Dual Extraction Process for the Utilization of an Acetone–Butanol

Jul 10, 2014 - ABSTRACT: One factor hindering the economic feasibility of butanol ... solvents in ABE extraction without fear of inhibiting the microb...
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Dual Extraction Process for the Utilization of an Acetone−Butanol− Ethanol Mixture in Gasoline Antti J. Kurkijar̈ vi* and Juha Lehtonen School of Chemical Technology, Department of Biotechnology and Chemical Technology, Aalto University, POB 16100, 00076 Aalto, Finland S Supporting Information *

ABSTRACT: One factor hindering the economic feasibility of butanol fermentation is the energy intensity of the product separation. In this study a new approach for liquid−liquid extraction is presented. Gasoline additives were tested in conjunction with isooctane as extraction solvents, and a continuous process utilizing a novel, dual extraction is proposed. This method enables the usage of effective, nonbiocompatible solvents in ABE extraction without fear of inhibiting the microbes. The product mixture of this process could be utilized as a gasoline additive. No product purification steps are necessary, and very low energy consumption is achieved. The best extraction solvent was either MTBE or ETBE, depending on the operational parameters of the process.

1. INTRODUCTION

The possibility to utilize the product mixture as such, without any energy consuming unit operations, could make the ABE fermentation more economical. Processes, where the extraction product mixture could be added directly to traffic fuel, have been reported for biodiesel,2,11,12 but according to our knowledge, before this no such processes have been reported for gasoline. Gasoline ethers enhance octane value of gasoline and reduce emissions by improving combustion.13 However, the utilization of these components as gasoline additives has been declining in recent years. These ethers have excellent fuel properties, and they could be used as extraction solvents in the ABE fermentation process to produce a biobased, high octane oxygenate mixture for gasoline. The aim of this work is to report a process for the utilization of ABE as gasoline oxygenates and biocomponents without product purification.

The resources of fossil fuels are depleting, and the need for alternative sources for chemicals and fuels is increasing. 1Butanol can be produced from biomass using bacteria of the Clostridium spp. The traditional separation method for the acetone−butanol−ethanol (ABE) fermentation products is distillation. This is not only a well-known and rigorous method but also energy intensive.1,2 Solvent recovery is not the most significant factor affecting the production costs in a conventional ABE plant, but its energy intensity is one reason which affects the economic feasibility of the ABE process.3,4 Because of this, literature covers a wide range of less energy consuming alternatives for butanol recovery. However, according to our knowledge, these have not been used in industrial scale. The profitability of the industrial ABE fermentation is sensitive to price fluctuation, but as the price of crude oil is increasing, the future of the ABE fermentation in industrial scale is promising.3,4 Furthermore, it has been predicted that demand for biofuels will grow rapidly in the future,4 and there is also the will to reduce our dependency on fossil fuels.1 It is not surprising that butanol has been suggested as a gasoline and diesel component, which could even replace ethanol, due to its superior fuel properties.5 Components with very high boiling points are often suggested as extraction solvents for ABE. Such components are for example ionic liquids,6 vegetable oils,7 C-20 guerbert alcohol,8 and oleyl alcohol.1,9 However, using these kinds of solvents is not very economical, as high temperatures and low pressures are needed in their regeneration, which is energy intensive. Other methods, such as flashing, offer only one ideal separation stage, which might not be enough to get to the desired product purity.10 Smaller, more polar components could be a more efficient option for ABE extraction. The advantage is that they have an extremely high butanol capacity, and, for that reason, less solvent is needed. The disadvantage is that they are generally considered to be toxic to the microbes. © 2014 American Chemical Society

2. DUAL EXTRACTION METHOD The dual extraction method, which allows the usage of nonbiocompatible solvents in ABE product extraction, utilizes two extraction units instead of one. In the first extraction, effective but nonbiocompatible solvents are used to extract ABE products. The second extraction removes traces of the toxic solvent from the broth, thus making its recycling back to the fermenter safe. This concept is reported in more detail in our previous work.14 To minimize the amount of water in the product mixture, the organic phases from the two extractions are combined in a decanter, and the separated water is recycled back to the process. If fuel compatible extraction solvents are used, then the recovery of ABE products from the extraction solvents and the solvent regenerations could be omitted. The product mixture Received: Revised: Accepted: Published: 12379

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operation costs, because the mass transfer of products to the solvent phase is lower and pumping is more energy intensive. BMON and BRON describe the behavior of the components as octane boosters. It can be seen that all the selected solvents have relatively low viscosity and very high octane numbers, and thus they are excellent octane boosters. 2.2.1. Ethanol and Butanol. Ethanol is the most common biocomponent in traffic fuels. However, butanol has some properties that favor its usage in traffic fuels over ethanol. Butanol has higher energy content than ethanol. Normal gasoline powered car engines can utilize a maximum of 15% of ethanol within gasoline without modifications, whereas butanol is compatible with any gasoline−butanol ratio. Moreover, butanol−gasoline blends will not form two separate phases in the presence of water like ethanol−gasoline mixtures. Butanol also presents lower bRvp than ethanol. Furthermore, with butanol the issues of material incompatibility are from very small to nonexistent and butanol is compatible with the existing fuel infrastructure.4,26−29 Other aspects concerning higher concentrations of ethanol in fuel are increased ozone and aldehyde emissions. These emissions are speculated to cause cancer, mortality, and hospitalization in people living in the cities with a lot of exhaust gas emissions.30 If a smaller concentration of ethanol is applied to fuel by replacing it with butanol or ABE product mixture, several of these problems could probably be avoided. Therefore, the obvious application for butanol is to blend it in gasoline or diesel. As butanol can be produced from more sustainable feedstock than biodiesel, it could also be a sustainable substitute for biodiesel.4,28 Zverlov, 2006, goes further and suggests that because butanol has superior fuel properties, it could gradually replace the fossil based gasoline as well as diesel.5 However, to achieve this, the production costs of biobutanol should be more competitive when compared to ethanol production.4 Currently the maximum amount of ethanol allowed in fuel is 10 vol-% by the European Standard EN 228:2012.31 The amount of butanol in fuel is set by the same standard under the name of “other oxygenates”; the maximum is set to 15 vol-%. 2.2.2. Acetone and Isopropyl Alcohol. Acetone has a very high octane number, but it has even higher vapor pressure than ethanol. Another factor affecting acetones suitability as a gasoline component is its possible effect on engine’s rubber and plastic parts. It is possible that the unfavorable effects of acetone would be even more severe than the effects of ethanol. The concentrations of acetone which have effects on these aforementioned parts is still to be determined. Removal of acetone would be one solution to the problem. However, according to our simulations a simple distillation is suitable only for TAEE, because the other gasoline ethers have azeotropes with the components present in the mixture. On the other hand, not enough experimental vapor−liquid equilibria (VLE) data is available for these components that this could be said for sure. Other possible ways to get an acetone free product are the usage of microbe that produces isopropyl alcohol instead of acetone or microbe with blocked metabolic pathway to acetone production. Another method would be selective acetone hydrogenation to isopropyl alcohol. The usage of a suitable strain of microbe is most likely to be applied in industrial scale, as it would not require any additional unit operations. If isopropyl alcohol producing microbe or selective hydrogenation of acetone would be used, the product, isopropyl alcohol, is acknowledged as a gasoline component by the European

from this process could be added to gasoline as such. This would mean an ABE process with extremely low energy consumption as no distillations would be needed. 2.1. Solvent Selection. Traditional fuels such as gasoline and kerosene have low distribution coefficients for butanol15,16 and are therefore not applicable as extraction solvents for the ABE process.17 On the other hand, there are gasoline components, which could be used as effective extraction solvents: 2-methoxy-2-methylpropane (methyl tert-butyl ether, MTBE), 2-ethoxy-2-methylpropane (ethyl tert-butyl ether, ETBE), 2-methoxy-2-methylbutane (tert-amyl methyl ether, TAME), and 2-ethoxy-2-methylbutane (tert-amyl ethyl ether, TAEE). The reason why these have not, according to our knowledge, been suggested as extraction solvents is that they are most likely toxic to the microbes used in the ABE process. If the dual extraction method is used, this toxicity aspect can be ignored. The ethers made from ethanol, namely ETBE and TAEE, offer the additional benefit that the ethanol used in their production could be biobased. This way the biocontent of this gasoline additive would be even greater. The second solvent was selected to be isooctane. This is because it is very nonpolar, it is readily available, and it is an extremely good gasoline component. It is also very likely to be nontoxic to the microbes, because no alkanes were detected to affect the microbial growth.15,18 In practice, the second solvent would probably not be pure isooctane but a mixture such as alkylate, which is a mixture of branched parafins. With these solvents the product mixture should be a high quality fuel additive. 2.2. Properties of ABE Products and the Extraction Solvents As Fuel Components. Table 1 presents water Table 1. Properties of the Suggested Extraction Solventsh

MTBE ETBE TAME TAEE isooctane

solvents solubility in water (m-%)

waters solubility in solvent (m-%)

viscosity (mPas)

2.46a 1.16b 0.59a 0.32c 0.0016f

1.10a 0.85b 0.83a 0.62c 0.017f

0.296d 0.387d 0.279d 0.396d 0.415d

BRON BMON 118e 118e 111e 106e 100e

100e 102e 98e 94e 100e

bRvp (bar) 0.7e 0.5e 0.2e 0.1e g

a

Reference 19. bReference 20. cReference 21. dReference 22. Reference 23. fReference 24. gData was not found. hData given at 37 °C and 1 bar, except bRvp, which is defined as vapor pressure at 37.78 °C (100 °F). e

solubilities, viscosities, blending motor octane numbers (BMON), blending research octane numbers (BRON), and blend reid vapor pressures (bRvp) of the suggested extraction solvents. As can be seen from Table 1, there are large differences in water solubilities of the solvents. From the gasoline ethers MTBE is most and TAEE the least soluble in water. For these components the difference is approximately 10-fold. The water solubility also affects the butanol capacity of the solvent, as higher capacity solvents are generally more polar. On the other hand, higher polarity generally increases the toxicity of the solvent to the microbes.25 Furthermore, the more polar a component is, the more difficult it is to extract from the aqueous phase. This increases the amount of isooctane needed in the second extraction. So an ideal solvent is somewhere between the polarity extremes. High viscosity affects the 12380

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Standard EN 228:2012.31 Its maximum amount is set to 12 vol%. 2.2.3. Gasoline Ethers and Isooctane. Gasoline ethers are excellent traffic fuel components. MTBE has been extensively used throughout the world as a gasoline component to enhance the octane number and reduce emissions as oxygenate. Despite that the usage of MTBE has decreased worldwide: the combined effect of several states in the U.S. banning its usage as groundwater pollutant and EU mandating the usage of ethanol as oxygenate in the European Standard EN 228:2012.31 As gasoline components the other ethers are equal to MTBE with the difference that they have significantly lower water solubility and thus lower potential to pollute groundwater as can be seen from Table 1. Furthermore, gasoline ethers have a low bRvp, which is beneficial, because of the increasing vapor pressure limits set to gasoline. The European Standard EN 228:2012 states that gasoline should not contain more than 22 vol-% of these ethers.31 Isooctane, on the other hand, does not suffer from such restrictions, and it is still widely used as an octane enhancer throughout the world. 2.2.4. Water. A small amount of water in gasoline does not affect the engine performance. It could even reduce NOx emissions, as it lowers the burning temperature inside the engine.32 However, water can have negative effects as well. It increases fuel consumption as it reduces the overall heating value of the gasoline. The amount of water is not directly specified in the European Standard EN 228:2012,31 but the maximum amount of oxygen in gasoline is set to 3.7 m-%. It would be best to have only a minimum amount, if any, of water in a gasoline. The remaining water from the product mixture could most likely be removed by using molecular sieves. This has, however, not been addressed in any more detail in this work.

to determine their water content. The aqueous phase water content was calculated from mass balance. For the calculation of each distribution coefficient the results from two GC-FID and one GC-MS analyses were combined. The reproducibility was ±5%. To make our results more reliable we averaged the results from four analyses. We can assume that evaporative losses and error from weighing are negligible when compared to the error from GC. The distribution coefficients for the components were calculated as mass fraction ratios in organic and aqueous phases Di =

xi ,Org xi ,Aq

(1)

where Di is the distribution coefficient for component i; xi,Org is the mass fraction of component i in organic phase; and xi,Aq is the mass fraction of component i in aqueous phase. 3.1.1. Analysis Methods. The GC-FID was a HewlettPackard 6890 Series gas chromatograph, with a 60 m long Zebron ZB Wax plus column, with 0.25 mm inner diameter and 0.25 μm phase thickness. The carrier gas was helium, the injection volume was 0.5 μL, and split ratio was 1:50. The temperature program began with a 10 min hold at 40 °C after which the temperature was elevated 10 °C min−1 to 230 °C followed by a hold of 2 min in that temperature. The GC-MS was manufactured by Agilent Technologies: the gas chromatograph was 7890A and the mass spectrometer was 5975C VL MSD. The used column was 30 m long Agilent Technologies Innowax with 0.25 mm inner diameter and 0.25 μm phase thickness. The carrier gas was helium, the injection volume was 0.5 μL, and split ratio was 1:50. The temperature program began with a 15 min hold at 40 °C after which the temperature was elevated 7.5 °C min−1 to 200 °C followed by a hold of 15 min. 3.2. Process Simulations. All simulations were carried out using Aspen Plus-software. To simulate LLE behavior, NRTL with interaction parameters regressed from experimental data measured in this study was used. All the extractions in this work, except those presented in Figure 1, were simulated with only four or five ideal extraction stages, which is probably close to a realistic industrial situation. The simulation consisted of a process where the aqueous broth is recycled back to the reactor after extractions. A continuous broth flow rate of 1000 kg h−1 with 8 g L−1 of butanol was assumed, and the mass ratio of the products was 3:6:1 for acetone, butanol, and ethanol,

3. MATERIALS AND METHODS 3.1. Liquid−Liquid Equilibrium (LLE) Experiments. MTBE, TAME, isooctane, butanol, and acetone were from Sigma-Aldrich, and their purities were 99.9%. The ethyl ethers, ETBE and TAEE, were obtained from Neste Oil Inc. with initial purities of 92.9 and 65.3%, respectively. Before usage these ethers were purified with water extraction and distillation to a purity of over 99%. The ethanol was purchased from Altia oyj, and its purity was 99.6%. Experiments were carried out to determine the LLE of the used components. An aqueous mixture containing 1.2, 0.6, and 0.2 m-% of butanol, acetone, and ethanol, respectively, was mixed with an organic phase consisting of one of the gasoline ethers. In another set of experiments, the aqueous phase was otherwise the same as above, but it contained an additional 1.0 m-% of one of the gasoline ethers and isooctane was used as the organic phase. In both cases three experiments were carried out with each extraction solvent: the organic to aqueous phase mass ratios were 1:10, 1:5, and 1:3. The experiments were carried out in 100 mL glass bottles. The bottles were filled so that the gas volume inside was small, approximately 5 mL. The relatively small gas volume ensured that any errors caused by VLE would be minimized. All the experiments were carried out at 37 °C with constant mixing for 24 h, after which the bottles were stored at 37 °C. The temperature was controlled within 0.05 °C. Both of the phases were analyzed using a gas chromatograph with flame ionization detector (GC-FID). The organic phases were further analyzed with a gas chromatograph with a mass spectrometer (GC-MS)

Figure 1. Butanol extracted with MTBE as a function of extraction stages at 37 °C and 1 bar. 12381

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Table 2. Experimental and Simulated Distribution Coefficients at 37 °C, 1 Bar, and an Organic to Aqueous Ratio of 1:5 ether as extractant

isooctane as extractant

exptla

NRTL parameters from Aspen Plus

NRTL after data regression

MTBE

DButanol DAcetone DEthanol DMTBE DWater

5.3 1.4 1.3 28.4 0.02

0.01 3.2 1.0 1.1 18.4

5.5 1.1 1.3 27.4 0.02

ETBE

DButanol DAcetone DEthanol DETBE DWater

3.0 1.4 1.1 77.0 0.01

1.0 1.0 1.0 1.0 1.0

3.0 1.4 1.1 74.4 0.02

TAME

DButanol DAcetone DEthanol DTAME DWater

2.8 1.3 1.0 85.0 0.01

0.01 2.9 1.3 1.0 54.9

2.8 1.3 1.0 82.5 0.01

TAEE

DButanol DAcetone DEthanol DTAEE DWater

2.3 1.3 1.0 371.0 0.007

1.0 1.0 1.0 1.0 1.0

2.3 1.3 1.0 365.9 0.008

DButanol DAcetone DEthanol DMTBE DWater DIsooctane DButanol DAcetone DEthanol DETBE DWater DIsooctane DButanol DAcetone DEthanol DTAME DWater DIsooctane DButanol DAcetone DEthanol DTAEE DWater DIsooctane

exptlb

NRTL parameters from Aspen Plus

NRTL after data regression

0.3 1.6 0.6 14.0 0.001 25339 0.3 1,6 0.6 90.8 0.001 110121.4 0.3 1,6 0.6 112.4 0.001 90572.1 0.3 1,6 0.6 651.1 0.001 403367.0

2.7 1.5 0.5 18.2 0.002 191646.0 2.4 1.5 0.5 0.2 0.0003 91922.2 2.7 1.5 0.5 55.3 0.0034 291608.3 2.4 1.5 0.5 0.2 0.0003 106863.4

0.3 1.7 0.6 14.0 0.0008 25303.1 0.3 1.7 0.6 90.8 0.0008 110116.0 0.3 1.7 0.6 111.5 0.0008 90573.3 0.3 1.7 0.6 651.0 0.0008 403441.1

a Aqueous composition: 0.6, 1.2, and 0.2 m-% for ABE, respectively. bAqueous composition: 0.6, 1.2, 0.2, and 1.0 m-% for ABE and ether, respectively.

these components are somewhat similar. Table 2 also reports the experimental distribution coefficients for isooctane extractions. As can be seen, the distribution coefficient for butanol is very small, which could have been predicted from the very small water solubility of isooctane. However, the gasoline ethers are extracted with isooctane quite readily. This indicates that practically all of the butanol extracted in this process will be from the first extraction. The second extraction protects the microbes by removing the nonbiocompatible components from the broth. As can be seen from Table 2, the differences in the ethers distribution coefficients in isooctane are very large, which means that significantly more isooctane is needed in the second extraction when using MTBE compared to the other ethers. According to our knowledge, these components have not been suggested as extraction solvents for ABE in the literature. For this reason no literature values for these components are available for comparison. 4.2. Parameter Estimations for LLE Simulations. As can be seen from Table 2, the experimental data and the simulated values, when using the NRTL parameters from Aspen Plus, are not in good agreement. In the ether extractions the NRTL method predicted no phase separation for ETBE and TAEE below organic to aqueous mass fraction of 1:4. Above this ratio all components separated evenly to both phases resulting in distribution coefficients for 1 for all components. For the methyl ethers, the simulations predicted that practically no butanol was extracted: distribution coefficients were 0.013 and 0.009 for MTBE and TAME, respectively. Also the distribution coefficients for the ethers and water were significantly different from the experimental data. The isooctane simulations with parameters from Aspen Plus correlated relatively well for other

respectively. The extraction simulations were performed isothermally at the assumed fermentation temperature of 37 °C. In the simulations the process operation was done by varying the amount of solvent in the first extraction. To ensure the biocompatibility of the recycled fermentation broth, the second extraction was adjusted to fix the amount of the nonbiocompatible gasoline ether in the recycled broth. The organic phases from the extractions were combined in a decanter, and the aqueous phase from the decanter was recycled back to the process. The decanter was simulated using NRTL model with parameters regressed from the isooctane extraction data measured in this study. 3.2.1. Case Simulations. Two simulation cases were compared to see how the solvents performed in the process. In the first case four ideal extraction stages were used, and the ether content of the recycled broth was restricted to 20 ppm. In the second case these were changed to five stages and 100 ppm. It can be assumed that these concentrations of gasoline ethers in the recycled broth are so low that they will not have an effect on the microbes.33

4. RESULTS AND DISCUSSION 4.1. Liquid−Liquid Equilibrium Experiments. Experimental distribution coefficients for ABE components and the gasoline ethers are reported in Table 2. As can be predicted from the water solubilities of the gasoline ethers, MTBE has the highest distribution coefficient for butanol, and TAEE the lowest. To achieve MTBE’s extraction efficiency with TAEE, approximately 2.37 times more solvent is needed. Acetone and ethanol are not extracted as readily as butanol, and the extraction efficiencies of the gasoline ethers for 12382

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Table 3. Product Mass Flows (kg h−1) and ABE m-% of the Product When 95 mass-% of Butanol Is Extracted with Different Ethers CASE1: 4 extraction stages, 20 ppm ether in recycle mWater (kg h−1) mAcetone (kg h−1) mButanol (kg h−1) mEthanol (kg h−1) mEther (kg h−1) mIsooctane (kg h−1) mSolvents (kg h−1) ABE m-%

MTBE

ETBE

TAME

TAEE

0.89 3.80 7.60 1.27 173 339 512 2.41

0.55 3.80 7.60 1.27 467 40.0 507 2.44

0.57 3.80 7.60 1.27 518 38.0 556 2.23

0.65 3.80 7.60 1.27 746 3.8 750 1.69

CASE2: 5 extraction stages, 100 ppm ether in recycle mWater (kg h−1) mAcetone (kg h−1) mButanol (kg h−1) mEthanol (kg h−1) mEther (kg h−1) mIsooctane (kg h−1) mSolvents (kg h−1) ABE m-%

components than ethyl ethers and butanol. Because very little literature LLE data was available for these components, new interaction parameters for NRTL model were fitted using data regression based on data measured in this study. As can be seen from Tables 2 and 3 the experimental data and the simulated values were in good agreement when using the regressed NRTL parameters. It can be concluded that simulations with the regressed NRTL parameters are accurate and reliable for the purpose of this work. 4.3. Process Simulations. The number of stages in the extraction column has an effect on the efficiency of the extraction. In Figure 1, the m-% of extracted butanol is presented as a function of extraction stages with different organic to aqueous mass ratios, when MTBE was the extraction solvent. Broth flow rate was 1000 kg h−1, and it contained 8, 4, and 1.33 kg h−1 of butanol, acetone, and ethanol, respectively. As can be seen from Figure 1, increasing the organic to aqueous ratio naturally increases the amount of extracted butanol. Similar effect can be seen with increasing number of ideal extraction stages. To achieve over 95 m-% butanol separation from broth with MTBE, the organic to aqueous ratio needs to be at least 0.3. One stage can be achieved in a simple decanter, and to achieve more stages usually a counter current extraction column is used. Generally, increasing the number of ideal stages increases the size and price of equipment. Because the price and profit margin of the ABE fermentation products are rather low,4 the investment costs of the product recovery unit should be minimized. For that reason all other simulations, except those reported in Figure 1, are simulated with only four or five ideal extraction stages. Figure 2 presents the performance of the different ethers as butanol extractants. The extraction was simulated with five stages, and the ratio between the aqueous and organic phase was varied. The broth flow rate was 1000 kg h−1, and it contained 8, 4, and 1.33 kg h−1 of butanol, acetone, and ethanol, respectively. As can be seen from Figure 2, of all the solvents in this work MTBE is the most effective extraction solvent for butanol. It extracts more than 90 m-% of butanol with an aqueous to organic mass ratio of 0.25, as the other ethers need 0.40, 0.45, and 0.55 for ETBE, TAME, and TAEE, respectively. Similarly 95 m-% butanol extraction occurs at the ratios of 0.30, 0.50, 0.55, and 0.65. Even though MTBE is the most effective extraction solvent for butanol, it does not mean that it is the best solvent for this process. The distribution coefficient for MTBE in the isooctane extractions was only 14.0, which means that large amounts of isooctane ares needed to make the broth biocompatible again after the extractions. So the solvents

MTBE

ETBE

TAME

TAEE

0.81 3.80 7.60 1.27 145 211 356 3.43

0.53 3.80 7.60 1.27 446 18.1 464 2.66

0.53 3.80 7.60 1.27 500 17.2 517 2.39

0.57 3.80 7.60 1.27 637 1.6 639 1.94

Figure 2. Extracted butanol as a function of different organic to aqueous mass ratios with 5 extraction stages at 37 °C and 1 bar.

butanol capacity is not the only criteria, when selecting the optimal solvent. 3.3.1. Comparison of the Two Cases. Table 3 presents the product distributions when 95 m-% of butanol is extracted with the different ethers. Two different cases are reported: one with four extraction stages and 20 ppm of toxic solvent in recycle and the second with five extraction stages and 100 ppm of toxic solvent in recycle. As can be seen from Table 3, the amounts of ethers and isooctane needed in the extractions varied strongly. If only the first extraction is considered, then MTBE is by far the best solvent in both cases presented. When both of the extractions are taken into account by combining the used solvents, ETBE is the best solvent in Case1 and MTBE in Case2. In both cases TAEE required the highest amount of solvents. This shows that the operational parameters determine the best solvent for this process. There are two variables in the comparison of Cases1 and 2: the number of ideal extraction stages and ether content in the recycle stream. There is one distinctive difference between these two: changing the number of ideal stages has an effect on both of the extractions, but the ether content only affects the second extraction. MTBE has a relatively small distribution coefficient for isooctane, when compared with the other ethers. Therefore, the amount of isooctane needed in the second extraction is approximately 10-fold, when compared with ETBE or TAME and 100-fold when compared with TAEE. This is the main reason why ETBE was the best extraction solvent in Case1. In all of the simulated cases the amount of ABE in the end product is low because of high volumes of extraction solvents. ABE concentration in the end product could be increased by limiting the percentage of 12383

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Figure 3. Mass flows, temperatures, and compositions of process flows in conditions where 95 m-% of butanol is extracted with ETBE. The concentration of ETBE in recycle is 100 ppm. A (acetone), B (butanol), E (ethanol), W (water), S1 (ETBE), S2 (isooctane).

extracted butanol. For example, reducing the amount of extracted butanol from 95 to 80 m-% reduces the amount of extraction solvents by half. On the other hand, doing this might reduce the productivity rate of the microbes in the fermenter. 4.3.2. Process Description. Figure 3 shows temperatures, mass flows, and flow compositions of the process when ETBE is used. Extractions are simulated with 5 stages, and the concentration of ether in the recycle stream is set to 100 ppm. Process was operated so that 95 m-% of butanol was recovered from the broth. As can be seen from Figure 3, the reactor is simulated by adjusting the stream ABE, which contains acetone, butanol, and ethanol in a mass ratio of 3:6:1. The desired concentration of butanol in the stream Broth is achieved by adjusting the flow rate of the stream ABE. This way the butanol concentration is the limiting factor of the solvent production. Similarly, the amount of water that is transferred out of the process is replaced with the stream Water. In real life, additional water would be introduced into the process within the sugar feed. Also the microbes would generate some water. If these are taken into account, some water has to be removed from the process. This is done with the stream Purge. The following assumptions were made: 30 m-% of sugar is transformed into solvents, 25 m-% of sugar is transformed into water, and the sugar feed solution is 500 g/L. By using these assumptions, the quantity of added water was 52.77 kg/h. When taken into account the water that is removed from the process with the products, the amount of water in the Purge flow was calculated to be 52.25 kg/h. Purge flow is also important, because it prevents accumulation of inerts in the process. What can also be seen from Figure 3 is that the fermentation broth is recycled back to the reactor after the extractions. This way all the unfermented nutrients, reaction intermediates, and remaining products will be transferred back to the reactor. The microbes

should not be allowed to travel from the fermenter to the extractions. The microbes would probably cause problems to the phase separation and the nonbiocompatible solvent would probably kill the microbes in the first extraction. This could be avoided by immobilization and filtration. These methods are not addressed in any more detail in this work. It can be observed that significantly more solvent is consumed in the first extraction than in the second. This is mainly because ETBE is much easier to extract from the broth than ABE; but as can be seen from the composition of stream Org2, isooctane is not effective for any other component in the broth. The concentrations of ethanol and acetone are slightly elevated in the stream Broth. This is because butanol is extracted more effectively than they are. However, the concentrations of ethanol and acetone remain at a level, where growth inhibition is unlikely to occur.34,35 In the decanter a phase separation occurs, and the aqueous phase is recycled back to the extractions. This decanter ensures that the amount of water in product flow remains low, which is important for high quality gasoline additive. Literature covers several ABE processes, where the product mixture is used directly as fuel or as a fuel additive. These processes have concentrated on using biodiesel as the extraction solvent.2,7,11,12 The reported distribution coefficients for butanol vary from 0.9 to 1.23. These values are so low that even with an organic to aqueous phase ratio of 1:1, only 40− 50% of butanol is extracted from the broth.2,11 This is very inefficient from both industrial and microbiological point of views. The distribution coefficients for ethanol, when using biodiesel, were reported to vary from 0.05 to 0.16.2,7,11,12 Similarly for acetone, values from 0.16 to 0.22 were achieved.7,11 In other words, ethanol and acetone are extracted only negligibly. This could cause an increase in their concentrations in the broth and could cause growth inhibition 12384

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in continuous operation. The gasoline ethers are more effective extraction solvents than biodiesel. The distribution coefficients for butanol vary from 2.5 to 5.3, for ethanol from 0.97 to 1.25, and for acetone from 1.25 to 1.44. The values for butanol are from three to four times higher than the corresponding values of biodiesel. Similarly the values for acetone and ethanol are approximately 10-fold. Furthermore, the ethers have lower viscosity than biodiesel, which will enhance the extraction rate as the mass transfer is faster. It can be concluded that the process proposed here is more effective than similar processes from the literature.

ASSOCIATED CONTENT

S Supporting Information *

Table of NRTL parameters regressed in this work. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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5. CONCLUSIONS A process utilizing gasoline components as extraction solvents in ABE fermentation without product purification steps is reported. This dual extraction method contains two extraction columns. The first extraction uses effective but nonbiocompatible solvents to extract ABE products. The second extraction makes the broth biocompatible by removing traces of the toxic solvent from the broth. This dual extraction process uses certain ethers with isooctane to produce a ready to blend gasoline mixture. The optimal solvent depends on the operational conditions. In the simulated situations ETBE and MTBE were the most effective solvents, followed by TAME and TAEE. We acknowledge that the ABE concentration is relatively low in the end product. However, when compared to similar processes from the literature, this process is more effective; but to make this process industrially attractive, the concentration of ABE in the end product has to be further increased.

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Article

AUTHOR INFORMATION

Corresponding Author

*Phone: 358 947022874. E-mail: antti.kurkijarvi@aalto.fi. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by Neste Oil, Stora Enso, and Tekes is gratefully acknowledged.

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ABBREVIATIONS ABE, acetone−butanol−ethanol; MTBE, methyl tert-butyl ether; ETBE, ethyl tert-butyl ether; TAME, tert-amyl methyl ether; TAEE, tert-amyl ethyl ether; GC-FID, gas chromatograph with flame ionizing detector; GC-MS, gas chromatograph with mass spectrometer detector; BMON, blending motor octane number; BRON, blending research octane number; bRvp, blend reid vapor pressure; LLE, liquid−liquid equilibrium; VLE, vapor−liquid equilibrium 12385

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