The differential solubility of sulfur in ethanol and water could be exploited to separate ethanol from water. The energy that could be produced by burning the separated ethanol would be more than that required in the separation process. In contrast, the separation of a small amount of ethanol (actually an ethanol/water solution poor in ethanol) from water by distillation requires more energy than can be produced by burning the resulting distillate. The proposed alcohol/water separation process could be exploited industrially to produce clean fuel from fermented vegetable matter.

In one version of this concept, sulfur would be added to an ethanol/water mixture: a slight amount of sulfur that depends on the temperature of the mixture would be dissolved by the ethanol. (All three forms of sulfur are insoluble in water, even at its boiling temperature, but the a form of sulfur is slightly soluble in ethanol and the b form is more soluble in ethanol, according to the CRC Handbook of Chemistry and Physics.) The sulfur/ethanol mixture would settle to the bottom of the container, where it could be bled off. This small part of the original mixture could then be heated to separate the volatile ethanol from the significantly less volatile sulfur. The hot sulfur left after the distillation could be added to another batch of the ethanol/water mixture.

In comparison with the energy consumed in the conventional distillation process, a significant amount of energy would be saved in this process because only the small bled-off portion of the original mixture would have to be heated. Because of its solubility in ethanol, the b form of sulfur would be used when the separation process was carried out at room temperature and atmospheric pressure. Finely divided sulfur that was not dissolved by the ethanol would float on the mixture.

In an alternative version of this concept, the ethanol/water/sulfur mixture would be placed in a retort, where it could be heated and pressurized to a temperature above the critical temperature and pressure of ethanol [243 °C and 63 atm (6.4 MPa), respectively] but below the critical temperature and pressure of water [374.1 °C and 218.3 atm (22.12 MPa), respectively]. The mixture would be retorted at a temperature slightly above 243 °C and at a pressure slightly above 63 atm (6.4 MPa), putting the ethanol in the supercritical state, in which it should easily dissolve all three forms of sulfur (including the γform, which is insoluble at ambient temperature and pressure). The water, on the other hand, would still be well below its critical state and still should not dissolve sulfur. The sulfur/ethanol mixture would settle to the bottom of the retort, where it could be piped away under pressure and at high temperature. The sulfur/ethanol mixture would then be expanded to a lower temperature and pressure at which not as much sulfur could be dissolved in the ethanol and at which ethanol would partially separate from the mixture. Further heating of the remaining mixture at a pressure of 1 atm (0.1 MPa) would separate most of the remaining ethanol and sulfur. The sulfur could be reused, and the high-pressure hot water could be used to cook more mash to be fermented or to preheat a charge going to another retort.

This second version is probably the most suitable for an industrial process, and could be aided by the addition of a centrifuge to separate the initial two-phase mixture. The role of sulfur in both versions could be played by another substance. However, the low toxicity and very low vapor pressure of sulfur at the boiling temperature of ethanol appear to make it the best candidate.

This work was done by Renaldo V. Jenkins of Langley Research Center. No further documentation is available. LAR-14894

NASA Tech Briefs Magazine

This article first appeared in the July, 1999 issue of NASA Tech Briefs Magazine.

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