(15g) CFD ANALYSIS of the Heat Transfer Coefficient During Hydrotreatment of Palm Oil

ABSTRACT Hydrotreatment transforms vegetables oils into a zero-sulfur liquid with properties that resemble those of diesel fuel. In a hydrotreatment reactor, high pressure addition of hydrogen reduces the size of the fatty acids molecules through isomerization and thermal cracking at temperatures close to 350°C (see Figure 1). Figure 1. Figure 1. Proposed process for crude palm oil hydrotreatment Hydrotreatment, an exothermic reaction, is strongly affected by heat transfer as high temperatures in hot spots could degrade the fuel to undesired small-chain hydrocarbons. A preliminary analysis of the hydrotreatment process suggests that a packed bed reactor, similar to those used during hydrotreatment of regular mineral oil, should be preferred to a packed-tube reactor as heat transfer in the former, if correctly designed, is enough to prevent the formation of hot spots. However, this analysis strongly depends on the heat transfer coefficient in the packed bed reactor and at the walls. As hydrotreatment of palm oil for biodiesel production is a relative new technology, there are no correlations in the refereed literature for the heat transfer during hydrotreatment of palm oil in packed-bed reactors. This study considers the use of computational fluid dynamics (CFD) as an alternative for the evaluation of heat transfer in this process. A review of the scientific literature showed that, even though there are no references that deal with CFD simulation of hydrotreatment reactors, there are several studies of CFD simulation of packed-tube reactors. 2D and 3D CFD analyses yielded encouraging results when modeling external air flow through a sphere (see Figure 2), a cylinder and a bank of tubes. The Nusselt numbers (Nu) calculated from the CFD simulations are close to those obtained from commonly used correlations. This CFD analysis was extended to the calculation of the variation Nu with Re for packed spheres and cylinders at conditions that resemble those of the hydrotreatment of palm oil. These correlations, when applied to the original packed-bed reactor design, suggests that an external jacket in a packed-bed reactor could effectively remove the heat produced during hydrotreatment of palm oil. Figure 2. Figure 2. Comparison of the predicted variation of Nu with Re for external flow around a sphere obtained with CFD with four different turbulent models (( ): Spalart-Allmaras; ( ): Reynolds-Stress; ( ): Standard K-Omega; ( ): Standard K-epsilon) with that obtained with the Whitaker Correlation (---). Bold lines represent 15% below and above the value predicted with the Whitaker Correlation.