Anhydrous recognised as an important biofuel [3]. The
Anhydrous ethanol has many uses within industry and serves
either as a raw material or chemical intermediates for various processes. In
the food and beverage industry, it is used to make liquor and enhance food
extract flavourings such as vanilla bean. It is also commonly found as an
ingredient in the pharmaceutical and medical industry for its antiseptic
properties. Due to its good solvating ability for water and organic compounds,
ethanol is a key solvent for the fabrication of paint, cosmetics, cleaning
agents etc. 1.
In 2017, about 118 million m3 of ethanol was produced worldwide.
With its growing demand for in many sectors, it is forecasted to increase by 9%
in the next 8 years 2.
Furthermore, ethanol is recognised as an important biofuel 3. The fuels industry
employs it as an additive to gasoline, usually with a blending percentage of 10
to 85% ethanol. As of current, only 3% of the gasoline market in the United
States is sold without the ethanol blend 4. Ethanol can be synthesised using
biomass such as sugarcane and corn via fermentation, therefore it is renewable
and can replace fossil crude. In 2017, the fuels industry shared 84% of the global
ethanol production 5.
The incorporation of ethanol into gasoline brings premium benefits such as
higher octane ratings, higher antiknock ability, and lower carbon footprint 4.
The desired purity of ethanol in industry ranges from 96 to
99.9 wt%, and is made according to the specific application 6.
In the fuels sector, there is a booming market for ethanol that has a standard purity
of 99.5 wt% (98.7 mol%) or more 7.
The high purity is essential as water can form immiscible mixtures with the
hydrocarbons fuel, and would result in a drop in fuel performance and engine
failure.
Azeotropes are formed when at least two different, but
close-boiling liquids are mixed. The mixture of ethanol and water forms a heterogeneous
minimum-boiling azeotrope, resulting in the mixture composition to be 95.6 wt% (89.5
mol%) at 78.15°C and 101.3 kPa 3.
Restricted by the identical compositions of ethanol in the vapour and liquid
phases, the purity achieved is limited and it cannot be separated from water by
conventional distillation. Hence, the azeotrope must be broken for ethanol to
achieve greater purity.
This limitation can be waived by using extractive
distillation with a solvent, which is also called an entrainer. The entrainer
enhances the separation by causing the relative volatility between the ethanol
and water to increase, therefore breaking the azeotrope. Extractive
distillation has been extensively studied in industry and literature, and is
established to be a reliable technology in azeotropic separation 7. It can yield good
process capacity and low energy usage 7
8.
The separation configuration to produce anhydrous ethanol
includes 3 units, as shown in Figure 1:
a pre-concentration distillation column for concentrating dilute ethanol
(<10 wt%) to near azeotropic composition; an extractive distillation column,
to further concentrate ethanol to 99.5%wt and above; and finally, a solvent
recovery column to recycle solvent used 9. The typical
solvents used are ethylene glycol, glycols, gasoline and benzene.
Solvent recovery
Extractive distillation
Pre-concentrator
Figure 1: Ethanol dehydration units configuration 9.
With the increasing ethanol production and awareness for
environmental sustainability, there is an incentive to ensure the ethanol
dehydration process is at its prime operability. For that matter, many authors are
focusing on the research for better solvents to replace the current conventional
solvents 10 11. This motivation
stems from the fact that these solvents, which are organic solvents, are hazardous,
flammable and toxic. As they have some degree of volatility, a clear separation
between ethanol and water cannot be achieved. Some of the solvent will be
evaporated to the top and contaminate the end-product. As a result, more
solvent and reflux are required to meet the demanded purity, leading to greater
operating costs 12.
Research and patent have shown that ionic liquids (ILs) are promising substitutions
13
14,
as they promote high separation and recovery as well as having zero emission.
ILs were first introduced in literature a century ago, but
has only been shown increasing amounts of interest in their potential uses in
the past 20 years 11.
They are molten salts with low melting points that are made of anions and
cations. Therefore, they generally exist as liquids at 100°C or below. Commonly used organic
cations in research and industry include imidazolium, pyridinium, ammonium,
phosphonium, pyrrolidinium etc. 15.
They are paired with either inorganic or organic anions, such as Cl, Br, AlCl4,
OAc, DCA, BF4, CH3COO etc. 16.
The different possible combinations of cations and anions allow properties of
ILs to be customised based on desired use, leading to the name 'designer
solvents'.
ILs have negligible vapour pressure, making them
essentially non-volatile, and therefore do not emit to the environment. Their
non-volatility allows ILs to be easily recovered in the bottom product of the
column and recycled back to the feed. Furthermore, they are non-flammable and are
stable towards heat and chemicals, allowing them to be safely used and
contained. Due to these unique and advantageous characteristics, they make good
candidates to substitute organic solvents whilst fulfilling performance and
sustainability aspects. For the extractive distillation of ethanol and water, the
use of ILs have been studied and proven to be feasible using process
simulations and lab experiments 8 17. Besides that, some
ILs are also found to be more effective than the conventional solvents, as they
imposed a lower heat duty, as demonstrated by Seiler et al. 13
Despite their remarkable features as solvents, ILs have several
drawbacks. Since they are relatively new to applications in industry,
large-scale bulk productions are scarce and hence prices are expensive (about 2
to 100 times more than organic solvents). The synthesis of ILs is also more complicated,
for they may have a long chain of complex precursors. In addition, there are
only few studies regarding ILs' environmental sustainability. Some of them may
actually be more toxic than conventional solvents, as seen in the life cycle
assessment (LCA) study of ILs by Alviz and Alvarez 18. Their toxicity may
pose health and environmental issues and are still currently under research to
find out about its effects and mechanism 19.
The lack of data limits ILs to be largely employed in industry. To bridge this
gap, it is essential to assess the IL's impacts on the environment. With no
existing sustainability study on the ILs used in ethanol dehydration, this
dissertation will be the first to address such a topic by conducting a LCA.
LCA is a method that incorporates life cycle thinking into the
evaluation of the environmental sustainability of a defined system, by using
quantitative study of environmental impacts. In a life cycle of a material, or
process, the complete system with defined boundary is grouped into 6 stages:
(1) extraction and derivation of raw materials, (2) manufacture, (3) storage, (4)
use, (5) disposal and waste handling, (6) transportation 20.
At each stage, mass and energy balances of each activity, or item, are used to
calculate the contributions towards environmental burdens with a standardised
functional unit. There are 4 important sections of a LCA study, which are goal
and scope definition, inventory analysis, impact evaluation, interpretation of
results. Using this methodology, impacts such as carbon footprint, eutrophication
potential, human toxicity etc. can be estimated and evaluated.
