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Systematic design of separation process for bioethanol production from corn stover

Systematic design of separation process for bioethanol production from corn stover Separation process is very crucial in bioethanol production as it consumes the highest energy in the process. Unlike other works, this research systematically designed a suitable separation process for bioethanol production from corn stover by using thermodynamic insight. Two separation processes, i.e., extractive distillation (case 2) and pervaporation (case 3), were developed and compared with conventional molecular sieve (case 1). Process design and simulation were done by using Aspen Plus program. The process evaluation was done not only in terms of energy consumption and process economics but also in terms of environmental impacts. It was revealed that pervaporation is the best process in all aspects. Its energy consumption and carbon footprint are 60.8 and 68.34% lower than case 1, respectively. Its capital and production costs are also the lowest, 37.0 and 9.88% lower than case Keywords: Bioethanol, Corn stover, Process design, Economic analysis, Life cycle analysis Introduction lignocellulosic resources for large scale ethanol produc- Bioethanol (C H OH) has been well-accepted and used tion [3]. 2 5 in the transportation sector worldwide. The first- Generally, the bioethanol production process from lig- generation bioethanol produced from food crops was nocellulosic biomass consists of four main steps, which found to be a suitable replacement for gasoline. It can are pretreatment, hydrolysis, fermentation, and separ- be fully or partially blended with gasoline effectively. ation [4]. The pretreatment step is used to remove lignin However, due to some considerable concern about com- and alter cellulose structures by increasing cellulose ac- petition with food crops [1], the second-generation cessibility for a further hydrolysis process. During this bioethanol produced from biomass such as agricultural step, hemicellulose inside the biomass is completely hy- wastes has gained much more attention in recent years. drolyzed into sugars, which can be directly converted to Lignocellulosic biomass has a great potential to be used ethanol in a later fermentation process [5, 6]. Furfural as feedstock for bioethanol production as it is cheap, and 5-hydroxymethyl furfural (HMF) that act as fermen- abundant, sustainable, and does not compete with food tation inhibitors are also formed in this step. [2]. There are enormous lignocellulosic biomass avail- The separation step is considered very important in able around the world, especially corn stover. Each year the design of the bioethanol production process as it corn stover is produced in vast quantities in many coun- consumes the highest energy in the process. Several tries. The management of this agricultural waste is es- novel separation techniques have been developed to sep- sential. Corn stover could be an excellent candidate for arate and purify ethanol more efficiently. In most cases, the distillation column is used as the critical method for * Correspondence: suksun.a@tggs.kmutnb.ac.th separation due to its performance and reliability. How- Chemical and Process Engineering Program, The Sirindhorn International ever, the purification method used for ethanol dehydra- Thai-German Graduate School of Engineering (TGGS), King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand tion may be various. The conventional method being Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 2 of 16 used commercially is a molecular sieve by the adsorption bioethanol production from corn stover was also done process [7, 8]. The principle of molecular sieve is based by Kazi et al. [19]. They designed and evaluated the eco- on the difference in molecular size between water and nomics of different process technologies for biochemical ethanol. Small molecules that can pass through the pores ethanol production from corn stover. Four pretreatment are adsorbed, while the larger molecules are not. Typic- technologies and three downstream process variations, ally, the molecular sieve for ethanol dehydration has a including Beer column, and pervaporation were investi- pore diameter of 3A°, capable of adsorbing water that gated by using available data from previous works. How- has a diameter of 2.5–2.8 A° but not ethanol that has a ever, they did not concern about the environmental diameter of 4–4.4 A° [9]. impact of the process. The use of pervaporation for Besides the conventional molecular sieve, extractive bioethanol production is still relatively new. Many works distillation is another method that can be used to pro- still need to be done to develop and prove its merit. duce anhydrous ethanol. The extractive distillation in- Thermodynamic insight is a methodology which em- volves two columns, which are extractive distillation ploys physicochemical properties of the substances to be column and recovery column [10]. A relatively non- separated and their relationships to determine appropri- volatile liquid solvent such as ethylene glycol is used to ate separation scheme. This methodology can be effect- change the relative volatilities of the components. In ively used to design and synthesize the appropriate Meirelles et al. work [11], extractive distillation using separation method [20]. Many research works have used ethylene glycol as a solvent was used for ethanol dehy- thermodynamic insight into the design process. For ex- dration. The experimental and simulation results showed ample, Holtbruegge [21] used the thermodynamic that extractive distillation could be used to achieve high insight for the conceptual design of flowsheet options purity of ethanol with low energy consumption. A com- that focus on process intensification. It was also used in parison of three ethanol dehydration techniques, includ- conjunction with the Analytical Hierarchy Process ing azeotropic distillation, extraction, and adsorption (AHP), heuristic synthesis approaches, and new hybrid was studied by Paola et al. [10]. It was revealed that ex- methodology to design and select suitable separation tractive distillation with ethylene glycol as a solvent is techniques by Li [22]. the best choice in terms of operation and economics. In this work, the thermodynamic insight was used as a The successful use of extractive distillation for bioetha- tool to determine and select an appropriate separation nol production has also been demonstrated by D. process for bioethanol production from corn stover. Un- Chuenbubpar et al. [12]. like other works, our designed processes are investigated Pervaporation is another promising separation tech- and evaluated in terms of both economics and environ- nique that can be used to produce anhydrous ethanol mental impacts. The design and simulation of the [13, 14]. Pervaporation is a kind of membrane separation process were performed by using the Aspen Plus simula- processes [15]. Principally, a liquid feed is separated into tion program. Two separation options, i.e., pervaporation two streams, which are permeate and retentate. The and extractive distillation, were obtained from using the water passes through the membrane as vapor permeate, thermodynamic insight. They were evaluated together while the ethanol remains in the liquid phase as reten- with the conventional molecular sieve process used in tate. The driving force of pervaporation is a pressure dif- commercials to make a fair comparison (https://www. ference created over the membrane. The vacuum is vogelbusch-biocommodities.com/process-units/dehydra- located on the permeate side while atmospheric pressure tion/molecular-sieve-process/). The evaluation was done is operated on the feed side, causing the pressure in terms of energy consumption, process economics, and difference. environmental impacts. The results of this study are sig- Hafrat et al. [16] developed a modeling equation of nificant as they can be used as supporting data for future pervaporation for dehydration of bioethanol by using design and development of optimal and more efficient Scilab software. The result showed that the pervapora- separation/purification schemes for bioethanol tion was able to break down the azeotrope of ethanol- production. water pointed out by Khan et al. [17]. Furthermore, Kunakorn et al. [18] compared azeotropic distillation Methods using entrainer (i.e., benzene, cyclohexane) with distilla- The production process in this work consists of two sec- tion followed by pervaporation by using Pro II. They tions: reaction and separation. The reaction occurred in found that the azeotropic distillation provided 99 wt.% the pretreatment step can be expressed by the following purity of ethanol, but its total cost was high due to high equations: The information of conversion of the reac- energy consumption. Thus, the hybrid system of distilla- tions is provided in Additional file 1. tion followed by pervaporation is the best in terms of Glucan þ H O→Glucose ð1Þ techno-economic point of view. Another work on Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 3 of 16 the thermodynamic insight method, while case 1 is Glucan þ H O→Glucose Oligomer ð2Þ based on the industrial separation process used in Glucan→HMF þ H O ð3Þ large-scale production. The simulation of all processes is performed by using the Aspen Plus simulation soft- Sucrose→Xylan þ H O→Xylose ð4Þ ware. Non-random two-liquid (NRTL) model was Xylan þ H O→Xylose ð5Þ employed to the activity coefficients [10, 24]. The de- sign basis is the plant capacity of approximately 24.5 Xylan þ H O→Xylose Oligomer ð6Þ tons/h, with the purity of product ethanol of 99% by Xylan→Furfural þ 2H O ð7Þ 2 weight. So raw material in case 1, case 2, and case 3 are required approximately 127 T/h, 104 T/h, and 104 Acetate→Acetic Acid ð8Þ T/h, respectively. The results of the simulation are Lignin→Soluble lignin ð9Þ evaluated and compared in terms of energy consump- tions, economics, and environmental impacts. The In the hydrolysis step, most celluloses are converted economic analysis is done based on a method of into fermentable sugars by some suitable enzymes such Guthrie [25], while environmental impacts assessment as Cellulase. This step is also known as enzymatic hy- is done by using LCSoft program [26]. drolysis process and has the reactions as follows: Process design Glucan þ H O→Glucose Oligomer ð10Þ In our design, we applied the thermodynamic insight Glucan þ 0:5H O→0:5Cellobiose ð11Þ method to identify appropriate separation techniques to be used in the separation section of the process. The Glucan þ H O→Glucose ð12Þ method employs physicochemical properties and their Cellobiose þ H O→2Glucose ð13Þ 2 relationships to separation techniques. Therefore, prop- erties of components are required, such as boiling point, The fermentable sugars obtained from the pretreat- vapor pressure, molecular diameter, molecular weight, ment and hydrolysis steps are sent to a fermenter kinetic diameter. where they are fermented in the presence of specific Different separations have different relationships be- microorganisms to produce ethanol. Then, the ethanol tween separation techniques and pure component is separated from other impurities and purified by ap- properties, as shown in Table 1. For example, distilla- propriate separation process [7, 23]. The reactions tion needs properties of boiling point and vapor pres- that take place during the fermentation can be shown sure, while molecular diameter (size) and molecular as below. weight are required for microfiltration. The computa- Glucose→2Ethanol þ 2CO ð14Þ tion of the binary ratio of properties of binary pairs is showninEq. 21. Glucose þ 2H O→2Glycerol þ O ð15Þ 2 2 Glucose þ 2CO →2Succinic Acid þ O ð16Þ p 2 2 A; j r ¼ ð21Þ ij B; j 3Xylose→5Ethanol þ 5CO ð17Þ 3Xylose þ 5H O→5Glycerol þ 2:5O ð18Þ 2 2 Xylose þ H O→Xylitol þ 0:5O ð19Þ 2 2 Where subscripts A and B represent the two compo- nents for the binary pair i. r is the binary ratio showing ij 3Xylose þ 5O →5Succinic Acid þ 2:5O ð20Þ 2 2 the feasible use of separation techniques. Note that P A,j Since this work focuses on the design of a suitable is component properties of component A which should separation process, the reaction section is not of the have a higher value than the component properties of primary concern. Therefore, we designed the reac- component B. Information of the binary ratio of proper- tion section of the bioethanol production from corn ties and a list of important pure component properties stover based on process data from work by Humbird and their classification are provided in Additional file 1. et al. [7]. All of those properties are considered thoroughly and For the separation section, there are three different compared with the recommended values for separation separation processes investigated in this work, namely, feasibility indices (see Additional file 1). If the value of a conventional molecular sieve (case 1), extractive distil- property for a particular separation technique is less lation (case 2), pervaporation (case 3). The separation than its recommended value, it is not appropriate or not process of case2 and case 3 are obtained from using feasible to use that technique for separation. Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 4 of 16 Table 1 The methodology for dehydration process Method for producing anhydrous ethanol Recommended values for separation feasibility indices Ratio of properties for Ethanol/Water Molecular sieve Kinetic diameter = 1.05 – Van der Waals volume = 1.07 1.71 Polarizability = 1.08 – Dipole moment = 1.05 1.09 Extractive distillation Vapor pressure = 0.00 2.49 Boiling point = 0.00 1.28 Pervaporation Molar volume = 3.20 3.23 Solubility parameter = 1.90 – After the fermentation is complete, the presence of distillation column to concentrate the ethanol to a a fraction of suspended solids and other residues in near azeotrope. Finally, the azeotropic mixture is sent the fermenter’s effluent cause low fluidity, subse- to dehydration to produce high purity ethanol of 99% quently decreasing the efficiency of ethanol separ- by weight. The feasible separation method for the de- ation. Therefore, it is necessary to remove the hydration in this research are identified from the suspended solids before the ethanol separation. Filtra- molar volume, dipole moment, vapor pressure, and tion was found to be an effective method to remove boiling point. Three feasible methods are chosen and solids and other residues in the effluent stream investigated in this work, which are pervaporation, ex- (https://www.vogelbusch-biocommodities.com/process- tractive distillation, and molecular sieve. These three units/dehydration/molecular-sieve-process/). After the methods were selected because the value of the ratio solids are removed by filtration, a flash operation is of properties for ethanol/water is higher than the rec- employed to separate gas phase components from the ommended values for separation feasibility indices, as liquid phase product. The flash operation can be used showninTable 1. From the binary ratio of molar ra- because the adjacent binary ratio of properties of tio between adjacent components, the ratio of water components of ethanol/CO is very high in terms of and ethanol is more dominant than vapor pressure. the boiling point and vapor pressure. Then, the liquid Thus, extractive distillation is preferable to simple product from the flash operation is fed to a distillation. From the binary ratio, ethanol and water Fig. 1 Block flow diagram of ethanol production process from corn stover – Case 1 (Conventional molecular sieve) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 5 of 16 are key components. So light components are ob- the cellulose inside the feed is converted into ferment- tained at the top of the column, and heavy compo- able sugars by using cellulose enzymes. So, this step is nents leave at the bottom stream. It should be noted also known as enzymatic hydrolysis [7, 31]. The hydroly- that the following assumptions were made for our de- sis is carried out at a temperature of 48 °C [7]. The main signs: steady-state operation, counting the number of components in the feed now are glucose and other stages from top to bottom, including condenser and sugars that are previously produced from the pretreat- reboiler. Also, the heat integration of distillation was ment step. The feed is then sent to fermenter operating not considered. at 32 °C to convert into ethanol by using suitable micro- organisms [32]. The liquid product containing ethanol Process description that is obtained from the fermentation is called beer Conventional process or broth. In addition to a small portion of ethanol, As depicted in Fig. 1, corn stover feed is mixed with the fermentation broth contains many impurities, water. Then the mixed feed is sent to pretreatment and therefore it is necessary to separate them by where it is exposed to dilute sulfuric acid at a proper separation methods. The state-of-the-art temperature of 158 °C. The hemicellulose inside the feed method is a series of distillation columns followed is converted to soluble sugars such as xylose, mannose, by a molecular sieve. First of all, the fermentation arabinose, and glucose via hydrolysis reaction. Fermenta- broth is sent to a Beer column. There, solid compo- tion inhibitors, such as furfural and 5-hydroxymethyl nents are removed at the bottom as stillage while furfural (HMF), are also formed in this step [13]. The gaseous components such as dissolved CO are re- feed is then sent to a solid/liquid separator to remove in- moved at the top. In the second distillation, called soluble solids. These two steps are the same as those in rectification column, water is separated and ethanol cases 1 and 2. After that, the feed is sent to detoxifica- is concentrated to near azeotrope [7, 33]. The prod- tion to eliminate the fermentation inhibitors. Over- uct that is obtained by this series of distillation pro- liming is employed as a means to remove the inhibitors. cesses is called hydrous ethanol, which has a purity Gypsum is produced as a by-product in this step [27– of ethanol about 95%. However, this purity is still 29]. However, it is essential to note that using the over- below the minimum purity requirement which is liming method has some drawbacks, as it can cause a typically 99%. Therefore, it is necessary to pass the sugar loss of about 10% through adsorption to lime [30]. hydrous ethanol to dehydration to purify it to an- Filtration is used to remove the produced gypsum in the hydrous ethanol. This is conventionally done by mo- feed before sending it to hydrolysis. In the hydrolysis, lecular sieve [7, 34, 35]. Fig. 2 Block flow diagram of ethanol production process from corn stover – Case 2 (Extractive distillation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 6 of 16 Alternative process It should be noted that the pervaporation model The production process of ethanol from corn stover be- used in this work was based on the work by Hafrat gins with the pretreatment of corn stover by using dilute et al. [16] since there is no pervaporation model sulfuric acid. After that, it is followed by a solid/liquid available in the Aspen Plus software. Thus, a calcula- separator to remove insoluble solids in the feed. Before tor block using a built-in Fortran code was created in entering the hydrolysis, it is vital to remove fermentation Aspen Plus to simulate the pervaporation model. In inhibitors from the feed, and this step is called detoxifi- principle, the diffusion passing through the membrane cation. It was found that using activated carbon is a very depends on the concentration of retentate and perme- effective method as it can remove 93% of furfural and ate, as shown in Eq. 22. 96% of 5-hydroxymethyl furfural (HMF), and, more im- portantly, without sugar loss [25, 36]. Hence, it is used Φ ¼ D C − C ð22Þ j i Rj Pj in our design cases 2 and 3. After the inhibitors are re- moved, the corn stover feed is sent to a neutralization Where Φ is a molar flux of component j, D is the j j unit to adjust the pH to a suitable condition. Then, it is diffusivity of component j, C is the concentration Rj passed to the hydrolysis and fermentation to convert cel- of component j in the liquid retentate, and C is the Pj lulose into fermentable sugars and ethanol, respectively. concentration of component j in the steam After the fermentation, the fermentation broth is sent to permeate. filtration to remove the suspended solids present in the The concentration of component j in the liquid fermentation broth. Then, it is followed by a flash retentate and concentration of component j in the operation to remove most of the dissolved CO at vapor permeate are calculated by Eqs. 23 and 24, low temperatures. After that, the liquid product is respectively. sent to the distillation column to concentrate the ethanol to near azeotrope. In the last step, the de- C ¼ w ð23Þ Rj Rj hydration, the concentrated ethanol is purified by pervaporation [37] to produce anhydrous ethanol (case 3). Alternatively, extractive distillation using Where C is the concentration of component j in the Rj ethylene glycol as a solvent [14] is investigated (case liquid retentate, ρ is the density of the liquid permeate 2). Block flow diagrams of the ethanol production (kg/m ), M is the molecular weight of component j (kg/ process from corn stover for both cases are illus- kmol), w is the mass fraction of compound j in the li- Ri trated in Figs. 2 and 3. quid retentate. Fig. 3 Block flow diagram of ethanol production process from corn stover – Case 3 (Pervaporation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 7 of 16 P consists of capital investment cost (such as fixed cap- C ¼ z ð24Þ Pj Pj ital cost and working capital cost), total production RT cost (such as manufacturing cost and general ex- Where C is the concentration of component j in the pj pense) [21]. For the impacts on the environment, life vapor permeate, P is the pressure of the permeate cycle assessment (LCA) is used as a method to evalu- through the membrane (atm), z is the mole fraction of Pj ate environmental impacts associated with all the compound j in the permeate, T is temperature (K), R is stages of a product’s life. For example, Kusolsongta- the ideal gas constant (R = 0.08206 L-atm/mol-K). wee et al. [39] evaluated the environmental impact of The coefficients of diffusivity for ethanol and water, the bioethanol production process from Ceratophyl- modified from Yeom and Huang [38], are obtained by lum demersum in terms of carbon footprint. This re- Eqs. 25 and 26, respectively. search analyzes the LCA by using LCSoft software − 9385 that is developed based on LCA methodology by D ¼ 2:028x10 e ð25Þ ethanol Kalakul et al. [26]. The software can estimate the im- − 9385 9 pact of a chemical process on the environment by D ¼ 1:158x10 e ð26Þ water using some input data, including energy consumption of each unit operation, type of energy sources, type of Process evaluation raw materials, amount of mass input, and amount of Sustainability of a process is determined by multiple mass output. The main result obtained from the soft- criteria, i.e., energy consumption, process economics, ware is presented in terms of carbon footprint (CO and environmental impacts. Economics of the eq.). Several other environmental impacts are also es- bioethanol production processes in this research is timated, which are human toxicity potential from in- carried out based on Method of Guthrie, which gestion and inhalation (HTPI), human toxicity Fig. 4 The production of ethanol production process from corn stover – Case 1 (Conventional molecular sieve) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 8 of 16 potential from dermal exposure (HTPE), aquatic toxic represented in the form of heat duty. They are sum- potential (ATP), Global warming potential (GWP), marizedand showninFig. 7. The energy consump- ozone depletion potential (ODP), photochemical oxi- tion of each unit is also provided in Table 4.For dation potential (PCOP), Acidification potential (AP), the reaction section, the energy consumptions of the Human toxicity carcinogens (HTC), human toxicity three cases are not much different. Furthermore, noncarcinogenic impact (HTNC), and ecotoxicological they are relatively much smaller compared to energy potential (ET) [22]. Noted that the system boundary usage in the separation section. However, cases 2 used in this work was gate-to-gate. Thus, plantation, and 3 were found to consume less energy than case transportation, and waste treatment are not 1 as a result of using activated carbon for the de- mentioned. toxification in place of the over-liming method. Nevertheless, the saving is not substantial and may Results and discussion even be neglected if we compare it with the total The ethanol production from corn stover with three energy saving. different separation processes was designed and simu- For the separation section, the energy consumption lated by using Aspen plus software. Process flow- in case 1 is much higher than the energy consump- sheets of the three processes are shown in Figs. 4, 5 tion in cases 2 and 3. The reason is that the binary and 6. The operating conditions used for each unit adjacent ratio of properties between ethanol and are summarized and shown in Tables 2 and 3.Inthe water of the molecular sieve is lower that of extract- next step, we analyze and compare the processes in ive distillation and pervaporation. Therefore, case 1 terms of energy consumption, process economics, and requires the Beer column and rectification column environmental impacts. to handle the ethanol separation before sending the ethanol to the molecular sieve. Moreover, because Energy consumption the relative volatility between water and ethanol is The results of energy consumptions consisting of closed to 1, azeotrope between ethanol and water is heating and cooling of the three processes are formed. It leads to a high reflux ratio of the columns Fig. 5 The production of ethanol production process from corn stover – Case 2 (Extractive distillation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 9 of 16 Fig. 6 The production of ethanol production process from corn stover – Case 3 (Pervaporation) and thus a high heat duty. From this viewpoint, the Table 2 The condition in the reactor and the separation distillation columns followed by the molecular sieve process is not a suitable separation process in terms of en- Type Condition ergy consumption. On the other hand, from the bin- Pretreatment Type: Dilute acid ary ratio, the molar volume gives the highest ratio in comparison with other properties. It indicates that Agent: H SO 2 4 the pervaporation would provide the highest driving Acid loading: 18 mg/g biomass forcefor theseparation. Theresults confirmedthat Temperature: 158 °C case 3 is the most appropriate process as its energy Pressure: 5.5 atm consumption is the lowest. Hydrolysis Temperature: 48 °C Specific energy consumption, defined as the ratio of Pressure: 1 atm the energy required per liter of ethanol produced, is de- termined. It was found that the specific energy con- Fermentation Agent: z. mobilis bacterium sumptions of case 1, 2, and 3 are 4599, 2352, and 2197 Temperature: 32 °C BTU/liter, respectively. It can be seen that both cases 2 Pressure: 1 atm and 3 are much more attractive than case 1 in terms of Beer column Number of stages: 32 energy efficiency. Since the energy consumptions for the Feed on stages: 4 reaction section are not much different, the high energy- Reflux ratio: 3 saving for cases 2 and 3 were obtained from the separ- ation section. To sum up, from the energy efficiency’s Rectification column Number of stages: 15 viewpoint, case 3 is more efficient for ethanol separation. Feed on stages: 7 It is also interesting to consider ethanol recovery from Reflux ratio: 2.5 different separation processes. Therefore, the ratios of Distillate rate: 530 kmol/hr ethanol product to ethanol feed to the separation section Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 10 of 16 Table 3 Operating conditions of equipment Equipment Parameters Operating conditions Distillation (case 1) Reflux ratio 2.5 Distillate rate 530 kmol/hr Number of stage 15 Feed stage 7 Flash (case 2,3) Temperature 32 °C Pressure 1 bar Extractive distillation (case 2) Reflux ratio 1 Distillate rate 445 kmol/hr Number of stage 12 Feed stage of crude ethanol 8 Feed stage of solvent 4 Solvent Ethylene glycol Ratio of solvent to feed 2 Recovery column (case 2) Reflux ratio 1 Distillate rate 82 kmol/hr Number of stage 9 Feed stage 4 Pervaporation (case 3) Temperature 129 °C Area of membrane 1950 m of the three cases were also determined and found to be To justify which case is more suitable, process econom- 0.64, 0.77, and 0.74, respectively. These results also sup- ics and environmental impacts need to be considered port that case 2 and 3 are more efficient processes as the further. ethanol recovery for both cases are significantly higher than the ethanol recovery of case 1. It can be noticed Economic analysis that the ethanol recovery of case 2 is slightly higher than The total capital investment costs of the three cases are case 3, even though its energy efficiency is slightly lower. illustrated in Fig. 8. It can be seen that the total capital Fig. 7 Energy consumption used in reaction and separation sections of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 11 of 16 Table 4 Energy consumption of each unit Unit Energy consumption (BTU/hr) Case 1 (base case) Case 2 Case 3 7 7 7 Pretreatment 1.47 × 10 1.47 × 10 1.47 × 10 4 4 4 Sep-01 1.29 × 10 1.29 × 10 1.29 × 10 7 7 7 DETOX 1.92 × 10 1.92 × 10 1.92 × 10 De-TOXIC 2.36 × 10 –– 7 7 7 Hydrolysis 1.34 × 10 1.93 × 10 1.93 × 10 7 6 6 Fermentation 2.29 × 10 2.40 × 10 2.40 × 10 Beer column-condenser 1.94 × 10 –– Beer column-reboiler 1.92 × 10 –– Rectification column-condenser 1.27 × 10 –– Rectification column-reboiler 1.27 × 10 –– Molecular sieve 3.16 × 10 –– 5 5 DETOC-AC – 1.05 × 10 1.05 × 10 6 6 Filtration – 1.97 × 10 1.97 × 10 −8 − 8 Flash – 8.81 × 10 8.81 × 10 Distillation column-condenser – 7.54 × 10 – Distillation column-reboiler – 7.70 × 10 – Extractive distillation-condenser – 3.71 × 10 – Extractive distillation-reboiler 4.69 × 10 Recovery column-condenser 6.56 × 10 Recovery column-reboiler 1.06 × 10 Pervaporation 2.33 × 10 Fig. 8 Total capital investment cost of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 12 of 16 investment cost of cases 2 and 3 are lower than the cost Table 5 Price of chemicals and utilities of case 1. The main reason is the use of filtration and Components Price References flash separator to remove to some suspended solids and Corn stover 59 ($/ton) [7] gaseous components from the fermentation broth before Sulfuric acid 250 ($/ton) [40] using the distillation column. In case 1, the Beer column Sodium hydroxide 300 ($/ton) [41] is used to perform these two tasks as well as to separate Lime 330 ($/ton) [41] ethanol. Since the column has to perform several tasks Ethanol 1.3 ($/gallon) [42] simultaneously, its sizing, together with the sizing of subsequent rectification column, are inevitably large. Ethylene glycol 1610 ($/ton) [43] The large sizing of Beer column and rectification col- Steam cost 4$/1000 gal [44] umn therefore resulted in very high total investment Cooling cost 0.05$/1000 gal [45] cost. Compared with case 3, case 2 uses the extractive distillation to purify the ethanol. This method, by nature, requires a high investment cost, too. Thus, it is unsur- prising that case 3 has the lowest total investment cost The reason that case 1 has a higher raw material cost among all, at 2.27 × 10 $. is that about 10% of sugar loss occurred during the The total production costs of the three cases, in- detoxification. This loss caused an increase in the re- cluding direct production costs and fixed charges, are quired amount of raw materials. For the utility con- illustrated in Fig. 9. Direct production costs include sumption, case 1 consumed very high utilities because raw materials, utilities, and other costs (i.e., the sum the sizings of the Beer column and rectification col- of labor, catalyst and solvent, maintenance and repair, umn are very large, to be able to handle the large and insurance). The prices of raw material and util- volume of feed. Hence, the heating (steam) and cool- ities are shown in Table 5. Fixed charge costs consist ing energy required to operate the reboilers and con- of depreciation, local taxes, and laboratory analysis. densers of the columns are consequently very high From the figure, it can be noticed that the raw mater- too. The total production cost of case 2 was found to ial cost is the most significant factor of the total pro- be slightly higher than in case 3. It is because case 2 duction cost as it contributes to the significant part uses the extraction distillation and recovery column of the cost. The economic calculation revealed that for ethanol purification. So, there is a need for solv- direct production costs contribute to 98–99% of the ent and higher utilities for the columns’ operation. It total production cost. In general, it can be seen that can be concluded that case 3 is the most economical case 1 has the highest total production cost, mainly process with the lowest total production cost of due to its high raw material and utility consumptions. 5.39 × 10 $. Fig. 9 Total production cost of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 13 of 16 Fig. 10 The amount of carbon footprint of different ethanol production processes Environmental impact carbon footprint in case 3 was found to be the low- It is widely accepted that carbon footprint can be est, at 0.575 CO eq. It is mostly generated from the used as a key parameter that represents the impact distillation column (T-301) that separates of a process on the environment. It is generally gen- wastewater. erated by the combustion of fuel, such as natural gas The comparisons of other environmental impact used to produce utilities required for the process. indicators of the three cases were summarized and Such combustion is, for example, steam production illustrated in Figs. 11, 12 and 13.Theyare HTPI in aboilerorchilled waterproductionthrough an (1/LD50),HTPE(1/TWA),ATP (1/LC50), GWP absorption chiller. The amounts of the carbon foot- (CO equivalent), ODP (CFC-11 equivalent), PCOP print of the three processes were shown in Fig. 10. (C H equivalent), AP (H equivalent), HTC (kg of 2 2 The results reveal that case 1 releases the highest benzene equivalent), HTNC (kg toluene equiva- amount of carbon footprint, at 1.816 CO eq. It is a lent), and ET (kg 2, 4-dichlorophenoxyacetic acid direct consequence of using the high amount of util- equivalent). Overall, it can be concluded that case ities in the Beer column and rectifying column. The 3 is the most environmentally friendly process as it Fig. 11 HTPI (1/LD 50), HTPE (1/TWA), ATP (1/LC 50), and ODP (CFC-11 equivalent) of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 14 of 16 Fig. 12 GWP (CO equivalent), AP (H equivalent), and HTNC (kg toluene equivalent) of different ethanol production processes has the lowest environmental impacts for most 3 is 60.8 and 6.6% lower, while the carbon footprint gen- categories. erated is 68.3 and 24.2% lower, respectively. The design of the pervaporation process is less complicated than the Conclusion other cases, resulting in the lowest total capital cost and Guided by thermodynamic insight, extractive distillation total production cost. (case 2) and pervaporation (case 3) were obtained as al- Even though pervaporation was found to be the best ternative separation processes for ethanol production separation process, more works can be done to improve from corn stover. Their performances were investigated the process performance. One potential improvement is in terms of energy consumption, economic analysis, and to recover the ethanol loss to wastewater at the perva- environmental impacts and compared with conventional poration. It may be done by installing a distillation col- molecular sieve (case 1). It was revealed that the perva- umn to separate water from the wastewater and send poration (case 3) is the best separation process. Com- back the concentrated ethanol to the pervaporator to re- pared to case 1 and case 2, the total energy used by case cover more ethanol. Fig. 13 PCOP (C H equivalent), HTC (kg of benzene equivalent), and ET (kg 2, 4-dichlorophenoxyacetic acid equivalent) of different ethanol 2 2 production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 15 of 16 Supplementary information 5. Lavaracka BP, Griffin GJ, Rodman D. 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Systematic design of separation process for bioethanol production from corn stover

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Abstract

Separation process is very crucial in bioethanol production as it consumes the highest energy in the process. Unlike other works, this research systematically designed a suitable separation process for bioethanol production from corn stover by using thermodynamic insight. Two separation processes, i.e., extractive distillation (case 2) and pervaporation (case 3), were developed and compared with conventional molecular sieve (case 1). Process design and simulation were done by using Aspen Plus program. The process evaluation was done not only in terms of energy consumption and process economics but also in terms of environmental impacts. It was revealed that pervaporation is the best process in all aspects. Its energy consumption and carbon footprint are 60.8 and 68.34% lower than case 1, respectively. Its capital and production costs are also the lowest, 37.0 and 9.88% lower than case Keywords: Bioethanol, Corn stover, Process design, Economic analysis, Life cycle analysis Introduction lignocellulosic resources for large scale ethanol produc- Bioethanol (C H OH) has been well-accepted and used tion [3]. 2 5 in the transportation sector worldwide. The first- Generally, the bioethanol production process from lig- generation bioethanol produced from food crops was nocellulosic biomass consists of four main steps, which found to be a suitable replacement for gasoline. It can are pretreatment, hydrolysis, fermentation, and separ- be fully or partially blended with gasoline effectively. ation [4]. The pretreatment step is used to remove lignin However, due to some considerable concern about com- and alter cellulose structures by increasing cellulose ac- petition with food crops [1], the second-generation cessibility for a further hydrolysis process. During this bioethanol produced from biomass such as agricultural step, hemicellulose inside the biomass is completely hy- wastes has gained much more attention in recent years. drolyzed into sugars, which can be directly converted to Lignocellulosic biomass has a great potential to be used ethanol in a later fermentation process [5, 6]. Furfural as feedstock for bioethanol production as it is cheap, and 5-hydroxymethyl furfural (HMF) that act as fermen- abundant, sustainable, and does not compete with food tation inhibitors are also formed in this step. [2]. There are enormous lignocellulosic biomass avail- The separation step is considered very important in able around the world, especially corn stover. Each year the design of the bioethanol production process as it corn stover is produced in vast quantities in many coun- consumes the highest energy in the process. Several tries. The management of this agricultural waste is es- novel separation techniques have been developed to sep- sential. Corn stover could be an excellent candidate for arate and purify ethanol more efficiently. In most cases, the distillation column is used as the critical method for * Correspondence: suksun.a@tggs.kmutnb.ac.th separation due to its performance and reliability. How- Chemical and Process Engineering Program, The Sirindhorn International ever, the purification method used for ethanol dehydra- Thai-German Graduate School of Engineering (TGGS), King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand tion may be various. The conventional method being Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 2 of 16 used commercially is a molecular sieve by the adsorption bioethanol production from corn stover was also done process [7, 8]. The principle of molecular sieve is based by Kazi et al. [19]. They designed and evaluated the eco- on the difference in molecular size between water and nomics of different process technologies for biochemical ethanol. Small molecules that can pass through the pores ethanol production from corn stover. Four pretreatment are adsorbed, while the larger molecules are not. Typic- technologies and three downstream process variations, ally, the molecular sieve for ethanol dehydration has a including Beer column, and pervaporation were investi- pore diameter of 3A°, capable of adsorbing water that gated by using available data from previous works. How- has a diameter of 2.5–2.8 A° but not ethanol that has a ever, they did not concern about the environmental diameter of 4–4.4 A° [9]. impact of the process. The use of pervaporation for Besides the conventional molecular sieve, extractive bioethanol production is still relatively new. Many works distillation is another method that can be used to pro- still need to be done to develop and prove its merit. duce anhydrous ethanol. The extractive distillation in- Thermodynamic insight is a methodology which em- volves two columns, which are extractive distillation ploys physicochemical properties of the substances to be column and recovery column [10]. A relatively non- separated and their relationships to determine appropri- volatile liquid solvent such as ethylene glycol is used to ate separation scheme. This methodology can be effect- change the relative volatilities of the components. In ively used to design and synthesize the appropriate Meirelles et al. work [11], extractive distillation using separation method [20]. Many research works have used ethylene glycol as a solvent was used for ethanol dehy- thermodynamic insight into the design process. For ex- dration. The experimental and simulation results showed ample, Holtbruegge [21] used the thermodynamic that extractive distillation could be used to achieve high insight for the conceptual design of flowsheet options purity of ethanol with low energy consumption. A com- that focus on process intensification. It was also used in parison of three ethanol dehydration techniques, includ- conjunction with the Analytical Hierarchy Process ing azeotropic distillation, extraction, and adsorption (AHP), heuristic synthesis approaches, and new hybrid was studied by Paola et al. [10]. It was revealed that ex- methodology to design and select suitable separation tractive distillation with ethylene glycol as a solvent is techniques by Li [22]. the best choice in terms of operation and economics. In this work, the thermodynamic insight was used as a The successful use of extractive distillation for bioetha- tool to determine and select an appropriate separation nol production has also been demonstrated by D. process for bioethanol production from corn stover. Un- Chuenbubpar et al. [12]. like other works, our designed processes are investigated Pervaporation is another promising separation tech- and evaluated in terms of both economics and environ- nique that can be used to produce anhydrous ethanol mental impacts. The design and simulation of the [13, 14]. Pervaporation is a kind of membrane separation process were performed by using the Aspen Plus simula- processes [15]. Principally, a liquid feed is separated into tion program. Two separation options, i.e., pervaporation two streams, which are permeate and retentate. The and extractive distillation, were obtained from using the water passes through the membrane as vapor permeate, thermodynamic insight. They were evaluated together while the ethanol remains in the liquid phase as reten- with the conventional molecular sieve process used in tate. The driving force of pervaporation is a pressure dif- commercials to make a fair comparison (https://www. ference created over the membrane. The vacuum is vogelbusch-biocommodities.com/process-units/dehydra- located on the permeate side while atmospheric pressure tion/molecular-sieve-process/). The evaluation was done is operated on the feed side, causing the pressure in terms of energy consumption, process economics, and difference. environmental impacts. The results of this study are sig- Hafrat et al. [16] developed a modeling equation of nificant as they can be used as supporting data for future pervaporation for dehydration of bioethanol by using design and development of optimal and more efficient Scilab software. The result showed that the pervapora- separation/purification schemes for bioethanol tion was able to break down the azeotrope of ethanol- production. water pointed out by Khan et al. [17]. Furthermore, Kunakorn et al. [18] compared azeotropic distillation Methods using entrainer (i.e., benzene, cyclohexane) with distilla- The production process in this work consists of two sec- tion followed by pervaporation by using Pro II. They tions: reaction and separation. The reaction occurred in found that the azeotropic distillation provided 99 wt.% the pretreatment step can be expressed by the following purity of ethanol, but its total cost was high due to high equations: The information of conversion of the reac- energy consumption. Thus, the hybrid system of distilla- tions is provided in Additional file 1. tion followed by pervaporation is the best in terms of Glucan þ H O→Glucose ð1Þ techno-economic point of view. Another work on Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 3 of 16 the thermodynamic insight method, while case 1 is Glucan þ H O→Glucose Oligomer ð2Þ based on the industrial separation process used in Glucan→HMF þ H O ð3Þ large-scale production. The simulation of all processes is performed by using the Aspen Plus simulation soft- Sucrose→Xylan þ H O→Xylose ð4Þ ware. Non-random two-liquid (NRTL) model was Xylan þ H O→Xylose ð5Þ employed to the activity coefficients [10, 24]. The de- sign basis is the plant capacity of approximately 24.5 Xylan þ H O→Xylose Oligomer ð6Þ tons/h, with the purity of product ethanol of 99% by Xylan→Furfural þ 2H O ð7Þ 2 weight. So raw material in case 1, case 2, and case 3 are required approximately 127 T/h, 104 T/h, and 104 Acetate→Acetic Acid ð8Þ T/h, respectively. The results of the simulation are Lignin→Soluble lignin ð9Þ evaluated and compared in terms of energy consump- tions, economics, and environmental impacts. The In the hydrolysis step, most celluloses are converted economic analysis is done based on a method of into fermentable sugars by some suitable enzymes such Guthrie [25], while environmental impacts assessment as Cellulase. This step is also known as enzymatic hy- is done by using LCSoft program [26]. drolysis process and has the reactions as follows: Process design Glucan þ H O→Glucose Oligomer ð10Þ In our design, we applied the thermodynamic insight Glucan þ 0:5H O→0:5Cellobiose ð11Þ method to identify appropriate separation techniques to be used in the separation section of the process. The Glucan þ H O→Glucose ð12Þ method employs physicochemical properties and their Cellobiose þ H O→2Glucose ð13Þ 2 relationships to separation techniques. Therefore, prop- erties of components are required, such as boiling point, The fermentable sugars obtained from the pretreat- vapor pressure, molecular diameter, molecular weight, ment and hydrolysis steps are sent to a fermenter kinetic diameter. where they are fermented in the presence of specific Different separations have different relationships be- microorganisms to produce ethanol. Then, the ethanol tween separation techniques and pure component is separated from other impurities and purified by ap- properties, as shown in Table 1. For example, distilla- propriate separation process [7, 23]. The reactions tion needs properties of boiling point and vapor pres- that take place during the fermentation can be shown sure, while molecular diameter (size) and molecular as below. weight are required for microfiltration. The computa- Glucose→2Ethanol þ 2CO ð14Þ tion of the binary ratio of properties of binary pairs is showninEq. 21. Glucose þ 2H O→2Glycerol þ O ð15Þ 2 2 Glucose þ 2CO →2Succinic Acid þ O ð16Þ p 2 2 A; j r ¼ ð21Þ ij B; j 3Xylose→5Ethanol þ 5CO ð17Þ 3Xylose þ 5H O→5Glycerol þ 2:5O ð18Þ 2 2 Xylose þ H O→Xylitol þ 0:5O ð19Þ 2 2 Where subscripts A and B represent the two compo- nents for the binary pair i. r is the binary ratio showing ij 3Xylose þ 5O →5Succinic Acid þ 2:5O ð20Þ 2 2 the feasible use of separation techniques. Note that P A,j Since this work focuses on the design of a suitable is component properties of component A which should separation process, the reaction section is not of the have a higher value than the component properties of primary concern. Therefore, we designed the reac- component B. Information of the binary ratio of proper- tion section of the bioethanol production from corn ties and a list of important pure component properties stover based on process data from work by Humbird and their classification are provided in Additional file 1. et al. [7]. All of those properties are considered thoroughly and For the separation section, there are three different compared with the recommended values for separation separation processes investigated in this work, namely, feasibility indices (see Additional file 1). If the value of a conventional molecular sieve (case 1), extractive distil- property for a particular separation technique is less lation (case 2), pervaporation (case 3). The separation than its recommended value, it is not appropriate or not process of case2 and case 3 are obtained from using feasible to use that technique for separation. Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 4 of 16 Table 1 The methodology for dehydration process Method for producing anhydrous ethanol Recommended values for separation feasibility indices Ratio of properties for Ethanol/Water Molecular sieve Kinetic diameter = 1.05 – Van der Waals volume = 1.07 1.71 Polarizability = 1.08 – Dipole moment = 1.05 1.09 Extractive distillation Vapor pressure = 0.00 2.49 Boiling point = 0.00 1.28 Pervaporation Molar volume = 3.20 3.23 Solubility parameter = 1.90 – After the fermentation is complete, the presence of distillation column to concentrate the ethanol to a a fraction of suspended solids and other residues in near azeotrope. Finally, the azeotropic mixture is sent the fermenter’s effluent cause low fluidity, subse- to dehydration to produce high purity ethanol of 99% quently decreasing the efficiency of ethanol separ- by weight. The feasible separation method for the de- ation. Therefore, it is necessary to remove the hydration in this research are identified from the suspended solids before the ethanol separation. Filtra- molar volume, dipole moment, vapor pressure, and tion was found to be an effective method to remove boiling point. Three feasible methods are chosen and solids and other residues in the effluent stream investigated in this work, which are pervaporation, ex- (https://www.vogelbusch-biocommodities.com/process- tractive distillation, and molecular sieve. These three units/dehydration/molecular-sieve-process/). After the methods were selected because the value of the ratio solids are removed by filtration, a flash operation is of properties for ethanol/water is higher than the rec- employed to separate gas phase components from the ommended values for separation feasibility indices, as liquid phase product. The flash operation can be used showninTable 1. From the binary ratio of molar ra- because the adjacent binary ratio of properties of tio between adjacent components, the ratio of water components of ethanol/CO is very high in terms of and ethanol is more dominant than vapor pressure. the boiling point and vapor pressure. Then, the liquid Thus, extractive distillation is preferable to simple product from the flash operation is fed to a distillation. From the binary ratio, ethanol and water Fig. 1 Block flow diagram of ethanol production process from corn stover – Case 1 (Conventional molecular sieve) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 5 of 16 are key components. So light components are ob- the cellulose inside the feed is converted into ferment- tained at the top of the column, and heavy compo- able sugars by using cellulose enzymes. So, this step is nents leave at the bottom stream. It should be noted also known as enzymatic hydrolysis [7, 31]. The hydroly- that the following assumptions were made for our de- sis is carried out at a temperature of 48 °C [7]. The main signs: steady-state operation, counting the number of components in the feed now are glucose and other stages from top to bottom, including condenser and sugars that are previously produced from the pretreat- reboiler. Also, the heat integration of distillation was ment step. The feed is then sent to fermenter operating not considered. at 32 °C to convert into ethanol by using suitable micro- organisms [32]. The liquid product containing ethanol Process description that is obtained from the fermentation is called beer Conventional process or broth. In addition to a small portion of ethanol, As depicted in Fig. 1, corn stover feed is mixed with the fermentation broth contains many impurities, water. Then the mixed feed is sent to pretreatment and therefore it is necessary to separate them by where it is exposed to dilute sulfuric acid at a proper separation methods. The state-of-the-art temperature of 158 °C. The hemicellulose inside the feed method is a series of distillation columns followed is converted to soluble sugars such as xylose, mannose, by a molecular sieve. First of all, the fermentation arabinose, and glucose via hydrolysis reaction. Fermenta- broth is sent to a Beer column. There, solid compo- tion inhibitors, such as furfural and 5-hydroxymethyl nents are removed at the bottom as stillage while furfural (HMF), are also formed in this step [13]. The gaseous components such as dissolved CO are re- feed is then sent to a solid/liquid separator to remove in- moved at the top. In the second distillation, called soluble solids. These two steps are the same as those in rectification column, water is separated and ethanol cases 1 and 2. After that, the feed is sent to detoxifica- is concentrated to near azeotrope [7, 33]. The prod- tion to eliminate the fermentation inhibitors. Over- uct that is obtained by this series of distillation pro- liming is employed as a means to remove the inhibitors. cesses is called hydrous ethanol, which has a purity Gypsum is produced as a by-product in this step [27– of ethanol about 95%. However, this purity is still 29]. However, it is essential to note that using the over- below the minimum purity requirement which is liming method has some drawbacks, as it can cause a typically 99%. Therefore, it is necessary to pass the sugar loss of about 10% through adsorption to lime [30]. hydrous ethanol to dehydration to purify it to an- Filtration is used to remove the produced gypsum in the hydrous ethanol. This is conventionally done by mo- feed before sending it to hydrolysis. In the hydrolysis, lecular sieve [7, 34, 35]. Fig. 2 Block flow diagram of ethanol production process from corn stover – Case 2 (Extractive distillation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 6 of 16 Alternative process It should be noted that the pervaporation model The production process of ethanol from corn stover be- used in this work was based on the work by Hafrat gins with the pretreatment of corn stover by using dilute et al. [16] since there is no pervaporation model sulfuric acid. After that, it is followed by a solid/liquid available in the Aspen Plus software. Thus, a calcula- separator to remove insoluble solids in the feed. Before tor block using a built-in Fortran code was created in entering the hydrolysis, it is vital to remove fermentation Aspen Plus to simulate the pervaporation model. In inhibitors from the feed, and this step is called detoxifi- principle, the diffusion passing through the membrane cation. It was found that using activated carbon is a very depends on the concentration of retentate and perme- effective method as it can remove 93% of furfural and ate, as shown in Eq. 22. 96% of 5-hydroxymethyl furfural (HMF), and, more im- portantly, without sugar loss [25, 36]. Hence, it is used Φ ¼ D C − C ð22Þ j i Rj Pj in our design cases 2 and 3. After the inhibitors are re- moved, the corn stover feed is sent to a neutralization Where Φ is a molar flux of component j, D is the j j unit to adjust the pH to a suitable condition. Then, it is diffusivity of component j, C is the concentration Rj passed to the hydrolysis and fermentation to convert cel- of component j in the liquid retentate, and C is the Pj lulose into fermentable sugars and ethanol, respectively. concentration of component j in the steam After the fermentation, the fermentation broth is sent to permeate. filtration to remove the suspended solids present in the The concentration of component j in the liquid fermentation broth. Then, it is followed by a flash retentate and concentration of component j in the operation to remove most of the dissolved CO at vapor permeate are calculated by Eqs. 23 and 24, low temperatures. After that, the liquid product is respectively. sent to the distillation column to concentrate the ethanol to near azeotrope. In the last step, the de- C ¼ w ð23Þ Rj Rj hydration, the concentrated ethanol is purified by pervaporation [37] to produce anhydrous ethanol (case 3). Alternatively, extractive distillation using Where C is the concentration of component j in the Rj ethylene glycol as a solvent [14] is investigated (case liquid retentate, ρ is the density of the liquid permeate 2). Block flow diagrams of the ethanol production (kg/m ), M is the molecular weight of component j (kg/ process from corn stover for both cases are illus- kmol), w is the mass fraction of compound j in the li- Ri trated in Figs. 2 and 3. quid retentate. Fig. 3 Block flow diagram of ethanol production process from corn stover – Case 3 (Pervaporation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 7 of 16 P consists of capital investment cost (such as fixed cap- C ¼ z ð24Þ Pj Pj ital cost and working capital cost), total production RT cost (such as manufacturing cost and general ex- Where C is the concentration of component j in the pj pense) [21]. For the impacts on the environment, life vapor permeate, P is the pressure of the permeate cycle assessment (LCA) is used as a method to evalu- through the membrane (atm), z is the mole fraction of Pj ate environmental impacts associated with all the compound j in the permeate, T is temperature (K), R is stages of a product’s life. For example, Kusolsongta- the ideal gas constant (R = 0.08206 L-atm/mol-K). wee et al. [39] evaluated the environmental impact of The coefficients of diffusivity for ethanol and water, the bioethanol production process from Ceratophyl- modified from Yeom and Huang [38], are obtained by lum demersum in terms of carbon footprint. This re- Eqs. 25 and 26, respectively. search analyzes the LCA by using LCSoft software − 9385 that is developed based on LCA methodology by D ¼ 2:028x10 e ð25Þ ethanol Kalakul et al. [26]. The software can estimate the im- − 9385 9 pact of a chemical process on the environment by D ¼ 1:158x10 e ð26Þ water using some input data, including energy consumption of each unit operation, type of energy sources, type of Process evaluation raw materials, amount of mass input, and amount of Sustainability of a process is determined by multiple mass output. The main result obtained from the soft- criteria, i.e., energy consumption, process economics, ware is presented in terms of carbon footprint (CO and environmental impacts. Economics of the eq.). Several other environmental impacts are also es- bioethanol production processes in this research is timated, which are human toxicity potential from in- carried out based on Method of Guthrie, which gestion and inhalation (HTPI), human toxicity Fig. 4 The production of ethanol production process from corn stover – Case 1 (Conventional molecular sieve) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 8 of 16 potential from dermal exposure (HTPE), aquatic toxic represented in the form of heat duty. They are sum- potential (ATP), Global warming potential (GWP), marizedand showninFig. 7. The energy consump- ozone depletion potential (ODP), photochemical oxi- tion of each unit is also provided in Table 4.For dation potential (PCOP), Acidification potential (AP), the reaction section, the energy consumptions of the Human toxicity carcinogens (HTC), human toxicity three cases are not much different. Furthermore, noncarcinogenic impact (HTNC), and ecotoxicological they are relatively much smaller compared to energy potential (ET) [22]. Noted that the system boundary usage in the separation section. However, cases 2 used in this work was gate-to-gate. Thus, plantation, and 3 were found to consume less energy than case transportation, and waste treatment are not 1 as a result of using activated carbon for the de- mentioned. toxification in place of the over-liming method. Nevertheless, the saving is not substantial and may Results and discussion even be neglected if we compare it with the total The ethanol production from corn stover with three energy saving. different separation processes was designed and simu- For the separation section, the energy consumption lated by using Aspen plus software. Process flow- in case 1 is much higher than the energy consump- sheets of the three processes are shown in Figs. 4, 5 tion in cases 2 and 3. The reason is that the binary and 6. The operating conditions used for each unit adjacent ratio of properties between ethanol and are summarized and shown in Tables 2 and 3.Inthe water of the molecular sieve is lower that of extract- next step, we analyze and compare the processes in ive distillation and pervaporation. Therefore, case 1 terms of energy consumption, process economics, and requires the Beer column and rectification column environmental impacts. to handle the ethanol separation before sending the ethanol to the molecular sieve. Moreover, because Energy consumption the relative volatility between water and ethanol is The results of energy consumptions consisting of closed to 1, azeotrope between ethanol and water is heating and cooling of the three processes are formed. It leads to a high reflux ratio of the columns Fig. 5 The production of ethanol production process from corn stover – Case 2 (Extractive distillation) Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 9 of 16 Fig. 6 The production of ethanol production process from corn stover – Case 3 (Pervaporation) and thus a high heat duty. From this viewpoint, the Table 2 The condition in the reactor and the separation distillation columns followed by the molecular sieve process is not a suitable separation process in terms of en- Type Condition ergy consumption. On the other hand, from the bin- Pretreatment Type: Dilute acid ary ratio, the molar volume gives the highest ratio in comparison with other properties. It indicates that Agent: H SO 2 4 the pervaporation would provide the highest driving Acid loading: 18 mg/g biomass forcefor theseparation. Theresults confirmedthat Temperature: 158 °C case 3 is the most appropriate process as its energy Pressure: 5.5 atm consumption is the lowest. Hydrolysis Temperature: 48 °C Specific energy consumption, defined as the ratio of Pressure: 1 atm the energy required per liter of ethanol produced, is de- termined. It was found that the specific energy con- Fermentation Agent: z. mobilis bacterium sumptions of case 1, 2, and 3 are 4599, 2352, and 2197 Temperature: 32 °C BTU/liter, respectively. It can be seen that both cases 2 Pressure: 1 atm and 3 are much more attractive than case 1 in terms of Beer column Number of stages: 32 energy efficiency. Since the energy consumptions for the Feed on stages: 4 reaction section are not much different, the high energy- Reflux ratio: 3 saving for cases 2 and 3 were obtained from the separ- ation section. To sum up, from the energy efficiency’s Rectification column Number of stages: 15 viewpoint, case 3 is more efficient for ethanol separation. Feed on stages: 7 It is also interesting to consider ethanol recovery from Reflux ratio: 2.5 different separation processes. Therefore, the ratios of Distillate rate: 530 kmol/hr ethanol product to ethanol feed to the separation section Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 10 of 16 Table 3 Operating conditions of equipment Equipment Parameters Operating conditions Distillation (case 1) Reflux ratio 2.5 Distillate rate 530 kmol/hr Number of stage 15 Feed stage 7 Flash (case 2,3) Temperature 32 °C Pressure 1 bar Extractive distillation (case 2) Reflux ratio 1 Distillate rate 445 kmol/hr Number of stage 12 Feed stage of crude ethanol 8 Feed stage of solvent 4 Solvent Ethylene glycol Ratio of solvent to feed 2 Recovery column (case 2) Reflux ratio 1 Distillate rate 82 kmol/hr Number of stage 9 Feed stage 4 Pervaporation (case 3) Temperature 129 °C Area of membrane 1950 m of the three cases were also determined and found to be To justify which case is more suitable, process econom- 0.64, 0.77, and 0.74, respectively. These results also sup- ics and environmental impacts need to be considered port that case 2 and 3 are more efficient processes as the further. ethanol recovery for both cases are significantly higher than the ethanol recovery of case 1. It can be noticed Economic analysis that the ethanol recovery of case 2 is slightly higher than The total capital investment costs of the three cases are case 3, even though its energy efficiency is slightly lower. illustrated in Fig. 8. It can be seen that the total capital Fig. 7 Energy consumption used in reaction and separation sections of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 11 of 16 Table 4 Energy consumption of each unit Unit Energy consumption (BTU/hr) Case 1 (base case) Case 2 Case 3 7 7 7 Pretreatment 1.47 × 10 1.47 × 10 1.47 × 10 4 4 4 Sep-01 1.29 × 10 1.29 × 10 1.29 × 10 7 7 7 DETOX 1.92 × 10 1.92 × 10 1.92 × 10 De-TOXIC 2.36 × 10 –– 7 7 7 Hydrolysis 1.34 × 10 1.93 × 10 1.93 × 10 7 6 6 Fermentation 2.29 × 10 2.40 × 10 2.40 × 10 Beer column-condenser 1.94 × 10 –– Beer column-reboiler 1.92 × 10 –– Rectification column-condenser 1.27 × 10 –– Rectification column-reboiler 1.27 × 10 –– Molecular sieve 3.16 × 10 –– 5 5 DETOC-AC – 1.05 × 10 1.05 × 10 6 6 Filtration – 1.97 × 10 1.97 × 10 −8 − 8 Flash – 8.81 × 10 8.81 × 10 Distillation column-condenser – 7.54 × 10 – Distillation column-reboiler – 7.70 × 10 – Extractive distillation-condenser – 3.71 × 10 – Extractive distillation-reboiler 4.69 × 10 Recovery column-condenser 6.56 × 10 Recovery column-reboiler 1.06 × 10 Pervaporation 2.33 × 10 Fig. 8 Total capital investment cost of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 12 of 16 investment cost of cases 2 and 3 are lower than the cost Table 5 Price of chemicals and utilities of case 1. The main reason is the use of filtration and Components Price References flash separator to remove to some suspended solids and Corn stover 59 ($/ton) [7] gaseous components from the fermentation broth before Sulfuric acid 250 ($/ton) [40] using the distillation column. In case 1, the Beer column Sodium hydroxide 300 ($/ton) [41] is used to perform these two tasks as well as to separate Lime 330 ($/ton) [41] ethanol. Since the column has to perform several tasks Ethanol 1.3 ($/gallon) [42] simultaneously, its sizing, together with the sizing of subsequent rectification column, are inevitably large. Ethylene glycol 1610 ($/ton) [43] The large sizing of Beer column and rectification col- Steam cost 4$/1000 gal [44] umn therefore resulted in very high total investment Cooling cost 0.05$/1000 gal [45] cost. Compared with case 3, case 2 uses the extractive distillation to purify the ethanol. This method, by nature, requires a high investment cost, too. Thus, it is unsur- prising that case 3 has the lowest total investment cost The reason that case 1 has a higher raw material cost among all, at 2.27 × 10 $. is that about 10% of sugar loss occurred during the The total production costs of the three cases, in- detoxification. This loss caused an increase in the re- cluding direct production costs and fixed charges, are quired amount of raw materials. For the utility con- illustrated in Fig. 9. Direct production costs include sumption, case 1 consumed very high utilities because raw materials, utilities, and other costs (i.e., the sum the sizings of the Beer column and rectification col- of labor, catalyst and solvent, maintenance and repair, umn are very large, to be able to handle the large and insurance). The prices of raw material and util- volume of feed. Hence, the heating (steam) and cool- ities are shown in Table 5. Fixed charge costs consist ing energy required to operate the reboilers and con- of depreciation, local taxes, and laboratory analysis. densers of the columns are consequently very high From the figure, it can be noticed that the raw mater- too. The total production cost of case 2 was found to ial cost is the most significant factor of the total pro- be slightly higher than in case 3. It is because case 2 duction cost as it contributes to the significant part uses the extraction distillation and recovery column of the cost. The economic calculation revealed that for ethanol purification. So, there is a need for solv- direct production costs contribute to 98–99% of the ent and higher utilities for the columns’ operation. It total production cost. In general, it can be seen that can be concluded that case 3 is the most economical case 1 has the highest total production cost, mainly process with the lowest total production cost of due to its high raw material and utility consumptions. 5.39 × 10 $. Fig. 9 Total production cost of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 13 of 16 Fig. 10 The amount of carbon footprint of different ethanol production processes Environmental impact carbon footprint in case 3 was found to be the low- It is widely accepted that carbon footprint can be est, at 0.575 CO eq. It is mostly generated from the used as a key parameter that represents the impact distillation column (T-301) that separates of a process on the environment. It is generally gen- wastewater. erated by the combustion of fuel, such as natural gas The comparisons of other environmental impact used to produce utilities required for the process. indicators of the three cases were summarized and Such combustion is, for example, steam production illustrated in Figs. 11, 12 and 13.Theyare HTPI in aboilerorchilled waterproductionthrough an (1/LD50),HTPE(1/TWA),ATP (1/LC50), GWP absorption chiller. The amounts of the carbon foot- (CO equivalent), ODP (CFC-11 equivalent), PCOP print of the three processes were shown in Fig. 10. (C H equivalent), AP (H equivalent), HTC (kg of 2 2 The results reveal that case 1 releases the highest benzene equivalent), HTNC (kg toluene equiva- amount of carbon footprint, at 1.816 CO eq. It is a lent), and ET (kg 2, 4-dichlorophenoxyacetic acid direct consequence of using the high amount of util- equivalent). Overall, it can be concluded that case ities in the Beer column and rectifying column. The 3 is the most environmentally friendly process as it Fig. 11 HTPI (1/LD 50), HTPE (1/TWA), ATP (1/LC 50), and ODP (CFC-11 equivalent) of different ethanol production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 14 of 16 Fig. 12 GWP (CO equivalent), AP (H equivalent), and HTNC (kg toluene equivalent) of different ethanol production processes has the lowest environmental impacts for most 3 is 60.8 and 6.6% lower, while the carbon footprint gen- categories. erated is 68.3 and 24.2% lower, respectively. The design of the pervaporation process is less complicated than the Conclusion other cases, resulting in the lowest total capital cost and Guided by thermodynamic insight, extractive distillation total production cost. (case 2) and pervaporation (case 3) were obtained as al- Even though pervaporation was found to be the best ternative separation processes for ethanol production separation process, more works can be done to improve from corn stover. Their performances were investigated the process performance. One potential improvement is in terms of energy consumption, economic analysis, and to recover the ethanol loss to wastewater at the perva- environmental impacts and compared with conventional poration. It may be done by installing a distillation col- molecular sieve (case 1). It was revealed that the perva- umn to separate water from the wastewater and send poration (case 3) is the best separation process. Com- back the concentrated ethanol to the pervaporator to re- pared to case 1 and case 2, the total energy used by case cover more ethanol. Fig. 13 PCOP (C H equivalent), HTC (kg of benzene equivalent), and ET (kg 2, 4-dichlorophenoxyacetic acid equivalent) of different ethanol 2 2 production processes Amornraksa et al. BMC Chemical Engineering (2020) 2:10 Page 15 of 16 Supplementary information 5. Lavaracka BP, Griffin GJ, Rodman D. 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Published: Oct 12, 2020

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