Gas maldistribution in fluidized beds

Resumo

Gas maldistribution is one of the most common problems in large-scale fluidized beds. High superficial gas velocities should be used to create a high pressure drop across the distributor plate to achieve an even gas distribution within the bed. However, higher pressure drop leads to higher power consumption in blowers, which means higher operational costs. In addition, the superficial gas velocity in large-scale reactors is seldom constant. As long as gas velocity perturbations lead to pressure drop variations, it is important to maintain the distributor pressure drop as low as possible. Maldistribution depends on the superficial gas velocity, the type of solids used as bed material and the distributor plate design. Low values of superficial gas velocity are more prone to generate gas maldistribution and therefore, several authors recommend operating beyond a critical value of gas velocity, at which all distributor plate orifices or tuyeres are considered active. The type of solids used as bed material determines the tendency of the bed to agglomerate. Since agglomeration could lead to maldistribution and defluidization, it is important to select the bed material properly. The distributor design affects mainly to the pressure drop, which is also dependent of other variables such the superficial gas velocity, the number of orifices and its diameter, the distributor plate thickness, the temperature. . . Pressure measurements and visual inspection of the bed surface are robust techniques commonly used in fluidized bed reactors for monitoring purposes. Therefore, these techniques were employed in this thesis to study maldistribution. Visual inspection technique was employed in a 3D cylindrical bubbling fluidized bed to detect maldistribution in the bed surface. To create a controlled induced maldistribution, the half of the distributor plate cross-section was covered. It was found that a high superficial gas velocity could overcome maldistribution at the bed surface, even though the maldistribution problem could still prevail at the bottom of the bed. According to that, pressure fluctuations measurements were investigated as a detection method for maldistribution. Several monitoring methods based on the pressure signal analysis were studied to evaluate the boundary of maldistribution grade that can be detected. The effect of the pressure probe location was also investigated and it was concluded that pressure probes should be located at 50-75% of the bed height for maldistribution detection purposes. A single pressure probe could be suficient to detect maldistribution in a lab-scale fluidized bed; however, several pressure probes should be placed in a large-scale fluidized bed reactor to cover all the bed cross-section. Visual inspection technique was also employed to develop a model for estimating the size of the stagnant zones created by covered parts of the distributor plate. A correlation was obtained using Digital Image Analysis and Particle Image Velocimetry of pictures taken in a pseudo-2D fluidized bed. The proposed correlation, coupled with correlations from the literature, was extrapolated to a 3D facility. The model was found to predict the size of the stagnant zones in a 3D fluidized bed with a maximum relative error of 20% and it could be used to estimate the size of the distributor cross-section affected by maldistribution. The distributor plate performance under operational conditions was also investigated. The effect of temperature on the distributor pressure drop was studied for two different distributor plates (i.e. multiorifice and tuyere type) in a Biomass Bubbling Fluidized Bed Gasifier. The results were employed to develop a methodology to design gas distributor plates at elevated temperature. The model predicts accurately the minimum distributor open area needed to satisfy a distributor to bed pressure drop ratio for a given temperature and operation conditions.