Ind. Eng. Chem. Res. 2005, 44, 6797-6803 6797 Solvent Extraction of Ethanol from Aqueous Solutions. II. Linear, Branched, and Ring-Containing Alcohol Solvents Richard D. Offeman,* Serena K. Stephenson, George H. Robertson, and William J. Orts U.S. Department of Agriculture, Western Regional Research Center, 800 Buchanan Street, Albany, California 94710 Distribution coefficients have been measured for the partitioning of ethanol and water from aqueous mixtures into 57 different alcohol solvents. The study has focused on the effects of systematic variations in chemical structure of the alcohols. Factors found to be important include chain length and hydroxyl position for the x-alcohols (i.e., 1-heptanol through 1-dodecanol, 2-heptanol through 2-undecanol, etc.), branch structure (e.g., methyl, ethyl, n-propyl, i-propyl, etc.) for a branch located on the hydroxyl carbon, location of the branch relative to the hydroxyl carbon, and multiple branching. Introduction this is the reason to search for solvents that will chemically react, hydrogen bond, or complex preferen- - Liquid liquid solvent extraction has long been of tially with the compound to be extracted. interest in recovering ethanol from dilute aqueous Many classes of chemicals have been evaluated as mixtures.1 It has the potential to be a more energy- solvents for the recovery of ethanol from dilute aqueous efficient alternative to distillation.1-4 Fermentation of mixtures. Roddy15 summarized his data in a general grain- and biomass-derived sugars is a major and ranking of solvent classes in order of increasing K , rapidly growing source of ethanol. In fermentation DE the ethanol distribution coefficient: hydrocarbons ) systems, the ethanol product is inhibitory to the micro- < < < < < organisms producing it,5,6 so gains in yield and fer- halocarbons ethers ketones amines esters alcohols ) phosphates, though specific exceptions are menter productivity can be realized by continuous 16 13 removal of the ethanol produced.2 Continuous solvent noted. Munson and King and Cabral made use of Lewis-acid/Lewis-base concepts to rank selectivity for extraction of ethanol-producing fermentation systems < < < has been demonstrated by a number of researchers.7-12 a given KDE as hydrocarbons ethers ketones amines < esters < alcohols < carboxylic acids. Souissi For successful implementation, many criteria must be 17 met by a potential extraction solvent. These criteria and Thyrion used the Hansen extension of the Hilde- include extraction performance, chemical stability, eco- brand solubility parameter for polar (δp) and hydrogen nomical product recovery and solvent regeneration, bonding (δh) contributions, observing that higher KDE minimization of solvent losses, safety and environ- values occur when a solvent’s δp and δh values are close mental risk from emissions, and, in the case of fermen- to those of ethanol. tation systems, biocompatibility with ethanol-producing Predictive models have been applied for solvent microorganisms.13 screening purposes. Poling et al.18 and Prausnitz and For the purpose of screening a variety of possible Tavares19 provide excellent overviews of the forms and extraction solvents, extraction performance can be evolution of a variety of thermodynamic models. The measured by two characteristics at the operating condi- thermodynamic group contribution models such as tions of interest: distribution coefficient KDE and sepa- UNIFAC and ASOG have been used in attempts to ration factor R.14 The equilibrium distribution coefficient estimate liquid-liquid equilibria (LLE) to predict the 20-26 for ethanol is defined as KDE ) [EtOH]org/[EtOH]aq, the extractive performance of solvents. The original ratio of the weight percent of ethanol in the organic UNIFAC model27 with LLE parameter tables,28 how- phase to the weight percent of ethanol in the aqueous ever, does not take positional information into account. phase. KDW, the equilibrium distribution coefficient for An examination of the literature data for alcohol water, is defined similarly: KDW ) [H2O]org/[H2O]aq. The solvents shows that positional information such as the separation factor is R)KDE/KDW, or the ratio of ethanol location of the hydroxyl group and its proximity to alkyl to water in the organic phase to that in the aqueous branches has a strong effect on the distribution coef- phase. The distribution coefficient indexes the solvent’s ficients and the separation factor, as has been noted by capacity for the extracted component, while the separa- others.15,16 Later extensions of the models add or modify tion factor is the solvent’s selectivity for one component UNIFAC groups to include some structural information over another. Unfortunately, it is often observed that (e.g., ref 29), but the accuracy of the current group solvents with a high separation factor generally have contribution methods is still inadequate for isomers.30 low ethanol distribution coefficients, and vice versa. A quantum chemistry approach, COSMO-RS,31 holds 14 King explains this effect in terms of solubility param- promise for predicting LLE information. Unlike the eters for physically interacting solvents and notes that group contribution models, a quantum chemical cal- culation is made for each specific molecule; hence, * To whom correspondence should be addressed. Tel.: (510) structural information is automatically included. Once 559-6458. Fax: (510) 559-5818. E-mail: [email protected]. the parameters for a molecule are calculated, estimation 10.1021/ie0500321 This article not subject to U.S. Copyright. Published 2005 by the American Chemical Society Published on Web 06/30/2005 6798 Ind. Eng. Chem. Res., Vol. 44, No. 17, 2005 of phase behavior for multicomponent mixtures is increased slope. The R values for the two 5-alcohols are relatively rapid. above, but with a slope parallel to, the 3- and 4-alcohols, Positional information is used in a correlation based and the R value of the one 6-alcohol is slightly higher on steric shielding of the hydroxyl oxygen by Munson than those of the 5-alcohols and is the highest of any of and King.16 On the basis of a cone-angle approach, they the alcohols studied. show a trend toward higher selectivity with increased The branched alcohols data is very scattered, with steric hindrance of the hydroxyl group, but several some data points falling above, and some below, the exceptions can be noted. All these approaches can make linear compounds with the same hydroxyl position. To qualitative predictions about solvent performance, but illustrate, for the eight branched 3-alcohols, two fall in the end, experimental data are still needed for final above, three on, and three below a line fitting the data solvent selection. for the linear 3-alcohols. Our objectives were to employ the solvent screening The ring compounds compare poorly to their nearest 32 method described in a companion paper to determine equivalent alkyl alcohols. For instance, 3-methylcyclo- the ethanol extractive performance of a wide range of hexanol (R)10.2) might be compared to 4-heptanol (R alcohol solvents, with focus on the systematic correlation ) 20.1) based on number of carbons and branching on to the chemical structure of the solvents. The factors the hydroxyl carbon. Similarly, 3-phenyl-1-propanol explored were the molecular weight, the location of the (9.3) might be compared to 1-nonanol (12.6). One pos- hydroxyl group, the location of branching relative to the sible reason for the comparatively poor performance of hydroxyl group, the type of branch (e.g., methyl, ethyl, the ring compounds is the lower mobility of the ring n-propyl, i-propyl, etc.), and the inclusion of an aromatic carbons compared to the alkyl chain carbons. or cyclohexyl ring. Alcohol solvents that have the greatest R for a given KDE are the 4-, 5-, and 6-unbranched-alcohols and some Experimental Section of the branched 3- and 4-alcohols such as 2-methyl-3- pentanol (but not 3-methyl-3-pentanol), 2,2-dimethyl- The extractions were carried out at 33 °C with an 3-hexanol, and 2,6-dimethyl-4-heptanol. aqueous-to-organic phase volume ratio of 2:1 and total The characteristic tradeoff between the separation liquid volumes of 7.5 mL. The mixtures were emulsified 14 factor and KDE has been discussed by King and multiple times at the extraction temperature to ensure Barton,38 who were able to derive this behavior from that the extraction system had reached equilibrium. The the Scatchard-Hildebrand equation for regular solu- mixtures were centrifuged at the extraction tem- tions, relating activity coefficients to molar volumes and perature, and the composition of each phase was solubility parameters. However, since hydrogen bonding analyzed by gas chromatography using an internal and other interactions are certainly taking place in the standard method. Distribution coefficients and separa- ethanol-water-polar solvent systems investigated here, tion factors were calculated from the compositions of the relationship between R and KDE is certainly more each phase. Specific details may be found in the complex than that of regular solutions. companion paper.32 Figure 2 presents the data for the unbranched alkyl The solvents that were investigated are listed in Table x-alcohols. The separation factor and the distribution 1. CAS number, source, purity, density, and solubility coefficients for ethanol and water are plotted against in water are included. Ethanol was 200 proof anhydrous the inverse of the molecular weight of each solvent. This grade, from Aaper Alcohol and Chemical Co. Anhydrous augments the work of Murphy et al.20 that showed K 1-butanol from Aldrich, 99.95%, was the organic-phase DE and KDW for the unbranched 1-alcohols are related to diluent and aqueous-phase internal standard. The the concentration of the solvent hydroxyl groups present. organic-phase internal standard, 1-hexanol, was anhy- In Figure 2a, each family of alcohols shows a linear drous 99.49% from Aldrich. Distilled water was used relationship of R with 1000/MW.