Some learning material on “distillation”

Thread Content

I. Definition of distillation: A process in which partial vaporization and partial condensation are carried out multiple times and simultaneously, in order to achieve a relatively complete separation of a mixture and obtain components that are nearly pure. Theoretically, multiple partial vaporizations in the liquid phase can yield highly pure non-volatile components, while multiple partial condensations in the gas phase can yield highly pure volatile components; however, a large number of intermediate components are produced, resulting in very low yields of the desired products and requiring large-scale equipment. The distillation process in industrial production is carried out in a distillation tower by organically combining both vaporization and condensation processes. II. Principles of distillation: By taking advantage of the differences in volatility among the various components in a mixture, and through the recycling of liquid and gas phases, the gas and liquid phases come into contact with each other in multiple stages in opposite directions. Under the influence of thermal energy and the constraints imposed by phase equilibrium, the more volatile components (light components) are continuously transferred from the liquid phase to the gas phase, while the less volatile components move from the gas phase to the liquid phase. This process enables the continuous separation of the mixture, and it is known as distillation. In this process, heat transfer and mass transfer occur simultaneously, which is governed by mass transfer control. Its distillation tower is shown in Figure 1. The feed enters the tower at an appropriate position in its middle, dividing the tower into two sections: the upper section is the distillation section, which contains no feed, while the lower section, which includes a feed plate, is the retention section. A condenser provides liquid-phase reflux from the top of the tower, and a reboiler provides gas-phase reflux from the bottom of the tower. Gas and liquid phase reflux are important characteristics of distillation. In Figure 1, during the distillation section, as the gas phase rises, its light components are continuously purified and become more concentrated in the gas phase, with the light component product being obtained at the top of the tower. In the stripping section, as the liquid phase moves downward, the light components are continuously stripped off, causing the heavy components to become increasingly concentrated in the liquid phase; thus, the heavy-component product is obtained at the bottom of the tower. The biggest difference between the distillation process and other distillation methods is that high-purity liquid and gas refluxes are provided at both ends of the tower, thereby creating the necessary conditions for mass transfer in the distillation process. It provides high-purity reflux, ensuring a certain mass transfer driving force at all times when achieving high-purity separation through distillation under the same number of theoretical plates. Therefore, as long as there are enough theoretical plates and a sufficient reflux rate, a high-purity light-component product can be obtained at the top of the tower, while a high-purity heavy-component product can be obtained at the bottom of the tower. III. Distillation Unit Process The typical distillation equipment is a continuous distillation unit, which includes a distillation tower, condenser, reboiler, etc. There are two types of tower equipment used for distillation: tray towers and packed towers, but tray towers are more commonly used. In continuous distillation, the feed liquid is continuously fed into the distillation column, while products (distillate and bottom residue) are continuously obtained from the top and bottom of the column; therefore, it is a steady-state operation process. IV. Function of the distillation unit: The distillation column is divided into two sections by a feed plate – the distillation section and the stripping section. 1. Function of the distillation section: The section of the tower above the feed plate is the distillation section, whose function is to increase, plate by plate, the concentration of the volatile components in the rising vapor phase. 2. Function of the stripping section: The following trays, including the feed tray, constitute the stripping section; its function is to extract the volatile components from the descending liquid phase, tray by tray. 3. Function of the tray: The tray is a place where mass and heat transfer occur between the gas and liquid phases. On each tray, mass transfer occurs in both directions between the gas and liquid phases; with a sufficient number of trays, it is possible to separate the mixture into two relatively pure components. 4. Function of the reboiler: Its role is to provide a certain flow rate of rising vapor stream. 5. Function of the condenser: Its role is to provide the liquid-phase product at the top of the tower and to ensure an appropriate amount of liquid-phase reflux. Reflux mainly replenishes the concentration of volatile components on the tray, and it is a necessary condition for the continuous and steady-state operation of distillation. Distillation is a distillation method that uses reflux to achieve high-purity separation of mixtures. This post was last edited by feng*nzi on 2009-2-7 15:56.]