Internal reflux in the distillation column

The reflux in the distillation column refers to the liquid flowing downward from the top of the distillation column, which condenses through reverse flow and encounters upward moving bubbles, and a large amount of heat and mass transfer is repeated, so as to improve the separation efficiency of the entire distillation column. Internal reflux is usually formed by a pump at the bottom of the column that draws a portion of the overhead liquid and re-injects it into the top of the column.

Internal reflux has the following advantages:

• Internal reflux can reduce the local concentration at the top of the column, thereby reducing the bubble point height and improving the stability of the whole distillation process;

• Internal reflux can also improve the efficiency of the distillation liquid, improve the separation effect, reduce the distillation time, and make the product get better quality and purity.

  
  

However, there are also some disadvantages of internal reflux:

• The pumps and pipelines required for internal reflux will increase the cost and maintenance expenses of the equipment.

• The amount of liquid extracted from the top of the tower by internal reflux must be properly controlled, otherwise it may lead to negative effects, such as the risk of "foam" and overflow of the tower bed.

External reflux in the distillation column

Column refers to the liquid flowing upwards from the bottom of the distillation column, also known as "gas-liquid two-phase flow". External reflux is usually achieved by a separator or level regulator, which can cause a large number of vacuoles to be generated at the bottom of the column and bring a thin layer of liquid into the top of the column to increase the separation efficiency of the column.

The external reflux has the following advantages:

• The external reflux can improve the separation efficiency of the whole tower, making the separation results more obvious and efficient;

• The external return does not need additional pumps and pipelines, so as to save equipment costs and maintenance costs.

However, external reflux also has some disadvantages:

• External reflux will increase the bubble point of the liquid, which may increase the requirements for heating and reduce the distillation efficiency.

• When the external reflux is too large, the liquid level at the top of the tower may be too high, resulting in problems in demulsification and yield.

  
  

In general, both internal and external reflow have their own advantages and disadvantages, and it is necessary to choose which reflow method to use according to the specific operational needs. For some demanding distillation operations, a combination of internal and external reflux is often used to achieve the best separation.