Improve energy efficiency with enhanced bundles in tubular heat exchangers
From 2008 to date, 45 twisted tube heat exchangers and bundles have been installed at BAYERNOIL refineries in different units and services, allowing higher heat recovery and contributing to BAYERNOIL’s energy targets.
From 2008 to date, 45 twisted tubea heat exchangers and bundles have been installed at BAYERNOIL refineries in different units and services, allowing higher heat recovery and contributing to BAYERNOIL’s energy targets. A number of these exchangers have undergone at least one turnaround (TAR) since their first installation, and have then been successfully brought back into service.
BAYERNOIL is a shareholder refinery owned by VARO Energy (45%), Ruhroel GmbH (25%), ENI Deutschland GmbH (20%) and BP Europa SE (10%). Two refineries (Neustadt and Vohburg in Germany) operate with a combined crude processing capacity of 10.3 MMtpy. As about half of its crude processing costs are energy costs, BAYERNOIL has set a target to increase energy efficiency and reduce energy costs. Increasing heat recovery in exchangers by installing twisted tube heat exchangers and bundles was one of several measures initiated. These measures target the reduction of the fired heater duties and a drop in natural gas imports. TABLE 1 shows a summary of the twisted tube heat exchangers and bundles installed at BAYERNOIL. The performances of the twisted tube heat exchangers and bundles in two refinery units in Neustadt—the mild hydrocracking unit (MHC) and Reformer 2—are reviewed, both from a design and operational perspective.
MHC plant in Neustadt
The MHC unit in Neustadt, which cracks various types of gasoil—such as vacuum gasoil (VGO), etc.—into mainly middle distillate products, is newly built and began operations in 2008. The net conversion of the unit is 60%–65%. A simplified flow diagram of the MHC plant is illustrated in FIG. 1.
FIG. 1. Simplified flow diagram of the MHC Plant.
The MHC unit has eight twisted tube heat exchangers and bundles (TABLE 2), with shell diameters that range from 850 mm to 1,370 mm, and straight tube lengths that range from 2,800 mm to 5,000 mm. Six exchangers are in the feed preheat section of the unit (EA-252A/B and EA-251A/D), and one exchanger is for preheating the stripper feed (EA-255). The seven exchangers were delivered at the end of 2007 and plant startup occurred in autumn 2008. During the 2011 TAR, the plain tube bundle of exchanger EA-254, which preheats treat gas, was replaced by a twisted tube bundle.
When the investment of the new grassroots MHC unit was evaluated in 2006, 10 high-pressure plain tube exchangers were necessary to achieve the required duty in this plant section. The adoption of the twisted tube heat exchanger technology allowed BAYERNOIL to save three high-pressure shells for the same duty. Consequently, a significant capital expenditure (CAPEX) savings was achieved, as well as a reduction in the plot space occupied by the exchangers.
Reformer 2 plant in Neustadt
The Reformer 2 unit is the smallest of the two reformers at the site. The unit’s purpose is to increase the octane number of the feed and produce hydrogen (H2) for the desulfurization units. Feed to the unit comes from the naphtha hydrotreater. A simplified flow diagram of Reformer 2 is shown in FIG. 2.
Fig. 2. Simplified flow diagram of the Reformer 2 plant.
The liquid feed flow is approximately 35 tph (tons per hour), and is mixed with reformer recycling gas before the feed/effluent heat exchangers EA-501/EA-506 and EA-521/EA-522. Heater BA-501 is installed in front of reactors DC-501, DC-502 and DC-503. After leaving the reactors, the effluent cools down in EA-501/EA-506 and EA-521/EA-522, and then enters into stabilizer column DA-501 via two flash drums. The bottom product of DA-501 is cooled down in EA-511/EA-512 against column feed.
The scope of the Reformer 2 heat exchanger revamping project (implemented during the 2011 TAR) was to replace the six conventional, plain tube bundles of the preheat train (EA-501/EA-506) with twisted tube bundles, and to add two new twisted tube heat exchangers to the train (EA-521/EA-522). The installation of units EA-521/EA-522 is depicted in FIG. 3. At the same time, the two conventional plain tube bundles of exchangers EA-511/EA-512 were also replaced by twisted tube bundles (TABLE 3). The shell diameters range from 486 mm to 914 mm, and the straight tube lengths range from 4,500 mm to 5,000 mm.
Fig. 3. Reformer 2 heat exchangers EA-521 and 522 during the January 2011 TAR.
Some exchangers in the preheat train were already Tubular Exchanger Manufacturers Association (TEMA)-style “F,” featuring two passes in the shell side. Exchangers that were originally TEMA-style “E” were converted to “F.”
FOCUS ON TECHNOLOGY
The twisted tube heat exchanger technology has been in industrial and commercial development since the early 1990s. More than 1,000 exchangers and bundles have been delivered and put into operation, covering a wide range of services, sizes, materials and design conditions.
Twisted tube bundles are manufactured from standard commercial round tubes. Each tube is first given a characteristic shape and is then assembled in a bundle. Tube-to-tubesheet joints are of standard type, depending on the specifications and process requirements, as the tube ends are left as circular.
Internally, a passing hole is present for cleaning and inspection purposes. Tubes are disposed in a triangular pattern and are carefully aligned to each other prior to fixing them to the tubesheet(s). As a result, an ordered pattern in the bundle is established.
This ordered tube pattern provides twisted tube bundles with shell-side cleaning lanes, which can be utilized for hydro-blast cleaning.
Increased heat transfer surface area per unit volume
Twisted tube bundles provide a larger heat transfer surface area in the same shell diameter. This result descends from the description above. First, tubes are arranged in a triangular pattern (not square or rotated square), while the presence of shell-side cleaning lanes is still granted. Second, as tubes are in contact with each other, the tube pitch itself is reduced and is usually smaller than the minimum value traditionally established by TEMA standards.
Welded tube-to-tubesheet joints of twisted tube bundles, though based on such reduced pitch, have been qualified and have proven to be fully acceptable in high-pressure and temperature designs (including hydrocracking applications) under the requirements of a large number of licensors, engineering, procurement and construction (EPC) companies, and end users.
The relative increase in heat transfer surface area, for a fixed shell diameter, depends on the tube diameter and pitch established by thermal engineers during the design phase. In refinery services, such increase can typically range from 30% to 65%.
Heat transfer enhancement
Fig. 4. Shell-side longitudinal flow in the twisted tube bundles.
The in-tube flow in twisted tube bundles is classified as “swirl flow.” Swirl flow induces fluid agitation and mixing, which, in turn, enhances the heat transfer coefficient. The enhancement effect is particularly evident in some applications, such as transition-flow regimes, boiling (in which film boiling effects tend to be suppressed) and mist flow.
The shell-side flow over the tubes is of “longitudinal” type (FIG. 4). Due to the external shape of the tubes, radial mixing of the fluid is generated, while axial mixing is inhibited due to the shrouded construction, which avoids bypass streams around the bundle. As a consequence, these types of heat exchangers realize a closer approach to “plug flow,” and can achieve a higher thermal effectiveness than baffled exchangers.
Typical TEMA-type “E” heat exchangers can be internally converted to type “F.” This is possible both for “U” and floating head (“S”) exchangers, and does not require any modification to existing nozzles or piping. The “F” arrangement, combined with two tube-side passes, provides a full counter-current option, thus maximizing the temperature difference that acts as the driving force for heat transfer.
The twisted tube heat exchanger technology has the potential to mitigate the fouling trend, due to the development of specific mechanisms—both tube-side and shell-side—that are able to attack some of the most common fouling phenomena.
Concerning the in-tube flow, higher values of the shear stress are generated at the wall, due to the secondary velocity components induced by the swirl flow. The higher shear stress is able to increase the rate of fouling removal from the wall.
On the shell side, the velocity field (and consequently the temperature field) of a twisted tube bundle are nearly uniform at any section. Conversely, in segmental-baffle exchangers, the velocity field inside each baffle compartment is far from uniform. For instance, the corners are areas of very low velocity, thus becoming prone to fouling buildup and, depending on the application, to temperature-induced fouling, as well.
The development of the new grassroots MHC around 2006 has already been discussed earlier in this article. It is interesting to focus on the execution of the Energy Conservation projects, which fall under the more general BAYERNOIL “EnCon” initiative.
In late 2009, after having gained approximately 1 yr of positive operating experience with the twisted tube heat exchanger design of the MHC plant, BAYERNOIL decided to evaluate the adoption of twisted tube bundles in a number of other exchangers in the company’s refineries.
First, a list of all the plants having a planned TAR at the beginning of 2011 was prepared. Second, the most promising opportunities for energy optimization were identified. For example, heat exchangers with a hot temperature approach higher than 50°C, or points of energy loss via air coolers and/or rundown, were recognized. The results from some preliminary evaluations were used as guidance through the process, as well.
Fig. 5. Task force for identification and quantification of energy conservation/energy-efficiency opportunities.
Once the selected number of exchangers were identified, BAYERNOIL provided Koch Heat Transfer with the necessary information to carry out the design of the twisted tube bundles. This information included geometrical and mechanical data about existing exchangers, stream properties and enthalpy curves, process data (flowrates, operating pressures and stream temperatures), fouling resistances and allowable pressure drops (FIG. 5). This part of the work was greatly facilitated by the fact that BAYERNOIL routinely performs detailed heat exchangers monitoring, based on a broad range of data consolidated into their data collection system (named “BORIS”).
For each exchanger under evaluation, the typical output from the design phase consisted of information such as the achievable heat recovery, the outlet stream temperatures and the calculated pressure drops. Thermal-hydraulic design was performed and integrated with the proprietary heat transfer and pressure drop correlations for the twisted tube heat exchanger technology.
These results had to be transferred back to BAYERNOIL for use in their process simulation tool, to take into account the various impacts on the process coming from the adoption of twisted tube bundles, which are characterized by different performances with respect to the plain tube bundles operating at that time.
In 2Q 2010, BAYERNOIL invested in a total of 22 twisted tube heat exchangers and bundles, which were to be installed during the TAR at four plants the following year. These plants included the MHC, CHD and Reformer 2 plants at the Neustadt site, and the hydrofiner plant at the Vohburg site.
All the bundles and heat exchangers were delivered between 4Q 2010 and the beginning of 2011, in time for installation into the relevant plants during the TAR. The different plants were regularly restarted after the TAR.
Over the next three years, BAYERNOIL selected additional heat exchangers to be converted from the conventional style to the twisted tube technology. Other exchangers are likely to be considered in the future for similar revamping activities, including other refinery units.
Considering the associated lead time, projects had to enter their execution phase approximately 8 mos to 10 mos prior to the TAR, depending on the materials of construction, to ensure a timely delivery.
BAYERNOIL has been continuously carrying out the monitoring of the heat exchangers’ performance throughout the years, thus enabling the direct evaluation of the results brought by twisted tube bundles in comparison with previous outcomes.
The exchangers originally supplied in 2007 are performing as expected. They underwent a TAR in January 2011, and another at the beginning of 2014, and were opened and cleaned (FIG. 6). After both TARs, performance of the exchangers in the feed preheat section were demonstrated to be nearly as good as after the startup of the unit in December 2008 (FIG. 7), thus providing an indication of the effective cleaning of the heat transfer surfaces.
Fig. 6. The twisted tube bundle being lifted during the January 2011 TAR.
Fig. 7. Heat exchangers EA-251 A/B fouling trend while utilizing the twisted tube heat exchanger technology.
Additional relevant results were achieved in the MHC due to the implementation of multiple energy conservation interventions during the 2011 TAR. For example, after the installation of the twisted tube bundle in EA-254, the efficiency of the exchanger has increased approximately 60%, and its heat recovery has significantly improved (FIG. 8). It is important to note that the measured performance is in line with predictions.
Fig. 8. Improved heat recovery in unit EA-254 after the installation of the twisted tube bundle.
Overall, the temperature in the feed drum of the preheat section could be raised by about 24°C, and furnace BA-251 could be completely switched off after the startup (FIG. 9). The overall reduction of the refinery’s energy intensity index1 has been 0.14, along with a reduction of CO2 emissions of 2.6 Mtpy.
Fig. 9. Effect on the BA-251 furnace duty coming from the interventions within the BAYERNOIL EnCon project implemented during the 2011 TAR. Furnace BA-251 could eventually be switched off.
Reformer 2 plant
Heat exchangers in the Reformer 2 plant are considered “clean services.” Therefore, after the installation of the twisted tube heat exchangers and bundles during the 2011 TAR, a shutdown and cleaning took place for an inspection in 2014.
The design case for these exchangers had the target to reduce the duty of heater BA-501 by 4.1 MW and, consequently, to reduce the refinery’s energy intensity by 0.37 and CO2 emissions by 7 Mtpy.
The furnace duty has been reduced by more than 4.5 MW at the design feedrate of 37 tph. The duty is consistent with the heat loss via the air coolers. The CO2 emissions have been reduced by up to 8.5 Mtpy, and the refinery’s energy intensity index has been reduced by up to 0.45.
The hot temperature approach of the preheat train is now similar to that obtained in the vertical feed/effluent heat exchangers of Neustadt’s Reformer 1 (also known as Texas Towers-type), while the investment cost has been much lower. FIG. 10 compares past data from Reformer 2—when the run was characterized by higher severity—with data after the installation of the twisted tube heat exchangers and bundles, as well as with typical data from Reformer 1.
Fig. 10. Hot approach of Reformer 2—before and after the installation of the twisted tube heat exchanger and bundles—and Reformer 1.
Another important target was reached in the stabilizer feed/effluent exchangers EA-511/512. In fact, the duty of reboiler EA-513 (at the bottom of column DA-501) could be reduced by about 1 MW at the Reformer 2 design feedrate of 37 tph. The hot oil supply to EA-513 has been reduced by half.
The systematic survey of a number of heat exchangers at BAYERNOIL confirms that the installation of twisted tube bundles achieved a higher heat recovery. This result relies on higher heat transfer surface area, improved or maintained heat transfer coefficient and optimized use of the available temperature difference, thanks to the internal conversion from TEMA type “E” to “F.”
The corresponding heaters’ duty reductions are in line with design anticipations. All projects were implemented within planned cost, and resulted in an energy intensity improvement for BAYERNOIL of about 1.5, an energy savings of approximately 16 MW, and a reduction of CO2 emissions of 27 Mtpy. These results contributed greatly to BAYERNOIL reaching its energy targets. Average payout time was less than 2 yr.
In all cases in which twisted tube bundles were inserted in place of conventional bundles, BAYERNOIL did not need to implement any modification to existing exchangers in terms of nozzles, piping and foundations. The insertion of the new bundles took place during a regular shutdown, and did not pose any additional time requirement to BAYERNOIL.
In a few units, the shells had to be substituted due to the fact that the twisted tube bundles were increasing the operating temperature beyond the design temperature of the existing pressure parts.
The normal TAR time for BAYERNOIL is 6 yr. This time may be shorter for dirty services and/or for catalyst change-out. BAYERNOIL did not observe any impact on TAR time due to the installation of the new heat exchangers and bundles. When the exchangers were cleaned, performance could be brought back to those of the first startup. A time-efficient cleaning of the twisted tube bundles was achieved along a learning curve, so that BAYERNOIL could learn how to properly handle the bundles and their components.
Twisted tube bundles emerge as a real “plug-and-play” tool that can effectively change refinery economics and environmental figures, while providing a short investment return time. HP
a TWISTED TUBE is a registered trademark of Koch Heat Transfer Company, LP in the US and may be registered in other jurisdictions.
- The Energy Intensity Index (EII) was developed by Solomon Associates.
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