Diffusion Mass Transfer in Fluid Systems

Extraction treats a feed with a liquid solvent to remove and concentrate a valuable solute. When the feed is a liquid, the process is called “liquid–liquid extraction,” or more commonly just “extraction.” When the feed is a solid, the process is called “solid–liquid extraction,” or more commonly “leaching.” In either case, the original solution is commonly called the feed; after the extraction, this stream is called the raffinate. Similarly, the second solvent is called the extract once it contains solute.

Extraction is almost never the first choice as a separation process. If the solute of interest is a gas, then we will first try gas absorption or stripping. If the solute of interest is volatile under convenient conditions, then we will attempt distillation. We will normally try extraction only after we fail at absorption and distillation. Still, we have included a separate chapter on extraction for two reasons. First, it is an important process, central to some petrochemical, pharmaceutical, and metallurgical processes. We discuss these in Section 14.1. Second and more importantly, extraction gives an extended example of the generalization of the analyses of absorption and distillation. When extraction is carried out in differential contactors like packed towers, its analysis is similar to gas absorption. When extraction is carried out in staged contactors, its analysis parallels staged distillation. Thus we can test our understanding of absorption and distillation by discussing extraction.