The effect of Prewetting on the Pressure Drop, Liquid Holdup and Gas-Liquid Mass Transfer in Trickle-Bed Reactors

Abstract

The prewetting of a trickle-bed reactor has important implications in the design and operation of these reactors. This is because the prewetting changes the flow morphology (shape and texture) of the liquid flowing through the bed and leads to the existence of multiple hydrodynamic states. The extent of this change in flow morphology can be seen in the effect the prewetting of the reactor has on the pressure drop, liquid holdup and gas-liquid mass transfer. The following prewetting procedures were used: -- Levec-wetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flow is reintroduced -- Kan-wetted: the bed is operated in the pulse flow regime and liquid and gas flow rates are reduced to the desired set point -- Super-wetted: the bed is flooded and gas and liquid flow are introduced once draining commences For the pressure drop: -- The different prewetting procedures resulted in two distinct regions (Upper region Kan and Super-wetted, Lower region Dry and Levec-wetted) -- There was no significant difference between the Dry and Levec-wetted beds -- The pressure drop in the Kan and Super-wetted beds can be as much as seven times greater than the pressure drop in the Dry and Levec-wetted beds For the liquid holdup: -- The different prewetting procedures resulted in four distinct regions (Kan-wetted, Super-wetted, Levec-wetted, Dry bed) -- The liquid holdup in the Kan-wetted bed can be as much as four times greater than the liquid holdup in the Dry bed -- The liquid holdup in the Levec-wetted can be as much as thirty percent lower than the liquid holdup in the Kan-wetted bed For the gas-liquid mass transfer: -- The different prewetting procedures resulted in three distinct regions (Kan and Super-wetted, Levec-wetted, Dry bed) -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as six times greater than the mass transfer coefficient in the Dry bed -- The volumetric gas-liquid mass transfer coefficient in the Kan and Super-wetted beds can be as much as two and a half times greater than the mass transfer coefficient in the Levec-wetted bed While an increase in the liquid flow rate results in an increase in the pressure drop, liquid holdup and gas-liquid mass transfer for all of the experiments, the effect of increasing gas flow on the measured variables were more pronounced for the prewetted beds. In a prewetted bed (Kan, Super and Levec-wetted) an increase in the gas flow rate causes an increase in the volumetric gas-liquid mass transfer coefficient and a decrease in the liquid holdup. The decrease in the liquid holdup is due to the fact that the increased gas flow rate causes the films around the particles to thin and spread out. In the dry bed the flow is predominantly in the form of rivulets and the increase in gas flow rate does not affect the liquid holdup. In the case of the volumetric gas-liquid mass transfer coefficient the increased gas flow rate causes an increase in the mass transfer coefficient regardless of the prewetting procedure. This increase is due to the effect that the gas flow rate has on the liquid holdup as well as the increase in the gas-liquid interfacial area due to the increased gas-liquid interaction. If the pulsing in the Kan-wetted bed is induced by increasing the gas flow rate and keeping the liquid flow rate constant the results are significantly different. The pressure drop in the gas-pulsing experiments was lower than the pressure drop in the recorded in the Kan and Super-wetted beds, but higher than the pressure drop in the dry and Levec-wetted beds. However, the liquid holdup in the gas-pulsing experiments was higher than the liquid holdup in any of the other beds. The volumetric gas-liquid mass transfer coefficient in the gas-pulsing experiments was lower than the mass transfer coefficients of the Kan and Super-wetted beds, but higher than the mass transfer coefficients in the dry and Levec-wetted beds. The multiple operating points obtained from the different prewetting procedures are by no means the only possible operating points. By simply decreasing the draining time in the Levec-wetted bed steady state operating points can be found between those of the Super and Levec-wetted beds. This alludes to the fact that the operating conditions determined from the different prewetting modes are only boundaries and that the actual operating point can lie anywhere between these boundaries. The existence of these multiple hydrodynamic states complicates things further when a correlation is developed to determine the pressure drop, liquid holdup or the volumetric gas-liquid mass transfer coefficient. No correlation tested was able to accurately predict the pressure drop, liquid holdup or volumetric gas-liquid mass transfer coefficient in the dry or prewetted beds.