Page 39 - Separation process principles 2

1.2 Mechanism of Separation 7

Product 1 reduction. A second technique, Figure 1.7b, is to introduce
7
the second phase into the system in the form of a solvent that
Feed mixture
selectively dissolves some of the species in the feed. Less
common, but of growing importance, is the use of a barrier,
Figure 1.7c, which restricts and/or enhances the movement
of certain chemical species with respect to other species.
Also of growing importance are techniques that involve the
Figure 1.6 General separation process.
addition of solid particles, Figure 1.7d, which act directly or
as inert carriers for other substances so as to cause separa-
constituent chemical species, is not a spontaneous process; it tion. Finally, external fields, Figure 1.7e, of various types are
requires an expenditure of energy. A mixture to be separated sometimes applied for specialized separations.
usually originates as a single, homogeneous phase (solid, For all the techniques of Figure 1.7, separations are
liquid, or gas). If it exists as two or more immiscible phases, achieved by enhancing the rate of mass transfer by diffusion
it is often best to first use a mechanical means based on of certain species relative to mass transfer of all species by
gravity, centrifugal force, pressure reduction, or an electric bulk movement within a particular phase. The driving force
and/or magnetic field to separate the phases. Then, appropri- and direction of mass transfer by diffusion is governed by
ate separation techniques are applied to each phase. thermodynamics, with the usual limitations of equilibrium.
A schematic diagram of a general separation process is Thus, both transport and thermodynamic considerations are
shown in Figure 1.6. The feed mixture can be vapor, liquid, crucial in separation operations. The rate of separation is
or solid, while the two or more products may differ in com- governed by mass transfel; while the extent of separation is
position from each other and the feed, and may differ in limited by thermodynamic equilibrium. These two topics are
phase state from each other and/or from the feed. The sepa- treated in Chapters 2, 3, and 4. Fluid mechanics and heat
ration is accomplished by forcing the different chemical transfer also play important roles, and applicable principles
species in the feed into different spatial locations by any of are included in appropriate chapters, particularly with respect
five general separation techniques, or combinations thereof, to phase separation, phase change, pressure drop, tempera-
as shown in Figure 1.7. The most common industrial tech- ture change, and entrainment.
nique, Figure 1.7a, involves the creation of a second phase The extent of separation achieved between or among the
(vapor, liquid, or solid) that is immiscible with the feed product phases for each of the chemical species present
phase. The creation is accomplished by energy (heat andlor in the feed depends on the exploitation of differences in
shaft-work) transfer to or from the process or by pressure molecular, thermodynamic, and transport properties of the

- Phase 2

Feed
Creation

Phase 2

Phase, 1 + MrrA , 1 +
Phase 1
7

Feed Barrier 7
Phase 1
1 , 1 bxl Force fie,; 1 Figure 1.7 General separation techniques:

or gradient
S$itt
phase 1
(a) separation by phase creation; (b) separa-
Phase 2 Phase 2 tion by phase addition; (c) separation by
barrier; (d) separation by solid agent;
(dl (el (e) separation by force field or gradient.