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1. What are phase harmony and phase equilibrium? Answer: A phase refers to a homogeneous portion within a system that possesses the same physical and chemical properties. There is often a phase boundary between different phases, which separates them from one another. The number of phases in a system is independent of the amount of substance. When water and ice are mixed together, water is in the liquid phase while ice is in the solid phase. Under normal conditions, the material in a distillation tower exists in both gas and liquid phases. At a certain temperature and pressure, if there are two or more phases in a material system, and the relative amounts of the material in each phase as well as the concentrations of various components within each phase remain constant over time, we say that the system is in equilibrium. At equilibrium, matter continues to move, but the amounts of each phase and the concentrations of various components remain constant over time. When conditions change, a new phase equilibrium is established; therefore, phase equilibrium is dynamic and relative, rather than static and absolute. For example, in a distillation system, when gas with a higher temperature and liquid with a lower temperature come into contact on the distillation column trays, heat and mass transfer occur; as a result, part of the gas condenses, and the concentration of high-boiling-point components in the resulting liquid phase increases continuously. The liquid portion on the tray vaporizes, and the concentration of the low-boiling-point components in the resulting gas phase continues to increase. However, this process of heat and mass transfer does not continue indefinitely; when the gas and liquid phases reach equilibrium, the composition of each component in both phases no longer changes over time. 2. What is the saturated vapor pressure? Answer: At a certain temperature, the pressure exerted by steam that is in equilibrium with the liquid (or solid) form of the same substance is called the saturated vapor pressure, and this pressure increases as the temperature rises. As is well known, the water in a cup gradually decreases as it keeps evaporating. If pure water is placed in a sealed container and the air above it is removed, as the water continues to evaporate, the pressure of the gas phase above the water surface, that is, the pressure exerted by the water vapor, increases continuously. However, when the temperature is constant, the vapor pressure will ultimately stabilize at a fixed value; this pressure is known as the saturated vapor pressure of water at that temperature. The saturated vapor pressure is an important property of a substance, and its value depends on the nature of the substance and the temperature. The higher the saturated vapor pressure, the more volatile the substance is. It should be noted that when the value of the vapor pressure reaches that of the saturated vapor pressure, water molecules in the liquid phase continue to vaporize, and water molecules in the vapor phase continue to condense into a liquid. It is only because the rate of vaporization of water equals the rate of condensation of water vapor that the amount of liquid does not decrease and the amount of gas does not increase; thus, a balance is achieved between the gas and the liquid. Therefore, when the pressure of the vapor of a liquid pure substance equals its saturated vapor pressure, phase equilibrium is achieved between the gas and liquid phases. 3. What is distillation, and what is its principle? Answer: The process of separating a liquid mixture into its desired components by subjecting the mixture to multiple partial vaporizations and simultaneously subjecting the resulting vapor to multiple partial condensations is called distillation. Why can a liquid mixture be separated into pure or relatively pure components by undergoing multiple partial vaporizations followed by multiple partial condensations? In the case of vaporization and condensation, due to the different boiling points of the components present in the liquid mixture, when it partially vaporizes at a certain temperature, the substances with lower boiling points tend to vaporize more easily; as a result, their concentration in the gas phase is higher than in the liquid phase. Conversely, the substances with higher boiling points have a higher concentration in the liquid phase than in the gas phase. This changes the composition of the gas-liquid phase. When part of the steam obtained from partial vaporization is partially condensed, since high-boiling-point substances tend to condense easily, the concentration of these substances in the condensed liquid is higher than in the gas phase; whereas the concentration of low-boiling-point substances in the condensed gas is higher than that in the condensed liquid. In this way, through partial vaporization and partial condensation, the mixture is initially separated as a result of changes in the concentrations of its various components. If this is repeated multiple times, essentially the high-boiling-point components will remain in the liquid phase, while the low-boiling-point components will remain in the gas phase. It can be seen that by carrying out multiple partial vaporizations and multiple partial condensations simultaneously, the mixture can be separated into pure or relatively pure components. Liquid vaporization absorbs heat, while gas condensation releases heat. To make rational use of heat, we can utilize the heat released during the condensation of gas to vaporize liquid, that is, by bringing the gas and liquid phases into direct contact to enable both heat transfer and mass transfer to occur simultaneously. To meet this requirement, in practice, this process of multiple partial vaporizations accompanied by multiple partial condensations takes place in plate-type equipment with counterflow operation. The so-called counterflow refers to the higher-temperature gas generated by the heating of the liquid, which flows upward in opposition to the lower-temperature return liquid (rich in low-boiling-point components) produced at the top of the tower due to condensation. The heat and mass transfer processes that occur inside the tower are as follows: 1) Heat is exchanged between the gas and liquid phases, with the heat contained in the gas mixture resulting from partial vaporization being used to heat the liquid mixture resulting from partial condensation ； 2) Mass exchange occurs simultaneously with heat exchange between the gas and liquid phases. The liquid mixture at a lower temperature is heated by the gas mixture at a higher temperature, causing it to vaporize in part. At this point, due to the difference in volatility (substances with low boiling points have higher volatility while those with high boiling points have lower volatility), substances with low boiling points evaporate more than those with high boiling points. As a result, the low-boiling-point components transition from the liquid phase to the gas phase, causing an increase in the concentration of volatile components in the gas phase ； Similarly, in a gas phase mixture with a higher temperature, the heating of the liquid mixture with a lower temperature causes part of the gas phase to condense. Again, due to differences in volatility, the components with higher boiling points transition from the gas phase to the liquid phase, resulting in an increase in the concentration of those less volatile components in the liquid phase. A distillation column is composed of multiple trays; the top part of the column is called the top, while the bottom part is called the bottom. In a tower, each tray undergoes partial vaporization and partial condensation only once; the more trays there are, the more times partial vaporization and partial condensation occur, resulting in better separation efficiency. Throughout the entire distillation process, high-purity volatile components are obtained at the top of the tower, while the bottom of the tower yields mainly non-volatile components. 4. What is dew point? Answer: A gas mixture is cooled at constant pressure; when it is cooled to a certain temperature, the first tiny droplets of liquid form. This temperature is known as the dew point temperature of the mixture at that pressure, or simply the dew point. A gas at its dew point temperature is called a saturated gas. The temperature of the gas evaporating from the top of the distillation tower is at the dew point temperature. It is worth noting that the first wild zone is not a pure component; it is the liquid phase in equilibrium with the gas phase at the dew point temperature of the column, and its composition is determined by the phase equilibrium relationship. It can be seen that gas mixtures with different compositions have different dew points for the tower. In 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 away, causing the heavier components to become increasingly concentrated in the liquid phase; the product containing these heavier components is obtained at the bottom of the tower. Changes in the composition of the feed directly affect the distillation process – when the concentration of heavier components in the feed increases, the load on the stripping section increases as well. For a tower with a fixed number of plates in the rectification section, this will result in the heavier components being carried to the top of the tower, causing the quality of the product at the top to be unsatisfactory. If the concentration of the light components in the feed increases, the load on the stripping section increases at that time. For a column with a fixed number of plates in the stripping section, this will result in incomplete evaporation of the light components in the stripping section, leading to increased loss of light components in the bottom liquid. At the same time, changes in the feed composition will also cause changes in the material balance of the entire tower and the process conditions. As the composition becomes lighter, the overhead distillate increases while the amount of liquid discharged from the reactor decreases. At this point, the temperature of the entire tower decreases while the tower pressure increases. The component becomes heavier; the situation is the opposite. When the composition of the feed changes, the following measures can be taken: (1) Improve the feed inlet; when the composition becomes heavier, move the feed inlet downward ； When the component becomes lighter, move the feed inlet upward. (2) Changing the reflux ratio: When the components become heavier, increase the reflux ratio ； When the component becomes lighter, reduce the reflux ratio. (3) Adjusting the refrigerant and heat input: Based on changes in the composition, the refrigerant used in the top condenser and the heat input at the bottom of the tower are adjusted accordingly to maintain constant product quality at both the top and bottom of the tower. (The distillation section is intended to remove the light components) ； The distillation section is used to refine and purify the light components; therefore, as the amount of light components in the feed increases, the load on the stripping section also increases ； As the Reorganized components increases, the burden on the distillation section increases. )