other industries, while difficult to quantify from an energy-use standpoint, probably added one to several percentage points to that number. About 60 percent of the total energy requirements of the chemical and petroleum processing industries are consumed by separation processes (DOE, 2005). Capital investments in separation processes are also a very important factor, with 40-70 percent of the total investments in various separation-intensive industries being consumed by these processes (Humphrey and Keller, 1997). Given that separation processes consume so much energy, it is clear that they also contribute very significantly to the nation’s output of greenhouse gases. Thus for three reasons—energy use, investment costs, and environmental considerations—the incentives to improve these processes, as well as to invent and develop new ones, are very great.
Box 5-1 portrays both the breadth of the separations field and the large number of disparate industries in which these processes are applied. Most chemical separation processes are based on thermodynamic equilibrium considerations. When, for example, a liquid stream containing two or more components is heated and forms a vapor phase in contact with the liquid, at least a partial separation of the components is possible if the resulting two phases at equilibrium have different compositions. Distillation is highly effective at separating compounds based on differences in their relative volatilities. From a design point of view, distillation-based processes are favored not only because their mechanical simplicity often leads to low investment costs but also because their design requires a much smaller set of phase-equilibrium data than all other separation options to quantify and optimize the efficiency of the separation. This fact accounts in large part for the historic preference for distillation over alternative methods.
Distillation, because it requires that the mixture be repeatedly vaporized and condensed, nonetheless consumes tremendous amounts of energy. Historically, energy consumption and its concomitant carbon dioxide release were not deemed to be of great concern, so chemical industries tended to design
Major Separation Processes and Industries That Depend Heavily on Chemical Separations