Activated carbon adsorption

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Research on the Use of Activated Carbon in Pressure Swing Adsorption for Benzene Hydrogenation Yuan Kangru, Liu Dinghua (Shanghai Baosteel Chemical Co., Ltd., Shanghai 200942), Liu Xiaoqin, Ma Zhengfei (School of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009). 1 Process Introduction Pressure swing adsorption (PSA) is a dry-process technology. Its basic principle relies on the differences in the adsorption capacity, adsorption rate, and adsorption driving force of different gases by the adsorbent, as well as the variation in the adsorbent’s capacity with pressure. Under pressure, the adsorption and separation of mixed gases take place, while regeneration of the adsorbent occurs under reduced pressure, thereby achieving gas separation and the recycling of the adsorbent. Activated carbon is a commonly used adsorbent for pressure swing adsorption decarbonization. It is a granular or powdered form of porous carbonaceous material with a highly developed microporous structure, which grants it strong adsorption capacity. Since the 1980s, extensive research has been conducted on the use of activated carbon in the removal of carbon dioxide. The benzene hydrogenation unit of the Chemical Division of Baoshan Iron and Steel Co., Ltd. utilizes process technology introduced from Japan in the 1980s. Its main operational procedure involves using crude benzene as raw material along with pure hydrogen produced by the hydrogenation unit; through a series of steps, pure benzene is obtained as the final product. The gases generated in this process are then processed by the unit’s hydrogen production and purification systems to produce more pure hydrogen, thus enabling a cycle of hydrogen reuse within the facility. The hydrogen purification system uses pressure swing adsorption technology. The alumina at the lower part of the adsorption tower is used to remove water from hydrogen, the activated carbon in the middle section of the tower is used to remove CO2 from hydrogen, and the molecular sieve at the top of the tower can eliminate impurities such as CO and CH4 from hydrogen, ensuring that the purity of the hydrogen exiting the tower exceeds 99%. Since the operating parameters of the equipment are determined during installation and commissioning, it is not possible to localize the equipment without conducting a systematic study on the pressure swing adsorption properties of imported activated carbon. Therefore, it is necessary to characterize the activated carbon by analyzing the imported activated carbon adsorbents, and to conduct thorough research and analysis covering its macroscopic and microscopic properties as well as its adsorption capacity, in order to proceed with subsequent work. 2 Characterization of imported activated carbon 2.1 Macroscopic physical properties of imported activated carbon The adsorption and separation process relies on the reversibility of physical adsorption of gases by the adsorbent to achieve the separation of mixtures; therefore, the physical properties of the adsorbent are generally more important than its chemical properties. The physical properties of activated carbon include particle size, density, wear resistance, mechanical strength, as well as specific surface area, pore volume, and pore size. These properties are often related to factors such as the raw materials used in its production, the manufacturing methods, the post-treatment conditions, and the shape and particle size of the activated carbon. In the experiment, the physical properties of imported activated carbon were analyzed and tested; the general macroscopic physical properties are shown in Table 1. Table 1 Relevant properties of imported activated carbon. Parameter: Bulk density, kg/m3; Shape wear%, %; Average compressive strength, N/cm2; Ash content, %; Specific surface area, m2/g; Pore volume, cm3/g. Design values: 480, ---, ≤810, 50–1150, –. Test values: 488.3, Irregular small pieces, 3.8, 122.8, 6.67, 1014.7, 0.44, 7.2. As can be seen from Table 1, the actual test values for the bulk density, ash content, and specific surface area of the imported activated carbon are in line with the design values provided by the Japanese side. Due to the frequent pressure changes in pressure swing adsorption, the wear of activated carbon and its average compressive strength are extremely important; relevant data have been obtained through experimental testing. 2.2 Microphysical properties of imported activated carbon (1) Pore size distribution. The pore sizes of activated carbon used for gas adsorption are mostly micropores (i.e., pore size < 2 nm, where 1 nm = 10 Å), and the commonly used liquid nitrogen adsorption method cannot accurately determine their pore size distribution. Therefore, the pore size distribution was determined using the adsorption-desorption method of CO2 at 273 K with ASAP2020. The models used to determine the pore size distribution are HK (Horvath-Kawazoe) and DFT (Density Functional Theory); since the adsorbate in this experiment is CO2, the DFT model provides more accurate results. The experimental results are shown in Figure 1. Figure 1 Pore size distribution of imported activated carbon. As can be seen from Figure 1, there is a significant concentration of pores with a size of around 6 Å, as well as some pores with sizes between 8 Å and 9 Å. Activated carbon possesses a well-developed pore structure, featuring a large number of micropores as well as a certain amount of mesopores and macropores, which ensures its excellent adsorption capacity. Generally, the micropore radius of activated carbon is around 0.6–0.8 nm, which is in the same order of magnitude as that of ordinary small molecules (the diameter of a CO2 molecule is 0.33 nm); therefore, it shows no selectivity in adsorbing these small molecules. The micropore volume is approximately 0.15–0.6 mL/g, and the specific surface area of the micropores can account for over 95% of the total specific surface area. (2) Electron spectroscopy (XPS) analysis. Electron spectroscopy analysis is one of the most effective methods for qualitative element analysis; the characteristic spectral lines appearing in the spectrum can be used to identify the type of element based on their position. XPS experiments on imported activated carbon were conducted using the PHI550 multi-functional electron spectrometer produced by PHI Corporation in the United States; the results are shown in Figure 2. Figure 2: XPS spectrum of imported activated carbon. The XPS spectrum of the imported activated carbon shows that its main components are carbon and oxygen, with few impurities; the ratio of carbon to oxygen is approximately 86.3:13.7. XPS primarily measures the surface composition and chemical state of materials; a small amount of oxygen adsorbed on the surface of activated carbon may cause XPS to detect an excess of oxygen elements. 3 Adsorption isotherms: The adsorption equilibrium relationship determines the direction and limits of the adsorption process, and it serves as the fundamental basis for this process. Static adsorption equilibrium data are very important references for the application of adsorbents. The experiment determined the adsorption isotherms of imported activated carbon at 273 K using an ASAP2020 (American-made) instrument. Since the activated carbon adsorbent is primarily used to remove CO2 from impurities during the hydrogen production process, it is necessary to test the adsorption capacity of the incoming activated carbon for CO2, that is, to determine its adsorption isotherm for CO2. The adsorption isotherm of carbon dioxide on the imported activated carbon is shown in Figure 3. Figure 3 Adsorption isotherms of imported activated carbon for carbon dioxide. As can be seen from Figures 2 and 3, the adsorption isotherms of the imported activated carbon belong to the preferential adsorption type; even at low partial pressures of the adsorbate, the amount of adsorption by the adsorbent remains high, which facilitates complete separation through adsorption. 4. Pressure swing adsorption tests on single towers: Although the equilibrium adsorption capacity of the activated carbon used as the inlet material for CO2 was determined, these data cannot be used directly for industrial design. What is more meaningful in practical applications is the dynamic adsorption capacity of the adsorbent in the adsorption bed. This is because the adsorption capacity is influenced not only by the partial pressure of the adsorbate but also by other components as well as the actual degree of desorption of the adsorbent. Dynamic adsorption involves examining the amount of adsorption while ensuring a high level of purification, which is what makes it truly reliable. Therefore, to evaluate the adsorption performance of the adsorbent, dynamic adsorption methods must be employed. In the experiments, a single-tower pressure swing adsorption unit was used to determine the dynamic adsorption capacity of the activated carbon adsorbent for mixed CO2 and N2 streams, thereby assessing the adsorption ability of this activated carbon for carbon dioxide under various adsorption conditions. The imported activated carbon was placed in a single tower with dimensions of Φ21×400mm, and its total weight was 61.8 g. Using a gas mixture of CO2 and N2 with a CO2 volume fraction of 13.6% as the feed gas, experiments on the single-tank pressure swing adsorption process were conducted under various conditions until complete breakthrough of the single tower occurred. The adsorption data for the penetration curves are listed in Tables 2 and 3. Table 2: Adsorption data of the mixture gas on activated carbon at different flow rates (adsorption pressure: 0.5 MPa)
Serial number | V, mL/min | Tb, min | md, min | Vb, mL/g | Vd, mL/cm3
1 | 100 | 144 | 452 | 38.7 | 18.92
2 | 200 | 70 | 194 | 38.3 | 18.73
3 | 300 | 421 | 383 | 35.6 | 17.4

Table 3: Adsorption data of the mixture gas on activated carbon at different adsorption pressures (mixture gas flow rate: 200 mL/min)
Serial number | P, MPa | Tb, min | md, min | Vb, mL/g | Vd, mL/cm3
4 | 0.2 | 309 | 822 | 2.3 | 10.95
5 | 0.5 | 701 | 943 | 38.3 | 18.76
6 | 0.8 | 961 | 705 | 50.5 | 24.7