Bed structure and its impact on liquid distribution in a trickle bed reactor

The work reported in this investigation involves the determination of the hydrodynamic properties of the Trickle Bed Reactor which has been loaded in various ways to mark the effect of the loading methodologies employed to pack the catalyst pellets. The bed structure of a packed three-phase reactor is critical to study as it provides the essential contact between the phases and provides the catalytic sites where the reaction takes place. Depending on the structural properties of the bed such as local void structure, liquid distribution, two-phase pressure drop, and holdup of fluids gets affected. The study aims to envelop the catalyst bed characteristics such as the local void structure, the length of the catalyst bed, flow characteristics such as liquid and gas flow rate, and liquid distributor at the top of the catalyst bed to gauge and quantify their effect on the hydrodynamics of a trickle bed reactor.

Liquid Distribution and its Effect on the Organic Removal in a Trickle Bed Bioreactor

Propylene glycol methyl ether was removed from wastewater in a trickling bed bioreactor under different liquid distribution conditions. A 0.3 m diameter column filled with two heights of 0.7 m and 1.4 m with 2 cm plastic spheres were used. The wastewater flow rate varied from 0.184 to .0918 kg/m₂.s. The effect of the initial liquid distribution was examined using two types of liquid distributors: a multipoint liquid distributor and a central single point liquid distributor. Over 96 hours of treatment period, the BOD₅ was reduced by 85% and 65% under the most uniform liquid distribution condition and the poor liquid distribution condition, respectively, achieved in this study. Increasing the liquid flow rate from 0.184 to 0.198 kg/m₂.s, it increased the dynamic liquid holdup by 53% and the apparent BOD₅ removal rate constant by 23% at 1.4 m bed height using the multipoint liquid distributor. Moreover, with the use of the multipoint liquid distributor, the apparent reaction when the liquid flow rate was increased from 0.184kg/m₂.s to 0.918 kg/m₂.s. In addition, it was found that the effect of an increase in the bed height on the percentage BOD₅ removal was not significant when initial liquid distribution was uniform. Under the uniform initial condition, only 4% increase in the percentage BOD₅ removal was observed when the bed height increased from 0.7 to 1.4 m whereas when the initial distribution was extremely non-uniform, the percentage of BOD₅ removal was increased by 20% with increasing bed height. The local distribution of the BOD₅ removal was not uniform across the bed cross-section and it was affected by the liquid flow distribution across the bed cross-section.

Liquid Distribution and its Effect on the Organic Removal in a Trickle Bed Bioreactor

Propylene glycol methyl ether was removed from wastewater in a trickling bed bioreactor under different liquid distribution conditions. A 0.3 m diameter column filled with two heights of 0.7 m and 1.4 m with 2 cm plastic spheres were used. The wastewater flow rate varied from 0.184 to .0918 kg/m₂.s. The effect of the initial liquid distribution was examined using two types of liquid distributors: a multipoint liquid distributor and a central single point liquid distributor. Over 96 hours of treatment period, the BOD₅ was reduced by 85% and 65% under the most uniform liquid distribution condition and the poor liquid distribution condition, respectively, achieved in this study. Increasing the liquid flow rate from 0.184 to 0.198 kg/m₂.s, it increased the dynamic liquid holdup by 53% and the apparent BOD₅ removal rate constant by 23% at 1.4 m bed height using the multipoint liquid distributor. Moreover, with the use of the multipoint liquid distributor, the apparent reaction when the liquid flow rate was increased from 0.184kg/m₂.s to 0.918 kg/m₂.s. In addition, it was found that the effect of an increase in the bed height on the percentage BOD₅ removal was not significant when initial liquid distribution was uniform. Under the uniform initial condition, only 4% increase in the percentage BOD₅ removal was observed when the bed height increased from 0.7 to 1.4 m whereas when the initial distribution was extremely non-uniform, the percentage of BOD₅ removal was increased by 20% with increasing bed height. The local distribution of the BOD₅ removal was not uniform across the bed cross-section and it was affected by the liquid flow distribution across the bed cross-section.

Liquid distribution and its effect on local mass transfer in a packed column of Pall rings

The spatial variations of liquid distribution and local mass transfer coefficient in a 0.30-m column of 25.4-m Pall rings were investigated. The data of liquid distribution was collected with a 39-cell liquid collector and a wall-flow tube from a doubled-wall section in the column at the packing-support level. The local mass transfer coefficients were measured via the electrochemical technique by individual cathodic nickel-coated Pall rings placed at various spatial positions. Both measurements were conducted at various fluid flow rates with three liquid distributor designs at different bed heights. Liquid distribution and local mass transfer coefficients observed were far from uniform in the column. The wall flow developed along the packed bed until a fully developed flow pattern was reached. With more uniform initial liquid distribution, the less packing height needed to reach the fully developed flow pattern along with higher the mass transfer efficiency in the column. Ladder-type liquid distributor (LLD) showed less angular effect in measurements. Increasing the liquid flow rate slightly improved the uniformity of liquid distribution and enhanced the mass transfer. No influence of gas flow rate on liquid distribution and mass transfer coefficient was found at the range of gas flow rates used. These gas flow rates were much lower than the loading point. Liquid maldistribution factor and mass transfer maldistribution factor decreased with increases in the uniformity of the initial liquid distribution. These values were 0.21(0.48). 0.16(0.26) and 0.14(0.22) for single-point liquid distributor (SPLD), cross-type liquid distributor (CLD) and LLD, respectively. By comparison, a good agreement was observed on the relation of liquid maldistribution factor and mass transfer maldistribution factor.

Liquid distribution and its effect on local mass transfer in a packed column of Pall rings

The spatial variations of liquid distribution and local mass transfer coefficient in a 0.30-m column of 25.4-m Pall rings were investigated. The data of liquid distribution was collected with a 39-cell liquid collector and a wall-flow tube from a doubled-wall section in the column at the packing-support level. The local mass transfer coefficients were measured via the electrochemical technique by individual cathodic nickel-coated Pall rings placed at various spatial positions. Both measurements were conducted at various fluid flow rates with three liquid distributor designs at different bed heights. Liquid distribution and local mass transfer coefficients observed were far from uniform in the column. The wall flow developed along the packed bed until a fully developed flow pattern was reached. With more uniform initial liquid distribution, the less packing height needed to reach the fully developed flow pattern along with higher the mass transfer efficiency in the column. Ladder-type liquid distributor (LLD) showed less angular effect in measurements. Increasing the liquid flow rate slightly improved the uniformity of liquid distribution and enhanced the mass transfer. No influence of gas flow rate on liquid distribution and mass transfer coefficient was found at the range of gas flow rates used. These gas flow rates were much lower than the loading point. Liquid maldistribution factor and mass transfer maldistribution factor decreased with increases in the uniformity of the initial liquid distribution. These values were 0.21(0.48). 0.16(0.26) and 0.14(0.22) for single-point liquid distributor (SPLD), cross-type liquid distributor (CLD) and LLD, respectively. By comparison, a good agreement was observed on the relation of liquid maldistribution factor and mass transfer maldistribution factor.

A Closed-Loop Optimized System with CFD Data for Liquid Maldistribution Model

For the simulation of a trickle-bed reactor (TBR) in coal and oil refining, modeling the liquid maldistribution of the gas-liquid distributor incurs enormous pre-processing work and bears a huge computational cost. A closed-loop optimized system with computational fluid dynamic (CFD) data is therefore proposed for the first time in this paper. A fast prediction model based on support vector regression (SVR) is developed to simplify the modeling of the liquid flow rate in TBRs. The model uses CFD simulation results to determine an optimized set of structural parameters for the gas-liquid distributor in TBRs. In order to obtain an accurate SVR model quickly, the particle swarm optimization (PSO) algorithm is employed to optimize the SVR parameters. Then, the structural parameters corresponding to the minimum liquid maldistribution factor are calculated using the response surface methodology (RSM) based on the hybrid PSO-SVR model. The CFD validation results show a good agreement with the values predicted by RSM, with liquid maldistribution factors of 0.159 and 0.162, respectively.