time during operation; the PSA-H2 process hydrogen extraction unit maintains at least two towers operating in parallel during the adsorption step at any time during operation.

8. A method for performing pressure swing adsorption (PSA) of vinyl chloride production tail gas using a pressure swing adsorption system as described in any one of claims 1 to 7, characterized in that, Includes the following steps: (1) The vinyl chloride distillation tail gas (101) is introduced into the PSA-1 process recovery unit (200) as raw material gas for the first stage of pressure swing adsorption to obtain vinyl chloride-rich product gas (205) and semi-purified gas (204). (2) The semi-purified gas (204) is introduced into the PSA-2 process purification unit (300) for the second stage of pressure swing adsorption to obtain product gas 2 (307) and purified gas (306) rich in vinyl chloride and acetylene. (3) The purified gas (306) is introduced into the hydrogen extraction unit (400) of the PSA-H2 process for third-stage pressure swing adsorption to obtain product hydrogen (406) and tail gas (407).

9. The pressure swing adsorption method for the tail gas of vinyl chloride production as described in claim 8, characterized in that: In step (1), the working pressure Pads of the raw gas entering the PSA-1 process recovery unit (200) is controlled to be 0.50~0.54 MPaG and the temperature is −25±5℃; the product gas (205) is pressurized and returned to the vinyl chloride monomer gas holder (206).

10. The pressure swing adsorption method for the tail gas of vinyl chloride production as described in claim 8, characterized in that: In step (2), the product gas 2 (307) is pressurized by the booster (305) and returned to the acetylene conversion system (308) for recycling reaction; in step (3), the purity of the output product hydrogen (406) is ≥99.9%, and the acetylene content in the product hydrogen is ≤10 ppm.

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