Distillation is a thermodynamic separation process, which uses the different boiling points of each component in the mixed liquid or liquid-solid system to evaporate the low boiling point components and then condenses to separate the whole component. it is a combination of evaporation and condensation unit operations.
When the liquid is heated, its vapor pressure Pi increases with the increase of temperature. When the vapor pressure increases to the same pressure as the external pressure applied on the liquid surface, there will be a large number of bubbles escaping from the liquid, that is, liquid boiling. The temperature at this time is called the boiling point of the liquid.
Distillation Operation
- Feeding
Carefully pour a certain volume of distilled liquid into the distillation bottle through a glass funnel. Add a few boiling aids and plug the thermometer casing and thermometer. Once again check whether the various parts of the instrument are connected tightly and properly.
- Heating
Turn on the condensate water switch, water flows into the water tank from the upper end of the condensate pipe, and then starts heating. When heating, it can be seen that the liquid in the distillation bottle gradually boils, the steam rises gradually, and the reading of the thermometer also rises slightly.
When the top of the vapor reaches the mercury bulb of the thermometer, the thermometer reading rises sharply. At this time, the heating rate should be appropriately reduced to heat the upper part of the bottleneck and the thermometer, so that the temperature of the liquid droplets and vapor on the mercury bulb reaches equilibrium.
Control the heating temperature and adjust the distillation rate, usually 1-2 drops per second is advisable. During the whole distillation process, the mercury bulb of the thermometer should be kept dripping with condensed liquid.
The temperature at this point is the temperature at which the liquid and vapor are in equilibrium. The temperature indicated by the thermometer is the boiling point of the liquid.
The heating flame during distillation should not be too large, otherwise it will cause overheating in the neck of the retort, and the boiling point read by the thermometer will be on the high side. Low or irregular readings cannot be caused by sufficient wetting of the distillate vapor.
- Before Distillation
Before distillation, at least two receiving flasks should be prepared. Because before the boiling point of the desired substance is reached, the liquid with a lower boiling point is often evaporated first. After the previous fraction is evaporated and the temperature tends to be stable, the purer substance is evaporated. At this time, a clean and dry receiving bottle should be replaced for acceptance. And write down the reading of the thermometer when this part of the liquid starts to pour out and the last drop, which is the boiling range of the distillate.
After the required distillate is distilled, if the original heating temperature is maintained, there will be no more distilled liquid, and the temperature will drop suddenly, which means that the distillation should be stopped. Even if the impurity content is very small, do not evaporate to dryness, so as not to break the distillation bottle and other accidents.
- Distillation Completed
After the distillation is completed, the heating should be stopped first, then the water flow should be stopped, and then the instrument should be dismantled. The order of dismantling the instrument is reverse to that of assembling, that is, remove the receiver first, then remove the liquid pipe, condenser pipe, distillation head and distillation bottle, etc.
- Boiling Range
The boiling range of a liquid can often represent its purity, and the boiling range of a pure liquid generally does not exceed 1~2°C.
