OPTICAL PROBING OF HYDROTHERMAL REACTIONS AND FLOW PATTERNS IN A HIGH-PRESSURE FLOW REACTOR

We describe experiments with an optical hydrothermal flow cell capable of containing aqueous solutions at temperatures to 600°C and pressures up to 40 MPa. Hydrothermal reactions of three low-molecular-weight alcohols, ethanol, 2-propanol and 2-methyl-2-propanol, in water at temperatures from 150-500°C and a constant pressure of 34.5 MPa were observed with spontaneous Raman spectroscopy. Reactants and products were identified in situ by their spectral signatures. At temperatures up to 450°C only dehydration products could be detected, whereas at 500°C cleavage was observed. A visual flow pattern analysis in the temperature range from 300 to 450°C and at pump flow rates from 0.1 - 10 ml/min was performed. We observed asymmetric velocity profiles, which were especially pronounced at near-critical densities where reflux flow patterns developed at low flow rates. These results are consistent with model calculations based on a finite grid technique indicating that in hydrothermal flow reactors, long-range radial and axial temperature as well as density gradients exist. Within a large density/temperature region the flow behaviour in our cell can not be adequately described by common reactor models relying on symmetric flow profiles.