Ethanol-water vapour-liquid equilibria

For non-ideal solutions, such as the ethanol-water system, at the intermediate ethanol concentrations found in the bed and the condensate during anaerobic gas-solid fluidized bed fermentafions, Raoult s law (equation 4.2) is inadequate and an activity coefficient ymust be introduced, so that the partial pressures of efhanol pe and water p over an ethanol-water solution are given by equation 6.11 

The methods most generally used for the calculation of activity coefficients at intermediate pressures are the Wilson (1964) and UNIQUAC (Abrams and Prausnitz, 1975) equations. Wilson s equation was used by Sato et al. (1985) to predict the composition of fhe condensate gas stripped from a packed bed fermenter at 30°C, whilst Beck and Bauer (1989) used the UNIQUAC equation, with temperature-dependent parameters given by Kolbe and Gmehling (1985), for their model of an anaerobic gas-solid fluidized bed fermenter at 36°C. In this case it was necessary to go beyond the temperature range of fhe source data down to 16°C in order to predict the composition of the fluidizing gas leaving the condenser. 

The presence of solutes other than ethanol might be expected to reduce the mole fractions of ethanol and water and influence the nonideality of the ethanol-water system. However, both Williams (1983), who modelled a batch wine fermentation, and Rottenbacher (1985), in ethanol sorption experiments with yeast pellets in a fluidized bed, established that the ethanol-water-yeast system behaves as if the water and ethanol content of the pellets were a simple ethanol-water solution supported by a solid matrix which influences neither mole fractions nor activity coefficients. 

Carbon dioxide build-up in the fermenter is ignored in the model and it is assumed that atmospheric pressure prevails at all points in the system. The partial pressures of ethanol and water vapour over the bed are evaluated at the bed temperature using 

Beck and Bauer (1989) used an empirical kinetic model (derived by Rottenbacher et al. (1985a) for the commercial baker s yeast strain DHW DZ in submerged culture) to define the maximum possible rate of ethanol production in the bed as a function of the dry mass of yeast in the bed and the mole fraction of ethanol in the liquid phase of the bed, thus 

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The value of Xe,bei depends upon the prevailing ethanol-water vapour-liquid equilibria in the bed and the condenser. Assuming that the bed produces ethanol at its maximum possible rate then... 

Partition coefficients can be determined by vapour-phase calibration (VPC) [54], by the phase-ratio variation (PRV) method [also known as the vapour-liquid equilibrium (VLE) method] [57] for many solvents in their aqueous solutions, and by VLE for ethanol in water. If two sample vials of different volume are both filled with the same sample, the partition coefficient, K, will be the same. In order to determine the solute s partition coefficient, K, each vial, at equilibrium, is subjected to headspace analysis in order to derive the slope of the linear equation (4.1). The concentrations of a solute in the two vials can be written as... 

Ethanol purchased from Verbiese (France) (purity > 99.77 %) is mixed with double distilled and deionized water (resistivity 18 MO/cm) for the preparation of aqueous solutions of concentration 0.6, 1, 2, 3, 4.5, 9 and 17 mol%. The solutions are placed in an insulated glass bulb connected to a vacuum line. The vapour-liquid equilibrium conditions are first obtained at 295 K before a contact between the gas phase in equilibrium above the liquid and the pre-cooled sample holder of a cryostage is established. The deposition takes place at 10 Torr and 88 K in 3-5 min. Using thermodynamic modelling and a condensation kinetic model (relation below), one derives the concentration XEton) of EtOH in the deposited solids as ... 

Zielkiewicz, J. Konitz, A. Vapour-liquid equilibrium in ternary system N,N-dimethyl-formtimide + water + ethanol at 313.15 K... 

Figure VI - 22. Distillation (vapour-liquid equilibrium) and perv xnation characteristics for an ethanol-water mixture at 20 C. Pervaporation was carried out using a polyacrylonitrile membrane [38].

Membrane distillation is one of the membrane processes in which the membrane is not directly involved in separation. The only function of the membrane is to act as a barrier between the two phases. Selectivity is completely determined by the vapour-liquid equilibrium involved. This means that the component with the highest partial pressure will show the highest permeation rate. Thus, in the case of an etbanol/water mixture where the membrane is not wetted at low ethanol concentrations, both components will be transported through the membrane but the permeation rate of ethanol will always be relatively higher. With salt solutions, for example NaCl in water, only water has a vapour pressure, i.e. the vapour pressure of NaCl can be neglected, which means that only water will permeate through the membrane and consequently very high selectivities are obtained. 

Gomis, V., Font, A., and Saquete, M.D., "Vapour-Liquid-Liquid and Vapour-Liquid Equilibrium of the System Water + Ethanol + Fleptane at 101 3 kPa," Fluid Phase Equilibria, Vol. 248, 2006, pages 206 to 210. This research reports that a ternary azeotrope is formed when the binary azeotrope (entry 456 in Ref 3, Table A 2-2, 48% alcohol) is contacted with water The boiling point of the ternary azeotrope is 3 C less. 

Zielkiewicz, J. Vapour + liquid equilibrium measurements and correlation of the ternary mixture (N-methylacetamide -1- ethanol + water) at the temperature 313.15. J. Chem. Thermodyn. 2000, 32, 55-62. 

Jaques and Furter [95] derived an equation for the salt effect on the water-vapour equilibrium in binary mixtures which correlates the temperature and the liquid concentration of the three components ethanol, water and salt. The equation has 6 constants. The theory of the salt effect has been discussed by Furter and Meranda [96]. On the basis of simplifying assumptions Sada et al. [97] have established a relation for the calculation of vapour-liquid equilibria for non-aqueous binary systems in which the salt is dissolved only in one component e.g., benzene-ethanol with lithium or calcium chloride). 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

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