Giuseppa Vella

Characterization of pressure retarded osmosis lab-scale systems

AbstractPower generation from salinity gradient is a viable alternative to produce energy from renewable sources. Pressure Retarded Osmosis (PRO) is one of the technologies proposed so far for the exploitation of such energy source. In the present preliminary work, two different geometry modules were tested under atmospheric pressure (i.e. forward osmosis or depressurized-PRO conditions). The first one is a conventional planar geometry cell. The second is a customized cylindrical membrane module, able to mechanically support the osmotic membrane along with the spacers. The latter, thanks to its design, allows membranes and spacers to be easily changed for testing purposes. A novel simplified procedure is proposed and employed in the planar geometry module to characterize an asymmetric membrane commercially available (i.e. assessing the water and salt permeability coefficients and the porous structure parameter). The parameters found were employed to mathematically estimate the permeate fluxes experimental...

Modelling and Simulation of Gas–liquid Hydrodynamics in a Rectangular Air-lift Reactor

Abstract Computational Fluid Dynamics is a quite well established tool for carrying out realistic simulations of process apparatuses. However, as a difference from single phase systems, for multiphase systems the development of CFD models is still in progress. Among the two-phase systems, gas–liquid systems are characterised by an additional complexity level, related to the fact that bubble sizes are not known in advance, being rather the result of formation and breakage-coalescence dynamics and therefore of complex phenomena related to flow dynamics and interfacial effects. In the present work, Euler–Euler Reynolds-averaged flow simulations of an air-lift reactor are reported. All bubbles are assumed to share the same size, and a simplified approach is adopted for modelling inter-phase momentum exchange, that involves bubble terminal velocity as the sole parameter needed. Good agreement between simulation results and literature experimental data is found for all the gas flow rates simulated. This result implies that, despite the many simplifications that have to be adopted in order to make them viable, fully predictive CFD simulations of gas–liquid systems can give rise to reasonably accurate predictions of reactor fluid dynamics.

Pressure retarded osmosis: A membrane process for environmental sustainability

Salinity Gradient Power (SGP) based on the controlled mixing between two solutions at different salinities is a viable alternative to produce power from renewable sources. Pressure Retarded Osmosis (PRO) is one of the most promising technologies proposed so far for the exploitation of such energy source. Apart from the typical source of salinity gradients, namely river water and seawater, more and more interest has been raised recently towards the use of non-conventional saline solutions. In this work, water originating from a sewage treatment plant is used as the dilute solution (feed solution), while brine exiting from a desalination plant is used as the concentrate (draw solution), thus being efficiently diluted before the discharge into the sea. Aim of this work is to investigate the performance of PRO modules arranged in series via a purposely developed process model based on transport and balance equations. Detrimental effects due non-ideal phenomena as salt diffusion through the membrane, concentration polarization were taken into account. Pumping power was also accounted for. Geometrical data were derived from existing spiral wound membrane modules typically employed in Reverse Osmosis. The effect of solution velocity, number of PRO units and operative hydraulic pressure (applied on the draw solution channel) on the process performance was investigated. Results show that the dependence of the net power on velocity and applied pressure is not monotonic thereby exhibiting a maximum: under optimized conditions, a maximum net power of ~1.5kW corresponding to a net power density of ~6.6 W/m 2 can be obtained with six identical PRO modules. Along with this power production, a simultaneous brine dilution of more than 22% was found at the same conditions. However, despite the potential shown by the model, the main limitation for a real process might be the real performances of the osmotic membranes in terms of mechanical robustness and resistance against fouling phenomena. In this regard, valuable contributions may come from nanotechnologies ad-hoc developed.

Measurement of Multiphase Flow Characteristics Via Image Analysis Techniques: The Fluidization Case Study

In recent years, thanks to the continuous development of digital imaging systems and digital image processing, a great number of researchers have chosen digital visual methods to be applied in the field of experimental fluid dynamics. These kinds of techniques play a fundamental role in analysis and data acquisition for multiphase flows such as gas-solid, gas-liquid, solid-liquid flows, where the observation of inter-phase boundaries is relatively simple.

Gas-Fluidization Characteristics of Binary Mixtures of Particles in 2D Geometry

The bubbling behaviour of fluidized beds has been thoroughly investigated in the last decades by means of several techniques, e.g. X-ray, Inductance, Resistance and Impedance based techniques, PIV. In recent years, Digital Image Analysis Techniques have shown their potential for accurate and cost effectively measurements.Most of the work related to bubble behaviour analysis deals with single-sized particles, while almost all industrial equipment operates with multi-sized particles. Although considerable work has been done in the past with focus on the analysis of the mixing-segregation behaviour and predictions of fluid dynamics regime transitions, a lack of knowledge still exists in the analysis of bubbles properties measurements for the case of polydispersed systems.In this work, digital image analysis has been adopted to accurately measure fundamental global parameters such as bubble hold up and bed expansion as well as average bubble hold-up distribution maps or bubble size distributions in bubbling fluidized beds of binary mixtures of particles. The experiments have been carried out at steady state conditions with binary mixtures of corundum particles, at various inlet gas velocities. This preliminary study has been performed with the aim to collect valuable data for future development of predictive models and validation of CFD codes.

Digital image analysis technique for mixing pattern measurements in Bi-dispersed 2D gas fluidized beds

This work concerns a new experimental technique based on Digital Image Analysis for the measurement of the mixing behaviour in a bi-dispersed 2D fluidized bed. Two different sets of particles are fluidized with the same density and different size. The technique is based on suitable color-field decomposition of the fluidized bed snapshots, where particles of different colors are employed. The technique here proposed can effectively measure the concentration field in the whole bed during operation and capture the mixing and segregation dynamics of powders. Notably, the technique here developed can be used for both gasor liquidfluidized beds.