International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
Effects of Temperature and Space Velocity on Product
Distribution during Ethanol Conversion over ZSM-5
Mutasim H. Elhussien1, Yusuf M Isa2
1
Chemistry Department, Faculty of Education, Nile Valley University, Atbara, Sudan
Chemical Engineering Department, Durban University of Technology, Durban, South Africa
2
Converting ethanol to ethylene and water and
subsequent transformation of ethylene to other
hydrocarbons involves a number of complex reactions such
as dehydration, oligomerization, cycling, cracking,
alkylation among others [5-8]. The product obtained during
catalytic conversion of ethanol is usually a function of
reaction conditions such as temperature, pressure, space
velocities and catalyst type. ZSM-5 has been known to
have high activity in the conversion of alcohols to other
fuel range hydrocarbons [9-11].In the present study, the
product distribution of catalytic conversion of ethanol was
investigated as a function of
temperature and weight
hourly space velocity.
Abstract— Commercially obtained ZSM-5 has been
successfully protonated and used as catalyst for the
conversion of ethanol to fuel range hydrocarbons. 100%
conversion was achieved in all conversion runs. At WHSV of
1h-1, the yield of liquid hydrocarbons was not affected much
by temperature. At 300 and 350°C and WHSV of 2h-1, the
highest yield of liquid hydrocarbons was obtained.
It is observed that both temperature and space velocities
can affect the selectivity to gasoline and diesel range
hydrocarbons during ethanol conversion over protonated
ZSM-5
Keywords—ZSM-5,
gasoline
range
selectivity, activity, conversion, ethanol.
hydrocarbons,
I. INTRODUCTION
II. MATERIALS AND METHODS
Stringent rules on the quality of fuels and their impacts
on the environment have led to more research in the area of
renewable fuels. Solar, bio, wind as well as tidal energy all
look promising as potential future energy sources that can
augment to the present contribution by fossil fuels.
However, despite many forms of energy being renewable
and environmentally friendly not all tend to have the
capacities of supplying exactly the same product slate
obtainable from crude oil. Ethanol, butanol and other
alcohols obtainable from fermentation are renewable and
are capable of reducing the net greenhouse gas emissions
drastically [1-4]. Presently alcohols particularly ethanol is
used in different places as a fuel blend for internal
combustion engines [2, 4]. Despite their good calorific
values, alcohols are disadvantaged as fuels in terms of their
hygroscopic nature and the need to modify engines that can
run on 100% ethanol. The latter reason has contributed
largely to the limitations of the amount of ethanol that can
be used as a blend in fuels. One of the possible routes of
exploiting the renewability of ethanol as a fuel without
necessarily having to forgo its universality as a fuel is to
find a way of transforming ethanol to products that are
chemically similar to the numerous products obtained from
crude oil.
ZSM-5 catalysts was obtained from zeolite international
in its sodium form. Ethanol was obtained from a local
supplier in Cape Town, South Africa. To obtain the
protonated form of the ZSM-5 catalyst, the required
amount of ammonium chloride was used for ion exchange
with the Na-ZSM-5 under reflux conditions. The obtained
HZSM-5 was filtered from the mother solution and allowed
to dry overnight after which it was calcined for a period of
6 hours at a temperature of 480°C in a tube furnace. The
prepared H-ZSM-5 was later used as a catalyst for ethanol
conversion. The catalyst was characterised using XRD,
SEM and EDS
In a typical conversion experiment, a HPLC pump was
used to supply ethanol into a stainless steels reactor with a
fixed bed catalyst. Quartz wool was used to hold the
catalyst in place. The reactor was heated in a tubular
furnace. The reactor effluent was collected after cooling
and the gases were separated from the liquids (water and
hydrocarbons) before the hydrocarbon products were taken
for analyses using a Varian 2014 GC. The weight hourly
space velocities were varied as well as the reaction
temperature.
29
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
It was observed that the morphology did not change
much after protonation of the commercially obtained ZSM5. It is however observed that the average particle size of
catalysts was 200 µm and better synthesis conditions could
favour the formation of nano materials
III. RESULTS
Both the catalysts synthesized as well as the distribution
products were characterised. The yield was determined
based on the amount produced and the theoretical amount
expected. The conversion was determined based on the
product analyses and a 100% conversion was considered if
there were no traces of the feed in the final products.
A. Catalyst characterisation
At a magnification of 5000 as shown on figure 1, the
energy dispersive spectroscopy (EDS) was used to estimate
the elemental composition of the protonated catalyst. Table
1, shows the detected elements and their respective signals.
The high amount of carbon is attributed to the fact that
carbon coating was used in preparing the samples for the
analyses. The results shows that the Silicon to aluminium
ratio was about 30.
Figure 2: Micrograph of HZSM-5
The X ray powder diffraction of the protonated catalysts
showed that the characteristic peaks for ZSM-5 (2theta = 79° and 22-25°) catalyst were maintained and no other form
of impurities were identified implying that the protonation
step did not result in a change in the phases that were
present in the commercially obtained ZSM-5
B. Conversion of ethanol over ZSM-5 catalyst.
The conversion was carried out at atmospheric pressure.
All reactor conditions resulted in 100% ethanol conversion.
However there was a change in the ratio of liquid to
gaseous hydrocarbons as the reaction conditions were
changed. At a WHSV of 1h-1, it was observed that ZSM-5
had a higher selectivity to gaseous hydrocarbons (C1-C5).
The liquid yield at different temperatures was not more
than 21% as shown on figure 1. This low liquid yield could
be attributed to the catalyst being less active in
oligomersiation reaction that could lead to the formation of
higher hydrocarbons, it is also possible that catalyst activity
decreases as a number of reactions proceed. Changing the
weight hourly space velocity (WHSV) to 2 and 5h-1
resulted in a change in yield of liquid hydrocarbons. It was
also observed that the change in reaction temperature
resulted in more significant changes in the yield when
compared to the data obtained at WHSV of 1h-1.
Figure 1: Micrograph of HZSM-5 used to determine elemental
composition of the catalyst
Table 1:
EDS analyses of HZSM-5
Element
C
O
Si
Al
Atomic%
50.98
40.75
8
0.26
The protonated ZSM-5 as shown on figure 2 is seen to
have particles interwoven and forming prism like
structures, particles were found to be between 190 nm and
400 nm.
30
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
For the purpose of these studies, the products of ethanol
conversion were classified into gaseous (C1-C5), gasoline
(C6-C9) and diesel (C10+) range hydrocarbons. Figure 5
shows the selectivity to gasoline and diesel range
hydrocarbons at different temperatures. It is seen that at
WHSV of 1 h-1, at all reaction temperatures, the selectivity
to gasoline range hydrocarbons in the liquid is more than
70%. Interestingly enough, as the temperature is increased,
the selectivity to diesel range hydrocarbons reaches a
maximum at 350 °C, this further confirms that not only
cracking reactions are predominant in the conversion of
ethanol over ZSM-5 catalysts, other reactions such as
aromatization, dehydrogenation that lead to formation of
other heavier hydrocarbons are likely to emerge and
compete with the cracking reactions.
Effect of temeperature on Liquid
hydrocarbon yield (WHSV =1h-1)
Yield (%)
19,6
19,4
300
19,2
350
19
400
18,8
18,6
300
350
400
Temperature (degrees celsius)
Figure3: Effect of temperature on Liquid hydrocarbon yield
(WHSV=1h-1)
Liquid Product distribution of ethanol
conversion (WHSV = 1 h-1)
At WHSV of 2h-1, there was a liquid hydrocarbon yield
of 23% at 300 and 350°C.However at a temperature of
400°C it was observed that the gaseous hydrocarbon yield
increased by 23%, we suggest that this increase in gaseous
hydrocarbon yield resulted from cracking reactions. This
trend was however not observed when the WHSV was
changed to 5h-1. The liquid yield increased at 400°C when
compared to the other temperatures (figure 4)
Selectivity(%)
100
80
60
Gasoline Range
40
Diesel Range
20
0
Liquid
hydrocarbons(w %)
Liquid hydrocarbon yield during
ethanol conversion at WHSV of 2 and 5
h-1
300
350
400
Temperature(°C)
Figure 5: Selectivity of ZSM-5 to liquid hydrocarbons at different
reaction temperatures.
25
20
15
10
5
0
300
Table 2
Product distribution of ethanol conversion over ZSM-5
350
400
2
Produc
t
5
WHSV=2 h-1
WHSV =5 h-1
300°C
350°C
400°C
300°C
350°C
400°C
C1-C5
67
67
94.5
98
91
86
C6-C9
19
26
5
1.6
7.6
11
C10+
14
7
0.5
0.4
1.4
3
-1
WHSV(h )
Figure 4: Liquid hydrocarbon yield during ethanol conversion over
HZSM-5
A direct relationship was observed between the Liquid
hydrocarbon yield and reaction temperature. It is suggested
that unlike the yields at WHSV of 2h-1, At WHSV of 5h-1,
the residence time is not sufficient even at higher
temperatures to further ensure that most of the higher
hydrocarbons are cracked into gaseous hydrocarbons.
Working at high WHSV can greatly favour the productions
of gaseous hydrocarbons from bioethanol.
31
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
At weight hourly space velocities of 2h-1 and 5h-1, a
change in temperature resulted in a change in liquid
hydrocarbon yield. Despite the low yield of liquid
hydrocarbons, operating conditions affects the selectivity to
gasoline and diesel range hydrocarbons during ethanol
conversion.
As shown on Table 2, C10+ hydrocarbons were least
selective at temperature of 300°C and WHSV of 5h-1. We
suggest that the temperature was too low for
oligomerisation to heavier hydrocarbons. At a WHSV of
5h-1, the residence time may as well not be sufficient to
yield the desired reactions. However it can be seen from
table 2 that the selectivity for C10+ is not a function of
residence time alone since with increase in temperature
from 300°C to 400°C, the selectivity of C10+ increased
from 0.4 to 3 wt.%. At WHSV of 2h-1, selectivity to
gasoline range hydrocarbons passes through a maximum of
26 % at 350°C. A further increase in temperature to 400°C
results in just 5% selectivity to gasoline range
hydrocarbons, we suggest that an increase in temperature
favours the formation of cracking products and mainly
gases, this is evident from the very high selectivity to
gaseous hydrocarbons and only 0.5% selectivity to diesel
range hydrocarbons.
At WHSV of 5h-1, the selectivity to gaseous
hydrocarbons decreases at the temperature increased. This
further confirms that an increase in temperature could
result in a number of reactions that can as well compete
with cracking reactions. The relatively high concentration
of gasoline range hydrocarbons in the liquid products
400°C suggests that oligomerisation reactions do not
dominate at higher temperatures. It should however be
noted that the low residence time could be a reason for the
low selectivity to diesel range hydrocarbons at 400°C.
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IV. CONCLUSION
Commercially obtained ZSM-5 have been converted to
their protonated form without a loss in phase
characteristics. The protonated catalysts without promotion
were used in a fixed bed reactor for the conversion of
ethanol.100% ethanol conversion was achieved in all runs.
Selectivity to gaseous hydrocarbons was more than that to
liquid hydrocarbons. Temperature did not have a
significant effect on liquid hydrocarbon yields at WHSV of
1h-1.
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