Activated carbon and zeolite molecular sieve to treat unsteady emissions of VOCs gas

Through the characterization of activated carbon and zeolite molecular sieves and dynamic adsorption/desorption experiments, the application prospects of the two types of adsorbent fixed-bed technology for discontinuous and unsteady type of large air volume and low concentration volatile organic compounds (VOCs) emission control are explored. The results show that the pore size distribution of zeolite molecular sieve is relatively concentrated, about 0.8nm, and the specific surface area is 393.76m~2˙g~(-1); the pore size distribution of activated carbon has a broad spectrum, and the micropores are concentrated between 1~2nm, and the specific surface area It is 1026.71m~2˙g~(-1). The equilibrium adsorption capacity of activated carbon for p-xylene is generally higher than that of zeolite molecular sieves, and its equilibrium adsorption capacity varies with the equilibrium concentration and temperature of the gas phase to be greater than that of zeolite molecular sieves. The results of dynamic adsorption breakthrough experiments show that the average mass transfer rate per unit length of the zeolite molecular sieve is about 1.42~1.66 times that of activated carbon. Under the same adsorption and desorption temperature conditions, the working capacity of activated carbon is greater than that of zeolite molecular sieve. Zeolite molecular sieve desorption is basically complete at 210 ℃, and can get more than 48 times the concentrated desorption gas. 50 repeated experiments of adsorption and desorption have little effect on the performance of zeolite molecular sieve. Zeolite molecular sieve as an adsorbent requires energy consumption per unit of xylene to desorb 2.9 to 4.2 times that of activated carbon. Activated carbon and zeolite molecular sieves can be applied to the purification of VOCs with low concentration, large air volume, and unstable emissions by using different processes.
 
  In recent years, the cost-effective control of volatile organic compounds (VOCs) has become a hot spot in environmental protection work [1-3]. There are many sources of VOCs in the atmosphere, among which industrial sources include petrochemical, packaging and printing, textile printing and dyeing, light industry, painting and fine chemical industries, and their production processes will emit a large amount of VOCs gas[4-6]. At present, the characteristics of VOCs produced by many processes are large air volume, low concentration, and many are intermittent emissions. Direct emissions without treatment can no longer meet the increasingly stringent emission standards. Activated carbon adsorption, hot air desorption and additional catalytic combustion processes have been used in the treatment of large air volume and low concentration organic gases under the scenario where the early emission requirements are not high and the treatment objects are mainly organic substances with low boiling points.[7] However, when dealing with some organic pollutants with higher boiling point or increasing the temperature of thermal desorption air to above 120℃ in order to improve the purification efficiency, there are greater safety hazards. In recent years, zeolite runners have been widely used in low-concentration and high-volume exhaust occasions due to their good safety[8-11], but from a technical and economic point of view, zeolite runners are more suitable for continuous discharge and relative concentration. Stable organic gas emission control.
 
  Compared with runner-type processes, fixed-bed adsorption has better stability and compliance with the exhaust gas with interstitial emissions and large fluctuations in pollutant concentration. In this study, xylene was used as the experimental object, through the adsorption/desorption performance experiment of granular activated carbon and granular zeolite molecular sieve for low concentration organic gases, combined with the characterization results of the adsorbent, the characteristics of the two types of adsorbents were discussed and compared, and the two types of adsorbents were explored. The fixed bed composed of non-continuous, non-steady type of large air volume, low concentration organic pollutant gas purification prospects.
 
1Materials and methods
 
1.1 Experimental materials
 
  The adsorption materials used in the experiment are zeolite molecular sieve and activated carbon. The zeolite molecular sieve is HiSiv1000 produced by Honewell, which is hydrophobic; the activated carbon is a coal column (commercially available) with a diameter of 4mm and a CTC (carbon tetrachloride adsorption value) of 61.32%; xylene is analytically pure (China National Pharmaceutical Group Chemical Reagent Co., Ltd.).
 
1.2 Characterization of adsorbent
 
  The nitrogen adsorption/desorption curve, specific surface area, pore size distribution and pore volume of activated carbon and zeolite molecular sieve were measured by ASAP2020C type adsorption instrument (Micromeritics, USA). The specific surface area of the adsorbent is calculated by the BET method, the specific surface area of the micropores is calculated by the T-Plot equation [12], and the pore volume of the micropores is calculated by the Horvath-Kawazoe equation [13-14].
 
1.3 Dynamic experiment process and equipment
 
1.3.1 Adsorption device and detection method
 
  The self-built experimental device is shown in Figure 1. The device consists of a gas distribution system, an adsorption system, and a detection system. The gas distribution system is composed of a fan, a drying device, a rotameter, a micro injection metering pump, a heater, and a buffer chamber. The adsorption system is composed of a heating constant temperature device and a fixed adsorption bed. The detection port is set before and after the adsorption system. The experiment is based on two In the experiment where toluene is a single detection object, a ppb-level RAE3000 analyzer (Huarui Company, USA) is used as the measurement equipment. The xylene standard curve R2 measured and drawn by all analyzers was above 0.99.
 
  This system is used in the adsorption process experiment of xylene on activated carbon and zeolite molecular sieve. After the dried clean air is adjusted and distributed in a certain proportion through the rotameter, the dilution gas and the xylene gas loaded with a certain concentration are merged into the buffer chamber, and after being completely mixed, it enters the heating constant temperature chamber, and the concentration, flow and temperature are set in the experiment. The stable gas enters the adsorption unit, and the xylene concentration is measured before and after the adsorption unit. The xylene adsorption process research device is the same as the adsorption isotherm experimental device, and the adsorbent filling height is 10cm.

 
1.3.2 Desorption experimental device
 
  The flow of the desorption experimental device is shown in Figure 2. The adsorbent filling height is 10cm. Different flow rates of high-temperature clean air are used to blow off the adsorbed adsorption unit at different temperatures, and a xylene sampling detection device is installed at the exhaust end of the system.
 
1.3.3 Determination of adsorption isotherm
 
  The adsorption isotherm of xylene on the adsorption unit was determined by dynamic adsorption method and verified by gravimetric method. That is, the xylene gas of the set concentration is continuously passed into the adsorbent, and the xylene concentration is detected before and after the adsorption system. When the outlet concentration reaches the inlet concentration, the adsorption amount of the adsorbent p-xylene under this working condition is obtained by integrating the concentration curve. , At the same time, the weight difference of the adsorption material before and after the comparison is checked and verified.
 
2 Results and discussion
 
2.1 Characterization of adsorbent
 
  The pore size distribution of zeolite molecular sieve and activated carbon is shown in Figure 3. It can be seen that the main pore size distribution of the molecular sieve is below 2.0nm, concentrated at about 0.8nm, which is a typical uniform microporous adsorbent; activated carbon reflects a broad spectrum, and while the micropores are more developed, it also contains certain mesopores. The pore diameter is concentrated around 1~2nm. Table 1 shows the pore structure parameters of zeolite molecular sieve and activated carbon. The specific surface area and pore volume of activated carbon are larger than those of zeolite molecular sieve, but the specific surface area of zeolite molecular sieve is as high as 85% of the total specific surface area.


 
2.2 Xylene molecular sieve and activated carbon adsorption isotherm equation
 
  Figure 4 shows the xylene adsorption isotherm of zeolite molecular sieve and activated carbon. In general, the equilibrium adsorption capacity of activated carbon for xylene is higher than that of zeolite molecular sieve, which is consistent with the results of specific surface area and pore volume. At the same time, compared with zeolite molecular sieves, the equilibrium adsorption capacity of activated carbon can reach 18% with the adsorption temperature of benzene, while it is only 6% at 80°C, which is only about 1/3 of the capacity at 30°C; At the same concentration, the adsorption equilibrium capacity of zeolite molecular sieve at 30°C is about 10%, and the adsorption equilibrium capacity at 80°C is about 6%, with little change, indicating that the zeolite molecular sieve has a small pore size, high adsorption potential, and strong adsorption capacity, but it is not easy Desorption and small capacity potential.

 
  For the adsorption equilibrium data obtained in the experiment, Langmuir adsorption isotherm and Freundlich adsorption isotherm were used to fit the results, as shown in Table 2. The R2 of the Freundlich adsorption isotherm equation is above 0.99, so it is considered that the adsorption isotherm equation of activated carbon and zeolite molecular sieve for xylene is more suitable to be expressed by Freundlich equation.

 
  Aiming at intermittent, large air volume, and low-concentration exhaust gas emissions, the adsorption concentration method is considered to be an ideal process route with both economy and reliability [7,15-16]. The adsorbents widely used in industry are mainly activated carbon and molecular sieves. The pore size of the adsorbent has a great influence on its adsorption and desorption performance, and there is a certain matching problem between the pore size of the adsorbent and the geometric size of the adsorbate [17-19]. Activated carbon is an amorphous carbon with irregular microcrystalline structure. Due to its large specific surface area and strong adsorption capacity, it is the most used adsorbent, but activated carbon is flammable and its adsorption performance is greatly affected by moisture. The shortcomings limit its application [20-22]. Hydrophobic subsieve is a kind of synthetic zeolite. It is a kind of microporous hydrated aluminosilicate crystals with framework structure. The pore size of zeolite molecular sieve is usually less than 1.0nm, due to its unique regular pore structure, selectivity and high Hydrothermal stability is widely used in catalysis, separation and other chemical and petrochemical fields. The activated carbon with porous structure has a higher equilibrium capacity for xylene adsorption, and the working capacity increases in a wide range with the increase of concentration. The specific surface area and pore volume of the molecular sieve are both low, which is reflected in the relatively low adsorption balance of p-xylene. However, when the adsorption temperature is greater than 80°C and the pollutant concentration is low, the adsorption balance capacity of the molecular sieve gradually exceeds that of activated carbon, and the adsorption The equilibrium capacity fluctuates little with the change of concentration, indicating that zeolite molecular sieve is more suitable for working conditions with low concentration and high exhaust temperature.
 
2.3 Xylene adsorption breakthrough curve of activated carbon and zeolite molecular sieve fixed bed
 
  Figure 5 shows the adsorption breakthrough curve when xylene gas passes through the adsorption column of activated carbon and zeolite molecular sieve of the same height. The wind speed of the selected adsorption bed section is about 0.3, 0.4, and 0.5 m˙s-1, respectively, the inlet concentration of the activated carbon dynamic adsorption experiment is about 288.2, 315.8, and 301.0 mg˙m-3; the inlet concentration of the dynamic adsorption experiment of the zeolite molecular sieve is respectively It is about 299.9, 301.2 and 296.7mg˙m-3, and the adsorption system has a constant temperature of 30℃.

  The results show that the penetration curves of zeolite molecular sieve and activated carbon under various wind speed conditions are typical S-shaped curves. Taking the outlet concentration as 5% of the inlet concentration as the penetration point, the penetration time of the zeolite molecular sieve under the working conditions of 0.3, 0.4 and 0.5m˙s-1 is about 15.5, 11.5 and 9.5h, respectively; the activated carbon is at 0.3, 0.4 The penetration time under working conditions of 0.5m˙s-1 and 0.5m˙s-1 is about 19, 12.5 and 9h, respectively. It can be seen that when the cross-sectional wind speed of zeolite molecular sieve is high, the adsorption penetration time has exceeded that of activated carbon, and it can also maintain a good adsorption speed. From the point of view of the time to reach the adsorption equilibrium, the adsorption saturation time of activated carbon is greater than that of zeolite molecular sieves, so the equilibrium adsorption capacity is also higher than that of zeolite molecular sieves. This is consistent with the aforementioned characterization results. Activated carbon is a porous structure containing micropores and mesopores, with a specific surface area. The larger the adsorption capacity. To a certain extent, activated carbon is more advantageous to adsorb xylene. Under the experimental conditions of 30℃ and 300mg˙m-3 of air intake, the average mass transfer per unit length of activated carbon and zeolite molecular sieve under different cross-sectional velocity is shown in Table 3. When the cross-sectional velocity is 0.3~0.5m At ˙s-1, the average mass transfer per unit length of the zeolite molecular sieve is about 1.42~1.66 times that of activated carbon, indicating that the adsorption rate of the zeolite molecular sieve is faster. With the increase of wind speed, the degree of turbulence in the adsorption bed increases, and the average mass transfer increases.

 
2.4 Xylene desorption performance of activated carbon and zeolite molecular sieve fixed adsorption bed
 
  Activated carbon has a wide pore size distribution. The desorption of adsorbents from the surface of activated carbon mainly depends on the diffusion resistance in the pores, while zeolite molecular sieves have a uniform pore structure. The main factor that determines the desorption of adsorbates from its surface is the adsorption strength.[23] . From the perspective of the safety of activated carbon desorption, this paper only conducted desorption experiments with different cross-sectional wind speeds on activated carbon and zeolite molecular sieve after the adsorption equilibrium was reached at 30℃ and 300mg˙m-3 under the desorption temperature of 90℃. The ratio of the cumulative average concentration at the outlet of the desorption experiment to the inlet concentration at the time of adsorption, that is, the concentration ratio.The result is shown in Figure 6(a). At this temperature, the concentration ratio is not high and the desorption time is longer. When the cross-sectional wind speed is 0.4 At m˙s-1, the peak concentration factor of activated carbon is about 4.5 times, and that of zeolite molecular sieve is about 2.1 times. Under the working condition of 90℃, it is more difficult to desorb xylene from the surface of zeolite molecular sieve than activated carbon.

 
  The adsorption isotherm shows that the equilibrium adsorption capacity of zeolite molecular sieve at 90℃ is more than 50% of the equilibrium adsorption capacity at 30℃. The desorption temperature of zeolite molecular sieve must be increased to achieve better desorption effect. The results of the 210°C desorption regeneration experiment on molecular sieves saturated with adsorption at a temperature of 30°C and an intake concentration of about 300mg˙m-3 are shown in Figure 6(b). Zeolite molecular sieves get relatively complete desorption.
 
  As shown in Figure 6(b), when the wind speed of the purge section gradually decreases from 0.3, 0.2 to 0.1 m˙s-1, the peak time of the desorption concentration ratio increases accordingly, and the peaks appear at the 8, 16 and 32 minutes respectively. Compared with the adsorption inlet concentration, the concentration is about 48-60 times. This desorption experiment has oven auxiliary heating, so the obtained concentration ratio is about the limit concentration ratio. Compared with the desorption of zeolite molecular sieve at 90℃, the concentration ratio increases significantly at this temperature.
 
  The change of the desorption amount of zeolite with time under different cross-section wind speeds is shown in Figure 7.The desorption amount gradually increases with time, roughly showing that the early growth rate is faster and the later period tends to be flat. For outstanding. Taking the recovery working capacity of 5% as an example, when the cross-sectional wind speed is 0.3m˙s-1, the required time is 10min, the shortest time, 0.2m˙s-1 requires about 16min, and the cross-sectional wind speed is 0.1m˙s-1. It takes 27 minutes and takes the longest time. The input clean air volume is 248.8, 241.3 and 203.6L respectively. In summary, as the wind speed increases in the experimental interval, the desorption speed also increases, but the required energy consumption increases.

 
  Zeolite molecular sieve is adsorbed and desorbed at 210°C. After repeating the experiment 50 times, no decrease in adsorption capacity is found. The test results of its specific surface area, pore volume, and pore size are shown in Table 4, indicating that repeated use of molecular sieve affects its performance. Not big.

 
2.5 Analysis of regeneration energy consumption based on adsorption isotherm
 
  Taking xylene as an example, the energy consumption of the low-temperature regeneration process of activated carbon and zeolite molecular sieve is analyzed and compared, that is, the difference in energy consumption of regenerated VOCs per unit mass is compared. The specific heat capacity of activated carbon is uniformly 0.84kJ˙(kg˙K)-1, and the specific heat capacity of zeolite molecular sieve is 0.92kJ˙(kg˙K)-1. The available working capacity is selected as two zeolite molecular sieves and activated carbon at 30℃ and 80℃ The difference in adsorption capacity between temperatures. Under different xylene inlet concentration, adsorption is saturated and desorption regeneration is performed under 80℃ working condition. It is assumed that the influence of heating shell and heat dissipation is calculated as 20% of the required heat.

 
  As shown in Figure 8, under low-concentration xylene inlet conditions (less than 500mg˙m-3), the energy consumption per unit mass of xylene regeneration from activated carbon is 0.5~0.7kJ˙g-1, zeolite molecular sieve The energy consumption is 1.5~2.1kJ˙g-1. Zeolite molecular sieve as an adsorbent requires energy consumption per unit mass of xylene to be 2.9 to 4.2 times that of activated carbon. In general, activated carbon has a greater working capacity in this range
, The regeneration energy consumption is lower when treating low-concentration xylene waste gas. The high temperature safety of zeolite molecular sieve is better than that of activated carbon. Therefore, if the zeolite molecular sieve is used as the adsorption material, regular high temperature desorption can be used, and the advantage of high concentration ratio can be used to burn the gas after desorption; when activated carbon is the adsorption material, 80~90℃ gas can be used for low temperature desorption. Attached and desorbed concentrated gas can be used mobile secondary adsorption, thereby improving the economic performance of the process, related results can be seen in previous studies [24].
 
3 conclusion
 
  1) The characterization results show that both zeolite molecular sieves and activated carbon are rich in uniform microporous structure, zeolite molecular sieve has uniform pore size, and activated carbon has a wider pore size distribution and is generally larger than zeolite molecular sieve.
 
  2) For xylene, the equilibrium adsorption capacity of zeolite molecular sieve is generally smaller than that of activated carbon, and the equilibrium adsorption capacity of zeolite molecular sieve is smaller than that of activated carbon with the equilibrium concentration and adsorption temperature.
 
  3) When the cross-sectional velocity is 0.3~0.5m˙s-1, the average mass transfer rate per unit length of the zeolite molecular sieve is about 1.42~1.66 times that of activated carbon.
 
  4) Desorption of xylene from the surface of zeolite molecular sieve is more difficult than activated carbon. When using low temperature desorption regeneration below 90℃, activated carbon has a larger working capacity than zeolite molecular sieve. Zeolite molecular sieve desorption is more complete at 210 ℃, and can get more than 48 times the concentrated desorption gas. 50 repeated experiments of adsorption and desorption have little effect on the performance of zeolite molecular sieve.
 
  5) The energy consumption per unit mass of adsorbate regenerated by activated carbon at 80°C is lower than that of zeolite molecular sieve. The thermal stability of zeolite molecular sieve makes it suitable for occasions with higher safety requirements.