What is the impact of feed conditions on distillation operations?

There are 5 feed conditions for distillation columns: cold feed； Sponge feeding ； Gas-liquid mixed feed ； Saturated steam feed ； Superheated steam feed. In general distillation, feed at the bubble point is used; in this case, the flow rates of the vapor rising in the distillation section and the stripping section are equal, which facilitates the design of the distillation tower. Methanol distillation also employs feed at the bubble point. In actual production, sometimes, due to a slight excess in the distillation section, a gas-liquid mixed feed is used while still ensuring the quality of methanol. This reduces the number of plates in the distillation section while allowing an increase in the number of plates in the stripping section, which aids in the concentration and recovery of heavier components. When the feed condition changes, it causes a redistribution of the stripping and rectifying sections, which inevitably leads to changes in the gas-liquid equilibrium and temperature within the tower; adjustments are necessary to achieve a new equilibrium.

There are five feeding states: (1) cold feeding. (2) Saturated liquid. (3) Gas-liquid mixture. (4) Saturated gas. (5) Superheated gas. For a column with a fixed feed, changes in the feed condition will affect product quality and losses. For example: if a tower is fed with saturated liquid, switching to a cold feed means that the liquid enters the tower and meets the vapor rising from the stripping section on the feeding tray, where it is heated to the saturation temperature. At the same time, part of the rising vapor condenses. If there are too many trays in the distillation section and not enough in the stripping section, this will result in increased losses in the bottom liquid. At this point, in terms of operation, the reboiler steam should be adjusted appropriately so that the reflux rate of the column returns to its original level.