Factors Affecting Distillation Production
Thread Content
Let me start by saying: First, the impact of the raw material composition. The actual composition differs greatly from the designed composition. Various impurities have different effects on product quality. 1. Ester impurities mainly affect the content of free base in the product. In practical operations, a 30% concentrated NaOH solution is added to the pre-tower to cause the esters to undergo a decomposition reaction, producing methanol and the corresponding sodium salts. These sodium salts dissolve in water and eventually become part of the wastewater that is discharged from the facility. The actual content of ester impurities is higher than the designed level, and this can be addressed by increasing the amount of alkaline solution. 2. Alcoholic impurities mainly affect the levels of free acid and ethanol in the product. Alcohols all have boiling points higher than methanol and are considered heavy components. There is an fusel oil extraction port in the middle of the atmospheric pressure column, used to extract alcoholic impurities. At the same time, since ethanol is a key component in the methanol distillation system, the actual ethanol content being **higher than the designed value poses difficulties in carrying out the operations. In practice, we prevent the accumulation of ethanol by controlling the top temperature of the wastewater stripping tower to be no higher than 66°C. However, a high ethanol content **reduces the operational flexibility of the methanol distillation system. 3. Alkane impurities mainly affect the water solubility of the product; all alkane impurities other than octane can be removed relatively easily in the pre-tower. The problems caused by actual values being higher than the design values can be resolved by increasing the steam supply to the pre-tower, raising the amount of water used in the pre-tower, and installing an oil-water separator. N-octane has a boiling point above 100°C, making it difficult to remove in the pre-column. In practice, the octane number often turns out to be much higher than the designed value, which can well explain the issue of substandard water solubility in the products from the atmospheric distillation column as well as the frequent wax formation at the bottom of the column. In practice, we address this issue by regularly draining and removing wax from the vapor leg of the wastewater cooler. However, too high a content of n-octane also **reduces the operational flexibility of the methanol distillation system**. 4. The inorganic impurity is mainly water, which primarily affects the water content of the product. The actual water content in the crude methanol is higher than the designed value; we resolved this by increasing the amount of steam used. However, this increases the energy consumption of the methanol distillation system. II. Influence of raw material quantity 1. At higher loads, the liquid flow velocity increases, the time for gas-liquid mass transfer shortens, and the efficiency of mass transfer declines; once a certain limit is reached, the quality of the product is affected. This limit is determined by both the design capacity of the tower and its operational flexibility; a reduction in operational flexibility inevitably leads to a decrease in this limit. Therefore, under high load, it is not very easy for us to achieve stability in our operations. 2. At low load levels, as the liquid flow rate decreases, the time required for gas-liquid mass transfer increases; this leads to an improved efficiency of mass transfer and ensures the quality of the product. However, the energy consumption is high in such conditions. III. Influence of the reflux ratio § Reflux ratio R = L/D, where L is the flow rate of the reflux liquid at the top of the tower and D is the flow rate of the liquid taken out from the top of the tower. § Reflux is a characteristic feature of distillation operations. Backflow is a necessary condition for gas-liquid mass and heat transfer. The selection of the reflux ratio has a significant impact on distillation operations. If the reflux ratio is too high, the temperature at the bottom of the tower decreases. To maintain a sufficient driving force for heat transfer, it is necessary to increase the amount of steam used, which inevitably leads to higher energy consumption. If the reflux ratio is too low, the efficiency of gas-liquid mass and heat transfer decreases, resulting in excessive levels of heavy components in the product at the top of the tower. Therefore, an appropriate reflux ratio is very important. IV. Effects of flooding: In a distillation column, flooding has a significant impact on the quality of the product. In a normal distillation process, the gas phase is the continuous phase, while the liquid phase is the dispersed phase. When the liquid flow rate increases to a certain level, the interaction between the gas and liquid phases creates a vicious cycle, eventually with the gas phase becoming the dispersed phase and the liquid phase becoming the continuous phase. At this point, the gas passes through the liquid layer in the form of bubbles; the tower is filled with liquid, the pressure drop increases sharply, liquid backmixing and entrainment of gas bubbles occur extensively, and the mass transfer efficiency is extremely poor, **which affects the quality of the product. When flooding occurs, we should disrupt the equilibrium point by quickly increasing or decreasing the reflux flow rate, and then establish a new gas-liquid equilibrium. This post was last edited by zhangyunx2003 on 2009-3-28 18:54.]
I’m working on a little project related to distillation simulation; I hope to see everyone’s valuable opinions – please leave comments
During the start-up of distillation, feeding material into the pressure column before the temperature of the pre-column has reached the appropriate level often results in methanol purity that does not meet the required standards. A too low reflux rate can also have an adverse effect. The pH value, which is related to the alkalinity, should be maintained between 9 and 11
What is the impact of changes in feed composition on distillation operations? Changes in the feed composition directly affect distillation operations; when the concentration of heavier components in the feed increases, the load on the distillation section rises. For a column with a fixed number of trays in the rectification section, this will result in the heavier components being carried to the top of the column, causing the quality of the product at the top to be substandard. If the concentration of the light components in the feed increases, the load on the distillation section increases at that time. For a column with a fixed number of plates in the stripping section, this will result in incomplete evaporation of the light components in that section, leading to an increased loss of light components in the bottom liquid. At the same time, 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 this point, the temperature of the entire tower decreases while the tower pressure increases. The composition becomes heavier; the situation is the opposite. When the feed composition changes, the following measures can be taken: (1) Improve the feed inlet; when the composition becomes heavier, move the feed inlet downward ； When the composition becomes lighter, move the feed inlet upward. (2) Changing the reflux ratio: When the composition becomes heavier, increase the reflux ratio ； When the composition becomes lighter, reduce the reflux ratio. (3) Adjusting the refrigerant and heat input amounts: Depending on changes in the composition, the refrigerant used in the top condenser and the heat input amount at the bottom of the tower are adjusted accordingly to maintain constant product quality at both the top and bottom of the tower. 5. What is the impact of changes in feed temperature on distillation operations? Changes in feed temperature have a significant impact 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. For example, when the feed temperature is too low and there is insufficient heating steam at the bottom of the tower, it will increase the content of light components in the distillate at the tower bottom. Changes in the feed temperature imply changes in the feed condition, and such changes affect the load 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. 6. What is 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 speed of the rising steam inside the tower directly affects the mass transfer efficiency. Generally speaking, the maximum upward steam velocity inside the tower should be slightly lower than the flooding velocity. In practice, the maximum allowable speed is often chosen to be 80% of the flooding speed. Too low a speed will significantly reduce the tray efficiency. The main factor affecting the upward steam velocity in the tower is the heating capacity of the evaporation kettle. With the boiler temperature remaining stable, as the heating amount increases, the speed of steam rising inside increases as well ； As the heat input decreases, the speed of steam rising inside also decreases. It should be noted that an excessively large or rapid adjustment range of the heating amount may cause flooding or leakage. 7. What is the impact of the reflux ratio on distillation operations? During operation, the quality of the product is ensured by adjusting the reflux ratio. 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 standards. When the light components from the distillation section move down to the stripping section, causing a decrease in temperature at the lower part of the tower, 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 within the tower, and may even lead to flooding, disrupting the normal operation of the tower. 8. What is the impact of the amount of coolant at the top of the tower on distillation operations? For columns operating with internal recirculation, the amount of coolant has a relatively significant impact on the distillation process ； It is also the main factor affecting the fluctuations in backflow volume. For columns that use external reflux, fluctuations in the amount of coolant will also affect the operation of the distillation column to varying degrees. For example, a reduction in the cold dose will weaken the performance of the condenser, resulting in less condensate; and 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 of these will increase the top temperature of the distillation column, raise the content of heavier components in the product at the top of the column, and degrade its quality. 9. What is 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 within 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, the heavy components are carried to the top of the tower, resulting in product quality that does not meet standards. 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 likewise leads to a decline in the quality of the product at the tower top. If the feed rate is increased while the draw rate at 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 vapor, and a greater pressure difference between the top and bottom of the tower; in severe cases, this can lead to flooding. 10. What is the impact of the bottom product flow rate on distillation operations? Maintaining a stable liquid level in the tower bottom is the primary condition for keeping the tank temperature constant. The change in the liquid level at the bottom of the tower is primarily determined by the amount of product taken out from the bottom of the tower. When the draw rate at the bottom of the tower is too high, it can cause the liquid level in the tower bottom to drop or even lead to evacuation. This will reduce the circulation rate of the liquid in the evaporation kettle, resulting in poor heat transfer; the light components cannot be vaporized, and the products at the top and bottom of the tower are both unsuitable. If a shell-and-tube evaporator is used, the low volume of circulating liquid causes the liquid in the evaporator to become superheated as it passes through the upper section of the tubes; this results in a higher gas temperature in the vaporization tubes while the temperature of the liquid in the evaporator 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 cause the tower to be flooded. This increases the resistance to the circulation of the liquid in the tower, leading 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. Additionally, maintaining a certain level of liquid in the kettle also serves as a liquid seal to ensure safe production.