Facility layout problem: Bibliometric and benchmarking analysis

Facility layout problem is related to the location of departments in a facility area, with the aim of determining the most effective configuration. Researches based on different approaches have been published in the last six decades and, to prove the effectiveness of the results obtained, several instances have been developed. This paper presents a general overview on the extant literature on facility layout problems in order to identify the main research trends and propose future research questions. Firstly, in order to give the reader an overview of the literature, a bibliometric analysis is presented. Then, a clusterization of the papers referred to the main instances reported in literature was carried out in order to create a database that can be a useful tool in the benchmarking procedure for researchers that would approach this kind of problems

Reverse electrodialysis – Multi effect distillation heat engine fed by lithium chloride solutions

Salinity Gradient Heat Engines (SG-HEs) have been proposed as a promising technology for converting low-temperature heat into electricity. The SG-HE includes two different processes: (i) a salinity gradient process where the salinity gradient between two solutions is converted into electricity and (ii) a thermal regeneration process where low-grade heat (T<100°C) is used to re-establish the original salinity gradient of the two streams. Among the proposed working solutions, aqueous solution of lithium chloride has been identified as one of the most promising thanks to its remarkable solubility and activity. In this work, a process model to study the performance of a SG-HE constituted by a Reverse ElectroDialysis (RED) unit coupled with a Multi Effect Distillation (MED) unit fed with lithium chloride solution is presented. The influence of the concentration of the inlet solution in the RED unit and the temperature difference in the evaporators of the MED unit on the performance were evaluated by considering ideal membranes. Furthermore, the impact of membrane permselectivity and resistance on the system performance was evaluated. Results showed promising system efficiencies, making this technology attractive for conversion of low-grade heat (<100°C) into electricity, but membrane properties should be enhanced

Fluid-structure interaction and flow redistribution in membrane-bounded channels

The hydrodynamics of electrodialysis and reverse electrodialysis is commonly studied by neglecting membrane deformation caused by transmembrane pressure (TMP). However, large frictional pressure drops and differences in fluid velocity or physical properties in adjacent channels may lead to significant TMP values. In previous works, we conducted one-way coupled structural-CFD simulations at the scale of one periodic unit of a profiled membrane/channel assembly and computed its deformation and frictional characteristics as functions of TMP. In this work, a novel fluid-structure interaction model is presented, which predicts, at the channel pair scale, the changes in flow distribution associated with membrane deformations. The continuity and Darcy equations are solved in two adjacent channels by treating them as porous media and using the previous CFD results to express their hydraulic permeability as a function of the local TMP. Results are presented for square stacks of 0.6-m sides in cross and counter flow at superficial velocities of 1 to 10 cm/s. At low velocities, the corresponding low TMP does not significantly affect the flow distribution. As the velocity increases, the larger membrane deformation causes significant fluid redistribution. In the cross flow, the departure of the local superficial velocity from a mean value of 10 cm/s ranges between -27% and +39%

Evaluation of the economic and environmental performance of low-temperature heat to power conversion using a reverse electrodialysis - Multi-effect distillation system

In the examined heat engine, reverse electrodialysis (RED) is used to generate electricity from the salinity difference between two artificial solutions. The salinity gradient is restored through a multi-effect distillation system (MED) powered by low-temperature waste heat at 100 ◦C. The current work presents the first comprehensive economic and environmental analysis of this advanced concept, when varying the number of MED effects, the system sizing, the salt of the solutions, and other key parameters. The levelized cost of electricity (LCOE) has been calculated, showing that competitive solutions can be reached only when the system is at least medium to large scale. The lowest LCOE, at about 0.03 €/kWh, is achieved using potassium acetate salt and six MED effects while reheating the solutions. A similar analysis has been conducted when using the system in energy storage mode, where the two regenerated solutions are stored in reservoir tanks and the RED is operating for a few hours per day, supplying valuable peak power, resulting in a LCOE just below 0.10 €/kWh. A life-cycle assessment has been also carried out, showing that the case with the lowest environmental impact is the same as the one with the most attractive economic performance. Results indicate that the material manufacturing has the main impact; primarily the metallic parts of the MED. Overall, this study highlights the development efforts required in terms of both membrane performance and cost reduction, in order to make this technology cost effective in the future

Optimization of net power density in Reverse Electrodialysis

Reverse Electrodialysis (RED) extracts electrical energy from the salinity difference between two solutions using selective ion exchange membranes. In RED, conditions yielding a large net power density (NPD) are generally desired, due to the still large cost of the membranes. NPD depends on a large number of physical and geometric parameters. Some of these, for example the inlet concentrations of concentrate and diluate, can be regarded as “scenario” variables, imposed by external constraints (e.g., availability) or chosen by different criteria than NPD maximization. Others, namely the thicknesses HCONC, HDIL and the velocities UCONC, UDIL in the concentrate and diluate channels, can be regarded as free design parameters and can be chosen so as to maximize NPD. In the present study, a simplified model of a RED stack was coupled with an optimization algorithm in order to determine the conditions of maximum NPD in the space of the variables HCONC, HDIL,UCONC, UDIL for different sets of “scenario” variables. The study shows that an optimal choice of the free design parameters for any given scenario, as opposed to the adoption of standard fixed values for the same parameters, may provide significant improvements in NPD

Cfd investigation of spacer-filled channels for membrane distillation

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models

On the Reduction of Power Consumption in Vortexing Unbaffled Bioslurry Reactors

Bioremediation of polluted soils via bioslurry reactors is an interesting option among those available nowadays, especially when recalcitrant pollutants are present. Vortexing unbaffled stirred tanks may be a valuable choice to this purpose as they were recently found to be more efficient than baffled vessels for solid suspension processes where mixing time is not a controlling factor. When operated at sufficiently high agitation speeds, the central vortex bottom reaches the impeller and air bubbles start to be distributed throughout the system, thus avoiding any sparger and related clogging issues. In the present work, a vortexing unbaffled stirred tank with solid loadings ranging from 2.5% w/w (weight of the solid/weight of the liquid) up to the very high 160% w/w was studied. Different turbine types including Rushton turbine, up- and down-pumping pitched blade turbines, and A310 were investigated. The minimum impeller speeds for complete particles' suspension (N-js) and system aeration (N-cr) along with the relevant power (P-js, P-cr) and specific power (per mass unit) consumptions (epsilon(js), epsilon(cr)) were assessed, in order to identify the geometrical configuration and operating condition providing the lowest power consumption. Results showed that the Rushton turbine and a solid concentration B of about 30% may be the most economically convenient arrangement to achieve system aeration and complete particles' suspension at the same time inside the reactor

CFD simulation of channels for direct and reverse electrodialysis

Flows within very thin channels, typically filled with spacers, can be often encountered in many processes such as electrodialysis (ED) and reverse electrodialysis (RED). Although the ED and the RED processes have been studied for a long time, the optimization of the fluid dynamics within the channels is still an open problem. In the present work, realized within the EU-FP7 funded REAPower project, computational fluid dynamics simulations were carried out in order to predict the fluid flow field inside a single ED/RED channel. Some different configurations were tested which includes: an empty channel, a channel provided with a spacer, and a channel filled with a purposely manufactured fiber porous medium. Two types of spacers were investigated: (1) a commercial type made of woven perpendicular filaments and (2) an overlapped perpendicular filament spacer. A sensitivity analysis concerning computational grid size and topology was performed. For the cases investigated, adopting the hybrid grids mainly composed of hexahedral volumes was found to be more reliable and less computational demanding than tetrahedral grids. As concerns the dependence of the pressure drops on the flow rate, the empty channel was found to guarantee the lowest pressure drops at a given fluid flow rate, as expected. Conversely, the woven spacer filled channel was found to provide larger pumping costs. The pressure drops along the channel filled with a porous medium even at low flow rate were very high thus suggesting that this specific configuration may be unsuitable and that further investigations should be devoted to this topic

An Ion Exchange Membrane Crystallisation reactor for Magnesium recovery from brines

CrIEM technology is a novel ion exchange application that allows reactive crystallization for separation of valuable species (e.g. Mg from brines) with a large flexibility in the choice of reactants