(46b) Retrofit of Refinery Heat Exchangers to Mitigate Fouling Based On An Advanced Thermo-Hydraulic Model

Stringent environmental policies and tightening refining margins in a highly competitive market are forcing oil companies worldwide to increase their focus on the efficient utilisation of energy. Fouling, the deposition of unwanted material on heat transfer surfaces is a ubiquitous problem in oil refineries and a major cause of costly inefficiencies.

Traditional design and retrofit methodologies rely on highly empirical, fixed, TEMA fouling factors. The use of these ?safety factors' has been largely criticized [1, 2] as it often results in over-designed heat transfer units that, in theory, should compensate for reduced thermal efficiency given by fouling deposition. Ultimately, however, this approach turns out to exacerbate fouling rather than mitigating it as a consequence of producing higher temperatures at start-up and lower fluid velocities than those for which the unit was designed [3]. The ability to capture at the design stage the dependence of fouling on process conditions and fouling evolution over time becomes therefore pivotal to achieve fouling?resilient designs and retrofits of heat exchangers.

A novel, sophisticated thermo-hydraulic mathematical model for shell-and-tube heat exchangers that takes into account local fouling rates and deposit ageing as a function of exchanger geometry, oil characteristics, process conditions and time was recently proposed [4]. The model has been validated against real plant data from four units in two refineries operated by major oil companies showing for all units an accuracy in predicting outlet temperatures (in ºC) over extended periods (i.e. 4?16 months) with an excellent accuracy of ±1% for the tube?side and ±2% for the shell?side [5]. The model has also been used to assess economic and environmental impact of fouling [6] and to evaluate retrofit configurations for the network structure [7] of refinery pre-heat trains.

In this paper, the model is used to assess alternative retrofit options for existing units. The thermo-hydraulic simulations allow capturing in a quantitative way the complex interactions between flow dynamics and fouling deposit growth over time, and establishing the trade-off between costs and benefits. Instant and integral energy losses caused by fouling can thus be accurately assessed for all the retrofit options proposed and configurations that mitigate fouling by design identified.

References

[1] Chenoweth, J. M., 1997, "The Tema Standards Fouling Section: Another Look," Proc. Fouling Mitigation of Industrial Heat-Exchange Equipment, C. B. Panchal, ed. San Luis Obispo, California (USA), pp. 61-71.

[2] Bennett, C. A., Kistler, R. S., Lestina, T. G., and King, D. C., 2007, "Improving Heat Exchanger Designs," Chemical Engineering Progress, 103(4), pp. 40-45.

[3] Butterworth, D., 2004, "Process Heat Transfer 2010," Applied Thermal Engineering, 24(8-9), pp. 1395-1407.

[4] Coletti, F., and Macchietto, S., 2009, "A Dynamic, Distributed Model of Shell-and-Tube Heat Exchangers Undergoing Crude Oil Fouling," Submitted for publication.

[5] Coletti, F., and Macchietto, S., 2010, "Validation of a Novel Model for Shell and Tube Heat Exchangers Undergoing Crude Oil Fouling," Proc. 14th International Heat Transfer Conference (IHTC14), Washington, DC (USA).

[6] Coletti, F., and Macchietto, S., 2009, "Refinery Pre-Heat Train Network Simulation: Assessment of Energy Efficiency and Carbon Emissions," Proc. International Conference on Heat Exchanger Fouling and Cleaning VIII, H. Müller-Steinhagen, et al., eds., Schladming, Austria, pp. 61-68.

[7] Coletti, F., Macchietto, S., and Polley, G. T., 2010, "Effects of Fouling on Performance of Retrofitted Heat Exchanger Networks; a Thermo-Hydraulic Based Analysis," Ischia, Naples, Italy, accepted.