Laboratory distillation operation and principle of the new distillation unit

- Dec 14, 2018-

Laboratory distillation operation and principle of the new distillation unit

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


Different component liquids have different boiling points and are the most basic theoretical basis for distillation. At the same temperature, the single-phase liquid of each component vapor above the liquid, when the liquid A and B are mixed in a certain proportion (the concentration of A is c1), after heating, reaches the boiling point line, and then passes through no difference. The gas-liquid equilibrium (from 1 to 2), in the resulting gas phase, a mixture of A having a content of c2 is obtained; the gas is collected by cooling and collected to obtain a liquid having a new composition (A concentration of c2). According to the legend, the A component on the left side is purified (because C2 is greater than c1).

Basic classification

Simple distillation, such as making distilled water to remove solid impurities dissolved therein; making distilled liquor to concentrate alcohol to remove part of the water; rectification, also called fractionation, multiple partial vaporization and partial condensation in one apparatus to separate the liquid mixture, For example, the fractionation of petroleum can separate various components such as gasoline, diesel, kerosene and heavy oil.

Simple distillation, such as making distilled water to remove solid impurities dissolved therein; making distilled liquor to concentrate alcohol to remove part of the moisture;

Distillation, also known as fractionation, performs multiple partial vaporizations and partial condensations in a single unit to separate liquid mixtures. For example, fractionation of petroleum can separate gasoline, diesel, kerosene, and heavy oil components.

Laboratory distillation operation

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

(1) Separating the liquid mixture to achieve a more effective separation only when there is a large difference in the boiling points of the components in the mixture;

(2) determining the boiling point of the pure compound;

(3) Purification, by purifying the substance containing a small amount of impurities, to improve its purity;

(4) The solvent is recovered, or a part of the solvent is distilled off to concentrate the solution.

Feeding: Carefully pour the liquid to be distilled into the distillation flask through a glass funnel, taking care not to allow liquid to flow out of the branch pipe. Add a few boil-offs and set up the thermometer. The thermometer should be installed at the side of the side that leads to the condenser. Once again, check that the parts of the instrument are tightly connected and properly connected.

Heating: When condensing the water with water, the cold water is slowly introduced from the lower end of the condensing pipe, and the water is discharged from the upper port to the water tank, and then heating is started. When heated, it can be seen that the liquid in the distillation flask gradually boils and the vapor gradually rises. The thermometer reading also increased slightly. When the tip of the vapor reaches the mercury ball of the thermometer, the thermometer reading rises sharply. At this time, the flame of the small gas lamp should be adjusted appropriately or the voltage of the heating electric furnace or the electric heating sleeve should be lowered, so that the heating speed is slightly slowed down, the steam tip stays in the original place, the upper part of the bottle neck and the thermometer are heated, and the temperature of the droplets and the vapor on the mercury ball are balanced. . Then slightly increase the flame and carry out the distillation. The heating temperature is controlled, and the distillation speed is adjusted, usually 1 to 2 drops per second. During the entire distillation process, the thermometer mercury spheres should always have condensed droplets. The temperature at this time is the temperature at which the liquid and the vapor are in equilibrium, and the reading of the thermometer is the boiling point of the liquid (distillate). The flame heated during distillation should not be too large, otherwise it will cause overheating in the neck of the distillation bottle, so that a part of the liquid vapor is directly subjected to the heat of the flame, so that the boiling point read by the thermometer will be higher; on the other hand, the distillation is also It cannot be carried out too slowly, otherwise the mercury ball of the thermometer cannot be sufficiently wetted by the distillate vapor to make the boiling point read on the thermometer low or irregular.

Observe the boiling point and collect the distillate: Prepare at least two receiving bottles before distilling. Because the liquid with a lower boiling point is first distilled out before reaching the boiling point of the desired material. This part of the distillate is referred to as "front fraction" or "distillate". After the distillation of the former fraction is completed and the temperature tends to be stable, the purer substance is distilled out. At this time, a clean and dry receiving bottle should be replaced, and the reading of the thermometer when the liquid starts to be distilled and the last drop is recorded. It is the boiling range (boiling point range) of this fraction. Generally, the liquid contains more or less high-boiling impurities. After the required fraction is distilled out, if the heating temperature is further increased, the reading of the thermometer will rise significantly. If the original heating temperature is maintained, it will not be repeated. When the distillate is distilled off, the temperature will suddenly drop. At this point, the distillation should be stopped. Even if the amount of impurities is extremely small, do not evaporate to avoid rupture of the distillation bottle and other accidents.

After the distillation is completed, the heating should be stopped first, then the water is stopped and the instrument is removed. The order in which the instruments are removed is reversed to the order in which they are assembled. Remove the receiver and remove the tail pipe, condenser, distillation head, and retort.

Pay attention to the operation: (1) Put a small amount of broken ceramics in the distillation flask to prevent the liquid from boiling. (2) The position of the thermometer mercury ball should be on the same horizontal line as the lower end of the branch pipe. (3) The liquid contained in the distillation flask should not exceed 2/3 of its volume, nor less than 1/3. (4) The cooling water in the condensing pipe enters from the lower mouth and the upper port. (5) The heating temperature shall not exceed the boiling point of the highest boiling point substance in the mixture.