Ind. Eng. Chem. Res. 2002, 41, 2745-2752
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Purification of Fermentation-Derived Acetic Acid By Liquid-Liquid Extraction and Esterification Sai P. R. Katikaneni and Munir Cheryan* University of Illinois, Agricultural Bioprocess Laboratory, 1302 W. Pennysylvania Avenue, Urbana, Illinois 61801
Liquid-liquid extraction and esterification were investigated for the purification of acetic acid. With model solutions, a solvent system containing equal volumes of Alamine-336 and 2-ethyl1-hexanol resulted in 85% extraction efficiency when the pH of the broth was below 3.5. At higher pH, Aliquat-336 could extract higher amounts. With the fermentation broth (pH 6-7), direct acidification with sulfuric acid gave yields of 66% at pH 3.4. Lowering the pH by bipolar electrodialysis was more effective and gave higher yields of 85%. Esterification with ethyl and butyl alcohols was optimum at a molar ratio of 3:1 alcohol to acetic acid. With model solutions (no water present), yields were 65-75% of theoretical. However, the large amount of water present in the fermentation broth inhibited esterification, giving yields of only 5-20%. Introduction Acetic acid is conventionally produced from petrochemical feedstocks by oxidation of acetaldehyde, the oxidation of liquid-phase hydrocarbons, or the carbonylation of methanol. Worldwide production of acetic acid is about 4 million tons per year, with U.S. demand alone being 1.9 million tons per year at a price of $600-800 per ton. Acetic acid can also be produced from renewable sources such as corn or biomass by fermentation. Of the two fermentation routes used for the production of acetic acid, our laboratory focused on anaerobic production using Clostridium thermoaceticum because of its potentially lower cost compared to aerobic fermentation.1 We studied the microbiology and improved the bacterial strain by adaptation and mutation,2,3 optimized fermentation parameters and lowered media costs,4,5 designed bioreactors and developed operating strategies to maximize productivity,5-7 and developed downstream processing methods based on membrane separations.8-10 The final phase was the development of methods for the isolation and purification of acetic acid from the fermentation broth. The literature shows several separation processes such as adsorption, distillation, membrane separation, and solvent extraction that have been explored for the selective recovery of low-molecularweight organic acids from industrial wastewaters and fermentation broths.11-21 In our case, simple distillation, although technically feasible, might not be economical as the fermentation broth typically contains 90-95% water. Figure 1 shows the two routes that we studied to purify acetic acid: esterification and liquid-liquid extraction (LLE). The product from the fermenters is a salt of acetic acid, such as ammonium acetate or sodium acetate, depending on the alkali used to neutralize the fermentation broth. Esterification converts acetic acid into an ester by reacting it with an alcohol in the presence of an acid catalyst. Subsequently, the ester can be hydrolyzed to produce the acid. We focused on ethanol and butanol for esterification, partly because * To whom correspondence should be addressed. Phone: (217)333-9332.Fax: (217)333-9592.E-mail:
[email protected].
Figure 1. Two possible routes for purification of acetic acid from fermentation broths.
ethanol is already being produced from corn-based substrates and partly because the esters (ethyl acetate or butyl acetate) have a market value and could be end products themselves, in addition to being an intermediate in the purification of acetic acid. The reactions are as follows:
Ethanol Esterification CH3COOH + C2H5OH a CH3COOC2H5 + H2O (1) Butanol Esterification CH3COOH + C4H10OH a CH3COOC4H10 + H2O (2) Judging from the stoichiometry of the reactions shown above, the yields should be 1.467 g of ethyl acetate per gram of acetic acid or 1.97 g of butyl acetate per gram of acetic acid. However, this reaction is reversible and generally reaches equilibrium when appreciable quantities of both reactants and products are present. Yields will be less than 100%. For example, if 1 mol of acetic acid is reacted with 1 mol of ethyl alcohol in the presence of sulfuric acid, the equilibrium is reached after 4-5 h. At the end of the reaction, the reaction mixture would consist of about two-thirds ester and water and one-third alcohol and acid. This equilibrium can be shifted forward by carrying out the reaction with
10.1021/ie010825x CCC: $22.00 © 2002 American Chemical Society Published on Web 04/24/2002
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an excess of either alcohol or acid, which facilitates the forward reaction. Esterification also depends on the reaction temperature, the presence of water, and the nature of the alcohol used. Hence, several reaction parameters have to be optimized. In this work, model solutions of acetic acid and ethanol were first used to establish reaction parameters, and then studies with the real fermentation broth were conducted. In LLE, an organic solvent that is immiscible in water is used to form a complex with the desired product. The product (acetic acid, in this case) moves from the aqueous phase into the organic phase. After phase separation, azeotropic distillation (for dilute aqueous feeds) can be used to separate the organic solvent for recycle. This technique has been used to recover acetic acid from the chemical manufacture of cellulose acetate and vinyl acetate and from industrial waste streams.11-14 However, the extraction solvents were usually ethers, ketones, or alcohols, which resulted in relatively high costs. Phosphorus-bonded oxygenated extractants15,16 and high-molecular-weight aliphatic amines are particularly attractive, because they have low solubilities in water and the distribution coefficients of acetic acid in amines are high.12,13 Tertiary amines are not soluble in water and do not form any byproducts through side reactions. Thus, tertiary amines are usually preferred over secondary or primary amines for the LLE of organic acids.17 LLE efficiency is influenced by the natures of the solvent and diluent, their relative concentrations, the properties of the acid to be extracted, the concentration of the acid present in the aqueous stream, the molar ratio of solvent to acid, and the temperature of the extraction. Some of these variables have been studied in detail for the extraction of organic acids using amines.13,17 With organic acids in wastewater, the extraction efficiency was found to decrease with increasing temperature and to display an optimum with respect to the feed acid concentration.13 In addition, the relative amounts of dissociated and undissociated acid in the feed solution is important. The extraction efficiency is high when the organic acid is present in the undissociated (acid) form.17 This, in turn, depends on the pH of the feed solution. Low pH increases the proportion of undissociated acid in the mixture. This is one limitation of using anaerobic fermentation to produce acetic acid. To increase yields during fermentation, the pH of the broth has to be kept high (pH > 6). This keeps the concentration of undissociated acid low, which, in turn, minimizes transport of the acid through the cell membrane of the microorganism and inhibition of the microorganism. However, keeping the pH high results in a low yield of acetic acid in the extraction step. It would be desirable to use an extractant that is effective at the higher pH of the fermentation broth. Another option is to acidify the fermentation broth or to develop an alternate technology for the selective removal of acetic acid from the fermentation broth. In the present work, various extractants were screened for separating acetic acid from the fermentation broth. The effect of lowering the pH of the fermentation broth, by direct addition of acid or by bipolar electrodialysis, was also studied. Materials and Methods Acetate. Model acetate solutions were prepared by diluting glacial acetic acid (100% acetic acid, Baker
Chemicals, Phillipsburrg, NJ) with deionized water to the required concentration. The solutions were adjusted as required to pH 4.3, 5.2, and 6.6 using solid sodium hydroxide pellets. In addition, all nutrients that are normally present in the fermentation broth1 were added to the model solutions. Extractants. Alamine-336, Alamine-304, and Aliquat-336 were obtained from Henkel Corporation, Kankakee, IL; Amberlite LA-2 from Fluka, Ronkonkoma, NY; and tributyl phosphate and trioctyl phosphine oxide from Aldrich Chemical Co., Milwaukee, WI. Diluents. Diisobutyl ketone (99.99%), octanol (99.99%), 2-ethyl-1-hexanol (99.99%), and 2-heptanone (99.99%) were from Aldrich Chemical Co., Milwaukee, WI. All reagents were used without further purification. Fermentation Broth. Fermentation of glucose was carried out using a mutant strain of C. thermoaceticum (ATCC 49707) as described earlier.1 A 2-L fermentation vessel was used. The temperature was controlled at 60 °C, and the pH of the broth was maintained with automatic addition of 10 N sodium hydroxide solution. Carbon dioxide was sparged through the mixture to maintain anaerobic conditions. Initially, the culture was grown in 3 L of growth medium for 24 h. The culture was then concentrated with a hollow-fiber microfiltration membrane (UFP500-E4, A/G Technology, Needham, MA) to a final volume of 800 mL. This concentrated culture was transferred to the fermentation vessel. The final acetic acid concentration in the fermentation broth was 45 g/L, and the acetate yield was 0.81 g of acetic acid per gram of glucose. The final pH of the fermentation broth was 6.5. The fermentation broth was microfiltered, and the filtrate was stored at 4 °C. Liquid-Liquid Extraction. The “solvent system” in this paper refers to the mixture of the extractant and the diluent. Extraction experiments were conducted in a 100-mL separatory funnel at room temperature. In a typical experiment, 25 mL of the aqueous feed solution containing acetate was mixed with 25 mL of the organic solvent system. The funnel was shaken for 5 min, and the mixture was equilibrated for 12 h at room temperature. The mixture was centrifuged at 10 000 rpm for 5 min to separate the aqueous and organic phases. The acetic acid concentration in the aqueous phase was determined as described below. The acid concentration in the organic phase was determined by the difference from the concentration in the original feed stream. The efficiency of extraction (E) is expressed as
E ) (1 - Ce/C0) × 100
(3)
where Ce is the concentration of acetic acid in the aqueous phase after extraction and C0 is the initial concentration of acetic acid in the aqueous phase (feed). An E value of 100% means that all of the acetic acid in the aqueous phase has been removed and is present in the organic phase of the mixture. If no acetic acid is extracted, Ce ) C0, and E ) 0%. This definition of extraction efficiency neglects any changes in the volumes of the organic and aqueous phases upon mixing; however, no changes were observed with Alamine 336 as the extractant.17 The various solvents used for the extraction studies are listed in Table 1. Each solvent system was studied in detail by changing variables such as the solvent/diluent ratio and organic/aqueous ratio. Esterification. These reactions were carried out in a 1-L round-bottom flask with a 0.75-L working volume. The reaction vessel was connected to a water condenser
Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2747 Table 1. Extractants and Diluents Used in the Solvent System for Liquid-Liquid Extraction Experiments system I
II III IV V VI
Table 2. Recovery of Acetic Acid from Model Feed Solutiona with Various Extractantsb
extractant
diluents
extractant
E (%)
Alamine-336 Alamine-336 Alamine-336 Alamine-336 Alamine-336 Aliquat-336 Aliquat-336 Alamine-304 tributyl phosphate trioctyl phosphine oxide amberlite
none diisobutyl ketone octanol 2-ethyl-1-hexanol 2-heptanone 2-ethyl-1-hexanol 2-heptanone 2-ethyl-1-hexanol 2-ethyl-1-hexanol 2-ethyl-1-hexanol 2-ethyl-1-hexanol
Alamine-304 Alamine-336 Aliquat-336 Amberlite-LA2 tributyl phosphate trioctyl phosphine oxide
32 85 55 42 68 72
to maintain the vessel under reflux conditions. The temperature was controlled with a hot plate, and the reaction medium was continuously agitated at 25-50 rpm, which was enough to keep the reactants wellmixed. The products were collected at the end of the reaction and analyzed by gas chromatography. Model solutions and the real fermentation broth obtained after bipolar electrodialysis were used for esterification with ethyl and butyl alcohols. The pH of the electrodialyzed fermentation broth was 2.87, and its acetic acid concentration was 46.4 g/L. Sulfuric acid was used as the catalyst at a level of 2% (w/w) with respect to acetic acid. The reaction was carried out at the boiling temperature of the alcohol (ethanol at 78 °C and butanol at 90 °C; butanol in the presence of water will boil at 90 °C rather than 110 °C.). The molar ratio of alcohol to acid was 3:1, and the reaction was conducted for a period of 4 h. At the end of 4 h, the product mixture was cooled to room temperature. The mixture separated into two layers, the upper phase being ester and the lower phase being the spent aqueous fermentation broth. The phases were separated by centrifugation (3200 G for 10 min), and both phases were analyzed for ester, acetic acid, alcohol, and acetamide. Electrodialysis. A laboratory-scale bipolar electrodialysis system using Aquatech membranes was used for pH modification of the fermentation broth. Analytical Methods. Glucose and acetic acid were analyzed using high-performance liquid chromatography (HPLC) using a BioRad (Richmond, CA) Aminex HPX-87H column and an RI detector. The mobile phase was 0.002 N H2SO4 at a flow rate of 0.8 mL/min, and the column temperature was maintained at 65 °C. Esterification products were measured with a HewlettPackard (6890 series) gas chromatograph equipped with a flame ionization detector. A capillary column (DB-wax, 30-m length, 0.25-mm i.d., J&W Scientific, Edison, NJ) was used for the analysis. The oven temperature was programmed from 40 to 230 °C at a rate of 5 °C/min. The column could determine acetic acid, ethanol, butanol, ethyl acetate, butyl acetate, and acetamide. Results and Discussion Liquid-Liquid Extraction. The first series of experiments focused on screening various solvents using model solutions containing only acetic acid at a concentration of 35 g/L at pH 3.3. Among all solvents studied, Alamine-336 was found to be the best solvent on the basis of E values (Table 2). Other solvent systems (Alamine-304, tributyl phosphate, trioctyl phosphine oxide, Amberlite and Aliquat-336) gave lower extraction efficiencies for acetic acid recovery. As shown in Figure 2, Alamine-336 is a nonpolar compound with a nitrogen
a Model feed solution contained 35 g/L acetic acid, pH 3.3. Diluent was 2-ethyl-1-hexanol in equal volumes. Ratio of solvent system to feed ) 2:1.
b
Figure 2. Structures of Alamine-336, ethyl hexanol, and the possible complex formed with acetic acid during extraction.
atom surrounded by long hydrocarbon chains. When an organic acid contacts the Alamine molecule, it forms an acid-amine complex, which is more polar in nature. Addition of a polar compound (the diluent) enhances complex formation, whereas nonpolar solvents inhibit formation of this complex. Typical polar solvents are alcohols, hydrocarbons, ketones, and esters. The effect of various diluents on the extraction efficiency of Alamine-336 is shown in Table 3. The diluents were selected on the basis of the polarity of the molecule that could influence the basicity of the solvent. At a 1:1 ratio of Alamine-336 to diluent, diisobutyl ketone gave an extraction efficiency of 62%, whereas 2-ethyl-1-hexanol gave an extraction efficiency of 85% under otherwise similar operating conditions and for the same concentration of acid in the aqueous feed solution. Ethyl hexanol (Figure 2) is a stronger polar diluent because of the π electrons of the hydroxyl group. This results in an increase in the dielectric constant surrounding the solvent, which would increase the basicity of Alamine-336. This results in better solvation by interaction of the acid with the π electrons. Ethyl hexanol also has an acidic hydrogen available for hydrogen bonding, whereas diisobutyl ketone does not. The hydrogen is probably involved in the acid-amine complex. On the other hand, without the diluent, Alamine-336 had a low extraction efficiency for acetic acid (36 vs 85% with ethyl hexanol). Other diluents, e.g., hexane and kerosene, gave poor results. Thus, 2-ethyl1-hexanol was chosen as the preferred diluent. Effect of Solvent Concentration. The selected solvent system (Alamine-336 in 2-ethyl-1-hexanol) was optimized with respect to the concentration of Alamine-336 in the solvent system. The same model feed solution containing 35 g/L acetic acid at a pH of 3.3 was used. As shown in Figure 3, the extraction efficiency showed an apparent optimum at a 1:1 ratio (50% Alamine-336). This optimum is probably related to the number of amine ion pairs formed with acetic acid. At low levels
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Table 3. Effect of Diluent on Recovery of Acetic Acid from Model Feed Solutiona Using Alamine-336 as the Extractant
diluent used no diluent diisobutyl ketone
1-octanol 2-ethyl-1-hexanol
2-heptanone
a
volume ratio of Alamine-336 to diluent
volume of Alamine-336 in reaction mixture (mL)
volume of diluent in reaction mixture (mL)
volume ratio of solvent system to model feed solution
volume of model feed solution in reaction mixture (mL)
E (%)
1:0 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:4 1:1 4:1 1:3 1:1 1:1 1:1
25 10 25 30 25 10 25 25 25 30 10 25 5 25 20 10 10 25 30
0 10 25 30 25 10 25 25 25 30 10 25 20 25 5 30 10 25 30
1:1 1:2 1:1 3:2 2:1 1:2 1:1 2:1 2:1 3:2 1:2 1:1 1:1 1:1 1:1 1:1 1:2 2:1 3:2
25 40 50 40 25 40 50 25 25 40 40 50 25 50 25 40 40 25 40
36 38 52 57 62 70 75 84 85 80 74 78 69 60 49 61 47 67 62
Model feed solution contained 35 g/L acetic acid in water, pH 3.3.
Figure 3. Effect of Alamine-336 concentration in the solvent system on extraction efficiency.
of Alamine (
