Yingshu Liu

Performance of mesoporous silicas (MCM-41 and SBA-15) and carbon (CMK-3) in the removal of gas-phase naphthalene: adsorption capacity, rate and regenerability

The adsorption isotherms of naphthalene on three typical mesoporous adsorbents, mesosilicas MCM-41 and SBA-15, and mesocarbon CMK-3 were determined by column tests at 125 °C, with feed concentrations ranging from 7.63 × 10−5 to 4.64 × 10−2 mol m−3 (1.88 to 1140 ppm). The Langmuir model and constant-pattern wave propagation model were found to well fit the isotherms and the breakthrough curves, respectively. Regenerabilities of the mesoporous samples and a benchmark activated carbon (AC) were characterized based on thermogravimetric analysis (TGA). The results show mesoporosity significantly reduced the internal mass-transfer resistance, contributing to facile desorption and to fast adsorption kinetics shown by high overall mass-transfer rate coefficient following the order of: CMK-3 > SBA-15 > MCM-41. Micropore–mesopore coexisting structures present in CMK-3 and SBA-15 facilitated the adsorption at very low concentrations due to micropore-filling, while greater surface hydrophobicity and micropore abundance on CMK-3 exhibited larger affinity for nonpolar naphthalene, rendering the highest adsorption capacity (1.014 mol m−3) among all sorbents including ACs. SBA-15 showed higher regenerability with a desorption temperature below 440 K, owing to the weaker binding and diffusion advantages contributed by the interconnectivity between primary mesopores.

Novel Wire-on-Plate Electrostatic Precipitator (WOP-EP) for Controlling Fine Particle and Nanoparticle Pollution

A new wire-on-plate electrostatic precipitator (WOP-EP), where discharge wires are attached directly on the surface of a dielectric plate, was developed to ease the installation of the wires, minimize particle deposition on the wires, and lower ozone emission while maintaining a high particle collection efficiency. For a lab-scale WOP-EP (width, 50 mm; height, 20 mm; length, 180 mm) tested at the applied voltage of 18 kV, experimental total particle collection efficiencies were found as high as 90.9-99.7 and 98.8-99.9% in the particle size range of 30-1870 nm at the average air velocities of 0.50 m/s (flow rate, 30 L/min; residence time, 0.36 s) and 0.25 m/s (flow rate, 15 L/min; residence time, 0.72 s), respectively. Particle collection efficiencies calculated by numerical models agreed well with the experimental results. The comparison to the traditional wire-in-plate EP showed that, at the same applied voltage, the current WOP-EP emitted 1-2 orders of magnitude lower ozone concentration, had cleaner discharge wires after heavy particle loading in the EP, and recovered high particle collection efficiency after the grounded collection plate was cleaned. It is expected that the current WOP-EP can be scaled up as an efficient air-cleaning device to control fine particle and nanoparticle pollution.

Proportion Pressure Swing Adsorption for Low Concentration Coal Mine Methane Enrichment

Using traditional Pressure Swing Adsorption (PSA) with a single adsorbent for low concentration coal mine methane (LCCMM) at a concentration of 30% or less can result in a final CH4 concentration very close to the explosion limit, increasing the risk of explosion. Proportion Pressure Swing Adsorption (PPSA) is a new and safer enrichment method suggested for LCCMM enrichment that uses a mixture of active carbon (AC) and carbon molecular sieves (CMS) as adsorbents. With this method, CH4 and O2 in LCCMM can be adsorbed simultaneously because CH4 is mostly adsorbed by active carbon and O2 is mostly adsorbed by the CMS. Therefore, the concentration of CH4 and O2 is well controlled and does not exceed the explosive limit during the adsorption and desorption processes. We have demonstrated the safety and feasibility of PPSA for obtaining 30% CH4 from LCCMM, with 20% CH4 in air as a feed stock. Our results show that the O2 concentration can be controlled well and does not exceed the explosive limit in both adsorption and desorption, and the CH4 concentration in the desorption gas can be increased to more than 30% by adjusting the bed length and mass ratio of the AC and CMS. Taking these results together, it appears that PPSA is a safe method for LCCMM enrichment.

Safe Separation of the Low-Concentration and Oxygen-Bearing Coal Mine Methane by Vacuum Pressure Swing Adsorption

Pressure swing adsorption (PSA) is a suitable method to enrich the concentration of methane in coal mine methane (CMM) separation processes. However, traditional PSA processes for low-concentration CMM can easily increase the concentration of methane to explosive limits, which subsequently increases the risk of explosion. In this paper, a novel PSA process for low-concentration CMM was studied. using a mixture of activated carbon (AC) and carbon molecular sieve (CMS) as the adsorbent, this method ensures that the gaseous mixture adsorbed does not reach the explosive limits by adsorbing methane and part of the oxygen simultaneously. using a two-bed vacuum PSA experimental apparatus, the low-concentration CMM was safely enriched from 20% to more than 30%, with CMS and AC mass ratio of 3.4. The results of experimental studies indicate that the mixture adsorbent (AC and CMS) has explosion-suppression and flameproof characteristics. The igniting resource (methane) will not explode even if it appears in the adsorbent layer and, in addition, the explosion will not propagate through the adsorbent layer if it happens in other areas.

Adsorption thermodynamics and desorption properties of gaseous polycyclic aromatic hydrocarbons on mesoporous adsorbents

Performances of mesoporous materials in removal of polycyclic aromatic hydrocarbons (PAHs) from hot gases were evaluated systematically. The adsorption and desorption natures for PAHs with different aromatic rings, naphthalene (Nap), phenanthrene (Phe) and pyrene (Pyr) on mesosilicas MCM-41 and SBA-15, and mesocarbon CMK-3, were studied. Adsorption equilibria were well described by Langmuir or Freundlich model, giving the order of adsorption capacity of CMK-3 > SBA-15 > MCM-41 and Pry > Phe > Nap. Temperature programmed desorption exhibited the order of ease of desorption of SBA-15 > MCM-41 > CMK-3, along with desorption kinetic triplet determined by combined model fitting analysis. The Johnson–Mehl–Avrami (JMA) rate equation was found to well describe PAH desorption kinetics, except for Phe/SBA-15 and Pyr/CMK-3 due to potential blockage and strong binding, respectively. MCM-41 with simple 1-D mesoporous structure provided smooth diffusion and consistent behavior but very low adsorption capacity for each PAH. SBA-15 with micropores/small mesopores as interconnectivity between primary mesopores not only showed high sorption capacities but also diffusion advantages in desorption. CMK-3 with great microporosity facilitated the adsorption at low concentrations due to micropore-filling, and with surface hydrophobicity rendered high affinities especially for the bulkier Pyr.

Low Concentration Coal Mine Methane Concentrated by Vacuum Pressure Swing Adsorption

In order to solve the current situation that low-concentration oxygen-bearing coal mine methane can’t be concentrated and utilized, the vacuum pressure swing adsorption process was employed to concentrate the oxygen-bearing coal mine methane gas with the methane concentration of 20%. To ensure the safety of the enrichment process, mixed adsorbents consist of active carbon and carbon molecular sieve was used innovatively, which can adsorb methane and oxygen simultaneously. So that the oxygen and methane concentration of both effluent gas and product gas of the process are both in safe range. The separation process of coal mine methane gas was carried out at room temperature under adsorption pressure no more than 320 kPa abs., and desorption pressure about 25 kPa abs. The effects of adsorber adsorption time, adsorbent’s quality ratio, adsorption pressure, purge time, purge time on the process were investigated. When the quality ratio of carbon molecular sieve and active carbon was 3.4, the methane can be concentrated to upper than 33% the concentration of methane and oxygen in effluent gas are lower than 2.4% and 10.8% respectively. When we added a purge step with a purge time of 5s, the product methane concentration decreased to 31%, and the concentration of methane and oxygen in effluent decreased to 1.6% and 9.5% respectively. This study shows that the low-concentration coal mine methane gas can be concentrated and utilized for industrial applications.

Desorption characteristics and kinetic parameters determination of molecular sieve by thermogravimetric analysis/differential thermogravimetric analysis technique:

The kinetics of the thermal desorption of CO2 adsorbed on zeolite 13X were obtained using a differential thermogravimetric analyser under two different carrier gas conditions. The varying heating rates were set as 8, 12, 16, and 20 K min−1, respectively. The desorption activation energy of the physisorption sites for this experiment evaluated by an integral method without prediction of the reaction order ranged from 12.15 to 14.12 kJ mol−1 (CO2 as the carrier gas) and 43.32 to 50.42 kJ mol−1 (Ar as the carrier gas), respectively. The desorption activation energy of the chemisorption sites ranged from 57.95 to 58.53 kJ mol−1 (CO2 as the carrier gas) and 74.02 to 79.92 kJ mol−1 (Ar as the carrier gas), respectively.