Basic knowledge of methanol distillation

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I. The significance of crude methanol distillation: During methanol synthesis, due to factors such as synthesis conditions like pressure and temperature, the composition of the syngas, and the performance of the catalyst, a series of side reactions occur alongside the reaction that produces methanol. In addition to methanol, the resulting product contains dozens of organic impurities such as water, ethers, aldehydes, ketones, esters, alkanes, organic acids, organic amines, higher alcohols, thiol compounds, methyl mercaptan, and iron carbonyl. As a basic raw material in the organic chemical industry, methanol is used to produce a wide variety of products; therefore, certain purity standards are required for it. By distilling crude methanol, high-purity methanol can be produced to meet various requirements, with various impurities reduced to levels below specified thresholds, thereby ensuring the quality of the high-purity methanol. II. Basic Concepts 1. What is distillation? What is the principle of distillation? 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 and 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 vaporize more easily; as a result, their concentration in the gas phase is higher than that 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 obtained from 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 that in the gas phase, whereas the concentration of low-boiling-point substances in the uncondensed gas is higher than that in the liquid phase. In this way, through partial vaporization and partial condensation, the mixture is initially separated by altering the concentrations of its various components. If this is repeated multiple times, essentially high-boiling-point components will remain in the liquid phase, while essentially low-boiling-point components will remain in the gas phase. It can be seen that both partial vaporization and partial condensation cause changes in the composition of the gas and liquid phases; by carrying out multiple instances of partial vaporization and partial condensation simultaneously, a 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 for the vaporization of liquid, that is, by bringing the gas and liquid phases into direct contact to enable mass transfer alongside heat transfer. To meet this requirement, in practice, this process of multiple partial vaporizations accompanied by partial condensations takes place in counter-current tower equipment. The so-called counterflow refers to the situation where, as a liquid is heated, gas with a higher temperature is generated; this gas flows upward and moves in the opposite direction to the reflux liquid with a lower temperature (rich in low-boiling-point components) that is produced at the top of the tower due to condensation. In other words, the reflux liquid moves downward and meets the rising vapor, allowing for mass and heat transfer to occur within the tower. The processes involved are as follows: (1) Heat exchange between the gas and liquid phases – the heat from the partially vaporized gas mixture is used to heat part of the liquid mixture that has condensed ； (2) During the heat exchange process, mass transfer occurs simultaneously 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 partially vaporize. At this point, due to differences in volatility, the components with lower boiling points evaporate more than those with higher 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 transfer 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. A single 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, the highly pure volatile components are ultimately obtained from the top of the tower (the tower overhead). What is obtained at the bottom of the tower are essentially the poorly volatile components. 2. What is Raoult’s law? Raoult’s law is an important rule derived from experiments. This law states that, at a constant temperature and in equilibrium between the vapor and liquid phases, the partial pressure of any component in the vapor phase above the solution is equal to the saturated vapor pressure of that component at the same temperature, multiplied by its mole fraction in the liquid phase. Expressed in mathematical terms as: pA = PAXA, where pA is the partial pressure of component A in the gas phase ； PA — the saturated vapor pressure of pure component A at that temperature ； XA — the molecular fraction of component A in the liquid phase. 3. What is Dalton’s law? Dalton’s law is a law that describes the relationship between the total pressure and partial pressures of an ideal gas mixture. Dalton’s law states that the total pressure of an ideal gas mixture is equal to the sum of the partial pressures of each constituent gas. According to Dalton’s law, a very important conclusion can be drawn: the partial pressure of each component gas in a mixed gas is equal to the total pressure of the mixed gas multiplied by the molecular fraction of that gas in the mixture. For example, the partial pressure of the i-th component gas can be expressed by the following formula: pi = P_total / Yi, where pi is the partial pressure of component gas i ； P_total – the total pressure of the gas mixture ； Yi — the molecular fraction of component gas I in the mixed gas. 4. Volatility and relative volatility: Volatility and relative volatility are among the important basic concepts used in the distillation process, and they are also frequently applied in the design of distillation columns. The volatility of a pure substance is generally described by the magnitude of its saturated vapor pressure. For different substances at the same temperature, those with a high saturated vapor pressure are considered volatile substances, while others are non-volatile substances. The saturated vapor pressure is the vapor pressure of a substance at its boiling point under external pressure; therefore, it is customary to use the boiling point as an indicator of volatility as well. At 101.325 kPa, the boiling point of water is 100°C, that of ethanol is 78.4°C, and that of methanol is 64.7°C; we can say that methanol vaporizes more easily than ethanol, and ethanol vaporizes more easily than water. For pure substances, both the saturated vapor pressure and the boiling point can be used to determine their degree of volatility. The distillation process deals with multi-component liquid mixtures, and the gas phase in equilibrium with the liquid is also a gas mixture composed of various components. At this point, the magnitude of the partial pressure of each component in the gas phase reflects the volatility of that substance. The degree of its volatility is related not only to the properties of the substance, but also, according to Raoult’s law, to the concentration of that substance in the liquid. To this end, the definition of volatility is specified as the ratio of the vapor pressure of a component to its concentration in the liquid phase. For a solution composed of substance a and substance b, Va = Vb =; where Va and Vb represent the volatilities of component a and component b, respectively ； Pa, Pb – the partial pressures of component a and component b in the gas phase ； Xa, Xb —— the mole fractions of component a and component b in the liquid phase. For an ideal solution that obeys Raoult’s law, Pa = PA·Xa and Pb = PB·Xb = PB(1 – Xa), where PA and PB represent the saturated vapor pressures of components a and b, respectively. It can be seen that, when the solution concentration remains constant, the volatility of a substance is related to its saturated vapor pressure, which in turn is related to temperature; the higher the temperature, the greater the volatility. When the temperature remains constant, the degree of volatility is related to the concentration. For ideal solutions, the higher the concentration, the greater the volatility, and the two are directly proportional to each other. To compare the volatilities of various components in a mixture and to facilitate calculations during distillation, the concept of relative volatility was introduced; it is defined as the ratio of the volatilities of the various components in the mixture. For a solution composed of substance a and substance b, its relative volatility is given by αab=; where αab represents the relative volatility of component a with respect to component b. 5. What is dew point? What is the bubble point? What is the boiling point? The gas mixture is cooled at constant pressure; when it reaches a certain temperature, the first tiny droplets form. This temperature is known as the dew point of the mixture at that specified 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 should be noted here that the first droplet is not a pure component; it is the liquid phase in equilibrium with the gas phase at the dew point temperature. Its composition is determined by phase equilibrium relations. It can be seen that gas mixtures with different compositions have different dew points. When a liquid mixture is heated to a certain temperature under a specific pressure, the first very small bubbles appear in the liquid, marking the onset of boiling. This temperature is referred to as the boiling point of that liquid at the given pressure, or simply the boiling point. A liquid at its bubble point temperature is called a saturated liquid, and the temperature of the liquid in the bottom of a distillation column is at the bubble point temperature. It should be noted that this first, very small bubble is not a pure component either; its composition is also determined by the phase equilibrium relations. When the saturated vapor pressure of a pure liquid equals the external pressure, the liquid boils; the temperature at this point is known as the boiling point of that liquid at the given pressure. It should be noted that the boiling point of a pure substance changes with external pressure. As external pressure increases, the boiling point rises ； As external pressure decreases, the boiling point decreases. For a liquid mixture, the material begins to boil when the sum of the partial pressures of its components equals the external pressure. Since the partial pressures of the various components change as their contents in the liquid phase vary, it does not have a constant boiling point. The liquid mixture is in a boiling state across the entire temperature range from the bubble point to the dew point, and the gas-liquid phase compositions vary at different temperatures. 6. Reflux ratio, full reflux, minimum reflux ratio: The reflux ratio is defined as the weight ratio of the amount of liquid returned to the system to the amount of liquid extracted. It is usually denoted by R, that is, R = L/D, where R represents the reflux ratio ； L — Volume of reflux liquid at the tower top per unit time, kilograms/hour ； D —— The amount of product removed from the top of the tower per unit time, in kilograms per hour ； Full reflux refers to a distillation operation in which the feed to the column, the bottom product of the column, and the top product of the column are all stopped, and the entire condensate from the top of the column is used as reflux liquid; this is what is known as full reflux. The minimum reflux ratio is the value obtained when, under specified separation requirements – that is, when the compositions of the materials taken from the top and bottom of the tower are fixed – the reflux ratio is gradually reduced; at this point, the number of theoretical plates required increases gradually. When the reflux ratio is reduced to a certain value, the number of theoretical plates required increases to infinity; this value of the reflux ratio is known as the minimum reflux ratio necessary to achieve the desired separation. It is one of the important data in the design calculations of distillation columns; typically, the actual reflux ratio during operation is 1.3 to 2 times the minimum reflux ratio. 7. Briefly describe the role of the reflux liquid in the distillation separation process: During distillation, the vapor generated by heating the mixture is drawn out from the top of the tower and enters the top condenser. Here, the vapor condenses (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 surfaces, 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. As the reflux liquid flows down along the tray, it undergoes multiple instances of partial vaporization and partial condensation in interaction with the steam that is continuously vaporizing and rising within the tower; this is a process of mass exchange, which increases the concentration of high-boiling-point components in the liquid returning back. Recirculation is a necessary condition for mass transfer through gas-liquid phase contact; without such phase contact, mass exchange cannot take place. Of course, the difference in the volatility of the components remains the basis of the distillation process. 8. Pressure drop of the distillation column: The pressure drop of a distillation column refers to the pressure difference between the top and bottom of the column, as commonly understood. For a plate tower, the plate pressure drop consists mainly of three 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 top and bottom of the tower is the sum of the pressure drops at the top and bottom of each tray in the entire tower. The so-called dry plate pressure drop refers to the pressure drop that occurs when rising gas (or steam) in a 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 tower, under normal operation, the pressure drop in the tower 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. 9. What is the empty tower velocity? What is its relationship with the pore velocity? The empty tower velocity refers to the ratio of the volume of vapor rising in the distillation column per unit time to the cross-sectional area of the column, that is, the distance traveled by the rising gas within the column per unit time. The unit is cubic meters per second per square meter, or meters per second. It can be expressed by the formula: W = Vs/Aa, where W represents the empty tower velocity, in meters per second ； Vs——Volume flow rate of rising vapor, m3/s ； Aa——total cross-sectional area of the tower, in m2. The above formula can also be expressed as W = Vs/0.785D2, where D is the inner diameter of the tower, in meters. Vapor velocity refers to the ratio of the volume of rising vapor that passes through a vapor rise channel per unit time to the total cross-sectional area of the channel; in other words, it is the flow rate of the rising gas through the vapor rise channel, with units of m3/s·m2 or m/s. It can be expressed by the formula: W_hole = Vs/AT, where W_hole is the hole velocity, in meters per second ； Vs——Volume flow rate of rising vapor, m3/s ； AT — total cross-sectional area of the aeration channels, in m2. Since the total cross-sectional area of the vapor rise channels is determined by the opening ratio of the tray, and if the opening ratio is denoted as φ, then the above formula can be expressed as: W_vents = Vs/0.785D²·φ = 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 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 flow of gas through the channels, and it may even be unable to hold back the liquid on the upper trays. The liquid on these trays can flow back to the lower trays via the upward-flowing channels; this phenomenon is commonly referred to as liquid leakage. When leakage is severe, it reduces the separation efficiency of the distillation tower, especially in tray towers, floating valve towers, and tongue-shaped towers. 10. What are the flow rate and velocity of a fluid? What kind of relationship exists between the two? Both the flow rate and velocity of a fluid can be divided into mass flow rate and mass flow velocity, as well as volume flow rate and volume flow velocity. Weight flow rate: The weight of the fluid that passes through any cross-section of a pipe or device (a cross-section perpendicular to the direction of flow) per unit of time. Weight flow rate is usually denoted by the symbol G, with the unit being kilograms per second. Volumetric flow rate: The volume of fluid that passes through any cross-section of a pipe or device (a cross-section perpendicular to the flow direction) per unit of time. Volumetric flow rate is usually denoted by the symbol V, with the unit of cubic meters per second. Weight flow rate: The weight of the fluid that passes through a unit cross-section of a pipe or device (a cross-section perpendicular to the flow direction) per unit of time. The weight flow rate is usually denoted by the symbol WG, with the unit of kilograms per second per square meter. Volumetric flow rate: The volume of fluid that passes through a cross-section of a pipe or device (a cross-section perpendicular to the flow direction) per unit of time. Volumetric flow rate is usually denoted by the symbol WV, with units of m3/s·m2 or m/s. The relationship between them is as follows: G = WG·F = WV·γ·F = V·γ, where F is the cross-sectional area of the pipe or device, in square meters ； γ — density of the fluid, kg/m3. In practical applications, since the change in liquid density with temperature and pressure is negligible, volumetric flow rate and volumetric flow velocity are often used when studying liquid flow. When gas is in flow, since its specific gravity changes significantly with temperature and pressure, it is more convenient to use mass flow rate and mass flow velocity. 11. What is the opening area of a tower? How is the porosity determined? In the distillation column, steam flows from bottom to top while liquid flows from top to bottom, and both must pass through each tray simultaneously. The channel through which gas passes in a tray is called the rising gas path, and the total cross-sectional area of these rising gas paths corresponds to the opening area of each tray. The opening area of a floating valve column or a tray column is the sum of the cross-sectional areas of all the floating valve holes or all the vapor rise holes on each tray. The selection of the opening cross-sectional area is determined based on 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 — opening cross-sectional area, in m2. Aa——total cross-sectional area of the empty tower, in m2. Sometimes, to accommodate the varying gas loads on different plates or sections within the tower, different opening ratios can be selected during design. 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 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. 12. What is flooding? How to handle it? In distillation operations, the liquid on the lower trays surges upward to the upper trays, disrupting the normal operation of the tower; this phenomenon is known as flooding. The cause of flooding is mainly that the speed of the rising vapor in the tower is too high, exceeding the maximum allowable speed. Furthermore, 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 gradually until the liquids on the upper and lower trays merge together, disrupting the normal operation of the tower; this is also a form of flooding. The above two phenomena both fall under the category of flooding, but their causes are different. When flooding occurs, whether in a plate tower or a packed tower, the feed rate should be stopped or reduced, the steam supply should be slightly decreased, the temperature of the reactor should be lowered, and the extraction from the top of the tower should be halted. Full reflux operation should be carried out to allow the poorly volatile components that have risen to the top or upper sections of the tower to gradually flow back to the bottom of the tower or to their normal position there. When production does not allow the cessation of feed, the reactor temperature can be maintained slightly below the normal operating temperature, the amount of product taken from the top of the tower can be increased (the quality of this product cannot be guaranteed), and the reflux ratio can be reduced. Once the pressure difference across the tower returns to normal levels, the operating conditions can be brought back to their normal state. 13. What is mist entrainment? Mist entrainment refers to the liquid droplets that are carried from the lower trays to the upper trays by the gas. During the mass transfer process, the entrainment of large amounts of mist can carry heavy components that should not reach the top of the tower into the product, thereby reducing product quality. It also reduces the concentration difference during mass transfer, leading to a decrease in the efficiency of the tower plates. For a given tower, the maximum allowable amount of mist entrainment (which is generally 10%, or 10 kilograms of liquid per 100 kilograms of gas) determines the upward velocity of the gas; this velocity is referred to as the maximum allowable velocity. At this point, the operational effects mainly manifest as an increase in the pressure difference across the tower and a rise in the content of heavier components in the overhead distillate; if the overhead is collected as a gas phase, obvious liquid entrainment can be observed. There are many factors that affect foam entrainment, such as 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 mist 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 spacing between the trays is increased to expand the separation space, the empty tower velocity can be correspondingly increased. 14. What is a liquid leak? The phenomenon in which the liquid on a tray flows back down to the lower tray through the upward gas channel is called leakage. In distillation operations, if the energy of 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-point 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 on 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 decreases. 15. What is operational flexibility? Operational flexibility refers to the range between the minimum allowable value of gas velocity (lower load limit) and the maximum allowable value (upper load limit). When the upward gas velocity varies within this range, the distillation column can maintain normal operation while achieving a certain level of separation efficiency. The upper limit on the load of the distillation tower is determined by ensuring that the amount of mist carried away by the rising vapor does not exceed 10% of the vapor flow rate ； The lower load limit is set such that the leakage amount of liquid on the tray does not exceed 10% of the amount of liquid on that tray. 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 be around 7 to 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. III. Classification of distillation: 1. How is distillation classified? Distillation can be classified from three perspectives. First, based on the different methods of distillation operation, it can be divided into batch distillation and continuous distillation. Second, depending on the conditions of distillation operation (such as pressure), it can be divided into pressure distillation, atmospheric distillation, vacuum distillation, etc. Third, based on the different principles of distillation separation, it can be divided into normal distillation and special distillation. Special distillation further includes azeotropic distillation, extractive distillation, steam distillation, and molecular distillation, among others. The methanol distillation in our plant is continuous and extractive distillation, using a process that combines pressurized distillation with atmospheric distillation. Principles of extractive distillation and selection of extractants: Extractive distillation involves adding a third component (known as an extractant or solvent) to a mixture in order to change the volatility of the original components, thereby enabling separation. Here, it is required that the boiling point of the extractant be much higher than that of the components, and that it does not form an azeotrope with them. Extractive distillation is commonly used to separate solutions in which the boiling points (volatilities) of the various components differ very little. For extractive distillation, many different extractants can often be chosen. Generally speaking, the main criteria for selecting an extractant are as follows: (1) The extractant should have high selectivity. The degree of relative volatility of the separated components in the extractant is known as the selectivity of the extractant. The relative volatility of the separated components in the extractant increases significantly, making separation easier; in other words, the selected extractant has high selectivity. Selectivity is the main criterion for choosing an extractant. Because the degree of selectivity determines the ease with which the more and less important components within the separated mixture can be distinguished from one another. Therefore, the number of plates and the reflux ratio (which affects the tower diameter) are also closely related to it. (2) The extractant should have a high solubility for the components to be separated, so that the liquid on the tray can remain homogeneous and avoid stratification. (3) The boiling point of the extractant should be much higher than that of the components to be separated; otherwise, the extractant is likely to volatilize and be lost from the top of the tower. (4) It should have good thermal and chemical stability, be non-toxic, and not cause equipment corrosion. (5) It is easy to recycle and is inexpensive and readily available. 3. Application of extractive distillation in crude methanol distillation: In the distillation of crude methanol, the boiling point of the azeotrope formed between methanol and alkanes is relatively close to that of methanol itself. Among them, the azeotempers of methanol-heptane, methanol-isooctane, methanol-nonane, and methanol-decane are 58.8°C, 58.3°C, 63.9°C, and 64.3°C respectively, which are closer to the boiling point of methanol at 64.7°C; therefore, it is difficult to separate them using conventional distillation methods. However, impurities such as C6–C15 alkanes are dissolved only in methanol at very high concentrations. Water and methanol are miscible with each other in any proportion. When an extractant—water—is added to the solution, reducing the methanol concentration, impurities such as alkanes precipitate out of the solution and can be separated. The boiling points of the separated heptane, isooctane, nonane, and decane are 98.4°C, 109.8°C, 150.7°C, and 174°C respectively, which are much higher than that of methanol. Thereby achieving the separation of impurities such as alkanes. IV. Factors Affecting Distillation Operations 1. What are the factors that affect distillation operations? Apart from equipment issues, the factors affecting the distillation process include the following: (1) the temperature and pressure of the tower (including the top, bottom of the tower, and certain trays of special significance) ； (2) Feed condition ； (3) Feed rate ； (4) Feed components ； (5) Feed temperature ； (6) Steam rising speed inside the tower and heating capacity of the evaporation kettle ； (7) Backflow volume ； (8) Top of tower cooling capacity ； (9) Top of tower yield ； (10) Bottom draw rate. 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. Furthermore, during the operation of extractive distillation and azeotropic distillation, changes in the addition temperature, purity, and amount of the extractant or azeotrope are also factors that affect the process. Distillation operators must overcome variations in various influencing factors during operation to prevent effects on the quantity and composition of the products at the top and bottom of the tower. 2. What is the impact of changes in the operating pressure of a distillation tower on distillation operations? The design and operation of a tower are based on a certain tower pressure; therefore, in general, a distillation tower always needs to maintain a constant pressure first. 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 vapor-liquid equilibrium on each tray. As the pressure increases, the concentration of heavier components in the gas phase decreases, thereby increasing the concentration of lighter components in the gas phase ； The content of light components in the liquid phase increased compared to before; this also altered the weight ratio between the gas and liquid phases, resulting in an increase in the amount of liquid phase and a decrease in the amount of gas phase. 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 pressure decreases, the amount of distillate at the top of the tower increases, while the concentration of light components decreases ； The volume of the kettle 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 heavy components in the product at the top of the tower, the pressure can be increased appropriately to ensure that the product quality remains satisfactory; but this will result in an increased loss of light components in the liquid in the reactor. (2) Changing the relative volatility between components: As pressure increases, the relative volatility between components decreases, resulting in a reduced separation efficiency; conversely, when the relative volatility between components increases, the separation efficiency improves. (3) Changing the tower’s production capacity: An increase in pressure and a higher density 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 under the condition that the tower pressure remains constant. When the column pressure changes, the bubble point and dew point of the mixture change, which in turn causes a change in the temperature throughout the column; the relationship between temperature and product quality also changes as a result. As can be seen from the above analysis, changing the operating pressure will affect the operation of the entire tower; therefore, a constant pressure (process parameter) should be maintained during normal operation. Only when the normal operation of the tower is disrupted can the pressure of the tower be adjusted appropriately, within the limits permitted by the process parameters, based on 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 pressure (for internal reflux towers), as well as tower clogging, can all cause fluctuations in tower pressure. In such cases, it is necessary to first analyze the causes of these pressure fluctuations and take appropriate action to restore normal operation. 3. What is the impact of feed conditions on distillation operations? There are five types of feed conditions: ① Cold feed ； ②Sponge feeding ； ③Gas-liquid mixed feed ； ④Saturated vapor feed ； ⑤Superheated steam feed. For ease of analysis, let δ = As can be seen from the above equation, δ > 1 for cold feed, δ = 1 for feed at the bubble point, 0 < δ < 1 for gas-liquid mixed feed, δ = 0 for feed of saturated vapor, and δ < 0 for feed of superheated vapor. Within the distillation tower, the area above the feed inlet is the distillation section, while the area below the feed inlet is the stripping section. The overall material balance for a distillation column is that the feed rate (F) per unit time is equal to the sum of the amount of distillate leaving the tower at the top per unit time (P) and the amount of residue remaining at the bottom of the tower per unit time (W). F = P + W. The material balance in the distillation section states that the amount of vapor rising to the distillation section per unit time (V) is equal to the sum of the amount of liquid falling to the stripping section per unit time (L) and the amount of distillate leaving the top of the tower per unit time (P). V = L + P. The material balance in the stripping section states that the amount of liquid that flows downward to this section per unit time (L′) is equal to the amount of vapor that flows upward to the distillation section (V′) plus the amount of liquid remaining at the bottom of the tower per unit time (W). L′= V′+ W. When the feed rate is F, the additional flow of liquid on the feed plate is δF; therefore, the amount of liquid that reaches the stripping section should be L′= L +δF. Moreover, since F = P + W, V = L + P, and L′= V′+ W. Then L′ = V′ + W = V′ + F – P = V′ + F – (V – L) = V′ – V + F + L. Since V′ – V + F + L = L + δF, it follows that V′ = V + (δ – 1)F. When the reflux ratio and the composition of the overhead distillate are set at specified values, changes in the feed conditions will also cause changes in the value of δ. As L′ = L + δF, this directly affects the amount of reflux liquid in the stripping section; ultimately, changes in δ will lead to changes in the number of theoretical plates, as well as in the distribution of plates between the rectifying section and the stripping section, thereby changing the position of the feed plate as well. For example, if a column is designed for feed at the bubble point, switching to feed at a cold liquid temperature results in too many trays in the distillation section and insufficient trays in the stripping section; as a consequence, the quality of the product at the top of the column may improve, but the evaporation of light components in the bottom stream is incomplete. If a gas-liquid mixed feed, or saturated steam or superheated steam feed is used, the number of trays in the distillation section becomes insufficient while the number of trays in the stripping section becomes excessive. As a result, the content of heavy components in the product at the top of the tower exceeds the specified level, and the content of light components in the bottom liquid is lower than the specified value. This also increases the consumption of coolant in the condenser at the top of the tower and reduces the consumption of heat medium in the bottom of the tower. In a distillation column, L′ = L only when the feed is saturated vapor (δ=0). Similarly, from V′ = V + (δ – 1)F, it can be seen that V′ = V only when the feed is a liquid at its boiling point (δ = 1). In production, bubble feeding is commonly used; in this case, the flow rates of the vapor rising in the distillation section and the stripping section are equal, so the tower diameter is the same, which also makes the design calculations simpler. 4. What is the impact of changes in feed composition on distillation operations? Changes in the feed composition have a direct impact on distillation operations; as the concentration of heavier components in the feed increases, the load on the distillation section rises. For a column with a fixed number of trays in the rectification section, this will result in the heavier components being carried to the top of the column, 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 distillation section rises 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 that section, leading to an 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 drops while the tower pressure increases. The composition becomes heavier; the situation is the opposite. When the feed composition changes, the following measures can be taken: (1) Improve the feed inlet; when the composition becomes heavier, move the feed inlet downward ； When the composition becomes lighter, move the feed inlet upward. (2) Change the reflux ratio: Increase the reflux ratio when the composition becomes heavier ； When the composition becomes lighter, reduce the reflux ratio. (3) Adjusting the refrigerant and heat input: Depending 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. 5. What is the impact of changes in feed temperature on distillation operations? Changes in feed temperature have a significant impact on distillation operations. Generally speaking, a decrease in feed temperature will increase the heat load on the evaporation vessel at the bottom of the tower and reduce the cooling load on the condenser at the top of the tower ； An increase in feed temperature increases the cooling load of the top condenser and reduces the heating load of the bottom evaporator. When the variation in feed temperature is too large, it usually affects the temperature of the entire tower, thereby altering the vapor-liquid equilibrium composition. For example, when the feed temperature is too low and there is insufficient heating steam at the bottom of the tower, it will increase the content of light components in the distillate at the tower bottom. Changes in the feed temperature imply changes in the feed condition, and such changes affect the load on the distillation section and the stripping section; as a result, both the product quality and the material balance are altered. Therefore, the feed temperature is one of the important factors affecting the operation of a distillation column. 6. What is the impact of the velocity of the rising steam inside the tower and fluctuations in the heating power of the evaporation kettle on distillation operations? The speed of the rising steam inside the tower directly affects the mass transfer efficiency. Generally speaking, the maximum upward steam velocity inside the tower should be slightly lower than the flooding velocity. In practice, the maximum allowable speed is often chosen to be 80% of the flooding speed. Too low a speed will significantly reduce the efficiency of the tray. The main factor affecting the upward steam velocity in the tower is the heating capacity of the evaporation kettle. With the reactor temperature remaining stable, an increase in heating amount leads to an increased speed of steam rising inside ； As the heating amount decreases, the speed of steam rising inside also decreases. It should be noted that an excessively large or rapid adjustment range of the heating amount may cause flooding or leakage. 7. What is the impact of the reflux ratio on distillation operations? During operation, the quality of the product is ensured by adjusting the reflux ratio. When the content of heavy components in the tower top distillate increases, it is common to increase the reflux ratio in order to reduce these heavy components and ensure that the product quality meets the requirements. When the light components from the distillation section move down to the stripping section, causing a decrease in temperature at the lower part of the tower, the temperature there can be raised by appropriately reducing the reflux ratio. Increasing the reflux ratio can improve the quality of the product obtained from the top of the distillation column; however, it reduces the column’s production capacity and increases consumption of water, electricity, and steam. An excessively high reflux ratio will result in an excessive circulation rate of the material inside the tower, and may even lead to flooding, disrupting the normal operation of the tower. 8. What is the impact of the amount of coolant at the top of the tower on distillation operations? For columns operating with internal recirculation, the amount of coolant has a relatively significant impact on the distillation process ； It is also the main factor affecting fluctuations in the return flow. For columns that use external reflux, fluctuations in the amount of coolant can also affect the operation of the distillation column to varying degrees. For example, a reduction in the cold dose will weaken the performance of the condenser, resulting in less condensate; and when the liquid phase yield of the product at the top of the tower is kept constant, the reflux flow will inevitably decrease. If the condenser also has a subcooling effect (i.e., what is commonly referred to as a condensing cooler), then the reduction in the amount of refrigerant will also cause an increase in the temperature of the return fluid. All of these will increase the top temperature of the distillation column, raise the content of heavier components in the product at the top of the column, and degrade its quality. 9. What is the impact of the amount of product taken from the tower top on distillation operations? There is a corresponding relationship between the amount of product extracted from the top of the tower and the feed rate to that tower; as the feed rate increases, the amount of product extracted should also increase. It is well known that only when the yield varies with the feed rate can a constant reflux ratio within the tower be maintained, ensuring the proper operation of the tower; otherwise, the gas-liquid equilibrium inside the tower will be disrupted. For example, when the feed rate remains constant, in a column that uses internal reflux, if the amount of product taken from the top of the column increases, the reflux ratio will inevitably decrease, resulting in less reflux liquid on each tray, poor gas-liquid contact, and a reduced mass transfer efficiency ； At the same time, the operating pressure will also decrease, and the gas-liquid phase compositions on each plate will change. As a result, the heavier components are carried to the top of the tower, resulting in product at the top having substandard quality. During forced reflux operation, if the feed rate remains constant but the amount taken from the top of the tower suddenly increases, it is easy to cause the reflux liquid tank to become empty. As soon as the reflux flow is interrupted, the top temperature rises, which also leads to a decline in the quality of the product at the tower top. If the feed rate is increased while the amount taken from the top of the tower remains unchanged, the result is an increase in the reflux ratio, more material inside the tower, an increased velocity of the rising steam, and a greater pressure difference between the top and bottom of the tower; in severe cases, this can lead to flooding. 10. What is the impact of the bottom product flow rate on distillation operations? Maintaining a stable liquid level in the tower bottom is the primary condition for keeping the tank temperature constant. The change in the liquid level at the bottom of the tower is primarily determined by the amount of product taken out from the bottom of the tower. When the draw rate at the bottom of the tower is too high, it can cause the liquid level in the tower bottom to drop or even lead to evacuation. This will reduce the circulation rate of the broth through the evaporation kettle, resulting in poor heat transfer; the light components cannot be vaporized, and the products at the top and bottom of the tower are both unsuitable. If a shell-and-tube evaporator is used, the low volume of circulating liquid causes the liquid in the evaporator to become superheated as it passes through the upper section of the tubes; this results in a higher gas temperature in the vaporization tubes while the temperature of the liquid in the evaporator remains low. If the draw rate at the bottom of the tower is too low, it will result in a high liquid level in the tower bottom; in severe cases, this level can exceed the vaporization pipe and even submerge the tower. This increases the resistance to the circulation of the liquid in the tower, which in turn leads to poor heat transfer and a decrease in the tower temperature. It should be particularly noted that for materials prone to polymerization, either an excessively high or low liquid level in the reactor will result in an increased residence time, thereby increasing the likelihood of polymerization. Additionally, maintaining a certain level of liquid in the kettle serves as a liquid seal to ensure safe production. 11. Summary of distillation column operation: For operating a distillation column, it is essential to master three types of equilibrium. (1) Material balance: F = D + W and FxFi = DxDi + WxWi, where F is the feed amount ； D —— Top discharge volume ； W —— Discharge volume at the bottom of the tower ； xFi——Feed composition ； xDi — composition of the overhead product; xWi — composition of the bottom product. Material balance reflects the production capacity of the tower, which is primarily regulated by the feed rate and the amounts taken out from the top and bottom of the tower. When the operation of the tower does not conform to the overall material balance, this can be observed from changes in the tower pressure difference: more input and less output result in an increase in the tower pressure difference. For a distillation column that is to be stabilized, the column pressure difference should remain within a certain range. An excessive pressure difference in the tower indicates that the speed of the rising steam inside the tower is too high, resulting in severe entrainment of mist, and even flooding, which disrupts the normal operation of the tower ； An excessively low pressure difference in the tower indicates that the velocity of the rising steam inside the tower is too low; as a result, the turbulence of vapor and liquid on the tray plates is insufficient, leading to poor mass transfer. This can also cause leaks in tray elements such as sieve trays, floating valves, bubble caps, and inclined holes, thereby reducing the efficiency of the tray plates. If the operation of the distillation column does not satisfy the material balance equation for a certain component, two effects will occur: ① DxDi > FxFi – WxWi. In this case, the amount of light component removed exceeds what is allowed by the material balance, resulting in a heavier composition of the material inside the column. The temperature throughout the column gradually rises, and the concentration of heavy components in the overhead distillate increases, leading to products that do not meet quality standards. For example, in methanol production, the reflux flow rate in the pressure tower is regulated at a fixed value; if this fixed value is too low, the amount of product extracted will be excessive, which will result in a reduced distillation volume of pure methanol, an increase in the dry point, and substandard quality. ②WxWi > FxFi – DxDi; this situation is the opposite of the first one, as the amount of the heavier components recovered exceeds the amount specified by the material balance. The composition of the materials throughout the tower gradually becomes lighter as operations proceed, the temperature of the tower decreases, especially the temperature of the reactor, while the concentration of the lighter components in the reactor liquid increases. It can be seen that poor control of material balance will lead to chaos in the operation of the entire tower, failing to achieve the desired objectives. Therefore, material balance is a key aspect in tower operation. Furthermore, if the normal material balance is disrupted, the gas-liquid equilibrium will not achieve the desired results, and as a consequence, the heat balance also needs to be adjusted. (2) Gas-liquid equilibrium: yi = piXi or yi = KiXi, where yi is the molar amount of component i in the mixture ； pi——saturation vapor pressure of pure component I at that temperature ； Xi —— the number of moles of component i in the solution ； Ki——gas-liquid equilibrium constant. Vapor-liquid equilibrium mainly reflects the quality of the product and the amount of loss. This is achieved by adjusting the operating conditions of the tower (temperature, pressure) and the gas-liquid contact on the tray. Because only when temperature and pressure are fixed is there a definite gas-liquid equilibrium composition. The operating temperature and pressure of a distillation column are determined by the column’s separation task (i.e., the degree of separation of the key components). When temperature and pressure change, the composition determined by gas-liquid equilibrium changes, as do the quality of the product and the amount of loss. However, the gas-liquid equilibrium composition is achieved through mass and heat transfer that occurs as a result of gas-liquid contact on each tray. In other words, gas-liquid equilibrium is closely related to material balance. When material balance is well managed, the velocity of the steam rising inside the tower is appropriate, and gas-liquid contact is good, then the mass transfer efficiency is high; the gas and liquid compositions on each tray become closer to the equilibrium compositions, which is what is referred to as high tray efficiency ； Conversely, it is low. Of course, temperature and pressure also change as the material balance changes. In short, the composition of gas-liquid equilibrium is inseparably related to material balance. Conversely, changes in temperature and pressure can cause changes in the relative amounts of vapor and liquid phase on the tray, thereby disrupting the original material balance. For example, in methanol production, if the temperature of the reactor is below the specified value, it leads to an increase in the liquid phase volume on the tray, a decrease in the vapor volume, an increase in the amount of liquid in the reactor, a downward movement of the methanol component, and a reduction in the amount of methanol at the top ； When the top temperature is higher than the specified value, it increases the amount of vapor on the tray, reduces the amount of liquid phase, increases the amount of product at the top, and decreases the amount of liquid in the reactor. All of these will disrupt the normal material balance. (3) Heat balance: Q_condensation = Q_vaporization (for each tray), and Q_in = Q_out + Q_loss (for the entire tower). Here, Q_condensation refers to the heat amount of condensation in the vapor phase on each tray ； Q_vaporation – the heat required to vaporize the liquid phase on each tray ； Q_in – Total heat introduced by the materials and total external heating amount ； Q_out – Total heat carried away by the material ； Q-loss – the heat lost throughout the entire tower. Heat balance is the foundation upon which material balance and gas-liquid balance are achieved. Without heating from the bottom of the tower, there is no rising steam; without condensation at the top of the tower, there is no reflux liquid, and thus the entire distillation process cannot take place. And heat balance is in turn dependent on mass balance and gas-liquid balance. For example, if the feed rate or composition changes, both the heat consumption at the bottom of the tower and the cooling load at the top of the tower should be adjusted accordingly. Otherwise, either the low reflux rate affects the quality of methanol, or an excessive reflux ratio leads to unnecessary waste. When the operating pressure and temperature of the tower change (i.e., the gas-liquid equilibrium composition changes), the heat released due to vapor condensation on each tray, as well as the heat absorbed due to liquid vaporization, also change. In summary, these changes are reflected in alterations in the heat supply at the bottom of the tower and the heat removal at the top of the tower. Conversely, changes in heat balance can also affect changes in material balance and gas-liquid balance. For example, if the heating supply to the reactor is insufficient, the temperature inside the reactor will not reach the specified value; as a result, the reflux ratio decreases, leading to disruptions in the operation of the tower. This causes: ① a disruption in material balance, an increase in the amount of liquid discharged from the reactor, and a decrease in the amount of distillate at the top of the tower. For the process of obtaining products at the tower’s top, this leads to a reduction in the tower’s production capacity. ②The gas-liquid equilibrium is disrupted, resulting in a decrease in the amount of vapor rising within the tower. This leads to poorer gas-liquid contact, reduced mass transfer efficiency. At the same time, the concentration of heavier components in the gas phase decreases, while the concentration of lighter components in the liquid phase increases, causing an increased loss of lighter components in the bottom stream. The proper operation of the crude methanol distillation process involves using control measures to maintain three balances. Generally, a certain amount of heat is supplied to the bottom of the tower based on its load, in order to establish a heat balance ； This leads to a certain gas-liquid equilibrium, after which material balance is used as a regular tool to maintain the stability of both the heat balance and the gas-liquid equilibrium. During operation, it is usually the material balance that changes first (load and composition); accordingly, by adjusting the heat balance (return flow rate and reflux ratio), gas-liquid equilibrium is achieved (including the quality of pure methanol, the alcohol content in the residue, and the degree of concentration of heavy components). Naturally, when a change in the heat supply to the bottom of the tower disrupts the heat balance, the heat supply must be adjusted (usually through automatic control) to restore balance, along with adjustments to the material balance (and even the tower load), so as to prevent serious disruption of the gas-liquid equilibrium within the tower. 12. What is the impact of auxiliary equipment on distillation operations? Distillation operations rely on components such as the bottom evaporator, top condenser, and transfer pumps. In normal operation, the performance of these devices must meet the requirements for distillation tower operation; otherwise, the production capacity of the distillation tower will be limited, and in severe cases, the entire operation process will not be able to proceed. For example, during the production process in the evaporation kettle, the tubes may become clogged due to the polymerization of certain unsaturated hydrocarbons, which reduces the heat transfer area and leads to a decrease in the tower’s production capacity or even forces it to stop operating. The top condenser also experiences reduced condensation efficiency due to poor water quality and tube fouling, which lowers the tower’s production capacity and prevents it from operating under normal process conditions. When the normal production process is disrupted, it is necessary to look for causes not only in the distillation column’s operating procedures but also to consider the condition of the auxiliary equipment. 13. What is the impact of tower installation on distillation operations? Different materials and different processing procedures impose varying requirements on tower equipment. However, it is generally desired that tower equipment have high applicability, large production capacity, and stable operation. For a standard tower unit, installation issues may prevent it from meeting the aforementioned requirements. Elements such as the tower body, tray, overflow weir, and downcomer, if not installed properly, can all have an impact on the distillation process. (1) Tower body: The tower body must be vertical; the inclination should generally not exceed one in a thousand, otherwise dead zones will form on the tray surface (as shown in Figure 4-1). For small distillation columns with a diameter of 600 millimeters or less, if the tray plates are installed in sections first and then assembled, the lack of verticality in the column body will directly affect the levelness of all the tray plates in the column, thereby reducing its efficiency. (2) Tray plates: These plates must be level, with their levelness measured using a level; the deviation should not exceed ±2 millimeters. If the tray is not level, it will result in uneven liquid layer heights on the tray surface. The rising steam inside the tower can easily pass through the areas where the liquid layer is shallower (as shown in Figure 4-2), making it difficult to ensure the efficiency of the tray. This has a particularly severe impact on sieve plate towers. (3) Overflow port: The distance between the overflow port and the lower tray (as shown in Figure 4-3) should be determined based on the production capacity and the height of the lower overflow weir. However, it is necessary to ensure that the overflow port is inserted into the liquid in the receiving tray in order to seal off the rising steam. If the overflow port is too close to the lower tray, it may cause backflow in the upper trays, as the liquid cannot flow smoothly into the lower trays. This leads to an increase in the liquid level in the upper trays and an increase in pressure in the lower trays; in severe cases, this can result in flooding. If the overflow port is too high, exceeding the height of the overflow weir, the rising steam takes a shortcut, rising directly to the upper tray through the overflow pipe; this prevents the formation of a liquid seal and affects the efficiency of the trays. During installation, various specific plate types have different requirements; failing to follow these requirements may result in a **decline in the tower’s production efficiency**. 14. What are the operational requirements for sieve plate towers? The tray structure of sieve plate towers features low operational flexibility, requiring relatively strict operating conditions. (1) The vapor velocity must not be lower than the minimum value (about 0.1 m/s); otherwise, the liquid is likely to leak through the sieve pores, reducing the mass transfer efficiency. (2) The vapor velocity should not be too high; otherwise, the liquid on the tray will be blown away by the vapor stream, resulting in a dry tray. On the one hand, this can lead to foam entrainment, and on the other hand, it prevents mass transfer on the tray surfaces, resulting in a decrease in distillation efficiency. (3) The levelness of the tray plates is subject to strict requirements; when the levelness is poor, the trays tilt significantly, which can lead to gases passing through the shallower areas of the liquid surface without fully contacting the liquid, thereby reducing the efficiency of distillation. 15. What is the function of the pre-distillation tower? The function of the pre-distillation tower. ①Removes light organic impurities such as dimethyl ether and methyl formate, as well as syngas dissolved in crude methanol. ②Water extraction is carried out to remove the light fractions with a boiling point similar to that of methanol, as well as to separate the methanol-alkane azeotropes with a boiling point close to that of methanol. After pre-distillation, the potassium permanganate value of the aqueous methanol is at least 1 point or more, with the pH value maintained between 8 and 9. ③When there are specific requirements for the ethanol content in pure methanol, the pre-distillation column helps to partially remove the azeotrope of ethanol. 16. What is the significance of controlling the condensation temperature at the top of the pre-distillation tower? Most of the organic impurities, which are primarily light components, pass through the overhead condenser of the pre-distillation tower without being condensed, and are then removed by venting them through the discharge tank. The condensation temperature acts as a threshold; its level has a direct impact on the components that are removed as impurities. Although the content and composition of crude methanol impurities primarily determine the choice of catalyst, changes in catalyst temperature and pressure have a significant impact on both the composition and total amount of these impurities. In the middle and later stages of the catalyst usage, as the reaction temperature and pressure increase, the components with medium to high boiling points in the impurities increase significantly in both quantity and proportion. Synthesis pressure/MPa 200°C 220°C 240°C 260°C 280°C 300°C 5 7 10 15 20 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.3 0.2 0.2 0.3 0.3 0.3 0.2 0.2 0.4 0.5 0.4 0.3 0.4 0.4 0.7 0.6 0.5 0.6 0.5 0.8 0.8 0.6 0.8 The table above shows the impurity content of crude methanol synthesized under different conditions (copper catalyst). At 200°C, the impurity content is 0.1% at 20 MPa, while at 300°C it is 0.8%. The total amount of impurities in crude methanol is usually estimated by determining the ethanol content in it; for example, the ethanol level is 150–200 mg/kg at the early stage of catalyst use, while it rises to over 1000–2000 mg/kg at later stages. The constant control of the condensation temperature at the top of the pre-distillation tower cannot meet the requirements resulting from changes in the composition and concentration of crude methanol. By also keeping the condensation temperature between 30 and 40°C, if the water solubility and stability of the catalyst in its early stages meet the requirements, they may not meet those requirements in later stages. Therefore, as the catalyst enters the middle to later stages and the temperature and pressure of the synthesis reaction increase, the condensation temperature should also be increased gradually to facilitate the effective removal of impurities. Naturally, controlling the condensation temperature at the top of the tower helps improve product quality while also preventing reduced distillation losses of methanol. 17. What are the advantages of controlling the temperature of the sensitive plate in the stripping section of a distillation column? In the plate-by-plate calculation of the distillation column, it can be observed that the composition of the liquid on a particular plate or over a certain range of plates changes significantly, which in turn results in substantial temperature variations. When changes in operation disrupt the material balance, the temperature of that plate or that section of plates is the most sensitive to such changes. In actual production, one of these plates is selected as the sensitive plate, using this temperature to control changes in the material. The sensitive plate in the stripping section of the distillation column is located between the 8th and 12th plates from the bottom; this temperature is referred to as the fusel oil recovery temperature, and it is generally controlled at 88–94°C. Advantages of controlling the temperature at this point: ① Sensitive to changes, with accurate adjustment. ②It is possible to anticipate the trend in material changes and make adjustments in advance. An increase in temperature indicates that the heavier components move upward, while a decrease in temperature indicates that the lighter components move downward. Especially when the temperature drops, it is necessary to increase the amount of product taken from the top of the tower or reduce the feed rate in advance. If necessary, increase the extraction of fusel oils to prevent methanol and medium-boiling components from reaching the bottom of the tower, which could otherwise lead to excessive levels of methanol and COD in the liquid discharged from the tower bottom. 18. What are the advantages of controlling the temperature of the sensitive plate in the distillation section of a distillation tower? Similar to the significance of the sensitive plate in the stripping section, a location highly sensitive to reactions is also selected as the temperature of the sensitive plate in the distillation section. As can be seen from Figure 4-4, the temperature difference from the top of the tower to its middle section is very small, and the variation in the top temperature is also minimal. It is only under conditions of significant material imbalance that such differences become apparent; this often leads to delays in regulation, resulting in large fluctuations. Moreover, heavier components are likely to end up in the product, affecting its quality. The temperature and concentration in the middle of the tower change significantly; as long as these are kept within certain limits, the temperature and composition at the top of the tower can be maintained ； Once the material balance is disrupted, the tower temperature here is the most sensitive to such changes; therefore, the temperature of the sensitive plates in the distillation section is set at the 26th to 30th plates from the bottom (some choose the 36th plate as well), with the temperature controlled between 76 and 80°C. Pre-adjustment is carried out to ensure the stability of the temperature throughout the tower, especially at its top. Under normal production conditions, maintaining this temperature is key to the material balance of the entire tower. 19. What is the significance of controlling the top temperature of a distillation tower? The top temperature of the distillation tower is an important factor determining the quality of methanol products. Essentially, it is the boiling point of pure methanol under operating pressure; in distillation columns at atmospheric pressure, this temperature is generally maintained between 66 and 67°C. If, under the condition of stable column pressure, the top temperature increases, it indicates an increase in the heavy components at the top of the column, resulting in values for boiling range and potassium permanganate that exceed the specified limits. At this point, it is necessary to determine whether it is due to process reasons or a leak in the equipment’s condenser. The former is often due to an increase in the heavy components inside the tower, while the latter is caused by moisture outside the tower being carried back to the top of the tower ； If it is due to process reasons, the steam volume and return flow rate should be adjusted ； If the reflux ratio is low, the amount of steam should be increased to raise the reflux ratio; if necessary, the extraction of pure methanol can be reduced or suspended, and product extraction should resume only once the top temperature returns to normal, in order to maintain material balance within the tower. If the equipment’s condenser leaks, the operation should be stopped to locate and fix the leak, after which normal production can resume. 20. What is the significance of controlling the temperature at the bottom of a distillation column? If the separation effect in the tower is good, the liquid at the bottom of the reactor is nearly a single-component water, with a boiling point of 106–110°C (depending on the pressure at the bottom of the tower). Maintaining a normal temperature at the bottom of the reactor helps prevent the loss of light components and improves the methanol recovery rate ； It can also reduce the contaminating effect of residual liquids. If the temperature at the bottom of the tower decreases, it is often due to light components being carried over into the residue, or a sudden reduction in heat load; it may also be caused by an excess of heavy components (azeotropes with boiling points lower than that of water) in the lower part of the tower. At this point, it is necessary to assess the situation and make adjustments, such as adjusting the reflux (by increasing the heat load), increasing the methanol recovery rate (taking into account the temperature of the sensitive plates in the distillation section), or increasing the recovery of heavier components; if needed, the feed rate should be reduced. V. Preparations before driving, driving, and parking 1. What preparations are required before starting up the distillation tower after its installation? After the distillation tower is installed, a strength pressure test must be carried out first, followed by purging of the tower and pipelines as well as a gas-tightness test. The process flow must also be verified to be functional; only after all these tasks are completed can the tower be accepted and put into use for production. It should be noted that the order of the strength pressure test and the airtightness test must not be reversed, otherwise it may lead to safety hazards. 2. What are the specific requirements for the strength pressure test of equipment? After the equipment is installed, a strength pressure test must be carried out first, with water generally being used as the testing medium. The test pressure shall be the pressure specified in the equipment’s design drawings. If there are no specific requirements stated in the drawings, generally, the following procedure can be followed: for equipment with a working pressure of 5 kilograms/cm2 or less, the test pressure is 1.5 times the working pressure (except for cast iron towers) ； For those with a working pressure of 5 kg/cm2 or higher, the test pressure is 1.25 times the working pressure ； When the working pressure is less than 2 kg/cm2, the test pressure is 2 kg/cm2. For containers operating at atmospheric pressure, a water filling leak test can be performed alone. During the test, the pressure of the equipment should be increased gradually. When a leak or other defect is detected, the pressure must be reduced to normal levels; it is not allowed to repair the equipment while it is under pressure to avoid accidents. After the maintenance, raise the voltage again and conduct another inspection. During the pressure testing process, the following points must be followed. (1) During the strength pressure test, after reaching the specified pressure, the pressure must be reduced for 5 minutes; thereafter it is lowered to the operating pressure. A hammer weighing 0.5–1.5 kilograms is then used to strike areas 150 millimeters away from both sides of the weld, in order to check for any leaks or deformation. (2) The allowable pressure drop within one hour is: when the volume of the equipment is 1 cubic meter or less, the allowable drop is less than 1% ； When the volume of the equipment is 1 to 3 cubic meters or less, the allowable decrease is less than 0.5% ； When the volume of the equipment is 3 m3 or more, the allowable decrease is less than 0.2% ； (3) Normal temperature water is generally used as the medium for strength pressure testing; the water should be injected at the lowest point of the equipment so that the gas inside can be released from the highest point of the equipment. 3. Why is it necessary to purge new equipment or equipment that has been overhauled, as well as the pipelines connected to it? The purpose of purging is to remove dust, welding rods, iron shavings, and other debris that remain in the equipment or pipelines during installation. Compressed air is generally used as the purge gas. The inspection method for cleaning is to use a white gauze to check at the end of the cleaning process; if no black spots are present, the cleaning is considered successful. The direction of the purging gas should be from the highest point of the equipment downward. 4. Why is a gas-tightness test required for the equipment before it is put into operation? What are the specific requirements? The purpose of the airtightness test is to ensure, through testing, that the equipment is airtight and leak-free; if any leaks exist, they can be rectified before operation begins. This prevents the leakage of toxic, flammable, and explosive materials, ensuring continuous and normal production. The airtightness test requirements are as follows. (1) The medium used for the airtightness test is most commonly air ； However, for equipment that must not have air present after testing, nitrogen should be used. If reducing nitrogen usage is possible by using materials, an air test can be conducted first; once it is successful, nitrogen can then be used to replace the air. Test the oxygen content in the air; production can only begin once it meets the required standards. (2) The pressure gauge used for testing should have an appropriately higher accuracy to facilitate the detection of leaks. (3) The pressure for the airtightness test is generally 1.1 times the operating pressure. For equipment with a working pressure of 5 kg/cm2 or higher, the test pressure is 1.05 times the working pressure ； The pressure reduction value is generally 2 kg/cm2; special requirements apply to equipment operating at high temperatures, and these should be determined based on the process requirements and the equipment design drawings. (4) Airtightness test of the system, requiring pressure to be maintained for 24 hours ； For the airtightness test of individual units, it is specified to maintain the pressure for 8 hours. (5) It is advisable to use two pressure gauges during the test, with measurements taken for each temperature separately, in order to enable comparison. (6) The passing requirement for the airtightness test is that the average leakage rate per hour must not exceed 0.25%. The calculation formula is as follows: A = ÷t×100%, where A represents the leakage rate per unit of time, in % ； P_initial, P_final – the absolute pressures of the initial and final tests ； T_initial, T_final – the initial and final test temperatures, expressed here in absolute temperature as 273+T ℃ ； t — total pressure testing time. 5. Why is it necessary to replace the air with an inert gas before the equipment is put into operation? When the materials to be processed are flammable or explosive, if the air inside the equipment – primarily oxygen – is not removed before it is put into use, there is a risk of fire and explosion after material feeding begins. Therefore, it is necessary to replace the air inside the equipment with an inert gas before it is put into operation. Inert gases remain after the displacement, and since they do not react chemically with the material being separated, potential accident risks can be eliminated. Inert gases can be gradually removed during startup. The commonly used inert gas is nitrogen. Before operation, the oxygen content in the equipment shall not exceed 2% (by volume). 6. What are the disadvantages of excessive water in the distillation tower during startup? When a distillation tower used for separating water-immiscible organic compounds is brought online, excessive water in the reactor tank can cause the temperature of the reactor tank to drop. Sometimes, even if the temperature rises, it may still fall again as water from the tray gradually returns to the bottom of the tower. At this point, it is necessary to drain water in order to raise the temperature of the kettle to normal levels. The reason for the low pot temperature can be considered from the principle of steam distillation. That is, at the bottom of the tower, water and the material form two immiscible phases. When heated, the boiling point of the mixture will be lower than that of any of its components. Due to this decrease in boiling point, the temperature at the bottom of the tower cannot reach the desired process parameters. 7. What preparations are required for a distillation tower before it is put into operation? The main preparatory work that needs to be done before starting up a distillation tower includes: ① Checking whether water, electricity, gas (air, nitrogen), and steam (water vapor) meet the process requirements ； ②Is the transmission equipment ready for use? ； ③ Are the equipment, instruments, and safety facilities complete and in good working condition? ； ④The valves used must be in a closed state ； ⑤Each water condenser (cooler) should be pre-cooled with a small amount of water, and the heating kettle should be pre-heated with a small amount of steam ； ⑥The oxygen content within the equipment should meet the requirements for feeding ； ⑦Ensure good coordination between the upstream and downstream units (or positions); in particular, pay attention to the supply of raw materials as well as the storage and transportation of products. Meanwhile, the laboratory should be prepared to take samples for analysis. 8. What should be noted when starting up a distillation tower? (1) Upon receiving the start-up command, immediately contact the relevant stations to proceed with startup. (2) Strictly adhere to the process procedures and operational guidelines, and strengthen routine inspections. (3) Fine-tune carefully. The feed rate should be steady; once a liquid level is observed in the bottom of the tower, the temperature should be increased slowly according to its heating rate until the process parameters are reached. As the tower pressure increases, the inert gas inside the tower is gradually removed, and the cooling capacity of the top condenser is progressively increased. Reflux is initiated once the liquid level in the reflux tank reaches more than 1/2 of its capacity. When the liquid level in the reactor reaches 2/3, it is possible to decide whether to remove the liquid from the reactor or to reduce or even stop the feed rate to the tower, depending on the temperature of the reactor; however, the liquid level in the tower reactor must always be maintained between 1/2 and 2/3. Once the operation is stable, material analysis should be conducted. Unqualified materials can be removed in small quantities or sent back through the process entirely; once the analysis shows that the materials are qualified, production can proceed on a continuous basis. (4) While driving, it is essential to adjust the valves and instruments frequently and slowly, and to use them in a proper manner. (5) If any abnormality is detected, the cause should be analyzed promptly and decisive action taken. 9. Why must the temperature of the slurry be increased slowly while driving? During charging to an empty tower, there is no reflux liquid, so the trays in the distillation section operate in a dry-tray condition. Due to the lack of gas-liquid contact, the poorly volatile components in the gas phase can be easily carried directly into the distillation section. If the temperature rises too quickly, the less volatile components will be carried in large quantities to the distillation section, where they are replaced by the more volatile components uselessly; as a result, the quality of the product at the top of the tower cannot meet the required standards, leading to a longer startup time. Once there is reflux liquid at the top of the tower and a liquid layer is formed on the tray, the heating rate can be increased appropriately. 10. How is a distillation tower shut down? The shutdown of a distillation tower can be divided into two types: temporary shutdown and long-term shutdown. Temporary shutdown: Upon receiving the shutdown command, feeding to the tower, extraction from the top of the tower, and extraction from the bottom of the tower are immediately stopped, and full reflux operation is initiated. Appropriately reduce the cooling load at the top of the tower and the heating load at the bottom, keeping the entire tower in a state of heat and pressure retention. If the parking time is short, it can be handled according to the specific conditions of the tower: only the feed to the tower should be stopped, while the product taken from the top of the tower can continue to be extracted, in order not to affect the production in subsequent processes; however, the extraction from the bottom of the tower should be stopped. This practice disrupts the normal material balance and should not be used for extended periods, otherwise the product quality will decline. For long-term parking, upon receiving the shutdown command, feed to the tower should be stopped immediately; however, product extraction can continue. If the analysis results are unsatisfactory, extraction should be halted, along with heating of the tower bottom and condensation at the tower top, after which the liquid in the bottom of the tower should be drained. For columns used to separate low-boiling-point materials, the draining of the bottom liquid should be carried out slowly to prevent excessively low temperatures resulting from throttling, which could cause the equipment materials to become brittle. After the discharge is complete, release the residual pressure inside the equipment and then purge it with nitrogen; maintenance can only be carried out once it meets the required standards. If it is necessary to enter the equipment for maintenance, the nitrogen must be replaced with air; maintenance personnel are only allowed to enter when the oxygen content in the gas inside the equipment reaches 19% (by volume) or more. VI. Specific Operations 1. Quality standards for pure methanol: GB338-92 (Industrial pure methanol)
Parameter | Grade 1 | Grade 2 | Qualified grade
Colorimetry (platinum-cobalt)/number ≥ 5 10
Density (20°C)/(g/cm3) 0.791–0.792 0.791–0.793
Temperature range (0°C, 101325 Pa)/°C 64.0–65.5
Boiling range (including 64.6°C ± 0.1°C)/°C ≤ 0.8 1.0 1.5
Potassium permanganate test/min ≥ 50 30 20
Water solubility test Clarity —
Acidity (as HCOO)/% ≤ 0.0015 0.0030 0.0050
Alkalinity (as NH3)/% ≤ 0.0002 0.0008 0.0015
Moisture content ≤ 0.10 0.15 —
Aldehyde content (as CH2O)/% ≤ 0.002 0.005 0.010
Evaporation residue content/% ≤ 0.001 0.003 0.005
2. How to adjust the pressure in the distillation process? What are the factors that affect tower pressure changes? The pressure in the tower is one of the main control parameters for a distillation tower. In the operation of any distillation column, the column pressure must be kept within specified limits in order to adjust other parameters accordingly. Excessive fluctuations in tower pressure can disrupt the material balance and gas-liquid equilibrium throughout the tower, resulting in a product that does not meet the required quality standards. Therefore, many distillation columns have specific measures to ensure that the column pressure remains within an appropriate range. For pressure control in atmospheric pressure towers, there are mainly the following three methods. (1) When the stability requirements for the top pressure are not high, there is no need to install a pressure control system; instead, a pipe leading to the atmosphere should be provided on the distillation unit (condenser or reflux tank) to ensure that the pressure inside the tower is close to atmospheric pressure. (2) When high stability requirements are imposed on the top pressure of the tower, or when the material to be separated cannot come into contact with air, the control of the top pressure can be achieved using the control method for the pressure in a pressurized tower. (3) Adjust the vapor pressure in the tower bottom by using steam to heat the tower bottom (as shown in Figure 6-1). For pressure control in pressurized towers, there are mainly the following three methods. (1) When the top condenser is a fractional condenser, the column pressure is generally adjusted based on the gas phase withdrawal rate (as shown in Figure 6-2). With its conditions remaining unchanged, the gas phase recovery increases while the pressure decreases ； The gas phase recovery decreases and the pressure rises. (2) When the top condenser is a total condenser, the column pressure is usually adjusted by controlling the amount of refrigerant, which is equivalent to adjusting the temperature of the reflux liquid (as shown in Figure 6-3). With all other conditions unchanged, increasing the amount of cooling agent lowers the temperature of the reflux liquid and reduces the tower pressure; whereas reducing the amount of cooling agent decreases %B