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Basic Knowledge of Distillation Operations 1. What are phases and phase equilibrium? Answer: A phase refers to a homogeneous portion within a system that has identical physical and chemical properties. There is usually 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 circumstances, 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, causing the concentration of low-boiling-point components in the resulting gas phase to increase continuously. However, this process of heat and mass transfer does not go on 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 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 eventually stabilize at a fixed value; this pressure is known as the saturated vapor pressure of water at that temperature. 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, resulting in an equilibrium state 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 vapor produced by partial vaporization is partially condensed, since substances with high boiling points tend to condense easily, the concentration of these high-boiling-point 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 phenomenon where, as a liquid is heated, gas with a higher temperature is generated, and this gas flows upward in the opposite direction to the return liquid with a lower temperature (rich in low-boiling-point components) that is produced at the top of the tower due to condensation. The heat and mass transfer processes that occur within 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 formed by 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 several 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 field 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 for gas mixtures with different compositions, the dew point of the tower varies. 5. What is the bubble point? Answer: The temperature at which the first very small bubbles appear in a liquid mixture when it is heated to a certain temperature under a specific pressure, that is, the temperature at the onset of boiling, is called the bubble point temperature of that liquid at that pressure; it is simply referred to as the bubble point. A liquid at its bubble point temperature is called a saturated liquid, which is the temperature of the bottom of a distillation column. It should be noted that this first, very small bubble is not a pure component either; its composition is also determined by phase equilibrium relationships. 6. What is the boiling point? Answer: When the saturated vapor pressure of a pure liquid equals the external pressure, the liquid boils; the temperature at this point is called the boiling point of that liquid at the specified pressure. The boiling point of a pure substance changes with external pressure. When external pressure increases, the boiling point rises; when external pressure decreases, the boiling point falls. For a pure substance, at a certain pressure, the bubble point, dew point, and boiling point are all the same value. 7 What is latent heat? Answer: The pure substance per unit weight undergoes a phase change (a change in the phase state of a substance without any chemical reaction taking place; this is referred to as a phase change). Processes such as water turning into ice or water vaporizing into steam are phase change processes. The heat absorbed or released during a process is called latent heat. The heat absorbed when 1 kilogram of water changes from a liquid state to steam as a result of heating is called the latent heat of vaporization of water, with the common unit being kilocalories per kilogram. It is worth noting that both temperature and pressure remain constant during phase change; otherwise, it cannot be called latent heat. Therefore, when stating the value of latent heat, it is necessary to specify under what temperature and pressure what phase transition process is taking place. For example, when 1 kilogram of water vaporizes at a pressure of 760 millimeters of mercury and at 100 degrees Celsius, the latent heat of vaporization is 539.6 kilocalories. On the contrary, under these conditions, the heat released by the condensation of water vapor is called latent heat of condensation, and its value is equal to the previous one. The latent heat of the mixture can be measured or calculated; its value depends not only on the properties of the components but also on their concentrations, and it is not a fixed value. 8. What is sensible heat? Answer: The heat absorbed or released by a pure substance due to a change in temperature, without any phase changes or chemical reactions occurring, is called sensible heat. 9 What is the reflux ratio? Answer: In the distillation process, the vapor generated by heating the mixture exits at the top of the tower and enters the top condenser. Steam condenses here (or partially condenses) into a liquid; part of this condensed liquid is returned to the top of the tower and flows downward along the tray plates, and this liquid is known as reflux ； Another portion of the condensate (or uncondensed vapor) is taken from the top of the tower as a product. The reflux ratio is the weight ratio of the amount of reflux liquid to the amount of product removed, and it is usually denoted by R. That is, R = L/D, where R is the reflux ratio, and L is the amount of liquid flowing back to the top of the tower per unit time, in kilograms per hour. D – The amount of product removed from the top of the tower per unit time, in kilograms per hour. 10. What is the minimum reflux ratio? Answer: Under the specified requirements for separation accuracy, that is, when the composition of the material taken from the top and bottom of the tower remains constant, gradually reducing the reflux ratio results in an increase in the so-called number of theoretical plates. When the reflux ratio is reduced to a certain value, the number of theoretical plates required increases to an infinite amount; this value of the reflux ratio becomes the minimum reflux ratio necessary to accomplish the desired separation task. In normal operation, the actual reflux ratio is taken as 1.3 to 2 times the minimum reflux ratio.