Page 40 - Separation process principles 2

8 Chapter 1 Separation Processes

species in the different phases present. Some properties of separation operations based on interphase mass transfer
importance are: between two phases, one of which is created by an ESA or i
added as an MSA. Graphic symbols that are suitable for 1
1. Molecular properties
process-flow diagrams are included in the table. Vapor and
Molecular weight Polarizability liquid and/or solid phases are designated by V, L, and S,
van der Waals volume Dielectric constant respectively. Design procedures have become fairly routine
van der Waals area Electric charge
for the operations prefixed by an asterisk (*) in the first col-
Molecular shape (acentric factor) Radius of gyration
umn of Table 1.1. Such procedures have been incorporated
Dipole moment
as mathematical models into widely used commercial com-
2. Thermodynamic and transport properties puter-aided, chemical-process simulation and design
Vapor pressure Adsorptivity (CAPD) programs for continuous, steady-state operations
Solubility Diffusivity and are treated in considerable detail in subsequent chapters
of this book. Batchwise modes of these operations are also
Values of these properties for many substances are available treated in this book when appropriate.
in handbooks, specialized reference books, and journals.
When the feed mixture includes species that differ widely
Many of these properties can also be estimated using in their tendency to vaporize and condense, partial conden-
computer-aided, process-simulation programs. When they sation or partial vaporization, Separation Operation (I) in
are not available, these properties must be estimated or deter-
Table 1.1 may be adequate to achieve the desired separation
mined experimentally if a successful application of the
or recovery of a particular component. A vapor feed is par-
appropriate separation operation(s) is to be achieved. tially condensed by removing heat, and a liquid feed is par-
tially vaporized by adding heat. Alternatively, partial vapor-
ization can be caused by $ash vaporization, Operation (2),
1.3 SEPARATION BY PHASE ADDITION j
by reducing the pressure of the feed with a valve. In both of
OR CREATION these operations, after partitioning of species by interphase 1
If the feed mixture is a homogeneous, single-phase solution mass transfer has occurred. the resulting vapor phase is en- I
(gas, liquid, or solid), a second immiscible phase must often riched with respect to the species that are most volatile (most I
be developed or added before separation of chemical species easily vaporized), while the liquid phase is enriched with re-
can be achieved. This second phase is created by an energy- spect to the least volatile species. After this single contact,
separating agent (ESA) and/or added as a mass-separating the two phases, which, except near the critical region, are of
agent (MSA). Application of an ESA involves heat transfer considerably different density, are separated by gravity.
and/or transfer of shaft work to or from the mixture to be Often, the degree of species separation achieved by a sin-
separated. Alternatively, vapor may be created from a liquid gle, partial vaporization or partial condensation step is inad-
phase by reducing the pressure. An MSA may be partially equate because the volatility differences among species in
immiscible with one or more of the species in the mixture. In the feed are not sufficiently large. In that case, it may still
this case, the MSA frequently remains the constituent of be possible to achieve a desired separation of the species in
highest concentration in the added phase. Alternatively, the the feed mixture, without introducing an MSA, by employ-
MSA may be completely miscible with a liquid mixture to be ing distillation, Operation (3) in Table 1.1, the most widely
separated, but may selectively alter the partitioning of utilized industrial separation method. Distillation involves
species between liquid and vapor phases. This facilitates a multiple contacts between countercurrently flowing liquid
more complete separation when used in conjunction with an and vapor phases. Each contact consists of mixing the two
ESA, as in extractive distillation. phases to promote rapid partitioning of species by mass trans-
Although separations that use an ESA are generally pre- fer, followed by phase separation. The contacts are often
ferred, an MSA can make possible a separation that is not made on horizontal trays (referred to as stages) arranged in a "
feasible with an ESA. Disadvantages of the use of an MSA vertical column as shown in the symbol for distillation in ;
are: (1) need for an additional separator to recover the MSA Table 1.1. Vapor, while flowing up the column, is increas- '
for recycle, (2) need for MSA makeup, (3) possible contam- ingly enriched with respect to the more volatile species. I
ination of the product with the MSA, and (4) more difficult Correspondingly, liquid flowing down the column is increas-
design procedures. ingly enriched with respect to the less-volatile species. Feed
When two immiscible fluid phases are contacted, inti- to the distillation column enters on a tray somewhere be-
mate mixing of the two phases is important in enhancing tween the top and bottom trays, and often near the middle of '
mass-transfer rates so that the thermodynamic-maximum the column. The portion of the column above the feed entry
degree-of-partitioning of species can be approached more is called the enriching or rectiJication section, and that
rapidly. After phase contact, the separation operation is com- below is the stripping section. Feed vapor starts up the col-
pleted by employing gravity and/or an enhanced technique, umn; feed liquid starts down. Liquid is required for making
such as centrifugal force, to disengage the two phases. contacts with vapor above the feed tray, and vapor is re-
Table 1. I is a compilation of the more common industrial- quired for making contacts with liquid below the feed tray.