Characterization of a new system for space-resolved simultaneous in situ measurements of hydrocarbons and dissolved oxygen in microchannels

A space-resolved in situ measurement technique based on laser Raman spectroscopy with high detection sensitivity is described. This method allows the simultaneous detection of the concentrations of dissolved molecular oxygen and of hydrocarbons as well as oxidation products in organic liquids in a microchannel during reaction. It can be used as a new tool for detailed kinetic studies of liquid-phase reaction. Raman spectra are produced using an argon ion laser at 488 nm with a continuous optical power of 100 mW. This radiation is coupled into a microscope and a microchannel. The arising Raman stray light is detected with a spectrometer and a sensitive CCD camera. Special optics were used to collect as much light as possible on the CCD detector. This results in high signals and low noise levels. In order to demonstrate the usefulness of the system, cyclohexane oxidation by oxygen was investigated. In a feasibility study for the products of the cyclohexane oxidation, a limit of detection of 0.05 % m/m for cyclohexanol and 0.01 % m/m for cyclohexanone was achieved. Molecular oxygen dissolved in cyclohexane could be detected at the relevant concentration ranges for carrying out the oxidation of cyclohexane with a limit of detection of 0.01 % m/m. An optically transparent microchannel reactor was built, which can be used up to temperatures of 503 K and pressures of 8 MPa. With this reactor and the in situ measurement technique, space-resolved studies with a measuring volume of 5 μm × 5 μm × 38 μm can now be realized. The spectral selectivity and sensitivity of the measurement technique applied to cyclohexane oxidation, and the characteristics of the spatially resolved measurement technique are discussed.

[1]  O. E. Potter,et al.  Mass Transfer and Solubility in Autocatalytic Oxidation of Cyclohexane , 1988 .

[2]  S. Haswell,et al.  Monitoring of chemical reactions within microreactors using an inverted Raman microscopic spectrometer , 2003, Electrophoresis.

[3]  Dusan Boskovic,et al.  Novel multifunctional microreaction unit for chemical engineering , 2004 .

[4]  W. Podgórska,et al.  Kinetic model of cyclohexane oxidation , 2001 .

[5]  S. Duke,et al.  Laser induced fluorescence measurements of dissolved oxygen concentration fields near air bubble surfaces , 2000 .

[6]  A. K. Suresh,et al.  Modeling Liquid-Phase Cyclohexane Oxidation , 2007 .

[7]  M. Duduković,et al.  Effect of Oxygen on Cyclohexane Oxidation: A Stirred Tank Study , 2009 .

[8]  Peter Ehrhard,et al.  Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels , 2009 .

[9]  Klavs F. Jensen,et al.  Silicon Micromixers with Infrared Detection for Studies of Liquid-Phase Reactions , 2005 .

[10]  Elias Klemm,et al.  Uncatalyzed selective oxidation of liquid cyclohexane with air in a microcapillary reactor , 2010 .

[11]  A. Leipertz,et al.  Simultaneous determination of the composition and temperature gradients in the vicinity of boiling bubbles in liquid binary mixtures using one-dimensional Raman measurements , 2011 .

[12]  R. Pohorecki,et al.  Kinetic model of uncatalyzed oxidation of cyclohexane , 2009 .

[13]  Modellbasierte Bestimmung lokal gültiger Kinetiken chemischer Reaktionen in Flüssigphase mittels Flachbettmikroreaktor , 2010 .

[14]  O. E. Potter,et al.  Uncatalysed oxidation of cyclohexane in a continuous reactor , 1997 .

[15]  Birgit Ungerböck,et al.  Online oxygen measurements inside a microreactor with modeling of transport phenomena , 2013 .

[16]  A. Manz,et al.  Micro total analysis systems. Recent developments. , 2004, Analytical chemistry.

[17]  Atsushi Urakawa,et al.  On-chip Raman analysis of heterogeneous catalytic reaction in supercritical CO2: phase behaviour monitoring and activity profiling. , 2008, The Analyst.

[18]  S. Kerschbaum,et al.  In situ Raman spectroscopy to monitor the hydrolysis of acetal in microreactors , 2011 .

[19]  I. Hermans,et al.  To the core of autocatalysis in cyclohexane autoxidation. , 2006, Chemistry.

[20]  O. E. Potter,et al.  Autocatalytic oxidation of cyclohexane—modeling reaction kinetics , 1988 .

[21]  Wenka Schweikert,et al.  Analysis and Improvement of Strong Exothermic Nitrations in Microreactors , 2003 .

[22]  P Hinsmann,et al.  Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution. , 2001, Lab on a chip.

[23]  J. Li,et al.  Mass Transfer Parameters of O2 and N2 in Cyclohexane under Elevated Pressures and Temperatures: A Statistical Approach , 1997 .