Anhydrous ethanol: A renewable source of energy

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Abstract

Anhydrous ethanol is one of the biofuels produced today and it is a subset of renewable energy. It is considered to be an excellent alternative clean-burning fuel to gasoline. Anhydrous ethanol is commercially produced by either catalytic hydration of ethylene or fermentation of biomass. Any biological material that has sugar, starch or cellulose can be used as biomass for producing anhydrous ethanol. Since ethanol–water solution forms a minimum-boiling azeotrope of composition of 89.4 mol% ethanol and 10.6 mol% water at 78.2 °C and standard atmospheric pressure, the dilute ethanol–water solutions produced by fermentation process can be continuously rectified to give at best solutions containing 89.4 mol% ethanol at standard atmospheric pressure. Therefore, special process for removal of the remaining water is required for manufacture of anhydrous ethanol. Various processes for producing anhydrous ethanol have been used/suggested. These include: (i) chemical dehydration process, (ii) dehydration by vacuum distillation process, (iii) azeotropic distillation process, (iv) extractive distillation processes, (v) membrane processes, (vi) adsorption processes and (vii) diffusion distillation process. These processes of manufacturing anhydrous ethanol have been improved continuously due to the increasingly strict requirements for quantity and quality of this product. The literature available on these processes is reviewed. These processes are also compared on the basis of energy requirements.

Introduction

Efforts are under way in many countries including India to search for alternatives of hydrocarbon-based fuels such as gasoline (petrol), diesel fuels, etc. The need for the alternative gasoline and diesel fuels arises mainly from the standpoint of preserving the global environment and the concern about long-term supplies of conventional hydrocarbon-based gasoline and diesel fuels. Among various alternatives, biofuels have been suggested as a blending component for gasoline and diesel fuels [1], [2], [3], [4], [5]. Anhydrous ethanol is one of the biofuels produced today. Other biofuels include biodiesel and biogas. Biofuels is a term used to describe raw biomass processed into a more convenient form to be used as a fuel. It is most commonly applied to liquid biofuels for transport but could also refer to gaseous fuels and solid such as wood pellets and briquettes. The use of biofuels enjoys benefits in the areas of environment, energy security and economic development. The use of biofuels instead of fossil fuels reduces net emissions of carbon dioxide, which are associated with global climate change. Biofuels are produced from renewable plant resources that recycle the carbon dioxide generated when biofuels are consumed. Biofuels also typically burn clearly in vehicle engines and reduce emissions of unwanted products, particularly unburned hydrocarbons and carbon monoxide. These characteristics contribute to improvements in local air quality. Biofuels help provide energy security for the countries that use them. Biofuels are produced from local and regional biomass resources and hence they are relatively isolated from the uncertainities of international political disruptions. Domestically produced biofuels also enhance national security by reducing net imports of petroleum and helping reduce international trade imbalances sometimes associated with oil imports. Biofuels create local and regional development opportunities; such developments frequently occur in rural areas where other options are very limited. The compatibility of biofuels with modern vehicles provides an option for replacing petroleum fuels in transportation. Current motor vehicles use technologies that permit a range of biofuel blends to be used for consumers. Most new vehicles today can readily accommodate biofuel blends up to about 20% and flexible fuel or dedicated fuel vehicles for high concentration blends or neat biofuels are also commercially available. As a result consumers have a variety of vehicle options available that will readily use biofuels.

Biofuels rely on biotechnology and are a subset of renewable energy [6]. Unlike other renewable energy forms, biofuels can be joined with production of chemicals, under the category called white biotechnology, as shown in Fig. 1.

Anhydrous ethanol, also known as absolute ethanol [7], [8], is a clear, colourless and homogeneous liquid free from suspended matter and consisting of at least 99.5% ethanol by volume at 15.6 °C. The maximum water content, percent by volume at 15.6 °C, determined by Karl-Fisher method [IS:2362-1963] should be 0.5. The maximum specific gravity at 15.6 °C /15.6 °C should be 0.7961. Table 1 shows the requirements of anhydrous ethanol for use in automotive fuel in India [8]. The Indian standard IS:15464-2004 makes specific mention of a list of prohibited denaturants, which it states have extremely adverse effects on fuel stability, automotive engines and fuel systems. These materials may not be used in automotive fuels in any circumstances. They are as follows: methanol, pyrroles, turpentine, ketones and tars (high molecular weight pyrolysis products of fossil or non-fossil vegetable matter).

Ethanol is produced by fermentation of biomass [9], [10], [11], [12], [13], [14], [15]. Energy consumption is likely to increase appreciably in the coming years, mainly in densely populated countries like India, China and South Africa. To enable populous countries to achieve a standard of living similar to the industrial nations, all resources available on the globe have to be utilized. In order to reduce the carbon dioxide release to the atmosphere and meeting growing energy demands, ethanol is to be produced from feed stocks based on whole plant and biomass [16]. Ethanol can be produced from any biological material that has sugar, starch or cellulose [10]. Various steps involved in the conversion of biomass into anhydrous ethanol are the following:

  • 1.

    Conversion of biomass into a useable fermentation feed stock (typically some form of sugar).

  • 2.

    Fermentation of the biomass intermediates using biocatalysts (micro organisms including yeast and bacteria) to produce ethanol.

  • 3.

    Processing of the fermentation product to yield anhydrous ethanol.

Ethanol can be produced from a large variety of carbohydrates (mono-, di-, polysaccharides). The most common disaccharides used for ethanol production is sucrose which comes from sugarcane, sugar beet and sweet sorghum. The extracted sugarcane juice contains water (84%) and sugar (14%). The fermentation of sucrose is performed using commercial yeast such as Saccharomyces cerevisiae. Chemical reaction is composed of enzymatic hydrolysis of sucrose followed by fermentation of simple sugars. First, invertase (an enzyme present in the yeast) catalyzes the hydrolysis of sucrose to convert it into glucose and fructose.C12H22O11Sucrose+H2OinvertaseC6H12O6Glucose+C6H12O6Fructose

Then, another enzyme (zymase), also present in the yeast, converts the glucose and the fructose into ethanol and CO2.C6H12O6Glucoseorfructosezymase2C2H5OHEthanol+2CO2

Monosaccharides (glyceraldehydes, xylose, ribose, glucose, fructose) consists of single sugars bound together with a general formula of (CH2O)n, where n = 3–7. The most common monosaccharides in nature are pentoses (n = 5, xylose) and hexoses (n = 6, glucose). One molecule of disaccharide results from a chemical reaction (dehydration synthesis) in which a new bond is formed between two monosaccharides after removal of water. Polysaccharides must be decomposed into disaccharides and/or monosaccharides through hydrolysis before fermentation.

Grains (corn, wheat or barley) mainly provide starch. For example, corn contains 60–70% starch. Starch stored in grains is long chains of α-glucose monomers, 1000 or more monomers for one amylase molecule and 1000–6000 or more monomers for amylopectin. Polymers of α-glucose are broken into glucose through a hydrolysis reaction with gluco-amylase enzyme.(C6H10O5)nStarch+nH2Ogluco-amylasenC6H12O6D-glucose

The resulting sugar is known as dextrose or D-glucose that is an isomer of glucose. The enzymatic hydrolysis is then followed by fermentation, distillation and dehydration to yield anhydrous ethanol.

Plants contain the cellulosic materials cellulose and hemicellulose. These complex polymers form the structure of plant stalks, leaves, trunks, branches and husks. They are also found in products made from plants, such as paper. Cellulosic feed stocks contain sugars within their cellulose and hemicellulose, but they are more difficult to biochemically convert into ethanol than sugar- and starch-based feed stocks. Cellulose resists being broken down into its component sugars. Hemicellulose is easier to break down, but the resulting sugars are difficult to ferment. The plant compound lignin also resists biochemical conversion. Significant progress has resulted in biochemical conversion processes to break down cellulose and hemicellulose and thermochemical conversion processes to break down lignin. Together, these processes could unlock the potential of cellulosic feed stocks for ethanol production.

Lignocellulose, which is the principal component of the plant cell walls, is mainly composed of cellulose (40–60% of the total dry weight), hemicellulose (20–40%) and lignin (10–25%). Cellulose molecules consist in long chains of β-glucose monomers gathered into micro-fibril bundles. The hemicelluloses can be xyloglucans or xylans depending on the types of plants. Backbone of the former consists of chains of β-glucose monomers to which chains of xylose (5-C sugar) are attached. The latter are mainly composed of xylose linked to a rabinose or/and other compounds that vary from one biomass source to the other. The hemicellulose molecules are linked to the micro-fibrils by hydrogen bonds. Lignins are phenolic compounds which are formed by polymerization of three types of monomers (p-coumaryl, coniferyl and synapyl alcohols). Lignin adds to the cell wall a compressive strength and stiffness. Cellulose is converted to simple sugars (monosaccharides), which are enzymatically hydrolyzed to yield ethanol under the following processes:

  • Dilute acid hydrolysis—Hydrolysis with a solution of sulfuric acid (0.5–1%) at about 160–190 °C for approximately 10 min occurs in two stages to maximize sugar yields from the hemicellulose and cellulose fractions of biomass. Liquid hydrolyzates are recovered from each stage, neutralized and fermented to ethanol. Dilute acid hydrolysis can be used to recover sugar from sugarcane bagasse.

  • Concentrated acid hydrolysis—This process is based on concentrated acid decrystallization of cellulose followed by dilute acid hydrolysis to sugars. Efforts are being made to commercially convert rice straw into ethanol and lignocellulosic components of municipal solid waste to ethanol.

  • Enzyme hydrolysis—Enzymatic hydrolysis of cellulose is achieved using cellulases, which are usually a mixture of groups of enzymes such as endoglucanases, exoglucanases and β-glucosidases acting in synergy for attacking the crystalline structure of the cellulose, removing cellobiose from the free chain ends and hydrolyzing cellobiose to produce glucose. Cellulases are produced by fungi, mainly Trichoderma reesei, besides Aspergillus, Schisophyllum and Penicillium. In order to reduce the cost of cellulase enzymes, simultaneous saccharification and fermentation (SSF) process for converting cellulose into ethanol has been introduced. In SSF process, cellulose, enzymes and fermenting microbes are combined, reducing the number of vessels and improving efficiency.

Cellulosic feed stocks suited to ethanol production include the following [5]:

  • Agricultural residue—Crop residues such as wheat straw and corn stalks, leaves and husks.

  • Forestry residue—Logging and mill residues such as wood chips, sawdust and pulping liquor.

  • Grasses— Hardy, fast-growing grasses such as switchgrass grown specifically for ethanol production.

  • Municipal and other wastes— plant-derived wastes such as household garbage, paper products, paper pulp and food-processing waste.

  • Trees—fast-growing trees such as poplar and willow grown specifically for ethanol production.

These feed stocks have many advantages over sugar- and starch-based feed stocks. They are much more abundant and thus can be used to produce more substantial amounts of ethanol to meet gasoline demand. They are waste products or, in the case of trees and grasses grown specifically for ethanol production, can be grown on marginal lands not suitable for other crops. Less fossil fuel energy is required to grow/collect and convert them to ethanol and they are not human food products.

Anhydrous ethanol is used as chemical reagent, organic solvent, raw material for many important chemicals and intermediates for drugs, plastics, lacquers, polishes, plasticizers, cosmetics. It is also used in pharmaceutical formulations, production of biodiesel (fatty acid ethyl esters), electronic and military industries.

Anhydrous ethanol is considered to be an excellent alternative clean-burning fuel to gasoline [1], [2], [3], [4], [5]. In properly designed automotive systems, ethanol has the potential to achieve very low emission levels. In addition, the combustion products from renewable fuels such as ethanol are considered by many to be environmentally safe from a greenhouse standpoint [17]. It is reported that the ethanol content of solution for blending in gasoline should be 99.35% by volume to eliminate phase separation problems during distribution, storage and use. To make effective use of ethanol as a substitute fuel the energy consumed to make anhydrous ethanol must be less than the energy obtained from ethanol. The use of anhydrous ethanol as a transportation fuel in countries like Brazil, USA and India has been promoted. While the main consideration for Brazil and India has been to reduce dependence on oil imports, USA has been promoting ethanol to promote agriculture and also from environmental considerations. Blending of anhydrous ethanol in gasoline up to 20% is practiced in Brazil. In some areas of the United States, ethanol is blended with gasoline to form an E10 blend (10% ethanol and 90% gasoline), but it can be used in higher concentrations such as E85 or E95. Original equipment manufacturers produce flexible-fuel vehicles that can run on E85 or any other combination of ethanol and gasoline. Recently, Government of India has decided to enforce blending of 5 percent anhydrous ethanol in gasoline [18]. This decision will lead to: (a) reduction in petroleum import bill and (ii) conservation of indigenous petroleum reserves.

Some of the properties [1], [2], [5], [19], [20], [21] of the ethanol are compared with gasoline in Table 2. Ethanol has high octane number, which makes it an excellent gasoline blending component. The motor octane number is of particular significance because it characterizes the performance of a fuel in a hot engine under conditions of full load. The high octane number helps to run vehicles more smoothly and keeps a vehicle's fuel system clean for optimal performance. The heat of evaporation of ethanol is more than that for gasoline. Therefore, ethanol-fuelled engine may develop cold-start problems. The flame temperature for ethanol is lower than that for gasoline. Thus the combustion of ethanol will yield considerable reductions in NOx emissions compared to gasoline. The oxides of nitrogen (NOx) arises from nitrogen and oxygen in the air when at temperatures of 1093 °C and higher. Theoretically, the hottest flame comes from stoichiometric air/fuel mixtures, however, NOx peaks at slightly leaner fuel/air ratios. Leaning of fuel/air ratio causes the flame temperature to be low enough to reduce NOx as well as other emissions. Carbon monoxide emissions are almost entirely determined by fuel/air ratio. Ethanol unlike gasoline contains 35 wt.% oxygen so that less air is needed for combustion. Ethanol reduces carbon monoxide, volatile organic compounds, toxics and respirable-particulate emissions that pose a health hazard, particularly to children and seniors. On account of increased oxygen content in the fuel, the oxidation stability of the blended fuel could be slightly poor which may require use of higher dosage of additive to keep the engine clean. The increased NOx emissions from four-stroke gasoline engines is another issue of concern as most of the studies indicate reduced CO and hydrocarbons but increased NOx with ethanol blends. In the case of two-stroke engines, which do not emit NOx, the use of ethanol–gasoline blends can result in substantial environmental benefits. The two-stroke engines in India from 2000 are also fitted with oxidation catalysts (catalytic converter). The studies indicate that strong potential of environmental improvement exists with use of ethanol–gasoline blends in the high population of two-stroke vehicles in Indian cities, particularly if they are retrofitted with oxidation catalyst while using the ethanol blended fuel [1]. The volumetric heating value of ethanol is less than that for gasoline. Per unit volume, ethanol has approximately 68% the energy of gasoline [5]. Ethanol has a lower Reid vapor pressure (RVP) than gasoline. A lower RVP makes cold-start ignition and operation of a spark-ignition engine difficult at low ambient temperatures. The possible solution of this problem is the on-board catalytic conversion of a portion of the ethanol fuel into diethyl ether and water [17]. Ethanol is quickly biodegradable in surface water, groundwater and soil. Since ethanol is a renewable fuel, it helps to reduce emissions of greenhouse gases that contribute to global warming. Ethanol helps to reduce pollution level.

Although anhydrous ethanol is completely miscible with gasoline at normal temperature, it is extracted by contact with small amounts of water, thus separating the blend into an upper gasoline-rich phase and a lower ethanol-rich phase. The separation can have highly undesirable effects. The lower phase is much greater in volume and lower in specific gravity than the original water and is more apt to be suspended and delivered to vehicles along with the upper fuel phase. The ethanol–water layer tends to pick up dirt or sediment; it can stall the engine upon reaching the carburetor and it is seriously corrosive to steel and to some metals commonly used for carburetor bodies and other fuel system parts. In addition, fuel in storage depleted of ethanol and lower phase with its high ethanol content would pose a disposal problem at distribution and marketing facilities. Most tanks in the commercial gasoline distribution system contain some water which would have to be rigorously excluded if gasoline in the system were to be replaced by ethanol–gasoline blends. The amount of water dissolved by gasoline–ethanol blend before breaking into two phases is dependent on the temperature, ethanol content and gasoline characteristics, particularly the aromatic content [2].

Brazil is the largest single ethanol producer, followed by USA, China, India, France, Russia, South Africa and UK. Sixty percent of the world ethanol production is from sugar crops. India is the largest producer of sugar in the world and has high potential for ethanol production. The major source of ethanol production in Brazil and India and other sugar-raising countries is sugar–molasses route. In European countries, sugar beet is preferred. Sweet sorghum can be cultivated in temperate and tropical regions, increasing its potential benefits. Other crops that can yield oligosaccharides (potatoes, cereals, grapes, etc.) are generally not much utilized for bioethanol production with the exception of corn in USA. It is claimed that a variety of sweet sorghum has been developed with potential to produce 2–4 kL/ha/year of ethanol.

India produces nearly 1.3 × 109  L of ethanol utilizing less than half of its total installed capacity [1], [22]. There are 295 alcohol distilleries in the country with an installed production capacity of 3.198 × 109 L. Ethanol is conventionally produced through fermentation processes from grains or other sugar bearing materials like sugarcane juice or molasses. Brazil produces most of its 11 × 109 L of ethanol from sugarcane. The USA produces ethanol mostly from corn starch. In India, molasses from sugar factories is the main source of ethanol production (Table 3a, Table 3b). Ethanol can be produced from abundant sources of biomass, including agriculture and forestry residues, municipal solid waste, rotten grains, etc.

Section snippets

Processes for production of anhydrous ethanol

Ethanol–water solution forms a minimum-boiling azeotrope of composition of 89.4 mol% ethanol and 10.6 mol% water at 78.2 °C and standard atmospheric pressure [23]. Ethanol boils at 78.4 °C, water boils at 100 °C, but the azeotrope boils at 78.2 °C, which is lower than either of its constituents. Indeed 78.2 °C is the minimum temperature at which ethanol–water solution can boil. When an azeotrope is partially boiled, the resulting vapor has the same ratio of constituents as the original mixture of

Energy requirements of anhydrous ethanol production

Anhydrous ethanol production requires energy for the preparation of useable fermentation feedstock, the fermentation of prepared feedstock and distillation/dehydration of dilute ethanol–water mixtures. The different sources of biomass have been compared on the basis of the net energy value (NEV) of ethanol, determined by subtracting the energy required to produce a liter of ethanol during the whole life cycle from the energy contained in a liter of ethanol. The NEV of ethanol from sugarcane,

Conclusions

Anhydrous ethanol is one of the biofuels produced today and it is a subset of renewable energy. It is considered to be an excellent alternative clean-burning fuel to gasoline. Any biological material that has sugar, starch or cellulose can be used as biomass for producing anhydrous ethanol. There are several sources of these biological materials, such as molasses, corn, bagasse. Since ethanol–water solution forms a minimum-boiling azeotrope of composition of 89.4 mol% ethanol and 10.6 mol% water

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    1

    Department of Chemical Engineering, UIET, CSJM University, Kanpur- 208024.

    2

    [email protected].

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