Two-fluid simulation of an airlift loop reactor with fluent
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Abstract
Airlift loop reactors are frequently applied in the chemical and biological industry. They are made of two sections interconnected at the top and bottom, with an ungassed downcomer and a gassed riser. In this type of reactor the density-difference between the sections supplies the needed driving force needed for the liquid circulation. Numerical simulations are recognised as a primary tool for improving the performance of process equipment for scaling up of the airlift loop reactors. Three dimensional simulations of two phase (gas and hquid) bubbly flow in a rectangular airlift loop reactor with two downcomer sections have been obtained using the CFD package Fluent 4.5.6. The simulations are based on a full two fluid model with a modified k - ? model for the turbulence. As the interfacial forces between the two phases, the drag force and virtual mass have been taken into account. The results are compared with an one-dimensional mechanical energy balance and are found to be in good agreement when a 'false' time step of 1 10^-2 s is used. The mean riser and downcomer gas fractions are too high in the simulations with time step of 1 10^-3 s, in comparison with this one-dimensional balance. Probably this is caused by the virtual mass. The interface turbulent momentum transfer terms (the turbulent diffusion terms), which can only be calculated for a dilute secondary phase are necessary for reahstic results. Outlets with a gas disengagement zone at the top can not be calculated well due to this limitation. An outlet type with fixed velocities on top has to be chosen, which works well but misses the flexibility for the gas throughput and velocities. After refining the grid the flow fleld changes and gives better results, but the calculation time increases. The minimal grid size has to be 30 x 10 x 80 (width x depth X height). Furthermore, two symmetry axis can used namely, one vertical plane through the riser and one vertical between the front and back wall through the riser and downcomer. With the use of both symmetry axis the calculation time is reduced signiflcantly.