Process flow determines coker heater performance
Uniform tube metal temperature resulting from a change in process flow enhances the run length of a delayed coker heater.
RAHUL C PATIL, PINAKIRANJAN PATRA, AJAY GUPTA, VED PRAKASH MISHRA and ASIT KUMAR DAS
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A delayed coker in a petroleum refinery processes vacuum residue from the vacuum distillation unit as feed and thermally cracks it into useful products including liquid petroleum gas, naphtha, gasoline, diesel, heavy gas oil, and petcoke. While it is desirable to have the majority of the cracking and coking taking place in the coker drum, a small amount of these reactions occurring inside the heater tubes is inevitable. The coke so produced inside the heater tubes provides additional resistance to heat transfer between the flue gas and the process fluid. Since the coil outlet temperature (COT) is to be kept constant, one may end up by firing more fuel to achieve the desired COT, thereby increasing the tube skin temperature or tube metal temperature (TMT) due to the additional resistance offered by coke. Coke depositing on heater tubes thus usually limits the run length by limiting the TMT which a heater tube can be allowed to experience in view of its metallurgy. For example, if the TMT at start of run (SOR) conditions is 550°C and the maximum allowable TMT is 650°C (generally governed by its metallurgy), for a rate of 2°C/day rise in TMT, the heater would run for 50 days. Therefore, it is desirable to restrict the rate of the TMT increase to a low value, which as such is a function of the rate of increase in the thickness of coke deposits. Once the limiting value of the TMT is reached, coke needs to be removed from the inner surface of the tubes, which normally would require downtime and consequent production loss. Coke is removed from the tubes by two processes: by utilising the difference in thermal expansion coefficient between coke and tube by applying sudden temperature variations (spalling); or by physical scraping of coke from the tube with the help of a moving pig (pigging). The typical time required for spalling and pigging is 1-1.5 days and 3-5 days, respectively. During this period, the throughput of the coker unit is reduced, which results in a loss in production. Further, the tubes are subjected to a harsh environment in both removal methods, and the frequency of coke cleaning eventually determines the life of the heater tubes. Thus it is desirable to increase the run length of the coker heater so as to increase the productivity of the delayed coker unit as well as the life of the coker heater tubes.
A coker heater (see Figure 1) consists of horizontal tubes where feed conventionally enters from the convection section to the radiant section of the heater (a down-flow configuration). The outlet temperature of the heater, the COT, is measured at the radiant section outlet. The burners are normally floor mounted at the bottom of the radiant section of the heater where they fire fuel with air. The radiative heat from the combustion of fuel gas in the radiant section is transferred to vacuum residue from the convection section. The remaining heat from the combustion gases is transferred to preheat vacuum residue in the convection section. In the process, vacuum residue cracks into lighter components. As pressure reduces in the heater tubes, the lighter components, typically from C1 to light naphtha, evaporate to form two phase flow in the heater. Along with cracking, asphaltenes in the vacuum residue contribute to the coking phenomena, and part of this coke deposits on the heater tubes to increase the TMT as time progresses. Besides feed characteristics such as asphaltenes, saturates, and residence time, coking inside the coker heater is a very strong function of the temperature it reaches in the tubes throughout the radiant section of the heater. Various measures are taken when designing the heat exchanger to eliminate or reduce the possibilities of localised peak temperatures.
Design considerations for a coker heater
Owing to the very strong dependence of coking reactions on temperature, it is particularly important to provide a uniform temperature profile along the length of the tube. Non-uniform coking is caused by an irregular temperature profile experienced by the process fluid. Proper burner placement along the tube ensures a uniform temperature profile along the length of the tube.
The other important factor which is considered in the latest designs of coker heater is uniform temperature distribution across the diameter of the tube. In earlier designs of coker heater, the burners are placed at the centre of the heater, with tubes placed horizontally along the wall. This arrangement of burner and tubes, termed a single fired coker heater, causes the half of the tube facing a burner flame to experience higher temperatures compared to the other half which faces refractory at a lower temperature. This results in a non-uniform temperature profile across the circumference of the tubes. Newer designs provide tubes at the centre of the heater facing flames from burners placed symmetrically on two sides of the tube. This arrangement of burners and tubes, termed a dual fired coker heater, causes both sides of the tube to experience the same temperature, unlike the single fired coker heater arrangement. The schematics (adopted from Golden & Barletta) of a single fired and dual fired coker heater are shown in Figure 2. While for a conventional single fired coker heater with an uneven temperature profile around the tube circumference, the peak heat flux is 1.8 times that of the average heat flux, for a dual fired coker heater this peak flux is only 1.15-1.2 times that of the average heat flux, thus offering a more even temperature profile around the circumference of the tube. These design changes have not only resulted in uniform circumferential TMTs but have enabled an increase in average heat flux value by as much as 50% for the same peak heat flux.
Strategies to improve the run length of a coker heater
Once the above provisions are made, a simple approach to increase the run length of the coker heater is to offload the radiant section by shifting its heat load to the convection section or redistributing its heat load locally to reduce non- uniformity in the temperature experienced by vacuum reside inside the radiant zone tubes. In a conventional delayed coker heater, the process fluid flows from top to bottom. Here, high fluid temperature inside the tubes meets high temperature flue gas from outside. This causes higher film formation as well as TMTs at which cracking and coking takes place. Ultimately, these tubes in the bottom section of the heater limit the run length of the coker heater. The temperature at which coking takes place can be reduced either by changing the process side temperatures (by varying the direction of the process fluid) or by changing the flue gas temperature in the fire box (using more excess air) or by varying the emissivity of the tubes (by varying the emissivity coating) so as to make it more uniform.
One way to evenly distribute the heat load across the radiant zone is to apply emissivity coatings to the outer surfaces of the tubes. The coatings are applied in such a way that the tubes with high fluid temperature in the bottom (the high temperature region) are covered with a low emissivity coating so as to absorb less heat, and the tubes with low fluid temperature in the top low-temperature region are smeared with a high emissivity coating so as to absorb more heat. Because of these different emissivity coatings, the heating rate experienced by hot fluid reduces and that experienced by cold fluid increases, which causes a more uniform cracking temperature. Such coatings have also been used to make the heating rate uniform across the circumference of the tube in a single fired heater.
In a new configuration of coker heater with respect to the flow of process fluid, termed upflow configuration (see Figure 3b), vacuum residue enters the radiant section from the bottom and exits at the top of the radiant section. This means that fluid at a lower temperature has a higher heating rate and fluid at a higher temperature has a lower heating rate compared to conventional heaters, leading to a more uniform cracking temperature. This design claims to give improved coker heater performance by migration of the hottest part of the process fluid to the upper portion with a lower heating rate, resulting in lower TMTs, reduced coking rates and higher run length.
This article attempts to study the effects of change in process flow direction on heat transfer distribution in the radiant and convection sections of a delayed coker heater. This was determined by applying a heater model employing commercial as well as newly developed tools.
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