Ethanol is generally considered to be a renewable energy source and is in principle a greener alternative to conventional fossil fuels. During the last few decades production of ethanol on a large scale has achieved remarkable escalation mainly due to its flexible application and high demand.
Nevertheless robust technologies are yet to be established focusing on the exploitation of lignocellulosic material and waste. Sugar cane processes for ethanol or sugar production provide—as waste bagasse—approximately — kg of bagasse per ton of sugar cane.
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This solid residue of the cane milling process has, in most cases, been misused so far. If it is not disposed to land, it is either sold as animal feed or used in a rather inefficient way to cover the domestic energy needs of the local residents. Thus, the proper exploitation of bagasse will enhance the sustainability of existing biorefineries and give an important economic boost to local communities. View Author Information. Cite this: Ind. Article Views Altmetric -. Citations 5. Cited By. The driving system with steam turbine is the most widely used, mainly in old industries.
The electric drive system consist of an electric motor attached to each roll of the mill unit. Even with the double transformation of energy, the overall efficiency of the electric drive is higher than the work done by a single-stage turbine. These become an issue for the sugar mill, since the surplus electricity becomes a profitable product for the mills.
After being milled, the juice contains several impurities, which must be removed prior to fermentation or concentration. These impurities are removed using a set of unit operations, which basically consist of heating, adjusting the pH, settling the precipitate formed in the body juice, and filtering. Only small changes on these processes were adopted during the last decades.
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In the heating, most industries prefer vertical shell-and-tube steam heaters. Despite the higher overall heat transfer coefficient in the horizontal one, the vertical one is easier to clean. The extracted sugarcane juice has a pH of about 5. For this, lime Ca OH is added, which is the most widespread process used.
For refineries that produce sugar in addition to ethanol, processes such as sulfitation, phosphatation, and carbonation [ 36 ] are also used, aiming to lower the color and turbidity. After the pH adjustment, the juice is sent to a clarifier to settle the insoluble part of the juice. Before the use of chemical products flocculants and polymer and instruments to control the flow and dosing, the most popular installation used multi-tray clarifiers.
The main advantages of the rapid clarifiers over the multi-tray ones are the lower installation costs, and the small retention time, which reduces the degradation of sugars [ 30 ]. There are not many research works carried out recently about juice treatment, and consequently, great changes in this process are not expected in a near future. Ethanol can be produced in an autonomous distillery, producing only ethanol, or in an attached distillery, which produces sugar and ethanol. In the second case, ethanol can be produced only from molasses, a residue from the sugar production process, or a mixture of molasses and juice, depending on the amount of juice diverted to produce ethanol.
This concentration is obtained using multiple-effect evaporators MEVs , which reduces the required steam to concentrate the juice, since each effect produces lower-pressure steam, which is used in the next effect to evaporate more water and so on. In this way, only the first-effect evaporator uses the exhaust steam from turbines. Saving the exhaust steam is crucial for industries that have condensation turbine installed or for refineries that want to save bagasse for other purposes, such as second-generation ethanol or just selling bagasse as a product.
When sugar and ethanol are produced in a refinery, there is a synergistic effect that reduces the total consumption of steam. Since large amount of water must be taken off from the juice in order to concentrate it in the crystallization process, this water is withdrawn as steam in an MEV. This steam from MEV is used in the ethanol process. In this manner, steam from MEV replaces the necessity of using exhaust steam. Combining ethanol and sugar production, results in an energy efficient refinery. Alcoholic fermentation is a biological process, which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as a side effect.
The overall chemical reaction for alcoholic fermentation is as follows:. Sucrose is a dimer of glucose and fructose molecules. In the first step of alcoholic fermentation, the enzyme invertase cleaves the glycolic linkage between the glucose and fructose molecules.
When the fermentation finishes, the fermented liquor is centrifuged to remove east Saccharomyces cerevisiae , which is recycled to fermentation. There are two types of process for alcoholic fermentation commonly used to produce ethanol. The majority of the sugar mills use a fed-batch fermentation process. Continuous process is also used by some industries; however, despite the lower installation cost, continuous fermentation results in lower efficiency in the production of ethanol.
The lower efficiency is a result of bacterial contamination since, in a continuous process, the fermenter cannot be as frequently cleaned as in a fed batch, in which cleaning can be carried out after each batch.
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The main disadvantage of the fed-batch process is related to the large size of the fermenters. Many technologies are under development to reduce this size.
There are many problems in the operation of such large equipment: high cost of installation, difficulty to control parameters, such as contamination, mixing, and temperature, which can cause temperature gradients and dead zones inside the fermenter. To reduce these problems, the total volume of broth under process must be reduced. Therefore, the proposed technologies aim to increase the concentration of reactants and products. Removing ethanol during the fermentation process is one possibility, since a high concentration of ethanol is toxic to yeast.
There are some studies of pervaporation [ 39 ] and stripping [ 40 ] to take ethanol off the fermentation broth during the ethanol fermentation. The high cost of pervaporation membranes and the difficulty in recovering ethanol from CO 2 make this technology unfeasible nowadays. Reducing the temperature of the fermentation broth using a chiller is also an option under development, and there are some commercial-scale units [ 41 ].
At a lower temperature, yeast would resist a higher concentration of ethanol, but reducing the temperature would also reduce the reaction rate. By now, the best available technology continues to be the fed batch cooled by cooling towers. The recent development in downstream process did not aim to improve the ethanol recovery efficiency but to save the energy demand by the process. Downstream consists of the separation of ethanol from the other components in the fermented wine; the first step is the centrifugation of wine, which recovers yeast to the next fermentation fed batch.
There are two main components at the centrifuged wine: water and ethanol. Due to the azeotropic point in the mixture ethanol-water, anhydrous ethanol cannot be obtained using a common distillation process. Anhydrous ethanol There are two common dehydration systems used in Brazil: azeotropic distillation with cyclohexane or monoethyleneglycol, and, more recently, absorption on molecular sieves. The main advantage of using molecular sieves is that steam consumption is about one-third of those in the azeotropic distillation. Pervaporation is a process that could significantly reduce the energy demand; however, the high cost of membranes makes it unfeasible to be used in a commercial scale.
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Vinasse is the final by-product of the ethanol distillation and is the main effluent of the ethanol process. About 12 liters of vinasse are produced per liter of hydrous ethanol. Most industries use the vinasse without any treatment as a fertilizer and, it is simultaneous used for irrigation due to its high amount of water fertirrigation. The vinasse produced in a distillery is a stream composed basically of water, organic matter, and inorganic salts. Therefore, there are many possibilities to utilize this vinasse: as biogas obtained by conversion of the organic matter into gas, and as fertilizer through using the inorganic salts phosphorus, potassium, and nitrogen to partially replace synthetic fertilizers derived from the petroleum industry.
Biogas has a great potential opportunity for using the vinasse. Many studies have been carried out about the bio-digestion of vinasse [ 42 — 44 ].
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Biogas is an easy to handle fuel since it can be transported in high-pressure cylinders, or by pipeline, and can fuel farm machinery and trucks to partially replace diesel [ 45 ]. Biogas can also be obtained from sugarcane trash bagasse and straw [ 46 ] in a bio-digester blended with vinasse. This is also an opportunity to the use of bagasse, that is, bagasse, as well as straw, can be converted into biogas instead of producing electricity or 2G ethanol.
There are also many sugar mills close to the gas pipeline network in Brazil, which would raise the feasibility of implementing a biogas facility. Besides the high cost of installation, a great challenge to implementing biogas facilities is to deal with an unstable vinasse supply. Vinasse is not produced continuously, since industry interrupts its operation relatively frequently due to the rain, which stops the harvest, or due to the maintenance or the braking of equipment. Another possibility is to concentrate vinasse and to recover water to be used in the process.
In this case, the concentrated vinasse can be transported to longer distances to be used as liquid fertilizer. Concentrated vinasse can also be burned into the boiler; in this case, the higher the concentration, the higher its net calorific value. The main disadvantage in this process is the steam demand to concentrate vinasse. After biodigestion, water can be withdrawn from vinasse using commercial technologies such as evaporator or ultra-filtration membrane. Reducing the use of water has a positive environmental impact, but the cost of these processes may be higher than the intake of water from natural sources mainly rivers.
So, only few refineries in Brazil are withdrawn water from vinasse to be used in the process. What makes ethanol from sugarcane superior to that from other feedstocks e. First-generation ethanol processes from sugarcane have a positive energetic balance, which means that it is not only self-sufficient on energy, but it can export the surplus energy usually as electricity.
Using the combined heat and power CHP process is the most efficient way to produce electricity. A high-efficient process, that is, a process which consumes small amount of thermal energy, results in surplus bagasse that can be used as feedstock to another process, such as second-generation ethanol, or to produce surplus steam—the steam produced by the boiler and not condensed in the process.
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