Reboiler Calculations Design Guide

PROCEDURE NO.

PTD-DGS-122 PREPARED BY

PROCESS TECHNOLOGY PROCEDURES

Edited from existing doc by H Andrawis

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April 30, 2000

APPROVED BY DEPARTMENT: SUBJECT:

PROCESS ENGINEERING

JRB

REBOILER CALCULATIONS DESIGN GUIDE

REVISION DATE

1/11/2006

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TABLE OF CONTENTS Section 1.0

SCOPE.............................................................................................................

2.0

RESPONSIBILITIES........................................................................................

3.0

DESIGN CRITERIA.......................................................................................... 3.1 3.2

4.0

CALCULATIONS............................................................................................. 4.1 4.2 4.3

5.0

Cold Side................................................................................................. Hot Side...................................................................................................

Horizontal Thermosyphon Reboiler Calculations................................... Kettle Reboiler Calculations.................................................................... Vertical Thermosyphon Reboiler Calculations........................................

PROCESS SPECIFICATION........................................................................... LIST OF ILLUSTRATIONS

Figure 1 2 3 4 5 6

Horizontal Thermosyphon Reboiler - Side Drawoff......................................... Horizontal Thermosyphon Reboiler - Bottom Drawoff..................................... Kettle Reboiler - Pressured Type.................................................................... Kettle Reboiler - Gravity - Flow Type.............................................................. Vertical Thermosyphon Reboiler - Bottom Drawoff......................................... Vertical Thermosyphon Reboiler - Side Drawoff.............................................

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1.0

SCOPE

This design guide1 covers the calculation of vessel skirt heights, pressure drops, line sizes, and calculation of driving head to determine required drawoff nozzle elevations for reboiler circuits. 2.0

RESPONSIBILITIES

The process engineer is responsible for determining vessel skirt heights, pressure drops, line sizes, and driving heads for reboiler circuits. 3.0

DESIGN CRITERIA

3.1

Cold Side

Lines carrying liquid to the reboiler shall be sized using a maximum liquid velocity of 4 to 5 feet per second. For lines drawing liquid from the side of a column, a velocity of 2 feet per second shall be used for a minimum length of 5 feet. 3.2

Hot Side

For a reboiler vapor outlet, ρ V2 should generally not exceed 500. The mixed-phase reboiler return line shall be sized using a minimum vapor velocity of 15 feet per second, even if this produces unstable flow in the vertical up. In cases where the density is low, higher velocities will occur, but these should not exceed 100 feet per second. The horizontal portion of the mixed-phase line may be larger (vapor velocity may be lower) than the vertical up portion of the line, if necessary to maintain stable flow in both the horizontal and vertical run of lines. 4.0

CALCULATIONS

The reboiler calculations are segregated on the following arrangements: 1. Horizontal Thermosyphon Reboiler Calculations a. Side drawoff (Figure 1) b. Bottom drawoff (Figure 2) 2. Kettle Reboiler Calculations a. Pressured type (Figure 3) b. Gravity flow type (Figure 4)

1

Rev. 0 of the Design Guide was adapted with minimal changes from a Pasadena document, DSG-MP8, 1/93

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REBOILER CALCULATIONS DESIGN GUIDE

3. Vertical Thermosyphon Reboiler Calculations a. Bottom drawoff (Figure 5) b. Side drawoff (Figure 6) 4.1

Horizontal Thermosyphon Reboiler Calculations

Horizontal reboilers, with natural circulation, have a simple circulation system. Liquid flows from a tower side-drawoff nozzle, tower bottom, or an elevated drum through a downcomer pipe to the bottom of the reboiler shell. The liquid is heated, partially vaporized, and leaves the reboiler in the return piping as vapor-liquid mixture, then flowing back to the tower or drum. The circulation is forced by the static head difference between the two fluid columns. Use the exchanger centerline as a reference line. P1 = ρ1H1 /144

(Eq. 4-1)

P2 = ρ 2H2 /144

(Eq. 4-2)

P1 − P2 = ∆P = ( 1/144 ) ( ρ1H1 − ρ2H2 )

(Eq. 4-3)

where: H1 = height of downcomer, in ft H2 = height of riser, in ft P1 = downcomer static pressure at reboiler center line, in pounds per square inch atmosphere (psia) P2 = riser static pressure at reboiler center line, in psia ∆ P = pressure drop, in pounds per square inch (psi)

ρ 1 = hot liquid density, in lb/cu ft ρ 2 = liquid/vapor density, in lb/cu ft This ∆ P is the driving force necessary for natural circulation. This is also the pressure available to overcome exchanger and piping friction losses. ∆ P ≥ ∆ Preboiler + ∆ Pdowncomer + ∆ Priser + ∆ P safety factor ∆ Preboiler generally ranges from 0.25 to 0.50 psi.

(Eq. 4-4)

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∆ Pdowncomer and ∆ Priser should be calculated using the Parsons Line Sizing and Optimization program and the design criteria in section 3.0. These ∆ Ps should include pipe inlet and outlet losses and acceleration losses. ∆ Psafety factor = 0.10 psi

(Eq. 4-5)

The riser pipe diameter is generally one or two sizes larger than the downcomer pipe diameter.

Figure 1 - Horizontal Thermosyphon Reboiler - Side Drawoff 4.1.1

Side Drawoff Nozzle Elevation

The minimum downcomer nozzle elevation above the horizontal reboiler centerline (H 1) may be found from the available DP Equation 4-3. The minimum (H1 - H2) is usually 3 feet. Insert H 2 = H1 - 3 into the available ∆ P equation and solve for H1: H1 =

144∆P − 3 ρ 2 ρ1 − ρ 2

(Eq. 4-6)

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where: ∆ P = ∆ Preboiler + ∆ Pdowncomer + ∆ Priser + 0.10 safety factor The value of H1 is useful when elevation adjustments are made to vessel heights during plant layout or when the vessel can be located at a minimum elevation. Many towers have a bottom drawoff pump. Pump net positive suction head (NPSH) requirements generally elevate the process vessel and the reboiler drawoff nozzle higher than that of the reboiler's minimum. This increases the static head in the vertical legs and also the driving force in the circuit. With the increased tower height, it is worthwhile to check the reboiler circuit for a possible reduction of liquid and return line sizes, especially where large diameter lines are required.

Figure 2 - Horizontal Thermosyphon Reboiler - Bottom Drawoff 4.1.2

Bottom Drawoff

At horizontal reboilers, whether the drawoff nozzle is elevated or located in the bottom of the tower, the hydraulic conditions are the same. To find the available energy, the ∆ P Equation 4-3 can be used.

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For the minimum downcomer length (H 1), the return line nozzle elevation (H 3) must be known. Insert H2 = H1 + H3 into DP Equation 4-3 and solve for H 1, as shown in Equation 4-7: H1 =

4.2

144∆P + ρ 2H3 ρ1 − ρ 2

(Eq. 4-7)

Kettle Reboiler Calculations

There are two general types of kettle reboilers: the pressured flow and the gravity flow.

Figure 3 - Kettle Reboiler - Pressured Type 4.2.1

Kettle Reboilers - Pressured Flow

In this type of kettle reboiler, the liquid level in the reboiler is generally controlled by a level controller. Excess liquid flows from the reboiler to a lower pressure destination through a level control valve. These reboilers are generally mounted at grade, and the hydraulics involved is the same as for any pressured flow from a vessel.

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Figure 4 - Kettle Reboiler - Gravity - Flow Type 4.2.2

Kettle Reboiler - Gravity Flow Type

In this type of kettle reboiler, the liquid flows by gravity from a tower side drawoff nozzle into the reboiler. In the reboiler, the liquid overflows a weir, which maintains the liquid level. The overflow is returned to the bottom of the tower by gravity. As in thermosyphon reboilers, the static head between the tower drawoff nozzle and the top of the reboiler weir must be large enough to overcome the reboiler and piping pressure drops.

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(Ho-Hw) specific gravity x 0.4331 psi > ∆ Preboiler + ∆ Pdowncomer + ∆ Priser (Eq. 48) where: Ho = elevation of tower downcomer outlet nozzle, in ft Hw = elevation of top of reboiler weir, in ft ∆ Preboiler generally ranges from 0.25 to 0.50 psi. ∆ Pdowncomer and ∆ Priser should be calculated using the Parsons Line Sizing and Optimization program and the design criteria in Section 3.0. These ∆ Ps should include pipe inlet and outlet losses. A kettle-type reboiler produces high evaporation rates and a large diameter return line might be necessary. The process flow diagram, of course, should indicate flowrates and the physical properties of the flowing fluid. The static head between the top of the weir in the reboiler and the tower high liquid level must be great enough to overcome the liquid return line pressure drops, including the pipe inlet and outlet losses. (Hw - HHLL) x specific gravity x 0.433 psi > ∆ P liquid return line

(Eq. 4-9)

where: Hw = elevation of top of the weir in the reboiler HHLL= elevation of high liquid level in the tower 4.3

Vertical Thermosyphon Reboiler Calculations

Liquid flows from a tower side drawoff nozzle, tower bottom, or an elevated drum through a downcomer to the bottom of the vertical reboiler shell. The liquid is heated and leaves the reboiler in the return piping as vapor or vapor-liquid mixture. It then flows back to the tower or drum. A simplified, conservative, and convenient assumption is made that density varies along the vertical reboiler length in a straight-line proportion.

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Figure 5 - Vertical Thermosyphon Reboiler - Bottom Drawoff 4.3.1

Vertical Thermosyphon Reboiler - Bottom Drawoff

Assuming that the fluid density in the reboiler will be an average of the liquid downcomer and return line densities we have

ρ3 =

ρ1 + ρ2 2

(Eq. 4-10)

The sum of the static heads gives the following equation: ∆P = ( 1/144 ) ( ρ1H '1 − ρ 2H2 − ρ3H4 )

(Eq. 4-11)

where: H'1 = relative elevation between the tower bottom tangent and bottom of the reboiler H2 = height of riser, in ft H3 = elevation between the tower bottom tangent and vapor return nozzles, in ft H4 = height of the vertical reboiler, in ft

ρ 1 = density of the liquid, in lb/cu ft

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ρ 2 = density of the vapor/liquid mixture, in lb/cu ft ρ 3 = average density of fluid in the reboiler, in lb/cu ft This ∆ P is the driving force for vertical reboilers. Friction losses should be smaller than this. The relative position between the tower bottom tangent and the bottom of the reboiler must be a minimum of H'1 feet. Expressing H'1 from the above equation and inserting H2 = H '1 + H3 − H4 or H '1 + H3 = H2 + H4 The relative elevation difference as shown in Equation 4-12: H '1 =

144∆P + ρ 2 ( H3 − H 4 ) + ρ3H4 ( ρ1 − ρ2 )

(Eq. 4-12)

Assuming that the tower bottom tangent line and the top tubesheet are on the same elevation, the above Equations 4-11 and 4-12 can be simplified, because H' 1 = H4 and H3 = H2. Consequently: ∆P = ( 1/144 ) H '1 ( ρ1 − ρ3 ) − ρ3H3 

(Eq. 4-13)

and H '1 =

144∆P + ρ 2H3 ( ρ1 − ρ3 ) , no safety factor has been included

(Eq. 4-14)

The H2 dimension can be greater than H 3. The minimum liquid level static head above the bottom tangent line can also be taken into account as an additional driving force.

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Figure 6 - Vertical Thermosyphon Reboiler - Side Drawoff 4.3.2

Vertical Thermosyphon Reboiler - Side Drawoff

For a vertical reboiler with a high drawoff nozzle, the driving force is: ∆P = ( 1/144 ) ( ρ 2H "1 − ρ 2H2 − ρ3H4 )

(Eq. 4-15)

where: H"1 = relative elevation between the tower drawoff nozzle and the bottom of the reboiler, in ft This ∆ P is the driving force for vertical reboilers. Friction losses should be smaller than this. The minimum height between the liquid drawoff nozzle and the vapor return nozzle is 3 feet. Therefore, the minimum drawoff nozzle elevation will be as shown in Equation 4-16: H "1 =

144∆P − ρ 2 (H4 + 3) + ρ3H4 , no safety factor has been included ρ1 − ρ2

(Eq. 4-16)

Once more the total system loss ( ∆ P), known densities, and exchanger length (H 4) will give the distance between the drawoff nozzle and the reboiler bottom tubesheet (H" 1). 5.0

PROCESS SPECIFICATION

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The process engineer is responsible for selecting the type of Reboiler required for the service. The process specification shall include the type, either by indication the type on the data sheet or better include a sketch, examples figure 1 through 6 , depicting the type of boiler specified.