The Benefits of 3D-Printed Photochemical Reactors

Image showing a photochemical reactor 3D printed by ACEO®

The University of Stuttgart, Germany, researched the advantages of 3D-printed photochemical reactors made out of silicone over conventional photochemical reactors in collaboration with ACEO® (Photo: University of Stuttgart)

Research is the fundament of innovation. Since its starting days, ACEO® has supported and launched several projects with scientists to further explore the application of 3D-printed silicones. The latest success story is a collaboration with the University of Stuttgart, Germany. Dr. Dirk Ziegenbalg and PhD student Ümit Tastan researched the advantages of 3D-printed photochemical reactors over conventional photochemical reactors. The results are summarized in the following report, which they kindly allowed us to share with the scientific community and additive manufacturing enthusiasts.

Photosynthesis is the source of live on earth, yielding carbohydrates as chemical energy storage as well as oxygen. While this kind of chemical energy conversion is of utmost importance in biological systems, mankind struggles with using light to drive reactions in an efficient manner. The challenges arise from the requirement of guiding light to the reactants. The amount of absorbed light depends on the optical path in a logarithmic manner. Hence, volumes, which are deeper in the reactor, receive far less light as volumes right beside the light source. To ensure a fast reaction progress, a large photon flux must be provided to the reaction solution. For this, a high transmittance of the window through which the reaction solution is irradiated must be ensured as well as a reactor geometry that fits the requirements of the considered reaction. The most important requirement for photochemical reactors is a good transmission for light of the used wavelength. While the typically used glass reactors can provide this, adapting the reactor geometry to the characteristics of the light source can be quite challenging. The availability of powerful light emitting diodes (LEDs) opens up a new chapter in the book of photochemical reaction engineering. The high energy efficiency, the narrow-band monochromatic emission as well as the availability of LEDs with different wavelengths yield new degrees of freedom for the design of photoreactors.

To overcome the limitations associated with the construction of glass reactors, additive manufacturing technologies are an attractive option that allow utilization of the degrees of freedom gained from the use of LEDs. The probably most popular kind of these manufacturing technologies is 3D-printing. While the possibilities are numerous, application of these technologies is still challenging for photochemical reactions since most available materials are not transparent for light. As an intermediate solution, glass windows might be used to allow an irradiation of the reaction solution. The disadvantage of this approach is that it requires a leak proof installation of the glass on the 3D printed reactor, raising new problems.

3D printing of optical transparent materials is the next step to unleash the full potential of additive manufacturing for the design of photochemical reactors. Recently, ACEO® presented the capabilities to use silicones in additive manufacturing processes to the market. These materials are transparent for visible and ultraviolet light, rendering them suited for the construction of photoreactors. The three-dimensional printing process allows for a large flexibility in the reactor design and the optical properties make the installation of e.g. a glass window unnecessary to allow irradiation of the reaction solution.

At the University of Stuttgart, the research group “Photochemical Reaction Engineering” works on intensifying photochemical processes. One key for this is a sophisticated design of the reactor as well as the reactor-light-source-setup. Only providing an efficient irradiation yields efficient photochemical processes. To gain a thorough understanding of the relevant parameters, rapid prototyping is used to get access to a large variety of different setups. This enables incremental investigation of the different parameter combinations. This approach was recently applied to investigations of Advanced Oxidation Processes (AOP). This kind of processes generate highly reactive hydroxyl radicals, which can be used to degrade persistent pharmaceuticals.

As benchmark reaction, the photocatalytic degradation of Diclofenac was investigated. Diclofenac is a popular nonsteroidal anti-inflammatory drug. Because it is only metabolized to a low degree in the human body, a large fraction is excreted. With this it gets into surface water bodies, causing damage to the fauna. As this drug is persistent, conventional waste water treatment plants are not able to remove it from the processed streams. Consequently, Diclofenac accumulates in water bodies. To overcome this problem, it is essential, to develop techniques that are capable of degrading Diclofenac. As the mentioned problems also apply for a large number of other drugs, the associated problems are even more relevant. Hence, to keep the operation costs low, the investigations at the University of Stuttgart aim on using sun light to drive the degradation. The work focusses on investigating the parameters relevant for designing a technical scale process.

The conventional reactor used to investigate AOPs at the University of Stuttgart was made of two parts of stainless steel, which were assembled prior to use. A flat gasket ensured leak tightness. The top part possessed a glass window through which the catalyst could be irradiated. The catalyst was deposited on a carrier plate, which could be inserted in an indentation in the bottom part of the reactor. The downside of this reactor was, that changing the catalyst carrier was not comfortable, as the reactor had to be opened every time. This caused issues with leak proofness as well as with the mechanical stability of the glass window.

To overcome these problems, the design of the conventional reactor was revised slightly to enable manufacturing by 3D printing. As material, transparent silicone was chosen to enable an optimal irradiation. Keeping the changes to the conventional design as small as possible maintained comparability. The main differences were the fluidic connections. For this, the usually used fittings were replaced by printed silicone tubes that could be pushed on the tubes connected with the pumps. With this, easy installation without the need of any additional parts became possible. After manufacturing, the reactor didn’t have a possibility to insert the catalyst carrier. This was desired to ensure a leak proof reactor first. After initial testing, the reactor was furnished with a slit to enable insertion of the catalyst carrier. Thanks to the properties of silicone, this could be realized with a knife. After this adaption, it was possible to insert the catalyst carrier by slightly pushing the sides of the reactor together. Releasing the sides sealed up the reactor again. For low pressures in the reactor, a binder clip is sufficient to avoid any leaks. A LED was used for irradiation and water containing Diclofenac was cycled with the help of a pump.

Comparative testing showed, that the degradation rates in the 3D printed reactor were comparable to the results obtained with the conventional reactor. This allows replacement of the conventional reactor. The benefits gained by the use of the 3D printed silicone version of the reactor can be summarized as follows: highly flexible design possibilities, fast manufacturing, very good optical properties, easy installation and handling of the reactor, access to tailor-made reactors in silicone, excellent mechanical properties and resistant to mechanical stress.