【2026 Distillation Technology】Adjustment of Distillation Column Process Parameters and Influencing Factors (2)
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This post was last edited by The one on 2026-5-24 18:42. How to adjust the tank temperature in distillation operations? 『2』 What are the factors that affect the fluctuation of the kettle temperature? The kettle temperature is determined by the kettle pressure and the material composition. During the distillation process, only by maintaining the specified reactor temperature can product quality be ensured. Therefore, the kettle temperature is one of the important control parameters in distillation operations. When the reactor temperature changes, the heating steam supply to the reboiler is usually adjusted to bring the reactor temperature back to normal. When the temperature of the reactor is below the specified value, the amount of steam used should be increased to boost the vaporization of the liquid in the reactor, thereby increasing the concentration of heavier components in it, raising its bubble point, and consequently increasing the reactor temperature. When the temperature of the reactor is higher than the specified value, the amount of steam used should be reduced in order to decrease the vaporization of the liquid in the reactor. This leads to an increase in the concentration of light components in the liquid, a reduction in the bubble point, and consequently a decrease in the reactor temperature. There are many reasons for fluctuations in kettle temperature. When the tower pressure suddenly rises, the kettle temperature increases accordingly and then decreases. This increase in the reactor temperature is caused by the rise in pressure, which in turn raises the bubble point of the liquid in the reactor. As a result, the amount of vapor rising inside the tower not only does not increase, but actually decreases due to the rise in pressure ； In this way, the evaporation of the light components in the bottom mixture of the tower is incomplete, which leads to a decrease in the bubble point of the liquid in the tank, and consequently the tank temperature also drops. Conversely, when the column pressure drops suddenly, the amount of vapor rising inside the column increases as a result of this drop in pressure, causing the liquid level at the bottom of the column to drop rapidly; as a result, the heavier components may be carried to the top of the column. As the components in the kettle liquid become heavier, the bubble point of the kettle liquid rises, and the kettle temperature also increases accordingly. It appears that tower pressure is an important factor causing changes in the reactor temperature. Therefore, during operation, only by first controlling the tower pressure at the required level can it be determined with certainty whether the reactor temperature meets the process requirements; otherwise, it will lead to incorrect operations. The kettle temperature also decreases as the concentration of light components in the feed increases, and increases as the concentration of heavy components increases. Furthermore, factors such as water in the reactor, polymerization of the material in the reboiler that blocks some of the tubes, fluctuations in the pressure of the heating steam, malfunctions of control valves, and disruptions to the balanced extraction of the material can all cause fluctuations in the reactor temperature. When the kettle temperature fluctuates, it is necessary to analyze the causes of these fluctuations and eliminate them. For example, the output at the top of the tower is too low, causing light components to be driven into the bottom of the tower and resulting in a drop in the bottom temperature. At this point, if the draw rate from the top of the tower is not increased and only the amount of heating steam supplied to the bottom of the tower is increased, it will have no effect on the temperature at the bottom of the tower; in severe cases, it may even cause flooding. For example, if the tubes of the reboiler become clogged due to material aggregation, which causes the temperature in the reactor to drop, the operation must be stopped to carry out maintenance on the equipment.
The following response is for reference — the top ten factors affecting distillation operations. The main factors affecting the distillation process include the following: feed entry point, column temperature, column pressure, feed state, feed rate, feed composition, feed temperature, upward vapor velocity within the column, heating rate of the reboiler, reflux rate, amount of cooling agent at the column top, overhead product flow rate, and bottom product flow rate, etc. 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 capacity 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 improving its separation capacity. Conversely, moving the feed plate upward increases the number of plates in the stripping section, thereby enhancing its separation capacity. In summary, 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 inside the tower from being disrupted after feeding, thereby ensuring stable operation.
2. Effect of changes in feed composition on distillation operations. Changes in feed composition have a direct impact 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 intensity 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 lowering 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 changing 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 at the tower bottom. For example, if a column is designed for feed at the bubble point, switching to feed at a cooler 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 mixture feed or saturated/overheated steam is used instead, the number of trays in the rectification section becomes insufficient, while the number of trays in the stripping section is excessive. As a result, the content of heavy components in the overhead product exceeds the specified limit, whereas the content of light components in the bottom liquid is lower than the specified value. Additionally, this leads to increased consumption of refrigerant in the overhead condenser and reduced consumption of heating medium in the reboiler. A change in the feed temperature implies a change in the feed condition, and this change in feed condition affects the loads 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 more ideal feeding condition is bubble-point feeding, which is the most commonly used feeding state.
4. The impact of changes in the operating pressure of a distillation tower on distillation processes. The design and operation of distillation towers are based on a certain tower pressure; therefore, it is generally necessary to maintain a constant pressure in such towers first. 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 changes 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 kettle liquid increases, the volume of the kettle liquid 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 amount of liquid in the kettle 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 this will result in 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 operations, 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 column pressure changes, the bubble point and dew point of the mixture change, causing a change in the temperature throughout the column; consequently, the relationship between temperature and product quality also changes. 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 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.
5. 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 velocity of the rising steam within the tower directly affects the mass transfer efficiency. Generally speaking, the maximum upward steam velocity in 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 an increased speed of the rising steam inside the tower ； The amount of heat input decreases, and the velocity of the rising steam within the tower decreases. It should be noted that an excessively large or rapid adjustment range of the heating amount may cause flooding or leakage.
6. The influence 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, 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 flow 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 inside the tower, and may even lead to flooding, disrupting the normal operation of the tower.
7. The effect of the amount of cooling agent at the top of the tower on distillation operations: For towers operating with internal reflux, the amount of cooling agent 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 refrigerant will also affect the operation of the distillation column to varying degrees. For example, a reduction in the cold dose will degrade the performance of the condenser and decrease the amount of condensate; 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 these factors cause the top temperature of the distillation column to rise, resulting in an increased content of heavy components in the overhead product and a deterioration in its quality.
8. The impact of the amount of product taken from the top of the tower on distillation operations: There is a corresponding relationship between the amount of product taken from the top of the tower and the feed rate to that 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 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 with 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, heavy components are carried to the top of the tower, and the quality of the top-product is substandard. 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.
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, and the liquid level at the bottom of the tower is typically used to control this material balance. Maintaining a stable liquid level in the tower kettle is a prerequisite for keeping the kettle temperature constant. The change in the liquid level in the column bottom is mainly determined by the amount of effluent drawn from the bottom of the column. 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. When a shell-and-tube reboiler is used, the volume of circulating liquid is too small; as a result, the kettle liquid forms superheated vapor when passing through the upper portion of the tubes. This manifests as a higher gas temperature in the vapor vent pipe, while the kettle temperature remains low. If the amount of product removed from 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 raising the likelihood of polymerization. Furthermore, maintaining a certain level of liquid in the kettle also serves as a liquid seal to ensure safe production.