Packing Of Drops Research Articles

CFD-PBM simulation of dense emulsion flows in a high-shear rotor–stator mixer

Computational fluid dynamics (CFD) is coupled to population balance modelling (PBM) for the simulation of turbulent drop dispersion and evolving rheology in concentrated oil–water emulsions flowing through a Silverson 150/250MS mixer. Unsteady Reynolds averaged Navier–Stokes (URANS) simulations on a sliding mesh were performed for the fluid dynamics, linking the k−ω SST turbulence model to the population balance equations. The quadrature method of moments (QMOM) approach is used to solve the population balance equations. Breakage kernels based on the multi-fractal theory of intermittent turbulence have been modified to include the effect of turbulent shear and close packing of drops at high phase volume. A previously developed rheological model is used to calculate the emulsion viscosity. Emulsion drop size is shown to be influenced by rotor speed and phase volume fraction. Flow curves characterising the rheology of the emulsion show shear thinning behaviour and viscosity build after a single pass through the mixer.

Electrostatic attraction of charged drops of water inside dropwise cluster

Based on the analytical solution of the Poisson-Boltzmann equation, we demonstrate that inside the electrically neutral system of charges an electrostatic attraction can occur between the like-charged particles, where charge Z ≫ 1 (in terms of elementary charge) and radius R > 0, whereas according to the literature, only repulsion is possible inside non-electrically neutral systems. We calculate the free energy of the charged particles of water inside a cluster and demonstrate that its minimum is when the interdroplet distance equals several Debye radii defined based on the light plasma component. The deepest minimum depth is in a cluster with close spatial packing of drops by type, in a face-centered cubic lattice, if almost all the electric charge of one sign is concentrated on the drops and that of the other sign is concentrated on the light compensation carriers of charge, where the charge moved by equilibrium carriers is rather small.

Yielding and fracturing of concentrated emulsions in narrow gaps

We used rheology and confocal microscopy techniques to characterise the flow of emulsions as the droplets were confined by increasing the drop volume fraction and reducing the distance between the shearing surfaces. Slip was minimised by matching the density of the oil and water phases. Attractive interactions between the drops caused them to flocculate. The contribution of the emulsion microstructure to its shear response becomes significant when the oil drop flocs almost span the distance between the surfaces. We found that confining the flow of droplet flocs causes a transition from a fluid phase with shear thinning flow behaviour into a jammed, solid-like material. The large deformations caused by flow at the maximum drop packing fraction induce droplet coalescence within highly localised regions of the emulsion.

Hydrodynamics of gas-agitated liquid-liquid extraction columns

Although the non-agitated extraction columns (spray column, packed column, perforated plate column, sieve plate column, etc) can handle high flow rates and are simple and cheap, there have been relatively few applications of these columns because they suffer from serious backmixing of the continuous phase. It was shown that the backmixing is reduced when the spray column is operated with dense packing of drops. Another way of increasing the efficiency of a non-agitated extraction column is to introduce an inert gas (air, nitrogen, oxygen) as a mixing agent in the two-phase liquid-liquid (L-L) system. This method of energy introduction increases the turbulence within the new three-phase gas-liquid-liquid (G-L-L) system, which causes an improved dispersion of droplets, and, consequently, a higher dispersed phase holdup and therefore a great mass transfer area. The present study reports the hydrodynamics in the non-agitated extraction columns, as well as the axial dispersion for the two- and three-phase systems.

Longitudinal Dispersion of the Dispersed Phase in Spray Extraction Columns

The longitudinal dispersion of the dispersed phase was studied in spray columns with mechanical dispersion of a heavy phase (water) in kerosene or a light phase (kerosene) in water. The columns were operated in the loose, close, and stirred close drop packing modes [1, 2]. The specific total liquid flow rate was varied in the range of Q = 15‐40 m 3 /(m 2 h); the peripheral velocity of the stirrers, in the range of u st = 0.690‐1.265 m/s. In the close drop packing mode, the peripheral velocity of the stirrers located in the close drop packing layer was varied from 0.6 to 1.0 m/s. In the Q , dispersed-phase holdup Φ , and mean drop diameter d 32 ranges examined, Pe disp = 4‐6 and, correspondingly, E disp = (100 - 150) 〈 10 —4 m 2 /s (see figure). The relatively low values of e disp and the high values of E disp indicate an intensive longitudinal dispersion of the dispersed phase in this drop packing mode. In conventional spray columns (with nozzle spraying) the Peclet number e disp is much higher [3], that is, there is only a slight longitudinal dispersion of the dispersed phase in the loose drop packing mode. The high intensity of longitudinal dispersion in columns with mechanical dispersion is apparently due to the formation of a more polydisperse emulsion, from which a large amount of small drops is carried away by the continuous phase. In the close drop packing mode, the entrainment of small drops is reduced owing to some enlargement of the drops and some narrowing of the drop-size range; the close-packing layer itself can prevent the small drops from being carried away by the continuous phase. In this case, we observe (figure) lower e disp values of 2.5‐4.0 ( E disp = (50—75) 〈 10 —4 m 2 /s), which indicate that the longitudinal dispersion intensity is lower in the close drop packing mode than in the loose one. Comparing these e disp and E disp values with the values of e ae ∪ 18 and E cont ∪ 6 〈 10 —4 m 2 /s [3] for the close drop packing demonstrates that the intensity of the longitudinal dispersion of the continuous phase is higher than that of the dispersed phase. Comparing the observed e disp values for the close and stirred close drop packings (figure) shows that

Limiting Liquid Flow Rates in Spray Extraction Columns

A physical model of the close packing of drops is developed, and the flooding conditions are found. Analytical expressions are obtained for the limiting liquid flow rates in spray extraction columns. The calculated and experimental results agree satisfactorily.

Longitudinal Dispersion of the Continuous Phase in Spray Extraction Columns

Longitudinal dispersion of the continuous phase was studied in spray extractors operated in the loose, close, and turbulized drop packing modes. The Peclet number was found to depend on the Reynolds number modified to take into account the hydrodynamic conditions in the working zone of the column. When the continuous phase is pulsed, the Peclet number is also governed by the modified Reynolds number, which accounts for the effect of pulsation on the longitudinal dispersion.

Hydrodynamique des écoulements en lit dense dans une colonne à pulvérisation Influence de la viscosité des deux liquides

Experimental results of a hydrodynamic study of dense packing of drops in a liquid-liquid spray column are presented. For fourteen pairs of liquid, immiscible or containing a solute at equilibrium, these results can be correlated with a simple relation between the relative velocity of the two phases, the dispersed phase hold up, the terminal velocity of a single drop and a constant m which only depends on the phase viscosity ratio μ C /μ D .

The Mechanism of heat transfer in a spray column heat exchanger: II. Dense packing of drops

AbstractTemperature profiles of water in a spray column heat exchanger, 15 cm. in diameter and 150 cm. long, operating with a dense packing of kerosene drops, were measured. The range of superficial velocities was 0 to 0.8 cm./sec. of water and 0.5 to 1.7 cm./sec. of kerosene. The bottom of the dense packing was either slightly above or 15 cm. below the bottom of the column proper.The physical picture of heat transfer is similar to that for dispersed packings of drops and emphasizes the dominant role of wakes in the heat transfer mechanism. The mathematical equations for dispersed packings of drops were modified to take into account the reduction of wake size at the interface of the two packings and the difference in the mixing patterns at the top of the column. An empirical allowance for the effect of bypassing is suggested. The volume of the wakes and the rate of wake shedding were estimated from the temperature profiles. General agreement was found between the theory and the experimental data of this and three other studies.

Residence time distribution of the dispersed phase in a spray column

AbstractA dye tracer technique was utilized to measure the residence time distribution of the drops in a spray column, operating with a dispersed or a dense packing of drops, for a wide range of flow rates, in a 15 cm.in diameter, 150 to 160 cm. long column proper.For dispersed packing of drops and for flow rates below the onset of coalescence, the flow of the drops was approximately plug flow, with a variance of the residence time distribution of 0.095. For flow rates above the onset of coalescence within the column, the variance increased sharply to 0.55 and decreased at higher flow rates.For dense packing of drops the variance of the residence time distribution was in the range of 0.1 to 0.5. This variance is attributed to the effect of bypassing water and kerosene near the wall of the column. A mathematical model based on this effect showed reasonable fit with the experimental results. This model predicts an approach to plug flow of the drops for large diameter columns.

Operation of a Spray Column with a Dense Packing of Drops

Operation of a Spray Column with a Dense Packing of Drops

The flow of Suspensions through tubes: IX. Particle Interactions in Pulsatile Flow

A study was made of the behavior of rigid spheres, fluid drops, and flexible fibers subjected to oscillatory and pulsatile flow in suspensions the concentrations of which ranged from 0.5% to 40% and at particle to tube diameter ratios ranging from 0.01 to 0.21. Visual observations of particle radial migration and interaction were combined with measurements of the changes in the displacement profiles and power dissipation in the tube with time. Larger rigid spheres at low concentrations were found to accumulate in groups approximately equally spaced in a central core in which the velocity profiles were blunted. At concentrations >10% and at all particle sizes, a particle-depleted layer developed at the wall. The displacement profiles, although initially blunted, became first more parabolic with time and then again blunted suggesting that a redistribution of spheres also occurred in the center of the tube. This was reflected in the power dissipation, calculated from the measured pressure gradient and phase lag, which exhibited a maximum before falling to a final value lower than the initial. In contrast, the power dissipation in suspensions of fluid drops decreased with time at all particle sizes and concentrations corresponding to an observed inward migration and packing of drops in the core of the tube where the displacement profiles were blunted. In dilute suspensions of pulp fibers, axial migration of particles resulted in a peripheral particle-free zone and mechanical entanglements between the fibers in the center of the tube. Whereas in very dilute suspensions and at low particle sizes, the displacement profiles and power dissipation were in good agreement with theory for Newtonian liquids, the observations made at higher concentrations and particle sizes could only be qualitatively interpreted.

The mechanism of heat transfer in a spray column heat exchanger

AbstractTemperature profiles of dispersed kerosene and water were measured in a spray column heat exchanger, 15 cm. in diameter and 160 cm. long. Dispersed packing of drops and narrow temperature ranges were used. The flow rates used were 0 to 50 liters/min. of water and 5 to 40 liters/min. of kerosene. The physical picture that emerges from the temperature profiles is that heat is transferred from fully mixed drops to fully mixed wakes while the wakes are formed, by shedding and renewal of elements of wakes in most of the column and by complete mixing of all streams at the water inlet at the top of the column. Mathematical equations were developed from the physical model. The volume of the wakes and the rate of shedding of wake elements were estimated from the temperature profiles and were used to calculate the temperature profiles for this and for other studies. The agreement of the calculated profiles with the experimental data is very good.