Desenvolvimento de técnica a laser para determinação da porosidade de um leito fluidizado sólido-líquido

Abstract

Obtaining reliable values for bed voidage is a crucial step in the design of fluidized beds. In this work, we studied different experimental methods for determination of a liquid-solid fluidized bed voidage. Four groups of particles with different properties were fluidized in water using laboratory-scale fluidization equipment. We obtained experimental values of bed voidage (𝜀) as a function of liquid superficial velocity (𝑈) using three different techniques. The first one uses a differential pressure sensor for obtaining the vertical pressure gradient (𝑑𝑃/𝑑𝑧) and relating it to the voidage using the force balance. The second one consists of visually obtaining the average bed height (𝐻) and relating it to the voidage using the definition of voidage and the bed volume. The third technique, developed in this work, consists in using a light-emitting diode (LED) and a LDR light sensor on the outside of the column, for determining the bed height with more precision and reproducibility. The luminosity profiles around the bed’s surface had a sigmoid shape, as expected. We could find the bed’s height with less than 5 mm difference between measurements. The voidage values obtained by the three techniques were remarkably similar, specially between the two techniques based on the bed’s height. We observed the biggest differences for smaller superficial velocities, showing that the methods have better agreement in less dense beds. The laser method was more precise than the visual method, with standard deviation being seven times smaller. The experimental curves agreed with Richardson-Zaki model. The values of parameters n and k showed that the two methods based on bed height are closer to the bed behavior reported in literature.