Sunun Limtrakul

Discrete particle simulation of solids motion in a gas-solid fluidized bed

Abstract The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newtons second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.

Mathematical modeling and simulation for gas-liquid reactors

Abstract Gas–liquid reactors are widely used in many industrial processes such as oxidation, hydroformylation, chlorination, etc. The paper develops comprehensive model for reactors using the mixing cell approach. It incorporates heat and mass transfer effects in the film and uses a boundary element method to solve the film model equations. The fluxes obtained at the interface are then directly used as the link to the reactor model. Simple isothermal and non-isothermal reactions were numerically tested. Application to two industrially important case studies, chlorination of butanoic acid and oxidation of cyclohexane are briefly illustrated. For the autocatalytic chlorination of butanoic acid, the yield of desired product, monochlorobutanoic acid, is favored by the high degree of mixing in the liquid phase. Therefore, this reaction should be carried out in a CSTR. A series of five bubble tanks with parallel gas reactant feed for cyclohexane oxidation was also simulated. It was found that the cyclohexane conversion is low while the oxygen conversion is relatively high and almost constant in each tank. Due to the complex multistep nature of this reaction scheme, oxygen is consumed in many steps of oxidation and selectivity of main products (which are intermediate products in the reaction scheme) depends on the critical control of over-oxidation in the kinetic mechanism.

DEM MODELING AND SIMULATION OF A DOWN-FLOW CIRCULATING FLUIDIZED BED

A mathematical model based on the distinct element method (DEM) was developed to investigate the hydrodynamics in a gas-solid down-flow circulating fluidized bed reactor (downer). The models consist of the equations of particle motion and fluid motion. The contact force is calculated by using the analogy of a spring, dashpot, and friction slider. Simulation results show that the radial solids holdup and particle velocity profiles are uniform in the core region. Near the wall, the solids holdup is higher with lower particle velocity. An increase in the particle size decreases the solids holdup and increases the particle velocity. The solids holdup decreases with superficial gas velocity but increases with solids circulation rate. Particle velocity increases with gas velocity and solids circulation rate. The solids holdup and particle velocity are almost uniform along the height of the downer except near the distributor. The hydrodynamic behavior from this simulation showed trends similar to those of the experimental results. The results obtained from this model fit better with the experimental results than Kimms and Bolkans models do.

Heat transfer effect in scaling-up a fluidized bed reactor for propylene polymerization

The effects of operating conditions and scaling-up on reactor temperature control and performance in propylene polymerization fluidized bed reactors were studied by phenomenological and CFD models. A phenomenological model with CFD hydrodynamics parameters predicts average information, while a CFD-based reactor model provides local information. Results suggest improved productivity and reactor temperature control by cautiously increasing catalyst feed rate, operating temperature, reactor size and superficial velocity, with consideration of hot spots and catalyst deactivation. High catalyst loading increases productivity but involves risk with regards to the control of oscillating temperature and hot spots. The model identifies an operating window to improve productivity and temperature control and to study operation details. Mixing effect is important to heat transfer but not to propylene conversion. Scaling-up cannot provide similarity of heat transfer. Keeping the same temperature when scaling up from 0.2 to 4 m in diameter requires heat transfer area multiplying factors of 2.43 to 5.26 or lowering the wall temperature by 7 to 18 K. Hot spots are detected with a temperature variation of 10 to 14 K. The results are useful for analyses of laboratory and industrial scale reactors and provide information on scale up.