Maria do Carmo Ferreira

A Hybrid Lumped Parameter/Neural Network Model for Spouted Bed Drying of Pastes with Inert Particles

The current study analyzed the suitability of a hybrid CST/neural network model to describe the highly coupled heat and mass transfer during paste drying in a spouted bed. In the present approach, the main information was the moisture content predictions in the powder. The model was based on global energy and water mass balances in the liquid and the gaseous phases. In this model, the inter-phase coupling term r, which reflects both water evaporation and particle coating, was described by an artificial neural network. Artificial neural networks are efficient computing models which are extensively used whenever theoretical models fail to properly represent a given phenomena and reliable data basis of the main variables involved is available. Simulations were done in MatLab. The drying experiments for model verification were carried out in a conical semi-pilot scale spouted bed, from which measurements of gas and solid phase moisture were done. The good agreement between calculated and measured powder moisture content suggested that the well-mixed hypothesis could be applied for paste drying in a spouted bed. The robustness of the model with respect to changes in feed flow rates and other operating conditions showed the merits of using a trained neural network.

A Review on Paste Drying with Inert Particles as Support Medium

An overview of the contributions of the literature in the field of drying with inert particles is presented. Recent experimental studies developed in conventional spouted and fluidized beds aimed at describing and understanding the effects of the presence of a liquid or paste on the global behavior of these types of dryers are approached. Advances in numerical simulation of drying with inert particles are discussed. Several open research issues and future perspectives are addressed.

Drying of coarse particles in a vertical pneumatic conveyor

In the present work, one-dimensional two-phase continuum models were applied to simulate the pneumatic drying of porous alumina and solid glass particles. Pressure profiles, gas and solid temperature, and gas and solids moisture profiles were obtained in a 53.4-mm conveying tube. For both particles, maximum values of gas-to-particle heat transfer coefficients were obtained at air velocities close to the minimum pressure gradient velocity. Experimental temperature and moisture profiles of gas and solids were compared to simulated predictions, showing that models based on the two-phase flow approach fail to predict all the observed physical phenomena in simultaneous momentum, heat, and mass transfer for pneumatic drying of coarse particles. However, using adequate correlations and constitutive equations to predict interaction forces and transport parameters, it was possible to obtain good predictions of gas and solid temperature profiles and of moisture content.

Drying of Pastes in Vibro-Fluidized Beds: Effects of the Amplitude and Frequency of Vibration

Abstract The dimensionless vibration number (Γ) is the parameter most widely applied to quantify the vibration energy of a vibro-fluidized bed. It is defined as a function of the amplitude (A) and of the frequency of vibration (f). Most works in literature present their results as a function of Γ, which is adopted as a single parameter to explain the results on fluid dynamic behavior of vibro-fluidized beds, independent on the magnitudes of A and f. From its definition, however, it is possible to obtain identical values of Γ just by combining different values of A and f, and further studies are necessary for a better analysis of fluid dynamic behavior of vibro-fluidized beds concerning their vibration energy. Therefore, in this work the effects of dimensionless vibration number on the fluid dynamic behavior of a vibro-fluidized bed will be investigated by choosing different combinations of A and f values arranged to provide identical values of Γ. The tests were carried out for dry and wet beds, for values of Γ = 0.0 (fluidized bed), 1.0, 2.0, and 4.0. The results indicate that Γ must be used carefully when applied as the only parameter to characterize the vibration effects in a vibro-fluidized bed since very different fluid dynamic behavior was detected at a fixed Γ at different amplitudes and frequencies.

Analysis of the Influence of Dimensionless Vibration Number on the Drying of Pastes in Vibrofluidized Beds

The purpose of this work is to evaluate the operation of a vibrofluidized bed of inert particles in the transient and steady-state regimes for water evaporation and for drying of domestic sewage sludge. The feeding rate was initially set on 10 mL/min and this value has been gradually raised at 4 mL/min increments until saturated moist conditions were obtained at the bed exit. The bed was operated at 80 and 100°C, with inlet air velocities 20 and 40% above the minimum fluidization velocity. The dimensionless vibration numbers (Γ) were 0.00 and 4.00, the latter one with amplitudes of 0.003 and 0.015 m. For the conditions investigated, the effect of Γ on the drying dynamics was observed to depend on the amplitude and frequency of vibration. The use of vibration with lower amplitude and higher frequency provided the highest powder production. For paste feeding rates from 14 to 38 mL/min, the moisture content of the dried sewage sludge powder varied from around 5 to 10% on a wet basis, tending to increase with the increase of paste feeding rates.

Effects of binary particle size distribution on the fluid dynamic behavior of fluidized, vibrated and vibrofluidized beds

In this work the effects of particle size distribution on the dynamics and segregation patterns in fluidized, vibrated and vibrofluidized beds were investigated. A binary particle size distribution and a reference one composed of glass spheres with a mean Sauter diameter of 2.18×10-3 m were tested. The experimental setup consisted basically of a circular glass chamber with a height of 0.50 m and a diameter of 0.114 m, operated in the fluidized bed mode (G = 0.00), in either vibrated or vibrofluidized bed modes (G = 2.00). The pressure drops in the fluidized and vibrofluidized beds were not significantly affected by the binary particle size distribution. Well-defined segregation patterns occurred in fluidized and vibrated beds with small particles concentrating at the top and large particles at the bottom in the first situation and the reverse in the second one. Segregation patterns in vibrofluidized beds depended on the values of vibration parameters. Segregation in vibrofluidized and vibrated beds was minimized by operating at a high amplitude of vibration.

Analysis of Fluid Dynamics and Thermal Behavior Using a Vertical Conveyor with a Spouted Bed Feeder

Fluid dynamic behavior of pneumatic conveying of dry and wet particles was investigated for a system operated with a spouted bed–type solid feeding system. Glass spheres of 2.7 mm diameter were conveyed in a riser of diameter equal to 53 mm, 4 m long. The performance of the spouted bed feeder was analyzed through plots of solid flow rates versus air velocity obtained at different values of z0, which is the distance between the air inlet and lower end of the riser. Pressure gradient versus air velocity plots were applied to identify the transition velocity from dilute to dense-phase flow in systems operated with spouted bed feeders. A fluid dynamic model originally proposed to describe particle flow in a draft tube spouted bed was successful in predicting pressure profiles and mean voidages in the pneumatic conveyor with a spouted bed feeder. Stable pneumatic conveying of wet particles could be accomplished for water flow rates from 20 mL/min up to 70 mL/min at inlet air temperatures of 100 and 120°C. For the conditions investigated, pressure profiles and inlet pressures were not significantly affected by the presence of a liquid phase. The results show the feasibility of performing water evaporation in pneumatic conveying with spouted bed feeders.

Homogeneous hydraulic and pneumatic conveying of solid particles

A discussion is provided of the difficulties and limitations involved in the application of the one-dimensional two-phase flow model to estimate pressure gradients associated with the transport of particles. Two particular cases are investigated, namely, hydraulic conveying and pneumatic transport. The validity of the model is checked by comparing experimental data with predicted values of pressure gradients and void fractions. New approaches for estimating the interaction forces in hydraulic and pneumatic transport are proposed. It is demonstrated that, despite its limitations, the two-phase flow model may provide good predictions for the pressure gradients.

Stability and performance of a spouted bed in drying skimmed milk: Influence of the cone angle and air inlet device

ABSTRACT This paper investigates the skimmed milk drying over inert particles in a 30 cm diameter spouted bed. Drying was evaluated for conical bases of three cone angles (45°, 60°, and 75°) operating with 4.5 kg of inert particles, and two air inlet devices with different geometries—a Venturi-type nozzle and a straight section pipe. The drying capacity was evaluated from the maximum paste feed rate that allowed stable operation and spouting dynamic behavior throughout drying. It was observed that drying capacity and dynamic stability were greatly affected by the cone angle and the inlet device geometry. From the conditions evaluated, the best configuration with regard to the drying capacity was obtained for the 45° conical base using the Venturi-type inlet device. In this dryer, a rate of 40 mL/min of skimmed milk was processed and a powder with a moisture content of about 10% was produced. The good performance on this configuration was attributed to several factors that affected positively the solid circulation rates and the dynamics stability. Such factors include the operation under a higher static-bed height, the spouting on conical configuration, and the improved aeration of the annular region provided by the inlet device. At this configuration, and for paste feed rates up to the maximum admissible value, the spouted-bed dynamic was almost not affected by the liquid and the wet bed operated under dynamic conditions very similar to those of the dry bed. Using a 75° conical base with the Venturi-type inlet device under a slightly lower drying capacity (35 mL/min) also resulted in quite stable dynamic conditions. The 60° conical base with the Venturi-type device was the configuration that performed worst with regard to drying capacity (30 mL/min).

ANALYSIS OF A ONE-DIMENSIONAL FLUID DYNAMIC MODEL FOR DILUTE GAS-SOLID FLOW IN A PNEUMATIC DRYER WITH A SPOUTED BED TYPE SOLID FEEDING SYSTEM

ABSTRACT The pneumatic bed with a spouted bed type solid feeding system reported by Ferreira and Freire (1992) allows stable solids conveying in a wide range of solids flow rates. This is being developed with the aim to dry products with high initial moisture contents. A one-dimensional model based on continuity and momentum balances is used to describe non-accelerating gas-solids flow and to predict pressure gradient data. Experimental data were obtained for the transport of glass spheres (db=1.00 to 2.BS mm) in galvanized iron pipes of four different diameters (Dt53.4 to 149.0 mm). Good predictions for the pressure gradients were obtained from the model. Estimated values of the solid-fluid interaction forces and slip velocities, however, were often not physically consistent. This result leads to the conclusion thac the one-dimensional model does not provide a good description of the physical phenomena and flow pattern involved in dilute pneumatic vertical transport.