Separating Ethanol From Water Via Differential Miscibility

The differential miscibility of castor oil in ethanol and water would be exploited to separate ethanol from water, according to a proposal. Burning the separated ethanol would produce more energy than would be consumed 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 the conventional process of distillation requires more energy than can be produced by burning the resulting distillate. As in the process described in the preceding article, "Separating Ethanol From Water Via Differential Solubility" (LAR-14894), the proposed alcohol/water separation process could be exploited industrially to produce clean fuel from fermented vegetable matter.

In one version of this process, castor oil would be added to an ethanol/water solution. The ethanol would mix freely with castor oil, which is insoluble in water. The resulting ethanol/castor-oil phase, which would contain less than 1 percent water, would collect as the top layer, the bottom layer being the remainder of the ethanol/water solution somewhat depleted in ethanol. Heating this two-layer mixture to a temperature slightly below the boiling temperature of ethanol (78.5 °C) would cause the partial pressure of ethanol above the top layer to be much greater than the partial pressure of either castor oil or water. This vapor-phase ethanol could be condensed in a relatively pure state.

Although heating an isolated ethanol/water solution like that in the bottom layer would normally raise the vapor pressure of both ethanol and water above the solution, this would not be the case in the presence of the top castor-oil/ethanol layer for the following reasons: The amount of water that could dissolve in the top castor-oil/ethanol layer would increase only slightly upon heating. On the other hand, ethanol could readily cross the interface between the two layers and enter the top layer. As long as the total mix was kept at a temperature below the boiling temperature of ethanol (thereby preventing agitation of the layers by boiling), the diffusion of water through the castor-oil/ethanol phase would be inhibited.

In an alternative version of this concept, the upper castor-oil/ethanol layer would be skimmed off and heated to obtain the ethanol. Once the ethanol was driven off, the castor oil could be returned to an ethanol/water solution to dissolve more ethanol to repeat the process. This concept could readily lend itself to a continuous process. Substances other than castor oil (one of its components perhaps, or another substance) could be used in this process or to extract other compounds from other mixtures by using this upper-of-two-phases vaporization technique.

This work was done by Renaldo V. Jenkins of Langley Research Center. No further documentation is available. LAR-14895

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CASTOR OIL SPECIFICATIONS