Hydrodynamic And Mass Transfer Characteristics In Three-Phase Fixed Bed Reactors Under Low Gas And Liquid Flow Rates

Three-phase fixed bed reactors are widely used in petroleum chemical industry. Comparisons of hydrodynamic and mass transfer characteristics in three-phase fixed bed reactors with gas-liquid concurrent upflow and downflow were studied in this investigation under low gas and liquid flow rates. The experiment was done in a Perspex column of 14 cm internal diameter. The gas flow rate ranges from 0.044 to 0.218 kg·m-2·s-1，while the liquid flow rate ranges from 1.8 to 7.2 kg·m-2·s-1. Three sizes of spherical particles (9.3 mm,4.0 mm, 1.9 mm) were used in this experiment for the investigation of particle size effect.The liquid holdup was measured by Electrical Capacitance Tomography system. It was found that the liquid holdup in the concurrent upflow is larger than downflow. An empirical correlation was developed for predicting the liquid holdup through dimensional analysis. The pressure drop was measured by pressure transmitter and the phenomenon of pressure drop hysteresis was noticed. The results showed that small particles have larger pressure drop hysteresis loop. Also, the residence time distribution was measured by pulse tracer method. According to the piston dispersion model, a comparsion of axial backmixing degree was made between concurrent upflow and downflow process, which indicated that the upflow process has larger mean residence time and bigger axial backmixing.The liquid volumetric mass transfer coefficient was measured using the physical absorption method, and the interfacial area was measured by chemical absorption system. The results indicated that both kLa and a are increasing with an increase of either gas or liquid flow rate as well as a decrease of the particle size.In particular, the mass transfer characteristics are significantly affected by the gas flow rate in upflow process, while they are obviously influenced by the liquid flow rate in downflow process. Respectively, at constant phase flow rate, the kLa is 45 percent higher and the interfacial area is 75 percent higher in upflow operation than that in downflow operation. Empirical correlations of kLa, obtained through dimensional analysis, fit the experimental data well. According to the stable bubble theoretical model, an empirical correlation for predicting the interfacial area in gas-liquid concurrent upflow process was also developed.