Distillation Column Tray Hydraulics-A Review

M.Tech ScholarDepartment of Chemical Engineering

I.I.T, Kanpur

Subhasish Mitra

Tray design-a real challenge:

� Numerous towers today are asked to “multi-task”and handle different feeds or varying feed mixtures.

� In addition, a number of towers in the industry need to meet different product purity specifications at different times of the year.

� Rigorous tray design thus requires to handle not only the flexibility of the process unit it is part of, but also the variations in liquid and vapor loads from the top to bottom tray under a single steady state operation condition.

Variation in Tray Load in an Oil Stabilizer:

Liquid load (kg/hr)Vapor load (kg/hr)Theoretical tray no

467571.533156835.64312 (Bottom)

445124.548134388.65711

425594.146114858.25610

408956.16498220.2739

388849.21978113.3288

358496.82747760.9377

174065.13780914.3016 (Feed)

165244.80379300.4235

162634.70176690.3204

158983.42173039.0413

160810.46167639.4302

151624.08058160.5191 (Top)

Classical tray hydraulic model:

� Liquid enters from the down-comer of the tray above.

� Liquid gets aerated with vapor from tray below and forms froth.

� Froth flows over the O/L weir where vapor is disengaged.

Simplified tray stability diagram:

Excessive vapor flow Jet flooding limit

Excessive liquid flow Down-comer flooding

100% weeping

Flooding mechanisms:

� In simple term, flooding is excessive accumulation of

liquid inside the column.

� Flooding on trays : Mechanisms are Spray

Entrainment Flooding & Froth Entrainment Flooding.

� Flooding in down-comer : Mechanisms are Down-

comer Back up flooding and Down-comer Choke

flooding.

Tray flooding mechanism (Contd):

� Spray regime :

At low liquid flow rate, most of liquid on trays stay in

form of droplets. With rise in vapor velocity, these

droplets get carried away on the tray above. Liquid thus

stay in the tray instead of flowing below.

� Froth regime :

Froth accumulates at higher liquid rate on tray. Froth

height accumulates with rise in vapor velocity. When

tray spacing is small, froth envelope touches the tray

above and entrainment rapidly increases. However when

tray spacing is high, spray mechanism invariably takes

over.

Change of regime:

At low liquid rate, entrainment diminishes with higher liquid load.

At high liquid rate, entrainment increases with liquid loads.

When most of the dispersion is in the form of a spray, entrainment diminishes with higher liquid load.

Transition from spray to froth regime.

Flooding mechanism (Contd):

� Down-comer back up flooding :

Aerated liquid is backed up into the down-comer

because of tray pressure drop, liquid height on the tray

and frictional losses in the downcomer apron. When

back-up of aerated liquid in downcomer touches the

tray above, flooding occurs.

� Down-comer choke flooding :

Velocity of aerated liquid inside downcomer increases

with liquid flow rate. When this velocity exceeds a

certain limit, friction losses in downcomer including

entrance become excessive and the frothy mixture can

not go down to below tray and flooding occurs.

Down-comer flooding illustration:

DC Choked floodingDC Back up flooding

Simplified flooding mechanism:

Low pressure favors higher vapor velocity hence spray regime prevails.

At high pressure , vapor and liquid separation in down-comer decreases which causes DC froth back up. High liquid flow also increases pr drop in DC.

Effect of design parameters on flooding:

� Tray spacing :

Low tray spacing enhances tendency of all types of

flooding except DC choke flooding. TS<18” can cause

both spray and froth entrainment flooding.

� Bubbling area:

Low bubbling area/low fractional hole area causes all type

of flooding except DC choke flooding.

Effect of design parameters on flooding (Contd.):

� Weir height & length:

High weir height & low length reduce tendency for spray

entrainment however increases height of froth envelope.

No effect on DC choke flooding.

� Down-comer area and clearance:

Low DC area increases velocity through DC along with

corresponding pressure drop while low DC clearance

causes head loss and results into DC back up flooding.

Effect of design parameters on flooding (Contd.):

Major tray design parameters:

� Vapor load:

Several correlations are available. Most used one is

� Liquid load:

Most accepted one is flux of liquid across tray (gpm/in),

Weeping:

� Weeping is descent of liquid through plate perforation. It occurs when liquid head on the tray exceeds the pr drop that holds the liquid on tray.

� Minor weeping can be tolerated without affecting tray efficiency.

� Large liquid rate, large fractional hole area and taller weirs cause weeping.

Major tray design parameters (Contd.)

� Down-comer load:

QD is the clear liquid velocity at down-comer entrance.

Alternatively, this load is also expressed in terms of

ft/sec.

Major tray hydraulics design guide:

� Flooding limit:

Several correlations available.

Fair’s Correlation:

Flooding limit : 80% – 85%

Csb = f( flow parameter, surface tension, tray spacing,

fractional hole area)

Major tray hydraulics design guide (Contd.):

Total pressure drop includes dry-hole + wet pr drop.

1.5” to 3” wcPressure drop

RemarksFiguresDesign parameter

More passes required for high liquid loading.

1 to 4Tray pass

Tray spacing decided based on tower diameter and maintenance.

18” to 24”Tray spacing

High FPL enhances tray efficiency while low FPL increases weir load.

16” to 18” minFlow path length

Depends on the system.Less value reported for highly foaming service. This is a safety margin on flooding limit.

0.9 to 1.0 for low foaming to non foamy service.

System factor

RemarksFiguresDesign parameter

Higher weir height causes excess tray pr drop and

leads to weeping.

25 to 50 mmOutlet weir height

5 to 10 mmDC Seal

Up to 20 gpm/in reported. Picket fencing may be required at lower weir load.

Min 2.5 gpm/in Weir loading

Velocity increases with TS but decreases with foaming tendency.

0.2 to 0.5 ft/secClear liquid velocity in DC

Residence time increases as foaming tendency goes up.

3 to 7 secDC residence time

Min of these two to be taken

5-8% of column dia10% of column area

DC width/Area

Major tray hydraulics design guide:

Major tray hydraulics design guide (Contd.):

Weir loading criteria

Tray operating region:

Tray types:

� Normally three major category -Valve, Sieve & Bubble Cap.

� Some popular valve-type trays from “Sulzer” widely used in Industry. Two sub-categories are floating type & fixed type.

RV SV SVG/MVG/MMVGBDH

Tray types (Contd.):

� Sieve type trays normally available in following sizes, 5-6 mm, 10-13 mm, 19 – 20 mm. Applied in both clean & fouling services. Low pressure drop, low cost & less TD.

� Bubble cap tray normally available in 3”, 4” & 6”sizes. Low liquid load & very high TD - costly.

� Various other types of tray available-e.g. Cartridge tray, Baffle tray, Ripple tray, Jet tray, MD tray and other High Performance Trays.

Tray performance comparison:

Some special type of trays:

Sulzer VG-Plus

High performance tray:

-Chordal high performance down-comer.

-Enhanced deck design for efficient vapor liquid contact.

-Optimized valve lay out.

Some special type of trays (Contd.):

Shell High Performance tray

Information courtesy:

� Various sources of information – IngeneroTechnology India Ltd, Mumbai &

Petrofac Engineering India Ltd, Mumbai.

� Technical documents – Sulzer Chemtech & Baretti

� Distillation design: Henry J Kister

Thanks for your attention!