Revamping atmospheric crude heaters
Revamp of an atmospheric crude unit heater suffering from coking caused by asphaltene precipitation resulted in an increased heater run length
Michael Whatley, Navajo Refining Company
Scott Golden and Jason Nigg, Process Consulting Services
Viewed : 10119
Refinery atmospheric crude heaters can experience rapid increases in tube metal temperature (TMT), requiring unplanned shutdowns for decoking. In the case of the Navajo Refining Company facility in Artesia, New Mexico, USA, rapid increases in the atmospheric crude heater’s TMT resulted in a shutdown every three to six months. In this case, as well as others observed in the industry, coke formation was initiated by asphaltene precipitation from unstable crudes.
Industry-wide, atmospheric crude heater coking is an unusual problem, with some heaters operating reliably at an average radiant section heat flux of 13–14 000 Btu/hr-ft2 or higher. However, some crude oils, including those produced from North American fields in West Texas, New Mexico, Ohio/Pennsylvania and Alberta are known to be unstable when there is asphaltene precipitation in certain areas of the crude unit. The equipment in these areas includes preheat exchangers, fired heaters and atmospheric column flash zone and stripping section internals.
Atmospheric crude heater coking
The Artesia refinery was experiencing chronic coking in its atmospheric column heater, with periodic shutdowns to remove coke at intervals as short as 90 days. Figure 1 shows the process flow scheme with the heater located downstream from the prefractionator column. Flashed crude is charged to the atmospheric crude heater, which feeds the atmospheric crude column. The heater was a vertical-tube hexagonal-shaped four-pass design with 12 burners. Prior to the revamp, the radiant section’s average heat flux rate was only 9400 Btu/hr-ft2 with an oil outlet temperature of just 635°F. Furthermore, the heater had oil mass flux rates of 250 lb/sec-ft2 and relatively poor flame stability. This, in combination with poor asphaltene stability, caused a very short heater run length even though the heater was operating at relatively mild conditions. Some atmospheric crude heaters operate at an average radiant section heat flux of 13–14 000 Btu/hr-ft2 and oil outlet temperatures of 730°F or higher, while meeting four- to five-year run lengths.
Coke forms because conditions in the shock or radiant tubes cause the oil to thermally decompose to coke and gas. The TMT increases as coke lays down on the inside of the tube. With rising TMTs, heater firing must decrease or the TMTs will progressively escalate until their limit is reached. The heater must then be shut down to remove the coke. Rapid coke formation is caused by a combination of high oil film temperature, long oil residence time and inherent oil stability. In the majority of cases where atmospheric heater coking occurs, the root cause is high average heat flux, high localised heat flux or flame impingement.
Oil thermal stability depends on crude type. For example, some Canadian and Venezuelan crude oils have poor thermal stability and begin to generate gas at heater outlet temperatures as low as 680°F. At outlet temperatures much above 700°F, these same crudes begin to deposit sufficient amounts of coke to reduce heater runs to two years or less. Another form of oil instability is asphaltene precipitation. As the oil is heated, the asphaltenes become less soluble, depositing in low-velocity areas, fouling crude preheat exchangers, heater tubes or atmospheric column internals. In some cases, the asphaltenes do not drop out until they reach the bottom of the atmospheric column or inside the vacuum heater. With some Canadian crude oils, especially the bitumen-based oil sands crudes, asphaltene precipitation occurs inside the vacuum heater tubes rather than in the atmospheric heater. The same heater design parameters that improve atmospheric heater performance also increase vacuum heater run length.
When asphaltenes separate from the crude oil, the material deposits inside the tubes. This increases heat-transfer resistance, raising asphaltene temperature and TMTs. Furthermore, when asphaltene deposits are widespread in the convection or radiant sections, heater firing must increase to meet the targeted heater outlet temperature. This leads to a higher localised heat flux, further raising the temperature of the asphaltenes deposited on the inside of the tubes. The temperature of these asphaltenes eventually exceeds their thermal stability, resulting in coke formation and even higher TMTs, because the coke layer has lower thermal conductivity than asphaltenes. Heater TMTs eventually exceed metallurgical limits, requiring a heater shutdown to remove the coke. In this example, heater run lengths were as low as 90 days between piggings.
Crude stability is a function of its source and highly variable. However, the designer can influence the process and equipment design to minimise the effect of poor asphaltene stability. In some cases, the material deposits inside the exchangers, piping, heater tubes or fractionation column. The lower the velocity, the more likely it is that asphaltenes will precipitate. In this example, the oil velocity inside the heater tubes was only 5.5–6 ft/s prior to the oil vapourising, which corresponds to a 250 lb/sec-ft2 oil mass flux. At these velocities, whether in an exchanger or heater tube, asphaltenes will likely drop out. Heat exchanger data gathered from hundreds of operating exchangers shows the rate of fouling and the ultimate fouling factor are to a large extent determined by the velocity of the crude flowing through the tubes (assuming no shell-side design problems).
Asphaltene precipitation in crude preheat exchanger tubes is common. Figure 2 shows asphaltene precipitation inside the channel head and tubes in a unit processing West Texas crudes. In this case, the oil velocity inside the tubes was less than 5 ft/s and severe fouling occurred. Exchangers operating at higher velocities in the same unit had less fouling. Moreover, the atmospheric heater downstream of the fouled exchanger had short heater runs, with TMTs increasing at 1°F/day. This rate of TMT rise is similar to a delayed coker heater. Crudes with poor asphaltene stability are especially difficult to process and the equipment must be carefully designed.
Maintaining a high velocity in the equipment minimises asphaltene precipitation. Crude preheat exchangers and heater tubes should be designed for oil velocities of 8–10 ft/s or higher. Experience shows significant improvements in crude preheat and heater reliability when velocities are high. Since many designers set the maximum allowable pressure drop through exchangers and the heater as design criteria, low velocities are often the result of meeting pressure drop specifications. Crude preheat exchangers and fired heater designs should be based on a higher velocity, with the pressure drop simply a result of the design.
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