Page 56 - Separation process principles 2

1.9 Selection of Feasible Separation Processes 21

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1,000,000,000
1.9 SELECTION OF FEASIBLE I I I I I I I I
SEPARATION PROCESSES 100,000,000 -
selection of a best separation process must be made from
10,000,000 -
among a number of feasible candidates. When the feed mix-
ture is to be separated into more than two products, a combi- 1,000,000 -
nation of two or more operations may be best. Even when
100,000 -
only two products are to be produced, a hybrid process of e
two or more different types of operations may be most eco- (A l0,OOO - -
g
nomical. Only an introduction to the selection of a separa- 'C
a
tion process is given here. A detailed treatment is given in 1,000 -
Chapter 7 of Seider, Seader, and Lewin [14].
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Important factors in the selection of feasible separation
,perations are listed in Table 1.9. These factors have to do -
with feed and product conditions, property differences that
-
can be exploited, and certain characteristics of the candidate
separation operations. The most important feed conditions 0.10 I I I I I I I I
are composition and flow rate, because the other conditions I 0.1 0.01 0.001 104 10" lo4 lo-' 104 IO-~
(temperature, pressure, and phase condition) can be altered We~ght fractior~ in substrate
by pumps, compressors, and heat exchangers to fit a particu- Figure 1.10 Effect of concentration of product in feed material
lar candidate separation operation. In general, however, the on price [9].
vaporization of a liquid feed that has a high heat of vapor-
ization, the condensation of a vapor feed with a refrigerant,
In general, as demonstrated by Sherwood, Pigford, and
andlor the compression of a vapor feed can add significantly Wilke [7] and updated recently, using additional data for
to the cost. Some separation operations, such as those based
biological materials from Dwyer [8], by Keller [9], the cost
on the use of barriers or solid agents, perform best on feeds
of recovering and purifying a chemical contained in a
that are dilute in the species to be recovered. The most im-
mixture can depend strongly on the concentration of that
portant product conditions are the required purities because,
chemical in the feed mixture. Keller7s correlation is given
again, the other conditions listed can be altered by energy
in Figure 1.10, where it is seen that the more dilute the
transfer after the separation is achieved.
chemical is in the mixture, the higher is its sales price.
When very pure products are required, either large differ-
Table 1.9 Factors That Influence the Selection of Feasible ences in certain properties must exist or significant numbers
Separation Operations of stages must be provided. It is important to consider both
molecular and bulk thermodynamic and transport properties,
A. Feed conditions
some of which are listed near the end of Section 1.2. Data
I. Composition, particularly concentration of species to be
and estimation methods for many bulk properties are given
recovered or separated
2. Flow rate by Poling, Prausnitz, and O'Connell [lo] and for both mole-
cular and bulk properties by Daubert and Danner [l 11.
3. Temperature
Some separation operations are well understood and can be
4. Pressure
readily designed from a mathematical model and/or scaled up
5. Phase state (solid, liquid, and/or gas)
B. Product conditions to a commercial size from laboratory data. The results of a sur-
vey by Keller [9], shown in Figure 1.11, show that the degree to
1. Required purities
which a separation operation is technologically mature corre-
2. Temperatures
3. Pressures lates well with its commercial use. Operations based on a bar-
rier are more expensive to stage than those based on use of a
4. Phase states
solid agent or the creation or addition of a second phase. Some
C. Property differences that may be exploited
separation equipment is limited to amaximum size. For capac-
1. Molecular
ities requiring a larger size, parallel units must be provided.
2. Thermodynamic
The choice of single or parallel units must be given careful con-
3. Transport
sideration. Except for size constraints or fabrication problems,
D. Characteristics of separation operation
the capacity of a single unit can be doubled for an additional in-
1. Ease of scale-up
vestment cost of only about 50%. If two parallel units are in-
2. Ease of staging
stalled, the additionalinvestment is 100%.Table 1. loisalistof
3. Temperature, pressure, and phase-state requirements
4. Physical size limitations the more common separation operations rar~ked according to
ease of scale-up. Those operations ranked near the top are fre-
5. Energy requirements
quently designed without the need for any laboratory data or