Ethanol or ethyl alcohol is a clear, colorless alcohol-based alternative fuel, produced by fermenting and concentrating starch crops, which have been converted into simple sugars.
It can be made not only from corn, barley, and wheat, but also from cellulose feedstock like corn stalks, rice straw, sugarcane bagasse, pulpwood, switchgrass, and municipal solid waste that have considerable amounts of sugar and other materials, which can be converted into starch or cellulose.
This biofuel can also be produced from 'cellulosic biomass', which include trees and grasses, and hence, is called bioethanol.
The production of ethanol from starch or sugar-based feedstock is considered to be one of the man's earliest ventures into value-added processing. Although, the basic steps remain the same, this process has been significantly refined in recent years, hence leading to a highly efficient manufacturing of this chemical.
There are two main production processes: wet milling and dry milling. The primary difference between them is the initial treatment of the grain. However, in this post, we will try to understand how is ethanol made from corn, which is rich in starch, and is relatively easy to convert into sugar.
The modern ethanol production plants employ dry mill method, which involves use of yeast to convert sugar into alcohol; it is the same old fermentation technique that has been used for centuries. In fact, on an average, a typical dry mill production plant produces up to 50 to 100 million gallons of ethanol per year. The steps in this process are as follows.
The entire corn kernel, feedstock, or other starchy grain is passed through a hammer mill, which grinds it into a course flour called meal.
The meal is mixed with water and alpha-amylase enzymes to form a 'mash', which is passed through cookers, where the starch is liquefied. Cookers with high temperature stages (120 - 150º Celsius) and a lower temperature holding period (95º Celsius) are used for liquefaction. These temperature ranges reduce the bacteria levels in the mash.
At this stage, the mash is cooled down, and the secondary enzyme (gluco-amylase) is added to turn the liquefied starch into fermentable sugars (dextrose).
The mash is cooled and transferred to fermenters where yeast is added, and fermentation of sugar to ethanol and carbon dioxide (CO2) begins. This fermenting mash is allowed to pass through several fermenters, until is it completely fermented, and it leaves the final tank.
The entire process usually takes about 40 to 50 hours, during which the mash is stirred up and kept cool to increase the yeast activity.
The fermented mash, now referred to as beer, comprises nearly 15% alcohol with all the non-fermentable solids from the corn and yeast cells. The beer is pumped to a continuous flow, multi-column distillation system, where the alcohol is separated from the solids and water with the help of heat.
In the final distillation column, the alcohol present at its top, leaves behind a highly concentrated ethanol. The residue mash from the distillation process called stillage, is then shifted from the base of the first column to the co-product processing area.
At this stage, the alcohol in the top of the last column is passed through a dehydration system, where the leftover water is removed. Commonly, a molecular sieve system is used, which is capable of capturing even the last remaining traces of water in ethanol. The alcohol product is now called anhydrous ethanol (pure, without water).
In this stage, ethanol is used as a fuel. It is denatured, i.e., made unhealthy for human consumption, by adding a small amount (2 - 5%) of gasoline.
Distillers grains and carbon dioxide are two main by-products formed during this process. The former ones, in either wet or dry form, are used as a valuable animal feed, while the latter is collected, compressed, and sold to be used in carbonated beverages, food processing, and other industries.
Ethanol is a high-octane premium fuel, which enhances engine performance, and prevents any 'break down' conditions. Hence, in its pure form, it can be used as a great substitute to gasoline in cars and vehicles.