Temperature control of the distillation tower

It is controlled through factors such as product yield and feed temperature. This post was last edited by lxzzl on 2009-4-16 10:10]

In addition to the product recovery rate and feed temperature, there are also the heat load of the bottom reboiler, the heat load of the top condenser, etc. This post was last edited by lxzzl on 2009-4-16 10:10]

The various parameters of a distillation tower influence one another, with material balance, phase balance, and heat balance being the key aspects to control.

How are the temperatures at the bottom and top of a distillation tower controlled, and what factors affect the control of the tower? Answer: The reflux ratio is the ratio of the rising vapor to the descending liquid. Control the evaporation rate or the reflux rate, but such control must be within the design range to ensure stable and continuous distillation. This post was last edited by lxzzl on 2009-4-16 10:10]

The amount of heat input to the reboiler is adjusted based on the temperatures at the measurement points (the top and bottom of the tower or the sensor plate), thereby ensuring the desired temperatures at these locations.

How are the temperatures at the bottom and top of a distillation tower controlled? The bottom of the tower temperature is controlled by regulating the material outlet temperature of the heating furnace, or the heating capacity of the reboiler. The temperature control at the top of the tower is achieved by controlling the number of air coolers in use, as well as the water cooling and recirculation rates. What factors affect the control of the tower? ⒈The weather is changing. Such as: rain, snow, strong winds, extreme temperatures, etc ； ⒉Power outage. Fluctuating power and outages cause the air-cooling system to stop operating, resulting in insufficient water supply. ⒊Fluctuations in gas composition, with liquid present ； ⒋Fluctuations in steam pressure and temperature of the reboiler. This post was last edited by lxzzl on 2009-4-16 10:10]

The temperatures at the bottom and top of a distillation column are primarily controlled by factors such as column pressure, the amount of heat input to the reboiler at the bottom of the column, the amount of coolant supplied to the condenser at the top of the column, and the reflux rate. The composition and temperature of the feed material, column blockages, and the heat transfer capacity of the reboiler and condenser also affect the control of the column. This post was last edited by lxzzl on 2009-4-16 10:11]

It mainly controls the temperature of the tower top, tower bottom, or sensitive plate. This post was last edited by lxzzl on 2009-4-16 10:11]

Based on the situation in our unit, it can be said that the distillation temperature is directly related to the steam in the tower bottom, the vacuum in the tower, as well as the condensate water at the top of the tower. This post was last edited by lxzzl on 2009-4-16 at 10:11

Composition, reboiler heating capacity, reflux rate, tower pressure. This post was last edited by lxzzl on 2009-4-16 10:12]

Both the steam flow rate and the reflux volume of the reboiler affect the temperature at the bottom and top of the tower. Fluctuations in tower pressure and changes in the amount of water in the condenser also have an impact on it.

The small batch distillation column used for laboratory analysis employs the pressure difference between the top and bottom of the column to control the heat input to the bottom of the column, thereby regulating the temperatures at both the bottom and the top of the column. This post was last edited by lxzzl on 2009-4-16 10:12]

The heating capacity of the reboiler is related to the reflux volume, reflux temperature, and tower pressure

The bottom of the tower temperature is controlled by regulating the material outlet temperature of the heating furnace, or the heating capacity of the reboiler. The temperature control at the top of the tower is achieved by controlling the number of coolers, water cooling, and the recirculation flow rate.

From the perspective of energy conservation, the feed temperature, as well as the heat loads of the reboiler at the bottom of the tower and the condenser at the top, all have an impact

The bottom and top temperatures of the tower are controlled by the load on the bottom reboiler and the top condenser. Tower pressure and feed composition have a significant impact on temperature. With constant column pressure, the temperatures of each tray reflect the lightness or heaviness of the composition. Weather conditions, equipment failures, and feed composition (such as a sudden increase in light components leading to vaporization and thus an increase in tower pressure) can also affect the tower pressure. Understanding where the interference comes from is essential for effective control.

The king also joined in the fun: (I’m a beginner; I’m here to learn*. Thank you all)

The bottom temperature of the tower is controlled by feed and a reboiler, while the top of the tower is controlled by reflux

Under normal continuous operation, the temperature at the bottom of the tower is closely related to the composition, pressure, and steam flow rate at that location. A high temperature at the bottom of the tower may be due to a high content of heavier components, requiring discharge from the tower. Under a certain pressure, the top temperature reflects the components at the tower top. Control the balance of incoming and outgoing materials

The use of temperature as an indirect mass indicator is based on the assumption that the tower pressure remains constant. Therefore, all the control schemes below assume that the tower pressure is already under a constant-value control system. 1. Temperature control in the distillation section: The temperature control in the distillation section is aimed at ensuring the quality of the product obtained there. Depending on the location of the temperature sensing points, there are various methods such as top-of-tower temperature control, sensitive plate temperature control, and intermediate temperature control. The control variable can be either the reflux rate or the top product withdrawal rate. The bottom stream yield of the tower can also be used as a control variable, but it is less commonly applied. Using the top temperature as the controlled variable allows for a direct reflection of product quality; however, due to the small temperature difference between the trays near the tower top, this control scheme places high demands on the temperature sensing devices, such as high precision and high sensitivity. Furthermore, impurities in the product affect its boiling point, causing disturbances in temperature. As a result, control schemes that use the top of the tower temperature to regulate the quality of the product at that level are rarely used; they are mainly employed for the rough separation of petroleum products based on their boiling points. Using the temperature of the sensitive plate in the distillation section as the controlled variable allows for rapid reflection of changes in product composition. A sensitive plate is the tray on which the temperature change is greatest under the influence of disturbances. Therefore, this tray has the greatest concentration gradient between it and the trays above and below it, resulting in a rapid dynamic response of the process. Figure 6.3-1 shows that the 11th tray is the sensitive tray, which exhibits relatively large gains when the direction of perturbation changes. The position of the sensitive plate can be determined through simulation calculations or actual measurements. Since the efficiency of the tray is difficult to estimate accurately, in practical applications, several temperature monitoring points can be installed above and below the calculated sensitive plate, with the choice being based on the actual operating conditions. Medium temperature usually refers to the tray slightly above or below the feed tray, or the temperature of the feed tray. Using medium temperature as the controlled variable allows for consideration of both the composition at the top and bottom of the tower, enabling timely detection of changes in the operating line. However, since it cannot promptly reflect the composition of the products at the top or bottom of the tower, it cannot be used in applications with high separation requirements and large variations in feed concentration. Distillation section temperature control is employed in cases where: ① stricter requirements are placed on the composition of the overhead product than on that of the bottom product ； ②All are gas-phase feedstocks ； ③The temperature at the bottom of the tower or in the stripping section does not reflect changes in the composition well; that is, when the composition changes, the temperature of the trays in the stripping section does not change significantly, or the feed contains heavy impurities that have a greater impact on the relationship between temperature and composition than the product at the bottom of the tower. 2. Temperature control in the stripping section: The temperature control in the stripping section is aimed at ensuring the quality of the product obtained there. Depending on the location of the temperature monitoring points, it can be divided into bottom of tower temperature control, sensitive plate temperature control, and medium-temperature control. The control variables can be the reboiler heating steam volume or the bottom product withdrawal rate. The tower top draw rate can also be used as a control variable, but it is less commonly applied. The control strategy is similar to that of distillation section temperature control. The situations where temperature control in the stripping section is employed are: ① When stricter requirements are imposed on the composition of the bottom product than on that of the top product ； ②All are liquid-phase feedstocks ； ③The temperature at the top of the tower or in the distillation section does not reflect changes in the composition well; that is, when the composition changes, the temperature of the trays in the distillation section does not change significantly, or the feed contains lighter impurities that have a greater impact on the relationship between temperature and composition than the product at the top of the tower ； ④When reflux control is used, the amount of reflux is high; minor changes in it have little impact on the composition of the product, whereas larger changes can affect the stable operation of the distillation column. 3. Pressure-compensated temperature is used as an indirect mass indicator. A constant column pressure is a prerequisite for controlling the temperature of a distillation column. When there are changes in column pressure or when high control requirements such as precise distillation are involved, slight pressure variations can affect the relationship between temperature and composition; therefore, pressure compensation for temperature is necessary. Common compensation methods include temperature difference control, dual temperature difference control, and compensation calculation control. 1) Temperature difference control: In a distillation column, composition is a function of temperature and column pressure; when the column pressure remains constant or changes only slightly, there is a one-to-one relationship between temperature and composition. However, during precision distillation, high purity of the product is required, and even slight changes in tower pressure can cause fluctuations in the composition. For example, during the separation of benzene and toluene, a pressure change of 6.67 kPa results in a boiling point change of 2°C for benzene. The principle of temperature difference control is based on maintaining the purity of the product at the top (or bottom) of the tower constant. Changes in tower pressure affect the temperatures on both trays, and the effect is almost identical in both cases; therefore, the temperature difference can remain constant. Typically, a temperature at which the temperature and composition of one tray remain essentially constant is chosen as the reference temperature; for example, the temperature at the top (or slightly below) of the tower or at the bottom (or slightly above) of the tower is selected. Another point is to select the sensor board temperature. Temperature difference control is often used in precision distillation where high separation requirements exist. For example, precise distillation of benzene-toluene-xylene, ethylene-ethane, propylene-propane, etc. When applying it, care should be taken to select the appropriate location for the temperature detection point, set the temperature difference threshold reasonably, and ensure that the operating conditions remain stable. 2) Dual temperature difference control: The drawback of temperature difference control in distillation columns is that changes in feed flow rate lead to changes in the composition within the column as well as changes in the pressure drop across the column. They all cause changes in temperature differences. The former reduces the temperature difference, while the latter increases it, resulting in a non-monotonic functional relationship between the temperature difference and the composition. The design concept of dual temperature difference control is that the influence of the feed on the temperature difference in the distillation section is the same as its influence on the temperature difference in the stripping section; therefore, dual temperature difference control can be used to compensate for the effects of changes in feed flow rate on these temperature differences. When applying it, in addition to properly selecting the location of the temperature detection points, the set values for the dual temperature differences also need to be adjusted reasonably. 3) Control of the temperature setpoint based on pressure compensation: When using a computer control system or DCS to control a distillation tower, thanks to the computer’s powerful computing capabilities, the effects of changes in tower pressure can also be addressed through pressure compensation calculations. The compensation formula is as follows: In equation (6.3-1), it represents the temperature setting corresponding to the component required by the product at the tower pressure ； It is the tower pressure measurement value ； It is the designed tower pressure value ； It is the temperature setpoint under actual tower pressure conditions. Therefore, a control system is formed to calculate the temperature setpoint based on the tower pressure model. When applying it, the coefficient terms in the compensation formula need to be set appropriately; generally, using quadratic powers is sufficient to meet the control requirements. When the precision does not meet the product purity requirements, the exponent can also be increased. Furthermore, the tower pressure signal needs to be filtered, the location of the temperature sensing points should be appropriate, and the compensation coefficients should be suitable.