Arielle Cruz
There are various methods to separate water from ethyl alcohol. The most common method is distillation, which involves heating the blended liquid to 80°C (176°F). Since alcohol has a lower boiling point than water, it will quickly turn to steam and can then be condensed into a separate container. Another method is freeze distillation, which relies on the different freezing temperatures of alcohol and water. This technique has been practised since the 7th century. Other methods include using salt, which decreases the solubility of ethanol in water, and pervaporation, which uses hydrophilic zeolite membranes to separate the two substances.
Characteristics and Values for Separating Water from Ethyl Alcohol
| Characteristics | Values |
|---|---|
| Heating | Heat the mixture to the lowest boiling point of 78°C, causing the ethanol to vaporize and condense separately |
| Distillation | Use a fractionating column to separate the alcohol and water, collecting the ethanol in a cooled vessel |
| Freezing | Freeze the mixture to partially remove the water, leaving a higher concentration of alcohol |
| Salting | Add sodium chloride (salt) to decrease the solubility of ethanol in water and separate the mixture |
| Pervaporation | Use hydrophilic zeolite membranes to separate ethanol and water mixtures |
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What You'll Learn
Fractional distillation
The process begins by placing the mixture in a distillation flask, which is then heated to the boiling point of ethyl alcohol, which is 78°C. As the mixture is heated, the alcohol evaporates, forming vapour in the air above the flask. This vapour then passes into a condenser, where it is cooled and condensed back into a liquid state. The condensed alcohol is collected in a separate vessel, leaving the water behind in the distillation flask.
To improve the efficiency of the process, the outside of the column can be insulated with materials such as wool or aluminium foil, or preferably with a vacuum jacket. The use of a reflux splitter can also enhance fractionation by returning condensate to the column. Additionally, the length of the fractionating column plays a crucial role in achieving successful separation. It should be long enough to ensure that the ethanol vapour is adequately condensed before reaching the receiving flask.
While fractional distillation is an effective method for separating ethyl alcohol from water, it is important to note that it has limitations. The maximum concentration of ethanol that can be achieved through simple fractional distillation is 95.6% due to the formation of an azeotropic mixture. To obtain absolute alcohol (100% ethanol), additional techniques such as extractive distillation or recycling from the product stream are required.
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Freezing
Freeze distillation is an ancient technique of separating water from ethyl alcohol that has been practised since the 7th century. It is also known as the Mongolian still. This method relies on the different freezing temperatures of alcohol and water. While the freezing point of water is 0 °C (32 °F), the freezing point of alcohol is −114 °C (−173 °F). This means that alcohol will never freeze under ordinary conditions.
To separate water from ethyl alcohol using freezing, start with a liquid that is 5%-15% alcohol. You will need a container that can be safely frozen and thawed, and a place (either a freezer or outdoor temperatures) that are below 0 °C (32 °F). Place the alcoholic liquid into the container, ensuring that the container is large enough to hold the expanded liquid without bursting, as water expands when it freezes.
Leave the container in the freezer or outside, and the water content of the liquid will expand and freeze, while the amount of alcoholic beverage will be much less due to the extraction of the water. The longer you leave your container in the freezer or outside, the higher the alcoholic content of your remaining liquid. For larger amounts, use larger containers, and be sure to use food-grade plastic containers, as lower-quality plastics may contaminate your beverage.
Remove the frozen material from the container. The frozen material will be mostly water, while the alcohol, which has a higher freezing temperature, will be left behind. The remaining liquid will be higher in alcoholic content and will also have a stronger flavour. This technique is popular for distilling hard apple cider (or apple jack), ale, or beer.
Another technique that uses freezing to separate water from ethanol-water solutions is progressive freeze-concentration. This technique involves freezing the solution in a stirring vessel and then using fractionated or controlled thawing to separate the ice crystals from the ethanol. The average yield of ethanol can be increased by 28% using this technique.
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Salting out
Because the salt ions are charged, they attract water molecules more strongly than alcohol molecules because alcohol is less polar than water. This means that when there is a high concentration of salt, all the water molecules will bond to the salt ions, leaving none to form bonds with the alcohol molecules. As a result, the alcohol becomes immiscible with water and forms a separate layer. This process is called "salt-induced phase separation".
To perform salting out, you will need a container, a pound of non-iodized table salt, and a baster with a reduced-size nozzle. First, fill the container with the water-alcohol mixture. Then, add a sufficient amount of salt—insufficient salt may result in incomplete separation. Close the container and shake vigorously to ensure the salt and water combine. Place the container on a level surface for 15 to 30 minutes to allow the alcohol and salt water to separate.
Finally, carefully open the container and use the baster to extract the top layer, which will be the alcohol. The remaining liquid will be higher in alcoholic content but will not be pure alcohol. It will also have a stronger flavour. This method can be used to separate isopropyl alcohol from water, and the resulting alcohol can be used as a fuel or for other purposes.
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Membrane pervaporation
Pervaporation membranes can be made from a variety of materials, including polymeric membranes, inorganic membranes, mixed-matrix membranes, and two-dimensional-material membranes. The choice of material depends on factors such as separation efficiency and the specific application. For instance, silicone rubber has been used, but its selectivity for alcohol is too low, so it is combined with a sorptive filler to improve both selectivity and flux.
Zeolite-filled membranes have been found to be effective in the pervaporation separation of ethanol and water mixtures. Zeolites are microporous, aluminosilicate minerals with a porous structure that allows for effective molecular transport and sieving. Different types of zeolites such as KA, NaA, CaA, and NaX zeolites have been used in combination with polyvinyl alcohol polymer to create composite hydrophilic membranes. These membranes have been tested at temperatures ranging from 20 to 50 °C, with the KA- and CaA-filled polyvinyl alcohol membranes showing better separation results than the ethanol-water system.
Another study by Shah et al. used hydrophilic zeolite NaA membranes for the pervaporation of ethanol-water mixtures. The total flux for the ethanol-water mixture varied from 2 to 0.05 kg/m2/h at 60 °C as the feed solvent concentration increased from 0 to 100 w%. The zeolite membranes exhibited high selectivities, with separation factors between 1000 and 5000 across the range of ethanol concentrations.
Pervaporation is a versatile process that can be combined with other separation techniques such as distillation to further enhance the separation of ethanol and water mixtures. The development of advanced pervaporation membranes with improved separation efficiency continues to be an active area of research.
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Hydrophilic pervaporation
The separation of ethanol and water mixtures is important for the production of ethanol from biomass. Pervaporation is an efficient membrane process for liquid separation. Hydrophilic pervaporation membranes allow for the dehydration of water/alcohol mixtures. The feed and retentate are both in liquid form, and the permeate leaves the membrane as a vapour. The feed is in contact with one side of the membrane, while a vacuum or sweep gas is applied to the other side. The driving force for mass transfer is the chemical potential gradient resulting from the difference in partial pressure across the membrane, allowing selective separation according to the different diffusion rates of the components through the membrane material.
The incorporation of NU-906, with its hydrophilicity and ordered porosity, significantly improved water selectivity and permeability in hybrid membranes. The optimal membrane with 5 wt.% NU-906 exhibited an impressive flux of 1086 g/m2h and a separation factor of 2651 for 90 wt.% ethanol dehydration at 76 °C. MOF-based MMMs offer unique advantages in terms of nanofiller synthesis and uniform filler dispersion within the polymer matrix, achieving thinner membrane layers.
Composite hydrophilic membranes, consisting of KA, NaA, CaA, and NaX zeolites, and polyvinyl alcohol polymer have been prepared and investigated for the separation of different alcohol-water systems. These membranes showed distinct improvements in molecular transport and the molecular sieving effect of the zeolites. Pervaporation-aided catalytic esterification of acetic acid with ethanol and methanol has been successfully carried out using these membranes, resulting in increased conversion and reduced reaction time due to the continuous removal of water through the membrane.
Overall, hydrophilic pervaporation is a promising technique for the separation of water from ethyl alcohol, offering high selectivity and efficient dehydration of water/alcohol mixtures.
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