High Capacity Deck Design They have smaller orifices which reduces the localized momentum of the vapor flowing They do not carry entrainment to the top section as the smaller streams can’t penetrate the froth in the tray deck area easily. It also lowers overall pressure drop leading to reduction of downcomer backup.
Columns and their hydraulic limits
Columns and their Hydraulic
Schematic Of Sieve Plate Operation
Fig. Normal Operation of Sieve Plate
Working of an Acid Gas Removal Column
• At high weir loads, trays operate predominantly in the froth regime,
where gas dispersion rises through continuous liquid, interfacial areas
are large and Liquid is highly agitated. This results in high absorption
of CO2 and H2S.
• Low weir load corresponds to spray regime, where liquid is dispersed
as large droplets that are projected through a continuous gas phase
as a spray. CO2 absorption is retarded while H2S removal improves.
Note: The liquid droplets (spray) behave like rigid spheres (or nearly stagnant liquid) and have extremely
small liquid-side mass transfer coefficients. Consequently, CO2 absorption is retarded because its rate is
controlled by the liquid-side resistance to mass transfer
General Design guidelines
• Turndown for a tray = 4:1
• Good Design
• Vapor loads approaching jet-flood limit
• Downcomers sized to approach the choke flood and back-up flood limit
• Maximum tray active area leading to smallest column diameter
System factor or System De-rating Factor
• Varies based on the correlation selected and the software selected for the
• For instance, Surface tension is a difficult transport property to predict
below 10 dyne/ct. It becomes much more difficult when it is less than 5
dyne/ct or when CO2 is present. The difference between 1 dyne/ct and 2
dyne/ct can generate loss in column capacity and lead to premature
• Care must be used when system factor and correlation is selected when
column design is pushed to 80 – 85 % limits and expected to be operated at
that point, especially in high pressure hydrocarbon/foaming applications
such as amine contactors and regenerators.
Parameters that identifies Hydraulic limits
• Jet Flooding %
• Downcomer Flooding %
• Downcomer Froth backup %
• Downcomer clear liquid, inch
• Weir loading (gpm/in)
• Dry Drop, mm H2O
• Pressure drop across MV trays, mm H2O
• Jet flooding: Flooding on the tray deck, typically caused by high vapour
flows resulting in excessive re-entrainment and back-mixing.
• Downcomer Flooding: Flooding in the downcomer, caused by high liquid
flows causing choking, or excessive liquid back-up in the downcomer.
• Weeping: Under extremely low vapour flow rates, liquid will weep through
the tray deck perforations / valves, reducing tray efficiency.
• Spraying: Under extremely low liquid flow rates, the hydraulic regime on
the tray deck changes from frothing to spraying. This typically results in low
• Vapour Channelling into the downcomer occurs because of low liquid flow
• Some vendors consider the maximum useful capacity to be 85% Jet
flood and 10% entrainment while some consider 80 or 90% jet flood
corresponding to 10 or 20% entrainment. These limits are mostly
considered arbitrarily and are subjective to different vendors.
• Near the Jet flood limit, entrainment rate is exponentially sensitive to
small changes in vapor loads. So it advisable not to operate in this
region as it leads to instability.
• One jet flood mechanism is by froth entrainment by gas
• As the gas flow rate decreases at a
fixed liquid rate, the tray begins to
weep (B). Further decrease in
liquid rate can lead to excessive
weep rates (D).
High Vapor Load Constraints
Jet Flooding or Entrainment Flooding
1. At high vapor rates, a mixture of spray or froth occupies the entire tray
2. It recirculates the liquid to the top section of column, contaminating the top
product with heavies and could cause downcomer flooding.
1. Increasing Active area of the tray
2. Increasing Tray spacing
3. Switching to high capacity deck design
Note: High Capacity Deck Design -They have smaller orifices which reduces the localized momentum of the vapor flowing.
They do not carry entrainment to the top section as the smaller streams can’t penetrate the froth in the tray deck area
easily. It also lowers overall pressure drop leading to reduction of downcomer backup.
As in figure (a), back up flood is determined
by the liquid depth on the tray, head loss for
flow of aerated liquid under downcomer
skirt and pressure drop across the tray.
• Back up flood is the easiest to avoid by
using the right downcomer escape area,
type of tray and fractional hole area of
From figure (b), it is understood that during
choking, liquid do not crest over the weir
instead a high volume flow of froth is
flowing at the maximum hydrodynamic
head. Once the froth covers the
downcomer mouth, liquid height over the
weir increases more than it should.
• The cross-sectional area of the
downcomer mouth is an important
parameter to avoid choking
High Liquid Load Constraints
At high liquid loads, downcomer backup flooding or downcomer choking occurs
1. When the downcomer backup is over the weir of the tray above, downcomer backup flooding
2. Downcomer choking is caused due to entrance conditions. When velocity of froth entering the
downcomer backup is high, it doesn’t provide enough time for vapor disengagement.
Therefore, a low density fluid is developed, a high volume froth cannot pass through the
downcomer leading to choking
Downcomer backup flooding
1. Increase tray spacing
2. Reduce Overall pressure drop
3. Decrease bottom downcomer frictional resistance
1. Increase downcomer top width