[2026 Distillation Technology] Adjustment of Distillation Column Process Parameters and Influencing Factors (4)

The following response is for reference — the top ten factors affecting the distillation process. The main factors affecting the distillation process include the following: feed position, column temperature, column pressure, feed state, feed volume, feed composition, feed temperature, upward steam velocity in the column, heating capacity of the reboiler, reflux rate, amount of cooling at the column top, amount taken from the column top, and amount taken from the column bottom. 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.

1. The influence of the feed plate position on distillation operations: The most suitable feed plate position is one that provides the highest separation efficiency under the same number of theoretical plates and identical operating conditions, or one that requires the fewest theoretical plates under the same operating conditions. In the chemical industry, most distillation columns are equipped with two or more feed plates, and the position of these feed plates is adjusted based on changes in the composition of the feed. When the proportion of light components in the feed is lower than normal, the position of the feed tray should be moved downward to increase the number of trays in the distillation section, thereby enhancing its separation capacity. Conversely, moving the feed plate upward increases the number of plates in the stripping section, thereby enhancing its separation capacity. In short, the content of light components in the feed on the feed plate should be less than that on the lowest tray in the distillation section, and greater than that on the highest tray in the stripping section. This prevents the material composition on each tray within the tower from being disrupted after feeding, thereby ensuring stable operation.

2. The impact of changes in feed composition on distillation operations. Changes in feed composition have a direct effect on distillation processes; when the concentration of heavy components in the feed increases, the load on the distillation section rises. In a tower with a fixed number of plates in the distillation section, this leads to the heavy components reaching the top of the tower, resulting in substandard quality of the product at the top. If the concentration of the light components in the feed increases, the load on the stripping section rises. For a column with a fixed number of plates in the stripping section, this results in incomplete evaporation of the light components in that section, as well as an increased loss of light components in the bottom liquid. 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 the same time, the temperature of the entire tower decreases while the tower pressure increases. The component becomes heavier; the situation is the opposite. When the feed composition changes, the following measures can be taken. 1) Adjust the feed inlet. As the component becomes heavier, the feed inlet moves downward ； As the component becomes lighter, the feed inlet moves upward. 2) Change the reflux ratio. Increase the reflux ratio as the component weight increases ； When the component becomes lighter, reduce the reflux ratio. 3) Adjust the temperatures at the top and bottom of the tower. Based on changes in the composition, the temperatures at the top and bottom of the tower are adjusted accordingly, and the product quality at these levels is maintained by adjusting the reflux temperature or volume of the liquid and gas reflux. Increasing the heating rate at the bottom of the tower can raise the temperature there; as a result, the content of light components in the product at the bottom of the tower decreases while the content of heavy components increases. However, this may lead to an increase in the heavy component content in the product at the top of the tower. In such cases, it is necessary to maintain the quality of the product at the top of the tower by reducing the temperature of the reflux stream or increasing its flow rate.

3. The impact of feed temperature changes on distillation operations: Changes in feed temperature have a significant effect 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. When the feed temperature is too low and there is no excess heating steam available at the bottom of the tower, it will increase the content of light components in the distillate from the tower bottom. 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 liquid is incomplete. If a gas-liquid mixed feed or saturated vapor/superheated vapor 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 overhead product 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 refrigerant in the overhead condenser and reduces the consumption of heat medium in the reactor bottom. A change in the feed temperature implies a change in the feed condition, and this change in feed condition affects 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 tower. The ideal feed condition is bubble-point feeding; this is the most commonly used feed state.

4. The effect of changes in the operating pressure of a distillation column on the distillation process. The design and operation of a distillation column are based on a certain column pressure; therefore, for most distillation columns, it is essential to maintain a constant pressure. Fluctuations in column pressure will have the following effects on column operation. 1) Affects product quality and material balance. Changing the operating pressure will alter the composition of the gas-liquid equilibrium on each tray. As the pressure in the reactor increases, the concentration of heavier components in the gas phase decreases; accordingly, the concentration of lighter components in the gas phase rises. The amount of lighter components in the liquid phase also increases, and this alters the weight ratio between the gas and liquid phases, resulting in an increase in the volume of the liquid phase and a decrease in the volume of the gas phase. The overall result is: the concentration of light components in the tower top distillate increases, but their quantity decreases relatively ； As the concentration of light components in the reactor broth increases, the volume of the reactor broth also increases. Similarly, as the pressure in the reactor decreases, the amount of distillate at the top of the tower increases, and 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 operation leads 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 meets standards; but in this case, there is an increased loss of light components in the liquid in the reactor. 2) Change the relative volatility between components. As the pressure in the reactor increases, the relative volatility between the components decreases, resulting in a decline in separation efficiency; conversely, an increase in the relative volatility between the components leads to an improvement in separation efficiency. 3) Change the production capacity of the tower. As the pressure in the reactor increases, the specific gravity of the components rises, thereby increasing the processing capacity of the tower. 4) Fluctuations in tower pressure. Fluctuations in tower pressure will cause confusion in the relationship between reactor temperature and composition. In practice, temperature is often used as an indirect criterion for measuring product quality, but this is only correct under the condition that the tower pressure remains constant. When the tower pressure changes, the bubble point and dew point of the mixture change, which in turn causes a change in the temperature throughout the tower; 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 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 refrigerant 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.

5. The impact of the velocity of the rising steam within the tower and fluctuations in the heating power of the evaporation kettle on distillation operations. The speed of the rising steam in the tower directly affects the mass transfer efficiency. Generally speaking, the maximum upward velocity of steam inside the tower should be slightly lower than the flooding velocity. In practice, the maximum allowable hole velocity is often chosen to be 80% of the velocity at which flooding occurs. 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 reboiler. With the reactor temperature remaining stable, an increase in heating capacity leads to a faster rise of steam inside the tower ； The heat input decreases, and the speed of the rising steam inside the tower slows down. It should be noted that an excessively large or rapid adjustment range of the heating amount may cause flooding or leakage.

6. The impact of the reflux ratio on distillation operations: In these operations, the reflux ratio is adjusted in order to ensure the quality of the product. When the content of heavy components in the tower top distillate increases, increasing the reflux ratio is often used to reduce these heavy components and ensure that the product quality meets the standards. When the light components from the distillation section move down to the stripping section and cause the temperature at the lower part of the tower to drop, 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 within the tower, and may even lead to flooding, disrupting the normal operation of the tower.

7. The impact of the amount of coolant at the top of the tower on distillation operations: For towers that use internal recirculation, the amount of coolant has a significant effect 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 refrigerant will 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 heavy components in the product at the top of the column, and degrade its quality.

8. 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 taken from the tower top and the feed rate to the tower; as the feed rate increases, the amount of product taken out should also increase. It is well known that only when the yield varies with the feed rate can a fixed reflux ratio in the tower be maintained to ensure its proper operation; otherwise, the gas-liquid equilibrium within 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. This leads to a reduction in the amount of reflux liquid on each tray, resulting in poor gas-liquid contact and a decline in 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 heavy components are carried to the top of the tower, resulting in product quality that does not meet standards. 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 likewise leads to a decline in the quality of the product at the tower top. If the feed rate is increased while the draw rate at 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.

9. The impact of the bottom product yield on distillation operations: In the operation of a distillation tower, it is necessary to maintain stability in the products at the top and bottom of the tower. Maintaining the material balance within the distillation unit is a prerequisite for its stable operation; typically, the liquid level at the bottom of the tower is used to control this material balance. 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 causes the liquid level in the tower bottom to drop or even become empty, which reduces the circulation volume of the liquid passing through the reboiler. As a result, heat transfer is impaired, the light components cannot be vaporized, and the products at both the top and bottom of the tower are of substandard quality. If a shell-and-tube reboiler is used, the low volume of circulating liquid causes the liquid in the reactor to become superheated gas as it passes through the upper part of the tubes; this results in a higher gas temperature in the vaporization tube while the temperature of the reactor liquid 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 cause the tower to be flooded. This increases the resistance to the circulation of the liquid in the tower, leading 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 raising the likelihood of polymerization. Additionally, maintaining a certain level of liquid in the kettle serves as a liquid seal to ensure safe production.