Questions for January 6, 2018; replies can earn 0–6 points (no scoring for replies sent more than 7 days after posting. Thank you for your replies)

The method of separating the various components in a liquid or gaseous mixture by taking advantage of their differences in volatility or boiling point is called distillation. Prerequisites for distillation: (1) The gas and liquid phases must be in thorough contact; the multiple layers of trays in a distillation column serve to ensure such thorough contact between the gas and liquid phases. (2) When the gas and liquid phases are in contact, the concentration of light components in the rising hot gas phase is higher than the equilibrium concentration, while the concentration of light components in the descending cold liquid phase is lower than the equilibrium concentration. Heat and mass transfer can occur due to the imbalance between the gas and liquid phases, as well as the presence of a temperature difference, thereby enabling distillation.

1. The temperature of the gas phase is higher than that of the liquid phase; 2. The concentration of low-boiling-point components in the liquid phase is higher than the concentration at equilibrium in the gas phase; 3. There is a place where the gas and liquid phases can come into full contact

The method of separating the various components in a liquid or gaseous mixture by taking advantage of their differences in volatility or boiling point is called distillation. Prerequisites for distillation: (1) The gas and liquid phases must be in thorough contact; the multiple layers of trays in a distillation column serve to ensure such thorough contact between the gas and liquid phases. (2) When the gas and liquid phases are in contact, the concentration of light components in the rising hot gas phase is higher than the equilibrium concentration, while the concentration of light components in the descending cold liquid phase is lower than the equilibrium concentration. Heat and mass transfer can occur due to the imbalance between the gas and liquid phases, as well as the presence of a temperature difference, thereby enabling distillation.

Prerequisites for distillation: (1) The gas and liquid phases must be in thorough contact; the multiple layers of trays in a distillation column serve to ensure such thorough contact between the gas and liquid phases. (2) When the gas and liquid phases are in contact, the concentration of light components in the rising hot gas phase is higher than the equilibrium concentration, while the concentration of light components in the descending cold liquid phase is lower than the equilibrium concentration. Heat and mass transfer can occur due to the imbalance between the gas and liquid phases, as well as the presence of a temperature difference, thereby enabling distillation.

1. The temperature of the gas phase is higher than that of the liquid phase; 2. The concentration of low-boiling-point components in the liquid phase is higher than the concentration at equilibrium in the gas phase; 3. There is a place where the gas and liquid phases can come into full contact

Prerequisites for distillation: (1) The gas and liquid phases must be in thorough contact; the multiple layers of trays in a distillation column serve to ensure such thorough contact between the gas and liquid phases. (2) When the gas and liquid phases are in contact, the concentration of light components in the rising hot gas phase is higher than the equilibrium concentration, while the concentration of light components in the descending cold liquid phase is lower than the equilibrium concentration. Heat and mass transfer can occur due to the imbalance between the gas and liquid phases, as well as the presence of a temperature difference, thereby enabling distillation.

1) The gas and liquid phases must be in full contact, and the multiple trays installed in the distillation tower serve to ensure such full contact between the gas and liquid phases; (2) When the gas and liquid phases are in contact, the concentration of light components in the rising hot gas phase is higher than the equilibrium concentration, while the concentration of light components in the descending cold liquid phase is lower than the equilibrium concentration. Heat and mass transfer can occur due to the imbalance between the gas and liquid phases, as well as the presence of a temperature difference, thereby enabling distillation.

1. The gas phase temperature is higher than the liquid phase temperature； 2. The concentration of low-boiling-point components in the liquid phase should be higher than the concentration at equilibrium with the oil and gas, and the concentration of high-boiling-point components in the gas phase should also be higher than the concentration at equilibrium with their liquid phase ； 3. A place where the gas and liquid phases can come into full contact.

The relative volatilities of the various components in the mixture differ, and this is the primary condition for distillation separation. The greater the difference in relative volatilities among the components, the easier it is to separate them; the smaller this difference, the more difficult separation becomes. If there is no difference at all, then distillation cannot be used for separation. b. The vapor and liquid phases must have differences in component concentrations as well as temperature differences between them; the concentration of light components in the liquid phase flowing to a particular tray must be higher than that of the equilibrium liquid phase on that tray. The temperature of the vapor in contact at the same time must be higher than the temperature of the equilibrium liquid phase; it is the presence of concentration differences and temperature differences between the phases that creates an unbalanced state, thereby generating a driving force for material and heat exchange between the phases. c. To create differences in the concentration of the various components as well as temperature differences, it is necessary to have countercurrent contact between the liquid phase and the vapor phase. Condensed product is introduced at the top of the tower, providing a liquid reflux with a high concentration of light components and a lower temperature. This enables liquid reflux with a high concentration of light components and a temperature lower than the equilibrium temperature to be generated in each layer of the tower. The reboiler at the bottom provides countercurrent contact between the rising vapor phase and the descending liquid phase; this liquid phase has a high concentration of heavy components and a higher temperature. Its concentration of heavy components is always higher than that of the equilibrium liquid phase, and its temperature is also higher than the equilibrium temperature, which facilitates the smooth progress of the distillation process. d. Facilities for full contact between vapor and liquid—such as tray plates—are necessary to ensure the occurrence of mass and heat transfer processes. e. The properties of liquid hydrocarbons determine that their separation must be carried out under the same conditions of pressure or cryogenic cooling in order to ensure proper separation.

Answer: (1) The distillation process relies primarily on multiple stages of vaporization and condensation to separate a liquid mixture; therefore, a significant difference in the relative volatility of the various components within the liquid mixture is a prerequisite for carrying out the distillation process. When the volatilities of the components in a mixture are very similar (such as in a C4 fraction mixture), separation can be achieved by adding a solvent to form an non-ideal solution, using azeotropic distillation or extractive distillation; at this point, there are significant differences in the relative volatilities of the various components within the resulting non-ideal solution. (2) Reflux liquid with a high concentration of light components is added at the top of the tower, while hot steam is generated at the bottom of the tower by heating or stripping. (3) The tower should be equipped with trays or packing, so that the steam with a higher temperature and a higher content of heavy components rising from the lower part comes into contact with the liquid phase with a lower temperature and a higher content of light components descending from the upper part, thereby enabling heat and mass transfer to occur. The heavier components in the steam are condensed into a liquid, and the heat released enables the lighter components in that liquid to vaporize. The vapor stream within the tower passes through multiple condensation stages from bottom to top, resulting in a decreasing concentration of light components; thus, a high-concentration heavy product can be obtained at the bottom of the tower.