System and method for fractional recovery of tar and phenolic substances from phenolic resin carbonization off-gas

Abstract

The application provides a system and method for grading recovery of tar and phenolic substances in phenolic resin carbonization tail gas, and relates to the technical field of industrial waste gas treatment and resource recovery. By first separating and collecting heavy components of tar from the gas phase at 500-900 DEG C using a ceramic membrane, then selectively absorbing phenolic compounds at 150-200 DEG C, and subsequently recovering phenol, cresol and dimethyl phenol through three-stage flash evaporation, the tar and phenolic substances are separated and recovered above their respective condensation / solidification temperatures, thereby avoiding the dual problems of tar condensation and deposition to block pipelines and phenolic crystallization to block equipment from the source, while achieving efficient grading recovery of tar and phenolic substances, and being suitable for industrial production scenarios of porous carbon materials prepared by using phenolic resin as a precursor.

Description

Technical Field

[0001] This invention relates to the field of industrial waste gas treatment and resource recovery technology, and in particular to a graded recovery system and method for tar and phenolic substances in the carbonization tail gas of phenolic resin. Background Technology

[0002] The carbonization process of phenolic resin produces a large amount of exhaust gas, which consists of high-boiling-point tar, phenolic compounds, low-carbon hydrocarbons, carbon monoxide, hydrogen and water vapor, making its composition extremely complex.

[0003] Currently, incineration processes, such as regenerative thermal oxidation (RTO) and direct-fired incinerators (TO), are commonly used to treat this type of exhaust gas. However, these incineration processes have the following drawbacks in practical applications: tar components in the exhaust gas are prone to condensation and deposition when the temperature decreases, clogging pipes and equipment and severely affecting the continuous operation of the system; tar and phenolic substances, as high-value-added chemical raw materials, are destroyed during incineration, resulting in resource waste; furthermore, existing technologies lack methods for the graded separation of exhaust gas components, allowing a large amount of condensable matter to enter subsequent incineration equipment, increasing the processing load.

[0004] Because tar is prone to condensation and blockage during cooling, those skilled in the art tend to avoid staged condensation of the exhaust gas in process design to prevent accelerated pipeline failure. This further reinforces the technological path dependence of direct incineration, resulting in the failure of the staged recovery approach to achieve substantial application in the treatment of phenolic resin carbonization exhaust gas to date.

[0005] Therefore, how to achieve efficient graded recovery of tar and phenolic substances from the carbonization tail gas of phenolic resin without causing tar deposition and blockage in pipelines and equipment is an urgent problem to be solved. Summary of the Invention

[0006] To address the aforementioned issues, this invention first separates and collects heavy tar components from the gas phase using a ceramic membrane at 500-900°C. Then, it selectively absorbs phenolic compounds using a high-boiling-point organic absorbent at 150-200°C. Subsequently, phenol, cresol, and xylenol are recovered through three-stage flash evaporation. Throughout the process, tar and phenolic substances are separated and recovered above their respective condensation / solidification temperatures. This avoids the dual problems of tar condensation and deposition clogging pipelines and phenolic crystallization precipitating and clogging equipment from the source, while simultaneously achieving efficient graded recovery of tar and phenolic substances.

[0007] One objective of this invention is to provide a staged recovery system for tar and phenolic substances in the carbonization exhaust gas of phenolic resin, comprising the following components connected sequentially in the direction of exhaust gas flow: A high-temperature ceramic membrane separation unit is used to intercept and collect the heavy tar components in the carbonized tail gas of phenolic resin at 500~900℃ to obtain the first tail gas. A high-temperature absorption unit is connected to the outlet of the high-temperature ceramic membrane separation unit and is used to absorb phenolic compounds in the first tail gas at 150~200℃ to obtain a phenol-rich liquid and a second tail gas; the high-temperature absorption unit includes a rich liquid outlet and a gas outlet. A three-stage flash evaporation unit is connected to the rich liquor outlet of the high-temperature absorption unit; it is used for staged flash evaporation of the phenol-containing rich liquor to recover phenol, cresol, and xylenol; the three-stage flash evaporation unit includes a primary flash evaporation unit, a secondary flash evaporation unit, and a tertiary flash evaporation unit connected in series; and, A gas combustion unit is connected to the gas outlet of the high-temperature absorption unit and is used to incinerate the second tail gas.

[0008] Preferably, the high-temperature ceramic membrane separation unit includes a retention side and a permeation side. The retention side is provided with a tail gas inlet and a tar outlet, and the permeation side is provided with a gas outlet. The phenolic resin carbonization tail gas enters through the tail gas inlet, the heavy tar components are retained on the retention side and discharged through the tar outlet, and the first tail gas is discharged through the gas outlet.

[0009] Preferably, the high-temperature ceramic membrane separation unit includes a high-temperature ceramic membrane separator; The membrane pore size of the high-temperature ceramic membrane separator is 0.1~0.5μm; and / or, The membrane material of the high-temperature ceramic membrane separator includes silicon carbide ceramic membrane or alumina-based ceramic membrane.

[0010] Preferably, the high-temperature absorption unit is filled with a high-boiling-point organic absorbent; The high-boiling-point organic absorbent includes at least one of dioctyl phthalate, 1-ethyl-3-methylimidazolium hexafluorophosphate, or 1-butyl-3-methylimidazolium chloride.

[0011] Preferably, the operating temperature of the first-stage flash evaporation unit is 160~170℃ and the pressure is 0.3~0.5MPa; the operating temperature of the second-stage flash evaporation unit is 180~190℃ and the pressure is 0.1~0.2MPa; and the operating temperature of the third-stage flash evaporation unit is 200~210℃ and the pressure is 0.08~0.12MPa.

[0012] Preferably, the graded recovery system for tar and phenolic substances in the carbonization tail gas of phenolic resin further includes: The first condenser is connected to the outlet of the first-stage flash unit and is used to condense and recover phenol; A second condenser, connected to the outlet of the secondary flash evaporation unit, is used to condense and recover cresol; and, The third condenser is connected to the outlet of the three-stage flash evaporation unit and is used to condense and recover xylenol.

[0013] Preferably, the condensing temperatures of the first condenser, the second condenser, and the third condenser are each independently 40~60℃.

[0014] Preferably, the gas combustion unit includes: An incineration device, connected to the gas outlet of the high-temperature absorption unit, is used to incinerate the second gas to obtain a ventable gas; and, A waste heat recovery device is connected to the outlet of the incineration device to recover heat from the exhaust gases.

[0015] Preferably, the graded recovery system for tar and phenolic substances in the carbonization tail gas of phenolic resin further includes an absorbent circulation pipeline, which is set between the high-temperature absorption unit and the three-stage flash evaporation unit, for sending the regenerated absorbent discharged from the three-stage flash evaporation unit back to the high-temperature absorption unit for recycling.

[0016] The second objective of this invention is to provide a method for the graded recovery of tar and phenolic substances from the carbonization tail gas of phenolic resin, comprising the following steps: The carbonized tail gas of phenolic resin at 500~900℃ is passed into the high-temperature ceramic membrane separation unit to intercept and collect the heavy tar components, thus obtaining the first tail gas. The first tail gas is passed into the high-temperature absorption unit, and the phenolic compounds in the first tail gas are absorbed at 150~200°C to obtain a phenol-rich liquid and a second tail gas. The phenol-rich solution is passed into the three-stage flash evaporation unit for graded flash evaporation to recover phenol, cresol and xylenol respectively. The second exhaust gas is passed into a gas combustion unit for incineration treatment before being discharged.

[0017] The beneficial effects of this invention are: This invention utilizes the molecular size sieving effect of a ceramic membrane to trap and collect heavy tar components in a high-temperature gas phase state (500-900℃), followed by absorption of phenolic compounds in a high-temperature absorption unit at 150-200℃. Throughout the process, tar and phenolic substances are separated and recovered above their respective condensation / solidification temperatures, thus avoiding the dual problems of tar condensation and deposition clogging pipelines and phenolic crystallization and precipitation clogging equipment. Furthermore, phenol, cresol, and xylenol are recovered sequentially through three-stage flash evaporation, achieving highly efficient graded recovery of tar and phenolic substances. Each product has high purity and can be directly used as a chemical raw material. Detailed Implementation

[0018] The present application will now be described in further detail with reference to embodiments. In the following description, certain specific details are included to provide a comprehensive understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments can be implemented without employing one or more of these specific details, but using other methods, components, materials, etc. Unless otherwise required by the present invention, the terms "comprising" and "including" should be interpreted in an open-ended, inclusive sense, meaning "including but not limited to". Throughout this specification, "an embodiment," "an embodiment," "a preferred embodiment," or "some embodiments" means that at least one embodiment includes a specific reference element, structure, or feature related to that embodiment. Therefore, the phrases "in an embodiment," "in an embodiment," "in a preferred embodiment," or "in some embodiments" appearing in different places throughout the specification do not necessarily all refer to the same embodiment. Furthermore, specific elements, structures, or features may be combined in one or more embodiments in any suitable manner.

[0019] According to a first aspect of the present invention, a staged recovery system for tar and phenolic substances in carbonized tail gas of phenolic resin is provided, comprising the following components connected sequentially in the direction of tail gas flow: A high-temperature ceramic membrane separation unit is used to intercept and collect the heavy tar components in the carbonized tail gas of phenolic resin at 500~900℃ to obtain the first tail gas. A high-temperature absorption unit is connected to the outlet of the high-temperature ceramic membrane separation unit and is used to absorb phenolic compounds in the first tail gas at 150~200℃ to obtain a phenol-rich liquid and a second tail gas; the high-temperature absorption unit includes a rich liquid outlet and a gas outlet. A three-stage flash evaporation unit is connected to the rich liquor outlet of the high-temperature absorption unit; it is used for staged flash evaporation of the phenol-containing rich liquor to recover phenol, cresol, and xylenol; the three-stage flash evaporation unit includes a primary flash evaporation unit, a secondary flash evaporation unit, and a tertiary flash evaporation unit connected in series; and, A gas combustion unit is connected to the gas outlet of the high-temperature absorption unit and is used to incinerate the second tail gas.

[0020] In this invention, the condensation temperature of heavy tar components in the carbonized tail gas of phenolic resin is relatively high, making them prone to precipitation and adhesion to the inner walls of pipes and equipment when the temperature decreases. If the tail gas is cooled before tar removal, the tar will condense and deposit during the cooling process, inevitably causing blockage. This invention places the ceramic membrane separation unit at the very beginning of the system and controls the operating temperature at 500~900℃. The purpose is to retain and separate the heavy tar components by utilizing the molecular size sieving effect of the ceramic membrane before the tail gas enters any cooling equipment, while the tar is still in a high-temperature state of gaseous or liquid droplets. The tar components do not undergo a cooling process from beginning to end, thus eliminating the possibility of condensation, deposition, and blockage in pipes and equipment from the source. This is completely different from the conventional approach of cooling the tail gas first to precipitate the tar before treatment.

[0021] In the first tail gas after separation by the high-temperature ceramic membrane separation unit, tar-heavy components have been removed, but phenolic compounds still need to be recovered. In existing technologies, phenolic recovery typically employs condensation or alkaline absorption. Condensation requires cooling the tail gas to below the phenol dew point, but this cooling process may cause residual tar to co-condense and cause blockage. While alkaline absorption can absorb phenols, it generates large amounts of high-salt, phenol-containing wastewater, resulting in secondary pollution. This invention places the high-temperature absorption unit after the high-temperature ceramic membrane separation unit, ensuring that the gas entering the high-temperature absorption unit is clean, phenol-containing gas free of tar-heavy components, thus avoiding tar contamination of the absorbent. The absorption temperature is controlled at 150~200℃, ensuring that the operating temperature is always above the freezing points of phenol, cresol, and xylenol, guaranteeing that phenols remain in gaseous form during absorption and do not crystallize within the equipment.

[0022] After high-temperature absorption, phenolic compounds accumulate in the absorbent, forming a phenol-rich solution. This invention employs a three-stage flash evaporation unit, utilizing the boiling point differences of phenol, cresol, and xylenol to recover these compounds in the first-stage, second-stage, and third-stage flash evaporation processes, respectively.

[0023] After absorption by the high-temperature absorption unit, the tar and phenols in the exhaust gas have been fully recovered at the front end. The second exhaust gas entering the gas combustion unit is a clean combustible gas mainly containing CO, H2, and CH4, and does not contain tar or phenolic components. Passing this gas into the gas combustion unit for incineration not only achieves complete decomposition of residual organic matter but also avoids the problem of tar clogging the heat storage medium. Tar clogging of the heat storage medium is a fatal flaw in existing RTO processes when treating tar-containing exhaust gases.

[0024] In a preferred embodiment of the present invention, the high-temperature ceramic membrane separation unit includes a retention side and a permeation side. The retention side is provided with a tail gas inlet and a tar outlet, and the permeation side is provided with a gas outlet. The phenolic resin carbonization tail gas enters through the tail gas inlet, the heavy tar components are retained on the retention side and discharged through the tar outlet, and the first tail gas is discharged through the gas outlet.

[0025] In a preferred embodiment of the present invention, the high-temperature ceramic membrane separation unit includes a high-temperature ceramic membrane separator; The membrane pore size of the high-temperature ceramic membrane separator is 0.1~0.5μm, for example, 0.1μm, 0.2μm, 0.3μm, 0.4μm or 0.5μm; and / or, The membrane material of the high-temperature ceramic membrane separator includes silicon carbide ceramic membrane or alumina-based ceramic membrane.

[0026] The tar heavy components in the carbonization tail gas of phenolic resin have relatively large molecular sizes, while the phenolic compounds and non-condensable gases have smaller molecular sizes. Controlling the membrane pore size to 0.1–0.5 μm allows phenolic compounds and light gases to pass smoothly through the membrane pores into the permeate side while effectively retaining large molecular weight heavy components and asphaltenes in the tar. If the pore size is too large, the tar heavy components easily permeate the membrane pores, leading to a decrease in separation efficiency; if the pore size is too small, the membrane flux decreases significantly, the system pressure drop increases, and the processing capacity is affected. Therefore, a pore size range of 0.1–0.5 μm is a balanced choice between ensuring separation accuracy and processing flux.

[0027] Regarding membrane materials, conventional organic polymer separation membranes in this field are prone to thermal deformation or degradation at high temperatures and cannot withstand operating temperatures of 500–900°C. Porous ceramic membranes, on the other hand, possess excellent chemical inertness, high mechanical strength, and high-temperature resistance, making them suitable for long-term operation under extreme conditions. Furthermore, alumina (Al2O3) ceramic membranes are among the most technologically mature ceramic membrane materials, widely used in water treatment and low-temperature flue gas treatment, exhibiting good separation performance and high cost-effectiveness. Silicon carbide (SiC) ceramic membranes offer even more outstanding advantages, including high-temperature resistance, thermal shock resistance, corrosion resistance, high throughput, and long service life. Their operating temperature can reach 800–1000°C, giving them a unique advantage in high-temperature flue gas treatment, effectively handling tar-containing syngas generated in biomass and waste gasification plants. Compared to alumina membranes, silicon carbide membranes offer superior throughput, corrosion resistance, and thermal shock resistance. This invention allows for flexible selection of either material, or a combination of both, depending on the exhaust gas conditions and cost requirements.

[0028] The preferred combination of a membrane pore size of 0.1~0.5μm and silicon carbide / alumina ceramic membrane material ensures efficient retention of heavy tar components and smooth permeation of phenolic components at high temperatures of 500~900℃, while also ensuring long-term stable operation of the high-temperature ceramic membrane separation unit in a high-temperature corrosive atmosphere. This provides clean phenol-containing gas free of tar for the subsequent high-temperature absorption unit to recover phenols.

[0029] In a preferred embodiment of the present invention, the high-temperature absorption unit is filled with a high-boiling-point organic absorbent. The high-boiling-point organic absorbent includes at least one of dioctyl phthalate, 1-ethyl-3-methylimidazolium hexafluorophosphate, or 1-butyl-3-methylimidazolium chloride.

[0030] The absorbent needs to possess both a high boiling point and thermal stability to withstand high-temperature operating conditions of 150–200°C. Dioctyl phthalate has a boiling point of 386.9°C, while 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium chloride, as ionic liquids, exhibit extremely low vapor pressures and excellent thermal stability. These absorbents remain liquid without decomposition or volatilization at operating temperatures of 150–200°C, allowing for long-term stable operation. If a low-boiling-point absorbent is selected, significant volatilization losses will occur under high-temperature operating conditions, increasing operating costs and potentially causing secondary pollution.

[0031] The absorbent needs to have high selectivity and high absorption capacity for phenolic compounds. Dioctyl phthalate, as a stationary phase in gas chromatography, has been proven to selectively retain and separate aromatic compounds, unsaturated compounds, and various oxygen-containing compounds. Phenolic compounds are hydroxyl-containing aromatic compounds, and there is a strong intermolecular interaction between them and dioctyl phthalate, allowing for selective absorption. Furthermore, nonpolar noncondensable gases such as H2, CO, and CH4 have extremely low solubility in dioctyl phthalate, thus achieving high-temperature selective separation of phenols from noncondensable gases. Regarding ionic liquids, imidazole-based ionic liquids have been proven to be highly efficient non-aqueous media for the extraction and separation of phenolic compounds from oils. Among them, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) has a strong extraction capacity for phenol. 1-Ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) has also been proven to be suitable for the extraction and separation of phenolic substances. Phenolic compounds are a class of weak protic acids, and their hydroxyl hydrogen atoms can react with anions of ionic liquids (such as Cl-). - PF6 - Hydrogen bonds are formed, and there is also a π-π stacking interaction between the imidazole ring cation and the phenolic aromatic ring. These multiple interactions give the imidazole ionic liquid an excellent selective absorption capacity for phenols.

[0032] The absorbent must be immiscible or have extremely low solubility with water and non-condensable gases to ensure its long-term cyclical use. Dioctyl phthalate has a solubility of less than 0.01% in water at 25°C, making it almost insoluble; 1-Butyl-3-methylimidazolium chloride is also insoluble in water. The hydrophobic properties of these absorbents prevent them from emulsifying or deteriorating due to water absorption in exhaust gas environments containing water vapor, ensuring their stability and cycle life. Simultaneously, non-condensable gases such as H2, CO, and CH4 have extremely low solubility in these absorbents and will not be absorbed, remaining in the rich solution, thus ensuring the purity of the phenolic products recovered through subsequent flash evaporation.

[0033] In summary, both dioctyl phthalate and imidazole ionic liquids possess characteristics such as high boiling point, good thermal stability, high selectivity for phenols, and low solubility in water and non-condensable gases, which can match the operational requirements of the high-temperature absorption of this invention at 150~200℃.

[0034] As an example, the high-boiling-point organic absorbent may be dioctyl phthalate; or 1-ethyl-3-methylimidazolium hexafluorophosphate; or 1-butyl-3-methylimidazolium chloride; or a combination of dioctyl phthalate and 1-ethyl-3-methylimidazolium hexafluorophosphate; or a combination of dioctyl phthalate and 1-butyl-3-methylimidazolium chloride; or a combination of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium chloride; or a combination of dioctyl phthalate, 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium chloride.

[0035] In a preferred embodiment of the present invention, the high-boiling-point organic absorbent is a compound absorbent of dioctyl phthalate and 1-ethyl-3-methylimidazolium hexafluorophosphate.

[0036] The high-boiling-point organic absorbent combines dioctyl phthalate with an ionic liquid, leveraging the respective performance advantages of each. Dioctyl phthalate exhibits excellent solubility and selectivity for phenols, while the ionic liquid further enhances the absorbent's affinity and absorption capacity for phenols. Furthermore, the combined system remains liquid without decomposition or volatilization at operating temperatures ranging from 150 to 200°C, ensuring long-term stable operation.

[0037] In a preferred embodiment of the present invention, the operating temperature of the first-stage flash evaporation unit is 160~170℃ and the pressure is 0.3~0.5MPa; the operating temperature of the second-stage flash evaporation unit is 180~190℃ and the pressure is 0.1~0.2MPa; and the operating temperature of the third-stage flash evaporation unit is 200~210℃ and the pressure is 0.08~0.12MPa.

[0038] The temperature settings for the three-stage flash evaporation are based on the boiling point differences between phenol, cresol, and xylenol. Phenol has a normal-pressure boiling point of 181.7–181.9 °C. Cresol exists in three isomers: ortho, meta, and para, with boiling points ranging from 191–203 °C. Specifically, ortho-cresol has a boiling point of 191 °C, meta-cresol 202.2 °C, and para-cresol 202 °C. Industrial xylenol is a mixture of six isomers, with a boiling range of 203–225 °C. These boiling point differences constitute the thermodynamic basis for the staged flash evaporation. The first-stage flash evaporation temperature was set at 160–170°C. At this temperature and under a pressure of 0.3–0.5 MPa, phenol possessed sufficient vapor pressure to preferentially vaporize and separate from the rich liquid, while cresol and xylenol, due to their higher boiling points, remained in the liquid phase. The second-stage flash evaporation temperature was set at 180–190°C, and the pressure was reduced to 0.1–0.2 MPa. Under reduced pressure, the boiling point of cresol decreased to this temperature range, and it vaporized and separated in this stage of flash evaporation. The third-stage flash evaporation temperature was set at 200–210°C, and the pressure was further reduced to 0.08–0.12 MPa. Xylenol vaporized and separated under these conditions. The three stages of temperature increased progressively, matching the boiling point gradient of the three phenolic substances, ensuring the purity of the flash evaporation products at each stage.

[0039] The temperatures of the three-stage flash evaporation are all higher than the freezing points of phenol, cresol, and xylenol. Phenol's freezing point is 40.6℃, the melting range of the cresol mixture is 11~35℃ (29.8℃ for o-cresol, 8~10℃ for m-cresol, and 34.69℃ for p-cresol), and the melting range of industrial xylenol is 20~76℃. The operating temperature of the three-stage flash evaporation in this invention (160~210℃) is much higher than the above freezing point range, ensuring that phenols remain in a gaseous state throughout the flash evaporation process and do not crystallize within the equipment. Combined with subsequent condensation using 40~60℃ warm water, this further ensures that the phenols remain in a liquid flow state during the condensation and collection process, eliminating the risk of phenol crystallization clogging pipes and equipment from the source.

[0040] The design with progressively decreasing pressure offers advantages in energy utilization. The first-stage flash evaporation operates at a relatively high pressure of 0.3–0.5 MPa, using pressurization to increase the boiling point of phenol, enabling its selective vaporization at 160–170 °C. The second-stage flash evaporation operates at 0.1–0.2 MPa, and the third-stage flash evaporation at 0.08–0.12 MPa, with pressures decreasing progressively. This allows for throttling and depressurization flash evaporation using the residual pressure from the previous stage, eliminating the need for additional compression work and resulting in more efficient energy utilization. Simultaneously, the progressively decreasing pressure helps lower the boiling points of cresol and xylenol, enabling the separation of components at relatively mild temperatures during each stage of flash evaporation, avoiding the risk of thermal degradation of the absorbent due to excessively high temperatures.

[0041] In summary, the temperature and pressure parameters of the three-stage flash evaporation unit described in this invention are matched based on the differences in boiling points and freezing point characteristics of phenol, cresol, and xylenol. The temperature increases stepwise to match the boiling point gradient of the three phenols, and the pressure decreases stepwise to achieve energy cascade utilization. At the same time, the pressure is kept above the freezing point of phenols throughout the process to prevent crystallization blockage. Together, these factors ensure the efficiency of staged recovery, product purity, and the reliability of system operation.

[0042] In this invention, the operating temperature of the primary flash evaporation unit is, for example, 160℃, 161℃, 162℃, 163℃, 164℃, 165℃, 166℃, 167℃, 168℃, 169℃, or 170℃; and the operating pressure is, for example, 0.3MPa, 0.32MPa, 0.34MPa, 0.36MPa, 0.38MPa, 0.4MPa, 0.42MPa, 0.44MPa, 0.46MPa, 0.48MPa, or 0.5MPa.

[0043] The operating temperature of the secondary flash evaporation unit is, for example, 180℃, 181℃, 182℃, 183℃, 184℃, 185℃, 186℃, 187℃, 188℃, 189℃, or 190℃; the operating pressure is, for example, 0.1MPa, 0.11MPa, 0.12MPa, 0.13MPa, 0.14MPa, 0.15MPa, 0.16MPa, 0.17MPa, 0.18MPa, 0.19MPa, or 0.2MPa.

[0044] The operating temperature of the three-stage flash evaporation unit is, for example, any value or a range between any two of 200℃, 201℃, 202℃, 203℃, 204℃, 205℃, 206℃, 207℃, 208℃, 209℃, or 210℃; the operating pressure is, for example, 0.08MPa, 0.084MPa, 0.088MPa, 0.09MPa, 0.092MPa, 0.096MPa, 0.1MPa, 0.104MPa, 0.108MPa, 0.112MPa, 0.116MPa, or 0.12MPa.

[0045] In a preferred embodiment of the present invention, the graded recovery system for tar and phenolic substances in the carbonization tail gas of phenolic resin further includes: The first condenser is connected to the outlet of the first-stage flash unit and is used to condense and recover phenol; A second condenser, connected to the outlet of the secondary flash evaporation unit, is used to condense and recover cresol; and, The third condenser is connected to the outlet of the three-stage flash evaporation unit and is used to condense and recover xylenol.

[0046] Preferably, the condensing temperature of the first condenser, the second condenser, and the third condenser is 40~60℃.

[0047] The choice of condensation temperature is based on the freezing point characteristics of phenol, cresol, and xylenol.

[0048] Phenol has a freezing point of 40.6°C; cresol exists in three isomers: ortho, meta, and para, with a melting range of 11-35°C; industrial xylenol is a mixture of six isomers with a melting range of 20-76°C. If the condensation temperature is lower than the freezing point of these substances, the phenolic vapors will directly condense into solids in the condenser, leading to tube blockage and a sharp decrease in heat transfer efficiency, requiring shutdown and unblocking in severe cases. The lower limit of the condensation temperature in this invention is set at 40°C, which is just higher than the freezing point of phenol and the highest freezing point of the cresol mixture, ensuring that phenol and cresol remain in liquid form throughout the condensation process. For the higher melting point isomers of xylenol, although 40-60°C is lower than the freezing point of individual isomers, industrial xylenol is a mixture and can still maintain a liquid or at least partially liquid flow state at 40-60°C. Furthermore, the proportion of high-melting-point isomers in the flash-evaporated xylenol vapor is low, and with a reasonable condenser structure design, blockage will not occur. The upper limit of the condensation temperature is set at 60℃ to provide a sufficient heat transfer temperature difference. If the condensation temperature is too high, the steam cannot be effectively condensed, and the recovery rate will decrease significantly.

[0049] Warm water at 40~60℃ is preferred as the condensing medium, which is feasible in industrial applications.

[0050] In a preferred embodiment of the present invention, the gas combustion unit includes: An incineration device, connected to the gas outlet of the high-temperature absorption unit, is used to incinerate the second gas to obtain a ventable gas; and, A waste heat recovery device is connected to the outlet of the incineration device to recover heat from the exhaust gases.

[0051] After treatment by the high-temperature ceramic membrane separation unit and the high-temperature absorption unit, although tar and phenols in the second tail gas have been fully recovered, they still contain combustible components such as CO, H2, and CH4. The purpose of setting up the incineration unit is to completely oxidize and decompose these residual combustible components into CO2 and H2O at high temperatures, achieving the final emission standard for the tail gas. More importantly, since easily clogging substances such as tar and phenols have been efficiently removed at the front end, the second tail gas entering the incineration unit is a clean combustible gas. This precisely solves the common industry problem of heat storage clogging faced by existing regenerative thermal oxidation (RTO) technologies when treating tar-containing waste gas.

[0052] The purified flue gas discharged from incineration units typically reaches temperatures of several hundred degrees Celsius, and direct emission would result in a significant waste of thermal energy. This invention incorporates a waste heat recovery device downstream of the incineration unit, which can recover the sensible heat of the high-temperature flue gas for purposes such as generating steam, preheating combustion air, or providing heating for the plant area, thereby improving the overall energy utilization efficiency of the system.

[0053] In a preferred embodiment of the present invention, the graded recovery system for tar and phenolic substances in the carbonization tail gas of phenolic resin further includes an absorbent circulation pipeline disposed between the high-temperature absorption unit and the three-stage flash evaporation unit, for sending the regenerated absorbent discharged from the three-stage flash evaporation unit back to the high-temperature absorption unit for recycling.

[0054] After the phenol-rich liquid undergoes three-stage flash evaporation, the phenolic substances in the absorbent are removed by stepwise vaporization separation. The regenerated absorbent is cooled and returned to the high-temperature absorption unit through the circulation pipeline to re-contact with the first tail gas for phenol absorption. A cooler and a circulation pump can be installed on the circulation pipeline to control the temperature (150~200℃) and flow rate of the returned absorbent.

[0055] High-boiling-point organic absorbents are expensive, but recycling can significantly reduce the replenishment and operating costs of absorbents. Furthermore, closed-loop recycling of absorbents avoids the discharge of waste absorbents, and the entire process generates no waste liquid, eliminating the risk of secondary pollution and meeting green environmental protection requirements. In addition, the absorbents are regenerated in situ through three-stage flash evaporation during the recycling process, eliminating the need for additional independent regeneration equipment and simplifying the process flow.

[0056] According to a second aspect of the present invention, a method for graded recovery of tar and phenolic substances from carbonization tail gas of phenolic resin is provided, comprising the following steps: The carbonized tail gas of phenolic resin at 500~900℃ is passed into the high-temperature ceramic membrane separation unit to intercept and collect the heavy tar components, thus obtaining the first tail gas. The first tail gas is passed into the high-temperature absorption unit, and the phenolic compounds in the first tail gas are absorbed at 150~200°C to obtain a phenol-rich liquid and a second tail gas. The phenol-rich solution is passed into the three-stage flash evaporation unit for graded flash evaporation to recover phenol, cresol and xylenol respectively. The second exhaust gas is passed into a gas combustion unit for incineration treatment before being discharged.

[0057] In a preferred embodiment of the present invention, the method for graded recovery of tar and phenolic substances in the carbonization tail gas of phenolic resin specifically includes: High-temperature exhaust gas (500-900℃) generated during the carbonization of phenolic resin is fed into a high-temperature ceramic membrane separation unit. The ceramic membrane in this unit has a pore size of 0.1-0.5μm, and the membrane material is selected from silicon carbide ceramic membranes or alumina-based ceramic membranes. The exhaust gas enters the stagnation side of the ceramic membrane. Under pressure, smaller molecular sizes of phenolic compounds, water vapor, low-carbon hydrocarbons, CO, H2, and CH4 permeate through the ceramic membrane pores into the permeate side, exiting as the first exhaust gas. Larger molecular sizes of tar heavy components and asphaltenes are retained on the stagnation side and discharged as liquid high-temperature tar from the tar outlet at the bottom of the stagnation side, where they are collected.

[0058] The first tail gas discharged from the permeation side is sent to a high-temperature absorption unit, which is filled with a high-boiling-point organic absorbent selected from at least one of dioctyl phthalate, 1-ethyl-3-methylimidazolium hexafluorophosphate, or 1-butyl-3-methylimidazolium chloride. The first tail gas enters from the bottom of the absorption tower and comes into countercurrent contact with the absorbent sprayed down from the top of the tower at a temperature controlled at 150-200°C. Phenolic compounds such as phenol, cresol, and xylenol in the first tail gas are selectively absorbed by the absorbent, forming a phenol-rich liquid that is discharged from the bottom of the tower; while non-condensable gases such as H2, CO, and CH4 are not absorbed and are discharged from the top of the tower as the second tail gas.

[0059] The phenol-rich liquid discharged from the bottom of the tower is sequentially sent to a three-stage flash evaporation unit for staged flash regeneration: First-stage flash evaporation: The phenol-rich liquid is fed into the first-stage flash evaporation unit and flashed at a temperature of 160~170℃ and a pressure of 0.3~0.5MPa. Phenol preferentially vaporizes under these conditions due to its low boiling point. The flashed phenol vapor is condensed in the first condenser with warm water at 40~60℃ as the condensing medium to obtain the phenol product. The remaining liquid phase is fed into the second-stage flash evaporation unit. Secondary flash evaporation: The remaining liquid phase after primary flash evaporation is sent to the secondary flash evaporation unit for flash evaporation at a temperature of 180~190℃ and a pressure of 0.1~0.2MPa. Cresol is vaporized under these conditions. The flash cresol vapor is condensed in the second condenser with warm water at 40~60℃ as the condensing medium to obtain the cresol product. The remaining liquid phase is sent to the tertiary flash evaporation unit. Three-stage flash evaporation: The residual liquid phase after the two-stage flash evaporation is sent to the three-stage flash evaporation unit for flash evaporation at a temperature of 200~210℃ and a pressure of 0.08~0.12MPa. Xylenol is vaporized under these conditions. The flash-evaporated xylenol vapor is condensed in the third condenser with warm water at 40~60℃ as the condensing medium to obtain the xylenol product. The residual liquid phase is the regenerated absorbent. After being regenerated, the absorbent is cooled and then sent back to the high-temperature absorption unit for reuse through the absorbent circulation pipeline.

[0060] The second tail gas discharged from the top of the tower is sent to the gas combustion unit. The combustion device first oxidizes and decomposes the combustible components such as CO, H2, and CH4 in the second tail gas into CO2 and H2O at high temperature, and then discharges them after meeting the standards. The high-temperature flue gas generated by combustion is sent to the waste heat recovery device for heat recovery. The recovered heat can be used to generate steam, preheat combustion air, or provide heating for the plant area.

[0061] Example 1 The exhaust gas source is the exhaust gas from the outlet of the phenolic resin carbonization furnace, with a flow rate of 1000 Nm³ / h and a temperature of approximately 750℃. Its composition is as follows: approximately 8 g / Nm³ of tar, approximately 3 g / Nm³ of phenol, approximately 1.5 g / Nm³ of cresol, approximately 0.8 g / Nm³ of xylenol, and the remainder being CO, H2, CH4, CO2, and water vapor.

[0062] Carbonized tail gas at 750℃ is fed into a high-temperature ceramic membrane separation unit. The ceramic membrane has a pore size of 0.2μm and is made of silicon carbide ceramic. The tail gas enters the retention side, where heavy tar components are trapped and discharged from the tar outlet for collection, yielding a high-temperature tar product. Phenolic compounds and light gases permeate through the membrane pores into the permeate side and are discharged as the first tail gas.

[0063] The first tail gas enters the high-temperature absorption tower from the bottom and comes into countercurrent contact with the 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) ionic liquid absorbent sprayed from the top of the tower. The absorbent temperature is controlled at 180°C, and the gas-liquid ratio (L / G, in volumetric flow rate) is 800. Phenol, cresol, and xylenol in the first tail gas are selectively absorbed by the absorbent, forming a phenol-rich liquid that is discharged from the bottom of the tower; non-condensable gases such as H2, CO, and CH4 are discharged from the top of the tower as the second tail gas.

[0064] The phenol-rich solution is sequentially fed into a three-stage flash evaporation unit. The first-stage flash evaporation is carried out at 165℃ and 0.4MPa, where the flashed phenol vapor is condensed in the first condenser (condensing medium: 50℃ warm water) to obtain the phenol product. The second-stage flash evaporation is carried out at 185℃ and 0.15MPa, where the flashed cresol vapor is condensed in the second condenser (condensing medium: 50℃ warm water) to obtain the cresol product. The third-stage flash evaporation is carried out at 205℃ and 0.1MPa, where the flashed xylenol vapor is condensed in the third condenser (condensing medium: 50℃ warm water) to obtain the xylenol product. The regenerated absorbent is cooled and recycled back to the high-temperature absorption unit for reuse.

[0065] The second exhaust gas is sent into the gas combustion unit, where it is oxidized and decomposed at high temperature to meet emission standards. The high-temperature flue gas generated by combustion is then cooled by a waste heat recovery device.

[0066] Results: The system operated continuously and stably for 500 hours, with no tar deposits or phenol crystals observed on the inner walls of pipes and equipment. Tar recovery rate ≥99%, phenol recovery rate ≥96%, cresol recovery rate ≥94%, xylenol recovery rate ≥92%, and purity of each product ≥98%.

[0067] Example 2 Unlike Example 1, the high-temperature ceramic membrane separation unit has a membrane pore size of 0.1 μm. All other conditions are the same as in Example 1.

[0068] Results: Tar recovery rate ≥99.5%, phenol recovery rate comparable to Example 1, system pressure drop slightly higher than Example 1, but still within acceptable range. The system ran continuously for 500 hours without clogging.

[0069] Example 3 Unlike Example 1, the high-temperature ceramic membrane separation unit has a membrane pore size of 0.5 μm and the membrane material is an alumina-based ceramic membrane. All other conditions are the same as in Example 1.

[0070] Results: Tar recovery rate was approximately 95%, and phenol recovery rate was comparable to that of Example 1. The system ran continuously for 500 hours without clogging.

[0071] Example 4 Unlike Example 1, the absorption temperature of the high-temperature absorption unit is 150°C, and the absorbent is 1-butyl-3-methylimidazolium chloride ([Bmim]Cl). All other conditions are the same as in Example 1.

[0072] Results: Phenol recovery rate approximately 95%, cresol recovery rate approximately 93%, and xylenol recovery rate approximately 90%. The system ran continuously for 500 hours without clogging.

[0073] Example 5 Unlike Example 1, the absorption temperature of the high-temperature absorption unit is 200°C. All other conditions are the same as in Example 1.

[0074] Results: Phenol recovery rate approximately 97%, cresol recovery rate approximately 95%, and xylenol recovery rate approximately 93%. The system ran continuously for 500 hours without clogging.

[0075] Example 6 Unlike Example 1, the absorbent in the high-temperature absorption unit is a compound absorbent of dioctyl phthalate and 1-ethyl-3-methylimidazolium hexafluorophosphate in a mass ratio of 1:1. All other conditions are the same as in Example 1.

[0076] Results: Phenol recovery rate was approximately 97%, cresol recovery rate was approximately 95%, and xylenol recovery rate was approximately 93%, slightly better than the single absorbent scheme. The system ran continuously for 500 hours without clogging.

[0077] Example 7 Unlike Example 1, the temperature / pressure of the three-stage flash evaporation was 160℃ / 0.3MPa, 180℃ / 0.1MPa, and 200℃ / 0.08MPa. All other conditions were the same as in Example 1.

[0078] Results: Phenol recovery rate approximately 94%, cresol recovery rate approximately 92%, xylenol recovery rate approximately 89%, product purity ≥ 97%. The system ran continuously for 500 hours without clogging.

[0079] Example 8 Unlike Example 1, the temperature / pressure of the three-stage flash evaporation was 170℃ / 0.5MPa, 190℃ / 0.2MPa, and 210℃ / 0.12MPa. All other conditions were the same as in Example 1.

[0080] Processing results: Phenol recovery rate approximately 97%, cresol recovery rate approximately 95%, xylenol recovery rate approximately 92%, product purity ≥ 98%. The system ran continuously for 500 hours without clogging.

[0081] Example 9 Unlike Example 1, the condensing temperature of the first, second, and third condensers is 40°C. All other conditions are the same as in Example 1.

[0082] Results: The recovery rates of all phenolic products were comparable to those in Example 1. The condenser operated normally without any crystallization blockage. The system ran continuously for 500 hours without blockage.

[0083] Example 10 Unlike Example 1, the condensing temperature of the first, second, and third condensers is 60°C. All other conditions are the same as in Example 1.

[0084] Results: The recovery rates of each phenolic product were slightly lower than in Example 1 by about 2-3 percentage points, but the condenser operated normally without crystallization blockage. The system ran continuously for 500 hours without blockage. Comparative Example Comparative Example 1 Unlike Example 1, the high-temperature ceramic membrane separation unit was omitted, and the carbonized tail gas of phenolic resin (750°C) was directly sent to the high-temperature absorption unit for absorption. The other conditions were the same as in Example 1.

[0085] Results: After 24 hours of operation, significant tar contamination appeared in the absorbent inside the high-temperature absorption tower. The absorbent's color changed from colorless and transparent to dark black, and its viscosity increased significantly. After 72 hours of operation, a large amount of tar adhered to the surface of the absorbent packing, the pressure drop inside the tower increased by 30%, the absorption efficiency decreased sharply, and the outlet phenol concentration rose to over 500 ppm. This indicates that tar in the exhaust gas, which was not separated by a ceramic membrane, caused serious contamination and deactivation of the absorbent.

[0086] Comparative Example 2 Unlike Example 1, the absorption temperature of the high-temperature absorption unit was controlled at 80°C (about 41°C below the freezing point of phenol, but high-temperature absorption of 150~200°C was not used), and the other conditions were the same as in Example 1.

[0087] Results: After 8 hours of operation, significant phenol crystals precipitated inside the high-temperature absorption tower and in the pipes, adhering to the tower walls and pipe inner walls; after 24 hours of operation, the pipe flow cross-section decreased significantly, and the system pressure drop increased; after 72 hours of operation, the pipes became severely blocked, forcing a shutdown for cleaning. This indicates that when the absorption temperature is below the freezing point of phenols, phenol crystallization and blockage are unavoidable.

[0088] Comparative Example 3 Unlike Example 1, the three-stage flash evaporation stage was omitted. The phenol-rich liquid was directly fed into a single-stage flash evaporator and flashed at 185°C and 0.15 MPa. The flash vapor was condensed and collected. The other conditions were the same as in Example 1.

[0089] Results: The mixed recovered phenolic products contained phenol, cresol, and xylenol, with a purity of only about 65%, making them unsuitable for direct use as single chemicals and requiring further distillation separation. This indicates that single-stage flash evaporation cannot achieve refined fractional recovery of phenols.

[0090] Comparative Example 4 Unlike Example 1, the condensing medium in the first, second, and third condensers is room temperature water at 20°C, while the other conditions are the same as in Example 1.

[0091] Results: After 4 hours of operation, phenol crystallization blockage occurred in the tube side of the first condenser, causing a sharp drop in heat transfer efficiency. After 12 hours of operation, the first condenser became completely blocked, forcing a shutdown for cleaning. The second and third condensers also subsequently developed varying degrees of crystallization blockage. This indicates that when the condensation temperature is below the freezing point of phenol, phenol crystallizes and precipitates in the condenser, causing blockages.

[0092] Comparative Example 5 Unlike Example 1, the operating temperature of the high-temperature ceramic membrane separation unit is controlled at 300°C (lower than the condensation temperature of tar in the exhaust gas), while the other conditions are the same as in Example 1.

[0093] Results: After the exhaust gas entered the ceramic membrane separation unit, the tar condensed and precipitated prematurely during the cooling process, adhering to the inner wall of the pipe and the surface of the ceramic membrane. After 6 hours of operation, the membrane flux decreased by 50%. After 24 hours of operation, the ceramic membrane surface was completely covered by a tar layer, resulting in loss of separation efficiency and necessitating shutdown for cleaning. This indicates that when the operating temperature of the ceramic membrane is lower than the tar condensation temperature, tar condensation and deposition on the membrane surface leads to membrane fouling and failure.

[0094] Comparative Example 6 Unlike Example 1, the gas combustion unit was omitted, and the second exhaust gas was directly discharged after simple water washing. The other conditions were the same as in Example 1.

[0095] Treatment results: The emission outlet detected CO concentrations of approximately 2000 ppm and CH4 concentrations of approximately 500 ppm, which do not meet the air pollutant emission standards. This indicates that the second exhaust gas still contains a large amount of combustible and harmful components, and must be incinerated to meet emission standards.

[0096] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A graded recovery system for tar and phenolic substances in the carbonization tail gas of phenolic resin, characterized in that, Including those connected sequentially according to the direction of exhaust gas flow: A high-temperature ceramic membrane separation unit is used to intercept and collect the heavy tar components in the carbonized tail gas of phenolic resin at 500~900℃ to obtain the first tail gas. The high-temperature ceramic membrane separation unit includes a high-temperature ceramic membrane separator; The membrane pore size of the high-temperature ceramic membrane separator is 0.1~0.5μm; The membrane material of the high-temperature ceramic membrane separator includes silicon carbide ceramic membrane or alumina-based ceramic membrane. A high-temperature absorption unit is connected to the outlet of the high-temperature ceramic membrane separation unit and is used to absorb phenolic compounds in the first tail gas at 150~200℃ to obtain a phenol-rich liquid and a second tail gas; the high-temperature absorption unit includes a rich liquid outlet and a gas outlet. A three-stage flash evaporation unit is connected to the rich liquid outlet of the high-temperature absorption unit; it is used to perform staged flash evaporation on the phenol-containing rich liquid to recover phenol, cresol and xylenol; the three-stage flash evaporation unit includes a first-stage flash evaporation unit, a second-stage flash evaporation unit and a third-stage flash evaporation unit connected in series. The operating temperature of the first-stage flash evaporation unit is 160~170℃ and the pressure is 0.3~0.5MPa; the operating temperature of the second-stage flash evaporation unit is 180~190℃ and the pressure is 0.1~0.2MPa; the operating temperature of the third-stage flash evaporation unit is 200~210℃ and the pressure is 0.08~0.12MPa. and, A gas combustion unit is connected to the gas outlet of the high-temperature absorption unit and is used to incinerate the second tail gas.

2. The system according to claim 1, characterized in that, The high-temperature ceramic membrane separation unit includes a retention side and a permeation side. The retention side is provided with a tail gas inlet and a tar outlet, and the permeation side is provided with a gas outlet. The carbonized tail gas of the phenolic resin enters through the tail gas inlet, the heavy tar components are retained on the retention side and discharged through the tar outlet, and the first tail gas is discharged through the gas outlet.

3. The system according to claim 1, characterized in that, The high-temperature absorption unit is filled with a high-boiling-point organic absorbent. The high-boiling-point organic absorbent includes at least one of dioctyl phthalate, 1-ethyl-3-methylimidazolium hexafluorophosphate, or 1-butyl-3-methylimidazolium chloride.

4. The system according to claim 1, characterized in that, Also includes: The first condenser is connected to the outlet of the first-stage flash unit and is used to condense and recover phenol; A second condenser, connected to the outlet of the secondary flash evaporation unit, is used to condense and recover cresol; and, The third condenser is connected to the outlet of the three-stage flash evaporation unit and is used to condense and recover xylenol.

5. The system according to claim 4, characterized in that, The condensing temperatures of the first condenser, the second condenser, and the third condenser are each independently 40~60℃.

6. The system according to claim 1, characterized in that, The gas combustion unit includes: An incineration device, connected to the gas outlet of the high-temperature absorption unit, is used to incinerate the second tail gas to obtain exhaustable gas; and, A waste heat recovery device is connected to the outlet of the incineration device to recover heat from the exhaust gases.

7. The system according to claim 3, characterized in that, It also includes an absorbent circulation pipeline, which is located between the high-temperature absorption unit and the three-stage flash evaporation unit, for returning the regenerated absorbent discharged from the three-stage flash evaporation unit to the high-temperature absorption unit for recycling.

8. A method for graded recovery of tar and phenolic substances from phenolic resin carbonization tail gas using the system described in any one of claims 1 to 7, characterized in that, Includes the following steps: The carbonized tail gas of phenolic resin at 500~900℃ is passed into the high-temperature ceramic membrane separation unit to intercept and collect the heavy tar components, thus obtaining the first tail gas. The first tail gas is passed into the high-temperature absorption unit, and the phenolic compounds in the first tail gas are absorbed at 150~200°C to obtain a phenol-rich liquid and a second tail gas. The phenol-rich solution is passed into the three-stage flash evaporation unit for graded flash evaporation to recover phenol, cresol and xylenol respectively. The second exhaust gas is passed into a gas combustion unit for incineration treatment before being discharged.