Columns and their Hydraulic Limits

Presented byGautham

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

Tray Operating Diagram

A

B

C

D

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

same service• 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 flooding.• 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

Hydraulic Limits• 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 tray efficiencies.• Vapour Channelling into the downcomer occurs because of low liquid flow

rates

Entrainment Flood• 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

Weeping• 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).

A

BC

D

High Vapor Load ConstraintsJet Flooding or Entrainment Flooding1. At high vapor rates, a mixture of spray or froth occupies the entire tray spacing.2. It recirculates the liquid to the top section of column, contaminating the top

product with heavies and could cause downcomer flooding.

Solution3. Increasing Active area of the tray4. Increasing Tray spacing5. Switching to high capacity deck designNote: 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.

Downcomer floodAs 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 the tray.

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 ConstraintsAt high liquid loads, downcomer backup flooding or downcomer choking occurs1. When the downcomer backup is over the weir of the tray above, downcomer backup flooding occurs2. 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

SolutionDowncomer backup flooding 1. Increase tray spacing2. Reduce Overall pressure drop3. Decrease bottom downcomer frictional resistanceDowncomer chocking4. Increase downcomer top width

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