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Project

Scaling up multiphase photoreactions utilizing repetitive structures

Photochemistry has many benefits such as high selectivity, mild reaction conditions and access to new reaction pathways. A high selectivity reduces the number of downstream processes required to obtain a product with the desired purity. Furthermore, it is expected that energy costs will increase, making energy-intensive processes, where high temperatures and pressures are used, less feasible. New reaction pathways can reduce the required reaction steps to synthesize a product. There is enough incentive for the industry to integrate photochemistry in their production sites. Despite all these advantages, there is a lack of industrial applications of photochemistry. There are numerous reasons for this. The first one being that photochemistry is less known by chemist and chemical engineers compared to thermochemical reaction schemes. The second one is that the industry has more knowledge and expertise about conventional reactors, which makes it a safer option to utilize the known reactors. The third reason is the limited scalability of photoreactors with the traditional reactor designs. Unlike thermochemical reactions, it is not feasible to scale-up photoreactions by increasing the dimensions of the reactor due to the exponential attenuation of light. This will inherently create a non-uniform light distribution and dark zones, leading to a lower selectivity and longer reaction time, which lowers the productivity. Furthermore, photoreactions are intrinsically fast, which can lead to mass transfer limitations, which in turn slows down the reaction.

Many successful attempts in literature can mitigate one or more issues. One reactor design, the microreactor, does allow uniform irradiation and provides a high surface area to volume ratio due to their small geometries. They have a low volume, which makes it difficult to utilize them in combination with an industrial throughput. Furthermore, they have a low energy efficiency. The first reason is that due to their small size it is difficult to find an appropriate light source with the same dimensions. When a larger light source is used, the whole reactor is irradiated and a large part of the energy is lost to the environment. Due to recent improvements in LED technology and research in how to design light sources, this is becoming less of an issue. However, due to the small path length of the reactor the majority of the light is not used, lowering the energy efficiency. One solution is to increase the concentration of the photosensitizer, but that would bring us back to the first situation where non-uniform light distribution and dark zones are present. Another solution would be to stack multiple microreactors, in a repetitive way, in one reactor, so that light that passes through one microreactor can be utilized in the next microreactor. When multiple microreactors are stacked in an organized structure, a monolith is obtained, which is used in this work to scale-up photoreactions. This does resolve the first issue of a microreacor, which is the lower volume and throughput.

Monoliths have been used in photochemistry but they had a low productivity and energy efficiency. These reactors were opaque and therefore the light was introduced via optical fibres. The issue with this is that light that is not used in one channel is absorbed by the reactor walls, lowering the energy efficiency. The solution is to make them translucent, which was done in this work. The guidelines to design this reactor are presented and a benchmark reaction (photo-oxidation of 9,10-diphenylanthracene) was used to compare it with other state-of-the art reactors. It was proven that translucent monoliths could be successfully used to scale-up liquid and gas/liquid photoreactions while keeping a high space-time yield and energy efficiency (photochemical space-time yield). Furthermore, it was shown how to intensify reactions by introducing a second phase into the system, which doubled the space-time yield. It was also observed that a wider monolith is usually more energy efficient than a smaller one, which is due to edge effects. When utilizing a wide reactor, most of the light will illuminate the reactor. However, when using a smaller reactor, a smaller portion of the light from the light source will contribute to the reaction. This is because there is always a fixed amount of power lost on the edges, and this lost power is relatively larger when using a smaller reactor.

The previous mentioned work does cover liquid and gas/liquid reactions. There are plenty of industries or applications in which gas/solid photoreactions can be used. Some examples include air purification and the production of hydrogen via photo-assisted methanol reforming. Hydrogen is considered a clean and promising energy carrier for the future, which can be an alternative for fossil fuels. However, the majority of hydrogen in the industry is produced via catalytic steam–reforming of hydrocarbons at high temperature and pressure (650-950°C and 15-45 bar). Therefore, the production of hydrogen can benefit from photochemistry. One example is the photo-assisted methanol reforming at lower temperatures and pressures. However, most works in literature focus on the reaction mechanisms of hydrogen generation or the improvement of the catalyst (ambient pressure and 100-300°C). It should be noted that the literature lacks reactors that can continuously produce hydrogen with a stream of liquid feedstock. Furthermore, it should be mentioned that many focus on liquid phase methanol reforming, which is important to understand the reaction pathways. However, it was shown that more hydrogen is produced in a gas phase reaction. This work tries to bridge these gaps by designing a reactor that can continuously use a liquid stream of methanol, that is converted to a gas phase in the system, to produce hydrogen. Different reactor packings, beads and plates, were used as catalyst support and optimized. This was done by transmission experiments, varying the number of repetitive structures (layers of beads or plates) and catalyst loading on the support. The best performing bead packing produced 12.0 mmol·g ·h-1 while the best performing plate produced 54.7 mmol·g-1 ·h-1 of hydrogen . When compared to literature, it was shown that the hydrogen production of the plate was at least 3.4 times higher compared to the rest. Furthermore, the modified photochemical space-time yield (a form energy-efficiency) was a factor of 633 higher compared to other systems with a similar catalyst. This flow setup can use a stream of liquid MeOH/H2O to produce a continuous supply of hydrogen, which is a promising way to power the future.

This PhD provides a basis to design multiple reactors to scale-up multiphase photoreactions with a high throughput utilizing repetitive structures such as a monolith, beads or plates.

Date:21 Sep 2018 →  26 Jan 2023
Keywords:Monolith
Disciplines:Catalysis and reacting systems engineering, General chemical and biochemical engineering, Process engineering, Transport phenomena, Other (bio)chemical engineering
Project type:PhD project