Design Specification of Thermosiphon Reboilers

1. Table of Contents

1. Table of Contents 1

2. Purpose 1

3. General 2

4. Shell and Tube Arrangement 2

4.1 Horizontal Thermosiphon 2

4.2 Vertical Thermosiphon 3

4.3 Stabbed-in 4

4.4 Immersed Kettle 5

4.5 Forced Circulation 6

5. Reboiler Type Selection 6

5.1 Reboiler Types 6

5.2 Selection Criteria 6

5.3 High FluxTM Tubing 7

6. Reboiler Hydraulics 9

6.1 Horizontal Thermosiphon 9

6.2 Vertical Thermosiphon 11

6.3 Kettle 12

7. Significance of Pressure for Thermosiphon Reboilers 13

7.1 Effects of low pressure on boiling heat transfer 13

7.2 Thermosiphon reboiler heat release curves 14

7.3 Specifying physical properties and enthalpy data 14

8. Revamp Considerations 15

8.1 Outlet Percent Vaporization 15

8.2 Reboiler Recirculation Rate at revamp conditions 15

8.3 Heat Transfer Coefficient at revamp conditions 16

9. Specification of Reboilers 16

9.1 Maximum Heat Flux Rates 16

9.2 Log Mean Temperature Difference 17

9.3 Strength-Welded Tubes 19

9.4 Design Conditions 19

Figure 1 20

Reboiler Types 20

Attachment 1 Horizontal Thermosiphon Reboiler Hydraulics Calculation Method 22

Attachment 2 Vertical Thermosiphon Reboiler Hydraulics Calculation Method 23

Attachment 3 Minimum Elevation of Reboilers and Fractionators 24

Attachment 4 Method of Feeding Liquid to Reboiler 25

2. Purpose

This procedure describes the Inflection Point Engineering practice and guidelines for the selection and design of thermosiphon reboilers on fractionators. Some guidelines are also given for the design of steam generators and forced circulation reboilers.

3. General

This procedure describes types and selection of shell and tube type thermosiphon reboilers. Design guidelines are provided for the specification and hydraulic design of thermosiphon reboilers and steam generators.

Reference Procedure for Inflection Point Engineering practice for heat exchangers, other than reboilers.

4. Shell and Tube Arrangement

4.1 Horizontal Thermosiphon

• The specified percent vaporization should not exceed 33 weight percent. Do not exceed 33 weight percent vapor to avoid excessive fouling and the risk of unstable operation. The reboiler percent vaporization is confirmed during final hydraulic evaluation based on final piping and exchanger design. See Section 8 for discussion of percent vaporization for revamps.
• The process side is the shell side and the heating medium is on the tube side. The horizontal reboiler is less easily upset by process changes than vertical reboilers.
• Compared with the vertical thermosiphon type, the capital cost may be higher because of more complex piping (a vertical thermosiphon is often directly connected to the column) and larger plot area requirements. However, the exchanger and its foundation are less expensive.
• This system is more easily maintained than the vertical thermosiphon type because it is located near grade, and the tube side is accessible for cleaning. If designed as a removable tube bundle, the shell side (cold side) is also accessible for cleaning.
• This system is better than vertical thermosiphons for thermal expansion when a U-tube bundle is used. TEMA type “S” and “T” return heads also allow for thermal expansion and bundle removal for cleaning. The U-tube design is the least expensive return head since the exchanger only requires a single tubesheet.
• This is the most commonly used reboiler type. Fractionator elevation may be excessive for high pressure fractionators where the density difference between liquid and vapor is low, and a kettle or stabbed-in type may be considered. Horizontal thermosiphons are recommended over vertical thermosiphons for systems operating at pressures higher than 350 psig on the boiling side (such as demethanizer bottoms reboilers).

4.2 Vertical Thermosiphon

• The specified percent vaporization should not exceed 33 weight percent. Do not exceed 33 weight percent vapor to avoid excessive fouling and the risk of unstable operation. The reboiler percent vaporization is confirmed during final hydraulic evaluation based on final piping and exchanger design. See Section 8 for discussion of percent vaporization for revamps.
• The process side is on the tube side, and the heating medium is on the shell side. The process side is not as easily fouled as other reboiler types because of the higher tube velocities.
• The hot side (heating medium in the shell) pressure drop is normally lower than in a horizontal thermosiphon (where the heating medium is in the tubes). This low pressure drop design makes vertical thermosiphons the preferred choice for the heat integrated Parex Unit reboilers.
• The radial tubeside outlet nozzle elevation is normally the same as the reboiler return nozzle on the fractionator, with a short straight run of piping making a direct connection, which minimizes pressure drop. Axial outlet nozzles are occasionally required to minimize pressure drop or increase the liquid driving force to meet outlet vapor fraction guidelines. Axial return nozzles may require additional column elevation.
• Maintenance and cleaning may be difficult because of the vertical arrangement of tube bundle and fixed tubesheet design. However, the tubeside is accessible for mechanical cleaning in place, and if the channel covers are removable (as in TEMA ‘N’ type), the piping does not need to be disturbed.
• Consider for use with clean heating medium in the shell side such as C3 Splitters, Parex Raffinate and Extract Columns, and steam heated reboilers.
• The vertical shell arrangement may require more skirt height, but requires less plot space than a horizontal thermosiphon.
• This arrangement often uses a drawoff well in the fractionator to provide a constant head for the liquid driving force and to reduce the required tangent length or skirt height of the fractionator. Because of the close coupling and drawoff well length, the fractionator tangent length and cost are increased; but foundation, structural, and plot requirements and costs are reduced.

A drawoff well does not allow a controllable liquid level that can be used to help solve reboiler operating problems. Furthermore, a drawoff well could be a problem due to the possibility of concentrating heavies and causing a temperature pinch. Fouling material may become trapped in the reboiler circuit when a drawoff well is used. A drawoff well may also complicate the locating of return nozzles, especially if multiple reboiler returns are required.

• For C3 Splitters, the drawoff well is not recommended due to the reasons stated in the previous point and because of very high liquid loadings.
• Vertical thermosiphon reboilers may encounter problems with stripping light materials from the reboiler liquid when specified for fluids with wide boiling ranges (boiling range greater than 50 °C). Horizontal thermosiphon reboilers should be considered for these services.
• Vertical thermosiphon reboilers my encounter wall dryout or film boiling at large temperature differences between the heating medium and the boiling fluid. Horizontal thermosiphon reboilers should be considered for services with temperature differences greater than 60 °C.

4.3 Stabbed-in

• This reboiler is internal to the fractionator. The percent vaporization is normally limited to a maximum of 90 weight percent to avoid excessive entrainment and buildup of debris.
• The process side is the shell side and the heating medium is on the tube side. The process side residence time may be high, which could lead to reboiler fouling. The net bottoms surge capacity is higher than the kettle type.
• The capital cost is lower, since a shell and process side piping are not required. Fewer flanged connections are required.
• A large reboiler nozzle is typically required. If the nozzle diameter is 30 inches or greater, a TEMA C-type head is recommended to avoid potential leakage problems.
• Limited tube length may result in poorer overall heat transfer rates than can be obtained in external exchangers. This limitation can be especially important in hot oil driven applications. Multiple tube passes may be required to achieve acceptable reboiler performance.
• This reboiler arrangement should be evaluated as a cross-flow type design (TEMA X-type design) as well as a kettle (TEMA K-type design), especially when a tub is used, to confirm the calculated heat transfer coefficient.
• Avoid using a tub when high heat flux or high vaporization designs are required. A tub will restrict the ability of column bottoms liquid to recirculate to the bundle and may keep the bundle from staying fully wetted.
• Stabbed-in reboilers may limit future revamp options due to space limitations and hydraulic limits of low liquid inventory.
• Reference Procedure for Inflection Point Engineering practices for stabbed-in reboilers.

4.4 Immersed Kettle

• This reboiler is external to the fractionator. The percent vaporization is normally limited to a maximum of 90 weight percent to avoid excessive entrainment and buildup of debris. Complete vaporization (100 weight percent vaporizations) is acceptable for systems known to be very clean such as boiling ethylene or propylene refrigerants.
• The process side is the shell side, and the heating medium is on the tube side. The process side residence time is high, which may lead to fouling. The net bottoms surge capacity is low, and the kettle needs a large diameter (1.4 to 2.0 times the bundle diameter) for disengagement of liquid.
• Kettle reboilers can be designed on liquid level control or with an overflow weir to maintain fixed liquid level. The weir height and the top of bundle dimensions are set at the vessel tangent line elevation to maintain tube immersion. The system does not require a large hydrostatic driving force.
• This is the most expensive type of reboiler, but kettle reboilers allow simple column internals and require the lowest fractionator elevation. With practically no static head, it is good for low pressure and vacuum service.
• This system may be considered where the density difference between liquid and vapor is low, or when the fractionator diameter is too small, despite swaging, to accommodate the stabbed-in type of reboiler, or when the customer does not allow stabbed-in reboilers.

4.5 Forced Circulation

• This system allows circulation independently of the liquid hydrostatic head. It is used on all fired heater reboilers and, to a limited extent, on revamps of thermosiphon reboilers and on viscous services.
• Typically the cold side fluid heat transfer becomes sensible liquid heating instead of vaporization. Vapor is generated when the heated process fluid is returned (flashed) back to the column pressure.
• Forced Circulation reboilers may be required for vacuum systems to achieve an acceptable reboiler design.

5. Reboiler Type Selection

5.1 Reboiler Types

Because of the restricted application to thermosiphon reboilers, the forced circulation method of feeding the reboiler is not considered further in this procedure. The advantages and disadvantages of the various reboiler systems discussed in Section 4 and Attachment 4 lead to the following types of reboilers being available for consideration:

• Thermosiphon (Horizontal and Vertical)—referred to as "simple"
• Absolute once-through (Horizontal and Vertical)
• Preferential once-through (Horizontal and Vertical)
• Immersed (Kettle and Stabbed-in)

Figure 1 shows simplified diagrams of each reboiler type, along with the forced circulation type for completeness.

5.2 Selection Criteria

Unless there are overriding process advantages for other types of reboiler or owner preferences, use the simple horizontal thermosiphon reboiler fed by bottoms liquid.

In situations where either the horizontal reboiler or the vertical reboiler may be used, select the horizontal for the following reasons:

In most cases the fractionator tangent length requirements are reduced.

Maintenance and cleaning are easier.

Inflection Point Engineering may consider vertical reboilers in situations where there is:

• Low available hot-side pressure drop (Parex Raffinate and Extract reboilers)
• High fouling boiling stream
• Limited plot space available

5.3 High FluxTM Tubing

Inflection Point Engineering offers High Flux Tubing through its PTE Separation & Heat Transfer Technology Group in , as a product for use in boiling services for various petrochemical and refinery applications where there are relatively clean boiling hydrocarbons in the C1 to C10 boiling range. The use of High Flux Tubing will typically increase the overall heat transfer coefficient by a factor of two to four over the conventional bare tube reboiler. When a large amount of surface is required for reboiling, (typically greater than 5000 ft2), High Flux Tubing may be used to reduce the surface area requirements and/or the number of reboiler shells required. The coating used to promote the higher overall heat transfer rate is applied to the surface where boiling is taking place. The tubing may, therefore, be ID or OD coated. When required, the High Flux boiling surface can be combined with an enhanced condensing surface such as ID finning (horizontal exchangers) or OD fluting (vertical exchangers). For more information about High Flux Tubing, contact a Heat Exchanger Specialist.

When High Flux Tubing is to be specified for a reboiler, contact the PTE Separation & Heat Transfer Technology Group in to coordinate the design of the reboiler and confirmation of the hydraulics. It is important that receives all revisions of the appropriate parts of the 401 Specification as soon as they are issued (or earlier) so that they can be best prepared to answer customer and contractor queries.

If reboiler hydraulic evaluation is required, should also be provided with a copy of the 301 and 307 Specification showing arrangement of the column bottoms and available elevations. It may be necessary to adjust elevations to satisfy the reboiler hydraulics.

a. Use in New Process Units

The following table is a partial list of reboilers where High Flux Tubing has been used. Decide whether or not to use High Flux Tubing during the Design Basis Phase, before the Design Phase starts. Consult the Technology Specialist for a particular process to see if High Flux Tubing is applicable.

Process Unit	Services	TEMA Type	OHTC (Btu/h-ft2-F)
BTX Fractionation and other Aromatics Units	Xylene Column Steam Generation Other Steam Generators (Benzene Column, Extract Column, Toluene Column, etc.) Benzene Column Reboiler Reb (hot oil driven) Reb (steam driven) Reformate Splitter Reboiler Toluene Column Reboiler Xylene Column Reboiler	H-AKU H-BKU V-NEN H-BHU H-BHU V-NEM H-AHU V-NEM H-AHU V-NEM	270 270 280 145 350 285 190 350 300 300
Parex	Raffinate and Extract Columns Finishing Col Reb (hydrocarbon) Finishing Col Reb (steam driven)	V-NEN H- H-	265 180 350
Oleflex / FCC	C3 Splitter	H-BHM	265
Recovery Plus	Chiller (liquid or vapor)	H-BKU	200

b. Use in Revamps

(1) High Flux Tubing has been shown to have good application in revamps, especially where its use keeps the shell, foundation, and piping from being replaced. High Flux Tubing has been successfully used in revamps for process units such as Parex, BTX Fractionation, Ethylene Plants, and naphtha splitters. Other refinery experience includes reboilers for DeC3, DeC4, DeC5, DeC6, DIH, Paraffin Stripper, Product Column, Recycle Column, Rerun Column, and Stabilizer Column. Consider using High Flux Tubing in revamps whenever a reboiler shell needs to be replaced, or when a stabbed-in bundle is limiting. Discuss potential applications with the Heat Exchanger Specialists and with .

(2) Tonawanda also has tools to check all other design limitations encountered in revamps such as nozzle limitations, thermosiphon hydraulics, instruments, pressure drops, bundle critical heat flux, etc., and will provide a TEMA specification sheet with a guaranteed thermal design. can provide guidelines for High Flux exchanger fabrication, storage, as well as recommendations for chemical cleaning.

6. Reboiler Hydraulics

6.1 Horizontal Thermosiphon

a. Calculation Method

The Horizontal Thermosiphon Reboiler Hydraulics Calculations are performed by NHP. A typical Overall Heat Transfer Coefficient (U-Value) for light hydrocarbon reboilers is 120 Btu/h-ft2-°F using steam as a heating medium, and 80-90 for hot oil heating mediums. Refer to the Process Specific Guidelines in the Documentation System for more specific U-Values. Consult the maximum heat flux criteria given in Section 9.1.

b. Elevations and Dimensions

Reference Attachment 3 for the minimum elevations of reboilers and fractionators. Estimate the reboiler shell diameter for large duties and use BEDQ information, such as maximum allowable bundle diameter or weight, to determine if multiple reboiler shells are required. If multiple reboiler shells are required, evaluate the use of High Flux Tubing to reduce the number of shells. On some systems, such as Amine Regenerators, it may be necessary to elevate the reboiler above the tangent line to ensure a Regenerator level is maintained.

The static head calculations are based upon the specific gravity of the two-phase reboiler return stream. The NHP calculations take into account liquid slippage in a two-phase vertical pipe, which may be significant.

High Liquid Level (HLL) and Low Liquid Level (LLL) conditions should be checked to verify sufficient MTD at High Liquid Level and sufficient circulation (outlet vapor mass fraction is not too high) at Low Liquid Level.

Reference Procedure for drawoff well velocities, surge volume, and residence times.

c. Pressure Drop

Inflection Point Engineering Standard Specification 4-11, “Tubular Exchangers Shell and Tube Type” requires that the exchanger designer include the exchanger entrance and exit losses in the guaranteed pressure losses. NHP includes equations for calculating this loss so that this number may be included in the total allowable pressure drop. The estimated exchanger bundle pressure loss, exclusive of entrance losses, exit losses, and bundle static head is typically 0.35 psi.

Note that the exchanger pressure drops shown on the 401 Specification for horizontal reboilers include the entrance and exit losses, frictional losses, momentum losses, and the static head losses in the reboiler. This is consistent with the pressure drops calculated by HTRI and other commercial software packages.

As the exchanger size increases, the entrance and exit losses may become a high proportion of the overall pressure losses. The total exchanger pressure loss, including entrance and exit losses, should not exceed 1.0 psi (this does not include bundle static head). If this occurs, consider the use of two exchanger inlets and/or two exchanger outlets as a means of reducing the entrance and exit losses.

Frequently, a reboiler will have two outlet nozzles because of size limitations relative to the reboiler shell. In this case, specify a TEMA type J shell. High Flux horizontal thermosiphon reboilers are commonly specified as TEMA H-type or X-type shells to minimize exchanger pressure drop and improve reboiler circulation. While TEMA X-type shells are acceptable for High Flux horizontal reboilers, they are not recommended for plain tube reboilers that typically depend on convective boiling heat transfer coefficients.

Note - Reboiler nozzle diameters shall not exceed 60 percent of the reboiler shell diameter.

6.2 Vertical Thermosiphon

a. Calculation Method

The method for calculating the vertical thermosiphon reboiler hydraulics is described in the worksheet in Attachment 2 and are performed by Tool . A typical Overall Heat Transfer Coefficient (U-Value) for light hydrocarbon reboilers is 120 Btu/h-ft2-°F using steam as a heating medium, and 80-90 for hot oil heating mediums. Refer to the Process Specific Guidelines in the Documentation System for more specific U-Values. Consult the maximum heat flux criteria given in Section 9.1.

b. Elevations and Dimensions

The reboiler often has a radial (side) outlet nozzle that allows a short, direct connection to the fractionator reboiler return nozzle. Sometimes an axial outlet nozzle is used to allow a larger liquid driving head or reduce return losses. Determine the number of reboilers and the tube length through preliminary reboiler sizing with an estimated U-Value from a Heat Exchanger Specialist. The tube length will normally be 10, 12, or 16 feet. Longer tube lengths may be required for very large duties, but consider this only in conjunction with the available pressure drop (liquid driving head). Typical tube-length to shell diameter (L/D) ratios for vertical reboilers are 3 to 4. If there are multiple vertical tube bundles, consider the number that may be mounted on the fractionator and the required internals to feed the reboilers. If multiple vertical reboiler shells are required, evaluate the use of High Flux Tubing to reduce the number of shells.

Complete the design of the bottom section of the fractionator, using appropriate residence time requirements for net bottoms and a maximum velocity of 0.5 ft/s in the reboiler drawoff well. The height of the drawoff well is fixed by the design of the fractionator internals. See Procedure for more information. Reference Attachment 3 for the minimum elevations of reboilers and fractionators.

Determine the static head driving force, i.e., the liquid height from the bottom of the reboiler tubes to the top of the liquid level in the drawoff well. Note that for preferential once-through and simple vertical thermosiphon reboilers the liquid is at the top of the drawoff well. For absolute once-through vertical thermosiphon reboilers, assume the liquid level to be at the 50 percent level in the drawoff well for design purposes.

If there is no drawoff well, High Liquid Level (HLL) and Low Liquid Level (LLL) conditions should be checked to verify sufficient MTD at High Liquid Level and sufficient circulation (outlet vapor mass fraction is not too high) at Low Liquid Level.

c. Pressure Drop

Use a design margin equal to the frictional pressure drop in the entrance and exit piping for the simple and preferential reboilers. Consider the remaining 50 percent of the drawoff well height to be the design margin for the absolute once-through reboilers. The static head driving force minus the resistance in the circuit equals the pressure drop available for the reboiler. Consult the Heat Exchanger Specialist to see if the available pressure drop is reasonable. It may be necessary for the Heat Exchanger Specialist to run the HTRI computer program to confirm if the available pressure drop is adequate.

Note that the exchanger pressure drops shown on the 401 Specification for vertical reboilers include the entrance and exit losses, tube frictional losses, momentum losses, and the static head losses in the reboiler. This is consistent with the pressure drops calculated by HTRI and other commercial software packages.

Check the exchanger exit pressure loss to ensure that the correct exit nozzle size has been selected. The exchanger exit pressure loss will typically not exceed 0.5 psi. For large size reboilers, where line size limitations require a higher exit loss, make sure that the exit nozzle cross-sectional area exceeds the total tube cross sectional area.

6.3 Kettle

a. Calculation Method

Check the hydraulics. Consult the maximum heat flux criteria given in Section 9.1.

b. Elevations and Dimensions

Set the weir at the tangent line of the vessel. Set the reboiler return line a minimum of 8 feet above the vessel tangent line. Reference Attachment 3 for the minimum elevations of reboilers and fractionators.

7. Significance of Pressure for Thermosiphon Reboilers

Systems with operating pressure below 30 psig on the boiling side are considered low pressure services. Special care is needed in properly specifying and designing thermosiphon reboilers at low pressure. However, the principles discussed here apply in varying degrees to thermosiphon reboilers at higher pressures.

7.1 Effects of low pressure on boiling heat transfer

Pressure has an important effect on the boiling of a fluid. At low pressures, typically less than 30 psig, the boiling temperature is sensitive to the pressure of the fluid. As the pressure decreases into vacuum, the boiling temperature sensitivity increases further. The vapor density also changes rapidly with typical reboiler pressure drops at low pressures. These changes can have significant effects on the proper design and operation of a thermosiphon reboiler.

Thermosiphon reboilers circulate the cold reboiling stream via the natural circulation that occurs due to the density difference between the liquid head in the column and the less dense two-phase stream in the reboiler and return piping. Below the bottom tray, at the top of the liquid level in the column, the liquid is saturated. At the exchanger inlet, the pressure is higher but the temperature is the same; therefore the liquid is subcooled. For low pressure reboilers, the boiling point rise due to this increased pressure can cause the subcooling to be significant.

The inlet pressure to a thermosiphon reboiler has two effects on heat transfer. The boiling temperature rise reduces the mean temperature difference available to drive heat transfer (lower MTD). The subcooled liquid also reduces the heat transfer coefficient at the inlet of the heat exchanger due to a sensible liquid heat transfer zone (single phase heat transfer). At low pressures, these effects are significant.

Both the MTD and the heat transfer coefficient are functions of the local pressures within the reboiler. The pressure drop within the heat exchanger, which is the total of static, friction, and momentum losses, and is a strong function of local density, may not be linear. Therefore, it is important to accurately determine the inlet pressure to the reboiler and provide relevant physical property, vaporization, and enthalpy information that bound the actual operating pressures of the reboiler.

7.2 Thermosiphon reboiler heat release curves

Historically, composite heat release curves (and physical property data), where the pressure gradient is included and assumed linear with the enthalpy change, were provided in Inflection Point Engineering heat exchanger specifications. At most pressures, where the pressure drop was a very small fraction of the absolute pressure, and for single phase fluids, this was an acceptable assumption.

For a low pressure thermosiphon reboiler stream, the pressure change included in a composite heat release curve causes an unusual result. As heat is transferred and the subcooled stream travels within the reboiler, the fluid temperature increases due to sensible heating and the pressure decreases due to the pressure drop of the reboiler. At some point the bubble point temperature is reached and true boiling starts. Depending on the composition of the bottoms fluid, as the pressure continues to decrease within the reboiler, the boiling temperature may correspondingly decrease, even though heat is being added. This causes the characteristic “hump” often seen in the heat release curve of low pressure thermosiphon reboilers.

Systems with designs for very small MTDs (less than 30 °F) are very sensitive to changes in boiling temperature. For example, a 2.0 °F increase in boiling temperature due to increased static head will result in a 7 percent reduction in reboiler performance. Care must be taken to ensure these sensitive systems are properly specified and modeled.

7.3 Specifying physical properties and enthalpy data

It is clearly wrong to model the pressure drop of a thermosiphon reboiler as zero by using the flawed simplification that the inlet and outlet pressure can both be considered to be the column pressure below the bottom tray. This assumption would provide incorrect physical properties and enthalpy data (lacking the subcooled zone) that would lead to incorrect calculations of MTD and heat transfer coefficients. As described in Section 7.1, the inlet pressure to a thermosiphon reboiler is higher than the column bottoms pressure.

Tables of isobaric physical property and enthalpy data, provided at multiple pressures, are the preferred method for specifying low pressure thermosiphon reboilers. This information allows the exchanger designer (and design software) to accurately determine the expected subcooling in the reboiler and properly calculate the MTD and local heat transfer coefficients for the design of the reboiler. Commercially available heat exchanger design software requires isobaric data to correctly handle the hump that occurs in a composite (pressure and enthalpy changing) heat release curve. These tables allow the software to interpolate for enthalpy or physical properties based on independent changes in temperature and pressure, permitting all local conditions to be properly determined.

For all reboilers, three isobaric physical property and enthalpy tables should be specified that bound the reboiler operating conditions. The highest pressure table should be at the pressure of the inlet piping upstream of the reboiler inlet. The lowest pressure table should include the pressure below the bottom tray (at the reboiler return). Some margin should be included in the selection of the maximum and minimum pressures such that operating pressure changes due to detailed hydraulic calculations and all operating liquid levels can be accommodated. The middle pressure table should be selected closer to the expected reboiler inlet pressure, at approximately 1/5 of the span between the inlet and outlet pressures. For non-vacuum reboilers, the mean pressure is an acceptable middle table pressure. For more information, including tools and methods for placing the proper information on the heat exchanger specification, or if there is some question whether multiple isobaric curves are necessary, consult a Heat Exchanger Specialist.

8. Revamp Considerations

8.1 Outlet Percent Vaporization

The maximum thermosiphon reboiler outlet vaporization for revamp conditions is 50 weight percent for hydrocarbon (non-aqueous) reboiler services (excluding kettles and stabbed-in reboilers which allow much higher vaporizations). Above this value dry out, unstable operation, and fouling become likely. An additional criterion that should be considered is to limit the vapor volume fraction to no more than 85-95 percent at the reboiler outlet (~10 weight percent for aqueous services). It is important to check the vapor volume fraction for low pressure and vacuum services.

A typical vaporization of 33 weight percent is recommended for new units (See Section 4; again excluding kettles and stabbed-in reboilers) to allow operational flexibility and to permit some margin for piping, elevation, and heat exchanger design details that may not be well known during the process design.

8.2 Reboiler Recirculation Rate at revamp conditions

Evaluating a thermosiphon reboiler for revamp conditions requires a determination of the circulation rate to calculate the outlet percent vaporization at the new process conditions. This can be done rigorously if the piping isometrics and exchanger vendor datasheet or fabrication drawings are available.

If sufficient information is unavailable, it is reasonable to estimate that the total circulation rate is the same as the original specification, even though there is a change in heat duty. A change in recirculation rate due to a heat duty change depends on whether the change in static head of the two phase flow in the reboiler circuit is greater or less than the change in frictional losses in the circuit.

An example best illustrates the reasoning, assuming original and revamp physical properties are similar. If the heat duty increases, the vapor mass flow rate generated by the reboiler will increase. If the total circulation rate is constant, the vapor mass fraction at the outlet of the exchanger increases, causing a larger differential between the liquid static head in the column and the static head in the reboiler circuit. The larger driving force for circulation is offset by the larger frictional losses caused by the increased vapor flow in the reboiler circuit, causing the circulation rate to stay similar to the original design, given the same liquid level in the column and similar physical properties. Again, this estimate is based on the assumption that the offsetting changes in static head and friction losses due to the change in heat duty are similar.

8.3 Heat Transfer Coefficient at revamp conditions

The heat transfer coefficient of reboilers is typically controlled by the boiling fluid heat transfer coefficient (except when High Flux Tubing is used). The boiling coefficient is a function of the subcooled, nucleate boiling, and convective boiling heat transfer coefficients. Since the static head driving force and circulation rate are likely constant, changes to the subcooled and convective boiling coefficients are likely small when comparing original to revamp operating conditions. The nucleate boiling coefficient is influenced by the temperature difference between the fluid and the heat source. Therefore, if the MTD is increased, there may be a modest improvement in the overall heat transfer coefficient. If a simulation using detailed heat exchanger design software is not possible, using the original overall heat transfer coefficient for the revamp conditions is a reasonable first guess.

9. Specification of Reboilers

9.1 Maximum Heat Flux Rates

In preliminary sizing of reboilers do not exceed the following heat flux rates in order to prevent vapor blanketing of the tube surfaces. These maximum heat flux values are only a preliminary guide, since the critical heat flux of a tube bundle is a function of the fluid physical properties, process conditions, and the bundle geometry. To determine the critical heat flux for a given reboiler, HTRI software as well as literature correlations are available; see an Exchanger Specialist for more information.

Hot oil heated reboilers have a lower maximum design heat flux because of the hot oil temperature gradient (from inlet to outlet), which causes high localized heat flux at the inlet.

Reference Section 9.2.a for information on minimum heat flux rates. Also, when boiling is on the shell side, the tube pattern shall be square pitch, except for low MTD or heat flux services such as C3 Splitter Column Reboilers.

Services	Maximum Design Heat Flux Rate (Btu/h-ft2)
Steam Heated Reboiler Hot Oil Heated Reboiler Steam Generator	15,000 12,000 80,000

9.2 Log Mean Temperature Difference

a. Range

Generally, the LMTD is fixed to the extent that heating medium side temperatures, steam pressures, and process side temperatures are all fixed. Inflection Point Engineering specifies reboilers with a LMTD of between 30 and 125 °F as calculated by the procedures outlined in this section. Obviously, the lower the LMTD is, the larger the reboiler will be. This may have a substantial impact on reboiler size, number of reboilers, reboiler type, swaging of the fractionator, etc., in the larger units.

When the LMTD is low, there is some risk that the heat flux will be too low to sustain stable boiling and/or good thermosiphon circulation. In this case, the target minimum heat flux is expected to be about 2500 Btu/h-ft2. Otherwise, increase the temperature of the heating medium to increase LMTD, or the use of High Flux tubing shall be considered.

b. Approach Temperature

Approach temperatures as low as 25F for steam heated reboilers and 50F for hot oil heated reboilers may be used for economical designs. If a lower approach temperature is required, consider the use of High Flux Tubing.

c. Process Side

For the types of reboilers listed below, use the process side outlet temperature as the process side inlet temperature for purposes of calculating LMTD to account for the backmixing of the boiling fluid. Project Specification 401 for the particular reboiler shall carry the following note "Base LMTD calculation on a cold side inlet temperature being equal to the cold side outlet temperature".

• Horizontal Thermosiphon
• Preferential Once-Through (Horizontal)
• Immersed (Kettle and Stabbed-in)

For kettle and stabbed-in reboilers, assume that the tube bundle is flooded with turbulent boiling liquid. Pressure drop through the other types of reboilers in the above list is generally low and back mixing may be significant. This practice provides some design margin and ensures that vendors quote the same MTD. Current heat exchanger design software by HTRI and HTFS properly calculates the back mixed boiling temperature, without user adjustment of the inlet temperature, when calculating the effective mean temperature difference.

For the types of reboilers listed below, use the process side inlet temperature and the process side outlet temperature for purposes of calculating LMTD in the conventional manner.

• Vertical Thermosiphon
• Absolute Once-Through (Horizontal and Vertical)
• Forced Circulation (Horizontal and Vertical)

For vertical reboilers, which are single pass, fixed tubesheet types, use the conventional practice. Also, for horizontal once-through or horizontal forced circulation reboilers with a type E shell, follow the conventional practice for LMTD, provided the pressure drop is increased to 2-5 psi.

d. Heating Medium Side – Hot Oil and Steam Heated Reboilers

When hot oil is the heating medium, thermal expansion effects may cause flange sealing problems for the front head if the hot oil temperature drop is excessive. To allow for throttling of the hot oil flow rate for reboiler control, use a maximum hot oil temperature drop of 125 °F at the process design heat duty. This applies to all horizontal reboilers, including stabbed-in and kettle type reboilers. Vertical thermosiphon reboilers do not have this limitation since the hot oil has a single pass on the shellside.

The thermal design of a hot oil exchanger should also be checked for minimum hot oil flow rates. The tubeside flow can transition to laminar flow (poor heat transfer coefficient) if the velocity drops low enough, or the viscosity of the oil becomes high enough at the outlet conditions. Turndown conditions should be considered when evaluating hot oil driven exchangers. High inlet temperature difference can also lead to local bundle critical heat flux limitations.

Typically, hot oil systems have an LMTD of approximately 75 ºF.

Reference Procedure ” for specifying the LMTD when superheated steam is used as the heating medium.

9.3 Strength-Welded Tubes

Some Inflection Point Engineering processes require strength-welded plus full rolled tube joints in certain reboiler and steam generator services. Reference Procedure for lists of pertinent processes and details.

9.4 Design Conditions

Reference Procedure ” for the methods for establishing design pressure and temperature for reboilers and heat exchangers. Generally, the ten-thirteenths rule does not apply to reboilers; reference Procedure for more information. This is based on the assumption that the heating medium is at a higher pressure than the process stream. Therefore, when a tube rupture occurs, the flow is into the process stream and the column’s relief system is designed for this contingency. If the heating medium is at a lower pressure than the process, the ten-thirteenths rule should be applied to the heating medium side. If the fractionator associated with a particular reboiler is designed for a vacuum condition, design the reboiler for a vacuum condition per Procedure

Figure 1

Reboiler Types

Simple Horizontal Thermosiphon Simple Vertical Thermosiphon

Absolute Once-Through (Horizontal) Absolute Once-Through (Vertical)

Preferential Once-Through (Horizontal) Preferential Once-Through (Vertical)

Figure 1

Reboiler Types

(continued)

Kettle Stabbed-in

Forced Circulation (Horizontal or Vertical)

Attachment 1

Horizontal Thermosiphon Reboiler Hydraulics

Calculation Method

The Horizontal Thermosiphon Reboiler Hydraulics Calculations are performed by NHP. For more information or hand calculation methods, see IPE-TM-510-02, or a Training Specialist in the Training, Systems & Service Skill Center or a Heat Exchanger Specialist.

Attachment 2

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). For more information or hand calculation methods, see IPE-TM-510-02, or a Training Specialist in the Training, Systems & Service Skill Center or a Heat Exchanger Specialist.

Attachment 3

Minimum Elevation of Reboilers and Fractionators

This discussion has been moved to IPE-TM-510-02.

Attachment 4

Method of Feeding Liquid to Reboiler

It is convenient to describe the reboiler systems in terms of how the fractionator bottoms liquid is fed to the reboiler. This is described in this section. Reboiler systems may also be described in terms of the horizontal or vertical shell and tube arrangement of the reboiler as described in Section 4.

a. Bottoms Liquid to Reboiler

• The column bottoms liquid that is fed to the reboiler is at its bubble point.

• The reboiler does not give a theoretical stage and the reboiler vapor is not in equilibrium with the net bottom liquid. The reboiler outlet temperature is higher than the net bottoms temperature unless the bottoms is a pure component.

• The arrangement is simple and requires no internals. If this system is used to feed vertical thermosiphon reboilers, a blind tray and drawoff well are typically used to minimize column length.

• This arrangement, with no drawoff well, is preferred for horizontal reboilers.

b. Bottom Tray Liquid to Reboiler (absolute once-through thermosiphon)

• The bottom tray liquid is fed to the reboiler at its bubble point on an once-through basis.

• The reboiler gives one theoretical stage and the reboiler vapor is in equilibrium with the net bottom liquid. The average reboiler boiling temperature is lowest for this arrangement.

• This system requires a drawoff well and a collector tray. The head in the drawoff well varies which changes the hydrostatic driving force through the reboiler. Start-up is more difficult because liquid may not be returned to the well without a pump.

• Similar reboiler types: kettle, stabbed-in

Attachment 4

Method of Feeding Liquid to Reboiler

(continued)

c. Mixed Bottom Liquid to Reboiler (preferential once-through thermosiphon)

• The bottom tray liquid is preferentially fed to the reboiler at its bubble point. Depending on the circulation rate, some reboiler outlet liquid is recirculated. The reboiler outlet temperature is the net bottoms bubble point temperature but the inlet temperature depends upon the amount of recirculation.

• The internal baffling provides a constant head for the hydrostatic driving force through the reboiler.

• This system requires a drawoff well, collector tray and blind tray.

d. Average Temperature Comparison

The following table shows the inlet and outlet temperatures for a debutanizer reboiler operating at 200 psig on a Unicracking Process Unit with the three different methods of feeding the bottoms liquid to the thermosiphon reboiler.

Method of Feeding Reboiler Liquid	a	b	c
Percent Vaporization	33.0	47.2*	33.0
Reboiler Temperature F
Inlet	594	479	510
Outlet	668*	594	594
Average of Inlet and Outlet	631	536	552

Note: The numbers shown in this example are for example purposes only and the numbers marked with a * are outside the range of normal Inflection Point Engineering criteria. The number of 47.2 percent vaporization and 668F are in this category. A fired heater would normally be used for this application. For once-through thermosiphon reboilers a percent vaporization higher than 33 percent may be used. A percent vaporization up to 45 percent may be used if a higher than usual reboiler pressure drop is specified. Also, the percent vaporization for a once-through system is determined by the reboiler duty and not by the hydraulics. Additionally, Inflection Point Engineering would normally limit a thermosiphon reboiler outlet temperature to 630F maximum to fit available hot oil system temperatures.