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Investigating the extraction of alcohol from agricultural wastes in Mauritius
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JECE 755 1–8 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
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Investigating the extraction of alcohol from agricultural wastes in Mauritius
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Sindra L. Summoogum-Utchanah* , Jena Swami
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Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Mauritius, Reduit, Mauritius
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 April 2015 Accepted 26 August 2015
Bioethanol production from lignocellulosic biomass has been found to be a potential alternative to mitigate the alarming situation of increasing energy demand, fast consumption of fossil fuels and climate change. The objective of the study was to investigate the optimum conditions for the alcohol extraction process from vegetables and fruit wastes, which make up a large portion of agricultural waste in Mauritius. Focus was made on the different process parameters affecting the hydrolysis and fermentation processes including acid concentration and biomass loading and yeast concentration, pH, temperature and retention time, respectively, to achieve maximum alcohol yield. Dilute acid hydrolysis was found to be a safer and more efficient process compared to concentrated acid hydrolysis, despite its high sugar yield. An optimum value of 4600 mg reducing sugar per 100 ml hydrolysate was achieved after treatment with 5% (w/w) sulphuric acid and at a biomass loading of 20% (w/w). Yeast growth and enzyme activities were found to be optimum at a pH of 5.0 and a temperature of 33 C when using 5% (w/v) SuperStart yeast (Saccharomyces cerevisiae), hence favouring ethanol production. Activated charcoal (10% w/v) was found to successfully remove inhibitors while keeping sugar degradation minimal. Therefore, under optimum extraction conditions, a maximum of 1.88% (v/v) ethanol yield could be produced after 80 h of fermentation. The study indicates that bioethanol production from agricultural waste could be a promising technology in Mauritius due to its low cost, abundance and renewable nature. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Agricultural waste Lignocellulosic biomass Ethanol Acid hydrolysis Fermentation Extraction
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Introduction
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With the growing energy demand and the rapid exhaustion of fossil fuel reserves, the quest for alternative energy resources is essential. A transition from the utilisation of petroleum-derived fuels in the transportation sector to renewable fuels, such as bioethanol, can be beneficial to society in many ways. Bioethanol has been found to be a prospective substitute to the polluting petroleum transportation fuel due to its renewable nature, reduction in combustion emissions and biodegradability [1]. Bioethanol is typically produced from various feedstock and technologies, which can be classified into first generation feedstock (sugar crops and cereal grains), second generation feedstock (lignocellulosic materials) and third generation feedstock (algae). Currently, bioethanol is mostly derived from expensive sucrose containing crops, such as sugarcane, in Brazil and starchy crops, like corn, in the USA which are known as first generation biofuel [2–4]. However, an increase in ethanol
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* Corresponding author. Fax: +230 4657144. E-mail address: [email protected] (S.L. Summoogum-Utchanah).
production from such crops would necessitate supplementary land. This would impact on the land available for food production and consumption, considering the rising demand for food as a result of the population boost. Moreover, the possible changes in climate may cause fluctuations in the price of fuel from these sources. Hence, the search for other potential raw material is becoming increasingly indispensable. As a result, an increase in ethanol production will necessitate the use of lignocellulosic biomass, which is cheap and not directly tied to the food chain. Lignocellulosic biomass is mostly composed of cellulose, hemicellulose and lignin and is considered to be the most promising feedstock for alcohol production due to its abundance and great potential of about 280 l/t biomass [5]. Likewise, agricultural waste, a lignocellulosic feedstock, is considered to be a low valued material, clean and renewable alternative to fossil fuels. The latter has the potential to be transformed from high volume waste disposal environmental problems to a variety of eco-friendly and sustainable products, with the second generation liquid biofuels being the leading ones. Agricultural wastes, especially those from market infrastructure, are more prone to spoilage due to their nature and composition, which produce an unpleasant smell if improperly
http://dx.doi.org/10.1016/j.jece.2015.08.021 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: S.L. Summoogum-Utchanah, J. Swami, Investigating the extraction of alcohol from agricultural wastes in Mauritius, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.08.021
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disposed. As a consequence, reusing these valuable resources for product recovery would bring better return, avoid wastage, and at the same time regulate the market, industries or farm infrastructures. According to the World Energy Outlook 2012, fossil fuel represents the major source of energy and is expected to remain so. However, when comparing the energy demand by fuel in 2010 with forecasts for 2035, it is assumed that countries will reduce their gas emissions by adopting more renewable energy sources [7]. The agricultural by-products can therefore play a crucial role to bring about the evolution to sustainable biofuels in order to reduce the burden on imported transportation fuels and solve disposal problem. Agricultural wastes represent a higher potential source for low cost ethanol production than sugar crops, but only after being hydrolysed to fermentable sugars [8]. The main barrier for the extraction of alcohol from agricultural wastes lies in their rigid structure which resists degradation. The major steps involved in agricultural waste to bioethanol conversion process are the pretreatment, hydrolysis, fermentation and distillation. Consequently, the main aim of the research focused in investigating the extraction of ethanol from vegetable and fruit wastes, which make up a large portion of agricultural waste in Mauritius, in an ecofriendly and profitable way. The objectives were the efficient pretreatment of the waste, optimisation of the hydrolysis and fermentation process using sulphuric acid and Saccharomyces cerevesiae respectively, and detoxification of the hydrolysate using activated charcoal to yield maximum alcohol, and separation and purification of the fermenting medium for alcohol recovery.
Materials and methods
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Feedstock collection and preparation
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The vegetable and fruit wastes were collected from the local market and stored in plastic bags prior to its use and preparation of the extract. They were properly washed with tap water to remove foreign objects such as rocks, impurities and dirt. The leftovers consisted of equal masses of beetroot leaves and stems, cauliflower and outer cabbage leaves, and peeling and topping of pineapples. These wastes were chosen because of their wide availability and non-consumable aspects. The mixtures were homogeneous and dried in hot air oven at 60 C for 24–48 h to aid preservation and storage.
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Feedstock characterisation
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Prior to the extraction, the wastes were processed for determination of approximate composition. The mixture was analysed for moisture content, amount of total solids, ash content, volatile matter, and lignin, hemicellulose and cellulose content. Each analysis was done in duplicate for comparison.
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Pretreatment
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The first step for alcohol production from agricultural wastes was size reduction. Shredding was found to give uniform pieces and improved the efficiency of the conversion process by
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Table 1 Milligrams of reducing sugars required to reduce 10 ml Fehling’s solution (Eynon and Lane’s Method). Mg reducing Sugar per 100 ml solution, when concentration of sucrose is— 0 0.5 g 1g 2g 3g 4g
5g
10 g
25 g
50 g
15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. Calculated by extrapolation
336 316 298 282 267 255 243 232 222 213 205 197 190 184 178 172 166 161 157 152 148 144 140 137 133 130 127 124 121 119 116 114 111 109 107 105
317 297 280 264 250 238 227 216 207 198 190 183 176 170 165 159 154 149 145 140 136 133 129 126 122 119 116 114 111 108 106 104 102 99 97 95
307 288 271 256 243 231 220 210 200 192 184 177 170 164 159 153 148 143 139 135 131 127 124 120 117 114 111 109 106 103 101 99 96 94 92 90
289 271 255 240 227 215 206 196 187 179 171 164 158 152 147 142 137 132 128 124 121 117 114 111 107 104 102 99 97 94 92 90 88 86 84 82
275 257 241 227 215 204 194 185 176 168 161 155 149 143 138 133 129 125 121 117 113 109 106 103 100 97 95 92 90 88 86 84 82 81 79 77
333 312 295 278 264 251 239 228 219 210 202 194 187 180 174 168 163 158 153 149 145 141 137 134 130 137 124 121 118 116 113 111 108 106 104 102
329 309 291 274 260 248 236 225 216 207 198 191 184 178 171 166 161 156 151 147 143 139 135 131 123 125 122 119 116 114 111 109 106 104 102 100
325 305 287 271 257 245 233 222 213 204 196 189 182 175 169 164 159 154 149 145 141 137 133 130 126 123 120 117 115 112 110 107 104 103 102 100
76 77 78 79 80 81 82 83
86 87 88 89
92 93
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Ml Sugar solution required
335 314 296 280 265 253 241 230 220 211 203 195 189 182 176 170 165 160 155 151 147 143 139 135 132 129 125 123 120 117 114 112 110 108 106 103
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322 301 284 268 254 242 230 220 210 202 194 186 179 173 167 161 157 152 147 143 139 135 131 128 124 121 118 116 113 110 108 105 103 101 100 98
Please cite this article in press as: S.L. Summoogum-Utchanah, J. Swami, Investigating the extraction of alcohol from agricultural wastes in Mauritius, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.08.021
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About 0.2 g urea and 2–3 drops of phosphoric acid were added to the hydrolysate after adjusting the pH [11]. Since yeast digests sugars in the absence of oxygen to produce ethanol and carbon dioxide, the fermenting flask was securely sealed with a rubber stopper to favour an anaerobic condition. However, to prevent built-up of pressure due to the formation of carbon dioxide gas during the fermentation process, plastic tubing was inserted in the stopper to allow the gas to move to a small bottle containing water. To maximise the alcohol yield, we determined the optimal conditions for the reaction to proceed at a favourable rate.
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Dilute acid hydrolysis The wastes were first treated with dilute sulphuric acid in the range of 1–5% (w/w) at a temperature of 121 C and a pressure of 1.2 bars for about 1.5 h to recover maximum hemicelluloses and celluloses in a standard autoclave. Cotton and aluminium foil were used to block the opening of the flask, thereby preventing evaporation at the high operating temperature of the autoclave. After the hydrolysis process, the mixture was then separated into liquid and solid. The solid phase was manually pressed for maximum liquid recovery, and was tested for reducing sugar, while the solid phase was discarded.
Yeast concentration The yeast concentration was varied at 1, 3 and 5% (w/v). Samples were then taken from the set-up to test for the amount of ethanol.
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pH
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pH was varied at: 3, 4, 5 and 6 at the optimum yeast concentration to yield maximum alcohol.
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Temperature Incubation temperature was then varied at: 28, 30, 33, 35, 37 and 40 C at optimum pH value and yeast concentration.
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Biomass loading Biomass loading may be defined as the variation in mass of dried waste mixture used. The dilute acid hydrolysis process was repeated using the optimum acid concentration but with varying biomass loading of 5%, 10%, 15%, 20% and 30% (w/w). Consequently, the concentrated acid hydrolysis was carried out using the optimum biomass loading.
Retention time The fermentation period was varied from 24 to about 96 h.
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Distillation
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Ethanol was recovered from the fermented mixture by distillation, which was achieved using a rotary-evaporator (‘Rotavap’). The apparatus works by boiling the liquid mixture of water and ethanol in a hot water bath maintained at 78–79 C. The distillate was collected and analysed for ethanol concentration using an ebulliometer as shown in Fig. 1.
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increasing the surface area and reducing cellulose crystallinity. Thus, the initially dried vegetable and fruit waste was mechanically pretreated using a pulveriser, and was sieved to a uniform size of about 2 mm. The resulting sample was then reserved for the hydrolysis process. Hydrolysis Acid hydrolysis was carried out using both dilute and concentrated sulphuric acid.
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Concentrated acid hydrolysis Concentrated acid hydrolysis was achieved in a heat resistant conical flask at atmospheric pressure. According to Balat, for concentrated acid hydrolysis, the range of acid concentration lies in the range of 10–30% [9]. Therefore, the pretreated biomass together with concentrated sulphuric acid in the range of 20–30% (w/w) was heated to about 100 C in a water-bath for 50 min using a hot plate. Subsequently, we allowed the sample to cool before the filtration process for the recovery of the hydrolysate which was tested for reducing sugar.
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Test for reducing sugar
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The Original Lane and Eynon method was used for estimating the amount of reducing sugar in the hydrolysate. The method is a short and rapid one for estimating the amount of reducing sugar. It is based on a determination of the volume of a test solution required to reduce completely a known volume of alkaline copper reagent. The end point is indicated by the use of an internal indicator, methylene blue. The amount of reducing sugar is read from Table 1.
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Detoxification
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The acid was neutralised and inhibitors were removed before the fermentation process by two methods which make use of lime and 10% activated charcoal, respectively, as reported by Canilha et al. [10].
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Fermentation
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The fermentation process was immediately carried out after the hydrolysis and detoxification process using the hydrolysate yielding the highest reducing sugar. It was accomplished biochemically by Saccharomyces cerevisiae yeasts (SuperStart).
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Fig. 1. Chart to read concentration of ethanol from temperature.
Please cite this article in press as: S.L. Summoogum-Utchanah, J. Swami, Investigating the extraction of alcohol from agricultural wastes in Mauritius, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.08.021
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Results and discussion
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Waste characterisation
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The moisture, ash, total solid and pH of the waste were determined and summarised as in Table 2. The results showed that the above mixture has a low solid content due to its high moisture. The high moisture content is mainly due to the increased organic composition of vegetable and fruits making them more prone to decomposition. Thus, the latter has to be dried prior to be used for the conversion process. Moreover, the sample exhibits a low ash content of about 12.48% which is near to the value (15–20%) stated by Mood et al. [4]. The high gross calorific value of 27, 445 kJ/kg of the sample indicates good potential for fuel conversion. Moreover, the high hemicellulose and lignin content show great potential for alcohol extraction energy production, respectively. Thus, about 64% of the waste can be converted to alcohol.
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Dilute acid hydrolysis
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Effect of acid concentration Fig. 2 shows the results of varying acid concentration on the amount of sugar liberated. It can be seen that the amount of sugar liberated increases considerably as the acid concentration is increased, though there is no significant difference in the value obtained with 2% and 3% (w/w) acid. However, the steep gradient indicates a significant increase in the amount of sugar produced from 0% to 2% (w/w) acid. This is followed by a partial stabilisation of the sugar formed from 2% to 3% (w/w). An increase in acid concentration above 3% (w/w) resulted in a quite significant increase in sugar produced again, as shown by the less steep gradient (397) from 3% to 5% (w/w). The hydrolysed material using 4% (w/w) acid could not be tested for reducing sugar since the sample was almost dry after the hydrolysis process. This might have occurred due to evaporation when exposed to the high temperature and pressure. It can also be observed from the plot that the optimum concentration of reducing sugar (about 4510 mg/ 100 ml solution) is produced from treatment with 5.0% (w/w) H2SO4. Further increase in acid beyond 5% (w/w) resulted in a decrease in the amount of reducing sugar liberated. This may be due to the severity of the acid which leads to the formation of undesired b yproducts like furfurals and HMF, and consequently leading to degradation of sugars. Due to the acidic medium of the hydrolysate, the formation of inhibitors could not be avoided. To minimise the latter and hence maximising ethanol yield, other methods which do not utilise chemicals could be considered. A control was set-up using distilled water instead of the acid to investigate this phenomenon. The control was maintained under similar operating conditions as those using acid. About 1329 mg reducing sugar per 100 ml solution sugar was liberated. The advantage is that there is no inhibitors formation and the rate of
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195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225
Fig. 2. Effect of acid variation on sugar concentration.
sugar degradation is negligible since the process makes use of no chemicals. However, as compared to those using acid the efficiency of the process was very low, about 3–4 times less. The hydrolysed solution from the control was found to be of lighter brown colour compared to those using acid. This can be explained by the fact that only part of the sugar was liberated. Also, the particle size of the substrate was observed to be unchanged compared to those using acid which resulted in finer particles after the hydrolysis process. Thus, filtration was difficult and time consuming. It can be concluded that acid concentration has an important relationship with the amount of sugar produced and also on the formation of inhibitors. As the acid concentration increases, the rate for both also increases. However, the effect of sugar degradation is highly noticeable when the acid concentration was increased at and beyond 5.0% (w/w).
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Effect of biomass loading Fig. 3 illustrates the effect of varying the mass of biomass while keeping the total working volume constant. It shows an increasing trend in the amount of reducing sugar liberated as the mass of biomass increases, up to the peak having the maximum amount of about 4600 mg/100 ml solution at 19–20% biomass loading. The amount of reducing sugar produced was found to decrease with further increase in biomass loading. This may be because with the 30% biomass loading the amount of acid used was insufficient to efficiently destroy the shell protecting the hemicellulose and cellulose. This result is similar to those reported by Binod et al. [12]. In addition, after hydrolysis in the autoclave the solid absorbed most of the acid. The results obtained with 5% and 10% have no significant differences and are low compared to those at 15% and 20% biomass. The low values may be because of the high amount of acid causing dilution of the reducing sugar produced. It can be concluded that a biomass to liquid ratio of about 1:10 yield to maximum reducing sugar which is the case with the
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Table 2 Summary of the main characteristics of the agricultural wastes. Characteristics
Value
Moisture content/% Total Solid content/% Ash content/% Volatile Solid content/% Heating value/kJ/kg Cellulose content/% Hemicellulose content/% Lignin content/%
88.52 11.48 12.48 87.52 27445 24.92 38.83 36.25 Fig. 3. Effect of biomass loading on reducing sugar concentration.
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20% biomass loading. This finding is in line with that stated by Balat for temperature less than 160 C [9]. The hydrolysis also caused a loss of biomass weight by removing the lignin and due to hydrolysis of the cellulose and hemicellulose. Weight loss was found to be different at varying acid concentration, as shown by Fig. 4. The higher the acid concentration, the more mass loss was recorded and vice-versa.
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Concentrated acid hydrolysis
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The amount of sugar liberated was found to differ, as shown in Fig. 5. The graph shows that the amount of reducing sugar liberated increases as the concentration of acid increases. The increase is significant when acid concentration is increased from 25% to 27% (w/w) to reach a maximum yield of 6122 mg/100 ml at 27%. This finding is close to the research done by Inranmahboob et al. [13], who obtained high sugar yield using 26% by weight sulphuric acid and is in line with Drapcho [14] who stated that maximum digestion should occur in the range of 20–30% acid concentration. However, any further increase in acid concentration was found to cause a significant decrease in amount of sugar produced. Binod et al. stated that the concentrated acid breaks the hydrogen bonding between the cellulose chains, changing its structure [12]. The cellulose is said to be decrystallised, and forms a homogeneous gelatin with the acid. This made the separation process difficult and time consuming. Moreover, due to the high acidity of the hydrolysate large amount of sodium carbonate solids and sodium hydroxide were needed for the neutralisation process prior to reducing sugar test and fermentation, respectively. Industrially, the acid has to be recovered, re-concentrated and recirculated to make the process economically feasible, which was not possible at laboratory scale. The high investment and maintenance costs involved in the process have made them commercially unattractive despite its higher yield.
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Fermentation
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Effect of detoxification process The detoxification process was carried out using two methods, overliming and absorption with activated charcoal. However, the one using lime resulted in the formation of a white precipitate (calcium sulphate). This in turn made the filtration process slow, and very little hydrolysate could be collected. Fig. 6 shows the effect of detoxifying the hydrolysate with 10% activated charcoal prior to the fermentation process using baker’s yeast. Sugar degradation is inevitable during acid hydrolysis. As a result, there is formation of by-products including furfurals and 5hydroxymethlyfurfural (HMF), which are known to have inhibitory effect on the rate of fermentation process. This is in line with other
Fig. 4. Mass loss as a result of hydrolysis.
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Fig. 5. Effect of concentrated acid variation on sugar concentration.
Fig. 6. Effect of detoxification of yield of ethanol.
researches, which state that inhibitors formation is favoured as acid concentration and temperature increase [15,16]. The bar chart shows that higher yield of ethanol was achieved after detoxification process. Therefore, this indicates that these compounds are harmful to fermenting microorganisms since they slow down their metabolism. In addition, Lenihan et al. stated that the by-products penetrate the cells and delay cell growth and reproduction [16]. Thus, the eliminations of these harmful compounds are important to increase the efficiency of the fermentation process and subsequently, ethanol yield. The solution was found to decolourise after treatment with activated charcoal. Moreover, the unpleasant smell of the solution disappeared. The efficiency of the common Baker’s yeast was compared with the latter after detoxification. The concentration of ethanol produced was found to increase from 0.46% to 0.97% (v/v) when SuperStart yeasts were used, as illustrated in Fig. 7. Consequently, further experiments were carried out using SuperStart yeasts.
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Effect of yeast concentration Fig. 8 depicts the effects of varying the yeast concentration on the amount of ethanol produced while maintaining other operating conditions constant. The temperature was maintained at 33 C at pH 5.0 throughout the process, since they were known to be optimum for enzyme growth and reproduction. The graph shows that an increase in yeast concentration leads to a significant increase in ethanol production. From the current trend, it can be predicted that the ethanol concentration would increase as the yeast loading would be increased until yeast metabolism is inhibited upon reaching a certain threshold value. The maximum yeast loading was therefore assumed to be 5% (w/v),
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The results showed that the pH of the medium had a noticeable effect on the final ethanol concentration which was found to be greatest when the pH of the reaction was at a lower acidic level. At pH 3.0, the amount of ethanol produced was very low. One reason could be that the low extracellular pH was destructive to the yeasts, as mentioned by Narendranath and Power [17]. There was a significant increase in the ethanol yield as pH was increased from a value of 3.0–4.0. As the pH level was further increased, the reaction yield also rose to an optimum value at pH 5.0, whereby the yeast reproduction and growth is favoured due to the slightly acidic environment. However, beyond pH 5.0 the reaction yield was observed to decrease by about 11.2%. This may be due to the small difference between the internal and external pH values, which inhibits yeast growth. The yeast cells require more energy to either pump in or pump out hydrogen ions with the aim to maintain an optimum intracellular pH. This finding is near to those observedby Lin et al. [18]. It can be concluded that a slightly acidic pH, around 5.0, is optimal for yeast reproduction and growth, and consequently fermentation. The low acidity ensures that the yeast functions under minimal internal stress, resulting in lower energy wastage and efficient growth. Therefore the sugars are fermented to ethanol more efficiently.
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Effect of pH on ethanol concentration It is very important for yeast to maintain a constant intracellular pH during growth for effective enzyme activities. The pH variation experiments investigated how the pH level of the system affected the production of ethanol from yeast. Since enzymes work best in acidic medium due to the acidophilic nature of the yeast and the optimum pH of the Superstart yeast lies between 3.5 and 6.0 (Medine), the pH values were varied within this range. Fig. 9 gives the plot of ethanol concentration against pH values.
Effect of temperature The temperature variation experiment investigated how the production of ethanol from yeast was affected by a range of temperature as illustrated in Fig. 10. The experiments suggested that the rate of fermentation reaction increased as temperature is increased up to about 33 C. There was no significant decrease in ethanol yield from 33 to 35 C. However, any increase in temperature beyond 33 C decreases the rate of reaction. This showed that at temperature between 33 and 35 C, the yeast was under minimal stress, therefore favouring growth and reproduction of cells. The decrease in concentration of ethanol at higher temperature shows that the ability of the yeast to ferment is affected under those conditions. At high temperature the yeast may be under stress, thereby slowing the rate of growth and reproduction. It can be predicted from the current trend that the yield would have further decreased if the temperature would have increased above 40 C. This would be because of the denaturation of enzymes which would restrain yeast metabolism. It can therefore be concluded that the optimum fermentation temperature is around 33 C, at which a maximum yield of 0.96% (v/v) ethanol after fermenting for 24 h, and higher temperature is unfavourable to the fermenting yeast. One reason may be due a lack of agitation causing the efficiency of the fermentation process
Fig. 9. Effect of pH on yield of ethanol.
Fig. 10. Effect of temperature on ethanol yield.
Fig. 7. Comparison between Baker’s and SuperStart yeast.
Fig. 8. Effect of yeast concentration on yield of ethanol. 333 334 335 336 337 338 339 340 341 342 343
which gave an approximate ethanol yield of 1.45% (v/v) after 48 h of fermentation.
Please cite this article in press as: S.L. Summoogum-Utchanah, J. Swami, Investigating the extraction of alcohol from agricultural wastes in Mauritius, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.08.021
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Fig. 11. Effect of retention time on ethanol yield.
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to be low. As a result, not all the yeast was able to come in contact with the sugars present. Effect of retention time The yield of ethanol production also depends on the fermentation period. The effect of retention time on the yield of ethanol is as shown in Fig. 11. The graph shows that as the number of fermentation hours is increased, the yield of ethanol is boosted due to a higher yeast metabolism. The concentration of ethanol rises until no more reducing sugar is left in the fermenting medium indicating end of the fermentation process. Therefore, any further augmentation in retention time will either keep the ethanol yield unchanged or could cause a possible decrease in ethanol yield due to contamination and formation of by-products. The optimum fermentation period was therefore found to be between 72 and 88 h, with maximum ethanol production of about 1.88% (v/v) after 80 h of fermentation.
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Environmental assessment
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Ethanol production from lignocellulosic biomass has several advantages which have dramatically increased the world ethanol production in recent years. Its oxygenated nature (35% oxygen) favours complete fuel combustion, thus producing cleaner emissions [19]. Unlike fossil fuels, the combustion of bioethanol does not contribute to the net increase of carbon dioxide in the atmosphere since they form part of the natural closed carbon cycle [20]. Walker also mentioned that bioethanol production and consumption needs a positive Net Energy Ratio (NER) in order to maintain environmental sustainability, which is the case for bioethanol produced from lignocellulosic biomass and other biowastes materials [19]. Bioethanol can either be used in its pure form or blended with gasoline in different proportions. The use of ethanol-blended fuel is one of the best pollution control strategies due to its high heat of vapourisation, high octane number and low cetane number [5]. The use of the high octane fuel shifts octane boosters, including carcinogen like benzene. In addition, bioethanol is biodegradable and contains no sulphur additives [21]. It produces cleaner combustion products unlike petroleum fuels; and reduces particulates, carbon monoxide, hydrocarbon, oxides of nitrogen and other toxic emissions. Thus the use of bioethanol significantly reduces death-causing illnesses like cancer, asthma and heartattacks because of its environmentally friendly nature. It can be said that an increased use of bioethanol will reduce greenhouse gases and help to fight against the alarming global warming.
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Economic assessment
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Lignocellulosic feedstock, being abundantly available and cheap, is considered to be the most promising feedstock, with the expectancy of reducing the production costs. The set-up of a biorefinery necessitates huge investment. As a consequence, several approaches exist in order to improve the economy of bioethanol production. One way is to make efficient use of the byproducts generated from the various processes involved. This involves the sale of waste streams, such as carbon dioxide and stillage. In addition, the price of the produced bioethanol should be set at a lower price compared to gasoline to find economic acceptance and be competitive. Chemicals like acids can be recovered and recycled, while the wastewater can be sent for treatment and reused. In addition, the non-fermentable lignin, which represents great potential for energy production due to its high energy value of 29.54 MJ/kg, can be burnt in boilers for coproduction of electricity and steam [23]. Both can meet the energy requirement of the entire plant, while the surplus can be sold to the local grid in the form of electricity thus increasing revenue. The lignin can also be further used for conversion into other valuable products like specialty polymers [24]. The cost of bioethanol production can be further reduced through process integration, making use of larger industrial facilities rather than smaller ones or even integrating the ethanol production to an existing plant [25,26]. Moreover, advancement in pretreatment process and fermenting microorganisms are crucial not only to bring down the cost of production, but also to increase ethanol yield.
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Conclusion
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Recently, there has been a growing interest in biofuels due to depletion of fossil reserves, rising liquid fuel prices, energy security and environmental problems. In light of this, bioethanol production from agricultural wastes is gaining popularity due to their abundance and low cost. Using these wastes is not only economically beneficial, but also solves waste disposal problem, thereby preventing the generation of infectious diseases. The study focused on extracting sugar from agricultural wastes, mostly vegetable and fruit wastes, and consequently fermented the released sugar to ethanol by the use of S. cerevesiae. Two chemical methods were used to achieve the hydrolysis process and the amount of sugar liberated was compared. It can be inferred that concentrated acid hydrolysis is a dangerous and uneconomical method, despite the high sugar yield, as it involves the use of concentrated sulphuric acid which is highly corrosive. Thus, dilute acid hydrolysis is safer and more cost effective when applied on a large scale. The effect of various parameters like acid concentration, biomass loading, temperature, pH, retention time, and yeast concentration on the final ethanol yield were examined, in order to determine the optimum condition for the extraction process. Optimisation of acid concentration showed that 5% (w/w) liberated the maximum amount of fermentable sugar during the hydrolysis process, at the expense of inhibitors formation. A maximum of 1.88% (v/v) ethanol was produced with 20% (w/w) biomass and 5% (w/v) yeast loading, after 80 h of fermentation at 33 C. Agricultural wastes are expected to lower the production cost of ethanol and provide stability to supply and price. However, to increase the yield of ethanol conversion, significant barriers need to be addressed. Proper and cost effective pretreatment and hydrolysis technologies, with the use of minimum chemicals and energy inputs, are important. Bioethanol being locally available will help in reducing the dependency on imported petroleum for automobiles. As future work, enzymatic hydrolysis, which is considered to the state-of-art technology for bioethanol
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production could be employed to compare its efficiency with acid hydrolysis. An array of research areas have to be brought together, such as science, engineering and economics, to make low-cost and environment-sound bioethanol production from agricultural wastes a reality in Mauritius in the near future.
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Uncited references
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[6,22]. References [1] E.D. Deenanath, S. Iyuke, K. Rumbold, The Bioethanol industry in sub-Saharan Africa: history, challenges, and prospects, J. Biomed. Biotechnol. 2012 (2012) , doi:http://dx.doi.org/10.1155/2012/416491. 22536020. [2] N. Sarkar, S.K. Ghosh, S. Bannerjee, K. Aikat, Bioethanol production from agricultural wastes: an overview, Renewable Energ. 37 (1) (2012) 19–27, doi: http://dx.doi.org/10.1016/j.renene.2011.06.045. [3] P.B. Hans, C.E. Thaddeus, S. Jurgen, Biofuels from Agricultural Wastes and Byproducts, first ed., Wiley-Blackwell, Iowa State University Press, Ames, USA, 201011–22. [4] S.H. Mood, A.H. Golfeshan, M. Tabatabaei, G.S. Jouzani, G.H. Najafi, M. Gholami, M. Ardjmand, et al., Lignocellulosic biomass to bioethanol, a comprehensive review with a focus on pretreatment. Elsevier, Renewable Sustain. Energ. Rev. 27 (2013) 77–93. [5] M. Balat, H. Balat, C. Öz, Progress in bioethanol processing, Prog. Energ. Combust. Sci. 34 (5) (2008) 551–573, doi:http://dx.doi.org/10.1016/j. pecs.2007.11.001. [6] L. Morello, Global CO2 Emissions from Fossil-Fuel Burning Rise into High-Risk Zone Sci. Am. 2012; Available from: http://www.scientificamerican.com/article.cfm?id=global-co2-emissions-from Accessed 5 October 2013 [7] R. Webster, Seven Essential Graphs from IEA0 s World Energy Outlook 2012; available from: http://www.carbonbrief.org/blog/2012/11/favourite-graphsfrom-iea [Accessed 8 October 2013] [8] Q.J. Iqbal, Microbiological Conversion of Waste Fruits and Vegetables into Ethanol, Department of Zoology, University of Punjab, Pakistan Research Repository, 2005. [9] M. Balat, Production of bioethanol from lignocellulosic materials via the biochemical pathway: a review, Energ. Convers. Manag. 52 (2) (2011) 858–875, doi:http://dx.doi.org/10.1016/j.enconman.2010.08.013. [10] L. Canilha, W. Carvalho, M.dG.A. Felipe, J.B.dA.e Silva, Xylitol production from wheat straw hemicellulosic hydrolysate: hydrolysate detoxification and carbon source used for inoculum preparation, Braz. J. Microbiol. 39 (2) (2008) 333–336, doi:http://dx.doi.org/10.1590/S1517-83822008000200025. [11] K.B. Noormamode, Potential production of ethanol From Ulva Recticulata Algae. Thesis, University of Mauritius, 2010.
[12] R. Sindhu, M. Kuttiraja, P. Binod, K.U. Janu, R.K. Sukumaran, A. Pandey, Dilute acid pretreatment and enzymatic saccharification of sugarcane tops for bioethanol production, Bioresour. Technol. 102 (23) (2011) 10915–10921, doi: http://dx.doi.org/10.1016/j.biortech.2011.09.066. 22000965. [13] J. Iranmahboob, F. Nadim, S. Monemi, Optimizing acid-hydrolysis: a critical step for production of ethanol from mixed wood chips, Biomass Bioenerg. 22 (5) (2002) 401–404, doi:http://dx.doi.org/10.1016/S0961-9534(02)00016-8. [14] C. Drapcho, Biofuels Engineering Process Technology, seventh ed., Oxford University Press, Oxford, United Kingdom, 2008. [15] M. Idrees, A. Adnan, S. Sheikh, Q. Ashraf, Optimization of dilute acid pretreatment of water hyacinth biomass for enzymatic hydrolysis and ethanol production, Excli J. 12 (2012) 30–40. [16] P. Lenihan, A. Orozco, E. O’Neill, M.N.M. Ahmad, D.W. Rooney, G.M. Walker, Dilute acid hydrolysis of lignocellulosic biomass, Chem. Eng. J. 156 (2) (2010) 395–403, doi:http://dx.doi.org/10.1016/j.cej.2009.10.061. [17] N.V. Narendranath, R. Power, Relationship between pH and medium dissolved solids in terms of growth and metabolism of Lactobacilli and Saccharomyces cerevisiae during ethanol production, Appl. Environ. Microbiol. 71 (5) (2005) 2239–2243, doi:http://dx.doi.org/10.1128/AEM.71.5.2239-2243.2005. 15870306. [18] Y. Lin, W. Zhang, C. Li, K. Sakakibara, S. Tanaka, H. Kong, Factors affecting ethanol fermentation using Saccharomyces cerevisiae BY4742, Biomass Bioenerg. 47 (2012) 395–401, doi:http://dx.doi.org/10.1016/j.biombioe.2012.09.019. [19] G.M. Walker, Bioethanol: Science and Technology of Fuel Alcohol, Ventus Publishing ApS, Frederiksberg, Denmark, 20108–101. [20] Colaneri K. AP: environmental impacts of ethanol may outweigh the benefits StareImpact 2013; available from: http://stateimpact.npr.org/pennsylvania/ 2013/11/12/ethanol/ [Accessed 11 December 2013] [21] Renewable Fuels Association (RFA) Policy Positions: Ethanol’s Positive Impact on the Environment & Air Quality 2013; available from: http://www.ethanolrfa.org/pages/policy-positions-environment. [Accessed 11 December 2013] [22] E. Gerden. On the Brink of Sweeping Change. Ethanol Producer Magazine 2011; available from: http://ethanolproducer.com/articles/7860/on-the-brink-ofsweeping-change. [Accessed 20 March 2014] [23] C. Cardonaalzate, O. Sancheztoro, Energy consumption analysis of integrated flowsheets for production of fuel ethanol from lignocellulosic biomass, Energy 31 (13) (2006) 2447–2459, doi:http://dx.doi.org/10.1016/j.energy.2005.10.020. [24] J.D. Gargulak, S.E. Lebo, Commercial use of lignin-based materials, ACS Symp. Ser. (1999) 304–320, doi:http://dx.doi.org/10.1021/bk-2000-0742.ch015. [25] A.K. Chandel, C. Es, R. Rudravaram, M.L. Narasu, L.V. Rao, R. Pogaku, Economics and environmental impact of bioethanol production technologies: an appraisal, Biotechnol. Mol. Biol. Rev. 2 (1) (2007) 014–032. [26] P. Dwivedi, J.R.R. Alavalapati, P. Lal, Cellulosic ethanol production in the United States: conversion technologies, current production status, economics, and emerging developments, Energ. Sustain. Dev. 13 (3) (2009) 174–182, doi: http://dx.doi.org/10.1016/j.esd.2009.06.003.
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Investigating the extraction of alcohol from agricultural wastes in Mauritius
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Sindra L. Summoogum-Utchanah* , Jena Swami
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Department of Chemical and Environmental Engineering, Faculty of Engineering, University of Mauritius, Reduit, Mauritius
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 April 2015 Accepted 26 August 2015
Bioethanol production from lignocellulosic biomass has been found to be a potential alternative to mitigate the alarming situation of increasing energy demand, fast consumption of fossil fuels and climate change. The objective of the study was to investigate the optimum conditions for the alcohol extraction process from vegetables and fruit wastes, which make up a large portion of agricultural waste in Mauritius. Focus was made on the different process parameters affecting the hydrolysis and fermentation processes including acid concentration and biomass loading and yeast concentration, pH, temperature and retention time, respectively, to achieve maximum alcohol yield. Dilute acid hydrolysis was found to be a safer and more efficient process compared to concentrated acid hydrolysis, despite its high sugar yield. An optimum value of 4600 mg reducing sugar per 100 ml hydrolysate was achieved after treatment with 5% (w/w) sulphuric acid and at a biomass loading of 20% (w/w). Yeast growth and enzyme activities were found to be optimum at a pH of 5.0 and a temperature of 33 C when using 5% (w/v) SuperStart yeast (Saccharomyces cerevisiae), hence favouring ethanol production. Activated charcoal (10% w/v) was found to successfully remove inhibitors while keeping sugar degradation minimal. Therefore, under optimum extraction conditions, a maximum of 1.88% (v/v) ethanol yield could be produced after 80 h of fermentation. The study indicates that bioethanol production from agricultural waste could be a promising technology in Mauritius due to its low cost, abundance and renewable nature. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Agricultural waste Lignocellulosic biomass Ethanol Acid hydrolysis Fermentation Extraction
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Introduction
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With the growing energy demand and the rapid exhaustion of fossil fuel reserves, the quest for alternative energy resources is essential. A transition from the utilisation of petroleum-derived fuels in the transportation sector to renewable fuels, such as bioethanol, can be beneficial to society in many ways. Bioethanol has been found to be a prospective substitute to the polluting petroleum transportation fuel due to its renewable nature, reduction in combustion emissions and biodegradability [1]. Bioethanol is typically produced from various feedstock and technologies, which can be classified into first generation feedstock (sugar crops and cereal grains), second generation feedstock (lignocellulosic materials) and third generation feedstock (algae). Currently, bioethanol is mostly derived from expensive sucrose containing crops, such as sugarcane, in Brazil and starchy crops, like corn, in the USA which are known as first generation biofuel [2–4]. However, an increase in ethanol
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* Corresponding author. Fax: +230 4657144. E-mail address: [email protected] (S.L. Summoogum-Utchanah).
production from such crops would necessitate supplementary land. This would impact on the land available for food production and consumption, considering the rising demand for food as a result of the population boost. Moreover, the possible changes in climate may cause fluctuations in the price of fuel from these sources. Hence, the search for other potential raw material is becoming increasingly indispensable. As a result, an increase in ethanol production will necessitate the use of lignocellulosic biomass, which is cheap and not directly tied to the food chain. Lignocellulosic biomass is mostly composed of cellulose, hemicellulose and lignin and is considered to be the most promising feedstock for alcohol production due to its abundance and great potential of about 280 l/t biomass [5]. Likewise, agricultural waste, a lignocellulosic feedstock, is considered to be a low valued material, clean and renewable alternative to fossil fuels. The latter has the potential to be transformed from high volume waste disposal environmental problems to a variety of eco-friendly and sustainable products, with the second generation liquid biofuels being the leading ones. Agricultural wastes, especially those from market infrastructure, are more prone to spoilage due to their nature and composition, which produce an unpleasant smell if improperly
http://dx.doi.org/10.1016/j.jece.2015.08.021 2213-3437/ ã 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: S.L. Summoogum-Utchanah, J. Swami, Investigating the extraction of alcohol from agricultural wastes in Mauritius, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.08.021
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disposed. As a consequence, reusing these valuable resources for product recovery would bring better return, avoid wastage, and at the same time regulate the market, industries or farm infrastructures. According to the World Energy Outlook 2012, fossil fuel represents the major source of energy and is expected to remain so. However, when comparing the energy demand by fuel in 2010 with forecasts for 2035, it is assumed that countries will reduce their gas emissions by adopting more renewable energy sources [7]. The agricultural by-products can therefore play a crucial role to bring about the evolution to sustainable biofuels in order to reduce the burden on imported transportation fuels and solve disposal problem. Agricultural wastes represent a higher potential source for low cost ethanol production than sugar crops, but only after being hydrolysed to fermentable sugars [8]. The main barrier for the extraction of alcohol from agricultural wastes lies in their rigid structure which resists degradation. The major steps involved in agricultural waste to bioethanol conversion process are the pretreatment, hydrolysis, fermentation and distillation. Consequently, the main aim of the research focused in investigating the extraction of ethanol from vegetable and fruit wastes, which make up a large portion of agricultural waste in Mauritius, in an ecofriendly and profitable way. The objectives were the efficient pretreatment of the waste, optimisation of the hydrolysis and fermentation process using sulphuric acid and Saccharomyces cerevesiae respectively, and detoxification of the hydrolysate using activated charcoal to yield maximum alcohol, and separation and purification of the fermenting medium for alcohol recovery.
Materials and methods
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Feedstock collection and preparation
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The vegetable and fruit wastes were collected from the local market and stored in plastic bags prior to its use and preparation of the extract. They were properly washed with tap water to remove foreign objects such as rocks, impurities and dirt. The leftovers consisted of equal masses of beetroot leaves and stems, cauliflower and outer cabbage leaves, and peeling and topping of pineapples. These wastes were chosen because of their wide availability and non-consumable aspects. The mixtures were homogeneous and dried in hot air oven at 60 C for 24–48 h to aid preservation and storage.
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Feedstock characterisation
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Prior to the extraction, the wastes were processed for determination of approximate composition. The mixture was analysed for moisture content, amount of total solids, ash content, volatile matter, and lignin, hemicellulose and cellulose content. Each analysis was done in duplicate for comparison.
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Pretreatment
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The first step for alcohol production from agricultural wastes was size reduction. Shredding was found to give uniform pieces and improved the efficiency of the conversion process by
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Table 1 Milligrams of reducing sugars required to reduce 10 ml Fehling’s solution (Eynon and Lane’s Method). Mg reducing Sugar per 100 ml solution, when concentration of sucrose is— 0 0.5 g 1g 2g 3g 4g
5g
10 g
25 g
50 g
15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. Calculated by extrapolation
336 316 298 282 267 255 243 232 222 213 205 197 190 184 178 172 166 161 157 152 148 144 140 137 133 130 127 124 121 119 116 114 111 109 107 105
317 297 280 264 250 238 227 216 207 198 190 183 176 170 165 159 154 149 145 140 136 133 129 126 122 119 116 114 111 108 106 104 102 99 97 95
307 288 271 256 243 231 220 210 200 192 184 177 170 164 159 153 148 143 139 135 131 127 124 120 117 114 111 109 106 103 101 99 96 94 92 90
289 271 255 240 227 215 206 196 187 179 171 164 158 152 147 142 137 132 128 124 121 117 114 111 107 104 102 99 97 94 92 90 88 86 84 82
275 257 241 227 215 204 194 185 176 168 161 155 149 143 138 133 129 125 121 117 113 109 106 103 100 97 95 92 90 88 86 84 82 81 79 77
333 312 295 278 264 251 239 228 219 210 202 194 187 180 174 168 163 158 153 149 145 141 137 134 130 137 124 121 118 116 113 111 108 106 104 102
329 309 291 274 260 248 236 225 216 207 198 191 184 178 171 166 161 156 151 147 143 139 135 131 123 125 122 119 116 114 111 109 106 104 102 100
325 305 287 271 257 245 233 222 213 204 196 189 182 175 169 164 159 154 149 145 141 137 133 130 126 123 120 117 115 112 110 107 104 103 102 100
76 77 78 79 80 81 82 83
86 87 88 89
92 93
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Ml Sugar solution required
335 314 296 280 265 253 241 230 220 211 203 195 189 182 176 170 165 160 155 151 147 143 139 135 132 129 125 123 120 117 114 112 110 108 106 103
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322 301 284 268 254 242 230 220 210 202 194 186 179 173 167 161 157 152 147 143 139 135 131 128 124 121 118 116 113 110 108 105 103 101 100 98
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About 0.2 g urea and 2–3 drops of phosphoric acid were added to the hydrolysate after adjusting the pH [11]. Since yeast digests sugars in the absence of oxygen to produce ethanol and carbon dioxide, the fermenting flask was securely sealed with a rubber stopper to favour an anaerobic condition. However, to prevent built-up of pressure due to the formation of carbon dioxide gas during the fermentation process, plastic tubing was inserted in the stopper to allow the gas to move to a small bottle containing water. To maximise the alcohol yield, we determined the optimal conditions for the reaction to proceed at a favourable rate.
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Dilute acid hydrolysis The wastes were first treated with dilute sulphuric acid in the range of 1–5% (w/w) at a temperature of 121 C and a pressure of 1.2 bars for about 1.5 h to recover maximum hemicelluloses and celluloses in a standard autoclave. Cotton and aluminium foil were used to block the opening of the flask, thereby preventing evaporation at the high operating temperature of the autoclave. After the hydrolysis process, the mixture was then separated into liquid and solid. The solid phase was manually pressed for maximum liquid recovery, and was tested for reducing sugar, while the solid phase was discarded.
Yeast concentration The yeast concentration was varied at 1, 3 and 5% (w/v). Samples were then taken from the set-up to test for the amount of ethanol.
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pH
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pH was varied at: 3, 4, 5 and 6 at the optimum yeast concentration to yield maximum alcohol.
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Temperature Incubation temperature was then varied at: 28, 30, 33, 35, 37 and 40 C at optimum pH value and yeast concentration.
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Biomass loading Biomass loading may be defined as the variation in mass of dried waste mixture used. The dilute acid hydrolysis process was repeated using the optimum acid concentration but with varying biomass loading of 5%, 10%, 15%, 20% and 30% (w/w). Consequently, the concentrated acid hydrolysis was carried out using the optimum biomass loading.
Retention time The fermentation period was varied from 24 to about 96 h.
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Distillation
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Ethanol was recovered from the fermented mixture by distillation, which was achieved using a rotary-evaporator (‘Rotavap’). The apparatus works by boiling the liquid mixture of water and ethanol in a hot water bath maintained at 78–79 C. The distillate was collected and analysed for ethanol concentration using an ebulliometer as shown in Fig. 1.
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increasing the surface area and reducing cellulose crystallinity. Thus, the initially dried vegetable and fruit waste was mechanically pretreated using a pulveriser, and was sieved to a uniform size of about 2 mm. The resulting sample was then reserved for the hydrolysis process. Hydrolysis Acid hydrolysis was carried out using both dilute and concentrated sulphuric acid.
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Concentrated acid hydrolysis Concentrated acid hydrolysis was achieved in a heat resistant conical flask at atmospheric pressure. According to Balat, for concentrated acid hydrolysis, the range of acid concentration lies in the range of 10–30% [9]. Therefore, the pretreated biomass together with concentrated sulphuric acid in the range of 20–30% (w/w) was heated to about 100 C in a water-bath for 50 min using a hot plate. Subsequently, we allowed the sample to cool before the filtration process for the recovery of the hydrolysate which was tested for reducing sugar.
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Test for reducing sugar
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The Original Lane and Eynon method was used for estimating the amount of reducing sugar in the hydrolysate. The method is a short and rapid one for estimating the amount of reducing sugar. It is based on a determination of the volume of a test solution required to reduce completely a known volume of alkaline copper reagent. The end point is indicated by the use of an internal indicator, methylene blue. The amount of reducing sugar is read from Table 1.
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Detoxification
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The acid was neutralised and inhibitors were removed before the fermentation process by two methods which make use of lime and 10% activated charcoal, respectively, as reported by Canilha et al. [10].
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Fermentation
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The fermentation process was immediately carried out after the hydrolysis and detoxification process using the hydrolysate yielding the highest reducing sugar. It was accomplished biochemically by Saccharomyces cerevisiae yeasts (SuperStart).
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132 133 134 135 136 137
141 142
146 147 148
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Fig. 1. Chart to read concentration of ethanol from temperature.
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Results and discussion
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Waste characterisation
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The moisture, ash, total solid and pH of the waste were determined and summarised as in Table 2. The results showed that the above mixture has a low solid content due to its high moisture. The high moisture content is mainly due to the increased organic composition of vegetable and fruits making them more prone to decomposition. Thus, the latter has to be dried prior to be used for the conversion process. Moreover, the sample exhibits a low ash content of about 12.48% which is near to the value (15–20%) stated by Mood et al. [4]. The high gross calorific value of 27, 445 kJ/kg of the sample indicates good potential for fuel conversion. Moreover, the high hemicellulose and lignin content show great potential for alcohol extraction energy production, respectively. Thus, about 64% of the waste can be converted to alcohol.
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Dilute acid hydrolysis
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Effect of acid concentration Fig. 2 shows the results of varying acid concentration on the amount of sugar liberated. It can be seen that the amount of sugar liberated increases considerably as the acid concentration is increased, though there is no significant difference in the value obtained with 2% and 3% (w/w) acid. However, the steep gradient indicates a significant increase in the amount of sugar produced from 0% to 2% (w/w) acid. This is followed by a partial stabilisation of the sugar formed from 2% to 3% (w/w). An increase in acid concentration above 3% (w/w) resulted in a quite significant increase in sugar produced again, as shown by the less steep gradient (397) from 3% to 5% (w/w). The hydrolysed material using 4% (w/w) acid could not be tested for reducing sugar since the sample was almost dry after the hydrolysis process. This might have occurred due to evaporation when exposed to the high temperature and pressure. It can also be observed from the plot that the optimum concentration of reducing sugar (about 4510 mg/ 100 ml solution) is produced from treatment with 5.0% (w/w) H2SO4. Further increase in acid beyond 5% (w/w) resulted in a decrease in the amount of reducing sugar liberated. This may be due to the severity of the acid which leads to the formation of undesired b yproducts like furfurals and HMF, and consequently leading to degradation of sugars. Due to the acidic medium of the hydrolysate, the formation of inhibitors could not be avoided. To minimise the latter and hence maximising ethanol yield, other methods which do not utilise chemicals could be considered. A control was set-up using distilled water instead of the acid to investigate this phenomenon. The control was maintained under similar operating conditions as those using acid. About 1329 mg reducing sugar per 100 ml solution sugar was liberated. The advantage is that there is no inhibitors formation and the rate of
180 181 182 183 184 185 186 187 188 189 190 191
195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225
Fig. 2. Effect of acid variation on sugar concentration.
sugar degradation is negligible since the process makes use of no chemicals. However, as compared to those using acid the efficiency of the process was very low, about 3–4 times less. The hydrolysed solution from the control was found to be of lighter brown colour compared to those using acid. This can be explained by the fact that only part of the sugar was liberated. Also, the particle size of the substrate was observed to be unchanged compared to those using acid which resulted in finer particles after the hydrolysis process. Thus, filtration was difficult and time consuming. It can be concluded that acid concentration has an important relationship with the amount of sugar produced and also on the formation of inhibitors. As the acid concentration increases, the rate for both also increases. However, the effect of sugar degradation is highly noticeable when the acid concentration was increased at and beyond 5.0% (w/w).
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Effect of biomass loading Fig. 3 illustrates the effect of varying the mass of biomass while keeping the total working volume constant. It shows an increasing trend in the amount of reducing sugar liberated as the mass of biomass increases, up to the peak having the maximum amount of about 4600 mg/100 ml solution at 19–20% biomass loading. The amount of reducing sugar produced was found to decrease with further increase in biomass loading. This may be because with the 30% biomass loading the amount of acid used was insufficient to efficiently destroy the shell protecting the hemicellulose and cellulose. This result is similar to those reported by Binod et al. [12]. In addition, after hydrolysis in the autoclave the solid absorbed most of the acid. The results obtained with 5% and 10% have no significant differences and are low compared to those at 15% and 20% biomass. The low values may be because of the high amount of acid causing dilution of the reducing sugar produced. It can be concluded that a biomass to liquid ratio of about 1:10 yield to maximum reducing sugar which is the case with the
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Table 2 Summary of the main characteristics of the agricultural wastes. Characteristics
Value
Moisture content/% Total Solid content/% Ash content/% Volatile Solid content/% Heating value/kJ/kg Cellulose content/% Hemicellulose content/% Lignin content/%
88.52 11.48 12.48 87.52 27445 24.92 38.83 36.25 Fig. 3. Effect of biomass loading on reducing sugar concentration.
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20% biomass loading. This finding is in line with that stated by Balat for temperature less than 160 C [9]. The hydrolysis also caused a loss of biomass weight by removing the lignin and due to hydrolysis of the cellulose and hemicellulose. Weight loss was found to be different at varying acid concentration, as shown by Fig. 4. The higher the acid concentration, the more mass loss was recorded and vice-versa.
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Concentrated acid hydrolysis
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The amount of sugar liberated was found to differ, as shown in Fig. 5. The graph shows that the amount of reducing sugar liberated increases as the concentration of acid increases. The increase is significant when acid concentration is increased from 25% to 27% (w/w) to reach a maximum yield of 6122 mg/100 ml at 27%. This finding is close to the research done by Inranmahboob et al. [13], who obtained high sugar yield using 26% by weight sulphuric acid and is in line with Drapcho [14] who stated that maximum digestion should occur in the range of 20–30% acid concentration. However, any further increase in acid concentration was found to cause a significant decrease in amount of sugar produced. Binod et al. stated that the concentrated acid breaks the hydrogen bonding between the cellulose chains, changing its structure [12]. The cellulose is said to be decrystallised, and forms a homogeneous gelatin with the acid. This made the separation process difficult and time consuming. Moreover, due to the high acidity of the hydrolysate large amount of sodium carbonate solids and sodium hydroxide were needed for the neutralisation process prior to reducing sugar test and fermentation, respectively. Industrially, the acid has to be recovered, re-concentrated and recirculated to make the process economically feasible, which was not possible at laboratory scale. The high investment and maintenance costs involved in the process have made them commercially unattractive despite its higher yield.
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Fermentation
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Effect of detoxification process The detoxification process was carried out using two methods, overliming and absorption with activated charcoal. However, the one using lime resulted in the formation of a white precipitate (calcium sulphate). This in turn made the filtration process slow, and very little hydrolysate could be collected. Fig. 6 shows the effect of detoxifying the hydrolysate with 10% activated charcoal prior to the fermentation process using baker’s yeast. Sugar degradation is inevitable during acid hydrolysis. As a result, there is formation of by-products including furfurals and 5hydroxymethlyfurfural (HMF), which are known to have inhibitory effect on the rate of fermentation process. This is in line with other
Fig. 4. Mass loss as a result of hydrolysis.
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Fig. 5. Effect of concentrated acid variation on sugar concentration.
Fig. 6. Effect of detoxification of yield of ethanol.
researches, which state that inhibitors formation is favoured as acid concentration and temperature increase [15,16]. The bar chart shows that higher yield of ethanol was achieved after detoxification process. Therefore, this indicates that these compounds are harmful to fermenting microorganisms since they slow down their metabolism. In addition, Lenihan et al. stated that the by-products penetrate the cells and delay cell growth and reproduction [16]. Thus, the eliminations of these harmful compounds are important to increase the efficiency of the fermentation process and subsequently, ethanol yield. The solution was found to decolourise after treatment with activated charcoal. Moreover, the unpleasant smell of the solution disappeared. The efficiency of the common Baker’s yeast was compared with the latter after detoxification. The concentration of ethanol produced was found to increase from 0.46% to 0.97% (v/v) when SuperStart yeasts were used, as illustrated in Fig. 7. Consequently, further experiments were carried out using SuperStart yeasts.
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Effect of yeast concentration Fig. 8 depicts the effects of varying the yeast concentration on the amount of ethanol produced while maintaining other operating conditions constant. The temperature was maintained at 33 C at pH 5.0 throughout the process, since they were known to be optimum for enzyme growth and reproduction. The graph shows that an increase in yeast concentration leads to a significant increase in ethanol production. From the current trend, it can be predicted that the ethanol concentration would increase as the yeast loading would be increased until yeast metabolism is inhibited upon reaching a certain threshold value. The maximum yeast loading was therefore assumed to be 5% (w/v),
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The results showed that the pH of the medium had a noticeable effect on the final ethanol concentration which was found to be greatest when the pH of the reaction was at a lower acidic level. At pH 3.0, the amount of ethanol produced was very low. One reason could be that the low extracellular pH was destructive to the yeasts, as mentioned by Narendranath and Power [17]. There was a significant increase in the ethanol yield as pH was increased from a value of 3.0–4.0. As the pH level was further increased, the reaction yield also rose to an optimum value at pH 5.0, whereby the yeast reproduction and growth is favoured due to the slightly acidic environment. However, beyond pH 5.0 the reaction yield was observed to decrease by about 11.2%. This may be due to the small difference between the internal and external pH values, which inhibits yeast growth. The yeast cells require more energy to either pump in or pump out hydrogen ions with the aim to maintain an optimum intracellular pH. This finding is near to those observedby Lin et al. [18]. It can be concluded that a slightly acidic pH, around 5.0, is optimal for yeast reproduction and growth, and consequently fermentation. The low acidity ensures that the yeast functions under minimal internal stress, resulting in lower energy wastage and efficient growth. Therefore the sugars are fermented to ethanol more efficiently.
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Effect of pH on ethanol concentration It is very important for yeast to maintain a constant intracellular pH during growth for effective enzyme activities. The pH variation experiments investigated how the pH level of the system affected the production of ethanol from yeast. Since enzymes work best in acidic medium due to the acidophilic nature of the yeast and the optimum pH of the Superstart yeast lies between 3.5 and 6.0 (Medine), the pH values were varied within this range. Fig. 9 gives the plot of ethanol concentration against pH values.
Effect of temperature The temperature variation experiment investigated how the production of ethanol from yeast was affected by a range of temperature as illustrated in Fig. 10. The experiments suggested that the rate of fermentation reaction increased as temperature is increased up to about 33 C. There was no significant decrease in ethanol yield from 33 to 35 C. However, any increase in temperature beyond 33 C decreases the rate of reaction. This showed that at temperature between 33 and 35 C, the yeast was under minimal stress, therefore favouring growth and reproduction of cells. The decrease in concentration of ethanol at higher temperature shows that the ability of the yeast to ferment is affected under those conditions. At high temperature the yeast may be under stress, thereby slowing the rate of growth and reproduction. It can be predicted from the current trend that the yield would have further decreased if the temperature would have increased above 40 C. This would be because of the denaturation of enzymes which would restrain yeast metabolism. It can therefore be concluded that the optimum fermentation temperature is around 33 C, at which a maximum yield of 0.96% (v/v) ethanol after fermenting for 24 h, and higher temperature is unfavourable to the fermenting yeast. One reason may be due a lack of agitation causing the efficiency of the fermentation process
Fig. 9. Effect of pH on yield of ethanol.
Fig. 10. Effect of temperature on ethanol yield.
Fig. 7. Comparison between Baker’s and SuperStart yeast.
Fig. 8. Effect of yeast concentration on yield of ethanol. 333 334 335 336 337 338 339 340 341 342 343
which gave an approximate ethanol yield of 1.45% (v/v) after 48 h of fermentation.
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Fig. 11. Effect of retention time on ethanol yield.
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to be low. As a result, not all the yeast was able to come in contact with the sugars present. Effect of retention time The yield of ethanol production also depends on the fermentation period. The effect of retention time on the yield of ethanol is as shown in Fig. 11. The graph shows that as the number of fermentation hours is increased, the yield of ethanol is boosted due to a higher yeast metabolism. The concentration of ethanol rises until no more reducing sugar is left in the fermenting medium indicating end of the fermentation process. Therefore, any further augmentation in retention time will either keep the ethanol yield unchanged or could cause a possible decrease in ethanol yield due to contamination and formation of by-products. The optimum fermentation period was therefore found to be between 72 and 88 h, with maximum ethanol production of about 1.88% (v/v) after 80 h of fermentation.
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Environmental assessment
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Ethanol production from lignocellulosic biomass has several advantages which have dramatically increased the world ethanol production in recent years. Its oxygenated nature (35% oxygen) favours complete fuel combustion, thus producing cleaner emissions [19]. Unlike fossil fuels, the combustion of bioethanol does not contribute to the net increase of carbon dioxide in the atmosphere since they form part of the natural closed carbon cycle [20]. Walker also mentioned that bioethanol production and consumption needs a positive Net Energy Ratio (NER) in order to maintain environmental sustainability, which is the case for bioethanol produced from lignocellulosic biomass and other biowastes materials [19]. Bioethanol can either be used in its pure form or blended with gasoline in different proportions. The use of ethanol-blended fuel is one of the best pollution control strategies due to its high heat of vapourisation, high octane number and low cetane number [5]. The use of the high octane fuel shifts octane boosters, including carcinogen like benzene. In addition, bioethanol is biodegradable and contains no sulphur additives [21]. It produces cleaner combustion products unlike petroleum fuels; and reduces particulates, carbon monoxide, hydrocarbon, oxides of nitrogen and other toxic emissions. Thus the use of bioethanol significantly reduces death-causing illnesses like cancer, asthma and heartattacks because of its environmentally friendly nature. It can be said that an increased use of bioethanol will reduce greenhouse gases and help to fight against the alarming global warming.
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Economic assessment
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Lignocellulosic feedstock, being abundantly available and cheap, is considered to be the most promising feedstock, with the expectancy of reducing the production costs. The set-up of a biorefinery necessitates huge investment. As a consequence, several approaches exist in order to improve the economy of bioethanol production. One way is to make efficient use of the byproducts generated from the various processes involved. This involves the sale of waste streams, such as carbon dioxide and stillage. In addition, the price of the produced bioethanol should be set at a lower price compared to gasoline to find economic acceptance and be competitive. Chemicals like acids can be recovered and recycled, while the wastewater can be sent for treatment and reused. In addition, the non-fermentable lignin, which represents great potential for energy production due to its high energy value of 29.54 MJ/kg, can be burnt in boilers for coproduction of electricity and steam [23]. Both can meet the energy requirement of the entire plant, while the surplus can be sold to the local grid in the form of electricity thus increasing revenue. The lignin can also be further used for conversion into other valuable products like specialty polymers [24]. The cost of bioethanol production can be further reduced through process integration, making use of larger industrial facilities rather than smaller ones or even integrating the ethanol production to an existing plant [25,26]. Moreover, advancement in pretreatment process and fermenting microorganisms are crucial not only to bring down the cost of production, but also to increase ethanol yield.
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Conclusion
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Recently, there has been a growing interest in biofuels due to depletion of fossil reserves, rising liquid fuel prices, energy security and environmental problems. In light of this, bioethanol production from agricultural wastes is gaining popularity due to their abundance and low cost. Using these wastes is not only economically beneficial, but also solves waste disposal problem, thereby preventing the generation of infectious diseases. The study focused on extracting sugar from agricultural wastes, mostly vegetable and fruit wastes, and consequently fermented the released sugar to ethanol by the use of S. cerevesiae. Two chemical methods were used to achieve the hydrolysis process and the amount of sugar liberated was compared. It can be inferred that concentrated acid hydrolysis is a dangerous and uneconomical method, despite the high sugar yield, as it involves the use of concentrated sulphuric acid which is highly corrosive. Thus, dilute acid hydrolysis is safer and more cost effective when applied on a large scale. The effect of various parameters like acid concentration, biomass loading, temperature, pH, retention time, and yeast concentration on the final ethanol yield were examined, in order to determine the optimum condition for the extraction process. Optimisation of acid concentration showed that 5% (w/w) liberated the maximum amount of fermentable sugar during the hydrolysis process, at the expense of inhibitors formation. A maximum of 1.88% (v/v) ethanol was produced with 20% (w/w) biomass and 5% (w/v) yeast loading, after 80 h of fermentation at 33 C. Agricultural wastes are expected to lower the production cost of ethanol and provide stability to supply and price. However, to increase the yield of ethanol conversion, significant barriers need to be addressed. Proper and cost effective pretreatment and hydrolysis technologies, with the use of minimum chemicals and energy inputs, are important. Bioethanol being locally available will help in reducing the dependency on imported petroleum for automobiles. As future work, enzymatic hydrolysis, which is considered to the state-of-art technology for bioethanol
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production could be employed to compare its efficiency with acid hydrolysis. An array of research areas have to be brought together, such as science, engineering and economics, to make low-cost and environment-sound bioethanol production from agricultural wastes a reality in Mauritius in the near future.
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Uncited references
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