SOR Nov 2016
SOR Nov 2020
100.00
600.00
90.00
500.00
80.00
Chemical cleaning
Chemical cleaning
70.00
400.00
FIXOTAL installation
60.00
50.00
300.00
40.00
200.00
30.00
20.00
100.00
10.00
0.00
0.00
Date
Duty shell side A-F GJ/hr
Duty shell side G-L GJ/hr
CRD charge kL/h
Figure 7 Trend of duty with Fixotal equipment on half of the exchangers (red) Study C
OHTC of the last six HXs consistently remained above the OHTC of the first six HXs but declined significantly to 350 kJ/h.m² °K late in September 2022 (period of chemical cleaning of the exchangers, vertical dot line) after two years of operation. The last six HXs were then delivering +75% OHTC compared to the reference run at the same duration, even though only the half of the HXs were equipped with the shear stress promoter. The evaluation of the duty exchanged before the chem - ical cleaning revealed that 66% of the train’s total duty was achieved through the last six HXs, compared to only 52% during the previous run. The gain in duty before the chemical cleaning was in the range of 14% of duty on the complete train, compared to the reference run for the same run duration. Knowing that the shear stress promoter will only influence the tube-side fouling rate and heat transfer coefficient, the performance of the last six HXs was impacted significantly by the fouling on the tube side of the first three HXs and the fouling on the crude oil shell side of the six HXs, which was the main contributor to the performance limitation. • From chemical cleaning: Chemical cleanings are typically performed after two years of operation, halfway through the four-year turnaround cycle. This cleaning consists of light aromatic gasoil recirculation on both tube and shell sides to soften the fouling material, followed by steaming to flush and remove part of the fouling material. It is well known that this type of operation does not allow a full recovery of performance, as the older deposits harden and age, so only mechanical cleaning would be efficient to fully recover heat exchanger performance. The evaluation of the OHTC after the chemical cleaning
revealed a significant recovery for both runs. However, the reference run still indicated a lower OHTC for the last six HXs. In contrast, for the run with the shear stress promoter, the last six HXs were still producing a significantly higher OHTC of 990 KJ/h.m² °K in comparison to the reference case at 660 KJ/h.m² °K +50%. This was proof that fouling on the crude shell side was the limiting factor and that the chemical cleaning significantly improved the heat transfer performance of the preheat train. At this stage, a compari - son of the duty of the complete train reveals a gain with the Fixotal of 9% of duty at 96.67 GJ/hr vs 88.5 GJ/hr. • End of run (EOR): The comparison of the two runs still shows better heat transfer for the last six HXs when the last three are equipped with the shear stress promoter (typically 50% higher than the reference run). After 1,035 days of operation, the last six HXs achieved 69% of the total duty of the train, which was typically 10% higher than the previous run (after the same duration). • Average on the complete run: By evaluating the com - plete run of 1,035 days, the run with the Fixotal inserts equipped in the last three exchangers of the preheat train generated, on average, 34% higher OHTC on the last six bundles and a total average increase in duty of 20% on the last six exchangers. Overall, the preheat train with the inserts was generating 3% more duty, equivalent to 56,600 GJ over 1,035 days of operation. The energy savings from reducing the firing of the fur - nace and, consequently, the reduction in CO2 emissions of the plant yielded economic benefits, as summarised in Table 6 . This is equivalent to 405 k€ per year, even though some heat transfer limitations were reached. As the residue on the tube side cooled down much faster
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PTQ Q1 2025
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