Experimental study on the treatment of volatile organic compound vapors using a photoreactor equipped with photocatalyst‐coated fabrics

In this study, methods were developed to enlarge the scope of traditional applications of titanium dioxide (TiO₂) and to increase the value of felted fabric by allowing volatile organic compound (VOC) degradation as well as dust filtration in a photoreacting fabric filter. In the past, when a V–Ti mixed catalyst was used, the application of felted fabric as a support material for the catalyst was difficult because the active temperature of the catalyst ranged from 250 to 400°C. Thus, in this study catalyst‐coated felted fabric was manufactured at normal temperature conditions, and then fundamental de‐VOC performance tests were conducted under irradiation condition to develop a fabric filter having a de‐VOC function in addition to a dust‐filtration function. Toluene vapor was selected as a sample VOC because it poses health hazards, has been widely used as an organic solvent, and has been known as a compound that is difficult to dissociate. To manufacture and use a fabric filter that degrades VOC_S, and removes dust particles through using photocatalyst‐coated fabrics and light sources, optimum operating conditions were obtained by observing the degradation attributed to varying the toluene‐vapor flow rate, the initial toluene concentration, flue gas humidity, TiO₂ loading onto the surface of the fabric, and the intensity and wavelength of a near ultraviolet light lamp. To keep the flue gas humidity at a relatively constant level, a system to automatically control the humidity was constructed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3174–3179, 2004     

Behavior of heavy metals in air pollution control devices of 2,400 kg/h municipal solid waste incinerator

We analyzed the behavior of heavy metals, such as Cd, Cr, Cu, Ni, Pb, Zn, and Hg, in air pollution control devices of a municipal solid waste incinerator. For this study, a municipal solid waste incinerator with a burning capacity of 2,400 kg/h was selected. A semi-dry reactor (SDR), fabric filter, and wet scrubber were installed to serve as air pollution control devices. Flue gas was sampled upstream and downstream of each air pollution control device to determine the heavy metal concentrations therein. Ash was collected from the furnace, boiler, SDR, and fabric filter to determine the heavy metal concentration in the ash produced by each device. Each heavy metal was found to have a different fate in the incinerator and air pollution control devices. Cd and Pb were mostly present in the fabric filter ash, whereas Cr, Cu, and Ni were most prevalent in the bottom ash of the furnace and boiler, and Zn was present in the bottom and fabric filter ash at a ratio of 7: 3. However, only a few percent of Hg was identified in the ash from the furnace, boiler, SDR, and fabric filter; the majority of Hg passed through the fabric filter and existed in an oxidized form. The wet scrubber exhibited high control efficiency for oxidized mercury, and the injection of commercial activated carbon at a rate of 0.2 g/Sm³ resulted in 93.2% mercury removal efficiency.     

Continuous toluene vapour photocatalytic deduction in a multi-stage fluidised bed

Continuous deduction of 1000 ppm toluene vapour in a multi-stage gas fluidised bed photoreactor was studied using TiO₂ coated activated carbon (AC/TiO₂) particles as the fluidising media. Due to the enrichment of the local toluene concentration, the efficiency of the toluene removal from the gas stream by the mass transfer-limited photocatalytic reaction was significantly improved using AC/TiO₂ particles as the fluidising media. In the stage addition studies, the results showed that the toluene removal efficiency at steady state increased approximate 3-4% for every stage addition. The three-stage design effectively utilised the freeboard region and improved the toluene removal efficiency for approximately 37% when comparing to the single-stage system. Using SiO₂ powders as fluidising media could enhance the light utilisation by light scattering and promotes the toluene deduction during the initial to steady toluene deduction period.     

Indoor air purification by photocatalyst TiO2 immobilized on an activated carbon filter installed in an air cleaner

The enhancement effect of using TiO₂ immobilized on activated carbon (TiO₂/AC) filter for removing indoor air pollutant at parts-per-billion (ppb) levels has been previously reported. To further evaluate the TiO₂/AC filter for practical application, it was installed in an air cleaner available in the commercial market and tested inside an environmental chamber. Nitrogen oxide (NO) and toluene were selected as target pollutant. Results showed that a higher removal efficiency of NO was achieved using shorter wavelength ultraviolet lamp than longer wavelength ultraviolet lamp. A higher NO removal was achieved using TiO₂/AC filter compared to TiO₂ filter only. The intermediate, NO₂, generated from the photodegradation of NO was also successfully suppressed from exiting the system using TiO₂/AC filter. A 25% higher of nitrogen oxides (NO_x) was achieved using TiO₂/AC filter compared to the TiO₂/AC only. The higher removal efficiency of using TiO₂/AC is owing to the large adsorption capacity provided by the activated carbon. The adsorbed NO is then transferred to the TiO₂ for photodegradation. The difference in toluene removal efficiency using TiO₂/AC filter compared to the TiO₂ filter is even more significant.     

The adsorption of PAHs, BTEX, and heavy metals on surfactant-modified desulfurization sorbents in a dry scrubber

The injection of dry Ca-based sorbents for removing acid gases had been investigated previously. However, the utilization of Ca-based sorbents for adsorbing other air pollutants was rarely examined. The objective of this study was to investigate the reduction of organic compounds polycyclic aromatic hydrocarbons (PAHs) and benzene, toluene, ethylbenzene, and xylene (BTEX) and heavy metals by using the desulfurization sorbents in a dry scrubber integrated with a fabric filter. Four surfactants, calcium lignosulfonate, sodium lignosulfonate, alkyl naphthalene sodium sulfonate and β-naphthalene sodium sulfonate condensates, were used as additives to modify the surface characteristics of desulfurization sorbent calcium hydroxide (Ca(OH)₂). Modifying the desulfurization sorbents with surfactants showed different effects on removal of PAHs, BTEX, and heavy metals, and exploited the potential of Ca-based sorbents for adsorbing air pollutants other than SO₂.     

NO and SO2 removal in non-thermal plasma reactor packed with glass beads-TiO2 thin film coated by PCVD process

We analyzed the NO and SO₂ removal in the non-thermal plasma discharge process combined with TiO₂ photocatalyst. The non-thermal plasmas were generated by dielectric barrier discharge with glass beads as dielectric materials. The TiO₂ thin films were coated on the glass beads uniformly without crack by a rotating cylindrical plasma chemical vapor deposition reactor. The NO and SO₂ removal efficiencies obtained in non-thermal plasma–TiO₂ photocatalysts hybrid system were higher than those in plasma process only, because of the additional removal of NO and SO₂ by photocatalysts. The NO and SO₂ removal efficiencies become higher, as applied peak voltage, pulse frequency and gas residence time increase, or as the initial NO and SO₂ concentrations decrease. The hybrid system of non-thermal plasma and photocatalyst thin film on glass beads prepared by PCVD process is quite efficient method to remove NO and SO₂.     

Modeling of SO2 removal in fabric filter

Fabric filters are involved in most semi-dry flue gas desulfurization process and represent ability of SO₂ removal. SO₂ removal efficiency in fabric filter after a semi-dry scrubber is investigated. Experimental results showed that SO₂ inlet concentration has little effect on SO₂ removal efficiency, SO₂ removal efficiency increases as flue gas inlet temperature increases and relative humidity affects SO₂ removal efficiency significantly. The kinetic model based on shrinking core theory has been presented. It is found that, in the beginning, when calcium hydroxide conversion ratio is less than 0.3, SO₂ removal process is mainly controlled by chemical reaction (Model-2); and when calcium hydroxide conversion ratio is greater than 0.3, SO₂ diffusion through product layer is rate limiting (Model-3). The experimental results in fabric filter are successfully correlated by Model-3.     

Development of air purification device through application of thin-film photocatalyst

TiO₂ photocatalyst was prepared by hydrothermal method using titanium isopropoxide as a precursor. This photocatalyst had higher content of anatase structure and larger surface area than commercial P-25 photocatalyst. The air purification device with a filter coated with photocatalyst film was excellent in its capabilities to decompose organic materials and to kill germs. When the device was installed in hospitals, it was powerful for the sterilization of floating germs in the air. In addition, the number of germs after 7 months operation was almost the same as that obtained in the first operation.     

Toluene vapor removal using an inorganic/organic double-network ion gel membrane

ABSTRACT

Tough gel membrane composed of a large amount of an ionic liquid and an inorganic/organic composite double-network (inorganic/organic DN ion gel membrane) was examined to remove toluene vapor from toluene vapor/N₂ mixed gas. The DN ion gel membrane with 80 wt% of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide showed higher toluene vapor permeability (more than 30,000 barrer) and toluene vapor/N₂ permselectivity (690) than the supported ionic liquid membrane. The toluene vapor permeability and toluene vapor/N₂ permselectivity were maintained under pressurized condition. It was confirmed that the toluene vapor permeation was limited by intramembrane diffusion of the dissolved toluene vapor.     

The synthesis of spherical activated carbons containing zinc and their photochemical activity

Spherical activated carbon (AC) containing photocatalyst was prepared using strong and weak acid ion-exchange resins as starting materials. These resins were treated with an aqueous solution of Zn²⁺ or [Zn(NH₃)₄]²⁺ and both treated resins and non-treated resins were carbonized and activated. The physicochemical characteristics of the AC samples were examined using XRD, SEM, EDS, BET, EPMA, ESR, compressive strength and zinc content. The photocatalytic activity was evaluated by measuring the humic acid (HA) removal efficiency using UV/photocatalyst system. When strong acid ion-exchange resins (SZ samples) were used to prepare AC containing zinc, the zinc was found to have ZnS crystal structures, whereas in case of the use of weak acid ion-exchange resins (WZ samples), the zinc was found to have ZnO crystal structure. Both the SZ and WZ samples had spherical shape except the WZ-A-900 and had a good BET specific surface area. In the HA removal test using a floating UV/photocatalyst system, the WZ samples showed higher HA removal efficiency than the SZ samples.