Era of Cheap, Easy Oil has ended. This blog is dedicated for alternative energy especially for alternative gasoline. It is bioethanol fuel.

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Monday, October 19, 2009

Bioethanol from Cellulosic Materials

Cellulosic resources such as paper, cardboard, wood, agricultural residues and other fibrous plant material are in general very wide spread and abundant. For example, forests comprise about 80% of the world's biomass. Ethanol can be produced from different kinds of raw materials. The raw materials are classified into three categories of agricultural raw materials: simple sugars, starch and cellulose. Cellulosic biomass materials can be supplied from a variety of resources at a low price. They can be classified in four groups based on type of resource: wood,municipal solid waste, waste-paper and crop residue resources.
Being abundant and outside the human food chain, makes cellulosic materials relatively inexpensive inputs for ethanol production. Cellulosic materials are comprised of lignin, hemicelluloses, and cellulose and are thus sometimes called lignocellulosic materials. Cellulose molecules consist of long chains of glucose molecules (6-carbon sugars) as do starch molecules, but have a different structural configuration. These structural characteristics plus the encapsulation by lignin makes cellulosic materials more difficult to hydrolyze than starchy materials. Hemicelluloses are also comprised of long chains of sugar molecules, but contain pentoses in addition to glucose.
Based on average 42% cellulose and 21% hemicelluloses in wood, the maximum theoretical yield of ethanol can be calculated to be 0.32 grams of ethanol per gram of wood. This calculation is based on a full conversion of cellulose and hemicelluloses to sugars, and conversion of sugars to ethanol at the theoretical yield of 0.51 g/g.
Since pentose molecules (5-carbon sugars) comprise a high percentage of the available sugars, the ability to recover and ferment them into ethanol is important for the efficiency and economics of the process. Recently, special microorganisms have been genetically engineered which can ferment 5-carbon sugars into ethanol with relatively high efficiency. Bacteria have drawn special attention from researchers because of their speed of fermentation.
Cellulose hydrolysis produces glucose, which is readily fermented with existing organisms in much the same way as has been done for centuries. Hemicelluloses hydrolysis produces both hexose and pentose sugars: mannose, galactose, xylose and arabinose that are not all fermented with existing strains. The hemicelluloses fraction typically produces a mixture of sugars including xylose,arabinose, galactose and mannose. These are both pentosans: xylose and arabinose, andhexosans: galactose and mannose. The quantities are dependent on the material and alsothe growing environment and storage history of the material.
Pretreatment methods refer to the solubilization and separation of one or more of the four major components of biomass (hemicellulose, cellulose, lignin, and extractives) to make the remaining solid biomass more accessible to further chemical or biological treatment.Hydrolysis (saccharification) breaks down the hydrogen bonds in the hemicellulose andcellulose fractions into their sugar components: pentoses and hexoses. These sugars can then be fermented into bioethanol. After the pretreatment process, there are two types of processes to hydrolyze the cel-lulosic biomass for fermentation into bioethanol. The most commonly applied methodscan be classified in two groups: chemical hydrolysis (dilute and concentrated acid hydro-lysis) and enzymatic hydrolysis. In addition, there are some other hydrolysis methods in which no chemicals or enzymes are applied. For instance, lignocellulose may be hydrolyzed by gamma ray or electron beam irradiation, or microwave irradiation. However,those processes are commercially unimportant. Both enzymatic and chemical hydrolyses require a pretreatment to increase the sus-ceptibility of cellulosic materials. In the chemical hydrolysis, the pretreatment and thehydrolysis may be carried out in a single step. There are two basic types of acid hydrolysis processes commonly used: dilute acid and concentrated acid, each with variations.
Dilute Acid Hydrolysis
The dilute acid process is conducted under high temperature and pressure, and has a reaction time in the range of seconds or minutes, which facilitates continuous processing. As an example, using a dilute acid process with 1% sulfuric acid in a continuous flow reactor at a residence time of 0.22 minutes and a temperature of 510 K with pure cellulose provided a yield over 50% sugars. In this case, 1000 kg of dry wood would yield about 164 kg of pure ethanol. The combination of acid and high temperature and pressure dictate special reactor materials, which can make the reactor expensive. The first reaction converts the cellulosic materials to sugar and the second reaction converts the sugars too their chemicals. Unfortunately, the conditions that cause the first reaction to occur also are the right conditions for the second to occur.The biggest advantage of dilute acid processes is their fast rate of reaction, which fa-\cilitates continuous processing. Since 5-carbon sugars degrade more rapidly than 6-carbon sugars, one way to decrease sugar degradation is to have a two-stage process. The first stage is conducted under mild process conditions to recover the 5-carbon sugars while the second stage is conducted under harsher conditions to recover the 6-carbon sugars. 
Concentrated Acid Hydrolysis
Hydrolysis of cellulosic materials by concentrated sulfuric or hydrochloric acids is arelatively old process. The concentrated acid process uses relatively mild temperatures,and the only pressures involved are those created by pumping materials from vesselto vessel. Reaction times are typically much longer than for dilute acid. This method generally uses concentrated sulfuric acid followed by a dilution with water to dissolve and hydrolyze or convert the substrate into sugar. This process provides a complete and rapid conversion of cellulose to glucose and hemicelluloses to 5-carbon sugars with little degradation. The critical factors needed to make this process economically viable are to optimize sugar recovery and cost effectively recovers the acid for recycling. The solid residue from the first stage is dewatered and soaked in a 30 to 40% concentration ofsulfuric acid for 1 to 4 hours as a pre-cellulose hydrolysis step. The solution is again dewatered and dried, increasing the acid concentration to about 70%. After reacting in another vessel for 1 to 4 hours at low temperatures, the contents are separated to recover the sugar and acid. The sugar/acid solution from the second stage is recycled to the first stage to provide the acid for the first stage hydrolysis. The primary advantage of the concentrated acid process is the potential for high sugar recovery efficiency. 
The acid and sugar are separated via ion exchange and then acid is reconcentrated via multiple effect evaporators. The low temperatures and pressures employed allow the use of relatively low cost materials such as fiberglass tanks and piping. The low temperatures and pressures also minimize the degradation of sugars. Unfortunately, it is a relatively slow process and cost effective acid recovery systems have been difficult to develop. Without acid recovery, large quantities of lime must beused to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense. 
Enzymatic Hydrolysis
Another basic method of hydrolysis is enzymatic hydrolysis. Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. There are two technological developments: enzymatic and direct microbial conversion methods.
The chemical pretreatment of the cellulosic biomass is necessary before enzymatic hydrolysis. The first application of enzymatic hydrolysis was used in separate hydrolysis and fermentation steps. Enzymatic hydrolysis is accomplished by cellulolytic enzymes. Different kinds of "cellulases" may be used to cleave the cellulose and hemicelluloses. A mixture of endoglucanases, exoglucanases, glucosidases and cellobiohydrolases is commonly used. The endoglucanases randomly attack cellulose chains to produce polysaccharides of shorter length, whereas exoglucanases attach to the non reducing ends of these shorter chains and remove cellobiose moieties, glucosidases hydrolyze cellobiose and other oligosaccharides to glucose. 
For enzymes to work efficiently, they must obtain access to the molecules to behydrolyzed. This requires some kind of pretreatment process to remove hemicelluloses and break down the crystalline structure of the cellulose or removal of the lignin toexpose hemicelluloses and cellulose molecules.


Biomass residues available from agricultural and forest processing constitute a potential source for production of chemicals such as ethanol, reducing sugars and furfural,using enzyme or acid-catalysed hydrolysis. Bioethanol can be produced from plentiful,domestic, cellulosic biomass resources such as herbaceous and woody plants, agricultural and forestry residues, and a large portion of municipal solid waste and industrial waste streams. To ensure that a low cost energy feedstock is available, researchers are examining dedicated energy crops, wood and grass species that have been selected to produce high yields. To produce bioethanol from cellulosic biomass, a pretreatment process is used to reduce the sample size, break down the hemicellulose to sugars, and open up the structure of the cellulose component. The cellulose portion is hydrolyzed by enzymes into glucose sugar that is fermented to bioethanol. The sugars from the hemicellulose are also fermented to bioethanol. The worldwide desire to reduce greenhouse gas emission will lead to an increased interest in renewable resources for energy production. Cellulosic biomass materials are among the candidates to be used as a renewable resource. Ethanol has very good characteristics to be used as a fuel either in a neat form or in a mixture with gasoline. Bioethanol is a domestically produced liquid fuel from cellulosic biomass resources. It is a high octane fuel that can contribute substantially to the automotive fuel supply. Ethanol is a potentially clean burning fuel that reduces smog and emissions of carbon monoxide. The use of gasohol (ethanol and gasoline mixture) as an alternative motor fuel has been steadily increasing around the world for a number of reasons. Domestic production and use of ethanol for fuel can decrease dependence on foreign oil, reduce trade deficits, create jobs in rural areas, reduce air pollution, and reduce global climate change carbon dioxide build up. Ethanol, unlike gasoline, is an oxygenated fuel that contains 35% oxygen, which reduces particulate and NOx emissions from combustion.
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Sugar Beet as a Raw Material for Bioethanol Production

Many papers have been focused on the total energy effectiveness of bioethanol production from sugar beet. Itis considered that energy needed for sugar beet processing to obtain ethanol is equal to 25–50% of the energy cost of produced ethanol. The costs depend on the technique of treatment and that is why it is possible to find variant numbers qualifying the total efficiency in a widerange from 67% (loss) to 300% (very optimistic estimation) .
From an economic point of view and in comparison with cereals, sugar beet and intermediates from beet processing are very good raw materials for alcohol productiondue to their content of fermentable sugars which can be directly used for fermentation without any modification. Molasses is a traditional raw material for distilleries inthe Czech Republic and about 90% of ethanol production comes from this raw material now a days. Molasses production has decreased every year since 1997 and the estimated last year’s production of 118 thousand tons is not sufficient for domestic alcohol production. This insufficiency was partly covered by an increased import.  Disadvantage of direct beet and beet pulp fermentation is a slow release of sugars from pulp into the fermented solution. The second aspect is a sort of problematic storability of beet that brings about sugar loss due to enzyme action.
Raw juice contains about 15–20% of dry solids. Raw juice purity ranges between 85 and 90% that means there are about 85–90% of sugars and 10–15% of nonsugars in dry matter. Considering these facts, raw juice can be used straight away after pH adjustment for fermentation. All these properties together with a relatively low price incomparison with other intermediates from beet processing make the raw juice a very profitable material for alco-hol production. Its only disadvantage is low storability and easy decomposition by the action of micro organisms. Contrary to molasses, raw juice contains all non sugarsthat are usually removed by purification process in further steps of sugar beet processing. Hence, these nonsug-ars remain in a broth and after the fermentative processpass to stillage making its composition very suitable for addition into fodder. Additional advantage is a low content of inorganic salts (especially potassium). Another suggestion how to decrease the volume of stillage from juice fermentation is based on a recycling of the part of distillation residue for sugarextraction from the raw material. Thin juice is very suitable for ethanol production butthe biggest disadvantage is a very small or hardly any possibility of its storage because the concentration of sugars is almost ideal for microorganism growth. For thesereasons it is necessary to supply the distillery very fluently with this material, which requires a contact between the distillery and the sugar plant. Thick juice is a relatively pure and highly concentrated sugar solution (RDS content 60–65%, polarization 55–65%,purity 90–95%) that is obtained by the concentration and thickening of thin juice on evaporators. This eliminates problems with storability that is comparable with molasses. On the other hand, the production of thick juice is very complicated and expensive, and it consequently influences the ethanol price, too.  (Source : Czech J. Food Sci., Vol. 19, No. 6: 224–234)


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Continuous Fermentation Through Fixed-yeast Fluidized-bed

The new fermentation technology through fixed yeast fluidized-bed is an advanced biological technique in the world of today. Comparing with batch fermentation and /or single concentration continuous fermentation, the new fermentation technology possesses the advantages of quick speed, shout fermentation cycle, high yield, few instruments, and it is easy to realize automation. The production capacity of the new technology is 10 to 20 times as high as that of batch fermentation. In order to develop the technology of refining alcohol from juice of sweet sorghum stem, to reduce equipment investment and to improve economical benefits, Shenyang Agricultural University and Institute of Applied Ecology, Academia Sinica, have jointly engaged in the research work on this new technology. A 286 ml CH-1 type glass reactor, a 2800 ml CH-2 type glass reactor and a SOL CH-3 type stainless steel reactor were designed successively. To observe inner reaction and grasp reaction mechanism, a SOL polymethyl methacrylate reactor was designed. In September, 1989, a 450L reactor and the complete set of technological equipment were designed again. Experimental result showed that the design requirements are satisfied.
Tests show that, since the contact area of fixed-yeast carrier and juice is large, the reaction rate is increased and the fermentation cycle is shorten to be 4 to 5 hours. Because fixed-yeast reproduces unceasingly in the carrier and is not easy to become ageing, the life span of yeast is prolonged, the discharge of organism is reduced, and the environmental pollution is alleviated. Moreover, because the biological reactor is a kink of dumbbell-shaped three-unit vertical equipment, the volume is small, the ground area it occupies is little, and the capital construction investment is greatly decreased. In order to cooperate with the project of "The Energy Integrated Demonstration Base for China's Cold Northeastern Region", technology and equipments of fixed-yeast, which achieves a daily yield of 400 kg alcohol, was designed by the author.
a. Technological Process
The technological process of refining alcohol through fixed-yeast fluidized-bed biological reactor is shown below :


It includes CO2 circulation system, juice flowing and filling system, beer removing system, cooling and warming system, measuring instruments, and particle-producing system.
CO2 circulation system is composed of (along the gas flow) 13 - foam collector, 14 - gas-liquid separator, 15-cooling and purifying tank, 17-gas compressor (No.1), 18-gas-storage bag, 9-gas compressor (No.2), 10-constant pressure gas storage tank, 11 -gas flowmeter, 12-gas chamber.
Juice flowing and filling system consists of 1-juice storage tank, 2 - juice pump, 33-juice filter, 3-high position tank, 4-juice flowmeter, 5 - three-unit fluidized-bed biological reactor.
Beer removing system contains 6-beer storage chamber, 7-solid-liquid separator, 8-beer trough.  37. First column 38. Second column.
An annular pipe is used to sprinkle water for cooling or warming.
Before normal operation of this system, fixed-yeast cells must reproduce. The prepared fixed-yeast particles are filled into the first unit of the reactor. Then, diluted juice of sweet sorghum is introduced into the reactor until it is full. At the same time, aseptic gas should be led in continuously to make fixed-yeast cells to reproduce. This process lasts about 50 hours. When cells reproduce from 106 to 108 per millilitre solution, and the sprouting rate rises from 5-10% to 15-30%, fermentation stage begins.
When normal operation begins, CO2 circulation system and juice flowing and filling system should be started simultaneously. CO2 in the gas bag is pumped into the constant pressure gas storage tank by the No.1 gas compressor. Then, CO2 is led into the gas chamber at the lower end of the biological reactor. Passing through the gas dispersing plank, it eventually enters the reactor and forms the reaction force. Thus, the force makes fixed-yeast particles react inside the third unit of the reactor. At the same time, juice of sweet sorghum inside the high position tank, under the action of pressure, injects into the reactor from two tangentially-mounted spouts in the first unit of the reactor and circulates around the axis. Acted simultaneously by the upward force formed by CO2, juice moves in a upward spiral way. The solid-liquid-gas three-phase flowing layer (shown in Fig.4.3.6) that was formed in the reactor makes the CO2 gas adhered to the surface of fixed-yeast particles be released.
As CO2 gas constantly contacts with fermentation liquid, vigorous reaction state is maintained and the reaction rate is greatly increased. Since juice entered from the tangential spouts in the first until of the reactor, it takes about 5 hours for juice to pass through all of the three units and carry out the reaction. Above 90% of sugar is converted into alcohol and CO2. The eventually formed fermentation beer enters beer storage chamber, and them flows into solid-liquid separator through the outlet. After broken remains of yeast particles have been removed, fermentation beer is introduced to beer storage trough to be distilled.
CO2 gas coming out from the third unit is pumped into CO2, storage bag through beer storage chamber, foam collector, gas-liquid separator and cooling and purifying tank by the compressor. It is prepared to be used next time.



More explanation can be seen on : http://www.fao.org/docrep/t4470E/t4470e07.htm


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