Binary Solid-liquid Fluidized Bed Research Articles

CFD modeling of segregation in binary solid-liquid fluidized beds: Influence of liquid viscosity and density

Liquid–solid fluidized beds (LSFBs) find several industrial applications, where a clear understanding of bed expansion demeanor, phase distribution, solid segregation and intermixing is inevitable. Here, a transient multi-Eulerian model with granular flow extension is presented to investigate the segregation characteristics in a polydisperse LSFB. Both segregated and intermixed zones were encountered in the expanded bed based on variability in particle diameters and densities. The influences of liquid superficial velocity, viscosity and density on particle segregation are elaborated. For the considered system, liquid density was found to render more discerning influence on particle segregation.

Mimicking layer inversion in solid-liquid fluidized beds in narrow tubes

This paper investigates experimentally and numerically the dynamics of solid particles during the layer inversion of binary solid-liquid fluidized beds in narrow tubes. Layer inversion can happen in solid classifiers and biological reactors, where different solid particles coexist and segregation by diameter and density occurs. The fluidized beds were formed in a 25.4 mm-ID pipe and consisted of alumina and aluminum beads with diameters of 6 and 4.8 mm, respectively. We placed initially the lighter particles on the bottom in order to force an inversion of layers, mimicking the layer inversion mechanism. In the experiments, we filmed the bed with a high-speed camera and tracked individual beads along images, while in numerical simulations we computed the bed evolution with a CFD-DEM (computational fluid dynamics - discrete element method) code. We found the distances traveled by individual particles during the inversion and the characteristic time for layer inversion.

Prediction of Layer‐Inversion Behavior of Down‐Flow Binary Solid‐Liquid Fluidized Beds

AbstractThe layer‐inversion behavior of down‐flow binary solid‐liquid fluidized beds is predicted using the property‐averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson‐Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up‐flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.

Voidage at the Layer Inversion Point in Binary-Solid Liquid-Fluidized Beds

On the basis of three well documented runs on the layer-inversion phenomenon in binary liquid-fluidized beds, it is shown that the best method, to date, for predicting the layer-inversion voidage, knowing the layer inversion velocity, is via the bed contraction equation of Westman (1936), with the adjustable parameter G of that equation as recently modeled.

Layer inversion phenomenon in binary-solid liquid-fluidized beds: Prediction of the inversion velocity

A number of theoretical models have appeared in the literature which predict the inversion phenomenon of liquid fluidization whereby smaller denser particles in a binary mixture report to the bottom at low superficial velocities, but to the top at higher velocities. This paper reviews and categorizes these models into five groups based on the mechanisms and approaches followed. Revisions are proposed in some models for greater accuracy or consistency. The predictions of inversion velocity, i.e. the velocity at which the bed is homogeneously mixed, are then compared to available experimental data. Unfortunately, the exact temperature was not specified in most of the experimental studies, and since the predictions are sensitive to the liquid viscosity, which depends strongly on temperature, this introduces significant uncertainty into the comparison. Agreement between predictions and experimental data is within ± 14 % on average for the best models.

Volume contraction behaviour of binary solid–liquid fluidized beds

Contrary to the widely held notion that the total bed height of a binary solid fluidized bed will be the sum of heights of the two individual mono-component beds fluidized at the same velocity, significant negative deviations have been observed in our experimental investigation. The negative deviations, signifying a contraction of the total volume of the binary solid fluidized bed, could sometimes be as high as 25% of the actual volume. The volume contraction has been found to depend mainly upon the degree of solids mixing prevailing in the bed irrespective of whether it is fully fluidized or not. The composition of the binary solid fluidized was another important factor that influenced the contraction behaviour of the bed besides the size ratio of the two constituent solid species of the binary present in the bed.

Study of Binary‐Solid Liquid Fluidized Beds

AbstractA proper characterization of the hydrodynamics of binary‐solid liquid fluidized beds is an important first step in its effective utilization. Of particular importance in this connection is to be able to predict the unique hydrodynamic phenomenon of the layer‐inversion, which is associated with the change of the stratification pattern of the two solid species in the fluidized bed brought about due to a change either in the liquid velocity or the bed composition. Past few years have witnessed the development of several models for the prediction of the layer‐inversion phenomenon. It is, therefore, important to analyze their predictions in the light of the growing wealth of the literature data. Such an exercise is important to examine the underlying assumptions and propose modifications to improve their predictive capability. Predictions of well‐known layer‐inversion models are compared with the experimental data reported in the literature including our own which comprises of solid species of widely different size, and therefore, provide an important test of predictive capability of models.

The complete segregation model for a liquid fluidized bed: formulation and related issues

The complete segregation model of Gibilaro et al. [Chem. Eng. Sci. 41 (1986) 379] has been widely used to predict the layer-inversion phenomenon in a binary solid–liquid fluidized bed. Important features of the model have been analyzed in this paper. Simplifications are proposed to reduce the original two-dimensional optimization problem to mainly a single implicit algebraic equation. A hybrid scheme using a combination of both property-averaging scheme as well as the serial model for evaluating the bed void fraction apparently improves the model predictions. Finally, it has been shown that a straightforward extension of the model from the binary solid to the ternary solid case is generally not tenable.

Effective Voidage Model of a Binary Solid−Liquid Fluidized Bed: Application to Solid Layer Inversion

A conceptual model based on the effective voidage around a test particle in a bidisperse solid-liquid fluidized bed has been proposed and has been used to interpret and predict its phase inversion behavior. The model visualizes the test particle to be surrounded by particles of average size. An average size particle has a liquid envelope of average thickness which is different from the thickness of the liquid envelope surrounding the test particle. The effective voidage (e eff ) enjoyed by a particle of this kind has been expressed as a function of the volume fractions of individual particles in the bed. The two equations for the effective voidage (e eff ) for the two types of particles can be solved numerically for the local concentration pairs in the bed. The solution leads to the prediction of the critical velocity or the inversion velocity of the system. It also allows computation of the particle concentration profiles as well as the heights of different sections of the binary solid-liquid fluidized bed. The computed critical velocities for 19 systems studied by different workers are in excellent agreement with reported experimental data. The model is fully predictive and does not necessarily require any experimental bed expansion data for the prediction of the inversion velocity and other parameters.

Prediction of the inversion velocity in the binary-solid liquid fluidized bed

Prediction of the inversion velocity in the binary-solid liquid fluidized bed

A simple predictive model of layer inversion in binary solid–liquid fluidized beds

A simple predictive model of layer inversion in binary solid–liquid fluidized beds

A simple predictive model of layer inversion in binary solid-liquid fluidized beds

A simple predictive model is presented to describe the layer inversion behaviour of binary solid-liquid fluidized beds. The model is based on the assumption that the concentrations of the two particle species in the binary bed when normalized with respect to their mono-component counterparts are linearly related. The comparison of the model predictions with the experimental data available in the literature confirms the applicability of this simple model to predict the inversion phenomenon.

An improved Richardson-Zaki formula for computing mixed layer composition in binary solid-liquid fluidized beds

In a liquid fluidized bed consisting of binary solid particles differing in size and density, the mixing of the two kinds of particles occurs and the degree of mixing is controlled by an equilibrium force balance relationship. A model to predict the volume fractions of particles in the mixed layer is developed in this paper on the basis of the force balance relationship of each particle species. In describing the drag forces of particle components 1 and 2 in a mixed layer, two hypothetical mono-component beds, bed (1∗) and bed (2 ∗), are introduced. The bed ( i∗) ( i=1,2) consists of only one particle component i, and it is assumed that the drag force acting on the particle component i in this bed is equal to that acting on component i in the mixed layer. This assumption, coupled with the Richardson and Zaki formula, leads to a modified Richardson and Zaki formula for computing the composition of the mixed layer in a binary solid-liquid fluidized bed. The validity of this model is tested against experimental data.

Mixing in segregated, binary-solid liquid-fluidized beds

The behaviour of liquid-fluidized binary-solid mixtures can be represented by a “complete segregation model”, where two superposed non-interacting zones are predicted to form. Deviations from this idealized situation may be predicted utilizing existing relationships for the total interaction force on a particle in a fluidized bed; experiments are reported that support the predictions.

A predictive model for the equilibrium composition and inversion of binary-solid liquid fluidized beds

The concept of complete segregation of a binary-solid particulate fluidized bed is generalized to accommodate cases where the bottom zone contains an equilibrium mixture of the two solid components. The model is fully predictive and provides a complete explanation of the “inversion phemonenon” in close agreement with recently published experimental studies.