Liquid Ring Vacuum Pump & Compressors for Bioethanol Production
Biofuel as an alternative fuel and energy source has been gaining traction. The world is moving away from primary power sourcing through fossil fuels to hinder environmental impact and increase cost efficiency through the recycling of renewable raw organic materials, such as sugar cossette and bagasse waste. In the United States, large amounts of corn are grown and processed solely for bioethanol production.
Ethanol is biodegradable and incredibly combustible. Although it is sufficient as a source of combustion on its own in general uses, it is often mixed with fuel to produce more complete combustion in automotive vehicles, increasing fuel efficiency and lessening greenhouse gas emissions. Bioethanol is produced mainly through the conversion of sugars within raw organic material through fermentation. Almost any plant can be used for production of bioethanol in the fermentation process, as any plant fiber is made of chains of sugars (polysaccharides). In the United States, the most common kinds of crops used for bioethanol production are starches: wheat, potatoes, and corn. Taking a look at corn, it has the highest sugar content of the three. Potatoes are priced at approximately 524 USD per tonne as of April 2022. Wheat is priced at 368 USD as of May this year, and corn is priced at 224 USD. Looking at the numbers, corn has the most glucose to be fermented into ethanol, and it is the cheapest to produce or import. With over 90 million acres of corn plantations in the US, 45% of the produce is used for bioethanol production as of 2018 according to the USDA.
First, the corn is milled, either by wet mill or dry mill.Wet milling separates fiber, starch, germ, and protein. Dry milling produces CO2, ethanol, and dried distiller grain.
In the wet mill process, corn is separated from its cob and washed to remove dust, dirt, and chaff. The remaining grains of corn are screened for cleanliness and loaded into stainless steel tanks.
These tanks are filled with a warm, extremely dilute sulfur dioxide solution for 30-40 hours, beginning the steeping process. The sulfur dioxide makes the solution acidic, discouraging bacterial growth, weakening the fibers of the grain, and swelling the grain so that the starches are more easily extractable. The steeped corn is ground while in the steep water only to gently and quickly remove the germ from the rest of the grain. The slurry moves to a germ separator, separating the germ from the rest of the grain. The germ holds the most oil out of any other part of the grain. The germ separator uses a cyclonic separator to float the germ from the slurry. The germ is low in density compared to the fibers. The oil is extracted using pressing and solvent extraction methods. The rest of the germ is used as livestock feed. The slurry is ground finely, extracting the gluten and starch from the fibers into the slurry. The slurry flows onto fine screens that catch fiber while allowing the starch and gluten liquid mixture to pass through. The fiber is processed again to extract any residual starch and gluten, and then added to livestock feed. The starch-gluten mixture (mill starch) is piped to a centrifuge. The slurry is full of water, and gluten has a much lower water-buoyant density than starch, which is composed of dense glucose monosaccharide units. The gluten is used as livestock feed. The starch is washed, diluted, and centrifugally separated another 8-14 times to remove residual proteins. Depending on the desired product (thin-boiling, bleached, maltodextrin, etc.) there is a different chemical modification process that brings the starch to react and form the product. Typically for the fermentation process that converts the starch into alcohol, the starch ideally needs to be broken down to basic dextrose units to reach maximum conversion. The starch is liquefied in a hot acidic solution. The heat makes the starch thicken, and the enzymes and acidic solution liquefy the viscous starch into a watery mixture by breaking the starch polymers. This liquid starch is piped to oxygen-deprived fermentation tanks for yeast bacteria like saccharomyces cerevisiae to convert glucose (dextrose) into ethanol and carbon dioxide. This batch process takes 2-3 days at a constant 86 degrees Fahrenheit. Almost 95% of all glucose is converted. The carbon dioxide helps to keep the mixture acidic, which discourages the growth of any bacteria that may hinder the process or cause contamination. The resulting liquid is distilled to extract hydrous alcohol and rectified in a membrane distillation chamber to extract anhydrous ethanol.
In the dry mill process, the grains are ground whole by a hammermill to increase the surface area through which the starch can be extracted. The ground corn is slurried in hot water to make a mash. The mash is cooked, thickening with the heat. As the mash continues to cook, water reacts with starch to form simpler glucose units (partial hydrolysis). Ammonia and sulfuric acid are added to the cooking tank depending on what the pH of the corn is. It needs to stay slightly acidic, 5.9-6.2 roughly. This acidic environment helps to break down the fibers of the corn and encourage the release of starch. This is also the more ideal environment for the addition and breakdown by enzymes, like -amylase and -amylase. Amylases are enzymes that break starches into simple sugars; they can be found in human saliva to perform that exact task for proper conversion of energy in the digestive system. -amylase is added and the mixture is raised to around 224 degrees Fahrenheit for 2-7 minutes. This accelerates the hydrolysis, sterilizes the mash, and creates a more ideal environment for the rest of the required amount of enzyme to be introduced into the mix. At this point, the starch is broken down into maltose and dextrins. These are much smaller than the starch polymers, but they are not dextrose, which has the best yield of ethanol by far. Thus, a saccharification step is performed by adding glucoamylase as an enzyme to further break these sugars down into the coveted dextrose units. The fermentation process is the same as with the wet mill fermentation, following the traditional yeast fermentation and distillation processes.
In the past decade, technological advancements have been applied to increase the efficiency of the bioethanol production process. Because the process emphasizes the control of temperature and purity, many vacuum applications have appeared. One of these is vacuum distillation, where a vacuum pump is connected to the reflux drum of the distillation column. This negative pressure environment decreases the boiling point of the fermented mixture of hydrous ethanol, heavier alcohols, acids, and water. The lowered boiling point translates to less operation costs by lowering required heat energy. Rectification is an extra distilling step that thoroughly separates hydrous ethanol. The hydrous ethanol is set under vacuum in a chamber. A membrane made of polytetrafluoroethylene is set vertically in the rectification chamber. Polytetrafluoroethylene is hydrophobic, thermally resistive, and porous. Heated hydrous ethanol splits due to the subatmospheric pressure; water vapor passes through the membrane. PTFE films can be treated to have 2.8 Angstrom-diameter pores, the perfect size for a water molecule while the 4.4-Angstrom ethanol is too large. The deep vacuum pressure that the liquid ring vacuum pump is able to achieve encourages the separation of the azeotropic hydrous ethanol to obtain purer anhydrous ethanol. Burning hydrous ethanol will create more carbon dioxide, which contributes to environmental harm and reduces the amount of energy created through ethanol combustion. Anhydrous ethanol burns much more cleanly and efficiently.
Because the process emphasizes the control of temperature and purity, many vacuum applications have appeared in the past decades to increase the efficiency of the production process. Of these technological advancements is vacuum distillation, where a vacuum pump is connected to the reflux drum of the distillation column. This negative pressure environment decreases the boiling point of the fermented mixture of hydrous ethanol, heavier alcohols, acids, and water. The lowered boiling point translates to less operation costs by lowering required heat energy. This method is further refined and researched every year, adding new apparati and other forms of technology that had not existed previously. As research in science and technology takes steps forward, so too does this ancient yet rapidly evolving technique. This is shown through distillation’s next step in the bioethanol production process, rectification. Rectification is an extra distilling step that thoroughly separates hydrous ethanol. The hydrous ethanol is set under vacuum in a chamber. A membrane made of polytetrafluoroethylene is set vertically in the rectification chamber. Polytetrafluoroethylene is hydrophobic, thermally resistive, and porous. Heated hydrous ethanol splits due to the subatmospheric pressure; water vapor passes through the membrane. PTFE films can be treated to have 2.8 Angstrom-diameter pores, the perfect size for a water molecule while the 4.4-Angstrom ethanol is too large. The deep vacuum pressure that the liquid ring vacuum pump is able to achieve encourages the separation of the azeotropic hydrous ethanol to obtain purer anhydrous ethanol. Burning hydrous ethanol will create more carbon dioxide, which contributes to environmental harm and reduces the amount of energy created through ethanol combustion. Anhydrous ethanol burns much more cleanly and efficiently.
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Applications of Liquid Ring Vacuum Pumps
The main two applications of vacuum in the corn bioethanol industry are distillation and rectification. Vacuum distillation of fermented ethanol mash requires the processing of water vapors and hydrous ethanol. The condensable vapors produced by vacuum distillation will create sludge when it interacts with oil lubrication in a vacuum pump. This sludge has the potential to hinder or completely interrupt the performance of the pump. A liquid ring vacuum pump has minimal lubrication because of the lack of moving metal-to-metal contact within the pump. Even if sludge enters, it is simply passed through the pump. Steam condenses because of the liquid ring, which absorbs the heat of condensation. This creates more space for the pump to draw gas in. This translates to an increased efficiency when pumping condensable vapors. The pump can be constructed for corrosion resistance against the acids used for liquefaction and saccharification with stainless steel, the same material that many other vessels in the process are constructed of.
Rectification requires deep constant vacuum pressure from a wet-running vacuum pump. A two-stage liquid ring vacuum pump is able to reach 28” Hg, so a two-stage liquid ring vacuum pump system will supply a strong enough vacuum pressure to disturb the dynamic equilibrium of the hydrous ethanol, separating the water and anhydrous ethanol and allowing the water to permeate through the membrane. The liquid ring vacuum pump is simple to repair and easy to maintain because there are few components. The chamber and impeller is robust, translating to a long service life. The water vapor will increase the mass transfer efficiency of the pump.
Our NL-6 and NL-7 liquid ring vacuum pumps have been in use for years for distillation and rectification of bioethanol.