Chemical Engineering Journal xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Kinetic study for burning regeneration of coked MFI-type zeolite and
numerical modeling for regeneration process in a fixed-bed reactor
Yuta Nakasaka a,⇑, Teruoki Tago a, Hiroki Konno a, Akihiro Okabe b, Takao Masuda a
a
b
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
Catalysis Science Laboratory, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan
h i g h l i g h t s
" Kinetic analysis of burning regeneration of coked MFI-type zeolite obtained by catalytic cracking of n-hexane was studied using multiple-reaction
model.
" Reaction rates of carbon and hydrogen combustion in coke were correlated using Arrhenius equation.
" Regeneration of coked MFI-type zeolite in the fixed bed reactor was numerically simulated using the reaction rates obtained from the kinetic analysis.
" The temperature near the outlet showed higher than other location in the reactor during regeneration.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Coke
MFI-type zeolite
Regeneration
Kinetic
a b s t r a c t
Kinetic analysis of burning regeneration of coked MFI-type zeolite obtained by catalytic cracking of nhexane was studied using a multiple-reaction model. Hydrogen contained in the coke was oxidized faster
than carbon, and subsequently carbon remaining after the hydrogen combustion was gradually oxidized.
Reaction rates of carbon and hydrogen correlated with the Arrhenius equation; and activation energies for
the combustion of carbon and hydrogen were 156 kJ/mol and 140 kJ/mol, respectively, regardless of the
coke loading. Regeneration of coked MFI-type zeolite in the fixed-bed reactor was simulated numerically
using reaction rates obtained from the kinetic analysis. The numerical result for changes in gas composition with time at the reactor outlet agreed well with the experimental results. Axial distribution of the
temperature and concentration of water vapor in the catalyst bed was obtained numerically and results
showed that the temperature and water vapor near the outlet were higher than other location in the
reactor.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
The demand for light olefins, such as ethylene and propylene, is
increasing. Light olefins are produced mainly by steam cracking of
light naphtha. Because the molar ratio of propylene to ethylene in
the steam cracking products depends on the thermodynamics,
making a major change in the product ratio is difficult. In addition,
steam cracking of light naphtha requires a large amount of energy.
Therefore, development of a new process for the selective production of propylene has been desired. Catalytic cracking of light
naphtha over zeolite is a promising technique. Some researchers
have reported catalytic cracking of light naphtha over zeolite for
selective propylene production. Inagaki et al. obtained 45–50% propylene selectivity through catalytic cracking of hexane over MCM68 zeolite [1]. Kubo et al. reported a 59.7 C-mol% ethylene + pro⇑ Corresponding author. Tel./fax: +81 11 706 6552.
E-mail address: [email protected] (Y. Nakasaka).
pylene yield with a propylene/ethylene ratio of ca. 0.72 through
catalytic cracking of n-heptane over HZSM-5 zeolite [2]. Mochizuki
et al. reported that the crystal size of HZSM-5 zeolite influenced
the catalytic cracking of n-hexane [3]. Konno et al. obtained
approximately 35 C-mol% of light olefin by catalytic cracking of
n-hexane at 823 K using MFI-type zeolite with a Si/Al ratio of
150, and reported that nano-sized MFI-type zeolite exhibited stable and high activity with low coke formation [4].
In contrast, during catalytic cracking of light naphtha over zeolite, coke is formed on the zeolite catalyst and thus deactivates the
catalyst. The combustion of coke is usually conducted to regenerate the deactivated catalyst. However, burning regeneration reactions are exothermic and the temperature of the catalyst bed
rises steeply during the reactions. In addition, water is produced
during the regeneration reaction due to hydrogen contained within
the coke. Dealumination of the zeolite framework occurs easily
under a hydrothermal atmosphere at high temperatures [5,6].
Therefore, kinetic information about the burning regeneration is
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.138
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
2
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Nomenclature
C 0c
concentration of carbon at any radial position in the
coke particle (mol m3)
initial concentration of carbon in the coke particle
C 0C0
(mol m3)
C 0H0
initial concentration of hydrogen in the coke particle
(mol m3)
concentration of component i in the bulk flow
Ci,b
(mol m3)
C i;p
concentration of component i in the catalyst pellet
(mol m3)
specific heat capacity of the catalyst pellet (J kg1)
cp
cpf
fluid specific heat capacity (J kg1)
C p;j
average molecular heat capacity of component j (j: H2,
O2, H2O, CO2, and CO) (J mol1 K1)
effective diffusivity of gases in the catalyst pellet
Deff
(m2 s1)
e
emissivity (blackness) of the coked catalyst pellet ()
h
heat transfer coefficient between the gas and pellet
(W m2 K1)
DHC
overall heat of reaction for the oxidation of carbon to
produce CO and CO2 (J mol1)
DHf ;CO ; DHf ;CO2 ; DHf ;H2 O standard heat of formation of the CO, CO2
and H2O at 298 K (J mol1)
DHH
heat of reaction for the oxidation of hydrogen (J mol1)
hw
heat transfer coefficient between the gas and reactor
wall (W m2 K1)
i
gas component (O2, CO2 + CO, H2O) ()
j
gas component (H2, O2, H2O, CO2, and CO) ()
rate constant for the oxidation of carbon to CO and CO2
kC
(Pa1 s1)
kC0
frequency factor in the rate constant of carbon
(Pa1 s1)
rate constant for the oxidation of hydrogen (m Pa1 s1)
kHs
kHs 0
frequency factor in the rate constant of hydrogen (m
Pa1 s1)
k1
rate constant for the oxidation of carbon to CO
(Pa1 s1)
k2
rate constant for the oxidation of carbon to CO2
(Pa1 s1)
important for design of the regeneration process of coke-deposited
catalysts obtained by catalytic cracking of light naphtha. The main
object of this work was to analyze the kinetic results of burning
regeneration of coked MFI-type zeolite obtained by catalytic cracking of n-hexane. Using the kinetic results, distributions of the temperature and gases produced (such as H2O and CO2) in a fixed-bed
reactor, as well as changes in these distributions during regeneration of the coked zeolite catalyst were obtained by numerical
simulation.
2. Experimental
2.1. Coked MFI-type zeolite
Table 1 lists the properties of coked MFI-type zeolites, which
were obtained by catalytic cracking of n-hexane using a fixedbed flow reactor. Temperatures employed for catalytic cracking
of n-hexane were 823 and 923 K. The mass ratio of coke to the
zeolite catalyst varied between 0.4% and 45%, depending on the
reaction time and reaction conditions. The H/C ratios of coke
Nc
Np
pA
pfeed
i
R0
r0
rCc
r0Cc
Rg
rH
rHc
r0Hc
ri
Tb
Tfeed
Tp
Tw
t
u
v
wc
XC
XH
Xt
number of coke particle per unit volume of the catalyst
pellet (m3)
number of pellet per unit volume of the catalyst bed
(m3)
partial pressure of oxygen (Pa)
partial pressure of component i in feed gas (Pa)
radius of the catalyst pellet (m)
radius of the coke particle (m)
reaction rate of carbon based on the volume of coke
(mol m3 s)
reaction rate of carbon based on a single coke particle
(mol s1)
gas constant (J mol1 K1)
radius of unreacted coke (m)
reaction rate of carbon based on the surface area of the
reaction interface (mol m2 s1)
reaction rate of hydrogen based on a single coke particle
(mol s1)
reaction rate of component i based on a single coke particle (mol s1)
temperature of the bulk flow in bed (K)
temperature of the feed gas (K)
temperature of the catalyst pellet (K)
temperature of the reactor wall (K)
time at which unreacted coke disappeared (s)
dimensionless time ()
gas velocity in the fixed-bed reactor (m s1)
mass ratio of coke to uncoked zeolite ()
overall conversion of carbon in the catalyst pellet ()
overall conversion of hydrogen in the catalyst pellet ()
overall conversion of coke in the catalyst pellet ()
Greek letters
a
kc0pAt
k
k2/(k1 + k2) ()
qf
fluid density (kg m3)
qp
apparent density of the catalyst pellet (kg m3)
qc
density of coke (kg m3)
were measured using a Micro Corder instrument (JM10, J-Science
Lab).
2.2. Reaction rate of carbon and hydrogen combustion in coke
Transient changes in the overall conversion (Xt) for coked MFItype zeolite were calculated from the decrease in weight of the
zeolite catalyst with coke under a stream of air diluted with nitrogen. The weight change in catalyst with coke was measured by
thermo gravimetric analysis (Micro-Thermobalance (TGA-50), Shimadzu). Approximately 10 mg of coked MFI-type zeolite were
placed in a quartz cell, which was suspended into the apparatus
and nitrogen was fed into the apparatus. The sample was subsequently heated to the experimental temperature. After reaching
the experimental temperature, air diluted with nitrogen was fed
into the apparatus and the weight change in coked MFI-type zeolite was measured. The reaction temperatures for burning regeneration ranged from 873 to 923 K, partial pressure of the oxygen in
the stream ranged from 5 to 21 kPa and gas flow rates ranged from
30 mL/min to 90 mL/min.
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
3
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 1
Properties of coked MFI-type zeolites.
Sample
Sample
Sample
Sample
Sample
Sample
a
1
2
3
4
5
6
Si/Al ratio (-)
Crystal size (nm)
Mass ratio of coke (wt%)
Temperature (K)a
WHSV (h1)a
50
50
50
110
110
150
150–200
150–200
1500
300
300
300
0.36
1.56
7.5
0.4
45
15
823
823
823
923
923
923
2.0
2.0
2.0
10
10
10
n-Hexane cracking conditions for obtaining coked MFI-type zeolite.
Fig. 1. Model for the oxidation of carbon and hydrogen in coke [7,8].
Kinetic analyses of coke combustion were investigated
using the multiple reaction model developed in our previous
study [7], where hydrogen in the coke is oxidized faster than
carbon. The reaction interface (rH) appears between the unreacted-core at the same composition as the initial coke (0 rH)
and the carbon remaining after hydrogen combustion (rH), as
shown in Fig. 1. rH (radius of unreacted coke) decreased from
r0 (radius of coke particle) to 0 as the reaction progressed. The
carbon remaining after hydrogen combustion, outside of the
reaction interface, was gradually oxidized. The stoichiometry
and reaction kinetics of the oxidations of hydrogen and carbon
are:
Stoichiometry
[Carbon]
ð1Þ
C þ O2 ! CO2
ð2Þ
ð3Þ
Reaction kinetics
[Carbon]
[Hydrogen]
Here, rCc and rHc are the reaction rates of carbon and hydrogen
in coke, respectively. In addition, kC and kHs are rate constant for
oxidation of carbon and hydrogen, respectively. k1 and k2 are rate
constant for oxidation of carbon to CO and CO2, respectively. C 0C
and C 0H0 indicate concentration of carbon and initial concentration
of hydrogen in coke, respectively, and pA is the partial pressure of
oxygen. The reaction rate for the carbon and hydrogen combustion
in coke was expressed as first order with respect to the oxygen partial pressure as shown in Eqs. (4) and (5).
A Transient change in the carbon (XC) and hydrogen (XH) conversion is expressed as [7]:
dX H
¼ 3ð1 X H Þ2=3
du
ð6Þ
u ¼ t=t and t ¼ r0 =kHs pA
ð7Þ
where
[Hydrogen]
rCc ¼ ðk1 þ k2 ÞC 0CpA ¼ kC C 0CpA
ð5Þ
[Hydrogen]
C þ 1=2O2 ! CO
H þ 1=4O2 ! 1=2H2 O
r Hc ¼ kHs C 0H0pA
[Carbon]
dX C
¼ aðX H X C Þ
du
ð4Þ
ð8Þ
where
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
4
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
a ¼ kC pA t ¼ kC r0 =kHs
ð9Þ
Here, t represents the time at which unreacted coke disappeared and u is a dimensionless time value.
Moreover, the transient change in the overall conversion for the
coked catalyst is represented in terms of XH and XC as [7]:
1 Xt ¼
n
12
ð1 X H Þ þ
ð1 X C Þ
12 þ n
12 þ n
ð10Þ
where Xt is the overall fraction of unreacted coke, and n is the molar
ratio of hydrogen to carbon in coke. By parameter fitting the overall
conversion of coke using the theoretical equations, rate constants of
carbon and hydrogen combustion contained in coke were obtained.
The details have been published previously [7].
2.3. Regeneration of coked MFI-type zeolite using fixed-bed reactor
The experimental regeneration reaction of coked MFI-type zeolite was conducted under the air flow after n-hexane cracking over
MFI-type zeolite at 923 K. The initial temperature of the fixed-bed
reactor for regeneration was 773 K. Inner diameter of the reactor
was 1 102 m, pressure of air flow was 0.1 MPa and air flow rate
was 200 mL/min. Outlet gas composition was detected by Infrared
Gas Analyzer (OGT-7000, Shimadzu). The gas analyzer can measure
only CO, CO2, O2 and CH4. Therefore, H2O was not measured in this
experiment. The average amount of coke loading on the zeolite was
76 wt%, calculated using the concentrations of CO2 and CO at the
outlet of the reactor.
Fig. 3. Transient change in the overall fraction of unreacted coke at different
temperatures.
3. Results and discussion
3.1. H/C ratio in coke
Coke is composed of carbon and hydrogen. The molar ratios of
hydrogen to carbon (H/C ratio) in the coke used for the kinetic
analyses of coke combustion depend on the coke loading [7]. In
addition, the H/C ratio of coke depends on the raw material used
in the reaction. Therefore, measurement of the H/C ratio of the
coke formed on MFI-type zeolite by catalytic cracking of n-hexane
is required for the kinetic analysis of coke combustion. Fig. 2 shows
the relation between the H/C ratio of coke and the coke loading.
The H/C ratio of coke decreased with increasing coke loading on
MFI-type zeolite.
Fig. 2. Relation between the H/C ratios of coke and the mass ratio of coke on zeolite.
Fig. 4. Effect of the gas flow rate in the bulk phase on the rate constant of carbon
and hydrogen combustion in coke.
3.2. Rate constants of carbon and hydrogen combustion in coke
Fig. 3 shows the transient change in the overall fraction of unreacted coke at different temperatures using MFI-type zeolite with
15 wt% coke. The burning regeneration time decreased with an increase in temperature. Calculated curves agreed well with the
experimental data at each temperature. In the initial stage of coke
combustion, the decrease in the overall fraction of unreacted coke
is relatively small. Coke is composed of carbon and hydrogen, but
the mass fraction of hydrogen in coke is small. Because hydrogen
in coke is oxidized faster than carbon, the decrease in the overall
fraction of coke in the initial stage is small.
Hashimoto et al. [9] reported that the combustion rate is constant and the overall fraction of unreacted coke (1 Xt) decreases
monotonously with time when most of the coke is deposited on
the outer surface of MFI-type zeolite. In contrast, the combustion
rate changes in the middle of the reaction and the change in the
overall fraction of unreacted coke (1 Xt) with time become a reverse-S-shape when coke deposited mainly on the surface of the
micro pores within the zeolite crystal, because coke in the straight
pores is preferentially oxidized, followed by oxidation of the coke
in the zig-zag pores. Transient changes in the overall fraction of
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Fig. 5. Effect of oxygen partial pressure on the rate constant of carbon and
hydrogen combustion in coke.
unreacted coke obtained in this study monotonously decreased
with time. Therefore, most of the coke is deposited on the outer
surface of MFI-type zeolite during catalytic cracking of n-hexane.
Oxygen molecules in the bulk phase diffuse across the laminar
film around the interface of a crystal and diffuse within the
macro-pores of the catalyst onto coke particles. Therefore, mass
transfer resistance affects the value of the rate constant obtained.
Fig. 4 shows the effect of gas flow rate on the rate constants for carbon and hydrogen combustion in coke. At a gas flow rate of 30 mL/
min, the rate constants for carbon and hydrogen were smaller than
those at gas flow rates greater than 40 mL/min. Mass transfer resistances in the laminar film and macro-pores of the crystals affect
the overall combustion rate of coke when the gas flow rate is less
than 30 mL/min. In contrast, at gas flow rates greater than 40 mL/
min, rate constants for carbon and hydrogen remained constant,
indicating that the mass transfer resistance was negligibly small.
The rate constants of carbon and hydrogen were lower at a gas flow
rate of 30 mL/min, indicating that the mass transfer resistances affect the overall combustion rate of carbon and hydrogen in coke.
Accordingly, a diluted air flow rate at 50 mL/min was used for
kinetic analyses in this study.
Fig. 5 shows the effect of the oxygen partial pressure on the rate
constants of carbon and hydrogen combustion in coke. The carbon
(a)
5
Fig. 7. Effect of the coke loading on the frequency factors in the rate constants of
carbon (kC0) and hydrogen (kHs0/r0).
and hydrogen rate constants were nearly the same regardless of
the oxygen partial pressure, indicating that the reaction rate for
carbon and hydrogen combustion is first order with respect to
the oxygen partial pressure, which supports the validity of Eqs.
(4) and (5). Figs. 4 and 5 show that the diffusion rate of the oxygen
from the bulk phase to coke is higher than the reaction rate constant of carbon and hydrogen combustion at a high diluted air flow,
and the oxygen concentration gradient between the outer zone of
coke and the bulk stream was small. Accordingly, the change in
oxygen concentration in the laminar film was not taken into account in the numerical simulation (shown in 3.3).
Fig. 6 shows Arrhenius plots of the rate constants of carbon and
hydrogen combustion in coke obtained from burning regeneration
using coked MFI-type zeolite. For comparison, the rate constants of
carbon and hydrogen for the combustion of coked SiO2–Al2O3 obtained by catalytic cracking of cumene [7] are also shown. The rate
constants of carbon combustion in coke showed a linear correlation for each coked zeolite. In contrast, the rate constants of hydrogen combustion in coke decreased with increasing coke loading.
The activation energies of the rate constants for carbon and hydrogen were 156 kJ/mol and 140 kJ/mol, respectively, and the activation energies obtained in this study were nearly equal to the
(b)
Fig. 6. Arrhenius plots of the rate constants of (a) carbon and (b) hydrogen in coke.
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
6
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
values of coked SiO2–Al2O3. The activation energy of carbon
combustion in coke has been reported to be between 109 kJ/mol
and 207 kJ/mol [10]. The activation energies obtained in this study
were nearly same as the reported values [7], regardless of the coke
loading. In Fig. 6b, the activation energies of reaction constant for
hydrogen obtained using the coked MFI-type zeolite with low coke
loading are slightly smaller than that with high coke loading,
which have source in the high ratio of aliphatic chains in the coke.
Fig. 7 shows the effect of the coke loading on the frequency factors of the rate constants of carbon and hydrogen. The frequency
factors for carbon were constant regardless of the coke loading.
In contrast, the frequency factors of hydrogen decreased with
increasing coke loading. It is considered that the rate constant for
hydrogen (kHs0/r0) depended on the coke particle size, because
the rate constant for hydrogen is expressed in terms of the coke
particle radius (r0).
Table 2
Numerical simulation conditions for burning regeneration of coked MFI-type zeolite.
Length of bed (L)
Pellet diameter (2R0)
Inner diameter of reactor (D)
Pressure of air for regeneration (p)
Specific heat capacity of the catalyst pellet (cp)
Specific heat capacity of fluid (cpf)
Emissivity of the coked pellet (e)
Activation energy of combustion of carbon (EC)
Activation energy of combustion of hydrogen (EH)
Apparent density of catalyst pellet (qp)
Apparent density of coke particle (qC)
Fluid density (qf)
Mass ratio of coke to the catalyst
Weight of catalyst (w)
Wall temperature (Tw)
2.5 102 m
1.6 103 m
1.0 102 m
0.1 MPa
1.5 kJ kg1 K1
1.0 kJ kg1 K1
0.9
156 103 J mol1
140 103 J mol1
900 kg m3
1600 kg m3
1.0 kg m3
10 76 wt%
0.75 g
773 K
3.3. Numerical simulation of coked MFI-type zeolite regeneration in a
fixed-bed
Assuming that a fixed-bed reactor is used for catalytic cracking
of light naphtha, coked catalysts are regenerated in the same
fixed-bed with air flow subsequent to the cracking. In burning
regeneration of coked catalyst in the fixed bed reactor, it is considered that the temperature of the catalyst bed rises rapidly because the combustion of coke is exothermic. Therefore, the
temperature and distribution of gas produced in the catalyst
bed must be predicted during regeneration of coked catalyst for
the process design. Using the kinetic results of the regeneration
reaction for coked MFI-type zeolite, changes in the temperature
in the catalyst bed and gas composition during regeneration is
obtained numerically using gPROMS (Process System Enterprise
Inc.) and the simulated result is compared with the experimental
result.
In the numerical simulation, the distribution of gas concentration in the radius direction of reactor was ignored because the
gas flow rate in the reactor was sufficiently large. Moreover, the
mass balance inside the zeolite pores was not considered in the
simulation because it is considered that the coke was deposited
mainly on the outer surface of the zeolite crystals during catalytic
cracking of n-hexane.
The mass balance equation for component i (i: O2, CO + CO2, and
H2O) in the catalyst bed and boundary conditions can be represented as:
@C i;b
@C i;b
@C i;p ¼ v
4pR20 Np Deff @t
@z
@r z¼z;r¼R0
pfi eed
Rg T
f eed
¼ C i;b
reactor entrance
ð11Þ
ð12Þ
Here, Ci,b and Ci,p indicate concentration of component i in bulk
flow and pellet, respectively. R0, v, Np and Deff indicate radius of catalyst pellet, gas velocity, number of pellet per unit volume of catalyst bed and effective diffusivity of gases in the catalyst pellet,
respectively. In addition, pfeed
, Rg and Tfeed indicate the partial presi
sure of component i in the feed gas, the gas constant, and the temperature of the feed gas, respectively.
The mass balance equation for component i (i: O2, CO + CO2, and
H2O) in the catalyst pellet can be represented as:
@C i;p Deff @
@C i;p
r2
¼ 2 NC ri
@r
@t
r
@r
ð13Þ
C i;p ¼ C i;b
ð14Þ
pellet surface
Fig. 8. (a) Simulated result and (b) experimental result for coked MFI-type zeolite
regeneration in a fixed-bed reactor (coke: 76 wt%).
@C i;p
¼ 0 center of pellet
@r
ð15Þ
Here, ri is the reaction rate of component i and Nc indicates the
number of coke particle per unit volume of catalyst pellet. The Nc
was calculated as:
4
3
qp wC ¼ pr30 qC NC
ð16Þ
where qp, qc and wc are apparent density of pellet, density of coke
and mass ratio of coke on uncoked zeolite, respectively.
The energy balance equation in the catalyst bed and boundary
conditions can be represented as:
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
7
Fig. 9. Axial distribution of (a) temperature and (b) water vapor in the catalyst bed (coke: 76 wt%).
4 3
@T
pR q c p p ¼
3 0 p @t
Z
0
R0
fðDHH Þ ðr0Hc Þ þ ðDHC Þ
ðr 0Cc ÞgNC 4pr 2 dr 4pR0 fhðT p T b Þ
þ 5:674 108 eðT 4p T 4w Þg
ð19Þ
where cp, DHH and DHC indicate the specific heat capacity of the
catalyst pellet, the heat of reaction for the oxidation of atomic
hydrogen and the overall heat of reaction for the oxidation of carbon to produce CO and CO2, respectively. The reaction heats for
hydrogen and carbon in the coke were calculated from following
equations:
[Hydrogen]
(b)
(a)
DHH ¼ 0:5fDHf ;H2 O þ ðT 298:2ÞðC p;H2 O C p;H2 0:5C p;O2 Þg
ð20Þ
[Carbon]
DHC ¼ kDHf ;CO2 þ ð1 kÞDHf ;CO þ ðT 298:2ÞðC p;CO2
þ ð1 kÞC p;CO C p;C 0:5ð1 þ kÞC p;O2 Þ
ð21Þ
DHf ;CO2 ; DHf ;CO ; DHf ;H2 O
where
are the standard heat of formation of
the CO2, CO, and H2O at 298 K and the values were -393.7, -110.6,
and 241.8 kJ/mol, respectively. C p;j is the average molecular heat
capacity of component j (j: H2, O2, H2O, CO2, and CO) over the temperature range from 298 K to the pellet temperature. In addition, k
in Eq. (21) can be expressed as follows [8]:
Fig. 10. Effect of the flow rate on the changes in (a) temperature and (b) conversion
of carbon in coke at reactor outlet (coke: 76 wt%).
qf cpf
@T b
@T 2hw
¼ qf cpf v
ðT b T W Þ þ 4pR0 Np fhðT p
@z
@t
R
T b Þ þ 5:674 108 eðT 4p T 4w Þg
T ¼ T feed
reactor entrance
ð17Þ
ð18Þ
Here, qf, cpf, hw, h and e indicate fluid density, fluid specific heat
capacity, heat transfer coefficient between the gas and the reactor
wall, heat transfer coefficient between the gas and the pellet and
emissivity (blackness) of the coked catalyst pellet, respectively.
In addition, Tb, Tp and Tw indicate the temperature of bulk flow, pellet, and reactor wall, respectively.
The energy balance equation in the catalyst pellet can be represented as [8]:
Fig. 11. Effect of the coke loading on the maximum temperature of the bulk flow of
bed during regeneration and the regeneration time up to 90% of coke burned.
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
8
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
k¼
k2
k1 þ k2
ð22Þ
The reaction rate of oxygen, hydrogen (r0Hc ) and carbon (r0Cc ) during the coke combustion can be shown as follows [8]:
[Oxygen]
1
1þk
ðr 0Cc Þ
r O2 ¼ r 0Hc þ
4
2
ð23Þ
[Hydrogen]
EH
p ð1 X H Þ2=3
r 0Hc ¼ 4pr 20 C H0 kH0 exp Rg T A
ð24Þ
[Carbon]
r 0Cc ¼
4 3
E
pr0 C C0 kC0 exp C pA ðX H X C Þ
3
Rg T
ð25Þ
The transient change of the carbon (Xc) and hydrogen (XH) conversion under non-isothermal and intrapellet diffusion-limited
conditions can be shown as [8]:
dX H 3kHs pA
ð1 X H Þ2=3
¼
dt
r0
ð26Þ
dX C
¼ kC pA ð1 X C Þ
dt
ð27Þ
Parameters used for the numerical simulation of regeneration of
coked MFI-type zeolite are listed in Table 2; fitting parameter was
not used for the numerical simulation. Fig. 8a shows the change in
gas components at the outlet of catalyst bed and the change in
temperature at the center and outlet of the bed obtained by the
numerical simulation. In addition, Fig. 8b shows the experimental
results for coked MFI-type zeolite regeneration in 0.1 MPa air flow.
In the experimental, produced CO was negligibly small. Therefore,
numerical simulation was conducted assuming k is 1. Fig. 8 shows
that numerical changes in gas composition were similar to the
experimental results. In the simulation, a constant coke loading
is assumed throughout the catalyst bed, whereas a distribution of
the coke loading exists in the axial direction after n-hexane cracking, leading to a slight difference between the experimental and
simulated results. Temperature in the catalyst bed increased steeply at the beginning of regeneration and reached a maximum,
when the composition of gas produced (CO2 + CO and H2O) reached
to the maximum value.
Sano et al. reported the dealumination rate of MFI-type zeolite
[5,6] and indicated that increasing the bed temperature and the
partial pressure of water vapor accelerated dealumination rate.
Therefore, the temperature distribution in the bed during regeneration affects the distribution of dealumination degree in the bed
after regeneration. Fig. 9 shows the axial distribution of temperature and water vapor in the bulk flow at regeneration times of
20, 50, 100, 200 and 300 min, obtained by the numerical simulation. The temperature and water vapor had maximum values near
the reactor outlet. Accordingly, dealumination of MFI-type zeolite
near the outlet of the catalyst bed, where the zeolite catalyst will
be exposed to water vapor at the highest temperature is important.
In addition, it is considered that a temperature increasing along the
gas flow in the fixed-bed during coke combustion is result from
heat convection. Therefore, the gas flow rate affects the change
in the temperature and conversion of coke in the reactor. Fig. 10
shows the effect of the flow rate on the changes in temperature
and carbon conversion in coke at reactor outlet. Because of the heat
convection in the reactor, the maximum temperature at the reactor
outlet decreased with increasing the flow rate. In other words,
reactor temperature due to the reaction heat of coke combustion
was suppressed by the convection. As shown in Fig. 10b, it takes
185, 260, and 360 min to reach the 98% carbon conversion in coke
at reactor outlet (nearly 1 wt% coke was remaining on zeolite at
reactor outlet, when zeolite with 76wt% coke loading was used)
when the flow rates are 150, 200, and 300 ml/min, respectively.
Higher flow rate makes the lower temperature in the reactor,
which led the longer regeneration time.
Next, the effect of the coke loading on the maximum temperature in the bed during regeneration and the regeneration time until
90% of coke burned, and the results are shown in Fig. 11. The regeneration time increased with the mass ratio of coke on the zeolite. In
contrast, the maximum temperature in the bed during regeneration increased with increasing coke loading, reached a maximum
temperature near 40 wt%, and then decreased, because the coke
particle size depended on the amount of the coke loading on the
zeolite catalyst.
4. Conclusion
Kinetic analysis of burning regeneration of coked MFI-type zeolite by catalytic cracking of n-hexane was examined. Coke deposited on the MFI-type zeolite was located mainly on the outer
surface. The reaction rates of carbon and hydrogen combustion in
coke correlated using Arrhenius equation and the activation energies of them were 156 kJ/mol and 140 kJ/mol, respectively. The frequency factors of carbon combustion in coke were constant to the
coke loadings. In contrast, the frequency factors of hydrogen combustion in coke were decreased to the coke loading because of the
coke particle growth. Regeneration of coked MFI-type zeolite obtained by catalytic cracking of n-hexane can be numerically simulated using the reaction rates obtained in this study, which
revealed that the temperature and concentration of water vapor
reached a maximum value near the outlet of reactor at each time.
This led to the most rapid dealumination rate in the bed, during
regeneration of coked MFI-type zeolite.
Acknowledgement
This work was supported by a Research Grant Program of the
New Energy and Industrial Technology Development Organization
(NEDO) of Japan.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cej.2012.06.138.
References
[1] Satoshi Inagaki, Kazuyoshi Takechi, Yoshihiro Kubota, Selective formation of
propylene by hexane cracking over MCM-68 zeolite catalyst, Chem. Commun.
46 (2010) 2662–2664.
[2] Kohei Kubo, Hajime Iida, Seitaro Namba, Akira Igarashi, Selective formation of
light olefin by n-heptane cracking over HZSM-5 at high temperatures,
Micropor. Mesopor. Mater. 149 (2012) 126–133.
[3] Hiroshi Mochizuki, Toshiyuki Yokoi, Hiroyuki Imai, Ryota Watanabe, Seitaro
Namba, Junko N. Kondo, Takashi Tatsumi, Facile control of crystallite size of
ZSM-5 catalyst for cracking of hexane, Micropor. Mesopor. Mater. 145 (2011)
165–171.
[4] Hiroki Konno, Takuya Okamura, Yuta Nakasaka, Teruoki Tago, Takao Masuda,
Effects of crystal size and Si/Al ratio of MFI-type zeolite catalyst on n-hexane
cracking for light olefin synthesis, J. Jpn. Petrol. 55 (2012) 267–274.
[5] T. Sano, N. Yamashita, Y. Iwami, K. Takeda, Y. Kawakami, Estimation of
dealumination rate of ZSM-5 zeolite by adsorption of water vapor, Zeolites 16
(1996) 258–264.
[6] T. Sano, H. Ikeya, T. Kasuno, Z.B. Wang, Y. Kawakami, K. Soga, Influence of
crystallinity of HZSM-5 zeolite on its dealumination rate, Zeolites 19 (1997)
80–86.
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138
Y. Nakasaka et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
[7] Kenji Hashimoto, Kouji Takatani, Hiroshi Iwasa, Takao Masuda, A Multiplereaction model for burning regeneration of coked catalysts, Chem. Eng. J. 27
(1983) 177–186.
[8] Kenji Hashimoto, Kouji Takatani, Takao Masuda, Transient changes in the
temperature and conversion of a single coked catalyst pellet during its burning
regeneration, Chem. Eng. J. 29 (1984) 85–96.
9
[9] Kenji Hashimoto, Takao Masuda, Tadatsugu Mori, Kinetics of combustion of
carbonaceous on HZSM-5 zeolites, Chem. Eng. Sci. 43 (1988) 2275–2280.
[10] Tadashi Hano, Fumiyuki Nakashio, Koichiro Kusunoki, J. Chem. Eng. Jpn. 8
(1975) 127–130.
Please cite this article in press as: Y. Nakasaka et al., Kinetic study for burning regeneration of coked MFI-type zeolite and numerical modeling for regeneration process in a fixed-bed reactor, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.138