Definition of Fractionation

Definition of Fractionation

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  • Time of issue:2021-01-07
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(Summary description)

Definition of Fractionation

(Summary description)

  • Categories:News
  • Author:
  • Origin:
  • Time of issue:2021-01-07 10:09
  • Views:

The difference in the volatility of each component in the liquid mixture is used to partially vaporize the liquid mixture and then partially condense the vapor, thereby achieving the separation of the components contained therein. It is a unit operation of mass transfer separation. It is widely used in oil refining, chemical industry, light industry and other fields.

The principle of the ordinary distillation device is to separate the two-component mixture as an example. The material liquid is heated to partially vaporize it, the volatile components are concentrated in the vapor, and the hardly volatile components are also concentrated in the remaining liquid, which realizes the separation of the two components to a certain extent. The greater the difference between the volatility of the two components, the greater the degree of concentration mentioned above. In industrial rectification equipment, the partially vaporized liquid phase is directly contacted with the partially condensed gas phase to carry out mass transfer between vapor and liquid phase. As a result, the hardly volatile components in the gas phase are partially transferred to the liquid phase. The volatile components are partially transferred to the gas phase, that is, partial vaporization of the liquid phase and partial condensation of the vapor phase are realized at the same time. Liquid molecules have a tendency to overflow from the surface due to molecular motion. This tendency increases with increasing temperature. If the liquid is placed in a closed vacuum system, the liquid molecules will continue to overflow and form vapor on the upper part of the liquid. Finally, the speed at which the molecules escape from the liquid is equal to the speed at which the molecules return to the liquid from the vapor, and the vapor remains constant. pressure. At this time, the vapor on the liquid surface is saturated, which is called saturated vapor, and the pressure it exerts on the liquid surface is called saturated vapor pressure. Experiments show that the saturated vapor pressure of a liquid is only related to temperature, that is, a liquid has a certain vapor pressure at a certain temperature. This refers to the pressure at which the liquid and its vapor are in equilibrium, regardless of the absolute amount of liquid and vapor in the system. Heating the liquid to boiling, turning the liquid into vapor, and then cooling the vapor to condense into a liquid. The combined operation of these two processes is called distillation. Obviously, distillation can separate volatile and non-volatile substances, as well as liquid mixtures with different boiling points. However, the boiling points of the components of the liquid mixture must differ greatly (at least 30°C) to obtain a better separation effect. When distillation is performed under normal pressure, because the atmospheric pressure is often not exactly 0.1 MPa, strictly speaking, a correction value should be added to the observed boiling point, but because the deviation is generally very small, even if the atmospheric pressure differs by 2.7KPa, this correction value It is only about ±1°C, so it can be ignored.

Bumping puts the flask containing the liquid on the asbestos net and heats it with a gas lamp underneath. Vapor bubbles are formed on the heated contact surface between the bottom of the liquid and the glass. The air dissolved in the liquid or the air adsorbed on the bottle wall in the form of a film contributes to the formation of such bubbles, and the rough surface of the glass also promotes it. Such small bubbles (called vaporization centers) can serve as the core of large vapor bubbles. At the boiling point, the liquid releases a large amount of vapor into the small bubbles. When the total pressure of the bubbles is increased to exceed the atmospheric pressure and is enough to overcome the pressure generated by the liquid column, the vapor bubbles rise and overflow the liquid surface. Therefore, if there are many small air or other vaporization centers in the liquid, the liquid can boil smoothly. If there is almost no air in the liquid, the bottle wall is very clean and smooth, and it is very difficult to form bubbles. When heated in this way, the temperature of the liquid may rise much above the boiling point without boiling. This phenomenon is called "overheating". Once a bubble is formed, because the vapor pressure of the liquid at this temperature far exceeds the sum of the atmospheric pressure and the liquid column pressure, the rising bubble increases very quickly, and even flushes the liquid out of the bottle. This abnormal boiling The phenomenon is called "bumping". Therefore, the boiling aid should be added before heating in order to introduce the gasification center to ensure stable boiling. Boiling aids are generally objects with loose and porous surfaces and air adsorbed, such as broken porcelain pieces and zeolite. In addition, several capillaries closed at one end can be used to introduce the vaporization center (note that the capillary has enough length so that the upper end can rest on the neck of the distillation flask, with the open end facing down). Under any circumstance, do not add the boiling aid to the liquid that has been heated to boiling, otherwise it will often cause a large amount of liquid to be ejected from the mouth of the distillation flask due to the sudden release of a large amount of vapor. If you forget to add a boiling aid before heating, you must remove the heat source when adding it, and wait until the heated liquid is cooled below the boiling point before adding it. If the boiling stops in the middle, a new boiling aid should be added before reheating. Because the initially added boiling aid expelled part of the air when heated, and absorbed the liquid when cooled, it may have failed. In addition, if the bath is used for indirect heating, keep the bath temperature not to exceed the boiling point of the distillate by 20°C. This heating method can not only greatly reduce the temperature difference between the parts of the distillate in the bottle, but also make the vapor bubbles not only from the flask When the bottom rises, it can also rise along the edge of the liquid, thus greatly reducing the possibility of overheating.

Process Pure liquid organic compounds have a certain boiling point under a certain pressure, but liquids with a fixed boiling point are not necessarily pure compounds, because some organic compounds often form binary or ternary azeotropic mixtures with other components , They also have a certain boiling point. The boiling point of the impure substance depends on the physical properties of the impurity and its interaction with the pure substance. If the impurity is not volatile, the boiling point of the solution is slightly higher than the boiling point of the pure substance (but during distillation, the actual measurement is not the boiling point of the impure solution, but the temperature when the escaped vapor and its condensation balance, that is Is the boiling point of the distillate, not the boiling point of the distillate in the bottle). If the impurities are volatile, the boiling point of the liquid will gradually increase during distillation or because two or more substances form an azeotropic mixture, the temperature can remain constant during the distillation process and stay within a certain range. Therefore, the constant boiling point does not mean that it is a pure compound. When distilling mixed liquids with a large difference in boiling point, the one with the lower boiling point will be steamed out first, the higher boiling point will be steamed out later, and the non-volatile ones will remain in the distiller. In this way, the purpose of separation and purification can be achieved. Therefore, distillation is one of the commonly used methods for separating and purifying liquid compounds. It is an important basic operation and must be mastered skillfully. However, when distilling a mixture with a relatively close boiling point, the vapors of various substances will be steamed out at the same time, but there are more low boiling points, so it is difficult to achieve the purpose of separation and purification, so we have to resort to fractional distillation. The boiling range of pure liquid compounds during distillation is very small (0.5~1℃). Therefore, distillation can be used to determine the boiling point. The method of boiling point determination by distillation is the constant method. This method uses a large amount of sample, more than 10 mL. If there are not many samples, the micro method should be used.

Definition of Fractionation: Fractionation is a method that uses a fractionating column to complete multiple vaporization-condensation processes in one operation. Therefore, fractional distillation is actually multiple distillation. It is more suitable for the separation and purification of liquid organic mixtures with similar boiling points. [3] The necessity of fractionation: (1) Distillation and separation are not complete. (2) The multiple distillation operation is cumbersome, time-consuming and wasteful. After the mixed liquid boils, the vapor enters the fractionating column and is partially condensed. The condensate contacts the rising vapor during the descending process, and the two exchange heat. The high-boiling components in the vapor are condensed, and the low-boiling components still rise and condense. The low-boiling point components in the liquid are heated and vaporized, and the high-boiling point components remain in the liquid state and drop. The result is an increase in low-boiling components in the rising steam, and an increase in high-boiling components in the falling condensate. After multiple heat exchanges in this way, it is equivalent to multiple continuous ordinary distillations. As a result, the vapor of low-boiling components continues to rise and is distilled out; high-boiling components continue to flow back into the distillation flask, thereby separating them.

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