National Cheng Kung University
From the SelectedWorks of Wei-Hsin Chen
October, 2012
One-step synthesis of dimethyl ether from the gas
mixture containing CO2 with high space velocity
Wei-Hsin Chen, National Cheng Kung University
Available at: http://works.bepress.com/wei-hsin_chen/97/
Applied Energy 98 (2012) 92–101
Contents lists available at SciVerse ScienceDirect
Applied Energy
journal homepage: www.elsevier.com/locate/apenergy
One-step synthesis of dimethyl ether from the gas mixture containing CO2
with high space velocity
Wei-Hsin Chen a,⇑, Bo-Jhih Lin a, How-Ming Lee b, Men-Han Huang b
a
b
Department of Greenergy, National University of Tainan, Tainan 700, Taiwan, ROC
Physics Division, Institute of Nuclear Energy Research, Taoyuan 325, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 11 December 2011
Received in revised form 8 February 2012
Accepted 29 February 2012
Available online 27 April 2012
Keywords:
Dimethyl ether (DME)
One-step and direct synthesis
Bifunctional catalyst
Cu–ZnO–Al2O3 catalyst and ZSM5
High space velocity
Synthesis gas and CO2
a b s t r a c t
Dimethyl ether (DME) has been considered as a potential hydrogen carrier used in fuel cells; it can also be
consumed as a diesel substitute or chemicals. To develop the technique of DME synthesis, a bifunctional
Cu–ZnO–Al2O3/ZSM5 catalyst is prepared using a coprecipitation method. The reaction characteristics of
DME synthesis from syngas at a high space velocity of 15,000 mL (gcat h)1 are investigated and the
effects of reaction temperature, pressure, CO2 concentration and ZSM5 amount on the synthesis are taken
into account. The results suggest that an increase in CO2 concentration in the feed gas substantially
decreases the DME formation. The optimum reaction temperature always occurs at 225 °C, regardless
of what the pressure is. It is thus recognized that the DME synthesis is governed by two different mechanisms when the reaction temperature varies. At lower reaction temperatures (<225 °C) the reaction is
dominated by chemical kinetics, whereas thermodynamic equilibrium is the dominant mechanism as
the reaction temperature is higher (>225 °C). For the CO2 content of 5 vol.% and the pressure of
40 atm, the maximum DME yield is 1.89 g (gcat h)1. It is also found that 0.2 g of ZSM5 is sufficient to
be blended with 1 g of the catalyst for DME synthesis.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, the applications of dimethyl ether (DME) in
industries are receiving considerable attention. In chemical industries, DME is a useful intermediate for producing chemicals, such as
methyl acetate, dimethyl sulfate and light olefins [1,2]. Meanwhile,
DME has been considered as a potential diesel substitute used in
compression ignition engines because of its high cetane number
(i.e. 55–60) [3,4]. DME also possesses the merit of environmentally
friendly properties [5]. A comparison to gasoline combustion,
burning DME in diesel engines emits much less air pollutants, such
as NOx, CO, hydrocarbons and particulate matters [6–8]. DME can
be efficiently reformed to hydrogen at lower temperatures so that
it is also considered as a promising feedstock for fuel cells [9,10].
Although DME is a volatile organic compound (VOC) with the
boiling temperature of 25 °C at the atmospheric pressure, it is
non-carcinogenic, non-teratogenic, non-mutagenic and non-toxic
[11]. This implies that the risk in damaging health from consuming
DME is slight.
DME synthesis plays a crucial role in extending its applications.
Conventionally, DME is produced through a two-step process or
indirect method. In this route, methanol is first synthesized from
⇑ Corresponding author. Tel.: +886 6 2605031; fax: +886 6 2602205.
E-mail address: [email protected] (W.-H. Chen).
0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.apenergy.2012.02.082
syngas (i.e. CO + H2); then, DME is produced from the dehydration
of methanol [12–15]. The reactions in the two-step process are expressed as:
Methanol synthesis
CO þ 2H2 $ CH3 OH DH0 ¼ 90:6 kJ=mol
ð1Þ
Methanol dehydration
2CH3 OH $ CH3 OCH3 þ H2 O DH0 ¼ 23:4 kJ=mol
ð2Þ
Apart from the two-step process, DME can also be generated
from syngas via a one-step process, namely, the direct method.
In addition to the methanol synthesis and dehydration, the direct
method is also related to water gas shift reaction (WGSR)
[13,16–19]. The reaction is given by:
Water gas shift reaction:
CO þ H2 O $ CO2 þ H2
DH0 ¼ 41:2 kJ=mol
ð3Þ
Accordingly, the net reaction in the direct method is
3CO þ 3H2 $ CH3 OCH3 þ CO2
DH0 ¼ 245:8 kJ=mol
ð4Þ
This method for the production of DME has attracted more and
more industrial interest because of its lower thermodynamic limitation as well as higher economic value and theoretical significance
[17,20].
93
W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
2. Experimental
H
I
F
E
D
G
J
C
A
B
Fig. 1. A schematic of the reaction system (A: feed gas; B: mass flow controller;
C: heating tape; D: reactor; E: catalyst bed; F: thermocouple; G thermocouple; H: back
pressure valve; I: microgas chromatography; J: recorder).
(a)
(b)
400
300
ZnO
CuO
cps
When DME is synthesized from syngas through the direct method, the development of bifunctional catalysts is a key factor in
determining the performance. As the name implies, two types of
active site are simultaneously contained in a bifunctional catalyst,
with one for methanol synthesis and the other for methanol dehydration [20,21]. Cu–ZnO–Al2O3 catalysts are the most commonly
employed catalysts for methanol synthesis in industry. In Cu–
ZnO-based catalysts, metallic copper clusters are the active sites
for methanol synthesis reaction, and ZnO plays a pivotal role in
maintaining the active copper metal in optimal dispersion [22].
In other words, the main function of ZnO is to increase Cu dispersion in the calcined sample, thus providing a high number of active
sites exposed to gaseous reactants. The purpose of addition of M3+
ions (e.g. Al3+) into Cu–ZnO-based catalysts is to increase their surface areas and copper dispersion; it is also able to inhibit the sintering of Cu particles at on-stream conditions [22,23]. In regard
to methanol dehydration, the commonly employed solid acid catalysts include c-Al2O3, silica–alumina and zeolites such as HZSM5, HY, HMCM-49, HMCM-22, SAPOs and Ferrierite. Among these
catalysts, HZSM5 has been extensively employed due to its high
catalytic activity for the conversion of syngas to methanol at certain reaction temperatures [24,25]. In the study of An et al. [26],
it was reported that more water was produced in the process of
CO2 hydrogenation compared to CO hydrogenation, and ZSM5
was not sensitive to the concentration of water. On the other hand,
Yang et al. [27] addressed that the methanol dehydration catalyst
(or zeolite catalyst), such as ZSM5, had more activity and stability
than c-Al2O3 catalyst. Moreover, Aguayo et al. [28] pointed out that
CuO–ZnO–Al2O3/NaHZSM5 catalyst had an excellent performance
and it was suitable for using it in uninterrupted reaction–regeneration cycles. For these reasons, ZSM5 zeolite was chosen in the
present study.
DME synthesis has been reported in some studies; however the
information concerning its production process remains insufficient. For instance, when syngas is produced from the gasification
of biomass and steam reforming of hydrocarbons and used as the
feedstock for DME production, CO2 is inevitably contained in the
feed gas [20,29,30]. However, relatively little research has been
performed on the role played by CO2 upon DME synthesis, especially at the conditions of high space velocity. From Eq. (4), it is
known that a higher CO2 concentration disadvantages DME formation. Nevertheless, if CO2 is removed prior to carrying out DME
synthesis, an extra operating cost for CO2 separation is required.
To provide an in-depth observation on DME synthesis, a bifunctional catalyst for directly producing DME from syngas will be prepared and tested in the present study. Particular attention is paid
to the influence of CO2 on DME formation at a high space velocity.
A comparison between the present study and others will be made
as well.
200
ZnO
CuCO3
100
ZnO
γ-Al2O3
Cu2O
CuO
CuO
2.1. Catalyst preparation
The catalyst used for synthesizing DME was a Cu–ZnO–Al2O3
catalyst which was prepared by a coprecipitation method. A solution of Cu(NO3)22.5H2O, Zn(NO3)26H2O and Al(NO3)39H2O and a
solution of Na2CO3 were individually prepared. Then, a beaker containing deionized water was heated to 70 °C. The two solutions
were simultaneously added drop wise to the beaker with the period of 60 min by controlling the flow rates of the two solutions at
pH = 7 under continuous stirring so that the coprecipitation was
achieved. The suspension was aged for 3 h followed by filtered
out, washed and dried at 120 °C for 12 h. Then the precipitate
was calcined in air by raising temperature to 350 °C with the heating rate of 1 °C min1 followed by soaking it at 350 °C for 2 h. The
CuO
0
20
30
40
50
60
70
80
2θ
Fig. 2. (a) SEM image (50,000) and (b) XRD pattern of the fresh prepared
CuO–ZnO–Al2O3 catalyst.
prepared Cu–ZnO–Al2O3 catalyst was mixed with ZSM5 to form the
bifunctional catalyst. In the prepared catalyst, the weight percentages of Cu, Zn and Al were 47.3, 23.9 and 2.9 wt.%, respectively.
Accordingly, the atomic ratio between the three metals Cu, Zn
and Al was 1:0.490:0.144. The BET surface area, pore volume and
94
W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
(a) Conversion
100
100
90
99.5
99
CO conversion
70
After test
98.5
60
TGA (wt%)
Conversion (%)
80
50
40
30
98
97.5
97
Before test
96.5
H 2 conversion
20
96
10
0
(a) TGA
0
10
20
30
40
50
60
70
80
90
95.5
100
Time (h)
95
100
200
400
500
600
700
800
Temperature (oC)
(b) DME yield
3
300
0.015
2.5
(b) FTIR
0.014
0.012
0.011
Absorbance height
Yield (g/gcath)
0.013
2
1.5
1
0.5
0.01
0.009
0.008
0.007
0.006
After test
0.005
0.004
0
0
10
20
30
40
50
60
70
80
90
100
Time (h)
0.003
0.002
Before test
0.001
0
(c) Selectivity
100
100
400
500
600
700
800
Fig. 4. Distributions (a) TGA and (b) FTIR of the catalyst before and after undergoing
durability test.
80
70
Selectivity (%)
300
Temperature (oC)
90
DME selectivity
ZSM5 was 0.05 wt.% and its BET surface area was 425 m2 g1. It
should be illustrated that the ZSM5 zeolite was not treated or modified with an alkali. In general, the formation temperature of aromatics with high molecular weight (e.g., 350–450 °C) is higher
than the temperature for DME synthesis (i.e. 200–300 °C) around
100 °C. In the experiments, the signals detected by the micro-GC
for hydrocarbon compounds above C3 were relatively small. Therefore, the formation of hydrogen compounds above C3 could be ignored in the study.
60
50
40
MeOH selectivity
30
20
10
0
200
0
10
20
30
40
50
60
70
80
90
100
Time (h)
Fig. 3. Temporal distribution of (a) CO and H2 conversion and (b) DME yield and (c)
DME and MeOH selectivities.
pore radius of the prepared catalyst were 68.7 m2 g1, 0.4 cc g1
and 9.77 nm, respectively. With regard to the adopted ZSM5, it
was a commercial zeolite supplied by Zeolyst (CBV5524G). In the
ZSM5, the SiO2/Al2O3 molar ratio was 50 and the nominal cation
form was ammonium. The weight percentage of Na2O in the
2.2. Reaction system
A schematic of the conducted reaction system is demonstrated
in Fig. 1. As can be seen in the figure, the system consisted of three
units, including a reactant feeding unit, a preheating and reaction
unit and a product gas analysis unit. In the feeding unit, the gas
mixture of four gases of H2, CO, CO2 and N2 were stored in a steel
cylinder. The flow rate of the gas mixture was controlled by a mass
flow controller (MKP, Model TSC-100). In the preheating and reaction unit, a heating tape, a reactor and a back pressure valve were
95
W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
98
96
150
94
92
0
CO
175
0.1
2
o
200
0.2
/CO 0.30.4
0.5 250
rat
io
e(
tur
225
era
p
m
Te
C)
DME selectivity (%)
95
94.5
150
CO
175
0.1
o
200
0.2
2 /C O
0.3
0.4
rat
0.5 250
io
e(
tur
a
r
225
e
mp
Te
75
70
65
150
60
55
0
175
0.1
95.18
95.09
95.00
94.91
94.83
94.74
94.65
94.56
94.47
94.38
94.29
94.21
94.12
94.03
93.94
C)
0.2
0.3
0.4
0.5 250
rat
io
6
5.75
5.5
5.25
5
4.75
0.5
DME yield
(DME mol/CO mol)
CO
250
0.4
CO
200
0.3
2 /C O
0.2
0.1
rati
o
150
2
200
0.2
/CO 0.30.4
0.5 250
rat
io
T
C)
225
175
0.1
225
e(
tur
a
r
pe
em
(d) MeOH selectivity
(e) DME yield
0.4
0.38
0.36
0.34
0.32
0.3
0
o
200
2 /C O
95.5
93.5
0
80
CO
(c) DME selectivity
94
(b) H 2 conversion
H 2 conversion (%)
100
99.96
99.42
98.89
98.35
97.82
97.28
96.75
96.21
95.67
95.14
94.60
94.07
93.53
93.00
92.46
M eOH selectivity (%)
CO conversion (%)
(a) CO conversion
o
e(
tur
225
era
p
m
Te
C)
175
0 150
Te
o
(
ure
rat
e
mp
C)
79.33
77.73
76.13
74.54
72.94
71.34
69.74
68.14
66.55
64.95
63.35
61.75
60.16
58.56
56.96
6.06
5.97
5.88
5.79
5.71
5.62
5.53
5.44
5.35
5.26
5.17
5.09
5.00
4.91
4.82
0.409
0.403
0.397
0.391
0.385
0.379
0.373
0.367
0.361
0.355
0.349
0.343
0.337
0.331
0.325
Fig. 5. Three-dimensional profiles of (a) CO conversion, (b) H2 conversion, (c) DME selectivity, (d) MeOH selectivity and (e) DME yield predicted from thermodynamic
analysis.
installed in series. A stainless steel tube (12.7 mm o.d.) was placed
in the reactor to carry out the chemical reactions. The back pressure valve was used to regulate the pressure of the reaction system. Meanwhile, two K-type thermocouples with one inside the
catalyst bed and the other outside the stainless steel tube were
mounted. The former and the latter were employed for measuring
bed temperature and providing the reference of set temperature,
respectively. The thermocouples were calibrated periodically and
their accuracy was within ±2 °C. In the gas analysis unit, it comprised an online gas chromatography (GC, Varian, CP-4990) and a
recorder to measure and monitor the concentrations of H2, CO,
CO2, methanol and DME.
2.3. Experimental procedure
To control the space velocity of the reactants, the flow rate of
the feed gas was adjusted by the mass flow controller. When the
feed gas flew through the heating tape, they were preheated at
the temperature of 120 °C. Soon after the feed gas past through
the reaction tube, chemical reactions were triggered. In each
experiment, 1 g of Cu–ZnO–Al2O3 catalyst was mixed with ZSM5
and packed in the reaction tube. ZSM5 was used to aid in removing
moisture when DME synthesis proceeded so as to improve the performance of chemical reactions. Prior to experiments, the catalyst
was sieved to the particle sizes in the range of 250–500 lm (i.e.
35–60 mesh) and the bifunctional catalyst was reduced in a gas
mixture (H2/N2 = 1/1) with the flow rate of 150 mL min1 under
320 °C for 2 h (heating rate 4 °C min1 to 320 °C). In the experiments, 0.2–0.4 g ZSM5 was blended with 1 g CuO–ZnO–Al2O3
catalyst. Hence the mass ratio of the catalyst and ZSM5 was between 5 and 2.5. After DME was produced, the product gas was
analyzed by means of the online GC. The electrical signals from
the mass flow controller, thermocouples, back pressure valve and
GC were sent into the recorder. Before the experiments were carried
W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
out, the reproducibility of the experiments has been tested. The
tests suggested that the experiments could be reproduced. Therefore, the quality of the experiments was reliable.
50
(a) Conversion
2.4. Indexes of performance of DME synthesis
The performance of DME synthesis can be represented by CO
and H2 conversions as well as DME and methanol selectivities
[12,31]. They are expressed as follows:
CO or H2 conversion ð%Þ ¼
DME selectivity ð%Þ ¼
Produced CO or H2 ðmolÞ
100
Initial CO or H2 ðmolÞ
Conversion (%)
40
3.1. Catalyst characteristics and performance
Fig. 2 first demonstrates the scanning electron microscope
(SEM, Hitachi S-4800) image (50,000) and the X-ray diffraction
(XRD, Bruker AXS, D8) pattern of the prepared catalyst. As can be
seen in Fig. 2a, the individual particles which agglomerate together
are in nanoscales. From the XRD pattern, the components of CuO,
ZnO and Al2O3 can be clearly identified, rendering the successful
preparation of the Cu–ZnO–Al2O3 catalyst. To figure out the reaction characteristics of the prepared catalyst, temporal distributions
of CO and H2 conversions, DME yield as well as DME and methanol
(MeOH) selectivities are shown in Fig. 3 where the reaction temperature, the compositions of the feed gas, ZSM5 amount and
tested duration are 220 °C, H2/CO/CO2/N2 = 61/30/5/4, 0.2 g and
90 h, respectively. It is found that the performance of the catalyst
decays progressively in that the CO and H2 conversions as well as
DME yield decreases with increasing time. A comparison to the results of Wang et al. [17], the trend of decaying in this work is
slower, suggesting the better performance of the prepared catalyst.
Even though the performance of the catalyst has a trend to decay
with time, the values of DME and MeOH selectivities are around
90% and 10%, respectively (Fig. 3c). It follows that the prepared
200
225
250
(b) Selectivity
100
90
Selectivity (%)
80
70
DME selectivity
60
50
40
MeOH selectivity
30
20
10
0
175
200
225
250
Temperature (oC)
(c) DME yield & bed temp
350
3
Bed temperature
2.5
o
306 C
290 oC
Yield (g/gcath)
In the present study, attention is paid to DME synthesis under
the conditions of reactants containing CO2; hence two different
gas mixtures with H2/CO/CO2/N2 volume ratios of 61/30/5/4 and
48/32/16/4 are selected as the feed gases for experiments. From
past literature [12,13,17,18,32,33], the reaction temperature of
DME synthesis was usually in the range of 200–280 °C. Thermodynamically, it is easier to synthesize DME at lower reaction temperatures because of the exothermic reaction involved. Hence, four
different reaction temperatures of 175, 200, 225 and 250 °C are taken into consideration. The space velocity means the volumetric
flow rate of feed gas in the catalyst with per unit weight. In reviewing past studies of DME synthesis [12,13,17,18,32,33], the space
velocity was controlled between 800 and 6000 mL (gcat h)1. In
the present study, a high space velocity of 15,000 mL (gcat h)1
serves as the basis of the current work.
175
o
Produced MeOHðmolÞ
2 Produced DMEðmolÞ þ Produced MeOH
3. Results and discussion
H 2 conversion
Temperature ( C)
ð6Þ
ð7Þ
20
0
100
100
30
10
ð5Þ
2 Produced DME ðmolÞ
2 Produced DME ðmolÞ þ Produced MeOH
MeOH selectivity ð%Þ ¼
CO conversion
300
2
275
o
260 C
1.5
250
o
227 C
1
225
0.5
0
325
200
175
200
225
250
Bed temperature (oC)
96
175
o
Temperature ( C)
Fig. 6. Profiles of (a) CO and H2 conversions, (b) DME and MeOH selectivities and (c)
DME yield and bed temperature of Case 1.
bifunctional catalyst does provide the high performance of DME
synthesis.
To proceed farther into the recognition of the decaying processes, the distributions of the thermogravimetric analysis (TGA,
PerkinElmer Diamond TG/DTA) and Fourier transform infrared
spectroscopy (FTIR, PerkinElmer/Spectrum 100) of the fresh and
tested catalysts are presented in Fig. 4. In the TGA, the flow rate
of air and the heating rate are 100 mL min1 and 10 °C min1,
respectively; the curve of FTIR is obtained from the wave number
of 2363 cm1, representing the intensity of CO2 emission [34]. For
the fresh catalyst, a monotonic decrease in TGA is exhibited
(Fig. 4a), resulting from the consumption of combustible constituents contained in the catalyst. Meanwhile, the FTIR curve suggests
97
W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
catalyst can be ignored. These results indicate that the descent in
the performance of the catalyst with time (Fig. 3a and b) is attributed to the behavior of coking occurring on the catalyst when it is
used for DME synthesis. This leads to the deactivation of the catalyst [38]. Though coke deposition causes catalyst deactivation, the
function of the active sites for DME synthesis is not affected. Therefore, the DME selectivity almost remains constant with time. Similar phenomenon has also been observed in the study of Stiefel
et al. [39]. Alternatively, the increase in the TGA curve for the temperature range of 160–413 °C is a consequence of oxidation of Cu
to CuO (Fig. 4a).
(a) Conversion
70
Conversion (%)
60
CO conversion
50
40
30
20
H 2 conversion
10
0
3.2. Thermodynamic analysis
175
200
225
250
o
Temperature ( C)
100
(b) Selectivity
90
Selectivity (%)
80
70
DME selectivity
60
50
40
30
MeOH selectivity
20
10
0
175
200
225
250
o
Temperature ( C)
(c) DME yield & bed temp
Bed temperature
Yield (g/gcath)
2.5
350
o
312 C
300
281 oC
2
275
1.5
250
o
1
231 C
225
0.5
0
325
296 oC
o
Bed temperature ( C)
3
200
175
200
225
250
175
Temperature (oC)
Fig. 7. Profiles of (a) CO and H2 conversions, (b) DME and MeOH selectivities and (c)
DME yield and bed temperature of Case 2.
that the formation of CO2 is fairly slight (Fig. 4b). In contrast, when
the TGA and FTIR curves of the catalyst undergoing the durability
test are examined, it can be seen that the weight loss is observed
when the temperature is higher than 413 °C. With further examining the curve of FTIR, it is realized that the weight loss is due to the
reaction of carbon with oxygen at the catalyst surface so that the
formation of CO2 grows with time. In the studies of Sierra et al.
[35,36], it was pointed out that there were two possible causes
of catalyst deactivation in DME synthesis; one was coke deposition
on the active sites of the metallic and acid functions, and the other
was sintering of the metallic function. When the temperature was
below 325 °C, the sintering in CuO–ZnO–Al2O3 catalysts would not
occur [37]. It will be described later that the bed temperature is below 325 °C in the present study. So the behavior of sintering in the
To evaluate the effect of CO2 on DME synthesis, the thermodynamic analysis is carried out using the commercial software HSC
Chemistry 7.0 where six species of H2, CO, CO2, steam, DME and
MeOH are simultaneously considered. In the analysis, the molar ratio between H2 and CO in the feed gas is fixed at 2 and the total
amount of CO is 1 mol. Meanwhile, the total pressure of the reaction system is 40 atm. When the CO2/CO ratio increases from 0
to 0.5, Fig. 5a and b depict that a lower reaction temperature is
conducive to CO and H2 conversions, as a result of exothermic reaction of DME synthesis involved. For higher temperatures such as
225 and 250 °C, the concentration of CO2 plays a significant role
in determining CO conversion because CO2 suppresses DME formation, as shown in Eq. (4). Alternatively, when the reaction temperature is lower, say, 150 °C, increasing CO2 concentration facilitates
the reverse WGSR, as shown in Eq. (3). This results in that the H2
conversion in DME synthesis tends to increase. With attention
shifted to the DME and MeOH selectivities, Fig. 5c and d reveal that
a higher temperature or CO2 concentration disadvantages DME formation, whereas it is conducive to MeOH generation. Despite an
unfavorable environment to DME formation at a higher temperature or CO2 concentration, overall, the selectivity of DME (>93%)
is much higher than that of MeOH (<7%). The profile of DME yield
is displayed in Fig. 5e where it is defined as the following
DME yield ðmol=mol COÞ ¼
Produced DME ðmolÞ
Initial CO ðmolÞ
ð8Þ
As a whole, the DME yield is in the range of 0.33–0.41 mol (mol
CO)1 and it is mainly affected by the reaction temperature. In contrast, the CO2/CO ratio just has slight influence on the yield. In the
study of Aguayo et al. [40], the characteristics of DME synthesis
with one-step process were studied kinetically and thermodynamically where two different gas mixtures of H2 + CO and H2 + CO2
were taken into account. In their thermodynamic analysis, the
DME yield from the gas mixture of H2 + CO2 was lower than that
from H2 + CO under the same operating conditions. They also found
that increasing temperature disadvantaged the formation of DME.
Basically, the observed results in the present study (i.e. Fig. 5) are
consistent with those of Aguayo et al. [40].
3.3. Reactions of direct DME synthesis
The reaction characteristics of DME synthesis through the direct
method at four different operating conditions named Cases 1, 2, 3
and 4 are examined in Figs. 6–9, respectively. Detained operating
conditions are listed in Table 1. For Case 1 where the operating
pressure is 20 atm and 5 vol.% of CO2 is contained in the gas mixture, Fig. 6 depicts that the better operation occurs at 225 °C in that
the CO and H2 conversions are relatively higher (Fig. 6a). However,
the CO and H2 conversions are 29.4% and 17.6%, respectively, implying that the performance of DME synthesis is low. With the aforementioned temperature, it can be seen that the DME selectivity
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W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
(a) Conversion
(a) Conversion
50
45
45
40
Conversion (%)
35
30
25
20
15
H 2 conversion
10
30
25
20
15
H 2 conversion
10
5
5
0
CO conversion
35
175
200
225
0
250
175
Temperature ( C)
100
90
90
Selectivity (%)
Selectivity (%)
70
DME selectivity
60
50
MeOH selectivity
40
30
70
DME selectivity
60
50
40
MeOH selectivity
30
20
20
10
10
0
175
200
225
175
250
Temperature ( C)
(c) DME yield & bed temp
3
o
251 C
250
o
219 C
225
0.5
Yield (g/gcath)
300
275
1.5
250
(c) DME yield & bed temp
2.5
325
o
o
2
225
Bed temperature
Bed temperature ( C)
311 oC
Bed temperature
289 C
Yield (g/gcath)
3
350
2.5
200
Temperature (oC)
o
0
250
80
80
1
225
(b) Selectivity
(b) Selectivity
100
0
200
Temperature (oC)
o
350
o
319 C
o
300
2
275
257 oC
1.5
250
o
1
226 C
225
0.5
200
200
175
200
225
250
0
175
175
and yield are 87% and 1.04 g (gcat h)1, respectively (Fig. 6b and c).
In examining the temperature distribution shown in Fig. 6c, when
the set temperatures is 175 °C, the temperature in the catalyst
bed is 227 °C, stemming from the exothermic nature of DME synthesis. For the set temperature of 250 °C, the bed temperature is
306 °C which is close to the calcined temperature (350 °C) of the
catalyst. This may be the reason that the performance of DME synthesis at 250 °C is somewhat lower than that at 200 °C.
When the reaction pressure increases to 40 atm (Case 2), the
better DME formation also takes place at 225 °C (Fig. 7a) and the
CO and H2 conversions are significantly enhanced. Specifically,
the values of the former and the latter are 57.1% and 33.1%, respectively, suggesting that they are approximately amplified by a factor
200
225
Temperature
Temperature (oC)
Fig. 8. Profiles of (a) CO and H2 conversions, (b) DME and MeOH selectivities and (c)
DME yield and bed temperature of Case 3.
325
299 C
250
o
Conversion (%)
40
CO conversion
Bed temperature ( C)
50
175
(oC)
Fig. 9. Profiles of (a) CO and H2 conversions, (b) DME and MeOH selectivities and (c)
DME yield and bed temperature of Case 4.
Table 1
A list of various operating conditions.
Case
Pressure (atm)
Gas mixture H2/CO/CO2/N2 (Vol.%)
ZSM5 (g)
1
2
3
4
20
40
40
40
61/30/5/4
61/30/5/4
48/32/16/4
48/32/16/4
0.2
0.2
0.2
0.4
of 2 when compared to Case 1. Meanwhile, the DME selectivity and
yield are 90% and 1.89 g (gcat h)1, respectively (Fig. 7b and c). By
virtue of more DME synthesized at 40 atm, more heat is generated
in the catalyst bed so that the bed temperature is 312 °C when the
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W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
(a) CO conversion ratio
(a) DME yield
2.5
2.25
2
80
Case 1
Case 2
Case 3
Case 4
Ratio (%)
Yield (g/gcath)
Case 1
60
Case 3
1.5
1.25
1
50
40
30
20
0.75
10
0.5
0
175
0.25
225
200
250
o
80
(b) CO2 increasing ratio
Case 1
Case 2
Case 3
Case 4
Case 2
Case 3
Case 4
50
0.8
0.7
40
30
20
0.6
0.5
10
0.4
0
175
0.3
200
225
250
Tempereture (oC)
0.2
0.1
80
0
175
250
Case 1
60
Ratio (%)
CO2 increasing ratio
0.9
225
(b) H 2 conversion ratio
70
1.2
1
200
Tempereture (oC)
Temperature ( C)
1.1
Case 2
Case 4
1.75
0
175
70
200
225
250
70
o
Temperature ( C)
Case 1
Case 2
60
Fig. 10. Profiles of (a) DME yields and (b) CO2 increasing ratio at various operating
conditions.
Case 3
Case 4
Ratio (%)
set temperature is 250 °C (Fig. 7c). The feature of increasing pressure
facilitating DME synthesis is consistent with the thermodynamic
equilibrium or Le Chatelier’s principle. In the study of Moradi
et al. [32] where CuO–ZnO–Al2O3 catalysts were prepared, with
the conditions of H2/CO = 64/32, 40 atm, 240 °C and the space
velocity of 1000, the obtained CO conversion and DME selectivity
were in the ranges of 2–42% and 20–68%, respectively. In the study
of Venugopal et al. [41], with the catalyst of Cu–Zn–Cr/c–Al2O3 and
the operating conditions of H2/CO = 60/40, 40 atm, 250 °C and the
space velocity of 6000, the CO conversion was between 19% and
35%. In Case 2 with the set temperature of 250 °C and the space
velocity of 15,000, the CO conversion and DME selectivity are
51% and 88%, respectively. This reveals that the performance of
DME synthesis in the present study is superior to the aforementioned study, even though the space velocity is as high as
15,000 mL (gcat h)1.
When the CO2 content in the gas mixture is increased to 16%
along with the conditions of 40 atm and 0.2 g ZSM5, namely, Case
3, a comparison to Case 2 it is apparent that the CO and H2 conversions decline markedly. It follows that the performance of DME
( c) DME yield ratio
50
40
30
20
10
0
175
200
225
250
Tempereture (oC)
Fig. 11. Profiles of (a) CO conversion ratio, (b) H2 conversion ratio and (c) DME yield
ratio at various operating conditions.
synthesis is obviously affected by CO2 contained in the feed gas.
The optimum temperature for DME synthesis and selectivity also
develops at 225 °C (Fig. 8a and b) where the bed temperature is
289 °C; however, the DME yield is decreased to 1.11 g (gcat h)1
(Fig. 8c). In Fig. 9, the influence of ZSM5 amount on DME synthesis
is examined where 0.4 g of ZSM5 is blended with the catalyst and
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W.-H. Chen et al. / Applied Energy 98 (2012) 92–101
the CO2 content and the pressure are 16% and 40 atm, respectively
(Case 4). When Figs. 8 and 9 are compared with each other, it can
be seen that the difference in the conversions, the selectivities and
the bed temperature is slight. For the set temperature of 225 °C,
the DME yield is 1.28 g (gcat h)1, approximately higher than Case
3 by 10%. It is thus addressed that more than 0.2 g of ZSM5 blended
with 1 g of catalyst just has slight influence in enhancing DME
synthesis.
3.4. Characterization of direct DME synthesis
According to the results shown in Figs. 6–9, the profiles of DME
yield and CO2 increasing ratio at the four different operating conditions are summarized in Fig. 10a and b, respectively. The CO2
increasing ratio is given by
CO2 increasing ratio ð%Þ ¼
Prodcued CO2 ðmolÞ
1 100
Feeding CO2 ðmolÞ
ð9Þ
Obviously, a decrease in pressure or an increase in CO2 concentration has a pronounced impact upon DME synthesis. In the four
cases, Case 2 provides the best performance of DME synthesis
(Fig. 10a) so that its CO2 increasing ratio is the highest (Fig. 10b).
Fig. 10b also reflects that 0.2 g of ZSM5 is sufficient to be blended
with 1 g of catalyst for DME synthesis in that the curves of Case 3
and Case 4 almost overlap together. Overall, the highest DME yield
is exhibited at 225 °C, regardless of what the case is tested
(Fig. 10a). At this temperature, the bed temperature is in the range
of 289–299 °C. Accordingly, it should be addressed that the reaction mechanisms can be divided into two different regimes from
the performance of the prepared catalyst. When the reaction temperature is lower than 225 °C, increasing temperature intensifies
DME yield, revealing that the chemical kinetics dominates DME
synthesis. Alternatively, once the temperature is higher than 225 °C,
thermodynamics is the dominant mechanism in determining
DME synthesis in that the yield decreases with increasing temperature and this behavior is consistent with Le Chatelier’s principle.
Eventually, the experimental results are compared with the thermodynamic predictions and shown in Fig. 11. It is evident that
the maximum CO and H2 conversions as well as DME yield in Case
2 are close to 60% of the theoretical ones. In the other cases, the
values are always below 40%. It is thus concluded that if one intends to promote DME synthesis, the higher the reaction pressure
and the lower the CO2 concentration in the feed gas, the better the
DME production.
4. Conclusions
In the present study, a bifunctional Cu–ZnO–Al2O3/ZSM5 catalyst for DME synthesis has been prepared using a coprecipitation
method. Four different cases by varying reaction pressure, CO2 concentration in the gas mixture and the blending ratio of ZSM5 to the
catalyst under the condition of a high space velocity of 15,000 mL
(gcat h)1 have been tested through the one-step process. A thermodynamic analysis has also been carried out to recognize the
DME synthesis behavior. The conclusions are drawn below:
1. The thermodynamic analysis indicates that increasing
reaction temperature and CO2 concentration leads to a
decrease in CO and H2 conversion as well as DME selectivity. Therefore, from the practically operating point of view
for DME production, an environment with lower temperature and CO2 content as well as higher pressure is better.
2. However, from the experimental results in the four cases,
it is found that the maximum DME yield always occurs
at 225 °C where the bed temperature is in the range of
289–299 °C. This clearly suggests that the reaction phenomena governed by chemical kinetics turns to be dominated by thermodynamics when the temperature is
higher than 225 °C. In Case 2 where the CO2 concentration
is 5% and the pressure is 40 atm, the DME yield can reach
up to 58% of the theoretical result.
3. For the influence of CO2 on DME synthesis, the DME yield
significantly drops when the CO2 concentration increases
from 5% to 16%. This reflects that CO2 should be captured
and separated after the syngas is produced and before it
is used as the feedstock of DME synthesis. The experiments also indicate that the blending ratio of ZSM5 to
the catalyst at 0.2 is an appropriate operating condition
for DME production.
Acknowledgements
The authors acknowledge the financial support of the National
Science Council, Taiwan, ROC, in this research.
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