challenges and opportunities in improving the production

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Review

Challenges and opportunities in improving the production ofbio-ethanol

Jan Baeyensa ,c, Qian Kang b ,c, *, Lise Appels b , Raf Dewil b, Yongqin Lvc, Tianwei Tan c

a University of Warwick, School of Engineering, Coventry, UKb KU Leuven, Department of Chemical Engineering, Process and Environmental Technology Lab, Sint-Katelijne-Waver, Belgiumc Beijing University of Chemical Technology, College of Life Science and Technology, Key Lab of Bioprocess, Beijing, China

a r t i c l e i n f o

Article history:

Received 25 June 2014Accepted 22 October 2014Available online 15 December 2014

Keywords:

Bio-ethanolCharacteristicsEnvironmental aspectsRaw materialsEnergy savingHybrid processAnhydrous ethanolAspen Plus V8.2

a b s t r a c t

Bio-ethanol, as a clean and renewable fuel, is gaining increasing attention, mostly through its majorenvironmental benets. It can be produced from different kinds of renewable feedstock such as e.g. sugarcane, corn, wheat, cassava (rst generation), cellulose biomass (second generation) and algal biomass(third generation). The conversion pathways for the production of bio-ethanol from disaccharides, fromstarches, and from lignocellulosic biomass are examined. The common processing routes are described,with their mass and energy balances, and assessed by comparing eld data and simulations. Improve-ments through 5 possible interventions are discussed, being (i) an integrated energy-pinch of condensersand reboilers in the bio-ethanol distillation train; (ii) the use of Very High Gravity (VHG) fermentation;(iii) the current development of hybrid processes using pervaporation membranes; (iv) the substitutionof current ethanol dewatering processes to >99.5 wt% pure ethanol by membrane technology; and (v)additional developments to improve the plant operation such as the use of microltration of thefermenter broth to protect heat exchangers and distillation columns against fouling, or novel distillationconcepts.

Whereas the benets of introducing these techniques are recognized, extensive research is still needed

to scientically and economically justify their application. The paper nally presents a tentative eco-nomic assessment, with production costs not only depending on the extent of applying process im-provements, but also on the raw material used in the process.

2014 Elsevier Ltd. All rights reserved.

Contents

1. Bio-ethanol: characteristics and worldwide potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

1.1. Ethanol and its characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

1.2. Worldwide production and research importance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.2.1. The recognized potential of bio-ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

1.2.2. The different generationsof bio-ethanol production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

2. The uses of bio-ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

2.1. Generalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.2. The uses of bio-ethanol as fuel and feedstock in chemicals' synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3. Bio-ethanol production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.1. Major raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.2. Main steps in biomass-to-ethanol processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

3.3. Bioethanol from disaccharides- and starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

3.4. Lignocellulosic biomass-to-ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

3.5. Fermentation of hexoses and pentoses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

* Corresponding author. Beijing University of Chemical Technology, College of Life Science and Technology, Key Lab of Bioprocess, Beijing, China. Tel.:86 1015210645421;fax:86 1064794689.

E-mail address:[email protected](Q. Kang).

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / p e c s

http://dx.doi.org/10.1016/j.pecs.2014.10.003

0360-1285/

2014 Elsevier Ltd. All rights reserved.

Progress in Energy and Combustion Science 47 (2015) 60e88
mailto:[email protected]://www.sciencedirect.com/science/journal/03601285http://www.elsevier.com/locate/pecshttp://dx.doi.org/10.1016/j.pecs.2014.10.003http://dx.doi.org/10.1016/j.pecs.2014.10.003http://dx.doi.org/10.1016/j.pecs.2014.10.003http://dx.doi.org/10.1016/j.pecs.2014.10.003http://dx.doi.org/10.1016/j.pecs.2014.10.003http://dx.doi.org/10.1016/j.pecs.2014.10.003http://www.elsevier.com/locate/pecshttp://www.sciencedirect.com/science/journal/03601285http://crossmark.crossref.org/dialog/?doi=10.1016/j.pecs.2014.10.003&domain=pdfmailto:[email protected]


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3.6. The problem of ethanol-inhibition in the first generation feedstock fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4. Traditional processing routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.1. Integrated saccharification/fermentation processes versus two-step processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.2. Basic data and energy requirement of the process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.2.1. Fermentation broth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.2.2. Simulation in Aspen Plus of the basic concept (no internal energy recycle) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5. Process improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.1. Energy integration within the current production processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.2. The use of VHG fermentation: principles and application results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 755.2.1. Introduction of very high gravity (VHG) fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.2.2. Effect of implementing VHG on the distillation thermal requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.3. The development of hybrid (pervaporation) systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.3.1. Introduction of hybrid operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.3.2. Effect of implementing pervaporation on the distillation thermal requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.4. Overall assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.5. The process of final ethanol dewatering to fuel grade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

5.5.2. Major processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.5.3. Production of anhydrous ethanol using hydrophilic membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.6. Additional developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.6.1. Cross-flow microfiltration of bio-ethanol fermentation broth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.6.2. Novel distillation concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.6.3. The improved bio-ethanol production plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

6. Preliminary economic assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 837. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

1. Bio-ethanol: characteristics and worldwide potential

1.1. Ethanol and its characteristics

Ethanol or ethyl alcohol (CH3CH2OH) is a colorless, volatile andammable liquid, with molecular weight of 46.07 g and density of789 kg/m3 at 294 K. Thermal properties are given inTable 1.

It burns with a smokeless blue ame, generally invisible innormal light.

The auto-ignition temperature is the lowest temperature atwhich ethanol will spontaneously ignite in a normal atmospherewithout an external source of ignition, such as a ame or spark.

Mixtures of water and ethanol are important throughout thebio-ethanol process. The knowledge of mixture ash points, aspresented inFig. 1,is needed for their safe handling, storage andtransportation: the ash point is one of the most important phys-ical properties used to determine the potential for re and explo-sion hazards of liquids, used for the classication and labeling ofdangerous substances and preparations. The ash point of a givenliquid is the experimentally determined temperature adjusted tostandard temperature and pressure at which a substance emits

sufcient vapor to form a combustible mixture with air. A lowerash point value indicates that a given liquid is more hazardousrelative to a different liquid with a higher value.

The physical properties of ethanol result from the presence ofboth the hydroxyl group and the shortness of the carbon chain. Thehydroxyl group is prone to hydrogen bonding, making ethanol

more viscous and less polar than organic compounds of similarmolecular weight. Ethanol is moreover miscible with water(unlike > C3 alcohol) and with many organic solvents, e.g. aceticacid, acetone, ether, ethylene glycol, glycerol, and toluene[1,2]. It isalso miscible with light aliphatic liquids, such as C5H12and C6H14,and with chlorinated aliphatics such as CH3CCl3and Cl2CHeCHCl2[2]. Mixing ethanol and water is slightly exothermic, releasing~0.78 kJ/mol[3]at 298 K. Mixtures of ethanol and water at atmo-spheric pressure form an azeotrope of ~89 mol% ethanol and~11 mol% water [4] at a temperature of 351 K. This azeotropicbehavior is a pronounced function of temperature and pressure andvanishes at temperatures below 303 K or pressures below about10 mbar[5].

Table 1

Primary properties of ethanol.

Boiling point 351.37 KFlash point 289.6 KAuto-ignition temperature 698 KHeat of combustion 26,800 kJ/kg

Fig. 1. Flash points of ethanole

water mixtures.

J. Baeyens et al. / Progress in Energy and Combustion Science 47 (2015) 60e88 61


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The ash point of pure ethanol is 289.6 K only, lower than theambient temperature[6]. The ash points of ethanolewater solu-tions are shown inFig. 1[7].

Ethanol for industrial synthesis reactions or as a solvent ismainly produced petrochemically through the acid-catalyzed hy-dration of ethylene[8]:

C2H4 H2O/CH3CH2OH

The catalyst is most commonly phosphoric acid [9,10], absorbedonto a porous carrier such as silica gel or diatomaceous earth.

Ethanol for use in alcoholic beverages, and the vast majority ofethanol for use as bio-fuel [9,10], is produced by fermentation.Certain species of yeast (e.g.,Saccharomyces cerevisiae) or bacteria(e.g.Zymomonas mobilis) metabolize sugars in oxygen-lean condi-tions and produce ethanol and carbon dioxide. The chemical re-actions for glucose/fructose and for sucrose as respective raw

materials are given below:C6H12O6/2CH3CH2OH2CO2; for glucoseand fructose (1.1)

C12H22O11 H2O/4CH3CH2OH 4CO2 ; for sucrose (1.2)

The general ow sheet of the fermentative bio-ethanol pro-duction, including the starting fermentation and successive processsteps, is illustrated inFig. 2.

The function of the different process steps, and the differencebetween 1st, 2nd and 3rd generation raw materials will be dis-cussed in Section 1.2. The possible introduction of membraneseparations is also indicated in the ow sheet, and will be detailedin Section5of this paper.

The liquid waste of the fermenter (stream 8) and the stillagerecovered from the distillation (stream 5) of Fig. 2 are valuablesources for anaerobic treatment. These streams contain moderateconcentrations of glucose and starch, respectively 1e20 g/L and upto 200 g/L[11]. The waste water production exceeds 8 m3/ton bio-ethanol, providing sufcient substrate for an efcient anaerobicdigestion. The biogas produced (~670 Nm3/ton bio-ethanol) has asufcient energy content to make a CHP (Combined Heat and Po-wer) application viable, generating sufcient electricity to partly

power the whole bio-ethanol plant, whilst also providing hot water(~353 K) to be used in the boiler.

The CO2 production of fermentation nearly equals the bio-ethanol yield. The CO2 produced is recovered, cleaned, and

Fig. 2. Schematics of the processes for bio-ethanol production (a) As currently applied; (b) With potential membrane applications (DDGS: Distillers Dried Grains with Solubles;

123: cellulosic materials; 23: starch or carbohydrate rich materials).

Fig. 3. Literature (2000e2013) concerning bio-ethanol SCOPUS with keywords ( Bio-ethanol General; Fuel-Application; Environment and Economics; Simulation and

Separation; Membrane Technology; Very High Gravity (VHG)).

J. Baeyens et al. / Progress in Energy and Combustion Science 47 (2015) 60e8862


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compressed/liqueed. A review paper[12]assessed the equipmentand operating costs needed to capture and liquefy CO2 for truckdelivery from an ethanol plant. Estimated costs are provided forfood/beverage grade CO2and also for less puried CO2suitable forenhanced oil recovery or sequestration. The paper includes pre-liminary plant and equipment designs and estimates major capitaland operating costs for each of the recovery options.

1.2. Worldwide production and research importance

1.2.1. The recognized potential of bio-ethanol

With the globalization of the increasing demand for energy,energy shortage is a common worldwide problem. It is predictedthat the growth in the production of easily accessible oil and gaswill not match the projected rate of demand by 2040e2050, sincethe level of oil demand will increase from 85 Mb$day1 in 2008 to105 Mb$day1 in 2030 and to higher values thereafter[13], whilstthe number of vehicles will increase to 1.3 billion by 2030 and to 2billion by 2050[14].In addition, concerns towards CO2emissions

and associated climate change[15]have instigated an acceleratedresearch and production of renewable energy resources. Bio-ethanol is considered as the alternative renewable fuel with thelargest potential to replace fossil-derived fuels, with a world pro-duction of 50 million m3 in 2007 and in excess of 100 million m3 in2012 [16], and with a potential for a signicant reduction ofgreenhouse gas emissions[17].

Brazil and the United States represent approximately 80% of theworld supply[16]. More than 20% of Brazilian cars are able to use100% ethanol as fuel, including ethanol-only engines and exefuelengines. Flexefuel engines in Brazil are able to use pure ethanol, allgasoline, or any mixtures of both. In the USA, exefuel vehicles canrun on 0%e85% ethanol (15% gasoline) with higher ethanol blendsnot yet allowed or deemed less efcient. Brazil produces ethanolfrom domestically grown sugar cane. Sugar cane not only has ahigher concentration of sucrose than corn (by about 30%), but isalso much easier to be extracted. The bagasse generated by theprocess is not wasted, but is used to produce process steam andelectricity. The US feed stock is mostly corn and wheat [16].

Table 2

Literature (>2010) about rst generation bio-ethanol in general, and cassava in particular.

Reference Objectives Main results

[19] Production of ethanol from cassava pulp via fermentation entation - Produced ethanol at 91% and 80% of theoretical yield from 5% to 10% cassava pulp(K7G strain fermented starch and glucose).

[20] Twosoil derived yeastsfor bio-ethanol production of Cassava starch -S. cerevisiaeCHY1011 and CHFY0901 have potential use in industrial bio-ethanolfermentation processes.

[21] Process optimization for bio-ethanol production from cassavastarch

- The reaction was completed within 48.5 h at 303 1 K.

[22] Non-thermal enzymatic saccharication of cassava pulp - More energy ef cient, and more fermentable sugar yields.[18,23e25] Bio-ethanol in China - Development of non-grain ethanol;

- Cassava fermentation achieves ethanol concentration of 120 g L1;- Cassava will be China's largest non-grain ethanol base over the next ve years.

[26] Ethanol fermentation of energy beets - Ethanol yields: 0.47 g/g (sugar) and volumetric productivity: 7.81 g L-1h1

(simultaneous saccharication and fermentation (SSF));- A suitable advanced bioenergy crop for producing industrial fermentation sugarsin the Mid-South of the U.S.

[27] Evaluation of whole Jerusalem artichoke for consolidatedbioprocessing ethanol production (CBP) - Suggested a cost-effective CBP strategy;- Produced 45.3 g L1 ethanol (30 h); -Productivity of 1.51 gL1h1, ethanol yield0.32 g ethanol/g fermentable sugars (60% fermentable sugar conversion efciency).

[28] Ultrasound and technical enzymes during bio-ethanol productionfrom fresh cassava root

- Increased the free sugar from 0.5% in raw material to 8%;- Upto 100%of starchcouldbe convertedin glucose,andup to11 vol%ethanol couldbe produced.

[29] Bio-ethanol production from rawjuiceas intermediate of sugar beetprocessing

- Optimal fermentation time: 30 h; sugar mass fraction in batch fermentation:12.30 wt%.

[30] Improvement of bio-ethanol production from corn by ultrasoundand microwave pretreatments

- Increased the maximum ethanol concentration produced in the SSF process by11.15% and 13.40% (ultrasound and microwave);- Maximum ethanol concentration of 9.87 wt% and ethanol yield of 90.80% wereachieved (microwave).

[31] Production of bioethanol by immobilizedSaccharomyces cerevisiaeonto modied sodium alginate gel

- The maximum concentration, productivity and yield of ethanol were 69.68 g L1,8.71 g L1 h1 and 0.697 g g1.

[32] Real-time monitoring of fermentation process applied to sugarcanebio-ethanol production

- Presented an automated, low-cost, real-time response and minimally invasivesolution for the optimization technology of bio-ethanol production.

[33] Cassava waste for bio-ethanol for application with dual-fuelabsorption refrigeration in Africa

- Proposed a simple and scalable two-step process that biologically converts thewaste streams into valuable products.

[34] Saccharication and fermentation of waste sweet potato for bio-ethanol production

- 91.5% of the starch and sucrose were converted to glucose and fructose;- Converted more than 90% of the total sugars into ethanol with 87.2% of thetheoretical yield and 49.07 g/Lnal concentration of ethanol (Z. mobilis8b).

[35] Utilization of microwave and ultrasound pretreatments in theproduction of bio-ethanol from corn

- Increased the glucose concentration obtained after liquefaction by 6.82 and 8.48%(ultrasonic and microwave);- Increased the maximum ethanol concentration by 11.15 and 13.40% (ultrasonicand microwave);- A maximum ethanol concentration of 9.91% (w/w) and percentage of theoreticalethanol yield of 92.27% were achieved.

[36] Optimal strain design ofSaccharomyces cerevisiaefor bio-ethanolproduction

- Conrmed the effectiveness and of a 20e30% improvement in ethanol productionduring glucose fermentation;- Three sequential gene knockout targets have been identied for eliminating over99% of the inefcient pathways for ethanol production through the network.

[37] Bio-ethanol production from intermediate products of sugar beetprocessing

- Attained efciency of sugar utilization was at least from 98 to 99 wt%;- Maximum productivity was achieved at ~1.8 g L1 h1 for all applied yeast strains.

[38] Duckweed starch accumulation for bio-ethanol production - Thechange of temperature difference betweenday andnight affects theduckweedstarch content.



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Table 3

Literature (>2010) about second generation bio-ethanol production.


[40] Optimal industrial symbiosis system to improve bioethanol production - Reduce bioethanol production and logistic costs;- 2nd generation biomass should be used for bioethanol production.

[41] Bio-ethanol production from dilute acid pretreated Indian bamboovariety by separate hydrolysis and fermentation

- Bio-ethanol yield of 1.76 vol% with an efciency of 41.69%;- Bamboo can be used as feed stock for the production of bio-ethanol.

[42] Fuel ethanol production from sweet sorghum bagasse using microwave

irradiation

- An ethanol yield based on total sugar of 480 g kg1was obtained;

- Ethanol produced on marginal land at 0.252 m3

ton1

biomass.[43] Ultrasonic-assisted simultaneous SSF of pretreated oil palm fronds for

bio-ethanol production- Maximal bioethanol concentration (18.2 g/L) and yield (57.0%).

[44] Convert sucrose and homocelluloses in sweet sorghum stalks intoethanol

- All sugars in sweet sorghum stalk lignocellulose were hydrolysed intofermentable sugars.

[45] An integrated optimization model for switchgrass-based bio-ethanolsupply chain

- Optimal to harvest switchgrass as loose chop; intended bioreneries with totalcapacity of 2270 ML yr1 of bio-ethanol in North Dakota.

[46] Low-intensity pulsed ultrasound to increase bio-ethanol production - Increase the production of bio-ethanol from lignocellulosic biomass to52 16%.

[47] Different process congurations for bio-ethanol production frompretreated olive pruning biomass

- Ethanol concentration of 3.7 vol% was obtained.

[48] Bio-ethanol production from water hyacinthEichhornia crassipes - YeastSaccharomyces cerevisiaeTY2 produced ethanol at 9.6 1.1 g/L.[49] Enhanced saccharication of biologically pretreated wheat straw for

ethanol production- Increase of the sugar yield from 33 to 54%, and reduction of the quantity ofenzymatic mixture by 40%.

[50] Ethanol production, purication, and analysis techniques - Utilization of lignocellulosic biomass for ethanol production is being studiedmore intentionally;- Ozonation degrades impurities, activated carbon removes impurities, and gas

stripping removes high volatile compounds without any heating.[51] Fermentation of biologically pretreated wheat straw for ethanolproduction

- The highest overall ethanol yield was obtained with the yeast Pachysolentannophilus: yielded 163 mg ethanol per gram of raw wheat straw (23 and 35%greater).

[52] Integration of pulp and paper technology with bio-ethanol production - Re-use existing assets to the maximum extent;- Keep the process as simple as possible;- Match the recalcitrance of the biomass with the severity of the pretreatment.

[53] Lignin and silica from the black liquor generated during the productionof bio-ethanol from rice straw

- Sulphuric acid is the best reagent;- Economically more suitable; the two-step treatment is more efcient inproviding a product superior in qualities and an efuent safer to dispose off.

[54] Pequi cake hydrolysis and fermentation to bio-ethanol - 61.6% conversion of its starch content to reducing sugars;- Produces 53 L of ethanol per ton of hydrolyzed pequi meal.

[55] Pretreatment technologies for an efcient bio-ethanol productionprocess based on enzymatic hydrolysis

- Chemical and thermochemical are currently the most effective technologiesfor industrial applications;- Efcient conversion and utilization of hemicellulosic sugars has become animportant task to increase sugar yields.

[56] Production of bio-ethanol by fermentation of lemon peel wastespretreated with steam explosion

- Reduces the residual content of essential oils below 0.025% and decreases thehydrolytic enzyme requirements;

- Obtained ethanol production in excess of 60 L/1000 kg fresh lemon peelbiomass.

[57] Sono-assisted enzymatic saccharication of sugarcane bagasse for bio-ethanol production

- The maximum glucose yield obtained was 91.28% of the theoretical yield andthe maximum amount of glucose obtained was 38.4 g/L (MTCC 7450);- The hydrolyzate obtained was 91.22% of the theoretical ethanol yield (MTCC89);- Decreases the reaction time;- The application of low intensity ultrasound enhanced the enzyme release andintensied the enzyme-catalyzed reaction.

[58] Status and barriers of advanced biofuel technologies - The major barriers for the commercialization of 2nd generation ethanolproduction are the high costs of pretreatment, enzymes used in hydrolysis, andconversion of C5 sugars to ethanol;- The residues need to be processed for byproducts through biorenery toimprove the economics of the whole process.

[59] Sugarcane bagasse hydrolysis using yeast cellulolytic enzymes - This enzyme extract promoted the conversion of approximately 32% of thecellulose;- C. laurentiiis a good b-glucosidase producer.

[60] Switchgrass for bioethanol - Glucose yields: 70%e90%, and xylose yields: 70%e100% after hydrolysis;- Ethanol yields range from 72% to 92% of the theoretical maximum.[61] Tween 40 pretreatment of unwashed water-insoluble solids of reed

straw and corn stover pretreated with liquid hot water to obtain highconcentrations of bioethanol

- Obtain a high ethanol concentration of 56.28 g/L (reed straw) and 52.26 g/L(corn stover);- Ethanol yield reached a maximum of 69.1% (reed straw) and 71.1% (cornstover).

[62] Enzymatic saccharication of algal biomass for bio-ethanol production(Chlorella variabilis)

- The enzyme addition resulted in the highest ethanol production byfermentation.

[63] Waste paper sludge as a potential biomass for bio-ethanol production - SSF using cellulase produced by A.cellulolyticusgave ethanol yield 0.208 (gethanol/g PS organic material);- Consolidated biomass processing (CBP) technology gave ethanol yield 0.19 (gethanol/g Solka oc).

[64] Assessment of combinations between pretreatment and conversioncongurations for bio-ethanol production

- The process based on dilute acid pretreatment and enzymatic hydrolysis andco-fermentation combination shows the best economic potential;- The cellulose hydrolysis based on an enzymatic process showed the bestenergy efciency.



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In developing economies, food-related feed stock is preferablyreplaced by non-food raw materials, such as sweet sorghum orcassava. Sweet sorghum can be used in ethanol fermentation, and issuitable for growing in dry conditions: the water requirement ofsweet sorghum is one-thirdthat of sugarcane on a comparable timescale, and it requires about 22% less water than corn does. Theworld's rst sweet sorghum-based ethanol production distillerybegan commercial ethanol production in 2007 in Andhra Pradesh,

India[14].Sorghum is now also an important feedstock for bio-ethanol

production in the USA Plains states. In some parts of Europe,particularly France and Italy, grapes have become a feedstock forfuel ethanol from wine surplus. In Japan, it has been proposed touse rice, normally converted into sake. Ethanol can be efcientlyproduced from starches, with cassava starchy crop having thehighest ethanol yield. Thailand already had a large cassava industryin the 1990s, for use as cattle feed and as a cheap admixture towheatour. Nigeria and Ghana are establishing cassava-to-ethanolplants. The world'srst large-scale cassava ethanol plant was builtin Guangxi (China) by COFCO in 2007, with an annual productioncapacity of 200,000 tons[18]. China operates several bio-ethanolproduction plants for a total annual production capacity of

~2,085,000 ton/year, using mostly corn and wheat (~65%) andcassava (~35%) as feedstock.

The main reasons for the enhanced development of bio-ethanolare related to its specic and advantageous properties, as discussedin detail in Section2 of the paper. Its use as a favorable and nearcarbon-neutral renewable fuel, thus reducing CO2 emissions andassociated climate change; its use as octane enhancer in unleadedgasoline; and its use as oxygenated fuel-mix for a cleaner com-bustion of petrol, hence reducing tailpipe pollutant emissions andimproving the ambient air quality, are the major driving forcesbehind the development. The importance and potential of bio-ethanol are highlighted in the literature, with recent exponentialresearch developments illustrated inFig. 3.

1.2.2. The different generations

of bio-ethanol productionFermentation of sugar-basedraw materialsis referred to as rst

generation bio-ethanol, whereas the use of lignocellulosic rawmaterials is commonly called second generationbio-ethanol. Thethird generation of algal bio-ethanol is at its early stages ofinvestigation. Recent references about rst and second generationbio-ethanol are summarized inTables 2 and 3, respectively.

In view of the marked research development since about 2008(in general) and since 2010 (specic novel technologies), we havelimited the literature survey to papers of the post-2010 period,whichmostly already assess previous work, whilst discussing state-of-science development.

Fermentation of sugars is referred to as the rst generationbio-ethanol process, where yeast is cultured under favorable thermal

conditions to convert sugars into ethanol at around 308e

313 K. The

toxicity of ethanol to yeast inhibits the conversion and limits theethanol concentration obtainable, as will be discussed in Section3.Although some expensive ethanol-tolerant strains of yeast areknown, they are not applied in producing fuel-grade ethanol due totheircost,butare appliedforwineand spiritproduction, e.g. Red StarPasteur Champagne and Lalvin EC-1118 wine yeasts, with inhibitionabove 18 vol% ethanol only, or Turbo Yeastfor concentrations of20 vol% or higher. To produce ethanol from starchy materialssuch as

cereal grains, the starch must rst be converted into sugars. Inbrewing beer, this is generally accomplishedby allowing thegraintogerminate, or malt, which produces the enzyme amylase. When themalted grain is mashed, the amylase converts the starches intosugars. For fuel ethanol, the hydrolysis of starch into glucose can beaccomplished more rapidly by treatment with dilute sulfuric acid[39], fungally produced amylase, or a combination of both. In bio-processes, amylase is the preferred additive[24,25].

As illustrated in Table 3, thesecond generationof bio-ethanol pro-cesses uses cellulose-released sugars[68], although the cost of theenzymes capable of hydrolyzing cellulose has been a limiting factor.

The Canadian company Iogen started the rst cellulose-basedethanol plant in 2004[69]. Development of this technology coulddeal with a number of cellulose-containing agricultural byproducts,

such as e.g. straw, wood trimmings, sawdust, bamboo and others.Lignocellulosic materials typically contain lignin, cellulose and

hemi-cellulose. Hydrolysis of hemicellulose yields mostly ve-carbon sugars such as xylose. S. cerevisiae, the yeast commonlyused for ethanol production, cannot metabolize xylose. Otheryeasts and bacteria are under investigation to ferment xylose andother pentoses into ethanol, and genetically engineered fungi thatproduce large volumes of cellulase, xylanase, and hemicellulaseenzymes, are under investigation. These could convert agriculturalresidues (e.g. corn stover, straw, sugar cane bagasse) and energycrops (e.g. switchgrass) into fermentable sugars[67].

Thethird generationof bio-ethanol processes is at early stages ofinvestigation.

Recent references are summarized inTable 4.

Algae contain lipids, proteins, carbohydrates/polysaccharides andhave thin cellulosic walls. Whereas algal lipids are mostly extractedand transformed into biodiesel, the left-over cake of starch (thestorage component) and cellulose (the thin wall component) can beconverted into bio-ethanol. Especially algae strains Glacilaria- andEuglena gracilis appear promising candidates. The resulting com-bined biodiesel/bio-ethanol process offers moreover an addedadvantage of producing fermentation CO2that can be captured andused. Despite these advantages, the alternative production ofnumerous valuable products from the algal cake (e.g. carrageenan,agar) offers signicant economic benets in comparison with low-priced bio-ethanol. Additional research is certainly required, andthe process cannot be considered for industrial application yet.

Whether rst, second or third generation feed stock is used,

fermentation produces an alcohol-lean broth only, as such unusable

Table 3 (continued )


[65] Combined use of gamma ray and dilute acid for bio-ethanol production - Increasing enzymatichydrolysisafter combined pretreatment is resulting fromor decrease in crystallinity of cellulose, loss of hemicelluloses, and removal ormodication of lignin.

[66] Bio-ethanol production from Lantana camara - 87.2% l ignin removal , and 80.0% saccharication;- 17.7 g/L of ethanol with corresponding yields of 0.48 g/g (Saccharomycescerevisiae).

[47] Different process congurations for bio-ethanol production frompretreated olive pruning biomass

- High ethanol concentration of 3.7 vol% was obtained.

[67] Ethanol production from lignocellulosic biomass (exergy analysis) - Lowest environmental impact for second-generation bio-ethanol production;- Highest exergy efciency (Steam Explosion Pre-treatment SSF Dehydration) reaching 79.58%.



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(5% maximum in Europe and in India, 10% in USA, 22e26%mandatory blends in Brazil) or as pure fuel (100%) of dry ethanol indedicated vehicles, as illustrated inTable 6.

When anhydrous bio-ethanol is blended with gasoline in a smallproportion (up to 15%), the inuence of its LHV has no signicanteffect. For higher blend levels, more fuel is required compared to

when using conventional gasoline due to the lower LHV of ethanol.Ethanol dedicated vehicles are adapted to improve the engine byrunning at higher compression ratios to take advantage of thebetter octane number of ethanol compared to gasoline: despite thelower HV, such optimized vehicles running on pure ethanol stillhave an operation range 75% of the range of gasoline fueled carson a volume basis. Finally, special Flexible Fuel Vehicles (FFV) areequipped with line sensors, which measure ethanol levels andadapt the airefuel ratio to maintain good combustion conditions.These vehicles can burn fuel containing ethanol (0e85%) in gaso-line and are becoming more and more frequent, with variousmanufactures developing such vehicles on a commercial scale withspecial modication of carburetor, fuel injector, fuel pump, fueltank and catalytic converter when using E10 to E25 blends, whistadditional modications of the engine itself, the intake manifold,exhaust system and cold-start are incorporated for E25 to E100(hydrous) blend. Apart from the advantages of using bio-ethanol inIC engines, a few disadvantages should be considered: whereas lowlevelsof ethanol blended with gasoline increase the vapor pressure,thus favoring evaporative emissions that contribute to smog for-mation, higher ethanol blend levels have a low vapor pressure thatcan lead to difculties in cold weather conditions. Straight hydrousethanol as an automotive fuel has been widely used in Brazil forneat ethanol vehicles and more recently for exible-fuel vehicles:the ethanol fuel is distilled close to the azeotrope mixture of95.63 wt% ethanol and 4.73 wt% water. At the end of 2012, therewere ~20 million exible-fuel vehicles running on Brazilian roads.Hydrous ethanol imposes a limitation on normal vehicle operation,

as the ethanol lower evaporative pressure (as compared to gaso-line) causes problemswhen cold starting the engine at temperaturebelow 288 K. For this reason, both pure ethanol and exefuel ve-hicles are built with an additional small gasoline reservoir insidethe engine compartment to help in starting the engine when coldby initially injecting gasoline. Once started, the engine is thenswitched back to ethanol. An improvedexefuel engine generationwas developed to eliminate the need for a gasoline tank bywarming the ethanol during starting and allowing them to start attemperatures as low as 268 K. The use of ethanol blends in con-ventional gasoline vehicles is restricted to low mixtures, as ethanolis corrosive and can degrade some of the materials in the engineand fuel system. The hygroscopicity of ethanol should moreover beconsidered in colder and humid climates, due to the possibleplugging of fuel lines by ice crystals, and due to the lower ash-points at higher water contents.

Due to its low cetane number, ethanol does not burn efcientlyby compression ignition. Moreover, ethanol is not easily misciblewith diesel fuel. To improve the use of ethanol in CI vehicles,measures can be taken, such as the addition of an emulsier inorder to increase the ethanolediesel miscibility; or the addition ofethylhexylnitrate or diterbutyl peroxide in order to enhance the

cetane number. Most blends of ethanol and diesel (E-diesel) arelimited to 15% ethanol and 5% emulsiers[88].

Alternative solutions are either a dual fuel operation in whichethanol and diesel are introduced separately into the cylinder, orthe modication of diesel engines in order to adapt their charac-teristics of auto-ignition and make them capable to use high blends,up to 95% ethanol[89].

2.2. The uses of bio-ethanol as fuel and feedstock in chemicals'

synthesis

The uses of bio-ethanol as a fuel or as raw material for thechemical synthesis of other products have been dealt with innumerous publications, some of the relevant and recent ones

(>2010) being summarized inTable 7.When specically discussed in the respective publications, the

environmental effects of using bio-ethanol as fuel are also includedin the summaries. From the literature survey, it appears that themain reasons of the success of bio-ethanol are the following: (i) itsuse as renewable energy to partially substitute oil and increasesecurity of supply; (ii) its use as octane enhancer in unleadedgasoline to replace metyl-tert-butylether (MTBE); (iii) its use asoxygenated compound for clean combustion of the gasoline, thusreducing the tailpipe emissions and improving the ambient airquality and (iv) its use as renewable fuel to reduce CO 2emissionsand its contribution to a reduced effect on climate change.

In addition to the few publications concerning the conversion ofbio-ethanol to H2, there is a recent revival of its conversion into

various organic chemicals with either a higher number of carbon-atoms in their molecules, or/and with an added-value. Fig. 4sum-marizes the different options.

Although these conversion pathways offer possibilities toupgrading bio-ethanol, their investigation is at an early stage ofresearch.

Specic recent literature about environmental effects has beenlisted inTable 8.

The environmental advantages of using ethanol as a fuel arerecognized since it reduces harmful tailpipe emissions of carbonmonoxide, particulate matter, and nitrogen oxides. Argonne Na-tional Laboratory analyzed the greenhouse gas emissions ofdifferent engine and fuel combinations. Comparing ethanol blendswith gasoline alone, they showed reductions of 8% with the bio-

diesel/petrodiesel blend known as B20, 17% with the conventional

Table 5

Energy content and Octane Number of some fuels compared with ethanol.

Fuel type[87] MJ/L MJ/kg RON

Dry wood (20% moisture) ~19.5Methanol 17.9 19.9 108.7Ethanol 21.2 26.8 108.6E85 (85% ethanol, 15% gasoline) 25.2 33.2 105Liqueed natural gas 25.3 ~55

Autogas (LPG) (60% propane 40% butane) 26.8 50Aviation gasoline (high-octane

gasoline, not jet fuel)33.5 46.8 100/130

(lean/rich)Gasohol (90% gasoline 10% ethanol) 33.7 47.1 93/94Regular gasoline/petrol 34.8 44.4 Min. 91Premium gasoline/petrol Max. 104Diesel 38.6 45.4 25

Table 6

Common ethanolepetrol mixtures.

Code Composition Countries Comments

E5 Max. 5% anhydrous ethanol,min. 95% petrol

Western Europe,India

Blends forregular cars


USA, Europe, China,India, South Africa


USA, cars >2000,South Africa


Brazil


USA, Europe Flexefuelvehicles

E100 Hydrous ethanol (~5.3 wt% water) Brazil



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E85 ethanol blend, and a reduction of emissions by 64% when usingcellulosic ethanol. Despite these positive effects, literature alsodraws our attention to the fact that ethanol combustion in an in-ternal combustion engine can increase the emission of products ofincomplete combustion, leading to a signicantly larger photo-

chemical reactivity that generates much more ground level ozone.

3. Bio-ethanol production

3.1. Major raw materials

Bio-ethanol can be produced from a large variety of carbohy-

drates (mono-, di-, polysaccharides). Monosaccharides (e.g. xylose,

Table 7

Literature (>2010) concerning the applications of bio-ethanol.

Ref. Objectives Main results

Bio-ethanol as fuel

[90] Investigation of using blends of diesel and ethanol - Engine power increases (up to 8.5%);- Fuel consumption increases (low caloric value of ethanol);- Unburned hydrocarbon combustion emissions decrease.

[91] Bio-ethanol injection in DISI engine for two wheels vehicles (2000

e4000 rpm)

- High injection pressure needed;

- Low tendency to soot formation.[92] Temperature dependent density of biodiesel-diesel-bioethanol blends - Modeling density;

- Recommendations on suggested blends.[93] Bioethanolediesel fueled engine - Reduced HC and CO emissions;

- Premixed combustion phasing decreases when bioethanol fraction increases.[94] Biofuels in HCCI engines - Biodiesel: reduction of NOx;

- Bioethanol: engine tolerance to exhaust gas recycle (EGR) was extended.[95] CHP using blends of gasohol in IC engine - E20 recommended;

- With increasing bioethanol, CO deceases, cylinder pressure and temperatureincrease.

[96] Automotive spark ignition engine using bioethanol - The spark ingnition (SI) engine efciency and power performance increase byusing bioethanol;- CO, HC, and NOxemissions decrease for bioethanol used in supercharged engine.

[97] Compression ignition engines using biofuels - Bioethanol: CO2and smoke decrease;- Diesel fuel (DF) and Rapeseed Methyl Ester (RME): NO xand CO2decreases;- Ethanol fraction in both fuels reaches 50%.

[98] China's bioethanol development and national application of E10 - Linear optimization model used to consider the economic cost of distributing

ethanol;- Cassava, sweet potato, sweet sorghum and sugar beet are promising feedstocks forbioethanol expansion.

[99] Bio-fuels for the gas turbine - Low emissions and substantial fuel exibility obtained in lean, premixed, pre-vaporized (LPP) combustion with ethanol;- NOxemission reduction from gas turbines.

[100] Bioethanol/REM/diesel blend in Euro5 automotive diesel engine - The closed loop combustion control (CLCC) adoption enables the performanceimprovement with the blend.

[101] Ethanol f ue l from biomass - Lignocellulose-to-ethanol is the most viable pathway (environmental view).[102] A lternative fue ls for gas turbine s - Brazilian ethanol has the lowest environmental perf ormance.[103] FT/Biodiesel/Bioethanol surrogate fuel oxidation in jet-stirred reactor - The kinetic modeling gave an overall good representation of the experimental

results.[104] A comparison of motor fuels, related pollution and technologies - Ethanol exhaust 2.14 times as much as gasoline exhaust;

- The pollution contributing to smog is 1.7 for ethanol.[105] Increasing the stability of bio-ethanol/gas oil emulsions - High stability bioethanol/gas oil emulsion produced with 5.0 v/v% biodiesel and

4.0% new chemical structure additive (20 C) in the 5.0 vol% water containingethanol.

[106] Ethanol-unleaded gasoline blends in a spark-ignition engine - Torque with E50 and E85 found to be higher than E0;

- The engine torque, power and fuel consumption increase; CO, NO xand HCemissions decrease with ethanol;- Compression ratio (CR) without knock occurrence increases with ethanolegasoline blends

Bio-ethanol as chemical feedstock or for use in fuel cells

[23] Effect of initial pH on thermophilic fermentative hydrogen productionfrom cassava ethanol wastewater

- Maximum hydrogen product;- The total amount of VFA/ethanol and the proportion of acetic acid in the VFAincrease with the increase of pH.

[107] Production of hydrogen from Bio-ethanol - High hydrogen plant ef ciency and a highly efcient heat integration obtainedwith typical ethanol feed.

[108] Vapor-fed PBI-based direct ethanol fuel cell - An improvement of the cell performance with an increase in temperature(enhanced kinetics and electrolyte conductivity);- The E/W weight ratio (0.25e0.5) was found to be suitable;- Ethanol crossover currents increases with the temperature and the E/W;- This cell can tolerate the use of the renewable bio-ethanol.

[109] Techno-economic and environmental assessment of bioethanol-basedchemical process

- Bioethanol-based processes have better cost saving and less global warmingpotential;- Bioethanol-based processes have great potential in ethyl acetate production.

[110] Butadiene production from bioethanol and acetaldehyde - The pore size of SBA-15-100 is optimum for the ETB catalyst.[111] CoAlZnand NiAlZn mixedoxides in hydrogen productionby bio-ethanol

partial oxidation- Selectivity to H2and COon NiAlZn was lower than on CoAlZn (673e773 K), but theselectivity trend reversed (>873 K);- ZnAl2O4spinel was less active in ethanol conversion and much less selective to H 2and CO;- NiAlZn is a better catalyst.



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glucose, fructose) consist of single sugars, represented by (CH2O)nwithn 3e7. Pentoses (n 5, xylose) and hexoses (n 6, glucose)are the most common monosaccharides in nature. Whereas glucoseis the most widespread sugar transport form in animal organisms,

sugar is often transported in the form of disaccharides in plants(sucrose, maltose, and lactose). A disaccharide is formed by dehy-dration coupling of two monosaccharides. Polysaccharides arecomposed of similar subunits (monomers), with e.g. starch andcellulose composed of monomers of glucose. Polysaccharides mustbe hydrolyzed into disaccharides and/or monosaccharides beforeethanol fermentation.

Large-scale carbohydrate-to-ethanol plants mostly use sugar-cane or sugar beet juice, corn or wheat. Ethanol is also commer-cially produced in the pulp and paper industry as a by-product.These carbohydrates-to-ethanol crops include both multipurpose

crops, also devoted to food markets; and dedicated ethanol crops.Whereas the latter are cultivated especially for ethanol productionon non-agricultural lands, the former provide almost all of thefeedstock now used to date for ethanol production (sugarcane in

Brazil and corn in the United States), where the development ofbiomass-to-ethanol conversion emerged as alternative markets forsugar, corn or wheat surpluses. In developing countries, thepossible competition with food is shifting from using agriculturalcrops towards non-food feedstock, e.g. cassava, sweet sorghum.

Lignocellulosic biomass can be considered as feedstock for bio-ethanol production in the medium and long termdue to its low costand high availability. Criteria and methodologies for assessing thesustainability of energy crops have been developed[122,123]. Asillustrated in the literature survey ofTable 3, the ethanol yieldswhen using lignocellulose biomass are only reported for lab-scale

Fig. 4. Conversion pathways of ethanol to different organic chemicals.

Table 8

Literature (>

2010) about the environmental effects.Reference Objectives Main results

[112] Emission abatement by bio-ethanol and diesel/bio-ethanol blends - Addition of bio-ethanol to diesel decreases particulate matter (PM) generation andincrease the diesel PM oxidation.

[95] CHP using various blends of gasoline in an IC engine - Cylinder pressure and fermentation increase at higher bio-ethanol blends;- CO emissions are reduced by up to 50%;- E20 provides maximum availability for heat recovery.

[113] Integrated bio-ethanol fermentation and CHP - All heat required in the plant can be generated from biogas produced by digestingthe stillage.

[114] Combustion performance of bio-ethanol at various blend ratios in agasoline direct injection engine

- Faster combustion, higher in-cylinder pressure;- Similar or reduced NOxemission;- Reduced CO emission;- Increased engine efciency.

[115] Bio-ethanol affects combustion and emission reduction in an SI engine - CO and volatile HC decrease;- NOxtends to increase as ambient air temperature increases.

[116] Adding ethanol in a small capacity Diesel engine - Optimum 15% of ethanol;- Unburned hydrocarbon emissions decrease;- Load smoke opacity is lowest at 14% ethanol.

[117] Use of bio-ethanol fueled buses by air pollution screening and on-roadmeasurements

- Higher emissions of acetaldehyde and acetic acid during driving conditions.

[118] Wet ethanol in HCCI engines with exhaust heat recovery - HCCI engines can use ethanol fuel with up to 20% H2O;- Low NOx, CO emissions at high intake pressures, high equivalence ratios, anddelayed combustion timings.

[119] NOx-PM trade-off in a single cylinder diesel engine by means of bio-ethanol and exhaust gas recirculation (EGR)

- Very low levels of NOxand PM (meeting 2009 Japanese Standards);- 50% ethanol blended diesel fuel and high EGR ratios are preferred.

[120] DI diesel engine fueled with bio-ethanol diesel emulsions - 5, 10 and 15% ethanol addition tested;- NO reduced by 4%;- Smoke emission cut by 20%;- 5% blend has best performance.

[121] Sulfur in bio-ethanol - Sulfur content of ethanol fuel must be


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investigations. As of 2013, the rst conmercial-scale plants havestarted operation. Multiple pathways for the conversion of differentfeedstocks are being used. In the next few years, associated costdata and their relative performance will become available. A pre-liminary assessment of the lignocellulosic bio-ethanol is given byLimayem and Ricke[124].

Carbohydrate-to-ethanol processes have been used for severaldecades, and conversion data are established. Per ton of bio-ethanol, the required amount of raw materials required rangesfrom 5 to 6.6 ton for cassava, 3e3.2 ton for corn, and 12e13.5 tonof sugarcane [125,126]. Similar feed stock requirements whenusing lignocellulosics are not yet available. The ethanol productioncost is moreover scale sensitive, with also costs of feedstockcollection and transport to be considered. Optimal sizes currentlyvary between 50 and 500 thousand ton per year, dening theamount of feedstock required for given or known conversionyields of the feedstock.

3.2. Main steps in biomass-to-ethanol processes

Once the feed stock is delivered to the ethanol plant, it needs to

be carefully stored and conditioned to prevent early fermentationand bacterial contamination. Through pre-treatment, simple sugarsare made available in proportions depending on the type ofbiomass used and the pre-treatment process. The main steps,different according to the feedstock, are summarized in Fig. 5,providing a general production owsheet for the different bio-ethanol generations.

The processes are equivalent from the fermenter onwards, andthen start with a dilute ethanol-in-water fermenter broth. Thehydrolysate, the yeasts, nutrients and other ingredients are addedfrom the beginning of the batch fermentation. Continuous pro-cesses, in which ingredients are constantly added and productsremoved from the fermentation, are also used[127]. The cell den-sities are kept high by recycling or immobilizing the yeasts to

improve their activity and enhance the fermentation yield. Thefermentation reactions occur at 298e314 K and last 1 to ~3 daysdepending on both the feedstock composition, and the type,amount and activity of the yeasts. Distillation will separate ethanoland water up to the azeotropic point. The residual ow from thedistillation column, known as vinasse or stillage, can either be usedto produce biogas by anaerobic digestion (toproduce process steamand electricity), or processed into animal food, fertiliser and othervaluable by-products.

3.3. Bioethanol from disaccharides- and starch

The most common disaccharide used for bio-ethanol productionis sucrose, i.e. the combined molecule of glucose and fructose. Itsfermentation is performed using commercial yeast such as S. cer-evisiae in a successive hydrolysis of sucrose followed by fermen-tation of simple sugars: invertase (an enzyme present in the yeast)catalyzes the hydrolysis of sucrose into glucose and fructose, andanother enzyme (zymase), also present in the yeast, subsequentlyconverts the glucose and the fructose into ethanol and CO 2.

The practical efciency of fermentation is about 90e92%. Su-

crose is mostly available in sugarcane and sugar beet, where theproduction of sugar is the main objective, and bio-ethanol is pro-duced from the molasses, the residue after sugar crystallization.

Starch consists of long chains ofa-glucose monomers, 1000 forone amylose molecule and 1000e6000 or more for amylopectin[128]. To convert starch to ethanol, the polymer ofa-glucose needsto be hydrolyzed into glucose by the gluco-amylase enzyme.

C6H10O5n nH2O!glucoamylasenC6H12O6 (3.1)The resulting sugar is known as dextroseor D-glucose, an isomer

of glucose. The enzymatic hydrolysis is then followed by fermen-tation and successive ethanol separation technique. Corn and cas-sava contain 60e70% starch. The milled raw material is hydrolyzedand the resulting sugar contained in the hydrolysate is converted toethanol while the remaining residue containing bre, oil and pro-tein is roasted and converted into a by-product known as DistillersDried Grains (DDG) or DDGS when it is combined to process syrupand sold mainly as animal feed additive (as molasses are alsocommonly used for). The CO2 produced can be sold for differentapplications (carbonated beverages or dry ice).

3.4. Lignocellulosic biomass-to-ethanol

Lignocellulose[128], the principal component of the plant cellwalls, is mainly composed of cellulose (40e60% of the total dryweight), hemicellulose (20e40%) and ligin (10e25%). Cellulosemolecules consist in long chains ofb-glucose monomers gathered

into micro-bril bundles. The hemicelluloses, mostly xyloglucansor xylans, are linked to the micro-brils by hydrogen bonds. Ligninsare phenolic compounds which are formed by polymerizationdifferent productions of three types of monomers (p-coumatyl,coniferyl and synapyl alcohols). Lignin adds compressive strengthand stiffness to the cell wall [129]. Lignocellulose is abundant innature does not compete with food, is available as agricultural andforestry residues, industrial wastes and dedicated woody crops(willow, poplar). Once the lignocellulosic biomass is pre-treatedand hydrolysed, the released sugars are fermented and the down-stream process is similar to that of rst generation feed stock.Pretreatment involves delignication of the feedstock[130]in or-der to make cellulose more accessible in the hydrolysis step, usingphysical, physico-chemical, chemical and biological treatment

(Table 9).Fig. 5.

Generations

of bio-ethanol production.



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Carbonic acid and alkaline extraction have the best perfor-mance. However, the most common methods are steam explosionand dilute acid pre-hydrolysis, which are followed by enzymatichydrolysis. Sulphuric acid or carbon dioxide are often added inorder to reduce the production of inhibitors and improve the sol-

ubilization of hemicellulose[131]. Steam explosion has a few lim-itations since the ligninecarbohydrate matrix is not completelybroken down; degradation products are generated that reduce theefciency of hydrolysis and fermentation steps; and a portion of thexylan fraction is destroyed.

The use of dilute sulphuric acid (0.5e1%; 433e463 K for 10 min)has the preference of the US National Renewable Energy Laboratory[134,135]: hemicellulose is largely hydrolysed releasing differentsimple sugars (xylose, arabinose, mannose and galactose) but alsoother compounds of the cellulosic matrix, that can however inhibitthe enzymatic hydrolysis and fermentation. Part of the acetic acid,much of the sulphuric acid and other inhibitors produced duringthe degradation of the materials need to be removed, andneutralization is performed before fermentation.

Enzymatic hydrolysis of cellulose is achieved using cellulases,usually a mixture of groups of enzymes such as endoglucanases,exoglucanases and b-glucosidases acting in synergy for attackingthe crystalline structure of the cellulose, removing cellobiose fromthe free chain-ends and hydrolysing cellobiose to produce glucose.Cellulases areproduced by fungi, mainly Trichoderma reesei, besidesAspergillus, Schisophyllum and Penicillium. High concentrations ofcellobiose and glucose inhibit the activity of cellulase enzymes andreduce the efciency of the saccharication. One of the methodsused to decrease this inhibition is to ferment the reduced sugarsalong their release. This is achieved by simultaneous saccharica-tion and fermentation (SSF), in which fermentation using yeasts (S.cerevisiae) and enzymatic hydrolysis are achieved simultaneouslyin the same reactor. The fermentation of the xylose released from

the pre-hydrolysis process can be carried out in a separate vessel orin the SSF reactor using a genetically modied strain from thebacteriumZ. mobilisthat can convert both glucose and xylose. Thelatter method is named simultaneous saccharication and co-fermentation (SSCF).

Compared to the sequential saccharication and fermentationprocess, SSCF exhibits several advantages like lower requirement ofenzyme, shorter process time and cost reduction due to economy infermentation reactors (only one reactor compared to the multiplesets). Disadvantages include the difference between optimal tem-peratures for saccharication (323e333 K) and fermentation(303 K), inhibition of enzymes and yeast by ethanol and theinsufcient robustness of the yeast in co-fermenting C5 and C6sugars. The efuent from the distillation column that contains mostof the lignin and other non-fermentable products is sent to a

combined heat and power (CHP) plant to produce process steamand electricity required by the ethanol plant. Depending on theproportion of lignin in the feedstock, excess electricity may beavailable for export sale.

Contrary to the conversion of disaccharides and starch toethanol which are mature technologies, the modern lignocellulose-to-ethanol process is still at a pilot and demonstration stage,although demonstration plants of NREL (USA) [135], Iogen Corpo-ration (Canada) [69], and ETEK (Sweden) [136], have built pilotplants capable of producing a few hundred thousand liters ofethanol per year.

3.5. Fermentation of hexoses and pentoses

From the prevous review of rst and second generation bio-ethanol processes, it is clear that a variety of microorganismssuch as yeasts, bacteria or fungi are required to biochemicallyconvert hexoses (C6, rst generation, second generation) andpentoses (second generation). The conversion reactions aredifferent:

C6H12O6/2C2H5OH 2CO2 (3.2)

3C6H10O5/5C2H5OH 5CO2 (3.3)

S. cerevisiae, a facultative anaerobic yeast, andZ. mobilis, a gram-negative bacterium are commonly used to convert C6 sugars. Bothare adapted to ethanol fermentation, with a high ethanol tolerance,and amenability to genetic modications. Both are however unableto ferment C5 sugars. Other yeasts and bacteria are under investi-gation to ferment xylose and other pentoses into ethanol, withCandida shehataeandPichia stiplisyeasts offering a good potential,despite a low tolerance to ethanol, a low ethanol yield and inac-tivity at low pH. Pachysolen tannophilus and Escherichia coli, bothgram-negative bacteria, are able to use both pentose and hexose

sugars. Kluveromyces marxianus, a thermophilic yeast, is able togrow at a temperature above 325 K; and is capable to ferment abroad spectrum of sugars. Excess of sugars affect its alcohol yield. Ithas however a low ethanol tolerance and poor xylose fermentationyield. Thermophilic bateria, e.g. thermoanaerobacterium sac-chaarolyticum, thermoanaerobacter ethanolicus, clostridium ther-mocellum, are extreme anaerobic bacteria, resistant to anextremely high temperature of 343 K. These bacteria can ferment avariety of sugars, display cellulolytic activity, but exhibit a lowtolerance to ethanol.

Genetically engineered microorganisms[69,137e139]that pro-duce large volumes of cellulase, xylanase, and hemicellulase en-zymes, are under investigation. These could convert agriculturalresidues (e.g. corn stover, straw, sugar cane bagasse) and energy

crops (e.g. switchgrass) into fermentable sugars [69]. The devel-opment of genetically modied fermentative and cellulolytic mi-croorganisms is recommended to increase the ethanol yield andproductivity under the stress conditions of high production bio-ethanol processes[140].

Genetic engineering has succeeded in altering the conventionalS. cerevisiae's capacity to ferment glucose and pentose sugarssimultaneously[141]. Almaida et al.[142]investigated a modiedS. cerevisiae, capable of co-fermenting saccharides but also gener-ating less furfural inhibitors. Z. mobilis remains an attractivecandidate due to its high ethanol yield and resistance to tempera-ture in the range of 313 K [137]. Numerous genes have beenintroduced and heterologous expression has been incorporatedinto Z. mobilis to extend its effectiveness toward other substrates

namely, xylose and arabinose [143]. Both the gene engineered

Table 9

Assessment of selected pre-treatment processes.

Pre-treatment process[132,133] Yield offermentablesugars

Wastes Investment

Physical or physico-chemical

Mechanical Low Very low LowSteam explosion High Low High

Ammoniaber explosion (AFEX) Moderate Very low HighCarbonic acid Very high Very low Low

Chemical

Dilute acid Very high High ModerateConcentrated acid Very high High HighAlkaline extraction Very high High LowWet oxidation High Low LowOrganosolv Very high Low Very high



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Z. mobilisand S. cerevisiae have proven a high ethanol yield andadaptability[144].

Enhancing ethanol production by pre-treatment involving fungi(e.g.T. reeseiandBasidiomycetes) with appropriate lignocellulolyticproperties at low pH and high temperatures is also a promising andadded-value step in ethanol bioconversion. While fungi act slowly,potential lignocellulolytic fungi have been produced by mutagen-esis, gene expression and co-culturing[139]. Some genera, such asCandida, Pichia and Dekkera were isolated from sugarcane molasses,but resulted in low ethanol concentrations and produced aceticacid, an inhibitor of the fermentative yeast. Some natural wild yeastspecies appear capable of replacing S. cerevisiae in second-generation bioethanol, but their low bio-ethanol yield and poorsurvival in the fermenter need further improvement.

3.6. The problem of ethanol-inhibition in therst generation

feedstock fermentation

The yield of the fermentation xes the total energy neededduring the downstream separation processes, mostly by distilla-tion. This yield is not only limited by the conversion rate, but also byprocess inhibition, Inhibition occurs when the concentration of achemical, either produced during the reaction or present in thereaction mix, reaches a toxic value, whereby the reaction rate ishampered or possibly even stopped by deactivation of enzymes orby the death of the microorganism. This phenomena is responsiblefor the low achievable ethanol concentrations in industrial fer-menters. The maximum concentration of ethanol attained in mostfermenters is around 11e12 wt%. Due to the fact that ethanol-inhibition is a very complex and difcult phenomena, a lot ofunknows about the problem and its solution exist. Ethanol inhibitsthe system in 3 ways, i.e. inhibition of the cell growth, inhibition ofthe fermentation and cell death. The rst two effects are illustratedbelow (Fig. 6), for fermentation of lactose by Candida pseudo-tropicalis(150 g/L)[145,146].

This inhibition is a consequence of the effect of ethanol on the

plasma membrane and the enzymes in the rst glycolysis step offermentation [146]. These enzymes, particularly hexokinase, arevery sensitive to high ethanol concentrations: their activity de-creases and the fermentation rate drops. Furthermore the highethanol concentration hampers the plasma membrane uidity,necessary for the transport of nutrients in and out of the microor-ganism, leading to reduced activity. A review of inhibition

investigations in the bio-ethanol fermentation is presented inTable 10.

To increase the critical inhibition concentration, adapted yeastscan be used. The most commonly used yeast isS. cerevisiae, with amoderate yield of fermentation. Research has been done on morepromising yeasts and bacteria:Z. mobilissucceeds to survive higherethanol concentrations in the fermenter. Not only this advantage,but also a high tolerance for acids and sugars, makes this a verypopular yeast for industrialapplication. The fermentation rateis alsohigher with Z. mobilis in comparison to Saccharomyces cerevisiae[146]The research on this baterium has lead to the development ofvery high gravity (VHG) fermentation, as described in Section 5.2.Z.mobilis is a bacteriumbelonging tothe genusZymomonas. Originallyisolated from alcoholic beverages like African palmwine or Mexicanpulque, it is also a contaminant of cider and beer in Europeancountries. The advantages ofZ. mobilis over S. cerevisiaewith respectto producing bio-ethanol: (1) higher sugar uptake and ethanol yield[152]; (2) lower biomass production; (3) higher ethanol tolerance,up to 16 vol%[165]; (4) amenability to genetic manipulations. Aninteresting characteristic of Z. mobilis is indeed that its plasmamembrane contains hopanoids, pentacyclic compounds similar toeukaryotic sterols, thus providing an extraordinary tolerance to

ethanol in its environment, around 16 wt%.However, in spite of these attractive advantages, its substrate

range is limited to glucose, fructose and sucrose. It cannot fermentC5 sugars like xylose and arabinose which are important compo-nents of lignocellulosic hydrolysates. Unlike yeast,Z. mobiliscannottolerate toxic inhibitors present in lignocellulosic hydrolysates suchas acetic acid and various phenolic compounds [159]. Concentra-tion of acetic acid in lignocellulosic hydrolysates can be as high as1.5 wt%, well above the tolerance threshold ofZ. mobilis.

Several attempts have been made to engineer Z. mobilis toovercome its inherent deciencies by metabolic engineering,mutagenesis or adaptive mutation to produce acetic acid resistantstrains ofZ. mobilis[143,160,161]. However, when these engineeredstrains metabolize mixed sugars in the presence of inhibitors, the

yield and productivity are much lower, thus preventing their in-dustrial application.

As mentioned before, albeit outside the scope of our rst gen-eration feedstock approach, other types of inhibition occur duringthe processing of lignocellulosics. They include sugar degradationproducts, lignin degradation products, compounds derived fromextractives and heavy metal ions. Sugar degradation products are

Fig. 6. Inhibition of the fermentation (a) and the cell growth (b).



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formed during hydrolysis and include mostly furfural from pen-toses and 5-hydroxymethylfurfural (HMF) from hexoses.

The use of a detoxication methodology for a specic feedstockis mandatory for attaining good results in 2nd generation bio-ethanol production.

4. Traditional processing routes

4.1. Integrated saccharication/fermentation processes versus two-

step processes

Fig. 5of Section 3.2illustrated the successive steps of the overallprocess, including the different enzymatic activities duringsaccharication and fermentation. Whereas some processes, e.g.Lurgi [162] separate both steps, other processes, e.g. Cofco [24]combine the dual enzymatic activity within the fermenter solely.Despite this difference, fermentation yields obtained are nearlyequal, and hence not affecting the subsequent bio-ethanol puri-cation train and/or waste treatment. The fermentation yield de-pends upon the conversion of starchor monosaccharides content in

the

rst place (mostly 60e

70%, as function of the

rst generation

raw material), coupled with the yield of fermentation itself,generally >90%. The mass balance of the processes can hence beestablished, and is illustrated inFig. 7for the cassava-based bio-ethanol production of the Cofco concept[24].

The combined Cofco process uses 6 fermenters in parallel, eachfermenter having a volume of 3025 m3, i.e. ~3025 ton. 865 ton of

cassava our is used per batch of 65 h. The production is basedupon a 70% starch-content of cassava and 90% starch-to-glucoseconversion. The amount of starch converted is hence 549 ton.Fermenter broths of all 6 fermenters are collected and dealt with ina single continuous distillation train.

The recycle to the fermenter is applied to keep the enzyme andyeast balance at the required level.

For the total Cofco process, using 6 parallel fermerters, the totalproduction capacity is ~200,000 tpa of anhydrous ethanol. Sincethis capacity is the average of current industrial capacities(50,000e400,000 tpa), we have selected 200,000 tpa as targetcapacity.

Several distillation owsheets have been presented in literature,with minor process-specic defferences (2 or 3 distillation col-

umns), but with an overall equivalent mode of operation to enrich

Table 10

Literature review of inhibition studies (>2008).


[147] Tolerance ofS. cerevisiaeand Z. mobilisto inhibitors of soybean meal

- Obtained both at 408 K, 2.0% H2SO4and 45 min, and at 408 K, 1.25% H2SO4and 45 minshowed inhibition in the growth of the tested microorganisms;- Spiked yeast medium (YM) broths could withstand the highest levels of inhibitors.

[148] Enhancement of ethanol fermentation inSaccharomyces cerevisiaesake yeast

- The maximum rate of CO2production and nal ethanol concentration generated by theatg32Dlaboratory yeast mutant were 7.50% and 2.12% higher than those of the parent

strain, respectively;- The mutant produced ethanol at a concentration that was 2.76% higherthan the parent strain;- The ethanol yield of the atg32Dmutant was increased, although its biomassyield was decreased.

[149] Adaptive evolution of an industrial strain ofSaccharomyces cerevisiaefor combined toleranceto inhibitors and temperature

-Saccharomyces cerevisiaeisolate (ISO12) is capable of growing and fermenting the liquidfraction of non-detoxied spruce hydrolysate at 312 K with an ethanol yield of0.38 g ethanol/g hexoses;- ISO12 shows a higher capacity to ferment hydrolysate at 312 K and higher viabilityduring heat-shock at 325 K than the industrial strain Ethanol Red (ER).

[150] Investigation of hemicellulase inhibition in theproduction of bioethanol

- A range of potential xylanase inhibitors observed in dilute-acid pretreatment slurriesand fermentation broths are dosed into washed insoluble solids (IS) and beechwoodxylan at varied concentrations prior to enzymatic hydrolysis.

[151] A ThermotolerantSaccharomyces cerevisiaeStrain(TT6) for bioethanol production

- The ethanol yield from corn by simultaneous saccharication and fermentation (SSF)with TT6 at 309 K was 91.7% of the theoretical yield.

[152] Microbial contamination of fuel ethanol fermentations - The contamination of bioethanol fermentations with lactic acid bacteria (LAB) and wildyeasts is a signicant industrial problem causing production loss of anywhere from 2 to 22%;

-Dekkera bruxellensis

has been cited as one of the main contaminant yeasts inethanol production.[153] Crucial yeast inhibitors in bio-e

thanol and improvement of fermentation at highpH and high total solids

- Fermentation yield of hydrolysates can be improved signicantly byincreasing pH of hydrolysates;- For hydrolysates at 25% and 30% total solids, pH of 8.0e9.0 yields optimal fermentationwith almost no bacterial contamination.

[154] Increasing the bioethanol yield in the presence of furfuralvia mutation of a native strain ofSaccharomyces cerevisiae

- The bioethanol yield in the presence of furfural increased via mutation of a nativestrain ofSaccharomyces cerevisiae;- A potent mutant was selected which produced 36.7% more bioethanol than the parentstrain at 0.2 vol% furfural.

[155] Selection of stress-tolerant yeasts for simultaneoussaccharication and fermentation (SSF) of very highgravity (VHG) potato mash to ethanol

- NFRI3225 produced ethanol from potato mash at the fastest rate and in thehighest volume (13.7 vol%), and the maximum productivity and ethanol yieldswere 9.1 g L1h1 and 92.3% respectively during the VHG-SSF process.- NFRI3225 would save cooling energy during the SSF process and heating energyduring distillation.

[156] The stress response ofSaccharomyces cerevisiaeimposed bystrong inorganic acid with implication toindustrial fermentations

- Low pH activates the general stress response (GSR), and mainly the heat shock response;- A ne regulatory protein kinase A (PKA) dependent mechanism might affect the cellcycle in order to acquire tolerance to an acid environment.

[157] Ethanol production byZymomonas mobilisCHZ2501from industrial starch feedstocks

- The volumetric productivities and ethanol yields were attained to 3.26 g L1h1 and93.5% for brown rice, 2.62 g L1h1 and 90.4% for barley, and 3.28 g L1h1 and 93.7% forcassava, respectively.

[158] Kinetic studies on alcoholic fermentation undersubstrate inhibition conditions

- The diffusion inside the biocatalyst particles avoids the inhibitory effects;- The obtained values ofVmaxand K0mwere found to be higher than those underethanol inhibition, but lower than those without inhibitory phenomena.



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the ethanol-lean broth to the ethanolewater azeotrope. Since elddata for the Cofco application are available and published, Cofco ismore readily assessed than other applications where eld data arenot available.

The ow sheet of the current distillation process of the Cofcocassava-based fuel ethanol production (200,000 tpa) is shown inFig. 8.

It consists of a sequence of distillation columns, heat ex-changers, pumps and a ash tank. The feed stream enters the crudecolumn C1501. The distillate from C1501 is fed to column C1502.

The side stream 4 is fed to the stripping section C1503-1 of thesecond column. C1503-2 is the rectifying section of the secondcolumn. C1503-1 has no condenser and C1503-2 has no reboiler.The bottom stream 10 enters C1502. Stream 15 is the ethanol-richphase. Simultaneously, the distillate from C1502 is separated viathe ash tank V1504, whose gas and liquid streams are both fed toC1503. The bottom streams (5, 9, 16) are directed to the wastewatertreatment unit and should therefore be ethanol-lean (0.05 wt%ethanol). Finally the distillate from C1503-2 is further dewateredusing molecular sieve adsorption to an anhydrousnished product

Fig. 7. Mass balance of the rst generation Cofco process of cassava-based bio-ethanol.

Fig. 8. Schematics of the Cofco distillation train.

Fig. 9. The distillation process of Lurgi (C1501-Crude column; C1502-the polishing column).



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of 99.7 wt% ethanol [163]. A reboiler saturated steam circuit at443K (8 bar) is available.Fig. 8 does not include condenser/reboilerheat recovery, as well be dealt with in Section5.1.

The distillation train of the Lurgi concept[162]is represented inFig. 9, and uses 2 distillation columns, 2 heat exchangers andassociated pumping. Distillation columns C1501 and C1502 usesteam of 373 K and 413 K, respectively.

The analogy between the illustrated Cofco and Lurgi designs isoutspoken,since therstCofcocolumn (C1501)does notconsiderablyenrich the top and side streams, but mostly eliminates the ne sus-pended solids of the broth, further evacuated in the bottom stream.

4.2. Basic data and energy requirement of the process

4.2.1. Fermentation broth

As example of the complete analysis, the large-scale cassava-based operation of Cofco[24,25] is examined as distillation feed.The composition of the cassava fermentation broth is shown inTable 11.

The feed ow rate is about 220,000 kg/h at atmospheric pres-sure, and 305 K. Since high boiling point components were notindividually analyzed, they are grouped as C

20H

16for use in the

subsequent simulations.Referring to Fig. 8 before and the different key-parts of the

distillation process, the number of the theoretical trays and oper-ating pressure of each column are taken from the eld data, andshown inTable 12.

The operating pressure increases along the distillation train, fromabout 0.5to about6 bar. Number of trays andreux ratio arecommonand important operation characteristics of the respective columns.

4.2.2. Simulation in Aspen Plus of the basic concept (no internal

energy recycle)

Aspen Plus V8.2 is a comprehensive chemical process modelingsystem, used by the world's leading chemical and specialty chem-

ical organizations, and related industries to design and improvetheir process plants. To simulate the process operation, the soft-ware Aspen Plus V8.2 was used[164]. Due to the presence of thehighly polar components, ethanol and glycerol, both the non-random two liquid (NRTL) and universal quasi-chemical (UNI-QUAC) thermodynamic/activity models were used to predict theactivity coefcients of the components in a liquid phase [165].Some unavailable interaction parameter coefcients were esti-mated using the UNIQUAC liquideliquid equilibrium module. Re-sults from the simulation towards operating temperatures in thedifferent columns were compared with the real values ofTable 12.The comparison ofTable 13 shows an excellent agreement, withsimulated column temperatures and ethanol concentration ofsimulation and plant measurements in fair agreement.

It conrms that the watereethanol distillation can adequatelybe designed on the basis of Aspen Plus simulations.

The total reboiler duty amounts to:

QREBkW 9000 18; 222 11; 604 38; 826 kW (4.1)

For a stream feed at 443 K and the different reboiler tempera-tures, a conservative average sensible and latent enthalpy of steamis taken at 2300 kJ/kg steam. For a total production of 25375 kg/h ofpure ethanol, the steam requirements are:

38; 826 360025; 375 2200

2:50 kg steam=kg bio ethanol (4.2)

A stream duty of about 0.5 kg steam/kg bio-ethanol must beadded for the molecular sieve separation (see Section5.5).

This high steam requirement of the fundamental solution,without in-process heat recovery should be reduced to as low valueas possible in order to improve the operation economy of the bio-ethanol production. This will be dealt with in Sections 5 and 6hereafter.

5. Process improvements

5.1. Energy integration within the current production processes

As can be seen from Table 12, the pressure is progressivelyincreased along the distillation train. It is hence possible to recovercondenser heat to feed the reboilers of previous columns. The heat

duty difference between the top of C1502 and the bottom of itsneighbor C1503 is sufcient to modify the system into a combinedreboiler/condenser distillation: the top high pressure condensate ofcondenser C1502 is used as the heat source for the reboiler ofC1503, and top high pressure condensate of C1503 is used as theheat source for the reboiler of C1501. In this energy recyclingscheme and accounting for a required thermal driving force in thereboilers of about 288e293 K, the remaining steam consumptionwill only be due to the bottom reboiler of C1502.

Referring toTable 13, the total reboiler heat duty of the heat-integrated distillation train will be

QREBkW 18; 222 kW (5.1)

At the average steam enthalpy of 2300 kJ/kg, the energy con-sumption of the integrated reboiler-condenser solution is

18; 222 360025; 375 2200

1:18 kg steam=kg bio ethanol (5.2)

In the current Cofco operation, this reboiler-condenser inte-gration is largely applied, with steam duties cited as ~1.3 kg steam/kg bio-ethanol, in fair agreement with the Aspen-simulated results.

5.2. The use of VHG fermentation: principles and application results

5.2.1. Introduction of very high gravity (VHG) fermentation

In traditional starch fermentation, Saccharomyces

challenges and opportunities in improving the production

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