Distillation Column Tray Hydraulics 3k5l4

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 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

1 to 4

More es 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 . -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!