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4.2 TRAY TOWERS
A typical tray tower is shown in Fig. 4.1. These are
cylindrical towers with trays
or plates with a downspout to facilitate the flow of liquid
from one tray to the other
by gravity. The gas passes upward through the openings of
one sort or another, in
the trays and then passes through the liquid to form froth
and subsequently
discharges from it and then passes on to the next tray
located above. Each tray of
the tower acts as a stage, since there is an intimate contact
between the gas phase
and liquid phase in each tray. In order to provide a longer
contact time, the liquid
pool on each tray should be deep so that the gas bubbles
will require relatively a
longer time to rise through the column of the liquid. When
the gas velocity is
relatively high, it is dispersed very thoroughly into the
liquid, which in turn is
agitated into froth. This provides large interfacial areas.
However, these lead to certain operational difficulties like
entrainment of
droplets of liquid in the rising gas stream and a high-
pressure drop for the gas in
flowing through the trays. The higher pressure drop also
results in high pumping
cost and hence a higher operating cost. Especially, in the
case of distillation, one
may need to maintain a higher pressure in the reboiler,
which also results in a
higher boiling point that may lead to decomposition of heat
sensitive compounds.
Sometimes a higher pressure drop also leads to a condition
of flooding in
which there will be a gradual build-up of liquid in each tray
and may ultimately
fill the entire space between the trays. The tower is said to
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be flooded and the liquid
may escape from the top position of the column through
the gas exit, which results
in lowering the efficiency.
In the case of gas–liquid systems, which tend to foam
excessively, high gas
velocities may lead to a condition of priming. In such case
the foam is present in
the space between trays and there is a great deal of liquid
getting entrained with
the gas. The liquid carried is recirculated between trays and
the added liquid
handling load gives rise to an increase in pressure drop
leading to flooding.
If the liquid rates are too low, the gas rising through the
openings of the tray
may push the liquid away, a phenomenon called coning
resulting in poor gasliquid contact. When the gas rate is too
low, much of the liquid may rain down
through the opening of tray, called weeping, thus failing to
obtain the benefit of
complete flow over the trays. At very low gas rates, none of
the liquid reaches the
downspouts and this is known as dumping.
4.2.1 General Features
Generally the towers are made of metals depending on the
nature of gas and liquid
being handled. Some of them are made of glass or at times
glass lined or made of
1. Gas out
2. Shell
3. Sieve tray
4. Liquid in
5. Downspout
6. Sidestream
withdrawal
7. Froth
8. Weir
9. Intermediate feed
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10. Gas in
11. Liquid out
Fig. 4.1 Schematic section of a sieve-tray tower.
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plastics. To facilitate the maintenance work, smaller towers
are fitted with hand
holes and larger towers with manways. Trays are also made
of metals or alloys and
are fastened suitably to the shell to prevent their
movement owing to surges of gas.
Tray spacing is chosen on the basis of expediency in
construction,
maintenance cost, flooding and entrainment. It varies from
15 cm. Tower diameter
should be sufficiently large to handle the gas and liquid
rates under satisfactory
operating conditions. It can also be decreased by the use of
increased tray spacing.
Hence, the cost of tower, which depends also on the height,
can be optimized with
suitable tray spacing.
The liquid is drawn to the next lower tray by means of
downcomers or
downspouts. These may be circular pipes or portion of the
tower cross section set
aside for liquid flow by vertical plates. Since the liquid is
agitated into froth on the
tray, sufficient time must be provided in the downspout, so
that the gas gets
detached from the liquid and the liquid also flows down to
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the next lower tray. The
legs of the downcomer will normally dip in the liquid in the
next lower tray, which
prevents short-circuiting of gas.
The depth of liquid on the tray required for gas contacting
is maintained by
overflow weir, which may or may not be a continuation of
the downspout plate.
Though straight weirs are common, V-notch weirs and
circular weirs are also used.
Weir length varies from 60 to 80% of tower diameter.
Having seen some of the constructional features of the
towers let us now
discuss the constructional features of trays.
4.3 TYPE OF TRAYS
4.3.1 Bubble Cap Trays
In these trays, chimneys or risers lead the gas through the
tray and underneath
caps surrounding the risers. The gas passes through a
series of slots cut into the rim
or skirt of each cap. The liquid depth is such that the caps
are fully covered by them.
4.3.2 Sieve Trays
These are trays with perforations and the gas flows through
them. The gas
dispersed by the perforations, expands the liquid into a
turbulent froth and
results in providing enormous interfacial area for mass
transfer. These trays are
subject to flooding because of backup of liquid in the
downspouts or excessive
entrainment. In comparison to bubble caps these are quite
simple and are also cost
effective.
4.3.3 Linde Trays
These are slotted trays which show an alteration in the
perforation pattern
to influence the flow of liquid. The slots distributed all over
the tray, not only
reduce the hydraulic gradient in large trays but also
influence the direction of
liquid flow and eliminate stagnant areas. Thus, the
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efficiency of these trays are very
high.
4.3.4 Valve Trays
These are sieve trays with large variable openings for gas
flow. The perforations
are covered with movable caps, which rise as the gas flow
rate increases. Though
the gas pressure drop is low, it is higher than sieve trays.
Due to small openings
the tendency to weep is also reduced.
4.3.5 Counter-flow Trays
In this liquid and vapour flow counter-currently through the
same openings.
Downspout is absent in these trays. They are more suited
for vacuum distillation
as the pressure drop is low.
"convert to easy to understand and exam preparation"
4.2 Tray Towers - Simplified Explanation for Exam Preparation
Tray towers are cylindrical columns used for gas-liquid contact in processes like distillation.
The tower has trays or plates with openings, and gas flows upward through these openings.
The liquid flows down from one tray to the next through downspouts. Each tray acts like a
stage, allowing gas and liquid to interact and transfer mass. The liquid pool on each tray
should be deep to provide longer contact time for the gas bubbles to rise through it.
Key points:
Gas flow: Gas rises through openings in the trays and contacts the liquid.
Liquid flow: Liquid flows down from tray to tray through downspouts.
Froth formation: Gas bubbles create froth in the liquid, which increases the surface area
for mass transfer.
Operational issues:
Flooding: High gas velocity or too much liquid can cause flooding, where liquid
builds up between trays, reducing efficiency.
Priming: Excessive foam between trays leads to liquid being carried with the gas,
increasing pressure drop and causing flooding.
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Coning: Low liquid flow can cause gas to push liquid away, reducing contact.
Weeping: Low gas flow can cause liquid to fall through tray openings, reducing
efficiency.
Dumping: Very low gas flow causes no liquid to reach the downspouts.
4.2.1 General Features of Tray Towers
Materials: Towers are made of metals, glass, or plastics, depending on the chemicals
involved.
Tray spacing: Tray spacing affects construction cost, flooding, and entrainment. It
typically ranges from 15 cm.
Tower diameter: Should be large enough to handle the gas and liquid flow rates. It can
be reduced by increasing tray spacing.
Downcomers: Liquid flows down through pipes or vertical plates called downcomers.
These prevent gas short-circuiting and help in liquid flow.
Weirs: Maintain the liquid depth on the tray. They can be straight, V-notch, or circular.
4.3 Types of Trays
1. Bubble Cap Trays:
Gas flows through risers and caps with slots.
Liquid covers the caps, ensuring gas passes through them.
These are effective but complex and more expensive.
2. Sieve Trays:
Have perforations that allow gas to flow through.
The gas creates a froth in the liquid, increasing surface area for mass transfer.
Simple and cost-effective, but can flood if liquid backs up.
3. Linde Trays:
Slotted trays with a pattern to control liquid flow.
They reduce stagnant areas and improve efficiency.
4. Valve Trays:
Sieve trays with adjustable caps that rise with increased gas flow.
These have a low pressure drop but are more expensive than sieve trays.
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5. Counter-flow Trays:
Gas and liquid flow in opposite directions through the same openings.
No downcomers are used.
Best for vacuum distillation due to low pressure drop.
Key Takeaways for Exam:
Flooding: Caused by too much liquid or high gas velocity.
Priming: Excessive foam leading to liquid carried by gas.
Coning and Weeping: Issues from low liquid or gas flow.
Tray Types: Different trays (bubble cap, sieve, valve, Linde, counter-flow) have specific
advantages based on cost, efficiency, and application.
By focusing on the operational issues, features of trays, and their types, you’ll be well-
prepared for questions on tray towers.
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