bioethanol from renewable sources congress proceedings/03_ivana bankovic.pdfar solution 2. using...

435

Introduction

Petroleum provides the single largest fraction of the world’s energy, accounting for about 37% of the total world energy used (US DOE, 2002). How-ever, for most countries, much of this petroleum has to be imported, and a large fraction (about 30%) comes from politically volatile locations in the Per-sian Gulf. The use of biofuels for transport is becom-ing of increasing importance for a number of reasons, such as environmental concerns relating to climate change, depletion of fossil fuel reserves, and reduc-tion of reliance on imports (EC Directive 2003/30/EC, Wingren et al. 2003). This is leading to interna-tional, national and regional focus upon alternative energy sources. In Europe, the European Commis-sion has proposed indicative targets for biofuel sub-stitution of 5.75% by 2010 (EC Directive 2003/30/EC). In order to satisfy European legislative pres-sures and energy demand, an environmentally sus-tainable approach required to change the current pe-troleum management strategies into the bioconver-sion of biomass fractions to bioethanol.

Bioethanol is considered one of the most im-portant renewable fuels due to the economic and en-vironmental benefits of its use. However, the pro-duction of bioethanol is a complicated process. The transformation of such biological resources as sug-

BIOETHANOL FROM RENEWABLE SOURCES

Ivana BANKOVIĆ-ILIĆ, Marija TASIĆ, Vlada VELJKOVIĆ & Miodrag LAZIĆ

Faculty of Technology, Bulevar oslobodjenja 124, 16000 Leskovac, Serbia

ABSTRACT

Banković-Ilić I., Tasić M., Veljković V. & Lazić M. (2008): Bioethanol from renewable sources. Proceed-ings of the III Congress of Ecologists of the Republic of Macedonia with International Participation, 06-09.10.2007, Struga. Special issues of Macedonian Ecological Society, Vol. 8, Skopje.

Ethanol obtained out of biomass (bioethanol) is a modern way of using energy and a significant substitute for fossil fuels and natural gas. Because of its purity, lesser volatility, low toxicity and biodegradibility in water and dirt, it is recommended for usage in engines with internal combustion. Raw materials used in the process for ob-taining bioethanol may be divided into three groups: sugars, starch and cellulose. In this work are listed raw mate-rials characteristics for bioethanol production. The technological process depending of the used material is also de-fined here. Within the scope of every process scheme that is offered there is an accent on process of the raw mate-rial, which goes before the main stages of fermentation and distillation.

Key words: bioethanol; feedstocks; flowsheet configurations

ars, starchy or lignocelluloses biomass requires the pretreatment of the feedstocks for fermenting organ-isms to convert them into ethanol. Then, the fermen-tation broth should be concentrated for obtaining hy-drous ethanol, which has to be dehydrated in order to be utilized. The complexity of this process explains why the development of cost-effective technologies for bioethanol production is a priority.

The design of a cost-effective process for bioethanol production implies not only the selec-tion of the most appropriate feedstock, and the se-lection and definition of a suitable process config-uration. The assessment of the utilization of differ-ent feedstocks (i. e. sucrose containing, starchy ma-terials, lignocelluloses biomass) is required consid-ering the big share of raw materials in bioethanol costs. The task of defining a proper configuration of the process requires the generation and assessment of many process flowsheets for finding those ones with improved performance indicators. The present work deals with the assessment of both the utiliza-tion of different feedstocks and the known configu-rations of the bioethanol production process.

Feedstocks for bioethanol production

Bioethanol, whether for use on its own or for blending with conventional fuels, can be produced

Стручен трудProfessional Article

Ivana BANKOVIĆ-ILIĆ et al.

436 Зборник на трудови од III Конгрес на еколозите од Македонија

by fermentation of different feedstocks, which are grouped according to the type of carbohydrate into sugars, starch and lignocellulose (Tab. 1).

Tab. 1. Feedstocks for bioethanol production

Sugars Starch LignocelluloseSugar caneSugar beetArtichokeSweet sorghumFruit

Corn, maize and wheat cropsPotatoSweet potato

WoodWaste paperAgricultural wastesMunicipal solid wastes

The nature of the biomass raw material is a major factor when calculating the cost and selecting the equipment required for a hypothetical bioethanol production. The simplest way is to start with biomass that already contains a satisfactory quantity of the monomeric sugars that can be directly fermented in-to ethanol, as in the case of sugar cane and sugar beet. More often, the sugar in the biomass is in a polymer-ic form that requires pre-treatment to transform it in-to an accessible monomeric form. Glucose polymers include starch and cellulose, while hemicellulose is a polymer that is largely composed of sugars containing five carbon atoms, such as xylose.

The most common sugar feedstocks for bio-ethanol production are sugar cane and sugar beet. Other biomass feedstocks rich in sugars include ar-tichoke, sweet sorghum, and various fruits. Howev-er, these materials are all in the human food chain and, except for some processing residues, are gener-ally too expensive to use for fuel ethanol production. Sugar cane contains 12-17% total sugars of which 90% is saccharose and 10% glucose and fructose. Either in the form of cane juice or cane molasses, sugar cane is the most important feedstock utilized in tropical and subtropical countries for producing bioethanol. Sugar beet grows in temperate or cold regions, so that it is an ideal crop for those parts of Europe where it represents the primary source of ed-ible sugar. The interest in the use of sugar beet for bioethanol production derives in higher sugar con-tent comparing to sugar cane, which are 20.4% and 16.5%, respectively (http://www.hort.purdue.edu/newcrop). The conversion of saccharose into etha-nol is easier compared to starchy and lignocellulos-ic materials since this disaccharide can be utilized by the yeast cells.

Starch is a natural polymer. Its constituent monomers are glucose molecules held together by glycosidic bonds between an oxygen atom on one molecule and a carbon atom on its neighbour. Those bonds may be α or β type depending on their stere-oisomerism to anomeric carbon. The world’s most important starch based ethanol comes from cereals and tubers, primarily corn and potato. In grains and tubers that contain it, starch takes the form of gran-ules each containing two main constituents: amylo-

se (about 20%) and amylopectin (about 80%), which are glucose polymers. Both polymers are easily hy-drolysed and fermented into ethanol.

Since most starchy and sugary biomasses are used as food attention has focused on the exploita-tion of the cellulose and hemicellulose sugars in li-gnocellulosic biomass to produce ethanol. Lignocel-lulosic biomass also contains lignin, which is struc-turally interwoven with the plant’s cellulose and hemicellulose polymers and provides the rigidi-ty of the plant. Cellulose generally accounts for 30-60% (higher in woods, lower in agricultural waste), hemicellulose content varies from 10% to 40%, and the lignin content from 10% to 25% of the biomass weight.

Municipal solid waste (MSW) could be a ma-jor source of raw material for the production of eth-anol. The paper, woody waste and organic matter in MSW contain cellulose and sugars that can be hy-drolysed and fermented simultaneously into eth-anol. The composition of MSW varies from place to place: generally the largest fraction is paper (20-40%) followed by garden wastes (10-20%), plastics, glass, metals and other materials. This source of bio-mass is now abundant, but since all of MSW is do-mestic in origin and can not be used as food it is the most promising one.

The increasing need to make use of renew-able resources in producing energy is focusing at-tention on a search for targeted crops whose prod-ucts or byproducts would be used exclusively for the production of biofuels. In terms of the design of bio-ethanol production process from biomass, the avail-ability and transport costs of the feedstock continues playing a crucial role when production facilities are being projected.

Basic steps in bioethanol production

Whatever the initial biomass, the production of bioethanol involves four main steps:

Treating the feedstock to obtain a sug-1. ar solutionUsing yeasts or bacteria to convert the 2. sugar into ethanol and CO2Distilling the ethanol out of the fermen-3. tation brothDehydrating the ethanol (if necessary).4.

Treating the feedstock to obtain a sugar solution. In order to get ethanol from starch and li-gnocellulosic biomasses, their carbohydrates must be broken down into monomeric sugars (hydrolysis) and then fermented using appropriate microorgan-isms. This can be achieved by either chemical or en-zymatic hydrolysis. Both procedures need suitable and effective feedstock pretreatments to enhance the susceptibility of feedstocks to the catalytic action. Basically, the pretreatments employed can be phys-

Bioethanol from renewable sources

437Proceedings of the III Congress of Ecologists of Macedonia

ical, thermal, chemical and biological, depending on the action mechanism applied to the substrate. Sometimes two or more of these are combined to produce synergetic effects. This step in bioethanol production from starchy and lignocellulosic materi-als is the most important, since alcohol fermentation and dehydration are common operations to all bio-mass-to-ethanol technologies.

Alcohol fermentation of hydrolyzate. What-ever the process employed to hydrolyse the biomass, the hydrolisate obtained is mainly constituted of glu-cose, xylose, arabinose, and cellobiose. The most widely employed microorganism in alcohol fer-mentation is Saccharomyces cerevisiae. In contrast to most other species it is able to perform alcohol-ic fermentation, and to grow under strictly anaerobic conditions. A major obstacle in using Saccharomy-ces yeasts is that there are unable to ferment xylose into ethanol. While technology to convert hexoses to ethanol is well established, the fermentation of pen-toses is problematical. In recent years, it has been sought for yeasts and fungi that can convert D-xylo-se into ethanol. Some thermophilic and mesophilic bacteria which are able to ferment xylose to etha-nol have been also identified. Among these, the most thoroughly investigated are: Clostridium thermohy-drosulphuric and Clostridium ethanolicus (Ogier et al. 1999). Recently, another recombinant bacterium Zymomonas mobylis CP4 (pZB5) has been proven to be effective in co-fermenting both xylose and glu-cose (Joachimsthal et al. 2000; Krishnan et al. 2000; Lawford et al. 2000).

There are five developed fermentation strate-gies in production of bioethanol: sequential fermen-tation of C5 and C6 sugars, separate hydrolysis and fermentation, simultaneous saccharification and fer-mentation, fermentation in co-cultures and simulta-neous saccharification and cofermentation. The cri-teria used for the implementation of a fermentation strategy include the highest productivity, the fast-est production rates, the limitation of toxic products

formed during the alcoholic fermentation and an ef-fective way to control the process.

Product separation (distillation and dehy-dration). The purpose of the distillation is to effect the bulk of the binary ethanol-water mixture separa-tion and to increase the concentration of ethanol in the distillate stream (tops product) to 90 wt.%. It is critical to reach this alcohol concentration via distil-lation to ensure the final 99.8 wt.% ethanol concen-tration can be reached in the exit stream of the fi-nal pervaporation module. This separation can not be achieved solely with standard distillation due to the existence of a minimum boiling azeotrope, 95.5 wt.% ethanol with a boiling point of 78.15°C in the vapour-liquid equilibrium characteristic of an etha-nol-water mixture. One option for the separation is to use a benzene entrainer to form a ternary azeo-trope allowing the ethanol-water mixture to be sep-arated. However added complications including po-tential benzene contamination of the ethanol prod-uct and the questionable sustainability of the use of benzene that is carcinogenic and highly flammable, present a risk to the environment, and plant oper-ators. A suitable alternative is to distil the ethanol-water mixture to a point close to the azeotrope and then use pervaperation to complete the dehydration. This method allows for a simpler column design and avoids the use of chemicals such as benzene.

Bioethanol production from sugar feedstocks

To produce ethanol from sugar feedstocks, it is necessary to obtain the sugar syrup from stipes or beets by either the counter-current extraction us-ing water as extractive agent or squeeze through in a ball mill. Milling extracts roughly 95% of the cane’s sugar content (Wheals 1999) leaving behind the sol-id cane fibre known as “bagasse”.

The actual procedures involved in bioetha-nol production from sugar cane depend on the type

Fig. 1. Flow chart of the sugar cane processing relative to a plant for both ethanol and sugar production.



of distillery. There are two types of distilleries: dis-tilleries that ferment all the cane juice extracted into ethanol and distilleries, which use some of the juice to make sugar. The flowchart in Figure 1 depicts the simultaneous sugar ethanol production process. The flowchart for ethanol production only is similar but without the sugar-producing lines. In the first type, the cane juice is heated to 110°C to reduce the risk of bacterial contamination, then decanted (in some cases after concentration by evaporation) and then fermented. In distilleries that produce both sugar and ethanol, the crystals formed by the concentra-tion process are centrifuged out, leaving behind the very thick syrup known as molasses, which contains up to 65% w/w sugars and is the part destined to fer-mentation. In both cases, the sugar content has to be adjusted to 14-18% for the ferments to work at max-imum efficiency. At that point the solution contains sufficient organic and mineral nutrients to ensure fermentation by S. cerevisiae, the yeast most com-monly used for the conversion of saccharose, glu-cose and fructose into ethanol.

To produce ethanol from beet roots molasses, roots have to be loaded into a flume, where they are separated from debris (Figure 2, http://www.made-how.com/Volume-5/Molasses.html). Once washed, they are sliced and loaded into cylindrical diffusers that wash the beet juice out with the aid of hot wa-ter. During this stage, the beet slices are placed in contact with the extraction medium (water or beet juice extracted later in the procedure) and held at a temperature of 70-80°C. Temperature is a crucial ex-

traction parameter, since it has to be high enough to break down the proteins in the cell walls enabling the sugar diffusion towards the extraction medium. Once the diffusion process is completed, the drained beet pulp is dried for sale as animal feed or sold to the chemicals and pharmaceuticals industries for use in the manufacture of chemicals like citric acid and its esters. The extracted juice is clarified by adding milk of lime and carbon dioxide, then it is heated and mixed with lime. The juice is filtered, producing a mud like substance called carb juice. Next, the carb juice is heated and clarified, causing the mud to set-tle and the clear juice to rise. Once again the mud is filtered out, leaving a pale yellow liquid called thin juice. The juice is pumped into an evaporator which separate the water from a syrup. The syrup is con-centrated through several stages of vacuum boiling.

Bioethanol production from starchy materials

The process used in producing ethanol from starch is, with a few minor adjustments, the standard procedure long used by the food-starch industry. To produce ethanol from starch it is necessary to break down the chains of this carbohydrate for obtaining glucose syrup (hydrolysis), which can be converted into ethanol by yeast. Acidic hydrolysis uses strong acids, such as hydrochloric acid, which hydrolyse amylose and amylopectin in starch gels into dextrins and finally mixtures of oligosaccharides and other simple sugars. The important advantages of the ac-

Fig. 2. Production of molasses from sugar beet



id hydrolysis process are the fast reaction rate, the simple pretreatment of starch feedstocks, the cheap and easily available acid catalyst and the relatively low reaction temperature with high acid concentra-tion (Tasic et al. 2006). Currently, the enzyme-cat-alyzed starch hydrolysis is preferred as it offers a number of advantages (for instance, milder reaction conditions).

The Danish Distilleries Ltd. of Aalborg T has developed a semi-continuous process for the pro-duction of ethanol from potatoes or grains consist-ing of washing and comminution in a pulper and enzyme saccharification (Figure 3, Rosen 1978). Though it demands more power, costs more and yields less ethanol, the wet milling process is indus-trially preferable because it delivers purer starch and higher value co-products (Wyman et al. 1996). Com-minution of the potatoes or grains and the blending of grains and water take place in a pulper. After en-zymatic treatment by Termamyl 60, the potatoes or grains are heated to 90-95°C by flash steam in a con-denser, and again heated in boiler tube to a temper-ature of approximately 150°C. Since the incoming starch is in a highly structured crystalline form, boil-ing (cooking) the mash performs an initial hydro-lysis of starch. This process, gelation, causes starch granules to adsorb water, swell, and break. After cooking, starch is in the form of amylase and amy-lopectin. Alpha-amylase catalyzes random hydroly-sis of 1, 4 bonds of these polymers. This is known as liquefaction. Therefore, the starch mixture is flashed to atmospheric pressure for liquefaction with com-mercial amylase preparations of bacterial origin. If pH regulation is required, it is accomplished with slaked lime. The mash is, then, cooled to 30 °C and is pumped to the yeast vessels, where batch fermen-tation to ethanol is carried out simultaneously with

saccharification. Prior to fermentation, β-amylase, which catalyses sequential hydrolysis of free ends of amylopectin, is added.

Bioethanol production from lignocellosic materials

A number of physical and chemical meth-ods can be used to separate cellulose from its protec-tive sheath of lignin and to increase the surface ar-ea of the cellulose crystallite available for catalytic hydrolysis. In lignocellulosic hydrolysates, the con-centration of sugars as well as the concentration of by-products depends on the hydrolysis conditions. Severe conditions can degrade sugars to furfural, le-vulinic acid and formic acid while only a minor part of the lignin is degraded, resulting in a range of ar-omatics. Depending on the available hexose sugars, ethanol yields varied from 74 to 89% of the theoret-ical value. Various reactor designs have been evalu-ated: percolation reactor, progressive batch-percola-tion reactor, counter-current and co-current reactors (Lee et al. 2000). Whatever reactor is employed, the glucose yield from this process does not usually ex-ceed 65-70%, which means the process is economi-cally unfeasible (Torget et al 2000). On the contrary, a critical step using concentrated acid (10-30%) to get sugars at near theoretical yields is the acid recov-ery and reconcentration. The development of cost ef-fective technologies to improve acid separation and recovery represents the way to open a new market for this process. Membrane separation is the most promising technology (Springfield et al., 1999).

According to Mendelsohn and Wettstein (1981), diluted acid hydrolysis was taken as the chemical pretreatment to broke down fibers and re-alize lignin on ethanol production from wood cheaps

Fig. 3. Semicontinuous production of alcohol from potatoes or grains



(Figure 4a). The reactors operate on wood cheaps at about 1 kPa and at 140-180 °C. Hemicellulose and cellulose from wood are hydrolyzed with dilute (0.6 wt.%) sulfuric acid. The solution of acid and sugars, the wort, is removed from the reactor and is collect-ed in a flash tank. The lignocellulose cake leaving the reactor is used as a fuel. The vaporized furfural is collected in a recovery section. The cooled wort is then neutralized with limestone (CaCO3) and the gypsum is separated. The wort is cooled to the tem-perature required for fermentation and the nutrients are added.

The same feedstock, can be hydrolysed using concentrated sulfur acid (Yu and Miller 1980) (Fig-ure 4b). Undried wood is first introduced into a pre-hydrolysis digester column in which dilute sulfuric acid is used to remove the hemicellulose. Lignin cel-lulose particles then enter a pressurized feeder and are transported by recycling strong acid hydrolysis solution to the top of the second digester. In the sec-ond countercurrent column, the cellulose is hydro-lyzed at room temperature by 70-80% sulfuric ac-id. The glucose-sulfuric acid solution leaving the top of the column is separated by electrodialysis mem-

Fig. 4a. Flowsheet for ethanol production by diluted acid hydrolysis (Mendelsohn and Wettstein 1981).

branes. The glucose retained by the membrane is neutralized and deionized before fermentation. The sulfuric acid permeated from the electrodialysis is evaporated and reconcentrated for recycle. Lignin is separated from the strong acid exiting the bottom of the second digester by filtration and washing.

Conclusions

This paper has focused on the technologi-cal aspects of the conversion of biomass to ethanol. Within the scope of every process schema that is of-fered there is an accent on process phase of the feed-stock preparation, which goes before the main phas-es of fermentation and distillation. As already out-lined, the most relevant worldwide technologies are the concentrated acid, the dilute acid and enzyme hy-drolysis. The acid technology is already mature and needs very few improvements in the process eco-nomics. By contrast, the enzymatic process needs further improvement if enzyme production costs are to be lowered. The literature survey shows that both biological (enzymatic) and chemical hydrolysis are under testing, as well different fermentation strat-

Fig. 4b. Flowsheet for ethanol production from wood using the strong acid hydrolysis method



egies on byproduct recovery. However, in order to get a more complete picture, fermentation and dis-tillation have also to be considered. Neverthless, it seems that a “standard” process for bioethanol pro-duction has been not yet assessed.

A rapid increase of the ethanol demand would lead to a price boosting of the sugar and grains, the feedstocks from which it is currently produced, mak-ing more difficult the competition with the fossil fu-els. These considerations point out that the bioetha-nol success is connected to the development of new processes able to produce it from alternative feed-stocks largely available and cheap, such as residual biomasses and the organic fraction of MSW.

References

EC (2003) Directive 2003/30/EC of the Europe-an Parliament and of the Council of 8 May 2003 on the promotion of the use of biofuels or other renewable fuels for transport. Offi-cial J Eur Union L 123:42-46.

http://www.hort.purdue.edu/newcrop/ (Center for New Crops and Plants Products, Pardue Uni-versity)

http://www.madehow.com/Volume-5/Molasses.htmlJoachimsthal, E. L. & Rogers, P. L. (2000). Charac-

terization of a high-productivity recombinant strain of Zymomonas mobilis for ethanol pro-duction from glucose/xylose mixtures. Appl. Biochem. Biotechnol. 84-86: 343-356.

Krishnan, M. S.; Blanco, M., Shattuck, C. K., Ng-hiem, N. P. & Davison, B. H. (2000). Ethanol production from glucose and xylose by im-mobilized Zymomonas mobilis CP4 (pZB5). Appl. Biochem. Biotechnol. 84-86: 525-541.

Lawford, H. G. & Rousseau, J. D. (2000). Compara-tive energetics of glucose and xylose metab-olism in recombinant Zymomonas mobilis. Appl. Biochem. Biotechnol. 84-86: 277-293.

Lee, Y. Y., Zhangwen, W. & Torget, R. W. (2000). Modeling of countercurrent shrinking bed-

reactor in dilute-acid total-hydrolysis of lig-nocellulosic biomass. Biores. Tecnol. 71: 29-39.

Mendelsohn, H. R. & Wettstein, P. (1981). Ethanol from wood. Chem. Eng. 88 (12): 62-65.

Ogier, J. C., Ballerini, D., Leygue, J. P., Rigal, L. & Pourqui, J. (1999). Production d’ethanol par-tir de biomasse lignocellulosique. Oil & Gas Science and Technology-Revue de l’IFP, 54: 67-94.

Rosen K., (1978). Continuous production of alcohol. Process. Biochem. 13 (5): 25

Springfield, R. M. (1999). Continuous Ion-Exclu-sion Chromatography System for Acid/Sugar Separation. Separat. Sci. Technol.34 (6/7): 1217.

Tasic, M., Bankovic-Ilic, I., Lazic, M., Veljkovic, V. & Mojovic, Lj. (2006). Bioethanol- State and Perspectives. Chem. Ind. 60: 1-9.

Torget, R. W., Kim, J. S. & Lee, Y. Y. (2000). Fun-damental Aspects of Dilute Acid Hydrolysis/Fractionation. Kinetics of Hardwood Carbo-hydrates. Cellulose Hydrolysis. Ind. Engin. Chem. Res. 39 (8): 2817.

US Department of Energy, 2002. Annual Energy Review 2001, Energy Information Agency, Washington, DC, p.279.

Wheals, A. E., Basso, L. C., Alves, D. & Amorim H. (1999). Fuel ethanol after 25 years. Focus., 17: 482-487.

Wingren, A., Galbe, M.& Zacchi, G. (2003). Tech-no-economic evaluation of producing etha-nol from softwood: comparison of SSF and SHF and identification of bottlenecks. Bio-technol. Prog. 19:1109-1117.

Wyman, C. E. et al., (1996) Handbook on bioetha-nol production and utilization, Taylor &Fran-cis ed,, 15: 338.

Yu, J. & Miller, S. F. (1980). Utilization of cellu-lose feedstock in the production of fuel gra-de ethanol. Ind. Eng. Chem. Prod. Res. Dev. 19:237-241.

bioethanol from renewable sources congress proceedings/03_ivana bankovic.pdfar solution 2. using...

Documents