double-effect distillation

To know it in detail, you have to check university textbooks. Simply put, double-effect distillation involves two columns: one operates at high pressure while the other operates at low pressure or even in a vacuum. The vapor from the top of one column is used directly as a heat source for the bottom of the other column, without the need for condensation. In this way, the condenser associated with the top of one column is eliminated, and the functions of the condenser and the reboiler of the second column are combined. Cooling water is required only at the condenser on the top of the second column. This arrangement allows for a savings of around 50% in primary steam usage, with roughly a 50% reduction in cooling water needs as well. Since distillation itself is a unit operation with high heat consumption, most of the heat added by the vapor at the bottom of the tower is carried away by the cooling water at the top of the tower; therefore, making use of the vapor from the top of the tower is a way to save energy. The salt production industry makes extensive use of multi-effect evaporation; the principle is the same as mentioned above. The reason for operating the towers at different pressures is to maintain a certain temperature difference for heat transfer, thereby preventing the heat exchanger area from becoming too large.

Double-effect evaporation is the simplest type among multi-effect evaporations; it utilizes the vapor from the first effect to supply heat energy to the second effect, thereby achieving energy savings. There are two types: parallel flow and counterflow. A balance must be maintained among steam, condensate, and vacuum. And maintain the temperature difference between the first and second effect stages.