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From Wikipedia

Fractional distillation

Fractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation. Generally the component parts boil at less than 25 Â°C from each other under a pressure of one atmosphere (atm). If the difference in boiling points is greater than 25 Â°C, a simple distillation is used.

Laboratory setup

Fractional distillation in a laboratory makes use of common laboratory glassware and apparatuses, typically including a Bunsen burner, a round-bottomed flask and a condenser, as well as the single-purpose fractionating column.



As an example, consider the distillation of a mixture of water and ethanol. Ethanol boils at 78.4 Â°C while water boils at 100 Â°C. So, by gently heating the mixture, the most volatile component will concentrate to a greater degree in the vapor leaving the liquid. Some mixtures form azeotropes, where the mixture boils at a lower temperature than either component. In this example, a mixture of 96% ethanol and 4% water boils at 78.2 °C, being more volatile than pure ethanol. For this reason, ethanol cannot be completely purified by direct fractional distillation of ethanol-water mixtures.

The apparatus is assembled as in the diagram. (The diagram represents a batch apparatus, as opposed to a continuous apparatus.) The mixture is put into the round bottomed flask along with a few anti-bumping granules (or a Teflon coated magnetic stirrer bar if using magnetic stirring), and the fractionating column is fitted into the top. As the mixture boils, vapor rises up the column. The vapor condenses on the glass platforms, known as trays, inside the column, and runs back down into the liquid below, refluxing distillate. The column is heated from the bottom. The efficiency in terms of the amount of heating and time required to get fractionation can be improved by insulating the outside of the column in an insulator such as wool, aluminium foil or preferably a vacuum jacket. The hottest tray is at the bottom and the coolest is at the top. At steady state conditions, the vapor and liquid on each tray are at equilibrium. Only the most volatile of the vapors stays ingaseous form all the way to the top. The vapor at the top of the column, then passes into the condenser, which cools it down until it liquefies. The separation is more pure with the addition of more trays (to a practical limitation of heat, flow, etc.) The condensate that was initially very close to the azeotrope composition becomes gradually richer in water. The process continues until all the ethanol boils out of the mixture. This point can be recognized by the sharp rise in temperature shown on the thermometer.

Typically the example above now only reflects the theoretical way fractionation works. Normal laboratory fractionation columns will be simple glass tubes (often vacuum jacketed, and sometimes internally silvered) filled with a packing, often small glass helices of 4 to 7 mm diameter. Such a column can be calibrated by the distillation of a known mixture system to quantify the column in terms of number of theoretical plates. To improve fractionation the apparatus is set up to return condensate to the column by the use of some sort of reflux splitter (reflux wire, gago, Magnetic swinging bucket, etc.) - a typical careful fractionation would employ a reflux ratio of around 10:1 (10 parts returned condensate to 1 part condensate take off).

In laboratory distillation, several types of condensers are commonly found. The Liebig condenser is simply a straight tube within a water jacket, and is the simplest (and relatively least expensive) form of condenser. The Graham condenser is a spiral tube within a water jacket, and the Allihn condenser has a series of large and small constrictions on the inside tube, each increasing the surface area upon which the vapor co

From Yahoo Answers

Question:My future father-in-law got into a debate with a coworker about this, and I was charged with finding the answer... Imagine an unbreakable, fully sealed container. We put a teaspoonn of water in the bottom, and then pump in air to really high pressure. Say, a million lbs per square inch. What happens to the system? Their specific question was 'is there a point at which the pressure holds the water in place?' I figured not, but said I would ask for other opinions. Now I'm curious about this, too. With water being an incompressible(ish) fluid, can air be compressed such that it is denser than water? Would the water then float on the air? Thanks for the help!

Answers:Although the volume of the water will (pretty much) not be reduced by the pressure, the pressure within your sealed container will be equally distributed between the air and the water. Unless there was enough pressure to change the state of the air from a gas to a liquid, the water would still be heavier, molecule for molecule than the air. (I'm not sure what would happen if the air turned into a liquid.) When we get to the point of talking about the interaction of two liquids we are no longer talking about pressure. We are then talking about specific gravity and buoyancy. In order to determine which would float, we need to know which is heavier: liquid air or liquid water? re: ppgpca -- I'm not sure about the water turning into a solid bit due to the fact that water EXPANDS when it freezes. .


Answers:Steam distillation is a common way to extract oils from natural materials (i.e. getting clove oil from cloves, which I remember doing as an undergraduate). The reason it's good is because it lowers the pressure in the flask so that you don't have to heat it as high; high temperatures can often decompose organic compounds, so chemists tend to avoid this when possible. The boiling point of limonene is 176C, but depending on the pressure you reduce to, you can boil it at an even lower pressure, which means you don't have to use as much heat to achieve the distillation. Check out the wikipedia site. Another distillation that wouldn't be bad is vacuum distillation, where instead of using steam you use a vacuum to reduce pressure.

Question:Methanol is injected into our process whenever a hydrate is plugging up a pipe or tower? What reaction is taking place between the methanol and the hydrate (water and hydrocarbon mixture - ethylene)? The hydrate is in the solid form and the methanol dissolves the solid and carries it out of the tower.

Answers:Alcohol is a Polar substance, as is water. Methanol etc..will mix with, and dissolve the hydrates to water and hydrocarbons and prevent blockage of small diameter tubing and equipment in the distillation system. The water will be vaporised and pass up the tower with the lighter fractions of hydrocarbons and separated out of the overhead distillate and disposed of. I used to work on a Low Temperature Separation unit where Hydrates were purposely produced by the refrigeration effect of expanding high pressure gas into a special separator vessel. The decreased pressure and temperature cause the Hydrate formation to take place and greatly reduced the water vapour content of the Natural Gas. The hydrates are then separated out of the gas and again further separated by melting them to water and liquid hydrocarbons by warm water coils and a series of baffles within the separator. Where a likelyhood of blockage was possible, as in the expansion valve and piping at the separator inlet, Methanol injection was available as a preventive measure.

Question:conditions.? 1.) much greater 2.)there is no difference 3.) much less 4.)depends on the type of gas. not sure please help. serious answers only please. also. how does it compare at low temperature and pressures.

Answers:An ideal gas assumes the incompressible molecules themselves occupy no volume. this is reasonable at low pressures but their effect on the total volume increases as the gas is compressed. the effect depends on the size of the molecules - large complex molecules taking up more space. At low pressures less effect. At low temperatures there is less molecular movement so to maintain pressure volume has to be reduced increasing the proportion taken up by the molecules.

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