What is the difference between stripping and distillation?

Stripping and distillation are both methods of separation of components that have differences in their relative volatility, or tendency to become a vapor as temperature rises. From a simple semantic point of view, stripping is a form of specialized distillation.

Distillation is a term that encompasses a variety of separation techniques where a system of two or more components are heated within a separation device, such as a flash drum or a distillation column. The vapor generated is condensed back into a liquid that is enriched with a higher composition of the lighter component and a lower composition of the less volatile, or heavier component. A single stage system would be a simple flash drum where a liquid system of two or more components is heated and the vapor flashes once and is condensed. A more complex system is a distillation column, where there are multiple, consecutive flashes (multiple stages of equilibrium) that further purifies lighter components from heavier components, thereby making each product stream richer in the desired component ratios.

To describe what stripping is, you are typically referring to an attempt to remove light components from a 'heavy' stream. Essentially, a stripper is a distillation column that 'strips' light material out of it by applying heat, and sometimes returning the cooled distillate (tops product) into the column to further purify the bottoms product, removing more incremental light material from the bottoms.

A typical stripper is a column where the feed tray enters at the top of the column, with few to zero stages (trays) above the feed tray. The heat source enters the column at the bottom, and the feed trickles down to the bottom trays and is essentially 'stripped' of light material by the vaporized reflux traveling up the column. The light material vaporizes and leaves in the overhead product, and the heavy material is purified all the way down to the heavier product stream.

A column that both strips and rectifies will have a feed tray in the middle of the column. The stripping section are all trays below the feed tray, and the rectifying trays are above the feed tray. The rectifying section is designed to allow the cool reflux to 'wash out' any entrained heavy material that flashes up into the trays above the feed tray.

Finally, a stripping method found in refineries employs injecting steam into hydrocarbons in much the same way heat is applied to the bottom of stripping tower. A stripping tower will take feed from a larger distillation column, and have its own trays with stripping steam entering at the bottom of the mini-column. The steam lowers the partial pressure of the light components; the heat from the heavier material heats up the light ends and flash them out of the mixture. The steam does not vaporize the light ends; it merely lowers the overall boiling point and allows the light material to flash away and travel back into the main column with the steam to be recovered in product streams higher up in the main column. The 'stripped' material leaves the small stripper as an independent product.