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Process model and economic analysis of ethanol production from sugar beet raw juice as part of the cleaner production concept
Bioresource Technology 104 (2012) 367–372
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Process model and economic analysis of ethanol production from sugar beet raw juice as part of the cleaner production concept Damjan G. Vucˇurovic´ ⇑, Siniša N. Dodic´, Stevan D. Popov, Jelena M. Dodic´, Jovana A. Grahovac Faculty of Technology, Department of Biotechnology and Pharmaceutical Engineering, University of Novi Sad, Boulevard cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
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Article history: Received 16 July 2011 Received in revised form 13 October 2011 Accepted 22 October 2011 Available online 4 November 2011 Keywords: Biofuels Ethanol Model Economics Fermentation
a b s t r a c t The batch fermentation process of sugar beet processing intermediates by free yeast cells is the most widely used method in the Autonomous Province of Vojvodina for producing ethanol as fuel. In this study a process and cost model was developed for producing ethanol from raw juice. The model can be used to calculate capital investment costs, unit production costs and operating costs for a plant producing 44 million l of 99.6% pure ethanol annually. In the sensitivity analysis the influence of sugar beet and yeast price, as well as the influence of recycled biomass on process economics, ethanol production costs and project feasibility was examined. The results of this study clearly demonstrate that the raw material costs have a significant influence on the expenses for producing ethanol. Also, the optimal percentage of recycled biomass turned out to be in the range from 50% to 70%. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ethanol has experienced unseen levels of attention due to its value as fuel alternative to gasoline, the increase of oil prices, and the climatic changes, besides being a renewable and sustainable energy source, efficient and safe to the environment (Mussatto et al., 2010). The processing of sugar beet toward bioethanol could be a convenient process, but is important to emphasize that, to be a viable alternative, bioethanol must present a high net energy gain, have ecological benefits, be economically competitive and able to be produced in large scales without affecting the food provision (Šantek et al., 2010). Although ethanol production process models by different techniques and from different resources such as sugarcane (Quintero et al., 2008), corn (Kwiatkowski et al., 2006; Li et al., 2010; Lin et al., 2011a,b), wheat (Francis, 2006), soybean (Sequeira et al., 2008), barley (Nghiem et al., 2011), lignocellulosic feedstock (Morales-Rodriguez et al., 2011) and even glycerol (Posada and Cardona, 2010) are more frequent, the favourable and most accessible resource for the region of Southeastern Europe is sugar beet (Icoz et al., 2009; Krajnc et al., 2007). According to Van der Poel et al. (1998) the sucrose content in raw juice is between 13.5% and 19.5%. However, the sugar content of raw juice from domestic sugar factories is significantly lover (11.0–14.0%) due to the bad quality of sugar beet, which is a result of inappropriate application of agrotechnical measures over the past few decades (Milic´ et al., 2006). For this reason significantly ⇑ Corresponding author. Tel.: +381 21 4853682; fax: +381 21 450413. E-mail address: [email protected] (D.G. Vucˇurovic´). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.10.085
greater amount of water is being used during the extraction of sugar from sugar beet cassettes and therefore a low concentration of sucrose in raw juice is inevitable (Hinkova and Bubnik, 2001). The Autonomous Province of Vojvodina has great potential for producing energy from renewable resources. Energy production from renewable resources in the Autonomous Province of Vojvodina is in the early stages and therefore requires an adequate approach and strategic planning (Dodic´ et al., 2010). Together with the technological modernisation and energy efficiency increase, the use of renewable energy resources is one of the priorities and the Autonomous Province of Vojvodina as part of the Republic of Serbia, which is a signatory to the Kyoto Protocol, has committed to increase the share of energy produced from renewable resources from 1% to 20% by 2012. The Republic of Serbia has about 5.092 million ha of agricultural land (0.68 ha per capita) of which 4.218 million ha is arable land (0.56 ha per capita), which is above the standards of European countries. About 10% of arable land belongs to the state and state-owned enterprises, while 90% is privately owned (Bulletin 523, 2010). Suitable soil and climatic conditions allow the development of diversified agricultural plants, i.e. the cultivation of cereals, industrial crops, fruits and vegetables, seed and planting material and medicinal plants. The total sugar consumption per capita is about 25–30 kg which is, in the Republic of Serbia, on an annual basis about 240,000 tons of sugar. However, production capacities of domestic sugar factories are much higher than the total sugar consumption. Since the estimates for 2011 for sugar beet yield are 48 Mg/ha, a total sugar beet production of 3206,919 Mg or about 480,000 Mg of produced
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sugar is expected (Report SRB233 PO15, 2011). This means that besides the annual sugar consumption of 240,000 Mg and the maximum annual export of sugar to the EU of 180,000 Mg there is still a surplus of 60,000 Mg of sugar, i.e. 520,000 Mg of raw juice that can be used for bioethanol production. The work presented here, thus, provides a simulation solution for a bioethanol production plant with minimal waste generation. A sensitivity analysis was also performed to examine the effect of sugar beet and yeast price on the final ethanol production cost. The effect of the amount of recycled biomass on process economics was also examined. The process and cost models were developed using SuperPro DesignerÒ software version 6.0 (Intelligen Inc., Scotch Plains, NJ), a simulation programme that is able to estimate both process and economic parameters. 2. Methods 2.1. Process overview The simplified process flow sheet of ethanol production from sugar beet raw juice is shown in Fig. 1. During campaign, the raw juice generated after the extraction of sugar beet cassettes in a sugar production plant is charged directly into the fermentor. Since sugar beet is harvested for a short period each year, long term storage is required to provide feed to the factory. However, raw juice is known for its low storability and easy decomposition by the action of microorganisms (Dodic´ et al., 2009) so raw juice needs to be concentrated by multiple effect evaporators to a high sugar concentration to reduce storage volume and inhibit microbial growth. These evaporators are taken into account in this process model (not shown in Fig. 1) even though they are part of the sugar factory, which is located near the ethanol production plant. Therefore, during the rest of the year, the concentrated raw juice needs to be diluted before use. Commercial baker’s yeast Saccharomyces cerevisiae is used as the biocatalyst, i.e. will ferment sugars to ethanol and other by-products. Six vessels are used in stagger mode as fermentors. After fermentation the broth is sent to centrifugation for biomass separation. The solid fraction, mostly containing yeast cells, is then split in two ways, one going back as inoculum for the following fermentation process and the other is sent to further processing.
The fractions of recycled biomass, that were considered in this work, are 10%, 30%, 50%, 70% and 90%. On the other hand, the liquid fraction of the centrifugation operation is sent to distillation and molecular sieve adsorption to recover ethanol and produce 99.6% ethanol. Distillation is accomplished in two columns. The first column removes the dissolved CO2 and most of the water, while the second concentrates the ethanol to a near azeotropic composition. All the water from the nearly azeotropic mixture is removed by vapour phase molecular sieve adsorption. Finally, the 99.6% pure ethanol vapour is cooled by heat exchange, condensed and pumped to storage. Fermentation vents (off gas containing mostly CO2, water and also some ethanol) as well as the beer column vent are scrubbed in a water scrubber, recovering nearly all of the carbon dioxide. The bottoms from the first distillation contain all the unused organic matter (sources of sugar, nitrogen, etc.). This stillage is concentrated in a multiple effect evaporator using waste heat from the distillation. The concentrated syrup from the evaporator is mixed with the portion of the split biomass not sent back to the fermentation section and fed to a rotary drum dryer where it is dried to produce the co-product which will be sold as animal feed. On the other hand, the evaporated condensate from the stillage concentration operation is mixed together with the bottoms from the second distillation, the liquid outlet of the molecular sieves and the liquid outlet of the scrubber, and reused as process water, due to the relatively low content of organic matter in these streams. One twentieth of this water is sent back to the scrubber, while the rest is used for diluting the concentrated raw juice (from storage) before fermentation. 2.2. Process design Input data and data on operating conditions of the examined process are obtained from literature, while equipment and process data are obtained directly from the SuperPro Designer software. The raw juice requirement for the plant is 300 Mg per batch, which is obtained by dividing the potential annual surplus of raw juice in Vojvodina (Report SRB233 PO15, 2011) with the maximum number of batches per year predicted by the model. The fermentable sugar content in the raw juice is set to 12.5% by diluting the concentrated storage juice, since this concentration proved to be
Fig. 1. Simplified process flow diagram of ethanol production from sugar beet raw juice.
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optimal for ethanol production from raw juice (Popov et al., 2010). Ash and protein content in the raw juice is set to 0.03% and 0.01%, respectively, according to the average composition of raw juice from several domestic sugar production plants (Popov et al., 2010). Initial inoculum concentration is set to be about 10% of biomass quantity in relation to the amount of cultivation media. Fermentation is conducted in six 385,000 L (16.5 m height and 5.5 m diameter) vessels. The process time is 20 h and the set temperature is 28 °C for the sugar fermentation. The reactions and conversions in the process of fermentation are defined by Popov et al. (2010). The effective ethanol concentration in the fermentation broth is 6.15% while the sugar concentration is 0.23%. Fermentation broth is sent to a disc-stack centrifuge in order to remove most of the biomass (throughput 157,000 L/h). The biomass stream is then split (throughput 20,000 kg/h) and one part (base case 50%) is sent to drying while the other is sent back to the fermentation section. After centrifugation the liquid fraction is sent to a distillation column with a height of 13.4 m and diameter 5.3 m, that operates in a mode to remove the CO2 and as little ethanol as possible overhead, while removing about 86% of water to the bottoms (Aden et al., 2002). The reflux ratio is 3:1. The overhead of the distillation column is vented and contains 91.61% CO2, 8.13% ethanol and 0.25% water. All of the CO2 and only 3.97% of ethanol are vented here. Over 96% of the ethanol fed is removed as a 33.6% mixture with water. The ethanol mixture with water is removed as a vapour side draw from the column and fed directly to the rectification column which has a working volume of 4900 L. A vapour overhead mixture of 92.5% ethanol is obtained while the composition of the bottoms is 0.31% ethanol. Only 0.6% of the ethanol fed is lost in this operation. Overhead vapour from the rectification column is fed to the molecular sieve adsorption unit. The adsorption column removes 95% of the water (Aden et al., 2002). The 99.6% pure ethanol is cooled to 20 °C by heat exchange with a heat transfer area of 280 m2 and finally pumped to storage. The heat transfer agent is chilled water while the heat transfer coefficient is 860.44 W/m2 K. A scrubber with a volume of 25,500 L (height 12.8 m and diameter 1.6 m) is used to remove ethanol and water from the fermentation and distillation vents so as to recover pure CO2. The stillage from the distillation column is concentrated in the evaporator with a heat transfer area of 75.6 m2 and saturated steam as transfer agent, removing over 90% of water which is used as process water after condensation. Part of the biomass sent to drying is mixed with the syrup obtained after evaporation (throughput 22,400 kg/ h) prior to being sent to drying. The drying is carried out in a rotary drum with a drying capacity of 996 kg/h, drum area 85.5 m2 and air as drying gas (Kwiatkowski et al., 2006). Rectification bottoms and molecular sieve liquid outputs are mixed with an operating throughput of 59,000 kg/h and then mixed with the evaporation condensate and the scrubber liquid outlet giving a total of 280,000 kg/h process water that can be recycled. One part (15,000 kg/h) of this water is reused in the scrubber, while the rest is used to dilute the concentrated raw juice for the following fermentation process. The design basis is 34.8 million kg (44 million l) of 99.6% pure ethanol per year, which is produced from 564,000 Mg of raw juice. The cost model reflects economic conditions in the second half of 2010. 2.3. Process analysis Economic analysis was performed for the proposed process model and is given as an executive summary of project indices. Sensitivity studies were also performed for this process model at various sugar beet costs and yeast costs. The cost of sugar beet se-
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lected for this study ranged from $0.017/kg to $0.029/kg. This range was selected because of the fluctuations of its purchase price in the period of 2000–2009 (Price Report, 2010). Yeast cost depends from the quantity needed, so its purchase price offered from a local yeast production plant ranged from $1.144/kg to $1.43/kg. Finally, the influence of the biomass split factor on economic parameters was examined.
3. Results and discussion 3.1. Economic analysis The results of the economic analysis are shown in Table 1. The individual components (capital investment cost, operating cost and unit production cost) are discussed in the following paragraphs. The total capital investment cost for this production plant is obtained by adding together the values of the direct fixed capital (DFC), working capital and start-up and validation costs. DFC represents the sum of the total plant cost (TPC) and costs for contractor’s fees and contingency (CFC). Further, the TPC consists of the total plant direct cost (TPDC) and the total plant indirect cost (TPIC). While the TPIC depends on the costs for engineering and construction, the TPDC expenses include the costs for equipment purchase, installation, process piping, instrumentation, insulation,
Table 1 Economic analysis of the proposed process model. Capital investment cost ($) Direct fixed capital DFC = TPC + CFC ($) Total plant cost TPC = TPDC + TPIC ($) Total plant direct cost TPDC ($) Equipment purchase cost ($) Fermentors ($) Disc-stack centrifuge ($) Batch distillation vessel ($) Scrubber ($) Rectification column ($) Molecular sieves ($) Evaporators ($) Rotary drum dryers ($) Condenser ($) Unlisted equipment ($) Installation ($) Process piping ($) Instrumentation ($) Insulation ($) Electrical ($) Buildings ($) Yard improvement ($) Auxiliary facilities ($) Total plant indirect cost TPIC ($) Engineering ($) Construction ($) Contractor’s fee & contingency CFC ($) Contractor’s fee ($) Contingency ($) Working capital ($) Start-up and validation cost ($)
68,778,000 64,080,000 55,722,000 34,826,000 10,896,000 3000,000 1475,000 478,000 57,000 525,000 2700,000 548,000 966,000 57,000 1090,000 4644,000 3269,000 4141,000 327,000 1090,000 4685,000 1634,000 4141,000 20,896,000 8707,000 12,189,000 8358,000 2786,000 5572,000 1494,000 3204,000
Operating cost ($/year) Raw materials ($/year) Yeast ($/year) Raw juice ($/year) Facility-dependent ($/year) Labour ($/year) Laboratory/QC/QA ($/year) Utilities ($/year) Electricity ($/year) Steam ($/year) Chilled water ($/year) Co-production
20,982,000 2037,000 424,000 1612,000 7068,200 6968,100 278,500 7427,200 4118,600 995,600 2313,000 2797,000
Unit production cost ($/kg)
0.603
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electrical, building, yard improvement and auxiliary facilities. The purchasing cost for major equipment items listed in Table 1 is based on offers from the suppliers of equipment or taken from the report of Aden et al. (2002). If the capacities of the equipment in the model were different from the equipment capacities offered by the suppliers, the cost was corrected by using the Rule of Sixtenths (Dysert, 2003). The following equation expresses this rule:
CB ¼ CA
0:6 SB SA
ð1Þ
where CB is the approximate cost ($) of equipment having size SB (L, m3, or whatever) and CA is the known cost ($) of equipment having corresponding size SA (same units as SB). Besides capacity, the type of construction material and specific characteristics of the equipment can change the purchasing equipment cost (Nghiem et al., 2011). The estimated capital cost for this case is $68,778,000, which is over $20 million more than the capital costs determined for a corn to ethanol production plant by Kwiatkowski et al. (2006). This difference exists because this group of authors had an industry feedback to estimate the total capital investment cost as approximately three times the sum of individual equipment purchase cost and installation expenses. By applying the same calculation method as Kwiatkowski et al. (2006) to this case the capital cost data would match, i.e. the capital investment would be nearly the same. Operating costs are calculated by summing the raw material and utility costs, costs that are facility-dependent (equipment availability, maintenance, insurance, local taxes, etc.), labour costs and laboratory, quality control (QC) and quality assurance (QA) expenses, and subtracting a credit for the sale of co-products (Table 1). Compared to the annual ethanol production costs from molasses (Sobocˇan and Glavicˇ, 2000), the operating costs of producing ethanol from raw juice are slightly lower. This difference exists due to the fact that molasses is a by-product, while raw juice is a intermediate product of sugar beet processing, i.e. molasses is obtained from raw juice after its purification (micro and ultra filtration), concentration (evaporation) and multistage crystallization. In other words, the operating costs are higher for molasses to ethanol production because of these additional operations. The primary feedstock for the facility is raw juice. Yeast that ferments the glucose into ethanol and carbon dioxide is also required. The quantities of materials required are provided by the process model and their unit costs have been incorporated into the model. Information about various feedstock prices can be collected from the Statistical Office of the Republic of Serbia. Electricity, steam and chilled water are the utilities required in the process. Utility requirements of the various equipment operations are calculated and summed within the programme. These utilities are treated as purchased utilities and the unit costs for each of them were set based on 5 year average market prices. The 5 year average market prices (2009–2005) of steam ($2.10/ Mg), chilled water ($0.30/Mg) and electricity ($0.10/kWh) are obtained from the annual reports of the Power Industry of Serbia1 and used in the process model. Three products are produced in the conversion of raw juice to ethanol. They are ethanol as the main product, and animal feed and carbon dioxide as co-products. The ethanol can be sold to be blended with fossil fuels. Current pricing for it is available from the Biomass Action Plan (2010). The assumed ethanol price in this work is 0.75 $/kg (0.59 $/L). The animal feed is sold and its value is strongly dependent on its protein content and is related to the value of other protein-based animal feeds. The animal feed pro1 Power Industry of Serbia, Annual Reports from 2005 till 2009. Database at http:// www.eps.rs/Pages/FolderDocs.aspx?list=Godisnji%20Izvestaji, accessed at June 2011.
duced in this process has a protein content of approximately 21% and its value, based on its proteins content, is estimated to be $0.07/kg (Belyea et al., 2004). The second co-product, carbon dioxide, is economically viable only if it is collected from the scrubber and sold to a third party at the plant site for further purification and distribution to the food processing companies and manufacturers of carbonated beverages. In this case its value to the ethanol producer is $15 per metric ton (Kwiatkowski et al., 2006). A breakdown of the costs to produce ethanol with the values used in this model case is illustrated in Fig. 2. It shows that the unit production cost of ethanol is approximately $0.603/kg, which corresponds with the results reported by Sobocˇan and Glavicˇ (2000). Fig. 2 also shows that the income from the sale of animal feed reduces the ethanol production costs, and is a major factor in operating costs. Since the cost of sugar beet can fluctuate due to market and weather conditions, the influence of sugar beet cost was also examined in the sensitivity study. 3.2. Sensitivity study For this process model the simulation software gave an executive summary of the project calculating the project indices shown in Table 2. Gross margin represents a company’s total sales revenue minus its cost of goods sold, divided by the total sales revenue, expressed as a percentage. This number represents the proportion of each dollar of revenue that the company retains as gross profit. For this process the gross margin is 18.01%, which means the company would retain $0.18 from each dollar of revenue generated. The ROI (Return of Investment) is a performance measure used to evaluate the efficiency of an investment. ROI is a very popular metric because of its versatility and simplicity. That is, if an investment does not have a positive ROI, then the investment should not be undertaken. The length of time required to recover the cost of an investment can be calculated on the basis of the ROI. Table 2 shows that the payback time is approximately 7.5 years, which is quite acceptable. NPV (Net Present Value) represents the difference between the present value of cash inflows and the present value of cash outflows. It is used to analyse the profitability of an investment through considering the time-value of the earned money, because a dollar that is earned in 5 years time has a lower value than a dollar earned today (Heinzle et al., 2006). If the NPV of a prospective project is positive (like in Table 2) with assuming a discount interest of 7%, it should be accepted and realised. The IRR (Internal Rate of Return) is a discount rate often used in capital budgeting that makes the NPV of all cash flows from a particular project equal to zero. In other words the higher a project’s IRR, the more desir-
Fig. 2. Ethanol production cost breakdown.
D.G. Vucˇurovic´ et al. / Bioresource Technology 104 (2012) 367–372 Table 2 Project indices of the ethanol production process model. Project indices
Value
Gross margin (%) Return on Investment (%) Payback time (years) Internal Rate of Return after taxes (%) Net Present Value at 7.00% ($)
18.01 13.41 7.46 7.73 2955,000
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able it is to undertake the project. This project yields an after-tax IRR of 7.73%, with income taxes set to 40%. Based on these results, with a project lifetime of 25 years and depreciation period of 10 years with a salvage value 5% DFC, this project represents a very attractive investment, i.e. the examined system is economically viable. The influence of sugar beet and yeast price on ethanol production costs is shown in Fig. 3. It can be seen that lower sugar beet and yeast costs result in lower unit production costs. Also, the sugar beet cost has a much higher impact on the expenses of producing ethanol. However, depending on the market conditions the management can regulate the split factor and decide how much biomass is going to drying (animal feed) or back to fermentation after centrifugation. The effect of the biomass split factor on economic parameters is shown in Fig. 4. By recycling more biomass reduces the quantity of yeast needed for fermentation and lowers the costs for purchasing yeast (even if the price is higher for smaller quantities) but on the other hand causes lower income for selling animal feed (coproduct). Still, the total revenues vary only by $1,000,000 per year depending on the split factor. However, the ethanol production costs are higher with the increase of recycled biomass. Because of the greater curve slope of the unit production cost for the values of the split factor in the range from 50% to 70% and based on the highest IRR this range would be optimal for varying the biomass split factor. 4. Conclusion
Fig. 3. The effect of sugar beet and yeast cost on ethanol production cost.
The process model developed here reflects the base case for a 44 million l/year fuel ethanol plant. A cost model has been developed for economic analysis of this ethanol production from sugar beet raw juice. A sensitivity study has been performed to examine the effects of sugar beet and yeast costs on ethanol production costs. The effect of the amount of recycled biomass on process economics was also examined. The benefits of obtaining lower unit production costs due to lower prices of sugar beet and yeast were clearly demonstrated. The optimal percentage of recycled biomass proved to be between 50% and 70%. Acknowledgements This study is part of the project (TR-31002) which is supported by the Ministry of Education and Science of Serbia. The authors are grateful for the financial support. References
Fig. 4. Process economics as a function of the amount of recycled biomass.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Process model and economic analysis of ethanol production from sugar beet raw juice as part of the cleaner production concept Damjan G. Vucˇurovic´ ⇑, Siniša N. Dodic´, Stevan D. Popov, Jelena M. Dodic´, Jovana A. Grahovac Faculty of Technology, Department of Biotechnology and Pharmaceutical Engineering, University of Novi Sad, Boulevard cara Lazara 1, 21000 Novi Sad, Serbia
a r t i c l e
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Article history: Received 16 July 2011 Received in revised form 13 October 2011 Accepted 22 October 2011 Available online 4 November 2011 Keywords: Biofuels Ethanol Model Economics Fermentation
a b s t r a c t The batch fermentation process of sugar beet processing intermediates by free yeast cells is the most widely used method in the Autonomous Province of Vojvodina for producing ethanol as fuel. In this study a process and cost model was developed for producing ethanol from raw juice. The model can be used to calculate capital investment costs, unit production costs and operating costs for a plant producing 44 million l of 99.6% pure ethanol annually. In the sensitivity analysis the influence of sugar beet and yeast price, as well as the influence of recycled biomass on process economics, ethanol production costs and project feasibility was examined. The results of this study clearly demonstrate that the raw material costs have a significant influence on the expenses for producing ethanol. Also, the optimal percentage of recycled biomass turned out to be in the range from 50% to 70%. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ethanol has experienced unseen levels of attention due to its value as fuel alternative to gasoline, the increase of oil prices, and the climatic changes, besides being a renewable and sustainable energy source, efficient and safe to the environment (Mussatto et al., 2010). The processing of sugar beet toward bioethanol could be a convenient process, but is important to emphasize that, to be a viable alternative, bioethanol must present a high net energy gain, have ecological benefits, be economically competitive and able to be produced in large scales without affecting the food provision (Šantek et al., 2010). Although ethanol production process models by different techniques and from different resources such as sugarcane (Quintero et al., 2008), corn (Kwiatkowski et al., 2006; Li et al., 2010; Lin et al., 2011a,b), wheat (Francis, 2006), soybean (Sequeira et al., 2008), barley (Nghiem et al., 2011), lignocellulosic feedstock (Morales-Rodriguez et al., 2011) and even glycerol (Posada and Cardona, 2010) are more frequent, the favourable and most accessible resource for the region of Southeastern Europe is sugar beet (Icoz et al., 2009; Krajnc et al., 2007). According to Van der Poel et al. (1998) the sucrose content in raw juice is between 13.5% and 19.5%. However, the sugar content of raw juice from domestic sugar factories is significantly lover (11.0–14.0%) due to the bad quality of sugar beet, which is a result of inappropriate application of agrotechnical measures over the past few decades (Milic´ et al., 2006). For this reason significantly ⇑ Corresponding author. Tel.: +381 21 4853682; fax: +381 21 450413. E-mail address: [email protected] (D.G. Vucˇurovic´). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.10.085
greater amount of water is being used during the extraction of sugar from sugar beet cassettes and therefore a low concentration of sucrose in raw juice is inevitable (Hinkova and Bubnik, 2001). The Autonomous Province of Vojvodina has great potential for producing energy from renewable resources. Energy production from renewable resources in the Autonomous Province of Vojvodina is in the early stages and therefore requires an adequate approach and strategic planning (Dodic´ et al., 2010). Together with the technological modernisation and energy efficiency increase, the use of renewable energy resources is one of the priorities and the Autonomous Province of Vojvodina as part of the Republic of Serbia, which is a signatory to the Kyoto Protocol, has committed to increase the share of energy produced from renewable resources from 1% to 20% by 2012. The Republic of Serbia has about 5.092 million ha of agricultural land (0.68 ha per capita) of which 4.218 million ha is arable land (0.56 ha per capita), which is above the standards of European countries. About 10% of arable land belongs to the state and state-owned enterprises, while 90% is privately owned (Bulletin 523, 2010). Suitable soil and climatic conditions allow the development of diversified agricultural plants, i.e. the cultivation of cereals, industrial crops, fruits and vegetables, seed and planting material and medicinal plants. The total sugar consumption per capita is about 25–30 kg which is, in the Republic of Serbia, on an annual basis about 240,000 tons of sugar. However, production capacities of domestic sugar factories are much higher than the total sugar consumption. Since the estimates for 2011 for sugar beet yield are 48 Mg/ha, a total sugar beet production of 3206,919 Mg or about 480,000 Mg of produced
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sugar is expected (Report SRB233 PO15, 2011). This means that besides the annual sugar consumption of 240,000 Mg and the maximum annual export of sugar to the EU of 180,000 Mg there is still a surplus of 60,000 Mg of sugar, i.e. 520,000 Mg of raw juice that can be used for bioethanol production. The work presented here, thus, provides a simulation solution for a bioethanol production plant with minimal waste generation. A sensitivity analysis was also performed to examine the effect of sugar beet and yeast price on the final ethanol production cost. The effect of the amount of recycled biomass on process economics was also examined. The process and cost models were developed using SuperPro DesignerÒ software version 6.0 (Intelligen Inc., Scotch Plains, NJ), a simulation programme that is able to estimate both process and economic parameters. 2. Methods 2.1. Process overview The simplified process flow sheet of ethanol production from sugar beet raw juice is shown in Fig. 1. During campaign, the raw juice generated after the extraction of sugar beet cassettes in a sugar production plant is charged directly into the fermentor. Since sugar beet is harvested for a short period each year, long term storage is required to provide feed to the factory. However, raw juice is known for its low storability and easy decomposition by the action of microorganisms (Dodic´ et al., 2009) so raw juice needs to be concentrated by multiple effect evaporators to a high sugar concentration to reduce storage volume and inhibit microbial growth. These evaporators are taken into account in this process model (not shown in Fig. 1) even though they are part of the sugar factory, which is located near the ethanol production plant. Therefore, during the rest of the year, the concentrated raw juice needs to be diluted before use. Commercial baker’s yeast Saccharomyces cerevisiae is used as the biocatalyst, i.e. will ferment sugars to ethanol and other by-products. Six vessels are used in stagger mode as fermentors. After fermentation the broth is sent to centrifugation for biomass separation. The solid fraction, mostly containing yeast cells, is then split in two ways, one going back as inoculum for the following fermentation process and the other is sent to further processing.
The fractions of recycled biomass, that were considered in this work, are 10%, 30%, 50%, 70% and 90%. On the other hand, the liquid fraction of the centrifugation operation is sent to distillation and molecular sieve adsorption to recover ethanol and produce 99.6% ethanol. Distillation is accomplished in two columns. The first column removes the dissolved CO2 and most of the water, while the second concentrates the ethanol to a near azeotropic composition. All the water from the nearly azeotropic mixture is removed by vapour phase molecular sieve adsorption. Finally, the 99.6% pure ethanol vapour is cooled by heat exchange, condensed and pumped to storage. Fermentation vents (off gas containing mostly CO2, water and also some ethanol) as well as the beer column vent are scrubbed in a water scrubber, recovering nearly all of the carbon dioxide. The bottoms from the first distillation contain all the unused organic matter (sources of sugar, nitrogen, etc.). This stillage is concentrated in a multiple effect evaporator using waste heat from the distillation. The concentrated syrup from the evaporator is mixed with the portion of the split biomass not sent back to the fermentation section and fed to a rotary drum dryer where it is dried to produce the co-product which will be sold as animal feed. On the other hand, the evaporated condensate from the stillage concentration operation is mixed together with the bottoms from the second distillation, the liquid outlet of the molecular sieves and the liquid outlet of the scrubber, and reused as process water, due to the relatively low content of organic matter in these streams. One twentieth of this water is sent back to the scrubber, while the rest is used for diluting the concentrated raw juice (from storage) before fermentation. 2.2. Process design Input data and data on operating conditions of the examined process are obtained from literature, while equipment and process data are obtained directly from the SuperPro Designer software. The raw juice requirement for the plant is 300 Mg per batch, which is obtained by dividing the potential annual surplus of raw juice in Vojvodina (Report SRB233 PO15, 2011) with the maximum number of batches per year predicted by the model. The fermentable sugar content in the raw juice is set to 12.5% by diluting the concentrated storage juice, since this concentration proved to be
Fig. 1. Simplified process flow diagram of ethanol production from sugar beet raw juice.
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optimal for ethanol production from raw juice (Popov et al., 2010). Ash and protein content in the raw juice is set to 0.03% and 0.01%, respectively, according to the average composition of raw juice from several domestic sugar production plants (Popov et al., 2010). Initial inoculum concentration is set to be about 10% of biomass quantity in relation to the amount of cultivation media. Fermentation is conducted in six 385,000 L (16.5 m height and 5.5 m diameter) vessels. The process time is 20 h and the set temperature is 28 °C for the sugar fermentation. The reactions and conversions in the process of fermentation are defined by Popov et al. (2010). The effective ethanol concentration in the fermentation broth is 6.15% while the sugar concentration is 0.23%. Fermentation broth is sent to a disc-stack centrifuge in order to remove most of the biomass (throughput 157,000 L/h). The biomass stream is then split (throughput 20,000 kg/h) and one part (base case 50%) is sent to drying while the other is sent back to the fermentation section. After centrifugation the liquid fraction is sent to a distillation column with a height of 13.4 m and diameter 5.3 m, that operates in a mode to remove the CO2 and as little ethanol as possible overhead, while removing about 86% of water to the bottoms (Aden et al., 2002). The reflux ratio is 3:1. The overhead of the distillation column is vented and contains 91.61% CO2, 8.13% ethanol and 0.25% water. All of the CO2 and only 3.97% of ethanol are vented here. Over 96% of the ethanol fed is removed as a 33.6% mixture with water. The ethanol mixture with water is removed as a vapour side draw from the column and fed directly to the rectification column which has a working volume of 4900 L. A vapour overhead mixture of 92.5% ethanol is obtained while the composition of the bottoms is 0.31% ethanol. Only 0.6% of the ethanol fed is lost in this operation. Overhead vapour from the rectification column is fed to the molecular sieve adsorption unit. The adsorption column removes 95% of the water (Aden et al., 2002). The 99.6% pure ethanol is cooled to 20 °C by heat exchange with a heat transfer area of 280 m2 and finally pumped to storage. The heat transfer agent is chilled water while the heat transfer coefficient is 860.44 W/m2 K. A scrubber with a volume of 25,500 L (height 12.8 m and diameter 1.6 m) is used to remove ethanol and water from the fermentation and distillation vents so as to recover pure CO2. The stillage from the distillation column is concentrated in the evaporator with a heat transfer area of 75.6 m2 and saturated steam as transfer agent, removing over 90% of water which is used as process water after condensation. Part of the biomass sent to drying is mixed with the syrup obtained after evaporation (throughput 22,400 kg/ h) prior to being sent to drying. The drying is carried out in a rotary drum with a drying capacity of 996 kg/h, drum area 85.5 m2 and air as drying gas (Kwiatkowski et al., 2006). Rectification bottoms and molecular sieve liquid outputs are mixed with an operating throughput of 59,000 kg/h and then mixed with the evaporation condensate and the scrubber liquid outlet giving a total of 280,000 kg/h process water that can be recycled. One part (15,000 kg/h) of this water is reused in the scrubber, while the rest is used to dilute the concentrated raw juice for the following fermentation process. The design basis is 34.8 million kg (44 million l) of 99.6% pure ethanol per year, which is produced from 564,000 Mg of raw juice. The cost model reflects economic conditions in the second half of 2010. 2.3. Process analysis Economic analysis was performed for the proposed process model and is given as an executive summary of project indices. Sensitivity studies were also performed for this process model at various sugar beet costs and yeast costs. The cost of sugar beet se-
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lected for this study ranged from $0.017/kg to $0.029/kg. This range was selected because of the fluctuations of its purchase price in the period of 2000–2009 (Price Report, 2010). Yeast cost depends from the quantity needed, so its purchase price offered from a local yeast production plant ranged from $1.144/kg to $1.43/kg. Finally, the influence of the biomass split factor on economic parameters was examined.
3. Results and discussion 3.1. Economic analysis The results of the economic analysis are shown in Table 1. The individual components (capital investment cost, operating cost and unit production cost) are discussed in the following paragraphs. The total capital investment cost for this production plant is obtained by adding together the values of the direct fixed capital (DFC), working capital and start-up and validation costs. DFC represents the sum of the total plant cost (TPC) and costs for contractor’s fees and contingency (CFC). Further, the TPC consists of the total plant direct cost (TPDC) and the total plant indirect cost (TPIC). While the TPIC depends on the costs for engineering and construction, the TPDC expenses include the costs for equipment purchase, installation, process piping, instrumentation, insulation,
Table 1 Economic analysis of the proposed process model. Capital investment cost ($) Direct fixed capital DFC = TPC + CFC ($) Total plant cost TPC = TPDC + TPIC ($) Total plant direct cost TPDC ($) Equipment purchase cost ($) Fermentors ($) Disc-stack centrifuge ($) Batch distillation vessel ($) Scrubber ($) Rectification column ($) Molecular sieves ($) Evaporators ($) Rotary drum dryers ($) Condenser ($) Unlisted equipment ($) Installation ($) Process piping ($) Instrumentation ($) Insulation ($) Electrical ($) Buildings ($) Yard improvement ($) Auxiliary facilities ($) Total plant indirect cost TPIC ($) Engineering ($) Construction ($) Contractor’s fee & contingency CFC ($) Contractor’s fee ($) Contingency ($) Working capital ($) Start-up and validation cost ($)
68,778,000 64,080,000 55,722,000 34,826,000 10,896,000 3000,000 1475,000 478,000 57,000 525,000 2700,000 548,000 966,000 57,000 1090,000 4644,000 3269,000 4141,000 327,000 1090,000 4685,000 1634,000 4141,000 20,896,000 8707,000 12,189,000 8358,000 2786,000 5572,000 1494,000 3204,000
Operating cost ($/year) Raw materials ($/year) Yeast ($/year) Raw juice ($/year) Facility-dependent ($/year) Labour ($/year) Laboratory/QC/QA ($/year) Utilities ($/year) Electricity ($/year) Steam ($/year) Chilled water ($/year) Co-production
20,982,000 2037,000 424,000 1612,000 7068,200 6968,100 278,500 7427,200 4118,600 995,600 2313,000 2797,000
Unit production cost ($/kg)
0.603
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electrical, building, yard improvement and auxiliary facilities. The purchasing cost for major equipment items listed in Table 1 is based on offers from the suppliers of equipment or taken from the report of Aden et al. (2002). If the capacities of the equipment in the model were different from the equipment capacities offered by the suppliers, the cost was corrected by using the Rule of Sixtenths (Dysert, 2003). The following equation expresses this rule:
CB ¼ CA
0:6 SB SA
ð1Þ
where CB is the approximate cost ($) of equipment having size SB (L, m3, or whatever) and CA is the known cost ($) of equipment having corresponding size SA (same units as SB). Besides capacity, the type of construction material and specific characteristics of the equipment can change the purchasing equipment cost (Nghiem et al., 2011). The estimated capital cost for this case is $68,778,000, which is over $20 million more than the capital costs determined for a corn to ethanol production plant by Kwiatkowski et al. (2006). This difference exists because this group of authors had an industry feedback to estimate the total capital investment cost as approximately three times the sum of individual equipment purchase cost and installation expenses. By applying the same calculation method as Kwiatkowski et al. (2006) to this case the capital cost data would match, i.e. the capital investment would be nearly the same. Operating costs are calculated by summing the raw material and utility costs, costs that are facility-dependent (equipment availability, maintenance, insurance, local taxes, etc.), labour costs and laboratory, quality control (QC) and quality assurance (QA) expenses, and subtracting a credit for the sale of co-products (Table 1). Compared to the annual ethanol production costs from molasses (Sobocˇan and Glavicˇ, 2000), the operating costs of producing ethanol from raw juice are slightly lower. This difference exists due to the fact that molasses is a by-product, while raw juice is a intermediate product of sugar beet processing, i.e. molasses is obtained from raw juice after its purification (micro and ultra filtration), concentration (evaporation) and multistage crystallization. In other words, the operating costs are higher for molasses to ethanol production because of these additional operations. The primary feedstock for the facility is raw juice. Yeast that ferments the glucose into ethanol and carbon dioxide is also required. The quantities of materials required are provided by the process model and their unit costs have been incorporated into the model. Information about various feedstock prices can be collected from the Statistical Office of the Republic of Serbia. Electricity, steam and chilled water are the utilities required in the process. Utility requirements of the various equipment operations are calculated and summed within the programme. These utilities are treated as purchased utilities and the unit costs for each of them were set based on 5 year average market prices. The 5 year average market prices (2009–2005) of steam ($2.10/ Mg), chilled water ($0.30/Mg) and electricity ($0.10/kWh) are obtained from the annual reports of the Power Industry of Serbia1 and used in the process model. Three products are produced in the conversion of raw juice to ethanol. They are ethanol as the main product, and animal feed and carbon dioxide as co-products. The ethanol can be sold to be blended with fossil fuels. Current pricing for it is available from the Biomass Action Plan (2010). The assumed ethanol price in this work is 0.75 $/kg (0.59 $/L). The animal feed is sold and its value is strongly dependent on its protein content and is related to the value of other protein-based animal feeds. The animal feed pro1 Power Industry of Serbia, Annual Reports from 2005 till 2009. Database at http:// www.eps.rs/Pages/FolderDocs.aspx?list=Godisnji%20Izvestaji, accessed at June 2011.
duced in this process has a protein content of approximately 21% and its value, based on its proteins content, is estimated to be $0.07/kg (Belyea et al., 2004). The second co-product, carbon dioxide, is economically viable only if it is collected from the scrubber and sold to a third party at the plant site for further purification and distribution to the food processing companies and manufacturers of carbonated beverages. In this case its value to the ethanol producer is $15 per metric ton (Kwiatkowski et al., 2006). A breakdown of the costs to produce ethanol with the values used in this model case is illustrated in Fig. 2. It shows that the unit production cost of ethanol is approximately $0.603/kg, which corresponds with the results reported by Sobocˇan and Glavicˇ (2000). Fig. 2 also shows that the income from the sale of animal feed reduces the ethanol production costs, and is a major factor in operating costs. Since the cost of sugar beet can fluctuate due to market and weather conditions, the influence of sugar beet cost was also examined in the sensitivity study. 3.2. Sensitivity study For this process model the simulation software gave an executive summary of the project calculating the project indices shown in Table 2. Gross margin represents a company’s total sales revenue minus its cost of goods sold, divided by the total sales revenue, expressed as a percentage. This number represents the proportion of each dollar of revenue that the company retains as gross profit. For this process the gross margin is 18.01%, which means the company would retain $0.18 from each dollar of revenue generated. The ROI (Return of Investment) is a performance measure used to evaluate the efficiency of an investment. ROI is a very popular metric because of its versatility and simplicity. That is, if an investment does not have a positive ROI, then the investment should not be undertaken. The length of time required to recover the cost of an investment can be calculated on the basis of the ROI. Table 2 shows that the payback time is approximately 7.5 years, which is quite acceptable. NPV (Net Present Value) represents the difference between the present value of cash inflows and the present value of cash outflows. It is used to analyse the profitability of an investment through considering the time-value of the earned money, because a dollar that is earned in 5 years time has a lower value than a dollar earned today (Heinzle et al., 2006). If the NPV of a prospective project is positive (like in Table 2) with assuming a discount interest of 7%, it should be accepted and realised. The IRR (Internal Rate of Return) is a discount rate often used in capital budgeting that makes the NPV of all cash flows from a particular project equal to zero. In other words the higher a project’s IRR, the more desir-
Fig. 2. Ethanol production cost breakdown.
D.G. Vucˇurovic´ et al. / Bioresource Technology 104 (2012) 367–372 Table 2 Project indices of the ethanol production process model. Project indices
Value
Gross margin (%) Return on Investment (%) Payback time (years) Internal Rate of Return after taxes (%) Net Present Value at 7.00% ($)
18.01 13.41 7.46 7.73 2955,000
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able it is to undertake the project. This project yields an after-tax IRR of 7.73%, with income taxes set to 40%. Based on these results, with a project lifetime of 25 years and depreciation period of 10 years with a salvage value 5% DFC, this project represents a very attractive investment, i.e. the examined system is economically viable. The influence of sugar beet and yeast price on ethanol production costs is shown in Fig. 3. It can be seen that lower sugar beet and yeast costs result in lower unit production costs. Also, the sugar beet cost has a much higher impact on the expenses of producing ethanol. However, depending on the market conditions the management can regulate the split factor and decide how much biomass is going to drying (animal feed) or back to fermentation after centrifugation. The effect of the biomass split factor on economic parameters is shown in Fig. 4. By recycling more biomass reduces the quantity of yeast needed for fermentation and lowers the costs for purchasing yeast (even if the price is higher for smaller quantities) but on the other hand causes lower income for selling animal feed (coproduct). Still, the total revenues vary only by $1,000,000 per year depending on the split factor. However, the ethanol production costs are higher with the increase of recycled biomass. Because of the greater curve slope of the unit production cost for the values of the split factor in the range from 50% to 70% and based on the highest IRR this range would be optimal for varying the biomass split factor. 4. Conclusion
Fig. 3. The effect of sugar beet and yeast cost on ethanol production cost.
The process model developed here reflects the base case for a 44 million l/year fuel ethanol plant. A cost model has been developed for economic analysis of this ethanol production from sugar beet raw juice. A sensitivity study has been performed to examine the effects of sugar beet and yeast costs on ethanol production costs. The effect of the amount of recycled biomass on process economics was also examined. The benefits of obtaining lower unit production costs due to lower prices of sugar beet and yeast were clearly demonstrated. The optimal percentage of recycled biomass proved to be between 50% and 70%. Acknowledgements This study is part of the project (TR-31002) which is supported by the Ministry of Education and Science of Serbia. The authors are grateful for the financial support. References
Fig. 4. Process economics as a function of the amount of recycled biomass.
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