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Apr-2023

Energy saving with tube inserts for heat exchangers

When it comes to fouling in crude oil preheat trains, tube inserts can maximise heat transfer and reduce heat exchanger fouling, with an average six-month payback.

Nicolas Aubin
Petroval

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Article Summary

Refiners face the challenge of continuously improving energy efficiency and reducing CO2 emissions. The energy efficiency of refinery and petrochemical plants is strongly influenced by the efficiency of their heat exchangers. The achievable level of heat recovery that can be reached represents significant margins estimated during the plant design phase. However, reality may be completely different once fouling occurs at any stage of the process. Fouling will lead to loss of heat recovery, pressure drop increase, and throughput reduction until a maintenance shutdown becomes mandatory.

The benefits of using tube insert technologies are already demonstrated in terms of increased heat transfer coefficient, reduced fouling rate, and stability of pressure drop. Against this backdrop, fouling in crude oil preheat trains caused by asphaltene deposition and/or coke formation on hot surfaces will be examined in detail.
In tests performed for crude oil preheat train fouling, heat exchangers forming part of preheat trains at two refineries were equipped with the proprietary Turbotal technology inserts for Study A, and the proprietary Spirelf technology inserts were equipped for Study B. Their performance was monitored over different periods, depending on the case, between two and four years and compared to the previous run durations at similar process conditions.

Study A – Heat transfer enhancement and fouling mitigation with Turbotal inserts
Turbotal technology is a rotating device hooked on a fixed head and attached to the tube end on the inlet side (see Figure 1). These inserts incorporate a continuous online cleaning device whose purpose is to reduce the fouling layer at the tube walls by means of mechanical effect. The technology uses the energy of the flow running in the tubes to effect a rotation of the device within the range of 1,000 rpm during the whole run duration with a limited impact on pressure drop.

The final pair of heat exchangers before the furnace were experiencing severe fouling over less than one year. The four heat exchangers were equipped with the inserts and operated in the same range of process conditions as previously discussed (see Table 1). The monitoring of the performance was then compared with the previous data. The comparative trend of the outlet temperature is presented in Figure 2.

The trend presented in Figure 2 shows an increased outlet temperature at SOR of 3°C (in clean condition). This gain is related to the extra turbulence generated by the rotation of the device, leading to an 80% increased heat transfer coefficient on the tube side compared to bare tubes.

The slope of the preheat decrease is almost three times slower with Turbotal technology compared to bare tubes. This phenomenon is due to the fouling mitigation during the run. The technology significantly reduces the fouling rate but cannot eliminate the fouling deposition from occurring. Previous studies detected that the fouling resistance generates an asymptotic profile corresponding to the distance between the tube wall and the device.

A payback analysis has been performed on this application to evaluate the energy gains compared to the cost of the device and installation. An average gain of more than 10°C was achieved during the first year, leading to a gain in energy of €1.4 million to compare with an investment in the range of €100,000.

The calculated payback is in the range of one month when factoring only the cost of energy. However, other cost sources would have to be considered, such as reduced maintenance cost (avoided mechanical cleaning), production losses (throughput reduction during partial shutdown for cleaning), and CO2 emissions costs from the furnaces (around 100 €/t today in Europe). Factoring in these additional cost reductions results in an actual payback period of less than one month vs just from energy savings at the furnace.

Study B – Extensive run duration with Spirelf technology
Spirelf technology is a vibrating device fixed on both tube ends by a fixing wire (see Figure 3). This system is also a continuous online cleaning device whose purpose is to reduce the fouling layer on the tube walls by means of mechanical effect. The technology uses the energy of the flow running in the tubes to convert it to vibrations of the device both radially and longitudinally. The extra pressure drop from the Spirelf is minimal and allows for a potential run duration of up to six years.

The last pair of heat exchangers just before the furnace was experiencing severe fouling over less than one year. The two heat exchangers were equipped with Spirelf technology and operated in the same range of original process conditions from Table 2. The monitoring of the performance was then compared with the previous data; the comparative trends of the heat transfer coefficients of each tube bundle and the crude flow rate can be seen in the results section (see Figure 4).

The trend presented in Figure 4 (blue and red trends) shows a first run period lasting for one year, for which the overall heat transfer coefficient (OHTC) decreases sharply from a clean design level to only 70% within a year. The flow rate across the heat exchangers remains close to the design value, which results in a decreasing coil inlet temperature (CIT) at the furnace, requiring extra consumption of combustible energy to compensate for this loss of preheat.

The Spirelf devices were implemented during the turnaround, and the performance of each tube bundle is represented on the same trend. The OHTC with Spirelf technology goes through the ceiling at 22% above design value and remains higher than or at design value for about 500 days.

Over this period, the crude flow rate was also pushed above the design value, gradually increasing from +10% to +25% at 900 days. As the performance of some other exchangers began declining, the feed rate was slightly reduced during the cleaning period for the other tube bundles without the need to stop the bundles equipped with Spirelf technology.

Once the complete train was back in service at 940 days, the feed rate was pushed back to +25% of design for 400 days, with the OHTC of both heat exchangers still above or at the level of design. A second maintenance period was carried out on other heat exchangers, again without stopping the heat exchangers equipped with Spirelf technology, as the duty was still acceptable.

After returning to full capacity, at 1,400 days the performance of the equipped heat exchangers began declining, and the crude flow rate was slightly reduced. The exchangers equipped with Spirelf technology were bypassed at 1,600 days for recirculation of hot gasoil over a short period of time. This operation was carried out without opening the heat exchangers, and the objective was to soften and remove some of the deposits that had accumulated over the years.

Immediately after this flushing operation, the heat exchangers were reintroduced in the process. Throughput was pushed back gradually to +35% of the design value, with the OHTC for each heat exchanger above the design by +22%. The heat exchangers were still in service at the time of writing. Implementing Spirelf technology in these heat exchangers has considerably increased the run duration from one year to more than four years.

This allowed for the removal of three maintenance periods on these heat exchangers based on production runs prior to the installation of Spirelf technology. The performance of the heat exchangers improved considerably and stabilised conditions compared to before the installation of the technology, resulting in significant energy savings. The unit flow rate was debottlenecked to +35% of design value while maintaining acceptable feed preheat.

Conclusions
These bespoke studies highlight significant improvements related to the use of tube inserts, and some concluding remarks can be drawn from this field data analysis.
For both applications, the run duration with the tube inserts is, at minimum, doubled compared to the same run with bare tubes without any mechanical alteration on the heat exchanger tubes. For each case, the performances of the heat exchangers saw increased heat transfer.

This improvement translated to increased outlet temperature (Study A), where the gain on the clean condition was 3°C and, on average, over the whole run duration, more than 10°C. For Study B, the gain on the OHTC was followed and reached +22% compared with -30% with bare tubes in less than a year.

Tube insert technology can be used to debottleneck an existing and production-limited preheat train (Study B). The capacity of the debottlenecked train to create more preheat can be used to increase the throughput of the unit. As identified in Study B, the capability to debottleneck is also a function of the cleanliness and availability of the whole train and not only the exchangers equipped.


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