Thermosiphon Reboilers

1. Purpose

This procedure provides guidelines for creating a hydraulic circuit in NHP only for a horizontal thermosiphon reboiler.

2. General

Detailed hydraulics for a horizontal thermosiphon reboiler are calculated by NHP. Thermosiphon reboiler hydraulics establish the elevation difference needed between the fractionator and the reboiler in order to maintain a desired reboiler circulation rate. If the distance between the vessel tangent and the centerline of the reboiler is too low, then the circulation rate will be lower than expected, causing too much vaporization and poor efficiency of the exchanger. If the distance between the vessel tangent and the centerline of the reboiler is too high, the circulation rate could be much higher than expected which can cause excessive velocities through the piping and add unnecessary expense to the cost of the installed plant. The hydraulics of a thermosiphon reboiler are based on a hydraulic balance between the static head of liquid to the reboiler against the friction loss and static head of two phase material returning back to the column. The higher density of the liquid to the reboiler provides the driving force for circulation into the reboiler, and the heat input, which vaporizes part of the liquid, creates the lower density two phase mixture returning to the column. The target weight percent vaporization is 33 percent. But since the flow is not controlled, the design of the system determines how close the actual operation approaches the desired percent of liquid and vapor at the given heat input. The variables available to adjust in the hydraulic design are the line sizes, nozzle arrangement/sizes, and elevation of the reboiler relative to the fractionator.

Mechanical clearance of the reboiler and its piping and the relative elevation of the reboiler to the column need to be checked to see if it is the governing consideration for determining the column skirt height.

Reference Procedure ”, for basic background information and design philosophy.

3. Setting Up Horizontal Thermosiphon Reboiler circuit

Determine the reboiler configuration and elevation using NHP. The line sizes and configuration can be easily varied and the results can be immediately reviewed to interactively converge on an optimum design. NHP will calculate the circuit hydraulics, including reboiler entrance and exit losses, friction losses, and static head driving force. The vessel elevation will be manually entered and the reboiler elevation will be calculated based on necessary static head driving force required. After creating the reboiler circuit in NHP, NHP will calculate the equipment inlet pressures and then export the reboiler into ABE.

If Tonawanda completed detailed sizing of the reboiler, the Tonawanda criteria override the design engineer’s design criteria. The design criteria specified by Tonawanda should be entered in the hydraulics program.

In the New Hydraulics Program, enter the following:

• 3.1 Before beginning, make sure that the vessel and cold side of the reboiler connectivity in the simulation match Figure 1. The connectivity must be vessel-line-splitter-line-exchanger-line-vessel. If not, modify the connectivity manually in the simulation and/or NHP.
• 3.2 Manually enter the vessel elevation on the vessel form. Make sure that this elevation satisfies any NPSH requirements.
• 3.3 Open the exchanger form. Select “Horizontal Reboiler” in the “TSR Type” combo box. It does not matter if this is selected from the hot side or cold side of the exchanger.
• 3.4 Select one of the nine reboiler configurations.
• If not sure which configuration to use, start with the top left configuration (one inlet line/nozzle and one outlet line/nozzle).
• A single column outlet nozzle should normally be used to feed multiple thermosiphon reboilers. On rare occasions, a customer request or other special situation may justify the use of separate column outlet nozzles.
• Separate column return nozzles are always required when multiple thermosiphon reboilers are used.

• 3.5 Select the button “Create Circuit”.
• 3.6 Enter the number of reboiler shells in parallel. This step is important since NHP will automatically set the stream factors for all pipes.

• 3.7 Select the CreateCircuit button.

• 3.8 Hit Ok button.
• 3.9 When NHP creates the reboiler circuit, the following occurs:
• The circuit starts at the column, includes the total bottoms stream, goes through the exchanger and back to the column. Attachment 1, Figures 1 and 2 provide examples of the thermosiphon reboiler circuit with a hydraulic tabulation. The float element in the circuit is the contingency.
• If multiple reboiler inlets and/or outlets are required, NHP will build these branches into the reboiler hydraulic circuit with the appropriate splitters and mixers, and flow factors for each division of the reboiler flow into parallel branches. If multiple nozzles are required on the outlet of the reboiler, NHP will make sure that the mixer is elevated above the reboiler shell.
• 3.10 On the Cold side of the exchanger, go to the “Pressure Drop” tab and enter a “Maximum Allowable Pressure Drop” of 0.35 psi for the bundle.

• 3.11 On the Thermosiphon Reboiler\Vessel Results tab, the “Recommended Contingency” default value is 0 psi. A different value can be entered upon request by the customer.

• 3.12 Go to the Thermosiphon Reboiler\Reboiler Information tab; manually enter the Shell Diameter from the .
• 3.13 Select the “Yes” radio button if the hot fluid is a condensing vapor on condensate flow control.
• Enter the hot fluid outlet pipe size.
• Enter the hot fluid outlet specific gravity.
• 3.14 NHP will calculate the two phase density taking into account liquid slippage. The user can manually enter a different two density to be used in the calculations, if required.

• 3.15 Keep the default Start\End Pressure Instruction set at “Previous”.
• 3.16 Run the reboiler circuit.

4. Running Horizontal Thermosiphon Reboiler circuit

• While running the circuit, NHP will perform the following calculations:

• 4.1 NHP will automatically set the reboiler elevation such that the distance from the column vessel tangent line to the reboiler centerline will achieve the desired circulation rate. The Net Static Head value will balance the Friction + Contingency (typically 0 psi) value. See Appendix A for formulas. NHP will issue a warning message when the distance between the fractionator tangent line and the reboiler centerline is not at its recommended value.

• 4.2 NHP will calculate the reboiler nozzle entrance and exit losses. See Appendix A for formulas.

• 4.3 NHP will calculate the thermosiphon reboiler hydraulics at 100% of the process flow rate for the normal case only.

• 4.4 NHP will use the column pressure calculated in the bottoms product circuit. NHP will then calculate the hydraulic profile from the column through the reboiler circuit, and back to the column and the correct inlet pressure shall appear on the exchanger specification sheet.

• 4.5 The static head in the reboiler outlet piping does take into account liquid slippage.

• 4.6 Providing the data was entered, NHP will determine if there is a need to account for additional elevation required for the reboiler due to tube side hydraulic requirements for steam condensate going to a flow meter.

5. Evaluating the Horizontal Thermosiphon Reboiler Circuit Results

5.1 The total of the entrance, bundle and exit losses should be less than 1.0 psi. If greater, then the number of inlets and/or outlets should be increased to decrease the entrance and exit losses. Delete the reboiler circuit and create a new circuit with a different configuration.

• 5.2 Make sure that the actual reboiler elevation is greater than the two calculated minimum reboiler elevations (reboiler inlet piping and condensing vapors). With an orifice on the bubble point condensate, it is necessary to elevate the reboiler high enough to ensure that the static head on the orifice plate is sufficient to exceed the pressure loss across the plate to prevent flashing.
• 5.3 The calculated contingency will equal the “Recommended Contingency” for only the governing case.
• 5.4 For the reboiler shell, the mechanical size limit for a nozzle connection is 60% of the reboiler shell diameter. NHP will confirm that the lines connected to the reboiler shell do not exceed 60% of the reboiler shell diameter. If the reboiler return line size exceeds 60% of the bundle diameter, it will be necessary to have two process outlet nozzles on the shell which combine into a common line returning to the column. Delete the reboiler circuit and create a new circuit with a different configuration.
• 5.5 Do not set any of the pipes in the reboiler circuit to “IsDummy”. If the circuit needs to be manipulated manually and a pipe requires zero pressure drop, enter an equivalent length such as 0.0001 ft.
• 5.6 Confirm that the column elevation is greater than the minimum requirement for piping clearance:

6. Deleting the Reboiler Circuit

6.1 Open Reboiler form

6.2 Select Thermosiphon Reboiler tab

6.3 Press the “Delete Circuit” button

7. Horizontal Reboiler with Inlet from a Well

All calculations and circuits are handled the same as a simple horizontal reboiler with one exception. You need to account for the normal liquid level in the reboiler well. Set the “Recommended Contingency” equal to the difference between the reboiler well normal liquid level and the vessel minimum liquid level that is shown at the beginning of the reboiler circuit. Be sure to enter this value on the reboiler form in the units of pressure.

8. Inflection Point Engineering Practices for Vertical Thermosiphon Reboilers

NHP does not do a thorough calculation for the vertical thermosiphon reboilers. Use PC program tools” to determine the reboiler elevation requirements. See Appendix B for detailed calculations. The vertical thermosiphon reboiler work process is listed below:

8.1 NHP

• Confirm that the correct “Pipe Service” has been selected for the reboiler inlet and outlet pipes in NHP.
• Hit the “calculate” button for both inlet and outlet pipes in order to calculate the line size.
• For the reboiler, set the “Side” to “Shell” for the hot side of the exchanger.
• It is not necessary to create a reboiler circuit in NHP.
• When exporting the exchanger from NHP to ABE, check the radio button “Export All Equipments” on the “Select Cases” tab.

8.2 T-401-09 “Vertical Thermosiphon Reboiler Calcs”

• In T-401-09, enter data in the yellow cells on the “Input” tab for sections:
• Reboiler Outlet Data
• Reboiler Inlet Data
• Reboiler and Column Internals Dimensions
• Choose the type of reboiler
• Hit “Calc Dp/100 ft using T-800-02” button
• For now, leave the defaults in section “Reboiler and Column Internals Dimensions”
• Under the “Results Summary”, if the “Available Head (psi)” is not equal to the “Reboiler Press Drop (psi)”, then hit the button “Estimate Total Circulation Rate”
• Determine if the calculated circulation rate through the reboiler is acceptable. If not, see a specialist for other design options.
• After the design is complete, go to the “Results” tab and manually enter the ”Column Bottoms Pressure (Below bottom tray)” around row 82.

8.3 ABE

• Manually enter the calculated “Reboiler Inlet Pressure” and “Reboiler Outlet Pressure” (from T-401-09) in the 401 calc tool in ABE.

Attachment 1 - Figure 1

Appendix A

Horizontal Thermosiphon Reboiler Hydraulics Formulas

• Reboiler Entrance Loss

V = liquid velocity in reboiler entrance nozzle

SGL = liquid specific gravity

• Reboiler Exit Loss

SGm =

V = mixture velocity in reboiler exit nozzle

SGm = average specific gravity of mixture (slippage of liquid is not considered)

• Optimum Distance From Column Bottom Tangent to Centerline of Reboiler

ASSUMPTIONS:

• From the bottom of the reboiler to the centerline, there is only liquid phase.
• From the centerline to the top of the reboiler, there is a two phase mixture.
• The net static head from the centerline of the reboiler and below is zero since the liquid flowing downward and the liquid flowing upward cancel each other out.

LM: minimum liquid level

L: distance from column bottom tangent to centerline of reboiler

H: distance from column bottom tangent to reboiler inlet piping

I: distance from column bottom tangent to reboiler return piping

SGL: specific gravity of the liquid phase

SGm: specific gravity of the 2-phase mixture

P: pressure at top of column

PC: pressure drop of the column

PfromCol: friction losses in piping exiting the column bottom

PToReb: friction losses in piping from the split to the reboiler

Pcont: contingency (typically 0 psi)

PInletNozz: friction losses in reboiler inlet nozzle

D: diameter of reboiler shell

PReb: pressure drop in reboiler shell

POutletNozz: friction losses in reboiler outlet nozzle

PfromReb: friction losses in piping from reboiler to column return nozzle

EQUATION FOR REBOILER CIRCUIT (starting and ending at top of column):

Simplify and solve for L:

where:

PFricLoss = PfromCol+PToReb+Pcont+PInletNozz+PReb+POutletNozz+PfromReb

• Net Static Head

• Nozzle Projection and Flange Thickness

Nominal Pipe size, in	Nozzle Projection, in	Cl. 150 Flange Thick., in
1	6	2.75
1.5	6	2.75
2	7	2.75
3	7	2.75
4	8	3
6	8	3.5
8	8	4
10	10	4
12	10	4.5
14	10	5
16	10	5
18	10	5.5
20	12	5.75
22	12	6
24	12	6
26	12	6
28	12	6
30	12	6
32	12	6
34	12	6
36	12	6
38	12	7.75
40	12	7.75
42	12	7.75

• Minimum Reboiler Elevation For Reboiler Piping Clearance

• Minimum Reboiler Elevation for Condensing Vapors

This section describes how to calculate the minimum reboiler elevation where a bubble point liquid is being pressured out of the hot side of a reboiler on flow control.

The static head on the orifice plate, represented as dimension "O", must be sufficient to exceed the pressure loss across the orifice plate to prevent flashing.

Typically, an orifice plate is calibrated for 50 inches of water. If the orifice plate was in condensing hydrocarbon service, the static head requirement would become 50 inches/SG of flowing liquid. This assumes no subcooling, no pressure recovery, and maximum flow rate and is conservative. The ΔP/100 ft should not exceed 0.15 psi and the total frictional pressure drop in the line to the orifice plate should not exceed 0.1 psi.

Inflection Point Engineering specifies the dimension "O" on the P&ID as 5 ft (1500 mm) for condensing services with a SG greater than 0.85. For specific gravity values less than 0.85, the dimension “O” must be increased to provide a minimum 1.8 psi of static head.

Appendix B Vertical Thermosiphon Reboiler Hydraulics Calculation Method

The Vertical Thermosiphon Reboiler Hydraulics Calculations are performed by Tool . (Note – This method does not apply to High Flux vertical thermosiphons due to differences in standard tube sizes and the tubeside pressure drop of the High Flux boiling surface. Inflection Point Engineering Tonawanda has special tools to evaluate hydraulics for vertical High Flux reboilers)

a. Static Head Driving Force

Note: All diameters and dimensions are in inches

BTh - Height of Blind Tray (from Computer Program P254.6 Vessel Design)

Df - Diameter of fractionator

Di - Diameter of reboiler inlet nozzle

Do - Diameter of reboiler outlet nozzle

L - Height of liquid level in drawoff well to reboiler tube inlet or Static Head Driving Force (for recirculating and preferential types only)

- Static Head Driving Force (for Absolute Once-Through type)

Tl - Tube Length (from Heat Exchanger Specialist)

Wh - Height of drawoff well (calculated)

L = Do + Tl – (0.5 Do + 24 + 0.05 Df + 9)

L = 0.5 Do + Tl – 0.05 Df –33

Wh = BTh – 0.5Di - 5

= Tl + (0.5Do + 24) – 0.5 Wh

Appendix B Vertical Thermosiphon Reboiler Hydraulics

Calculation Method (continued)

b. Column Exit Loss

Pipe I.D. = feet

V1 = liquid velocity in column exit pipe, ft/s

SG1 = liquid specific gravity in column exit pipe

CSA1 = 0.785(pipe ID)2 = ______ ft2

= ______ psi

c. Column Entrance Loss

V2 = velocity of mixture in column entrance pipe, ft/s

SGm= average specific gravity of mixture

CSA2 = 0.785 (pipe ID)2 = ______ ft2

= ______ psi

d. Friction Loss to Reboiler

= ______ psi

Notes:

This equation assumes constant line diameter from column to reboiler with a single reboiler inlet nozzle.

Appendix B Vertical Thermosiphon Reboiler Hydraulics

Calculation Method

(continued)

e. Friction Loss to Column

= ______ psi

Notes:

This equation assumes constant line diameter from reboiler to column with a single reboiler outlet nozzle.

f. Design Margin (Sum of piping friction loss)

Normally d + e

No design margin for absolute once-through type reboilers (f = 0) = ______ psi

g. Total Pressure Piping Loss

b + c + d + e + f = R (Excluding reboiler) = ______ psi

h. Static Head Driving Force

Note: Use L or from part a. in inches, as appropriate.

= ______ psi

i. Pressure Drop Available for Reboiler

Sh – R = ( ) - ( ) = ______ psi

Appendix B Vertical Thermosiphon Reboiler Hydraulics

Calculation Method

(continued)

j. Total Reboiler Pressure Loss

V3 = velocity of liquid in reboiler entrance pipe, ft/s

CSA3= entrance nozzle cross section area, ft2

= ______ psi

V4= velocity of mixture in reboiler exit pipe, ft/s

CSA4= exit nozzle cross section area, ft2

= ______ psi

(Exit Loss should be less than 0.5 psi)

Tube Loss:

Tube Length, ft	Tube Loss, psi (frictional, momentum, and static head losses only; does not include entrance and exit losses)
12	2
16	3
20	4

= ______ psi

Total Reboiler Loss = ______ psi

Verify that the pressure drop available for reboiler (i) is greater or equal to the estimated total reboiler pressure loss (j).