20th European Symposium on Computer Aided Process Engineering – ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) © 2010 Elsevier B.V. All rights reserved.

Separation of butanol from acetone-butanol-ethanol fermentation by a hybrid extraction-distillation process Korbinian Kraemer, Andreas Harwardt, Rob Bronneberg, Wolfgang Marquardt Aachener Verfahrenstechnik, RWTH Aachen University, 52056 Aachen , Germany

Abstract The alternative fuel butanol can be produced via acetone-butanol-ethanol (ABE) fermentation from renewable resources, i.e. biomass. Expensive feedstocks and the high costs for the separation of ABE from the dilute fermentation broth in the downstream processing have so far prohibited the industrial-scale production of bio-butanol. The low productivities and butanol yields of ABE batch fermentation can be increased by continuous fermentation with cell recycle and integrated product removal. In order to facilitate an effective and energy-efficient product removal, we suggest to apply a hybrid extraction-distillation process with ABE extraction in an external column. The removal of ABE outside the fermenter in an extraction column is favored from an operational point of view. By means of computer-aided molecular design (CAMD), mesitylene has been identified as a new solvent for ABE extraction from the fermentation broth. The solvent properties of mesitylene are compared to those of oleyl alcohol, which is the most common solvent for ABE extraction. Subsequently, we propose a hybrid extraction-distillation downstream process for product removal and purification. It is shown that the specific energy demand of this process is significantly lower when mesitylene is used as extraction solvent instead of oleyl alcohol. Keywords: butanol, extraction, solvent, biofuels, CAMD

1. Introduction Butanol has been identified as a possible fuel from renewable resources. Compared to ethanol, butanol offers several advantages as a bio-fuel such as higher energy content, lower vapor pressure, and lower hygroscopy [1]. Bio-butanol is produced via ABE fermentation from renewable feedstocks using Clostridium acetobutylicum or C. beijerinckii in anaerobic conditions. ABE fermentation ranked second only to ethanol fermentation in the first part of the 20th century, but disappeared in the second part due to the rise of the petrochemical industry. With the depletion of fossil fuels ABE fermentation becomes interesting again. The main challenges which need to be tackled in order to make ABE fermentation economically viable are [1,2] (i) expensive feedstocks, (ii) high product inhibition especially by butanol (typically 20 g/L ABE with a mass ratio of 3:6:1 is achievable), (iii) low productivity (up to 0.6 g L-1 h-1) and ABE yields (0.3) in batch fermentation and (iv) expensive downstream processing. Efforts are being made to use agricultural residues and energy crops such a switchgrass to reduce the cost of feedstock (i), since the butanol-producing cultures are able to catabolize a wide variety of carbohydrates [1]. To address product inhibition (ii), hyper-butanol-producing strains were developed, including C. beijerinckii BA101 which produces ABE up to 33 g/L with a 50% productivity threshold at about 12.5 g/L butanol [3]. Genetic engineering of butanol producing strains should allow for even lower

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product inhibition, enhanced productivity and butanol yield (iii) in the future [4]. Aside from advanced butanol producing strains, productivity and yield has also been improved by continuous fermentation processes with cell recycle membrane reactors, immobilized cells reactors or packed bed reactors [5,6,7]. Continuous fermentation processes enable the use of concentrated sugar solutions, decrease product inhibition by integrated product removal, and lower the cost of waste water treatment. ABE productivities of up to 15.8 g L-1 h-1 have been achieved in immobilized cell reactors [6]. Various authors also report improved productivities by staged fermentation processes in effect cascades where the fermentation conditions are adapted to the respective cell stadium [8,9]. Despite the accomplished advances of the ABE fermentation, the expensive product removal from the dilute fermentation broth (iv) still prohibits the industrial production of bio-butanol. Since butanol has a higher boiling point than water, pure distillation processes suffer from a high energy demand. A variety of alternative hybrid separation processes have therefore been proposed: These are based on gas stripping, liquid-liquid extraction, pervaporation, perstraction, and adsorption [5,10]. Besides reduced energy demands for butanol separation, these methods also offer the advantage that they can be applied inside the fermenter to decrease product inhibition. [10] suggest that adsorption and extraction combined with distillation are the most energy efficient alternatives. According to [5], hybrid processes with pervaporation or extraction are most attractive for integrated product removal. [9] prefer extraction in combination with distillation since these techniques are conventional unit operations. Although these innovative hybrid processes constitute a leap in energy efficiency from the pure distillation process, the specific energy demand is still considerably higher than 10% of the energy content of butanol, which has been stated as the target for energy efficiency [11]. Since the high energy cost for ABE removal is still the bottleneck in industrial bio-butanol production, it is the scope of this work to explore possibilities to further lower the energy demand. Considering the above mentioned promising experiences of other authors and the preference in industry for established unit operations, we chose to study the energy savings potential of hybrid extraction-distillation processes.

2. Solvent screening for extraction of solvents in external column Most publications on ABE removal via liquid-liquid extraction study extractive batch fermentation. Here, the fermentation products are removed in situ, i.e. inside the fermenter, into an organic solvent phase. Various authors conduct extensive solvent screenings [11,12,13,14,15]. Suitable solvents are selected based on the following criteria: non-toxicity to cells, immiscibility with water, high distribution coefficient towards butanol, low viscosity and different density as water, commercially available at low cost. Two main groups of solvents were identified: Alcohols and alkanes. While alcohols exhibit high distribution coefficients (D>5) towards butanol, they have relatively low selectivities (Dbutanol/Dwater < 350). Alkanes, on the other hand, offer large selectivities (2500-4300) but suffer from low distribution coefficients (D<0.5). Many authors choose oleyl alcohol as extracting agent due to its non-toxicity towards the microorganisms and its relatively high distribution coefficient for butanol (D=3.8). Oleyl alcohol has therefore become the solvent of choice for extractive fermentation and many authors report enhanced cell productivity and butanol yields for extractive fermentations with the help of oleyl alcohol [12,17,19,20]. However, most studies were carried out as batch fermentations on a lab-scale level. Under these circumstances, some disadvantages of oleyl alcohol for continuous large-scale production have little effect: The high boiling point (360°C) hinders a separation of the product from oleyl alcohol

Separation of butanol from acetone-butanol-ethanol fermentation by a hybrid extraction-distillation process

via distillation in a large-scale process. In addition, the low distribution coefficient for acetone (D=0.34) requires that a large amount of solvent is used in order to prevent an accumulation of acetone in the fermentation. The required amount of solvent is therefore determined by the removal rate of byproducts rather than butanol itself [16]. Some authors also indicate that extractive fermentation with in situ product removal may not be suitable for large-scale production due to various reasons: · difficult process control [11] · slow mass transfer into solvent phase (slower than butanol production) [17,18] · formation of emulsions through agitation [5,14,18,19] · cell inhibition by solvent (interface toxicity) and loss of cells at interface [19] · physical shielding by attraction of cells to interface: real distribution coefficients in experiments lower than in experiments without cells [20] · precipitates carry water into the solvent phase [14] For these reasons, external product removal in an extraction column with recycle of solvent-lean broth seems to be better suited for large-scale production of bio-butanol [11,17,18]. When the cells are retained in the fermenter by immobilization or ultrafiltration, powerful but toxic solvents can be used in an external extraction column as long as their solubility in water is low. Hence, we performed a solvent screening where we did not exclude toxic solvents but emphasized a low solubility in water and paid attention to operational constraints like a boiling point which allows for an economic distillation to remove the products from the solvent. In addition, we emphasized high distribution coefficients not only for butanol but also for acetone and favored an optimal balance between distribution coefficient and selectivity. The solvent screening was performed with the help of the software package ICAS [21], which uses a generate and test approach to screen molecules. First, thresholds for the desired properties are specified by the user. Then, meaningful molecules are generated by Computer-Aided Molecular design (CAMD) and tested for the desired properties based on thermodynamic group contribution methods, i.e. UNIFAC. These tested molecules can then be ranked and checked against a database to exclude non-existing molecules. The best solvent properties were predicted for methylbenzenes with more than three methyl groups, i.e. tri-, tetra-, and pentamethylbenzene. From our knowledge of the literature, these solvents were never considered in solvent screenings for ABE removal from fermentation broth before. This is probably due to the expected toxicity to the cells when applied in situ and the relatively low distribution coefficient at room temperature compared to fatty alcohols. We excluded pentamethylbenzene because of its melting point at around 50°C. Tri- and tetramethylbenzene exhibit similar properties as solvents. We chose to study 1,3,5-trimethylbenzene (mesitylene) in more detail, since it is most commonly used as a solvent in industry and research. To validate the properties predicted in ICAS by the UNIFAC group contribution method, we measured the distribution coefficients for acetone, butanol, and ethanol in systems of water and mesitylene. We also determined the solubility of mesitylene in water experimentally. Table 1 lists the solvent properties of mesitylene (UNIFAC and measured) and gives a comparison to oleyl alcohol, which is the common solvent choice in literature. The first column contains the properties for the new solvent mesitylene predicted by UNIFAC, which led to the selection in the solving screening procedure. Note that the distribution coefficients D for butanol and acetone are predicted to be very similar. This is beneficial since not only butanol needs to be removed from the broth but also a considerable amount of acetone. Mesitylene is also predicted to exhibit a very large selectivity (Dbutanol / Dwater) and low solubilities for solvent in water and vice versa. The distribution coefficients which we measured at 25°C are considerably lower than

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the predicted coefficients, particularly for acetone and ethanol. However, at 80°C we measured significantly higher distribution coefficients. Note that ethanol is the least inhibitory product and, therefore, the relatively low distribution coefficient for ethanol should not be detrimental. Table 1 shows that the UNIFAC predictions are rather inaccurate. We have therefore used the measured data in the simulation in Section 3.

Table 1. Comparison of solvent properties.

mesitylene oleyl alcohol

UNIFAC 25°C

measured 25°C

measured 80°C

[15] 30°C

D butanol (kg/kg) 1.3 0.76 2.2 3.8 D acetone (kg/kg) 1.4 0.43 0.83 0.34 D ethanol (kg/kg) 0.14 0.03 0.1 0.28 selectivity 7620 1650 4760 330 solubility water in solvent (wt%) 0.017 0.046 0.112 1.14 solubility solvent in water (wt%) 0.0027 5.2e-3

[22] 0.0019

viscosity (mPa s) 0.66 26 melting / boiling point (°C) -45 / 165 13-19 / 330-360

Oleyl alcohol offers an even higher distribution coefficient for butanol than mesitylene. Nevertheless, a larger amount of oleyl alcohol needs to be used for extraction compared to mesitylene at 25°C and at 80°C, since the distribution coefficient for acetone is considerably lower. Furthermore, oleyl alcohol exhibits a substantially lower selectivity than mesitylene due to the higher solubility for water. This results in noticeable amounts of water in the organic phase, which raises the cost for the downstream purification. In the following, additional advantages of the solvent mesitylene are noted. Whereas oleyl alcohol removes the valuable intermediates butyric acid (D=3.7) and acetic acid (D=0.35) from the broth [16], mesitylene leaves these intermediates (D = 0.58 and 0.06, respectively) in the broth such that they can be catabolized in the fermenter. Groot et al. [5,14] report fouling inside the extraction column when they use oleyl alcohol as solvent due to its non-toxicity. The anticipated toxicity of mesitylene, however, will presumably reduce the issues with fouling. Both solvents have a density that allows for an efficient phase separation (0.85 g/cm3), but the higher viscosity of oleyl alcohol results in a diffusion coefficient of only 1.1e-10 m2/s [5] which will lead to a large height of the extraction stages. The melting and boiling points also favor mesitylene as solvent. The high boiling point of oleyl alcohol prohibits a separation of the products in a simple distillation column at normal pressure. The melting point just below room temperature can complicate large-scale production as well.

3. Simulation of hybrid extraction-distillation downstream process As a consequence of the above mentioned favorable solvent properties of mesitylene, it is expected that the use of mesitylene as solvent in hybrid extraction-distillation downstream processes can significantly reduce the separation costs. In order to quantify the energy savings, we have modeled the entire downstream processes for the solvents mesitylene and oleyl alcohol and a pure distillation process in ASPEN PLUS. We assume a broth flowrate of 1 m3/h with a butanol concentration of 8 g/L. This concentration is below the threshold for butanol inhibition and has been reached in

Separation of butanol from acetone-butanol-ethanol fermentation by a hybrid extraction-distillation process

continuous fermentations in the literature [6,18]. The concentration of acetone in the broth (first column in Table 2) is determined from a mass balance around the extraction column assuming that the mass ratio of butanol and acetone in the saturated solvent stream is 2:1, which is consistent with the ratio they are produced by the cells in the fermentation [24]. For oleyl alcohol, the total concentration of ABE would then exceed 25 g/L at the minimal solvent flowrate for butanol removal. Therefore, oleyl alcohol demands a higher solvent flow than necessary for butanol removal.

SOLVENT

W-OUT

EXTRACT

BAEW

EXTRACT

COL1

1

HEX1

DECANT

HEX2HEX3

BROTH-IN

RECYCLE

COL2 COL3

W-E

ACET ONE

BUTANOL

Fig.1. Process flowsheet for hybrid extraction-distillation process. The flowsheet of the process with the new solvent mesitylene is shown in Figure 1. The fermentation broth is passed through a filter (not shown), heated by the recycles to 80°C, and given into the extraction column. The extraction is modeled with the measured distribution coefficients and solubilities at 80°C. The extraction column is assumed to consist of 10 equilibrium stages. 87.5 % of the butanol is extracted such that the product-lean fermentation broth leaves the column with a butanol content of 1 g/L. The heat of the product-lean fermentation broth is recuperated in heat exchanger Hex1. We assume a temperature difference of 2°C for this heat exchanger. While this may seem low, it results in a reasonable heat exchange area of 29 m2. Before the cool broth is recycled back to the fermenter it is sent into a decanter where remains of mesitylene are recovered at lower temperatures. The saturated solvent stream is preheated by the solvent recycle in heat exchanger Hex3 and purified from the fermentation products in distillation column Col1. After passing through heat exchangers Hex3 and Hex2, the solvent recycle is fed into the extraction column at 80°C. The distillate product of Col1, which contains ABE and remains of water, is further split up into its pure components in columns Col2 and Col3. Note that Col3 operates at a pressure of 0.7 bar where this separation can be performed more efficiently. The vapor-liquid-equilibrium in the distillation columns is modeled by the UNIFAC (Col1) or the NRTL model (Col2 and Col3) with parameters from ASPEN. The resulting energy demands for the solvents mesitylene and oleyl alcohol and the pure distillation process are shown in Table 2. The process with the new solvent mesitylene demands significantly less energy than both the process with solvent oleyl alcohol and the pure distillation process. The main reasons for the relatively large energy demands for oleyl alcohol are the higher solvent flowrate due to a lower distribution coefficient for acetone and the large content of water in the distillate of Col1 (45 wt%). It still needs to be determined in further experiments, whether the nutrients in the broth are extracted into the solvent in considerable amounts. In addition, possible inhibition of the cells by traces of mesitylene in the fermentation broth needs to be tested in experiments.

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Table 2. Comparison of energy demands (energy content of butanol: 36 MJ/kg).

energy demand conc. in broth (g/L ABE)

solvent flow Col1 Col 2 Col 3 total

solvent mesitylene 10/8/5 383 kg/h 9.1 kW 0.4 kW 1 kW 5.7 MJ/kg

butanol solvent oleyl alcohol 12/8/5 738 kg/h 22 kW remaining columns

6.1 kW 15 MJ/kg butanol

pure distillation 4/8/2 4 columns

38.2 kW 19.4 MJ/kg butanol

4. Conclusions The new solvent mesitylene for the removal of fermentation products of continuous ABE fermentation compares favorably to the solvent oleyl alcohol, which is commonly used for ABE extraction. We have therefore proposed a hybrid extraction-distillation downstream process, where the fermentation products are removed from the broth with the help of mesitylene in an external extraction column. The entire downstream process including product purification exhibits a specific energy demand of 5.7 MJ/kg butanol produced, which is 16% of the energy content of butanol. This is a significant reduction compared to the extraction with oleyl alcohol (15 MJ/kg butanol), compared to the pure distillation process (19.4 MJ/kg), and compared to the most energy efficient process reported in the literature (8.2 MJ/kg via adsorption-distillation [10,11] ). Note that pure acetone, which is retrieved in a weight ratio of 1:2 (A:B), is a valuable product as well. Future research will be directed towards a rigorous optimization of the hybrid process, possibly bringing further down the energy demand and taking into account capital costs. Financial support by the cluster of excellence ‘Tailor-Made Fuels from Biomass’ is gratefully acknowledged.

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