JULIO / DICIEMBRE 2020 - VOLUMEN 30 (2)
/ ISSN electrónico: 2215-2652
Esta obra está bajo una Licencia de Creative Commons. Reconocimiento - No Comercial - Compartir Igual 4.0 Internacional
DOI 10.15517/ri.v30i2.39236
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica
Heterogeneous Catalytic Ozonation of Phenol over Iron-based
Catalysts in a Trickle Bed Reactor
Ozonación catalítica heterogénea de fenol utilizando catalizadores de
hierro en un reactor de lecho percolador
Luis Briceño Mena,
School of Chemical Engineering,
University of Costa Rica, San José, Costa Rica
luis.bricenomena@ucr.ac.cr
ORCID: https://orcid.org/0000-0003-3684-4232
J. Esteban Durán Herrera,
School of Chemical Engineering,
University of Costa Rica, San José, Costa Rica
esteban.duranherrera@ucr.ac.cr
ORCID: https://orcid.org/0000-0001-7382-0454
Recibido: 7 de octubre 2019 Aceptado: 14 de febrero 2020
_________________________________________________________
Abstract
The use of continuous reactors for heterogeneous catalytic ozonation is yet to be investigated in order to
develop a viable technology for industrial applications. This paper presents hydrodynamic and degradation
studies on the use of a co-current down flow trickle bed reactor for heterogeneous catalytic ozonation of
phenol (as model pollutant) over Fe-Diatomite pellets and Fe-coated glass beads. It was found that the reactor
can operate under trickle or pulsing flow regimes, promoting mass transfer augmentation. Residence time
distribution data, tted with n-CSTR and axial dispersion (ADM) models, showed low axial dispersion and
high flow distribution. Just the Fe-diatomite pellets showed important phenol adsorption (16 %). Degradation
experiments demonstrated that phenol conversion was substantial when using both catalysts, up to 19,7 %
pollutant conversion with liquid-phase space times of just 6 s. Compared to direct ozonation, the use of
the Fe-diatomite pellets and Fe-coated glass beads enhanced the reactor performance by 48 % and 23 %
respectively. It was conrmed that mass transfer is an important factor that restricts this reaction system
performance; consequently, further improvement in mass transport rate is necessary for system optimization.
Keywords:
Heterogeneous catalytic ozonation; trickle bed reactor; emerging contaminants; advanced oxidation processes
Resumen
El uso de reactores continuos para la ozonización catalítica heterogénea es un tema que debe aún ser
investigado con el objetivo de desarrollar una tecnología viable para aplicaciones industriales. Este artículo
presenta estudios hidrodinámicos y de degradación utilizando un reactor de lecho percolador de flujos
descendentes en co-corriente para la ozonización catalítica heterogénea de fenol (como contaminante modelo)
Esta obra está bajo una Licencia de Creative Commons. Reconocimiento - No Comercial - Compartir Igual 4.0 Internacional
DOI 10.15517/ri.v30i2.39236
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica
sobre gránulos de Fe-diatomita y perlas de vidrio recubiertas con Fe. Se determinó que el reactor puede
operar bajo regímenes de flujo percolador o pulsante, promoviendo el aumento de transferencia de masa.
Los datos de distribución de tiempo de residencia, ajustados a los modelos de n-CSTR y de dispersión axial
(ADM), mostraron baja dispersión axial y alta distribución de flujo. Solamente los gránulos de Fe-diatomita
mostraron una adsorción de fenol (16 %) importante. Los experimentos de degradación demostraron que la
conversión de fenol fue sustancial cuando se usaron ambos catalizadores, hasta un 19,7 % de conversión del
contaminante con espacio-tiempos de la fase líquida de solo 6 s. En comparación con la ozonización directa,
el uso de los gránulos de Fe-diatomita y las perlas de vidrio recubiertas con Fe mejoraron el rendimiento
del reactor en un 48 % y un 23 % respectivamente. Se conrmó que la transferencia de masa es un factor
importante que restringe el rendimiento de este sistema de reacción; en consecuencia, se necesita una mejora
adicional en la velocidad de transporte másico para la optimización del sistema.
Palabras clave:
Ozonización catalítica heterogénea; reactor de lecho percolador; contaminantes emergentes; procesos de
oxidación avanzada.
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica DOI 10.15517/ri.v30i2.39236
3
1. INTRODUCTION
The presence of persistent organic pollutants (POPs) in water is a key environmental issue
and one of the biggest challenges in wastewater and drinking water t reatment (Peña-Guzmán et
al. 2019), mainly because POPs are toxic and unresponsive to traditional treatment techniques.
Occurrence of such pollutants can be related, for example, to petroleum, chemical products, pesti-
cides, paints, and pharmaceuticals industries. One example of POPs is phenol, which can be found
in many industrial wastewaters. Phenol is a highly soluble, stable, and toxic molecule that even at
low concentrations can cause health problems. Also, at concentrations above 400 mg/L, biological
oxidation in conventional water treatment plants is not efcient due to phenol bactericide properties
(Zeng et al., 2012). Because of all these issues, phenol is regulated in the USEPA Priority Pollutant
List and its removal from industrial wastewater is important.
Among the different strategies for the abatement of POPs from wastewaters, those that involve
the use of highly oxidative chemical species (particularly the hydroxyl radical) are of special interest.
These technologies, known as advanced oxidation processes (AOPs), have proven to be effective
for the degradation of phenolic compounds and other recalcitrant pollutants (Babu, Srivastava, Nid-
heesh & Kumar, 2019). Specically, ozone-based processes, in which ozone is used as the source of
hydroxyl radicals, are an attractive alternative mainly due to the high degradation rates achievable
at temperatures and pressures near to ambient conditions (Maugans & Akgerman, 2003). Further-
more, by the use of proper catalysts, heterogeneous catalytic ozonation (HCO) has been conside-
red as a way to improve ozonation processes, increasing pollutant and total organic carbon (TOC)
removal rates and lowering ozone and energy consumption (Zhao, Ma, Zhizhong & Zhai, 2008).
Most of the HCO research reported in the literature has been carried out using batch or semi-
batch reactors, utilizing high liquid/gas ratios and small catalyst particle size, mainly to promote
favorable mass transfer conditions (Li, Xu, Zhu, Ding & Mahmood, 2010); however, those ope-
ration congurations are not suitable for industrial applications. In order to develop a continuous
HCO-based technology for water treatment, three-phase phenomena and catalyst effects (such as
diffusion, adsorption, pH changes, and surface reaction) must be addressed. Thus, mass transfer
and fluid dynamics characterization of the solid-gas-liquid contact system is relevant for properly
design such continuous reactors.
Some researchers have proposed the use of trickle bed reactors for three-phase advanced oxi-
dation processes (e.g., catalytic wet air oxidation), pointing out some benets such as low liquid
to solid ratio and effective liquid-gas interaction (Maugans & Akgerman, 2003). For example,
Pintar, Batista & Tisler (2008) reported TOC removal up to 98 % for phenol solutions at 453 K. In
their experiments, catalyst deactivation was found to be signicant (TOC conversion dropped to
41 % after 28 hours). Other authors (Singh, Pant & Nigam, 2004) achieved up to 50 % of phenol
degradation in a pilot plant trickle bed reactor using a copper oxide catalyst supported on alumina
(60 cm bed length), nding a strong dependence of the system performance on the operation con-
ditions (temperature, pressure, gas, and liquid flow rates).
BRICEÑO Y DURÁN: Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts...
4
This paper investigates the use of a co-current down flow trickle bed reactor for heterogeneous
catalytic ozonation of phenol. Fluid dynamics characterization (residence time distribution (RTD)
and flow regime mapping) and phenol degradation studies were performed under different mass
transfer conditions and geometrical congurations of the reactor. Two new catalysts (iron oxide-
diatomite pellets and iron oxide coated glass beads) were evaluated and compared against non-
catalytic bed packings with similar geometries.
2. MATERIALS AND METHODS
2.1 Reagents
For the residence time distribution experiments, laboratory grade NaCl solutions were used as
tracer. 3 % m/m Na
2
SO
4
solution was used to quench the ozonation reaction, and NaOH solution
was used to adjust pH for phenol determination. 2 % m/m KI solutions were utilized for gaseous
ozone capture and determination.
2.2 Catalyst preparation
Iron-diatomite catalyst pellets (CPFe) were prepared using a variation of the method described
by Maugans & Akgerman (2003). Diatomite powder (Celite
R
) was mixed with Ludox binder
colloidal silica (Sigma Aldrich) in a 1:1,5 m/v ratio to obtain a consistent paste. It was observed
that such ratio minimizes the use of binder while keeping the needed consistency in the mixture.
Cylindrical pellets were formed and calcinated a 325 ºC for 4 hours. The pellets were loaded with
iron by incipient impregnation method using Fe
2
SO
4
solutions. After 2 hours, impregnated pellets
were calcinated for 4 hours at 325 ºC resulting in 6.38 mm equivalent diameter pellets with 0,77 %
m/m iron load.
Iron-coated glass beads (CBFe) were prepared as follows: Glass beads (5 mm) were acid
washed for 1 hour using 40 % sulfuric acid solution. After thoroughly rinsing with distilled water,
glass beads were dry at ambient conditions. For coating with iron oxide, the beads were submer-
ged in Fe
2
SO
4
solution (6 % F
2
SO
4
/mass of glass beads) for 2 hours, dried at 65 ºC for 2 hours, and
calcinated at 325 ºC for 4 hours.
2.3 Experimental Setup
The experimental setup is comprised of a modied Reef Octopus Ozone Reactor (OR150),
equipped with an interchangeable acrylic column (39,0 cm height and 2,5 cm or 5,0 cm diameter)
and packed with either cylindrical pellets or glass beads. The experimental setup also included gas
and liquid flowmeters with regulation valves, ozone generator (Ozomax 1VTT), exhaust ozone des-
truction system, syringe for tracer input, and phenol and ozone sampling points at the liquid outlet
(see Figure 1). The reactor was able to operate in the range from 0 L/min - 30 L/min gas flowrate
and 0,2 L/min - 0,5 L/min liquid flowrate (STP).
5
Figure 1. Experimental setup: (a) air compressor, (b) ozone generator, (c) submergible pump, (d) feed solution
5 L tank, (e) 2,5 cm internal column, (f) tracer injection point, (g) gas-liquid separator, (h) ozone sampler,
(i) ozone off-gas destructor. F indicates in-line flowmeters and P manometers.
2.4 Flow regime experiments
Trickle bed reactor flow regime map was obtained by increasing gas flow rate (0 L/min - 30 L/
min) for a xed liquid flow rate (0,2 L/min - 0,5 L/min) until flow pulses were observed. All fluid
dynamics experiments were performed using water as liquid and air as gas phase.
2.5 RTD experiments
The residency time distribution analysis was done following the pulse method described by
Fogler (2008). For this, NaCl solution (25100 mg/L) was spiked at the reactor inlet and its concen-
tration was measured at the outlet using a conductivity meter (Cole-Parmer CON 410).
2.6 Phenol adsorption experiments
Catalyst samples were sprayed with distilled/deionized water and then submerged in phenol
solutions (380 mg/L - 410 mg/L) in a 1:2 solid-liquid bulk volume ratio. Preliminary runs showed
that, when using dry catalyst, air occluded in the particles affected negatively catalyst wetting.
Dilution effect of spraying the pellets with water was considered negligible. In all adsorption expe-
riments, phenol concentration was measured for 8 hours using a BioMate3 spectrophotometer at
270 nm wavelength. Before measurements, all samples pH was adjusted to 6,84 ± 0,05 to block
bathochromic shift effects or -OH group hydrolyzation.
BRICEÑO Y DURÁN: Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts...
6
2.7 Ozonation experiments
In prior ozonation experiments, catalyst particles were left in contact with phenol stock solutions
(400 mg/L - 450 mg/L) overnight to reach adsorption equilibrium; in this way, any reduction of
phenol concentration during reactor operation could be attributed to chemical (ozone) degradation.
Initial phenol concentrations of around 400 mg/L were used so that experimental results could be
comparable with other studies (Pintar, Batista & Tisler, 2008). Liquid and gas flows in the reactor
were let to reach stability before the ozone generator was turned on. For all ozonation experiments,
samples were taken during steady state operation. Liquid outlet samples were taken for dissolve ozone
(Ozone Pocket Colorimeter, HACH) and phenol analysis, and gas outlet samples were collected for
ozone determination by iodimetric titration (Rakness et al., 1996).
3. RESULTS AND DISCUSSION
3.1 Catalysts
Mechanical strength of the iron-diatomite catalyst pellets was adequate to withstand column pac-
king weight and operation; thus, no fracture or particle deformation was observed. Iron content was
measure for both CPFe and CBFe catalysts by ICP-MS analysis, resulting in (0,77
0,01) % m/m
for the CPFe pellets and (0,0079
0,0010) % m/m for the CBFe beads. Iron loading in CBFe beads
was expected to be much lower since iron is deposited only on the external surface of glass beads,
whereas pellets are made of a porous material and deposition takes place inside the pellets too. Iron
lixiviation was not observed during reactor operation. Equivalent diameter of the cylindrical pellets
was 6,38 mm: this particle size was small enough for having good liquid distribution, low pressure
drop, and trickle bed flow conditions as showed in the hydrodynamic studies. pH changes in phenol
solutions suggest a correlation between acidic sites in the catalyst surface and solid-liquid interac-
tion since a signicant pH change was observed as soon as phenol solution and catalyst were put in
contact, varying from 6,2 to 4,0. This behavior was not observed when using spherical glass beads.
3.2 Flow regime map
Typically, in a trickle bed reactor, four different flow regimes (spray, trickle, pulsing, and
bubbling) can be observed, characterized by liquid or gas phase continuity. Flow regime present in a
specic reactor depends on the throughput of the phases, bed and particle geometrical characteristics,
and fluids physical properties. In most industrial applications, trickle bed reactors are operated under
trickle or pulsing flow conditions because high mass and heat transfer rates can be achieved in such
fluid flow conditions. Stagnant liquid holdup mobilization in pulsing flow has been reported to
enhance reactor performance, increase catalyst usage, and reduce axial dispersion (Gunjal, Ranade
& Chaudhari, 2003).
Phase continuity and pulse presence were studied to construct a flow regime map for the trickle
bed reactor used; in this way, proper operation conditions were identied. As shown in Figure 2,
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica DOI 10.15517/ri.v30i2.39236
7
the reactor can be operated in either trickle or pulsing flow regimes. It was found that increasing
bed diameter leads flow regime transitions at higher gas velocities and that particle geometry plays
an important role as the effect of diameter was bigger for the pellets. These observations are in
accordance with those reported by Boelhouwer, Piepers & Drinkenburg (2002).
As it can be observed, 0,025 m diameter column presents pulsing flow in most of its operation
range. These results suggest that this reactor conguration presents favorable conditions for HCO
since pulsing flow conditions are suitable for fast reactions (Boelhouwer, Piepers & Drinkenburg,
2002).
Figure 2. Flow regime map for glass beads and pellets with column
diameters of 0,025 m (D1) and 0,05 m (D2).
3.3 RTD Analysis
In order to establish the effect of geometry on the hydrodynamic behavior of the trickle bed
reactor, a residence time distribution analysis was performed using two column diameters (0,025
m and 0,050 m), and two particle geometries (5 mm diameter glass beads and 5 mm equivalent
diameter acrylic pellets), operating the reactor under pulsing flow regime. The n-CSTR model (Eq.
1) and the axial dispersion model (ADM) (Eq. 2) (Wanchoo, Kaur, Bnasal & Thakur, 2007) were
tted to the experimental data as shown in Figure 3.
(1)
BRICEÑO Y DURÁN: Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts...
8
(2)
Figure 3. RTD data and n-CSTR and ADM model predictions for the 0,025 m (D1) and 0,050 m (D2) diameter columns, lled
with CPFe pellets ((a) and (b)) and CBFe glass beads ((c) and (d)). Liquid flow rate = 0.3 L/min and gas flow rate = 8,0 L/min.
Table 1 shows the Péclet number (Pe), N parameter, experimental mean residence time ( ),
variance (
), and skewness (s
3
) for the studied congurations. Table 2 shows the standard error and
R
2
values for the tted models. When the obtained Pe values are compared with those reported by
Perry & Green (2008) for pilot plant reactors and by inspecting the sharpness of the curves (see
also the standard deviations), it can be deducted that axial dispersion is low in the experimental
system. This behavior is in accordance with that expected for trickle bed reactors operating under
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica DOI 10.15517/ri.v30i2.39236
9
pulsing flow conditions because of the reduction of stagnant liquid holdup and increase of radial
mixing (Boelhouwer, Piepers & Drinkenburg, 2002).
Table 1. RTD moments, Pe and N values obtained for the 0,025 cm (D1) and 0,050 cm (D2) columns.
Geometry τ
m
(min) Pe
ADM
95% CI (Pe)
N
N-CSTR
95% CI (N) s
3
CBFe D1 0,073 36,22 6,75 9,09 0,06 0,0006 0,0110
CBFe D2 0,116 58,32 0,02 15,13 0,08 0,0009 0,0082
CPFe D1 0,097 24,31 0,04 8,11 0,05 0,0012 0,0112
CPFe D2 0,198 32,50 0,03 19,00 0,02 0,0020 0,0042
Table 2. Standard error and R
2
values obtained for the ADM and N-CSTR model tting.
Geometry SE
ADM
SE
N-CSTR
R
2
ADM
R
2
N-CSTR
CBFe D1 0,873 1,306 0,985 0,963
CBFe D2 3,027 2,550 0,815 0,898
CPFe D1 3,765 0,938 0,904 0,976
CPFe D2 0,751 0,514 0,950 0,983
Mean residence times showed the effect of geometry (particle size and shape, and column
diameter) on the hydrodynamic behavior. It was found that increases when increasing the column
diameter and that it is higher for the pellets than the beads. This result suggests that higher mean
residence times can be achieved when increasing bed tortuosity (cylinder vs. spherical particles),
leading to higher contact times and promoting pollutant conversion for the same column diameter.
However, when comparing the beads against the pellets for the same column diameter, Pe number
is consistently higher, suggesting lower axial dispersion. In this case, the more regular geometry of
beads allows a more even flow through the reactor. This behavior is also observed when increasing
the column diameter for the same particle geometry, possibly due to decreased wall effects.
BRICEÑO Y DURÁN: Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts...
10
3.4 Phenol degradation by catalytic ozonation
In the ozonation experiments, the three processes through which phenol disappears from
solution were evaluated independently for better comparing the catalysts performance (i.e., phenol
adsorption over the solid catalyst, phenol degradation by direct ozonation, and phenol degradation
by heterogeneous catalytic ozonation). For evaluating degradation only by direct ozonation, the
reactor was lled and operated with a non-catalytic packing of comparable size and shape as the
catalyst pellets and beads.
Phenol adsorption measurements for the CPFe pellets are shown in Figure 4. As can be seen,
phenol adsorption over the diatomite pellets was important with nearly 16 % drop in its concentration
after 8 hours. On the other hand, phenol adsorption on the CBFe beads was negligible and therefore
the data are not shown. Based on these results, for all the following ozonation runs, phenol solutions
were left in contact overnight (12 hours) with the catalysts, allowing adsorption equilibrium to be
reached prior to the reaction with ozone. In this way, it can be assumed that all phenol abatement
in the ozonation experiment can be attributed to either direct or catalytic ozonation.
Figure 4. Phenol adsorption on CPFe catalyst pellets in batch mode. Initial phenol
concentration = 387 mg/L with a 1:2 solid-liquid bulk volume ratio.
Error bars represent standard deviation (n=3).
Ozonation experiments were performed using four different congurations: spherical non-
catalytic packing, spherical catalytic packing, cylindrical non-catalytic packing (acrylic cylinders),
and cylindrical catalytic packing (iron-loaded diatomite pellets). All runs were performed under the
same flow conditions: 8,0 L/min gas and 0,3 L/min liquid owrates ( =0,0965 min for the cylindrical
packing, =0,0730 min for the spherical packing). From the results shown in Figure 5, it can be stated
that the presence of iron oxide on any packing leads to an improvement in phenol conversion: from
13.3 % to 19.7 % for CPFe and from 15.9 % to 19.5 % for CBFe. This result demonstrates a better
performance of catalytic over direct ozonation (48,1 % for the case of CPFe). Moreover, when both
particles containing iron (CPFe and CBFe) are compared, no signicant difference was observed;
this suggests that the presence of iron oxide in the particle plays a greater role than its geometry.
Ingeniería 30 (2): 1-13, julio-diciembre, 2020. ISSN: 2215-2652. San José, Costa Rica DOI 10.15517/ri.v30i2.39236
11
Figure 5. Phenol conversion under steady state operation for different column packings.
Liquid flowrate = 0,3 L/min, gas flowrate = 8,0 L/min, initial phenol concentration = 431 mg/L,
diameter column = 0,025 m. Error bars represent standard deviation (n=3).
Figure 6. Dependence of phenol overall mass transfer coefcient with liquid mass flux for the 0,025
m diameter column packed with CPFe and CBFe. Error bars represent standard deviation (n=3).
In order to investigate the effect of mass transfer on the reactor performance, experiments
were carried out varying the liquid flowrate. No signicant change in phenol conversion was
observed under different flowrates. This result evidences that mass transfer effects are important
in this system; therefore, the overall mass transfer coefcient from bulk liquid to the particle sur-
face (k
vL
) was determined for better analysis. This overall coefcient was calculated as presented
by Fogler (2008), assuming pseudo rst order reaction for the catalytic ozonation and considering
BRICEÑO Y DURÁN: Heterogeneous Catalytic Ozonation of Phenol over Iron-based Catalysts...
12
that the liquid phase was saturated with ozone gas throughout the column. Figure 6 shows that for
the porous catalyst (diatomite pellets) an increase in flowrate leads to an increase in the overall
mass transfer coefcient. For the glass beads, on the other hand, the change of k
vL
with the liquid
flowrate is almost negligible. It is theorized that pulsing flows induce higher internal mass transfer
in porous media due to capillary pressure effects. Such phenomena would not be present in the case
of non-porous particles. These results suggest that, under certain flow conditions, the performance
of the reactor could be signicantly improved when using porous pellets.
4. CONCLUSIONS
In this contribution, the results on the use of a trickle bed reactor for the heterogeneous catalytic
ozonation of phenol were shown. It was found that:
• Fluid dynamics studies showed that the reactor could operate under trickle and pulsing flow
regimes.
• RTD data showed that under pulsing flow regime, axial dispersion was low when compared
with other reactors. Experimental data were better described by the n-CSTR model.
• The synthetized Fe-diatomite catalyst showed important phenol adsorption (16 %).
•
Trickle bed reactor effectively decreases phenol concentration (19,7 % conversion with a
liquid-phase space time of just 6 seconds).
• The use of the Fe-diatomite catalyst enhanced the reactor performance by 48 % when com-
pared to direct ozonation.
ACKNOWLEDGMENTS
The authors would like to acknowledge the help and support from: Research Center of Electro-
chemistry and Chemical Energy (CELEQ) and Research Vice-rectory of the University of Costa Rica.
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