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Industrial Preparative Photochemical Processes

The electrical energy used worldwide for photochemistry is estimated to several tens of megawatts. Nevertheless, mainly due to industrial secrecy, very little information about industrial applications of photochemistry can be found in literature.

Due to the availability of Hg lamps with an electrical power of up to 100 kW, these are the most frequently used light source for large scale applications. Excimer lamps only represent an alternative for small plant sizes. However, when their use is technical feasible, these light sources can be advantageous due to a quasi monochromatic emission. LEDs possess this advantage as well, however, until now only application in surface treatment, like film curing, became public.

Industrial photochemical applications can be classified into three parts: i) waste water treatment and disinfection, ii) surface treatment and iii) preparative photochemistry. Photochemical steps are used in waste water treatment for the complete oxidation and mineralization of organic residues in combination with the use of H2O2 or O3/H2O2. These approaches have in common, that hydroxyl radicals are produced as active species. Production of hydroxyl radicals can also be achieved by the Fenton reaction. More detailed information can be found in the article Advanced Oxidation Process. Plants of this type with an installed electrical power of up to 200 kW are known. For the disinfection of air, vacuum-ultraviolet radiation (λ < 200 nm) and TiO2 photocatalysis are used to decompose pollutants. The objective of disinfection is the destruction or at least inactivation of potential harmful microorganisms. Irradiation with hard UV light not essential causes the death of the organism, but deactivation to prevent infections.

Surface treatment or coating curing processes are nowadays common in many industrial branches like automotive or printing industries. UV-curing can be used to adjust of the coating, like durability or flexibility. As a consequence of the suited absorption characteristics of the curing agents, this photochemical application is the first one with a widespread use of UVLEDs as light source. The demand for cured surfaces in most cases is limited to objects with a size in the lower cubic meters, hence the size of the curing lamps is comparably small to the lamps used in preparative photochemical processes. The technique of surface treatment is well distributed over the world and applied in small industries as well. Thus, the electrical power for photochemical surface treatment is hard to estimate.

Still, up to the present day in chemical industry thermally induced processes are mostly favored over photochemical pathways. This can not only be attributed to the high development status or massive research in thermal processes, that often require the use of a catalyst. Instead, some drawbacks of preparative photochemistry can be reasoned, which are still present. Most of these disadvantages of preparative photochemical processes concern the utilized light source. As mentioned above, in industry mainly Hg lamps are used. Such emitters usually have a low electrical efficiency. In the worst case up to 90% of the applied electrical energy are transformed to waste heat, making expensive cooling devices necessary. Furthermore, the polychromatic emission spectrum of Hg lamps can induce side reactions. Consequently, this has to be avoided by the use of filters, which leads to additional expenses.[1;2]

Nevertheless, there are some crucial points that make preparative photochemistry attractive for industrial applications. By using photochemical synthetic pathways the number of process steps can significantly be reduced. Often, purification steps, which are necessary for most thermal processes, can be omitted. This is a direct consequence of high selectivities, that can be achieved by optimal reaction control. Additionally, some chemical end products are even only accessible through photochemical synthetic ways.[1]

The economy of a photochemical process for industrial application depends on multiple factors. A main parameter is the quantum yield Φ of the applied photochemical reaction. With information about this reaction property, it is possible to calculate the costs arising from the required electrical energy. The amount of the desired product n produced per energy can be estimated with the equation below when a monochromatic irradiation is assumed as first approximation:

                                                          n = 0.0302 mol kW1  h1  λ ·η ·Φ· f                                                              

λ is the emitted wavelength in nm. η is the power-to-light efficiency at this wavelength. Φ is the quantum yield of the photoreaction. f is the fraction of light absorbed by the product. The prefactor can be derived from the PLANCK-EINSTEIN correlation when the amount of photons per kW h is calculated.[2]

To assess the economic efficiency of a photochemical industrial process, it has to be checked if the quantum yield of the reaction is above or below 1. In this context, photoreactions with quantum yields 1 (e.g. radical chain reactions) are favored for industrial use. For such reactions the number of induced reaction steps per absorbed photon is very high. The accruing energy costs for the operation of the light source have very low influence on the total production costs. When the reaction quantum yield is below 1, the inverse is true. In this case the light source efficiency η becomes the crucial parameter for the economy of the process.[2]

The absorption factor f can be deduced from the Beer-Lambert-law. For reactants with large extinction coefficients, low concentrated solutions, which are irradiated in reactors with small depths, are favorable. On an industrial scale this is only achieved in thin-film/falling film reactors that have a lower space-time-yield compared to conventional photoreactors. Furthermore, low concentrations demand higher amounts of solvent. This directly affects the process efficiency due to the additional steps required for solvent recycling.[2] In practice, a compromise between the technically realizable layer depth and the possible reactor performance has to be made. For reactants with small absorption coefficients, a batch reactor with large optical depths can be appropriate. In contrast, for reactants possessing large absorption coefficients, reaction control is only maintained when the optical depth is sufficiently small.

In addition to theses factors to some practical aspects have to be considered. The purity of the reactants and solvents must be high enough to avoid unwanted light extinction as well as quenching of photochemical processes. Furthermore, precipitation of product or side products on the light source has to be prevented.[2]

Several photoreactions introduced in this document are conducted on an industrial scale by various, companies such as Philips, Bayer or BASF. Prominent examples are photochlorination and photonitrosylation. A detailed description of these reactions can be found in the particular articles.

 

  1. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie: Konzepte, Methoden, Experimente, im kolophon: milton keynes: lightning source, 2010 edn., 2010, Wiley-VCH, Weinheim, ISBN 978-3-527-29545-6.
  2. BECKER, H.; BÖTTCHER, H.; DIETZ, F.; REHOREK, D.; ROEWER, G.; SCHILLER, K.; TIMPE, H.-J.: Einführung in die Photochemie, 3. bearb. aufl. edn., 1991, Deutscher Verlag der Wissenschaften GmbH, ISBN 978–3–3260–0604–8.


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