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HEAT TRANSFER
How to select the best reboilerfor your processing operationGuidelines streamline choosing a heat exchanger to maximizeprocess efficiencies
B. TAMMAMI, Fluor Corp., Houston, Texas
R eboilers are used ilirougliout refineries and are cricicalto reliable plant operation. They generate vapor, whichdrives fractional distillation separacion. In traditional
fractional distillation services, all vapor driving separation comesfrom the reboiler. Therefore, proper reboiler operation is vitalto effective distillarion.
The most critical element ot reboiler design is selecting theproper type. Heat-transfer equipment, including stab-ins, plate-Bns and spiral-place, can be used for specific services. But themost common type used is the shell-and-tube reboiler. Severalexamples review the characteristics oi various shell-and-tubereboiler designs and examine selection criteria for particularprocessing operations.
It is possible to operate multiple reboilers in one distillationcolumn. Reboilers with the same operating temperature shouldbe identical, installed in parallel formation with identical pipingand balance line. 1 he presented concepts will not go into thedesign details of various reboilers; we will define the character-istics of shell-and-tube reboilers and examine selection criteriathat favor one configuration over another.
TYPES OF REBOILERSTable 1 lists the various shell-and-tube reboilers that can be
applied in various heat transfer operations in any hydrocarbonprocessin^ facility.
Kettle reboilers. Historically, the most common reboilerused in the refining industry is the kettle type. They oftenrequire pumping of the column bottoms liquid to deliver theliquid into the reboiler. Pool-boiling is a typical characteristic ofkettle reboilers with 20% to 100% boil-up range. However, itis recommended to keep the vaporization rate at 80% and thusminimize excessive fouling.
in this reboiler type, steam flows through the tube bundleand exits as condensate (see Fig. 1). Liquid from the bottomof the tower, commonly called the bottoms, flows through theshell side. The reboiler construction may include a retainingwall, commonly referred to as "weir." I he weir separates thetube bundle from the reboiler section, where residual reboiledliquid (bottoms product) is withdrawn. The liquid level shouldbe maintained over the tube bundle, using a weir or controlsto assure maximum heat-transfer efficiency and to avoid tubeexposure by extreme heat that may result in dry out and tubedamage.
Reboiler vapor to tower
Steam
Bottomsproduct
A typical steam-heated kettle reboiler.
Size of the kettle reboiler depends on the required vapordisengagement and acceptable liquid cntrainment, which is afunction of corresponding vapor velocity (vertical and horizon-tal), liquid level, vapor load, etc'
With the larger capital cost, we inherit a natural heat andthermal capacitance that withstands large process variations.That is the principal reason applied to select a kettle reboilerwhen future turndowns or turn ups are predicted and inevi-table.
Thermosyphon reboilers. This reboiler does not requirepumping of the column-bottoms liquid into the reboiler (seeTable I). Natural circulation is obtained by using the densitydifference between the reboiler inlet column bottoms liquid andreboiler outlet liquid-vapor mixture. This provides sufficientliquid head to deliver the tower bottoms into the reboiler.
Thermosyphon reboilers are more complex than kettle reboil-ers and require careful design and more attention from theplant operators (Fig. 2). There are many types of thermosyphonreboilers, which include vertical (updraft in-tube vaporization)or horizontal (in-shell vaporization). Reboilers can be once-through or recirculating in operations. The basic approacb fordesigning thermosyphons is to:
• Estimate the fraction vaporized• Determine the circulation rate from the piping layout and
pressure drop• Calculate the heat transfer rates.
HYDROCARBON PROCESSING MARCH 2008 91
BOniUSREPORT HEAT TRANSFER
V = VaporL = Liquid
Steam
Bottom section ofdistiilation column
Liquid + vapor mixture
Ttiermosyphonreboiler
Down comerTray liquid
Liquid
Condensate
Bottoms liquid
Liquid and vapor mixture to tower Steam
Condensate
Tower bottoms
Bottoms product
Bottomsproduct
A typical hort7ontal thermosyphon reboiler
The calculations arc repeated until the estimared vapor frac-tion and calculated fraction converge. The fraction vaporized iskept around 25% for vertical units; higher rates may be requiredfor horizontal reboilers. The objective is to control the bottomsflowrate through the thermosyphon and vaporization rate. Ingeneral, higher vaporization results in a lower heat transfer coef-ficient due to lower vapor heat transfer rate.
Once-through thermosyphons can be used for heat-sensitivebottom.s product recovery, but the vaporization rates are con-trolled better by using circulation operation. Accurate thermody-
Pump
A typical steam-heated forced circulation reboiler.
namics and transport properties of the boiling liquid are essentialfor proper sizing of thermosyphons.
Forced-circulation reboilers. These reboilers use a pumpto torce the liquid through rhe exchanger. They can be eithervertical or horizontal with one or multi-pass construction(Fig. 3). The main process characteristic ot a forced reboiler islow percentages ot vaporization with high liquid circulation.Forced circulation is used especially when the viscosity otliquid is higher than 2 cp. The efficiency of forced reboilers isidentical to that ot natural circulation thermosyphon providedthe liquid circulation is small, which is not always the case inmost operations.
T A B L E 1 Summary of the advantages and disadvantages for kettle and thermosyphon reboilers
Kettie reboilers
Advantages
nsensitive to hydrodynamics
Easy to size
Requires the lowest liquid driving head
Good performance in deep vacuum and near critical
Enhanced surfaces are most effective
Could be used as waste-heat-recovery unit
Virtualfy no tuhe vibration
:asy to operate
Disadvantages
Dirt collector; requires more frequent cleaning
Concentration effects may cause severe fouling or corrosion problems
Wide boiling range mixtures and large bundles inhibit performance
Prone to vapor blanketing with high heat fluxes
Liquid entrainment with high mass fluxes
Higher capital cost
Horizontal thermosyphon reboilers
Advantages Disadvantages
Excellent performance at low temperature differences
deal for wide boiling range mixtures
.ower liquid driving head requirements than VTS*
Higher circulation rates forthe same liquid head
Fouling on shell-side; hence difficult to clean
Vapor blanketing and localized dryout possible at high fluxes
Large units require multiple nozzle and expensive manifold piping
Improper exit piping design could lead to operational problems
Vertical thermosyphon reboilers
Advantages Disadvantages
High sheer rates reduce fouling tendencies
Fouling on tube-side; hence easier to dean
nexpensive shell and piping
Better design possible with sensible heating mediums
Difficult to operate in deep-vacuum and near-critical conditions
Prone to instability at low-pressure and high heat fluxes
Mist flow at high exit vapor fraction or low circulation rate
Poor performance for low temperature difference
• Vertical Itiermosyphon reboilers
92 MARCH 2008 HYDROCARBON PROfESSING
HEAT TRANSFER
TABLE 2. Reboiler selection
Process conditions
Operating pressure
Moderate
Near critical
Dppii v,icuum
Design AT
Moderate
Large
Small
Very small
Fouling
Clean
Moderate
Heavy
Very heavy
Mixture
Pure component
Narrow
Wide
Very wide (viscous liquid)
based on process conditions
Reboiler selection
Kl
OE
B-OE
G
OE
B
G-F
F-P
G
RO
P
P
G
G
F
F-P
Reboiler type*
HTS
G
R
R
G
R
G
F
G
G
RD
P
G
G
B
G-RD
VTS
B
RD
RD
B
G-RD
RD
P
G
B
B
RD
G
B
G
P
FRF
OE
OE
OE
OE
OE
RD
P
OE
OE
G
B
OE
OE
OE
G
FAF
OE
R
B
OE
P
G
B
OE
G
G
G-RD
G
G
G-R
G-R
START
i
Yes
Yes
Forcedcirculation
Kettlereboiler
Is vapor > 30%?
Yes
Is % viscosity >2cp?
•,No
Is area >approximate 600
ft?
No
Horizontalthermosyphon
No
Yes
Is there abottoms pump?
No
Yes
,No Yes
Can towerbe elevated?
•Yes
Is viscosity >0.5 cp?
Yes
.No
Is tube length >approximate 20 ft?
,No
Verticalthermosyphon
Category abbreviations B—beit 6—.good operation F—fair bui better choice possibleRD—nsky unless carefully designed R—risky due to insutficieni dataP—poor operation OE—operable but unnecessarily expensive
Reboiler abbreviations Kl—kettle or internal HTS—horizontal thermosyphonVTS— v̂ertical thermosyphon RFR^—forced flow rAF—falling film
FIC. 4 " Reboiler selection flovuchart for varying conditions.
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HEAT TRANSFER
Falling film reboilers. This type of reboiler or evaporatorcan be vertical or horizontal:
Vertical tube falling film reboiler. The recirculating liquid isincroduced at the top of a vertical tube bundle and it falls In a thinfilm down inside the tubes. The liquid absorbs heat from steamcondensing on the outside of tubes and water, in liquid state, isvaporized. This evaporator type is usually selected for higher vis-cosity liquids and for concentrating heat-sensitive solutions chatrequire low residence times. Evaporation occurs on the highlyturbulent tllm and not on the cube sutface. This heat-transferoperacion requires that temperature differences be low. The mainconcetn wich falling film units is that liquids must be disctibutedevenly to all tubes.
Horizontal tube spray film reboiler. The recirculating liquoris heated and sprayed over che outside ot a horizontal tube bundlecarrying low-pressure steam, thus condensing watet vapor insidethe Cube. Vapor from the evapotacor chamber can be used as steamin a subsequenc effect, or mechanically compressed and reused asthe heating medium in the stage where it was generated.
MATERIALS OF CONSTRUCTIONI he selection of materials ol conscruccion for reboilers is simi-
lar to the methodology used for standard heat exchangers. Mildsteel alloys have excellent heat-transfer coefficient and long servicelife. Coppet alloys are not generally recommended due to processcompatibility concerns. Occasionally, operating companies mayrequire using stainiess steel tube bundles to reduce the possibil-ity of cube failure from corrosion. This material may impact the
design since che heat-transfer efficiency of stainless steel alloy islower than mild steel alloys, especially when reboilets are operat-ing at maximum throughput.
S e l e c t i n g a reboi ler . Many factors influence reboiler typeselection. In che end, ;ill chese factors reduce co maximum effi-ciency, maincenance and economics. Every plane will weight chetrade-offs between these factors difFerencly as shown in Fig. 4. No"one-size-type Res all" selection exists. Table 2 summarizes theadvantages and disadvantages for shell-and-tube reboilers. HP
LITERATURE CITED' Tammani, B., "Simplifying reboiler entrainment calculations," Oil & GasJournal. ]u\y 15, 1985.
T a m m a m i has over 29 years of experience in process,heat-transfer engineering and construction, as well as equipmentspecification and fabfication. As design engineer and engineeringmanager with shell-and-tube and pressure wessef fabricators, hegained the bulk of his experience in equipment design, estimating
and manufactufing. He also has extensive engineering experience with several majorengineering and construction firms on a variety of projects including gas, power andpetrochemical projects. His heat-transfer experience includes nearly 6,500 shell-and-tube and air-cooter designs, over 2,700 of which are in operation worldwide.Mr. Tammani has authored several technical papers regarding heat transfer and fluidflow. He has developed several software products including thermal design and costestimating for shell-and-tube heat exchangers. He holds a BS degree in chemicalengineering from the University of Tulsa. He is a principal engineer with Fluor Corp.He is a member of AIChE and the heat transfer division of ASME Mr, Tammani is aregistered professional engineer in Texas
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