Biochemical Conversion Process of Producing Ethanol

Plant biomass contains approximately 75% polysaccharides, a rich source of sugars. Production of ethanol from this biomass (which is no different from that produced from first generation processes) by fermentation is, however, significantly more complicated than its production from first generation feedstocks such as sugarcane/beet and starch crops such as wheat grain.

The process can be divided into three phases: pretreatment of biomass to unmask the contained carbohydrate polymers, hydrolysis of carbohydrate polymers to sugar monomers and finally fermentation of sugars to ethanol.

Schematic Representation of the Biochemical Conversion Process

1.Pretreatment: The purpose of the pretreatment step is to further increase the surface area of the lignocellulosic material, disrupt the structure of the lignocellulose such that the cellulose component is accessible to hydrolyzing agents and reduce the crystallinity of the cellulose to further facilitate hydrolysis. Depending on the nature of the pretreatment technology selected, this step can also include solubilisation of the lignin or the hemicellulose component. Due to the recalcitrance of the lignocellulose structure these treatments are generally severe in nature and are combinations of either physical, chemical, biochemical or thermal treatments. Consequently, the pretreatment process represents a significant cost element of the whole lignocellulosic bioethanol process.

Feedstock Size Reduction - Before pretreatment, the first stage in the production of ethanol from biomass is cleaning followed by chipping or milling to reduce its size. Size reduction is necessary to provide pumpable slurry and to increase the biomass surface area so that mass transfer effects are minimised during the downstream processes. Techniques for size reduction include hammer, disk and knife milling and are well established.

2. Hydrolysis: In the hydrolysis reaction, the complex chains of sugars that make up the hemi-cellulose are broken, releasing simple sugars. The complex hemi-cellulose sugars are converted to a mix of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. The rest of hemicelluloses are degraded to weak acids, furan derivates, and phenolics. These compounds, however, are potential fermentation inhibitors. By the action of dilute acids, concentrated acids, and/or enzymes (cellulase), the glucose yields of cellulose hydrolysis often exceed 90%, but hydrolysis without preceding pretreatment yields typically less than 20% only.

The cellulose hydrolysis reactions can be simply represented as

The pretreated feedstock can be hydrolysed by two methods:

1. Acid hydrolysis:

- Dilute acid hydrolysis.

- Concentrated acid hydrolysis.

2. Enzyme hydrolysis

3. Fermentation: The fermenting of the biomass is conducted under standard fermenting conditions and will utilize all the major biomass. Yeast is the most commonly used microorganism in fermentation processes. Yeasts are minute, often unicellular, fungi.

The yeasts used are typically brewers' yeasts. Examples of yeast capable of fermenting the decaying biomass include, but are not limited to, Saccharomyces cerevisiae and Saccharomyces uvarum. Non-Sacharomyces yeasts, also known as non-conventional yeasts, are also used to make a number of commercial products. Some examples of non-conventional yeasts include Kuyberomyces lactis, Yarrowia lipolytica, Hansenula polymorpha and Pichia pastoris.

Microorganisms other than yeast can also be useful in making fermentation products. For example, cellulosic ethanol production also utilizes fungi and bacteria. Examples of these cellulolytic fungi include Trichoderma reesei and Trichoderma viride. One example of a bacteria used in cellulosic ethanol production is Clostridium Ijungdahlii.

Mid- to long-term technology under development are expected to improve the fermentation efficiency of the organism, producing higher yields in less time, and an organism requiring less detoxification of the hydrolysate.

4. Distillation: Separation of ethanol from the fermentation solution refers to the stage in which once ethanol begins to form during fermentation, it is isolated from the fermentation solution. The fermentation solution is likely to contain water, ethanol, and the remaining biomass. Separation can be achieved by any known methods, such as distillation. The separated ethanol, which will generally not be fuel-grade, can be concentrated to fuel grade (e.g., at least 95% ethanol by volume) via a second distillation.

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