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A Dual CompartmentChimney Tray
Wayne Chung and Dick Nielsen
Fluor Daniel Aliso Viejo, California
Presented to AIChE 2001 Spring National Meeting
April 26, 2001 at Houston, Texas
A Dual Compartment Chimney Tray
A Highly Effective Technique for Adding A Side Reboiler to
an Existing Column
History of the Hydrocracker Unit
• Built in 1962 for a capacity of 8,800 BPD• Revamped in 1971 to a capacity of 11,200
BPD• Revamped in 1992 to a capacity of 17,500
BPD• Now running at 22,000 BPD
• Reused the existing 3-pass trays in the bottom section of the Debutanizer
• Added a pumped side reboiler loop to recover heat from the reactor effluent
• Installed 3 x 12” draw-off nozzles at tray 25 and piped to side reboiler pump suction
• Installed 3 x 8” distributors and one 8” and one 10” return nozzle at tray 25
Debutanizer Revamp in 1971
Tray 24
Tray 25
Tray 26
Return Nozzle 10" Return Nozzle 8"
Drawoff Nozzles2 x 12"
24”
24”
8"8"
DrawoffNozzle
12"
9"
15"
Side View of the Side Reboiler Drawoff and Return Nozzles Installed in 1971
14" 27"
14"x8"Reducer14"x10"
Reducer
10"Return Nozzle
8"Return Nozzle
13-2-1/2" Dia.Holes, Equal.
Spaced
34-1-3/4" Dia.Holes, Equal.
Spaced
15-2-5/8" Dia.Holes, Equal.
Spaced
Top View of the Side Reboiler Return Distributors and Nozzles Installed in 1971
• Side reboiler pump cavitates at 2/3 of the rated flow of 42,000 BPD, or at about 28,000 BPD
• Debutanizer performance doesn’t meet the 1971 revamp requirements
• Side reboiler feed stream is a “mixed” stream consisting of side reboiler return and liquid from tray 24 (loss of LMTD)
Problems
• Poor Drawoff Tray Design- Uneven drawoffs in a 3-pass tray- Inadequate degassing of the boiling liquid
• Poor Side Reboiler Return Design- Uneven side reboiler return to each flow pass- Inadequate segregation of cold and hot fluids
• Poor Mixing of Fluids- Fluid mixing on the deck and troughs- No assurance of fluid flow direction
Causes of Side Draw Limitations
Tray 24
Tray 25
Tray 26
ReturnNozzle 10" Return Nozzle 8"
Drawoff Nozzles2 x 12"
2'
2'
8"8"
Drawoff Nozzle
12"
9"
15"
‘Idea’ Hot Fluid Flow Direction
Drawoff Rate < 28,000 BPD
Tray 24
Tray 25
Tray 26
ReturnNozzle 10" Return Nozzle 8"
Drawoff Nozzles2 x 12"
2'
2'
8"8"
Drawoff Nozzle
12"
9"
15"
‘Actual’ Hot Fluid Flow Direction
Drawoff Rate > 28,000 BPD
Design a Dual Compartment Chimney Tray to perform the following tasks:• Cold fluid surge for pump drawoff• Hot fluid surge for liquid stabilization and
degassing prior to flowing to tray below• Side-reboiler return vapor/liquid separation• Isolation of the cold fluid for a better LMTD• Vapor chimney for vapor from the tray below
Process Design Objectives
• Limited free vertical space for a chimney tray and pump liquid surge.
• How to control a 90 to 95% drawoff of cold fluid without a level or flow controller?
• How to prevent the cold fluid from being mixed with the hot side reboiler return fluid?
• How to safeguard the pump?
Column Design Limitations
A Dual CompartmentChimney Tray with
A Special Double Wall
Innovative Solution
• Install a special double wall partition for separating the cold and hot compartments
• Isolate the cold fluid and allow excess cold fluid to sweep inside the double wall
• Reverse hot fluid flow as an emergency backup for pump protection
• Use temperature sensors to indicate the fluid flowdirection on the chimney tray
Process Innovations
• A new 20” pump draw nozzle designed for automatic degassing
• A new 10” U type side reboiler return distributor and a new nozzle
• A vapor chimney of 27” X 40” X 43”(High)• A 32”(High) double wall• Two tray 26 downcomer chutes• Two 29” (High) liquid dams to tray 27
Chimney Tray Features
Return Nozzle
Draw-off Nozzle
ColdFluidCold
Fluid
Tray 25
Tray 26
Tray 27
HotFluid
HotFluidVapor Chimney
Cold Compartment (front)Hot Compartment (rear)
5'
20"
Front View of the Chimney Tray
Tray 27
5'
20" Draw-off Nozzle
Cold SideHot Side
Tray 25
Tray 26
Return Nozzle
Tray 26Seal Pan
ColdFluid
Vapor Chimney
Hot Fluidto Tray 27
Vapor
Distributor
Liquid Dam
Double Wall
Splash Plate
Side View of the Chimney Tray
Partial View of the Chimney Tray
The Double Wall -Heart of The Design
Tray 26 Downcomer Chute
Double Wall & Tray 26 Chute
Reboiler Return Distributor
Partial View of the Chimney Tray
Tray 26 Deck
Partial View of the Chimney Tray
Return Nozzle (above)Draw-off Nozzle (below)
Cold SideLiquid Surge
Double Walloverflow
Vapor fmTray 27
Liq. toTray 27(below)
Liq. fm Tray 26(above)
Liq. toTray 27(below)
Liq. fmTray 26(above)
Hot SideLiquid Surge
overflow
Normal Flow Path
Return Nozzle (above)Draw-off Nozzle (below)
Cold SideLiquid Surge
Double Wall reverse flow
Vapor fmTray 27 Liq. to
Tray 27(below)
Liq. fm Tray 26(above)
Liq. fmTray 26(above)
Liq. toTray 27(below)
Hot SideLiquid Surge
reverse flow
Reverse Flow Path
Side Draw and Return Flow Schematic
From Fired HeaterReboiler
To Fired HeaterReboiler Pump
Side ReboilerPump
529°F
566°F468°F
468°F
520°F
dTC
17
18
19
26
27
30
H
537°F
700°F 630°F
551°F
TI
Feed
Reactor EffluentReactor Effluent
FTFFCFT
TT
TT
• Hot fluid reserves flow to the cold compartment during initial startup as anticipated
• Draw rate increases to 30,000 BPD with a peak rate of 35,000 BPD
• The system has been operating successfully since initial startup
Operating Results
• Maintain the coldest possible temperature to the side reboiler
• Maximize the side reboiler LMTD• Minimize the column space loss, i.e., 2 tray loss (or
less) for the chimney tray• Indicate or control the flow direction of the cold or
the hot fluid with temperature sensors• Isolate and insulate the cold compartment with
excess cold fluid flowing through the double wall
Conclusion: Benefits of the Double Wall
