Design and analysis of fuel ethanol production from raw glycerol

Energy 35 (2010) 5286e5293

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Design and analysis of fuel ethanol production from raw glycerol J.A. Posada, C.A. Cardona* Departamento de Ingeniería Química, Universidad Nacional de Colombia sede, Manizales, Cra. 27 No. 64-60, Manizales, Colombia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2009 Received in revised form 27 May 2010 Accepted 24 July 2010

Three configurations for fuel ethanol production from raw glycerol using Escherichia coli were simulated and economically assessed using Aspen Plus and Aspen Icarus, respectively. These assessments considered raw glycerol (60 wt%) purification to both crude glycerol (88 wt%) and pure glycerol (98 wt%). The highest purification cost (PC) was obtained using pure glycerol due to its higher energy consumption in the distillation stage. In addition, the remaining methanol in the raw glycerol stream was recovered and recycled, decreasing the purification costs. The E. coli strain is able to convert crude glycerol (at 10 g/L or 20 g/L), or pure glycerol (at 10 g/L) to ethanol. Among these three glycerol concentrations, the lowest bioconversion cost was obtained when crude glycerol was diluted at 20 g/L. Purification and global production costs were compared with the commercial prices of glycerol and fuel ethanol from corn and sugarcane. Purification costs of raw glycerol were lower than previously reported values due to the methanol recovery. Global production costs for fuel ethanol from glycerol were lower than the reported values for corn-based production and higher than those for cane-based production. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Economic assessment Fuel ethanol Glycerol bioconversion Process simulation

1. Introduction Fuels and polymers have been widely produced from fossil resources. However, the inevitable reduction of oil reserves as well as the increased demand has augmented its costs. In addition, sustainability and policy concerns around fossil fuels have demonstrated during last decades a progressive change from petrochemical refineries to specialized biorefineries using energy crops and residues as feedstocks. In this way, some technologies for producing fuels and materials from renewable carbon sources have been developed. The most important technological platforms using biomass as feedstock are based on sugar and vegetable oils, with bioethanol and biodiesel as examples of their commercial products, respectively. The growing biodiesel production has created a surplus of glycerol [1,2], which has reduced the glycerol price in the market and generated environmental concerns associated with contaminated glycerol disposal. In general, biodiesel production generates 10% wt of crude glycerol. Therefore, new uses for glycerol must be proposed. Although glycerol could be burnt directly as fuel, it is a potentially important feedstock which can be processed to added valuable components. The glycerol molecule (1,2,3-propanetriol) is a highly reactive tri-alcohol which has two primary and a secondary hydroxyl

* Corresponding author. Tel.: þ57 6 8879300x50417; fax: þ57 6 8879300x50199. E-mail address: [email protected] (C.A. Cardona). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.07.036

groups. Glycerol is a water soluble, colorless, odorless, viscous, and hygroscopic liquid with a specific gravity of 1.261 g mL
1, melting temperature of 18.2  C, and a boiling temperature of 290  C (accompanied by decomposition). Chemically, glycerol is available for reacting with a stable alcohol under most operation conditions, and it is basically non-toxic to human health and to the environment. The key feature of its usefulness is the particular combination among its physicochemical properties, compatibility with other substances, and easy handling. Due to these particular properties glycerol has found more than 1500 end-uses or large volume applications. Glycerol is abundant in nature as the structural component of many lipids. It is produced by yeasts during osmoregulation to decrease extracellular water activity due to its compatible solubility [3]. Wide glycerol occurrence in nature allows different kinds of microorganisms to metabolize it as a sole carbon and energy source. Glycerol can substitute traditional carbohydrates, such as sucrose, glucose, and starch, in some industrial fermentation processes [4e6]. One of many promising applications to take advantage of the glycerol surplus is its bioconversion to high value compounds through microbial fermentation. Bioconversion is a cheap way to obtain reduced chemicals (e.g., succinate, ethanol, xylitol, propionate, hydrogen, etc.), at higher yields than those obtained from sugars [7]. Moreover, glycerol offers an important opportunity to produce hydrogen [8] or energy [9]. In this way, the ethanol production processes from crude and pure glycerol are designed and economically assessed. These processes consider the glycerol fermentation to ethanol using E. coli. Furthermore, total

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Microorganisms as Klebsiella, Citrobacter, Enterobacter, Clostridium, Lactobacillus, Bacillus, Propionibacterium, and Anaerobiospirillum have been reported for glycerol degradation in a fermentative way. However, these microorganisms present diverse problems for their industrial use such as pathogenicity level, strict anaerobic conditions, and complex cultivation media. In this way, it is necessary to search anaerobic systems available to metabolize glycerol without pathogenic effects. One alternative is the E. coli stain which is able to use glycerol as carbon source without any external electrons receiver. This process is regulated by GldA dehydrogenase and DHAK dihydroxyacetone kinase to obtain ethanol, succinate, acetate, and formate [10]. This above mentioned process is resumed in Fig. 2. Recently, deletions in E. coli were carried out to increase formate and ethanol yields from glycerol at 10 g/L [11]. Thus, using glycerol dehydrogenase (gldA) and dihydroxyacetone kinase (dhaKLM) overexpression, an ethanol yield of 95% from glycerol was achieved. Other authors developed a metabolic characterization to evaluate succinate, acetate, formate, lactate, and ethanol yields [12]. The highest ethanol yield was achieved with 12.51 g ethanol/g acetate.

Nomenclature PC BCC GPC

5287

purification cost bioconversion cost global production cost

ethanol production costs from glycerol are compared with reported values for ethanol production from corn, cane, and glycerol. 2. Metabolic pathways of glycerol fermentation Glycerol is a molecule of high reducing power that can be used as energy source in microorganisms. Some microorganisms like E. coli have showed the ability to metabolize it in the presence of external electron acceptors. The glycerol degradation process begins with the GlpF gene incorporation into the cytoplasm. A later phosphorylation process is carried out which is catalyzed by the GlpK kinase. The phosphorylated carbohydrate (i.e., glycerol 3-phosphate) starts an oxide-reduction process which is accelerated by different enzymes. The anaerobic process is catalyzed by the GlpC, GlpB, and GlpA dehydrogenases, meanwhile the aerobic process is catalyzed by GlpD. This dehydrogenation process produces dihydroxyacetone 3-phosphate and then the glycolysis pathway takes place to obtain pyruvate as shown in Fig. 1.

3. Ethanol production from glycerol Ethanol can be produced from sugarcane [13], corn starch [13], sugar [14], molasses [15], cassava [16], wheat [17] or lignocellulosic biomass [18e21]. On the other hand, a mixture of ethanol and formate

glycerol transport

Transport of sn-glycerol-3-phosphate

OH HO

OH

glycerol

a glycerophosphodiester glycerophosphoryl diester phosphodiesterase, cytoplasmic: ugpQ

H 20 glycerol kinase: glpK 2.7.1.30 an alcohol

glycerophosphoryl diester phosphodiesterase, periplasmic: glpQ HO

OH P

HO

O

sn-glycerol-3-phosphate glycerol-3-phophatedehydrogenase, anaerobic: UQ8 glpC glpB glpA glycerol-3-phophatedehydrogenase, aerobic: glpD ubiquinol-8 1.1.99.5 OH OH

O O

P

OH

O

dihydroxy-acetone phosphate

glycolysis I Fig. 1. Schematic representation of glycerol degradation process on the part of E. coli, on non fermentative process.

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ATP

OH OH

OH

Glycerol (42/3) GldA (gldA)

O OH

OH

NADH Biomass (43/10)

NADH

Dihydroxyacetone

DHAK (dhaKLM)

Phosphoenolpyruvate Pyruvate O

Dihydroxyacetone OH Phosphate

O P

OH OH

O

NADH

ATP OH O OH

P

Phosphoenolpyruvate O

O

CO2 + NADH

OH

PYK (pykF) ATP

O

O

O OH

O

OH

OH OH

O

HO

Pyruvate

Fumarate

Succinate (31/ 2)

O

PFL (pf lB)

H

O

FRD (f rd ABCD) NADH/H2

OH

Formate (2) FHL (f dhF, hycB-I) H2 CO2

O CoA

Acetyl-Coenzyme-A H2

NADH ADH (adhE)

ADH (adhE) O

PTA (pta)

CO2 O

O

OH

Ethanol (6)

NADH Acetaldehyde

O

P

ACK (ackA) OH OH

acetyl-phosphate

O

OH

ATP Acetate (4)

Fig. 2. Main metabolic pathways for fermentative degradation of glycerol by E. coli.

can be produced by glycerol fermentation using Klebsiella planticola isolated from the red deer rumen [22]. Dharmadi et al. [7] reported the glycerol fermentation by E. coli, where pH-dependence and CO2 availability were analyzed. Ito et al. [23] showed that glycerol at 10 g/L was almost completely consumed within 84 h and the main products were ethanol and succinic acid with molar yields of 86% and 7%, respectively. According to these authors, E. coli is already a good biocatalyst for glycerol conversion into ethanol and hydrogen. Enterobacter aerogenes can be used for ethanol production at high yields from biodiesel wastes containing glycerol. In this way, an 80 mM glycerol synthetic medium containing biodiesel wastes was analyzed and it was observed that glycerol was consumed in 24 h producing 0.89 mol of H2 and 1.0 mol of ethanol per mol of

glycerol [23]. Table 1 shows the main aspects for different microorganisms used for glycerol fermentation to ethanol. A comparison between ethanol production in terms of manufacturing requirements, feedstocks, and operational costs, from corn or glycerol (including H2eCO2 or formic acid as co-products) was presented by Yazdani and González [11]. Fig. 3 shows a comparative scheme for ethanol production from corn and glycerol. The ethanol production scheme is more complex for corn than for glycerol and also a higher capital investment is required. Moreover, as discussed by Yazdani and González [11] it is expected that the operational costs are almost 40% lower for ethanol production from glycerol, even when no credit was given to the formate or hydrogen co-produced with ethanol.

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Table 1 Used microorganisms for ethanol production from glycerol. Yeast

Glycerol conc. (g/L)

Cervus elaphus

40*

Enterobacter aerogenes

10

Yield (mol/mol)

Ref

Comments

18.5

[22]

80e85

[23]

The isolate bacterium was identified as a strain of the species Klebsiella planticola based on phenotypic characterization. Also formate and ethanol are produced at equimolar levels. The yields decrease with an increase in the concentrations of crude and pure glycerol, also the rates of H2 and ethanol production from crude glycerol were much lower than those at the same concentration of pure glycerol, due to a high salt content in the wastes. Glycerol fermentation is analyzed in a pHdependent manner, being linked to the availability of CO2, which is produced under acidic conditions by the oxidation of formate by the enzyme formate hydrogen lyase. High ethanol yields were achieved by minimizing the synthesis of by-products succinate and acetate through mutations that inactivated fumarate reductase (DfrdA) and phosphate acetyl transferase (Dpta), respectively. Crude glycerol (88 wt% was evaluated at two concentrations) and refined glycerol was evaluated at only one concentration.

Escherichia coli

10*

86

[7]

Escherichia coli Escherichia coli Escherichia coli

10* 10** 20**

104 97 102

[24] [24] [24]

*Pure glycerol as fermentation substrate. **Crude glycerol as fermentation substrate.

4. Simulation aspects Glycerol fermentation by E. coli results in a mixture of compounds containing predominantly ethanol, acetate, and

succinate. Low amounts of formate could also be produced [7]. Succinate and acetate are competitive by-products which could eventually decrease the ethanol yield (Fig. 2). Thus, glycerol can be converted into ethanol and either hydrogen or formate. The

Corn

Glycerol

Grinding Fermentation Cooking Enzyme

Liquefaction

Enzyme

Saccharification

CO2

Fermentation

Ethanol

Product recovery (distillation) Whole

Distillers dried grains

H2-CO2 or Formic acid

Product recovery (distillation/others )

Stillage

Centrifugation Thin

Nutrients water

Ethanol

Stillage

Evaporation

Distillers solubles Fig. 3. Comparison of ethanol production from corn-derived sugars (based on a dry milling process) and ethanol production from glycerol.

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resulting mixture can be easily purified due to the significant physicochemical differences between its compounds. This paper is based on the results presented by Yazdani and González, about glycerol conversion to ethanol by E. coli SY04 (pZSKLMgldA) [24]. Their experimental study used two approaches: (i) ethanol and H2 co-production, and (ii) ethanol and formate co-production. It was found that the maximum theoretical yield in both cases was 1 mol of ethanol plus 1 mol of either formate or hydrogen per each mol of consumed glycerol. As additional information to perform the simulation, the crude glycerol composition (wt/v) was 84% glycerol, 5% salts, 0.02% methanol, and water. The above mentioned values were obtained at 37  C and pH 7.5. The average molecular formula of CH1.9O0.5N0.2 for E. coli was used [25]. Three different possibilities for ethanol production from glycerol were considered. The first and second possibilities use crude glycerol (88 wt%) in a fermentation stage at a dilution of 10 g/L and 20 g/L, respectively. Meanwhile, the third possibility considered pure glycerol (98 wt%) at 10 g/L. In the three cases a raw glycerin with the following composition was used: 32.59 wt% methanol, 60.05 wt% glycerol, 2.62 wt% NaOCH3, 1.94 wt% fats, and 2.8 wt% ash, which is a typical stream of crude glycerin in the biodiesel production [26]. Under these process conditions a pretreatment of glycerol stream is required. Two final glycerol concentrations were considered. The first one was 88 wt% of glycerol (crude glycerol) and the second one was 98 wt% of glycerol (pure glycerol). Fig. 4 shows the flowsheet for the glycerol purification process in both cases (88 and 98 wt%). The mass flow feed for both purification processes was 1000 kg/h of raw glycerol. For glycerol purification the feed mixture is initially evaporated, where 90% of methanol at 99 wt% is recovered. Glycerol is the unique impurity in this stream and then the recovered anhydrous methanol is appropriate to be reused in the transesterification process. The bottom stream from evaporator 1 is neutralized using an acid solution. The salts produced during the neutralization process and the remaining ashes are retired by centrifugation. This product is washed with water using a weight ratio of 2.4 (water/glycerol stream). Aqueous glycerol stream has free salts and solids, with a low concentration of methanol and triglycerides. Thus, more than 90% of water and the remaining methanol are retired by evaporation, where glycerol losses are 0.2%. So, the glycerol purity reached is 80 wt%. Then, the glycerol stream is purified through a distillation column to reach the required purity, either 88 wt% or 98 wt%. Results of glycerol purification process are shown in Table 2. The flowsheet of these three simulated bioprocess for fuel ethanol production from glycerol using E. coli is shown in Fig. 5. In all cases the flowsheet is the same, but the operational conditions are different. Obtained glycerol from the purification process (88 wt % or 98 wt%) was cooled at 37  C and diluted (10 g/L or 20 g/L) in fresh water at 37  C. Then the glycerol fermentation process was carried out by E. coli SY04 (pZSKLMgldA) [24] and a mixture of ethanol, formate, and cells was produced. Cells were withdrawn by

Table 2 Simulation results for raw glycerol purification process. STREAM

Temperature ( C) Mass flow (kg/h) Mass fraction: Triglycerides Methanol Water Glycerol NaOCH3 Ash mass flow (kg/h)

Methanol

Glycerol (88%)

Glycerol (98%)

25 973.30

144.2 301.98

104.7 665.25

0,02 0.335 0 0.617 0.027 27.6

0 0.99 0 0.01 0 0

0.015

189.2 596.60 0.014 0016

0.105 0.88 4.3 ppm 0.003

0004 0.98 4.3 ppm 0003

centrifugation and an aqueous stream of ethanol and formate was obtained. This stream was distilled and ethanol was concentrated in two distillation columns with 40 and 30 stages respectively. Then, an ethanol stream between 93 wt% and 94 wt% of purity was obtained (concentration near to ethanolewater azeotrope 95.6 wt %). Finally, ethanol was dehydrated in a molecular sieve and fuel ethanol was obtained at 99.5 wt%. The main results of this simulation are shown in Table 3. Economic assessments for the purification and bioconversion processes were carried out using Aspen Icarus [18]. The economic assessment for the purification process considers two scenarios. In the first one, the retired methanol from raw glycerin stream is considered as a waste, and in the second scenario the methanol is recycled to the transesterification process, which contains 99 wt% of methanol and 1 wt% of glycerol. Therefore, in the second case methanol is obtained as a co-product. Current price of methanol and international prices of different qualities of glycerin were taken from ICIS [27] as described in Table 4. The used values for glycerin sale prices according to its origin and quality were: 2 US cts/Lb for raw glycerin, 10 US cts/Lb for glycerol at 88 wt%, and 50 US cts/Lb for glycerol at 98 wt%. 5. Results Economic assessment results for the purification process of raw glycerol to 88 wt% and 98 wt% are shown in Table 5, where the costs were discriminated by services, operatives, depreciation, and products and co-product sales. Column 1 shows the purification cost (PC) in US$/L for each purification process, and column 2 shows the percentage of each item in the PC. In general terms for most industrial processes the cost of raw material represents near 50% of the total production costs. However, raw glycerin in bioethanol production represented only 30% of the total costs, because of its low price in the international market. Transportation costs were not considered in the economic assessment since the purification step was assumed to be adjacent

Water waste 1

Methanol

1

Raw Glycerol

2

3

Water

4

5

6

Water waste 2

Raw Glycerol Solids

Organic Phase

Aqueous Glycerol

Glycerol

Fig. 4. Simplified flowsheet of glycerol purification process (88 and 98 wt%). 1. First evaporation column, 2. Neutralization tank, 3. Centrifuge, 4. Decantation tank, 5. Second evaporation column, 6. Distillation column.

J.A. Posada, C.A. Cardona / Energy 35 (2010) 5286e5293

Water

2

1

Broth

3

4

Glycerol Diluted Glycerol

Solids

5291

5

6

Distillate 1

Distillate 2

Water waste 1

Ethanol

Adsorbate Water waste 2

Fig. 5. Simplified flowsheet of fuel ethanol production from glycerol at 88 wt% and 98 wt%. 1. Mixed tank, 2. Fermentation tank, 3. Centrifuge, 4. First distillation column, 5. Second distillation column, 6. Molecular sieves.

Table 3 Simulation results for fuel ethanol production from glycerol.

Table 4 Commercial prices of glycerol.

STREAM Diluted glycerol From crude glycerol at 10 g/L 37 Temperature ( C) Mass flow (kg/h) 57,633.551 Mass fraction Water 0.99 Glycerol 0.01 E. coli 0 Ethanol 0 Formate 0 From crude glycerol at 20 g/L 37 Temperature ( C) Mass flow (kg/h) 28,896.25 Mass fraction Water 0.98 Glycerol 0.02 E. coli 0 Ethanol 0 Formate 0 From pure glycerol at 10 g/L 37 Temperature ( C) Mass flow (kg/h) 57,530.195 Mass fraction Water 0.99 Glycerol 0.01 E. coli 0 Ethanol 0 Formate 0

Broth

Distillate 2

Ethanol

37 57,624.871

77.9 292

77.9 273.072

0.9899 0.0002 0.0004 0.0048 0.0046 37 28,881.1230

0.06 0 0 0.94 0 77.9 313

0.9801 0.0000 0.0006 0.0103 0.0089 37 57,538.901

0.067 0 0 0.933 0 77.9 317

0.9896 0.0000 0.0004 0.0053 0.0047

0.07 0 0 0.93 0

0.005 0 0 0.995 0 77.9 290.508 0.005 0 0 0.995 0 77.9 293.383 0.005 0 0.995 0

to the biodiesel production process. On the other hand, utilities and capital costs represent the highest cost on the purification process (i.e., between 20% and 30%). Also, the final quality of glycerol increases mainly the utility costs. PCs of raw glycerol to glycerol at 88 wt% are 0.1574 US$/L (scenario I) and 0.0984 US$/L (scenario II) when the methanol price is considered. Moreover, when glycerol at 98 wt% is used the PCs are 0.1782 US$/L (scenario I) and 0.1124 US

Glycerol quality

Localization

USCts/Lb

Crude (88 wt%)

US EU

5e15 5e10

Pure (vegetable, 98 wt%)

US EU ASIA

80e90 42e57 38e45

Pure (tallow, 98 wt%)

US EU

70e80 38e46

Refined (USP)

e

110e140

$/L (scenario II). Approximate costs for refining crude glycerol have been reported to about 0.15 US$/lb or 0.26 US$/L [28], which are higher than the PCs obtained in this paper, but near to the obtained PCs in the scenario I. PCs obtained are lower than the sale price of each product, which are 0.28 US$/L for glycerol at 88 wt%, 1.39 US$/L for glycerol at 98 wt% from vegetable oil and 1.11 US$/L for glycerol at 98 wt% from tallow. Then a decrease in the whole fuel ethanol production costs from glycerol can be expected due to the purification process. The economic assessment carried out for the glycerol bioconversion process to fuel ethanol does not consider the raw material cost because it is involved in the purification costs. Table 6 shows the bioconversion costs (BCCs) obtained using Aspen Icarus. The lowest BCC was obtained for crude glycerol (88 wt%) when it was diluted at 20 g/L, since it uses a lower quantity of water than the other two processes. In this way, equipment size and utilities are modified in each case. On the other hand, when pure glycerol (98 wt%) is used a higher water quantity is necessary then increasing sizing and utilities. Finally, global production costs (GPCs) for raw glycerol bioconversion to fuel ethanol are obtained adding by PCs and BBCs, like shown in Table 7. In all cases the PCs are near 35% and the BCCs are near 65%. Furthermore, these obtained GPCs from crude glycerol

Table 5 Purification costs (PC) of raw glycerol. Item (US$/L) Raw materials Utilities Operating labor Maintenance and operating charges Plant overhead and general and administrative costs Capital depreciation Co-products credit Product production cost (US$/L)

PC Glycerol 88 wt%

Share (%)
88%

PC Glycerol 98 wt%

Share (%)
98%

0.05539 0.03741 0.00378 0.01193

35.19 23.76 2.40 7.58

0.05539 0.05608 0.00378 0.01313

31.09 31.48 2.12 7.37

0.01093

6.94

0.01179

6.62

0.03797
0.05900 0.09841

24.12

0.03798
0.06574 0.11240

21.32

100

100

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Table 6 Bioconversion costs (BCC) for fuel ethanol production form raw glycerol. Item (US$/L)

Crude glycerol (10 g/L)

Share (%)

Crude glycerol (20 g/L)

Share (%)

Pure glycerol (10 g/L)

Share (%)

Utilities Operating labor Maintenance and operating charges Plant overhead and general and administrative costs Capital depreciation Product production cost (US$/L)

0.0599 0.0188 0.0205 0.0266 0.0625 0.1883

31.82 9.97 10.89 14.14 33.18 100.00

0.0503 0.0188 0.0154 0.0309 0.0556 0.1710

29.41 10.99 9.02 18.05 32.54 100.00

0.0975 0.0188 0.0193 0.0410 0.0596 0.2361

41.28 7.96 8.17 17.37 25.23 100.00

Table 7 Global production costs (GPCs) for fuel ethanol production from raw glycerol. Costs

Crude glycerol (10 g/L)

Share (%)

Purification costs Bioconversion Costs Global costs

0.0984

34.32

0.0984

36.53

0.1124

32.26

0.1883 0.2867

65.68 100.00

0.1710 0.2694

63.47 100.00

0.2361 0.3485

67.74 100.00

are lower than those reported by Quintero et al. [13] for fuel ethanol production from corn (0.3381 US$/L), where using crude glycerol at 10 g/L and 20 g/L could represent a saving of 15% and 20%, respectively. The obtained GPCs are higher than those reported by Quintero et al. [13] for fuel ethanol production from sugarcane (0.2153 US$/L). Nevertheless, these obtained GPCs are lower than the international prices for fuel ethanol ranging from (0.4552 US$/L [27] to 0.6057 US$/L [28]). Although a rigorous analysis of ethanol fuel market and its prices should be carried out, the obtained results indicate that the production process of ethanol fuel from raw glycerol using E. coli can be as profitable as those using sugar cane or corn as feedstocks. 6. Conclusions Due to the low cost of raw glycerol, methanol recovery from glycerol implies low PCs. Meanwhile, the three possibilities assessed for glycerol bioconversion showed that the GPCs of fuel ethanol from raw glycerol are lower than the commercial price of fuel ethanol. These facts show the potential for raw glycerol bioconversion to fuel ethanol using E. coli. Also, the comparison carried out with a previous paper (which considers the fuel ethanol production from sugarcane and corn in the Colombian case [13]), shows that the GPCs of fuel ethanol from raw glycerol can be as profitable as the production of fuel ethanol from conventional raw materials as sugarcane. The latter is a completely developed industry in Colombia. Acknowledgements The authors express their acknowledgments to the National University of Colombia at Manizales for funding this research. References [1] Bournay L, Casanave D, Delfort B, Hillion G, Chodorge JA. New heterogeneous process for biodiesel production: a way to improve the quality and the value of the crude glycerin produced by biodiesel plants. Catal Today 2005;106(1e4):190e2. [2] Ramadhas AS, Jayaraj S, Muraleedharan C. Biodiesel production from high FFA rubber seed oil. Fuel 2005;8(4):335e40. [3] Wang ZX, Zhuge J, Fang H, Prior BA. Glycerol production by microbial fermentation: a review. Biotechnol Adv 2001;19:201e23. [4] Solomon BO, Zeng AP, Biebl H, Schlieker H, Posten C, Deckwer WD. Comparison of the energetic efficiencies of hydrogen and oxychemicals formation in Klebsiella pneumoniae and Clostridium butyricum during anaerobic growth on glycerol. J Biotechnol 1995;39:107e17.

Crude glycerol (20 g/L)

Share (%)

Pure glycerol (10 g/L)

Share (%)

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