Methanol distillation

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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 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 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 gas phase pressure reaches that of the saturated vapor pressure, water molecules in the liquid phase continue to vaporize, and water molecules in the gas 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. ;h+ H3 m" \5 e" K/ q 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 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 more easily, the concentration of these high-boiling-point substances in the condensed liquid is higher than in the gas phase. Conversely, 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, ultimately, the components with high boiling points will remain in the liquid phase, while those with low boiling points 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. m8t5 ~0 Q9 \( When a liquid vaporizes, heat is absorbed; when a gas condenses, heat is released. To make rational use of heat, we can utilize the heat released during the condensation of gas to power the vaporization of liquid, that is, by bringing the gas and liquid phases into direct contact to enable both heat transfer and mass transfer. To meet this requirement, in practice, this process of multiple partial vaporizations accompanied by multiple partial condensations takes place in counter-current plate-type equipment. 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 within the tower are as follows: 1) Heat is exchanged between the gas and liquid phases, with the heat from the partially vaporized gas mixture being used to heat the partially condensed liquid mixture ； 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 it 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. 9a& {# T; E% h T A distillation column is composed of several trays; the topmost part of the column is called the top, while the bottommost 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. -?( r( U& x$ ]0 D- j% @ 4. What is dew point? * S. l+ v5 ] S: @ Answer: Cool the gas mixture 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? " D7 f d$ T& n 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 known as the boiling point of that liquid at that pressure; it is simply referred to as the boiling point. A liquid at its boiling point is called a saturated liquid, which corresponds to 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 given pressure, the bubble point, dew point, and boiling point are all the same value. 5U! L/ G8 A2 H- g- L( J 7What 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 vapor turning 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 is heated from a liquid state to steam 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 that of the previous value. 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? , i2 R) y* C3 ] 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. /]5 a. x( h. A 9What is the reflux ratio- J4 R) ^: m# D) T) n% { 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, 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 extracted; it is usually denoted by R, where R = L/D. Here, R represents the reflux ratio; L is the amount of reflux liquid at 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? ,\+ W, O2 s7 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, reducing the reflux ratio gradually 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. t:w- u( b0 V! {1 F 11、What is full reflux? Answer: In distillation operations, the practice of stopping feed to the tower, as well as discharge from the bottom and top of the tower, and using all the condensate at the tower top as reflux is known as full reflux. Full reflux operation is mostly used at the beginning of the start-up of a distillation column, or in the self-circulation operation of the distillation column when production is abnormal. 12. How is the optimal reflux ratio determined? Answer: For processes with fixed separation requirements, reducing the reflux ratio will lower the operating costs (primarily reflected in the heating required at the bottom of the tower and the cooling required at the top of the tower); however, it will increase the number of trays needed, thereby raising the capital investment for the tower ； Conversely, increasing the reflux ratio reduces the number of trays but increases operating costs. Therefore, during design, an optimal reflux ratio should be selected to minimize the sum of capital costs and operating costs under specific economic conditions; this reflux ratio is known as the optimal reflux ratio. The most appropriate reflux ratio is 1.3 to 2 times the minimum reflux ratio. 13. What is the pressure drop in a distillation column? Answer: The pressure drop in a distillation column refers to the pressure difference between the bottom and top of the column, as commonly understood. For plate towers, the plate pressure drop consists of three main components: the dry plate pressure drop, the liquid layer pressure drop, and the pressure drop required to overcome the surface tension of the liquid. The pressure difference between the tower bottom and the tower top is the sum of the pressure drops across each tray in the entire tower. The so-called dry plate pressure drop refers to the pressure drop that occurs when the rising gas (or steam) inside the distillation column passes through trays where there is no liquid present ； The pressure drop that occurs as the gas passes through the liquid layer on each tray is called the liquid layer pressure drop ； The pressure drop generated by a gas as it overcomes the surface tension of a liquid is called the surface tension pressure drop. For a fixed column, under normal operation, the column pressure drop mainly varies with the flow rate of the rising gas; experience shows that the column pressure drop is proportional to the square of the gas flow rate. %k5 T N; G$ l7 }: @ 14. What is the empty tower velocity? What is its relationship with pore velocity? %O; j, {; q5 L r$ o& @5 X Answer: The empty tower velocity refers to the ratio of the volume of vapor rising in the distillation tower per unit time to the cross-sectional area of the tower, that is, the distance that the vapor rises within the tower per unit time. The unit is cubic meters per second. m2 or m/s. The formula is: W = VsAa, where W is the empty tower velocity, in meters per second ； Vs—Volume flow rate of rising steam, m3/s ； Aa—total cross-sectional area of the tower, in m2. ∵Aa=0.785D2 (D is the inner diameter of the tower, in meters). Therefore, W=Vs/0.785D2+. The hole velocity refers to the ratio of the volume of steam rising through the vapor rise holes per unit time to the total cross-sectional area of those holes; in other words, it is the flow rate of the rising gas through the vapor rise holes, measured in cubic meters per second. m2 or m/s; formula: W_hole = Vs/AT. Where: W_hole is the hole velocity, in m/s ； 0 S( M/ I9 g( g' `0 AT—total cross-sectional area of the stomatal pores, in m2. Since the total cross-sectional area of the vapor rise channels is determined by the opening ratio of the tray, let the opening ratio be Φ; then the formula is: W_vents = Vs / (0.785 × D² × Φ) = W / (Φ × 1)². The empty tray velocity is one of the important factors affecting distillation operations. For a tower that has already been designed, increasing the empty tower velocity within the allowable range can enhance its production capacity. When the empty tower velocity is increased to a certain level, the gas-liquid two phases have too short a contact time on the tray plates, which leads to severe foam entrainment and disrupts the normal operation of the tower. Generally, the empty tower velocity is determined based on the mist entrainment level not exceeding 10%, which is referred to as the maximum allowable velocity. When the velocity of the liquid in the tower is too low, it hinders the gas from passing through the pores; moreover, it is not possible to retain the liquid on the upper trays. The liquid on these trays can flow back to the lower trays through the rising vapor pores, and this phenomenon is known as liquid leakage. When the leakage is severe, it reduces the separation efficiency of the distillation column, especially in tray columns, floating valve columns, and tongue-shaped columns. 15. What is the opening area of a tower? How is the opening ratio determined? Answer: In a distillation tower, steam flows from bottom to top while liquid flows from top to bottom, and both must pass through each tray at the same time. The channel through which gas passes in a tray is called the rising gas channel, and the total cross-sectional area of these rising gas channels corresponds to the opening area of each tray. The opening area of a floating valve column is the sum of the cross-sectional areas of all the floating valve holes. The selection of the opening cross-sectional area is determined based on the production load and the allowable steam velocity. The so-called opening ratio is the ratio of the selected opening area to the total cross-sectional area of the empty tower, denoted by Φ. That is: Φ = (AT/Aa) × 100%. Here, AT represents the total cross-sectional area of the openings in square meters, while Aa represents the total cross-sectional area of the empty tower in square meters. Sometimes, in order to accommodate different gas loads on various plates or sections within the tower, different opening ratios can be chosen during the design process. The mass transfer efficiency varies depending on the porosity. Furthermore, the porosity also has a significant impact on the tower’s processing capacity. In the same tower diameter, the processing capacity increases accordingly as the opening ratio rises ； For the same processing capacity, an increase in the porosity allows the tower diameter to be reduced; therefore, porosity is one of the important parameters in design. #B’ A. a4 V, c’ l6 @ 16. What is flooding? 7 T1 p6 M, I* `4 _ Answer: In distillation operations, the liquid on the lower trays rises to the upper trays, disrupting the normal operation of the tower; this phenomenon is known as flooding. The cause of flooding is mainly due to the steam rising inside the tower at an excessive speed, exceeding the maximum allowable speed. Additionally, in distillation operations, it is common to encounter a situation where the liquid load is too high, causing the liquid level in the overflow pipe to rise to such an extent that the liquids on the upper and lower trays become connected, thereby disrupting the normal operation of the tower; this is also a form of flooding. Both of the above phenomena are types of flooding, but their causes are different. 5{7 j C7 T8 x% a