Multiscale mathematical models for simulation and scale-up of green processes in the perspective of industrial sustainability

The present work presents research studies aimed at developing tools useful to design engineering solutions moving in the direction of industrial sustainability. The investigations hereinafter discussed regard an extraction process of active compounds – polyphenols – from agro-food industry wastes (olive and grape pomaces) and a biorefinery exploiting waste frying oil, solid organic wastes and algal biomass to produce biofuels. In particular, for the former topic, a procedure aimed at the evaluation of the technological feasibility at pilot scale of said process is discussed. The proposed approach takes into consideration the extended kinetic route coupled with mathematical simulation. Detailed physically-based dynamic mathematical models, taking into account mass and energy balance equations, are adopted to describe both the lab-scale and the pilot-scale reactors. Chemical physical parameters appearing in the models are estimated from the experimental data at lab-scale or are partially taken from literature. Different heating systems are designed for the pilot scale reactor and their performance is tested by simulation. Characteristic times are evaluated also during start-ups and different control loops are analyzed in order to set-up the best process and operating variables. Average yields in polyphenols are finally evaluated for both the batch and the continuous operated pilot reactor, by considering feed variability and fluctuations of process parameters. For what concerns the biorefinery, special attention was devoted to the modeling of the airlift reactor, its most delicate and complex component. In fact, to optimize this interesting microalgae cultivation system, a precise description of the moving interfaces formed by the liquid and gas phase is critical. In this study, coupled front capturing methods (standard and conservative level set methods) and finite difference method are used to simulate gas bubbles dynamics in a pilot-scale external loop air-lift photobioreactor in which microalgae are used to capture CO2 from flue gas and to treat wastewater. Numerical simulations are carried out on rectangular domains representing different sections of the vertical axis of the riser. The data employed was either acquired from previous experimental campaigns carried out in the airlift reactor or found in the literature. The rise, shape dynamics and coalescence process of the bubbles of flue gas are studied. Moreover, for each analyzed applications, a procedure based on Buckingham π-theorem to perform a rigorous scale-up is proposed. In this way, scale-invariant dimensionless groups describing and summarizing the considered processes could be identified. For the research focused on the scale-up of photobioreactors used to cultivate Chlorella Vulgaris, an experimental campaign at three levels was designed and carried out to evaluate the characteristic dimensionless numbers individuated by the theoretical formulation. Since scale-up regards both geometrical dimensions and type of reactor, passing from lab-scale stirred tanks to pilot scale tubular and airlift, particular attention was devoted to define characteristic lengths inside the dimensionless numbers.