Comparisons Nowadays, many incentives are in place to encourage the reduction of CO2 emissions at every level, and the industry sector is one of the largest contributors.5 Refineries and chemical plants are mainly operating with shell and tube heat exchangers for heat recovery. Reducing their CO2 emissions can be done right now thanks to these techni- cal solutions, even as longer-term projects are ongoing for large-scale impacts. These technologies are available and can be retrofitted to any shell and tube exchanger within a few weeks. Comparing the three technologies would be a difficult exercise, as they are not designed to operate on the same type of feed, the same level of flow conditions, and do not have the same mechanical lifetime. However, whenever it is possible, and if fouling mitigation is the driving force to use inserts, priority should be given to selecting the inserts that provide a mechanical cleaning effect (Turbotal and Spirelf). Although there are already a wide range of potential applications, exploring benefits in different flow conditions or with various types of fluids and processes could be very interesting. Future technological developments could focus on implementing new technologies for dual-phase flows to improve heat transfer at minimum cost on pressure drop.
Impact on energy savings and CO2 em issi ons on heat exchangers used in Study C* **
With Fixotal
Gain on energy recovery (Gcal/yr) Gain on energy recovery (TOE/yr)
4,500
450
Energy savings
270 k€ 1,350 135 k€
Gain on CO2 emissions (tons first yr)
Reduction in CO2 emissions
Table 6
than during the reference run, there was less potential for heat recovery through the first six HXs. In addition, the total crude flow was 8% lower during the run with Fixotal (due to some unit upsets), which reduced the heat trans- fer performance as a result of the lower Reynolds number. Nevertheless, the unit achieved better overall heat transfer performance. Combined benefits Significant improvements related to the use of tube inserts were highlighted by the three studies presented, and some concluding remarks can be drawn from these field data analyses. For applications A and B, the run lengths with the tube inserts were, at minimum, doubled compared to the same run with bare tubes without any modifications of the heat exchanger tubes. In Study C, the operator is constrained by regulation to shut down and inspect the whole plant every four years. It is not possible to target extended run length since a single chemical cleaning in the middle of the run is sufficient to recover enough heat transfer capacity. The implementation of Fixotal was, therefore, used to optimise heat recovery, even though the fouling on shell side was predominant. In each case, the performances of the heat exchangers were increased in terms of heat transfer. This improvement was translated in OHTC (Study A) with both an increased and a stabilised level of heat transfer over the run. For Study B, the benefit was directly expressed in duty, with an average increase of 25% during the run, signifi - cantly reducing the firing of the downstream furnace by about 100 tons of gas per month. For study C, the increase of OHTC with only three HXs equipped with Fixotal out of 12 unbalanced the preheat train performance and allowed an increase in duty and heat recovery even though the operating conditions were unfavourable compared to the reference run. The benefits achieved in the three applications demon - strate the potential improvements achievable with stand- ard shell and tubes heat exchangers when limitations come from either tube side or shell side film coefficient or from fouling deposition in either tube or shell side. The complete range of operation must then be evaluated to highlight the main contributors to the thermal resistances of the exchangers and assess if these limitations can be tackled with these inserts. This must be done by comparing the effect of tube inserts with bare tubes at different levels of throughput, but also taking into account the level of fouling typically reached in these flow conditions at SOR and EOR.
Nomenclature HXs
Heat exchangers
BWG
Tube wall thickness in Birmingham wire gauge
OD CIT
Outside diameter of tube (mm)
Coil inlet temperature
OHTC
Overall heat transfer coefficient kcal/h.m² °C or kJ/h.m² °K
SOR EOR TOE
Start of run End of run
Ton of oil equivalent = 10 Gcal
(* )
Cost of energy considered €600 per TOE Taxes on CO2 emissions = €100 per ton
( ** )
References 1 Petitjean E., Aquino B., Polley G.T., 2007, Observations on the Use of Tube Inserts to Suppress Fouling in Heat Exchangers, Hydrocarbon World 2007 , Ed. Touch Briefings, pp47-51. 2 Bories M., Patureaux T., 2003, Preheat train crude distillation foul - ing propensity evaluation by Ebert and Panchal model, Proc. Heat Exchanger Fouling & Cleaning: Fundamentals & Applications , Santa Fe, July 2003. 3 Pouponnot F., Krueger A. W., 2004, Heat Transfer Enhancement and fouling mitigation by heat exchanger tube inserts, Int Conf. On Heavy Organic Depositions, Los Cabos, Mexico. 4 Aquino B., Derouin C., Polley G.T., 2007, Towards an understanding of how tube inserts mitigate fouling in heat exchangers, Int Conf on Heat Exchanger fouling and Cleaning , Tomar, Portugal. 5 . Aubin N., Joung J., 2023, Quick win on carbon footprint by improv - ing existing assets, AFPM 2023 Summit, Dallas, Tx, USA. Nicolas Aubin is Technical and R&D Manager at Petroval, based in Saint-Romain-de-Colbosc, France. He has 20 years of experience and holds a master’s in chemical engineering. Email: n.aubin@petroval.com
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