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Scale Up Aspects

When it comes to scale-up of photoreactors, several crucial aspects have to be considered. The follwing remarks will give a short overview of relevant aspects of the scale-up rather than a complete guide for scaling-up photochemical processes. In this section a well evaluated lab scale photoreactor is assumed with a defined average volumetric rate of photon absorption Lp,Va (AVRPA), defined as the amount of absorbed photons per volume:

scaleupeq1

With the absorption coefficient ε and the concentration c of the reaction mixture AVRPA is specific to a certain reaction and the chosen setup. The AVRPA is the technical equivalent to the time yield (STY). The only difference is the neglect of the actual chemical reaction. This procedure is possible, because the photon flux and with this the technical characteristics of the light source determine the rate of the reaction primarily. For continuous systems, the theoretical productivity and space time yield (STY) of the setup can be determined with the flow rate and the quantum yield of the reaction.W hen an increase of the production height is desired, the first point to check is, whether the reactor is scalable. The central element of the reactor is the light source. The type of light source directly determines the achievable increase in production. The popular medium or high pressure Hg lamps can be purchased with an electrical power from 100 W up to 60 kW.[1] In addition, the size of the Hg lamps scales linear with its power and the diameter of the emitting arc and with that the diameter of the entire lamp changes little compared to the length of the lamp. These technical characteristics can be related to the AVRPA. Because the emitted radiant power scales with the length, what also applies for the irradiated surface, AVRPA does not change significantly when more powerful Hg lamps are used. On the first sight, the scale-up of photochemical processes using Hg lamps seems very simple. Nevertheless, this only applies when neglecting all other aspects of a scale-up, e.g. heat management, mass transport, technical feasibility, safety measures, and so on. At this point the classical scale-up problems associated with a different scaling of surface and volume come into play. Consequently, challenges arise almost all "‘realistic"’ photochemical processes.

A perfect example underlining the complexity of a scale-up is the use of a Xenon arc lamp for irradiation. The emitting arc has a length of only a few millimeters. Irrespective of the power of the lamp, it will be a quasi punctual light source. Consequently, the irradiated surface of the reactor will not scale that easy with the size of the lamp. It will be likely that the lamp will over irradiate the reaction and probably, significant work has to be invested to design a process yielding the desired quality of the product. Another aspect is the operational mode of the reactor. In the last decade, microphotoreactors gained a large amount of interest. The high surface to volume ratio is one reason for that.[2] In addition, it is proclaimed, that a numbering-up instead of a scale-up is possible and reasonable for microreactors, implying a simpler increase of production capacity. It is obvious, that the overall costs for technical equipment will be larger than with for a “normal” scale-up, because economy of scale does not apply anymore. Despite this, all reaction conditions are guaranteed to remain constant and an increase of the production height can be achieved during a very short period due to simple replication of the equipment.[3]

 

  1. AILLET, T.; LOUBIÈRE, K.; DECHY-CABARET, O.; PRAT, L.: Microreactors as a tool for acquiring kinetics data on photochemical reactions, 2015, n/a–n/a, DOI: 10.1002/ceat.201500163.
  2. GÜNTHER PESCHL: Informationen zu Hg-Mitteldruckstrahlern.
  3. KASHID, M. N.; RENKEN, A.; KIWI-MINSKER, L.: Microstructured devices for chemical processing, 2015, Wiley-VCH, ISBN 9783527331284.

 


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