Page 302 - Distillation theory

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276 Synthesis of Separation Flowsheets

bottom and side flows from the first column into the second one (Fig. 8.3b), side
flow from the top section of the first column into the second one (Fig. 8.3c), and
side flow from the bottom section of the first column into the second one (Fig.
8.3d).
The simplified method similar to method Fenske–Underwood–Gilliland was
developed for calculation of distillation complexes. The comparison of different
sequences was made at the example of the mixture obtained at the unit of alky-
lation. The most interesting result was obtained from sequence: (1 : 2,3,4,5) →
(2,3 : 4,5) → (4 : 5) → (2 : 3) while uniting the second and third columns into the
complex shown in Fig. 8.3b. It turned out that this sequence is better by 30% in
terms of expenditures than the best simple sequence.

8.3. Thermodynamically Improved and Thermally Integrated
Separation Flowsheets

8.3.1. Thermodynamic Losses and Their Decrease

The comparison of adiabatic and reversible distillation determines several sources
of thermodynamic losses leading to increase of energy expenditures compared
with separation work at the reversible process (Eq. [8.2]).

1. Thermodynamic losses caused by unequilibrium of liquid and vapor flows
mixed at each tray ( 1 ). Two ways to decrease 1 are decrease of excess
reflux factor to σ = 1,05 ÷ 1,1 and usage of intermediate along the height
of the column input or output of heat (see Sections 6.2 and 6.8). The sec-
ond way is efficient if a wide-boiling mixture is subjected to separation.
An example of its application is the usage of pumparounds in petroleum
refining (see Section 6.8). A modified variant of this method is stepped
condensation of vapor feeding and input of the formed liquid fractions
into different cross-sections along the height of the column. The examples
can be low-temperature units of obtaining ethylene and propylene from
pyrolysis gases (Fig. 8.4).
A decrease in 1 is always connected with an increase of necessary tray
numbers and total capital costs. Therefore, it is justified only in the case of
massive energy expenditures.
2. Thermodynamic losses caused by mixing of flows of different composition
in the feed cross-section of the column ( 2 ). These losses always arise
at separation of multicomponent mixture at any split without distributed
components. The losses are absent only at the preferable split when the
compositions of the liquid and vapor parts of feeding coincide (in the mode
of minimum reflux) or are close (at the reflux bigger than minimum) to the
composition of the liquid flow from the top section of the column and
to the composition of vapor flow from the bottom section of the column,
respectively.