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About Ethyl Alcohol / Ethanol Production

December 21, 2008

Ethyl Alcohol Production

Introduction

Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates include soluble sugars such as sucrose from sugarcane, storage carbohydrates such as starch from grains and tubers, and structural carbohydrates which make up the plant cell wall of grasses and wood, such as cellulose, hemicellulose, and pectin. The principal carbohydrates contained in lignocellulose resources are the structural carbohydrates. These carbohydrates, along with proteins and lignin, form the complex matrix of plant cell walls that give plants structural stability and protection from the environment.

Ethanol production processes vary by type of feedstock. Production from corn primarily uses the Dry Grind Process or, to a lesser extent, the more capital intensive Wet Milling Process, which produces a greater variety of marketable co-products. Production of ethanol from sugarcane juice or molasses uses technologies similar to those used to produce ethanol from corn.

Production of ethanol from cellulosic biomass such as crop residues, grasses or wood involves intensive pretreatment to break down the biomass (hydrolysis) into fermentable sugars. A number of pretreatment approaches are currently under evaluation. However, scaling pretreatment processes to commercial sizes and the associated reactor design issues so far remain a major barrier to the commercial production of fuel ethanol from lignocellulosic biomass.

On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, the glucose consumed to generate additional yeast cells does not result in the production of ethanol. Most industrial fermentation processes therefore operate at 90 to 95% of the theoretical yield.

Raw Materials

The three basic types of feedstock  used in Ethanol Production are:

a.       SACCHARINE (sugar containing) materials in which the carbohydrate (the actual substance from which the alcohol is made) is present in the form of simple, directly fermentable six and twelve carbon sugar molecules such as glucose, fructose, and maltose. Such materials include sugar cane, sugar beets, fruit (fresh or dried), citrus molasses, cane sorghum, whey and skim milk.

b.      STARCHY MATERIALS that contain more complex carbohydrates such as starch and insulin that can be broken down into the simpler six and twelve carbon sugars by hydrolysis with acid or by the action of enzymes in a process called malting. Such materials include corn, grain sorghum, barley, wheat, potatoes, sweet potatoes, Jerusalem artichokes, cacti, manioc, arrowroot, and so on.

c.       CELLULOSE MATERIALS such as wood, wood waste, paper, straw, corn stalks, corn cobs, cotton, etc., which contain material that can be hydrolyzed with acid, enzymes or otherwise converted into fermentable sugars called glucose.

Production of Ethanol in Dry – Milling Process


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Grain Preparation

Preparation of the corn grain involves cleaning and conditioning steps, as well as generating an aqueous solution high in simple sugars. Generally, enough grain is stored on site in bins to meet facility needs for 8-12 days of operation (Kwiatkowski, 2006). Broken corn kernels and foreign materials (metal, dirt, cobs, etc.) are removed by blowers and screens. The cleaned corn is then ground in hammer mills fitted with screens with openings ranging between 3.2 and 4.8 mm in diameter, which provides grain particles of a more uniform size: more than 90% of the ground corn, by weight, has a diameter between 0.5 to 2 mm (Rausch, 2005). Grinding serves to break the tough outer coating of the corn kernel and increases the surface area of exposed starch.

Liquefaction involves combining the ground corn with process water to form a slurry which is approximately 30% solids by weight (Kwiatkowski, 2006). Ammonia and lime are added at this step to adjust the pH of the slurry to 6.5. The ammonia, which contains nitrogen, also serves as a nutrient for the yeast in the subsequent fermentation step. The slurry is heated to 88°C by direct steam injection using a “jet-cooker”. A thermostable enzyme (alpha-amylase) is added to cleave the starch molecules at random points along the middle of the polymer chain and to break the starch into smaller water soluble fragments called dextrins. After approximately one hour, the output from the first step of liquefaction is combined with “backset”, which is recycled water from the end of the ethanol distillation process. The backset accounts for approximately 15% of the final volume of the corn mash (McAloon, 2000). Critical nutrients for the yeast are also carried in the backset. As the liquefied slurry is cooled to 60°C, the heat is recovered and used to heat new, incoming slurry going to the jet-cooker (Kwiatkowski, 2006). A new enzyme technology developed by Genencor allows for the rapid hydrolysis of granular starch and eliminates the need for gelatination of the starch slurry by jet-cooking, thus significantly lowering the energy requirements for ethanol production from corn (Shetty, 2005).

Following liquefaction, sulfuric acid is added to the slurry to lower the pH to 4.5. An additional enzyme, glucoamylase (also called beta-amylase) is added to break the starch and dextrins into glucose via a stepwise hydrolysis of glucose from the end of the molecules. The slurry is held at 60°C for 5-6 hours as the glucoamylase hydrolyzes the dextrins to fermentable glucose (Schenck, 2002). Most of the dextrins are converted to glucose during this step, however the glucoamylase remains active throughout the fermentation step and will continue to hydrolyze any residual dextrins during fermentation. After saccharification, the slurry (now called mash) is cooled to 32°C with the heat recovered and transferred to other process streams. The cooled mash then enters the fermentation tanks. A popular alternative to mash-presaccharification is to add glucoamylase during the filling of the fermentor and to saccharify and ferment the starch simultaneously (SSF, Simultaneous Saccharification and Fermentation). An additional advantage to this approach is that reversion reactions (re-polymerization of glucose) are much less likely to occur (Power, 2003).

Sugar Fermentation

Fermentation uses microorganisms (the yeast Saccharomyces cerevisiae, which is also used for brewing beer and baking bread) to convert sugars to ethanol, and results in the production of ethanol and carbon dioxide, as well as additional yeast cells (from cell division) and heat. One molecule of glucose yields 2 molecules of ethanol and 2 molecules of carbon dioxide.

On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, glucose consumed to generate additional yeast cells (cell mass) does not result in the production of ethanol. Most industrial fermentation processes operate at 90 to 95% of the theoretical yield.

In the fermentation step, yeast grown in seed tanks is added to the corn mash to ferment the simple sugars (glucose) to ethanol.  The other components of the corn kernel (protein, oil, etc.) remain largely unchanged during the fermentation process, though the corn oil helps to prevent foaming during the fermentation.

In most dry-grind ethanol plants, the fermentation process occurs in batches. A fermentation tank is filled and ferments completely before being drained and refilled with a new batch. The up-stream processes (grinding, liquefaction, and saccharification) and downstream processes (distillation and recovery) occur continuously. Thus, dry-grind facilities using batch processing usually have three or more fermentors, with one fermentor filling, one fermenting (for approximately 46-68 hours), and one emptying and resetting for the next batch all at the same time. While the exact size of each fermentor varies between plant designs, common fermentor sizes range between 300,000-500,000 gallons (1-2 million liters) each (Kwiatkowski, 2006). A major advantage of batch fermernations is that there are fewer opportunities for contamination, provided they are properly sanitized between runs. Bacteria, especially species of Lactobacillus, can infect yeast fermentations and produce organic acids that lower ethanol yields and interfere with the Saccharomyces (Graves, 2006).

While batch fermentation is more common in dry-grind facilities, continuous fermentation processes are used in some facilities. In this process, fermentation occurs through a series of cascading tanks where the liquid continuously flows through the process. New fermentation media is continuously added at the front end and fermented product is continuously removed from the back end. While continuous fermentation has greater reactor productivity because it is continuously operating with high yeast loads, much more care needs to be exercised to prevent contamination (Bayrock, 2001).

In addition to ethanol, carbon dioxide is also produced during fermentation. Usually, the carbon dioxide is not recovered as a sellable product. If recovered, this carbon dioxide can be cleaned, compressed and sold for carbonation of soft drinks or frozen into dry-ice for cold product storage and transportation. If the carbon dioxide is not recovered, it passes through a water scrubber to remove evaporated ethanol and other volatile organic compounds (VOCs) carried in the gas. The water from the scrubber, containing the recovered ethanol, is sent to the distillation system. The cleaned carbon dioxide is vented to the atmosphere.

Heat is generated during fermentation: approximately 12000 kJ per kilogram of ethanol; 516 BTU per pound (Kwiatkowski, 2006). This heat must be continuously removed from the fermentors and is accomplished either by passing water through a cooling coil contained within the fermentor, or by continuously pumping the fermenting mash through a large heat exchanger where the heat is transferred to cooling water before the mash is returned to the fermentor. Failure to remove the heat causes a rise in temperature sufficient to kill the yeast. The optimal temperature for ethanol fermentation is between 27 and 32°C.

After the fermentation is nearly complete, the fermented corn mash (now called beer) produced in individual batches is emptied from the fermentor into a beer well where it is stored enabling a continuous stream to be supplied to the ethanol recovery system. The beer contains 8-10% ethanol by weight.

Ethanol Recovery

Separation and recovery of the ethanol is accomplished through a continuous process involving several steps (figure 4). In the first step, the beer is processed through a beer column where steam is used to strip off almost all of the ethanol, along with some water, from the slurry. The ethanol and water vapor exit the top of the beer column and the whole stillage (containing less than 0.1% ethanol by weight) exits from the bottom. The overhead vapor flows to a rectifier column where the ethanol is concentrated from 45% to 91% through fractional distillation. The bottoms from the rectifier pass through a stripping column to remove residual ethanol.  Liquid exiting the bottom of the stripper has less than 0.1% ethanol by weight, and is recycled as process water for slurrying the ground corn.  The overhead vapor from the rectifier (91% ethanol by weight) is superheated and passes through molecular sieves. The final product from the molecular sieve system is ethanol vapor that is at least 99.6% pure. This vapor is condensed and mixed with a denaturant (e.g. gasoline) to render it as non-potable fuel ethanol. Generally, 8 to 12 days worth of denatured fuel ethanol production is stored on site (Kwiatkowski, 2006).

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All industrial fuel ethanol production uses continuous-feed distillation column systems. Distillation is a common chemical separation method that is based upon differences in volatility (Wankat, 1988). If a mixture of ethanol and water are placed in a container at a given temperature and pressure, after time, the mixture will reach equilibrium. At equilibrium, some of the ethanol will be a vapor in the gas above the liquid and some will be in the liquid phase. Similarly, some of the water will be in the vapor phase and some will be in the liquid phase. Because ethanol is more volatile than water (boils at a lower temperature), the ratio of ethanol to water in the vapor phase is greater than the ratio of ethanol to water in the liquid phase. This characteristic allows for the separation of the ethanol from the water.

Through subsequent vaporization of the mixture, condensation, re-vaporization, and re-condensation, the mixture becomes higher and higher in ethanol content because the vapor at each vaporization step has higher ethanol concentration than the liquid from which it was vaporized. Thus, multiple fractionation steps can be used to purify ethanol from water. However, the basic principle by which this occurs – difference in boiling points between water and ethanol – ceases to exist when the mixture is 95.6 wt% ethanol (4.4% water). At this point (the azeotrope), the ethanol and water both vaporize to the same degree and cannot be further fractionated by distillation. Either a third solvent can be introduced to break the azeotrope (e.g. beneze) or an alternate seperation method can be used such as absorption of water using molecular sieves.

Molecular sieves used to dry ethanol are crystalline metal zeolites (aluminosilicates) with a 3-dimensional porous structure of silica and alumina tetrahedra. Zeolites strongly and preferentially adsorb water from vapor/gas mixtures. The adsorbed water can be removed by increasing the temperature of the zeolite and passing dry gas over the particles, thus allowing this rather expensive desiccant (~$10/lb) to be reused (Al-Asheh, 2004).

While this drying property was discovered with naturally occurring zeolites, commercial molecular sieves are synthetically produced to have highly uniform pores within a tight size distribution. Industrial molecular sieve drying systems consist of multiple columns each filled with a bed of uniform sized zeolite sieves. The ethanol/water vapor mixture leaving the fractional distillation system is super-heated and forced through the molecular sieve bed. The water vapor is selectively adsorbed to the particles while ethanol passes through the column, where it is recovered and condensed to liquid at high purity. After the capacity of the zeolites to adsorb water from the ethanol vapor is reached, feed vapor is stopped and the flow through the column is reversed. Dry gas (usually CO2 produced by the fermentation process) passes over the zeolites while the system is placed under a slight vacuum to drive the desorption of the water from the solid particles. The water vapor and residual ethanol vapor exiting the column is condensed and returned to the stripper in the distillation system to re-vaporize the residual ethanol which improves the overall efficiency of ethanol recovery by the plant (Ladisch, 1979).

To achieve continuous processing, molecular sieve dehydration systems consist of pairs of beds. As the first bed in the pair processes wet ethanol, a second molecular sieve bed undergoes regeneration to remove the adsorbed water. When the capacity of the first column to remove water is filled, the duties of the columns are switched so that the wet column begins regeneration and the fresh column continues to process wet ethanol vapor.

An alternative to zeolite molecular sieves is using ground corn (corn grits) in a packed bed, similar in design to conventional molecular sieve beds (Chang, 2006; Neuman, 1986; Westgate, 1992). In this system, currently in commercial use by Archer Daniel Midlands, water is selectively adsorbed to corn starch from a water-ethanol vapor mixture (Beery, 1998; Beery, 2001; Ladisch, 1979). Similar in design to the zeolite molecular sieve beds, the corn grit system operates in pairs of beds with one drying ethanol vapor while the other(s) are undergoing regeneration to remove the adsorbed water. The major advantages of the bio-based adsorbents that they are easily available, less expensive than molecular sieves, mechanically stable, and easily disposable (Ladisch, 1997).

Industrial fractional distillation to produce fuel ethanol is one of the major energy inputs for the production of fuel ethanol. Process improvements that capture and recycle energy from the process have greatly reduced the cost (Gulati, 1996; Ladisch, 1979).

Co-product production and handling

The principal co-product from ethanol production using the dry-grind process is distiller’s dried grains and solubles (DDGS). The whole stillage leaving the bottom of the beer column contains approximately 15% solids. Centrifugation of the whole stillage removes approximately 83% of the water and results in wet distillers grains (also called wet cake) which is 35 to 40% solids (figure 3). The liquid stream, called thin stillage, is partially recycled as backset to the second stage of the liquefaction process. The remaining thin stillage passes to a surge tank which supplies a steady feed to the evaporators, where it is concentrated.

Thin stillage passes through a multiple effect evaporator which removes a significant amount of the water as steam, which is used to vaporize the ethanol during the recovery process (in the reflux of the rectifier). The steam, now condensed to liquid, is mixed with other condensates and added to the ground corn at the beginning of the grain preparation phase. The concentrated product of the evaporators is a syrup containing 55% solids by weight (McAloon, 2000) which is mixed with wet distillers grains and sent to a large rotary drum dryer where the mixture is dried from 64 to 9-10% moisture to form the DDGS. The hot gas from the dryers is processed prior to venting to the atmosphere to remove volatile organic compounds (VOCs) released during drying. Thermal oxidation is commonly used to convert VOCs to carbon dioxide and water (Vij, 2003).

Distillers dried grains and solubles are currently used as livestock feed as they contain relatively high quantities of protein (table 2). They have long been used in cattle feed rations at rates up to 40% (Ham, 1994; Peter, 2000), but not as widely used in feed rations for non-ruminants (swine and poultry). Recommended inclusion rates (based on amino acid content) for swine are up to 20% of the ration (Whitney, 2006), but are less than 10% for laying hens (Lumpkins, 2005). As fuel ethanol production from corn dry-grind technology continues to increase, efficient use and capturing the value of the co-products becomes increasingly important (Rausch, 2006; Belyea, 2004).

Production of Ethanol using Wet Milling Process

Producing fuel ethanol from grains using the wet mill process involves four major steps-preparation of the grain, fermentation of the sugars, recovery of the ethanol, and handling of the co products. The principal differences between the ethanol dry-grind process and the ethanol wet mill process are the grain preparation steps and the numbers and types of co-products recovered. Once the starch has been recovered the process of converting it to fuel ethanol and recovering the ethanol is similar in both wet mill and dry-grind facilities. But, the wet mill process is designed to fully fractionate the grain so that the major constituents (carbohydrates, lipids, and protein) can be efficiently recovered and purified for the production of value-added products (figure 3).

Grain preparation
The upstream processing steps of corn (steeping and fractionation) are common to all wet mills. Steeping, the first step in the wet milling process, is what differentiates this process from dry milling (for cereal processing) and dry-grinding (for ethanol production) processes. In the steeping process, the corn kernels are soaked in water acidified with sulfur dioxide (SO2), typically for 22-50 hours at 52°C (125°F). Steeping softens the kernels by thoroughly wetting the grain and increaseing the moisture content from 15% to 45%. The sulfur dioxide (typically at 0.12 – 0.20% of the water) and possibly lactic acid produced by bacteria in the mix, break down the protein-starch matrix contained in the endosperm (figure 4) of the grain, which aids in the separation of the starch from the protein and other components of the grain.

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Source: May, 1987.

The corn kernel is further fractionated by first coarse grinding the grain followed by several separation steps. The coarse grinding mill consists of a stationary disk and a rotating disk, each with knobs that break up the kernels. They are adjusted so that few kernels exit without being broken while the seed germ remains undamaged (May, 1987). Following coarse grinding, the oil-rich germ is separated from the slurry based on differences in densities. Hydrocyclones are used to separate solids from a liquid slurry using centrifugal force. The slurry is fed tangentially at the top of a cylindrical entry section. The germ floats to the top and exits through a large opening located on the cylinder. Water and any remaining slurry exit through a smaller opening at the apex of a cone located at the bottom of the cylinder. The separated germ is dewatered and dried before removal of the corn oil by solvent extraction.

The material leaving the bottom of the cyclone passes through a wedge-wire screen to separate the hull and the fiber from the starch and protein (i.e., gluten). Approximately 30-40% of the starch is recovered in this step (May, 1987). The recovered fiber, containing attached starch, is next milled using either an entoleter or a disk mill. An entoleter mill forces the fiber against pins at high speed to shear the starch from the fiber. Disk mills, similar to the coarse mill, have knobs or grooves that remove the starch from the fiber. Both milling processes are adjusted to maximize starch seperation from the fiber, while keeping the fiber intract. The starch is separated a second time by passing it through a screen sized to prohibit passage of the fiber. The processed fiber is then pressed to remove water (to 40-60% moisture), mixed with evaporated stillage, and dried to produce corn gluten feed. Corn gluten feed contains less protein and has a lower energy content than distiller’s dried grains and solubles produced in the dry-grind process.

The starch/protein slurry that is fractionated from the fiber is further processed to separate the starch and the protein. Gluten (the protein) is lower in density than starch (1.06 specific gravity compared with 1.6 specific gravity for starch) permitting efficient separation by centrifugation or hydrocyclones (May, 1987). The fractionated protein is dewatered and dried to produce corn gluten meal which is used as livestock feed. The remaining starch contains 3-5% protein and may be further purified depending on the final use of the starch. For fuel ethanol production, this primary starch slurry usually undergoes liquefaction and saccharifaction processes similar to those used in the ethanol dry-grind process.

Liquefaction involves adding ammonia and lime to the starch slurry to adjust the pH to 6.5. The ammonia, which contains nitrogen, also serves as a nutrient for the yeast in the subsequent fermentation step. The slurry is heated to 88°C by direct steam injection using a “jet-cooker”. A thermostable enzyme (alpha-amylase) is added to cleave the starch molecules at random points along the middle of the polymer chain and to break the starch into smaller water soluble fragments called dextrins. After approximately one hour, the output from the first step of liquefaction is combined with “backset”, which is recycled water from the end of the ethanol distillation process. The backset accounts for approximately 15% of the final volume of the corn mash (McAloon, 2000). Critical nutrients for the yeast are also carried in the backset. As the liquefied slurry is cooled to 60°C, the heat is recovered and used to heat new, incoming slurry going to the jet-cooker (Kwiatkowski, 2006). A new enzyme technology developed by Genencor allows for the rapid hydrolysis of granular starch and eliminates the need for gelatination of the starch slurry by jet-cooking, thus significantly lowering the energy requirements for ethanol production from corn (Shetty, 2005).

Following liquefaction, sulfuric acid is added to the slurry to lower the pH to 4.5. An additional enzyme, glucoamylase (also called beta-amylase) is added to break down the starch and dextrins into glucose via a stepwise hydrolysis of glucose from the end of the molecules. The slurry is held at 60°C for 5-6 hours as the glucoamylase hydrolyzes the dextrins to glucose (Schenck, 2002). Most of the dextrins are converted to glucose during this step, however the glucoamylase remains active throughout the fermentation step and will continue to hydrolyze any residual dextrins during fermentation. After saccharification, the slurry (now called mash) is cooled to 32°C with the heat transferred to other process streams. The cooled mash then enters the fermentation tanks. A popular alternative to mash-presaccharification is to add the glucoamylase during the filling of the fermentor and to saccharify and ferment the starch simultaneously (SSF, Simultaneous Saccharification and Fermentation). An additional advantage to this approach is that reversion reactions (re-polymerization of glucose) are much less likely to occur (Power, 2003)

Sugar fermentation
Fermentation uses microorganisms (the yeast Saccharomyces cerevisiae, which is also used for brewing beer and baking bread) to convert sugars to ethanol and results in the production of ethanol and carbon dioxide, as well as additional yeast cells (from cell division) and heat.

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On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, glucose consumed to generate additional yeast cells (cell mass) does not result in the production of ethanol. Most industrial fermentation processes operate at 90 to 95% of the theoretical yield.

The fermentation of the saccharified starch in a wet mill is similar to the process used in dry-grind facilities with the only major differences being the presences of fewer insoluble solids in the fermentation liquid (due to the removal of the fiber and germ), and the use of steep water. The liquid used in the steeping process contains a significant amount of protein and micronutrients from the corn (Rausch, Thompson, Belyea, & Tumbleson, 2003) and is sometimes sold in a condensed form as a complex nitrogen/nutrient source for industrial fermentations (Kapen, 1996). For fuel ethanol production, the steep water is typically added to the saccharified starch to dilute it to the desired concentration (16 – 22 wt% sugar) before adding the yeast.

The yeast, which is grown in seed tanks, is added to the saccharified starch to ferment the simple sugars (glucose) to ethanol.

Fermentation processes involve either batch or continuous processing methods. In a batch process, a fermentation tank is filled and ferments completely before being drained and refilled with a new batch. The up-stream processes (grain preparation and separation, liquefaction, and saccharification) and downstream processes (distillation and recovery) occur continuously. Facilities using batch processing usually have three or more fermentors, with one or more fermentor filling, one or more fermenting (for approximately 46-68 hours), and one or more emptying and resetting for the next batch operating at the same time. While the exact size of each fermentor varies between plant designs, common fermentor sizes range between 300,000-500,000 gallons (1-2 million liters) each (Kwiatkowski, 2006). A major advantage of batch fermernations is that there are fewer opportunities for contamination, provided they properly santized between runs. Bacteria, especially species of Lactobacillus, can infect yeast fermentations and produce organic acids that lower ethanol yields and interfere with the Saccharomyces (Graves, 2006).

Continuous fermentation processes involve a series of cascading tanks where the liquid continuously flows through the process. New fermentation media is continuously added at the front end and fermented product is continuously removed from the back end. While continuous fermentation has greater reactor productivity because it is continuously operating with high yeast loads, much more care needs to be exercised to prevent contamination (Bayrock, 2001).

In addition to ethanol, carbon dioxide is also produced during fermentation. Carbon dioxide can be captured, cleaned, compressed and sold for carbonation of soft drinks or frozen into dry-ice for cold product storage and transportation. If the carbon dioxide is not recovered, it passes through a water scrubber to remove evaporated ethanol and other volatile organic compounds (VOCs) carried in the gas. The water from the scrubber, containing the recovered ethanol, is sent to the distillation system. The cleaned carbon dioxide is vented to the atmosphere.

Heat is generated during fermentation (approximately 12000 kJ per kilogram of ethanol; 516 BTU per pound) (Kwiatkowski, 2006). This heat must be continuously removed from the fermentors and is accomplished either by passing water through a cooling coil contained within the fermentor, or by continuously pumping the fermenting mash through a large heat exchanger where the heat is transferred to cooling water before the mash is returned to the fermentor. Failure to remove the heat causes a rise in temperature sufficient to kill the yeast. The optimal temperature for ethanol fermentation is between 27 and 32°C.

After the fermentation is nearly complete, the fermented corn mash (now called beer) is emptied from the fermentor into a beer well where it is stored enabling a continuous stream to be supplied to the ethanol recovery system. The beer contains 8-10% ethanol by weight.

Ethanol Recovery

Separation and recovery of the ethanol is accomplished through a continuous process involving several steps (figure 5). In the first step, the beer is processed through a beer column where steam is used to strip off almost all of the ethanol, along with some water, from the slurry. The ethanol and water vapor exit the top of the beer column and the whole stillage (containing less than 0.1% ethanol by weight) exits from the bottom. The overhead vapor flows to a rectifier column where the ethanol is concentrated from 45% to 91% through fractional distillation. The bottoms from the rectifier pass through a stripping column to remove residual ethanol.  Liquid exiting the bottom of the stripper has less than 0.1% ethanol by weight, and is recycled as process water for slurrying the ground corn.  The overhead vapor from the rectifier (91% ethanol by weight) is superheated and passes through molecular sieves. The final product from the molecular sieve system is ethanol vapor that is at least 99.6% pure. This vapor is condensed and mixed with a denaturant (e.g. gasoline) to render it as non-potable fuel ethanol. Generally, 8 to 12 days worth of denatured fuel ethanol production is stored on site

(Kwiatkowski, 2006).

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All industrial fuel ethanol production uses continuous-feed distillation column systems. Distillation is a common chemical separation method that is based upon differences in volatility (Wankat, 1988). If a mixture of ethanol and water are placed in a container at a given temperature and pressure, after time the mixture will reach equilibrium. At equilibrium, some of the ethanol will be a vapor in the gas above the liquid and some will be in the liquid phase. Similarly, some of the water will be in the vapor phase and some will be in the liquid phase. Because ethanol is more volatile than water (boils at a lower temperature), the ratio of ethanol to water in the vapor phase is greater than the ratio of ethanol to water in the liquid phase. This characteristic allows for the separation of the ethanol from the water.

Through subsequent vaporization of the mixture, condensation, re-vaporization, and re-condensation the mixture becomes higher and higher in ethanol content because the vapor at each vaporization step has higher ethanol concentration than the liquid from which it was vaporized. Thus, multiple fractionation steps can be used to purify ethanol from water. However, the basic principle by which this occurs – difference in boiling points between water and ethanol – ceases to exist when the mixture is 95.6 wt% ethanol (4.4% water). At this point (the azeotrope), the ethanol and water both vaporize to the same degree and cannot be further fractionated by distillation. Either a third solvent can be introduced to break the azeotrope (e.g. beneze) or an alternate seperation method can be used such as adsorption of water using molecular sieves.

Molecular sieves for drying ethanol are crystalline metal zeolites (aluminosilicates) with a 3-dimensional porous structure of silica and alumina tetrahedra. Zeolites strongly and preferentially adsorb water from vapor/gas mixtures. The adsorbed water can be removed by increasing the temperature of the zeolite and passing dry gas over the particles, thus allowing this rather expensive desiccant (~$10/lb) to be reused (Al-Asheh, 2004).

While this drying property was discovered with naturally occurring zeolites, commercial molecular sieves are synthetically produced to have highly uniform pores within a tight size distribution. Industrial molecular sieve drying systems consist of multiple columns each filled with a bed of uniform sized zeolite sieves. The ethanol/water vapor mixture leaving the fractional distillation system is super-heated and forced through the molecular sieve bed. The water vapor is selectively adsorbed to the particles while ethanol passes through the column, where it is recovered and condensed to liquid at high purity. After the capacity of the zeolites to adsorb water from the ethanol vapor is reached, feed vapor is stopped and the flow through the column is reversed. Dry gas (usually CO2 produced by the fermentation process) passes over the zeolites while the system is placed under a slight vacuum to drive the desorption of the water from the solid particles.  The water vapor and residual ethanol vapor exiting the column is condensed and returned to the stripper in the distillation system to re-vaporize the residual ethanol which improves the overall efficiency of ethanol recovery by the plant (Ladisch, 1979).

To achieve continuous processing, molecular sieve dehydration systems consist of pairs of beds. As the first bed in the pair processes wet ethanol, a second molecular sieve bed undergoes regeneration to remove the adsorbed water. When the capacity of the first column to remove water is filled, the duties of the columns are switched so that the wet column begins regeneration and the fresh column continues to process wet ethanol vapor.

An alternative to zeolite molecular sieves is using ground corn (corn grits) in a packed bed, similar in design to conventional molecular sieve beds (Chang, 2006; Neuman, 1986; Westgate, 1992). In this system, currently in commercial use by Archer Daniel Midlands, water is selectively adsorbed to corn starch from a water-ethanol vapor mixture (Beery, 1998; Beery, 2001; Ladisch, 1979). Similar in design to the zeolite molecular sieve beds, the corn grit system operates in pairs of beds with one drying ethanol vapor while the other(s) are undergoing regeneration to remove the adsorbed water. The major advantages of the bio-based adsorbents are that they are readily available, less expensive than molecular sieves, mechanically stable, and easily disposed (Ladisch, 1997).

Industrial fractional distillation to produce fuel ethanol is one of the major energy inputs for the production of fuel ethanol.  Process improvements that capture and recycle energy from the process has greatly reduced the cost of this step (Gulati, 1996; Ladisch, 1979).

Co-product production and handling

The wet milling of corn produces ethanol and three co-products-germ, corn gluten meal, and corn gluten feed. The germ, which is separated from the other kernel constituents after the coarse grind, contains most of the oil contained in the grain. It can be processed to extract the oil, which can then be further refined into food-grade corn oil. This may be done in the same facility as the ethanol plant or sold to a secondary processor.

Corn gluten meal is the protein rich (about 60%) product separated from the gluten-starch slurry and sold as a high-protein animal feed (Ham, 1994). The residual corn fiber separated from the starch-gluten slurry after the second milling step is mixed with evaporated light stillage and dried to produce corn gluten feed, a medium-grade protein product (20-26% of dry matter), which is also used as animal feed (Loe, 2006). Corn gluten feed is similar in fiber content, lower in protein content, and much lower in oil content compared to Distiller’s Dried Grains and Solubles (DDGS) produced in the corn ethanol dry grind process (Ham, 1994). Approximately 2.5 gallons of ethanol, 16.4 pounds of carbon dioxide, 2.1 pounds of oil, 2.6 pounds (dry mass) of corn gluten meal, and 11.2 pounds (dry mass) of corn gluten feed are produced per bushel of corn using the wet milling process (May, 1987).

Production of Ethanol using Cellulose Resources

Feedstock preparation

The stability and chemical complexity of cellulose increase the difficulty of breaking it down into glucose, a situation frequently referred to as the recalcintrance of cellulose. A number of pretreatment approaches are being explored to overcome this problem. Pretreatment is the general term used to describe the processing steps preceding hydrolysis of cellulose and hemicellulose into fermentable sugars. The goal of any pretreatment technology is to alter or remove structural and compositional factors present in plant biomass that hinder the breakdown (hydrolysis) of cell wall polysaccharides (polymers of simple sugars) into the fermentable simple sugars (Mosier, 2005a, 2005b; Grohmann, 1984; Lynd, 1999; McMillan, 1994). Pretreatment methods often involve harsh conditions and non-selectively hydrolyze the polysaccharides in the plant material. Sugars that are produced during pretreatment are also subject to further degradation to form compounds that act as inhibitors to the subsequent fermentation process (Klinke, 2004; Olsson, 1996; Palmqvist, 2000; Taherzadeh, 1997).

Pretreatment methods are usually either physical or chemical, although some approaches incorporate both (Hsu, 1996; McMillan, 1994). Chemical approaches use acids or bases to promote the hydrolysis of cellulose by removing the hemicellulose and/or lignin during pretreatment. The most commonly used acids and bases are sulfuric acid (H2SO4), sodium hydroxide (NaOH) or ammonia. Other chemicals used include cellulose solvents such as alkaline H2O2, ozone, organosolv (Lewis acids, FeCl3, and Al2SO4 in aqueous alcohol), glycerol, dioxane, phenol, or ethylene glycol, which disrupt the cellulose structure and promote hydrolysis (Wood, 1988).

Dilute sulfuric acid is commonly used as a pretreatment method, but increases cost, due to the need for reactors constructed of expensive steel materials, and results in the formation of unwanted salts that require neutralizing agents (Hinman, 1992; Lynd, 1996; Lynd, 1999;  Thompson, 1979). Liquid hot water pretreatment with pH control effectively dissolves hemicellulose and lignin, while minimizing degradation of monosaccharides without the need for costly and potentially dangerous pretreatments and neutralizing agents (Kim, 2005; Weil, 1998). Other pretreatment methods include steam explosion (Abatzoglou, 1992; Heitz, 1991; Ramos, 1992), ammonia fiber expansion (AFEX) (Gollapalli, 2002; Teymouri, 2005; Wang, 1998), and other chemical solvents (Hsu, 1996; McMillan, 1994). Few of these pretreatment processes have been fully commercialized or tested at industrial scales (Mosier, 2005a, 2005b). Scaling pretreatment processes to commercial sizes and the associated reactor design issues remains a major barrier to the commercial production of fuel ethanol from lignocellulosic biomass (Lynd, 1999; Mosier, 2005a, 2005b). A summary and evaluation of a number of pretreatment processes can be found in Lynd (1999), Mosier (2005a, 2005b), and Kim (2005).

Following pretreatment, plant cell wall polysaccharides are more susceptible to chemical or enzymatic hydrolysis that breaks them into monomeric (single) sugars (saccharification) that can be fermented into ethanol (Lynd, 1999). Depending on the type and effectiveness of the pretreatment method, hydrolysis takes 24-48 hours to complete (Lin, 2006; Olsson, 1996). In an effort to reduce the overall time needed to produce ethanol, the hydrolysis and fermentation processes are being combined, rather than conducted sequentially, in a process called simultaneous saccharification and fermentation (SSF) (Lynd, 1999). In SSF, the fermenting microorganism (e.g., yeast) and enzymes that hydrolyze the polysaccharides are added at the same time so that the sugars are fermented as soon as they are available in a just-in-time process. SSF reduces ethanol production time and reduces the amount of enzyme used when a separate hydrolysis step is conducted, due to the formation of fewer compounds that inhibit ethanol production (Lin, 2006).

Fermentation of sugars

Fermentation involves microorganisms which consume sugars as a food source. Ethanol fermentation results in four major products: additional yeast cells (cell division), ethanol, carbon dioxide, and heat. One molecule of glucose will yield, stoichiometrically, 2 molecules of ethanol plus 2 molecules of carbon dioxide (Figure 3). On a mass basis, one kilogram of glucose will theoretically produce 0.51 kilogram of ethanol and 0.49 kilogram of carbon dioxide. However, glucose consumed to generate additional yeast cells (cell mass) does not result in the production of ethanol. Most industrial fermentation processes operate at 90-95% of the theoretical yield of ethanol from glucose fed to the yeast.

62

The fermentation process for producing fuel ethanol from cellulose is similar to that from corn grain. The fermentation media contains insoluble solids from the plant biomass, yeast, and soluble products from the pretreatment and saccharification steps. Hexoses (the six carbon sugars glucose and galactose) produced by hydrolysis of cellulose and hemicellulose can be fermented to ethanol using existing industrial strains of Saccharomyces cerevisiae.

The major difference between using cellulosic feedstocks and starch feedstocks lies in the existence of relatively large amounts of pentose sugars (five carbon sugars such as xylose and arabinose) contained in the hemicellulose. These sugars also need to be fermented to make the overall process economically feasible (Eggeman, 2005; Nagle, 1999). Existing strains of S. cerevisiae cannot directly ferment xylose, but can ferment xylulose through the pentose phosphate pathway (figure 4). Other yeasts and bacteria can ferment xylose or xylitol; efforts utilizing the tools of biotechnology are underway to develop industrial microorganisms capable of efficiently converting xylose to ethanol. A number of approaches are being explored.

Ethanol recovery

The recovery of ethanol produced in cellulosic processes is expected to use technology similar to that used in corn ethanol or sugar cane ethanol facilities. These technologies include repeated distillation (vaporization) and condensation of the ethanol-water mixtures produced during fermentation until the azeotrope point is reached (the point at which the difference between the boiling points of water and ethanol ceases to exist, which occurs when the mixture is 95.6% ethanol and 4.4% water by weight). At this point, the ethanol and water both vaporize to the same extent and cannot be further fractionated by distillation and final purification will by performed by the use of molecular sieves (zeolites which adsorb water from a vapor/gas mixture) (Al-Asheh, 2004; Kwiatkowski, 2006; Ladisch, 1979; Wankat, 1988).

Co-product production and handling

Unlike processes that produce ethanol from corn, the residual solid material produced in lignocellulose processes has little value as an animal feed as it is high in lignin while low in fiber and protein (McAloon, 2000). The material can be used to produce heat, steam, and electricity needed to run the ethanol facility, with the excess electricity sold to the electrical grid (Nagle, 1999).

Production of Ethanol using Sucrose Resources

Ethanol is produced by fermenting plant carbohydrates with yeast. Plant carbohydrates are grouped as soluble sugars (such as sucrose from sugarcane), storage carbohydrates (such as starch from grains and tubers), and structural carbohydrates which make up the plant cell wall (such as cellulose, hemicellulose, and pectin). Sucrose, commonly called table sugar, is composed of two simple sugars (fructose and glucose). It is the main sugar extracted from sugar cane and sugar beets, and is highly soluble in water. Industrial strains of yeast such as Saccharomyces can be used to hydrolyze sucrose into fructose and glucose, and ferment the sugars into ethanol. Ethanol production from sucrose predominates in tropical regions with sugar cane production, such as Brazil, which produced 4.2 billion gallons (15 billion liters) of fuel ethanol in 2004 (Barros, 2005).

Sugar is produced by first squeezing the juice out of the stems. The raw juice is clarified, impurities and solids removed, and thickened, followed by a series of crystallization steps to produce sugar crystals, which are removed. The remaining syrup is molasses. Approximately 3 gallons of molasses are produced for every 100 pounds of raw sugar produced (Shapouri, 2006). Fermentation of sucrose from sugar cane can be conducted using the juice directly obtained from squeezing the sugar cane stalks, or from the molasses. Most ethanol plants in Brazil produce ethanol from molasses, typically molasses A, which has had the sugar removed by a single crystallization step. The ethanol concentration in the fermented molasses is about 9% v/v (7% w/w). As sugarcane molasses is naturally low in free nitrogen, urea is typically added as a nitrogen source to insure proper yeast performance. Depending upon the quality of the molasses, other nutrients such as phosphorous, biotin, pantothenic acid, and inositol may also be added (Piggot, 2003). From each gallon of sugar cane molasses, 0.41 gallons of ethanol can be produced. If the raw sugar and molasses in sugar cane is used to produce ethanol, 19.6 gallons of ethanol can be produced per ton of harvested sugar cane (Shapouri, 2006).

The major processes involved in the production of ethanol from cane molasses are:

a. Fermentation of the sugar in the raw materials into alcohol and Carbon dioxide and;

b. Concentrations and purification of the aqueous alcohol by distillation

Fermentation of Sugar

Starts with the gradual propagation of pure yeast in the laboratory, which is done by growing the yeast in the sugar solution that takes about 24 hrs. The yeast is then transferred into a bigger cultivator until the yeast in sufficient quantity is produced in the pre-fermenter. Ample air is used in all the yeast cultivation stages to speed up this yeast growing cycle.

When the yeast is fully matured in the pre-fermenter, it is transferred to the first fermenter, which is previously cleansed. Diluted molasses from the continuous molasses mixer are mixed together with the yeast and fermentation commences as shown by the evolution of carbon dioxide. During the process, heat is evolved and it is being cooled down by recirculating the fermented liquor or “beer” into plate heat exchangers. This process of cooling lasts for about 16 to 24 hours.

After fermentation is complete, the “beer” that contains about 8% alcohol by volume is immediately transferred to the beer receiving tank ready for distillation.  This is done to allow the early cleaning of the fermenter to be ready for distillation.

Distillation

The beer from the receiving tank is pumped to the beer still column via a heating system:

a. Beer is first pre-heated by heat from hot spent wastewater to save on the usage of  steam, and;

b. The hot spent wastewater (slops) originally at 105°C is cooled to around 50°C before it   is introduced into the slops biological digester

The purpose of the beer still is to recover all the alcohol from the beer including all the volatile components of the beer. The hot crude alcohol vapor is partially condensed and the resulting condensate is again pumped to the midpoint of the purifying column. The purpose of the Purifying column is to separate alcohol from the other volatile component of the crude alcohol by virtue of the addition of hot pure water at the top of the column. Due to the interaction of the ascending hot water in the column, the volatile component of the ascending vapor separates from the alcohol.

The bottom product of the purifying column is fed to the rectifying column for final concentration and purification.

The rectifying column contains numerous rectification trays that cause the gradual increase in ethanol concentration by virtue of the difference in temperature.

Therefore, the bottom product of the rectifying column is almost pure water and the overhead product of the column, which is the more volatile compound, is high-grade ethyl alcohol.

Ethanol Recovery

Ethanol recovery processes for sugarcane juice or molasses use technologies similar to those used to produce ethanol from corn, thus distillation and molecular sieves are used to purify the ethanol following fermentation of the sucrose (Rosillocalle, 1986). The stillage left after the distillation of ethanol is used as a fertilizer for sugarcane fields.

The fibrous plant material that remains after the juice has been squeezed out of the sugarcane stalk is called bagasse. Approximately 0.3 lb of wet bagasse (about 50% moisture) is produced per 1 lb of wet sugarcane (Shapouri, 2006). Bagasse is rich in cellulose, hemicellulose, and lignin (Okano, 2006; Pandey, 2000; Silva, 2005), but has no food value. In most processing facilities, bagasse is burned to produce the heat and steam used to evaporate the water from the sugar in the crystallization process, and to distill the ethanol. Bagasse is also used to produce the electricity used by the plant with excess electricity sold to the power grid (Bhatt, 2001; Goldemberg, 2004). However, bagasse could be used as a cellulosic biomass resource to produce ethanol, other biofuels, and bioproducts.

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One comment

  1. I like the information you have provided. How much it will cost to produce 1 liter of Ethanol (96 % v/v). Please consider all the utility charges and operating cost. Plant is molasses based.
    Thanks,
    Atul.



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