Model-based Optimization of a LED-based photocatalytic reactor
Fatemeh Khodadadian1, Zonghan Li1, J. Ruud van Ommen3, Andrzej Stankiewicz1, Richard Lakerveld2
1 Department of Process & Energy, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, the Netherlands
2Department of Chemical and Biomolecular Engineering, Hong Kong University of Science & Technology, Hong Kong
3Chemical Engineering Department, Faculty of Applied Science, Delft University of Technology, the Netherlands
The great potential of TiO2-assisted heterogeneous photocatalysis for implementation of redox reactions such as water splitting to produce hydrogen  , reduction of CO2 to hydrocarbons  , water and air purification [3, 4] has raised a considerable interest in recent years. Nevertheless, its industrialization has been hindered due to the low overall efficiency of the process . One of the main challenges is the design a reactor in which both mass transfer and photon transfer are optimized [6-8]. Poor photon utilization leads to a large reactor volume and, in case when using artificial light, large operational costs.
The photon distribution in the reactor depends on several factors including the type and geometry of the light source . Using conventional UV-lamps such as low pressure mercury lamp, which are rigid cylindrical lamps, constrains the reactor design. Moreover, fragility, toxicity, gas leakage and disposal issues are other disadvantages of mercury lamps . Alternatively, Light Emitting Diodes (LEDs) are promising light sources for photocatalysis applications [10-12]. LEDs are robust, energy-efficient, non-toxic and long-lasting light sources. Moreover, LEDs can be positioned flexibly within each reactor configuration due to their small size and ability to deliver a range of intensities giving a large design space. This calls for a systematic design approach of LED-based photocatalytic reactors instead of trial-and-error methods for optimization. Such a systematic design approach should optimize the configuration of a LED array simultaneously with the reactor design to optimize the overall reactor performance.
In this study, the optimization of a mathematical model of a LED-based photocatalytic reactor is investigated. An annular reactor geometry with the light sources at the center has been selected as the basic configuration. The LEDs are positioned on the outer wall of the inner tube. Toluene degradation in the gas phase was chosen as the model reaction. The reactor model is based on balances for momentum and mass combined with a model for the photon radiation field. An objective function representing the trade-off between capital and operational cost has been defined. The developed optimization program minimizes the objective function with a constraint on conversion of toluene and involves both integers (number of LEDs) and continuous variables. The simulation and optimization were conducted using gPROMS Modelbulider (©PSE Limited).
First, the minimum reactor cost by varying the reactor length was found for a given number of LEDs and a given set of economic data (Figure 1). The result shows that an optimum number of LEDs exists that minimizes the reactor cost. Second, the number of LEDs was used as a degree of freedom creating a mixed-integer optimization problem. The optimum number of LEDs could be retrieved by solving this mixed-integer optimization problem, which demonstrates the potential of the proposed method. Second, the power of the LEDs was added as a decision variable to the optimization problem. An optimum number of LEDs and power were found to minimize the reactor cost. In summary, the proposed method allows for the design of LED-based photocatalytic reactors in which both the light source and reactor design are optimized simultaneously using more degrees of freedom that LEDs offer for design compared to the design of a photocatalytic reactor with a conventional light source. Current work focuses on adding more degrees of freedom to the optimization problem such as reactor geometry, and experimental investigation of the effect of design parameters on the reactor performance by using a dedicated experimental set-up (Figure 2).
Figure SEQ Figure \* ARABIC 1. reactor cost($/s) as a function of the number of LEDs and the optimized reactor length
Figure SEQ Figure \* ARABIC 2. Annular-LED-based photocatalytic reactor
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