Design, construction and commissioning of an automated optical fibre catalyst coating process for use in photocatalytic reactor systems

Harrisankar, Naomi

Design, construction and commissioning of an automated optical fibre catalyst coating process for use in photocatalytic reactor systems

Master Thesis

2020

Abstract

Climate change is one of the greatest challenges facing humanity. Fossil fuels are the primary source of energy on Earth. Since the global economic growth is closely linked to the global energy demand, fossil fuel usage remains the largest contributor to the steadily increasing atmospheric carbon dioxide concentration (CO2). CO2 mitigation through carbon capture and conversion are of great interest. Capturing CO2 from point source emitters is possible by absorption in a basic, sodium hydroxide (NaOH) containing solution, which is then converted into sodium bicarbonate (NaHCO3). Conversion of CO2 is thermodynamically demanding as it will require a large amount of energy, which renders currently used technologies infeasible. A promising alternative is the conversion of captured NaHCO3 into useful hydrocarbons at moderate operating conditions using solar energy, by a process called photocatalysis. Photocatalysis is the acceleration of a photo-induced reaction in the presence of a catalyst. Photocatalytic reactors have not yet been commercialised due to suboptimal catalyst and reactor designs. The typically low catalyst activity has to be countered by efficiently loading a large amount of catalyst in the reactor. This results in a problem regarding the photon transfer limitations to the catalytically active site, which limits illumination of the catalyst in the reactor. This can be overcome by using optical fibre to guide photons, which are coated with the photocatalyst. However, it is estimated that a reactor containing ca. 1 g of catalyst will require ca. 1.8 km of identically coated optical fibre. The aim of the project is to design, construct and commission an automated controllable process to increase the production volume of catalyst coated optical fibre using either a solgel suspension or a slurry containing P25 (TiO2). A multi-step optical fibre coating process was developed to achieve the desired coated optical fibre as a product. It consists of 6 major units that process raw (polymer-coated) optical fibre into catalyst coated optical fibre. The steps include the 4 essential steps required for optical fibre preparation by-hand, these steps are stripping, washing, coating and heat treatment. This automated optical fibre catalyst coating process (AOFCCP) can make the coating of optical fibres time-efficient and controllable. The latter can be achieved by controlling the effect various process parameters affecting the coating thickness and homogeneity of the coating, such as pH, heat treatment, catalyst slurry concentration as well as pulling speed. The AOFCCP produced coating thicknesses ranging from 0.47 µm - 0.59 µm and 0.37 µm - 0.46 µm for the P25 slurry and sol-gel coating methods respectively. The pH of the P25 slurry was found to have a negligible effect on both the coating thickness and surface morphology, therefore is no longer regarded as a process variable in the AOFCCP. The thickness of the coating increased with an increase in P25 slurry concentration with a maximum achievable coating thickness of 0.87 µm using a slurry concentration of 20 wt.-%. The temperature of heat treatment which was tested showed different relationships between the coating methods. For the sol-gel coating method, the increase in temperature resulted in a decrease in coating thickness possibly due to the decrease in porosity whereas for the P25 slurry method the increase in temperature showed an increase in coating thickness possibly due to the higher evaporation rates. An increase in the pulling speed in the AOFCCP resulted in an increase in coating thickness on the optical fibre independent of the coating method; coating thicknesses ranging from 0.41 µm - 0.71 µm and 0.23 µm - 2.14 µm were obtained using the P25 slurry and sol-gel coating methods, respectively, by varying the pulling speed. The critical cracking thickness is defined as the thickness of the film, produced by the sol-gel method, at which coating deformations become observable which was found to be 0.37 µm at 600 °C, and 0.77 µm at a pulling speed of 2.30 mm.s -1 . The results obtained from the commissioning experiments showed that the AOFCCP can produce coated optical fibre with controllable thickness. The controllability was discovered to be in the adjustment of the process variables investigated which showed a significant effect on the coating thickness, except for pH. Based on the statistical analysis that was performed, it was confirmed that the results obtained from the system were repeatable and that the coating was uniform for all process variables that were investigated except for sol-gel coating at high speeds of 2.88 mm.s -1 – 3.46 mm.s -1 . The system was able to produce fibre with coating thickness's between 0.4 – 1.1 µm. It is recommended that a combination of the process variables be used in order to achieve better controllability in the process and to achieve thicker coating layers. Furthermore, the operating ranges of the process variables should be increased in order to determine the extent of the relationship between the process variable and the coating thickness and surface morphology.

Keywords

Chemical Engineering

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