Superficial Gas Velocity Ug Research Articles

CFD Modeling of Bubble Column Reactor Including the Influence of Gas Contraction

AbstractIn this paper a CFD model for a bubble column reactor undergoing a first order reaction A → B is developed. The reactor operates in the homogeneous bubbly regime and has a diameter DT = 1 m and height HT = 5 m. The incoming gas stream contains inerts, varying in proportion from 10 % to 90 %. Three‐dimensional transient Eulerian simulations were carried out for an inlet superficial gas velocity UG = 0.04 m/s. Due to the consumption of A, the gas phase suffers contraction along the height of the reactor and as a consequence there is a significant change in the gas velocity along the column height; this variation in gas velocity is stronger when the incoming gas contains a smaller proportion of inerts. The CFD simulations show that there is a considerable influence of gas contraction on both the bubble column hydrodynamics and on the reactor conversion. None of the conventionally used reactor models is capable of describing the reactor performance in the case of high gas phase contraction.

CFD Modeling of a Bubble Column Reactor Carrying out a Consecutive A → B → C Reaction

AbstractIn this paper, we develop a CFD model for describing a bubble column reactor for carrying out a consecutive first‐order reaction sequence A → B → C. Three reactor configurations, all operating in the homogeneous bubbly regime, were investigated: (I) column diameter DT = 0.1 m, column height HT = 1.1 m, (II) DT = 0.1 m, HT = 2 m, and (III) DT = 1 m, HT = 5 m. Eulerian simulations were carried out for superficial gas velocities UG in the range of 0.005–0.04 m/s, assuming cylindrical axisymmetry. Additionally, for configurations I and III fully three‐dimensional transient simulations were carried out for checking the assumption of cylindrical axisymmetry. For the 0.1 m diameter column (configuration I), 2‐D axisymmetric and 3‐D transient simulations yield nearly the same results for gas holdup ϵG, centerline liquid velocity VL(0), conversion of A, χA, and selectivity to B, SB. In sharp contrast, for the 1 m diameter column (configuration III), there are significant differences in the CFD predictions of ϵG, VL(0), χA, and SB using 2‐D and 3‐D simulations; the 2‐D strategies tend to exaggerate VL(0), and underpredict ϵG, χA, and SB. The transient 3‐D simulation results appear to be more realistic. The CFD simulation results for χA and SB are also compared with a simple analytic model, often employed in practice, in which the gas phase is assumed to be in plug flow and the liquid phase is well mixed. For the smaller diameter columns (configurations I and II) the CFD simulation results for χA are in excellent agreement with the analytic model, but for the larger diameter column the analytic model is somewhat optimistic. There are two reasons for this deviation. Firstly, the gas phase is not in perfect plug flow and secondly, the liquid phase is not perfectly mixed. The computational results obtained in this paper demonstrate the power of CFD for predicting the performance of bubble column reactors. Of particular use is the ability of CFD to describe scale effects.

Heat Transfer Model for Regenerative Beds

Regenerative catalytic oxidizers (RCO) are an economic and effective means of controlling volatile organic compounds with concentrations exceeding 1,200 mg/m3 in a gas stream. However, factors influencing the performance of RCO when treating volatile organic compounds in gas streams have seldom been addressed. Therefore, this study presents a convection-dispersion model with an effective thermal diffusivity αe as a parameter to simulate the performance of RCO. To verify the effectiveness of the proposed model, a pilot-scale RCO was constructed with two 20 × 200 cm (inside diameter × height) regenerative beds. Gravel was used as the thermal regenerative solid material. Experimental results indicated that the model with an αe of 2.0–3.8 × 10−6 m2/s can be used to describe the time variation of solids temperature with the packing height at superficial gas velocities Ug of 0.080–0.382 m/s. Values of αe for the bed are closer to those for the gravel solids (αs = 1.0 × 10−6 m2/s) than for air (αg = 54 × 10−6 m2...

Measurement and Prediction of Axial Distribution of Immobilized Glucose Oxidase Gel Beads Suspended in Bubble Column.

The axial distribution of gel beads suspended completely in a bubble column was measured by sampling the three-phase dispersion through a vertical pipe located at any height of the column axis under various operating conditions of superficial gas velocity UG, average gel beads holdup eS and unaerated slurry height HD. The gel beads used were prepared by entrapping glucose oxidase as well as fine palladium particles within calcium alginate gel to catalyze the air oxidation of glucose for efficient production of calcium gluconate. The conventional sedimentation-diffusion model, which includes the gel beads dispersion coefficient EP and their settling velocity vP to describe the axial distribution, is found to give unreasonable EP values being at least twice as high as than those predicted from the published correlations, and also twice as high as the estimated values of the liquid dispersion coefficient EL in the three-phase bubble column. This was assumed to be ascribed to the reduced apparent values of vP, since the intrinsic EP value should be nearly equal to the EL value. In view of the fact that the gel bead has almost the same density as the liquid phase and the size is comparable to the mean bubble size, the model was modified by introducing an additional parameter, the rise velocity of the gel beads swarm um representing an effect of rising air bubbles on settling gel beads. The values of um were determined from the observed axial distributions by assuming that the EP values are given by the reported correlation of EL in suspension of the inert calcium alginate gel beads, and empirically correlated with UG and eS. The axial distributions calculated from the modified model well agreed with the observed ones for the different HD.

Heat transfer in horizontal plug and slug flow for gas-liquid and gas-slurry systems.

Heat transfer coefficients and flow characteristics were measured in a horizontal loop of 51.5 mm diam. pipe. The viscosities of liquid and slurry were changed in the range of 0.8-55 mPa •.s. The experiments were carried out in the regimes of plug and slug flow. The pressure drop in the straight sections was smaller than that predicted by the Lockhart-Martinelli correlation and was correlated by a newly proposed equation. The gas holdup increased with increasing gas velocity and decreasing liquid or slurry velocity. A correlation of the gas holdup is presented. The heat transfer coefficients, h, increased with increasing liquid velocity and decreased with increasing liquid viscosity. In the range of the superficial gas velocity Ug=0.2-1 m•s-1, h was nearly independent of the gas velocity. In the range of Ug=1-10m•s-1, however, h increased with increasing gas velocity. The values of h in the present experiments as well as those in the literature are well described by the equation proposed in this work.