3. How Ethanol Is Made

The production of ethanol or ethyl alcohol from starch or sugar-based feedstocks is among man’s earliest ventures into value-added processing. While the basic steps remain the same, the process has been considerably refined in recent years, leading to a very efficient process. There are two production processes: wet milling and dry milling. The main difference between the two is in the initial treatment of the grain. As of January 2007, dry mill facilities account for 82% of ethanol production and wet mills 18%.

The Ethanol Production Process - Dry Milling

In dry milling, the entire corn kernel or other starchy grain is first ground into flour, which is referred to in the industry as “meal” and processed without separating out the various component parts of the grain. The meal is slurried with water to form a “mash.” Enzymes are added to the mash to convert the starch to dextrose, a simple sugar. Ammonia is added for pH control and as a nutrient to the yeast.

The mash is processed in a high-temperature cooker to reduce bacteria levels ahead of fermentation. The mash is cooled and transferred to fermenters where yeast is added and the conversion of sugar to ethanol and carbon dioxide (CO₂) begins.

The fermentation process generally takes about 40 to 50 hours. During this part of the process, the mash is agitated and kept cool to facilitate the activity of the yeast. After fermentation, the resulting “beer” is transferred to distillation columns where the ethanol is separated from the remaining “stillage.” The ethanol is concentrated to 190 proof using conventional distillation and then is dehydrated to approximately 200 proof in a molecular sieve system.

The anhydrous ethanol is then blended with about 5% denaturant (such as natural gasoline) to render it undrinkable and thus not subject to beverage alcohol tax. It is then ready for shipment to gasoline terminals or retailers.

The stillage is sent through a centrifuge that separates the coarse grain from the solubles. The solubles are then concentrated to about 30% solids by evaporation, resulting in Condensed Distillers Solubles (CDS) or “syrup.” The coarse grain and the syrup are then dried together to produce dried distillers grains with solubles (DDGS), a high quality, nutritious livestock feed. The CO₂ released during fermentation is captured and sold for use in carbonating soft drinks and beverages and the manufacture of dry ice.

The Ethanol Production Process - Wet Milling

In wet milling, the grain is soaked or “steeped” in water and dilute sulfurous acid for 24 to 48 hours. This steeping facilitates the separation of the grain into its many component parts.

After steeping, the corn slurry is processed through a series of grinders to separate the corn germ. The corn oil from the germ is either extracted on-site or sold to crushers who extract the corn oil. The remaining fiber, gluten and starch components are further segregated using centrifugal, screen and hydroclonic separators.

The steeping liquor is concentrated in an evaporator. This concentrated product, heavy steep water, is co-dried with the fiber component and is then sold as corn gluten feed to the livestock industry. Heavy steep water is also sold by itself as a feed ingredient and is used as a component in Ice Ban, an environmentally friendly alternative to salt for removing ice from roads.

The gluten component (protein) is filtered and dried to produce the corn gluten meal co-product. This product is highly sought after as a feed ingredient in poultry broiler operations.

The starch and any remaining water from the mash can then be processed in one of three ways: fermented into ethanol, dried and sold as dried or modified corn starch, or processed into corn syrup. The fermentation process for ethanol is very similar to the dry mill process described above.

The Production of Ethanol from Cellulosic Biomass

This process flow diagram shows the basic steps in production of ethanol from cellulosic biomass. Note that there are a variety of options for pretreatment and other steps in the process and that several technologies combine two or all three of the hydrolysis and fermentation steps within the shaded box. Chart courtesy of the National Renewable Energy Lab.

Methods of producing ethylic alcohol

Ethanol is produced using wither methods of synthesis or fermentation. In synthetic production, ethanol is taken from petroleum products and in fermentation method, food stuff and saccharine-containing plants are used. The major part of ethanol global production (about 93%) is produced through fermentation and only 7% is produced by the synthetic method.
It is to be mentioned that from the total ethanol produced in the world (through fermentation method), around 65% is produced by fermenting sugar industries by-products and the other 35% is produced through fermentation of saccharine-containing materials such as wheat, maize, date, etc.

Ethanol production processes are described as below:

1. Synthetic method

Synthetic ethanol is produced through direct and indirect hydration. Indirect hydration is older than the direct one and goes back to more than 100 years ago. In this method, first, the hydrocarbon feed containing 35-95% ethylene is exposed to 95-98% sulphuric acid and mono and di-ethylene sulphate is produced.
Then, the mixture is hydrolyzed using water and the 50-60% sulphuric acid solution is gained.
Next, ethanol is detached from thin sulphuric acid in separation tower and the acid returns to the process after thickening Direct hydration was first used in industries in 1947. In this process, the gas filled with ethylene is mixed with water and trns into ethanol after passing through catalyst.
There are several processes for synthesis of ethanol but they are not industrially significant.

2. Fermentation

Fermented ethanol can be produced from several raw materials. These materials are categorized into three groups of sweet, starchy and cellulosic compounds in the following table.