Separation Methods: MSA and Stripping

Separation by Mass Separating Agent (MSA)

• Separating agent: Liquid absorbent (MSA)
• Example: Separation of carbon dioxide from combustion products by absorption with aqueous solutions of an ethanolamine

Stripping

• A liquid mixture is separated, generally at elevated temperature and ambient pressure, by contacting liquid feed with a stripping agent
• Separating agent: Stripping vapor (MSA)
• Example: Stream stripping of naphtha, kerosene, and gas oil side cuts from crude distillation unit to remove light ends

Azeotropic Distillation

• Formation of minimum-boiling azeotropic mixtures
• Separating agent: Liquid entrainer (MSA) and heat transfer (ESA)
• Example: Separation of acetic acid from water using n-butyl acetate as entrainer to form azeotrope with water

Liquid-Liquid Extraction

• Widely used when distillation is impractical, especially when the mixture to be separated is temperature-sensitive
• The solvent selectively dissolves only one or a fraction of the components in the feed mixture
• Separating agent: Liquid solvent (MSA)
• Example: Recovery of Aromatics

Liquid-Liquid Extraction (Two Solvents)

• Each solvent has its own specific selectivity for dissolving the components of the feed mixture
• Separating agent: Two liquid solvents (MSA1 and MSA2)
• Example: Use of propane and cresylic acid as solvents to separate paraffins from aromatics and naphthenes

Leaching

• Referred to as solid-liquid extraction
• Widely used in the metallurgical, natural product, and food industries
• To promote diffusion of the solute out of the solid and into the liquid solvent
• Separating agent: Liquid solvent
• Example: Extraction of sucrose from sugar beets with hot water

Separation by Barrier

• The use of microporous and nonporous membranes as semipermeable barriers for difficult and highly selective operations
• For microporous membranes, separation is effected by differing rates of diffusion through the pores
• For nonporous membranes, separation occurs because of differences in both solubility in the membrane and the rate of diffusion through the membrane

Osmosis

• Involves transfer, by a concentration gradient, of a solvent through a membrane into a mixture of solute and solvent
• The membrane is almost nonpermeable to the solute
• Separating agent: Nonporous membrane

Reverse Osmosis

• Transport of solvent in the opposite direction and is effected by imposing a pressure, higher than osmotic pressure on the feed side
• Separating agent: Nonporous membrane with pressure gradient
• Example: Desalination of sea water

Dialysis

• Transport, by concentration gradient, of small solute molecules, sometimes called crystalloids, through a porous membrane
• Separating agent: Porous membrane with pressure gradient
• Example: Recovery of caustic from hemicellulose

Microfiltration

• Retention of molecules typically in the size range from 0.02 to 10 μm
• Separating agent: Microporous membrane with pressure gradient
• Example: Removal of bacteria from drinking water

Ultrafiltration

• Retention of molecules typically in the size range from 1 to 20 nm
• Separating agent: Microporous membrane with pressure gradient
• Example: Separation of whey from cheese

Pervaporation

• The species being absorbed by and transported through the nonporous membrane are evaporated
• Uses lower pressures than RO
• Separating agent: Nonporous membrane with pressure gradient
• Example: Separation of azeotropic mixtures

Gas Permeation

• Separation of gas mixtures through membranes, using pressure as the driving force
• Separating agent: Nonporous membrane with pressure gradient
• Example: Hydrogen enrichment

Separation by Solid Agent

• Usually in the form of granular material or packing
• Acts as an inert support for a thin layer of absorbent or enters directly into the separation operation by selective adsorption of, or chemical reaction with, certain species in the mixture
• The active separating agent eventually becomes saturated with solute and must be regenerated or replaced periodically