Ethanol: Production Overview

Ethanol production is one of the earliest chemical technologies known to man. Production of Ethanol from grain is a simple process that involves several different processing areas. Over the years the technology has evolved to incorporate improved methods to maximize yield, lower costs, and provide for recovery of valuable by-products. It involves the following steps:

Grain Receiving / Handling

Grain is unloaded via rail car or bulk truck for storage as required maintaining continuous operations.

Milling

There are a variety of milling options including dry, wet or modified. Dry milling is the most commonly used method.

The grain feedstock passes through hammer mills, which grind it into a fine meal.

Conversion

The meal is mixed with water to form a mash that passes through a high temperature (~200-300°F) cooker for a brief period of time, followed by a lower temperature (~140°F) holding period with an alpha-amylase enzyme, where the starch is liquefied. Heat is applied to release bundled starch molecules so that they can be broken down by the enzyme in the liquefaction process. The high temperatures also reduce bacteria levels in the mash.

Saccharification

After liquefaction, the mash is further cooled and the secondary enzyme (gluco-amylase) is added to convert the liquefied starch to fermentable sugars (dextrose).

Fermentation

Yeast is added to the mash to ferment the sugars to Ethanol and carbon dioxide (CO₂). If continuous fermentation is used, the mash flows, or cascades, through several fermenters in series until the mash is fully fermented and then leaves the final tank. In a batch fermentation process, the mash is held in one fermenter for about 48 hours to complete the fermentation. Fermentation heat must be removed to maintain optimum temperature for yeast activity.

Distillation

The fermented mash, now called “beer”, contains about 12-15% alcohol, by volume, as well as all the nonfermentable solids from the grain and the yeast cells. The mash is pumped to a continuous flow distillation tower where the alcohol is removed from the solids and the water. The alcohol leaves the top of the tower at about 95% strength (190 Proof), and the residue mash, called stillage, is transferred from the base of the tower to the co-product processing area.

Dehydration

The 95% alcohol from the top of the column must be dehydrated in order to be suitable for blended Ethanol fuel. There are a few different ways to dehydrate Ethanol; including molecular sieves, grits and azeotropic (entrainers). However, molecular sieves are the most popular method in North America.

Dehydration captures the last bit of water in the Ethanol. The alcohol product at this stage is called anhydrous (without water) Ethanol and is 99.5%+ Ethanol.

Denaturing

Ethanol intended for use as fuel is denatured with a small amount (2%-5%) of some product such as gasoline, to make it unfit for human consumption.

Co-products

Ethanol production is a no-waste process that adds value to the feedstock by converting it into more valuable products. Two main co-products — Carbon Dioxide (CO₂) and distillers grains — are created during Ethanol production. CO₂ is released during fermentation. Many Ethanol plants collect the CO₂, clean it of any residual Ethanol, compress it, and sell it for use in carbonated beverages or to flash freeze meat (“Dry Ice”). By-product DDGS (Dried Distillers Grains with Solubles) is high in protein and other nutrients and is used extensively in animal feed. Production of DDGS utilizes centrifuges to separate the liquid and solid fractions. The liquid is evaporated to syrup that is added back to the solids in the final drying step. Some Ethanol plants may sell the syrup separate from the distiller’s grain.

Wet Milling Facilities

Wet milling of corn is used in the manufacture of high-fructose corn sweetener. Dextrose, which is produced as an intermediate product, may be diverted to Ethanol production during the seasonal downturn in the sweetener market. This provides the flexibility to produce the more value-added product as the market changes.