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DISTILLATION THEORY The object of distillation is the separation of the alcohol from the other ingredients in the beer, mostly water. In making fuel alcohol it is necessary to get all of the alcohol and water separated if the alcohol is going to be mixed with gasoline, and most of the alcohol and water separated if the alcohol is going to be burned in a converted engine. As will be seen, the purer the alcohol, the harder it is to make. The separation of the alcohol and water by distillation is made possible by the fact that alcohol boils at about 173 degrees F. and water at 212 degrees F. When the mixture of water and alcohol is boiled, vapors with a greater concentration of alcohol will be formed and liquid with a lesser concentration of alcohol will remain behind. However, because water and alcohol do not form what is called an "ideal" mixture, the separation cannot be done in one clean step. Figure 11 -1: SIMPLE DISTILLATION APPARATUS

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Page 1: Distillation Labware

DISTILLATION THEORY

The object of distillation is the separation of the alcohol from the other ingredients in the beer, mostly water. In making fuel alcohol it is necessary to get all of the alcohol and water separated if the alcohol is going to be mixed with gasoline, and most of the alcohol and water separated if the alcohol is going to be burned in a converted engine. As will be seen, the purer the alcohol, the harder it is to make.

The separation of the alcohol and water by distillation is made possible by the fact that alcohol boils at about 173 degrees F. and water at 212 degrees F. When the mixture of water and alcohol is boiled, vapors with a greater concentration of alcohol will be formed and liquid with a lesser concentration of alcohol will remain behind. However, because water and alcohol do not form what is called an "ideal" mixture, the separation cannot be done in one clean step.

Figure 11 -1: SIMPLE DISTILLATION APPARATUS

Figure 11-1 illustrates a simple distillation apparatus using laboratory-type equipment. Note that the equipment consists basically of a container for the liquid to be distilled (still pot), a heat source, and a condenser to turn the distilled vapors back into liquid form. The thermometer is necessary to monitor the temperature of the vapors.

Figure 11-2: BOILING POINT COMPOSITION for LIQUID and VAPOR PHASES

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In Figure 11-2, the heavy solid curve represents the composition of the liquid phase of water/ethanol mixtures plotted against the temperature at which the mixture boils. The dotted curve represents the vapor phase. Using the apparatus illustrated, and starting with a concentration of 8% alcohol in water, the liquid will boil at about 200 degrees F. Reading across, the vapors will contain about 43% alcohol. Clearly, for fuel purposes, a purer product is needed. To this end, we must redistill the condensed vapors from the first distillation which contain 43% alcohol and 57% water. This mixture will boil at about 181 degrees F. and the vapors will contain about 68% alcohol. Each time the condensed vapors are redistilled, they will be slightly purer, but many separate distillations are needed to produce relatively pure alcohol. Fortunately, a type of distillation apparatus, called a reflux (or rectifying column), in effect, performs simultaneous distillations and will be described later.

However, with the equipment described, no matter how elaborate, the purest alcohol that can be produced is 95.6%. The remaining 4.4% water is impossible to remove because at this ratio, water and alcohol form a constant boiling mixture (called an "azetrope") whose boiling point is a fraction of a degree below that of pure alcohol , and separation by ordinary distillation is impossible. Special techniques that can remove this residual water are outlined later in Chapter 12.

THE REFLUX COLUMN

Figure 11-3 illustrates a reflux column installed on the simple apparatus described in Figure 11-1. In this laboratory version, the reflux column consists merely of a glass tube filled with packing material. In this case, the packing is short lengths of small-diameter glass tubing. The purpose of the packing is to provide as large an internal surface area as possible.

Figure 11-3: REFLUX DISTILLING APPARATUS

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As the vapors from the still pot ascend through the column, they condense on the packing material and drip downward. Additional ascending vapors contact the descending liquid (called "reflux") and revaporize it. Thus, as the vapors slowly work their way up the column, they become richer and richer in alcohol until, when they reach the top, they are relatively pure. Meanwhile, the descending liquid is stripped of its alcohol. The overall effect is that many "distillations" are performed simultaneously and the liquid in the still pot is stripped of its alcohol in one continuous operation.

Reflux columns can be constructed to operate on either a batch or continuous basis and are described in Chapter 14.

Chapter 14

DISTILLATION EQUIPMENT

Distillation equipment can be divided into several categories. Simple apparatus, such as illustrated in Figure 11-1, can be constructed on a large scale. However, such a simple "pot still" would achieve less than 100 proof on the first run from an average beer and many separate distillations would be necessary to achieve 190 proof or 95% alcohol. For the production of fuel alcohol such a unit is not economical either in terms of energy input or labor. The only exception would be if the alcohol were used in an injection system where low proof is acceptable.

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Rectifying columns, on the other hand, can achieve 190 proof on the first run. Stills incorporating rectifying columns can be designed to run either on a batch or continuous basis. A batch operation still is simply a reflux rectifying column attached to a suitable boiler. The boiler is loaded with beer, the alcohol is distilled out of the beer, and then the still is shut down while the stillage is emptied from the boiler and a fresh batch of beer run in. Continuous operation stills, on the other hand, do not have to be shut down periodically. They can be operated 24 hours a day and, with proper automatic controls, require very little attention.

Figure 14-1: CROSS SECTION of LARGE COLUMN

The basic packed reflux column described in Chapter 11 is only efficient up to a certain point. Figure 14-1 illustrates a cross section of a larger apparatus that operates on the same principle. Here, ascending vapors rise through holes in a perforated plate. The descending liquid flows downward from plate to plate through down-pipes. The liquid does not flow through the holes in the plate because of the pressure exerted by the ascending vapor. Thus, a certain amount of liquid is "trapped" on each plate and, as the vapors bubble through it, alcohol is removed from the descending liquid. The effect is similar to the packed reflux column in that a separate "distillation" is performed at each plate. The plate design shown is only one of several possibilities. Various forms of bubble caps, for example, are used on larger columns.

Figure 14-2: CONTINUOUS STILL

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Figure 14-2 is a continuous distillation apparatus incorporating the plates described in Figure 14-1. The apparatus illustrated consists of two columns, although there is no reason why the unit could not be constructed as a single column. The liquid to be distilled (beer) is pumped into the first column near the top. Steam is piped in at the bottom. As the beer descends, the steam "strips" it of alcohol. The alcohol vapors pass over to the next column, and the alcohol-free liquid (stillage) exits from the base. The next column also contains plates similar to those in the first. In this column the alcohol vapors are stripped of most of the remaining water (or "rectified") and exit as 190 proof (95%) alcohol.

The distillation equipment described so far uses the principle of adding heat to boil the beer and provide vapor for the distillation process. Alternately, vapor can also be produced by reducing pressure. In a vacuum, it is easily possible to boil ice water at 32 deg F. Similarly, alcohol/water mixtures can also be boiled at "room" temperature and below simply by reducing pressure. The equipment consists of a vacuum pump, condenser, and a still pot built to withstand the external pressure created by the vacuum. Although the energy required to run the vacuum pump is probably equal to the amount of energy required to operate a conventional still, this type of equipment merits consideration.

The selection of distillation equipment is largely a matter of economics. Continuous operation stills must be properly engineered and are costly to construct. However, the advantages of automated operation and low labor requirements make them very attractive. For operations producing a large amount of fuel, a continuous still clearly makes sense. Batch stills, although

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labor intensive, can be built by the layman with relative ease and for a small amount of money. The balance of this chapter describes the construction and operation of such equipment.

Note that solar energy can be used in several ways to provide heat for the distillation process. Solar stills are discussed in Chapter 15.

SIMPLE REFLUX COLUMN

Figure 14-3 shows a simple rectifying reflux column. The column is simply a length of pipe filled with a packing material to provide a large internal surface area. Aside from the pipe to hold the packing, some sort of screen or retainer is needed at the base of the column to keep the packing from falling into the boiler. The thermometer at the top of the column is necessary to check the temperature of vapors going to the condenser.

Figure 14-3: SIMPLE REFLUX COLUMN

A 3-inch diameter column of this design should be about 4 feet long. It would be capable of producing about 1 gallon per hour, depending on the initial concentration of the beer. Similarly, a 4-inch column should be about 6 feet long and should deliver about 2 gallons per hour; a 6 inch column should be at least 10 feet in length and should be capable of 5-6 gallons per hour. Columns of this design do not work well in diameters above 6 inches.

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CONDENSERS

The top of the column will have to be connected to a condenser to cool the vapors back into the liquid form. The condenser can be a coil of soft copper pipe inside a suitable container as illustrated in Figure 14-4. Here water is used as the heat exchange medium. On small stills, air cooled condensers are also possible. An old automobile radiator should work very well. The main thing is that the condenser be large enough to cool all of the vapors from the still below 100 deg F. and, preferably, to about 60 deg F.

Figure 14-4: CONDENSER CONSTRUCTION

Also, if the vapors going into the condenser are impeded in any great degree, pressure could build up inside the column and boiler. Therefore, the diameter of tubing in a condenser for a 3-inch column should be no smaller than 3/8 inch diameter. For a 4-inch column, 1/2 inch tubing would be the absolute minimum, and for a 6-inch column, the minimum would be 3/4 inch diameter. Note that the effective diameter of a condenser can be increased by connecting two or more condensing coils in parallel.

BOILERS

To complete the basic apparatus, you will need a boiler or "still pot". A 55-gallon drum can be adapted to this purpose. The drum is best placed on its side to allow maximum surface area for heating and the production of vapor. Alternately, a small column could be attached to the cooking and mashing drum illustrated in Figure 13-1. A more efficient boiler could be made from a hot water heater. Hot water heaters are available that can be fired with wood, coal, electricity, or gas. They are usually glass lined and insulated. Serviceable units can often be found in a junk yard and rebuilt. Other types of boilers can also be adapted for use as a still pot. An example might be certain types of home or commercial hot water furnaces.

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REFLUX CONTROL

Successful operation of the simple reflux column described earlier depends on careful regulation of the amount of vapor going to the column. However, it is often difficult to control the amount of vapor produced by boilers that are fueled by wood, coal and similar materials. The reflux control coil illustrated in Figure 14-5 can be added to the column and used to regulate temperatures at the still head with a great degree of accuracy. The reflux control unit is simply a condensing coil placed in the column and used to control the amount of reflux. It is constructed by wrapping soft copper tubing around a suitable form. The copper coil is then placed inside a section of the column.

Figure 14-5: REFLUX CONTROL COIL

In use, cooling water is circulated through the coil to condense a portion of the ascending vapors and, thus, increase the amount of reflux. Adjustment of cooling water in the reflux coils must be very precise. Small needle valves designed for precision metering of liquid should be used. Semi-automatic operation of the still could be achieved by replacing the cooling control needle valves with solenoid valves controlled by temperature sensors within the column.

HYDROMETER SUMP

There are several improvements that can be added to the basic still. One of them is a hydrometer sump as illustrated in Figure 14-6. It is constructed of ordinary pipe and fittings. In use, the product from the condenser is admitted at the bottom and flows out of the top. The hydrometer and thermometer allow a constant check of the proof of the alcohol being produced by the still. The valve at the bottom of the sump allows the unit to be drained.

Figure 14-6: HYDROMETER SUMP

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CONSTRUCTION OF A REFLUX COLUMN

Figure 14-7 illustrates a reflux column incorporating two reflux control coils, a hydrometer sump, and related plumbing. The layout of the condenser, pipes, valves, etc. is for clarity in the illustration and not necessarily the best configuration.

Figure 14-7: IMPROVED REFLUX COLUMN

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The column can be constructed from copper, iron, or steel pipe and fittings. Aluminum is not suitable because it can react chemically with the alcohol. The use of certain types of rubber and plastic that are attacked by the alcohol should also be avoided. Copper pipe and fittings, although expensive, are easy to work with and are recommended.

To construct, for example, a 4-inch column, begin by cutting a 6 foot length of pipe for the rectifying section, a 2-3 foot length for the stripping section, and two lengths about 1 foot long for the reflux control sections. Note that the use of two reflux control sections allows very precise control of temperatures within the column. However, if an easily controlled heat source, such as gas or electricity, is used for the boiler, the lower coil can be eliminated. The reflux control coils are soft copper tubing, 3/16 to 3/8 inches in diameter wound around a suitable form and placed inside the short sections of pipe.

Both the rectifying and stripping sections should be packed. The packing can be marbles, pebbles, broken glass, short pieces of metal or glass tubing, or whatever. Anything that won't rust or react with the alcohol will work. However, the best packing for this type column is probably copper or stainless steel scouring pads. Ordinary steel wool will not work because it will quickly rust. Some sort of screen or retainer is needed at the base of each packed section to keep the packing material from falling into the boiler. The simplest retainer is a section of coarse screening cut to the column diameter and soldered into place.

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Once packed, the entire unit can be assembled. The drawing shows the sections joined by pipe couplers.

Figure 14-3 shows the thermometer (or other temperature sensing device) being held in place by a cork. This is intended as a safety device. Excessive pressure will pop the cork. If some other arrangement is used to hold the temperature sensor, a pressure relief valve, such as used on hot water heaters, should be added either in the column or the boiler.

A worthwhile improvement would be the use of remote sensing thermometers (thermocouples or thermistors) with the read-out located in some convenient place, for example, near the reflux control needle valves. As mentioned earlier, the use of solenoid valves, controlled by remote temperature sensors, in place of (or preferably in conjunction with) the reflux control needle valves would allow semi-automatic operation. A thermometer to measure vapor temperature in the boiler (not shown) is also necessary.

The three-way valve located below the hydrometer sump allows the impure product to be run back into the boiler for redistillation. In addition, two collection tanks, one for high proof product and one for low proof residue, are needed. As will be explained in the section on still operation, the low proof tank should be set up so that its contents can be run back into the still pot at the beginning of each successive run.

OPERATION OF THE STILL

In operation, the boiler or still pot is filled to no more than the 3/4 level with the beer to be distilled. As the liquid begins to boil, vapors rise in the column. After a while, the column will come up to temperature, and an equilibrium will be established. For a normal beer concentration of about 8%, the initial temperature of the vapors in the still pot will be about 200 deg F. The vapors will cool as they rise in the column and, at the top, they should stabilize at 173 deg F which is the approximate boiling point of the water/alcohol azeotrope.

If the boiler is supplying more vapor than the column can handle, the temperature at the still head will rise above 173 deg F and the proof of the product going to the condenser will be reduced. Conversely, if the boiler is not producing enough vapor, the temperature at the still head will be low and no vapors will be going to the condenser. On a still without reflux control coils, the heat source at the boiler must be adjusted to keep the amount of vapor within the range that can be handled by the column. On stills with reflux control1, the boiler is adjusted to produce an excess amount of vapor. The lower reflux control coil is adjusted so that the thermometer located just above it reads 180 deg F and the top reflux control coil is adjusted to maintain the still head temperature at exactly 173 deg F.

After the column has stabilized, the 3-way valve is opened to let the product flow to the high proof tank. Note that when you begin a distillation run, certain low boiling vapors may come over, and a small amount of liquid may come out of the condenser that is not ethanol. This liquid is composed of substances in the beer that have a lower boiling point than ethanol. However, they do not affect fuel value and, for all practical purposes, can be ignored.

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As the distillation progresses, the vapors in the still pot will contain a greater and greater percentage of water and a correspondingly lesser proportion of alcohol. The still pot vapor temperature will rise. Eventually, a point will reached where there is too little alcohol in the vapor for the column to achieve effective separation. The temperature at the still head will rise slightly and the proof of the product will be lower. At this point, it is best to collect the product coming from the condenser in the low proof container mentioned earlier.

The distillation should be continued until the temperature at the still head equals the temperature of the vapors in the boiler, which will be near 208-212 deg F depending on altitude, atmospheric pressure, and the amount of dissolved material in the beer. Note that when switching to the low proof phase of the distillation, the reflux control coils should be turned off and the column temperatures allowed to rise naturally.

When all the alcohol has been removed from the beer, as indicated by boiler and still head vapor temperatures, as mentioned before, the distillation is complete. The beer, now called "stillage", is drained from the boiler, and a fresh charge of beer run in. The low proof "tail" from the previous run is added to the fresh charge of beer, and the still is ready for another run.

CAUTION

There are several inherent dangers in the construction and use of the equipment described in this chapter. Alcohol, and alcohol vapors, are flammable. Mixtures of alcohol vapors and air can be explosive. The equipment should be located in an area that receives adequate ventilation, and preferably outdoors or in a simple shed away from other buildings. The still should be electrically grounded to prevent the buildup of static electricity. Above all, reasonable care and common sense should be exercised. If you are not sure about something that could be potentially dangerous, find out before you proceed!

SOLAR STILLS

GENERAL DISCUSSION

Solar stills can be simple or elaborate. The solar analogy to a simple pot still is easy to construct and operate. Solar stills that incorporate stripping and reflux sections are also possible, but they are difficult to operate without sophisticated instrumentation and controls. This is because there is a delicate balance between temperature, feed rates, and the amount of vapor in the column. The slightest variation in the input of solar radiant energy will destroy this balance. To operate complex solar stills, all of the variables must be sensed and controlled within very close tolerances. This, unfortunately, means expensive electronic sensors, controls and valves. The overall efficiency of a complex solar still is low and, to offset the cost of instrumentation, the still must be very large.

It is usually better, for larger installations, to use solar power indirectly. For example, a solar installation could be used to preheat the water used for cooking or a solar boiler could be used to supplement a regular boiler for the production of steam.

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Simple solar stills have the disadvantage of producing low proof alcohol. However, they are entirely adequate when used to produce fuel for an injection system. An example of a simple fermentation set-up and solar still to produce fuel for an alcohol injected engine is described in Chapter 17. In this application, the solar still makes a lot of sense.

PRINCIPLE OF OPERATION

Figure 15-1: BASIC SOLAR STILL

Figure 15-1 illustrates a solar still of the type used in survival gear to distill fresh water from sea water. The sun's rays pass freely through the transparent dome and are absorbed by the dark bottom of the still. The Iiquid in the bottom of the still is heated. Vapors rise, contact the inside of the transparent dome (which remains relatively cool), and condense. The distilled liquid collects in the trough around the rim and is collected through the attached tube.

The still illustrated could just as easily distill alcohol as water. Such stills are available from various suppliers and are fairly inexpensive. However, before purchasing one of these units make sure that the alcohol vapors will not damage whatever plastic is used in the still's construction.

CONSTRUCTION OF SOLAR STILLS

Figure 15-2: PASSIVE SOLAR STILL

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Figure 15-2 shows a cross section of an easily constructed solar still. The box is made of plywood and is about 6 inches deep. The overall height is 2-3 feet. It can be constructed in any convenient length. It is important that the box be vapor tight. Otherwise, the alcohol vapors will escape. Therefore, the inside of the box should be sealed and painted with flat black, chemical resistant, epoxy paint. The glazing should be sealed with a gasket of silicone. A cheaper alternative to glass is translucent fiberglass used in the construction of greenhouses. It is available from most building supply houses in rolls that are 48 inches wide. A valve should be provided to fill and drain the beer trough. The drain opening in the lower alcohol trough must be left open to relieve internal pressure. To prevent uncondensed vapor loss, a simple "U" trap, as illustrated in Figure 15-3, can be installed. The optimum angle for the box as far as the sun is concerned is your latitude plus 15 deg in the winter and minus 15 deg in the summer. In the spring and fall the optimum angle is equal to your latitude.

Figure 15-3: VAPOR TRAP DRAIN

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In operation, the beer travels up the black-dyed burlap by capillary action, the alcohol evaporates, condenses on the glass, and collects in the lower trough. The temperature in the box is self-regulating or "passive" because as the box heats up, more alcohol will evaporate. This has a cooling effect. Conversely, if the temperature in the box drops, less alcohol will evaporate, and the temperature will rise. The vapor temperature inside the box should, therefore, remain relatively stable. Typically, the initial temperature will be about 175 deg F. and the proof of the distillate will be about 100-120, depending on the alcohol concentration of the beer. As the alcohol is removed from the beer, the temperature inside the tank will rise and the proof of the alcohol collected will decrease. Therefore, after about 1/2 to 2/3 of the alcohol has been collected, the distillate can be run into a second container for redistillation with the next run.

With slight modification the still in Figure 15-2 can be converted from passive batch operation to a more or less continuous operation. A drip tube is installed across the top of the still instead of the beer flowing up the burlap by capillary action, it is allowed to drip down the burlap from above. Figure 15-4 shows a solar still set up for this type of operation. Notice that the height of the still is much greater, about 8 feet. The object is to have all of the alcohol evaporated from the beer by the time it reaches the lower trough. This is not always possible. Therefore, some means of recirculating the beer usually must be provided. Control of the temperature within the still is no longer completely "passive". To a greater extent, it must be "actively" controlled by adjusting the flow rate of the beer. An increased flow rate will lower the temperature inside the box and a reduced rate will raise the temperature. An alternate to controlling the flow rate would be to install louvers or a shade, activated by a thermal sensor, to control internal temperature. The closer the temperature to 173 deg F., the higher the proof of the alcohol. Because the sun's energy is not consistent, some means of automatically controlling the flow rate to maintain optimum temperature is almost mandatory.

Figure 15-4: ACTIVE SOLAR STILL

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The passive system has the advantage of being self-regulating as far as temperature is concerned and the disadvantage of producing lower and lower proof as the distillation continues. Because of the fixed volume of the beer trough, it can only process a limited amount of beer at one time. The active system, if properly regulated, is capable of a higher overall proof on the first run and, with a recirculating tank, can process a larger amount of beer. It has the disadvantage of requiring active control of the internal still temperature.

Both stills produce a high enough proof for injection systems. However, if the fuel is to be burned in place of gasoline, the alcohol must be redistilled to achieve an acceptable proof. There is no reason, however, why several stills can not be connected in series with the product from the first being fed to the second, and so on. Also, although overall efficiency is low, the energy from the sun is "free" and stills of this design can be built as large as desired.

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Laboratory display of distillation: 1: A heating device 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirring means e.g.(shown), boiling chips or mechanical stirrer 16: Cooling bath.[1]

Idealized distillation model

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure in the liquid, enabling bubbles to form without being crushed. A special case is the normal boiling point, where the vapor pressure of the liquid equals the ambient atmospheric pressure.

It is a common misconception that in a liquid mixture at a given pressure, each component boils at the boiling point corresponding to the given pressure and the vapors of each component will collect separately and purely. This, however, does not occur even in an idealized system. Idealized models of distillation are essentially governed by Raoult's law and Dalton's law, and assume that vapor-liquid equilibria are attained.

Raoult's law assumes that a component contributes to the total vapor pressure of the mixture in proportion to its percentage of the mixture and its vapor pressure when pure, or succinctly: partial pressure equals mole fraction multiplied by vapor pressure when pure. If one component changes another component's vapor pressure, or if the volatility of a component is dependent on its percentage in the mixture, the law will fail.

Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture. When a multi-component liquid is heated, the vapor pressure of each component will rise, thus causing the total vapor pressure to rise. When the total

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vapor pressure reaches the pressure surrounding the liquid, boiling occurs and liquid turns to gas throughout the bulk of the liquid. Note that a mixture with a given composition has one boiling point at a given pressure, when the components are mutually soluble.

An implication of one boiling point is that lighter components never cleanly "boil first". At boiling point, all volatile components boil, but for a component, its percentage in the vapor is the same as its percentage of the total vapor pressure. Lighter components have a higher partial pressure and thus are concentrated in the vapor, but heavier volatile components also have a (smaller) partial pressure and necessarily evaporate also, albeit being less concentrated in the vapor. Indeed, batch distillation and fractionation succeed by varying the composition of the mixture. In batch distillation, the batch evaporates, which changes its composition; in fractionation, liquid higher in the fractionation column contains more lights and boils at lower temperatures.

The idealized model is accurate in the case of chemically similar liquids, such as benzene and toluene. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. These compounds, when heated together, form an azeotrope, which is a composition with a boiling point higher or lower than the boiling point of each separate liquid. Virtually all liquids, when mixed and heated, will display azeotropic behaviour. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate vapor-liquid equilibrium data is by measurement.

It is not possible to completely purify a mixture of components by distillation, as this would require each component in the mixture to have a zero partial pressure. If ultra-pure products are the goal, then further chemical separation must be applied. When a binary mixture is evaporated and the other component, e.g. a salt, has zero partial pressure for practical purposes, the process is simpler and is called evaporation in engineering.

Batch distillationMain article: Batch distillation

A batch still showing the separation of A and B.

Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid which contains a

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mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the liquid: the ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This in turn means that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e. more enriched in B than the starting liquid).

The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which in turn results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an increasing proportion of B in the gas phase). This results in a slowly changing ratio A : B in the distillate.

If the difference in vapor pressure between the two components A and B is large (generally expressed as the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and when component A has distilled off, the boiling liquid is enriched in component B.

Continuous distillationMain article: Continuous distillation

Continuous distillation is an ongoing distillation in which a liquid mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation. Continuous distillation produces at least two output fractions, including at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid. There is always a bottoms (or residue) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor.

Continuous distillation differs from batch distillation in the respect that concentrations should not change over time. Continuous distillation can be run at a steady state for an arbitrary amount of time. For any source material of specific composition, the main variables that affect the purity of products in continuous distillation are the reflux ratio and the number of theoretical equilibrium stages (practically, the number of trays or the height of packing). Reflux is a flow from the condenser back to the column, which generates a recycle that allows a better separation with a given number of trays. Equilibrium stages are ideal steps where compositions achieve vapor-liquid equilibrium, repeating the separation process and allowing better separation given a reflux ratio. A column with a high reflux ratio may have fewer stages, but it refluxes a large amount of liquid, giving a wide column with a large holdup. Conversely, a column with a low reflux ratio must have a large number of stages, thus requiring a taller column.

General improvements

Both batch and continuous distillations can be improved by making use of a fractionating column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as

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long as possible. The column can even consist of small subsystems ('trays' or 'dishes') which all contain an enriched, boiling liquid mixture, all with their own vapor-liquid equilibrium.

There are differences between laboratory-scale and industrial-scale fractionating columns, but the principles are the same. Examples of laboratory-scale fractionating columns (in increasing efficiency) include:

Air condenser Vigreux column (usually laboratory scale only) Packed column (packed with glass beads, metal pieces, or other chemically inert material) Spinning band distillation system.

Laboratory scale distillation

Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a still, consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapour is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also distillation types).

Simple distillation

In simple distillation, all the hot vapors produced are immediately channeled into a condenser that cools and condenses the vapors. Therefore, the distillate will not be pure – its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law.

As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C),[13] or to separate liquids from involatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose.

Fractional distillationMain article: Fractional distillation

For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification.[14]

As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-

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condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component.[15] In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.[16]

Steam distillationMain article: Steam distillation

Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive.[17] The temperature of the steam is easier to control than the surface of a heating element, and allows a high rate of heat transfer without heating at a very high temperature. This process involves bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water.

Steam distillation of various aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The essential oils are often used in perfumery and aromatherapy while the watery distillates have many applications in aromatherapy, food processing and skin care.

Dimethyl sulfoxide usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.

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Perkin triangle distillation setup1: Stirrer bar/anti-bumping granules 2: Still pot 3: Fractionating column 4: Thermometer/Boiling point temperature 5: Teflon tap 1 6: Cold finger 7: Cooling water out 8: Cooling water in 9: Teflon tap 2 10: Vacuum/gas inlet 11: Teflon tap 3 12: Still receiver

Vacuum distillationMain article: Vacuum distillation

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.

Molecular distillation is vacuum distillation below the pressure of 0.01 torr.[18] 0.01 torr is one order of magnitude above high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically

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by suspending a hot plate covered with a film of feed next to a cold plate with a line of sight in between. Molecular distillation is used industrially for purification of oils.

Air-sensitive vacuum distillation

Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "cow" or "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.

The Perkin triangle, has means via a series of glass or Teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.

Short path distillation

Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path; 1: Still pot with stirrer bar/anti-bumping granules 2: Cold finger – bent to direct condensate 3: Cooling water out 4: cooling water in 5: Vacuum/gas inlet 6: Distillate flask/distillate.

Short path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure.[19] A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. The advantage is that the heating temperature can be considerably lower (at reduced pressure)

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than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.

Zone distillation

Zone distillation is a distillation process in long container with partial melting of refined matter in moving liquid zone and condensation of vapor in the solid phase at condensate pulling in cold area. The process is worked in theory. When zone heater is moving from the top to the bottom of the container then solid condensate with irregular impurity distribution is forming. Then most pure part of the condensate may be extracted as product. The process may be iterated many times by moving (without turnover) the received condensate to the bottom part of the container on the place of refined matter. The irregular impurity distribution in the condensate (that is efficiency of purification) increases with number of repetitions of the process. Zone distillation is a distillation analog of zone recrystallization. Impurity distribution in the condensate is described by known equations of zone recrystallization with various numbers of iteration of process – with replacement distribution efficient k of crystallization on separation factor α of distillation. (Literature: Kravchenko, A.I. Zone distillation: a new method of refining // Problems of atomic science and technology, 2011. – N. 6 – Series: “Vacuum, pure materials, superconductors” (19). – P. 24-26. [in Russian]. – [1])

Other types

The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup.

Catalytic distillation is the process by which the reactants are catalyzed while being distilled to continuously separate the products from the reactants. This method is used to assist equilibrium reactions reach completion.

Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane.

Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.

Flash evaporation (or partial evaporation) is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations, being equivalent to a distillation with only one equilibrium stage.

Codistillation is distillation which is performed on mixtures in which the two compounds are not miscible.

The unit process of evaporation may also be called "distillation":

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In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a water aspirator or a membrane pump.

In a kugelrohr a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump.

Other uses:

Dry distillation or destructive distillation, despite the name, is not truly distillation, but rather a chemical reaction known as pyrolysis in which solid substances are heated in an inert or reducing atmosphere and any volatile fractions, containing high-boiling liquids and products of pyrolysis, are collected. The destructive distillation of wood to give methanol is the root of its common name – wood alcohol.

Freeze distillation is an analogous method of purification using freezing instead of evaporation. It is not truly distillation, but a recrystallization where the product is the mother liquor, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively. It is also used to produce applejack. Unlike distillation, freeze distillation concentrates poisonous congeners rather than removing them.

Azeotropic distillationMain article: Azeotropic distillation

Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in a constant-boiling azeotrope which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95.6% at 78.1 °C.

If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically removing or sequestering the impurity. For example, to purify ethanol beyond 95%, a drying agent or a (desiccant such as potassium carbonate) can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well.

Immiscible liquids, such as water and toluene, easily form azeotropes. Commonly, these azeotropes are referred to as a low boiling azeotrope because the boiling point of the azeotrope is lower than the boiling point of either pure component. The temperature and composition of the azeotrope is easily predicted from the vapor pressure of the pure components, without use of

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Raoult's law. The azeotrope is easily broken in a distillation set-up by using a liquid-liquid separator (a decanter) to separate the two liquid layers that are condensed overhead. Only one of the two liquid layers is refluxed to the distillation set-up.

High boiling azeotropes, such as a 20 weight percent mixture of hydrochloric acid in water, also exist. As implied by the name, the boiling point of the azeotrope is greater than the boiling point of either pure component.

To break azeotropic distillations and cross distillation boundaries, such as in the DeRosier Problem, it is necessary to increase the composition of the light key in the distillate.

Breaking an azeotrope with unidirectional pressure manipulation

The boiling points of components in an azeotrope overlap to form a band. By exposing an azeotrope to a vacuum or positive pressure, it's possible to bias the boiling point of one component away from the other by exploiting the differing vapour pressure curves of each; the curves may overlap at the azeotropic point, but are unlikely to be remain identical further along the pressure axis either side of the azeotropic point. When the bias is great enough, the two boiling points no longer overlap and so the azeotropic band disappears.

This method can remove the need to add other chemicals to a distillation, but it has two potential drawbacks.

Under negative pressure, power for a vacuum source is needed and the reduced boiling points of the distillates requires that the condenser be run cooler to prevent distillate vapours being lost to the vacuum source. Increased cooling demands will often require additional energy and possibly new equipment or a change of coolant.

Alternatively, if positive pressures are required, standard glassware can not be used, energy must be used for pressurization and there is a higher chance of side reactions occurring in the distillation, such as decomposition, due to the higher temperatures required to effect boiling.

A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.

Pressure-swing distillationFurther information: Pressure-Swing Distillation (section on the main Azeotrope page)

This section may be confusing or unclear to readers. Please help clarify the section; suggestions may be found on the talk page. (May 2009)

Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both positive and negative pressures may be employed.[clarification

needed]

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This has an important impact on the selectivity of the distillation and allows a chemist[citation needed] to optimize a process such that fewer extremes of pressure and temperature are required and less energy is consumed. This is particularly important in commercial applications.

Pressure-swing distillation is employed during the industrial purification of ethyl acetate after its catalytic synthesis from ethanol.

Industrial distillation

Typical industrial distillation towers

Main article: Continuous distillation

Large scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants and natural gas processing plants.

Industrial distillation[14][20] is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 centimeters to 16 meters and heights ranging from about 6 meters to 90 meters or more. When the process feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column and are often called the bottoms.

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Diagram of a typical industrial distillation tower

Industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation tower. The more reflux that is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux that is provided for a given desired separation, the fewer the number of theoretical plates required.

Such industrial fractionating towers are also used in air separation, producing liquid oxygen, liquid nitrogen, and high purity argon. Distillation of chlorosilanes also enables the production of high-purity silicon for use as a semiconductor.

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Section of an industrial distillation tower showing detail of trays with bubble caps

Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method [14] [21] or the Fenske equation [14] can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as "plates" or "trays") used in distillation towers are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation tower needs more trays than the number of theoretical vapor-liquid equilibrium stages.

In modern industrial uses, a packing material is used in the column instead of trays when low pressure drops across the column are required. Other factors that favor packing are: vacuum systems, smaller diameter columns, corrosive systems, systems prone to foaming, systems requiring low liquid holdup and batch distillation. Conversely, factors that favor plate columns are: presence of solids in feed, high liquid rates, large column diameters, complex columns, columns with wide feed composition variation, columns with a chemical reaction, absorption columns, columns limited by foundation weight tolerance, low liquid rate, large turn-down ratio and those processes subject to process surges.

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Large-scale, industrial vacuum distillation column[22]

This packing material can either be random dumped packing (1–3" wide) such as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium, the vapor-liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns, it is useful to compute a number of "theoretical stages" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

Another factor in addition to the packing shape and surface area that affects the performance of random or structured packing is the liquid and vapor distribution entering the packed bed. The number of theoretical stages required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct in the packed bed and the required separation will not be achieved. The packing will appear to not be working properly. The height equivalent of a theoretical plate (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. Liquid mal-distribution is more frequently the problem than vapor. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform to it maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be found in references.[23][24] Considerable work as been done on this topic by Fractionation Research, Inc. (commonly known as FRI).[25]

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Multi-effect distillation

The goal of multi-effect distillation is to increase the energy efficiency of the process, for use in desalination, or in some cases one stage in the production of ultrapure water. The number of effects is proportional to the kW·h/m3 of water recovered figure, and refers to the volume of water recovered per unit of energy compared with single-effect distillation. One effect is roughly 636 kW·h/m3.

Multi-stage flash distillation Can achieve more than 20 effects with thermal energy input, as mentioned in the article.

Vapor compression evaporation Commercial large-scale units can achieve around 72 effects with electrical energy input, according to manufacturers.

There are many other types of multi-effect distillation processes, including one referred to as simply multi-effect distillation (MED), in which multiple chambers, with intervening heat exchangers, are employed.

Distillation in food processing

Distilled beveragesMain article: Distilled beverage

Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol, including water, esters, and other alcohols, are collected in the condensate, which account for the flavor of the beverage.

Gallery

Chemistry on its beginnings used retorts as laboratory equipment exclusively for distillation processes.

A simple set-up to distill dry and oxygen-free toluene.

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Diagram of an industrial-scale vacuum distillation column as commonly used in oil refineries

A rotary evaporator is able to distill solvents more quickly at lower temperatures through the use of a vacuum.

Distillation using semi-microscale apparatus. The jointless design eliminates the need to fit pieces together. The pear-shaped flask allows the last drop of residue to be removed, compared with a similarly-sized round-bottom flask The small holdup volume prevents losses. A pig is used to channel the various distillates into three receiving flasks. If necessary the distillation can be carried out under vacuum using the vacuum adapter at the pig.

FRACTIONAL DISTILLATION OF IDEAL MIXTURES OF LIQUIDS

This page explains how the fractional distillation (both in the lab and industrially) of an ideal mixture of liquids relates to their phase diagram. This is the second page in a sequence of three pages.

Important:  If you have come straight to this page from a search engine and are looking for simple factual information about fractional distillation, this is probably not the page for you! It deals with the theory behind fractional distillation.

Again, if you have come straight to this page, you won't make much sense of it unless you first read the page about phase diagrams for ideal mixtures.

You will find a link at the bottom of that page which will bring you back here again.

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Using the phase diagram

On the last page, we looked at how the phase diagram for an ideal mixture of two liquids was built up. I want to start by looking again at material from the last part of that page.

The next diagram is new - a modified version of diagrams from the previous page.

If you boil a liquid mixture C1, you will get a vapour with composition C2, which you can condense to give a liquid of that same composition (the pale blue lines).

If you reboil that liquid C2, it will give a vapour with composition C3. Again you can condense that to give a liquid of the same new composition (the red lines).

Reboiling the liquid C3 will give a vapour still richer in the more volatile component B (the green lines). You can see that if you were to do this once or twice more, you would be able to collect a liquid which was virtually pure B.

The secret of getting the more volatile component from a mixture of liquids is obviously to do a succession of boiling-condensing-reboiling operations.

It isn't quite so obvious how you get a sample of pure A out of this. That will become clearer in a while.

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Fractional distillation in the lab

The apparatus

A typical lab fractional distillation would look like this:

Some notes on the apparatus

The fractionating column is packed with glass beads (or something similar) to give the maximum possible surface area for vapour to condense on. You will see why this is important in a minute. Some fractionating columns have spikes of glass sticking out from the sides which serve the same purpose.

If you sketch this, make sure that you don't completely seal the apparatus. There has to be a vent in the system otherwise the pressure build-up when you heat it will blow the apparatus apart.

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In some cases, where you are collecting a liquid with a very low boiling point, you may need to surround the collecting flask with a beaker of cold water or ice.

The mixture is heated at such a rate that the thermometer is at the temperature of the boiling point of the more volatile component. Notice that the thermometer bulb is placed exactly at the outlet from the fractionating column.

Relating what happens in the fractionating column to the phase diagram

Suppose you boil a mixture with composition C1.

The vapour over the top of the boiling liquid will be richer in the more volatile component, and will have the composition C2.

That vapour now starts to travel up the fractionating column. Eventually it will reach a height in the column where the temperature is low enough that it will condense to give a liquid. The composition of that liquid will, of course, still be C2.

Note:  As you will see shortly, that is an oversimplification because "our" vapour will become mixed with other vapours generated by various other reboilings happening in the column. I can't see any way around this simplification!

So what happens to that liquid now? It will start to trickle down the column where it will meet new hot vapour rising. That will cause the already condensed vapour to reboil.

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Some of the liquid of composition C2 will boil to give a vapour of composition C3. Let's concentrate first on that new vapour and think about the unvaporised part of the liquid afterwards.

The vapour

This new vapour will again move further up the fractionating column until it gets to a temperature where it can condense. Then the whole process repeats itself.

Each time the vapour condenses to a liquid, this liquid will start to trickle back down the column where it will be reboiled by up-coming hot vapour. Each time this happens the new vapour will be richer in the more volatile component.

The aim is to balance the temperature of the column so that by the time vapour reaches the top after huge numbers of condensing and reboiling operations, it consists only of the more volatile component - in this case, B.

Whether or not this is possible depends on the difference between the boiling points of the two liquids. The closer they are together, the longer the column has to be.

The liquid

So what about the liquid left behind at each reboiling? Obviously, if the vapour is richer in the more volatile component, the liquid left behind must be getting richer in the other one.

As the condensed liquid trickles down the column constantly being reboiled by up-coming vapour, each reboiling makes it richer and richer in the less volatile component -

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in this case, A. By the time the liquid drips back into the flask, it will be very rich in A indeed.

So, over time, as B passes out of the top of the column into the condenser, the liquid in the flask will become richer in A. If you are very, very careful over temperature control, eventually you will have separated the mixture into B in the collecting flask and A in the original flask.

Finally, what is the point of the packing in the column?

To make the boiling-condensing-reboiling process as effective as possible, it has to happen over and over again. By having a lot of surface area inside the column, you aim to have the maximum possible contact between the liquid trickling down and the hot vapour rising.

If you didn't have the packing, the liquid would all be on the sides of the condenser, while most of the vapour would be going up the middle and never come into contact with it.

Fractional distillation industrially

There is no difference whatsoever in the theory involved. All that is different is what the fractionating column looks like. The diagram shows a simplified cross-section through a small part of a typical column.

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The column contains a number of trays that the liquid collects on as the vapour condenses. The up-coming hot vapour is forced through the liquid on the trays by passing through a number of bubble caps. This produces the maximum possible contact between the vapour and liquid. This all makes the boiling-condensing-reboiling process as efficient as possible.

The overflow pipes are simply a controlled way of letting liquid trickle down the column.

If you have a mixture of lots of liquids to separate (such as in petroleum fractionation), it is possible to tap off the liquids from some of the trays rather than just collecting what comes out of the top of the column. That leads to simpler mixtures such as gasoline, kerosene and so on.

LABORATORY SETUPS

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LEFT: Classic vacuum filtration setup. RIGHT: Standard distillation apparatus at STP.

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LEFT: Reflux apparatus with drying tube. RIGHT: Reflux apparatus with electric stirrer.

Classic vacuum distillation apparatus.

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LEFT: Advanced reaction apparatus. RIGHT: Advanced reaction apparatus.

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LEFT: Apparatus for collecting nitric acid. RIGHT: Reaction apparatus with alkaline trap.

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Reaction apparatus with gas inlet tube and dying tube.

Simple DistillationA simple distillation apparatus is shown in Figure 1.2 below.  Distillation involves selectively volatilizing (converting from the liquid phase to the gas phase) a component in a mixture.  When a compound  in the distilling flask is heated to its boiling point temperature, a phase change from the liquid state to the gas state is induced.  The compound, in the gas phase, moves out of the distilling flask up into the other parts of the distilling apparatus, leaving behind the less volatile (higher boiling) components.  When the gas vapors encounter the cold condenser tube (below the boiling point temperature) of the distilling apparatus, the gaseous compound reverts back to the liquid phase and drips into the collection flask, effectively separating the compound from the mixture.  A simple distillation apparatus is depicted in Figure 1.2.   To set up the distillation apparatus, set a stirrer/hot plate and ring stand in the hood. Place a 50ml heating mantle on the stirrer. Insert a 50ml round bottom flask (distilling flask) into the heating mantle and clamp the neck of the flask to the ring stand. Be sure to position the flask over the center of the stirrer plate.  Measure out exactly 20 ml of commercial mouthwash using an Eppendorf pipe and weigh it.  Record the weight.  Dispense the

  Figure 1.2:  Simple Distillation Apparatus

(adapted from Aikens et. al, p.147)

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liquid to be distilled into the distilling flask.  Add a magnetic stir bar to the flask and continue to set up the simple distillation apparatus as shown in Figure 2.  Attach a distilling head, thermometer adapter and thermometer.  Position the thermometer bulb just below the “Y” of the distilling head.  Place a second clamp on the apparatus at the joint between the distilling head and the thermometer adapter.  (Never clamp anywhere except at the joints!  It will crack the glassware.) Set up a second ring stand.  Attach a condenserand vacuum adapter using Keck clamps.  Attach a 25 ml round-bottomed collection flask and place a third clamp (clamped to a second ring stand)at the joint between the vacuum adapter and the collection flask.  Be sure all the joints fit snuggly together, otherwise the apparatus will leak and reduce the efficiency of the distillation.  Connect the water hoses to the condenser, with water “in” at the bottom, and draining “out” to the sink at the top end of the condenser.  Connect the heating mantle to the Variac, and set the Variac at approximately 50V.  Check your apparatus against the diagram in Figure 1.2, and the set-up in the lab before you continue.Carefully turn the water on, turn on the magnetic stirrer and the Variac (heat).  As the apparatus is heated, the liquid in the distillation flask will begin to bubble.  The temperature reading on the thermometer will not rise immediately.  The actual distillation will begin when the entire apparatus is saturated with the distillate vapor.  As the vapor rises through the distilling head and encounters the condenser (which is cold from the water running through it) the distillate will condense back to the liquid phase and drip into the collection flask.  Monitor the thermometer temperature as the distillate drips into the collection flask.  Distillate should be collected when the thermometer reaches the boiling point temperature. 

Fractional DistillationFractional distillation is used to separate components of a mixture that have boiling points that are very similar, typically within 10C.  The set up for fractional distillation is essentially the same as simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with a packing material such as glass, steel wool, or plastic beads. The packing material increases the surface area and  improves the separation between the liquids being distilled. The reason that fractional distillation gives better separation between the liquids is because the packing material in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compound over and over. The more volatile liquids will tend to push towards the top of the fractionating column, while lower boiling liquids will stay towards the bottom, giving a better separation between the liquids. Of course, the more theoretical plates that you add to a column (the more surfaces or beads), the longer the

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distillation will take (typically), and the more energy required to keep reevaporating liquid in the fractionating column.

http://www.floragenics.com/img/products/50gallon.jpg

The 50 Gallon (190 Liter) Distillation System

This is a special order item -- please allow 8-10 weeks for delivery.

Larger systems and special systems can also be fabricated, please email or call for quote.

Prices are subject to change without notice.

Please call for a price quote: 1-877-446-3567

This system includes:

Electric boiler, 8 kw, 220/240 Volt, single phase, 60 Hz, 35 amps Retort is food grade 304 stainless steel Condenser is borosilicate glass

Page 47: Distillation Labware

Oil separator, glass, continuous operation Necessary hoses and fittings

The boiler is set to provide a continuous steam supply maintaining constant water level and pressure. Make-up and cooling water is supplied thru a standard garden hose (owner supplied).

The retort (Stock pot), is mounted on a steel frame with wheels to facilitate portability and provide easy dumping and cleaning. The system basically operates at atmospheric pressure and is complete with relief valves set at 5psi (0.34Bars).

The condenser can be furnished either in glass, which allows visual inspection of the process, or copper. The continuous operating separator is supplied in glass.

Options: (quoted on request)

Additional retorts can be added to provide additional capacity. The glass and copper components can be fabricated from stainless steel. The system can also be operated using a gas boiler. The boiler can be furnished in varying supplied voltages and frequencies.