Why vacuum unit fired heaters coke
A description of the internal workings of vacuum heaters and the causes of coke formation within them
Tony Barletta, Process Consulting Services
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Some refinery vacuum heaters have chronic problems with coking and short run-lengths. Several of these heaters operate at coil outlet temperatures of only 750–760ºF and average radiant section heat flux of 8000Btu/hr-ft2-ºF or less. Why do these seemingly mild operations have run-lengths less than two years between decokings, while others operate for four years at coil outlet temperatures of 790ºF and average flux of 11000Btu/hr-ft2-ºF? Common heater monitoring parameters such as coil outlet temperature, average heat flux, and fired duty are generally of little value in determining why a heater develops hot spots. Hot spots are typically localised phenomena. Often, they are a consequence of decisions made to reduce the heater initial investment.
When revamping, the designer should apply fundamental design principles to meet short term product yield targets and long term run-length objectives. Common heater design considerations that affect the rate of coke lay-down are radiant section tube layout, process coil design, and burner performance. This article reviews how heater design influences localised conditions that promote rapid coke formation. Two case studies show how fundamental principles can be applied to eliminate hot spots and increase run-length.
Coke forms because conditions in the shock or radiant tubes cause the oil to thermally decompose to coke and gas. Coke lay-down on the inside of the tube increases the tube metal temperatures (TMT). As tube metal temperatures increase, the heater firing must be reduced or TMT will progressively increase until the tube metallurgical temperature limit is reached. Then the heater must be shutdown 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.
Heater design affects the localised coke formation rates through its influence on oil residence time and film temperature. The lower velocity oil film flowing along the tube wall will be 25ºF to over 200ºF higher than the oil temperature. For instance, the oil film temperature in the outlet tube may be over 950ºF even though the bulk oil temperature is only 790ºF. Coke formation begins in the oil film flowing on the inside tube wall because its temperature is higher.
Oil film temperature is highest at the front of the tube facing the burner and lowest on the rear of the tube facing the refractory. This peak oil film temperature is where coking starts. The temperature rise through the oil film depends on a number of design factors. Heater tube layout, process coil design, and burner performance all have an effect on the oil film temperature.
Figure 1 represents the relationship between peak oil film temperature, oil residence time, and the rate of coke formation. Operating above the cracking line will cause rapid coke and gas formation that eventually leads to hot spots. Oil stability will move this line up or down. Heater tube layout, process coil design, and burner performance control localised peak film temperatures and oil residence time. Peak film temperature can vary significantly on a single tube due to fire box flue gas temperature gradients.
Vacuum heaters are typically cabin, box, or vertical cylindrical type design with firing on one side of the heater tube. Occasionally double-fired designs are used in tar sand, high bitumen crude, or hydrocracking vacuum residue services where oil stability is poor. Although vertical cylindrical designs are common, they should be avoided because the vertical tubes cause the oil to flow repeatedly through the high heat flux zone. In addition, the sizing of the last two to three tubes in each pass is complicated by pressure variations in the up-flow and down-flow tubes. Box and cabin type heaters are the most common in refinery vacuum units.
Most cabin or box type heaters have four or six passes in a single radiant cell. The height-to-width ratio (L/D) varies from 2.2 to 3.5. The number of burners, flame length, and the distance from the burners to the tubes are all design variables. Figure 2 shows a six-pass box heater with the coils stacked along each wall. Oil flows downward in each pass. This six-pass design will be used to review what happens inside a heater and some of the design considerations (stacked tubes, oil down-flow or up-flow, tube size, location of coil outlets, etc) that influence the rate of coking.
There are many different heater designs. Pass layout, process coil design, and burner performance vary from one heater to the next. While computer models are necessary tools to design and troubleshoot vacuum heaters, these models need to correctly represent actual heater operation. Often, heater models assume ideals that do not exist in the real world. Heater model results and the application of basic fired heater design principles should be used when revamping a vacuum heater.
Fired heater basics
Fired heaters consist of a convection and radiant section (Figure 3). The convection section recovers heat from the flue gas leaving the radiant section (bridgewall) and transfers it to the cold process fluid in the tubes. Convection duty depends on the equipment design, bridgewall temperature, and the flue gas rate. Maximising convection section duty decreases the radiant section duty, which always reduces the rate of coke formation. Once the convection section design is set, the radiant section must provide the remaining heat needed to meet the required coil outlet temperature.
The box heater shown in Figure 2 has horizontal tubes on the radiant section side walls. Three passes are located on each wall. Each pass consists of a number of tubes with the oil flowing downward in each pass from the convection section outlet. Most cabin and box heaters have the passes stacked because this reduces initial heater cost. However, stacking the tubes always results in heat absorption differences between the individual passes.
Today, most refiners vary pass flow rates to achieve equal coil outlet temperature. Hence, high pass flow rate variation indicates large heat absorption differences. Stacked tubes and other factors contribute to localised coking conditions.
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