IJUP08 - Universidade do Porto

Wet Peroxide Oxidation and Wet Oxidation of Nitrophenols in
Aqueous Streams
S. Martins 1,2 , A. Ribeiro 2 , A.M.T. Silva 1 , P. Araújo 2 , J.L. Figueiredo 1 , and J.L. Faria 1
1 Laboratório de Catálise e Materiais, Departamento de Engenharia Química, Faculdade de
Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
2 CUF – Químicos de Portugal, S.A., Quinta da Indústria, 3860-680 Estarreja, Portugal
The treatment of nitrophenols, typically found in the effluents from the process of
nitrobenzene synthesis, was carried out by wet oxidation (WO) and wet peroxide oxidation
(WPO). These processes are based in the principles of wet air oxidation, an efficient
process for treatment of high strength effluents which cannot be treated by conventional
methods [1]. In this process the organics are oxidized in the liquid phase at temperatures
around 300ºC and pressures of 5 to 200 bar, in presence of an oxygen-containing agent
(usually air). This process has low operating costs and minimal air pollution discharges,
the main limitations being the safety implications
associated with a system operating at such
conditions [2]. Under the usual conditions, the
organic compounds are mainly oxidized into
carbon dioxide and water. Our previous studies
show that this process is efficient above 200ºC
and 7.0 bar of O2 in the degradation of low
dinitrophenol (DNP) and trinitrophenol (TNP)
concentrations (below 0.4 g/L) [3]. However,
these operating conditions are quite severe for
industrial scale-up and higher concentrations of
Figure 1 – Evolution of the C/C0 ratio to
DNP and TNP.
nitrophenols (above 10 g/L) are usually found in real effluents. Therefore, the use
hydrogen peroxide (H2O2) as additional oxidizing agent was investigated in this work (wet
peroxide oxidation process) aiming to obtain a high efficiency at lower temperatures and
pressures. In a wide range of concentrations of nitrophenols the use of mild conditions for
temperature and O2 pressure (without H2O2) resulted in conversions up to 99.3 and 98.5%
for DNP and TNP, respectively (Fig.1). An initial period of 30 min was necessary to
achieve the working temperature. At the same temperature, with an oxygen pressure of 0.8
bar with the higher peroxide concentration used, a complete destruction of nitrophenols
was observed. Reducing temperature, keeping the oxygen pressure, but reducing peroxide
concentration to 56.1 g/L, resulted in maximum conversions of 92.1 and 99.8% for DNP
and TNP, respectively. Global TOC removal efficiencies ranged from 57 to 97%.
Therefore, the WPO is strongly recommended in the degradation of nitrophenols.
References:
[1] – Mishra, V.S, Mahajani, V.V. and Joshi J.B. (1995) Wet Air Oxidation, Industrial &
Engineering Chemistry Resaerch 34(1), 2-48.
[2] – Kolaczkowski, S. T., P. Plucinski, et al. (1999). Wet air oxidation: a review of process
technologies and aspects in reactor design, Chemical Engineering Journal 73(2), 143-160.
[3] – Apolinário, Â .C., Silva, A.M.T., Machado, B.F., Gomes, H.T., Araújo, P.P., Figueiredo, J.L.,
Faria, J.L. (2007), Wet air oxidation of nitro-aromatic compounds: reactivity on single- and multi
component systems and surface chemistry studies with a carbon xerogel, Applied Catalysis B,
Environmental, (in press, doi:10.1016/j.apcatb.2007.12.018).
C/C0
1,6
1,2
0,8
0,4
0
DNP
TNP
-30 -15 0 15 30 45 60 75 90 105 120
t (min)
159