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- Basic Ethanol Production
- The following quick review of the chemical manufacturing processes involved in alcohol production is excerpted from Chapter 3 of Fuel from Farms, which is available from the National Technical Information Service. Corn is used as the feedstock example. While molasses is currently the predominate ethanol feedstock in Louisiana, corn and milo are expected to dominate any expansion.
- Basically, fermentation is a process in which microorganisms such as yeasts convert simple sugars to ethanol and carbon dioxide. Some plants directly yield simple sugars; others produce starch or cellulose that must be converted to sugar. The sugar obtained must be fermented, and the resulting "beer" must then be distilled to obtain fuel grade ethanol.
- 1. Feedstock preparation
- Feedstocks can be selected from among many plants that either produce simple sugars directly (sugarcane, sweet sorghum) or produce starch (corn, grain sorghum). Feedstock preparation will vary with the feedstock, but some features are universal:
- - sugarcane or sorghum must be crushed to extract their simple sugars.
- - Starchy and cellulosic materials must be physically broken down by milling or grinding to break starch walls so that the material is available to water. Later steps break down the individual cell walls of the starch.
- - Cooking
- Starches are converted to sugars in two stages, liquefaction and saccharification, by adding water, enzymes, and heat (enzymatic hydrolysis). The choice of enzymes will determine the supervision the cooking stage will require. (Detailed information on suitable temperature, pressure, and pH for a particular enzyme appears on the manufacture's label.)
- Liquefaction, or the breakdown of starch to complex sugars, requires:
- - thoroughly mixing prepared feedstock with water;
- - adjusting pH of the mixture to a level suitable for the enzyme being used;
- - thoroughly mixing in the appropriate proportions of liquefaction enzyme (alpha-amylase) for the quantity of starch to be converted; and
- - heating the grain mash. This breaks the cell walls of the starch. The free starch will gelatinize as the temperature increases, forming a thick mash. As the mash reaches the enzyme's optimum temperature, the enzyme chemically breaks down the starch to complex sugars (dextrins). When this liquefaction stage is complete, the mash appears soupy, as it did before gelatinization.
- Saccharification, or the breakdown of complex sugars to simple sugars involves:
- - cooling the mash to the optimum temperature for the saccharifying enzyme;
- - adjusting the pH of the mash to the level required by the enzyme;
- - mixing the appropriate proportions of saccharifying enzyme (glucoamylase) needed to convert the available sugar; and
- - holding the pH and temperature (122 - 140°F) in the optimum range and stirring constantly until saccharification is complete, which is determined by testing for sugar content.
- 2. Fermentation
- At this point the starch has been broken down to the simple sugar glucose and is now in a form which microorganisms called yeasts can feed on. Yeasts, in metabolizing glucose, produce ethanol and carbon dioxide. As with the enzymes, yeasts have an optimum temperature range.
- - The mash is transferred to the fermentation tank and cooled to the optimum temperature (around 80 - 90°F). Care has to be taken to assure that no infection (other organisms that compete with the yeast for the glucose) occurs.
- - The appropriate proportion of yeast is added.
- - The yeast will begin producing alcohol and should turn the mash into a "beer of 8-12 percent alcohol and then become inactive as the alcohol content becomes too high".
- The mash is now ready for distillation. Separating the liquid beer from the solids of the mash stillage at this stage will help prevent possible clogging problems during distillation.
- 3. Distillation
- Distillation separates the ethanol from the beer, which is mostly water and ethanol. (in some alcohol plants, distillation takes place in one, very tall column; the process diagrammed above uses two separate columns, a stripper column and a rectifying column).
- Ethanol boils at 172°F ( at sea level), while water boils at 212°F. By heating the beer to 172°F, the ethanol can be boiled off and the vapor captured and condensed to produce 192-proof (96 percent) ethanol concentration producible by conventional distillation. 200-proof (anhydrous) alcohol (which is required for blending gasohol) can be obtained through additional dehydration steps. Lower-grade ethanol (170-190 proof) can be used by itself in vehicles modified for alcohol use.
- Source:
- Fuel from Farms - A Guide to Small Scale Ethanol Production, Solar Energy Research Institute (SERI), 1617 Cole Boulevard, Golden, CO 80401.
BRIEF ENERGY BALANCE FOR TYPICAL
GRAIN FUEL ALCOHOL PLANT
| PROCESS STEAM AND ELECTRICAL POWER BREAKDOWN | ||
| IDENTIFICATION | PROCESS STEAM | ELECTRICAL |
| * Receiving, Storage, & Milling | 0.0% | 6.1% |
| * Mash Cooking & Saccharification | 30.5% | 2.6% |
| * Fungal Amylase Production | 0.7% | 20.4% |
| * Fermentation | 0.2% | 4.0% |
| * Distillation | 58.5% | 1.6% |
| * DDG Recovery | 6.4% | 27.1% |
| * Storage & Denaturing | 0.0% | 0.7% |
| * Utilities | 2.7% | 27.0% |
| * Buildings | 1.0% | 0.5% |
| Totals | 100% | 100% |
- Total fuel requirements = 41,700 BTU/gallon of 200 proof ethanol
- Total electrical energy requirements = 1.32 KWH/gallon of 200 proof alcohol
- Source:
- A Guide to Commercial-Scale Ethanol Production and Financing, Solar Energy Research Institute (SERI), 1617 Cole Boulevard, Golden, CO 80401
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This page updated June 1989
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