INTRODUCTION
In our previous studies, MTBE and ETBE were synthesized at the atmospheric
condition by the reaction between methanol or ethanol with tert-butyl
alcohol which is a co-product of propylene oxide synthesis from iso-butane
and propylene see Matouq and Goto (1993), Matouq et al. (1993,
1994) and Yin et al. (1995). The reactive distillation was developed
by our previous studies to enhance the production rate of MTBE (combined
with pervaporation) and ETBE to achieve the purpose of continuous production
as mentioned by Matouq et al. (1996).
As a successive to our previous studies, our aim here is to synthesize
TAME using a micro-channel reactor technique. Microreactors have some
advantages over the conventional batch reactors, such as: the low residence
time avoids undesirable side reactions, a continuous-flow system can be
created which allows for the use of the same microdevices for large quantity
production and save energy, due to its lower size as reported by Doku
et al. (2005). Thus, it is anticipated to synthesize TAME while
avoiding the undesired reaction of dehydration of tert-amyl alcohol.
Micro-reactors are composed of channels with sub-micrometer to sub-millimeter
dimension in width and depth, fabricated by using micro-technology and
precision engineering. As one of the applications, micro-channels are
noticeable since they are suitable for multiphase reactions. Many kinds
of micro-reactors were proposed and applied to various liquid phase reactions,
which were summarized by Hessel et al. (2004). Among various advantages
of micro-reactors, if there is laminar flow inside the reactor will help
to separate each phase at the outlet, which will eliminate the post-treatment
of reaction mixtures for further separations, according to Ehrfeld et
al. (2000). The small channel dimension makes larger surface area
compared to volume ratio, which can reduce the mass transfer limitations
between two-phase systems such as organic-aqueous reaction systems as
reported by Emig and Liauw (2002). In order to apply these reaction systems
to micro-reactors, it is important to obtain stable multiphase laminar
flow pattern inside the reactor (Tagawa et al., 2007). Tosoh Corporation
proposed to make guideline structure in the micro-channel in order to
stabilize the laminar flow of multi liquid phase system as reported by
Maruyama et al. (2004). Partition walls, the so called guideline
structure, were fabricated by a photolithographic wet etching method in
the centre of the channel. The promotion effect of this guideline structure
was tested in the solvent extraction and a small turbulence was proposed
after each guideline wall, which promoted diffusion between organic-aqueous
interfaces as mentioned by Yamakawa et al. (2004).
TAME (tert-amyl methyl ether, 2-methoxy-2-methylbutane) is synthesized
in an acid-catalyzed equilibrium reaction of iso-amylenes (2-methyl-1-butene
and 2-methyl-2-butene) with methanol (MeOH), as reported by Oost and Hoffmann
(1996). Tertiary ethers are used in gasoline as octane-enhancing agents.
In addition to the increase in octane ratings, there are other improvement
to gasoline quality which are obtained by blending oxygenates. With increasingly
strict legislation, the need for tertiary ethers has grown steadily and
has turned the focus from MTBE (methyl tert-butyl ether, 2-methoxy-2-methylpropane)
towards heavier ethers like TAME (tert-amyl methyl ether, 2-methoxy-2-methylbutane),
since the supply of isobutene is limited and the future of MTBE is uncertain
due to banning in certain regions in the USA from the beginning of 2004.
In this study, micro-channels with guideline structure, were tested for
the synthesis in liquid phase using sulphuric acid. Partition walls or
so called guideline structure were designed in the middle of micro-channel
with adequate intervals in order to stabilize the laminar flow.
C5H12O (l) +
CH3OH (l) → C2H5C(CH3)2OCH3
(l) + H2O (l) |
TAME synthesis follow the above chemical reaction equation. This reaction
provides a good model for high selectivity in the presence of strong acid
catalyst, compared to those of results for batch reactor in homogeneous
phase. This experiment is difficult to conduct in a conventional batch
reactor due to the high reaction rate for dehydration of TAA, soon upon
adding the liquid catalyst smoke of dehydrated TAA will come up (iso-amylenene)
and hence the etherification reaction yield will be low if this reaction
take place as batch one. To our knowledge, the application of micro-channels
for this type of etherification reaction is not studied yet. Therefore,
in this study we aim at using this technique to synthesize tert-amyl
methyl ether (TAME) in liquid phase from the reaction between methanol
(MeOH) and tert-amyl alcohol (TAA) at a continuous process.
MATERIALS AND METHODS
The microchannel reactor type TSR 018B-05 was used. This reactor with
a guideline structured for three phase flow system is fabricated and supplied
by the Japanese Tosoh Corporation using photolithographic wet etching
method on a Pyrex glass plate. The detail of the guideline with its design
aspects is demonstrated in Fig. 1. As is shown in Fig. 1, partition walls
were constructed at the middle of the channel with adequate intervals.
This structure was called guideline structure of 100 μm length and
100 μm intervals.
The manufactured Pyrex glass micro-channel reactor was fixed between
two stainless steel holders equipped with three punched wells at the end
of their two edges. The three wells were connected to three HPLC pumps
type JASCO-PU980. One pump was assigned to feed sulphuric acid (H2SO4)
with different molarity concentration. Other two HPLC pumps were assigned
to feed TAA and MeOH separately. The outlets were collected in the three
punched wells. The collected materials from the wells were directly analyzed
using gas chromatograph, type GL Science 353, packed with TC1-GL science
capillary column.
Specification of the tested reactor is as follows: type TSR-018-B-05,
length = 30.0 mm, width = 282.6 μm, depth = 42.0 μm. Two guidelines
have length = 100 μm, interval 100 μm.
Analyses were conducted by using a gas chromatograph, type GL Science
353, packed with TC1-GL science capillary column. The specification of
the column was 30 m long and internal diameters of 0.25 mm. The column
temperature was fixed at 372 K and injection at 453 K.
The MeOH (99%) and TAA (98%) supplied by Wako were used without any further
purification as reactants. TAME (98%) supplied by Aldrich was used for
calibration curve construction. All experiments were viewed under OlympusCX41
type microscope equipped with digital camera for taking picture.
Three JASCO-PU980 type-high pressure pumps (HPLC) were attached to the
reactor by a capillary Teflon tube. Each inlet was drawn at a specified
flow intake. When the desired amount of the flow of the three inlets was
adjusted digitally, start on button was pushed to allow the flow to go
inside the reactor with a moderate pressure (3-5 kg cm-2).
When the flow started (this was confirmed under microscope) stopwatch
start button was pushed.
|
| Fig. 1: |
Experimental reaction apparatus and the details of micro-channel
with guideline structure |
This was considered as the initial time for the reaction. Each 30 min
0.5 mL samples from the three outlets were collected. One sample of the
collected amount was soon injected inside the column and analyzed, while
others were kept at refrigerator. These precautions were taken to avoid
any assumption of further reaction while waiting for the next analysis.
All samples were stopped after 2 h and 30 min. During and after analysis
by GC the steady state was completely assured.
RESULTS AND DISCUSSION
After several experimental runs, where the ultimate goal was to obtain
real three phase of reactants flow inside the microchannel reactor`s lane,
three flow rates combination were obtained and recorded as follows: H2SO4
= 0.05 mL min-1, TAA = 0.09 mL min-1 and MeOH =
0.20 mL min-1. At these flowrates the best distribution of
the reactants with the catalyst was observed inside the reactor and between
the guidelines Fig. 2 shows this pattern. These conditions were considered
as standard, while other experimental runs were conducted by changing
these flowrates as variables.
MeOH and TAA, the fixed flowrates for the reactants, were investigated
while changing the feed position of the catalyst on the TAME production.
Catalyst H2SO4, was introduced as the third component
from the third inlet of the reactor at it specified flow rate. The three
components will flow inside the reactor in the three lanes, no turbulent
occurred and each lane can clearly be seen without an overlap (apparently
laminar streams). Guidelines here played an important role to keep each
component flowing in its own lane. All experiments were conducted at room
temperature 298 K, while exposing the upper side of the micro-reactor
to hot air flowing at the top surface of the reactor. The measured temperature
at the ambient close to the upper side surface of the reactor was found
equal to 343 K. The conducted experiments at different parameters are
summarized in Table 1. Other related parameters were changed based on
these conditions, such as the position of the inlet of the catalyst and
its concentrations.
To study the effect of the position of the catalyst feed to the reactor
on reaction conversion, two-catalyst flow inlets position were changed.
First part of experiments deals with the effect of introducing the catalyst
at the top inlet of the reactor with two different concentrations. While
the second part deals with the effect of introducing the catalyst at the
middle inlet of the reactor with two different concentrations. These selections
will allow the MeOH to flow in the bottom lane and TAA in the top lane
of the reactor and vice versa.
|
| Fig. 2: |
Flow pattern inside the microreactor at standard conditions: flowrate
of TTA = 0.09 mL min-1, at Top lane, MeOH = 0.2 mL min-1
at bottom and 0.05 H2SO4 = 0.05 mL min-1
with 0.5 M in middle lane, (a) inlet of the reactor, (b) outlet of
the reactor and (c) middle of the reactor |
To investigate the effect of the flowrates on the conversion at the same
pattern of flow, catalysts with two different concentrations 1 and 0.5
M were chosen.
The selected flowrates (Runs no.1-8) were given at a stable flow pattern
inside the reactor and no turbulent flow was noticed in all conducted
runs. The mole fraction for this standard condition was 22% for MeOH and
37% for TAA. However this will not give an equimolar ratio for the reactant.
This is due to the fact that at equimolar flowrate of MeOH and TAA reactants,
there were no distinctively three lanes inside the reactor compared to
the standard flowrate condition shown in Fig. 3. This is due to the low
flowrate of the reactants in comparison with the standard condition, therefore
the reactant and the catalyst will not fill the lane adequately and will
widen the middle lane and decrease the contents of the reactants in the
top and bottom lanes. Even though to assure the effect of equimolar ratio
on the production of TAME two runs of experiments was conducted (Run No.
9 and 10).
| Table 1: |
Experimental conditions (TAA = tert-amyl alcohol, MeOH =
Methanol) |
 |
|
| Fig. 3: |
Inlet flowrate pattern at equimolar flow rate of TAA = 0.09 and
MeOH = 0.032 mL min-1 |
Effect of the feeding position of catalyst and concentration: Figure
4 shows the result of TAME production when the feeding position of the acid
was varied between top and middle inlets of the reactor at the standard flowrates
of reactants.
|
| Fig. 4: |
The effect of catalyst flowrate on TAME production for the three
top, middle and bottom outlets of the micro-channel reactor |
 |
| Fig. 5: |
Forming micelle at the top lane of the reactor when acid is introduced
through top inlet, while TAA and MeOH form one layer at the bottom
and middle lanes |
When H2SO4 flowed through the top lane of the reactor
(Run 3), the TAME production was higher if compared with Run 1, in which
the acid was introduced in the middle inlet of the reactor. This behavior
can be explained as feeding the acid from the top inlet and keeping TAA
and MeOH at middle and bottom lanes, exactly below the acid flow lane,
will give the chance for the two reactants to be mixed together and pull
the acid towards the middle lane of the reactor and mix to react there.
Therefore, we can consider the middle lane as the core of the reaction
zone phase, while the other lanes as separation zones. It was also noticed
that during the experimental work, when acid (H2SO4)
was introduced through the top lane of the channel, the catalyst distributed
itself within the TAA and MeOH which forms one lane layer and another
zone of micelle at the top lane with acid (Fig. 5).
|
| Fig. 6: |
The effect of changing the catalyst concentration from 1-0.5 M on
TAME production for the three top, middle and bottom reactor outlets |
|
| Fig. 7: |
The effect of reducing reactants flowrates on the TAME production
at 1 M acid concentration to half of its standard conditions |
When the flow was reduced to half of the standard condition the yield was
noticed to be higher. This may be attributed to the fact that lower flow
rate, will increase the residence time inside the reactor and more reaction
will develop.
Figure 6 shows the effect of reducing the catalyst
concentration from 1-0.5 M at the same standard flowrates conditions.
Comparing Fig. 4 and 6 clearly shows
that the production of TAME was reduced significantly. The mole fraction
has almost reduced ten times while the flow-rates of all other components
have been kept the same. This is expected because the reaction between
two alcohols is highly dependent on acid existence and concentration it
is a strong acid catalytic reaction.
Effect of the reactants flowrate on TAME production: Figure
7 shows the result of reducing the reactants flowrates to half of
its flow in its standard condition on TAME production, while keeping acid
concentration at 1 M. The figure shows that there is a slight reduction
in TAME mole fraction at the top and the middle, while there is a big
reduction for TAME at the bottom outlet of the reactor compared to Fig. 4.
|
| Fig. 8: |
The effect of acid concentration reduction from 1-0.5 M, on TAME
production when the reactants flowrate reduced to half of its standard |
 |
| Fig. 9: |
Effect of equimolar ratio for reactants on the TAME production |
This behavior can be attributed to the fact that when the flow rate is reduced
the bottom lane (which is MeOH) will be less in its thickness and hence
the produced TAME will be distributed in that layer lowering its concentration
in the bottom lane. Keep in mind that the production of TAME will be lower
when the flowrates reduces.
Figure 8 shows that when the acid concentration was
reduced to its half molarity, there was a clear significant reduction
on TAME mole fraction in all the top, middle and bottom reactor outlets.
This is clear when it is compared to Fig. 6. This behavior has been explained
before.
Equimolar feed flow rate: Figure 9 shows the
mole fraction of TAME at three outlets of the reactor, when the reactants
fed at equimolar ratio. As is shown the mole fraction is much lower when
reactants fed in equimolar base. This is due to the fact that, when the
feed is adjusted according to its molar ratio the flow rates will not
give the reactant the chance to fill each lane in its proper way. Therefore,
if we adjust in molar ratio it is difficult to get a well distributed
three phase lanes inside the reactor as mentioned before in Fig. 3. Therefore
the TAME production will be less in the outlets compared to previous results.
CONCLUSIONS
The microchannel reactor was sued here successfully to produce TAME in
the liquid phase reaction between methanol and ter-amyl alcohol in the
presence of strong acid. The criteria here are to investigate the best
flowrate for the reactants meanwhile the catalyst is an important element
to keep the three lanes of the reactor fill adequately with reactants.
After that the molar ratio between the reactants was adjusted. The best
flowrates were calculated as follows: TTA = 0.09 mL min-1,
MeOH = 0.2 mL min-1 and H2SO4 = 0.05
mL min-1. The flow rates of the reactants shows a significant
effect on the production of TAME, as the flow rate of the reactants increases,
TAME will decrease. This can be explained mainly with considering the
distribution of the reactants in the three lanes of the reactor. This
behavior has been clearly noticed when equimolar flowrates of reactants
was fed to the reactor. Although it is expected to have the highest conversion
when equimolar ration was fed, but a drawback of conversion was noticed
as the flow pattern inside the reactor was not distributed well. This
will lead to a solid conclusion that it is necessary to determine the
best flowrate inside this kind of reactor and then calculate the molar
ratio based on this flow.
In this study, it was clear that the concentration of acid is very important,
due to the fact that it decreases the production of TAME will also decrease.
This is a normal behavior as the concentration of catalyst decrease the
conversion will also decrease in such catalytic reaction.
It is noticed here that there is a strong relation in the feeding place
between the catalyst and the conversion on the reactants. When the catalyst
is introduced at the top of the inlet of the reactor the TAME mole fraction
in the outlets is higher than when the catalyst is introduced in the middle
inlet of the reactor. This is due to the dynamics behavior of the fluid
flow. When the catalyst is introduced in the top inlet of the reactor
it will give the chance for the TAA and MeOH to be mixed in the middle
lane of the reactor and serve as reaction and separation zone at the same
time, while introducing the catalyst in the middle will reduce the chance
for the TAA and MeOH to be mixed in the middle lane and hence the middle
lane will serve as mixing and then reaction. This behavior explains why
the TAME mole fraction is the highest at the middle outlets in all the
experimental runs.
ACKNOWLEDGMENTS
This research will not see the light without the contribution of many
people and organizations. First, Japan Student Services Organization (JASSO)
whose financial support as a fellowship grant to the correspondence author
during his stay in Nagoya University from the period 1st July-30 July
2006, is really appreciated. Second, Mr. Masakazu Shimizu from JASSO organization
(Exchange and Followup Division) and Ms. Shina Kawazu from Nagoya University
whose efforts for administration follow up and support is really appreciated.
Third, Mr. Itoh from Nagoya University who the authors are indebted to
for his technical support. Finally the correspondence author is grateful
to Nagoya University for hospitality.
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