Function Of Superficial Velocity Research Articles

Investigation of the Pressure Drop Across Packed Beds of Spherical Beads: Comparison of Empirical Models With Pore-Level Computational Fluid Dynamics Simulations

This study uses novel methods, combining discrete element method (DEM) simulations for packing and computational fluid dynamics (CFD) modeling of fluid flow, to simulate the pressure drop across rigid, randomly packed beds of spheres ranging from 1 to 3 mm in diameter, with porosities between 0.34 and 0.45. This modeling approach enables the combined effect of void fraction and particle size to be studied in more depth than that has been previously possible and is used to give insight into the ability of the well-established Ergun equation to predict the pressure drop behavior. The resulting predictions for pressure drop as a function of superficial velocity were processed to yield coefficients α and β in the Ergun equation and were found to be in keeping with equivalent data in the literature. Although the scatter in α with structural variations was very small, the scatter in β was large (±20%), leading to inaccuracies when used to predict pressure drop data at velocities beyond the Darcy regime. Evaluation of the packed particle structures showed that regions of poor packing, in samples with high porosity and large particle sizes, lead to lower β values. The findings bring strong support to the belief that a generalized model, such as that by Ergun, cannot yield a unique value for β, even for identical spheres.

Performance of a sand filter in removal of micro-algae from seawater in aquaculture production systems

In this study, a sand filter was used to remove micro-algae from seawater feeding aquaculture ponds. A lab-scale sand filter was used to filter 30,000 cells/mL of Heterocapsa triquetra suspension, a non-toxic micro-alga that has morphological and dimensional (15–20 μ m) similarities with Alexandrium sp., one of the smallest toxic micro-algae in seawater. Removal efficiency and capture mechanisms for a fixed superficial velocity (3.5 m/h) were evaluated in relation to size distribution and mean diameter of the sand. Various sands (average diameter ranging between 200 μ m and 600 μm) were characterized and used as porous media. The structural parameters of the fixed beds were evaluated for each medium using experimental measurements of pressure drop as a function of superficial velocity over a range of Reynolds numbers covering Darcy's regime and the inertial regime. For a filtration cycle of six hours, the best efficiency (E=90%) was obtained with the following sand characteristics: sieved sand with a range of grain diameter of 100 and 300 μm and a mean grain diameter equal to 256 μ m. Results obtained show the influence of the size distribution of sand on the quality of retention of the micro-algae studied.

Removal of Oil from Water by Inverse Fluidization of Aerogels

Surface-treated hydrophobic aerogel (Nanogel) granules of sizes between 500 and 850 μm, 1.7 and 2.3 mm, and 0.5 and 2.3 mm are fluidized by a downward flow of oil-contaminated water in an inverse fluidization mode. Aerogel particles are nanostructured, extremely light and porous, have a very large surface area per unit mass, and are sufficiently robust to be fluidized. Their hydrophobic surface gives them a strong affinity for oil and other organic compounds, with the exclusion of water. These desirable properties make them an ideal sorbent or filter media for the removal of oil from wastewater. The hydrodynamic characteristics of inverse fluidized beds of aerogel granules of different size ranges were studied by measuring the pressure drop and bed expansion as a function of superficial velocity. The oil removal efficiency and capacity of the aerogel granules in the inverse fluidized bed were found to depend mainly on the size of the granules, the initial height of the bed (amount of powder used), the void fraction, and the fluid velocity. Among the advantages of the process are the extremely low energy consumption (low pressure drop) during oil removal and the large absorption capacity. Oil concentrations of about 2000 mg/L in water could be reduced to less than 10 mg/L by the inverse fluidization process.

A NEW EXPERIMENTAL CORRELATION USING A CURVE-SHAPED CAPACITANCE SENSOR TO PREDICT LIQUID HOLDUP IN VERTICAL GAS-CONDENSATE PIPELINES

The performance of a curve-shaped capacitance sensor for measuring the mean liquid holdup of the two-phase mixture of gas-condensate and nitrogen in a vertical pipeline was studied experimentally. The sensor consists of two electrodes placed on the external wall of a cylindrical test duct. The calibration curves for bubble, slug, and plug flow regimes were developed for vertical flow and the sensitivity of the sensor to flow pattern was also investigated. Based on experimental observations, different calibration curves must be used for different flow regimes to have an acceptable accuracy in holdup measurement. Moreover, a new empirical correlation for estimating liquid holdup in vertical gas-condensate pipelines in the dynamic condition was developed as a function of superficial velocities, viscosities, and densities of the gas and liquid. Furthermore, a flow pattern identification map for vertical pipeline is also presented.

A unified model for particulate expansion of fluidised beds and flow in fixed porous media

A model is derived for a fluidised bed that enables its steady state particulate expansion to be predicted as a function of superficial velocity from the initial (packed bed) condition to the final fully expanded (single suspended particle) state. These predictions are in good agreement with the empirical correlations and functional dependencies found by Richardson and Zaki[1,2] over the full range of flow conditions. The derivations incorporat revisions in the form of the pressure drop correlations for flow in porous media that are justified on theoretical and empirical grounds.