Distillation Column Tray
Hydraulics-A Review
M.Tech Scholar
Department of Chemical Engineering I.I.T, Kanpur
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 467571.533 156835.643 12 (Bottom) 445124.548 134388.657 11 425594.146 114858.256 10 408956.164 98220.273 9 388849.219 78113.328 8 358496.827 47760.937 7 174065.137 80914.301 6 (Feed) 165244.803 79300.423 5 162634.701 76690.320 4 158983.421 73039.041 3 160810.461 67639.430 2 151624.080 58160.519 1 (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
Simplified tray stability diagram:
Excessive vapor flow Jet flooding limit
Excessive liquid flow Down-comer flooding
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-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 flooding DC 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:
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” wc Pressure drop Remarks Figures Design parameter
More passes required for high liquid loading.
1 to 4 Tray 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” min
Flow 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
Remarks Figures
Design parameter
Higher weir height causes excess tray pr drop and
leads to weeping. 25 to 50 mm
Outlet weir height
5 to 10 mm DC 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/sec
Clear liquid velocity in DC
Residence time increases as foaming tendency goes up.
3 to 7 sec DC residence time
Min of these two to be taken
5-8% of column dia 10% of column area DC width/Area
Major tray hydraulics design guide
(Contd.):
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/MMVG BDH
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.
Some special type of trays:
Sulzer VG-Plus
High performance tray:
-Chordal high performance down-comer.
-Enhanced deck design for efficient vapor liquid
contact.
Some special type of trays (Contd.):
Information courtesy:
Various sources of information – Ingenero
Technology India Ltd, Mumbai &
Petrofac Engineering India Ltd, Mumbai.