Modeling and Simulation of the Biodisel Production in a Pilot Continuous Reactor

Abstract

The finite elements method (FEM) is a numerical method especially useful for solve coupled engineering problems where the analytical solution is not possible due its high complex. In this paper, the liquid-phase biodiesel production in a pilot continuous reactor at the Pilot Plant of Reaction, Chemical Engineering Dept. (UNCPBA) was studied. The objectives were: a) develop a program based on FEM to simulate the transfer of momentum and heat in a pilot tubular reactor, b) couple the kinetic reaction of catalytic transesterification of a non-conventional oil with an alcohol into the general program, with the aim to optimize the process, c) to use the computational tool Aspen-HYSYS to simulate the process of biodiesel production and estimate useful data for the FEM model, d) to validate the numerical model with experimental data. This work presents the study and simulation of the catalytic reaction of transesterification of palm oil with methanol employing an alkaline catalysts to obtain methyl esters (biodiesel) and glycerol under different conditions (oil/methanol feed rates, reaction temperatures, catalyst concentrations) at the pilot plant reaction (PPR). The properties of the streams of reactants and products were obtained from the NRTL fluid package of Aspen-HYSYS. The reaction was defined as kinetics of first order for both oil and methanol. The kinetics constant for the forward reaction was expressed in Arrhenius form equation. The activation energy was determined from literature (Cheng et al., J Oil Palm Res, 16(2):19-29, 2004) and resulted 60737 J/mol with a frequency factor of 9.153109 m3/(mol-s) for the condition of maximum velocity of biodiesel production (using 0.125 mol NaOH/oil kg at 60ºC with oil:methanol feed rate of 1:10). Solving the mass balance of an isothermal plug flow reactor under these conditions, it was estimated that the pilot reactor (assumed ideal) could reach 99% conversion in 5 minutes. For modeling the process of biodiesel production in the PPR, the mass balance, energy and momentum were planed, along with other equations, conforming a system of coupled differential equations. For its resolution, a CFD model previously developed to describe the flow patterns in the mixer-reactor system through the Navier-Stokes module for incompressible fluids in stationary stage of COMSOL Multiphysics 3.5a was used. This model was coupled with the mass Convection and Diffusion, and the Heat Transfer by Convection and Conduction modules to the CFD model, including the reaction kinetics of transesterification of tripalmitin and esterification of palmitic acid. A 2D domain was considered which was discretized with Lagrange quadrangular elements. First, the transfer of momentum problem was solved; for that, fluid properties (density, viscosity) were defined based on Aspen-HYSYS. The initial conditions of null velocity and pressure were imposed. The boundary conditions were: fixed normal fluid velocity at the reactor inlet, no slip at lateral walls, and null pressure at the reactor outlet. The Direct (UNFPACK) solver was used. Convergence of solution was obtained for different feed conditions (velocity, Reynolds number, temperature), allowing to determine the velocity and pressure profiles which were stored for later solve the Convection and Diffusion, and Heat Transfer modules with the fluid moving inside the reactor. The reaction velocities of transesterification and esterification were defined such as global expressions and the diffusivities of the species were assumed identical. As results from the resolution of the whole coupled MEF model, conversions at the outlet reactor were corresponding with experimental data.