Performance Analysis And Geometric Optimization Of Photocatalytic CO₂ Reduction By Optical Fiber Monolith Reactor

Human development is restricted by energy crisis and greenhouse effects. Photocatalytic CO2 reduction by solar energy seems potential to mitigate greenhouse gas emissions and produce fuel methanol. As the barrier of photocatalytic CO2 reduction reaction, optical fiber monolith reactor is popular for high efficiency and great mass transfer.Based on optical fiber monolith reactor and selecting appropriate reaction kinetics, transport and diffusion models, the reaction was studied in numerical experiments. The results were anastomosed with present data, proving the feasibility of the numerical experiments.Monolith reactor with multiple optical fibers was proposed by inserting multiple fibers into a reaction unit and changing the position of lights. The new light distribution equation was obtained by analyzing and deriving the present models. The variations of CH3OH concentration with the fiber location and operation parameters were obtained by means of numerical simulation. With the increase of the distance between the fiber and the monolith center, the average CH3OH concentration increases. The average CH3OH concentration also rises as the light input and water vapor percentage increase, but declines with increasing the inlet velocity. The inlet velocity and the average CH3OH concentration are inversely proportional. The highest outlet CH3OH concentration in the model is 2.32×10-5 mol·m-3,31.1% higher than the previous model,1.770×10-5 mol·m-3. The maximum conversion rate and quantum efficiency in the model are 0.235μmol·g-1·h-1 and 0.0177% respectively, both higher than previous internally illuminated monolith reactor (0.16μmol·g-1·h-1 and 0.012%).By adding up the reaction surfaces of optical fiber monolith reactor to expand reaction area, monolith reactor with multiple reaction surfaces was put forward. Monolith reactor with multiple reaction surfaces and fibers was proposed by optimizing multiple fibers model further. Light power reaching reaction surfaces and production rate per input light power were defined to better analyze the performance of different structures. The linear relation between the light power obtained by the integration of light intensity and CH3OH concentration was discovered, in the condition of the same velocity and fiber number. Outlet CH3OH concentration reaches 3.004×10-5 mol·m-3 and 3.685×10-5 mol·m-3 in the two structures, increasing by 69.7% and 108.2% respectively, comparing with previous experimental results 1.770×10-5 mol·m-3. The maximum transmission efficiency of the light power arrives at 2.297% for one optimized model, and the highest production rate reaches 3.018×10-5 mol·s-1·W-1 in another optimized model. The optimal structure parameters of monolith reactor with multiple fibers and reaction surfaces are that the radius of the middle tube is 1.2 mm and reactor length is 50 mm by analyzing the concentration filed, average outlet concentration and light intensity for various radii and lengths, which is the best choice for the photocatalytic CO2 reduction.