Plate Column Design
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PLATE COLUMN DESIGN
Distillation maybe carried out in plate columns in which each
plate constitutes a single stage, or in packed columns where
mass transfer is between a vapor and liquid in continuous
countercurrent flow.
Actual design of the column (height, diameter, plate details
etc.) all need to be determined in order to specify the column
properly.
Columns typically vary in diameter from about 65 centimeters
to 16 meters with up to 100 plates. Plate spacings vary from
around 15 cm to 1 m.
The common tray (or plate) designs are bubble caps (now less
common), sieve or perforated plates and valve trays. 134
Packed column is used for distillation when separation is
relatively easy and the required column diameter is not very
large.
- less expensive than tray columns and have lower pressure
drop.
- disadvantage: difficult to get good liquid distribution
The number of theoretical stages required to effect a required
separation is actually rely on the following factors:
1) The type of plate or tray
2) The vapor velocity, which is the major factor in
determining the diameter of the column.
3) The plate spacing, which is the major factor fixing the
height of the column when the number of stages is known.
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Types of Tray/Plate
The main function of the tray/plate is to achieve intimate
mixing between the liquid and vapor phases.
It is achieved by creating a hold up (delay or obstruction) of
liquid on the plate, while at the same time allowing the vapor
to move up through openings on the plate into the liquid.
Better vapor-liquid contact means better separation at each
tray, hence better column performance.
Less trays will be required to achieve the same degree of
separation, less energy usage and lower construction costs.
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There are three main types of tray/plate achieve these
objectives in slightly different ways:
1) Sieve or perforated tray
2) Valve tray
3) Bubble cap tray
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1) Sieve or perforated tray
The same tray is used in distillation
and absorption.
The tray simply has holes in it,
which are small enough to prevent
the liquid from weeping through
them, but large enough to achieve
the necessary contact.
Vapor bubbles up through simple
holes in the tray through the
flowing liquid.
The overflow liquid flows into the
downspout to the next tray.
Holes size range from 3 to 12 mm in
diameter, and 5 mm is the common
size. Figure 36 sieve or perforated 138
tray
2) Valve tray
It is the modification of sieve tray,
consists of an opening in the tray
and a lift-valve cover with guides
to keep the cover properly
positioned over the opening.
This provides a variable open area
which is varied by the vapor flow
inhibiting leakage of fluid down
the opening at lower vapor rates.
Therefore, this type of tray can
operate over a greater range of
rates than the sieve tray, with a
cost of only about 20% more than
the sieve tray.
Figure 37. Valve tray
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3) Bubble-cap tray
In the bubble tray, the cap fits
over a larger hole, but in such a
way as to make it difficult for the
liquid to its way down against the
up-flowing vapor
the vapor or gas rises through the
opening in the tray into the
bubble caps.
Then, the gas flows through slots
in the periphery of each cap and
bubbles upward through the
flowing liquid.
Figure 38. Bubble-cap tray
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Figure 39. Tray Columns
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Figure 40. Tray Deck
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Figure 41. Valve Tray Deck
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Figure 42. Major Tray Damage
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Figure 43. Fouled Bubble Cap Tray
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Overall tray efficiency
The overall tray efficiency, o is simple to use but is the least
fundamental.
It is defined as the ratio of the number of ideal trays (number
of theoretical trays) needed in an entire column to the number
of actual trays, i.e.:
Ideal number of trays …..(51)
ηo 1
actual number of trays
For example, if eight theoretical steps are needed and the
overall efficiency is 60%, the number of theoretical trays is
eight minus a reboiler, or seven trays. Therefore, the actual
number of trays is:
7
Actual number of trays 11.7 trays
0.60
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Murphree tray efficiency
The Murphree tray efficiency, M is defined as follows:
yn yn 1
M * …..(52)
yn yn 1
where:
yn = average actual concentration of vapor leaving plate n
yn+1 = average actual concentration of vapor entering plate n
y*n = concentration of vapor in equilibrium with liquid
leaving downpipe from plate n
(refer to Figure 44)
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In other words, the Murphree tray efficiency refers to the
change in vapor composition from one tray to the next,
compared to the change that would occur under ideal
(equilibrium) conditions.
It is therefore a measure of how closely the real situation
approaches equilibrium.
It assumes that the liquid phase is completely mixed (i.e.:
uniform composition across the tray and in the downcomer).
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Figure 44: Vapor and liquid compositions on a sieve tray and tray efficiency.
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Local (point) efficiency
The local or point efficiency, MP is defined as follows:
yn' yn' 1
MP * ' …..(53)
y 'n yn 1
where:
y’n = concentration of vapor leaving specific point on
tray n
y’n+1 = concentration of vapor entering the tray n at the
same point
y’*n = concentration of vapor in equilibrium with liquid
at same point
(refer to Figure 44) 150
Since y’n cannot be greater than y’*n , therefore the local
efficiency cannot be greater than 1.00 or 100%.
For small columns where the liquid on each tray is well-mixed,
(i.e.: the composition leaving the tray is the same as every
where else on the tray), the local efficiency will be equal to
the Murphree efficiency.
For large columns though, mixing of the liquid occurs on the
tray is incomplete and there is a gradient from liquid inlet to
liquid outlet. Hence, the Murphree efficiency is greater than
the local efficiency.
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