Trickle bed reactor engineering for aqueous phenol oxidation over
activated carbon catalyst
N. Gabbiye, J. Font, A. Fortuny, C. Bengoa, A. Fabregat, F. Stüber
Departament d’Engineyeria Química, ETSEQ, Universitat Rovira i Virgili, Paisos Catalanns
26, 43007, Tarragona, Catalunya, Spain: e-mail: frankerich.stuber@urv.cat
The increasing release of hazardous organic wastewater, which cannot be amendable by
biological treatment, attracted many researchers to investigate catalytic wet air oxidation
(CWAO) of phenolic model pollutants in Trickle Bed Reactors (TBR)[1]. However, the
complex interaction of the inter-phase and intra-particle mass and heat transfer, kinetics and
hydrodynamics in TBRs imposes many engineering challenges to be solved. Important
operating factors that affect the overall reactor performance are hydrodynamics (flow pattern,
catalyst wetting), liquid and gas flow rates, oxygen partial pressure, temperature and pollutant
inlet concentration. Besides catalyst activity, its stability is also of major concern. In
particular, more recently developed activated carbon catalysts are found to deactivate over
time on stream by simultaneous burn-off and surface deposition of oxidation coupling
products at typical CWAO conditions [2].
This study attempts thus to assess in detail the performance of activated carbon and TBR
operation for the CWAO of phenol. Different start up procedures, hydrodynamic
configurations and main operating variables (PO2, T, CPh,0, FL and FG) were investigated.
Finally, catalyst impregnation (Fe) or dynamic reactor operation is proposed to improve the
catalyst stability. Start up procedures included external and fast in-situ saturation of activated
carbon (AC) or unforced saturation in either oxic or anoxic environments have been tested. It
was observed that the catalyst activity is significantly affected by the way how the three
phases were initially contacted. Forced saturation and unforced saturation of AC with phenol
under nitrogen led to lower although stable phenol conversions over 50 h of operation.
Different hydrodynamic tests (varying bed length, catalyst dilution, varying flow rates,
catalyst pre-wetting) yielded similar phenol conversions implying that performance of our
lab-scale TBR is mostly unaffected by flow irregularities or liquid maldistribution. Overall,
excellent initial catalytic activity (98% phenol conversion) was obtained at 160ºC and 2 bar of
oxygen partial pressure, however, AC burn-off was also greatly enhanced and fast activity
decay could not be avoided at these conditions. A solution to improve stability may offer the
use of Fe impregnated AC or TBR operation under gas feed composition modulation. On one
hand, the impregnated catalyst performs high enough and stable activity at a lower
temperature (140ºC), where AC burn-off is minimized. On the other hand, periodic TBR
operation allows working at higher temperature (160ºC) with reduced carbon consumption
leading to almost stable activity. These results reveal that addition of iron enhance the
catalytic activity of AC while operating TBR in gas feed modulation minimizes the
simultaneous AC burn-off. Hence operating TBR periodically with gas feed composition
modulation using Fe/AC catalyst can be a promising combination for the sustainable use of
AC catalyst in CWAO of phenolic pollutants.
[1] F. Stüber, J. Font, A. Fortuny, C. Bengoa, A. Eftaxias, A. Fabregat, Carbon materials and
catalytic wet air oxidation of organic pollutants in wastewater. Topics in Catalysis,
2005, 33, 3-50.
[2] A. Fortuny, C. Miró, J. Font, A. Fabregat, Three-phase reactor for environmental
remediation: catalytic wet oxidation of phenol using active carbon. Catalysis Today,
1999, 48, 323-328.