| Trays and Plates |
The
terms "trays" and "plates" are used interchangeably.
There are many types of tray designs, but the most common ones
are :
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Bubble cap trays
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A
bubble cap tray has riser or chimney fitted over each hole,
and a cap that covers the riser.
 The cap
is mounted so that there is a space between riser and cap
to allow the passage of vapour. Vapour rises through the chimney
and is directed downward by the cap, finally discharging through
slots in the cap, and finally bubbling through the liquid
on the tray.
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-
Valve
trays
-
In valve trays, perforations are covered by
liftable caps. Vapour flows lifts the caps, thus self creating
a flow area for the passage of vapour. The lifting cap directs
the vapour to flow horizontally into the liquid, thus providing
better mixing than is possible in sieve trays.
-
 
Valve trays (photos courtesy of Paul Phillips)
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-
Sieve trays
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Sieve
trays are simply metal plates with holes in them. Vapour passes
straight upward through the liquid on the plate. The arrangement,
number and size of the holes are design parameters.
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Because
of their efficiency, wide operating range, ease of maintenance
and cost factors, sieve and valve trays have replaced the once
highly thought of bubble cap trays in many applications.
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| Liquid and Vapour Flows in a Tray
Column |
|
The
next few figures show the direction of vapour and liquid flow
across a tray, and across a column.
 
Each
tray has 2 conduits, one on each side, called ‘downcomers
’. Liquid falls through the downcomers
by gravity from one tray to the one below it
.
The
flow across each plate is shown in the above diagram on the
right.
A
weir on the tray ensures that there
is always some liquid (holdup)
on the tray and is designed such that the the holdup is at a
suitable height, e.g. such that the bubble caps are covered
by liquid.
Being
lighter, vapour flows up the column and is forced to pass through
the liquid, via the openings on each tray. The area allowed
for the passage of vapour on each tray is called the
active
tray area.
The picture on the left
is a photograph of a section of a pilot scale column equiped
with bubble capped trays. The tops of the 4 bubble caps on the
tray can just be seen. The down- comer in this case is a pipe,
and is shown on the right. The frothing of the liquid on the
active tray area is due to both passage of vapour from the tray
below as well as boiling.
As
the hotter vapour passes through the liquid on the tray above,
it transfers heat to the liquid. In doing so, some of the vapour
condenses adding to the liquid on the tray. The condensate,
however, is richer in the less volatile components than is in
the vapour. Additionally, because of the heat input from the
vapour, the liquid on the tray boils, generating more vapour.
This vapour, which moves up to the next tray in the column,
is richer in the more volatile components. This continuous contacting
between vapour and liquid occurs on each tray in the column
and brings about the separation between low boiling point components
and those with higher boiling points.
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| Tray Designs |
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A
tray essentially acts as a mini-column, each accomplishing a fraction
of the separation task. From this we can deduce that the more
trays there are, the better the degree of separation and that
overall separation efficiency will depend significantly on the
design of the tray. Trays are designed to maximise vapour-liquid
contact by considering the
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liquid
distribution and
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vapour
distribution
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on
the tray. This is because better vapour-liquid contact means better
separation at each tray, translating to better column performance.
Less trays will be required to achieve the same degree of separation.
Attendant benefits include less energy usage and lower construction
costs.
 
Liquid distributors - Gravity (left), Spray (right)
(photos courtesy of Paul Phillips)
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| Packings |
|
There
is a clear trend to improve separations by supplementing the
use of trays by additions of packings.
Packings
are passive devices that are designed to increase the interfacial
area for vapour-liquid contact. The following pictures show
3
different types of packings.
  
These
strangely shaped pieces are supposed to impart good vapour-liquid
contact when a particular type is placed together in numbers,
without causing excessive pressure-drop across a packed section.
This is important because a high pressure drop would mean that
more energy is required to drive the vapour up the distillation
column.
Structured packing
(photo courtesy of Paul Phillips)
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| Packings versus
Trays |
|
A
tray column that is facing throughput problems may be de-bottlenecked
by replacing a section of trays with packings. This is because:
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packings
provide extra inter-facial area for liquid-vapour contact
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efficiency
of separation is increased for the same column height
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packed
columns are shorter than trayed columns
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Packed
columns are called continuous-contact columns while trayed
columns are
called staged-contact columns because of the manner in
which
vapour and liquid are contacted.
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