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In a conventional distillation system designed according to traditional principles, each column is equipped with a reboiler and a condenser, as shown in the three-component two-conventional-distillation process in Figure 1. In this process, since the actual heat transfer between the cold and hot fluids through the walls of the heat exchanger is irreversible, a sufficient temperature difference is required to enable the process to proceed. The greater the temperature difference, the greater the loss of exergy, and thus the lower the thermodynamic efficiency. The heat-coupled distillation tower is a new type of energy-saving distillation developed based on this principle. ▲Figure 1: Conventional three-component distillation process. Principle of heat-coupled distillation: as shown in the process in Figure 2. The material in the secondary tower is preliminarily divided into two mixtures: A and B, as well as B and C. The light component A is completely distilled off at the top of the tower, while the heavy component C is completely removed from the bottom of the tower. After entering the main tower, the material is further separated; product A is obtained from the top of the tower and product C from its bottom. In the middle section of the tower, the liquid concentration of component B reaches its maximum, and the intermediate product is extracted here. The secondary tower does not use a condenser or a reboiler, allowing for heat coupling; hence it is referred to as heat-coupled distillation. ▲Figure 2 Heat-coupled distillation process. Applications of heat-coupled distillation: In Figure 2, it is assumed that the relative volatilities between components A and B, as well as between B and C, are all 3. With a reflux ratio of 1 to 3, feeding at the bubble point, and a target product purity of 90%, heat-coupled distillation can save 20% in energy consumption with the same number of trays or slightly more. By eliminating the reboiler and condenser in the upstream tower, it is possible to reduce the investment in heat exchange equipment as well as the consumption of steam cooling water; thus, its economic benefits are high, and it can be applied to the separation of multi-component systems. Thermocoupled distillation is the most thermodynamically ideal system configuration, as it allows for energy savings as well as reduced equipment investment. Calculations show that heat-coupled distillation can save 20% to 40% in energy consumption compared to distillation using two conventional columns. Therefore, this new type of energy-saving distillation technology attracted widespread attention in the West during the energy crisis of the 1970s, and many studies were conducted on it. However, since it is difficult to maintain the design values for gas-liquid distribution between the main and secondary columns during operation, and the greater the separation difficulty, the greater the sensitivity to deviations in gas-liquid distribution, making it hard to stabilize the operation. Moreover, due to control issues and the lack of design methods, heat-coupled distillation has not been widely used in industry for over 20 years. Thermocoupled distillation is only recommended for separable systems with boiling points that are close to each other; however, care must also be taken during design to ensure that the gas and liquid flow rates in the main and secondary columns meet the required levels. Application scope of the heat-coupled distillation process: The heat-coupled distillation process is not suitable for all chemical separation processes, and its application has certain limitations. Although such towers possess the most ideal system structure from a thermodynamic perspective, they rely primarily on the \"reuse\" of heat supplied to the distillation tower; as a result, this process is greatly limited when the heat provided by the reboiler is very large or when the condenser needs to cool the stream to very low temperatures. Furthermore, the heat-coupled distillation process has certain requirements regarding the purity of the substances to be separated, the feed composition, the relative volatility, and the operating pressure of the column: (1) Product purity. The purity of the intermediate products obtained through a heat-coupled distillation process is higher than that achieved by the side-stream outputs of conventional distillation towers; therefore, when high-purity intermediate products are desired, a heat-coupled distillation process can be considered. If the purity requirement for the intermediate product is not high, it can be directly obtained from the side stream of a conventional distillation tower. ⑵Feed composition. If the three components A, B, and C are to be separated with increasing relative volatilities, when using this type of column, the amount of component B in the feed mixture should be the highest, while the amounts of components A and C should be similar. ⑶Relative volatility. When component B is the main component in the feed, the energy-saving advantage of using heat-coupled distillation is most evident only when the ratio of the relative volatility of component A to that of component B is equal to the ratio of the relative volatility of component B to that of component C. If component A and component B are very easy to separate (compared to component B and component C), it is less energy-efficient from an energy-saving perspective to use a conventional two-column process. ⑷The operating pressure of the tower and the pressure throughout the separation process cannot be changed. When it is necessary to change the pressure, only the conventional double-tower process can be used.