H. LI, Hualu Engineering & Technology Co. Ltd., Xi’an, China
The distillation process is a widely used process throughout the chemical processing industries. Distillation separates liquid mixtures by exploiting differences in component volatility. The column, reboiler and condenser are core components: the reboiler generates vapor for separation, while the condenser recovers distillate. Control systems must ensure product quality, minimize energy consumption and maintain stability during disturbances such as feed composition changes or pressure fluctuations. Key challenges include managing lag in temperature response and mitigating the interactions between variables (e.g., liquid level, heat input).
The first principle of the distillation process is to use the different boiling points in the liquid mixture of components, and heat them under a constant pressure until it boils. The low boiling point components that are easy to vaporize gather in the vapor, while the high boiling point components that are difficult to vaporize remain in the liquid. If the vapor produced is condensed, the condensate is the low boiling ends; therefore, to achieve the separation of components in the mixture, the feed is usually in the middle of the tower.
The whole tower is divided into two sections by a feeding tray: the feeding tray above is called the distilling section, while the feeding tray below is called the stripping section. In the distilling section, the high boiling point component of the rising vapor is converted into liquid. The low boiling point component in the liquid is converted into vapor to complete the distillation of the low boiling point component in the rising vapor. In the stripping section, the low boiling point component in the downstream liquid is converted into vapor, and the high boiling point component in the vapor is converted into liquid to complete the concentrating of the high boiling point component in the downstream liquid.
The process control requirements of the distillation tower include:
An onsite presence to ensure qualified product quality
The highest recovery rate
The lowest energy consumption of the tower.
The distillation process is carried out under certain constraints, so the necessary control system can be set up comprising four aspects: quality index, product output, energy consumption and constraint conditions. The key components of the top products (light key components) are volatile, while the key components of the bottom products (heavy key components) are not volatile. The purity of the product is not “the higher the better.” The higher the purity, the higher the control precision requirements of the control system, and operating costs and the price of the product are not proportional to the increase—purity requirements should be adapted to the requirements of use. Therefore, some key parameters in the production should be controlled automatically, so they can be automatically returned to the specified value range when they deviate from the normal state due to the influence of any external disturbance. To this end, an automatic control system should be implemented.
The factors affecting distillation performance include tower pressure; feed rate, composition and temperature; added and removed heat; bottom product rates; and distillate rates, among others. Without a corresponding automatic control system, it is difficult to achieve the separation requirements. Therefore, three temperature control schemes for a distillation column under constant pressure are discussed in the sections below.
CONTROL STRATEGIES
1. Cascade control of reboiler heating steam flow and tray temperature. In the configuration shown in FIG. 1, a primary temperature regulator (outer loop) sets the setpoint for a secondary steam flow regulator (inner loop). This configuration compensates for steam pressure fluctuations and reduces lag in temperature response, and is suitable for processes where the temperature is critical and deviations must be minimized (e.g., high-purity distillation).
In the configuration shown in FIG. 1, there are two regulators that receive the measured signals from different parts of the process equipment: the output of one regulator as a given value of another regulator, and the output of the latter to control the regulator to change the adjustment parameters from the structure of the system. The two regulators work in series, so this system is called a cascade control structure. The primary regulator is the outer ring, and the secondary regulator is the inner ring. The main loop is a constant value regulation system, and the secondary loop is a follow-up system. The main characteristic of this scheme is that the temperature regulator serves as the main regulator, and its output serves as the given value of the flow regulator (sub-regulator), namely the cascade regulation scheme. It can overcome the influence of steam pressure fluctuations early and improve the sensitivity of temperature control. The cascade adjustment is suitable for the main parameter and is an important indicator of the process operation. Its allowed fluctuation range is very small, and off-set is not permitted. The introduction of the secondary loop ensures the stability of the main parameter, which itself can vary over a wide range.
2. Cross-regulation of temperature and liquid level. The temperature and liquid level controllers interact (e.g., a temperature decrease triggers reduced bottom product withdrawal), while a rising liquid level increases reboiler heat input (FIG. 2). Advantages include a reduced lag compared to sequential control, and it is effective for columns with small bottom product rates. However, it is unsuitable for columns with large bottom product flows due to feed rate sensitivity.
In FIG. 2, the temperature regulation system is associated with the liquid level regulation system. For example, when the content of the light component of the bottom tray of the column increases, it will lead to the reduction of the temperature of the bottom of the tower. The temperature regulator will turn down the valve located in the draw-off line from the bottom of the tower, reducing the discharge of poor-quality bottom products, and the liquid level will rise. The liquid level regulator drives the steam regulating valve to increase the heat so that the content of the bottom tray is reduced.
This system is mainly used at the bottom of the tower to remove the residue because the bottom product rates is smaller than distillate product. The control of the liquid level cannot be achieved by changing the discharge.
It is worth noting that this cross regulation has the advantage of less lag and deviation than the general regulation system, and takes advantage of the relationship between temperature regulation and liquid level regulation. Generally, the correlation between adjustment systems is often unfavorable and will cause controller lag due to interaction. However, here, the correlation between temperature and liquid level regulation mutually promotes the regulation quality.
When the feed rate suddenly increases, the liquid level rises quickly, and it will quickly open the regulating valve to increase the heating as the amount of feed rate increase. Meanwhile, as the liquid level rises, the temperature will also decrease, and the temperature regulator will also drive the regulating valve to close down so that the discharge amount of poor-quality products is reduced and the liquid level rises. The rise of the liquid level further opens the heating medium regulating valve so that the heating can be changed more quickly to reduce the adjustment lag and reduce the temperature fluctuation.
However, if the general control scheme is adopted in which heat is added to control according to the temperature and control discharge according to liquid level, due to the large lag in temperature response, the increase in heat is not timely. The significant rise in liquid level makes the liquid level adjuster quickly open the discharge valve, causing a large amount of poor-quality products to flow out. Such a structure is, of course, not as good as cross-regulation.
Although cross-adjustment has its advantages, it is not difficult to see that it is only suitable for the situation where the bottom product is very small (compared to the distillate rates). If the bottom product is small (i.e., the distillate rate is large) for this kind of tower, a large amount of heat is generally needed, and the change of heat will have a significant impact on the liquid level. Therefore, for liquid level regulation, it is not advisable to change the discharge amount, but it is appropriate to change the heat. Conversely, for larger bottom discharge (compared to the distillate rates), the fluctuation of the feed rate and bottom with the amount of change has a great influence on the liquid level, and the impact of changing the heat on the liquid level is small. If there are large lags, then by cross regulation, if the liquid level will cause large fluctuations, this is clearly not appropriate.
3. Temperature difference control. This configuration, shown in FIG. 3, measures temperature differences between two trays to infer composition changes, bypassing pressure-related temperature noise. It is critical for high-purity or close-boiling mixtures where pressure fluctuations mask composition effects, and requires stable column pressure for high-purity products.
The temperature difference control can be considered in precision distillation. Precision distillation (close-boiling fractionation) is used when the product is of high purity and the relative volatility difference between components is small, so the temperature change caused by the composition change is smaller than the temperature change caused by the pressure change. In other words, during the operation of the distillation, the main factor causing the temperature change is no longer the fluctuation of composition but is rather the fluctuation of pressure. In most distillation column control systems, whether conventional or advanced, it can be assumed that the tower operates at a constant pressure;1 however, this does not eliminate a potential small fluctuation of pressure. When the pressure fluctuation leads to the change of temperature—one of the more obvious changes, of course—it can destroy the corresponding relationship between the temperature and composition. Usually in precision distillation, this indicates that the composition will not achieve good results in terms of temperature.
In the rectifying section of the distillation column shown in FIG. 3, two trays are in different positions, and the crude product purity of the tray is relatively high. When the final product purity further increases, the concentration difference between the two trays is smaller, and so the bubble point temperature difference between the two trays is also small. Conversely, if the purity of the final product is reduced, the concentration difference between the two trays will increase, as will the temperature difference. Therefore, the purity of the final product can be controlled by measuring the temperature difference. It should be noted that, regarding the temperature difference control, the operation of the distillation must be stable without pressure fluctuation, particularly in the heating steam of the reboiler.
Additionally, when the temperature difference control is used in the two-component distillation, at least one end (the top or bottom of the tower) of the product must be relatively pure. The indicating element of the temperature gauge should be located near this end. Because the component is relatively pure in this area, it is favorable to reflect the composition by temperature difference. During temperature difference control in the stripping section, there should be no impurities heavier than those in bottom products of tower, and no impurities lighter than those in the top products of the tower during the temperature difference control in rectifying section.
Takeaways. In distillation operations, many parameters must be adjusted as well as various combinations, so numerous control configurations are possible.2 No matter the scheme, distillation control systems must balance precision, efficiency and robustness. Cascade control excels in minimizing temperature deviations, while cross-regulation improves response speed for columns with small bottom product rates. Temperature difference control is indispensable for high-purity separations but demands stringent pressure stability. Future work should explore hybrid strategies and advanced predictive models to further optimize energy use and adaptability. HP
LITERATURE CITED
Sloley, A. W., “Effectively control column pressure,” Chemical Engineering Progress, January 2001.
Buckley, P., J. Shunta and W. Luyben, Design of distillation column control systems, Publishers Creative Services Inc., New York, New York, 1985.
LI HAN is a Professor Senior Engineer and the Deputy Chief Engineer at Hualu Engineering & Technology Co. Ltd. of CNCEC China National Chemical Engineering Co. Ltd. (CNCEC). He holds an MSc degree in chemical engineering from Northwest University in Xian, China, and has more than 34 yr of experience in process engineering and project management. He is senior member of AIChE. He lives in Xi’an, Shaanxi Province, China. The author can be reached at lh1720@chinahualueng.com.