Page 304 - Distillation theory

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278 Synthesis of Separation Flowsheets

y i
.
10 t x D
x 1,2
,
a) 0.9 s 2 t b)
x
0.8 c b r 2 , a
0.7 x D 1 , = x 3 , B d x t r
0.6 a b
1,2,3
0.5 x F
x b a c
0.4 F 1 , x = x = x
t d 2 , F 2 , D 2 , B x t
0.3 x s
c r 1 , d
0.2 x 3 , F x t
s 3,
0.1 c
b 2,3
0 x i x B
.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
x = x t
t
s 1, r 3,
Figure 8.5. McCabe-Thiele plots for all components of a ternary
mixture under minimum reflux for the preferable split. Operating
(solid) and equilibrium (dotted) lines for parts of column − a,b,c,d.
t t
(b) Parts of column a,b,c,d and pinches x F, x , x . x F,1, x F,2, x F,3,
r s
x D,1, x D,2, x D,3, x B,1, x B,2, x B,3 , concentrations of components 1, 2,
t
t
3 in feed, overhead product, bottom product, respectively; x , x ,
r,1 r,2
t
t
t
t
x , x , x , x , concentrations of components 1,2,3 in pinches,
r,3 s,1 s,2 s,3
respectively; regions between operating and equilibrium lines for a
and d parts of column are shaded.
separation. To decrease 4 , it is necessary to apply contact devices with
low hydraulic resistance, for example, regular packing.
5. Thermodynamic losses caused by temperature difference between the heat
source and flow from reboiler and between flow from condenser and the
heat sink ( 5 ). These losses play especially significant role in two cases:
in that of separation of narrow-boiling mixtures, and at low-temperature
separation. In the first case, the differences of temperatures between
the heat source and flow from reboiler and between flow from the con-
denser and the heat sink can considerably exceed that between the bottom
and the top of the column. This leads to an increase of term (1/T sink −
1/T sours ), which is to replace the term (1/T con − 1/T reb ) in Eq. (8.2) and to
lower thermodynamic efficiency, for example, of units for isotopes separa-
tion – 0,01% (London, 1961). In the second case, expensive cold sources
(heat sink) are used and, therefore, decrease of losses 5 leads to big
economy.
To decrease 5 , it is necessary to bring together temperatures of the
heat source and the sink with T reb and T con . There are several ways to do
this.
One of the ways consists of maximum use of hot or cold flows going
out from the unit as heat sources and cold sinks along with heat carriers
and refrigerants. The general approach to the solution of this task –“pinch
technology” (Linnhoff & Hindermarsh, 1983) – uses construction of curves
“temperature – enthalpy” for reboilers, condensers, hot and cold flows
going out from the unit, and heat carriers, and refrigerants.