(558g) Carbon-Based Chemical Filters Enhanced with Photocatalysis for Removing VOCs From Indoor Air
Chemical filters are common designed for the purpose of purifying indoor air, especially for controlled environments demanding low tolerance of contamination, exemplified by their extended use in cleanrooms for microelectronics and optoelectronics devices, as well as for pharmaceutical products. In the ventilation control system in these critical environments, chemical filters must be installed for the removal of gaseous contaminants from various possible sources, including the outside air, the recirculation air, and the point-of-use sub-environments which require purification of specific contaminants.
Activated carbons in granular (GAC) or fibrous (ACF) structure are typically used as the base materials and adsorbent for the removal of organic contaminants in air. Activated carbons can also be impregnated with either phosphoric (or citric) acid for the absorption of basic gases, or potassium permanganate (or hydroxide) for the absorption of acidic gases. The common problem with chemical filtration is that the sorbent materials will eventually be saturated beyond a critical point at which filter replacement becomes necessary to maintain the stringent requirement of air cleanliness in the manufacturing environment. One of the possible methods to prolong the useful life of a chemical filter is to incorporate photocatalysts in the filter structure. This study therefore investigates the effect of GAC-based chemical filter combining with photocatalysis in three different forms,namely (i) chemical filter containing GAC-loaded TiO2 powders; (ii) chemical filter preceded with a sheet of TiO2-loaded non-woven filter; and (iii) chemical filter pressed with a TiO2-coated non-woven filter.
The test reactor consisting of a cylindrical pyrex column (i.d. 80 mm) having two removal segments, namely a photocatalysis section (length 150 mm) and an outlet section (length 55 mm). The filter testing pieces were grooved between the two sections and airtight secured by an O-ring. A quartz tube (OD 45 mm, length 140 mm) in concentric with the column contained a 13-W ultraviolet lamp (Sankyo, Japan) irradiating at a primary wavelength of 365 nm. As it will be discussed in this paper, the test piece was deemed ?inactive? in area which was constricted by the flow path around the quartz tube in the center of the reactor. The estimated active area of the test piece was 34 cm2. The irradiation intensity on the perimeter of the testing piece was 0.20 mW/cm2. Toluene and isopropanol were targeted as the test contaminant and their vapors were generated from a syringed pump in a heated three-way glass branch. The air was humidified by passing through a water vessel before mixing with the toluene vapor and metered into the test reactor. The standard air flow rate into the test reactor was 1.0 L/min, corresponding to a face velocity of 0.005 m/s.
Experimental results showed that the chemical filter preceded with a separate sheet of TiO2-coated non-woven fabrics performed with superior results than directly loading TiO2 powders on carbon granules. Also, we have demonstrated that, by pressing the two filter sheets together, a possible synergistic effect occurred to further enhance toluene photocatalytic degradation to extend the adsorption breakthrough. The photocatalytic sheet serves to achieve pre-decomposition of organic contaminants into lower concentrations and/or smaller molecules for the subsequent adsorption by chemical filter, and oxidization of the adsorbed organic pollutants to re-open adsorption sites on the chemical filter. However, high concentration of toluene would lead to catalyst "poisoning" as a results of accumulation of intermediate products. These intermediate products may also promote competitive oxidation and adsorption with their parent contaminants, resulting in reduced photocatalyst oxidation efficiency, and thus shortening the expected breakthrough times.