Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 16

Table 2.1 Exploitable properties used in separation processes.

Equilibrium properties Rate properties

Vapor pressure Diffusivity

Partitioning between phases Ionic mobility

solubility of a gas in a liquid Molecular size and shape

sorption of a solute in a fluid onto a sorbent
Chemical reaction equilibrium
Electric charge
Phase change
solid/liquid
liquid/gas

Table 2.2 Examples of separating mechanisms and separating agents.

Separation process Separation mechanism Separating agent

Distillation Vapor pressure Heat

Extraction Partitioning between phases Immiscible liquid

Adsorption Partitioning between phases Solid sorbent

Absorption Partitioning between phases Non-volatile liquid

Filtration Molecular size and shape Membrane

Ion exchange Chemical reaction equilibrium Solid ion-exchanger

Gas separation Diffusivity and phase partitioning Membrane

Electrodialysis Electric charge and ionic mobility Charged membrane/electric field

(c) The magnitude of the property difference. Obviously, the larger the difference be-
tween the components to be separated, the easier the separation. How large is good
enough? This answer is based on experience and the value of the components to be
separated. Preliminary calculations for various separation processes (using various
property differences) can be very useful.
(d) Chemical behavior under process conditions. Will the process fluids chemically attack
the separation equipment (corrosion, morphological changes such as swelling, etc.)
and/or react themselves (polymerize, oxidize, etc.)? This is a very important consid-
eration as it affects the lifetime and reliability of the process.
(e) Quantities and phases which need to be processed. This is an economic consideration
as the cost of implementing various property differences (energy input as heat, for
example) is a function of the scale of the process. The phases (gas vs liquid, for
example) also affect the equipment size and material handling considerations.
(f) Separation criteria. What are the concentrations of the various components in the feed
stream? What purity and recovery are needed? How many components in the feed
need to be separated?