Common problems in distillation tower operation

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Distillation technology is widely used in the production of various chemicals, and distillation towers are also one of the common devices in chemical plants. 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 vapor-liquid equilibrium throughout the tower, resulting in products that fail to meet the required quality standards. Therefore, many distillation columns have specific measures to ensure that the column pressure remains within an appropriate range. I. Regarding the tower pressure of a pressurized tower, there are mainly the following two methods of adjustment: (1) When the top condenser is a fractional condenser, the tower pressure is generally adjusted based on the amount of gas taken out. With all other conditions unchanged, the gas-phase recovery increases while the column pressure decreases ； The gas-phase recovery rate decreases, and the tower pressure rises. (2) When 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 unchanged, increasing the amount of cooling agent lowers the temperature of the reflux liquid and reduces the tower pressure ； If the amount of cold dose is reduced, the temperature of the reflux liquid rises and the tower pressure increases. II. Regarding the pressure control of the vacuum distillation tower, there are mainly the following two methods: (1) When the vacuum in the tower 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. (2) When using an electric vacuum pump for vacuum creation, the control valve is installed on the return line of the vacuum pump; by adjusting the opening degree of this valve, the amount of exhaust gas removed from the system can be controlled, thereby regulating the vacuum level in the tower. III. For pressure control in atmospheric pressure columns, there are mainly the following three methods: (1) When high stability requirements are not imposed on the top pressure of the column, it is not necessary to install a pressure control system; instead, a 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. (2) When 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. (3) Adjust the vapor pressure in the reboiler by regulating the amount of heating steam supplied to it. IV. How to adjust the kettle temperature during distillation operations? 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 boiler temperature is one of the important control parameters in distillation operations. When the kettle temperature changes, it is customary to adjust it back to normal by changing the amount of heating steam supplied to the evaporator kettle. 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 the heavier components in it, raising its bubble point, and consequently increasing the reactor temperature. When the kettle temperature exceeds the specified value, the steam usage should be reduced to decrease the amount of vaporization of the kettle liquid. This increases the proportion of light components in the kettle liquid, lowers its bubble point, and in turn reduces the kettle temperature. V. What are the factors that affect fluctuations in the kettle 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 only fail to increase, but will actually 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 within 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 boiler liquid become heavier, the bubble point of the boiler liquid rises, and the boiler 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 to the required level can one accurately determine whether the kettle temperature meets the process requirements; otherwise, it may 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 there are fluctuations in the kettle temperature, it is necessary to analyze the causes of these fluctuations and eliminate them. VI. How to adjust the reflux ratio in distillation operations? The reflux ratio is determined based on the separation requirements for the raw materials. 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 can raise 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 and heating energy at the bottom of the tower. 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 be adjusted only when the normal operating conditions of the tower are disrupted or the product quality is substandard. VII. The methods for adjusting the reflux ratio include the following: (1) Reducing the amount taken from the tower top to increase the reflux ratio. (2) When 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. (3) Forced 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. VIII. How to adjust the pressure difference across the column during distillation operations? The column pressure difference is a major factor in measuring the gas load within the column; it is also one of the important indicators for determining whether the feed and output in a 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 tray levels, thereby causing variations in the tower pressure difference. In distillation operations, adjustments must be made in response to the factors that cause changes in the tower pressure difference. There are three common methods for this purpose: (1) With the feed rate remaining constant, the tower pressure difference is adjusted by changing the amount of liquid taken from the top of the tower. When more product is extracted, the flow rate of the steam rising in the tower decreases, and the pressure difference across the tower falls ； The yield decreases, the flow rate of the rising steam inside the tower increases, and the pressure difference across the tower rises. (2) With the extraction amount remaining constant, use the feed rate to adjust the column pressure difference. The feed rate increases and the pressure difference across the tower rises ； As the feed rate decreases, the tower pressure difference drops. (3) Within the range permitted by the process parameters, the tower pressure difference is adjusted by changing the reactor temperature. Increasing the kettle temperature causes the column pressure difference to rise ； Lowering the kettle temperature reduces the tower pressure difference. For pressure difference changes caused by equipment issues, each case should be handled on a case-by-case basis; in severe cases, the equipment should be shut down for maintenance. IX. How to adjust the top temperature in distillation operations? The top temperature of the tower is an important factor determining the quality of the product at the tower top. With the column pressure remaining constant, an increase in the top temperature leads to an increase in the content of heavier components in the product at the top of the column, 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. Due to the increasing size of production facilities, and considering production stability, methods for regulating the reflux rate have been widely adopted. The specific methods for adjusting the tower top temperature are as follows: (1) Control the top temperature using the reflux flow. The reflux flow increases and the top temperature decreases; this control method is often used when the tower top is a total condenser. (2) When the refrigerant used at the tower top undergoes a phase change during heat transfer, the top temperature can be controlled through cascade regulation using the evaporation pressure of the refrigerant and the top temperature. As the evaporation pressure decreases, the corresponding evaporation temperature also drops, resulting in a decrease in the top temperature. This method can change the reflux rate when the top-of-tower condenser functions as a dephlegmator ； When there is subcooling in the top condenser, it can also be used to adjust the reflux temperature. (3) When 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. (4) Adjust the top temperature using the heat exchange area of the top condenser. Raising the refrigerant level increases the heat exchange area and lowers the top temperature. This method can change both the reflux volume and the reflux temperature. (5) When the material concentration in the distillation section is relatively 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 rises, and the top temperature decreases.