What Is Extractive Distillation?

Extractive distillation is a chemical commonly used experimental method for separating liquid mixtures. The principle is to use compounds with different solubility or partition coefficients in two mutually incompatible (or slightly soluble) solvents to transfer compounds from one solvent to another. In a solvent. After repeated extractions, most of the compounds were extracted. The instruments used for extractive distillation include alcohol lamps, iron stands, asbestos nets, conical flasks, horn tubes, beakers, condensers and other commonly used laboratory instruments. At present, extraction and distillation have been widely used in daily life and industrial production.

Extractive distillation

Extraction, also known as

Distillation is a thermodynamic separation process. It utilizes different boiling points of components in a mixed liquid or liquid-solid system to evaporate low-boiling components and then condense to separate the entire component. It is an evaporation and condensation operation Union of unit operations. Compared with other separation methods, such as extraction and absorption, it has the advantage that it does not require the use of solvents other than system components, thereby ensuring that no new impurities are introduced.

Definition of extractive distillation

Refers to the mass transfer process that separates components by utilizing the differences in the volatility of each component in the liquid mixture. A process of evaporating and condensing the vapor produced by boiling a liquid into a condensate tube to cool and condense it into a liquid. Distillation is an important operation technology for separating mixtures, especially for the separation of liquid mixtures.

Extractive distillation characteristics

1. Through the distillation operation, the required product can be directly obtained, and absorption and extraction also require other components.

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

3. The energy consumption is large, and a large amount of gas phase or liquid phase is generated in the production process.

Extractive distillation classification

1. Divided by mode: simple distillation, equilibrium distillation, rectification, special rectification

2. According to operating pressure: normal pressure, pressurization, decompression

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

4. Divided by operation mode: batch distillation, continuous distillation

Extractive distillation main instrument

Distillation flask, thermometer, condenser tube, horn tube, alcohol lamp, asbestos net, iron stand, conical flask, rubber stopper

Extractive distillation history

In the ancient Greek era, Aristotle once wrote: "Through distillation, water is first turned into steam and then liquid, which makes seawater potable." This shows that people discovered the principle of distillation at that time. The ancient Egyptians used distillation to make spices. In the early Middle Ages, the Arabs invented the distillation of wine. In the tenth century, a philosopher named Avicenna described the still in detail.

Extractive distillation principle

By using the difference in the volatility of each component in the liquid mixture, the liquid mixture is partially vaporized and the vapor is partially condensed, so as to achieve the separation of its contained components. It is a unit operation that belongs to mass transfer separation. Widely used in oil refining, chemical, light industry and other fields.

The principle is based on the separation of a two-component mixture. The material liquid is heated to partially vaporize it, and the volatile components are concentrated in the vapor, and the non-volatile components are also concentrated in the remaining liquid, which achieves 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 enrichment described above. In industrial distillation equipment, a part of the vaporized liquid phase is brought into direct contact with a part of the condensed gas phase for vapor-liquid interstitial mass transfer. 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 achieved at the same time.

Liquid molecules tend 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 continue to overflow and form vapor on the liquid surface. At the end, the speed at which the molecules escape from the liquid is equal to the speed at which the molecules return from the vapor to the liquid, and the vapor remains constant. pressure. At this time, the vapor on the liquid surface reaches saturation, 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, the liquid has a certain vapor pressure at a certain temperature. This is the pressure at which a liquid is in equilibrium with its vapor, and is independent of the absolute amount of liquid and vapor in the system.

The operation of heating the liquid to boiling, turning the liquid into a vapor, and then cooling the vapor to condense the liquid into a liquid, is called distillation. Obviously, distillation can separate volatile and non-volatile materials, as well as liquid mixtures with different boiling points. However, the boiling points of the components of the liquid mixture must be very different (at least 30 ° C or higher) to obtain better separation results. When performing distillation at normal pressure, the atmospheric pressure is often not exactly 0.1 MPa, so strictly speaking, the correction value should be added to the observed boiling point, but the deviation is generally small, even if the atmospheric pressure differs by 2.7KPa, this correction value It is only about ± 1 ° C, so it can be ignored.

The flask containing the liquid was placed on an asbestos net, and the bottom was heated by a gas lamp. Vapor bubbles were formed on the bottom of the liquid and the heated contact surface of the glass. The air dissolved in the liquid or the air adsorbed on the bottle wall in the form of a film helps the formation of such bubbles, and the rough surface of the glass also promotes it. Such small bubbles (called gasification centers) can serve as the core of large vapor bubbles. At the boiling point, the liquid releases a large amount of vapor into small bubbles, and the total pressure of the bubbles will increase to more than atmospheric pressure, which is sufficient to overcome the

At the pressure generated by the liquid column, the vapor bubbles rise and escape 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, 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 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 atmospheric pressure and liquid column pressure, the rising bubble increases very quickly, and even the liquid overflows the bottle. This abnormally boiling This phenomenon is called "bumping." Therefore, a boiling aid should be added before heating in order to introduce the gasification center to ensure a smooth boiling. Boilers are generally porous, porous, and air-absorbed objects, such as broken porcelain and zeolites. It is also possible to introduce several capillaries with closed ends to introduce the gasification center (note that the capillaries are of sufficient length to allow the upper end to rest on the neck of the retort with the open end facing down). In any case, it is forbidden to add boiling aids to liquids that have been heated to near boiling, otherwise, a large amount of liquid is often ejected from the mouth of the distillation bottle due to the sudden release of a large amount of vapor, which is dangerous. If you forget to add the boiling aid before heating, you must remove the heat source before adding, and wait until the heated liquid is cooled below the boiling point before adding. If the boiling has stopped halfway through, add a new boiling aid before reheating. The boiling aid that was added initially expelled part of the air when heated, and absorbed the liquid when cooled, so it may have failed. In addition, if you use indirect heating of the bath, keep the bath temperature not exceeding the boiling point of the distillate 20 & ordm; C. This heating method can not only greatly reduce the temperature difference between the various parts of the distillate in the bottle, but also make the vapor bubbles not only from the flask It can also rise along the edge of the liquid, which can greatly reduce the possibility of overheating.

Pure liquid organic compounds have a certain boiling point under a certain pressure, but liquids with a fixed boiling point are not necessarily all 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 an impure substance depends on the physical properties of the impurity and its interaction with the pure substance. If the impurities are non-volatile, the boiling point of the solution is slightly higher than the boiling point of the pure substance (but during the distillation, the boiling point of the impure solution is not actually measured, but the temperature at which the escaped vapor balances with its condensation, ie 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 the distillation or due to the azeotropic mixture of two or more substances, the temperature can remain unchanged during the distillation and stay within a certain range. Therefore, the constant boiling point does not mean that it is a pure compound.

When distilling a mixed liquid with a large difference in boiling point, the one with the lower boiling point is distilled off first, and the one with the higher boiling point is then distilled off, leaving the non-volatile in the still, so that the purpose of separation and purification can be achieved. Therefore, distillation is one of the commonly used methods for the separation and purification of liquid compounds. It is an important basic operation and must be mastered. However, when a mixture with a relatively close boiling point is distilled, the vapors of various substances will be distilled out at the same time, but the lower boiling point is more, so it is difficult to achieve the purpose of separation and purification, and only by means of fractional distillation. Pure liquid compounds have a small boiling range during distillation (0.5 to 1 ° C). Therefore, distillation can be used to determine the boiling point. The method of determining the boiling point by distillation is a constant method. This method requires a large amount of sample, which should be more than 10 mL. If there are not many samples, the micro method should be used.

Fractionation experiment principle

Definition: Fractionation is a method that uses a distillation column to complete multiple gasification-condensation processes in one operation. Therefore, fractional distillation is actually multiple distillation. It is more suitable for separating and purifying liquid organic mixtures with little difference in boiling points.

Necessity of fractionation: Distillation is not complete. Multiple distillation operations are tedious, time consuming, and extremely wasteful.

After the mixed liquid is boiled, the vapor enters the fractionation column and is partially condensed. The condensate is in contact with the steam that continues to rise during the decline. The two exchange heat. The high-boiling components in the steam are condensed. The low-boiling components in the liquid are heated and gasified, and the high-boiling components still fall into a liquid state. 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 continuous ordinary distillation. As a result, the vapors of the low-boiling components keep rising and are distilled out; the high-boiling components keep flowing back into the distillation flask to separate them.

Extractive distillation

When the solution containing non-volatile components is distilled, the solvent vapor is led out by the condenser tube, and the non-volatile components are left in the residual liquid in the bottle, and most of the solvent can be distilled off in one simple distillation, thereby achieving 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 times the molar fraction of the solvent in the solution:

p solvent = po solvent x solvent

Where p solvent and po solvent are the vapor pressure of the solvent in the solution and the vapor pressure of the pure solvent, respectively;

Since the x solvent in the solution is <1, the vapor pressure of the solvent in the solution is always lower than that of the pure solvent. Distillation curve of sucrose aqueous solution. Curves 1 and 2 represent temperature-vapor pressure curves of water and aqueous sucrose solutions, respectively. The presence of sucrose molecules in the solution reduces the density of water molecules on the surface of the solution, thereby reducing the vapor pressure of the solution. So the vapor of the solution at the same temperature

The pressure (point B) is lower than the vapor pressure of water (point A). Water will boil (at 1 atmosphere) at 100oC, while the solution will not boil. Only at higher temperatures (point B ) will the solution boil. For this type of solution, distillation is used either to recover pure solvents or to obtain solid solutes.

Classification of extractive distillation methods

Flash distillation. After heating the liquid mixture, it undergoes a partial vaporization separation operation.

Simple distillation. A separation operation in which the mixed liquid is gradually vaporized and the vapor is condensed in time to be collected in stages.

Distillation. The use of reflux to achieve high purity and high recovery separation operations is the most widely used. For mixed liquids with equal or similar volatility of each component, in order to increase the relative volatility of each component, solvents or salts can be added during rectification separation. This type of separation operation is called special distillation, which includes azeotropic distillation. Distillation, extractive distillation and salt-added distillation; and those that undergo a chemical reaction between the components of the mixed liquid during rectification are called reactive distillation.

Extractive distillation laboratory distillation operation

Distillation is a commonly used experimental technique in chemical experiments and is generally applied in the following areas:

Separate a liquid mixture, only when the boiling points of the components in the mixture have a large difference can achieve a more effective separation;

Determine the boiling point of pure compounds;

(3) Purification, which improves the purity by distilling substances containing a small amount of impurities;

Recover the solvent, or distill off part of the solvent to concentrate the solution.

Adding: Carefully pour the liquid to be distilled through the glass funnel into the distillation bottle, taking care not to let the liquid flow out of the branch pipe. Add a few boiling aids, and install the thermometer. The thermometer should be installed at the side of the mouth leading to the condensation tube . Check again that the parts of the instrument are tightly and properly connected.

Heating : When using a water condensing tube, first slowly pass cold water from the lower opening of the condensing tube, flow from the upper opening to the water tank, and then start heating. When heating, you can see that the liquid in the distillation flask gradually boils and the vapor gradually rises. The thermometer reading also rose slightly. When the tip of the vapor reaches the mercury bulb of the thermometer, the thermometer reading rises sharply. At this time, you should appropriately reduce the flame of the gas lamp or reduce the voltage of the heating furnace or heating jacket to slow down the heating rate slightly. The top of the steam stays in place, the upper part of the bottleneck and the thermometer are heated, and the droplets and steam temperature on the mercury ball reach equilibrium. . Then slightly increase the flame and perform distillation. Control the heating temperature and adjust the distillation speed, usually 1-2 drops per second is appropriate. Condensed droplets should always be present on the mercury ball of the thermometer throughout the distillation process. at this time

Extractive distillation

The temperature is the temperature at which the liquid and vapor are in equilibrium, and the thermometer reading is the boiling point of the liquid (distillate). The flame to be heated during distillation must not be too large, otherwise it will cause overheating in the neck of the distillation flask, which will cause part of the liquid vapor to directly receive the heat of the flame, so that the boiling point read by the thermometer will be higher; on the other hand, distillation also Do not proceed too slowly, otherwise the thermometer's mercury ball cannot be fully infiltrated by the distillate vapor, so the boiling point read on the thermometer is low or irregular.

Observe boiling point and collect distillate : Prepare at least two receiving bottles before performing distillation. Because the lower boiling liquid evaporates before the boiling point of the desired substance is reached. This part of the distillate is called the "front cut" or "distillation head". After the first distillate has been distilled and the temperature has stabilized, the purer substance is distilled off. At this time, a clean and dry receiving bottle should be replaced to accept it. Record the thermometer readings when this part of the liquid begins to distill and the last drop. Is the boiling range (boiling point range) of the fraction. Generally, the liquid contains more or less high-boiling impurities. After the required fractions are distilled off, if the heating temperature is further increased, the reading of the thermometer will increase significantly. If the original heating temperature is maintained, it will no longer be Distillate evaporates and the temperature suddenly drops. Distillation should stop at this time. Even if the content of impurities is very small, do not evaporate it to avoid the distillation bottle rupture and other accidents.

After the distillation is completed, the heating should be stopped first, then the water flow should be stopped, and the instrument should be removed. The order of disassembling the instrument is the reverse of the order of assembly. Remove the receiver first, and then remove the tail tube, condenser tube, distillation head, distillation bottle, etc.

Note during operation : Put a small amount of broken porcelain pieces in the distillation flask to prevent the liquid from boiling. The position of the thermometer's mercury ball should be on the same horizontal line with the lower edge of the branch mouth. (3) The liquid contained in the distillation flask should not exceed 2/3 of its volume, nor should it be less than 1/3. The cooling water in the condensate tube enters from the lower port and exits from the upper port. The heating temperature must not exceed the boiling point of the highest boiling substance in the mixture.

The first distiller for extractive distillation

Archaeologists unearthed more than 440 Han tombs on the east side of Zhangjiabao Square in Xi'an. One of the higher-special Western Han dynasty tombs found a cup of strangely crafted copper still, which may be the earliest in history Distiller.

This cup of copper still has a height of 36 centimeters, and consists of a cylindrical shape, a copper spoon and a bean-shaped lid. Among them, the bottom of the cylinder has a Miguel-shaped cymbal, which is used as an interlayer in ancient cooking utensils. There is a small tubular flow at the bottom, copper-shaped three-hoof foot, the upper part of the bean-shaped device cover is disc-shaped, and the joint is a tenon-rivet structure, which can move freely within a certain range. Placed in an orderly manner when unearthed, the copper cymbals were placed in a cylinder, and the bean-shaped lid was placed on the copper cymbals. Such a combined still has never been discovered before, and although its working principle is not clear, from the structural point of view, it should be used as a distilling medicine and wine.

It is understood that previously, a bronze distiller from the Eastern Han Dynasty was unearthed in China, and the copper distiller found in the Han Tomb of Zhangjiapu in Xi'an was earlier. Cheng Linquan, deputy director of the Xi'an Institute of Cultural Relics and Archeology, said that the excavation provided valuable materials for the study of diet and medical technology in the Han Dynasty.

In addition, in the tomb number M115 where the distiller was unearthed, archeologists also unearthed more than 200 artifacts, including five large bronze tripods and four large glazed pottery tripods. According to Zhou Li, Emperor Tian was buried with Jiuding during the Western Zhou Dynasty. The tombowner of M115 admired the Zhou Dynasty etiquette and used Jiuding's funeral to show his special status. Jiuding and another funeral imitation copper-glazed pottery are real physical evidence of Wang Mangtuo's ancient reformation, which has extremely important academic value and historical significance.

More than 440 Han tombs excavated this time are located on the east side of Han Changan City, only 2,500 meters away from Changan City. A total of nearly 3,000 various cultural relics such as pottery, copper, iron, lead, jade, bone, etc. Pieces. These tombs are mainly small Han tombs, of which three medium-sized tombs from the late Western Han Dynasty to the New Mang period are the most important. Not only are common artifacts such as red painted pottery,

Enameled pottery, tripods, boxes, pots, warehouses, urns, stoves, etc. In the tomb M110, fragments of jade clothes were also unearthed. Large exquisite glazed pottery unearthed from tomb M114 is also rare in the tombs of the Han Dynasty in Xi'an. Experts said that this excavation provided important materials for studying the social life of the Han Dynasty and the layout of the Chang'an City in the Han Dynasty, and helped further interpret the formation and development of the Han culture.