Page 57 - Separation process principles 2

22 Chapter 1 Separation Processes

Table 1.10 Ease of Scale-up of the Most Common Separation Operations
- - -- - -
Operation in Decreasing Ease of Scale-up Ease of Staging Need for Parallel Units
Distillation Easy No need
Absorption Easy No need
Extractive and azeotropic distillation Easy No need
Liquid-liquid extraction Easy Sometimes
Membranes Repressurization required Almost always
between stages
Adsorption Easy Only for regeneration cycle
Crystallization Not easy Sometimes
Drying Not convenient Sometimes

Although propylene and propane have close boiling points, they are
Use
asymptote Y
traditionally separated by distillation. Representative conditions are
Distillation. 1 shown in Figure 1.12, where it is seen that a large number of stages is
needed and the reflux and boilup flow rates compared to the feed flow
Gas absorption 1 rate are also large. Accordingly, considerable attention has been
Ext./azeo. dist. given to the possible replacement of distillation with a more eco-
Crystallization nomical and less energy-intensive separation operation. Based on the
Ion exchange .Solvent ext. factors in Table 1.9, the characteristics in Table 1.10, and the list
,- .Adsorption: gas of species properties that might be exploited, given at the end of
4 t .Adsorption: liquid feed 4 Section 1.2, propose some feasible alternatives to distillation to
produce products from the feed in Figure 1.12.
Supercritical .Membranes: gas feed
gas abs.lext.
Membranes: liquid feed SOLUTION
Liquid 'chromatography: liquid feed
First, note that the component feed and product flow rates in Fig-
- induced se~arations
'~ffinity separations ure 1.12 satisfy (I-I), the conservation of mass. Table 1.1 1 com-
First
application I I I I I I I I I pares properties of the two species, taken mainly from Daubert and
Invention Technology Danner [ll], where it is seen that the only listed property that might
Technological maturity asymptote be exploited is the dipole moment. Because of the asymmetric
Figure 1.11 Technological and use maturities of separation location of the double bond in propylene, its dipole moment is sig-
processes [9]. nificantly greater than that of propane, making propylene a polar
compound, although weakly so (some define a polar compound as
one with a dipole moment greater than 1 debye). Separation opera-
pilot-plant tests. Operations near the middle usually require tions that can exploit this difference are:
laboratory data, while operations near the bottom require pilot- 1. Extractive distillation with a polar solvent such as furfural or an
plant tests on actual feed mixtures. Also included in the table is aliphatic nitrile that will reduce the volatility of propylene (Ref.:
an indication of the ease of providing multiple stages and to U.S. Patent 2,588,056, March 4, 1952).
what extent parallel units may be required to handle high
capacities. Adetailed discussion of the selection of alternative
Table 1.11 Comparison of Properties for Example 1.2
techniques for the separation of components from both homo-
geneous and heterogeneous phases, with many examples, is Property Propylene Propane
given by Woods [12]. Ultimately, the process having the low-
Molecular weight
est operating, maintenance, and capital costs is selected.
van der Waals volume, m3/kmol
van der Waals area, m2/krnol x lo-'
Acentric factor
EXAMPLE 1.2
Dipole moment, debyes
Propylene and propane are among the light hydrocarbons produced Radius of gyration, m x 10''
by thermal and catalytic cracking of heavy petroleum fractions. Normal melting point, K
Propane is valuable as a fuel by itself and in liquefied natural gas
Normal boiling point, K
(LPG), and as a feedstock for producing propylene and ethylene.
Critical temperature, K
Propylene is used to make acrylonitrile monomer for synthetic rub-
Critical pressure, MPa
ber, isopropyl alcohol, cumene, propylene oxide, and polypropylene.