The use of membrane filtration in the cellulose to ethanol process

High oil prices and the slowdown in grain based ethanol production due to the elevated cost of corn have contributed in creating the "perfect storm" of interest in producing ethanol from renewable sources such as cellulose.

Along with the passage of the Renewable Fuels Standard (RFS) included as part of H.R.6, the Energy Independence & Security Act signed in December, 2007 calling for significant increase in ethanol usage nationwide from both grain based and cellulosic based sources, there is tremendous interest in developing efficient, cost-effective manufacturing processes to produce ethanol from cellulosic feed stocks.

Membrane filtration has proven to be an extremely versatile separation technology that can play an important role in many steps within the Cellulose To Ethanol (C.T.E.) process.

The production of fuel grade ethanol has exploded in recent years due to the push to get away from the reliance on fossil fuels such as gasoline. Until recently, the primary source for the majority of ethanol has been corn. However, ethanol derived from cheaper and more replenishable sources such as cellulosic biomass like corn stover, switchgrass, cereal straws and wood chips has emerged as a major factor in this industry.

The main potential advantages of ethanol produced from cellulosic biomass sources are:

• Cellulosic sources are typically waste products that are easily available and renewable
• Burning of the residual lignin produces steam and electricity
• Up to 90% reduction in green house gases compared to gasoline

The process to access the cellulose from biomass and hydrolyze the sugars required for the fermentation and ethanol production involves many steps. Membrane filtration offers the opportunity to improve the efficiency, and therefore reduce the overall processing costs, in many of these steps.

Ethanol represents a cleaner and renewable energy source alternative to gasoline. The increasing use of, and reliance on, ethanol as a fuel requires that many types of biomass feedstocks can be used for ethanol production. The biochemical process involves the breakdown and conversion of the sugar polymers in biomass to ethanol. Advancements in the biochemical process and related equipment technologies continue to improve the economical feasible, resource efficiency, and environmentally impact. Membrane filtration technologies such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis can provide important process solutions in various steps of the biomass processes.

There are four types of organic materials that can be used in a typical biomass to ethanol biochemical conversion process: monomeric sugars, starch, cellulose, and hemicellulose. Monomeric sugars (as found in sugarcane, sugar beets, fruits, etc.) can easily be fermented into ethanol. However, the cost of production and/or competition from other uses makes these sources of biomass either too expensive or too low in profit in the United States. An objective of the processing of the other materials (starch, cellulose, hemicelluloses) is their efficient breakdown into sugars for fermentation. Starch is a biopolymer of glucose and can be easily broken down chemically and/or enzymatically to glucose and then fermented into ethanol. Starch derived from corn is the feedstock for the production of ethanol using the dry grind process and the wet milling process.

The basic steps in the biochemical production process of ethanol from cellulosic (cellulose or hemicellulose) biomass are:

The basic steps of the biochemical process are shown in the diagram of below:

Biomass Grasses Agricultural Crops Agricultural Residue Wood Chips		Pre-Treatment Physical Chemical Biological		Hydrolysis Enzymatic Dilute Acid Concentrated Acids
Ethanol Recovery Distillation Molecular Sieves Ethanol Non-Ethanol Solids		Fermentation Physical Chemical Biological		Separation Lignin Power Generation

Membrane filtration technology can be used to facilitate or improve various aspects of the above steps. Examples include:

In addition, membrane filtration can also have environmental benefits in the ethanol process as well when applied to water recovery and waste water treatment applications, thereby minimizing fresh water usage and waste discharges.

Ethanol production plants use roughly 4 gallons of water to make each gallon of fuel, putting tremendous pressure on water supplies in many regions throughout the country. Membrane filtration, particularly reverse osmosis, can be utilized to purify and recover up to 90% of the water used in some of the process steps, significantly lowering the overall volume of water needed.