What issues should be considered in distillation operations?

That’s quite general, isn’t it? There are so many things to keep in mind: processes, equipment, instruments, and so on. Which one did you ask about?

Reflux: Throughout the entire distillation process, the highly pure, volatile components are obtained at the top of the tower and distilled out, while the bottom of the tower contains mainly the non-volatile components. The difference between distillation and rectification lies in reflux, which includes the reflux of the liquid phase at the top of the tower as well as the reflux of the liquid phase resulting from the partial vaporization at the bottom of the tower. Backflow is a necessary condition for mass and heat transfer through gas-liquid phase contact. Without contact between the gas and liquid phases, no mass exchange can take place. Of course, the difference in the volatility of components remains the basis of the distillation process. The vapor generated by the heating of the mixture during the distillation process emerges from the top of the tower, enters the top separator, and condenses into a liquid. Part of this condensed liquid is returned to the top of the tower and flows downward along the tower plates; this liquid is known as reflux. A portion of the condensate is taken from the top of the tower as a product, and the reflux ratio is the ratio of the liquid reflux in the distillation section to the amount of liquid taken out. A high reflux ratio results in good separation and high product quality. However, if the reflux ratio is too high, production capacity decreases and energy consumption increases. The reflux ratio has a significant impact on distillation operations; it affects the material concentration on each tray within the tower as well as the temperature distribution, which ultimately influences the separation efficiency. 7. Adjustment of the reflux ratio: The reflux ratio for the main distillation tower is 2.0–2.5. The basis for this adjustment lies in the load on the tower and the quality of the purified methanol. When the tower’s load is low, there is a surplus of tray capacity, allowing for a lower reflux ratio, which is more economical. To ensure the quality of the purified methanol, the temperature of the sensitive trays in the distillation section can be kept slightly lower. Conversely, if a higher quality of purified methanol is required, the reflux ratio is increased; meanwhile, to maintain the temperature at the bottom of the tower and that of the sensitive trays, these temperatures can be kept slightly higher. In the distillation of pure methanol, an excessively high or low reflux ratio can affect the economic efficiency of the distillation process as well as the quality of the pure methanol. The reflux ratio is adjusted in cases of changes in load, when normal operating conditions are disrupted, or when the product does not meet the required standards. After adjustment, strive to keep the heating rate at the bottom of the tower stable in order to maintain a constant reflux ratio. While adjusting the reflux ratio, it is necessary to take into account the operating characteristics of the plate tower in order to prevent flooding and severe liquid leakage, as both can lead to disruptions in the operating temperature within the tower. 8. Effect of feed rate: Changes in the feed rate and composition of the distillation tower affect the material balance and gas-liquid equilibrium within the tower, resulting in fluctuations in tower temperature. If these changes are not adjusted in a timely manner, it can lead to poor quality of refined methanol or increased losses of methanol. Generally, when the feed rate fluctuates within the operating conditions of the tower and within the capacity limits of the associated equipment, as long as adjustments are made in a timely manner, it will not have a significant impact on the temperatures at the top and bottom of the tower; it only affects the velocity of the steam inside the tower. However, such changes in volume should be made gradually, otherwise, due to the operational characteristics of the tray system, rapid changes in temperature at the top and bottom of the tower can occur, which in turn affects the quality of purified methanol and may lead to losses. Handling method: After the feed rate changes, it is necessary to consider adjusting the heat load based on the reflux ratio. When the feed rate increases, the speed of the steam rises; this is generally favorable for mass transfer, but the steam speed must remain below the flooding velocity. When the feed rate decreases, the steam speed drops, which is unfavorable for mass transfer. Therefore, the steam velocity should not be too low. Sometimes, in order to maintain the separation efficiency of the tray, the reflux ratio is intentionally increased to an appropriate level; this also raises the steam rise velocity within the tower, thereby improving mass transfer. Of course, this approach is not economical, which is why distillation towers should not be operated at low loads. 9. Pressure drop: The pressure difference between the bottom and top of the tower. Distillation production is closely related to the pressure drop in distillation. For plate towers, the pressure drop across the plates consists of three components: (1) the pressure drop on the dry plates ； (2) Liquid layer pressure drop ； (Pressure drop to overcome liquid surface tension). The pressure difference between the bottom and top of the tower is the sum of the pressure differences across each tray in the tower. The pressure difference across a dry tray refers to the pressure drop caused by the rising vapor within the distillation tower as it passes through trays where there is no liquid present. The liquid layer pressure drop is the pressure drop caused by gas passing through the liquid layer on each tray of the tower. The pressure drop due to liquid surface tension is the pressure drop exerted by the liquid surface tension on the gas. Fluctuations in pressure drop can disrupt the corresponding relationship between temperature and components; in our operations, we often use temperature as an indirect indicator to assess product quality. But this is only true under the condition of a normal and constant column pressure; when the column pressure increases, the boiling point of the mixture rises as well, causing the temperature throughout the column to change. The relationship between temperature and product quality will also change. An increase in pressure drop leads to a decrease in the volatility of the components, resulting in reduced separation efficiency. As the pressure drop rises, the concentration of heavier components in the gas phase decreases, while the concentration of lighter components increases accordingly. This leads to an increase in the amount of liquid phase and a decrease in the amount of gas phase. The overall effect is an increase in the concentration of lighter components in the overhead distillate, although their quantity decreases relatively. The concentration of lighter components in the bottom stream also increases, along with the volume of that stream. Changes in pressure drop can have a significant impact on the stable operation of the tower; therefore, it is necessary to maintain the pressure drop within its normal range during tower operation. 10. Hazards of increased pressure drop and its handling: In a distillation tower, if the speed of the vapor rising inside the tower exceeds the maximum allowable value and rises to a certain level, the liquid is prevented from flowing downward by the gas; it accumulates more and more, and may even overflow from the top of the tower. This phenomenon is known as flooding, and it results in a sharp increase in the tower’s pressure drop. Whether it is a plate tower or a packed tower, the feed rate should be reduced or stopped, the amount of steam supplied to the reboiler should be slightly decreased, the temperature of the reactor vessel should be lowered, and distillation at the top of the tower should be ceased in order to achieve full reflux. This allows the components that are difficult to vaporize to flow back slowly to the bottom of the tower and to their normal positions. If production does not permit the cessation of feeding, the temperature of the reactor vessel can be maintained slightly below the normal operating temperature; the amount of product distilled from the top of the tower can be increased (at this point, the pure methanol produced may not meet quality standards), and the reflux ratio can be reduced. Once the pressure difference in the tower returns to normal, normal operating conditions can be gradually restored. 11. Control of condensation temperature: Most organic impurities, which are primarily light components, pass through the condenser at the top of the distillation tower; those that are not condensed are then released through the liquid seal tank. The condensation temperature acts as a watershed. Controlling the temperature level has a direct impact on the components of impurities removed. The content and composition of impurities in crude methanol are primarily determined by the choice of synthesis catalyst. Changes in catalyst temperature and pressure have a significant effect on both the composition and total amount of impurities. Especially in the later stages of operation, as temperature and pressure increase, the proportion and amount of high-boiling-point organic impurities rise significantly. With copper-based catalysts, the amount of ethanol, a by-product, is approximately 150–200 milligrams per kilogram in the early stages; however, this amount can rise to 1000–2000 milligrams per kilogram in later stages. Since there are few by-products with copper-based catalysts in the early stages, the condensation temperature at the top of the pre-tower can be controlled at 30–40°C. If the catalyst is used for a long time, the quality of methanol may not meet the standards. Therefore, as the temperature and pressure of the catalytic reaction increase, the condensation temperature also rises accordingly, which facilitates the effective removal of impurities. 12. Selection of the sensitive plate temperature: In distillation, a sensitive plate is chosen in order to better regulate the distillation temperature. During operation, it is observed that the composition of the liquid on a certain plate changes significantly, which leads to substantial temperature variations; such changes disrupt the material balance. The temperature changes are most sensitive in this plate and in the plate sections of that area. In actual production, one of them is selected as the sensitive plate, using this temperature as a reference to control changes in the material. The sensitive plates of the main tower are selected at the eighth to twelfth layers from the bottom up. The temperature at which fusel oil is extracted is generally controlled between 88–94°C. The advantages of controlling this temperature include: (1) sensitive response and accurate adjustment ； (2) It is possible to anticipate the trend of 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; if needed, the amount of fusel oils taken out should also be increased, in order to prevent methanol and medium-boiling components (fusel oils) from moving to the bottom of the tower, which could result in excessive levels of residue in the liquid discharged from the bottom of the tower. The pre-tower sensitivity plates are the 26th to 36th plates from top to bottom, with the temperature controlled at 76–80°C. Due to the small temperature difference in the middle of the tower top, the variation in the tower top temperature is minimal. It can only be clearly reflected in cases of severe material imbalance. At this time, if the temperature at the bottom of the tower is adjusted, there is often a delay in the adjustment, resulting in significant fluctuations. The temperature and concentration in the middle of the tower change significantly; therefore, by keeping the temperature within a certain range, it is possible to maintain the temperature and composition at the top of the tower. Once the material balance is disrupted, the temperature in this area of the tower reacts most sensitively, and by making prior adjustments, it is possible to ensure stability in the temperature of the entire tower, especially at its top. Maintaining the appropriate temperature is key to preserving the material balance throughout the tower. 13. Control of the top temperature of the tower: The top temperature of the distillation main tower is an important factor determining the quality of methanol products; essentially, it represents the boiling point of pure methanol at the operating pressure. Typically, the temperature at the top of the tower is maintained at 66–67°C; when the tower pressure remains stable, an increase in this temperature indicates an increase in the heavy components at the tower top. Of course, it is necessary to clearly determine whether it is due to process operation issues 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 to the top of the tower by the reflux stream. If it is due to operational issues in the process, adjust the amount of steam and the reflux volume; if the reflux ratio is low, increase the amount of steam and raise the reflux ratio. If necessary, reduce or stop the extraction of pure methanol until the temperature at the top of the tower returns to normal, in order to maintain material balance within the tower. If there is a leak in the equipment’s condenser, the system should be shut down for repair. 14. Control of the bottom temperature of the tower: Controlling the temperature at the bottom of the tower is an important aspect; if the separation efficiency within the tower is good, the liquid in the main bottom tank will consist of almost a single component – water – whose boiling point is around 106–110°C. Maintaining a normal temperature at the bottom of the tank can prevent methanol loss. 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 the heat load; it could also be caused by an excess of heavy components in the lower part of the tower. In such cases, it is necessary to determine the cause and take appropriate adjustments, such as adjusting the reflux rate, increasing the heat load, increasing the extraction of methanol, increasing the extraction of heavy components, and reducing the feed volume. 15. Influence of the reboiler liquid level change on distillation. The stability of the reboiler liquid level is a key requirement for maintaining a constant reboiler temperature; changes in this liquid level are primarily determined by the amount of liquid discharged from the bottom of the tower. When the discharge volume at the bottom of the tower is too high, it causes the liquid level in the tower bottom to drop to the point of evacuation. This reduces the circulation rate of the liquid through the evaporator, thereby affecting the heat transfer efficiency of the entire heating tank. The evaporation rate in the tower bottom decreases, and the steam velocity drops, which in turn disrupts the heat balance and mass transfer processes within the tower. If the discharge volume at the bottom of the tower is too low, it will result in a high liquid level in the tower bottom, increasing the resistance to the circulation of the liquid there; this in turn leads to poor heat transfer and a decrease in the temperature of the liquid. The proper liquid level should be maintained below the lower edge of the vaporization tube. The factors that affect the change in the liquid level at the bottom of the tower from a process perspective include: (1) changes in the composition of the liquid in the tank. At constant pressure, lowering the temperature of the tank alters the gas-liquid equilibrium at its bottom, resulting in an increase in the amount of liquid and in the concentration of light components within it. If the amount of liquid discharged from the tank remains unchanged, its level will rise; in such cases, the temperature of the tank needs to be raised again. (2) Changes in the feed rate: if the content of water and heavy components in the feed increases, the volume of liquid in the reactor also increases. If the discharge of this liquid from the reactor is not increased, raising the reactor temperature will cause the heavy components to be carried to the top of the tower. When the feed is increased, the discharge volume of the reactor liquid also increases; otherwise, the level of the reactor liquid rises. In the early stages of operation, due to the small amount of liquid on the trays, proper gas-liquid contact has not yet been established, allowing a large amount of light components to enter the bottom of the tower. The amount of vaporization is not sufficient to meet the heat requirements inside the tower at the moment. Therefore, for a tower that has just started operating, the reboiler should be preheated simultaneously with feed addition, and appropriate heat should be supplied once a liquid level is established in the tower bottom. If the bottom temperature of the tower is not increased in a timely manner, the liquid level in the tank rises, the amount of liquid discharged from the tank increases, and as a result, methanol loss increases. If the feed rate is high while the amount taken from the top of the tower is low, this will disrupt the existing material balance and the composition of the gas and liquid phases. The reflux ratio increases, more material accumulates inside the tower, the methanol concentration in the bottom liquid rises, the concentration of the rising vapor increases, and the pressure difference between the top and bottom of the tower increases; in severe cases, this can lead to flooding. As the feed rate increases, the yield also increases. If the feed rate remains constant but the yield increases, then the concentration of heavier components rises, the reflux ratio decreases, and the amount of reflux liquid on each tray decreases. This leads to poor gas-liquid contact, resulting in reduced efficiency in mass and heat transfer. At the same time, the operating pressure drops, which in turn causes changes in the gas-liquid composition on each tray; the heavier components are carried to the top of the tower. In particular, the temperature of the sensitive trays in the distillation section is the first to reflect this increase.

A few personal thoughts: 1. Pay attention to the composition of the feedstock； 2. Pay attention to the temperature of the feed ； Both of the above considerations can be addressed by adjusting the system before the distillation tower and the feed location in order to achieve stable system operation.

Answer 3 is very detailed; good. Sometimes, attention also needs to be paid to the corrosion at individual tower tops

I read the post on floor 3 and learned a lot from it; thanks to floor 3 :)

Backflow, feed rate, pressure, condensation temperature, sensitivity plate temperature, top of tower temperature, steam consumption, and more!

The person on the 3rd floor explained it in quite detail. However, I would like to add that the reflux ratio also needs to be determined based on the specific process. A reflux ratio of 2.0~2.5 is not necessarily suitable for all distillation columns. I personally think it may not necessarily apply to most distillation columns. I have experienced many, many distillation column processes with a reflux ratio of 1 or 10.

I read the post on floor 3 and learned a lot from it; thanks to floor 3

Material balance; the operation must be carried out slowly....

The explanation on the 3rd floor was quite detailed; I learned a lot and gained valuable insights. Thank you

The person on the 3rd floor is an expert; they really understand the issues deeply! Learned* it