Page 279 - Distillation theory

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7.5 Design Calculation of “Petlyuk Columns” 253

in a number of works to decompose the column with side strippings into a se-
quence of columns with indirect split (Hengstebeck, 1961; Glinos & Malone, 1985;
Carlberg & Westerberg, 1989; Liebmann, Dhole, & Jobson, 1998). In the latter
work, it is proposed, besides that, to design distillation columns and the system of
heat exchangers simultaneously using pinch analysis (Dhole & Linnhoff, 1993).
Thecalculationofeachtwo-sectioncolumniscarriedoutwiththehelpofsimplified
methods that examine the following modifications: (1) absence of thermal cou-
pling between two-section columns (absence of liquid flows between two-section
columns), (2) its availability, (3) steam stripping, and (4) stripping with the help
of a steam-heated reboiler. This method considerably decreases expenditures of
energy for separation (approximately by 20%).
We propose a method of design calculation based on decomposition of the
column with side strippings into a system of two-section columns, on rigorous
design calculation of each two-section column and on simultaneous design of
distillation columns and of system of heat exchange.
The main task of designing columns with side strippings is the determina-
tion of necessary number of plates in each section, of optimal thermal duties on
pumparounds and of the rates of steam for stripping or thermal duties on reboilers.
The main element of design calculation is the determination of necessary tray
numbers in each two-section column at fixed-mode parameters. In each two-
section column entering into the column with side strippings, intermediate sep-
aration without distributed product components decreases the number of steps
of design calculation compared with calculation of Petlyuk columns. Two-section
columns entering into the column with side strippings are calculated consecutively
from the column to where the initial mixture arrives. Calculation of two-section
columns is carried out at set compositions of pseudoproducts that are determined
by the set purity of the corresponding products. The composition at the end of the
two-section column where it joins the following two-section column corresponds
to the maximum concentration of the heavy key component of the following two-
section column. If, for example, split 1, 2 ... k : k + 1, ... l is in the following
two-section column, then the mentioned key component is component k + 1.
Ratios of the flow rates of liquid and vapor in two-section columns are deter-
mined by reasoning from the calculated mode of minimum reflux and specified set
of values of reflux excess coefficients, taking into consideration the output of heat
by pumparounds decreasing the vapor flow rate passing from one two-section col-
umn into another. The amount of heat withdrawn by pumparounds is determined
through application of the pinch method (Liebmann et al., 1998).
The same method is used to choose the method of stripping: with the help of
steam, reboiler, or a combination of steam and reboiler. Application of reboilers
compared with steam stripping, on the one hand, requires additional investment
cost and heat expenditures, but on the other hand, it leads to a decrease of vapor
flow in the column (i.e., to a decrease of necessary diameter and to an increase of
the temperature of stripping product used for heating the crude oil). Besides that,
theapplicationofreboilersisinadmissibleforheavyproductsbecausetheresulting
increase of the product’s temperature may lead to its chemical decomposition. In
the case when application of reboilers is admissible, the choice between reboilers,