Massimo Poletto

Almond oil extraction by supercritical CO2: experiments and modelling

Oil from crushed almond seeds was extracted with supercritical CO2 at 350 bar and 40°C. Almond particles of three different mean sizes were tested. Extraction of the smaller particles was performed at two different solvent flow rates. Oil yields were obtained with asymptotic values at large extraction times that were close to the values obtained by Soxhlet extraction. An extraction model based on the physical evidence of broken and intact oil cells has been developed. It accounts for a former equilibrium regime and a latter finite mass transfer regime. All model parameters except the internal mass transfer coefficient and the oil concentration at solvent saturation have been determined with independent experiments. The model solution was calculated with a finite difference numerical technique. A good agreement was obtained between model curves and the experimental data for an internal mass transfer coefficient of 7.5×10-9m/s. Solute concentration profiles within the extractor were evaluated with our model.

Hydrolysis of Lactose in a Fluidized Bed of Zeolite Pellets Supporting Adsorbed ß - Galactosidase

The purpose of this work is to verify the possibility of using a simple physical adsorption technique to improve enzyme efficiency in performing lactose hydrolysis. Immobilization experiments of ß -galactosidase on zeolite pellets showed that this process is fairly slow and can be accomplished with contact times shorter than few hours. Comparison between homogeneous and heterogeneous hydrolysis experiments in mixed batch reactors indicates that the enzyme distribution within the zeolite pellets is concentrated in a thin shell under the pellet surface. Continuous conversion experiments were carried out on a purposely-built fluidization column. Results were fitted with a mathematical model for fluidized bed conversion without the use of any further adjustable parameter than those used in the heterogeneous batch conversion. Furthermore, no change of the parameter values was necessary. Scale-up predictions given by the model showed that fluidized columns few meters high of pellets supporting b-galactosidase are sufficient for an industrially suitable process of lactose hydrolysis. which can be performed on columns of few meters and, therefore, these are applicable to relatively small process scales as those found in traditional Italian cheese making factories.

Gas pressure measurements inside an aerated hopper

A new technique, based on the determination of local gas pressure profiles, is proposed to characterize the flow properties of fine aerated powders. Gas pressure measurements were carried out in an aerated hopper discharging an aeratable powder (FCC) and a cohesive powder (corn starch). Solids discharge rates, gas pressure local time series and gas pressure profiles were determined. Experimental discharge rates confirm that the use of aeration promotes the motion of these powders. Pressure profiles and time series are analysed in order to discriminate between different mechanisms that were suggested for these kinds of powders and to elucidate the role of cohesive forces in the flow of fine powders. A classification of the regimes observed as a function of aeration and powder cohesion has been attempted. Pressure profiles were used to evaluate gas flow rates within the hopper during the discharge and to verify literature correlations on solids discharge rates. Some considerations on the relative significance of the gas rates during discharge allowed to obtain a simplified version of the De Jong and Hoelen (De Jong, J.A.H. and Hoelen, Q.E.J.J.M., 1975, Powder Technol., 12: 201–208) equation which can be used in a predictive way with reasonable approximation.

Distribution of gas pressure inside a hopper discharging fine powders

In this paper the mechanism of discharge of fine powders from hoppers is examined. Gas pressure measurements were taken in the proximity of the hopper outlet, mainly along the axis, while discharging fine powders. Different types of pressure traces were detected depending on the solid properties. A simple model was developed to account for the main features of pressure profiles. The main hypothesis relies on the presence of a surface near the hopper outlet which defines a discontinuous change in the voidage of the discharging solids. From the fluid dynamic point of view, the solid above this surface behaves as a permeable compact; the solid below, instead, can be considered as a moving suspension of solids in gas. This region appears to be the major responsible of the pressure gradients inside the hopper. A good agreement between model and results indicates that, in spite of its simplicity, the models accounts for the main phenomena involved.

Analysis of the dynamics of heat transfer between a hot wire probe and gas fluidized beds

Abstract Hot wire anemometry used inside air fluidized beds of glass (175 μm), FCC (75 μm) and silica (85 μm) powders (Archimedes numbers of 510, 29 and 16, respectively) allowed the measurement of the time-resolved local heat transfer coefficient. Time averages of this coefficient reproduce the same behaviour found by other authors with different experimental techniques. A stochastic model for the heat transfer rate has been developed on the basic hypothesis that heat transfer fluctuations are due to the continuous renewal of packets of solid particles along the wire. The most relevant simplifying hypothesis is that the contact time between the wire and the packet is much shorter than the characteristic heating time of the packets. With this model, probability density distributions of the heat transfer coefficient are evaluated. Comparison between experimental and theoretical results is fairly good in all experimental conditions relative to fully developed aggregative fluidization. The model is less reliable in conditions of incipient and homogeneous fluidization, where the simplifying hypotheses may not apply. Calculated values of packet to particle size ratios, λ / d p , are around 8 for glass, between 14 and 36 for FCC and between 17 and 32 for silica. The increasing number of particles inside a packet seems, therefore, to be correlated, on one hand, to the decreasing Archimedes number, and on the other, to an apparently reduced particle mobility of powders belonging to the Group A of the Geldart [D. Geldart, Types of gas fluidization, Powder Technol., 7 (1973) 285–292] classification.

Characterization of Woody Biomass Flowability

In this paper a simple method for characterizing the flowability of consolidated samples of woody biomass is presented. The apparatus used consists in a cylindrical bin provided with a circular orifice at the centre of its flat bottom. Biomass samples were consolidated in the bin by applying loads on the material bed surface while the bin orifice was closed by a plug. After the consolidation phase the loads were removed and the orifice was opened to assess if an arch or a rathole had formed. Critical values of consolidation loads for the formation of a stable arch were determined for different orifice sizes. These results obtained with two different samples of sawdust were compared in terms of material strength with those of flow functions obtained with a conventional Schulze shear tester.

Flow Properties of Moisturized Powders in a Couette Fluidized Bed Rheometer

This work investigates on the effect of air humidity on the flow properties of powders. The moisture of a powder sample (50 µm glass beads) was conditioned by fluidization with humid air. Air humidity was kept between 0 and 70% at ambient temperature. A Schulze shear cell was used to measure the bulk flow properties of the moisturized samples. A Couette fluidized bed rheometer was used to measure the torque necessary for the rotation of the inner cylinder when the fluidized powder had been moisturized with the same procedure. These experiments show a certain continuity of the results below and above the minimum fluidization velocity, suggesting a similar continuity of the role that interparticle interactions play in the fixed and in the fluidized bed. Experiments below the minimum fluidization velocity were interpreted with a rheological model in which the variable load along the vertical direction in the Couette was calculated with a modified Janssen equation. In this approach the apparent weight of the powder is given by the difference between the gravity and the upward body force determined by the rising gas flow. The agreement between the model and the experiments supports the proposed approach.

The particle velocity field inside a two-dimensional aerated hopper

A two-dimensional lab-scale hopper with a flat bottom used for aeration and transparent glass front wall was used to visualize the flow pattern of fine particles (FCC particles of 64 μm and glass beads of 147 μm) near the hopper outlet at different airflow rates. Particle traces near the hopper outlet are recorded with a digital video camera and analyzed with a semiautomated image analysis procedure to yield the particle velocity field. This procedure has been verified and set up with a preliminary analysis in which the effects of several variables and in particular the frame exposure time have been addressed. Results indicate that the quality of the solids flow changes considerably with the aeration rate. At zero or low gas rates, the flow is of the funnel flow type with the formation of a well-defined channel around the hopper axis which is of the same size of the outlet. The solids on the sides are stationary. This channel widens with increasing aeration rate and solids away from the hopper axis also move. Results have been discussed and compared with pressure measurements carried out in previous work [3rd World Congress on Particle Technology (1998)].

Preliminary Assessment of a Simple Method for Evaluating the Flow Properties of Solid Recovered Fuels

The increasing interest in combustion of solid recovered fuels (SRF), whose physical properties may significantly differ from those of conventional solid fuels, draws attention to the handling of these materials. In this article a simple method for evaluating particulate solids flow properties based on a solid extrusion procedure is assessed. The effects of the main geometrical parameters of the tester and of the testing operating condition are investigated. Results are critically discussed by comparing them with the flow properties evaluated in a Schulze ring shear tester.

Techno-economic analysis of power and hydrogen co-production by an IGCC plant with CO2 capture based on membrane technology

Abstract The techno-economic analysis of an IGCC plant for power and hydrogen production with CO 2 capture by means of Pd-based H 2 membranes was carried out. The case studies were referred to the modifications of the existing 330MWe Integrated Gasification Combined Cycle (IGCC) plant of ELCOGAS based on entrained flow gasification of a dry mixture of coal and pet-coke. The pre-combustion section consisting in a sour water gas shift reactor integrated with Pd-based H2 selective membranes and CO 2 selective membranes was simulated by process simulation techniques. Heat integration of the new process section was also addressed to minimize the energy loss. Power and energy penalty were evaluated as a function of the CO 2 capture percentage and of the electricity and hydrogen output. Economic assessment of the additional capital and production costs was also performed to evaluate the mitigation cost of the carbon capture and storage (CCS) based on membrane technology. Sensitivity analysis was carried out to derive the breakeven price and the threshold performance of hydrogen membranes. The results on the cost of electricity, calculated without accounting for the revenues of the hydrogen sales, provided preliminary data for the economic feasibility of H 2 membranes in the IGCC process.