(562az) Modeling and Kinetics Study of Methyl Orange Degradation over a Cu/?-Al2O3 Fenton-like Catalyst in Continuous Fixed Bed Reactor

Catalytic
wet peroxide oxidation (CWPO) is recognized as one of the most promising and
environmental-friendly techniques for reclamation of wastewater containing
toxic and bio-refractory compound. It takes advantage of the great oxidative
capacity of the hydroxyl radical (HO∙) driving from hydrogen peroxide (H₂O₂)
at moderate conditions (< 373 K, 0.1 − 0.5 MPa). Compared with the batch
reactor, continuous fixed bed reactor (FBR) was a superior configuration for industrial
application of CWPO process ^{[1, 2]}. However, a lack of study on the
diffusion and kinetics for this liquid-solid system in the continuous FBR results
in poor understanding of this process, which further hinders its scale-up and
commercial use.

In
this work, copper supported on alumina pellets (Cu/¦Ã-Al₂O₃)
were synthesized by impregnation method, and the average diameter of pellets
size was 2.3 mm. The as-prepared catalyst was denoted as 2.3Cu/¦Ã-Al₂O₃.
The morphology of pellets exhibits good sphericity with a rough surface (Fig.
1(a)), however, the hardening phenomenon was more serious for the surface
comparing with the inside of the pellet (Fig. 1(b − d)). In addition, Al, O and
Cu elements are well-dispersed for the whole pellet (Fig. 1(e)).

A
series of performance tests were conducted in an up-flow FBR for methyl orange
(MO) degradation. The 2.3Cu/¦Ã-Al₂O₃ exhibited high
activity and excellent leaching resistance during 200 h testing (leaching
amount of Cu < 0.6 mg/L). The
conversion of MO and COD equal to 97% and 92% was achieved over 2.3Cu/¦Ã-Al₂O₃
catalyst (reaction conditions: pH₀ = 8.2, flow rate = 2 mL/min, [catalyst]
= 9.9 g, [MO]₀ = 30 ¦ÌM, [H₂O₂]₀ =
29 mM, and 50¡æ). The apparent reaction kinetics was investigated on the basis
of the pseudo-first order simplification for the MO degradation and H₂O₂
decomposition, and the apparent E_a were calculated to be 21.0 and
13.8 kJ/mol, respectively. Compared with the intrinsic Ea studied for MO
degradation (57.8 kJ/mol) and H₂O₂ decomposition (47.7
kJ/mol) in batch reactor, the mass transfer was significant for MO/H₂O₂/2.3Cu/¦Ã-Al₂O₃
system in the FBR.

Fig.1 SEM and EDX
images of 2.3Cu/¦Ã-Al₂O₃ catalysts: (a − b) SEM images of 2.3
Cu/¦Ã-Al₂O₃ pellet and corresponding enlarged image, (c − b)
SEM images of cross-section for 2.3Cu/¦Ã-Al₂O₃ pellet and corresponding
enlarged image, (e) EDX mappings of Al, Cu and O elements for cross-section of 2.3Cu/¦Ã-Al₂O₃
pellet.

In
addition, the apparent kinetic study for MO oxidation and H₂O₂
decomposition were modeled with account for the external and internal diffusion
limitation, which were quantified by measuring the apparent kinetic rate
constant by changing the pellets sizes (mean d_p = 2.3, 3.5, 4.2 mm) and
flow rate (1 − 20 mL/min). As shown in Fig. 2, modified experimental
correlation (Sh = 1.5Re^1.10Sc^0.42) was proposed for the
calculation of external mass transfer coefficient. Additionally, intrinsic
kinetics was studied in the batch reactor to determine the effectiveness factor
(¦Ç) in FBR to provide accuracy description of the experimental data.

Fig. 2 (a)
Values of k_c for MO oxidation, ([H₂O₂]₀
= 29 mM, [MO]₀= 30 ¦Ìm, flow rate = 2 − 20
mL/min, mean d_p = 2.3 mm, and [Catalyst] = 9.9 g). (b) Fitting
results of the relationship between Sh, Re and Sc. (c) Calculated ¦Ç and ¦µ for
H₂O₂ decomposition and MO oxidation (blue square: H₂O₂,
red triangle: MO)

In
summary, Cu/¦Ã-Al₂O₃ pellet catalysts
prepared by impregnation method showed outstanding activity and stability
against leaching for H₂O₂ decomposition and MO degradation
in the FBR. Quantified analysis showed that H₂O₂
decomposition and MO degradation in the FBR was both controlled by the external
and internal mass transfer resistance, especially the latter. The geometric
size of the pellet catalysts exert a pronounced effect on the apparent kinetic
of the process.

For
the predication of apparent kinetic rate in the FBR, a combined diffusion-kinetic
model was proposed. A modified empirical correlation for the external mass
transfer at low Re was determined by fitting the experimental data in FBR and the
effective factor (¦Ç) for internal mass transfer resistance was calculated according
to the intrinsic kinetics measured in the batch reactor. This diffusion-kinetic
model provided a reasonable description of the experimental data and can be
expanded to other pollutants degradation, which provided a good solution for
the CWPO process analysis and reactor design.

References

[1]
Ioannou LA, Puma GL.Fatta-Kassinos D. Treatment of
winery wastewater by physicochemical, biological and advanced processes: A
review. J. Hazard. Mater. 2015; 286,343-368.

[2] Esteves BM, Rodrigues CSD.Madeira LM, in
Applications of Advanced Oxidation Processes (AOPs) in Drinking Water
Treatment, eds. Gil, Galeano,Vicente, Springer International Publishing, Cham,
2019,211-255.