Separations as unit operations - Principles of Chemical Separations with Environmental Applications - page 27

Slow kinetics translates to additional time required for the extraction and/or release step.
Sixth, and last, there can be no partitioning of the complexing agent into the feed or
receiving phases. This effect would continuously decrease the amount of complexing
agent available as the separation process is cycled.

2.10 Selection of a separation process

This section is normally included at the end of a textbook on separations; however, it is
included here to give the reader some “food for thought” in deciding how they might use
the material in the subsequent chapters. For additional perspectives on this topic, consult
references [2-11]. The following should be taken as a heuristic or guide.
1 Assess the feasibility. What are the property differences that you plan to exploit for the
separation(s)? Which processes use this property difference as their primary separating
mechanism? What operating conditions are associated with the feed stream (flowrate,
T , P , pH, reactive components, etc.)? Are these conditions “extreme” relative to normal
operating conditions for a given separation process?
2 Determine the target separation criteria. What purity and recovery are needed for the
various components in the feed stream? For a feasible separation process, what is the
separation factor based on the property difference chosen? For an equilibrium-based
process, this would be the separation factor for one stage.
There are various molecular properties which are important in determining the value
of the separation factor for various separation processes.
(a) Molecular weight. Usually, the heavier a compound is, the lower the vapor pressure.
Molecular weight is also related to molecular size, which affects diffusion rates and
access to the interior of porous materials.
(b) Molecular volume . This is a measure of density since there is an inverse relationship
between density and volume. As will be seen in the analysis of various separation
processes, density can be a significant variable.
(c) Molecular shape . The molecular shape can certainly affect the access of certain
molecules to pores and chemical binding sites. The shape also will affect how the
molecules order in a liquid or solid phase.
(d) Intermolecular forces . The strength of these forces can affect the vapor pressure and
solubility in certain solvents. One property is the dipole moment which is a measure
of the permanent charge separation within a molecule (polarity). Another property
is the polarizability which is a measure of a second molecule's ability to induce a
dipole in the molecule of interest. The dielectric constant is a physical property that
can be used as a measure of both the dipole moment and polarizability.
(e) Electrical charge . The ability of a molecule to move in response to an electric field
is a function of the electrical charge.
(f) Chemical complexation. Separations involving selective chemical reactions can
impart higher selectivity (separation factors) than the use of a physical property