Page 168 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 157

Tray Balance

A tray is in balance when it operates with acceptable effi-
ciency under conditions at or very near those of design.

Tray Flexibility

A tray is flexible when it operates with acceptable effi-
ciency under conditions which deviate significantly from
those established for design. The usual changes affecting
flexibility are vapor and/or liquid loading. A tray may
operate down to 50% and up to 120% of vapor load, and
down to 15% and up to 130% of liquid load and still be
efficient. Beyond these points its efficiency may fall off,
and the flexible limits of the tray would be established.

Tray Stability

VAPOR LOAD PER CAP, CFS A tray is stable when it can operate with acceptable effi-
ciencies under conditions that fluctuate, pulse, or surge,
Figure 8-102. Effect of vapor load on slot opening, slot load, and developing unsteady conditions. This type of operation is
pressure drop. Used by permission, Bolles, W. L., Pet. Processing, difficult to anticipate in design, and most trays will not
Feb. thru May (1 956).
operate long without showing loss in efficiency.

Flooding

CAP PRESSURE DROP, SLOT LOAD, A bubble tray tower floods when the froth and foam in
G INCHES OF LlOUlO % OF CAPACITY- the downcomer back up to the tray above and begin accu-
mulating on this tray. The downcomer then contains a
mixture of lower density than the clear liquid, its capacity
becomes limited, disengagement is reduced, and the level
rises in the downcomer. This level findig extends onto the
tray above, and will progress to the point of filling the col-
umn, if not detected and if the liquid and vapor loads are
not reduced. Flooding is generally associated with high
liquid load over a rather wide range of vapor rates. The
foaming tendencies of the liquid influence this action on
the tray. The design condition for height of clear liquid in
the downcomer for flooding is usually set at 0.60 to 0.80 of
(S, + hw). See Figure 8-63.

A bubble tray pulses when the vapor rate is low and
unsteady, when the slot opening is low (usually less than M
in.), and when the liquid dynamic seal is low. With irregu-
VAPOR LOAD PER CAP, CFS
lar vapor flow entering the caps, the liquid pulses or
Figure 8-103. Effect of liquid gradient on vapor distribution with 0.5 surges, even to the point of dumping or back-flowing liq-
vapor distribution ratio. Used by permission, Bolles, W. L., Pet. Pro- uid down the risers. The best cure is a steady vapor rate
cessing, Feb. thru May (1 956). and good slot opening to allow for reasonable upsets.