Page 303 - Distillation theory

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

8.3 Thermodinamically Improved and Thermally Integrated Separation 277

C 1

T 4
y F4
T 3
Figure 8.4. The demethanizer with mul-
tiple feed streams. T 1 ÷ T 4 , tempera-
x F3
ture since condencers, x F1 ÷ x F3 , liquid
T 2
compositionssincecondencers; y F4 ,vapor
x F2 composition since condencer.
C ,C + T 1
1 2
x F1

C 2 +

Such separation takes place in flowsheets with prefracionator and in
Petlyuk flowsheets (Fig. 6.12c ÷ f,6.13 α ÷ c).
3. Thermodynamic losses caused by input into the column of unequilibrium
flows of reflux from condenser and of vapor from reboiler ( 3 ). To exclude
3 , it is necessary to replace condenser and reboiler by the input of liquid
and vapor from the other columns (i.e., to turn from the flowsheet in Fig.
6.12d to the flowsheets in Fig. 6.12c,e,f). At such passage, parts of section
1
1
trajectories x D → S and x B → S , at which nonequilibrium of liquid and
r s
vapor flows being mixed at the trays is especially big, are excluded. It is very
clearly seen in Fig. 8.5 (Petlyuk & Platonov, 1965), which shows working
and equilibrium lines for each of three components at the preferable split
and mode of minimum reflux (α 13 = 5; α 23 = 2; x F1 = 0.1; x F2 = 0.6; x F3 =
0.3; x B1 = 0.0001; x D3 = 0.0004; L min /F = 0,25). As is evident from the figure,
1
1
the nonequilibrium at the end parts of the column x D → S and x B → S ,
s
r
if working with a condenser and a reboiler (the shaded regions correspond
to them), exceeds many times the nonequilibrium in the middle part of the
1
1
column at parts S → x f−1 and S → x f .
s
r
4. Thermodynamic losses caused by hydraulic resistance of the trays ( 4 ).
These losses consist of considerable parts of summary thermodynamic
losses if the pressure drop along the column is commensurable with the
absolute pressure at its top end (vacuum columns, columns for separation
of narrow-boiling mixtures – isomers, isotopes). The increase of pressure
at the bottom end of the column plays a twofold negative role: it decreases
relative volatilities of the components and increases the top-bottom differ-
ence of temperatures in the column. Decrease of relative volatilities of the
components leads to the necessity of increasing the reflux number and ex-
penditure of heat energy (Q in Eq. [8.2]), and the increase of top-bottom
temperature difference means an increase of term (1/T con − 1/T reb )in
this equation. Both facts lead to an increase of expenditures of energy for