Distillation column pressure adjustment

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How is the pressure in a distillation column adjusted during operation? What are the factors that affect changes in tower pressure? In the operation of any distillation column, the column pressure must be kept within specified limits in order to adjust other parameters accordingly. Excessive fluctuations in tower pressure can disrupt the material balance and gas-liquid equilibrium throughout the tower, resulting in a product that does not meet the required quality standards. Therefore, many distillation columns have specific measures to ensure that the column pressure remains within an appropriate range. (1) Regarding the tower pressure in a pressurized tower, there are mainly two adjustment methods: ① When the overhead condenser functions as a partial condenser, the tower pressure is generally adjusted by controlling the amount of vapor withdrawn. With all other conditions remaining unchanged, an increase in the gas-phase output leads to a decrease in tower pressure ； The gas-phase recovery decreases and the tower pressure rises. ②When the top condenser is a total condenser, the column pressure is usually adjusted by the amount of refrigerant used, which is equivalent to adjusting the temperature of the reflux liquid. With all other conditions remaining unchanged, increasing the amount of cooling agent results in a lower temperature of the reflux liquid and a lower tower pressure ； If the amount of cold medium is reduced, the temperature of the reflux liquid rises and the tower pressure increases. For the pressure control of a vacuum distillation column, there are mainly two methods: ① When the vacuum in the column is achieved using a jet pump, the vacuum level can be adjusted by modifying the amount of refrigerant or the temperature of the refrigerant in the top condenser, thereby changing the volume of the exhaust gas. When the separated material is allowed to come into contact with air, in this control scheme, the steam jet pump operates at its maximum capacity. A control valve is installed on the pipeline leading to the atmosphere, and by adjusting the opening degree of this valve, the amount of exhaust gas drawn from the system can be controlled, thereby regulating the vacuum level in the tower. ②When using an electric vacuum pump for vacuum creation, a control valve is installed on the return line of the vacuum pump; the degree of opening of this control valve is used to adjust the amount of exhaust gas removed from the system, thereby controlling the vacuum level in the tower. For pressure control in atmospheric distillation columns, there are mainly three methods: ① When the stability requirements for the column top pressure are not stringent, there is no need to install a pressure control system. Instead, a vent pipe leading to the atmosphere should be provided on the distillation equipment (condenser or reflux drum) to ensure that the pressure inside the column remains close to atmospheric pressure. ②When high stability of the top pressure is required, or when the separated material must not come into contact with air, the control of the top pressure can be achieved using the control method for the pressure in a pressurized tower. ③The vapor flow rate at the bottom of the tower is adjusted by regulating it, thereby controlling the gas phase pressure at that location. How to adjust the kettle temperature in distillation operations? What are the factors affecting the fluctuations in kettle temperature? The reactor temperature is determined by the reactor 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 temperature of the reactor changes, it is usually done by adjusting the amount of heating steam supplied to the evaporation reactor in order to bring the temperature back to normal. When the kettle temperature falls below the specified value, the amount of steam supplied should be increased to raise the vaporization rate of the liquid in the kettle. This increases the proportion of heavy components in the liquid, thereby raising the bubble point and the kettle 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 thus a decrease in the reactor temperature. There are many reasons for fluctuations in the pot temperature. When the tower pressure suddenly rises, the kettle temperature also rises, and then decreases again. This is because the increase in the kettle temperature is caused by the rise in pressure, which in turn raises the bubble point of the liquid in the kettle. Therefore, the amount of rising steam in the tower will not increase; instead, it will decrease 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 steam rising inside the column increases as a result of this pressure drop, 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 kettle, polymerization of the material in the evaporation kettle which 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 kettle temperature. When the kettle temperature fluctuates, it is necessary to analyze the causes of these fluctuations and eliminate them. For example, if the amount of product taken from the top of the tower is too low, light components are pushed into the bottom of the tower, causing the temperature there to drop. At this point, if the distillate taken 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 in the evaporation kettle become clogged due to material aggregation, which causes the temperature of the kettle to drop, it is necessary to arrange for maintenance work on the equipment. How to adjust the reflux ratio in distillation operations? The reflux ratio is determined based on the separation requirements for the feedstock. An excessively high or low reflux ratio can affect the economic efficiency of distillation operations as well as the quality of the product. Increasing the reflux ratio raises the concentration of light components in the overhead product; however, it reduces the tower’s production capacity and increases the consumption of cooling energy at the top of the tower as well as heat energy at the bottom. During normal operation, an appropriate reflux ratio should be maintained; on the premise of ensuring product quality, the best economic results should be achieved. The reflux ratio can only be adjusted when the normal operating conditions of the tower are disrupted or the product quality is substandard. For example, in top products, the content of heavy components increases and the quality deteriorates; thus, the reflux ratio should be appropriately increased. The column load (feed rate) is too low; to maintain a certain upward vapor velocity within the column, the reflux ratio must also be increased appropriately. For large-scale production units, when there are conflicts between different types of tray structures, the requirements regarding steam rise velocity, as well as the design range of instruments and the actual production volume, it is possible to appropriately adjust the reflux ratio accordingly. Take the floating valve tray as another example: when the processing capacity is only 50-60% of the designed capacity, in order to keep the floating valves operating within an appropriate range and to ensure that the instruments’ measurement ranges stay within the proper limits, it is necessary to increase the reflux ratio in order to maintain stable production. This is extremely important for large-scale production facilities to sustain stable operations. 4. The methods for adjusting the reflux ratio include the following: ① Reducing the amount taken from the tower top to increase the reflux ratio. ②When the top condenser is a fractional condenser, the amount of coolant used at the top of the tower can be increased to raise the amount of condensate and thus increase the reflux ratio. ③Forced reflux with a return liquid intermediate tank can temporarily increase the reflux flow rate to raise the reflux ratio, but the reflux tank must not be emptied. 5. How is the pressure difference in the tower adjusted during distillation operations? The tower pressure difference is a key factor in measuring the gas load within the tower, and it is also one of the important indicators for determining whether the feed and product flows in the distillation operation are balanced. With the feed and discharge kept in balance and the reflux ratio unchanged, the tower pressure difference remains essentially constant. When the normal material balance is disrupted, or when the temperature and pressure inside the tower change, it leads to changes in the velocity of the rising steam stream within the tower as well as in the liquid level height on the trays, thereby causing variations in the tower pressure difference. In distillation operations, adjustments must be made according to the causes of changes in column pressure difference. There are three commonly used methods: ① With a constant feed rate, the column pressure difference is adjusted by controlling the liquid draw-off from the top of the column. When more product is extracted, the flow rate of the steam rising in the tower decreases, and the pressure difference across the tower drops ； The yield decreases, the flow rate of the rising steam inside the tower increases, and the pressure difference across the tower rises. ②With the production volume remaining constant, the tower pressure difference is adjusted using the feed rate. The feed rate increases and the tower pressure difference rises ； As the feed rate decreases, the tower pressure difference drops. ③Within the limits permitted by the process parameters, the tower pressure difference is adjusted by changing the reactor temperature. Increasing the kettle temperature raises the tower pressure difference ； Lowering the kettle temperature reduces the column pressure difference. Regarding changes in pressure difference caused by equipment issues, each case should be handled individually; in severe cases, the equipment must be shut down for maintenance. 6. How is the temperature at the top of the tower adjusted in distillation operations? The top of the tower temperature is an important factor in determining the quality of the product at the tower top. With constant column pressure, as the top temperature rises, the content of heavy components in the overhead product increases, resulting in a decline in its quality. There are mainly two methods for adjusting the tower top temperature: one is to keep the reflux flow rate constant and adjust the reflux temperature ； One method is to fix the reflux temperature and adjust the reflux flow rate. As production facilities become increasingly large, methods for adjusting the reflux volume have been widely adopted to ensure production stability. The specific adjustment methods are as follows: ① Use the backflow rate to control the top temperature. The reflux flow increases and the top temperature decreases; this control method is often used when the tower top is a total condenser. ②When the refrigerant used at the tower top undergoes a phase change during the heat transfer process, the top temperature can be controlled by cascade regulation of the refrigerant’s evaporation pressure and the top temperature. The evaporation pressure decreases, and the corresponding evaporation temperature also drops, resulting in a decrease in the top temperature. This method can change the reflux flow rate when the top condenser is a demister ； When there is subcooling in the top condenser, it can also be used to adjust the reflux temperature. ③When there is no phase change in the refrigerant at the top of the tower during heat transfer, the top temperature can be controlled through cascade control of the refrigerant flow rate and the top temperature. If the flow rate increases, the top temperature decreases. This method can change both the backflow rate and the backflow temperature. ④Adjust the top temperature by using the heat transfer area of the overhead condenser. Raising the refrigerant level increases the heat exchange area and lowers the top temperature. This method can change both the backflow rate and the backflow temperature. ⑤When the material concentration in the distillation section is high, the temperature at the top of the column can be adjusted using the temperature difference between two plates. As the temperature difference increases, the amount of reflux liquid increases, and the top temperature decreases.