Simulation of the Process for Producing Butanol from Corn

May 6, 2009 - This study focuses on the simulation of a complete process for producing ... its commercial production through acetone–butanol–ethan...
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Ind. Eng. Chem. Res. 2009, 48, 5551–5557

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Simulation of the Process for Producing Butanol from Corn Fermentation Jiahong Liu, May Wu,* and Michael Wang Center for Transportation Research, Argonne National Laboratory, Argonne, Illinois 60439

This study focuses on the simulation of a complete process for producing butanol via acetone, butanol, and ethanol corn fermentation. The simulation, which begins with grain processing and proceeds through product purification, represents the first attempt to simulate such a complete process. Energy use for the production process is highlighted and compared to that for the conventional corn ethanol process. The simulation results are utilized in a lifecycle assessment for butanol as a potential transportation fuel. The lifecycle assessment study is conducted using the transportation full lifecycle assessment model, Greenhouse Gases, Regulated Emissions and Energy Use in Transportation (GREET), that has been developed by Argonne National Laboratory. A variety of key parameters are examined, such as the state of the art of the unit operations included in the process and their key process parameters, as well as their effects on the total energy consumption and greenhouse gas emissions in the lifecycle of butanol. 1. Introduction Acetone, butanol, and ethanol (ABE) fermentation by Clostridium acetobutylicum was the major route to producing butanol prior to the 1950s. This process began to phase out when the more economical petrochemical routes emerged. Currently, almost all butanol is produced from petrochemical feedstocks. Nevertheless, research interest recently has been rekindled toward the development of viable ABE fermentation processes, relative to pursuing non-fossil-based butanol production for potential transportation applications. For the past 20 years, experimental and computational engineering attempts to improve the efficiency of the ABE fermentation process have involved various aspects of the procedure. These attempts have included the development of new fermentation strains to improve product yield; the design of new schemes to minimize butanol inhibition through new fermenter configurations, new downstream processing (product isolation and purification) methods and flowsheeting; and the integration of fermentation and downstream processing. For instance, Huang et al.1 reported an experimental process that used continuous immobilized cultures of Clostridium tyrobutyricum and Clostridium acetobutylicum to maximize the production of hydrogen and butyric acid and convert butyric acid to butanol in two separate steps. This process produces butanol at a productivity of 4.64 g/(L h) and utilizes 42% glucose, in comparison with the potential 25% glucose utilization found in traditional ABE fermentation by Clostridium acetobutylicum alone. Qureshi et al.2 and Parek et al.3 published their experimental and pilot-scale ABE fermentation processes by Clostridium beijerinckii. With the various downstream processing schemes that they developed, the glucose utilization reaches 95.1%.4 Some recent studies on the integration of fermentation and downstream processing also have shown promising results.2,4-8 An exhaustive survey specific to major research results for ABE downstream processing can be found in the work of Liu.9 Computer simulation, as a major tool in process engineering to scale up experimental results and provide meaningful predictions on the performance of manufacturing plants, also has been adopted in the development of biobutanol production * To whom correspondence should be addressed. Tel.: 1-630-2526658. Fax: 1-630-252-3443. E-mail address: [email protected].

processes. The earliest efforts on ABE fermentation downstream processing simulation were reported in the work of Marlatt et al.10 and Dadgar et al.,11 based on which of the economics of their processes were evaluated. More recent studies were published in the work of Liu and co-workers9,12,13 In these studies, downstream processing systems are synthesized, simulated, and optimized, in terms of cost. The number of publications regarding ABE fermentation process simulation is limited. In the current study, the simulation of ABE fermentation and downstream processing is performed using Aspen Plus (Aspen Technologies, Inc., Cambridge, MA). The model, which was developed by the U.S. Department of Agriculture (USDA)14 for the upstream processing of the ethanol fermentation process, is then integrated into this simulation for the sections before the ABE fermentation, including corn grain processing up to saccharification. In the remainder of this paper, we first present the basis of the simulation, including plant capacity, product specifications, and scope of the simulation. The simulated process is described thereafter, together with the input parameters for the simulation. The upstream raw material processing also is included to provide the context of the simulated process. Energy uses in the fermentation, the downstream processing sections, and the entire production process are compared with those for the conventional corn ethanol production plant. Based on the simulation result, the lifecycle assessment for butanol as a potential transportation fuel is performed separately and the result is presented here briefly. The lifecycle assessment study is conducted using the transportation full lifecycle assessment model, Greenhouse Gases, Regulated Emissions and Energy Use in Transportation (GREET),15 which was developed by Argonne National Laboratory. Various key parameters are discussed, such as the state of the art of unit operations included in the process and their key process parameters, as well as their effects on the overall energy consumption and greenhouse gas (GHG) emissions during the lifecycle. The lifecycle assessment result for butanol is also compared with transportation fuel ethanol. 2. Basis of the Simulation The current simulation of ABE fermentation and downstream processing for producing butanol from corn starch by ABE fermentation is performed using the Aspen Plus simulation on

10.1021/ie900274z CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

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Figure 1. Schematic for the process of grain receiving, liquefaction, and saccharification.

a cluster of five Dell Pentium 4 CPU computers (3.40 GHz, 2.0 GB RAM). The simulation is based on the research results from a pilot-scale ABE fermentation plant4 and from other process engineering studies.9,12,13 The butanol production capacity of the simulated plant is set at 150 000 t/yr. The operation runs 315 days a year. Product and byproduct purity specifications are as follows: butanol, 99-99.5 wt %; acetone purity, 99 wt %; and ethanol purity, >99.5 wt %. 3. Overview of the ABE Process The butanol production from ABE fermentation simulated in this study is based on a process described by Qureshi and Blaschek.2,5-7 In this process, butanol, acetone, and ethanol are produced by a hyper-butanol-producing strain, C. beijerinckii BA101, that is able to tolerate the toxicity that results from a high butanol concentration. Extensive studies and pilot testing have been performed to characterize this strain.3 The model that was developed by the USDA16 for the upstream processing of the ethanol fermentation process is integrated into this simulation for the sections before ABE fermentation. Corn is first fed into a conventional corn dry grind for conversion to glucose through liquefication and saccharification. The glucose is fermented to ABE through a fed-batch system with recycling of fermentation gases CO2 and H2. After fermentation, acetone, butanol, and ethanol are removed by in situ gas stripping, using a fermentation gas mixture of CO2 and H2. Products are recovered through three-stage distillation and molecular sieve adsorption that separates acetone, butanol, ethanol, and water. Solids and biomass that are removed from the fermenter undergo centrifugation and drying. Excess CO2 and small amounts of H2 gases are vented to the atmosphere. The distillers dried grains and solubles (DDGS) generated from the fermentation and distillations are used as animal feed. Given herein is a detailed description of the process, which is comprised of three main parts: grain pretreatment, fermentation, and downstream processing. 3.1. Grain Receiving, Liquefaction, and Saccharification. This section describes part of the so-called “conventional dry grind ethanol process”, beginning with grain receiving and proceeding through saccharification, as modeled in the previously mentioned USDA Aspen Plus simulation of the corn ethanol process.16 Figure 1 shows the major steps in this segment. As indicated earlier, we use data derived from this

part of the corn ethanol process as if it were for the ABE fermentation process, except for capacity scaling from 40 million gallons of ethanol per year (MMGY) to 150 000 t (equivalent to a capacity of 89 MMGY) of butanol per year in the current work. Corn brought into the plant site is first separated from finer particles and foreign objects, using a blower and screens. The cleaned dry corn is ground in a hammer mill and weighed to control the feed rate to the process. The ground corn is then mixed with water, thermostable R-amylase, ammonia, and lime in a slurry tank. A steam injection heater is used to gelatinize starch, and the system is held at pH 6.5 for 60 min at 88 °C with agitation. This step is called liquefaction. The starch is then hydrolyzed by R-amylase into oligosaccharides. Slurry viscosity sharply increases in the gelatinization step and then rapidly decreases during the hydrolysis. The stream is then “cooked” and held at 110 °C for 15 min and subsequently is transferred to the saccharification tank, where the oligosaccharides are converted by glucoamylase to glucose. First, the pH in this tank is reduced to 4.5 by sulfuric acid and the slurry is allowed to stand for 5 h. Next, glucoamylase is added at 0.11% (dry base), and the oligosaccharides are hydrolyzed into glucose at a temperature of 61 °C. During this step, almost all the oligosaccharides are converted to glucose. The slurry is then cooled to 35 °C and transferred to the fermenter. 3.2. Fermentation and In Situ Gas Stripping. Fermentation is the process that converts glucose to butanol, acetone, ethanol, carbon dioxide, hydrogen, and other chemicals under anaerobic conditions. This section describes the fermentation process based on the research results from Blaschek and co-workers2,4-8 for their pilot-scale ABE fermentation testing plant. The slurry from the saccharification is introduced to an evaporator, where the glucose concentration is increased to 500 g/L (from 24 g/L). The product stream from the evaporator is fed to the oxygen-free fermentation vessel, where glucose is inoculated with C. beijerinckii BA101. The temperature is controlled at 35 °C, and no agitation or pH adjustment is applied during the process. The fermentation proceeds for 22 h. When ABE reaches ∼5 g/L, gas stripping is applied. In the condenser above the fermenter, the ABE vapors are cooled to 2 °C and condensed to liquids, leaving uncondensed carbon dioxide and hydrogen in the vapor phase, which are sent through the carbon dioxide scrubber prior to venting. The gas stripping captures most of the butanol, acetone, and ethanol produced during fermentation. The selectivity of the stripping for butanol is defined as selectivity )

y/(1 - y) x/(1 - x)

where x is the concentration of butanol in fermentation broth (in wt %) and y is the concentration of butanol in the condensate (in units of wt %). Here, the selectivity value is set at 20.6,7 Accordingly, 25 wt % water is present in the condensate. It will be removed from the product and byproducts in the downstream processing stage. The extent of fermentation conversion is set according to experimental and process data. Glucose utilization is 95.1%, with 4.9% glucose converted to other solids (yeast cells). The butanol, acetone, ethanol, acetic acid, and butyric acid yields are 0.303, 0.155, 0.0068, 0.0086, and 0.0084 g/(g glucose), respectively. Major reactions involved in the glucose fermentation include the following: C6H12O6 f C4H10O (butanol) + 2CO2+H2O

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C6H12O6 + H2O f C3H6O (acetone) + 3CO2+4H2 C6H12O6 f 2C2H5O (ethanol) + 2CO2+H2 C6H12O6 f C4H8O2 (butyric acid) + 2CO2+2H2 and C6H12O6 f 3C2H4O2 (acetic acid) 3.3. Downstream Processing. The liquid fermentation broth from the condenser is comprised of water, butanol, acetone, and ethanol, with boiling points of 100, 118, 56, and 78 °C, respectively. The broth is subject to a series of distillation operations where, based on their volatilities, the stream is separated into three product streams that are 99 wt % pure butanol, 99 wt % pure acetone, and 99.5 wt % pure ethanol, respectively. In a mixture that contains butanol, ethanol, and water, a homogeneous ethanol-water azeotrope and a heterogeneous water-butanol azeotrope are formed. In descending order of their volatilities, and including the two azeotropes, the components in the stream to be separated by the downstream processing are acetone, an azeotrope of ethanol and water, ethanol, an azeotrope of water and butanol, water, and butanol. The components in this sequence can possibly or plausibly be separated simply by fractional distillation, fulfilled by various simple and/or complex column configurations, into any two or more subsequences of components between the adjacent components whose volatilities differ sufficiently. The fermentation broth is first fed to the distillation tower, where a compositional cut is made between the three components (acetone, ethanol, and water, which are withdrawn in the top stream) and the component butanol, together with a trace amount of water, which goes to the bottom stream as a product with a butanol purity of 99 wt % (that specified for transportation fuel use). The top stream is sent to the second distillation tower, where acetone is concentrated, purified, and then sent outward from the top. The bottom stream contains mainly ethanol and water. It is subject to further separation in the third distillation tower, where the azeotropic vapor of 94.4 wt % ethanol and

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5.3 wt % water (and 2 wt % acetone) is withdrawn from the top and sent to an adsorption unit. Water stream from the bottom is sent back to the fermenter. Water in the top stream is adsorbed in this unit, and ethanol purity in the product stream reaches 99.5%, above which the purity of ethanol is required for ethanol to be blended with gasoline and used in vehicles. The adsorption unit consists of two adsorption columns that run cyclically between the adsorption and desorption phases. Molecular sieves are packed in the column as adsorbents. They are composed of a microporous substance, designed to separate small polar molecules from larger nonpolar molecules via a sieving action. Water molecules are trapped and adsorbed inside the microporous beads, while the ethanol molecules flow around them. Molecular sieves are then regenerated by heat and carrier air. The schematic of the adsorption and adsorbent regeneration process is presented in Figure 2. The heat use for the regeneration of adsorbents is calculated as the total amount required to bring the system to the regeneration temperature of 500 °F, which includes the adsorption column, the molecular sieves, the adsorbed water, and the carrier gas. Cooling of the system is fulfilled by exchanging the heat with the process streams fed to the distillation towers. Therefore, there is no external energy used to cool the system. The weight of water that the molecular sieves adsorb is set at 22% of the weight of the molecular sieve. The peak concentration of water in the carrier air is set at 1.7 wt %. The adsorption and regeneration cycle time is set at 8 h. The amount of heat required to bring the column to the specified temperature is calculated by the method based on the logarithmic mean temperature difference and the scaling of experimental data. The radiation loss factor is set at 5%. 4. Process Flow Sheet and the Simulation Figure 3 shows the process simulation flow sheet from the grain mill to the downstream processing. Data for the simulation of fermentation and gas stripping unit operations are based on the research results from Blaschek et al., at the University of

Figure 2. Schematic representation of alternately operated adsorption and adsorbent regeneration.

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Figure 3. Aspen Plus process flow sheet. Table 1. Main Input Parameters of Fermentation and Gas Stripping for the Aspen Plus Simulationa input parameter fermenter operating temperature fermenter operating pressure glucose utilization in fermentation yield acetone butanol ethanol ABE (total) acetic acid butyric acid hydrogen carbon dioxide ABE productivity gas stripping carrier gas gas stripping selectivity for butanol gas recycle rate condensation temperature a

value used in the simulation

Table 2. Parameters for the Calculation of Energy Use for Adsorbent Regeneration

value range reported in literature

35 °C 1 atm 500.1 g/L

30-67 °C n/a 45.4-500.1 g/L

0.155 g/(g glucose) 0.303 g/(g glucose) 0.0068 g/(g glucose) 0.465 g/(g glucose) 0.0086 g/(g glucose) 0.0084 g/(g glucose) 0.021 g/(g glucose) 0.6954 g/(g glucose) 1.16 g/(L h) fermentation gas 20.0

n/a n/a n/a 0.27-0.47 g/(g glucose) n/a n/a n/a n/a 0.17-1.16 g/(L h) fermentation gas 40.0-30.5

3.00 L/(L min) 2 °C

0.67-10.00 L/(L min) -60-4 °C

Data taken from refs 4, 6, and 7.

Illinois at Urbana-Champaign, for their ABE fermentation facility. The main parameters for fermentation and gas stripping input to the Aspen Plus simulation are summarized in Table 1. The downstream processing flow sheet in the simulation is based on the most cost-effective flow sheet rigorously generated from an exhaustive list of plausible processing equipment and unit operations by the work of Liu and co-workers.9,12,13 According to their study, this optimal flow sheet consists of a gas stripper that isolates a liquid product stream from the fermentation broth; an adsorption unit that removes the majority of water; and a set of distillation columns that purify the butanol, ethanol, and acetone products. Their study shows that the cost of this flow sheet is at least 12.5% lower than that of any other alternative flow sheet. Calculation for the energy consumption of the regeneration of the molecular sieve adsorbents is performed through a Fortran subroutine in the Aspen Plus simulation. Table 2 summarizes the parameters and results. The current simulation integrates the new fermentation and gas stripping unit operations into the existing downstream processing system. Shortcut methods that highlight the mass

water adsorbed inside molecular adsorption molecular sieves sieves column amount (lb/h) Cp (Btu/(lb °F)) T1 (°F) T2 (°F) ∆T (°F) Q (Btu/h) total (Btu/h)

55.6 1.0 175.0 500.0 325.0 18053.8

2020.0 0.2 175.0 500.0 325.0 150995.0 810227.9

n/a n/a 175.0 500.0 325.0 343706.1

carrier gas: air 3333.0 0.2 175.0 500.0 325.0 258890.8

and energy balances are chosen to simulate most of the unit operations in the process. Heat integration between the two distillation columns is taken into consideration. Not included in this simulation are stream recycling and simple unit operations, such as valves. Different distillation column configurations are not examined. 5. Results and Discussions In this section, we obtain the mass and energy balances from the Aspen Plus simulation and calculate the process fuel use for the energy generation in the fermentation and downstream processing steps. Also presented are the mass and energy balances for the complete production process, for which the calculation and assumptions can be found in the work of Wu et al.17 The mass and energy balances are compared to those of a conventional corn ethanol plant. Based on these data, we calculate the GHG emissions in the lifecycle of butanol as a potential transportation fuel, using the GREET model. The results also are compared to those for the corn ethanol plant. We qualitatively analyze the effect of product yield and the ratio on energy use. 5.1. Product Yield and Energy Use in the Corn Butanol Plant. Major chemicals produced from the ABE process include butanol and acetone at a ratio of the former to the latter at 2:1. An appreciable amount of ethanol is also produced in this process. 5.1.1. Product Yield and Output Energy. The product yield and output energy (contained in the products) per unit of corn input for each major product, as well as the energy output share among the products, are summarized in Table 3. The output

Ind. Eng. Chem. Res., Vol. 48, No. 11, 2009 Table 3. Yields of Acetone, Butanol, and Ethanol from the Bio-Butanol Plant product

yield (gal/bushel corn)

energy (Btu/bushel corn)

energy output share (%)

acetone butanol ethanol

0.868 1.495 0.037

69525 149267 2828

31.4 67.4 1.2

total

2.4

221621

100

Table 4. Process Fuel Use for ABE Fermentation and Downstream Processing process step

steam (Btu/h)

evaporation fermenter agitation condensation gas pumping gas stripping adsorption feed pumping distillation 1 distillation 2 distillation 3 distillation 2 feed pumping distillation 3 feed pumping adsorbent regeneration

278 653 091

electricity (kW) 192.73 3105.40 61.04

1 307 400 0.24 43 220 100 62 665 700 5 730 860 0.40 0.03 237.33

-729 205

Table 5. Total Thermal Energy Consumption for the Corn-Based Butanol Production Plant Thermal Energy (Natural Gas) process step

(Btu/bushel corn)

(Btu/gal butanol)

cooking drying ABE fermentation and downstream processing

27 236 29 458

18 216 19 703 77 195

total

115 114

Table 6. Total Electricity Consumption for the Corn-Based Butanol Production Plant Electricity process step sections excluding ABE fermentation and downstream processing ABE fermentation and downstream processing total

(kW/bushel corn) (kW/gal butanol) 1.84

1.23 0.53 1.76

energy for each product is calculated based on its lower heating value. The lower heating value, not including the heat obtained by the condensation, is used because the product would be utilized in an internal combustion engine in an automobile that exhausts water vapor produced by combustion without condensing it. The detailed calculation and analysis can be found in the work of Wu et al.17 5.1.2. Energy Use. Table 4 lists the energy use, in terms of steam use and electricity use, in each major process step of fermentation and downstream processing. Tables 5 and 6 present the thermal energy and electricity consumptions for each step or section in the entire production process and their total amounts, respectively. As indicated in Table 4, steam is mainly used for distillation towers (i.e., the reboilers for the distillation towers), whereas electricity is mainly consumed for condensation. As presented in Tables 5 and 6, in the entire production process, ABE fermentation and downstream processing accounts for ∼67% of thermal energy (natural gas) consumption and 30% of electricity consumption, respectively. This implies that enhancement in the energy efficiency in the ABE fermentation and downstream processing could lead to a more significant

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savings in natural gas consumption than in electricity consumption. 5.1.3. Effect of Product Yield and Ratio. The product yields obtained in the simulation reflect one of the ABE fermentation and downstream processing technologies currently available at a small scale.2,4-8 Lower yields are expected at a commercial scale. The enhancement of the product yields and the alteration of the relative amounts of product are dependent on the development of a new strain for fermentation and a new fermentation process, integration of the fermentation process, as well as adherence to product purity requirements. Among the three major products, butanol and ethanol are energy products; the former can potentially be used in transportation fuel, whereas the latter is currently used as a transportation fuel. To increase the energy product yield without flushing the current market for acetone, it is ideal to adopt a process that favors butanol and ethanol production while minimizing the production of acetone. This can be accomplished through the development of new strains and utilization of alternative sugar substrates. For instance, it is reported that, using the mixture of hexose and pentose sugar, the ratio between butanol and acetone yield can reach greater than 3:1.18 Furthermore, butanol has substantially advantageous features over ethanol, such as lower vapor pressure; lower corrosion, which makes pipeline transportation possible; and higher energy content. Therefore, a process that can maximize the yield of butanol is most desired. This again calls for discoveries and breakthroughs in genetic engineering. Energy use is dependent on the product purity requirements for end use, as well as the composition of product in fermentation broth. The distillation towers that consume the most natural gas are used to separate butanol, acetone, and ethanol. Given the specific fermentation broth composition, the energy use of the distillation towers can be reduced appreciably (as high as 30%) if the requirement on the purity of butanol is reduced to 99 wt % from 99.5 wt %. In addition, if the purity of ethanol is reduced to 96 wt %, at which it could be used in a vehicle engine that solely uses ethanol, the adsorption could be totally eliminated, thus leading to substantial savings in both energy and capital costs. On the other hand, if the ideal process can be developed with the maximum production of butanol and a minimum production of acetone and ethanol, the energy uses of Distillation 2 (for purifying acetone) and Distillation 3 and adsorption (for purifying ethanol) would all be minimized. To the extreme, if the production of acetone and ethanol could be totally eliminated, leaving butanol the only product of the process, Distillation 2, Distillation 3, and adsorption would be unnecessary, resulting in the ultimate maximal savings in natural gas consumption. 5.2. Comparison with the Conventional Corn Ethanol Plant. The output energy in products, process fuel use, thermal energy, and electricity consumption for the corn butanol plant also are compared to those for the conventional corn ethanol plant. The results are summarized in Tables 7 and 8. A detailed comparison can be found in the work of Wu et al.17 5.2.1. Yield and Output Energy in Products. As indicated in Table 7, neither the yield of energy products (ethanol and butanol) nor the output energy per bushel of corn in the current corn butanol plant is as high as in the corn ethanol plant. While 2.60 gallons of ethanol can be produced per bushel of corn in the conventional corn ethanol plant, only 1.54 gallons of butanol and ethanol combined can be produced per bushel of corn in the current corn butanol plant. Accordingly, in the conventional corn ethanol plant, 198 458 Btu per bushel of corn is output in

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Table 7. Comparison with the Corn Ethanol Plant: Yield and Output Energy (Contained in Products) Corn Butanol Plant butanol

ethanol

total of butanol and ethanol

ethanol

0.87 69525

1.50 149267

0.04 2828

1.54 152095

2.60 198458

yield (gal/bushel corn) output energy (Btu/bushel corn)

Table 8. Comparison with the Corn Ethanol Plant: Process Fuel Use corn corn ratio of butanol butanol plant ethanol plant plant to ethanol plant natural gas (Btu/bushel corn) electricity (kW/bushel corn)

Corn Ethanol Plant

acetone

172108

94320

1.82

2.63

1.97

1.34

ethanol; however, in the corn butanol plant, only 152 095 Btu per bushel of corn is output in butanol and ethanol combined. The improvement in yield and output energy for the current corn butanol plant can mainly be achieved through the development of new fermentation schemes, such as new strains and utilization of substrates. 5.2.2. Energy Use. As shown in Table 8, the current corn butanol plant uses 172 108 Btu of natural gas to process 1 bushel of corn, while the amount for the conventional corn ethanol plant is 94 320 Btu of natural gas. This represents 82% more natural gas consumption. As for electricity use, Table 8 presents a moderate 34% greater use in the current corn butanol plant than in the conventional corn ethanol plant. The increase in natural gas consumption in the corn butanol plant can be attributed partially to the evaporation, which accounts for ∼70% of the total natural gas usage, whereas the increase in electricity consumption is mainly due to the use of gas stripping and condensation, which accounts for ∼90% of the total electricity usage. According to Liu,9 the use of gas stripping and condensation is advantageous over other alternatives. Energy use would not be reduced by replacing it with other currently available technologies. It is worth noting that the adoption of the evaporation reduced the volume of stream to be treated further downstream at the same time, resulting in a reduction in energy use and capital cost. Therefore, the elimination of the evaporation would not necessarily lead to a savings in energy use. Based on the comparison with the corn ethanol plant, the current corn butanol plant consumes more raw materials and energy to produce a unit of energy products. For the current corn butanol plant to be as competitive, substantial enhancement in the process of the ABE plant must be accomplished through further advances in science and engineering. Advances throughout the process, such as development of novel fermentation strains, utilization of more effective substrates, adoption of efficient schemes for downstream processing, and process integration, could contribute to achieving this goal. 5.3. Lifecycle Assessment of Corn Butanol as a Potential Transportation Fuel. To place the current corn butanol process into greater perspective, we present the results of energy use and GHG emissions in the lifecycle of butanol as a potential transportation fuel, based on the simulation result obtained using the transportation full lifecycle GREET model.15 The detailed lifecycle corn butanol study and the comparative studies with corn ethanol and petroleum gasoline can be found in the work of Wu et al.17 Briefly, the lifecycle of petroleum gasoline includes feedstock (petroleum) processing (recovery), feedstock (petroleum) transportation, fuel (gasoline) production (petroleum refinery), fuel (gasoline) transportation and blending, and gasoline vehicle operation. The lifecycle of both corn butanol and ethanol includes feedstock (corn) processing (farming), fertilizer production, machinery manufacturing, feedstock (corn)

transportation, fuel (butanol and ethanol) production, fuel (butanol and ethanol) transportation and blending, and gasoline vehicle (for butanol) or flexible fuel vehicle (for ethanol) operation. Figure 4 illustrates the fossil energy use at different stages of the butanol lifecycle. As shown in the diagram, the butanol production plant consumes 73% of the total fossil energy in the corn butanol lifecycle. This indicates that butanol production is the bottleneck, in terms of fossil energy use. It implies that even a moderate improvement in the efficiency in the butanol production would lead to a noticeable reduction in the total fossil energy use in the corn butanol lifecycle, which would evidently enhance the potential of butanol as a viable new environmentally preferable transportation fuel. Figure 5 presents the comparison of GHG emissions by species during the lifecycle of corn butanol and ethanol as transportation fuels. Case 3 for butanol and Case 6 for ethanol are referenced as the two cases of the lifecycle assessment study

Figure 4. Breakdown of fossil energy use in the corn butanol lifecycle.

Figure 5. Comparison with corn ethanol: lifecycle GHG emissions breakdown.

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in the work of Wu et al. In Case 3, natural gas is considered as the process fuel for corn butanol production; acetone and dried distillers grains with solubles (DDGS) are regarded as energy products and, thus, are credited by allocating energy among butanol, acetone, ethanol, and DDGS. In Case 6, natural gas is considered as the process fuel for corn ethanol production by the dry grind process; DDGS are regarded as an energy product and credited by allocating energy between ethanol and DDGS. Detailed explanations about the two cases can be found in Wu et al.17 As indicated in Figure 6, using butanol as a potential transportation fuel would generate more than 50 000 g of CO2 emission per million of Btu input. This is in stark contrast to ∼30 000 g of CO2 emission per million of Btu input, in the case of using ethanol as a transportation fuel. In addition, as also demonstrated in Figure 5, slightly more emissions of N2O and CH4 occur for corn butanol than for corn ethanol as transportation fuels. 6. Concluding Remarks The simulation of a complete process for corn butanol production via corn fermentation is performed in the current study. It involves the complete process, from corn grain processing through downstream processing. This study represents the first attempt to simulate the production process in its entirety. The energy use in the current corn butanol production plant is compared to that of a conventional corn ethanol production process. Furthermore, the energy use at different stages of the lifecycle and production process is analyzed. The effects of a variety of production process parameters on the reduction of the overall energy use of the production process are reviewed. Based on the analysis, discussion is presented on how to improve energy efficiency. The simulation result is applied as one of the bases for assessing the potential benefit in energy use and the GHG emissions derived from exploiting corn to produce butanol and utilizing butanol as a potential transportation fuel (i.e., in its full lifecycle). The lifecycle assessment provides comprehensive insights into the viability of corn butanol as a transportation fuel. Based on the lifecycle assessment result, the bottleneck of utilizing corn butanol is identified. The lifecycle assessment result is further compared with that for conventional corn ethanol. The current study shows that the acetone, butanol, and ethanol (ABE) corn fermentation process produces less liquid fuels (butanol and ethanol) (energy basis) per bushel of corn than the conventional corn ethanol production process. In addition, it consumes more process fuel in the production stage than the conventional corn ethanol production. The lifecycle assessment indicates that, with the current technology, corn butanol is not viable as a transportation fuel. Furthermore, the assessment also demonstrates that, in the lifecycle of corn butanol as a potential transportation fuel, butanol production is the most fossil-energyintensive stage. Various schemes at different steps of butanol production could lead to an effective reduction in the overall energy use of the butanol production process and, eventually, improved viability of corn butanol as a transportation fuel. Acknowledgment Argonne National Laboratory’s work was supported by the U.S. Department of Energy, Assistant Secretary for Energy

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Efficiency and Renewable Energy, FreedomCAR and Vehicle Technologies Program of the Office of Energy Efficiency and Renewable Energy (under Contract No. DE-AC02-06CH11357). Valuable advice on the Aspen Plus simulation from Dr. Urmila Diwekar (Vishwamitra Research Institute, Westmont, IL) is also thankfully acknowledged. Literature Cited (1) Huang, W. C.; Ramey, D. E.; Yang, S. T. Continuous Production of Butanol by Clostridium Acetobutylicum Immobilized in a Fibrous Bed Bioreactor. Appl. Biochem. Biotechnol., Part A 2004, 113-116, 887–898. (2) Qureshi, N.; Blaschek, H. P. Economics of Butanol Fermentation Using Hyper-Butanol Producing Clostridium beijerinckii BA 101. Trans. Inst. Chem. Eng., Part C 2000, 78, 139–144. (3) Parekh, M.; Formanek, J.; Blaschek, H. P. Pilot-Scale Production of Butanol by Clostridium beijerinckii BA 101 Using a Low-Cost Fermentation Medium Based on Corn Steep Water. Appl. Microbiol. Biotechnol. 1999, 51, 152–157. (4) Ezeji, T. C.; Qureshi, N.; Blaschek, H. P. Acetone Butanol Ethanol (ABE) Production from Concentrated Substrate: Reduction in Substrate Inhibition by Fed-Batch Technique and Product Inhibition by Gas Stripping. Appl. Microbiol. Biotechnol. 2004, 63, 653–658. (5) Qureshi, N.; Blaschek, H. P. Economics of Butanol Fermentation Using Hyper-Butanol Producing Clostridium beijerinckii BA 101. Trans. Inst. Chem. Eng., Part C 2000, 78, 139–144. (6) Qureshi, N.; Blaschek, H. P. Recovery of Butanol from Fermentation Broth by Gas Stripping. Renew. Energy 2001, 22, 557–564. (7) Qureshi, N.; Blaschek, H. P. ABE Production from Corn: A Recent Economic Evaluation. J. Ind. Microbiol. Biotechnol. 2001, 27, 292–297. (8) Ezeji, T. C.; Karcher, P. M.; Qureshi, N.; Blaschek, H. P. Improving Performance of a Gas Stripping-Based Recovery System to Remove Butanol from Clostridium beijerinckii Fermentation. Bioprocess Biosyst. Eng. 2005, 27, 207–214. (9) Liu, J. Downstream Process Synthesis for Biochemical Production of Butanol, Ethanol, and Acetone from Grains, Ph.D. Dissertation, Kansas State University, Manhattan, KS, 2003. (10) Marlatt, J. A.; Datta, R. Acetone-Butanol Fermentation Process Development and Economic Evaluation. Biotechnol. Prog. 1986, 2, 23– 28. (11) Dadgar, A. M.; Foutch, G. L. Improving the Acetone-Butanol Fermentation Process with Liquid-Liquid Extraction. Biotechnol. Prog. 1988, 4, 36–39. (12) Liu, J.; Fan, L. T.; Seib, P.; Friedler, F.; Bertok, B. Downstream Process Synthesis for Biochemical Production of Butanol, Ethanol, and Acetone from Grains: Generation of Optimal and Near Optimal Flowsheets with Conventional Operating Units. Biotechnol. Prog. 2004, 20, 1518–1527. (13) Liu, J.; Fan, L. T.; Seib, P.; Friedler, F.; Bertok, B. Holistic Approach to Process Retrofitting: Illustration with Downstream Process for Biochemical Production of Organics. Ind. Eng. Chem. Res. 2006, 45, 4200– 4207. (14) McAloon, A. J.; Taylor, F.; Yee, W. C. A Model of the Production of Ethanol by the Dry Grind Process. In Proceedings of the Corn Utilization & Technology Conference, Indianapolis, IN, June 7-9, 2004. (15) Wang, M. et al. Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model. Available via the Internet at http://www.transportation.anl.gov/software/GREET/, 2007. (16) Taylor, F.; Kurantz, M. J.; Goldberg, N.; McAloon, A. J.; Craig, J. C. Dry-Grind Process for Fuel Ethanol by Continuous Fermentation and Stripping. Biotechnol. Prog. 2000, 16, 541–547. (17) Wu, M.; Wang, M.; Liu, J.; Huo, H. Life-Cycle Assessment of CornBased Butanol as a Potential Transportation Fuel, Argonne National Laboratory report, November 2007. (Available via the Internet at http:// www.transportation.anl.gov/pdfs/AF/448.pdf) (18) Blaschek, H. P. Butanol: A Second Generation Biofuel; Presented at the University of Illinois, Champaign-Urbana, IL, April 14, 2006.

ReceiVed for reView February 18, 2009 ReVised manuscript receiVed April 17, 2009 Accepted April 22, 2009 IE900274Z