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Photoreactions in the Laboratory Course of Technical Chemistry

General Information

The laboratory course of Technical Chemistry is an obligatory part of the chemistry studies in the 5th semester as a part of the Bachelor grade. It consists of six different experiments to get basic knowledge on mechanical and thermal unit operations as well as reaction engineering. The students carry out the experiments by themselves under the supervision of an assistant after passing the introductory colloquy to check their preceding preparation. After the experiments, the students have to write a protocol, in which they have to answer several specific questions.

Fallfilm  Spektrometer

Figure 1: left: falling film reactor, right top: spectrometer, right bottom: light source for both spectrometers.

The experiment referring to photochemistry is dedicated to the investigation and evaluation of two different photo reactor types. Mercury vapor lamps with different power are used as light sources to drive the reaction. The same lamp type was chosen for both reactor setups to exclude differences in the emission spectrum. Hence, only the power of the light source and the reactor setup influence the photon flux received in the reactor. Using the example of the degradation of rose bengal, a falling film photoreactor (figure 1) and a microphotoreactor (figure 2) are operated in three consecutive runs with different flow rates. The concentration of rose bengal is measured by UV/VIS spectrometry during the experiment. Measurements for the microreactor are conducted in a flow through cell and for the falling film reactor a cuvette is used for the subsequently taken samples (figure 1). Both reactor types are characterized in terms of reaction engineering aspects. This includes handling of figures of merit for reactor characterization like productivity and space time yield. Additionally to such general numbers, the features of photochemical reactors are evaluated. In this context, the energy efficiency and the external photon efficiency are introduced. The energy efficiency ξel is defined as the ratio of the change of molar flow of the product before and after the irradiation ˙noutP n˙inP and the electrical power Pel consumed by the lamp:

photpraktikumTCeq1

(1)

Therefore, the unit of the electrical efficiency is mol J−1. The other characteristic factor mentioned is the external photon efficiency. This number transfers the characterization to the photon level by comparing the molar product flow with the emitted photon flux of the light source ˙nem:

photpraktikumTCeq2


(2)

The external photon efficiency is a dimensionless number.

IMG_7966 Bild_Mikroreaktor 

Figure 2: Setup of the whole apparatus of the UV-Cube (left) and the implemented micro reactor plate (right).

A well-grounded interpretation of the experimental results including a comparison of both reactor types is expected in the protocol to show the gained knowledge of the students. Furthermore, the students become familiar with transferring the figures of merit to other reactor conditions by estimating the effect of different operation parameters and methods.

Experimental Procedure

The degradation of rose bengal (figure 3, left) will be investigated in two different reactor types: a falling film photoreactor and a microstructured photoreactor. Both setups are available from Peschl Ultraviolet GmbH in Mainz and feature medium pressure mercury lamps with 700 W and 150 W, respectively. The solution containing rose bengal will be cycled between the reactor and a reservoir vessel in both setups. Concentration measurements are conducted with two spectrometers and a UV/VIS lamp from Avantes BV, The Netherlands. An exemplary absorption spectrum at the beginning of the reaction is shown in figure 0.3 on the right.

Two different reaction solution with concentrations of 20µM (V = 1 L) and 5µM (V = 25 mL) are required for the experiments and have to be prepared first. The 20 µM solution will be used for the falling film reactor and the 5 µM solution for the microstructured reactor.

 

rose_bengal
 rose_bengal_spectrum

Figure 3: Chemical structure of rose bengal (left) and its absorption spectrum (right).

Preparation of the Spectrometer

The transmission of the reaction solution will be followed at a wavelength of 540nm. A flow through cell will be used for monitoring the concentration in the microstructured reactor. The concentrations in the falling film reactor are followed by sampling and consecutive measurement of the solution in a cuvette. For this, the following steps are required for both cells to prepare the analytic method for the experiments:

  1. Turn on both lamps (deuterium and halogen lamp) of the UV/VIS light source at least 15min before the first measurements or saving the reference spectrum to allow the lamps to stabilize. The TTL-switch should be switched to “off”. Both lamps must not switched off during the experiments. Only the TTL-switch should be used to control the illumination of the cells.
  2. Start the program “AvaSoft8” from the desktop.
  3. Choose the measure mode scope “S”. Save a dark spectrum before switching on the TTL-switch by clicking on the “Dark” button.
  4. Put a cuvette filled with purified water in the cuvette holder and rinse the flow through cell with purified water.
  5. Save the reference spectrum for both cells by clicking the “Reference” button.
  6. Change to measure mode transmission by clicking on “T”.

Therewith, both UV/VIS cells are prepared for the measurements and should show 100% transmission in both spectra.

UV/VIS Measurements

The spectra of the initial reaction solutions have to be recorded first. For this, the microstructured photoreactor is rinsed with the 5 µM reaction solution and the cuvette is filled with the 20 µM solution. The cuvette must be placed in the cuvette holder. The spectra are recorded in single mode by clicking the “Start” button and save in folder “Experiments”. As a guide for the eyes, these spectra can be displayed during the experiments by opening the respective tab.

Repetitive measurements after a given time can be automatically measured by the software. Therefore, the mode “TimeSeries” has to be chosen first. This opens a new tab. To enable simultaneous measurements for both reactors, two tabs are required. The routines for repetitive measurements are saved as functions “fallfilm_peak_540” and “mikro_peak_540”. One of these functions has to be loaded in one of the tabs. The routines measure the transmission at a wavelength of 540 nm every minute for the microstructured reactor and every 10 minutes for the falling film reactor. To enable correct measurements in the cuvette, the cuvette has to be filled with a current sample every 10 minutes manually. Wearing suited safety glasses is mandatory during sampling! The measurements are started by clicking on the “Start” button. The results are exported to an Excel sheet after the measurements are stopped by clicking the “Stop” button.

Reactor Startup

At the beginning of the course day, the circulating thermostat has to be switched on. The cooling water temperature has to be set to 20C. The cooling water tubes have to be checked for leaks and a sufficient cooling water flow rate in the UV-Cube, the reactor plate and the falling film reactor. When no leaks are detected, the pumps can be turned on and the given flow rates can be set.

Only after the pumps are running, the mercury vapor lamps are switched on by the assistant.

The following flow rates have to be investigated:

  • microstructured reactor: 0.5 mL min−1, 0.75 mL min−1  and 1 m L min−1
  • falling film reactor: 240 mL min−1, 360 mL min−1 and 480 mL min−1

Evaluation of Experimental Data

Based on the experimental data (time resolved transmission values) gathered, the following figures of merit should be calculated for all process conditions investigated:

  • conversion,
  • space-time-conversion (equals the space-time-yield, but based on conversion)
  • productivity,
  • energy efficiency.

It should be discussed, which parameter is influenced by which change of the process conditions.

With these numbers at hand, the results should be compared and the investigated reactors as well as the different process conditions should be characterized. What conclusions can be drawn from the results? The compiled graphs should be discussed with respect to the different coherences of the falling film and the microstructured reactor. What behavior of the curves can be expected for longer reaction times?

The influence of different mode of operation (batch, semi-batch, continuous) on the calculated figures of merit should be estimated based on the experience made with thermally initiated reactions. What reaction conditions should be chosen for which mode of operation? Do other relevant parameters exist, which influence the choice of the mode of operation?

List of Equipment

The following equipment is used in this experiment:

  • two medium pressure mercury lamps (700W and 150W) including the power supply units,
  • a circulating thermostat with an automatic alarm,
  • a PMMA-shielded falling film reactor system and a microreactor system, both from Peschl Ultraviolet,
  • a peristaltic (falling film) and an Ismatec rotary piston pump (microreactor),
  • a flow through cell and a cuvette in a cuvette holder connected to an Avantes spectrometer and the Avantes light source with two connections,
  • a desktop computer with installed Avantes software to measure and save the spectra,
  • UV-light suited safety glasses for each student.

 

 

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