A VOCs exhaust gas desorption treatment system and method

Abstract

The application discloses a VOCs waste gas desorption treatment system and method, which comprises an absorption rectification unit, a water recycling system, two groups of activated carbon adsorption units, a vacuum desorption unit and an exhaust tower; the absorption rectification unit is used for carrying out absorption separation and purification recovery of water-soluble VOCs of waste gas, and organic solvent products are output; the first purified tail gas enters the activated carbon adsorption unit to carry out deep adsorption of insoluble VOCs; the vacuum desorption unit is used for carrying out low-temperature vacuum extraction regeneration on saturated activated carbon; and the desorption gas is returned to the inlet of the absorption rectification unit after gas-liquid separation to form a closed loop circulation; the application realizes resource recycling through absorption rectification, greatly reduces energy consumption and saves drying and cooling equipment through vacuum desorption, eliminates secondary pollution through closed loop reflux, and realizes graded and qualitative treatment of VOCs and efficient and low-consumption regeneration.

Description

Technical Field

[0001] This invention relates to the field of VOCs waste gas treatment technology, and in particular to a VOCs waste gas desorption treatment system and method. Background Technology

[0002] One of the mainstream technologies for treating VOCs waste gas in industry is adsorption. This method uses adsorption materials such as activated carbon, macroporous resin, or porous molecular sieves to capture organic components in the waste gas. After the adsorption is saturated, the adsorption is regenerated by hot air or high-temperature steam to restore the adsorption capacity of the adsorption material. At the same time, the desorbed organic matter is condensed and recovered or incinerated.

[0003] First, most adsorption treatment systems only have simple alkaline or water washing pretreatment at the front end, mainly used to remove acidic gases or dust particles from waste gas. They cannot perform graded and differentiated treatment of the diverse organic components in the waste gas. Especially when the waste gas contains both water-soluble and insoluble VOCs, existing pretreatment methods struggle to efficiently separate the water-soluble components, forcing subsequent adsorption equipment to treat all organic matter simultaneously. This results in a high adsorption load, short lifespan of the adsorption material, and frequent regeneration, increasing operating costs. Second, the currently widely used desorption method is high-temperature steam desorption. While this method has high desorption efficiency, it consumes a large amount of steam energy. After desorption, a large amount of moisture remains inside the adsorption material, occupying the pore space and severely affecting its re-adsorption efficiency. Third, existing technologies necessitate the addition of hot air drying and cooling equipment to remove moisture from the adsorption material and cool it to a suitable adsorption temperature. This not only complicates the system structure and increases equipment investment but also further increases energy consumption during the drying and cooling processes. Fourth, the mixed organic vapors produced by steam desorption contain a large amount of water after condensation, resulting in organic solvents with high water content that are difficult to reuse directly. They usually require complex refining processes for dehydration, making resource utilization less economical.

[0004] Therefore, in response to the problems mentioned above, this invention proposes a VOCs waste gas desorption treatment system and method. Summary of the Invention

[0005] To overcome the problems of weak pretreatment and classification capabilities, high energy consumption of steam desorption, and high water content of desorption products that make direct reuse difficult in existing VOCs waste gas treatment systems, this invention proposes a VOCs waste gas desorption treatment system and method to achieve graded and differentiated treatment of water-soluble and sparingly soluble components in waste gas. Through closed-loop coupling of vacuum desorption and pre-absorption distillation unit, the system aims to reduce energy consumption, simplify the system, and improve resource recovery rate.

[0006] The technical solution of this invention is: a VOCs waste gas desorption treatment system, comprising: An absorption distillation unit is used to receive and treat raw VOCs waste gas. This unit includes an absorption tower, a distillation tower, a condenser, and a reflux tank. The absorption tower has an absorbent spray layer and a gas phase outlet at the top, a rich liquid outlet at the bottom, and a waste gas inlet on the side or bottom. The feed inlet of the distillation tower is connected to the rich liquid outlet of the absorption tower. The distillation tower has a wastewater outlet at the bottom, a gas phase outlet at the top, and a water replenishment port and a reflux port at the bottom. When the liquid level inside the distillation tower is insufficient, it is replenished through the water replenishment port. The condenser is connected to the gas phase outlet of the distillation tower. The reflux tank is connected to the outlet of the condenser. The liquid outlet of the reflux tank is divided into two paths: one path is connected to the top of the distillation tower via a reflux pump, and the other path serves as an organic solvent product collection pipeline. The non-condensable gas outlet of the reflux tank is connected to the waste gas inlet of the absorption tower. The water recycling system includes a water circulation pump, circulation pipeline, and condenser. The inlet of the condenser and the water circulation pump is connected to the bottom wastewater outlet of the distillation column. The outlet of the water circulation pump is connected to the absorbent spray layer of the absorption column through the circulation pipeline for recycling the absorbent water discharged from the bottom of the distillation column. At least a first activated carbon adsorption unit and a second activated carbon adsorption unit, each activated carbon adsorption unit is filled with one or more adsorption materials selected from granular activated carbon, honeycomb activated carbon or activated carbon fiber felt, and is respectively provided with a waste gas inlet, a purified gas outlet and a vacuum extraction port. The vacuum desorption unit includes a vacuum pump and a gas-liquid separator. The inlet of the vacuum pump is connected to the vacuum extraction ports of the first activated carbon adsorption unit and the second activated carbon adsorption unit via a main vacuum pipeline and corresponding branch valves, respectively. The outlet of the vacuum pump is connected to the inlet of the gas-liquid separator. The gas outlet of the gas-liquid separator is connected to the exhaust gas inlet of the absorption tower or to the exhaust tower via a return pipeline. The liquid outlet of the gas-liquid separator is used to recover the desorbed organic liquid. The exhaust tower is connected to the purified gas outlets of the first activated carbon adsorption unit and the second activated carbon adsorption unit, respectively. A demister is installed between the absorption distillation unit and the first and second activated carbon adsorption units to remove liquid droplets entrained in the first purified exhaust gas. The control valve assembly, including a PLC controller, allows one activated carbon adsorption unit to perform adsorption while the other performs vacuum desorption. The two activated carbon adsorption units are periodically switched according to a preset time sequence to achieve a continuous "one adsorption, one desorption" operation mode.

[0007] This invention proposes a method for VOCs waste gas desorption treatment, comprising the following steps: S1. The raw VOCs waste gas is passed into the absorption distillation unit, where the water-soluble VOCs components in the waste gas are absorbed by an absorbent, which is water or a dilute solution containing 5-15 wt% organic solvent. The rich liquid after absorption enters the distillation column for distillation separation. The operating pressure of the distillation column is atmospheric pressure or slightly positive pressure, the theoretical number of plates is 15-30, and the reflux ratio is 1:1 to 5:1. Organic solvent products with a purity ≥99.5% and non-condensable first purified tail gas are obtained. The absorbent wastewater at the bottom of the distillation column is condensed by a condenser and recycled back to the absorption column. S2, the first purified exhaust gas is introduced into the demister through the control valve group to remove entrained droplets, and then enters the first or second activated carbon adsorption unit in the adsorption state. The activated carbon is used to adsorb and purify the residual insoluble VOCs in the exhaust gas to obtain the second purified exhaust gas that meets the standards, and then it is discharged through the exhaust tower. At the same time, the organic vapor distilled from the top of the distillation tower enters the condenser, and after condensation, it enters the reflux tank. Part of the liquid in the reflux tank is taken out as product, and part of it is returned to the top of the distillation tower as reflux liquid. The non-condensable gas in the reflux tank is directly introduced into the exhaust gas inlet of the absorption distillation unit. S3, when the activated carbon adsorption unit in the adsorption state reaches saturation, the control valve group switches it to the desorption state, closes its exhaust gas inlet and purified gas outlet, and turns on the vacuum pump in the vacuum desorption unit to control the absolute pressure inside the activated carbon adsorption unit between 1-20 kPa and the desorption temperature between 30-60℃. No external heating is required, so that the VOCs adsorbed on the activated carbon are decomposed and extracted to form a desorption gas flow. S4. The desorbed gas flow generated in step S3 is introduced into the gas-liquid separator for gas-liquid separation to recover the liquid organic components. The remaining non-condensable gas is returned to the exhaust gas inlet of the absorption tower in step S1 through the reflux pipe. After mixing with the fresh exhaust gas, it re-enters the absorption distillation unit for further processing.

[0008] The beneficial effects of this invention are: 1. This invention pre-treats raw VOCs waste gas by setting up an absorption distillation unit, and uses an absorption tower to efficiently capture water-soluble organic components and purify them through a distillation tower. It can directly produce organic solvent products with a purity of ≥97.2% and a water content of ≤0.5%, realizing the resource recovery of waste gas pollutants and creating economic benefits. It solves the problem that the existing technology pretreatment can only remove acidic gases but cannot recover water-soluble organic matter, resulting in resource waste.

[0009] 2. This invention uses a vacuum desorption unit to regenerate saturated activated carbon through low-temperature vacuum extraction. The absolute desorption pressure is controlled between 1-20 kPa, and the desorption temperature is maintained within the range of 30-60°C. This completely eliminates the need for high-temperature water vapor, significantly reducing desorption energy consumption. Furthermore, the internal moisture content of the desorbed activated carbon is less than 2%, allowing for direct reuse without additional hot air drying or cooling equipment. This greatly simplifies the system structure and reduces operating costs.

[0010] 3. This invention connects the gas outlet of the gas-liquid separator to the original waste gas pipeline before the waste gas inlet of the absorption tower via a return pipeline. This allows the non-condensable gas containing residual VOCs generated by vacuum desorption to return to the inlet of the absorption distillation unit, mix with fresh waste gas, and re-enter the treatment process, forming a closed-loop circulation system. This ensures that there are no harmful gases bypassed under any operating conditions, solving the problem that the desorption tail gas in the prior art needs to be incinerated or adsorbed separately, which poses a risk of secondary pollution.

[0011] 4. This invention utilizes an absorption distillation unit for highly efficient removal of water-soluble VOCs (single-stage removal rate ≥98%), significantly reducing the total VOCs concentration in the first purified tail gas entering the activated carbon adsorption unit to 200 mg / m³. 3 The following significantly reduces the adsorption load on activated carbon and extends the activated carbon replacement cycle by more than 50%. At the same time, the two-stage treatment each exerts its advantages in treating water-soluble and insoluble components. Even if the composition of the waste gas fluctuates, it will not cause single-stage exceedance. The operational stability is significantly better than that of a single treatment process. Attached Figure Description

[0012] Figure 1 The diagram shown is a schematic of the system pipeline architecture of the present invention; Figure 2 The diagram shown is a schematic representation of the operation method of the present invention. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] Please see Figure 1 Example 1: This invention provides a VOCs waste gas desorption treatment system, including an absorption distillation unit, a water recycling system, a first activated carbon adsorption unit, a second activated carbon adsorption unit, a vacuum desorption unit, an exhaust tower, a demister, and a control valve group.

[0015] The absorption distillation unit is used to receive raw VOCs waste gas and absorb, separate, purify, and recover water-soluble organic components. This unit includes an absorption tower, a distillation tower, a condenser, and a reflux tank.

[0016] The absorption tower adopts a packed tower or plate tower structure. It has a waste gas inlet located at the lower side of the tower body, an absorbent spray layer and a gas phase outlet at the top, and a rich liquid outlet at the bottom. The raw VOCs waste gas, after being collected by the collection system, enters the waste gas inlet of the absorption tower through a pipeline. It comes into countercurrent contact with the absorbent sprayed from the top. Water-soluble VOCs in the waste gas are absorbed by the absorbent, and the purified gas is discharged from the top of the absorption tower as the first stage of purification tail gas for subsequent treatment. The rich liquid (the absorbent that has absorbed water-soluble VOCs) flowing out from the bottom of the absorption tower enters the middle feed inlet of the distillation tower through a rich liquid conveying pipeline.

[0017] A distillation column is a distillation device operating at atmospheric or slightly positive pressure. It contains 15-30 theoretical trays or high-efficiency packing. The column has a wastewater outlet at the bottom and a vapor outlet at the top. The rich liquor undergoes distillation separation within the column. Low-boiling-point water-soluble organic components are distilled off from the top and enter the condenser via the vapor outlet. High-boiling-point water (i.e., the absorbent) and a small amount of heavy components are concentrated at the bottom of the column.

[0018] The condenser is connected to the vapor outlet of the distillation column and uses circulating cooling water or low-temperature refrigerant as the cooling medium to condense the vapor at the top of the column into liquid. The condensate then enters the reflux tank.

[0019] The reflux tank is connected to the outlet of the condenser. The liquid outlet of the reflux tank is divided into two paths. One path is connected to the reflux port at the top of the distillation column via a reflux pump, returning a portion of the condensate as reflux liquid to the distillation column to maintain gas-liquid balance and separation efficiency within the column. The other path serves as the organic solvent product collection pipeline, transporting high-purity organic solvent products to the product storage tank. The reflux tank is also equipped with a non-condensable gas outlet. The gas discharged from this outlet contains a small amount of unabsorbed or uncondensed VOCs components, which is returned to the exhaust gas inlet of the absorption tower via a pipeline.

[0020] The water recycling system is used to recycle the absorbent, reducing water consumption. The system includes a water circulation pump, circulation piping, and a condenser. The inlets of the condenser and water circulation pump are connected to the bottom wastewater outlet of the distillation column via pipes, and the outlet of the water circulation pump is connected to the absorbent spray layer of the absorption column via circulation piping. The wastewater discharged from the bottom of the distillation column mainly consists of absorbent (water or dilute organic solution), which may contain a very small amount of incompletely separated organic matter, but overall still maintains a high absorption capacity. After cooling the wastewater in the condenser, it is pressurized by the water circulation pump and sent back to the top spray layer of the absorption column for reuse as absorbent. When the absorbent dosage is insufficient due to evaporation or discharge, a small amount of fresh water needs to be added from the water inlet at the bottom of the distillation column.

[0021] The first and second activated carbon adsorption units are arranged side-by-side, with identical structures, to alternately perform adsorption and desorption operations, ensuring continuous system operation. Each activated carbon adsorption unit is a fixed-bed adsorber filled with adsorbent material. Depending on the characteristics of the waste gas, the adsorbent material can be selected from one or more of granular activated carbon, honeycomb activated carbon, or activated carbon fiber felt. Each adsorption unit is equipped with a waste gas inlet, a purified gas outlet, and a vacuum extraction port. The waste gas inlet is connected to the outlet of the demister via a pipe and a control valve to receive the first purified tail gas from the absorption distillation unit; the purified gas outlet is connected to the exhaust tower via a pipe and a control valve to discharge the second purified tail gas after adsorption has met the standards; the vacuum extraction port is connected to the vacuum desorption unit via a pipe and a control valve to create a vacuum inside the adsorption unit during the desorption stage.

[0022] The demister is installed between the absorption distillation unit and the first and second activated carbon adsorption units. Specifically, the inlet of the demister is connected to the non-condensable gas outlet of the reflux tank, and the outlet of the demister is connected to the exhaust gas inlets of the first and second activated carbon adsorption units, respectively.

[0023] The demister can remove tiny droplets entrained in the first purified exhaust gas, thereby preventing droplets from entering the activated carbon adsorption unit and causing the activated carbon to become damp or clogged, thus protecting the adsorption performance of the activated carbon.

[0024] The vacuum desorption unit is used to regenerate saturated activated carbon. This unit includes a vacuum pump and a gas-liquid separator.

[0025] The vacuum pump's inlet is connected to the vacuum extraction ports of the first and second activated carbon adsorption units via a main vacuum pipeline and corresponding branch valves. The vacuum pump can reduce the absolute pressure inside the adsorption units to the range of 1-20 kPa, thereby decomposing and adsorbing VOCs adsorbed on the activated carbon at room temperature or lower. The vacuum pump's outlet is connected to the inlet of the gas-liquid separator, sending the desorbed mixed gas into the separator.

[0026] A gas-liquid separator is used to separate the desorbed gas stream into gas and liquid components. Internally, it may incorporate baffles, wire mesh, or cyclone separation structures. The liquid outlet of the gas-liquid separator is used to recover the desorbed organic liquid, which contains a high concentration of VOCs and can be directly reused as an industrial crude solvent or further refined. The gas outlet of the gas-liquid separator is connected via a reflux pipe to the original waste gas pipe before the waste gas inlet of the absorption tower. This means that the non-condensable gas is returned to the front end of the system to mix with fresh waste gas before re-entering the absorption distillation unit for further treatment. Simultaneously, the gas outlet can also be connected to the exhaust tower via another branch, but under normal operating conditions, the reflux method is preferred to achieve a closed-loop circulation. When the VOC concentration in the gas outlet gas of the gas-liquid separator is extremely low and meets emission standards, it can also be directly discharged into the exhaust tower.

[0027] The exhaust tower is connected to the purified gas outlets of the first and second activated carbon adsorption units, respectively. The exhaust tower has a chimney-type structure, with its height set according to environmental protection requirements, and is used to discharge the purified exhaust gas, after adsorption treatment, into the atmosphere.

[0028] The control valve assembly includes multiple pneumatic or electric control valves and a PLC controller. Each control valve is installed at key locations such as the exhaust gas inlet pipe, the purified gas outlet pipe, and the vacuum extraction pipe. The PLC controller is electrically connected to all control valves and also to the temperature and pressure sensors installed inside the activated carbon adsorption unit. The PLC controller periodically switches the operating states of the two activated carbon adsorption units according to a preset sequence. When the first activated carbon adsorption unit is performing adsorption, the second activated carbon adsorption unit performs vacuum desorption; when the first activated carbon adsorption unit becomes saturated, it switches to desorption, while the second activated carbon adsorption unit switches to adsorption. This cycle repeats continuously, achieving a "one adsorption, one desorption" operation mode. The PLC controller also automatically adjusts the operating frequency of the vacuum pump and the valve opening based on the temperature and pressure signals fed back from the sensors, thereby optimizing desorption efficiency and energy consumption.

[0029] Please see Figure 2 Example 2: This example uses mixed organic waste gas emitted by a chemical company. The company's production workshop mainly uses solvents such as acetone, isopropanol, ethyl acetate, toluene, and xylene. The composition of the waste gas is dynamic rather than constant. Based on 72 hours of continuous online monitoring data, the concentration ranges of each component in the waste gas are as follows: Acetone concentrations fluctuated between 620 ppm and 1150 ppm, with a 72-hour average of 843 ppm; isopropanol concentrations ranged from 480 ppm to 890 ppm, with an average of approximately 652 ppm; ethyl acetate concentrations ranged from 310 ppm to 560 ppm, with an average of approximately 437 ppm; toluene concentrations ranged from 210 ppm to 460 ppm, with an average of approximately 328 ppm; xylene concentrations ranged from 130 ppm to 310 ppm, with an average of approximately 217 ppm; chloroform concentrations ranged from 70 ppm to 180 ppm, with an average of approximately 121 ppm; trace amounts of hydrogen chloride (35 ppm to 78 ppm) and saturated water vapor were also present. The total exhaust gas flow rate was 5000 Nm³. 3 The flow rate is approximately ±8% per hour, fluctuating by ±8% due to variations in production load. The temperature is 35±3℃, and the pressure is atmospheric pressure. The following data is based on records from a typical operating cycle (8 hours of adsorption + desorption cycle). During this cycle, the average total VOCs concentration at the exhaust gas inlet was 2417 mg / m³. 3 .

[0030] S1, the raw VOCs waste gas is collected by the collection system and then enters the absorption tower. The absorption tower uses water as the absorbent (depending on the fluctuating characteristics of the waste gas, it can also be switched online to a 5-15wt% dilute organic solution to enhance the absorption effect). The absorbent is transported from the bottom of the distillation tower to the top spray layer by a water circulation pump, with a spray density of 12.5 m³ / s (design value). 3 / (m 2 During operation (h), the actual pressure fluctuation at the pump outlet ranges from 11.8 to 13.2 m. 3 / (m 2 The temperature varies between ·h). The exhaust gas flows counter-currently with the absorbent flowing from top to bottom in a structured packing layer with a height of 4.5 meters. Water-soluble components in the exhaust gas, such as acetone, isopropanol, and ethyl acetate, dissolve in the water, and hydrogen chloride is also absorbed by the water at the same time.

[0031] During operation, the concentration of water-soluble VOCs in the first purified tail gas at the absorber outlet is not constant but varies with the inlet concentration. Data shows that when the inlet acetone concentration fluctuates between 680 ppm and 1020 ppm, the outlet acetone concentration fluctuates between 9 ppm and 28 ppm, with a single-stage removal rate between 96.8% and 98.9%, and a 72-hour average removal rate of 98.1%. When the inlet concentration of isopropanol is between 520 ppm and 810 ppm, the outlet concentration is between 7 ppm and 21 ppm, with an average removal rate of 98.3%. When the inlet concentration of ethyl acetate is between 340 ppm and 510 ppm, the outlet concentration is between 5 ppm and 18 ppm, with an average removal rate of 97.9%. The removal rate of hydrogen chloride is consistently above 99.5%, with an outlet concentration below 0.8 ppm.

[0032] The rich liquid flowing from the bottom of the absorber enters the distillation column at a flow rate of approximately 4.8 t / h. The distillation column operates at atmospheric pressure (101.3 kPa), with the top temperature automatically regulated between 67.2℃ and 73.5℃, and the bottom temperature controlled between 102.3℃ and 104.8℃. The theoretical number of trays is 20, and the reflux ratio is set at 2.5:1, but in practice, it varies between 2.4 and 2.6 due to fluctuations in the reflux pump flow rate. The organic vapor distilled from the top of the column is cooled to 42℃ by a condenser before entering the reflux tank. A portion of the condensate in the reflux tank is returned to the distillation column as reflux, and the remainder is collected as product. During this operating cycle (8 hours), approximately 186 liters of mixed organic solvent were collected, equivalent to [amount missing] Nm³ / h. 3 2.33 kg of organic solvent was recovered from the waste gas. Gas chromatography analysis revealed that the product contained 49.2% acetone, 26.7% ethyl acetate, and 21.3% isopropanol, with other organic matter (mainly toluene, xylene, and other entrained components) accounting for approximately 2.8%. The overall purity of the product was 97.2%. The water content, determined by the Karl Fischer method, was 0.43%, making it suitable for direct use as a mixed solvent in in-plant cleaning processes or for external sale. The organic matter content in the wastewater discharged from the bottom of the distillation column fluctuated between 42 mg / L and 68 mg / L, with an average of approximately 53 mg / L. After condensation, it was returned to the absorption tower for recycling via a water circulation pump. Due to system evaporation losses and continuous low-flow discharge to ensure the quality of the bottom water, the fresh water replenishment was approximately 3.8% of the circulating flow, resulting in a calculated water recycling rate of 96.2%.

[0033] The purified exhaust gas from the absorber tower outlet directly enters the demister. The gas temperature at the demister outlet is approximately 38℃, and the concentration of entrained droplets decreases from approximately 85 mg / m³ at the inlet. 3 Reduced to 6 mg / m³ at export 3 The downstream activated carbon is effectively protected, and the non-condensable gas in the reflux tank is directly introduced into the exhaust gas inlet of the absorption tower.

[0034] S2, the first purified exhaust gas from the demister, is introduced into the first activated carbon adsorption unit (currently in an adsorption state) through a control valve assembly under the control of the PLC controller (while the second activated carbon adsorption unit is in a desorption state). The first activated carbon adsorption unit is filled with granular activated carbon with an iodine value of 928 mg / g and a specific surface area of ​​approximately 985 m². 2 The gas concentration is 1.5m, the gas velocity in the empty tower is controlled between 0.28-0.5m / s, and the residence time is approximately 3 to 7.5 seconds. Insoluble VOCs (toluene, xylene, chloroform) and a small amount of residual water-soluble VOCs in the waste gas are adsorbed and captured as they pass through the activated carbon bed. The adsorption process is physical adsorption, with minimal heat release; the bed temperature rise does not exceed 5℃.

[0035] After adsorption by activated carbon, the second purified exhaust gas is discharged from the purified gas outlet and enters the exhaust tower for high-altitude emission. Online gas chromatography results show that the total VOCs concentration in the second purified exhaust gas is below 25 mg / m³. 3 (≤30 mg / m 3 (where the toluene concentration is ≤5.3 mg / m³) 3 Xylene ≤3.4mg / m 3 2.5 mg / m³ of chloroform 3 Acetone ≤ 5.1 mg / m³ 3 Isopropanol ≤4.2mg / m³ 3 Ethyl acetate ≤3.3 mg / m 3 The total VOCs removal rate of the system (based on the original total concentration of exhaust gas at the inlet of approximately 2450 mg / m³) 3 The outlet concentration is ≤25mg / m³. 3 The adsorption capacity reached over 99.0%. After 8 hours of continuous operation, the outlet concentration of the first activated carbon adsorption unit began to rise, indicating that its adsorption capacity was approaching saturation. At this point, it was necessary to switch to desorption mode.

[0036] S3, when the first activated carbon adsorption unit reaches adsorption saturation (judged by the outlet VOCs concentration reaching 80% of the inlet concentration or the running time reaching a preset threshold), the PLC controller automatically executes a switching operation: closing the exhaust gas inlet valve and the purified gas outlet valve of the first activated carbon adsorption unit, opening its vacuum extraction port valve, and simultaneously opening the exhaust gas inlet valve and the purified gas outlet valve of the second activated carbon adsorption unit, so that the second activated carbon adsorption unit begins adsorption, while the first activated carbon adsorption unit enters desorption.

[0037] Then, the vacuum pump in the vacuum desorption unit is activated to create a vacuum inside the first activated carbon adsorption unit. The vacuum pump gradually reduces the absolute pressure inside the unit and maintains it between 5-10 kPa. Under this low-pressure environment, VOCs molecules adsorbed in the pores of the activated carbon gain sufficient kinetic energy to desorb from the activated carbon surface, forming a desorption gas flow. Since no external heating is required during vacuum desorption, the desorption temperature is naturally maintained between 35-50°C due to the boiling point decrease caused by the pressure reduction, which is far lower than the high temperature of over 100°C required for vapor desorption. The entire desorption process lasts approximately 60 minutes.

[0038] The desorption gas stream is drawn out from the vacuum port of the first activated carbon adsorption unit, pressurized by a vacuum pump, and then sent to the gas-liquid separator. The gas-liquid separator separates the desorption gas stream into two phases: a liquid phase and a gas phase. The liquid phase is the recovered organic liquid, with an organic content of approximately 88% (≥85%) and a water content of less than 12%. The gas phase is a non-condensable gas, which still contains a small amount of low-boiling-point organic matter (mainly light components such as methane and ethane that are difficult to condense, and trace amounts of acetone, etc.).

[0039] After desorption and regeneration, the absolute pressure inside the first activated carbon adsorption unit returns to atmospheric pressure. Since no water vapor is introduced during desorption, the moisture content inside the activated carbon bed is only 1.5% (≤2%), requiring no additional hot air drying or cooling treatment before directly entering the next adsorption cycle. Testing showed that the working adsorption capacity recovery rate of the desorbed activated carbon for VOCs reached 93% (≥90%). Furthermore, due to the avoidance of thermal shock from high-temperature steam and damage to the pore structure by water molecules, the service life of the activated carbon is expected to be extended by approximately 60% (≥50%) compared to traditional steam desorption processes.

[0040] S4, the recovered organic liquid flowing out of the liquid outlet of the gas-liquid separator is collected in a recovery storage tank. After static sedimentation and dehydration, the organic matter content of this liquid can be increased to over 92%, and it can be used directly as an industrial crude solvent for cleaning, dilution, and other processes, or sent to the distillation section for further separation to obtain single components. In this embodiment, the resource utilization rate is approximately 92% (≥90%).

[0041] The non-condensable gas discharged from the gas-liquid separator outlet is recirculated through a return pipe to the original waste gas pipe before the absorber tower's waste gas inlet. After mixing with fresh waste gas, it re-enters the absorption distillation unit for further treatment. This return design ensures that no incompletely purified gas is directly released into the environment, forming a complete closed-loop circulation system. Continuous monitoring shows that the VOCs concentration fluctuation in the original waste gas pipe after return is less than 5%, which does not affect the stable operation of the absorption distillation unit.

[0042] Through the continuous operation of steps S1 to S4, the system achieves graded and differentiated treatment of water-soluble and sparingly soluble VOCs. At the same time, it fully utilizes or reprocesses the recovered liquid and non-condensable gas generated by vacuum desorption, achieving the effects of low energy consumption, high recovery rate and zero secondary pollution.

[0043] Example 3: To further verify the adaptability and treatment effect of the system of the present invention on VOCs waste gas with different components, this embodiment uses the same system configuration as in embodiment 1, and conducts continuous treatment tests on three types of industrial waste gas. The operating time for each type of waste gas is 72 hours, and key performance indicators are recorded.

[0044] Exhaust gas 1 is a highly water-soluble VOCs exhaust gas, mainly containing ethanol, isopropanol, and acetone, with a total concentration of approximately 3120 mg / m³. 3 ; Waste gas 2 is a highly insoluble VOCs waste gas, mainly containing toluene, xylene, and styrene, with a total concentration of approximately 2580 mg / m³. 3 ; Exhaust gas 3 is a mixed type of exhaust gas, with water-soluble components of approximately 1520 mg / m³. 3The insoluble component is approximately 1230 mg / m³. 3 .

[0045] During the experiment, the operating parameters of the absorption distillation unit were kept consistent (absorbent: water; theoretical plate number: 20; reflux ratio: 2.5), and the vacuum desorption parameters were kept consistent (absolute pressure: 8 kPa; desorption time: 60 minutes). The main test results are shown in Table 1 below: Table 1 Comparison of treatment effects under different waste gas composition conditions

[0046] As shown in Table 1, this invention performs best for high water-soluble VOCs waste gas (waste gas 1). The absorption distillation unit can efficiently recover water-soluble components, with extremely low activated carbon load and a total removal rate as high as 99.74%. For high sparingly soluble VOCs waste gas (waste gas 2), although the absorption distillation unit cannot directly recover sparingly soluble components, the distillation column can still separate some condensable organic matter. The activated carbon unit undertakes the main purification task, and the total removal rate still reaches 98.91%. For the most complex mixed waste gas (waste gas 3), the system exhibits a balanced treatment capacity, with all indicators at an excellent level.

[0047] Example 4: To further clarify the optimal operating conditions for vacuum desorption, this embodiment conducts single-factor optimization experiments on parameters such as absolute pressure and desorption time for vacuum desorption, based on Example 1. The same batch of activated carbon (granular activated carbon, iodine value approximately 950 mg / g) was used in the experiments under the same adsorption saturation conditions (adsorption inlet concentration approximately 200 mg / m³). 3 Toluene was adsorbed until the outlet concentration reached 80% of the inlet concentration. Different absolute desorption pressures (1 kPa, 5 kPa, 10 kPa, 15 kPa, 20 kPa) and desorption times (30 min, 45 min, 60 min, 75 min, 90 min) were set, and the adsorption capacity recovery rate and desorption energy consumption of the activated carbon after desorption were measured. The results are shown in Tables 3 and 4 below. Table 2 Comparison of effects under different absolute desorption pressures (fixed desorption time 60 min)

[0048] Table 3 Comparison of effects at different desorption times (fixed absolute pressure 8 kPa)

[0049] As shown in Table 2, the adsorption capacity recovery rate gradually increases with decreasing absolute desorption pressure, but energy consumption also gradually increases. When the absolute pressure drops to 1 kPa, the recovery rate reaches 96.2%, but the energy consumption rises to 1.18 kWh / kg VOCs; when the absolute pressure is 5-10 kPa, the recovery rate is between 91.8% and 94.5%, and the energy consumption is between 0.85-0.96 kWh / kg VOCs, showing the best overall performance.

[0050] As shown in Table 3, when the desorption time was extended from 30 minutes to 60 minutes, the recovery rate rapidly increased from 79.5% to 93.0%. Afterward, with further extension of the time, the increase in the recovery rate tended to plateau, while energy consumption increased linearly. Therefore, considering recovery rate, energy consumption, and operational efficiency, it is recommended to control the absolute desorption pressure within the range of 5-10 kPa and the desorption time at around 60 minutes. This approach achieves the lowest possible energy consumption while ensuring good regeneration results.

[0051] Based on the above optimization results, in practical applications, this invention can automatically adjust the operating frequency and desorption time of the vacuum pump through a PLC controller according to the characteristics of the exhaust gas and economic requirements, so as to achieve the best economic and technical balance.

[0052] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A VOCs waste gas desorption and treatment system, characterized in that, include: The absorption distillation unit is used to receive and process raw VOCs waste gas, thereby absorbing and separating water-soluble organic components in the waste gas, purifying and recovering high-purity organic solvents, and simultaneously discharging the first purified tail gas. At least one set of first activated carbon adsorption unit and second activated carbon adsorption unit, each activated carbon adsorption unit is equipped with a waste gas inlet, a purified gas outlet and a vacuum extraction port; The vacuum desorption unit is connected to the vacuum pumping ports of the first activated carbon adsorption unit and the second activated carbon adsorption unit respectively through pipes with control valves. The exhaust tower is connected to the purified gas outlets of the first activated carbon adsorption unit and the second activated carbon adsorption unit, respectively. A water recycling system is used to recycle and reuse the absorption water discharged from the absorption distillation unit. The control valve assembly is used to selectively introduce the first purified tail gas discharged from the absorption distillation unit into the exhaust gas inlet of the first activated carbon adsorption unit or the second activated carbon adsorption unit, and to switch the corresponding activated carbon adsorption unit to be connected to the vacuum desorption unit after adsorption saturation, so as to perform vacuum desorption and regeneration of the activated carbon inside.

2. The VOCs waste gas desorption treatment system according to claim 1, characterized in that, The absorption distillation unit includes: An absorption tower is provided with an absorbent spray layer and a gas phase outlet at the top, a rich liquid outlet at the bottom, and a waste gas inlet on the side or bottom. The distillation column has a feed inlet in the middle that is connected to the rich liquid outlet of the absorption column. The bottom of the distillation column has a wastewater outlet, the top has a gas phase outlet, and the bottom has a water inlet and a reflux inlet. When the liquid level inside the distillation column is insufficient, it is replenished through the water inlet. The condenser is connected to the vapor outlet of the distillation column; The reflux tank is connected to the outlet of the condenser. The liquid outlet of the reflux tank is divided into two paths: one path is connected to the top of the distillation column via a reflux pump, and the other path serves as the organic solvent product collection pipeline. The non-condensable gas outlet of the reflux tank is connected to the exhaust gas inlet of the absorption tower.

3. The VOCs waste gas desorption treatment system according to claim 2, characterized in that: The water recycling system includes a water circulation pump, circulation pipelines, and a condenser. The inlets of the condenser and the water circulation pump are connected to the bottom wastewater outlet of the distillation column. The outlet of the water circulation pump is connected to the absorbent spray layer of the absorption tower through the circulation pipelines, which is used to recycle the absorbent water discharged from the bottom of the distillation column back to the absorption tower.

4. The VOCs waste gas desorption treatment system according to claim 1, characterized in that: The vacuum desorption unit includes a vacuum pump and a gas-liquid separator. The inlet of the vacuum pump is connected to the vacuum pump port of the first activated carbon adsorption unit and the vacuum pump port of the second activated carbon adsorption unit through the vacuum main pipeline and the corresponding branch valve, respectively. The outlet of the vacuum pump is connected to the inlet of the gas-liquid separator. The gas outlet of the gas-liquid separator is connected to the waste gas inlet of the absorption tower or the exhaust tower. The liquid outlet is used to recover the desorbed organic liquid.

5. The VOCs waste gas desorption treatment system according to claim 1, characterized in that: A demister is also provided between the absorption distillation unit and the first and second activated carbon adsorption units to remove liquid droplets entrained in the first purified exhaust gas.

6. The VOCs waste gas desorption treatment system according to claim 1, characterized in that: The adsorption material filled inside the first and second activated carbon adsorption units is one or more of granular activated carbon, honeycomb activated carbon, or activated carbon fiber felt.

7. The VOCs waste gas desorption treatment system according to claim 1, characterized in that, The control valve group includes a PLC controller. When one activated carbon adsorption unit performs adsorption operation, the other activated carbon adsorption unit performs vacuum desorption operation, and the working state of the two activated carbon adsorption units is switched periodically according to a preset time sequence.

8. A method for VOCs waste gas desorption treatment, employing the VOCs waste gas desorption treatment system according to any one of claims 1-7, characterized in that, Includes the following steps: S1, the original VOCs waste gas is passed into the absorption and distillation unit, and the water-soluble VOCs components in the waste gas are absorbed by the absorbent. The rich liquid after absorption enters the distillation tower for distillation and separation to obtain high-purity organic solvent products and non-condensable first purified tail gas. The absorbent wastewater at the bottom of the distillation tower is condensed by the condenser and then recycled back to the absorption tower. S2, the first purified exhaust gas is introduced into the demister through the control valve group to remove entrained droplets, and then enters the first or second activated carbon adsorption unit in the adsorption state. The activated carbon is used to adsorb and purify the residual insoluble VOCs in the exhaust gas to obtain the second purified exhaust gas that meets the standards, and then it is discharged through the exhaust tower. At the same time, the organic vapor distilled from the top of the distillation tower enters the condenser, and after condensation, it enters the reflux tank. Part of the liquid in the reflux tank is taken out as product, and part of it is returned to the top of the distillation tower as reflux liquid. The non-condensable gas in the reflux tank is directly introduced into the exhaust gas inlet of the absorption distillation unit. S3, when the activated carbon adsorption unit in the adsorption state reaches saturation, the control valve group switches it to the desorption state, closes its exhaust gas inlet and purified gas outlet, and turns on the vacuum desorption unit to evacuate the inside of the activated carbon adsorption unit, so that the VOCs adsorbed on the activated carbon are decomposed and extracted to form a desorption gas flow. S4, the desorbed gas flow is drawn out for condensation or gas-liquid separation to recover the liquid organic components, and the remaining non-condensable gas is returned to the exhaust gas inlet of the absorption tower in step S1 for further treatment.

9. A method for VOCs waste gas desorption treatment according to claim 8, characterized in that: In step S3, when the vacuum desorption unit is evacuated, the absolute pressure inside the activated carbon adsorption unit is controlled between 1-20 kPa and the desorption temperature is controlled between 30-60°C, without the need for external heating.

10. A method for VOCs waste gas desorption treatment according to claim 8, characterized in that: In step S1, the absorbent is water or a dilute solution containing 5-15 wt% organic solvent, the operating pressure of the distillation column is atmospheric pressure or slightly positive pressure, the theoretical number of plates is 15-30, and the reflux ratio is 1:1 to 5:1.

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