Purification Technology Edition: Daily Question [2017-11-19]

The reflux ratio has a significant impact on distillation operations; it is directly related to changes in the material concentration on each tray within the tower as well as to the temperature distribution. These factors ultimately affect the tower’s separation efficiency, making the reflux ratio one of the important operational parameters. For methanol distillation, the reflux ratio ranges from 2.0 to 2.5. This ratio is adjusted based on the tower’s load and the quality of the purified methanol – a lower reflux ratio can be used when the tower load is low, while a higher ratio is necessary in other cases

A high reflux ratio can improve product quality, but it correspondingly increases the cooling load on the top condenser and the heating load on the bottom of the tower. A low reflux ratio can affect product quality.

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 the consumption of water, electricity, and steam (heat sources). This leads to an excessive circulation rate of the material within the column, which may even result in flooding and disrupt the normal operation of the column. Nonetheless, the light components in the distillation section are purified, while the bottom of the column contains a higher amount of these light components. If the reflux ratio is too low, the liquid phase on the tower plates decreases, resulting in poor gas-liquid mass transfer. This causes the heavier components at the bottom of the tower to be carried to the top, and in severe cases, it leads to substandard quality of the product at the top of the tower

In air distillation, the reflux ratio generally refers to the ratio of the amount of liquid flowing downward in the column to the amount of vapor flowing upward; it is also known as the liquid-to-gas ratio. In chemical production, the reflux ratio generally refers to the ratio of the amount of liquid flowing downward in the tower to the amount of liquid distilled at the top of the tower. The purity of the distillate, under a constant number of tower plates, depends on the reflux ratio. When the reflux ratio is high, the purity of nitrogen in the gas phase is high, while the purity of oxygen in the liquid phase is low. When the reflux ratio is low, the purity of nitrogen in the gas phase is low, while the purity of oxygen in the liquid phase is high. This is because the hotter rising gas mixes with the cooler liquid flowing downward on the tray; after heat and mass exchange, their temperatures can, in an ideal situation, approach each other, that is, reach the same temperature. This temperature lies between the original gas and liquid temperatures. If the reflux ratio is high, that is, if there is more cold liquid flowing downstream or less vapor rising, then the gas-liquid mixture temperature will tend to be on the side of the colder liquid. As a result, the temperature drop of the rising vapor is greater, and more vapor condenses. Since oxygen is a poorly volatile component, more of it condenses, which in turn causes the nitrogen concentration in the gas rising away from the tray to increase rapidly. This is the case for each tray, which is why the gas obtained at the top of the tower has a high nitrogen purity. On the other hand, since the gas-liquid mixing temperature leans toward the side of the cold liquid, the temperature rise of the liquid flowing downstream is small, and less liquid evaporates; as a result, less nitrogen component evaporates from the liquid. Consequently, the oxygen concentration in the liquid flowing downstream and leaving the tray increases more slowly. This is the case for each tray, and as a result, the oxygen concentration of the liquid obtained at the bottom of the tower is low. When the reflux ratio is low, the situation is the opposite of what was described above, so it will not be repeated. Adjusting the distillation conditions essentially involves changing the reflux ratio at various sections within the tower. When operators talk about a high temperature in the distillation column, what they actually mean is a low reflux ratio ； A low tower temperature means a high reflux ratio.

The size of the reflux ratio has a significant impact on both the separation efficiency and the economic viability of the distillation process.

When the reflux ratio is high, the purity of nitrogen in the gas phase is high, while the purity of oxygen in the liquid phase is low. When the reflux ratio is low, the purity of nitrogen in the gas phase is low, while the purity of oxygen in the liquid phase is high

The reflux ratio has a significant impact on distillation operations; it is directly related to changes in the material concentration on each tray within the tower as well as the temperature distribution. This ultimately affects the tower’s separation efficiency, making it one of the important operational parameters. The reflux ratio for methanol distillation is between 2.0 and 2.5. This ratio is adjusted based on the tower’s load and quality – a lower load allows for a smaller reflux ratio, while a higher load requires a larger one

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 the consumption of water, electricity, and steam (heat sources), which leads to an excessive circulation rate of the material inside the column

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 the consumption of water, electricity, and steam (heat sources). This leads to an excessive circulation rate of the material within the column, which may even result in flooding and disrupt the normal operation of the column. Nonetheless, the light components in the distillation section are purified, while the bottom of the column contains a higher amount of these light components. If the reflux ratio is too low, the liquid phase on the tower plates decreases, resulting in poor gas-liquid mass transfer. This causes the heavier components at the bottom of the tower to be carried to the top, and in severe cases, it leads to substandard quality of the product at the top of the tower

The purity of the distillation product, with a constant number of tower plates, depends on the reflux ratio. A high reflux ratio results in a higher purity of the gaseous product, while the purity of the liquid product is lower; conversely, a low reflux ratio yields the opposite outcome.