Falling-Film Photoreactors
From laboratory to tailor-made industrial production
The efficient irradiation of liquids with low transmittance was an unsolved challenge for the development of some photochemical processes in former times. In case of a lack of the penetrative depth, photons are absorbed within shortest distance at the boundary surface. The optical path of conventional reactors is too large and the majority of the volume to be irradiated passes the reactor without exposition to the incident radiation. The photochemical process remains inefficient. Furthermore, radical intermediates, mostly macromolecular secondary products, may form and may deposit at the boundary surface of the immersion lamp system ('filming'). These deposits additionally absorb the photons emitted by the lamp and, in extreme cases, may lead to overheating of a conventional photoreactor.

The Peschl Falling-Film photoreactor has a special design that permits efficient irradiation of liquids with low transmission in the form of an evenly falling-film with high turbulence (tumbling) and low layer thickness. Deposits on the jacket tube are prevented and the liquid film has no physical contact to the immersion lamp system. A minimum flow rate is required to produce a constant and equilibrated falling-film with a turbulence characterized by Reynolds number of Re > 400. The unique design of the overflow edge and liquid distribution prevents that the film is tearing off. This simplifies considerably the levelling of the equipment.
In direct comparison to an annular thin layer photoreactor, the advantage of the falling film photoreactor is obvious:
The residence time is severly limited by the high flow required to achieve turbulence, while in the falling film photoreactor, the residence time depends on the length of the falling film. The turbulence depends on the thickness of the falling film within given limits on the flow of the solution to be irradiated.
The Peschl Falling-Film photoreactor has been designed in such a way, that the incident radiant power density can be adjusted and optimized for a given photochemical reaction by varying the surface of the irradiated film. In this design, highly efficient photoreactors are offered – tailor-made for each process and additionally equipped with a temperature control system, which can be supplied in the range from -80ºC to +120ºC. With the falling film photoreactor from Peschl Ultraviolet GmbH, photochemical reactions of very strongly absorbing substrates can now be realized successfully.


In the laboratory, falling film photoreactors are used which have a specially structured glass overflow edge and a vortex system developed by Peschl to ensure homogeneous and stable film formation.
This means that even highly viscous products and products with a high absorption coefficient can be irradiated. Examples include oils, latex and monomers.
Two types of Modular Photochemical Development Systems (MPDS®) are used in the laboratory, the MPDS®BASIC system and the MPDS®EVO.


Even in pilot sizes, glass reactors are sometimes used to monitor the development of film formation and viscosity changes (visual process control of hydrodynamic behavior). Furthermore, the possible formation of deposits and color changes can also be detected in this way.
In pilot sizes, photoreactors up to a maximum diameter of 50 cm are usually offered in glass. Beyond this, metal reactors or glass-lined reactors are used.


Large-scale falling film photoreactors are supplied with CIP cleaning systems and can be equipped with camera monitoring, as shown in this example.
Cleaning and inspection openings simplify service and maintenance work and ensure industrial-strength and robust operation.
What services we offer
- Engineering and supply of industrial-sized photochemical reactors
- Scale-up of your process from lab-size to industrial-size
- Scale-down of your process from industrial-size to mini-plant-size
- Photochemical process consultancy
- HAZOP studies for safe operation
- Commissioning support, on-site service
Typical Processes
- Vitamin D production (prone of filming if high radiant power densities are applied)
- Preparative photochemistry in general implying strongly absorbing substrates and/or radical imtermediates
- Recovery of precious metals
- Photooxidations implying radical intermediates (e.g. AOP)
- Bleaching
- Aqueous dispersion of polymers that can be solidified into rubber (Latex)
- Monomer modification
- API synthesis


Technical Features of Falling-Film Photoreactors
- Designed to irradiate homogeneous and heterogeneous (liquid/liquid) reaction systems containing strongly absorbing substrates
- Separation from lamp and product to prevent filming on the boundary surface
- Appreciable turbulence and high molecule exchange rate at low flux
- Prevention of over-irradiation
- Fixed residence time in the irradiated reactor zone
- Optimized irradiance for a given photochemical reaction
- Film equally distributed allover the irradiated surface by special reactor design
- High cooling efficiency through the reactor wall
- Ex-proof design of all components
- Electric power consumption up to 60 kW per lamp
- Gas-discharge, Excimer and LED lamps
- Process temperatures from -80°C up to +120°C
- Robust and reliable reactor designs for safe production
- Special construction materials (reactor tank: titanium, ceramics or special glass-lining for aggressive process conditions
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Davide Onesti
Customer Service,
Purchase & Sales
service@peschl-ultraviolet.com