Vacuum distillation and fractional distillation operations-------Experimental training

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Some time ago, the factory asked us to provide training on experimental procedures for new employees. I prepared some material and would like to share it with everyone. If there are any mistakes in what I’ve written, I hope everyone can offer their feedback! Your support is my greatest motivation! Principle of vacuum distillation: The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure; therefore, the boiling point changes as the external pressure changes. By using a vacuum pump to reduce the pressure within the system, it is possible to lower the boiling point of the liquid, and this is the theoretical basis for vacuum distillation. Vacuum distillation is one of the common methods for separating and purifying organic compounds. It is particularly suitable for substances that undergo thermal decomposition, oxidation, or polymerization before reaching their boiling point during atmospheric distillation. Prerequisite: You need to have a thorough understanding of the properties of what you are distilling. The boiling point of a substance at a certain pressure is fixed. The lower the pressure, the lower the boiling point generally is. Therefore, depressurization is used to lower the boiling point of the substance to be processed, allowing it to boil and evaporate at a lower temperature. When steaming something, the temperature depends first on the pressure; once the pressure is determined, the corresponding boiling point can be found. Then, taking into account factors such as the speed of distillation and the purity of the substance to be distilled, a temperature lower than its boiling point is chosen. Additionally, some mixtures form azeotropes, so it’s advisable to have access to phase diagrams for reference. Specific methods for vacuum distillation: 1) Gathering glassware: Similar to normal pressure distillation, the difference is that vacuum distillation requires a 3-port or 4-port adapter. 2) Preheat the oil bath or heating jacket. If the boiling point of the distillate is unknown, this step should be omitted. Remember, in most cases, the temperature of the heat source needs to be 20–30°C higher than the boiling point of the distillate. Note: Due to thermal decomposition and potential fire hazard, the oil bath should only be used when the heating temperature is below 200°C. 3) Record the weight of the labeled receiving bottle. 4) Place the material to be distilled in a round-bottom flask equipped with a stirrer (the stirrer is used to prevent bumping). It is very important to choose the size of the round-bottom flask. It is advisable to fill the bottle with the solvent to 1/2 to 2/3 of its capacity; if the liquid level is too high, boiling will occur prematurely, while if it is too low, distillation will take too long. 5) Assemble all glassware, making sure to apply grease to all joints. Be careful to conserve vacuum grease, as it is quite expensive; moreover, you certainly don’t want it to end up in your products. 6) Insulation of the distillation column. When using Wiegler columns, the columns should be wrapped with glass wool or aluminum foil. If no insulation is applied, distillation will take a very long time. 7) Connect the condenser tube to the water supply pipe, turn on the faucet, and check for leaks. 8) Do not start heating! ! ! 9) Slowly evacuate the distillation apparatus. You should be able to see the liquid starting to bubble. Don’t worry, everything is fine. At room temperature and under reduced pressure, the residual solvents and low-boiling-point impurities will be quickly vaporized. 10) Once the foam decreases, or slows down to almost stop, you can start heating it. 11) Lower the fume hood baffle. This helps to prevent accidental injuries, and it also protects the distillation apparatus from being affected by the laboratory’s air conditioning and ventilation systems. Air conditioning and cold air will lower the temperature of the distillation apparatus and prolong the distillation time. 12) Do not heat too quickly! ! ! Patience is the key to distilling success. 13) Gradually increase the temperature of the heater until the solution begins to reflux. 14) Wait and observe the changes in the distillation thermometer. If no temperature change is observed after 10 minutes, the temperature should be increased slightly. 15) Repeat step 14 until a change is observed on the thermometer. Once there is a change, prepare to collect the fraction. 16) Maintain a constant temperature in the distillation apparatus. Keep the recorded distillation temperature within a fluctuation of at most 5°C. 16) Collect the fractions until a temperature change occurs. Usually, when the distillation of a fraction is complete, the temperature shown on the distillation thermometer will drop. At this point, you should replace the receiving bottle or stop distillation altogether. 17) Release the vacuum. Even after you have collected the desired products, you still cannot lower the temperature of the heating device. First, you must release the vacuum. But before doing this, make sure that all receiving bottles are secured to the device using clips, interface clamps, or your hand. You don’t want to see the product receiving bottle shatter after the vacuum is removed, right! If everything is ready, introduce nitrogen into the device, then remove the heat source and allow the device to cool to room temperature. 18) After all the items have cooled down, weigh the receiving bottle to determine the weight of the product. Precautions for vacuum distillation: 1. A capillary tube is used to serve as the vaporization point; zeolite is not very effective for this purpose. Of course, for substances that are prone to oxidation, nitrogen can be passed through the capillary tube to provide protection. 2. A magnetic stirrer-equipped oil bath can also be used; it is convenient, ensures stable heating, and allows control over the stirring speed. 3. For simple vacuum distillations, a liquid nitrogen cold trap can be added at the back; for more complex cases, as mentioned earlier, paraffin columns, calcium chloride, and sodium hydroxide columns are needed to protect the vacuum pump. 4. If the temperature of the distillate keeps rising during the distillation process and purification is not possible, an additional fractionation column can be used; if that doesn’t work, a rectification column should be employed ； To control the rate of temperature increase, use gradient heating ； Create a vacuum before distillation; once the vacuum is stable, gradually increase the temperature. Precautions for operating the oil-control pump in vacuum distillation: The connection sequence of the vacuum distillation apparatus is as follows: distillation unit interface, vent bottle, U-shaped pressure gauge, absorption bottle, cold trap, calcium chloride drying tower, alkali tower, paraffin sheet drying tower, and oil pump. Among them, the absorption bottle can be omitted if it is not necessary. The liquid used in the commonly employed absorption flask is concentrated sulfuric acid. (It can absorb a large amount of water and organic matter!) I think it’s easier to replace the absorption bottle than to replace the three drying towers containing calcium chloride, alkali, and paraffin tablets, so that’s what I usually do. 1. At the end of distillation, don’t open the vent immediately. Instead, use medical hemostatic forceps (or a C-clamp if none are available) to clamp between the cold trap and the calcium chloride drying tower, then slowly open the vent. The purpose of this is to prevent too much air from entering the oil pump, thereby protecting the oil pump ； B is opened directly without being clamped; due to the high inlet velocity, it is easy for the liquid in the gas scrubber bottle and cold trap to be flushed into the next bottle. Once the situation ahead stabilizes, do not move the hemostat; instead, pull it out from the location of the calcium chloride drying tower (a tee fitting can be installed at this location to make the operation easier). Allow air to flow in, and then turn off the oil pump immediately. 2. If the product obtained through distillation is unstable – for example, acyl chlorides are sensitive to water, and other substances that require protection under nitrogen – follow the same procedure at the end of distillation. Before opening the vent, connect an air inlet of the vent to a bag filled with high-purity nitrogen (I used medical oxygen bags; if a high-purity nitrogen cylinder is used, it’s difficult to control the gas pressure, which can easily cause the stoppers and thermometers to be blown off, but this does not happen when using nitrogen bags). Nitrogen can be used in place of air. At this point, hemostatic forceps (or clip-type tools) are essential, and they should be tightened as much as possible to avoid wasting nitrogen. By doing this, cooling and nitrogen protection are achieved. 3. If there is a large amount of low-boiling substances that can be easily pumped away by the oil pump, 2–3 cold traps can be installed ； To achieve good cooling effects, cryopumps are usually cooled using ice-salt. For high-vacuum pumps, which require proper protection, cooling with dry ice-propane (or liquid nitrogen) can also be considered. 4. The cold trap of the oil pump’s cooling system can be cooled by a freezer, which is economical and cost-effective. There is no need for complicated methods such as ice salt baths or dry ice + propane either. The guy in my lab next to me often uses high-boiling-point solvents such as DMSO and DMF, so he uses a freezer to cool the cold trap. A general temperature of -30 degrees is sufficient. It works very well! ! 5. The cold trap must be cleaned promptly. It’s best to check after using it each time. If there is liquid inside, it should be cleaned up promptly to prevent solvents from entering the oil pump and affecting the distillation process. 6. The oil pump also needs to have its oil changed regularly. If, during experiments, the same temperature is observed and the same oil pump is used yet the distillation effect is poor, it is likely that the oil in the pump is no longer effective. Changing the oil in a timely manner will prevent it from affecting your experiments. 7. When using an oil pump, it is also important to take care of its maintenance; do not use it for half a day straight. It needs to rest in between as well; even people can’t keep working non-stop, and the same goes for her. Fractionation: The basic principle of fractionation is the same as that of distillation; it utilizes the differences in volatility among the various components in a liquid mixture. The mixture is partially vaporized and then the vapor is partially condensed, thereby enabling the separation of the constituent components. It is a unit operation that falls under the category of mass transfer separation processes. The difference is that distillation, with the help of a distillation column, allows a series of distillation steps to be completed in just one operation, rather than having to be repeated multiple times (distillation itself involves multiple distillation steps). Its scope of application also differs: for separation to be possible, the boiling points of the various components in the mixed liquid must differ by more than 30°C, while for complete separation, this difference needs to be over 110°C. Fractionation can be used to separate and purify mutually soluble liquid mixtures with similar boiling points (even those with a difference of only 1–2°C in boiling point). Precautions for distillation: (1) Distillation must be carried out slowly, with a constant distillation rate of 1–2 drops per second; this will yield better distillation results. (2) To allow a considerable amount of liquid to flow back into the flask along the column, it is necessary to select an appropriate reflux ratio, so that the rising gas stream and the descending liquid can exchange heat effectively, enabling the more volatile components to rise while the less volatile components descend as much as possible, thereby improving the distillation efficiency. (3) Heat loss and fluctuations in the fractionation column must be minimized as much as possible. The perimeter of the column can be wrapped with asbestos rope, which helps to reduce the heat loss from within the column. It also minimizes the impact of wind and room temperature, thereby reducing heat loss and fluctuations and ensuring uniform heating as well as a smooth distillation process. Common experimental phenomena and solutions: 1. The temperature is very high, but no distillate is produced. Check whether the apparatus is leaking air (until it is properly sealed and the vacuum level meets the requirements); verify that the apparatus is well insulated (use cotton for insulation, and turn off the fume hood, air conditioner, windows, etc.). Issues such as too little material, a distillation flask that is too large, or an excessively high efficiency of the distillation column may also be present – in such cases, rebuild the apparatus by using smaller, more suitable glass instruments. Other solutions include using a heating mantle to carry out initial high-temperature distillation (suitable for substances with a complex composition), followed by another round of distillation. 2. Shock material. Follow the procedures after distillation is complete: stop the distillation, transfer the material that has been flushed back into the distillation flask, use clean condensers and receiving flasks, and start distilling again. 3. Power outage. Under normal circumstances, at the beginning of a power outage, there is negative pressure throughout the entire device, and the power outage has no effect on it. However, it is important to prevent the oil pump from sucking in air even when it is idle, and to ensure that all receiving containers are properly placed. 4. The condensate solidifies at the mouth of the condenser tube. Preheat the outer wall of the condenser tube with a hair dryer to melt the solid; this can be done by using air for condensation or by circulating hot water inside the condenser tube. Make sure the receiving bottle is cooled to ensure the product is fully received and not drawn away.