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 tem - perature (CIT) at the furnace, requiring extra consump - tion of combustible energy to compensate for this loss of preheat. The Spirelf devices were implemented during the turn - around, 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 bun - dles 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 stop - ping the heat exchangers equipped with Spirelf technology, as the duty was still acceptable. After returning to full capacity, at 1,400 days the perfor - mance of the equipped heat exchangers began declining, and the crude flow rate was slightly reduced. The exchang - ers 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 dura - tion from one year to more than four years. This allowed for the removal of three maintenance peri - ods on these heat exchangers based on production runs prior to the installation of Spirelf technology. The perfor - mance 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 temper - ature (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. 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 If proprietary Turbotal and Spirelf technologies cannot be implemented for any reason, an alternative solution can also be provided using proprietary Fixotal technology; even if the technology does not provide a mechanical cleaning effect, it still improves heat transfer. This return on invest - ment (ROI) could be even more significant as the set of pro - prietary Fixotal technology can be reused for several run periods. The benefits of using these technologies are evident in extending run duration between cleaning shutdowns, increasing heat transfer coefficient, reducing fouling factor, and stability of pressure drop. From an economic viewpoint, the payback is generated by four improvements: energy saved in the preheat train in service (by the increase in heat transfer), reduction in maintenance cost (reduced cleaning frequency), increased run throughput, and reduced CO2 emissions, taking into consideration the cost of CO2 emis - sion, which is around 100 €/t. Although these technologies may be implemented in new exchangers where fouling issues are already foreseen from the design phase, the use of these technologies in existing heat exchangers will allow refinery and petrochemical com - plexes to improve their energy efficiency and reduce their carbon footprint for a sustainable future. Nicolas Aubin is Technical and R&D Manager, Petroval, providing technical support on catalyst change-outs, turnarounds, and energy efficiency projects involving tube inserts and infrared thermography surveys. Email: n.aubin@petroval.com
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PTQ Q2 2023
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