Author: Chan, Mo-chee
Title: Mathematical modelling of the performance of photocatalytic decomposition of defined gaseous pollutants
Degree: Ph.D.
Year: 2012
Subject: Air -- Purification -- Photocatalysis.
Titanium dioxide.
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Structural Engineering
Pages: xxiv, 281 leaves : ill. ; 30 cm.
Language: English
Abstract: The photodegradation of formaldehyde by the photocatalayst titanium dioxide was studied using mathematical modelling and the physical application of photocatalytic oxidation (PCO) reactors in air conditioning systems. As most previous research work was only carried out in the laboratory with low concentration of formaldehyde, which was not suitable for generation of useful design data for commercial PCO unit. Therefore, field PCO reactors (FM1 and FM2) were constructed that were suitable for the installation within common commercially available fan coil units within heating and cooling ventilation air conditioning (HVAC) systems, such as manufacturers McQuay, Carrier, York and Trane. Due to the application within air conditioning systems the parameters chosen were between the temperatures 15-25°C and relative humidity 55% based upon traditional comfort levels common in Hong Kong. The further approximation of the Colebrook equation-specifically the Darcy-Weisbach equation, allowed for the calculation of the friction factor f which led to evaluation of the turbulent pipe flow pressure loss in a straight duct. By analysing the data collected by our mathematical models and field reactors (FM1 and FM2-of which FM1 had larger dimensions) it was found that within the volumetric flow rate ranges (289-1876m³/h according to the commercial fan coil units used) and face velocity (which was less that 10m/s in low pressure duct work according to DW142) the pressure drop is less than 40 Pa which had an insignificant change on the air balance of the fan coil unit in the HVAC system. The calculations which determined the modelling predictions were calibrated using the laboratory data. The calibrated modelling result was further validated by the field data which ensured the consistency of the data. Each test was repeated thrice and reviewed thoroughly. The pressure drop of the field test was slightly higher than the modelling data in high flow region. It was concluded that this activity was the result of vibrations of the TiO₂ coated plates of stainless steel within the PCO reactor. This could be due to a fabrication inaccuracy made in the alignment of the fins within the reactor where the fins may not be completely parallel to each other. This could be meliorated by either (but not restricted to) improving the rigidity of the fins or altering the shape of the fins to be more structurally sound. It was also appeared that the copper earthing mesh was required and placed five centre meters away from the PCO reactor which made it possible to prevent the static electricity build up on the titanium dioxide coating plate surface. It could be a potential safe guide device for limiting catalytic poison during the photo degrading process The FM1 and FM2 PCO reactors could be used in common commercial fan coil unit in the existing HVAC system. The calibration and validation of the computing modelling were also used for the design optimisation, operation and control of the PCO system.
During the elimination of variables, it was found that the titanium dioxide coating on the aluminium supporting metal has no photocatalytic ability. It was also found that the UV light in PCO unit needs to be turn off for at least 30 min after 2 hours operation, then further photocatalysis can be obtained.
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