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Biobutanol production from municipal solid waste: Technical and economic analysis
Bioresource Technology 308 (2020) 123267
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Biobutanol production from municipal solid waste: Technical and economic analysis
T
Parisa Nazemi Ashania,b, Marzieh Shafieic, Keikhosro Karimia,d,
⁎
a
Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Food, Agricultural and Biological Engineering, The Ohio State University, Wooster, OH, USA c Biofuel Research Team, Karaj, Iran d Industrial Biotechnology Group, Research Institute for Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Acetone-butanol-ethanol Techno-economic analysis Municipal solid waste Gas stripping Pervaporation
Novel processes for the production of acetone-butanol-ethanol (ABE) from municipal solid waste (MSW) were developed and simulated using Aspen Plus®. In scenario 1, a conventional distillation system was used, while a gas stripping system was coupled with a fermenter in scenario 2. In scenario 3, pervaporation (PV) and gas stripping systems right after the fermentation reactor were applied. Gas stripping increased the total ABE produced while the addition of the PV module decreased the number of distillation columns from 6 to 2 as well as created 6.4% increments in the amount of butanol in comparison with scenario 1. Economical evaluation resulted in having payout periods of 15.9, 4.4, and 2.9 years for scenarios 1 to 3, respectively. These results show that using MSW as an inexpensive sugar-rich feedstock together with gas stripping PV system is a promising solution to overcome the major obstacles in the way of the ABE production.
1. Introduction With the rapid industrialization and globalization during recent decades, the demand for fossil fuels significantly increased, resulting in the fast reduction of nonrenewable fuel resources and increases environmental problems. This ever-increasing need for fuel has attracted considerable attention to finding alternative ways to provide renewable ⁎
fuels. These resources have to be safe for the environment to reduce the rate of air pollution and, subsequently, avoid contributing to climate change (Agrawal et al., 2007; Asif and Muneer, 2007; Jacobson, 2009). Biofuels, e.g., biobutanol, biogas, and bioethanol, earned a reputation as promising candidates for alternatives of liquid fuels (Amiri et al., 2015; Bringezu et al., 2009; Silva Braz and Pinto Mariano, 2018; Tao et al., 2014). After developments in bioethanol production, the
Corresponding author at: Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. E-mail address: [email protected] (K. Karimi).
https://doi.org/10.1016/j.biortech.2020.123267 Received 19 January 2020; Received in revised form 26 March 2020; Accepted 27 March 2020 Available online 30 March 2020 0960-8524/ © 2020 Elsevier Ltd. All rights reserved.
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introduction of biobutanol was a great step towards finding an advanced fuel. Biobutanol has many advantages over bioethanol, e.g., higher energy content and insolubility in water, which prevent many drawbacks like underground water pollution and corrosion (Lujaji et al., 2011). Nevertheless, certain obstacles still exist on the way of making biobutanol commercially competitive with gasoline or bioethanol. Problems such as low yield of fermentation and production of byproducts lead to higher energy demand for separation processes. These obstacles resulted in higher production costs, making biobutanol uncompetitive on a commercial scale (Gottumukkala et al., 2013). Biobutanol production is based on employing the Clostridium species for the fermentation of sugars and through acetone, butanol, and ethanol (ABE) fermentation (Dürre, 2007). While butanol is the main product, the production acetone and ethanol as byproducts improve the process economy. The first commercial plants for biobutanol production began in the 1910 s using molasses and corn as substrate. The process supplied about two-thirds of butanol and one-tenth of acetone needed all around the world. However, in the 1960 s, butanol started to lose its commercial popularity due to increases in the prices of molasses and corn along with development of less expensive production of butanol in petrochemical plants (Xue et al., 2013). In recent decades and after introduction of biofuels, attention returned to biobutanol production and finding technical routes to make ABE production commercially competitive with other fuels. Many studies have shown that the price of the process feedstock is the most important parameter affecting the production cost of biobutanol (Amiri and Karimi, 2018; Green, 2011; Karimi et al., 2015; Karimi and Pandey, 2014; Qureshi and Blaschek, 2001). Introducing municipal solid waste (MSW) as a rich source of pentose and hexose sugars is a novel idea to reduce the operating costs of the biobutanol production process. Furthermore, galloping increases in population have encouraged the production of MSW at an ever-increasing pace, facing societies with serious problems for the management of such a great amount of waste. This issue has the potential to evolve into a real threat to pollution of the soil, water, and air (Braber, 1995; Gottumukkala et al., 2013; Suocheng et al., 2001). Currently, the most common strategies of MSW disposal are landfilling, combustion, and composting. Statistics in 2013 show that in the United States alone, 258 million tonnes of MSW was produced in 2014, and about 20 million tonnes of MSW was produced in Iran (Hosseini et al., 2013; Maiti et al., 2017). Taken together with the nearly zero price of this golden dirt, these factors have attracted attention towards employing MSW as the best choice for the ABE production process. Gas stripping and pervaporation are advanced techniques that can help the economy of butanol production. Gas stripping is a proven method for the improvement of butanol production. In this method, the produced solvents are removed continuously from the fermentation broth; as a result, there always exists a driving force regarding the production of more solvents. It has been reported that application of gas stripping is operationally and economically feasible (Cai et al., 2016). Pervaporation is a high-tech method that separates a large amount of water from solvents, and consequently makes it easier to purify acetone, butanol, and ethanol in distillation columns. Pervaporation is an assistive technique in the separation of products, and it has been coupled with other methods in many studies (Azimi et al., 2019; Buchaly et al., 2007; Cai et al., 2016; Gorak, 2007; Huang et al., 2014; Qureshi et al., 2014). Recently, an experimental study on ABE production from MSW showed that MSW has a high potential for producing ABE (Farmanbordar et al., 2018). Since there is no techno-economic study on ABE production from MSW, it is important to evaluate the process’s economic aspects. This study is aimed at simulation and economic evaluation for the production of butanol, as the main product, and acetone and ethanol, as valuable byproducts. In addition, to evaluate whether the process is
economically feasible, a techno-economic assessment can identify the process bottlenecks and their possible remedies. In this regard, this study compares three different scenarios, using reported experimental data for ABE production from MSW, in which each scenario has a different downstream process that is searching for the minimum costs of ABE production. In the following section, these scenarios are described in detail. A part of this study’s novelty is designing the separation process with coupled gas stripping and pervaporation, which are potentially effective in coping with ABE production obstacles. 2. Process simulation and description (Methodology) MSW is mostly waste produced in households. It can be separated into wet and dry waste. The wet part of solid waste is also known as organic waste, including foods, fruit peels, and vegetables. This part of MSW is biologically degradable and called biodegradable MSW (BMSW). Dry waste mainly includes paper, plastics, glass, and metal. Commonly, solid waste is recycled, burnt, or landfilled. According to the U.S. Energy Information Administration, of the total amount of MSW generated in the United States in 2014, 52% was landfilled, 35% was recycled or composted, and 13% was burned for energy (Basheer et al., 2018). In Iran, different cities have different processes for the management of MSW. In Isfahan, MSW is separated into two groups of wet and dry waste, where the BMSW is composted, and the dry waste is separated into plastic, textile, and metals, and the residue is landfilled. In this study, three scenarios were designed, simulated, and focused mainly on the downstream separation unit of the ABE process. All three scenarios use the same amount of BMSW as substrate, 200,000 tonnes/ year. 2.1. Plant capacity and location It is assumed that each biorefinery plant is placed near the waste processing plant in Isfahan, Iran, where 1000 tonnes of waste is gathered daily and different fractions of waste are separated mainly using magnetic separator, shredder, sieve, and manual selection. According to the reported waste analyses, about 65% of the total amount of the waste is biodegradable (Demirbas et al., 2011). Therefore, assuming 200,000 tonnes/year of substrate for the Isfahan plant is reasonable. Further, the separated biodegradable fraction is converted to compost through a straightforward process. Now, this compost is ready to be used as the feedstock of the ABE production process. Although organic compost itself can be sold as a product, it is much more profitable when used as feedstock and changed into a high-value chemical like butanol. The process of ABE production was simulated using Aspen Plus® V8.8, according to the best experimental results reported in a recent study of (Farmanbordar et al., 2018), in which ABE was produced from the BMSW of Isfahan, Iran. 2.2. Process simulation overview The simulated scenarios mainly have several units, including feed handling, pretreatment, hydrolysis, fermentation, and purification (Fig. 1). The scope of designs is the production of acetone, butanol, and ethanol with 99.8%, 99.7%, and 95.5% purities, respectively. Each unit of the plant is described accordingly. 2.2.1. Feed handling Milled BMSW with particle sizes of less than 6 cm in length is delivered to this unit. Then, the materials are milled to a size of less than 0.5 mm. BMSW samples, gathered from the Isfahan municipal solid waste processing plant, contain 20.0 ± 0.3% lignocellulosic components, 58.6 ± 0.4% starch, 10.1 ± 0.8% pectin, 6.5 ± 0.1% lipid, 8.2 ± 0.2% protein, and 1.4 ± 0.1% ash (Farmanbordar et al., 2018). BMSW contains a high concentration of tannins, which are phenolic compounds and severely inhibit ABE fermentation (Heidari et al., 2
Bioresource Technology 308 (2020) 123267
P.N. Ashani, et al.
BMSW (200,000 ton/h)
Make up Ethanol Feed handling Lime
H2SO4, %0.5 w/w (179.29 ton/h)
Pretreatment
Liquid
H2SO4, %98 w/w
Detoxification Ethanol
Solid
(1.67 ton/h)
Hydrolysis and fermentation
Separation and purification
Butanol
(4.48 ton/h)
Enzyme
Microorganism
Acetone
(0.63 ton/h)
Fig. 1. Block flow diagram of ABE production process, numbers show the mass balance for scenario 3.
pretreatment unit is sent for hydrolysis using a cellulase enzyme complex (Cellic® CTec2, Novozymes, Denmark) at 45 °C for 72 h. During this step in the prepared environment, cellulose and other polysaccharides are converted to glucose and other sugar monomers. Therefore, the hydrolyzed material is ready to be processed in the fermentation tank. The fermentation process is separate hydrolysis and fermentation (SHF), and Clostridium acetobutylicum NRRL-B591 is used as a microorganism in all three scenarios. In the SHF process, the hydrolysis and fermentation occur in separate tanks; thus, the hydrolysis tank is designed to operate at relatively high temperatures to increase enzymatic activity. The produced sugars are sent to the fermentation tank, where the condition is optimal, at 37° C temperature for 72 h, to increase the fermentation yield (Cheng et al., 2012).
2016). Thus, tannin removal prior to pretreatment is an essential stage in ABE production from BMSW (Farmanbordar et al., 2018). Experimental data approved the efficiency of using ethanol as a solvent, which can eliminate the negative effects of tannins. To reduce the costs of buying raw materials, the needed ethanol was supplied from the ethanol produced in the plant. Moreover, ethanol from this section was recovered and recycled through this part of the process to minimize the expense of ethanol consumption. In addition to tannin compounds, this step of the feed processing removes other undesirable components like fats, proteins, and part of the existing pectin. 2.2.2. Pretreatment Lignocellulosic components are a major part of BMSW; thus, a pretreatment step is necessary for production of fermentable sugars. Based on experimental data, dilute sulfuric acid (0.5% v/v) at 140 °C for 60 min was used as pretreatment conditions. During this step, in addition to starch liquefaction, a major part of the crystalline structure of cellulose breaks into oligomers and glucose monomers that dissolve into the water (Farmanbordar et al., 2018). After pretreatment, the solid and liquid fractions are washed and separated with a belt filter. The solid fraction is directly sent to the hydrolysis reactors, while the liquid stream is sent to the detoxification unit.
2.2.5. Purification Purification of ABE is the most expensive and difficult part of the process because of the low concentrations of the products in the ABE fermentation. Accordingly, the main differences between the three scenarios are in the downstream purification of ABE. Therefore, each scenario’s downstream process is described separately. 2.2.5.1. Scenario 1. The separation system in scenario 1 is a simple method for purifying the products, using only a distillation system in which six distillation columns are used operating at atmospheric pressure (Cheng et al., 2012). The first column, usually called the beer column, separates the ABE from the produced biomass and other insoluble contents together with a large amount of water. The two subsequent columns are mainly for distribution of the three products and specifically for purifying the acetone. Acetone with 99.8% purity leaves the top of second column, and butanol and water are the main components of the bottom product. Furthermore, a side stream containing low purity acetone together with ethanol and water is drawn in the second column. This side stream is fed to the third column that recycles acetone back to the second column. A mixture of ethanol and water is the bottom product of the third column. The recycled
2.2.3. Detoxification Liquid detoxification is performed to remove the toxic, inhibitory contents produced during pretreatment, like hydroxymethylfurfural (HMF) and furfural. In this method, the mixture’s pH is increased to 11 using lime. The reaction of inhibitors with sulfuric acid results in the production of gypsum in solid form, which should be separated by filtration. The pH of the filtered liquid is further reduced for the fermentation step to the desired pH of 6.8 via sulfuric acid. 2.2.4. Enzymatic hydrolysis and fermentation In the hydrolysis and fermentation unit, solid residue from the 3
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P.N. Ashani, et al.
Fig. 2. Downstream section of scenarios 1 and 2 for the purification of acetone, butanol, and ethanol.
Recycled Gases
Flash Column
Flash Col umn
Flash Column
1990 ton/h
Fermenter
Purged Gases CO2 9.65 ton/h H2 0.43 ton/h
ABE rich flow to Separation
Microorganism
Water 168.70 ton/h Acetone 1.66 ton/h Butanol 4.49 ton/h Ethanol 0.77 ton/h
Fig. 3. Schematic of the gas stripping technique in the second and third scenarios.
acetone is fed to the second distillation column to be separated. The fourth column is designed to purify ethanol from water, up to a purity of 95% w/w. As mentioned, the bottom product of the second column contains water and butanol, which makes a heterogeneous azeotrope at the operating temperature and pressure. A combination of two distillation columns (T5 and T6 in Fig. 2) coupled with a decanter is the typical solution for separating water and butanol to the highest purification (99.9%). The lighter organic phase from decanter contains 58 mol% butanol. This stream is fed to the fifth column that an azeotropic mixture of butanol and water and a pure butanol stream are the top and bottom products of this column. The heavier aqueous phase from decanter contains 97% water, which is fed to the sixth column. An azeotropic stream and a pure stream of water are the top and bottom products of the last column, respectively. The two product streams containing azeotropic mixtures in column five and six are recycled to the decanter (Fig. 2).
hydrogen and carbon dioxide in the fermentation reactor makes it feasible to utilize these components as carrier gases to purge the produced ABE in the form of vapor out, while the microbial cells and fermentation nutrients stay inside the fermenter. This process, referred to as gas stripping, is very beneficial for the improvement of fermentation and purification as it decreases the need for supplying microorganisms and nutrients for fermentation and reduces the costs of solvent separation (Huang et al., 2015). Gas stripping can also significantly affect the rate of butanol production since butanol concentration close to 14 g/L in the fermenter is toxic and acts as an inhibitor for the butanol production. The continuous removal of butanol from the fermentation medium keeps its concentration below the toxic level. Subsequently, using this technique results in higher yields of fermentation reaction and production of more ABE (Chen et al., 2014; El-Zanati et al., 2006; Liu et al., 2014; Valentínyi et al., 2013; Xue et al., 2012). To separate ABE from the carrier gases, the flow passes through a four-stage compressor. Each compression stage is followed by a condenser and a flash drum to separate the condensed ABE from the gas stream (Fig. 3). Each compressor stage increases the pressure to about three times of the point at which temperature does not exceed the upper limit of the facilities’ heat endurance. The compressor outlet flow passes through a cooler, followed by a flash
2.2.5.2. Scenario 2. The second scenario applies the gas stripping technique coupled with the fermentation step. During fermentation, acetone, butanol, ethanol, hydrogen, and carbon dioxide are produced in the ratio of 3:6:1:12:23 (Luo et al., 2015). The abundant quantity of 4
Bioresource Technology 308 (2020) 123267
P.N. Ashani, et al.
A
B
Fig. 4. Process for the third scenario using gas stripping coupled with the pervaporation system.
column. This is the last condensation step for the separation of ABE from H2 and CO2. The bottom flows of the flash columns are collected and sent to the separation unit. After separation of ABE from the carrier gases, a part of H2 and CO2 is purged, and the remaining flow is returned to the fermenter (Fig. 3). The purification unit of this scenario is the same as that of the first scenario, and the only difference with the former scenario is in the amount and the concentration of the produced ABE (Fig. 2).
from about 80% w/w in the inlet stream to about 30% w/w in the permeate stream (Azimi et al., 2019; Cai et al., 2016; Yi et al., 2015). The purification unit thereby is dramatically simplified, and the number of distillation columns is reduced from six to two. The first column separates acetone from the ethanol–water-butanol mixture. The bottom stream of the first distillation column enters a decanter after cooling down. The decanter yields two organic and aqueous phases. The organic phase, mainly containing butanol, ethanol, and small amounts of water, is sent to the second distillation column to be further separated; the aqueous phase, containing water with a trace amount of alcohols, is sent to the wastewater treatment section (Fig. 4). The experimental data for the gas stripping and pervaporation sections were adopted from the recent study of Cai et al. (Cai et al., 2016).
2.2.5.3. Scenario 3. Similar to scenario 2, a gas stripping system is coupled with the fermentation process (Fig. 4.). However, in scenario 3 a pervaporation module is used right after the gas stripping system. Thus, the ABE flow, which contains a large amount of water, enters the pervaporation module. The pervaporation system is based on membrane technology, applying vacuum pressure on both sides of the module, which separates a large fraction of water from the ABE. Several studies have shown that polydimethylsiloxane (PDMS) membrane can effectively separate ABE from water (El-Zanati et al., 2006; Liu et al., 2014; Valentínyi et al., 2013). This step reduces the fraction of water
2.3. Economic evaluations Aspen Process Economic Analyzer (Aspen PEA®) V8.8 software was employed to calculate discounted cash flow and to evaluate profitability for each scenario. Appropriate assumptions were needed to 5
Bioresource Technology 308 (2020) 123267
P.N. Ashani, et al.
Table 1 Production rate and product purity of ABE in scenarios 1–3.
Butanol rate, kg/h purity, %w/w Acetone rate, kg/h purity, %w/w Ethanol rate, kg/h purity, %w/w
Scenario 1
Scenario 2
Scenario 3
4212 98.8 1703 97.2 711.5 95
4460 100 1680 96.2 740.5 95
4482 100 1676.8 98.3 635.5 95
Productioncost Totalproductioncost ($/yr ) Totalbyproductcredits ($/yr ) = kg Totalmainproductrate ( h )
(1)
For the three simulated scenarios, the produced acetone and ethanol are considered the byproducts of the process. 3. Results and discussion The designed biorefinery employs BMSW as a feedstock and utilizes 200,000 tonnes of substrate per year. Simulation of the process in the Aspen Plus® eased accessing the operating results of each process, which were theoretically defined based on the experimental studies. Simulation of the first scenario, as the base case, was used as a set point in order to improve comparisons between scenarios. The simulation results showed that applying the gas stripping system increased the productivity of ABE by overcoming the sugar and butanol inhibition in the fermenter, from 4212 kg/h butanol in the first scenario to 4460 kg/h and 4482 kg/h in the second and third scenarios. Table 1 summarizes the production rates of acetone, butanol and ethanol in all three scenarios. Compared to the first and second scenarios, in which six distillation columns are required, the third scenario employs two distillation columns with 17 and 35 trays. Table 2 presents the specifications of the six distillation columns of processes. Many studies have shown employing a pervaporation module with high selectivity of solvents in the presence of fermentation broth helps to remove undesired side products, metabolic compounds, and microbial cells (Li et al., 2019; Rom and Friedl, 2016; Zhang et al., 2019). However, feeding the whole fermentation broth to the pervaporation system would cause the fouling of the membrane, resulting in a notable decrease in separation yield. Coupling gas stripping system with pervaporation module would prevent fouling by removing components with larger particle sizes and only let ABE and water pass through the module. Furthermore, PDMS membrane has a higher selectivity for ABE than water. This membrane drastically helps in reducing the number of
achieve reasonable results. The most important assumptions used in the software are as follows: - The plant capacity in all scenarios is 200,000 tonnes of feed per year based on dry weight. - The location of the plant is assumed to be in Isfahan, Iran, and thus a location factor of 0.8 was assumed for capital investments - The plant is depreciated over 15 years via the straight-line depreciation method, and the salvage value is zero at the end of 15th year - The income tax rate is zero due to the plant’s adherence to Iran’s constitutional law, which supports developing the biofuel industry - The contingency factor assumed to be 15% of total investment - The interest rate of 20% was assumed - All costs for chemicals, utilities, and equipment are indexed to the 2016 USD. The price of raw materials, utilities and products used for economic evaluation are as follows: - Utilities: Electricity: 0.068 $/kWh; Water: 0.03 $/m3; Steam (690 kPa): 17.91 $/tonne - Raw materials: BMSW:0.005 $/kg; Sulfuric acid: 0.4 $/kg; Lime: 0.11 $/kg; Enzyme: 1 $/kg - Products: Acetone 1.1 $/kg; Butanol: 1.6 $/kg; Ethanol: 0.95 $/kg In the above data, utility costs are adopted from the Fajr and Mobin petrochemical plants in Iran. Costs of chemical compounds are the average value from the data gathered from (Humbird et al., 2011) and local suppliers. Using the above data and sizing the equipment according to the designed scenarios in Aspen PEA, major investment capital, including total project capital investment, total operating cost, total raw material cost, total product sales, and costs of utilities, were evaluated. Using capital and these costs, the profitability parameters (MIRR, NRR, ARR, payout period, and PI) are calculated for each scenario. Each parameter is an index showing the economic attractiveness of a scenario, and based on these indices, the best scenario is identified. The detailed formulas for calculation of these parameters are presented in the Appendix A.
Table 2 Specifications of distillation columns for scenarios 1–3. No. of Columns
1st
2nd
3rd
4th
5th
6th
10 2 5 10
30 34 5, 16a 20
30 4 19 55
45 4.5 36 74.9
20 3 3 91.4
10 1 5 91.7
1
1
1
0.9
1
1
Partial 9 2.5 3 89.9
Partial 40 30 3, 15b 47.5
Total 20 9 16 45.2
Total 33 5 29 68.6
Total 20 3 3 87.8
Total 10 5 5 98.1
1
0.8
0.7
0.7
0.9
1
Partial 35 2 11 52.2
Partial 17 2.5 7 77.4
Total – – – –
Total – – – –
Total – – – –
Total – – – –
0.9
1
–
–
–
–
Total
Total
–
–
–
–
Column specification Scenario 1
Scenario 2
2.3.1. Production price The production price is calculated by considering the production costs, including manufacturing costs and general expenses. According to Peters at al., manufacturing costs are considered the sum of direct production costs, fixed charges, and overhead plant expenses. General expenses include administrative expenses, distribution and marketing, and research and development costs (Peters et al., 2003). A reasonable margin between production costs and the market price of the products is desirable in order to contribute toward profitability. The lower the production price, the more unswerving the process will remain under the changes in economic situations, prices, and inflation. The production price is calculated by Eq. (1):
Scenario 3
No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser
a The main feed stream enters the column on the 16th tray and the flow from the third column enters on the 5th tray. b The main feed stream enters the column on the 15th tray and the flow from the third column enters on the 3rd tray.
6
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with more significant effects on the economy of a project, in order to evaluate the profitability range of a process. In this study, among all parameters, the prices of raw material and utilities were tracked. Having the major share of operating costs in the scenarios leads us to select H2SO4 and steam as the most important factors affecting product prices. The effect of changes in the prices of these two parameters on the production cost and profitability index (PI) for scenarios 1 through 3 are presented in Fig. 6A and 6.B. According to Fig. 6A, a 40% increment in the H2SO4 price makes the production cost of butanol exceed its market price (1.6 $/kg) and consequently is unprofitable. For the two other scenarios, any increment in H2SO4 price up to 500% results in a production cost, which is less than the market price and the project remains profitable. In Fig. 6A, all increments are approximately linear and scenario 3 has the lowest production costs even after a 500% increase in the H2SO4 price. Based on these results, any increment in the price of steam for the first scenario will reduce the PI to less than 1, which indicates an unprofitable process. This means that scenario 1 is unlikely to be profitable. In Fig. 6B, the curves for the second and third scenarios illustrate better results than scenario 1, as the PI is greater than that of scenario 1 (1.1 and 1.35 for the second and third scenarios, respectively, with 300% increase in steam price). A 400% increase in steam price makes the PI of second scenario reach 1, which means scenario 2 will no longer be profitable after any further increment in steam price. The economic feasibility of a process depends on various parameters, including factory location, raw material cost, production capacity, and interest rate. Techno-economic studies consider certain parameters to evaluate the whole process, including different feedstocks, the annual capacity of butanol production, interest rate, and different separation systems, leading to a variable range of butanol production costs and total production costs. Studies reveal that the production capacity can effectively help in the reduction of butanol production cost (Okoli and Adams, 2014; Mariano et al, 2013). On the other hand, choosing a feedstock with a lower price is shown to have a direct effect on the final product cost (Bhatia et al., 2020; Jiang et al., 2019). All in all, this study gives an overall assessment with marginal capacity, the lowest butanol production cost, and a relatively low total production cost. Using MSW as a low-cost feedstock, a high technology separation system, and choosing marginal capacity leads to a more economical process that has overcome the major bottlenecks of the biobutanol production process.
Table 3 A summary of capacity, total investment, and operating costs for each scenario. Scenario
Sc. 1
Sc. 2
Sc. 3
Capacity, butanol (tonnes/year) Internal rate of return % Investments (million $) Total Project Capital cost Total Operating Costa Total Raw Materials Costa Total Utilities Costa Total Product Salesa
36,900 20
39,000 54
39,200 80
38.7 62.2 3.48 53.4 64.6
38.9 54.3 3.48 45.9 77.4
31.0 44.7 3.48 37.0 77.4
a
Data are million $/year
required distillation columns by removing a significant fraction of water prior to using distillation system (Ismail and Matsuura, 2018). 3.1. Economic results Table 3 lists the main capital requirements obtained from the Aspen PEA calculation of each scenario. Processing 200,000 tonnes of BMSW per year, the designed scenarios are for relatively small plants with low capital investments both in costs and credits. A comparison between the reported results shows that for the second scenario, the capital raised is predictable, as it supports the gas stripping facilities in addition to all the equipment used in the first scenario. However, for the third scenario, gas stripping coupled with the pervaporation system decreases the capital costs. Although pervaporation is an advanced technology and an expensive system, its application results in lower total capital investments due to the consumption of fewer distillation columns than other scenarios (two columns instead of six). The differences in the utilities costs for these three scenarios are also the same as the changes in capital costs. Table 3 shows that the operating expenses are reduced from $62.2 million in the first scenario to $44.7 million in the third scenario with coupled gas stripping and pervaporation systems. As expected, employing the gas stripping system has led to the production of more ABE, and consequently, more credits in the second and third scenarios. To bring together all results, besides considering the economic indices such as payout period as an important index, it can be concluded that in the scenario coupled with gas stripping and pervaporation system has the best results as the payout period for this scenario is about three years, which is much lower compared to 16 and 4 years for the first and second scenarios, respectively. The total investment for this scenario was $31 million, compared to that of the base case, which was $38.7 million. A comparison of the total direct costs for each of the plants’ units can spot the most effective parameters of the total costs. Fig. 4 shows the cost distribution between different parts. As can be seen in all three scenarios, the unit for hydrolysis and fermentation has the greatest share of the plant’s total direct cost, because it employs the huge hydrolyzation and fermentation tanks that raised the expenses of this unit.
3.4. Butanol price compared with gasoline Calculating gasoline equivalent for a fuel is a tangible way to compare the prices of new fuels. Doing so primarily enables consumers to determine whether it is economical to shift from gasoline to them or not. Butanol gasoline equivalent is calculated based on the proportion of the lower heating value (LHV) for gasoline and butanol at standard pressure and temperature. This parameter is equal to 1.15 for butanol and means that the combustion of 1.15 L of butanol releases the same amount of energy as when 1 L of gasoline is burned. Considering 9% value-added tax (in Iran), the gasoline equivalent prices for biobutanol were in the range of 1.65 – 0.76 $/L (Table 4), which are less than biobutanol prices available as a chemical in the local market. However, the gasoline equivalent price for biobutanol is still higher than the gasoline price in Iran or the Persian Gulf countries. In Iran, petrochemical plants sell gasoline to the national Iranian oil products distribution company at FOB (Freight on Board) price of Persian Gulf, i.e., 0.31$/L – 0.40 $/L in 2016 (data from(www.platts.com, April 2019). However, the low-quality gasoline price in the market, i.e., 0.24 $/L, was provided by the subsidies from the Iranian government. In 2018, the Persian Gulf FOB price of gasoline had increased to 0.51 $/L. Accordingly, although zero income tax for biofuel production is used based on the Iran’s constitutional law, it is obvious that there is still a gap between the gasoline market price and the gasoline equivalent
3.2. Production costs Calculations showed the production of each kilogram of butanol costs $1.51, $0.95, and $0.70 for scenarios 1 to 3, respectively. These prices exclude value added tax (VAT). Thus, the difference between the production cost and the market price of butanol is relatively low for the first two scenarios, making these scenarios unreliable as profitable processing routes. However, for scenario 3, this difference is impressive, insofar as even with a 50% decrease in the price of butanol, the scenario remains profitable (Fig. 5). 3.3. Sensitivity analyses Sensitivity analyses are often studied along with those parameters 7
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Sc 3 Sc 2 Sc 1 -
1,00,00,000
2,00,00,000
3,00,00,000
4,00,00,000
Cost ($) Feed Handling
Pretreatment
Detoxification
Hydrolyze and fermentation
ABE Separation Fig. 5. Comparison of fixed capital investment of three scenarios for different units.
2
A
1.8 Production price, USD
Table 4 Comparison of butanol and biobutanol price with gasoline in different markets. Product price
Taxes/ Subsidies
Final Price
Final Price (gasoline equivalent)
Butanol (Chemical) Gasoline (Iran) Production price Biobutanol (Sc. 1) Biobutanol (Sc. 2) Biobutanol (Sc. 3) Gasoline (Sweden)f E85 (Sweden)f
1.6a 0.39b
0.14 −0.15c
1.74 0.24
2.00 0.24d
1.51 0.95 0.70 0.60 0.74
0.14e 0.09e 0.06e 1.14 0.58
1.65 1.04 0.76 1.74 1.32
1.89 1.19 0.88 1.74 1.90
1.6 1.4 1.2 1 0.8
a
0.6 0.4
b
Butanol market price 0
100
200
300
c
400
d
500
e f
Percentage of increment in price of H2SO4, % 1.6
B
1.5 1.4 1.3 1.2 1.1
4. Conclusions
1 0.9 0.8
Butanol price from local suppliers in 2016. Gasoline price in 2016 data from www.platts.com Subsidies imposed Gasoline price in 2016 which was sold at constant price in Iran Taxes imposed Data from: https://www.ingo.se/ [Accessed: March 2019]
chemicals in Iran. Yet, the biobutanol product can be an attractive renewable fuel in the international market. This is because biofuels still require financial support from the governments for being economically comparable to fossil fuels. An example of such supports in Sweden is presented in Table 4. In this country, addition of carbon dioxide and energy taxes to the gasoline price increases its price to 1.74 $/L (prices obtained from www.ingo.se, April 2019). Consequently, in Sweden, biofuels can compete with fossil fuels. The detailed calculations about different taxes in Sweden were presented by (Shafiei et al., 2014).
1.7
Profitability Index (PI)
Cost ($/L)
MSW is an environmental treat and widely available substrate with a negative price. The techno-economic evaluation for the production of high purity butanol, acetone, and ethanol from MSW revealed that the process could be economically feasible. However, employing gas stripping coupled with a pervaporation module had a significant impact on energy usage at the plant. It overcame the major obstacles of the process, making biobutanol production economically feasible. Nevertheless, water and energy optimizations are required to drive the biobutanol production through a more sustainable production process.
PI = 1 0%
100%
200%
300%
400%
500%
Change in steam price Fig. 6. (A) The effect of increase in the price of H2SO4 on butanol production cost (all the prices exclude VAT), and (B) Effect of changes in the price of steam on profitability indices for scenarios 1 (●), 2 (▲), and 3 (■).
CRediT authorship contribution statement
price for butanol. This gap is due to the high technology systems employed to produce butanol. Thus, the biorefineries in scenario 2 and 3 are economically profitable when their products were sold as bulk
Parisa Nazemi Ashani: Investigation, Modeling, Simulation, Techno-Economic Analysis, Software, Data Curation, Writing - original 8
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draft. Marzieh Shafiei: Investigation, Methodology, Resources, Validation, Software, Supervision, Visualization, Writing - review & editing. Keikhosro Karimi: Conceptualization, Methodology, Project administration, Supervision, Funding acquisition, Writing - review & editing.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Formulas for calculation of 1- Modified Internal Rate of Return (MIRR), Payout period, Net Rate of Return (NRR), Accounting Rate of Return (ARR) and Profitability Index (PI) are presented as supplementary data. E-supplementary data of this work can be found in online version of the paper. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2020.123267. References Agrawal, R., Singh, N.R., Ribeiro, F.H., Delgass, W.N., 2007. Sustainable fuel for the transportation sector. Proc. Natl. Acad. Sci. 104, 4828–4833. Amiri, H., Karimi, K., 2018. Pretreatment and hydrolysis of lignocellulosic wastes for butanol production: Challenges and perspectives. Bioresour. Technol. 270, 702–721. Amiri, H., Karimi, K., Bankar, S., Granström, T., 2015. Biobutanol from lignocellulosic wastes. In: Karimi, K. (Ed.), Lignocellulose-based bioproducts. Springer International Publishing, Switzerland, pp. 289–324. Asif, M., Muneer, T., 2007. Energy supply, its demand and security issues for developed and emerging economies. Renew. Sustain. Energy Rev. 11, 1388–1413. Azimi, H., Tezel, H., Thibault, J., 2019. Optimization of the in situ recovery of butanol from ABE fermentation broth via membrane pervaporation. Chem. Eng. Res. Des. 150, 49–64. Basheer, S., Rashid, H., Nasir, A., Parveen, Z., Ashgar, M.Z., 2018. EIA guidelines for landfill site in faisalbad using an integrated solid waste management approach. Ekoloji 27, 1467–1477. Bhatia, S.K., Jagtap, S.S., Bedekar, A.A., Bhatia, R.K., Patel, A.K., Pant, D., Banu, J.R., Rao, C.V., Kim, Y.G., Yang, Y.H., 2020. Recent developments in pretreatment technologies on lignocellulosic biomass: effect of key parameters, technological improvements, and challenges. Bioresour. Technol. 300, 122724. Braber, K., 1995. Anaerobic digestion of municipal solid waste: a modern waste disposal option on the verge of breakthrough. Biomass Bioenergy 9, 365–376. Bringezu, S., Schütz, H., ÓBrien, M., Kauppi, L., Howarth, R.W., McNeely, J., 2009. Towards sustainable production and use of resources: assessing biofuels, Global Environmental Change. International Panel for Sustainable Resource Management (UNEP), [Accessed: March 2019], http://www.unep.fr/shared/publications/pdf/ WEBx0149xPA-AssessingBiofuelsSummary.pdf. Buchaly, C., Kreis, P., Górak, A., 2007. Hybrid separation processes-Combination of reactive distillation with membrane separation. Chem. Eng. Process 46, 790–799. Cai, D., Chen, H., Chen, C., Hu, S., Wang, Y., Chang, Z., Miao, Q., Qin, P., Wang, Z., Wang, J., Tan, T., 2016. Gas stripping-pervaporation hybrid process for energy-saving product recovery from acetone-butanol-ethanol (ABE) fermentation broth. Chem. Eng. J. 287, 1–10. Chen, Y., Ren, H., Liu, D., Zhao, T., X. Shi, Cheng, H., al., e. 2014. Enhancement of nbutanol production by in situ butanol removal using permeating-heating-gas stripping in acetone-butanol-ethanol fermentation. Bioresour. Technol. 164, 276-284. Cheng, C.-L., Che, P.-Y., Chen, B.-Y., Lee, W.-J., 2012. Biobutanol production from agricultural waste by an acclimated mixed bacterial microflora. Appl. Energy 100, 3–9. Demirbas, M.F., Balat, M., Balat, H., 2011. Biowaste-to-biofuels. Energy Convers. Manag. 52, 1815–1828. Dürre, P., 2007. Biobutanol: An attractive biofuel. Biotechnol. J. 2, 1525–1534. El-Zanati, E., Abdel-Hakim, E., El-Ardi, O., Fahmy, M., 2006. Modeling and simulation of butanol separation from aqueous solutions using pervaporation. J. Memb. Sci. 280, 278–283. Farmanbordar, S., Karimi, K., Amiri, H., 2018. Municipal solid waste as a suitable substrate for butanol production as an advanced biofuel. Energy Convers. Manag. 157, 396–408. Gorak, A., 2007. Reactive and hybrid separations of chemicals and bioactive substances: Modeling and optimization. Comput. Aided Chem. Eng. 24, 7–8. https://doi.org/10. 1016/S1570-7946(07)80024-1. Gottumukkala, L.D., Parameswaran, B., Valappil, S.K., Mathiyazhakan, K., Pandey, A.,
9
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Biobutanol production from municipal solid waste: Technical and economic analysis
T
Parisa Nazemi Ashania,b, Marzieh Shafieic, Keikhosro Karimia,d,
⁎
a
Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran Department of Food, Agricultural and Biological Engineering, The Ohio State University, Wooster, OH, USA c Biofuel Research Team, Karaj, Iran d Industrial Biotechnology Group, Research Institute for Biotechnology and Bioengineering, Isfahan University of Technology, Isfahan 84156-83111, Iran b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Acetone-butanol-ethanol Techno-economic analysis Municipal solid waste Gas stripping Pervaporation
Novel processes for the production of acetone-butanol-ethanol (ABE) from municipal solid waste (MSW) were developed and simulated using Aspen Plus®. In scenario 1, a conventional distillation system was used, while a gas stripping system was coupled with a fermenter in scenario 2. In scenario 3, pervaporation (PV) and gas stripping systems right after the fermentation reactor were applied. Gas stripping increased the total ABE produced while the addition of the PV module decreased the number of distillation columns from 6 to 2 as well as created 6.4% increments in the amount of butanol in comparison with scenario 1. Economical evaluation resulted in having payout periods of 15.9, 4.4, and 2.9 years for scenarios 1 to 3, respectively. These results show that using MSW as an inexpensive sugar-rich feedstock together with gas stripping PV system is a promising solution to overcome the major obstacles in the way of the ABE production.
1. Introduction With the rapid industrialization and globalization during recent decades, the demand for fossil fuels significantly increased, resulting in the fast reduction of nonrenewable fuel resources and increases environmental problems. This ever-increasing need for fuel has attracted considerable attention to finding alternative ways to provide renewable ⁎
fuels. These resources have to be safe for the environment to reduce the rate of air pollution and, subsequently, avoid contributing to climate change (Agrawal et al., 2007; Asif and Muneer, 2007; Jacobson, 2009). Biofuels, e.g., biobutanol, biogas, and bioethanol, earned a reputation as promising candidates for alternatives of liquid fuels (Amiri et al., 2015; Bringezu et al., 2009; Silva Braz and Pinto Mariano, 2018; Tao et al., 2014). After developments in bioethanol production, the
Corresponding author at: Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran. E-mail address: [email protected] (K. Karimi).
https://doi.org/10.1016/j.biortech.2020.123267 Received 19 January 2020; Received in revised form 26 March 2020; Accepted 27 March 2020 Available online 30 March 2020 0960-8524/ © 2020 Elsevier Ltd. All rights reserved.
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introduction of biobutanol was a great step towards finding an advanced fuel. Biobutanol has many advantages over bioethanol, e.g., higher energy content and insolubility in water, which prevent many drawbacks like underground water pollution and corrosion (Lujaji et al., 2011). Nevertheless, certain obstacles still exist on the way of making biobutanol commercially competitive with gasoline or bioethanol. Problems such as low yield of fermentation and production of byproducts lead to higher energy demand for separation processes. These obstacles resulted in higher production costs, making biobutanol uncompetitive on a commercial scale (Gottumukkala et al., 2013). Biobutanol production is based on employing the Clostridium species for the fermentation of sugars and through acetone, butanol, and ethanol (ABE) fermentation (Dürre, 2007). While butanol is the main product, the production acetone and ethanol as byproducts improve the process economy. The first commercial plants for biobutanol production began in the 1910 s using molasses and corn as substrate. The process supplied about two-thirds of butanol and one-tenth of acetone needed all around the world. However, in the 1960 s, butanol started to lose its commercial popularity due to increases in the prices of molasses and corn along with development of less expensive production of butanol in petrochemical plants (Xue et al., 2013). In recent decades and after introduction of biofuels, attention returned to biobutanol production and finding technical routes to make ABE production commercially competitive with other fuels. Many studies have shown that the price of the process feedstock is the most important parameter affecting the production cost of biobutanol (Amiri and Karimi, 2018; Green, 2011; Karimi et al., 2015; Karimi and Pandey, 2014; Qureshi and Blaschek, 2001). Introducing municipal solid waste (MSW) as a rich source of pentose and hexose sugars is a novel idea to reduce the operating costs of the biobutanol production process. Furthermore, galloping increases in population have encouraged the production of MSW at an ever-increasing pace, facing societies with serious problems for the management of such a great amount of waste. This issue has the potential to evolve into a real threat to pollution of the soil, water, and air (Braber, 1995; Gottumukkala et al., 2013; Suocheng et al., 2001). Currently, the most common strategies of MSW disposal are landfilling, combustion, and composting. Statistics in 2013 show that in the United States alone, 258 million tonnes of MSW was produced in 2014, and about 20 million tonnes of MSW was produced in Iran (Hosseini et al., 2013; Maiti et al., 2017). Taken together with the nearly zero price of this golden dirt, these factors have attracted attention towards employing MSW as the best choice for the ABE production process. Gas stripping and pervaporation are advanced techniques that can help the economy of butanol production. Gas stripping is a proven method for the improvement of butanol production. In this method, the produced solvents are removed continuously from the fermentation broth; as a result, there always exists a driving force regarding the production of more solvents. It has been reported that application of gas stripping is operationally and economically feasible (Cai et al., 2016). Pervaporation is a high-tech method that separates a large amount of water from solvents, and consequently makes it easier to purify acetone, butanol, and ethanol in distillation columns. Pervaporation is an assistive technique in the separation of products, and it has been coupled with other methods in many studies (Azimi et al., 2019; Buchaly et al., 2007; Cai et al., 2016; Gorak, 2007; Huang et al., 2014; Qureshi et al., 2014). Recently, an experimental study on ABE production from MSW showed that MSW has a high potential for producing ABE (Farmanbordar et al., 2018). Since there is no techno-economic study on ABE production from MSW, it is important to evaluate the process’s economic aspects. This study is aimed at simulation and economic evaluation for the production of butanol, as the main product, and acetone and ethanol, as valuable byproducts. In addition, to evaluate whether the process is
economically feasible, a techno-economic assessment can identify the process bottlenecks and their possible remedies. In this regard, this study compares three different scenarios, using reported experimental data for ABE production from MSW, in which each scenario has a different downstream process that is searching for the minimum costs of ABE production. In the following section, these scenarios are described in detail. A part of this study’s novelty is designing the separation process with coupled gas stripping and pervaporation, which are potentially effective in coping with ABE production obstacles. 2. Process simulation and description (Methodology) MSW is mostly waste produced in households. It can be separated into wet and dry waste. The wet part of solid waste is also known as organic waste, including foods, fruit peels, and vegetables. This part of MSW is biologically degradable and called biodegradable MSW (BMSW). Dry waste mainly includes paper, plastics, glass, and metal. Commonly, solid waste is recycled, burnt, or landfilled. According to the U.S. Energy Information Administration, of the total amount of MSW generated in the United States in 2014, 52% was landfilled, 35% was recycled or composted, and 13% was burned for energy (Basheer et al., 2018). In Iran, different cities have different processes for the management of MSW. In Isfahan, MSW is separated into two groups of wet and dry waste, where the BMSW is composted, and the dry waste is separated into plastic, textile, and metals, and the residue is landfilled. In this study, three scenarios were designed, simulated, and focused mainly on the downstream separation unit of the ABE process. All three scenarios use the same amount of BMSW as substrate, 200,000 tonnes/ year. 2.1. Plant capacity and location It is assumed that each biorefinery plant is placed near the waste processing plant in Isfahan, Iran, where 1000 tonnes of waste is gathered daily and different fractions of waste are separated mainly using magnetic separator, shredder, sieve, and manual selection. According to the reported waste analyses, about 65% of the total amount of the waste is biodegradable (Demirbas et al., 2011). Therefore, assuming 200,000 tonnes/year of substrate for the Isfahan plant is reasonable. Further, the separated biodegradable fraction is converted to compost through a straightforward process. Now, this compost is ready to be used as the feedstock of the ABE production process. Although organic compost itself can be sold as a product, it is much more profitable when used as feedstock and changed into a high-value chemical like butanol. The process of ABE production was simulated using Aspen Plus® V8.8, according to the best experimental results reported in a recent study of (Farmanbordar et al., 2018), in which ABE was produced from the BMSW of Isfahan, Iran. 2.2. Process simulation overview The simulated scenarios mainly have several units, including feed handling, pretreatment, hydrolysis, fermentation, and purification (Fig. 1). The scope of designs is the production of acetone, butanol, and ethanol with 99.8%, 99.7%, and 95.5% purities, respectively. Each unit of the plant is described accordingly. 2.2.1. Feed handling Milled BMSW with particle sizes of less than 6 cm in length is delivered to this unit. Then, the materials are milled to a size of less than 0.5 mm. BMSW samples, gathered from the Isfahan municipal solid waste processing plant, contain 20.0 ± 0.3% lignocellulosic components, 58.6 ± 0.4% starch, 10.1 ± 0.8% pectin, 6.5 ± 0.1% lipid, 8.2 ± 0.2% protein, and 1.4 ± 0.1% ash (Farmanbordar et al., 2018). BMSW contains a high concentration of tannins, which are phenolic compounds and severely inhibit ABE fermentation (Heidari et al., 2
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BMSW (200,000 ton/h)
Make up Ethanol Feed handling Lime
H2SO4, %0.5 w/w (179.29 ton/h)
Pretreatment
Liquid
H2SO4, %98 w/w
Detoxification Ethanol
Solid
(1.67 ton/h)
Hydrolysis and fermentation
Separation and purification
Butanol
(4.48 ton/h)
Enzyme
Microorganism
Acetone
(0.63 ton/h)
Fig. 1. Block flow diagram of ABE production process, numbers show the mass balance for scenario 3.
pretreatment unit is sent for hydrolysis using a cellulase enzyme complex (Cellic® CTec2, Novozymes, Denmark) at 45 °C for 72 h. During this step in the prepared environment, cellulose and other polysaccharides are converted to glucose and other sugar monomers. Therefore, the hydrolyzed material is ready to be processed in the fermentation tank. The fermentation process is separate hydrolysis and fermentation (SHF), and Clostridium acetobutylicum NRRL-B591 is used as a microorganism in all three scenarios. In the SHF process, the hydrolysis and fermentation occur in separate tanks; thus, the hydrolysis tank is designed to operate at relatively high temperatures to increase enzymatic activity. The produced sugars are sent to the fermentation tank, where the condition is optimal, at 37° C temperature for 72 h, to increase the fermentation yield (Cheng et al., 2012).
2016). Thus, tannin removal prior to pretreatment is an essential stage in ABE production from BMSW (Farmanbordar et al., 2018). Experimental data approved the efficiency of using ethanol as a solvent, which can eliminate the negative effects of tannins. To reduce the costs of buying raw materials, the needed ethanol was supplied from the ethanol produced in the plant. Moreover, ethanol from this section was recovered and recycled through this part of the process to minimize the expense of ethanol consumption. In addition to tannin compounds, this step of the feed processing removes other undesirable components like fats, proteins, and part of the existing pectin. 2.2.2. Pretreatment Lignocellulosic components are a major part of BMSW; thus, a pretreatment step is necessary for production of fermentable sugars. Based on experimental data, dilute sulfuric acid (0.5% v/v) at 140 °C for 60 min was used as pretreatment conditions. During this step, in addition to starch liquefaction, a major part of the crystalline structure of cellulose breaks into oligomers and glucose monomers that dissolve into the water (Farmanbordar et al., 2018). After pretreatment, the solid and liquid fractions are washed and separated with a belt filter. The solid fraction is directly sent to the hydrolysis reactors, while the liquid stream is sent to the detoxification unit.
2.2.5. Purification Purification of ABE is the most expensive and difficult part of the process because of the low concentrations of the products in the ABE fermentation. Accordingly, the main differences between the three scenarios are in the downstream purification of ABE. Therefore, each scenario’s downstream process is described separately. 2.2.5.1. Scenario 1. The separation system in scenario 1 is a simple method for purifying the products, using only a distillation system in which six distillation columns are used operating at atmospheric pressure (Cheng et al., 2012). The first column, usually called the beer column, separates the ABE from the produced biomass and other insoluble contents together with a large amount of water. The two subsequent columns are mainly for distribution of the three products and specifically for purifying the acetone. Acetone with 99.8% purity leaves the top of second column, and butanol and water are the main components of the bottom product. Furthermore, a side stream containing low purity acetone together with ethanol and water is drawn in the second column. This side stream is fed to the third column that recycles acetone back to the second column. A mixture of ethanol and water is the bottom product of the third column. The recycled
2.2.3. Detoxification Liquid detoxification is performed to remove the toxic, inhibitory contents produced during pretreatment, like hydroxymethylfurfural (HMF) and furfural. In this method, the mixture’s pH is increased to 11 using lime. The reaction of inhibitors with sulfuric acid results in the production of gypsum in solid form, which should be separated by filtration. The pH of the filtered liquid is further reduced for the fermentation step to the desired pH of 6.8 via sulfuric acid. 2.2.4. Enzymatic hydrolysis and fermentation In the hydrolysis and fermentation unit, solid residue from the 3
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Fig. 2. Downstream section of scenarios 1 and 2 for the purification of acetone, butanol, and ethanol.
Recycled Gases
Flash Column
Flash Col umn
Flash Column
1990 ton/h
Fermenter
Purged Gases CO2 9.65 ton/h H2 0.43 ton/h
ABE rich flow to Separation
Microorganism
Water 168.70 ton/h Acetone 1.66 ton/h Butanol 4.49 ton/h Ethanol 0.77 ton/h
Fig. 3. Schematic of the gas stripping technique in the second and third scenarios.
acetone is fed to the second distillation column to be separated. The fourth column is designed to purify ethanol from water, up to a purity of 95% w/w. As mentioned, the bottom product of the second column contains water and butanol, which makes a heterogeneous azeotrope at the operating temperature and pressure. A combination of two distillation columns (T5 and T6 in Fig. 2) coupled with a decanter is the typical solution for separating water and butanol to the highest purification (99.9%). The lighter organic phase from decanter contains 58 mol% butanol. This stream is fed to the fifth column that an azeotropic mixture of butanol and water and a pure butanol stream are the top and bottom products of this column. The heavier aqueous phase from decanter contains 97% water, which is fed to the sixth column. An azeotropic stream and a pure stream of water are the top and bottom products of the last column, respectively. The two product streams containing azeotropic mixtures in column five and six are recycled to the decanter (Fig. 2).
hydrogen and carbon dioxide in the fermentation reactor makes it feasible to utilize these components as carrier gases to purge the produced ABE in the form of vapor out, while the microbial cells and fermentation nutrients stay inside the fermenter. This process, referred to as gas stripping, is very beneficial for the improvement of fermentation and purification as it decreases the need for supplying microorganisms and nutrients for fermentation and reduces the costs of solvent separation (Huang et al., 2015). Gas stripping can also significantly affect the rate of butanol production since butanol concentration close to 14 g/L in the fermenter is toxic and acts as an inhibitor for the butanol production. The continuous removal of butanol from the fermentation medium keeps its concentration below the toxic level. Subsequently, using this technique results in higher yields of fermentation reaction and production of more ABE (Chen et al., 2014; El-Zanati et al., 2006; Liu et al., 2014; Valentínyi et al., 2013; Xue et al., 2012). To separate ABE from the carrier gases, the flow passes through a four-stage compressor. Each compression stage is followed by a condenser and a flash drum to separate the condensed ABE from the gas stream (Fig. 3). Each compressor stage increases the pressure to about three times of the point at which temperature does not exceed the upper limit of the facilities’ heat endurance. The compressor outlet flow passes through a cooler, followed by a flash
2.2.5.2. Scenario 2. The second scenario applies the gas stripping technique coupled with the fermentation step. During fermentation, acetone, butanol, ethanol, hydrogen, and carbon dioxide are produced in the ratio of 3:6:1:12:23 (Luo et al., 2015). The abundant quantity of 4
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A
B
Fig. 4. Process for the third scenario using gas stripping coupled with the pervaporation system.
column. This is the last condensation step for the separation of ABE from H2 and CO2. The bottom flows of the flash columns are collected and sent to the separation unit. After separation of ABE from the carrier gases, a part of H2 and CO2 is purged, and the remaining flow is returned to the fermenter (Fig. 3). The purification unit of this scenario is the same as that of the first scenario, and the only difference with the former scenario is in the amount and the concentration of the produced ABE (Fig. 2).
from about 80% w/w in the inlet stream to about 30% w/w in the permeate stream (Azimi et al., 2019; Cai et al., 2016; Yi et al., 2015). The purification unit thereby is dramatically simplified, and the number of distillation columns is reduced from six to two. The first column separates acetone from the ethanol–water-butanol mixture. The bottom stream of the first distillation column enters a decanter after cooling down. The decanter yields two organic and aqueous phases. The organic phase, mainly containing butanol, ethanol, and small amounts of water, is sent to the second distillation column to be further separated; the aqueous phase, containing water with a trace amount of alcohols, is sent to the wastewater treatment section (Fig. 4). The experimental data for the gas stripping and pervaporation sections were adopted from the recent study of Cai et al. (Cai et al., 2016).
2.2.5.3. Scenario 3. Similar to scenario 2, a gas stripping system is coupled with the fermentation process (Fig. 4.). However, in scenario 3 a pervaporation module is used right after the gas stripping system. Thus, the ABE flow, which contains a large amount of water, enters the pervaporation module. The pervaporation system is based on membrane technology, applying vacuum pressure on both sides of the module, which separates a large fraction of water from the ABE. Several studies have shown that polydimethylsiloxane (PDMS) membrane can effectively separate ABE from water (El-Zanati et al., 2006; Liu et al., 2014; Valentínyi et al., 2013). This step reduces the fraction of water
2.3. Economic evaluations Aspen Process Economic Analyzer (Aspen PEA®) V8.8 software was employed to calculate discounted cash flow and to evaluate profitability for each scenario. Appropriate assumptions were needed to 5
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Table 1 Production rate and product purity of ABE in scenarios 1–3.
Butanol rate, kg/h purity, %w/w Acetone rate, kg/h purity, %w/w Ethanol rate, kg/h purity, %w/w
Scenario 1
Scenario 2
Scenario 3
4212 98.8 1703 97.2 711.5 95
4460 100 1680 96.2 740.5 95
4482 100 1676.8 98.3 635.5 95
Productioncost Totalproductioncost ($/yr ) Totalbyproductcredits ($/yr ) = kg Totalmainproductrate ( h )
(1)
For the three simulated scenarios, the produced acetone and ethanol are considered the byproducts of the process. 3. Results and discussion The designed biorefinery employs BMSW as a feedstock and utilizes 200,000 tonnes of substrate per year. Simulation of the process in the Aspen Plus® eased accessing the operating results of each process, which were theoretically defined based on the experimental studies. Simulation of the first scenario, as the base case, was used as a set point in order to improve comparisons between scenarios. The simulation results showed that applying the gas stripping system increased the productivity of ABE by overcoming the sugar and butanol inhibition in the fermenter, from 4212 kg/h butanol in the first scenario to 4460 kg/h and 4482 kg/h in the second and third scenarios. Table 1 summarizes the production rates of acetone, butanol and ethanol in all three scenarios. Compared to the first and second scenarios, in which six distillation columns are required, the third scenario employs two distillation columns with 17 and 35 trays. Table 2 presents the specifications of the six distillation columns of processes. Many studies have shown employing a pervaporation module with high selectivity of solvents in the presence of fermentation broth helps to remove undesired side products, metabolic compounds, and microbial cells (Li et al., 2019; Rom and Friedl, 2016; Zhang et al., 2019). However, feeding the whole fermentation broth to the pervaporation system would cause the fouling of the membrane, resulting in a notable decrease in separation yield. Coupling gas stripping system with pervaporation module would prevent fouling by removing components with larger particle sizes and only let ABE and water pass through the module. Furthermore, PDMS membrane has a higher selectivity for ABE than water. This membrane drastically helps in reducing the number of
achieve reasonable results. The most important assumptions used in the software are as follows: - The plant capacity in all scenarios is 200,000 tonnes of feed per year based on dry weight. - The location of the plant is assumed to be in Isfahan, Iran, and thus a location factor of 0.8 was assumed for capital investments - The plant is depreciated over 15 years via the straight-line depreciation method, and the salvage value is zero at the end of 15th year - The income tax rate is zero due to the plant’s adherence to Iran’s constitutional law, which supports developing the biofuel industry - The contingency factor assumed to be 15% of total investment - The interest rate of 20% was assumed - All costs for chemicals, utilities, and equipment are indexed to the 2016 USD. The price of raw materials, utilities and products used for economic evaluation are as follows: - Utilities: Electricity: 0.068 $/kWh; Water: 0.03 $/m3; Steam (690 kPa): 17.91 $/tonne - Raw materials: BMSW:0.005 $/kg; Sulfuric acid: 0.4 $/kg; Lime: 0.11 $/kg; Enzyme: 1 $/kg - Products: Acetone 1.1 $/kg; Butanol: 1.6 $/kg; Ethanol: 0.95 $/kg In the above data, utility costs are adopted from the Fajr and Mobin petrochemical plants in Iran. Costs of chemical compounds are the average value from the data gathered from (Humbird et al., 2011) and local suppliers. Using the above data and sizing the equipment according to the designed scenarios in Aspen PEA, major investment capital, including total project capital investment, total operating cost, total raw material cost, total product sales, and costs of utilities, were evaluated. Using capital and these costs, the profitability parameters (MIRR, NRR, ARR, payout period, and PI) are calculated for each scenario. Each parameter is an index showing the economic attractiveness of a scenario, and based on these indices, the best scenario is identified. The detailed formulas for calculation of these parameters are presented in the Appendix A.
Table 2 Specifications of distillation columns for scenarios 1–3. No. of Columns
1st
2nd
3rd
4th
5th
6th
10 2 5 10
30 34 5, 16a 20
30 4 19 55
45 4.5 36 74.9
20 3 3 91.4
10 1 5 91.7
1
1
1
0.9
1
1
Partial 9 2.5 3 89.9
Partial 40 30 3, 15b 47.5
Total 20 9 16 45.2
Total 33 5 29 68.6
Total 20 3 3 87.8
Total 10 5 5 98.1
1
0.8
0.7
0.7
0.9
1
Partial 35 2 11 52.2
Partial 17 2.5 7 77.4
Total – – – –
Total – – – –
Total – – – –
Total – – – –
0.9
1
–
–
–
–
Total
Total
–
–
–
–
Column specification Scenario 1
Scenario 2
2.3.1. Production price The production price is calculated by considering the production costs, including manufacturing costs and general expenses. According to Peters at al., manufacturing costs are considered the sum of direct production costs, fixed charges, and overhead plant expenses. General expenses include administrative expenses, distribution and marketing, and research and development costs (Peters et al., 2003). A reasonable margin between production costs and the market price of the products is desirable in order to contribute toward profitability. The lower the production price, the more unswerving the process will remain under the changes in economic situations, prices, and inflation. The production price is calculated by Eq. (1):
Scenario 3
No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser No. of trays Reflux ratio Feed plate Column temperature, °C Column pressure (condenser), bar Type of condenser
a The main feed stream enters the column on the 16th tray and the flow from the third column enters on the 5th tray. b The main feed stream enters the column on the 15th tray and the flow from the third column enters on the 3rd tray.
6
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with more significant effects on the economy of a project, in order to evaluate the profitability range of a process. In this study, among all parameters, the prices of raw material and utilities were tracked. Having the major share of operating costs in the scenarios leads us to select H2SO4 and steam as the most important factors affecting product prices. The effect of changes in the prices of these two parameters on the production cost and profitability index (PI) for scenarios 1 through 3 are presented in Fig. 6A and 6.B. According to Fig. 6A, a 40% increment in the H2SO4 price makes the production cost of butanol exceed its market price (1.6 $/kg) and consequently is unprofitable. For the two other scenarios, any increment in H2SO4 price up to 500% results in a production cost, which is less than the market price and the project remains profitable. In Fig. 6A, all increments are approximately linear and scenario 3 has the lowest production costs even after a 500% increase in the H2SO4 price. Based on these results, any increment in the price of steam for the first scenario will reduce the PI to less than 1, which indicates an unprofitable process. This means that scenario 1 is unlikely to be profitable. In Fig. 6B, the curves for the second and third scenarios illustrate better results than scenario 1, as the PI is greater than that of scenario 1 (1.1 and 1.35 for the second and third scenarios, respectively, with 300% increase in steam price). A 400% increase in steam price makes the PI of second scenario reach 1, which means scenario 2 will no longer be profitable after any further increment in steam price. The economic feasibility of a process depends on various parameters, including factory location, raw material cost, production capacity, and interest rate. Techno-economic studies consider certain parameters to evaluate the whole process, including different feedstocks, the annual capacity of butanol production, interest rate, and different separation systems, leading to a variable range of butanol production costs and total production costs. Studies reveal that the production capacity can effectively help in the reduction of butanol production cost (Okoli and Adams, 2014; Mariano et al, 2013). On the other hand, choosing a feedstock with a lower price is shown to have a direct effect on the final product cost (Bhatia et al., 2020; Jiang et al., 2019). All in all, this study gives an overall assessment with marginal capacity, the lowest butanol production cost, and a relatively low total production cost. Using MSW as a low-cost feedstock, a high technology separation system, and choosing marginal capacity leads to a more economical process that has overcome the major bottlenecks of the biobutanol production process.
Table 3 A summary of capacity, total investment, and operating costs for each scenario. Scenario
Sc. 1
Sc. 2
Sc. 3
Capacity, butanol (tonnes/year) Internal rate of return % Investments (million $) Total Project Capital cost Total Operating Costa Total Raw Materials Costa Total Utilities Costa Total Product Salesa
36,900 20
39,000 54
39,200 80
38.7 62.2 3.48 53.4 64.6
38.9 54.3 3.48 45.9 77.4
31.0 44.7 3.48 37.0 77.4
a
Data are million $/year
required distillation columns by removing a significant fraction of water prior to using distillation system (Ismail and Matsuura, 2018). 3.1. Economic results Table 3 lists the main capital requirements obtained from the Aspen PEA calculation of each scenario. Processing 200,000 tonnes of BMSW per year, the designed scenarios are for relatively small plants with low capital investments both in costs and credits. A comparison between the reported results shows that for the second scenario, the capital raised is predictable, as it supports the gas stripping facilities in addition to all the equipment used in the first scenario. However, for the third scenario, gas stripping coupled with the pervaporation system decreases the capital costs. Although pervaporation is an advanced technology and an expensive system, its application results in lower total capital investments due to the consumption of fewer distillation columns than other scenarios (two columns instead of six). The differences in the utilities costs for these three scenarios are also the same as the changes in capital costs. Table 3 shows that the operating expenses are reduced from $62.2 million in the first scenario to $44.7 million in the third scenario with coupled gas stripping and pervaporation systems. As expected, employing the gas stripping system has led to the production of more ABE, and consequently, more credits in the second and third scenarios. To bring together all results, besides considering the economic indices such as payout period as an important index, it can be concluded that in the scenario coupled with gas stripping and pervaporation system has the best results as the payout period for this scenario is about three years, which is much lower compared to 16 and 4 years for the first and second scenarios, respectively. The total investment for this scenario was $31 million, compared to that of the base case, which was $38.7 million. A comparison of the total direct costs for each of the plants’ units can spot the most effective parameters of the total costs. Fig. 4 shows the cost distribution between different parts. As can be seen in all three scenarios, the unit for hydrolysis and fermentation has the greatest share of the plant’s total direct cost, because it employs the huge hydrolyzation and fermentation tanks that raised the expenses of this unit.
3.4. Butanol price compared with gasoline Calculating gasoline equivalent for a fuel is a tangible way to compare the prices of new fuels. Doing so primarily enables consumers to determine whether it is economical to shift from gasoline to them or not. Butanol gasoline equivalent is calculated based on the proportion of the lower heating value (LHV) for gasoline and butanol at standard pressure and temperature. This parameter is equal to 1.15 for butanol and means that the combustion of 1.15 L of butanol releases the same amount of energy as when 1 L of gasoline is burned. Considering 9% value-added tax (in Iran), the gasoline equivalent prices for biobutanol were in the range of 1.65 – 0.76 $/L (Table 4), which are less than biobutanol prices available as a chemical in the local market. However, the gasoline equivalent price for biobutanol is still higher than the gasoline price in Iran or the Persian Gulf countries. In Iran, petrochemical plants sell gasoline to the national Iranian oil products distribution company at FOB (Freight on Board) price of Persian Gulf, i.e., 0.31$/L – 0.40 $/L in 2016 (data from(www.platts.com, April 2019). However, the low-quality gasoline price in the market, i.e., 0.24 $/L, was provided by the subsidies from the Iranian government. In 2018, the Persian Gulf FOB price of gasoline had increased to 0.51 $/L. Accordingly, although zero income tax for biofuel production is used based on the Iran’s constitutional law, it is obvious that there is still a gap between the gasoline market price and the gasoline equivalent
3.2. Production costs Calculations showed the production of each kilogram of butanol costs $1.51, $0.95, and $0.70 for scenarios 1 to 3, respectively. These prices exclude value added tax (VAT). Thus, the difference between the production cost and the market price of butanol is relatively low for the first two scenarios, making these scenarios unreliable as profitable processing routes. However, for scenario 3, this difference is impressive, insofar as even with a 50% decrease in the price of butanol, the scenario remains profitable (Fig. 5). 3.3. Sensitivity analyses Sensitivity analyses are often studied along with those parameters 7
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Sc 3 Sc 2 Sc 1 -
1,00,00,000
2,00,00,000
3,00,00,000
4,00,00,000
Cost ($) Feed Handling
Pretreatment
Detoxification
Hydrolyze and fermentation
ABE Separation Fig. 5. Comparison of fixed capital investment of three scenarios for different units.
2
A
1.8 Production price, USD
Table 4 Comparison of butanol and biobutanol price with gasoline in different markets. Product price
Taxes/ Subsidies
Final Price
Final Price (gasoline equivalent)
Butanol (Chemical) Gasoline (Iran) Production price Biobutanol (Sc. 1) Biobutanol (Sc. 2) Biobutanol (Sc. 3) Gasoline (Sweden)f E85 (Sweden)f
1.6a 0.39b
0.14 −0.15c
1.74 0.24
2.00 0.24d
1.51 0.95 0.70 0.60 0.74
0.14e 0.09e 0.06e 1.14 0.58
1.65 1.04 0.76 1.74 1.32
1.89 1.19 0.88 1.74 1.90
1.6 1.4 1.2 1 0.8
a
0.6 0.4
b
Butanol market price 0
100
200
300
c
400
d
500
e f
Percentage of increment in price of H2SO4, % 1.6
B
1.5 1.4 1.3 1.2 1.1
4. Conclusions
1 0.9 0.8
Butanol price from local suppliers in 2016. Gasoline price in 2016 data from www.platts.com Subsidies imposed Gasoline price in 2016 which was sold at constant price in Iran Taxes imposed Data from: https://www.ingo.se/ [Accessed: March 2019]
chemicals in Iran. Yet, the biobutanol product can be an attractive renewable fuel in the international market. This is because biofuels still require financial support from the governments for being economically comparable to fossil fuels. An example of such supports in Sweden is presented in Table 4. In this country, addition of carbon dioxide and energy taxes to the gasoline price increases its price to 1.74 $/L (prices obtained from www.ingo.se, April 2019). Consequently, in Sweden, biofuels can compete with fossil fuels. The detailed calculations about different taxes in Sweden were presented by (Shafiei et al., 2014).
1.7
Profitability Index (PI)
Cost ($/L)
MSW is an environmental treat and widely available substrate with a negative price. The techno-economic evaluation for the production of high purity butanol, acetone, and ethanol from MSW revealed that the process could be economically feasible. However, employing gas stripping coupled with a pervaporation module had a significant impact on energy usage at the plant. It overcame the major obstacles of the process, making biobutanol production economically feasible. Nevertheless, water and energy optimizations are required to drive the biobutanol production through a more sustainable production process.
PI = 1 0%
100%
200%
300%
400%
500%
Change in steam price Fig. 6. (A) The effect of increase in the price of H2SO4 on butanol production cost (all the prices exclude VAT), and (B) Effect of changes in the price of steam on profitability indices for scenarios 1 (●), 2 (▲), and 3 (■).
CRediT authorship contribution statement
price for butanol. This gap is due to the high technology systems employed to produce butanol. Thus, the biorefineries in scenario 2 and 3 are economically profitable when their products were sold as bulk
Parisa Nazemi Ashani: Investigation, Modeling, Simulation, Techno-Economic Analysis, Software, Data Curation, Writing - original 8
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draft. Marzieh Shafiei: Investigation, Methodology, Resources, Validation, Software, Supervision, Visualization, Writing - review & editing. Keikhosro Karimi: Conceptualization, Methodology, Project administration, Supervision, Funding acquisition, Writing - review & editing.
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