Method for removing xylene in waste gas

Abstract

The invention provides a method for removing xylene in waste gas, and belongs to the technical field of gas purification. The method comprises the following steps: firstly, determining the saturated adsorption time t of dimethylbenzene only selectively adsorbed on a strong adsorption site on the transition metal modified Y molecular sieve, and then adsorbing the waste gas containing dimethylbenzene by using the transition metal modified Y molecular sieve; xylene is adsorbed at a strong adsorption center in a transition metal modified Y molecular sieve only in a strong bonding manner, so that the desorption temperature of xylene is higher than the oxidized temperature, and no xylene molecule escapes in the in-situ oxidation process; when the adsorption time is greater than t, the xylene adsorbed in the pore channels of the Y molecular sieve is oxidized before escaping by controlling the temperature rising rate of the programmed temperature rising, and meanwhile, the xylene adsorbed in a strong adsorption center can be oxidized; therefore, the problem of secondary pollution caused by xylene escape in the in-situ oxidation process in the prior art is solved.

Description

Technical Field

[0001] The invention relates to the technical field of gas purification, and in particular to a method for removing xylene from waste gas. Background Art

[0002] Volatile organic compounds (VOC) are one of the main pollutants causing air pollution. Xylene is a common VOC that is very harmful to the environment and human body. Exhaust gas from production plants such as petrochemical, packaging printing, coating and textile printing and dyeing industries is the main source of xylene in the environment. Xylene in the air is harmful to the human body. Short-term exposure to high concentrations of xylene will anesthetize the central nervous system and may even cause convulsions or coma; long-term exposure to xylene will easily lead to neurasthenia syndrome. Therefore, removing xylene molecules from exhaust gas is of great significance to human health and improving air quality.

[0003] Taking xylene as an example, common removal methods include physical adsorption and catalytic oxidation. For example, the Chinese patent with publication number CN109692659A prepared a binder-free spherical molecular sieve adsorbent. The obtained adsorbent has high strength, high bulk density and low macropore ratio, and has improved xylene adsorption selectivity and fast mass transfer rate. However, this method only selectively adsorbs and aggregates xylene, not completely eliminates it. Subsequent oxidation treatment is required to convert the adsorbed xylene into harmless products, such as CO x and water; however, due to the weak binding force between xylene and the adsorbent in this method, desorption is prone to occur during the process of converting the adsorbed and accumulated xylene into harmless products, thereby causing secondary pollution.

[0004] It can be seen that although the existing method can adsorb and enrich xylene in the exhaust gas, there is a problem that xylene is easily desorbed when converting the adsorbed and enriched xylene into harmless products, thereby causing secondary pollution. Summary of the invention

[0005] The object of the present invention is to provide a method for removing xylene from waste gas, which can not only adsorb xylene in the waste gas, but also prevent the adsorbed xylene from being desorbed before being converted into harmless products.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions: The present invention provides a method for removing xylene from waste gas, comprising the following steps: (1) Adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve to obtain a transition metal-modified Y molecular sieve that adsorbs xylene; (2) subjecting the transition metal-modified Y molecular sieve adsorbing xylene obtained in step (1) to in-situ oxidation by programmed temperature increase; When the adsorption time in step (1) is less than t, the heating rate of the programmed temperature in step (2) is 5-100°C / min; When the adsorption time in step (1) is greater than t, the heating rate of the programmed temperature in step (2) is greater than 40°C / s; The t is the saturated adsorption time of xylene on the transition metal-modified Y molecular sieve, where xylene is selectively adsorbed only on the strong adsorption sites.

[0007] Preferably, the preparation method of the transition metal-modified Y molecular sieve is an isovolumetric impregnation method.

[0008] Preferably, the transition metal in the transition metal-modified Y molecular sieve includes one or more of Fe, Ag and Cu.

[0009] Preferably, the mass of the transition metal in the transition metal-modified Y molecular sieve is 1-10 wt % of the mass of the Y molecular sieve.

[0010] Preferably, the method for determining t comprises the following steps: adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve, recording the amount of adsorbed xylene at regular intervals to obtain a TPD curve; and determining, based on the TPD curve, the saturated adsorption time t at which xylene is selectively adsorbed only on strong adsorption sites on the transition metal-modified Y molecular sieve.

[0011] Preferably, when the adsorption time in step (1) is less than t, the heating rate in step (2) is 10° C. / min.

[0012] Preferably, when the adsorption time in step (1) is less than t, the end temperature of the programmed temperature increase in step (2) is 500°C.

[0013] Preferably, when the adsorption time in step (1) is greater than t, the heating rate in step (2) is 50° C. / s.

[0014] Preferably, when the adsorption time in step (1) is greater than t, the terminal temperature of the programmed temperature increase in step (2) is 500°C.

[0015] Preferably, the in-situ oxidation atmosphere is air, RH=0%~100%.

[0016] The present invention provides a method for removing xylene from waste gas, comprising the following steps: (1) adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve to obtain a transition metal-modified Y molecular sieve adsorbing xylene; (2) subjecting the transition metal-modified Y molecular sieve adsorbing xylene obtained in the step (1) to in-situ oxidation by programmed temperature increase; when the adsorption time in the step (1) is less than t, the heating rate of the programmed temperature increase in the step (2) is 5-100°C / min; when the adsorption time in the step (1) is greater than t, the heating rate of the programmed temperature increase in the step (2) is greater than 40°C / s; t is the saturated adsorption time of xylene on the transition metal-modified Y molecular sieve only selectively adsorbed on strong adsorption sites.

[0017] The present invention uses a transition metal-modified Y molecular sieve to adsorb waste gas containing xylene. The adsorption of xylene on the transition metal-modified Y molecular sieve includes two adsorption modes: first, by utilizing the property that the transition metal can form a strong bond with xylene, a strong adsorption center is formed in the transition metal-modified Y molecular sieve, and these strong adsorption centers can preferentially react with xylene, such as electron transfer and formation of new chemical bonds, thereby generating new intermediate species with high adsorption intensity, so that the desorption temperature of xylene is higher than the oxidation temperature; second, the Y molecular sieve can allow xylene to enter the molecular sieve pores, and allow xylene to be adsorbed in the pores of the Y molecular sieve by physical adsorption, and the xylene in this adsorption mode is easy to escape; according to the saturated adsorption time t of xylene only selected to be adsorbed on the strong adsorption site on the transition metal-modified Y molecular sieve, different heating rates are selected during programmed temperature in-situ oxidation, when the adsorption time is less than t, xylene is preferentially adsorbed on the strong adsorption center in the transition metal-modified Y molecular sieve by a strong bond, so that the desorption temperature of xylene is higher than the oxidation temperature, and the present invention The invention controls the heating rate of the programmed heating to be 5-100°C / min. Under this heating rate, a conventional heating device or a fast heating device can be used to achieve in-situ oxidation of xylene before its desorption. No xylene molecules escape during the in-situ oxidation process, thereby solving the problem of secondary pollution caused by the escape of xylene during the in-situ oxidation process in the prior art. When the adsorption time is greater than t, since the strong adsorption centers of the transition metal-modified Y molecular sieve are all occupied by xylene, part of the xylene is adsorbed in the pores of the Y molecular sieve by physical adsorption. The invention controls the heating rate of the programmed heating to be greater than 40°C / s. This heating rate can ensure that the transition metal-modified Y molecular sieve that adsorbs xylene is quickly heated, and the oxidation ability of the catalyst can be rapidly improved (i.e., the oxidation rate of xylene), and the xylene adsorbed in the pores of the Y molecular sieve by physical adsorption can be oxidized before escaping. At the same time, it can also ensure that the xylene adsorbed in the strong adsorption center is oxidized, thereby solving the problem of secondary pollution caused by the escape of xylene during the in-situ oxidation process in the prior art. The present invention uses transition metal-modified Y molecular sieve as an adsorbent, and by controlling the storage time (i.e., adsorption time) and the heating rate of the programmed temperature increase, the problem of secondary pollution caused by the escape of xylene during the in-situ oxidation process existing in the prior art can be solved. The results of the embodiment show that after the method provided by the present invention adsorbs xylene at different adsorption times, no xylene escapes when the adsorbent adsorbed with xylene is subjected to in-situ oxidation. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : is the TPD curve of xylene adsorbed at different adsorption times in Example 1 of the present invention; Figure 2 This is the infrared lamp heating regeneration performance curve in Example 1 of the present invention; Figure 3 This is the in-situ thermal regeneration performance curve in Comparative Example 1 of the present invention; Figure 4 is the in-situ thermal regeneration performance curve in Example 2 of the present invention; Figure 5 is the xylene oxidation performance curve in Example 2 of the present invention; Figure 6 The TPD curves of xylene adsorbed at different heating rates in Examples 3-4 of the present invention and Comparative Examples 2-3 are shown. Implementation

[0019] The present invention provides a method for removing xylene from waste gas, comprising the following steps: (1) Adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve to obtain a transition metal-modified Y molecular sieve that adsorbs xylene; (2) subjecting the transition metal-modified Y molecular sieve adsorbing xylene obtained in step (1) to in-situ oxidation by programmed temperature increase; When the adsorption time in step (1) is less than t, the heating rate of the programmed temperature in step (2) is 5-100°C / min; When the adsorption time in step (1) is greater than t, the heating rate of the programmed temperature in step (2) is greater than 40°C / s; The t is the saturated adsorption time of xylene on the transition metal-modified Y molecular sieve, where xylene is selectively adsorbed only on the strong adsorption sites.

[0020] The invention uses a transition metal-modified Y molecular sieve to adsorb waste gas containing xylene to obtain a transition metal-modified Y molecular sieve that adsorbs xylene. The invention uses a transition metal-modified Y molecular sieve to adsorb waste gas containing xylene, and the transition metal-modified Y molecular sieve has two adsorption modes for xylene: selective adsorption at a strong adsorption center and physical adsorption at the Y molecular sieve pores.

[0021] In the present invention, the transition metal in the transition metal-modified Y molecular sieve preferably includes one or more of Fe, Ag and Cu. The present invention utilizes the property of strong bonding between transition metal and xylene, and can make xylene selectively and firmly bonded to the transition metal-modified Y molecular sieve; when the transition metal is of the above type, it has a strong bonding ability with xylene.

[0022] In the present invention, the transition metal in the transition metal-modified Y molecular sieve is preferably uniformly distributed on the surface and in the pores of the Y molecular sieve in the form of one or more of metal ions, simple substances and metal oxides. In the present invention, the transition metal is present in the Y molecular sieve in the above manner, and can form uniformly distributed strong adsorption centers in the molecular sieve.

[0023] In the present invention, the Y molecular sieve is preferably purchased from Shandong Hefa Environmental Protection Technology Co., Ltd., and the silicon-aluminum ratio is 5, 11, 40 and 100. In the present invention, when the Y molecular sieve is of the above type, on the one hand, it can allow xylene molecules to enter the pores of the Y molecular sieve, thereby allowing the xylene molecules to be adsorbed by the molecular sieve; on the other hand, the above molecular sieve has abundant acidic sites, so that the transition metal can be located at the cationic site in the pores of the molecular sieve or be highly dispersed in the pores of the molecular sieve in a metallic state. The Y molecular sieve modified by the transition metal can form a strong bonding with xylene, so that xylene can be selectively and strongly bonded to the Y molecular sieve.

[0024] In the present invention, the mass of the transition metal in the transition metal-modified Y molecular sieve is preferably 1-10wt% of the mass of the Y molecular sieve, and more preferably 2-8wt%. In the present invention, when the mass content of the transition metal in the transition metal-modified Y molecular sieve is within the above range, more strong adsorption centers can be provided for the transition metal-modified Y molecular sieve.

[0025] In the present invention, the preparation method of the transition metal modified Y molecular sieve is preferably an isovolumetric impregnation method. The present invention does not specifically limit the operation method of the isovolumetric impregnation method, and the isovolumetric impregnation method well known to those skilled in the art can be used. In the present invention, the isovolumetric impregnation method preferably includes the following steps: dripping a transition metal salt solution into the Y molecular sieve drop by drop, allowing it to stand, to obtain a saturated adsorption Y molecular sieve; drying and calcining the saturated adsorption Y molecular sieve in turn, to obtain a transition metal modified Y molecular sieve.

[0026] In the present invention, the transition metal salt solution is preferably dripped into the Y molecular sieve drop by drop, and the mixture is allowed to stand to obtain a saturated adsorption Y molecular sieve.

[0027] In the present invention, the transition metal salt solution is preferably a nitrate solution of a transition metal, and the concentration of the transition metal salt solution is preferably 0.463 mol / L. In the present invention, the concentration of the metal salt solution used in the equal volume impregnation method depends on the content of the required loaded metal and the amount of saturated adsorbed water of the molecular sieve, and then a salt solution of a certain concentration is prepared; when the concentration of the transition metal salt solution is within the above range, it is more conducive to controlling the loading amount of the transition metal in the transition metal-modified Y molecular sieve.

[0028] In the present invention, the volume of the transition metal salt solution is preferably the same as the volume of water required for the saturated water absorption of the Y molecular sieve. In the present invention, when the volume of the transition metal salt solution is the above usage amount, equal volume impregnation can be achieved.

[0029] The present invention has no particular limitation on the dripping rate of the transition metal salt solution dripping into the Y molecular sieve, and can be adjusted according to the experimental process. In the present invention, the transition metal salt solution is dripped into the Y molecular sieve preferably under stirring. In the present invention, the stirring can promote the Y molecular sieve to fully absorb the transition metal salt solution.

[0030] In the present invention, the standing time is preferably 8 to 24 hours, more preferably 12 hours. In the present invention, the standing time can allow the transition metal in the transition metal salt solution to be fully absorbed by the Y molecular sieve.

[0031] After obtaining the saturated adsorption Y molecular sieve, the present invention preferably sequentially dries and calcines the saturated adsorption Y molecular sieve to obtain the transition metal-modified Y molecular sieve.

[0032] In the present invention, the drying temperature is preferably 60-110°C, more preferably 65°C; the drying time is preferably 2-12h, more preferably 2-4h. In the present invention, when the drying temperature and time are within the above ranges, water in the transition metal salt solution can be removed. The present invention does not specifically limit the drying device, and a drying device well known to those skilled in the art can be used. In the present invention, the drying device is preferably a drying oven.

[0033] In the present invention, the calcination atmosphere is preferably air; the calcination temperature is preferably programmed from room temperature to the end temperature. In the present invention, the heating rate from room temperature to the end temperature is preferably 1-20°C / min, more preferably 5-10°C / min. In the present invention, when the heating rate is within the above range, it is possible to prevent metal migration and aggregation to form large metal particles, thereby reducing the number of strong adsorption centers in the transition metal-modified Y molecular sieve.

[0034] In the present invention, the terminal temperature is preferably 450-500°C, more preferably 450°C; the holding time at the terminal temperature is preferably 3-5h, more preferably 4h. In the present invention, when the terminal temperature and the holding time are within the above ranges, the transition metal salt can be fully decomposed to form transition metal particles.

[0035] In the present invention, the waste gas containing xylene preferably includes xylene and air. The present invention does not specifically limit the content of xylene in the waste gas containing xylene, and the method provided by the present invention can remove xylene from the waste gas. In the present invention, the content of xylene in the waste gas containing xylene is preferably 1-500ppm, more preferably 80-200ppm. In the present invention, when the content of xylene in the waste gas containing xylene is within the above range, it is a low-concentration xylene waste gas in the industry. Conventionally, the removal of xylene above 1000ppm in the industry generally adopts a catalytic oxidation method, that is, the organic waste gas is heated to its complete oxidation temperature by a reaction furnace, and then the xylene in the waste gas is oxidized in the oxidation chamber to generate carbon dioxide and water; but for low-concentration xylene, the energy consumption required for the catalytic oxidation reaction is relatively high; and the method provided by the present invention can selectively adsorb xylene in the transition metal-modified Y molecular sieve, so it can adsorb low-concentration xylene in the waste gas, and when the adsorbed xylene is oxidized, it will not cause xylene to escape.

[0036] The present invention does not specifically limit the composition of the air in the waste gas containing xylene, and conventional air components can be used. In an embodiment of the present invention, the composition of the air in the waste gas is preferably 21% O2 / N2, that is, the oxygen content is 21%; the relative humidity is preferably 0-100% (25°C), more preferably 50-100% (25°C). In the present invention, when the composition of the waste gas is in the above range, it is closer to conventional air components.

[0037] The present invention does not specifically limit the operating method of adsorbing the waste gas containing xylene by the Y molecular sieve modified with a transition metal, and the operating method of adsorbing the waste gas containing xylene by a catalyst well known to those skilled in the art can be used. In the present invention, the operating method of adsorbing the waste gas containing xylene by the Y molecular sieve modified with a transition metal is preferably carried out in a fixed bed. The present invention does not specifically limit the total flow rate and mass space velocity of the waste gas containing xylene during the adsorption, and can be adjusted according to experimental needs. In the present invention, the total flow rate of the waste gas containing xylene is preferably 100 mL / min, and the mass space velocity is preferably 92,000 mL / g / h. In the present invention, when the total flow rate and mass space velocity of the waste gas containing xylene are in the above range, they are the currently commonly used experimental parameters for fixed bed adsorption of pollutants in waste gas.

[0038] After the adsorption is completed, the present invention performs in-situ oxidation on the transition metal-modified Y molecular sieve that adsorbs xylene by programmed temperature increase.

[0039] In the present invention, the method of subjecting the transition metal-modified Y molecular sieve adsorbing xylene to in-situ oxidation by temperature programming preferably comprises: placing the transition metal-modified Y molecular sieve adsorbing xylene in a heating device, performing temperature programming, and raising the temperature in the device from room temperature to the end temperature. By adopting the above method, in the process of temperature programming, xylene is in-situ thermally oxidized to generate CO x and water.

[0040] In the present invention, when the adsorption time is less than t, the heating rate of the programmed temperature rise is 5~100℃ / min, preferably 10~50℃ / min. In the present invention, t is the saturated adsorption time of xylene on the transition metal-modified Y molecular sieve, which is only selectively adsorbed on the strong adsorption sites; when the adsorption time is less than t, when the heating rate is within the above range, both conventional heating devices and rapid heating devices can oxidize the adsorbed xylene without causing xylene to escape. The present invention does not specifically limit the in-situ oxidation device, and a heating device familiar to those skilled in the art can be used to achieve the above heating rate. In the present invention, the in-situ oxidation device is preferably a tubular furnace.

[0041] In the present invention, the terminal temperature of the programmed temperature rise is preferably 500°C; in the present invention, the temperature of the in-situ oxidation is 380-400°C. When the terminal temperature of the programmed temperature rise is within the above range, xylene can be fully oxidized to CO during the programmed temperature rise of the transition metal-modified Y molecular sieve adsorbing xylene. x and water.

[0042] In the present invention, when the adsorption time is greater than t, the heating rate of the programmed temperature rise is greater than 40°C / s, preferably 40~50°C / s, and more preferably 50°C / s. In the present invention, when the adsorption time is greater than t, the heating rate is in the above range, which can rapidly heat up the transition metal-modified Y molecular sieve that adsorbs xylene, and its temperature can reach 500°C within 10s, which is far higher than the complete oxidation temperature of the adsorbed xylene, and can rapidly improve the oxidation capacity of the catalyst (i.e., the oxidation rate of xylene), so that xylene is oxidized before desorption, thereby preventing xylene from escaping. The present invention does not specifically limit the in-situ oxidation device, and a heating device familiar to those skilled in the art can be used to achieve the above heating rate. In the present invention, the in-situ oxidation device is preferably an infrared heating furnace.

[0043] In the present invention, the in-situ oxidation atmosphere is preferably air, and the relative humidity of the air is preferably 0-100% (25° C.). In the present invention, the above atmosphere can realize the regeneration of the transition metal-modified Y molecular sieve that adsorbs xylene in the air range, and the operation is simple.

[0044] In the present invention, the method for determining t preferably includes the following steps: adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve, recording the outlet concentration of xylene at regular intervals, and then calculating the amount of adsorbed xylene to obtain a TPD curve; determining the saturated adsorption time t of xylene on the transition metal-modified Y molecular sieve based on the TPD curve, at which xylene is only selectively adsorbed on strong adsorption sites.

[0045] In the present invention, the operation method of adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve is the same as the operation method and parameters for removing xylene from waste gas described in the above technical solution, and will not be repeated here. In the present invention, the determination method of t is the same as the operation method and parameters for adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve when removing xylene from waste gas, which can provide guidance for the adsorption time when removing xylene from waste gas, thereby facilitating the control of the adsorption form of xylene in the transition metal-modified Y molecular sieve, and then matching the in-situ oxidation conditions according to the adsorption form, so as to prevent xylene from escaping at different adsorption times.

[0046] In the present invention, when a new desorption peak begins to appear on the TPD curve at a low temperature (about 270°C), it indicates that the strong bonding site has been saturated, indicating that xylene is selectively adsorbed only on the strong adsorption center and reaches adsorption saturation. At this time, the corresponding time is the saturated adsorption time t of xylene selectively adsorbed only on the strong adsorption site on the transition metal-modified Y molecular sieve. The present invention does not specifically limit the range of t, and it can be read from the TPD curve according to the amount of the transition metal-modified Y molecular sieve used and the content of xylene in the exhaust gas.

[0047] The method provided by the present invention first determines the saturated adsorption time t of xylene on the transition metal modified Y molecular sieve only selectively adsorbed on the strong adsorption site. When the adsorption time is less than t, xylene is adsorbed on the strong adsorption center in the transition metal modified Y molecular sieve only by strong bonding. This adsorption mode makes the desorption temperature of xylene higher, which can reach 413°C, and then makes the desorption temperature of xylene higher than the oxidation temperature. The present invention controls the heating rate of the program heating to be 5-100°C / min. This heating rate can make no xylene molecules escape during the in-situ oxidation process, and can be oxidized before desorption, which can solve the problem of secondary pollution caused by the escape of xylene during the in-situ oxidation process in the prior art; when the adsorption time is greater than t When all the strong adsorption centers of the transition metal-modified Y molecular sieve are occupied by xylene, the excess xylene can be adsorbed in the pores of the transition metal-modified Y molecular sieve by physical adsorption. The desorption temperature of xylene in this adsorption mode is relatively low (about 270°C). The present invention controls the heating rate of the program heating to be greater than 40°C / s. This heating rate can ensure that the transition metal-modified Y molecular sieve that adsorbs xylene heats up quickly, thereby enabling the xylene adsorbed in the pores of the Y molecular sieve by physical adsorption to be oxidized before escaping. At the same time, it can also ensure that the xylene adsorbed in the strong adsorption center is oxidized, thereby solving the problem of secondary pollution caused by the escape of xylene in the in-situ oxidation process in the prior art. The present invention uses the transition metal-modified Y molecular sieve as an adsorbent and controls the combination of the adsorption time and the heating method to solve the problem of secondary pollution caused by the escape of xylene in the in-situ oxidation process in the prior art.

[0048] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Example

[0049] Preparation method of transition metal modified Y molecular sieve: silver is loaded by equal volume impregnation method, 1 mL of 0.463 mol / L silver nitrate solution is added dropwise to 1 g of Y-11 molecular sieve, stirred while adding, so that it becomes toothpaste-like, left to stand for 12 h, dried in a vacuum drying oven at 65 ° C for 2 h, then calcined in a muffle furnace, heated to 450 ° C at a heating rate of 5 ° C / min and maintained for 4 h, to obtain transition metal modified Y molecular sieve, referred to as Ag. x / Y-11(x=5).

[0050] The method for determining the saturated adsorption time t of xylene in this embodiment, which selectively adsorbs only on the strong adsorption sites on the transition metal-modified Y molecular sieve, is as follows: the transition metal-modified Y molecular sieve (Ag prepared in this embodiment) x / Y-11) was placed in the waste gas containing xylene, the content of xylene in the waste gas containing xylene was 90ppm, the composition of air in the waste gas was 21% O2 / N2, and the relative humidity was 50% (25℃); the total flow rate containing xylene was controlled to be 100mL / min, the mass space velocity was 92,000mL / g / h, the adsorption temperature was 30℃, and adsorption was carried out. The outlet concentration of xylene was tested every 6s, and the amount of adsorbed xylene was obtained by calculation, and the TPD curve of adsorbed xylene at different adsorption times was plotted, as shown in FIG. Figure 1 As shown, from Figure 1 It can be seen that the saturated adsorption time t of xylene on the transition metal-modified Y molecular sieve is 20 min only on the strong adsorption sites. When the adsorption time exceeds t, the xylene molecules are not only selectively adsorbed on the strong adsorption centers, but also adsorbed in the pores of the molecular sieve in the form of physical adsorption.

[0051] The method for removing xylene from the waste gas is as follows: Y molecular sieve modified with transition metal (Ag prepared in this embodiment) x / Y-11) was placed in a waste gas containing xylene for adsorption for 35 minutes, wherein the content of xylene in the waste gas containing xylene was 90ppm, the composition of air in the waste gas was 21% O2 / N2, and the relative humidity was 50% (25°C); the total flow rate containing xylene was controlled to be 100mL / min, the mass space velocity was 92,000 mL / g / h, and the adsorption temperature was 30°C, to obtain a transition metal-modified Y molecular sieve that adsorbs xylene; The transition metal-modified Y molecular sieve for adsorbing xylene was placed in an infrared heating furnace, the atmosphere was air (21% O2 / N2), the relative humidity was 50% (25°C), the total flow rate was 100 mL / min, the mass space velocity was 92,000 mL / g / h, and the temperature was raised from room temperature to 500°C at a heating rate of 50°C / s for heating regeneration. During the heating regeneration process, the infrared lamp heating regeneration performance curve was obtained when the adsorption time was 35 min. Figure 2 shown.

[0052] from Figure 2It can be seen that when the adsorption time is 35 min, which is greater than the saturated adsorption time t at which xylene is only adsorbed on the strong adsorption sites on the transition metal-modified Y molecular sieve, under this adsorption condition, xylene is in a form of coexistence of physical adsorption and chemical adsorption. The present invention adopts an infrared heating furnace for heating and controls the heating rate, so that the oxidation rate of xylene can be rapidly increased in a short time, and the desorption temperature of xylene can be increased, so that the desorption temperature is higher than the oxidation temperature. In the thermal oxidation process, xylene is not desorbed in advance, and the adsorbed xylene can be completely thermally regenerated.

[0053] The only difference from Example 1 is that the equipment used for heating regeneration is a tubular furnace, and the heating rate is 10°C / min from room temperature to 500°C for heating regeneration. The remaining steps are the same as those in Example 1. The in-situ thermal regeneration performance curve when the adsorption time is 35 min is obtained as shown in Figure 3 As shown. Figure 3 It can be seen that when the adsorption time is 35 minutes, under this adsorption condition, xylene is in a form of coexistence of physical adsorption and chemical adsorption. During the thermal oxidation process, part of the xylene is desorbed prematurely and complete thermal regeneration cannot be achieved. Example

[0054] The difference from Example 1 is that: the method for removing xylene from the waste gas is: the transition metal modified Y molecular sieve (Ag prepared in this example) x / Y-11) was placed in a waste gas containing xylene for adsorption for 15 minutes, so that the xylene molecules were selectively stored only in the strong adsorption center, wherein the xylene content in the waste gas containing xylene was 90ppm, the composition of air in the waste gas was 21% O2 / N2, and the relative humidity was 50% (25°C); the total flow rate containing xylene was controlled to be 100mL / min, the mass space velocity was 92,000 mL / g / h, and the adsorption temperature was 30°C, and a transition metal-modified Y molecular sieve for adsorbing xylene was obtained; The transition metal-modified Y molecular sieve for adsorbing xylene was placed in a tube furnace, the atmosphere was air (21% O2 / N2), the relative humidity was 50% (25°C), the total flow rate was 100 mL / min, the mass space velocity was 92,000 mL / g / h, and the temperature was raised from room temperature to 500°C at a heating rate of 10°C / min for heating regeneration, and the in-situ thermal regeneration performance curve of xylene molecules selectively stored only in the strong adsorption center was obtained as shown in Figure 4 As shown. Figure 4 It can be seen that the adsorption form changes from xylene to other intermediates (benzaldehyde or benzyl alcohol), the adsorption intensity is enhanced, the desorption temperature is higher than the oxidation temperature of the stored species, and the carbon balance is 100%; this indicates that the adsorbed xylene molecules can be directly thermally oxidized to CO in situ without desorption. x and water.

[0055] In the process of heating from room temperature to 500℃, the conversion rate curve of xylene is as follows: Figure 5 As shown. Figure 5 It can be seen that in the process of programmed temperature rise, when the temperature rises to 380°C, xylene can be completely oxidized. The present invention uses transition metal-modified Y molecular sieve to adsorb xylene by controlling the storage time, so that xylene is adsorbed on the strong adsorption center in the transition metal-modified Y molecular sieve only by strong bonding. At this time, the desorption temperature of xylene is higher than the oxidation temperature, so that xylene is oxidized in situ in the process of programmed temperature rise, and no xylene molecules escape in this process. Example

[0056] The only difference from Example 1 is that in the method for removing xylene from the exhaust gas, the transition metal-modified Y molecular sieve that adsorbs xylene is placed in an infrared heating furnace, the atmosphere is air (21% O2 / N2), the relative humidity is 50% (25°C), the total flow rate is 100 mL / min, the mass space velocity is 92,000 mL / g / h, and the temperature is raised from room temperature to 500°C at a heating rate of 50°C / min for heating regeneration, and the TPD curve of adsorbed xylene is obtained as shown in FIG. Figure 6 As shown, the desorption peak temperature is 410 °C. Example

[0057] The difference from Example 3 is that in the method for removing xylene from waste gas, the heating rate of the programmed temperature is 20°C / min, and the remaining steps are the same as those in Example 3. The TPD curve of adsorbed xylene is as follows: Figure 6 As shown, the desorption peak temperature is 300 °C.

[0058] The difference from Example 3 is that in the method for removing xylene from waste gas, the heating rate of the programmed temperature is 10°C / min, and the remaining steps are the same as those in Example 3. The TPD curve of adsorbed xylene is as follows: Figure 6 As shown, the desorption peak temperature is 255 °C.

[0059] The difference from Example 3 is that in the method for removing xylene from exhaust gas, the heating device is a tubular furnace, the heating rate of the programmed temperature is 10°C / min, and the remaining steps are the same as those in Example 3. The TPD curve of adsorbed xylene is as follows: Figure 6 As shown, the desorption peak temperature is 243 °C.

[0060] from Figure 6It can be seen that with the increase of the heating rate, the desorption peak temperature gradually moves to high temperature, which means that by increasing the heating rate, a wider reaction temperature window can be provided for the adsorbed xylene molecules. Therefore, when the method provided by the present invention is used to treat waste gas containing xylene, the adsorption time and the regeneration heating rate can be controlled to perform in-situ oxidation, and then the regenerated transition metal-modified Y molecular sieve is continued to be used to treat waste gas containing xylene until the xylene in the waste gas is fully removed, which is conducive to realizing the "storage (adsorption)-in-situ catalytic oxidation" cycle purification until the xylene in the waste gas is fully removed and the xylene is fully purified.

[0061] It can be seen from the above experimental results that the method provided by the present invention first determines the saturated adsorption time t of xylene on the transition metal-modified Y molecular sieve, and then uses the transition metal-modified Y molecular sieve to adsorb xylene in the waste gas containing xylene, and then heats and regenerates the transition metal-modified Y molecular sieve that adsorbs xylene, so that the enriched xylene can be oxidized, and different heating and regeneration conditions are controlled when the adsorption time is less than t and greater than t, so that the enriched xylene will not escape during the oxidation process, thereby solving the problem of secondary pollution of oxidized xylene in the prior art.

[0062] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for removing xylene from waste gas, comprising the following steps: (1) Adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve to obtain a transition metal-modified Y molecular sieve that adsorbs xylene; (2) subjecting the transition metal-modified Y molecular sieve adsorbing xylene obtained in step (1) to in-situ oxidation by programmed temperature increase; When the adsorption time in step (1) is less than t, the heating rate of the programmed temperature in step (2) is 5-100°C / min; When the adsorption time in step (1) is greater than t, the heating rate of the programmed temperature in step (2) is greater than 40°C / s; The t is the saturated adsorption time of xylene on the transition metal-modified Y molecular sieve, where xylene is selectively adsorbed only on the strong adsorption sites.

2. The method for removing xylene from waste gas according to claim 1, characterized in that: The preparation method of the transition metal modified Y molecular sieve is an equal volume impregnation method.

3. The method for removing xylene from waste gas according to claim 1 or 2, characterized in that: The transition metal in the transition metal-modified Y molecular sieve includes one or more of Fe, Ag and Cu.

4. The method for removing xylene from waste gas according to claim 1 or 2, characterized in that: The mass of the transition metal in the transition metal-modified Y molecular sieve is 1-10 wt % of the mass of the Y molecular sieve.

5. The method for removing xylene from waste gas according to claim 1, characterized in that: The method for determining t comprises the following steps: adsorbing waste gas containing xylene with a transition metal-modified Y molecular sieve, recording the amount of adsorbed xylene at regular intervals to obtain a TPD curve; and determining, based on the TPD curve, the saturated adsorption time t at which xylene is selectively adsorbed only on strong adsorption sites on the transition metal-modified Y molecular sieve.

6. The method for removing xylene from waste gas according to claim 1, characterized in that: When the adsorption time in step (1) is less than t, the heating rate in step (2) is 10°C / min.

7. The method for removing xylene from waste gas according to claim 1 or 6, characterized in that: When the adsorption time in step (1) is less than t, the terminal temperature of the programmed temperature rise in step (2) is 500°C.

8. The method for removing xylene from waste gas according to claim 1, characterized in that: When the adsorption time in step (1) is greater than t, the heating rate in step (2) is 50°C / s.

9. The method for removing xylene from waste gas according to claim 1 or 8, characterized in that: When the adsorption time in step (1) is greater than t, the terminal temperature of the programmed temperature increase in step (2) is 500°C.

10. The method for removing xylene from waste gas according to claim 1, characterized in that: The atmosphere of the in-situ oxidation is air, RH=0%~100%.

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