The difference between extraction and distillation distillation is a thermodynamic separation process, which uses the different boiling points of components in mixed liquid or liquid-solid system to evaporate the low-boiling components and then condense them to separate all the components. Distillation is a combination of evaporation and condensation. Compared with other separation methods, such as extraction, filtration and crystallization. Its advantage is that it does not use other solvents other than system components, thus ensuring that no new impurities will be introduced.
Extraction, also known as solvent extraction or liquid-liquid extraction, is a unit operation to separate the mixture by using the solubility of each component in the solvent. That is, the method of transferring solute substances from one solvent to another by using the difference of solubility or distribution coefficient of substances in two mutually insoluble (or slightly soluble) solvents. Widely used in chemical, metallurgical, food and other industries, generally used in petroleum refining industry. In addition, the operation of separating two immiscible liquids after extraction is called liquid separation.
Solid-liquid extraction, also known as leaching, is to separate the components in the solid mixture with solvents, such as leaching sugar from beets with water; Soak soybean oil in soybean with alcohol to increase oil yield; Extracting effective components from traditional Chinese medicine with water to make fluid extract is called? Leaching? Or? Leaching? .
Although extraction is often used in chemical experiments, its operation process does not change the chemical composition (or chemical reaction) of the extracted substance, so the extraction operation is a physical process.
Extraction is one of the methods used to purify and purify compounds in organic chemistry laboratory. By extraction, the desired substance can be extracted from a solid or liquid mixture.
Introduction of distillation classification Distillation can be divided into tower distillation and tower-less distillation.
Judging from the history of world distillation development, there was a requirement for separation and purification in liquor production 3000-5000 years ago.
However, the long-term alcohol content is around 15-20 degrees. After the research of countless inventors, the prototype of the separation device came out. The alcohol content of 42-56 degrees is a purification peak, which is the current content range of liquor.
More than 200 years ago, a French inventor produced 95% ethanol in a vertical distillation tower, which was recognized by the distillation community. More than 30 years later, a British inventor invented a rectifying tower based on the distillation vertical tower, which produced 99-99.9% ethanol for the first time. Alcohol? The word meaning is the essence of wine. Distillation columns are arranged in methanol or ethanol production plants, with a height of 20-120m and a diameter of 0.3-13.5m, all of which originated from French and British inventors. The maximum annual output of distillation column can reach 500,000-300,000 tons, which is the main purification method of organic solvents.
Since 2005, Anyang Haichuan Institute of Chemical Technology (formerly the Institute of Contemporary Chemical Engineering in Anyang High-tech Zone) has started its unique purification research, which many domestic users call tower-free distillation (rectification) and tower-free refining. 20 10 12 tower-less refining equipment came out and obtained patent certificate; 2065438+0 1 March-August, no-tower distiller and no-tower distiller came out, and the patent was granted1,which passed the preliminary examination. A large pharmaceutical production company in Shangqiu City, Henan Province carried out an industrial purification test of 20% ultra-low content waste methanol, and the initial fraction content was 86%. From October to August, 2065438+0/KLOC-0, a large pharmaceutical company in Anyang City, Henan Province conducted an industrial purification test on 82% of waste ethanol. The distillation range without tower is increased to 98%, and the content of methyl acetate in ethanol is 0.65438 0%, which is obviously better than the separation effect of 0.26% in the distillation tower of this factory. After 20 10, users attach great importance to tower-less distillation equipment in the purification of many organic solvents. The institute is not only committed to the research and development of separation equipment, but also strives to raise the accumulated rich purification experience to the theoretical level.
Overview of distillation principle
Using the difference of volatility of each component in the liquid mixture, the liquid mixture is partially vaporized, and then the vapor is partially condensed, thus realizing the separation of its components. Belonging to the unit operation of mass transfer and separation. Widely used in oil refining, chemical industry, light industry and other fields.
Its principle takes the separation of two-component mixed liquid as an example. When the feed liquid is partially vaporized by heating, the volatile components are enriched in the vapor, and the non-volatile components are also enriched in the remaining liquid, which realizes the separation of the two components to some extent. The greater the difference of volatilization ability between the two components, the greater the concentration degree. In industrial distillation equipment, partially vaporized liquid phase directly contacts with partially condensed gas phase for gas-liquid mass transfer. As a result, the hard volatile components in the gas phase are partially transferred to the liquid phase, and the volatile components in the liquid phase are partially transferred to the gas phase, that is, partial vaporization of the liquid phase and partial condensation of the vapor phase are simultaneously realized.
Due to molecular movement, liquid molecules are easy to overflow from the surface. This trend increases with the increase of temperature. If the liquid is placed in a closed vacuum system, the liquid molecules will overflow continuously and form vapor on the upper part of the liquid surface. Finally, the speed at which the molecules leave the liquid is equal to the speed at which the molecules return from the vapor to the liquid, and the vapor will maintain a certain pressure. At this time, the vapor on the liquid surface reaches saturation, 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 liquid is only related to temperature, that is, liquid has a certain vapor pressure at a certain temperature. This refers to the pressure when the liquid and its vapor are in equilibrium, regardless of the absolute amount of liquid and vapor in the system.
The combined operation of heating a liquid to boiling, turning it into steam, and then cooling and condensing the steam into liquid is called distillation. Obviously, distillation can separate volatile and nonvolatile substances, as well as liquid mixtures with different boiling points. However, the boiling points of each component in the liquid mixture must be very different (at least above 30℃) in order to get better separation effect. When distillation is carried out under normal pressure, the atmospheric pressure is often not exactly 0. 1MPa, so strictly speaking, the observed boiling point should be corrected, but because the deviation is generally small, even if the atmospheric pressure differs by 2.7KPa, this correction value is not enough? 1℃, so it can be ignored.
boiling
A flask filled with liquid is placed on an asbestos net, and the bottom is heated by a gas lamp to form a steam bubble on the contact surface between the liquid bottom and the glass. The air dissolved in the liquid or 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 plays a role in promoting it. Such a small bubble (called gasification center) can be used as the core of a large steam bubble. At the boiling point, the liquid releases a large amount of steam into small bubbles. When the total pressure of the bubbles increases above atmospheric pressure and is enough to overcome the pressure generated by the liquid column, the steam bubbles rise and overflow the liquid surface. Therefore, if there are many small air or other gasification centers in the liquid, the liquid can boil smoothly. If there is almost no air in the liquid and the bottle wall is very clean and smooth, it is difficult to form bubbles. When heated in this way, the temperature of the liquid may rise to much higher than the boiling point without boiling. This phenomenon is called. Overheating? . Once bubbles are formed, because the vapor pressure of liquid at this temperature far exceeds the sum of atmospheric pressure and liquid column pressure, the rising bubbles increase very quickly, and even the liquid overflows the bottle. This abnormal boiling phenomenon is called. Boiling? . Therefore, boiling AIDS should be added before heating to introduce the gasification center to ensure the stability of boiling. Boilers are generally objects with loose and porous surfaces and air adsorption, such as broken porcelain pieces and zeolite. In addition, several closed capillaries can be used to introduce the gasification center (note that the capillaries are long enough so that their upper ends can lean against the bottleneck of the distillation bottle and the open ends are downward). In any case, it is forbidden to add boiling AIDS to the liquid that has been heated to near boiling, otherwise it is often dangerous to spray a large amount of liquid from the distillation bottle because of the sudden release of a large amount of steam. If you forget to add the boiling aid before heating, you must remove the heat source before adding it, and you can't add it until the heated liquid cools below the boiling point. If boiling stops in the middle, new boiling AIDS should be added before reheating. Because the initially added boiling aid drives out some air when heating and absorbs liquid when cooling, it may have failed. In addition, if indirect heating in water bath is adopted, and the temperature of water bath should not exceed the boiling point of distilled liquid by 20℃, this heating method can not only greatly reduce the temperature difference of various parts of distilled liquid in the bottle, but also make steam bubbles rise not only from the bottom of the flask, but also 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 fixed boiling points are not necessarily pure compounds, because some organic compounds often form binary or ternary boiling mixtures with other components, and they also have a certain boiling point. The boiling point of impure substances depends on the physical properties of impurities and their interaction with pure substances. If the impurities are nonvolatile, the boiling point of the solution is slightly higher than that of the pure substance (but in distillation, the actual measurement is not the boiling point of the impure solution, but the temperature at which the escaping vapor and its condensation reach equilibrium, that is, the boiling point of the distillate rather than the boiling point of the distillate in the bottle). If the impurities are volatile, the boiling point of the liquid will gradually increase during the distillation process, or because two or more substances form a * * * boiling point mixture, the temperature can remain unchanged and stay in a certain range during the distillation process. So the constant boiling point does not mean that it is a pure compound.
When distilling mixed liquids with different boiling points, the lower boiling point is distilled first, the higher boiling point is distilled later, and the nonvolatile liquid remains in the distiller, thus achieving the purpose of separation and purification. Therefore, distillation is one of the common methods to separate and purify liquid compounds, and it is an important basic operation that must be mastered skillfully. When the mixture with close boiling point is distilled, the vapors of various substances will be distilled at the same time, but the boiling point is much lower, so it is difficult to achieve the purpose of separation and purification, so we have to resort to fractionation. 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 for determining boiling point by distillation is constant method, and the sample consumption of this method is large, exceeding 10 mL. If there are few samples, trace method should be used.
separately
Definition: Fractionation refers to multiple gasification by fractionating tower? A method of completing the condensation process at one time. So fractionation is actually multiple distillation. It is more suitable for the separation and purification of liquid organic mixtures with similar boiling points.
Necessity of fractionation: (1) incomplete distillation separation. (2) Repeated distillation operation is tedious, time-consuming and wasteful.
After the mixed liquid boils, the steam enters the fractionator and partially condenses. Condensed water contacts with rising steam in the process of falling, and they exchange heat. The high-boiling components in the steam are condensed, while the low-boiling components in the condensate still rise in the form of steam, while the low-boiling components in the condensate are heated and gasified, and the high-boiling components still fall in the liquid state. As a result, the low-boiling components in the ascending steam increase, while the high-boiling components in the descending condensate increase. After many times of heat exchange, it is equivalent to continuous ordinary distillation. So that the steam of the low-boiling component rises continuously and is distilled out; Components with high boiling point are continuously refluxed to the distillation bottle for separation.