Page 32 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 21

1.1 3 1.0
X X
Total Reflux Thermal Condition of
Feed to Column

1.0

2.

0

Minimum Reflux Partial Condensation
Abnormal Equilibrium of Overhead Vapors

Figure 8-15. Operating characteristics of distillation columns.

is approximately the amount of heat required to vaporize As an alternate to locating the “q” line, any value of xi
one mol of feed at the feed tray conditions, divided by the may be substituted in the “q” line equation below, and a
latent heat of vaporization of the feed. corresponding value of yi determined, which when plot-
ted will allow the “q” line to be drawn in. This is the line
Ls = Lr + qF (8-26) for SV - I, V - I, PV - I, BP - I and CL - I of Figure 8-15.

yi=-- xi+- XF (8 - 28)
1-q 1-q
The slope of a line from the intersection point of the
feed composition, XF, with the 45” line on Figure 8-2 is Total Reflux, Minimum Plates
given by q/ (q - 1) = - q/ (1 - 9). Physically this gives a
good approximation of the mols of saturated liquid that Total reflux exists in a distillation column, whether a
will form on the feed plate by the introduction of the feed, binary or multicomponent system, when all the overhead
keeping in mind that under some thermal conditions the vapor from the top tray or stage is condensed and
feed may vaporize liquid on the feed plate rather than returned to the top tray. Usually a column is brought to
condense any. equilibrium at total reflux for test or for a temporary plant
Liebert [218] studied feed preheat conditions and the condition which requires discontinuing feed. Rather than
effects on the energy requirements of a column. This topic shut down, drain and then re-establish operating condi-
is essential to the efficient design of a distillation system. tions later, it is usually more convenient and requires less