Tray Column Internals and Design

Mass Transfer and Separation processes

2

Dr. Ahmad Wafiq

February 15th, 2017

Columns mainly used for Absorption/Distillation

Columns

Tray (Plate) Packed

 Classification of Tray Towers

Tray Columns

Counter-
Cross-Flow
Current Flow

Bubble-Cap Sieve Valve

What is happening inside a Tray Column?

y
y
T y

x
x
x

0% A 100% A
100%B
x,y 0%B
What is happening inside a Tray Column?
What is the function of the main tower internals?
Weir:
To maintain a desired liquid level on the tray. It increases the
residence time of liquid-vapor contact on the tray.

Downcomer:
To guide liquid (froth) flow from an upper tray to a lower tray. It is
also the place where vapor-liquid disengagement takes place.

Calming zone:
To help have partial vapor-liquid disengagement before the
downcomer.

Tray Spacing:
To minimize liquid entrainment in the rising vapor stream.
 Cross-Flow Trays

Sieve Valve Bubble Cap

Sieve Trays
 Bubble Cap Trays

• Oldest tray type, and was almost exclusively used between 1920-1950.
• Its high cost is currently limiting its application (unless high turndown
ratio is required)
Valve Trays
 Valve Trays

• The valves (movable disks) will move up or down in response to

changing vapor flow rates. At normal flow rate, the valve is roughly
in the middle position.
• At low vapor rates , the disk settles over the perforation and
covers it to avoid liquid weeping. The valves should be heavy
enough to prevent excessive opening at low vapor flow rates.
• As the vapor rate is increased, the disk rises vertically, the upward
movement of the disk is restricted by retaining legs.
• Sticking of disk on tray may take place if there is sticky deposition
on tray.
 Comparison between Tray Types

Bubble Cap Sieve Valve

Flexibility
Highest Lowest Higher
(Turn Down Ratio*)
Efficiency Good Good Good

Capacity Lowest Higher Higher

Pressure Drop Highest Lowest Higher

Erosion/Corrosion Lower Lower Highest

Cost Highest Lowest Higher

* Turn down ratio is the ratio of the design vapor flow rate to the minimum vapor flow
rate.
 Types of Gas-Liquid Dispersion
• Froth (mixed) regime is
the preferred one since the
liquid phase is large
enough to prevent spraying
and not too large to cause
shearing for the gas
bubbles.
 How to Calculate Tray’s Pressure Drop

ΔPP Δh
= h p = h d + β (h w + h ow + )
ρLg 2
where:-
h p = pressure dropthrough plate (mm).
2
vgh ρg
h d =dry pressure drop = 2 × (mm).
2g ρ L
Qg
vgh = velocity of gas through holes =  ----(m/s)
area of holes
Qg = gas volumetric flow rate (m3 /s).
β = relative foam density = 0.4 - 0.6  0.5
 How to Attain High Tray Efficiency?
This is simply attained by having:
• Long time of contact High liquid level depth on tray
• High interfacial area between the 2 phases High gas velocity
• High intensity of turbulence

However, by experience, it was found that conditions leading to high

tray efficiencies will eventually lead to operational difficulties !

What are the operational difficulties that may occur in a tray tower?!
 Operational Problems to Avoid

1) Excessive Entrainment:
• Entrainment refers to the liquid carried by vapor up to the tray
above.
E= moles of liquid entrained
Ltray (moles of liquid on tray) + moles of liquid entrained
• This takes place at high vapor flow rates (high gas velocities)
• Entrainment leads to reduction in tray efficiency…Why?!
• Excessive entrainment takes place if E > 0.1
• Excessive entrainment can lead to flooding.
• How can you minimize it ?!
 Operational Problems to Avoid

2) Flooding:
• This may occur as a result of excessively high vapor flow pushing
liquid above in the column (entrainment flooding).
• This may also occur if the liquid flow rate is too high, so there is
no enough time for vapor-liquid disengagement in downcomer.
This leads to froth accumulation which reaches the above tray
(downcomer flooding).
• This may damage the downstream equipment.
• This is accompanied by huge pressure drop along the tower.
• The tower capacity is accordingly reduced.
• Separation efficiency will be deteriorated.
 Operational Problems to Avoid

3) Weeping/Dumping:
• Weeping occurs when some liquid start to leak through the holes.
• Dumping occurs when all Liquid start to leak through the holes.
• This occurs when vapor flow Is low compared to that of liquid.

How does
hole size
affect
that?!
Safe Operational Area
 Limitations of the Tray Types Discussed
• Liquid and/or vapor maldistribution that can reduce tray efficiency
and lead to entrainment flooding.

• Less-than-optimal downcomer design that can ultimately result in

downcomer flooding.

• Limited Capacity
 How to Improve Tray Performance?
• Advanced downcomer technology
• Active area enhancements
• Inlet Liquid area enhancements

So, now the world of trays is divided

into:
• Conventional Trays
• High Performance Trays

Inlet Liquid Area

 Advanced Downcomer Technologies
• Sloped Downcomer • Stepped Downcomer

• Hanging Downcomer

All designs aim to

provide high area for
disengagement zone,
and enhance the active
area of the tray below
 Active Area Enhancements
1) New Valve Types (e.g. V-grid fixed Valves)

Here the vapor is forced to move in lateral

direction; hence, providing the following
advantages:
- Lower froth height and less entrainment
- Increased capacity and improved
turndown compared to sieve trays
- High efficiency due to improved
vapor-liquid contact
- Reduced mechanical wear
When to use whom?
 Active Area Enhancements
2) Liquid Flow control
a) Vapor Directional Valves (optimized liquid push/flow guided holes)

Gas-flow through the guided holes transfers the

momentum to the liquid flowing flatly on the
trays, pushes the liquid forward instead of having
liquid back mixing.

b) Using Mechanical Baffles

Baffle will equalize flow on tray deck,

and reduce liquid momentum.
 Inlet Area Enhancements
Using Bubble Promoters:
- The main objective is to prevent weeping in inlet area.
- Froth is started earlier, so contact is enhanced.
 High Capacity Trays
How to enhance the tray’s capacity without high investments?
• Increase tray spacing
• Use sloping/swept/hanging downcomers
• Enlarging hole area: This reduces pressure drop so capacity
increases. However, this will cause lower tray efficiency.
• Lengthening downcomer weir “swept”
 High Capacity Trays
What if liquid flow is very high?
Multi-pass (2-6 pass) trays may be the
only solution
 High Capacity Trays (Multi-Downcomer)
Multiple downcomer will cause significantly more weir length, so
height over weir decreases, pressure drop decreases and capacity
increases.
 High Capacity Trays (Multi-Downcomer)
• No liquid receiving pan (no imperforate areas )
• Downcomer means do not extend downward to the next tray
• Very short liquid flow path
• Successive downcomers perpendicular or parallel to each others
 High Capacity Trays (Lattice Multi-Downcomer)
• Better Liquid Distribution
 Tray Column Revamping

So, What are your ideas in case of revamping ?!

Thank you !