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Reactor Types

Conventional Reactor Types

To a large extent the design of a photoreactor is determined by the geometric characteristics of the light source. Due to the fact that the geometry of powerful light sources such as Hg vapor lamps is often elongated numerous reactor designs derivate from the general concept of immersion reactors.

The general design of an immersion photoreactor is shown in figure 1a. The light source is centered in the middle of the reactor and surrounded by a cooling jacket. Depending on the power of the light source an additional jacket might be required to provide a sufficient thermal insulation or a liquid filter. The light source is immersed into the reaction mixture. Depending on the reaction conditions, additional heat exchangers or static mixing elements can be integrated into the reactor. In some cases it can be appropriate to “immerse” multiple lamps into the reaction mixture to adjust the ratio between irradiated surface and volume of the reaction mixture. Considering the fundamental coherencies of absorption, it reveals that the immersion reactor actually is not a well suited reactor type for photochemical reactions. The reason for its popularity for conducting photochemical reactions is the ease of use and set up. Because the rate of photochemical reactions depends on the rate of photon absorption, it becomes clear that the reaction rate decreases with increasing optical path length.

A reactor type with a very high surface to volume ratio is the so called falling film reactor. By use of different technical solutions, the reaction mixture is guided along the elongated dimension of the light source as a thin film. 1b shows an example if a simple falling film reactor. The reaction mixture is pumped onto the inner wall of a glass jacket. The resulting falling film is the dashed line in 1b. The thickness of the falling film can be adjusted in a range of several hundred µm by the physical properties and the flow rate. In addition to the large surface to volume ratio, which is favorable for reactions with high absorption, falling film reactors can also be used beneficially for photochemical gas/liquid reactions. Due to the large surface area and the good mixing, a very good gas/liquid mass transport can be achieved.

An alternative to immerse lamps into the reaction mixture is the possibility to surround the reaction mixture with lamps. This can be done for example for weak absorbing compounds, when a very high intensity of light is needed. This concept is applicable for a pipe which is flowed through continuously by the reaction mixture or a flask which is put into the center space of the lamps (batch). The horizontal cross section of a so called rayonet setup is schematically shown in Figure 2.

 

LS7a LS7b

 

(a)                                           (b)

Figure 1: (a) Immersion photoreactor.[1] (b) Annular falling film photoreactor.[1]

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Figure 2: Horizontal cross section of a schematic rayonet reactor.

Microstructured Photoreactor

One important aspect of the design of photoreactors is the realization of a good irradiation of the reactants. For this, the exponential decay of the light intensity caused by the interaction with matter has to be considered. According to the Lambert?Beer?law, a large fraction of the light is already absorbed in a short distance from the reactor wall. Consequently, it is possible that volumes in a large distance from the reactor wall are not irradiated. Such not irradiated volumes are unfavorable due to two reasons: In these parts of the reactor, sidereactions can occur, which influence the selectivity of the photochemical process negatively. Furthermore, photochemical reactions do not occur, in these regions, causing a decrease of the overall performance of the reactor. In principle, this can be considered a wrong adaption of the reactor to the process conditions. Such situations are frequently found in laboratories, because the setup of the reactor and the light source can often not easily be adapted to the requirements of the process conditions. An additional point might be the fact, that a reactor optimized to the process conditions might not be required for laboratory purposes.

As an answer to the points raised above, the use of microstructures reactors became popular during the last decade.[27] Microstructured reactors possess at least one reactor dimension with a length of less than 1 mm. These short optical depths allow a good irradiation of the reaction solution and with this an improved reaction control. Furthermore, conducting photoreactions in microstructured reactors profits from the general advantages of microstructured devices. Among others these advantages are the possibility of continuous processing and the possibility to use miniaturized light sources, the improved heat and mass transport as well as the improved process safety due to the low reactor volumes. Consequently, all relevant photochemical reaction types have been successfully conducted in microstructured photoreactors. This includes photosubstitutions, photoadditions, photoisomerizations, photorearrangements, photochemical redox-reactions and photochemical cleavage. With this, it is possible to use the whole spectrum of photochemical reactions in microstructured reactors. This also applies to heterogeneous reaction systems such as gas-liquid or gas-liquid-solid reactions.

The application of microstructured reactors is often referenced as flow chemistry. With respect to photochemical reactions, this nomenclature falls short, because the possibility to utilize continuous processing is only one aspect that makes this reactortype interesting for photochemistry. The combination of high-power light sources with short optical depths gives access to operation conditions with very high intensities without the drawbacks of not irradiated volumes. The continuous operation allows transformation of the time axis to a length axis and with this it is possible to precisely adjust the reaction times. Hence, the degree of reaction control can be increased. This point becomes even more relevant, when flow conditions are chosen, under which the reactor shows plug flow behavior. In this case, the residence time distribution is very narrow and consequently the reaction time distribution, too. Furthermore, back mixing is minimized, what is beneficial for reaction networks with consecutive reactions. Consecutive (photochemical) reactions can be quenched just after a certain selectivity threshold value is reached. Most frequently, this will be the point where maximum selectivity is reached. Additional benefits arise from the possibility to couple a continuously operated photoreactor to a continuously operated downstream processing and/or online analytics. This combination allows a further intensification of the overall photochemical process.

The simplest way of using microstructured reactors is the use of the capillaries with an inner diameter of less than 1 mm. To conduct photochemical reactions, it has to be ensured, that the reactor material is transparent to the light which is required to drive the reaction. Polymeric capillaries, made of e.g., FEP or PFA can be used as well as capillaries made of glass or fused silica. Glass capillaries have a better transparency, especially in the deep UV region but are more difficult to handle due to the risk of breaking. Breakage is not an issue for polymeric capillaries, which explains their frequent use in laboratories. Of course, this is only possible if the material is transparent to the relevant wavelength region. Specialized microstructured photoreactors made of glass, fused silica or combinations of transparent materials with metals for the reactor scaffold are also commercially available.

Although, the performance of microstructured photoreactors is typically much better than that of conventional photoreactors, the absolute output of a single microstructured reactor is comparably low. Hence, concepts are required to achieve high absolute productivities. A classical scale up of the reactors is not possible, because the benefits gained by microstructured nine cannot be maintained on a microscopic scale. As a consequence, the concepts of scale-out and numbering-up are used for microstructured reactors.

During a scale-out, all dimensions of the microstructured reactor are increased, which do not contribute to the beneficial effects required to achieve a high performance.[8;9] For photoreactors the optical depth is the most important parameter and should not marriage to a large extent during scale-out. The two remaining dimensions are less critical and can be subject to scaling. Numbering-up aimes on replicating the relevant structures several times. This can be done externally, meaning that all parts, including pumps, periphery, the reactor, connecting cubes, etc. are used several times. It is obvious, that this approach is simple, but associated with a large investment. A more elegant way is internal numbering-up. For this, only the parts of the reactor which require microstructuring are numbered up in a dedicated reactor housing and all other parts are scaled up classically. Although, the design of the internally numbered-up reactor is more challenging, the overall costs of the system to realize a certain production, will be lower than for external numbering-up.

  1. BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 10.1002/ange.19921041147.
  2. COYLE, E. E.; OELGEMÖLLER, M.: Micro-photochemistry: photochemistry in microstructured reactors. The new photochemistry of the future?, Photochem. Photobio. S., 2008, 7 (11), 1313–1322, DOI: 10.1039/b808778d.
  3. OELGEMÖLLER, M.; SHVYDKIV, O.: Recent Advances in Microflow Photochemistry, Molecules, 2011, 16 (9), 7522–7550, DOI: 10.3390/molecules16097522.
  4. OELGEMÖLLER, M.: Highlights of Photochemical Reactions in Microflow Reactors, Chem. Eng. Technol., 2012, 35 (7), 1144–1152, DOI: 10.1002/ceat.201200009.
  5. GRIESBECK, A. G.; OELGEMOLLER, M.; GHETTI, F.: CRC Handbook of Organic Photochemistry and Photobiology, Third Edition - Two Volume Set, 3 edn., 2012, Taylor & Francis Inc, ISBN 978-1-4398-9933-5.
  6. SU, Y.; STRAATHOF, N. J. W.; HESSEL, V.; NOEL, T.: Photochemical Transformations Accelerated in Continuous-Flow Reactors: Basic Concepts and Applications, Chem. Eur. J., 2014, 20 (34), 10562–10589, DOI: 10.1002/chem.201400283.
  7. CAMBIÉ, D.; BOTTECCHIA, C.; STRAATHOF, N. J. W.; HESSEL, V.; NOËL, T.: Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment, Chemical Reviews, 2016-September, 116 (17), 10276–10341, DOI: 10.1021/acs.chemrev.5b00707.
  8. SUGIMOTO, A.; SUMINO, Y.; TAKAGI, M.; FUKUYAMA, T.; RYU, I.: The Barton reaction using a microreactor and black light. Continuous-flow synthesis of a key steroid intermediate for an endothelin receptor antagonist, Tetrahedron Lett., 2006, 47 (35), 6197–6200, DOI:10.1016/j.tetlet.2006.06.153.
  9. SUGIMOTO, A.; FUKUYAMA, T.; SUMINO, Y.; TAKAGI, M.; RYU, I.: Microflow photoradical reaction using a compact light source: application to the Barton reaction leading to a key intermediate for myriceric acid A, Tetrahedron, 2009, 65 (8), 1593–1598, DOI: 1016/j.tet.2008.12.063.

 


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