Need to increase production
In this application, Petro-Chem’s client wanted to boost the output of a heater. The primary challenge was that increasing its output raised the risk of overheating the tubes that are designed to conduct heat to the oil flowing through them. If the temperature of the tubes rises above a certain level, the oil could solidify and block the passage through the tubes. If the temperature rises to an even higher level, the tubes could become so hot that they begin to melt. Engineers ran experiments that showed that increasing the output of the heater to the levels required to obtain the production increase would raise the temperature of the tubes beyond acceptable limits. Performing a series of physical experiments on the actual heater to evaluate alternate design configurations was out of the question, however, because it would require an extended period of downtime at the refinery. On the other hand, building a scale model would have been expensive and taken a considerable amount of time. Physical experiments, either on the actual heater or a scale model, would have also made it difficult to determine the cause of the problem since it would have been very difficult to measure the operating conditions inside the heater.
The refinery asked Petro-Chem to help solve the problem based on their extensive experience in the use of computer simulation, and particularly CFD, to help optimize hydrocarbon and chemical processing. A CFD analysis provides fluid velocity, pressure and temperature values throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, a researcher may change the geometry of the system or the boundary conditions such as inlet velocity or temperature and view the effect on fluid flow patterns. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop prototype equipment and thus reduce design cycle times and costs. According to Deng, Petro-Chem engineers selected FLUENT CFD software from Fluent Incorporated, Lebanon, New Hampshire, because “the program is very easy to use and FLUENT offers the widest range of physical models in the industry, which allows us to handle virtually any type of problem.”
Analysis provides understanding of problem
Petro-Chem engineers began by modeling the existing heater configuration. The engineers defined the geometry of the heater using Fluent’s GAMBIT pre-processor software, then allowed the software to automatically generate a mesh consisting of about 500,000 cells. They used hexahedral elements for the majority of the model and tetrahedral elements for areas with complicated geometry, such as the areas surrounding the tubes. The results showed that a flue gas inversion zone exists in the middle of the radiant chamber, forcing the flame, mostly hot flue gas, towards the tubes against the wall, and causing the tubes to be overheated.
“The visualization of the problem gained from the simulation helped us to quickly reach a solution,” Deng said. “We realized that we needed to break up the inversion zone and ideally create a single recirculation zone that would cover all or most of the heater. Since the variable that was under our control was the position of the burners, we tried a number of different configurations and compared the results. We ended up with an ideal arrangement. Rerunning the analysis with this burner arrangement showed that a single recirculation zone developed in the entire heater, with hot flue gas rising in the middle of the radiant chamber and cold flue gas moving from the roof to the bottom of the heater behind the tubes, providing a much more uniform heat distribution, and thereby reducing the temperature of the tubes.”
Petro-Chem engineers showed the results of the simulation to the client and they were convinced that the new design would make it possible for production to be increased. During the next turnaround, the heater configuration was changed as recommended by Petro-Chem and the new design worked exactly as predicted, enabling an output gain of 20% without damage to the tubes. “The key to the success of this application was the use of CFD to model the flue gas flow patterns inside the heater and to evaluate the performance of various burner configurations,” Deng said. “The information that we gained from the analysis was far more detailed than what we could have obtained from physical testing. The results that we achieved in this application are similar to those that we have seen in many other hydrocarbon and chemical processing applications. As a result, CFD has become our primary tool for solving customer problems and optimizing refinery performance.”
The flue gas velocity profile at the center plane.
The flue gas velocity profile with velocity vectors to show the circulation patterns.
The flue gas temperature profile at the center and bottom planes.