Page 296 - Distillation theory

P1: JPJ/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c08 CB644-Petlyuk-v1 June 11, 2004 20:20

270 Synthesis of Separation Flowsheets

the section 1,2,3 → 2,3 is intermediate. If for each bottom intermediate section in
the sequence of simple columns the reboiler is replaced by vapor flow from the bot-
tomsectionoftheothercolumn,thenwegetacolumnwithsiderectifiers.Similarly,
if in each top intermediate section the condenser is replaced by liquid flow from
the top section of other column, then we get the column with side strippings.
If a sequence of simple columns has minimum number of columns (n − 1) and
a summary number of reboilers and condensers 2(n − 1), then the column with
side strippings or rectifiers (partially coupled sequence) has a summary number
of reboilers and condensers n.
Wenotethatsectionscanbegroupedintocolumnsinvariousways.Forexample,
at separation of ternary mixture 1,2,3 in a partially coupled sequence, the sections
can be grouped into a column with side rectifier (i.e., sections 1,2,3 → 1, 1,2,3 →
2,3 and 2,3 → 3 can be grouped into the main column and section 2,3 → 2 can
be placed in the side column). Another variant of grouping of section is sections
1,2,3 → 1 and 1,2,3 → 2,3 are in one column and sections 2,3 → 2 and 2,3 → 3 are
in the other one. From a thermodynamic point of view, both of these variants of
grouping of sections are equivalent.
We can turn from partially coupled sequences to completely coupled ones (to
Petlyuk columns). For this purpose, the reboilers and condensers connected with
the sections, products of which are components with intermediate volatilities,
should be excluded. It is possible if each excluded condenser or reboiler is replaced
byflowofliquidorvaporfromanothersection.Toensurethisflow,itisnecessaryto
supplement the network with an additional node (i.e., with one top and one bottom
section). We note that inclusion of additional nodes is equivalent to inclusion of
columns with distributed components into the sequence. For example, in the side
rectifier 2,3 → 2 condenser can be replaced if network 1,2,3 → 1, 1,2,3 → 2,3,
2,3 → 2, 2,3 → 3 is supplemented with the node 1,2, (i.e., we get a new network
1,2,3 → 1,2, 1,2,3 → 2,3, 1,2 → 1, 1,2 → 2, 2,3 → 2, 2,3 → 3). This network has
only one reboiler connected with the section where the heaviest component 3 is
obtained and one condenser connected with section where the lightest component
1 is obtained. We have a classical Petlyuk column for ternary mixture separation.
Four thermodynamically equivalent groupings of sections into two columns are
possible for this column:
1. 1,2,3 → 1,2 and 1,2,3 → 2,3 (1st column), 1,2 → 1; 1,2 → 2;
2,3 → 2 and 2,3 → 3 (2nd column)
2. 1,2,3 → 1,2; 1,2,3 → 2,3 and 2,3 → 3 (1st column), 1,2 → 1;
1,2 → 2 and 2,3 → 2 (2nd column)
3. 1,2,3 → 1,2; 1,2,3 → 2,3 and 1,2 → 1 (1st column), 1,2 → 2;
2,3 → 2 and 2,3 → 3 (2nd column)
4. 1,2,3 → 1,2; 1,2,3 → 2,3, 1,2 → 1 and 2,3 → 3 (1st column),
1,2 → 2 and 2,3 → 2 (2nd column)
We note that for groupings 2 and 3 movement of vapor between columns is
unidirectional, which makes easier their practical realization (grouping 3 is shown
in Fig. 6.12e).