Equilibrium data on ethanol-water-solvent ternaries

Equilibrium data on ethanol-water-solvent ternaries

I. Kirbaslar, S. Cehreli, D. Ustun and E. Keskinocak

Department of Chemical Engineering, School of Engineering, Istanbul University,

34850 Avcilar, Istanbul, Phone: [phone]/1211,

Fax: [phone], Turkey.

E-mail: [e-mail]

(Received: September 20, 1999 ; Accepted: March 20, 2000)

Abstract - Experimental liquid-liquid equilibria of water-ethanol-1-nonanol and water-ethanol-1-decanol systems were investigated at 303.16± 0.20 K. The reliability of the experimental tie-line data was ascertained by using Othmer-Tobias and Hand plots. Distribution coefficients (D_i) and separation factors (S) were evaluated for the immiscibility region. It is concluded that the solvents with high boiling point, 1-nonanol and 1-decanol, are suitable separating agents for dilute aqueous ethyl alcohol solutions.

Keywords: Ethanol, 1-nonanol, 1-decanol, liquid-liquid extraction

INTRODUCTION

It is well known that alternative energy resources can significantly alleviate the demand for and consumption of the world’s petroleum supplies. Ethanol has several advantages because it utilises a renewable source, provides another economic use for current agricultural products, and can be utilised directly in current internal combustion engines with little or no adjustment (Zhang and Hill, 1991). The fermentation process produces dilute aqueous ethanol due to end-product inhibition. In order to concentrate the ethanol, traditional distillation processes are frequently used, but these processes are very energy intensive.

Recently some researchers have reported on the use of liquid-liquid extraction to selectively remove ethanol from water. Solvent extraction techniques have the potential for tremendous energy savings in the recovery of fermentation products. These savings will have a direct impact on the economics of the entire fermentation process (Sola et al., 1986, Miner and Goma, 1982; Dadgar and Foutch, 1985). The equilibrium condition between the organic and aqueous phases is a very important aspect of the liquid-liquid extraction process. Four desirable characteristics for a solvent are as follows: a) a high distribution coefficient for ethanol, b) a high selectivity for ethanol over water, c) a low solubility in water, and d) a low toxicity to yeast.

Higher values of the equilibrium coefficient indicate the ability of the solvent to recover relatively large quantities of solute, thereby reducing the solvent requirement, and corresponding material cost of the extraction equipment. The ability to selectively remove ethanol from aqueous solution may be described by relative selectivity, which is the ratio of the equilibrium distribution coefficient for ethanol over that for water. Toxicity can be important even for separate liquid-liquid extraction vessels because viable yeast could be used in recycled fermentation (Jassal et al., 1994).

This study is part of a research program on the recovery of ethanol from dilute aqueous solutions using solvents with high boiling points. The availability of the relevant liquid-liquid equilibrium data (LLE) is necessary for both the design and the simulation of the extraction processes (Arce et al., 1984; Arenson et al., 1990). In this paper, we report liquid-liquid equilibrium results for the ternary systems water-ethanol-1-nonanol and water-ethanol-1-decanol, for which no such data have previously been published.

EXPERIMENTAL

Ethanol, 1-nonanol and 1-decanol were purchased from Merck Co. The purities of ethanol, 1-nonanol and 1-decanol were 99.5 %, 96 % and 99 %, respectively. Water was deionised, distilled once with alkaline potassium permanganate and subsequently twice distilled.

Seven different weighed ternary mixtures with compositions within the heterogeneous gap were prepared for each ternary. Each of these mixtures was poured into the cell and stirred vigorously for 1 hour under isothermal conditions. After the stirrer was turned off, the contents were immediately allowed to enter the vertical settler also equipped with an isothermal jacket; after the complete separation of the phases, a suitable amount of each layer was removed for analysis. Temperature was controlled using a Nuve thermostat with a precision of 0.02 K. All mixtures were prepared by weighing with a Mettler scale, accurate to within ± 0.000 01 g.

The composition of each phase was determined by gas chromatography (HP, 6890 Series). The column was an HP-innowax (polyethylene glycol), 30 m x 320 mm x 0.5 mm film thickness. Two detectors, a thermal conductivity detector (TCD) and a flame ionisation detector (FID), were installed in series, for the determination of ethanol, water, 1-nonanol, and 1-decanol concentrations. The injector and detector were set at 230 °C. Oven temperature was set isothermally at 250 °C. Nitrogen, at a flow rate of 0.8 mL/min, was used as the carrier gas. Concentrations were determined by comparing 0.5 mL (Split ratio 10/1) injections with a calibration curve for each component.

RESULTS AND DISCUSSION

The experimental tie lines of ethanol-water-1-nonanol and ethanol-water-1-decanol ternaries at 303.16± 0.20 K are plotted, and they are shown in Figure 1 and Figure 2, respectively. Distribution coefficients, D_i, for ethanol (i=2) and water (i=1) and separation factors, S, were determined as follows:

(1)

(2)

W_i3 and W_i1 are the weight fractions in the solvent and aqueous phases, respectively. The weight fraction results are given in Tables 1 and 2. The tie-line data are given in Tables 3 and 4.

Othmer-Tobias and Hand Correlations

The reliability of experimentally measured tie-line data can be ascertained by applying the Othmer-Tobias and Hand equations. The Othmer-Tobias and Hand equations are given as Equation (3) and Equation (4), respectively.

(3)

(4)

The plots are shown in Figures 3 through 6. The linearity of the plot indicates the degree of consistency of the related data.

It is concluded that the solvents with high boiling points (1-nonanol and 1-decanol) are suitable separating agents for dilute aqueous ethanol solutions.

NOMENCLATURE

a₁ Othmer-Tobias equation constant a₂ Hand equation constant b₁ Othmer-Tobias equation slope constant b₂ Hand equation slope constant D_I distribution coefficient of the i th component i Component number of water (1), ethanol (2) and solvent (1-nonanol, 1-decanol) (3) S separation factor W_I mass fraction of the i th component W₁₁ mass fraction of water (1) in the aqueous phase W₂₁ mass fraction of ethanol (2) in the aqueous phase W₃₁ mass fraction of solvent (3) in the aqueous phase W₁₃ mass fraction of water (1) in the solvent-rich phase W₂₃ mass fraction of ethanol (2) in the solvent-rich phase W₃₃ mass fraction of solvent (3) in the solvent-rich phase

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Publication Dates

• Publication in this collection
  06 July 2000
• Date of issue
  June 2000

History

• Accepted
  20 Mar 2000
• Received
  20 Sept 1999