The group Photochemistry (PHO) investigates mixing processes and chemical reactions within micromixers and microreactors using optical methods. Mainly laser Raman spectroscopy is used. This allows space- and time-resolved concentration measurements of educts and products within microchannels. It is the aim to get a thorough understanding of the physical and chemical processes within microchannels. This will be the basis for an optimization of micromixers and microreactors, which is important for energy-efficient processes, chemical reactions with improved selectivity, higher yield and lower waste.
The following processes are investigated using these laser Raman systems:
- Biodiesel production with supercritical methanol (3rd Party Funded Research)
- Hydrolysis of acetal (EEC)
- CO2 absorption in a Taylor-flow (DFG-SPP1740)
- Cyclohexane oxidation single phase (EEC)
- Isobutane oxidation single phase and Taylor-flow (HGF)
Laser Raman spectroscopy
Electrons can be raised from the ground state to a virtual energy level by laser radiation. From this level they can fall back and emit light at the laser wavelength (Rayleigh scattering). However, they can also reach a vibrational energy level and emit Raman stray light. This light may be shifted to longer wavelengths (stokes lines) or at higher temperatures to shorter wavelengths (anti-stokes lines). All these lines are characteristic for each molecule and can be used to determine the concentrations of these molecules, selectively.
The following picture (left) illustrates the Raman effect for stokes lines in the energy level scheme. On the right side the principle setup is shown: The radiation of a laser is leaded through a dichroitic mirror and the microscope and focused into the microchannel. The molecules in the microchannel emit Raman stray light, which is leaded back via the microscope and the dichroitic mirror to a spectrometer with sensitive CCD camera. By using a focused laser spot and a confocal optics a space resolution of 15 µm is possible.
The following picture shows a test rig and a home-made laser Raman system. The test rig is capsulated with protective plates and attached to a fume cupboard. It can be equipped with vessels, pumps, mass-flow controllers and sensors, depending on the chemical reactions. Furthermore, a microscope is mounted on a vibration-isolated table, which is decoupled from the fluidics. The right side of the picture shows the large pulsed Nd:YAG laser (532 nm) and above a cw argon ion laser (488 nm).
For stationary processes the 100 mW cw laser (continuous-wave) is used. Its radiation (blue ray in picture) is leaded via mirrors, a dichroitic filter and the microscope into the microchannel. The stokes Raman stray light goes back the same way, but is separated in the dichroitic filter from the back-scattered laser radiation and Rayleigh scattering of the molecules. It is focused into a multi-mode fiber, which acts as pinhole for a confocal arrangement, to achieve a good depth resolution. The fiber ends at a spectrometer with reflection gratings and a cooled CCD detector array.
In order to examine non-steady processes in microchannels, too, e. g. Taylor-flow, no continuous-wave laser can be used, because the measuring time has to be very short. Otherwise, an averaging of concentrations would result and gas bubbles would disturb the measurements. For these applications a custom-design Nd:YAG laser was developed. Standard Nd:YAG lasers produce very short pulses of 10 ns by using Q-switching. Because Raman spectra without averaging require high laser energies to get good Raman spectra, the optical peak power would be too high for the microchannel and its window. Therefore, an oscillator-amplifier principle was used, which is shown in the picture: as seed laser a stable cw laser at 1064 nm is used. Its radiation is cut into the desired pulses by an acousto-optical modulator and amplified in several amplifiers. These consist of Nd:YAG rods which will be optically pumped by high-energy flash lamps. Nonlinear crystals accomplish a frequency doubling of the laser radiation to a wavelength of 532 nm. This laser system can produce laser pulses of 50 mJ at 5 Hz with a pulse duration between 1 µs and 10 µs. Such laser pulses can be focused up to 30 µm to a glass plate at the microchannel.
Generally, these laser Raman systems allow the measurements of concentrations of liquid compounds with high resolution in space and time. Steady-state or non-steady state processes in microreactors are possible. Raman spectroscopy as infrared spectroscopy is very selective, because they are sensitive to differences of vibrating molecules. In contrast to infrared spectroscopy also homonuclear molecules can be detected, e. g. N2 or O2. The combination of this optical technique with process engineering allows an in-situ measurement even at high temperatures (at the moment up to 400 °C) and high pressure (up to 150 bars).