What is side-stream extraction? What does “side line” mean?

In distillation operations, when the column pressure remains constant, temperature serves as an indicator of the concentration of light components. As long as the temperature is well controlled, a particular temperature value generally corresponds to a product with specific component proportions; by setting side draws at appropriate locations, the product can be extracted from the column. The gas-liquid compositions at each tray level of the tower are different, and since distillation is generally not a simple distillation of two components, there exist various material states of different components between the different tray levels! Take oil refining as an example: crude oil contains many components with different properties such as boiling points, and when a specific component is needed, it must be extracted from a particular tray! And this specific tray is not necessarily at the top or bottom of the tower; therefore, it often needs to be taken out via a side stream. Moreover, distillation is a process of mass and heat transfer (the temperature decreases from the top to the bottom of the tower, and the light components gradually decrease), and taking material out via a side stream also allows for the distillation of those components that have not been extracted There might also be issues related to energy conservation! It depends on the product you need; the products obtained from different sections are not the same. For example, in a atmospheric distillation tower for crude oil, gasoline, kerosene, light diesel, and heavy diesel are obtained sequentially from the top of the tower and its side lines.

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