Distillation Column Tray Hydraulics-A Review
Subhasish Mitra 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: Theoretical tray no
Vapor load (kg/hr)
Liquid load (kg/hr)
1 (Top)
58160.519
151624.080
2
67639.430
160810.461
3
73039.041
158983.421
4
76690.320
162634.701
5
79300.423
165244.803
6 (Feed)
80914.301
174065.137
7
47760.937
358496.827
8
78113.328
388849.219
9
98220.273
408956.164
10
114858.256
425594.146
11
134388.657
445124.548
12 (Bottom)
156835.643
467571.533
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 Downcomer 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 Back up flooding
DC Choked flooding
Simplified flooding mechanism: Low pressure favors higher vapor velocity hence spray regime prevails.
At high pressure , vapor and liquid separation in downcomer 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.): Figures
Remarks
0.9 to 1.0 for low foaming to non foamy service.
Depends on the system. Less value reported for highly foaming service. This is a safety margin on flooding limit.
16” to 18” min
High FPL enhances tray efficiency while low FPL increases weir load.
1.5” to 3” wc
Total pressure drop includes dry-hole + wet pr drop.
Tray spacing
18” to 24”
Tray spacing decided based on tower diameter and maintenance.
Tray pass
1 to 4
More passes required for high liquid loading.
Design parameter
System factor
Flow path length
Pressure drop
Major tray hydraulics design guide: Design parameter
Figures
Remarks
5-8% of column dia 10% of column area
Min of these two to be taken
3 to 7 sec
Residence time increases as foaming tendency goes up.
0.2 to 0.5 ft/sec
Velocity increases with TS but decreases with foaming tendency.
Weir loading
Min 2.5 gpm/in
Up to 20 gpm/in reported. Picket fencing may be required at lower weir load.
DC Seal
5 to 10 mm
DC width/Area DC residence time
Clear liquid velocity in DC
Outlet weir height
25 to 50 mm
Higher weir height causes excess tray pr drop and leads to weeping.
Major tray hydraulics design guide (Contd.): Weir loading criteria
Tray operating region:
Tray types:
BDH
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
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 – Ingenero Technology India Ltd, Mumbai & Petrofac Engineering India Ltd, Mumbai.
Technical documents – Sulzer Chemtech & Baretti
Distillation design: Henry J Kister
Thanks for your attention!
