1 The role of reaction kinetics and mass transfer on the selective 1 catalytic reduction of NO with NH 3 in monolithic reactors 2 3 Emilio Muñoz, Pablo Marín, Salvador Ordóñez*, Fernando V. Díez 4 Department of Chemical and Environmental Engineering, University of Oviedo, Facultad de 5 Química, Julián Clavería 8, 33006 Oviedo, SPAIN 6 7 Abstract 8 Background 9 Environmental regulations are moving to a tighter control of NOx emissions produced at 10 both stationary and mobile sources. Selective catalytic reduction (SCR) of NOx with NH 3 is an 11 efficient treatment technique capable of operating at high gas flow rates (e.g. using 12 monolithic catalysts) and a wide range of NO concentrations. The aim of this work is to 13 provide guidelines for designing this kind of reactors taking into account both intrinsic 14 kinetics and mass transfer. 15 Results 16 The experiments have been done in lab-scale (0.5 g) and bench-scale (430 g) reactors 17 operating at different conditions: temperature (150-320ºC), space velocity (WHSV 5360- 18 16100 mol h -1 kg cat -1 ), oxygen concentration (0-21%) and NH 3 /NO ratio (0.2-1.2). 19 Temperature has a great influence in the reaction rate, and at least 300ºC is required at a 20 WHSV of 16100 mol h -1 kg cat -1 . Oxygen is required in the feed because participates as 21 reactant in the SCR. 22 Conclusions 23 Data obtained in the lab-scale reactor in the absence of mass transfer limitations have been 24 used to fit an intrinsic kinetic model of the SCR reaction. A more complex model has been 25
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1
The role of reaction kinetics and mass transfer on the selective 1
catalytic reduction of NO with NH3 in monolithic reactors 2
3
Emilio Muñoz, Pablo Marín, Salvador Ordóñez*, Fernando V. Díez 4
Department of Chemical and Environmental Engineering, University of Oviedo, Facultad de 5
Química, Julián Clavería 8, 33006 Oviedo, SPAIN 6
7
Abstract 8
Background 9
Environmental regulations are moving to a tighter control of NOx emissions produced at 10
both stationary and mobile sources. Selective catalytic reduction (SCR) of NOx with NH3 is an 11
efficient treatment technique capable of operating at high gas flow rates (e.g. using 12
monolithic catalysts) and a wide range of NO concentrations. The aim of this work is to 13
provide guidelines for designing this kind of reactors taking into account both intrinsic 14
kinetics and mass transfer. 15
Results 16
The experiments have been done in lab-scale (0.5 g) and bench-scale (430 g) reactors 17
operating at different conditions: temperature (150-320ºC), space velocity (WHSV 5360-18
16100 mol h-1 kgcat-1), oxygen concentration (0-21%) and NH3/NO ratio (0.2-1.2). 19
Temperature has a great influence in the reaction rate, and at least 300ºC is required at a 20
WHSV of 16100 mol h-1 kgcat-1. Oxygen is required in the feed because participates as 21
reactant in the SCR. 22
Conclusions 23
Data obtained in the lab-scale reactor in the absence of mass transfer limitations have been 24
used to fit an intrinsic kinetic model of the SCR reaction. A more complex model has been 25
2
used for the bench-scale reactor accounting for reaction kinetic and mass transfer (internal 1
effectiveness factor was determined) in the monolithic catalyst. 2
3
Key words: 4
NOx emissions, air pollution control, monolithic catalyst, transient methods, mass transfer in 5
monolithic reactors. 6
7
Notation 8
List of symbols 9
a specific surface area (m2 mbed-3) 10
c gas mole concentration (mol m-3) 11
Dh hydraulic diameter (m) 12
Dim mixture molecular diffusion coefficient (m2 s-1) 13
Diz effective axial dispersion coefficient (m2 s-1) 14
Ea activation energy (J mol-1) 15
k kinetic constant (m3 mol-1 s-1 or s-1 ) 16
Kg mass transfer coefficient (m s-1) 17
n catalyst ammonia adsorption capacity (mol kgcat-1) 18
Qg gas flow rate (m3 s-1) 19
r reaction rate (mol kgcat-1 s-1) 20
Sh Sherwood number (-) 21
t time (s) 22
T temperature (K) 23
tsw switching time (s) 24
v gas interstitial velocity (m s-1) 25
w relative weight of catalyst (-) 26
W total weight of catalyst (kg) 27
z spatial coordinate (m) 28
bed porosity (-) 29
int internal effectiveness factor 30
solid fraction of adsorbed specie (-) 31
c catalyst density (kg m-3) 32
33
Sub indexes and super indexes 34
in inlet 35
3
ads adsorption 1
red reduction 2
s catalyst surface 3
4
Acronyms 5
BAT best available techniques 6
FIC flow indicator and controller 7
PI pressure indicator 8
SCR selective catalytic reduction 9
SNCR selective non-catalytic reduction 10
TIC temperature indicator and controller 11
WHSV gas-hourly space velocity (mol h-1 kgcat-1) 12
13
14
15
4
Introduction 1
Nitric oxides (NOx), including nitrogen oxide (NO), nitrogen dioxide (NO2) and nitrous oxide 2
(N2O), are emitted to the atmosphere mainly as a result of transportation and industrial 3
processes. They are among the most dangerous air pollutants, as they contribute to the 4
greenhouse effect, participate in photochemical reactions that cause acid rain and the 5
formation of troposphere ozone ('photochemical smog'), and have an important role in lakes 6
and rivers eutrophication. In addition, they are harmful to human health, as they can 7
damage the respiratory system.1-3 8
Due to their quantitative importance and harmful nature, NOx abatement has been studied 9
widely during the last decades,4-7 and their emissions in industrialized countries are 10
restrictively regulated, e.g. in the UE by Directive 2001/81/EC. 11
NOx emissions can be reduced by means of primary or secondary measures. Primary 12
measures decrease NOx formation, e.g. by controlling temperature, excess air or mixing in 13
combustion process, while secondary emissions separate or destroy NOx from effluents. 14
Selective Catalytic Reduction (SCR) and Selective Non Catalytic Reduction (SNCR) are 15
secondary processes considered in the EU as Best Available Techniques (BAT) for treating 16
NOx in emissions from large stationary sources, such as large combustion plants.8 NOx 17
selective reduction processes are based on the reaction of NOx with a reducing agent 18
(ammonia or urea), forming molecular nitrogen and water. When performed in the presence 19
of a catalyst (SCR), the process takes place at lower temperature, is able of working with 20
load variations or variable fuel quality, and is substantially more efficient than when the 21
process is non-catalytic (SNCR). On the other hand, the inversion required for SNCR is lower. 22
The main reactions taking place for ammonia, the most common SCR reducing agent, are: 23
4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O (1) 24
2 NH3 + NO + NO2 → 2 N2 + 3 H2O (2) 25
8 NH3 + 6 NO2 → 7 N2 + 12 H2O (3) 26
5
Since NO/NO2 ratio in NOx emissions is larger than 10, the first reaction, called 'standard 1
SCR', is largely the most important. When temperature is low, the third reaction, called 'fast-2
SCR', should be also taken into consideration. 3
Different catalysts have been used for ammonia SCR, including noble metals, metal oxides 4
and zeolites,3, 9-11 operating temperature being an important factor for catalyst selection. For 5
high temperatures (345-590ºC), zeolites are more durable, active and SO2 tolerant.12 6
Operating at lower temperature (typically 150-300ºC), allows saving fuel used for re-heating 7
the flue gas in some installations. Noble metals have shown better performance at low 8
temperature, but are expensive, the temperature operation range is narrow and present low 9
sulphur tolerance. Pd and Ag are both noble metals that present good performance for the 10
SCR reaction.12, 13 Pd is usually supported on perovskites or zirconia. Ag supported on Al2O3 11
shows high efficiency, particularly when using hydrocarbons. 12
Metal oxide catalysts (oxides of copper, iron or vanadium, either unsupported or supported 13
on alumina, silica or titania) 2, 8 are cheaper, and work well in the typical temperature range 14
of industrial applications. One of the most common industrial SCR catalyst types consists on 15
vanadium oxide, promoted by MoO3 or WO3, and supported on titania (in the anatase form). 16
WO3-V2O5/TiO2 catalysts show tolerance to SO2 poisoning and provide high NOx conversion, 17
the TiO2 support providing high surface area and increased catalyst activity.8 In the last 18
years, the research in this field has focused in the development of vanadium-free catalysts 19
with high activity, selectivity and stability.12, 13 Fe containing mixed oxides and Fe-exchanges 20
zeolites were found to present high activity. The selection of an adequate support is critical 21
for the activity and stability of the catalyst.12, 14 Under rich oxygen conditions, Cu/ZSM-5 was 22
found to present reduction capability using hydrocarbons.13, 15 23
SCR catalysts are generally used as structured beds, as this type of bed produces lower 24
pressure drop through the reactor, helps to keep process conditions uniform along the bed, 25
and presents higher resistance to attrition and lower tendency to fly ash plugging. A recent 26
review about the catalyst and reactor configuration is presented by Cheng et al.16 27
Several mechanisms have been proposed for ammonia SCR on vanadia catalysts, including 28
Langmuir-Hinsewood, Elay-Rideal and Mars Van Krevelen.17-23 Most researchers have found 29
that Elay-Rideal mechanisms fit better experimental results. Such models suppose that in 30
6
one step of the reaction mechanism, NO from the gas phase reacts with chemisorbed 1
ammonia, but published works differ on the nature of the involved active sites and reaction 2
intermediates.24-26 Topsoe et al.27, 28 observed in FTIR studies that, while at reaction 3
conditions NO adsorbed is negligible, ammonia adsorbs in large amounts on both Lewis and 4
Bronsted acid sites. Based on these studies, they proposed a mechanism according to which 5
ammonia adsorbs in equilibrium on V5+- OH sites: 6
NH3 + V5+- OH V5+- ONH4 (4) 7
Adsorbed ammonia is activated by adjacent V5+=O sites: 8
Table 1 Model for the lab-scale fixed-bed reactor. 4
Table 2 Model for the bench-scale fixed-bed reactor. 5
Table 3 Parameters of the kinetic model obtained by fitting of experimental data of 6
the lab-scale fixed-bed reactor. 7
8
9
10
11
12
13
14
32
Table 1 1
2
Gas phase mass balances Boundary conditions
Solid phase mass balance Boundary conditions
Kinetic model
A = NO, B = NH3 3
4
5
33
Table 2 1
2
Gas phase mass balances Boundary conditions
Solid phase mass balance Boundary conditions
Interphase equations
Kinetic equations
A = NO, B = NH3 3
4
5
6
34
Table 3 1
2
Kinetic parameters
0.113 mol NH3 kgcat-1
2.75 m3 mol-1 s-1
0.0278 s-1 at 280ºC
9.8 kJ mol-1
260 m3 mol-1 s-1 at 280ºC
92.8 kJ mol-1
3
4
5
35
1
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