Prediction of overall heat transfer rates in trickle-bed reactors

MARÍA J. TAULAMET; NÉSTOR J. MARIANI; OSVALDO M. MARTÍNEZ; GUILLERMO F. BARRETO

Acapulco

Congreso; International Mexican Congress on Chemical Reaction Engineering (IMCCRE 2014); 2014

Fixed bed reactors with downflow of gas and liquid, frequently called trickle-bed (TBR), have been widely employed for years in the petroleum, petrochemical and chemical plants. Also, new applications as biochemical and electrochemical processes and effluent treatment have arisen in recent years (Ranade et al., 2011). Despite its widespread use, the simulation of these units is far from being carried out confidently, being the complex fluid-dynamics one of the main reasons that introduces uncertainties, in particular because of its strong influence upon heat and mass transport processes. In several applications, such as MIBK (methyl isobutyl ketone) synthesis and SMDS process (Shell Middle Distillates Synthesis) converting natural gas to synthetic hydrocarbons by Fischer-Tropsch synthesis, it is necessary to exchange heat with an external fluid by using a multi-tubular TBR. In these cases the heat transfer process becomes paramount for the overall performance of the reactor. Heat transfer also plays a relevant role in laboratory and bench scale TBR?s when studying the behavior of a given catalyst or when dealing with catalyst screening. It is fairly usual to require an isothermal reactor operation because this condition leads to a simpler analysis on experimental data (Ancheyta, 2011). Then, it is essential to correctly evaluate beforehand the overall heat transfer capacity of the bed. Preliminary designs and simulations for efficient operation of TBR`s can be performed using computer models. 1D models are especially suitable, even when they disregard radial gradients of temperature and concentration, because they are easily formulated and run on computationally simple platforms, while giving adequate results if precise and reliable correlations for parameters are provided. In this context, the goal of this contribution is to report experimental results on heat transfer from a packed bed with cocurrent two-phase downflow to the tube-wall in trickling and pulsing regime. A 1D pseudo-homogeneous plug-flow model with a single adjustable parameter (overall heat transfer coefficient, hG) is proposed to account for the observed heat transfer rates. The effect of gas and liquid flow rates, particle shape and size, and also aspect ratio (tube to particle equivalent diameters ratio), is analysed with the final purpose of developing a reliable correlation.