(369b) Model-Based Optimization of a LED-Based Photocatalytic Reactor

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 [1] ,
reduction of CO₂ to hydrocarbons [2]
, 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 [5]. 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 [8]. 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 [9]. 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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