Basic knowledge of distillation operations

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Basic Knowledge of Distillation Operations 1. What is phase formation 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; thus, phase equilibrium is dynamic and relative, rather than static and absolute. For example, in a distillation system, when gases with higher temperatures and liquids with lower temperatures 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 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 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 steam produced by partial vaporization is partially condensed, since substances with high boiling points tend to condense easily, the concentration of such substances in the condensed liquid is higher than in the gas phase; whereas the concentration of substances with low boiling points 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. 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 countercurrent flow. 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 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 parts. 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, a gas phase mixture with a higher temperature causes itself to partially condense by heating the liquid mixture with a lower temperature; 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 the 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: By cooling a 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 droplet is not a pure component, but rather the liquid phase in equilibrium with the gas phase at the dew point temperature, whose composition is determined by the phase equilibrium relations. 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 bubble appears 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 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 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. The boiling point increases when external pressure rises, and it decreases when external pressure 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 emerges from 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 extracted; it is usually denoted by R, where R = L/D. In this formula, R represents the reflux ratio, and L represents 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. 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 natural 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 energy required at the bottom of the tower and the cooling energy needed at the top of the tower); however, it will increase the number of trays required, thereby raising the capital investment for the tower ； Conversely, increasing the reflux ratio reduces the number of plates 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 referred to as the optimal reflux ratio. The most suitable 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 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 pressure drop across the column mainly varies with the flow rate of the rising gas; experience shows that the pressure drop is proportional to the square of the gas flow rate. 14. What is the empty tower velocity? What is its relationship with the pore velocity? 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 rising inside the tower travels per unit time. The unit is m3/s. m2 or m/s. The formula is: W = Vs/Aa, where W is the empty tower velocity, in m/s ； Vs—rising steam volume flow rate, m3/s ； Aa—total cross-sectional area of the tower, m2. ∵Aa=0.785D2 (D is the inner diameter of the tower, in meters). Therefore, W=Vs/0.785D2. The pore velocity refers to the ratio of the volume of steam rising through the pore channels per unit time to the total cross-sectional area of those channels; in other words, it is the flow rate of the rising gas through the pore channels, measured in m3/s. m2 or m/s; formula: W_hole = Vs/AT. Where: W_hole is the hole velocity, in m/s ； AT—total cross-sectional area of the aeration pores, 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 becomes: W_vents = Vs / 0.785D²Φ = W/Φ. 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 increases to a certain level, the gas-liquid two phases have too short a contact time on the tray, which leads to severe foaming and entrainment, disrupting the normal operation of the tower. Generally, the empty tower velocity is determined by setting the mist entrainment rate at no more than 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 vapor rise holes, 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 channels through which gas passes in the tray are called rising gas channels, 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 size of the production load and the allowable steam velocity. The so-called porosity is the ratio of the selected open area to the total cross-sectional area of the empty tower, denoted by Φ. That is: Φ = AT/Aa × 100%. Here, Φ represents the porosity; AT is the total cross-sectional area of the openings, in m2, while Aa is the total cross-sectional area of the empty tower, also in m2. Sometimes, in order to accommodate the different gas loads on various plates or sections within the tower, different values of porosity can be chosen during design. The mass transfer efficiency varies depending on the porosity. Furthermore, the porosity also has a significant impact on the processing capacity of the tower. In the same tower diameter, the processing capacity increases correspondingly as the opening ratio increases ； 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. 16. What is flooding? 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 mix together, disrupting the normal operation of the tower; this is also a form of flooding. Both of the above phenomena are types of flooding, but they are caused by different reasons. 17. What is mist entrainment? Answer: Mist entrainment refers to liquid droplets that are carried from the lower tray to the upper tray by the gas. During the mass transfer process, the large amount of mist entrained can carry heavy components that should not reach the top of the tower into the product, thereby reducing the quality of the product. It also reduces the concentration difference during mass transfer, resulting only in a decrease in the efficiency of the tower plates. For a given tower, the maximum allowable amount of mist entrainment limits the upward velocity of the gas. There are many factors that affect the amount of mist entrained, such as the tray spacing, empty tower velocity, weir height, liquid flow velocity, and the physicochemical properties of the material. It must also be noted that the amount of mist entrained is highly dependent on the structure of the capture device. Although there are many factors that affect the amount of foam entrainment, the main influencing factors are the empty tower velocity and the gas-liquid separation space between two tray levels. For a fixed tower, the mist entrainment amount increases mainly as the empty tower velocity increases. However, if the distance between the trays is increased to expand the separation space, the empty tower velocity increases accordingly. 18. What is a liquid leak? Answer: The phenomenon in which the liquid on a tray flows back down to the lower tray through the rising gas channel is called leakage. In distillation operations, if the energy possessed by the rising gas is not sufficient to penetrate through the liquid layer on the tray, or is even lower than the potential energy of that liquid layer, the liquid cannot be held in place and leakage occurs. The lower the empty tower velocity, the more severe the leakage. As a result, some of the liquid flows to the lower trays without coming into contact with the rising gas, and the low-boiling components that should not remain in the liquid fail to evaporate, thereby reducing the efficiency of the trays. Therefore, the minimum operating empty tower velocity for the tray is limited by the liquid leakage rate; under normal operation, it is required that the leakage rate of the tray not exceed 10% of the amount of liquid on the tray. The amount of leakage is also one of the characteristics used to evaluate the performance of a tray. Strainer trays, floating valve trays, and tongue trays are more prone to leakage when the upward gas velocity in the tower is low. 19. What is operational flexibility? Answer: Operational flexibility refers to the range between the minimum allowable value (lower load limit) and the maximum allowable value (upper load limit) of the rising gas velocity. When the upward gas velocity varies within this range, the distillation column can maintain normal operation while achieving a certain level of separation efficiency. As mentioned earlier, the upper limit on the load of a distillation tower is determined by ensuring that the amount of mist carried by the rising steam does not exceed 10% of the steam flow rate ； The lower load limit is set such that the liquid leakage from the tray does not exceed 10% of the liquid flow rate. Generally speaking, float valve towers have the greatest operating flexibility; some tests show that the ratio of the upper load limit to the lower load limit can reach around 7–9. Bubble cap towers come next, while tray towers have the lowest flexibility. It should be noted that as the upward gas velocity changes, the tray efficiency changes, which leads to variations in the separation performance. 20. What is backmixing? Answer: On trays with downcomers, the liquid flows across the tray in counterflow to the gas, and the concentration of the volatile components in the liquid gradually decreases in the direction of flow. However, when the rising gas creates vortices in the liquid on the tray, the liquid with a high concentration mixes with that of low concentration, disrupting the concentration gradient along the flow direction; this phenomenon is known as backmixing. Backmixing can lead to a decline in separation efficiency. The occurrence of backmixing is influenced by many factors, such as residence time, liquid flow conditions, channel length, the levelness of the tray, and hydraulic gradient. 21. What is material balance? What is its significance in distillation processes? Answer: Material balance is the application of the laws of conservation of mass and transformation in chemical engineering. For any production process, on a weight basis, the amount of material converted should equal the sum of the amount of product produced and the amount of material lost. Through material balance, it is possible to understand how raw materials are converted into products and the extent of losses, in order to find ways for improvement. Material balance applies equally to the entire process or to a certain stage of it ； It is possible to account for all the substances involved in the process, as well as for any individual component. It is a most basic and essential requirement for an operator to keep track of the material balance in the distillation tower at all times during operation. If material balance is not properly managed, it can be due to either too much feed and too little output ； Another option is less feed and more extraction. Neither of these two situations represents normal operation, and they affect the quality and quantity of the products. Therefore, in order to operate the distillation tower under optimal conditions, to improve the quality and quantity of the product, reduce energy consumption, and minimize processing losses, it is necessary to conduct regular material balance calculations and adjust the operations in practice. 22. What is the most suitable position for the feed plate? Answer: The most suitable position for the feed plate is the one that provides the highest separation efficiency under the same number of theoretical plates and identical operating conditions, or the one that requires the fewest theoretical plates under the same operating conditions. In the chemical industry, most distillation columns are equipped with two or more feed plates, and the position of these feed plates is adjusted based on changes in the composition of the feed. When the proportion of light key components in the feed is lower than under normal operation, the position of the feed tray should be moved downward to increase the number of trays in the distillation section, thereby enhancing its separation capacity. Conversely, moving the feed plate upward increases the number of plates in the stripping section, thereby enhancing its separation capacity. In short, the content of the light key component in the feed components on the feed plate should be less than that on the lowest tray in the distillation section, and greater than that on the highest tray in the stripping section. This prevents the material composition on each tray within the tower from being disrupted after feeding, thereby maintaining stable operation. 23. What is tray efficiency? Answer: In the actual operation of a distillation column, due to limitations in mass transfer time and mass transfer area, a gas-liquid equilibrium state cannot be achieved. That is, the concentration of low-boiling-point components in the vapor on a tray is lower than that in the vapor when it is in equilibrium with the liquid phase; therefore, the effectiveness of an actual tray is always less than that of an ideal theoretical tray. Based on this concept, tray efficiency can be expressed as the ratio of the theoretical number of trays to the actual number of trays. The main factors affecting tray efficiency are: (1) the speed of gas-liquid exchange ； (2) Degree of gas-liquid mixing on the tray ； (3) The amount of liquid droplets entrained by the rising steam into the upper tray levels and the liquid leakage from the trays. The aforementioned three factors are in turn influenced by the design and arrangement of the trays, operating conditions, as well as the physical properties of the material being processed. The design and arrangement of trays include: the geometric dimensions of the trays, the tray spacing, the height of the overflow weir, the open area ratio, and the arrangement of the vapor lift holes. Operating conditions include: rising steam velocity, liquid residence time on the tray, temperature, and pressure, etc. The physical properties of materials that are relevant to their processing include relative volatility, the viscosity and specific gravity of vapor and liquid, diffusion coefficients, and surface tension. It should be noted that the physical properties of the material vary depending on the operating temperature and pressure in the tower. In production, when selecting the type of tray, the tray must meet the following requirements: (1) high tray efficiency ； (2) High production capacity, meaning that both the allowable gas and liquid phase loads are high, allowing smaller towers to handle large-scale production tasks ； (3) Stable operation with good operational flexibility ； (4) Economically durable, with low steel consumption ； (5) Easy to operate and maintain. 24. What is the equivalent height of the packing? Answer: The equivalent height of the packing is also known as its equivalent plate height. It corresponds to the packing height required for a level of separation equivalent to one theoretical plate, and can also be described as the packing height equivalent to the effect of one theoretical tray. 25. What is the spray density of the filler? Answer: The spray density of the filler is also known as the mass velocity of the liquid. The mass of liquid per kilogram per square meter of the tower’s cross-section, per hour. 26. What is the liquid holdup in a packing layer? Answer: During the operation of a packing layer, the amount of liquid that accumulates in the gaps between the packing elements and on their surfaces is referred to as the liquid holdup. Its unit of measurement is (m3 of liquid per m3 of tower volume). The liquid holdup can be divided into two parts: (1) Static liquid holdup: The amount of liquid remaining in the tower after the liquid spraying is stopped and dripping is ceased is referred to as static liquid holdup. The net holding capacity depends only on the properties of the filler and those of the liquid, and is independent of the liquid spray rate. (2) Flow rate after shutdown: The amount of liquid discharged when the packed tower stops spraying liquid is called the flow rate after shutdown. The momentum flux is related not only to the properties of the filler and the liquid but also to the spray density of the liquid; however, below the critical point, it is independent of the gas velocity. The sum of the static retention volume and the dynamic retention volume is the total retained liquid volume. 27. What is a packed tower? What are the advantages and disadvantages? Answer: A packed tower is a mass transfer device featuring a certain height of packing inside the tower, enabling continuous gas-liquid contact. The rising steam inside the tower flows from bottom to top through the pores of the packing ； The liquid remaining at the top of the tower flows downward along the surface of the packing. The transfer of matter and heat between the gas and liquid phases occurs through the formation of a thin liquid film on the surface of the packing. The prominent advantages of packed towers are: low resistance to fluid flow, simple structure, reduced use of steel, low cost, ease of installation and maintenance, and the possibility of using corrosion-resistant materials for the packing. 28. What are the requirements when using fillers? Answer: First, since the gap between the packing and the tower wall is larger than the gaps within the packing layer, liquid tends to flow toward the tower wall, which affects the mass transfer efficiency; this phenomenon is commonly referred to as the edge effect. It was previously believed that the larger the filler diameter, the more severe the edge wall effect. Generally, for packed towers, the ratio of the tower diameter D to the packing diameter d is required to be greater than 10:1. For tall towers, the sidewall effect is more severe; therefore, segmented filling or liquid redistribution is commonly used to address this issue. Under normal circumstances, a ratio of the packing height H to the diameter of the packed tower of 2 to 6 is considered appropriate. Secondly, there are also requirements regarding the sorting of fillers. When D/d is greater than 8, it is best to arrange the packing neatly. When the packing is piled up randomly, water should be added to the equipment first, and then the packing placed in the water to prevent it from breaking and causing blockages; after the packing is installed, the water can then be drained. Third, after the packing tower is shut down, its packing should be inspected for damage and contamination, to determine whether cleaning or replacement is necessary. 29. What is the structure of a floating valve tray? How does it work? Answer: The floating valve tower is a new type of mass transfer equipment for enterprises that has been widely used in China over the past decade or so. It is currently widely used in the petrochemical industry, achieving satisfactory results. The structure of a floating valve tower is relatively simple; its main components include a liquid receiving tray, downcomer pipes, an overflow weir, floating valves, and tray plates. 30. What are the advantages and disadvantages of floating valve trays? Answer: The performance of floating valve trays combines the advantages of bubble cap trays and perforated trays, while overcoming their disadvantages. Due to the fixed gap width of the bubble tray, its adaptability to changes in steam load is poor. At low gas velocities, the gas-liquid contact is poor; at high gas velocities, the steam tends to blow the liquid away. Although porous tray plates have a simple structure and high processing capacity, they offer limited operational flexibility; in the case of floating valve trays, the opening degree of the valve plates varies with the vapor velocity. At low gas velocities, the valve disc falls automatically under the force of gravity to reduce leakage. Therefore, the floating valve tower has high efficiency and great operational flexibility, enabling it to adapt well to changes in feed volume. Experiments have shown that the ratio of its maximum load to minimum load can reach around 7–9. The floating valve tower has a simple structure and a large free cross-sectional area; its cost is 12–15% lower than that of a bubble column tower, while its processing capacity is about 20–40% higher. Since the steam in the floating valve tray is introduced horizontally into the liquid layer, gas-liquid mixing is excellent, mist entrainment is minimal, the contact time is long, and mass transfer is efficient; its efficiency is 15% higher than that of the bubble cap tray. The main drawback of floating valve trays is that steam is ejected along the perimeter of the rising vapor apertures, resulting in reverse mixing of liquid, which thus reduces the mass transfer efficiency. Additionally, the valve disc can easily get stuck, rusted, or stuck together, affecting its operation. 31. What is the impact of tower height and tower diameter on yield and quality? Answer: Tower diameter mainly affects production capacity, while tower height primarily influences product purity. The relationship between tower diameter and production capacity can be expressed by the following formula. D=（v/0.785w）1/2 Where D----is the diameter of the tower, in meters ； v----Volumetric flow rate of steam in the tower, m2/s; w----Flow velocity in an empty tower, m/s. For a given tower, there are certain limits on the empty tower flow rate. At a certain empty tower velocity, the greater the volumetric flow rate of steam in the tower, the larger the diameter of the tower required ； Similarly, the larger the tower diameter, the greater the allowable steam load within the tower, which means a higher production capacity; therefore, the tower diameter is the main factor affecting production capacity. The height of the tower, once the tray efficiency and tray spacing are determined, determines the actual number of trays. And the actual number of theoretical plates is determined by the minimum number of theoretical plates. The greater the minimum theoretical number of plates, the greater the actual number of plates as well. The impact of tower diameter and tower height on production is dialectical and cannot be separated entirely. For example, increasing the tower height can reduce the reflux ratio, thereby improving production capacity ； Increasing the tower diameter allows for an increase in the reflux ratio, thereby achieving the goal of reducing the tower height. 32. What are the factors that affect distillation operations? Answer: Apart from equipment issues, the main factors affecting the distillation process include the temperature and pressure of the tower (including the top, bottom of the tower, and certain trays that are of particular significance) ； Feeding status ； Feed rate ； Feed composition ； Feed temperature ； Steam rise velocity in the tower and heating capacity of the evaporation kettle ； Backflow volume ； Top condenser cooling capacity ； Top tray yield and bottom tray yield. The operation of the tower involves adjusting these influencing factors in accordance with the composition requirements of the products at the top and bottom of the tower. 33. What is the impact of changes in the operating pressure of a distillation tower on the distillation process? Answer: The design and operation of a tower are based on a certain pressure level; therefore, in a typical distillation tower, it is necessary to maintain a constant pressure at all times. Fluctuations in column pressure will have the following effects on column operation. (1) Impact on product quality and material balance: Changing the operating pressure will alter the composition of the gas-liquid equilibrium on each tray. As the pressure increases, the heavier components in the gas phase decrease, thereby increasing the concentration of the lighter components in the gas phase ； The increase in the content of light components in the liquid phase also alters the weight ratio between the gas and liquid phases, resulting in an increase in the liquid phase volume and a decrease in the gas phase volume. The overall result is: the concentration of light components in the tower top fraction increases, but their quantity decreases relatively ； The concentration of light components in the reactor broth increases, and the volume of the reactor broth increases as well. Similarly, as the pressure decreases, the amount of distillate at the top of the tower increases, while the concentration of light components decreases ； The volume of the reactor liquid decreases, and the concentration of light components drops. Under normal operation, a constant pressure should be maintained; however, if abnormal operations lead to an increase in the concentration of heavier components in the product at the top of the tower, it is possible to raise the operating pressure appropriately to ensure that the product quality remains satisfactory, but this will result in an increased loss of lighter components in the liquid phase. (2) Changing the relative volatility between components: As pressure increases, the relative volatility between components decreases, resulting in a reduced separation efficiency; the opposite is true as well. (3) Changing the tower’s production capacity: An increase in pressure and an increase in the specific gravity of the components lead to an increased processing capacity of the tower. (4) Fluctuations in tower pressure will cause confusion in the corresponding relationship between temperature and composition. In our operations, we often use temperature as an indirect criterion for measuring product quality, but this is only correct when the tower pressure remains constant. When the tower pressure changes, the dew point and bubble point of the mixture change, which leads to a change in the temperature distribution throughout the tower; as a result, the relationship between temperature and product quality also changes. Based on the above analysis, changing the operating pressure will affect the performance of the entire tower; therefore, a constant pressure should be maintained during normal operation. Only when the normal operation of the tower is disrupted can the pressure be adjusted appropriately, within the limits permitted by the process parameters, in accordance with the analysis outlined above. It should be noted that during distillation operations, changes in the feed rate, feed composition, and feed temperature; changes in the heating steam supply to the bottom of the tower; changes in the reflux volume, reflux temperature, and coolant supply at the top of the tower; as well as blockages in the tower trays, can all cause fluctuations in tower pressure. In such cases, we must first analyze the causes of these pressure fluctuations and take timely action to restore normal operation. 34. What is the impact of feed conditions on distillation operations? Answer: There are five types of feed conditions (1) cold feed ； (2) Bubble-point feeding ； (3) Gas-liquid mixed feed ； (4) Saturated steam feed ； (5) Superheated steam feed. For ease of analysis, let δ be the heat required per kilomole of feed liquid to become saturated vapor, where δ equals the latent heat of vaporization per kilomole of feed. It can be seen from the above equation that δ > 1 for cold feed, δ = 1 for feed at the bubble point, and 0 for gas-liquid mixed feed