| Trays and Plates |
The terms
"trays" and "plates" are used interchangeably. There are many types of
tray designs, but the most common ones are :
-
Bubble
cap trays
-
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.
|
-
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) |
-
Sieve
trays
-
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.
|
|
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. |
| 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. |
| Tray Designs |
|
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 |
 |
liquid
distribution and |
|
|
vapour
distribution |
|
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) |
| 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) |
| 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: |
|
|
packings
provide extra inter-facial area for liquid-vapour contact |
|
|
efficiency
of separation is increased for the same column height |
|
|
packed
columns are shorter than trayed columns |
|
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. |
