Page 198 - Distillation theory

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172 Distillation Trajectories in Infinite Complex Columns and Complexes

location of stationary points of these section trajectory bundles does not differ
from their location for two-section columns.
To describe geometrically the distillation process in complex columns and com-
plexes, we use, as for simple columns, the conception of sharp separation, and then
we turn from a sharp separation to a quasisharp one. We need to examine the con-
ditions of joining intermediate sections to the top and bottom ones.
In contrast to simple columns, complex columns and complexes have greater
degrees of freedom of designing, which complicates their calculation and de-
signing.
For the main types of complex columns and complexes, we discuss their sphere
ofapplication,historyofinvestigation,geometricdescriptionoftrajectorybundles,
and methods of calculation.

6.2. Columns with Intermediate Inputs and Outputs of Heat:
“Pinch Method”
Columns under consideration are columns of nonadiabatic distillation (that can
also be used in simple two-section columns, in complex columns, and in distillation
complexes). The application of simple nonadiabatic columns for separation of
azeotropic mixtures was examined in Chapter 5, Section 5.7, when separation in
adiabatic columns is unfeasible.
Here we examine another application of nonadiabatic columns – to decrease
energy consumption in separation. Nonadiabatic columns are widely used for this
purpose in petroleum refining (heat output by “pumparounds”).
In the mode of minimum reflux adiabatic sections trajectories intersect re-
2
versible distillation trajectories in points S . Therefore, the separation process
2
between product point and point S can be carried out in principle, maintaining
phase equilibrium between meeting flows of vapor and liquid in the cross-section
at the height of the column by means of differential input or output of heat. We
call such a separation process, with the same product compositions as at adiabatic
distillation, a partially reversible one. A completely reversible process is feasible
only for the preferable split that is rarely used in practice. Nonadiabatic distil-
lation used in industry is a process intermediate between adiabatic and partially
reversible distillation. Summary input and output of heat at nonadiabatic and adi-
abatic distillation are the same, and the energetic gain at nonadiabatic distillation
is obtained at the transfer of a part of input or output heat to more moderate
temperature level, which uses cheaper heat carriers and/or coolants.
We examine the column with one intermediate input of heat in the bottom
section and one intermediate output of heat in the top section. Figure 6.2a shows
the change of internal liquid flows along the height of such a nonadiabatic column,
depending on the value inverse to absolute temperature (1/T). Figure 6.2b shows
the distillation trajectory of nonadiabatic column:
(1) int(2) (2) (3) (3) int(3) (2) (2)
x → x ⇓⇒ x ← x
D → N r → S r f −1 f ← N s ← S s B .
Reg D Q int Reg t r Reg sh Reg sh Q int Reg t s Reg B
con
reb