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CFD Simulation of Forced Recirculating Fired Heated Reboilers

By Dr. Alon Davidy

In the framework of this research, an advanced algorithm has been developed in order to analyze the performance of the re-boiling process of crude oil flowing inside reboilers tubes. Reboilers are applied to provide the vapor flux to feed the bottom tray of the distillation column. The liquid exiting from the bottom of the column is partially vaporized in the heated fired furnace. It is assumed that the heating medium is flue gaseous mixture produced by Heptane burning. There are four principal types of reboilers utilized in the distillation columns:  

1) Fired Reboilers

2) Thermosiphon Natural Circulation Reboiler

3) Kettle type

4) Forced Recirculating Fired Heated Reboiler (see Figure 1)

Distillation bottom crude oil liquids are often mixtures having substantial different boiling ranges.  Thermo-physical properties of the liquid and vapor exhibit large variations throughout the reboiler. Thermodynamic calculations are required to determine the phase compositions and other properties within the reboiler.

Figure 1: Schematics of forced recirculating fired heater reboiler [1].

The proposed theoretical model is composed from the Heptane fire heater and a tube array. The heat flux produced from the burner is transferred to the crude oil flowing inside the tube. The computational model is composed of two phases — Simulation of fire by using Fire Dynamics Simulator software (FDS) version 5.0 and then a Nucleate boiling computation of the crude oil. The FDS code is formulated based on CFD of fire heater. The FDS numerical solution has been carried out by using Large Eddy Simulation (LES) method. The temperature field at t = 80.5 s is shown in Figure 2. 

Figure 2. Temperature field (°C) inside the burner at t = 80.5 s.

It can be seen from this figure that the maximal temperature at time = 80.5 s reaches to 915 °C. The peak temperature results obtained by FDS software (915°C) has been compared to the literature. It is claimed that typical post-flashover room fire will have more commonly a peak temperature of 900 to 1,000 °C (1,652 to 1,832 °F). When heat is gradually transferred to the crude oil, several processes are taking place. As the temperature of the crude oil exceeds the boiling point, it begins to evaporate. At first individual bubbles form at the heating surface (saturation state). When they reach a particular diameter, they are breaking away and as a result, there is intensive agitation of the boundary layer and the whole mass of boiling liquid. As the heat flux increases the number of crude oil bubbles formed at the heating surface increases. This leads to a further increase in turbulence, and a rapid rise in the heat transfer coefficient. The boiling regime occurring during bubble generation in the vicinity of the tube wall surface is called “nucleate boiling.” In this boiling regime, high values of convective heat transfer are obtained with relatively small temperature gradient. Nucleate boiling is intensified as the wall temperature increases but is suppressed as the fluid velocity increases. As the forced convection component increases, the nucleate boiling component decreases (by suppression).  The thermal heat transfer to evaporating two-phase crude oil mixture occurs by bubble generation at the wall (nucleate boiling) and has been calculated by using Chen correlation. It has been assumed that overall convective heat transfer coefficient is composed from the nucleate boiling convective coefficient and the forced turbulent convective coefficient. The former is calculated by Forster Zuber empirical equation. The latter is computed from the Dittus-Boelter relationship. In order to validate the nucleate boiling heat transfer coefficient, a comparison has been performed to nucleate boiling convective coefficient obtained by Mostinski equation. The relative error between the nucleate boiling convective heat-transfer coefficients is: 10.5%. Designing a reboiler requires knowledge of thermodynamic and thermo-physical properties crude oil such as: boiling temperature, pressure, heat capacity, thermal conductivity, diffusivity and density. Heat exchanger design depends on enthalpies, thermal conductivities and viscosity of the flowing streams (the crude oil and the steam). The thermo-physical properties (such as: thermal conductivity, heat capacity, surface tension, viscosity) of the crude oil have been estimated by using empirical correlations. The external convective heat transfer across banks of tubes has been calculated by Grimison empirical correlation. The required transport properties of the flue gaseous mixture were calculated by using STANJAN software. The radiative heat flux emitted by the mixture flue gaseous and the soot particles has been also evaluated. A thermal structural analyses study has been performed in order to analyze the influence of the radiative and convective heat flux on the reboiler tube structural integrity. The COMSOL software has been loaded with the convective and radiative thermal loads. Figure 3 shows the 3D temperature field inside the steel tube.

Figure 3. 3D plot of the reboiler tube temperature field.

From Figure 3 it can be seen that temperature gradient inside the steel is small. This is because the thermal conductivity of the steel is high. This work has been further extended to include also the structural integrity aspects of the reboiler metal pipe by using COMSOL Multiphysics software. It was found out, that the calculated stress is less than the ultimate tensile strength of the AISI 310 Steel alloy. 

More information is available in the following paper:

[1] Davidy, A. CFD Simulation of Forced Recirculating Fired Heated Reboilers. Processes, 2020, 8, 145.

Note: The views, thoughts, and opinions expressed in the content above belong solely to the author and do not necessarily reflect the opinions and beliefs of Refining Community or its parent company, CRU Group. 

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