Enhancing and optimising the reaction rate of thin film photocatalysts in a photocatalytic spinning disc reactor

• Main Author: Wan Mansor, Wan Salida
• Other Authors: Patterson, Darrell ; Patterson, Emma
• Published: University of Bath 2017
• Subjects: 660
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760906

Process intensification can be described as any chemical engineering development that leads to a substantially smaller, cleaner and more energy efficient technology. The principle of spinning disc reactor (SDR) technology is based upon the formation of a thin liquid film (20−300 μm) on a rotating disc induced by centrifugal and shear forces. This creates a high surface-to-volume-ratio. For instance, the suitability of the spinning disc reactor for polymerization reactions, organic synthesis, photocatalysis, or precipitation reactions was investigated. The photocatalytic spinning disc reactor (pSDR) overcomes many of the aforementioned problems with chemical catalysts. The catalyst is immobilised, so does not need to be recycled and/or separated. Different flow can form across the surface of the disc depending on the flow rate, rotational speed, and liquid properties. To address these limitations, this research continues the development and optimisation of the pSDR by introducing textured discs (meshes and grooves as a macrostructure) to both increase the potential photocatalyst surface area on the disc as well as improve mixing. More photocatalyst can be coated on the increased disc area also. In doing this, this work therefore significantly extends the previous work on the pSDR through investigating the use of a range of different photocatalyst textured macrostructures (meshes and grooves in the photocatalyst disc) in order to increase the micromixing and photocatalyst surface area, to increase the overall degradation rate in the pSDR The effect of rotating speed, disc surface structure (meshes/grooves/smooth), oxidant input amount (oxygen added or not) and volumetric flowrate were investigated and related to four key reaction/reactor parameters: reaction rate (using photocatalytic degradation of water soluble organics - methylene blue and ibuprofen), thin film hydrodynamics (by high speed photography of the liquid thin films), residence time distribution (measured by tracer analysis) and micro-mixing (measured by the Villermaux-Dushman reaction). A titanium dioxide (TiO2) sol gel coated glass disc with coated additional macrostructures were used as the photocatalyst. Results demonstrated that reaction rate can be optimised through identification of the optimal rotating speed, disc surface structure (meshes/grooves/smooth), oxidant input and volumetric flowrate. Higher disc spinning speeds make the residence time distributions more plug flow in behaviour and increase micro-mixing, which is known to increase reaction rate. For methylene blue degradation, an intermediate disc speed of 300 rpm and 20 mL/s inlet flow for a mesh/textured disc with a square grooved pattern was found to give the highest reaction rate (4.546x10-7 mol/L.s) without added oxygen (the oxidant for the photocatalytic degradation). For ibuprofen degradation an intermediate disc speed of 300 rpm and 10 mL/s inlet flow for smooth disc was found to give the highest reaction rate (1.580x10-5 mol/L.s) also without added oxygen (the oxidant for the photocatalytic degradation). This indicates that the self-aeration of oxygen by the pSDR spinning liquid film contacting air produces sufficient and more optimal oxidant input for both compounds. Analysis shows that using a patterned/mesh disc is optimal since it smooth out the surface flow across the disc, which both increases local UV penetration (less scattering) and provides more uniform volumetric utilisation of the catalyst. This effect appears to dominate the other positive effects of higher spinning speeds. Overall this work shows that reactor and photocatalyst surface macrostructure and design plays an important role in increasing photocatalytic treatment efficiency. Moreover, the process intensification (higher reaction rates and photonic efficiency) and self-aeration demonstrated here, further demonstrates the significant advantages of the pSDR over other photocatalytic reactors.