Reaction Kinetics of Ethane Partial Oxidation To Acetic Acid
Reaction Kinetics of Ethane Partial Oxidation To Acetic Acid
Reaction Kinetics of Ethane Partial Oxidation To Acetic Acid
https://doi.org/10.1007/s13203-018-0195-8
ORIGINAL ARTICLE
Received: 6 February 2018 / Accepted: 13 February 2018 / Published online: 7 March 2018
© The Author(s) 2018. This article is an open access publication
Abstract
The partial oxidation of ethane to ethylene and acetic acid on supported MoVNbPd/TiO2 (P25 of Degussa) has been investi-
gated. Pd was added in a nano-metallic form. The catalyst composition was also different from similar studied catalysts. This
results in a better selectivity towards acetic acid formation. The reaction was carried out in a tubular reactor at temperature
range 225–275 °C, total pressure range 0–200 psig and oxygen percentage in the feed gas of 10–40%. The feed gas contains
ethane and oxygen. In this work, we develop a kinetic model for the reaction for the developed catalyst. In this model, we
assume that oxidation reactions take place on different sites; ethane oxidation takes place on one site, ethylene oxidation
on another site, and CO is oxidized to CO2 on a third site. The model exhibits good agreement with the experimental data.
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30 Applied Petrochemical Research (2018) 8:29–38
decomposes to ethylene and water. Bare metal sites M reacts (0.0025–0.01 wt%) as a physical mixture of separate 0.3
with water to form MOH which reacts with ethylene to give (wt%) Pd/SiO2 led to the near complete depletion of eth-
an ethoxide MOC2H5. The ethoxide is oxidized to acetic ylene and to a significant increase in acetic acid synthesis
acid. rate.
For acetic acid production, a pressure of about 300 psig A suitable kinetic model for the catalytic oxidation of
gives acetic acid selectivity about 20%, and ethylene selec- ethane to acetic acid on MoVNb catalyst must be able to
tivity about 70%. In addition, the use of high pressure is explain the following key observations:
advantageous in reducing contact time for a specified ethane
conversion and thus reducing the catalytic reactor bed. 1. Rate of depletion of ethane is first order with respect to
To avoid the formation of inflammable mixture, the feed ethane as noted by Thorsteinson et al. [1]. Burch and
to the reactor contains low percentage of oxygen limiting Swarnakar [12] determined the rate of reaction to be
ethane conversion. This requires the recycle of unreacted 0.8–1.0 with respect to ethane depending on tempera-
ethane. ture.
The kinetics of Thoresteinson et al. has been used for 2. Rate of depletion of ethane is a fraction close to zero
simulation of fixed bed reactor [2], and fluidized bed reac- with respect to oxygen as noted by Thorsteinson et al.
tor [3]. To avoid the formation of inflammable mixture, El- [1]. Burch and Swarnakar [12] determined the rate of
Sherehy et al. [4] studied the use of distributed oxygen along reaction to be 0.07–0.5 with respect to oxygen.
the reactor bed. 3. High pressure favors the formation of acetic acid.
Karim et al. [5] showed that the addition of Pd to the 4. Temperature increase makes the reaction more selective
MoVNb catalyst greatly increases the selectivity to acetic to ethylene with respect to acetic acid.
acid to about 80% and completely oxidizes CO to CO2. 5. The strong acceleration of the rate of the ethylene oxida-
Fakeeha et al. [6] obtained the kinetics for MoVPO cata- tion to acetic acid by the presence of water (Linke et al.
lyst. A model based on Eley–Rideal and Mars and Van Krev- [7, 8]; Rahman et al. [9]). Water, whether formed as a
elen redox mechanisms were found adequate. byproduct or added with C 2H6–O2 reactants, increases
The mechanism and kinetics of the reaction with a acetic acid selectivity by promoting the desorption of
Mo1V0.25Nb0.12Pd0.0005Ox catalyst were studied by Linke adsorbed acetate species as acetic acid. Thus, the pro-
et al. [7, 8]. It has been shown that ethylene inhibits ethane moting effect of water on acetic acid selectivity reflects
oxidation through depletion of lattice oxygen (O*). The con- a specific increase in the rate at which ethane converts
secutive oxidation of ethylene to acetic acid is itself cata- to acetic acid via direct pathways. The absence of water
lyzed by the palladium oxide in a heterogeneous analogue led to larger (ethylene/acetic acid) ratios in products and
of the Wacker process. to slightly higher COx selectivity.
Their kinetic model contains some negative activation 6. Palladium catalyzes the oxidation of CO to C O2. Ber-
energy which is physically wrong. They have indicated that lowitz et al. [13] have shown that for some range of
the C–H bond activation in ethane by the oxygen-saturated operating conditions the rate of CO oxidation is first
catalyst surfaces is the rate limiting step [7]. Water is also order in oxygen and negative first order in CO.
believed to increase acetic acid selectivity by promoting the
desorption of acetate species as acetic acid [7]. The catalyst used in our experiments has the composi-
0
The kinetics of MoV type catalyst was re-investigated by tion Mo16V6.37Nb2.05OxPd0.0037 /TiO 2 (P25).The catalyst
Rahman et al. [9] but the catalyst composition was not men- loading on Titania is 30%. Pd was added. The method of
tioned. Most probably it contains Nb and Pd. Rahman et al. preparation, catalyst characterization and the effect of oper-
[9] developed a two-site Eley–Rideal-Redox (ERR) model ating parameters for the partial oxidation reaction of ethane
to predict the partial oxidation of ethane to ethylene and the such as temperature, space time and feed composition are
partial ethane oxidation to acetic acid over the catalyst. To be described in references [15, 16]. The experimental tubular
used in acetic acid plant design, we need to know how much reactor used to carry out the experiments is also described
CO is produced. However, their model assumes that the oxi- in these two references. This catalyst composition was opti-
dation of ethylene and acetic acid to CO is insignificant. mized for maximum acetic acid yield, and thus it is required
Also the oxidation of ethane to CO and CO2 is negligible. to develop a kinetic model for its use in the partial oxidation
Li and Iglesia [10, 11] found that precipitation of Mo, reaction of ethane to acetic acid.
V and Nb salts solution in the presence of colloidal T iO2 In the next section, the experiments carried out and the
(titania P25 from Degussa) led to a tenfold increase in results are presented. This is followed by the kinetic model
ethylene and acetic acid rates (per active oxide) with- suggested. Parameter estimation results are then presented.
out significant changes in selectivity relative to unsup- The results are then discussed and reaction mechanism is
ported samples. The introduction of trace amounts of Pd suggested. Finally, conclusions are presented.
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Applied Petrochemical Research (2018) 8:29–38 31
1. At pressure 200 psig, and flow-rate 10 ml/min; Here we attempt to obtain kinetic expression based on
(a) One experiment at 225 °C and 10% O2 known mechanisms that are usually used in partial oxi-
(b) Two experiments at 225, 240 °C and 20% O2 dation reactions. These are Langmuir–Hinshelwood (LH)
(c) One experiment at 225 °C and 40% O2 kinetics assuming reaction taking place between adsorbed
2. At pressure 200 psig, and flow-rate 20 ml/min; species, Eley–Rideal kinetics assuming reaction taking
(a) Two experiments at 225, 240 °C and 10% O2 place between adsorbed species and gaseous reactant, and
(b) Three experiments at 225, 240, 250 °C and 20% Mars–Van Krevelen redox mechanism in which the oxi-
O2 dized catalyst react with ethane and thus is reduced and is
(c) Three experiments at 225, 250, 275 °C and 40% reoxidized with molecular oxygen.
O2 For the reaction
3. At pressure 200 psig and flow-rate 40 ml/min
1
(a) Two experiments at 225, 250 °C and 10% O2 A + O2 → B + H2 O, (1)
2
(b) Four experiments at 225, 250 °C and 20 and 40%
O2 where A could be a hydrocarbon and B is the corresponding
4. At pressure 100 psig and flow-rate 10 ml/min dehydrogenated hydrocarbon.
(a) One experiment at 225 °C and 10% O2 Langmuir–Hinshelwood kinetics takes the form
(b) Two experiments at 225, 250 °C and 20% O2
kPm1 Pm2
(c) Three experiments at 225, 250, 275 °C and 40% Rate of oxidation reaction = [
A O2
]m3 .
O2 1 + K1 PO2 + K2 PA + K3 PB + K4 Pw
5. At pressure 100 psig and flow-rate 20 ml/min, (2)
(a) One experiment at 225 °C and 10% O2 While the redox kinetics take the form
(b) Two experiments at 225, 250 °C and 40% O2.
6. At pressure 100 psig and flow-rate 40 ml/min, nine kPA PO2
experiments at 225, 250, 275 °C and 10, 20 and 40% O2.
Rate of oxidation reaction = . (3)
PO2 + KPA
7. At pressure 0 psig, twenty-seven experiments at 225,
250, 275 °C and 10, 20 and 40% O2, flow-rates of 10, 20 The reaction network is assumed in its most general form
and 40 ml/min. of Fig. 1 (ri’s denote reaction rates of different species).
This assumes that acetic acid can be obtained from
The results are shown in Table 1. ethane and ethylene and that all reactants and products
Some observations can be made from testing results: are oxidized to CO and CO2 and CO is converted to CO2.
We assume the conversion of ethane is X 1, the con-
1. Acetic acid is not formed at atmospheric pressure. version to CO is X2, CO2 is X3, and CH3COOH is X4. In
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32 Applied Petrochemical Research (2018) 8:29–38
200 10 10 225 99.49 4.04 2.06 82.79 0.00 15.15 161.26 1.87
200 10 20 225 74.98 6.37 1.08 83.81 0.31 14.8 228.80 1.38
200 10 20 240 99.32 9.25 0.37 81.79 0.09 17.74 324.23 0.69
200 10 40 225 42.68 9.52 0.83 77.01 0.28 21.88 235.64 1.19
200 20 10 225 66.09 3.26 6.71 77.84 0.67 14.78 244.69 9.84
200 20 10 240 99.42 4.89 4.08 79.47 0.4 16.05 374.72 8.98
200 20 20 225 42.09 4.80 2.62 81.76 0.47 15.15 336.38 5.03
200 20 20 240 72.71 7.31 1.43 79.75 0.32 18.50 499.68 4.18
200 20 20 250 97.16 9.56 0.82 78.82 0.22 20.15 645.86 3.14
200 20 40 225 24.31 6.44 2.59 82.00 0.56 14.85 339.47 5.00
200 20 40 250 62.54 15.18 0.73 71.2 0.23 27.84 694.79 3.32
200 20 40 275 99.94 21.96 0.15 64.64 0.16 35.06 912.51 0.99
200 40 10 225 30.68 1.78 12.76 71.61 1.32 14.32 245.82 20.44
200 40 10 250 79.12 3.50 4.99 76.65 0.52 17.84 517.37 15.72
200 40 20 225 17.12 2.21 10.29 76.17 1.36 12.18 288.57 18.19
200 40 20 250 49.97 6.12 2.67 78.79 0.54 18.00 826.60 13.07
200 40 40 225 12.21 3.01 6.99 74.77 1.26 16.99 289.35 12.62
200 40 40 250 27.21 7.93 2.44 74.1 0.59 22.88 755.48 11.61
100 10 10 225 75.53 2.88 2.30 82.83 0.15 14.718 115.01 1.49
100 10 20 225 43.54 4.53 1.95 81.9 0.17 15.98 159.00 1.77
100 10 20 250 99.04 11.08 0.76 82.45 0.17 16.62 391.50 1.68
100 10 40 225 21.34 6.15 1.67 77.46 0.15 20.72 153.12 1.54
100 10 40 250 61.19 14.95 0.72 73.97 0.17 25.14 355.44 1.62
100 10 40 275 99.96 23.14 0.24 68.41 0.11 31.24 508.81 0.83
100 20 10 225 21.82 2.14 11.89 75.19 2.06 10.87 155.16 11.45
100 20 40 225 11.03 3.57 6.92 74.61 0.89 17.57 171.23 7.41
100 20 40 250 31.62 8.38 2.32 71.00 0.50 26.18 382.48 5.83
100 40 10 225 12.16 0.50 84.34 0.00 0.97 14.69 0.00 37.95
100 40 10 250 42.77 1.20 27.22 45.30 2.27 25.22 104.83 29.40
100 40 10 275 96.39 4.02 16.47 57.54 1.13 24.86 446.09 59.59
100 40 20 225 7.26 1.63 27.52 61.33 2.05 9.10 171.37 35.89
100 40 20 250 25.57 2.61 16.14 59.04 2.31 22.51 264.16 33.70
100 40 20 275 76.45 8.18 3.39 70.68 0.6 25.33 991.11 22.18
100 40 40 225 5.82 1.20 31.99 45.25 3.59 19.17 69.81 23.03
100 40 40 250 14.77 4.80 6.96 72.73 1.53 18.78 448.84 20.05
100 40 40 275 43.92 11.96 1.77 67.05 0.70 30.48 1031.01 12.70
0 10 10 225 15.25 0.98 100.00 0.00 0.00 0.00 0.00 22.05
0 10 10 250 41.76 1.98 86.15 0.00 2.54 11.31 0.00 38.38
0 10 10 275 90.01 3.56 58.51 0.00 6.89 34.60 0.00 46.87
0 10 20 225 8.35 1.01 100.00 0.00 0.00 0.00 0.00 20.20
0 10 20 250 21.85 2.02 80.07 0.00 1.34 18.59 0.00 32.35
0 10 20 275 58.42 3.75 49.13 0.00 7.11 43.76 0.00 36.85
0 10 40 225 3.12 1.30 91.42 0.00 0.00 8.58 0.00 17.83
0 10 40 250 11.75 2.50 74.55 0.00 2.66 22.78 0.00 27.96
0 10 40 275 35.46 4.80 37.33 0.00 9.07 53.60 0.00 26.88
0 20 10 225 6.83 0.52 100 0.00 0.00 0.00 0.00 23.40
0 20 10 250 16.61 1.13 90.22 0.00 0.00 9.79 0.00 45.88
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Applied Petrochemical Research (2018) 8:29–38 33
Table 1 (continued)
Pressure Flow O2 Temp. °C O2 conv. % C2H6 conv. % Selectivity (%) Space time yield
psig ccm/min In feed % gm/kg h
Ethylene Acetic acid CO CO2 Acetic acid Ethylene
dX4
= rCH3 COOH (8)
d𝜏
With the initial conditions at
W
𝜏=
FC2 H6
= 0, X1 = X2 = X3 = X4 = 0, (9)
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34 Applied Petrochemical Research (2018) 8:29–38
C2 H6 + 21 O2 → C2 H4 + H2 O
k3 × PC2 H6 × PO2
r3 = (16) −X1 − 21 X1 X1 X1
PO2 + K1 × PC2 H6
C2 H4 + O2 → CH3 COOH
k4 × PC2 H6 × PO2 −X4 − X4 X4
r4 = (17)
PO2 + K1 × PC2 H6
C2 H4 + 3O2 → 2CO2 + 2H2 O
− 12 X3 − 23 X3 X3 X3
k5 × PC2 H4 × PO2
r5 = (18)
PO2 + K2 × PC2 H4 Moles of ethane = 1−X1 (25)
1 3
Moles of oxygen = Y1 − X1 −X2 − X3 −X4 (26)
k6 × PC2 H4 × PO2 2 2
r6 = (19)
PO2 + K2 × PC2 H4
1 1
Moles of ethylene = X1 − X2 − X3 − X4 (27)
2 2
k7 × P2C H × PO2 × PH2 O
r7 = 2 4
(20) Moles of CH3 COOH = X4 (28)
PO2 + K3 × PC2 H4
Moles of CO = X2 (29)
k8 × PCH3 COOH × PO2 Moles of CO2 = X3 (30)
r8 = (21)
PO2 + K3 × PC2 H4
Moles of H2 O = X1 + X2 + X3 (31)
1 1
k9 × PCH3 COOH × PO2 Total moles (nt) = 1 + Y1 + X1 + X2 − X4 . (32)
r9 = (22) 2 2
PO2 + K3 × PC2 H4
Thus, the partial pressures of the different compounds are
as follows:
k10 × PCO × PO2
r10 = , (23) (1 − X1 )
(PCO + K4 PO2 )2 PC2 H6 = Pt (33)
nt
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Applied Petrochemical Research (2018) 8:29–38 35
(X1 + X2 + X3 )
PH2 O = Pt , (39) Parameter estimation results
nt
where Pt = total pressure Preliminary screening led us to conclude that the rate of
Parameter estimation was done by the minimization of oxidation of ethane to acetic acid and CO is negligibly small,
weighted residual squares of the conversions. The resulting i.e., r2 = 0 and r3 = 0 . In addition, the rate of oxidation of
non-linear equations are solved by Marquardt method. The ethylene to CO and C O2 is negligibly small, i.e., r5 = 0
method used is described in details by Hosten et al. [14]. and r6 = 0 . We obtain the following parameter estimates
The computer package for carrying out estimation calcu- (Table 2).
lations provides statistical data. They are F value for judg-
ing goodness of fit. t values for significance of parameters
The values of the rate of reactions considered in this work are based on the units of normal cm3/min g
They must be multiplied by 0.00268 to transform them to the more usual units of mol/h g catalyst
n.a. not applicable
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36 Applied Petrochemical Research (2018) 8:29–38
Discussion from the second route is not significant. The first route involv-
ing water needs modification. Let Y be a catalytic center for
We were not able to fit our experimental results with the ethylene oxidation to acetic acid. Consider the following
model of Rahman et al. [9]. Thus, the model needs modifi- scheme;
cations for our case since we are using a different catalyst. 2Y + 2O2 ⇄ 2YO2
Parameter estimation results indicate that the main reactions
taking place are the oxidation of ethane to ethylene and C O2 C2 H4 + Y ⇄ C2 H4 Y
and the oxidation of ethylene to acetic acid. Some of acetic
acid is also oxidized to CO and C O2. CO is oxidized to C O2. 2C2 H4 + YO2 ⇄ (CH3 CH)2 OYO
The final reaction network takes the form of Fig. 2.
(CH3 CH)2 OYO + H2 O ⇄ (CH3 CH)2 OY(OH)2
The results indicate that oxidation reactions take place
on different sites. Ethane oxidation takes place on one site. (CH3 CH)2 OY(OH)2 → (CH3 CHO)2 Y + H2 O (RDS)
Ethylene oxidation in presence of water to acetic acid takes
place on another site. CO is oxidized to CO2 on a third site. (CH3 CHO)2 Y + YO2 → 2CH3 COOH + 2Y (fast).
The estimation results indicate that a redox mechanism This leads to the following rate equation
is more suitable to explain the reactions taking place for the
oxidation of ethane to ethylene. Equation (14) for ethane k7 × P2C × PO2 × PH2 O
2 H4
oxidation to ethylene takes the familiar form of redox kinet- r7 = k
1 + k(PO2 + K3 × PC2 H4 + negligibe adsorption terms … .)
.
ics. The parameters associated with the oxidation of ethane
to ethylene are determined with large t values giving high For large k this equation simplifies to Eq. (20).
confidence in their values. For the oxidation of ethane to For the oxidation of acetic acid to CO and C O2, we suggest
ethylene, our results agree with Linke et al. [7]. For the oxi- the following scheme
dation of ethylene to acetic acid our results also agree with
3 3 3
that of Linke et al. [7] that acetic acid is related to the forma- Y + O2 ⇄ YO2
2 2 2
tion of a hydroxyl group from water on the active site. There
are some controversy of whether ethane or ethylene oxidizes
1 1 1
to CO and C O2. Burch and Swanakar [12] suggested that Y + O2 ⇄ YO
CO and C O2.are formed to a large extent from ethane. On 2 4 2
the contrary Thorsteinson et al. [1] suggested that ethylene
CH3 COOH + YO2 → CH3 COOHYO2 (RDS)
oxidizes into CO and CO2. Our results suggest that CO2 is
mainly obtained from ethane and acetic acid whereas CO is 1 1
obtained from acetic acid. This CO is then oxidized to C O2 2
CH3 COOHYO2 + YO2 → CO2 + H2 O + Y (fast)
2
on sites containing Pd. For reactions involving ethylene, ace-
tic acid and CO a LH kinetic model is suggested. 1 1
For the oxidation of ethylene to acetic acid, Rahman et al. 2
CH3 COOHYO2 + YO → CO + H2 O + Y (fast),
2
[9] suggested two routes one involves water and the second
does not involve water. We found that acetic acid formation which leads to the following rate equations
k8 × PCH3 COOH × PO2
r8 = k ,
1 + k(PO2 + K3 × PC2 H4 + negligibe adsorption terms … .)
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Applied Petrochemical Research (2018) 8:29–38 37
1
ZO + O2 ⇄ ZO2
2
CO + Z ⇄ COZ
COZ + ZO2 → CO2 Z + ZO (RDS)
Fig. 3 Observed and calculated conversion for ethane partial oxida- Fig. 5 Observed and calculated values of acetic acid yield for partial
tion oxidation of ethane
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38 Applied Petrochemical Research (2018) 8:29–38
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port provided by King Abdul Aziz City for Science and Technology tive oxidation of ethane to acetic acid catalyzed by Mo–V–Nb
(KACST) for this research under Grant number AR-29-256. oxides. Appl Catal A Gen 334(1–2):339–347
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