Vacuum distillation

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Vacuum distillation is a method of distillation performed under reduced pressure. Like distillation, this technique separates compounds based on different boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. The pressure drop decreases the boiling point of the compound. Reduction of boiling points can be calculated using pressure-temperature nomographs using Clausius-Clapeyron_relation.

Video Vacuum distillation

Aplikasi skala-laboratorium

In laboratory vacuum distillation is used to purify the compound or remove the solvent.

Distillation is used to separate compounds based on different boiling points. Vacuum distillation enables this purification technique to be used in compounds with high boiling point, or those that are sensitive to air. Compounds with a boiling point lower than 150 ^o C can usually be distilled without reducing the pressure. Using fractionation columns in the set-up increases the separation of the mixture, and may allow the separation of compounds with the same boiling point. With equipment under reduced pressure, atmospheric exposure is minimized and equipment can be filled with an inert atmosphere when the distillation is complete.

For better results or for highly air sensitive compounds, either a Perkin triangle distillation arrangement or a short-path distillation set-up may be used.

Perkin triangular distillation set-up

The set-up of the Perkin triangle (Figure 5) uses a series of Teflon valves to allow the distilled fraction to be isolated from the distillation flask without the main part of the distillation removed from the vacuum or heat source, and thus can remain in a reflux state.

To do this, the distillate receiving vessel is first isolated from the vacuum by means of a Teflon valve.

The vacuum above the sample is then replaced by an inert gas (such as nitrogen or argon) and the distillate receiver can then be blocked and removed from the system.

Vacuum distillation setup using short path head

Air/sensitive enough air/vacuum fluid distillation can be performed using standard Schlenk-line techniques (Figure 6). When assembling the set-up equipment, all the connecting lines are clamped so they can not get out.

Once the equipment is assembled, and the liquid to be distilled is in pot still, the desired vacuum is established in the system by using a vacuum connection on the short path distillation head. Care is taken to prevent potential "crashing" as the liquid in the pot is still degases.

When building the vacuum, the coolant flow starts through the short path distillation head. Once the desired vacuum is established, heat is applied to the pot still.

If necessary, the first distillate section can be removed by cleaning with inert gas and replacing the distillate receiver.

When the distillation is complete: heat is removed, the vacuum connection is closed, and the inert gas is purged through the distillation head and the distillate receiver. While under cleaning inert gas, remove the distillate receiver and cover with an airtight cap. The distillate receiver may be stored under vacuum or under inert gas by using the sides of the armature on the distillation flask.

Rotary evaporation

Rotary evaporation is a common technique used in a laboratory to concentrate or isolate a compound from a solution. Many volatile solvents can be easily evaporated using rotary evaporation. Even fewer volatile solvents can be removed by vapor evaporation under high vacuum and by heating. It is also used by environmental regulators to determine the amount of solvent in paints, coatings and inks.

Security considerations

Safety is an important consideration when using glass equipment as part of the arrangement. All glass components should be checked carefully for scratches and cracks that may cause implosions when vacuum is applied. Wrapping as many glasses with ribbons as practically helps to prevent harmful shards of broken glass in the event of an explosion.

Maps Vacuum distillation

Industrial-scale applications

Industrial-scale vacuum distillation has several advantages. Boiling boiling mixture may require many equilibrium stages to separate the main components. One tool to reduce the number of stages required is to utilize vacuum distillation. The vacuum distillation columns (as depicted in FIGS. 2 and 3) are commonly used in oil refineries having diameters ranging up to about 14 meters (46 feet), altitudes ranging up to about 50 meters (164 feet), and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

Vacuum distillation increases the relative volatility of key components in many applications. The higher the relative volatility, the more separable are the two components; this implies fewer stages in the distillation column to affect the same separation between overhead and subordinate products. Lower pressure increases the relative volatility in most systems.

The second advantage of vacuum distillation is the reduced temperature requirement at lower pressures. For many systems, the product undergoes degradation or polymerization at high temperatures.

Vacuum distillation may increase the separation by:

• Prevention of product degradation or polymer formation due to reduced pressure causing lower tower base temperature,
• Reduced product degradation or polymer formation due to reduced average residence time especially in column using packing rather than tray.
• Increase capacity, yield, and purity.

Another advantage of vacuum distillation is the reduced capital cost, at the expense of slightly larger operating costs. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of the distillation column.

Refining vacuum in petroleum refining

Petroleum crude oil is a complex mixture of hundreds of different hydrocarbon compounds that generally have 3 to 60 carbon atoms per molecule, although there may be small amounts of hydrocarbons outside the range. Crude oil refining begins by refining the crude oil into a so-called atmospheric distillation column that operates at a pressure slightly above atmospheric pressure.

The vacuum distillation can also be referred to as "low temperature distillation"

In refining crude oil, it is important not to subdue crude oil to temperatures above 370 to 380 ° C because the high molecular weight component in crude oil will experience thermal crack and form petroleum coke at above temperature. The formation of the coke will result in a tube plugging in the furnace that heats the feed stream to the crude oil distillation column. Entering will also occur in the pipe from the furnace to the distillation column as well as in the column itself.

Limitations imposed by limiting the column inlet crude oil to temperatures of less than 370 to 380 ° C produce residual oil from the bottom of the atmospheric distillation column composed entirely of boiling hydrocarbons above 370 to 380 ° C.

To further distill the remaining oil from the atmospheric distillation column, distillation should be carried out at an absolute pressure as low as 10 to 40 mmHg (also called Torr) so as to limit the operating temperature to less than 370 to 380 ° C.

FIG. 2 is a simplified process diagram of a petroleum refinery vacuum distillation column illustrating the internal column and FIG. 3 is a photograph of a large vacuum distillation column in a petroleum refinery.

The absolute pressure of 10 to 40 mmHg in the vacuum distillation column increases the volume of vapor formed per volume of the distilled liquid. The result is that the column has a very large diameter.

Distillation columns such as those in Figures 1 and 2 may have a diameter of 15 meters or more, a height of about 50 meters, and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

The internal vacuum distillation column shall provide a good steam-liquid contact while, at the same time, maintaining a very low pressure increase from the top of the top column downward. Therefore, the vacuum column uses the distillation tray only when pulling the product from the side of the column (referred to as the draw side ). Most columns use a packing material for steam-liquid contact because the packaging has a lower pressure drop than the distillation tray. The packing material may be a structured metal sheet or a randomly disposed package such as a Raschig ring.

The absolute pressure of 10 to 40 mmHg in the vacuum column is most often achieved by using several stages of jet ejector jet.

Many industries, in addition to the petroleum refining industry, use vacuum distillation on a much smaller scale.

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Molecular distillation

The molecular distillation is a vacuum distillation under a pressure of 0.01 torr (1,3 Pa). 0.01 torr is an order of magnitude above a high vacuum, where liquid is in free molecular flow regime, ie the average molecular free path is proportional to the size of the apparatus. The gas phase no longer gives significant pressure on the substance to evaporate, and as a result, the rate of evaporation is no longer dependent on pressure. That is, since the continuum assumption of fluid dynamics is no longer valid, mass transport is regulated by molecular dynamics rather than fluid dynamics. Thus, a short path between hot and cold surfaces is required, usually by suspending hot plates covered with a feed film next to a cold plate with a line of sight between them. Molecular distillation is used industrially for oil purification.

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Gallery

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See also

• Continuous distillation
• Fractionation column
• Fractional distillation
• Kugelrohr

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References

This article incorporates material from the Citizendium article "Vacuum Distillation", licensed under the Attribution-ShareA 3.0 License Permission without Creative Commons License but not under the GFDL.

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External links

• D1160 Vacuum Distillation
• How vacuum suction works
• Temperature-pressure nomograph
• Short path distillation, including tables that compare the
method

Source of the article : Wikipedia