Feb-2025

Optimising shell and tube heat exchanger operation

Case studies show how inserts improve heat transfer coefficients, mitigate fouling and reduce end-of-run pressure drop, as demonstrated with the preheat train of a CDU.

Nicolas Aubin
Petroval

Article Summary

Several studies published by Total Energies and Petroval have examined the improvements that can be obtained from tube inserts in heat exchangers. The benefits of using a combination of tube insert technologies are manifested in extended run lengths between cleaning shutdowns, an increased heat transfer coefficient, a reduced fouling rate, and stability of pressure drop.

From an economic viewpoint, the payback is achieved within a few months from four sources of improvements: the preheat train energy saved (by the increase in the heat transfer), the reduction in maintenance cost (reduced cleaning frequency), the increased throughput, and positive environmental impact stemming from the reduction in CO₂ emissions (as a consequence of the better heat transfer performance). Indeed, a very substantial benefit can be obtained if a unit is bottlenecked by a heat transfer limitation or the furnace.

Technology limits
Limiting the carbon footprint is now essential to achieve net-zero emissions in the oil industry by 2050. This ambitious target will require large investments in new technologies for heat transfer efficiency and carbon capture. However, the technologies that will be required in the future are not yet available at an industrial scale; time is needed for their maturation, investment, and timely operation.

The oil industry relies mainly on preheat shell and tube heat exchangers to reduce the amount of firing required in fired furnaces. However, the performance of these exchangers is often limited by fouling and mechanical designs not upgraded to the required level of operation. There are tube insert technologies available on the market that offer a quick solution to enhance the performance of shell and tube exchangers. These provide immediate improvements in heat transfer from start of run (SOR) with no modifications to the exchangers or the operating conditions.

The benefits of using tube insert technologies were previously demonstrated in terms of an increased heat transfer coefficient^1,3, reduced fouling rate,² and stability of pressure drop. This current study will only consider fouling in crude oil preheat trains caused by asphaltene deposition and/or coke formation on hot surfaces.

In these tests, heat exchangers forming part of preheat trains in three refineries were equipped with new inserts. Their performances were monitored over two to four years, depending on the circumstances, and compared to the durations of previous runs in similar process conditions. The improvements in heat transfer and the impact on CO2 emissions will be further highlighted.

Fouling reduction case studies
Case A - Rotational effect
The Turbotal rotating device is hooked onto a stationary head and installed at the inlet end of the heat exchanger tube (see Figure 1). This system is a continuous online cleaning device, the purpose of which is to reduce the fouling layer at the tube walls by means of a mechanical effect.

The device uses the energy of the flowing medium in the tubes to achieve rotation at around 1,000 rpm during the whole run duration. This rotation speed is determined at the design stage by the mechanical design of the Turbotal and issued from correlations determined on experimental skids.

The extra pressure drop generated is typically in the range of 100 millibar per pass at a flow velocity of 1.0 m/s, with a lifetime limited to three years due to mechanical erosion of the parts. The last two pairs of heat exchangers just before the furnace were suffering from severe fouling over a period of less than one year.  All four heat exchangers were equipped with Turbotal and operated in the same range of process conditions as previously (see Table 1). The monitoring of the performance was then compared to the previous data; the comparative trend of the outlet temperature will be presented in the results section.

Case B - Vibrational effect
The Spirelf vibrating device is fixed on both tube ends by a fixing wire (see Figure 2). This system also serves as a continuous online cleaning device, reducing the fouling layer on the tube walls by means of a mechanical effect.

The vibrating device uses the energy of the flowing medium in the tubes to convert it into vibrations of the device, both radial and longitudinal. The extra pressure drop generated by the device is typically in the range of 200 millibars per pass for a flow velocity of 1.0 m/s. The lifetime of the device is limited to six years since it must be removed and replaced at each turnaround for internal cleaning and inspection of the heat exchanger tubes.

The last pair of heat exchangers, just before the furnace, suffered from severe fouling over a period of less than one year. The two heat exchangers were equipped with Spirelf and operated in the same range of process conditions as previously (see Table 2). The monitoring of the performance was then compared to the previous data. The comparative trends of the duty achieved and the flow rates will be presented in the results section.

Case C - Heat transfer effect
To promote turbulence at the inside tube surface, the Fixotal system significantly increases shear stress at the wall, preventing product stagnation in the boundary layer adjacent to the tube. The purpose of this fixed device is mainly to increase the rate of heat transfer by renewing the boundary layer at the tube wall, with an appreciable side effect on fouling mitigation, including certain types of fouling linked to wall temperature, such as polymerisation, paraffin solidification, scaling, and crystallisation.

The extra pressure drop generated by the device is typically in the range of 200 millibars per pass for a flow velocity of 1.0 m/s. An example of Fixotal installed in a tube bundle is presented in Figure 3 to illustrate the device once installed.

The chosen case study will review the performance of a complete preheat train of 12 heat exchangers that are all operated with the same fluids. Crude is flowing on the shell side from the desalter to the furnace, and atmospheric residue is flowing counter-current on the tube side from the tower towards the beginning of the hot train.

Only the last three exchangers out of the 12 were equipped with Fixotal technology and operated in the same range of process conditions as previously (see Table 3). The monitoring of the performance was then compared with the previous data; the comparative trends of the overall heat transfer coefficient (OHTC) and duty will be presented in the results section.

Due to a lack of instrumentation, only three temperature measurements points were available on each flow pass: at the inlet, in the middle (after six bundles), and at the outlet. Consequently, the improvements achieved in the last three heat exchangers were mitigated with the normal performance of the other three that were not equipped between the two temperature indicators.

Case A results
The trend presented in Figure 4 shows the OHTC of the four heat exchangers in operation on comparative runs. The reference run in blue lasted only 183 days, with a significant loss of performance as the OHTC dropped from 230 kcal/h.m² °C at SOR to 87 kcal/h.m² °C within this six-month period. After this a shutdown and mechanical cleaning were required to recover heat transfer on these exchangers.