IPE-TM-400 Heat Exchangers
Stabbedin Reboilers
IPE-TM-400-02
1. Table of Contents
2. Purpose
This procedure emphasizes the inherent safety and economic aspects of stabbed-in reboilers and presents design guidelines thereby promoting the use of this type of heat exchanger.
3. Description
A stabbed-in reboiler is a U-tube bundle installed into a column through a nozzle in the shell. Attached to the tubesheet and column nozzle by bolts is a channel with a removable cover for hot oil heating or a bonnet with an integral cover for steam heating. The tube bundle is submerged in a tub with an overflow weir above the top of the tube bundle and parallel to the axis of the tube bundle. A collector tray diverts liquid from the bottom contacting tray through a chordal downcomer, parallel to the tube bundle, into the bottom of the reboiler tub. This liquid flows across the tubes, the vapor rises, and the unvaporized liquid flows over the weir into the bottom of the column. See Inflection Point Engineering Standard Drawings 3-270, 3-272, 3-346, 3-348, and 3-350 to 3-353 for more information.
4. Guidelines for Use
Inflection Point Engineering practice is to select whatever type of reboiler is appropriate for a given process service. Typically, horizontal thermosiphon reboilers are used unless there are overriding process advantages for other types of reboilers or Owner preferences. Reference Procedure ” for more information and for Inflection Point Engineering practice and guidelines for reboilers.
Stabbed-in reboilers have been used in limited applications in some process units. A list of typical past applications is given in Attachment 1. In general, consider a stabbed-in reboiler in any application where a horizontal thermosiphon reboiler may be used, provided, that from a practical viewpoint, it can be installed in the processing vessel as outlined below:
Determine Column Nozzle Diameter
Determine the column nozzle diameter necessary to accommodate the tube bundle using and described in Design Manual . The initial input for tube length shall be the inside diameter of the column at the location the tube bundle will be installed. (The effective tube length in the neck of the nozzle is approximately equal to the clearance between the end of the tube straight length and the vessel shell.) After an initial bundle diameter has been determined, revise the tube length based on the initial bundle diameter, and recalculate the required bundle diameter using TZ-400-01. The revised tube length equals the column ID + 10 inches - bundle diameter / 2. This procedure assumes that the stabbed-in bundle is located on the column centerline. If the bundle is in an off-center location, then the appropriate cord length must be used to determine the bundle length. See Attachment 3 for typical single bundle and two-bundle stabbed in reboiler arrangements using TEMA “B” type heads.
The procedure described above recommends that the tube length be revised once, based on the radius of the U-bend, which is equal to 1/2 the bundle diameter. Revising or iterating once on the tube length allows a more accurate bundle diameter to be calculated. The equation for the revised tube length comes from geometry of the U-bend, an assumed 12" nozzle projection, and a 2" clearance between the U-bend and the vessel wall.
U-values should be obtained from the appropriate process specific guideline tools. If specific guidelines are not available, estimated U-values for bare tubes are as follows:
for hot oil - 70 Btu/h - ft2 - °F
for steam - 120 Btu/h - ft2 - °F
Use High Flux tubes for clean light service with estimated U-values as follows:
for hot oil - 100 Btu/h - ft2 - °F
for steam - 350 Btu/h - ft2 - °F
Generally, review these U-values with an Exchanger Specialist for specific applications. TZ-400-01 produces the inside diameter for the shell of a shell and tube exchanger. Determine the column nozzle nominal diameter for a stabbed-in tube bundle by adding 2 inches to the inside diameter of the shell of the shell and tube exchanger. If the result is an odd number, increase the nozzle diameter to the next even diameter as listed in Attachment 2.
The nozzle diameter for the stabbed-in tube bundle shall not exceed one-half of the inside diameter of the column at the nozzle location, with a practical maximum limit of 60 inches. Some refiners have maximum limits for bundle diameters and weights. This information shall be available in the Basic Engineering Design Questionnaire (BEDQ) along with their preferred tube diameter.
If the nozzle diameter for the stabbed-in tube bundle exceeds 30” (762mm), then specify a TEMA “C” head. If the diameter is on the cusp of being more than 30”, show the TEMA type front head as “B or C” and show materials of construction for a TEMA “B” head. See Design Manual ” for more information about specifying TEMA “C” heads.
4.2 Consider Swaging the Column
If the nozzle diameter required for a single stabbed-in tube bundle exceeds the limits in Section 4.1, resolve the situation by swaging the bottom of the column. A practical minimum increase in column diameter is 1.5 feet, and a maximum increase is based on engineering judgment. This would permit both a larger tube bundle diameter and length. See Section 5.5.
4.3 Consider Dual Tube Bundles
If the nozzle diameter required for a single stabbed-in tube bundle exceeds the limits in Section 4.1, resolve the situation by using two parallel tube bundles (see Attachment 3), where each is designed for one-half of the total reboiler duty. See Inflection Point Engineering Standard Drawing 3-272 for arrangement details, and see IPE-TM-300-08 and F-PSD-16 for downcomer sizing. Dual stabbed-in tube bundles normally cannot be accommodated in column diameters less than 12 feet. Determine the minimum column diameter as the sum of the following:
Dual Tube Bundles – Minimum Column Diameter Estimation
Side Downcomer Inlet to Reboiler Tub
| Clearance between reboiler nozzle flanges | = | 6 inches |
|---|---|---|
| 2 * (0.5 times reboiler nozzle flange O.D., Attachment 2) | = | nozzle flange O.D. |
| 2 * (0.5 times reboiler nozzle nominal) diameter) | = | nozzle diameter |
| 2 * (2 inch clearance between reboiler nozzle and collector tray downcomer or reboiler tub weir) | = | 4 inches |
| 2 * (chord height of collector tray downcomer, where the liquid velocity does not exceed 0.6 ft/sec. This also equals the chord height of the reboiler tub overflow area.) | = | 2 times the chord height of the collector tray downcomer |
| Minimum column inside diameter | = | nozzle diameter + nozzle flange O.D.+ 2 times the chord height of the collector tray downcomer + 10 inches. |
Dual Tube Bundles – Minimum Column Diameter Estimation
Center Downcomer Inlet to Reboiler Tub
| center downcomer width of collector tray to reboiler tub | = | center downcomer width |
|---|---|---|
| 2 * (reboiler nozzle diameter) | 2 times nozzle diameter | |
| 4 * (2 inch clearance between reboiler nozzle and collector tray downcomer or reboiler tub weir) | = | 8 inches |
| 2 * (chord height of overflow weir locations) | = | 2 times chord height of overflow weir |
| minimum column inside diameter | = | center downcomer width + 2 times nozzle diameter + 2* chord height of overflow weir + 8 inches |
5. Advantages
The advantages of stabbed-in reboilers are as follows:
5.1 Less Piping and Flanges
A stabbed-in reboiler replaces four flanged connections with one large connection, and eliminates the process piping to and from the column and the external reboiler. The reduction in flanged connections reduces the potential for leaks and possible fires, although the one large flange may cause some leakage issues. A second benefit is the elimination of the reboiler piping. Failure of the process piping due to erosion and/or corrosion is considered by some refiners to be a greater potential for catastrophe than the flange leakage.
No Hydraulic Problems
A stabbed-in reboiler eliminates the hydraulic problems inherent with an external horizontal thermosiphon reboiler circuit. As the difference in specific gravity between liquid and vapor decreases, the distance increases between the column bottom tangent and the external reboiler in order to provide the necessary driving force for the circuit. In some situations this requires a higher column elevation than is required to satisfy the net positive suction head (NPSH) for the column net bottoms pump, and in most situations it is greater than the minimum clearance between the column bottom and the foundation. The elimination of the process piping removes all pressure drop and slug flow problems associated with an external reboiler circuit.
Similar to kettle type reboilers, stabbed-in reboilers are often used in vacuum column services where minimal reboiler circuit pressure drops are desirable. Also like kettle type reboilers, the liquid covered bundle arrangement of stabbed-in reboilers allows designs with high outlet vapor fractions (up to approximately 90 weight percent vapor at outlet).
5.3 Reduced Number of Trays
A stabbed-in reboiler is equivalent to one theoretical equilibrium stage. With a stabbed-in reboiler, the reboil vapor is in equilibrium with the net bottoms liquid. With an external thermosiphon reboiler, the reboil vapor is not in equilibrium with the net bottoms liquid but is in equilibrium with the reboil liquid, which is heavier than the net bottoms liquid. The external thermosiphon reboiler is equivalent to approximately one-third of a theoretical equilibrium stage. On this basis, the use of a stabbed-in reboiler may reduce the required number of theoretical equilibrium stages by two-thirds of a theoretical stage, which may be reflected in the actual number of trays specified for the column. The stabbed-in reboiler will also have a lower inlet temperature than “simple” (not absolute once-through or preferential once-through) thermosiphon reboilers.
5.4 Less Plot Area
A stabbed-in reboiler is contained within a column so that no plot space is required for the location of equipment as for an external reboiler. Provide an area allocated for the removal of the tube bundle.
5.5 Lower Capital Cost
A stabbed-in reboiler with the advantages described above may result in a lower capital cost than an external reboiler in many cases. If a single stabbed-in tube bundle can be accommodated in a column without swaging the shell, the cost of the stabbed-in reboiler circuit is approximately 90 percent of the cost of an external horizontal thermosiphon reboiler circuit. If the column must be swaged or dual tube bundles used, the cost of the stabbed-in reboiler circuit is approximately the same as the external reboiler circuit. If the column must be swaged and dual tube bundles used, the cost is approximately 110 percent of the external reboiler circuit. The cost of a smaller High Flux tube bundle is approximately the same as a larger bare tube bundle for the same duty.
6. Disadvantages
The disadvantages of stabbed-in reboilers are as follows:
6.1 Large Diameter Flange for Tube Bundle
A stabbed-in reboiler, due to its shorter tube length, has a larger diameter flange for the tube bundle than a comparable external horizontal thermosiphon reboiler. As the diameter of the flange increases so does the potential for leakage and fire.
Both the stabbed-in and external reboilers have the same problem at the flanged tubesheet joint. The problem is potentially more severe with the stabbed in reboiler because of the larger diameter flange. The tubesheet has a very complex temperature distribution. While the range of temperatures in the tubesheet is nearly the same as in the flanges, the relative distribution is quite different. Most of the tubesheet is at a relatively high temperature, while only a small outer ring is at a lower temperature. In the flanges almost the opposite is true. Most of the flange is at a relatively low temperature, while a small portion at the interior of the flange is at an elevated temperature. These temperature distributions are responsible for the difference in the thermally induced radial growth between the tubesheet and flange. The expansion of the tubesheet is greater than that of the flange, subjecting the gasket to what is called a scuffing condition. Due to the temperature distribution in the tubesheet, the magnitude of the radial scuffing is not constant around the circumference of the flange. The amount of the scuffing versus the ability of the gasket to reliably seal is an engineering judgment decision by the designer.
A second problem at the flanged tubesheet common to both the stabbed-in and external reboilers is the elongation of the flange bolts during the circuit start-up and operation. The bolts reach their final temperature some time after the flanges. The thermal expansion of both the flanges and tubesheet causes the bolts to be stressed beyond the yield point of the metal and elongation results. Due to the temperature distribution in the tubesheet and flanges, the elongation of the bolts is not equal around the circumference of the flange. A reduction of temperature at any time during the processing cycle may cause the flanges and tubesheet to shrink, the gasket to become unsealed, and leakage to occur. See Reference 4 for more information.
When these two problems are properly analyzed during the design stage, a relatively safe joint can result. Correct severe leakage problems as follows:
- To control the scuffing problem, use a weld-ring gasket in place of a spiral-wound gasket.
- To control the bolt elongation problem, use carbon steel bolts with disk spring washers to control the maximum bolt load in place of stainless steel bolts.
6.2 Re-circulation of Column Bottoms
A stabbed-in reboiler for most situations is submerged in a tub and is separated from the liquid inventory in the bottom of the column. When the column slumps during emergency shutdown, the light material drains to the bottom of the column and overflows the reboiler tub. In some situations this light material, if allowed to go out with the bottoms material, may be harmful to downstream processing and/or equipment. To prevent this, a means of recirculating the bottoms liquid back to the reboiler tub or to some other cleanup processing is necessary. This may require the addition of a pump specifically for this purpose, if there is no net bottoms pump.
Re-circulation may also be necessary in the situation where most of the liquid to the stabbed-in reboiler is vaporized and there is only a small net bottoms rate. However, a stabbed-in reboiler with continuous recycle of net bottoms would no longer be equivalent to one theoretical stage of separation.
More Difficult Maintenance
A stabbed-in reboiler is elevated higher than an external reboiler and in most installations there is no permanent structure in place for removing the stabbed-in tube bundle. This makes maintenance of the tube bundle more difficult.
Stabbed-In Reboiler Modeling and Design Considerations
7.1 Evaluating Revamp Conditions for Existing Stabbed-In Reboilers
There are times when existing stabbed-in reboilers require evaluation for revamp conditions. It can be difficult to model stabbed-in reboilers because they are not listed as one of the TEMA Standard shell types that are commonly evaluated by commercial heat exchanger programs. It is recommended that HTRI’s IST computer program be used with a “K” or Kettle shell type and the following comments to model stabbed-in reboilers.
Nucleate Boiling is expected to be augmented (and partially suppressed in the process) by potentially dominant two-phase convective up-flow through the stabbed-in reboiler bundle. A single-phase liquid head pushing down around the stabled-in bundle drives the flow. The liquid head must balance against the two-phase friction, static, and momentum losses through the bundle to determine the equilibrium recirculating flow rate (and therefore the relative values of the different boiling coefficient components) for a given set of heating medium process conditions. HTRI’s IST program models kettle type reboilers in this manner.
The IST program considers kettle nozzles for pressure drop, while a stabbed-in reboiler bundle has no process side nozzles. HTRI IST software versions 5.0 and later allow the specification of “zero” nozzles to exclude calculation of nozzle pressure drops. The "Inlet Pressure" should be specified as the pressure (including any liquid head and any bundle submergence depth) at the bottom of the bundle. This inlet pressure specification is particularly relevant for any vacuum case, where bundle head (and any excess submergence head) might have a critical effect on vapor-liquid equilibrium and the calculation of the mean temperature difference.
7.2 Cross Flow Modeling of Stabbed-In Reboilers
There are cases when the flow and heat transfer in a stabbed-in reboiler behaves more like a cross-flow type heat exchanger than an internally recirculating kettle type heat exchanger as described in 7.1. This is typically true for cases where the inlet flow rate is relatively large and/or the difference between liquid density and vapor density is small, causing lower natural internal recirculation. When these cases occur, HTRI will provide a warning message stating “The calculated internal kettle circulation rate is the same as the feed rate. Please check the kettle reboiler geometry, process conditions, and physical properties. If no problems are found with the input data, rerun the case as a TEMA X shell to check the performance. It appears that the kettle is operating as a crossflow exchanger and not a kettle reboiler.”. It is recommended that the stabbed-in reboiler is also modeled as a TEMA “X” type shell to evaluate the potential performance of cross flow boiling.
The main differences between an X shell model and a K shell are a result of the recirculation assumption for the kettle model:
- The kettle model will have lower MTD for fluids with wide boiling ranges, since the outlet fluid is back-mixed within the bundle.
- The kettle model will also have lower vapor mass fractions at the top of the bundle, again due to the liquid recirculation.
- The kettle model may have a higher pressure drop from bottom to top of the bundle, due to the higher static head of the typically lower vapor mass fraction flow (especially at the top).
- The kettle model may have higher cold side heat transfer coefficients due to enhancement from the recirculation.
- When modelling as an X shell, it is essential to provide the correct inlet pressure which includes the pressure drop of the bundle, since the pressure at the top of the bundle (outlet pressure) is the known value. Certainly, the X shell does not include the same static head as the kettle, and if there is some additional liquid level above the bundle, the static head would be more than the pressure drop calculated by HTRI.
7.3 High Outlet Vapor Mass Fraction and High Heat Flux Cases
When the vapor mass fraction at the outlet of the bundle exceeds 0.9, it is possible that local tube dryout, film boiling, fouling, and unstable operation may occur. The result can be lower than expected heat transfer coefficients. These cases should be modeled carefully, to determine if sufficient internal bundle recirculation is predicted to occur, and also to check if local film boiling is predicted. To alleviate this situation, consider a pumped recycle of the bottoms stream to the reboiler as described in Section 6.3.
At high heat fluxes (above 12,000-15,000 Btu/hr-ft²) local tube dry-out, film boiling, fouling, and unstable operation may also occur. High mean temperature differences (greater than 90 °F or 50 °C) can often cause high heat flux situations. See section 9.1 of for more information.
For both high outlet vapor fractions and high heat flux cases, it is important to allow liquid to readily re-circulate through the bundle. Removing restrictions around the bundle such as a reboiler tub, or expanding the tub volume should be considered.
7.4 Reboiler Tub Considerations
As described in Section 3, a reboiler tub is often specified with a stabbed-in reboiler. For stabbed-in side reboilers, a tub is required to provide a liquid pool for the stabbed-in reboiler bundle. See IPE-TM-300-08 and F-PSD-16 for design guidelines. For column bottoms stabbed-in reboilers, a tub may not be necessary, and the following points should be considered:
“Pros”:
- Reboiler tubs provide constant level control for the stabbed-in bundle due to the use of an overflow weir.
- Level control instrumentation of the liquid bottoms product level at the bottom of the column is simplified since vapor has been disengaged from the liquid by the internals above.
- The reboiler is in a once-through configuration, often providing separation and mean temperature difference improvements.
- Liquid from the bottom tray is better directed to pass over the reboiler bundle, avoiding liquid bypass of the bundle to the column bottoms outlet nozzle.
“Cons”:
- The tub reduces the liquid inventory available to the stabbed-in bundle.
- The reduced volume of liquid around the bundle may result in dry-out of the upper rows of tubes, potentially causing unstable operation and fouling. At high heat flux or high outlet vapor fractions this becomes more important.
7.5 Hot Oil Driven Reboiler Considerations
The hot oil side (tubeside) of a stabbed-in reboiler must be designed for a reasonable temperature drop (typically less than 125 °F) to avoid mechanical issues related to thermal stresses. See section 9.2d of " for more information.
8. References
- Inflection Point Engineering Standard Drawings 3-270, "Stabbed-in Reboiler Tub For One Reboiler", and 3-272, "Stabbed in Reboiler Tub For Two Reboilers”.
- Inflection Point Engineering Standard Drawings 3-346 to 3-352, "Collector Trays".
- Jacobs, J. K., Hydrocarbon Processing & Petroleum Refiner, 1961, 40 (7), 189-196.
- Martens, D.; Porter, M. A. Developments in Pressure Vessels and Piping; ASME: 1994; PVP-Vol 278; pp 133-143.
Attachment 1 Inflection Point Engineering Typical Applications for Stabbed-In Reboilers
| Detergent Alkylate Acid Regenerator (no tub) Paraffin Column Light Alkylate Column Benzene Column | Molex Extract Column Raffinate Column | |
| HF Alkylation Isostripper Depropanizer HF Stripper | Oleflex Depropanizer | |
| Sulfuric Acid Alkylation Propane Drying Column (no tub) | Pacol Product Stripper | |
| Catalytic Condensation (Cumene) Depropanizer | Phenol Finished Acetone Column | |
| Catalytic Condensation (Motor Fuel) Propane Stripper | Platforming Deethanizer stripper Depropanizer | |
| Detal Paraffin Column Rerun Column Recycle Column Benzene Column (no tub) | Sulfolane Recovery Column Solvent Regenerator | |
| Fractionation Finishing Column | Unibon/Unicracking Stabilizer | |
| Gas Concentration Drying Column Deethanizer |
Attachment 2 Nozzle Flange Outside Diameters
| NOZZLE DIA. | ASME B16.5 | ASME B16.5 | ASME B16.5 | ASME B16.5 | ASME B16.5 | ASME B16.5 |
|---|---|---|---|---|---|---|
| NPS | CL 150 | CL 300 | CL 600 | CL 900 | CL 1500 | CL 2500 |
| 1 | 4.25 | 4.88 | 4.88 | 5.88 | 5.88 | 6.25 |
| 1 | 5.00 | 6.12 | 6.12 | 7.00 | 7.00 | 8.00 |
| 2 | 6.00 | 6.50 | 6.50 | 8.50 | 8.50 | 9.25 |
| 3 | 7.50 | 8.25 | 8.25 | 9.50 | 10.50 | 12.00 |
| 4 | 9.00 | 10.00 | 10.75 | 11.50 | 12.25 | 14.00 |
| 6 | 11.00 | 12.50 | 14.00 | 15.00 | 15.50 | 19.00 |
| 8 | 13.50 | 15.00 | 16.50 | 18.50 | 19.00 | 21.75 |
| 10 | 16.00 | 17.50 | 20.00 | 21.50 | 23.00 | 26.50 |
| 12 | 19.00 | 20.50 | 22.00 | 24.00 | 26.50 | 30.00 |
| 14 | 21.00 | 23.00 | 23.75 | 25.25 | 29.50 | |
| 16 | 23.50 | 25.50 | 27.00 | 27.75 | 32.50 | |
| 18 | 25.00 | 28.00 | 29.25 | 31.00 | 36.00 | |
| 20 | 27.50 | 30.50 | 32.00 | 33.75 | 38.75 | |
| 24 | 32.00 | 36.00 | 37.00 | 41.00 | 46.00 | |
| ASME B16.47 | ASME B16.47 | ASME B16.47 | ASME B16.47 | ASME B16.47 | ASME B16.47 | ASME B16.47 |
| 26 | 34.25 | 38.25 | 40.00 | 42.75 | ||
| 28 | 36.50 | 40.75 | 42.25 | 46.00 | ||
| 30 | 38.75 | 43.00 | 44.50 | 48.50 | ||
| 32 | 41.75 | 45.25 | 47.00 | 51.75 | ||
| 34 | 43.75 | 47.50 | 49.00 | 55.00 | ||
| 36 | 46.00 | 50.00 | 51.75 | 57.50 | ||
| 38 | 48.75 | 46.00 | 50.00 | 57.50 | ||
| 40 | 50.75 | 48.75 | 52.00 | 59.50 | ||
| 42 | 53.00 | 50.75 | 55.25 | 61.50 | ||
| 44 | 55.25 | 53.25 | 57.25 | 64.88 | ||
| 46 | 57.25 | 55.75 | 59.50 | 68.25 | ||
| 48 | 59.50 | 57.75 | 62.75 | 70.25 | ||
| 50 | 61.75 | 60.25 | 65.75 | |||
| 52 | 64.00 | 62.25 | 67.75 | |||
| 54 | 66.25 | 65.25 | 70.00 | |||
| 56 | 68.75 | 67.25 | 73.00 | |||
| 58 | 71.00 | 69.25 | 75.00 | |||
| 60 | 73.00 | 71.25 | 78.50 |
Attachment 3 Typical Stabbed-in Reboiler Bundle Arrangements
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