TABLE 5-1 Examples of Mass-Separating Agents and Their Applications

Mass-Separating Agent


Zeolite molecular sieve

Oxygen from air, hydrogen recovery, isomer separations, glucose-fructose separation, CO2 removal from gas streams, water removal from ethanol

Activated carbon

Removal of trace organics from water and air, of color from petroleum fractions, and of odor and taste bodies from water

Ion exchange resin

Removal of specific ions from various, usually aqueous, streams

Functionalized solvent

Separation of derivatized organics from simpler organics


Separation of ions and polar organics from organic phases

Polymer membrane

Nitrogen from air, hydrogen recovery, water removal from gases, water purification, CO2 recovery, desalination, biological materials separations


Removal of solids from gases and liquids

Flocculating agent

Concentration of fine particles and biological agents in aqueous streams

separation systems based on distillation if it was a viable option, turning to other options only if it was not. This approach remains dominant, even though most of the alternatives to distillation would require less energy and produce less CO2. Given that distillation is by far the most common separation process, used in as much as 80 percent of all the chemical separations listed in Box 5.1, optimization of phase equilibria will remain an important grand challenge for the chemical separations industry.

It is also true that distillation is sometimes not an effective option. Instead, mass-separating agents (MSAs)—solvents, absorbents, adsorbents, membranes, and so on—are often added to amplify the separating capability for these more intractable systems, while potentially providing for more economical, environment-friendly solutions. Some examples of MSA-based processes are given in Table 5-1, which we amplify by focusing on two examples of their use that have broad societal implications.

Example 1:
Pure Oxygen from Air

Even though oxygen is already produced inexpensively on a massive scale, the number of uses and overall volume produced could grow substantially if its price were cut even more. Some of the existing and potential applications include the following:

  • Feeding oxygen instead of air to power plant furnaces to reduce the volume of flue gas produced and to increase the percentage of carbon dioxide, sulfur oxides, and nitrogen oxides in the flue gas, dramatically reducing the cost of their recovery. Whether this use comes about is highly dependent on the need to sequester the carbon dioxide.

  • Feeding oxygen to gasification reactions such as occur in next-generation, integrated gasification combined cycle (IGCC) coal-based power plants, which may be the wave of the future.

  • Feeding oxygen instead of air to aerobic waste-treatment processes, thereby reducing equipment size and costs.

  • Feeding oxygen to a large number of organic oxidation processes to improve selectivities and reduce energy costs.

The savings would have to be larger than the capital and energy costs of producing the oxygen in order for these applications to be realized and grow. The secret to lowering oxygen costs would appear

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