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Nowadays, pyrohydrolysis techniques are widely applied for regeneration of spent pickling liquors providing an excellent environmental and economical strategy to the problem of waste disposal/recovery, also thanks to the high acid recovery efficiencies (>99%) achieved. In fact, in these processes, iron chlorides are converted into iron oxides and hydrogen chloride at high temperature in spray roasting or fluidized bed reactors. Though the state-of-the-art technologies have been successfully applied only to large-scale plants, the development of small-scale units, able to perform a delocalized regeneration of spent solutions where these latter are actually produced, would be strongly needed in order to minimize the transportation of hazardous spent pickling and fresh HCl solutions to/from pickling factories from/to large centralized regeneration plants. In the present work, a steady-state simulation model for regeneration of spent pickling liquor via spray roasting has been developed, aiming at the analysis of performance of a small-scale unit. The simulation and optimization of the process has been carried out using Aspen Plus simulator. A simplified model of the spray roaster, based on the data collected from studies reported in the literature, has been implemented and exported from Aspen Custom Modeler. Solid recovery from roaster off-gases has been obtained by a cyclone and a Venturi scrubber, where the spent pickling liquor is concentrated prior to be sprayed into the reactor. Hydrogen chloride is absorbed in an adiabatic absorber, where a sub-azeotropic (18–20 wt%) hydrochloric solution is obtained. Prior to the stack, a scrubber is required to wash off the exhaust gases. Several parametric studies have been performed. Different designs of solid separators have been compared in order to investigate the particulate separation efficiency of the process, while absorption column’s packing types and rinse water flow rates have been varied in order to investigate their effects on HCl recovery and acid gas emissions.

Reverse electrodialysis (RE) is a promising technology for electric power generation from controlled mixing of two differently concentrated salt solutions, where ion-exchange membranes are adopted for the generation of ionic currents within the system. Channel geometry strongly influences fluid flow and thus crucial phenomena such as pressure drop and concentration polarization. Profiled membranes are an alternative to the more commonly adopted net spacers and offer a number of advantages: avoiding the use of non-conductive and relatively expensive materials, reducing hydraulic losses and increasing the active membrane area. In this work, Computational Fluid Dynamic simulations were performed to predict the fluid flow and mass transfer behaviour in channels with profiled membranes for RE applications. In particular, channels equipped with pillars were simulated. The influence of channel geometry on fluid flow and concentration polarization was assessed by means of a parametric analysis for different profile geometries. The unit cell approach along with periodic boundary conditions was adopted to simulate fully developed boundary conditions. Transport equations, valid also for concentrated solutions, were obtained from the rigorous Stefan–Maxwell equation along with the assumptions of binary electrolyte and local electroneutrality. Simulation results show that, in the geometries investigated here, the pumping power consumption is much lower than in a conventional net spacer and very close to that of the empty channel, while calm zones are generated by the profiles, which may accentuate polarization phenomena.

Technologies for the exploitation of renewable energies have been dramatically increasing in number, complexity and type of source adopted. Among the others, the use of saline gradient power is one of the latest emerging possibilities, related to the use of the osmotic/chemical potential energy of concentrated saline solutions. Nowadays, the fate of this renewable energy source is intrinsically linked to the development of the pressure retarded osmosis and reverse electrodialysis technologies. In the latter, the different concentrations of two saline solutions is used as a driving force for the direct production of electricity within a stack very similar to the conventional electrodialysis ones. In the present work, carried out in the EU-FP7 funded REAPower project, a multi-scale mathematical model for the Salinity Gradient Power Reverse Electrodialysis (SGP-RE) process with seawater and concentrated brines has been developed. The model is based on mass balance and constitutive equations collected from relevant scientific literature for the simulation of the process under extreme conditions of solutions concentration. A multi-scale structure allows the simulation of the single cell pair and the entire SGP-RE stack. The first can be seen as the elementary repeating unit constituted by cationic and anionic membrane and the relevant two channels where dilute and concentrate streams flow. The reverse electro-dialysis stack is constituted by a number of cell pairs, the electrode compartments and the feed streams distribution system. The model has been implemented using gPROMS , a powerful dynamic modelling process simulator. Experimental information, collected from the FUJIFILM laboratories in Tilburg (the Netherlands), has been used to perform the tuning of model formulation and eventually to validate model predictions under different operating conditions. Finally, the model has been used to simulate different possible scenarios and perform a preliminary analysis of the influence of some process operating conditions on the final stack performance.

Salinity Gradient Power by Reverse Electrodialysis (SGP-RE) technology allows the production of electricity from the different chemical potentials of two differently concentrated salty solutions flowing in alternate channels suitably separated by selective ion exchange membranes. In SGP-RE, as well as in conventional ElectroDialysis (ED) technology, the process performance dramatically depends on the stack geometry and the internal fluid dynamics conditions: optimizing the system geometry in order to guarantee lower pressure drops (DP) and uniform flow rates distribution within the channels is a topic of primary importance. Although literature studies on Computational Fluid Dynamics (CFD) analysis and optimization of spacer-filled channels have been recently increasing in number and range of applications, only a few efforts have been focused on the analysis of the overall performance of the process. In particular, the proper attention should be devoted to verify whether the spacer geometry optimization really represents the main factor affecting the overall process performance. In the present work, realized within the EU-FP7 funded REAPower project, CFD simulations were carried out in order to assess the effects of different parameters on the global process efficiency, such as the choice of spacer material and morphology, and the optimization of feed and blowdown distribution systems. Spacer material and morphology can affect the fluid dynamics inside each channel. In particular, the appropriate choice of net spacer material can influence the slip/no-slip condition of the flow on the spacer wires, thus significantly affecting the channel fluid dynamics in terms of pressure drops. A Unit Cell approach was adopted to investigate the effect of the different choices on the fluid flow along the channel. Also, the possibility of choosing a porous medium to substitute the net spacer was theoretically addressed. Such investigation focused on the porosity and the fiber radius required to respect the process constrains of pressure drops and mechanical stability. On the other hand, the overall pressure drops of a SGP-RE or ED stack can be considered as resulting from different contributions: the pressure drop relevant to the feed distributor, the pressure drop inside the channel, and the pressure drop in the discharging collector. The choice of the optimal stack geometry is, therefore, strongly related to the need of both minimizing each of the above terms and obtaining the most uniform feed streams distribution among the stack channels. In order to investigate such aspects, simulations were performed on a simplified ideal planar stack with either 50 spacer-less or 50 spacer-filled channels. The effect of the distribution/collector channel thickness and geometry on single-channel flow rates and overall pressure drops in the system was analyzed and a significant influence of distributor layout and size on the overall process performance was found.