Butanol-Water System

The interest in w-butanol as a biofuel has increased in recent years owing to its superior fuel qualities compared to ethanol. These include a higher octane number, lower heat of vaporization, higher energy density (energy/volume), and lower vapor pressure. However, in the traditional ABE (acetone-butanol-ethanol) fermentation process, the concentration of n-butanol coming from the fermenter is lower than that achieved in ethanol fermentation. In addition, acetone and ethanol are also produced. Recent studies to improve yield and increase w-butanol concentration have explored fed-batch systems with stripping, adsorption, liquid-liquid extraction, distillation, and/or pervaporation to recover products. 

The purificalion of n-butanol involves removing the acetone and ethanol, and separating the n-butanol from the water. This separation carmot be achieved in a single distillation column because of the presence of an azeotrope. 

There are four isomers of butanol n-butanol, isobutanol, 2-butanol, and tert butyl alcohol. The butanol produced in fementation is n-butanol. Water and n-butanol form a heterogeneous azeotrope at atmospheric pressure and 364.6 K with a composition of 76.33 mol% water. 

Qurishi et al., Ladisch, and Phillips and Humphrey discuss some of the issues concerning the separation of the fermenter products. Doherty and Malone indicate that a two-column distillation system in conjunction with a decanter can be used to separate the heterogeneous binary n-butanol-water azeotrope. Sticklmair and Faii discuss the separation of the ternary acetone-water-n-butanol system in a two-column system. Pucci et al. suggest that the ternary separation can be achieved in a single column in which a side decanter is used to remove the water. 

Liquid from the condenser flows to a decanter in which the aqueous and organic hquid phases separate. The aqueous phase is returned to the top of the first column Cl. The organic phase is fed to the top of the second column C2. 

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The first partial chiral resolution reported in CCC dates from 1982 [120]. The separation of the two enantiomers of norephedrine was partially achieved, in almost 4 days, using (/ ,/ )-di-5-nonyltartrate as a chiral selector in the organic stationary phase. In 1984, the complete resolution of d,l-isoleucine was described, with N-dodecyl-L-proline as a selector in a two-phase buffered n-butanol/water system containing a copper (II) salt, in approximately 2 days [121]. A few partial resolutions of amino acids and dmg enantiomers with proteic selectors were also published [122, 123]. 

Early examples of enantioselective extractions are the resolution of a-aminoalco-hol salts, such as norephedrine, with lipophilic anions (hexafluorophosphate ion) [184-186] by partition between aqueous and lipophilic phases containing esters of tartaric acid [184-188]. Alkyl derivatives of proline and hydroxyproline with cupric ions showed chiral discrimination abilities for the resolution of neutral amino acid enantiomers in n-butanol/water systems [121, 178, 189-192]. On the other hand, chiral crown ethers are classical selectors utilized for enantioseparations, due to their interesting recognition abilities [171, 178]. However, the large number of steps often required for their synthesis [182] and, consequently, their cost as well as their limited loadability makes them not very suitable for preparative purposes. Examples of ligand-exchange [193] or anion-exchange selectors [183] able to discriminate amino acid derivatives have also been described. 

Mixtures of 2-butanol (.sec-butanol) and water form two-liquid phases. Vapor-liquid equilibrium and liquid-liquid equilibrium for the 2-butanol-water system can be predicted by the NRTL equation. Vapor pressure coefficients for the... 

Figure 2 Effect of spinning rate on the interfacial tension of r -butanol/water system at 30°C.

At atmospheric pressure, the n-butanol-water system exhibits a minimum boiling azeotrope and partial miscibility, and hence a binary heterogeneous azeotrope. Figure 1.8 shows the Tyx and Pyx phase diagrams for l-propanol(l)-water(2) azeotropic mixture obtained from the Aspen Plus simulator using the NRTL activity coefficient model. 

FIG. 13-8 Vapor-liquid equilibrium data for an n-butanol-water system at 101.3 kPa (1 atm) phase splitting and heterogeneous-azeotrope formation. 

The vapor-liquid equilibrium data for the 3-methyl-l-butanol-water system are shown in Table III and Figure 4. The boiling point measurements agreed with those reported in Timmermans (13). The value of a = 0.45 as suggested by Renon and Prausnitz (8) for alcohol-water systems was not suitable. Various other values of a were tried, and a value of a. = 0.3 was found to agree best. This fit can be established by using the method described to test the consistency of the equations—i.e., the... 

AH° and AS0 vs. solvent composition for the ethanol-water solvent systems (see Figures 5 and 6) show a more complex behavior at about 60% ethanol than the corresponding plots for the tert-butanol-water solvent systems (see Figures 6 and 8 at about 60% terf-butanol. The ethanol-water system shows a slight minimum and then a maximum in the 0 to 60% ethanol range, while tert-butanol-water system shows only a maximum in the 0 to 60% terf-butanol range. 

Fig. 12. Convenient diagram to synthesize a separation process for the n-butanol/water. system.

Limited information is available concerning the KBI s for systems with phase separation. The KBI s for the 1-butanol— water system were calculated in ref 7. The behavior of the KBI s in all butanol (1-, 2-, and iso-)—water systems is similar (Figures 7—9). In these systems, the GyS change rapidly and become infinite at the phase separation point... 

Figure Z Correlation radius Rc against xi for the tert-butanol—water system. The broken hue is for the alcohol-rich cluster and the sohd line is for the water-rich cluster experimental data from , ref 14 (T = 301.15 K) O, ref 21 T = 293.15 K) , ref 23 T = 293.15 K).

Figure 7. Local composition in the tert-butanol—water system. The solind line is calculated with the NRTL and the broken line with the Wilson equation , eq 27 with = 100 cmVmol , eq 27 with Vjor = 175 cmVmol A, eq 27 with = 500 cmVmol O, eq 27 with = 1000 cm /mol.

Koga, Y. (1984). A SAXS study of concentration fluctuations in tert-butanol-water system. Chemical Physics Letters, 111, 176-180. 

Schreiber, E. Schuettau, E. Rant, D. Schuberth, H. Extent to which a metalchloride can influence the behavior of isothermal phase equilibrium in n-propanol-water and n-butanol—water systems. Z. Phys. Chem. (Leipzig) 1971, 247, 23-40. 

We only give basic directions for the choice of a solvent system. If the polarities of the solutes are known, the classification established by Ito [1] can be taken as a first approach. He classified the solvent systems into three groups, according to their suitability for apolar molecules ( apolar systems), for intermediary polarity molecules ( intermediary system), and for polar molecules ( polar system). The molecule must have a high solubility in one of the two immiscible solvents. The addition of a third solvent enables a better adjustment of the partition coefficients. When the polarities of the solutes are not known. Oka s [8] approach uses mixtures of n-hexane (HEX), ethyl acetate (EtOAc), n-butanol (n-ButOH), methanol (MeOH), and water (W) ranging from the HEX-MeOH-W, 2 1 1 (v/v/v) to the n-BuOH-W, 1 1 (v/v) systems and mixtures of chloroform, methanol, and water. These solvent series cover a wide range of hydrophobicities from the nonpolar n-hexane-methanol-water system to the polar n-butanol-water system. Moreover, all these solvent systems are volatile and yield a desirable two-phase volume ratio of about 1. The solvent system leading to partition coefficients close to the unit value will be selected. 

Oka et al. [5] proposed a choice of various solvent systems to purify antibiotics. They have to fulfill various criteria. The settling time of the solvent system should be shorter than 30 s to ensure the satisfactory retention of the stationary phase. The partition coefficient of the target compounds should be close to 1, and the separation factor (a) between the compounds must be larger than 1.5. Two series of solvent systems can provide an ideal range of the K values for a variety of samples n-hexane-ethyl acetate-n-bu-tanol-methanol-water and chloroform-methanol-water. These solvent series cover a wide range of hy-drophobicity, continuously, from the nonpolar n-hexane-methanol-water system to a more polar n-butanol-water system. 

Chemical structures of sporaviridins are described in Fig. 1. They are only soluble in polar solvents such as water, methanol, and n-butanol. Therefore, a two-phase solvent system containing n-butanol was examined. A nonpolar solvent such as diethyl ether has been added to the n-butanol-water system to decrease the solubility of molecules in n-butanol and to obtain partition coefficients close to 1. The partition coefficients, K, are defined as the ratio of the solute concentration in the upper phase (butanol rich) to its concentration in the lower one (water rich). A two-phase solvent system oin-butanol-diethyl ether-water (10 4 12, v/v/v) was selected because it allows one to obtain the almost equally dispersed partition coefficients among six components (C2, B2, A2, Cl, Bl, Al). The preparative separation of six components from sporaviridin complex by HSCCC was performed in 3.5 h (500 mL of elution volume). The six components were eluted in the order of their partition coefficients, yielding pure components Al (1.4 mg), A2 (0.6 mg), Bl (0.7 mg), B2 (0.5 mg). Cl (1.1 mg), and C2 (1.4 mg) from 15 mg of the sporaviridin complex. 

T.W. Kim, F. Kleitz, B. Paul, and R. Ryoo, MCM-48-like Large Mesoporous Silicas with Tailored Pore Structure Facile Synthesis Domain in a Ternary Triblock Copolymer-Butanol-Water System. J. Am. Chem. Soc., 2005, 127, 7601-7610. 

Figure 13.14. Composition and efficiency profiles in distillation of ethanol-l-butanol-water system. Calculations by Aittamaa (1981).

Typical non-ideal binaries forming two liquid phases is the n-butanol-water system at 1 atmospheric pressure. A solution with approximately 2 mole% n-butanol in water exists at equilibrium with another liquid phase with approximately 38 mole% n-butanol in water. The fugacity of n-butanol in both phases is about 0.48. A phase diagram of this binary is illustrated in Figure 1.16. The curves, which closely match the experimental data, are based on calculations using the NRTL equation for activity coefficients. 

The solubility of water in the alkanols does not fall off so drastically, and Figure 5 shows a plot of solubility vs. chain length. Furthermore, the water-in-alcohol portion of Figure 4 shows the small change in solubility over a wide range of temperature in fact, this curve (for the 1-butanol-water system) is binodal. This behavior reflects the delicate balance between the association and the ideal mixing tendencies in alcohol-water mixtures. 

Krishna et al. (1977) showed that when the vapor mole-fraction driving force of a component (call it A) is small compared to the other components in the mixture, the transport rate of A is controlled by the other components, with the result that Emg for A is anywhere in the range from minus infinity to plus infinity. They confirmed this theoretical prediction by conducting experiments with the ethanol/ferf-butanol/water system and obtained values of EMG for /er/ butanol ranging from -2978% to +527%. In addition, the observed values of EMG for ethanol and water sometimes differed significantly. 

Unsubstituted benzyl 4-(9-benzyl-2,3-dideoxy-2,3-epimino-(3-D-/yv o- (341) and -ot-D-n )o-pyranoside (342) have been used by Paulsen and Patt in 1981 in cleavage reactions with azide. By the action of a mixture of sodium azide and ammonium chloride in a butanol-water system, the aziridine-ring cleavage proceeded exclusively at C-3, thus exhibiting anti-Furst-Plattner regioselectivity for both epimines. No explanation for this preference was given in the paper. 

The VLB diagram for the butanol/water system and the high values of y for both butanol in water and water in butanol show that the fractionating approach to the azeotrope is very easy from both directions so that the columns required for the continuous separation need only a few plates. 

Let us now study a system in which there is more dissimilarity of the molecules by looking at the -butanol/water system. The normal boiling point of -butanol is 398 K, and that of water is 373 K, so water is the low boiler in this system. The azeotrope search results are shown in Figiure 1.16, and the Txy diagram is shown in Figure 1.17. Notice that Vap-Liq-Liq is selected in the Phases under the Property Model. ... 

For a group of compounds, the dependence of K on temperature would be determined by the properties of the selected sol vent-water systems, which determine A77o and AH. For example, when the polarity of the organic solvent becomes increasingly close to that of water, as in the butanol-water system (65), the value of AHq — be relatively small and the partition coefficients of... 

The following data were obtained for the butanol-water system at 25°C ... 

Tobias type, and except in the case of isopropanol excellent straight lines are obtained up to plait-point concentrations. These authors have tested this coordinate system with a large number of systems and have shown it to be very useful generally. Figure 2.28, with the Hand type of coordinates, also shows excellent rectilinearity even up to the plait points for all data, with the exception again of the iso-propanol distribution. Here the uncertainty of the single point in the elhanol-n-butanol-water system is very clear. This plot has been tested with over sixty systems with similar consistently good results (56). 

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