How is fuel ethanol produced?

Fuel ethanol or absolute alcohol is produced by dehydration of rectified spirit or industrial alcohol and can be produced from
a) Industrial grade alcohol
b) Rectified spirit (RS)
c) Extra-neutral alcohol (ENA)

Commercially available technologies for dehydration of rectified spirit can be classified in to two broad headings.
1. Molecular sieve technology
2. Pervaporation (Membrane technology)
3. Azeotropic distillation

Molecular sieve technology:
Molecular sieve technology works on the principle of pressure swing adsorption wherein water is first removed by adsorbing on surface of 'molecular sieves' and then cyclically removing it under different conditions including steaming.

Molecular sieves are synthetic zeolites typically 3A zeolite. Zeolites are synthetic crystalline aluminosilicates. This material has strong affinity for water. They adsorb water in cold condition and desorb water when heated. This principle is used to dehydrate ethanol. The crystalline structure of zeolites is complex and gives this material the ability to adsorb or reject material based on molecular sizes. Water molecule can enter the sieve and be adsorbed, but larger alcohol molecule will not be retained and will go through the bed.

There can be two beds in parallel. Once a particular bed is saturated with water, it is heated with steam so that adsorbed water is desorbed from the bed. Till that time, other bed is used for dehydration.

Low steam consumption and low power consumption as compared to distillation characterise this type of system.

Pervaporation:
It is nothing but a form of ultra-filtration where a hydrophilic membrane is used to filter out water from the mixture of alcohol and water.

Here, membranes are used to filter out water out of the ethanol - water mixture. Ultra filtration, nano-filtration and reverse osmosis principles are used here. Also, the size of the membrane depends on the flux of water molecules through it. Hence, if higher purity of ethanol (>99.2 per cent), is required, we get two streams - absolute alcohol stream and weak alcohol stream. The weak alcohol stream has to be distilled to recover ethanol.

Azeotropic distillation:
Rectified spirit of industrial alcohol has around 5 per cent v/v water content. This cannot be removed further by simple binary distillation. This is because ethanol forms azeotrope with water at that temperature at atmospheric pressure and temperature.

To dehydrate ethanol further, a third substance called as entrainer (cyclohexane, benzene, toulene, ether, ketone etc) is added to the mixture of ethanol and water.

Entrainer brakes the azeotropic point of ethanol and water, i.e. it alters the relative volatility of water making it more volatile. A typical distillation assembly consists of two distillation columns namely, Dehydration column and Recovery column and a decanter.

During distillation of such a mixture, entrainer forms a ternary azeotrope with water and ethanol and comes at the top of the dehydration column. Water free ethanol comes out from bottom of the dehydration column. The ternary azeotrope coming out from top of the column is heterogeneous in nature and separates into two layers in the decanter. The organic layer contains predominantly entrainer.

It is recycled back to the dehydration column as reflux. The aqueous layer contains some dissolved ethanol and small amount of entrainer. It is send to the recovery column for recovery of this dissolved ethanol and entrainer. Thus the total entrainer is recycled back and only small make up is required to account for losses in vent and drain. This configuration is energy intensive.

Raw Material for Alcohol Production
Alcohol or Ethanol can be produced from feedstocks containing sugar or starch. The sugar substrates include sugarcane juice, filtrate, cane molasses, sugar beet (molasses and juice), sweet sorghum. Starch-based raw materials include grains like wheat, corn, sorghum, rye, millet, rice etc, and tubers like cassava.

Cellulose is another upcoming source of alcohol. Cellulose can be found in straw, bagasse, waste such as wood chip, paper pulp etc. This waste, if properly handled, could yield almost 40 billion gallons of ethanol annually. However, the process of converting cellulose to ethanol is yet to be made commercially viable. Many agencies including PRAJ, are involved in developing economical methods for converting cellulose.

Alcohol Production Process
Whilst in the case of sugarcane juice and cane molasses there are no major preparatory step involved, in the case of sugar beet, sweet sorghum and starch based raw materials, one has to extract the starch/sugar from the raw material prior to the alcohol conversion process.

The preparatory methods vary for each raw material. For example, in the case of grain, milling precedes liquefaction (adding water) and saccharification (conversion of starch into sugar through enzymatic process). In the case of sugar beet, one has to clean and prepare the beet into pulp and then introduce the liquefaction and saccharification steps.

The mash thus produced is then introduced to fermentors. Fermentation is a biochemical process wherein the action of yeast in the presence of oxygen leads to the production of alcohol and carbon dioxide. This alcohol is then fed to the distillation section of the plant.

Distillation is a separation technique to separate alcohol from other products of fermentation (usually termed as congeners) with respect to the boiling points of the mixture.

A distillation system includes distillation columns or towers or pot stills containing different types of internals, which assist in the process of separation. There could be different configuration of towers in terms of numbers, type of internals and material of construction depending upon the impurities or congeners to be removed and the end product desired.

Distillation is an energy intensive process.Technology providers like PRAJ are constantly finding more energy efficient methods for effective separation of impurities. The ECOFINE MPR system by Praj Industries is one such example.

Distillation plants can be designed for production of different grades of alcohol. There are three main grades of alcohol - industrial, potable and fuel ethanol.

Industrial alcohol is the same as rectified spirit containing 95.5 per cent alcohol. The remaining are impurities and water. In most countries industrial alcohol is denatured in order to make it unfit for human consumption.

Separating the impurities, which are not fit for human consumption, produces potable alcohol. Again, the composition of potable alcohol could vary depending upon the liquor being produced. For example in Rum one can have some of the congeners as they lend a unique flavour to Rum.

Fuel Ethanol or dehydrated alcohol is produced by removal of water molecules from the rectified spirit.

Developments in Ethanol

Cellulose to Ethanol
A commercial plant using cellulose from agricultural or municipal wastes as a feedstock, instead of the more traditional feedstock, has yet to be developed. The ethanol from the cellulose process is complex and costly and is still currently in the research and development stage. In recent years, pilot plants have proven the technical feasibility. Several North American companies have considered building factories that would convert the cellulose from organic plant wastes into ethanol.

Flex Fuel Vehicles
A flexible fueled vehicle (FFV) has a single fuel tank, fuel system, and engine. The vehicle is designed to run on unleaded gasoline and an alcohol fuel (usually ethanol) in any mixture. The engine and fuel system in a flex-fuel vehicle must be adapted slightly to run on alcohol fuels because they are corrosive.

There must also be a special sensor in the fuel line to analyze the fuel mixture and control the fuel injection and timing to adjust for different fuel compositions. The flex-fuel vehicle offers its owner an environmentally beneficial option whenever the alternative fuel is available. Some 270,000 of the flex-fuel "green" vehicles are expected to be sold in Brazil in 2004, according to industry forecasts.

When ethanol is more than 40 per cent cheaper than gasoline, it becomes attractive to drivers.

Ethanol Fuel Cells
Fuel cells work by combining hydrogen and oxygen in a chemical reaction to create electricity, without the noise and pollution of conventional engines. In principle, a fuel cell operates like a battery. Unlike a battery, however, a fuel cell does not run down or require recharging. It will produce energy in the form of electricity and heat as long as fuel is supplied.

Because the fuel cell relies on chemistry and not combustion, emissions are much lower than those from the most efficient internal combustion engines and consist primarily of water and steam. Fuel cells offer a unique combination of efficiency and ultra-low emissions, which is why every major automaker has a fuel cell development program in place. Many of these fuel cell programs involve hundreds of millions of dollars and hundreds of ethanol is a hydrogen-rich liquid, which overcomes both the storage and infrastructure challenges of hydrogen for fuel cell applications.

There are no technical barriers to the use of ethanol in fuel cells. Because ethanol is far easier to transport and store than hydrogen, fuel reforming - which uses a chemical process to extract hydrogen from fuel - offers a practical solution to the challenge of providing hydrogen to fuel cells onboard vehicles or for remote or stationary applications. In addition, ethanol is easier to reform than gasoline and most alternative fuels because of its relatively simple molecular structure.