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Distillation is a chemical process used to separate components with different boiling points in a mixture. It achieves separation through heating and condensation, that is, by subjecting the mixture to multiple cycles of partial vaporization and partial condensation in a distillation tower, so that the light components (those with lower boiling points) move into the gas phase and rise to the upper part, while the heavy components (those with higher boiling points) remain in the liquid phase and sink to the lower part. This process can be controlled by changing temperature and pressure to achieve separation. The operating parameters of a distillation tower generally include tower pressure, number of trays, feed position, reflux ratio, and distillate volume. But in reality, there is another factor that affects the operation of the distillation tower and its separation efficiency, and that is the feeding method of the distillation tower. Based on their thermal conditions, there are five types of feed for distillation columns: subcooled liquid phase, saturated liquid phase (bubble-point feed), gas-liquid mixture, saturated vapor phase (dew-point feed), and superheated vapor phase. Different feeding methods carry different amounts of heat into the tower, which in turn changes the temperature distribution within the distillation column. When designing a tower, if we try to examine the effects of changing the thermal conditions of the feed, we will find that in order to achieve the same separation efficiency, the number of tray levels, the feed position, as well as the loads on the condenser and reboiler may all change. 1. Subcooled liquid feed: This refers to the use of a liquid or gas at a temperature below its saturation point as the feed material in the distillation tower. This prevents boiling from occurring inside the tower, thereby reducing mass transfer resistance and improving distillation efficiency. 2. Saturated liquid phase (bubble-point feed): This refers to using a liquid that has already reached its saturation temperature as the feed material in the distillation column. 3. Gas-liquid two-phase: In this case, the feed consists of both gas and liquid, which are mixed and separated in the distillation tower. 4. Saturated vapor phase (dew point feed): This refers to using gas that has reached its saturation temperature as the feed material in the distillation column. 5. Superheated vapor phase: This refers to feeding gas at a temperature higher than the saturation temperature into the distillation tower, thereby enabling heat exchange within the tower and further improving the purity of the product. The choice of these feed conditions depends on the quality of the product and the desired degree of separation. The graphical method for calculating the number of theoretical plates in distillation columns, as well as the influence of feed heat conditions on the operating line equation in the stripping section, are all discussed in the chapters on distillation in textbooks on chemical engineering principles. The figure below shows the operating line equation for the distillation section and the feed equation (the q-line equation). It can be seen that once the reflux ratio and the purity of the overhead distillate are determined, the operating line equation for the distillation section remains constant. The feed equation is related to the heat condition of the feed, with different values of q representing various heat conditions of the feed. q is referred to as the thermodynamic parameter of the feed state; numerically, q is approximately equal to the ratio of the heat required to convert 1 kmol of feed into saturated steam at the temperature of the feed plate to the latent heat of vaporization of 1 kmol of feed at that same temperature. This ratio is determined by performing a heat balance on the feed plate. The value of q indicates the proportion of saturated liquid contained in the feed liquid. When q=1, it indicates that the feed is a saturated liquid, meaning 100% of the feed consists of saturated liquid. When q=0, it indicates that there is no saturated liquid in the feed, meaning 100% of it is saturated vapor. When 0