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 I he weir separates thetube bundle from the reboiler section, where residual reboiledliquid (bottoms product) is withdrawn.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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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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