Page 278 - Distillation theory

P1: JPJ/FFX P2: JMT/FFX QC: FCH/FFX T1: FCH
0521820928c07 CB644-Petlyuk-v1 June 11, 2004 20:18

252 Trajectories of the Finite Columns and Their Design Calculation

split 1, 2 ... k, k + 1, ... l : k + 1, ... l, l + 1, ... n component (k + 1) is a light key
component of the side product and l is a heavy key one).

Purity of intermediate pseudoproducts D and B are important variables at the

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calculationoftraynumbers.Inseparatingthree-componentmixture,itisnecessary
to determine the concentration of heavy component in top pseudoproduct D
1

and of light component in bottom pseudoproduct B . These concentrations are
1
determinated by the means of series of calculations of three columns of distillation
complex from the condition of equality of compositions at the tray of output
of the side product obtained at calculation of the second and third columns for
given purity of the side product. Concentrations of key components of the side
product in pseudoproducts D and B at separation of multicomponent mixture

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are determined in the same way.
The general algorithm of design calculation of Petlyuk columns includes the
following stages:

1. Determination of optimal distribution coefficient D /B , pseudoproduct

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compositions of the first column, and calculation of the mode of minimum
reflux in first column (a) from the preferable split in the case of separation
of a three-component mixture (see Section 6.8) or (b) by means of the
solution of the Underwood equation system and up-to-date calculation in
the case of sharp separation of a multicomponent mixture with several
distributed components.
2. Calculation of tray numbers of three two-section columns beginning with
the first column by algorithms of design calculation of two-section columns

at fixed parameters L/V and fixed purity of pseudoproducts D and B of
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the first column (see Section 7.3).

3. Determination of purity of pseudoproducts D and B reasoning from the

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equality of compositions in the cross-section of output of the side product
obtained at calculation of the second and third columns for given purity of
the side product by means of the series of calculations at different purities
of pseudoproducts.
r1
4. Calculation of tray numbers at different values of parameters (L/V) ,
r2
r3
(L/V) ,(L/V) and choice of design values of these parameters.
7.5.2. Design Calculation of Columns with Side Sections
Columns of this type with side strippings were used for petroleum refining for
many decades. Design of petroleum refining units is based on usage of the existing
experience and on simulation trials. The procedure of designing is nonsystematic.
As a rule, distillation columns are designed at the beginning and then the system
of heat exchangers used for heating of petroleum and for recuperation of the heat
withdrawn from the distillation columns is designed. The procedure of designing
is described in old works (Nelson, 1936; Packie, 1941; Watkins, 1979). This pro-
cedure includes correlations based on empirical data about tray numbers, reflux,
sharpness of separation, and amount of water vapor for stripping. It was proposed