Extraction distillation extraction

Extraction, also known as solvent extraction or liquid-liquid extraction, also known as extraction, is often used in petroleum refining industry. Extraction, also known as solvent extraction or liquid-liquid extraction (as distinguished from solid-liquid extraction, that is, leaching), also known as extraction (often used in petroleum refining industry). It is a mass transfer and separation process that uses liquid extractant to treat immiscible two-component or multi-component solutions to realize component separation. This is a widely used unit operation. Based on the principle of similar miscibility, there are two extraction methods:

Liquid-liquid extraction is the separation of a component in a liquid mixture with a selected solvent. The solvent must be immiscible with the extracted mixed solution, with selective solubility, good thermal stability and chemical stability, and low toxicity and corrosiveness. For example, benzene is used to separate phenol from coal tar; Separating olefins from petroleum fractions with organic solvents; Extraction of Br2 from water with carbon tetrachloride.

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; The effective components in traditional Chinese medicine are leached with water to make fluid extract, which 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 compound can be extracted from a solid or liquid mixture. The commonly used liquid-liquid extraction is introduced here. Using the difference of solubility or partition coefficient of compounds in two insoluble (or slightly soluble) solvents, the compounds are transferred from one solvent to another. After repeated extraction, most of the compounds were extracted.

Distribution law is the main basis of extraction theory, and substances have different solubility in different solvents. At the same time, soluble substances can be dissolved in two solvents by adding them into two mutually immiscible solvents. The experiment shows that the ratio of the compound in the two liquid layers is constant at a certain temperature, and it does not decompose, electrolyze, associate and solvate with the two solvents. This is true no matter how much substance is added. This is a physical change. Expressed by formula.

CA/CB=K

California. CB represents the mass concentration of a compound in two immiscible solvents. K is a constant called "distribution coefficient".

Organic compounds are usually more soluble in organic solvents than in water. Extracting compounds dissolved in water with organic solvents is a typical example of extraction. When extracting, if a certain amount of electrolyte (such as sodium chloride) is added to the aqueous solution, the "salting-out effect" can be used to reduce the solubility of organic matter and extraction solvent in the aqueous solution, and the extraction effect can often be improved.

To completely extract the desired compound from the solution, it is usually not enough to extract it once, and it must be extracted several times. Using the relationship of distribution law, the residue of the compound after extraction can be calculated.

Let v be the volume of the original solution.

W0 is the total amount of compounds before extraction.

W 1 is the residue of the compound after one extraction.

W2 is the residue of the compound after the second extraction.

W3 is the residue of the compound after n times of extraction.

S is the volume of the extract.

After one extraction, the concentration of the compound in the original solution is w1/v; The concentration of the compound in the extraction solvent is (w0-w1)/s; The ratio of the two is equal to k, that is:

w 1/V =K w 1=w0 KV

(w0-w 1)/S KV+S

Similarly, after the second extraction, there is still

W2/V =K, that is

(w 1-w2)/S

w2=w 1 KV =w0 KV

KV+S KV+S

Therefore, after n extractions:

wn=w0 ( KV)

KV+S

When a certain amount of solvent is used, it is desirable that the amount remaining in water be as small as possible. On the other hand, KV/(KV+S) is always less than 1, so the greater N, the smaller wn. That is to say, instead of extracting with all the solvents once, it is better to divide the solvents into several times and extract for many times. However, it should be noted that the above formula is applicable to solvents that are almost insoluble in water, such as benzene and carbon tetrachloride. However, a small amount of solvents, such as diethyl ether, are miscible with water. The above formula is only approximate. But you can still qualitatively point out the expected results.

Instrument: separatory funnel

Common extractant: water, benzene, carbon tetrachloride.

Requirements: The extractant and the original solvent are immiscible with each other.

The extractant and solute will not react with each other.

The solubility of solute in extractant is much greater than that in original solvent.

Relevant laws: organic solvents are soluble in organic solvents, polar solvents are soluble in polar solvents, and vice versa.

Audislee will tell you simply: extraction uses the solubility difference between the two. The principle of extraction and dissolution, for example, now that A and B are mixed together and there is a solvent C, which is soluble with A and insoluble with B, then we can add C to the mixed solution of AB, and then A is dissolved in C and separated from B, which is called stratification in chemistry. An extractant insoluble (at most partially soluble) in the solution to be separated is added to form two liquid phases. Using the difference of solubility (including dissolution after chemical reaction) between the original solvent and extractant, they are unevenly distributed in the two liquid phases, and then the separation of the two liquid phases is realized. For example, the aqueous solution of iodine is extracted with carbon tetrachloride, and almost all iodine moves into carbon tetrachloride, so that iodine can be separated from a large amount of water.

The most basic operation is single-stage extraction. During the mixing process, the feed liquid is in close contact with the extractant, and the extracted components enter the extractant through the interphase interface until the distribution of components between the two phases is basically balanced. Then standing and settling, and separating into two layers of liquid, namely, the extraction liquid transformed by extractant and the raffinate transformed by feed liquid. When the single-stage extraction reaches the phase equilibrium, the phase equilibrium ratio of the extracted component B is called the distribution coefficient k, that is:

K=yB/xB

In this formula, yB and xB are the concentrations of component B in the extract and raffinate, respectively. The expression method of concentration needs to consider various existing forms of components and calculate according to the same chemical formula.

If another component D in the feed liquid is also extracted, the ratio of the partition coefficients of component B and component D, that is, the separation factor of B and D, is called the selectivity coefficient α, that is:

α=KB KD=yB xD/(xB yD)

When α "1,component b is preferentially extracted; α= 1 indicates that the distribution of the two components in the two phases is the same, and this extractant cannot be used to separate the two components.

The extraction rate of a given component by single-stage extraction (the ratio of the extracted component in the extraction liquid to the initial amount in the raw material solution) is low, which often cannot meet the technological requirements. In order to improve the extraction rate, many methods can be used: ① Multi-stage cross-flow extraction. The feed liquid and raffinate of each stage contact with fresh extractant, which can achieve higher extraction rate. However, the dosage of extractant is large, and the average concentration of extractive solution is low. ② Multi-stage countercurrent extraction. The feed liquid and extractant are added from two ends of the cascade (or plate tower) respectively, and flow reversely between the stages, and finally become raffinate and extraction liquid, which leave from the other end respectively. The feed liquid and extractant are extracted many times, so the extraction rate is high and the concentration of extracted components in the extract is also high, which is a common process of industrial extraction. ③ Continuous countercurrent extraction. In the differential contact extraction tower (see extraction equipment), the feed liquid and extractant contact and transfer mass in the countercurrent process, which is also a common industrial extraction method. Among the feed liquid and extractant, the high density is called the heavy phase, and the low density is called the light phase. Light phase enters from the bottom of the tower and overflows from the top of the tower; The heavy phase is added from the top of the tower and led out from the bottom of the tower. When the extraction tower is running, the liquid phase filling the whole tower is called continuous phase; Another liquid phase is usually dispersed in it in the form of droplets, which is called dispersed phase. The dispersed phase liquid is dispersed when entering the tower, and then condensed and layered before leaving the tower. It is necessary to consider the operation and process requirements of the tower to choose which feed liquid and extractant are dispersed phase. In addition, reflux extraction and fractional extraction can achieve a higher degree of separation. 1. By distillation, the desired product can be obtained directly, and other components are needed for absorption and extraction.

2. Distillation separation is widely used and has a long history.

3. The energy consumption is high, and a large amount of gas phase or liquid phase is generated in the production process. 1, divided into simple distillation, equilibrium distillation, distillation and special distillation.

2. According to the working pressure: normal pressure, pressurization and decompression.

3. According to the components in the mixture: two-component distillation and multi-component distillation.

4. According to the operation mode, batch distillation and continuous distillation make use of the volatility difference of each component in the liquid mixture to partially vaporize the liquid mixture and then partially condense the vapor, thus realizing the separation of the components contained therein. 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. In atmospheric distillation, the atmospheric pressure is often not exactly 0. 1MPa, so strictly speaking, a correction value should be added to the observed boiling point. However, due to the small deviation, even if the atmospheric pressure difference is 2.7KPa, the correction value is only about 65438 0℃, which can be ignored.

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 lot of steam to form small bubbles, and the total pressure of the bubbles increases to exceed the atmospheric pressure, which is enough to overcome the problem caused by

Under the pressure generated by the liquid column, the steam bubble rises and escapes from 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 much above the boiling point without boiling, which is called "overheating". Once bubbles are formed, because the vapor pressure of the liquid at this temperature far exceeds the sum of atmospheric pressure and liquid column pressure, the rising bubbles increase very quickly and even flush the liquid out of 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 into 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 the water bath is used for indirect heating, the temperature of the water bath should not exceed the boiling point of distillate by 20 & ordmc. This heating method can not only greatly reduce the temperature difference in different parts of the distilled liquid in the bottle, but also make the 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.

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 similar boiling points is distilled, the vapors of various substances will be distilled at the same time, but there are many low boiling points, 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.

Principle of fractionation experiment

Definition: Fractionation is a method to complete multiple gasification-condensation processes in one operation by using a fractionator. 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) distillation separation is not complete. ⑵ Repeated distillation 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. When the solution containing non-volatile components is distilled, the solvent vapor is led out of the condenser tube, and the non-volatile components remain in the residual liquid in the bottle. Most of the solvents can be distilled by simple distillation, so as to achieve the purpose of separation. According to Raoul's law, under a certain pressure, the vapor pressure of a solvent in a dilute solution is equal to the vapor pressure of a pure solvent multiplied by the molar fraction of the solvent in the solution:

P solvent =po solvent x solvent

In the formula, the P solvent and the po solvent are the vapor pressure of the solvent in the solution and the vapor pressure of the pure solvent respectively;

Because that solvent x in the solution

The pressure (point B) is lower than the vapor pressure of water (point A). Water will boil at 100℃ (1 atmospheric pressure), but the solution will not boil. Only at a higher temperature (point B') will the solution boil. For this solution, distillation operation is either used to recover pure solvent or to obtain solid solute. ① Flash evaporation. The liquid mixture is heated, and then a partial evaporation separation operation is carried out.

② Simple distillation. The separation operation of gradually vaporizing the mixed liquid and condensing the steam in time to collect it in stages.

3 rectification. Reflux is the most widely used method to achieve high purity and high recovery separation operation. For mixed liquids with equal or similar volatility, in order to increase the relative volatility between components, solvents or salts can be added during distillation separation. This separation operation is called special distillation, including azeotropic distillation, extractive distillation and salting distillation. In the process of distillation, chemical reactions also occur between the components of the mixed liquid, which is called reactive distillation. Distillation operation is a commonly used experimental technology in chemical experiments, which is generally used in the following aspects:

For the separation of (1) liquid mixture, only when the boiling points of the components in the mixture are quite different can more effective separation be achieved;

⑵ Determining the boiling point of pure compounds;

(3) purification, that is, distilling substances containing a small amount of impurities to improve their purity;

(4) Recovering the solvent, or evaporating part of the solvent to concentrate the solution.

Feeding: Carefully pour the liquid to be distilled into the distillation bottle through the glass funnel, and be careful not to make the liquid flow out of the branch pipe. Add some boiling AIDS and install a thermometer, which should be installed at the side opening leading to the condenser tube. Check again whether all parts of the instrument are connected tightly and correctly.

Heating: When using the condenser tube, cold water is slowly introduced from the lower mouth of the condenser tube, then flows out from the upper mouth to the sink, and then heating is started. When heating, you can see that the liquid in the distillation bottle gradually boils and the vapor gradually rises. The reading of the thermometer also rose slightly. When the top of the steam reaches the mercury ball of the thermometer, the thermometer reading rises sharply. At this time, the flame of the gas lamp should be turned down properly or the voltage of the electric heating furnace or electric heating jacket should be reduced, so that the heating speed is slightly slower and the top of the steam stays in place, so that the upper part of the bottleneck and the thermometer are heated, so that the temperature of the droplets and steam on the mercury ball can be balanced. Then slightly increase the flame and distill. Generally, it is appropriate to control the heating temperature and adjust the distillation speed at 1 ~ 2 drops per second. During the whole distillation process, there should always be condensed water droplets on the mercury ball of the thermometer. The temperature at this time is the temperature when liquid and vapor are in equilibrium, and the reading of thermometer is the boiling point of liquid (distillate). When distilling, the heated flame should not be too large, otherwise the bottleneck of distillation will be overheated, so that some liquid vapor will be directly heated by the flame, and the boiling point read by thermometer will be high; On the other hand, distillation should not be carried out too slowly, otherwise the boiling point reading on the thermometer is low or irregular because the mercury ball of the thermometer cannot be fully penetrated by distillation steam.

Observe the boiling point and collect the distillate: at least two receiving bottles should be prepared before distillation. Because before reaching the boiling point of the expected substance, the liquid with lower boiling point is steamed out first. This fraction is called "pre-fraction" or "distillate". After the front distillate is steamed, the temperature tends to be stable and it is a relatively pure substance. At this time, a clean and dry receiving bottle should be replaced for acceptance, and the thermometer reading of the liquid at the beginning and the last drop of distillation should be recorded, which is the boiling range (boiling range) of distillate. Generally, liquids contain some impurities with high boiling point more or less. After distilling off the required fraction, if the heating temperature continues to rise, the reading of the thermometer will increase significantly. If the original heating temperature is maintained and the distillate is no longer distilled, the temperature will suddenly drop. At this point, distillation should be stopped. Even if the impurity content is small, don't evaporate it to dryness, so as to avoid accidents such as cracking of distillation bottles.

After distillation, stop heating first, then stop water supply, and dismantle the instrument. Disassembly of the instrument is in the reverse order of assembly. First remove the receiver, then remove the tail pipe, condenser pipe, distillation head and distillation bottle.

Attention should be paid to the operation: (1) put a small amount of broken porcelain pieces in the distillation bottle to prevent the liquid from boiling. ⑵ The mercury ball position of thermometer should be on the same level with the lower edge of branch nozzle. (3) The liquid in the distillation bottle should not exceed 2/3 of its volume and should not be less than 1/3. (4) The cooling water in the condenser tube enters from the lower mouth and is discharged from the upper mouth. 5] The heating temperature should not exceed the boiling point of the substance with the highest boiling point in the mixture. Archaeologists excavated more than 440 Han tombs in the east of Zhangjiabao Square in Xi 'an. Among them, a copper distiller with peculiar technology was found in a tomb in the period of Wang Mang in the Western Han Dynasty, which may be the earliest distiller in history.

This copper distiller is 36 cm high and consists of a cylindrical container, a copper pot and a bean-shaped cover. Among them, there is a MiG-shaped grate at the bottom of the cylinder, which is used as a sandwich in ancient cookers. There is a small tube flow at the bottom, with three hooves. The upper part of the bean-shaped cover is disc-shaped, and the joints are tenon-nail structure, which can move freely within a certain range. When unearthed, it is placed in an orderly manner. The copper pot is placed in a cylindrical container, and the bean-shaped lid is placed on the copper pot. This kind of combined distiller has never been found before. Although its working principle is not clear, it should be used as distillation medicine and wine in structure.

It is understood that a bronze distiller from the Eastern Han Dynasty was unearthed in China before, and a copper distiller was also found in the Han Tomb in Zhangjiabao, Xi 'an. Cheng Linquan, deputy director of the Xi Institute of Cultural Relics Protection and Archaeology, said that its excavation provided very valuable information for the study of diet and medical technology in the Han Dynasty.

In addition, in the tomb with the distiller number M 1 15, archaeologists also unearthed more than 200 cultural relics, including 5 large bronze ding and 4 large glazed pottery ding. According to Zhou Li, in the Western Zhou Dynasty, the emperor was buried with Jiuding. The owner of the tomb of M 1 15 respected the ritual system of the Zhou Dynasty and buried it with Jiuding, which shows its special status. Jiuding and another funerary object, the imitation bronze glazed pottery tripod, are the real material evidence of Wang Mang's political reform, which has extremely important academic value and historical significance.

More than 440 Han formations excavated this time are located in the east of Chang 'an in Han Dynasty, only 2,500 meters away from Chang 'an, and nearly 3,000 cultural relics such as pottery, copper, iron, lead, jade and bones have been unearthed. These tombs are mainly small Han tombs, among which three medium-sized tombs from the late Western Han Dynasty to Xin Mang are the most important, which not only unearthed the common objects of tombs in the Western Han Dynasty, such as red pottery,

Glazed pottery, ding, boxes, cans, warehouses, pots, furnaces, etc. In tomb M 1 10, fragments of jade clothes were also unearthed. Large-scale exquisite glazed pottery unearthed from tomb M 1 14 is also very rare in Han tombs in Xi 'an. Experts said that the excavation provided important information for studying the social life of the Han Dynasty and the layout of Chang 'an City, and helped to further understand the formation and development of Chinese culture.