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Mathematical modeling in chemical engineering: a tool to analyse complex systems
Full text linkMathematical modeling is an attempt to describe a slice of reality in mathematical terms.
In Chemical Engineering, mathematical modeling is used for simulation, control and optimization of a process and it is also a tool to design the industrial devices.
Mathematical modeling is a technique commonly in place also in both theoretical and experimental studies of chemical processes.
In the present chapter mathematical modelling applications to complex systems as a consequence of structure heterogeneity and involved various physical-chemical phenomena are presented. Particular attention will be focused on improving the quantitative understanding of the basic phenomena of a process that can come from the use of mathematical models.
Specific task is also demonstrating how, through the use of information coming from experimental investigations and simulations, it is possible checking on the validity of the assumptions made and fine tuning the predictive mathematical model capability.
The possibility of analyzing and quantifying the role played by each single step of the process is examined in order to define the relevant mathematical expressions. The latter allows getting useful indications about the impact of different operating conditions on the role of each single step and at the very end it gives indication about the efficiency of the process itself.
Next step focuses on the estimation of the significant parameters of the process. In complex systems the determination “a priori” of some parameters is not always feasible and they are therefore determined as a comparison of experimental and simulation data.
The final result is therefore the availability of a tool, the verified and validated (V&V) mathematical model, that can be used for simulation, process analysis, process control, optimization, design.
Specific reference will be made to the use of the proposed methodology on a system whose behaviour, on varying the agitation level, was quantified and validated against the results of an experimental investigation in a pilot plant.
A second application will allow to analyse the effect of transport phenomena in multi-phase heterogeneous systems in order to detect the conditions at which production plant efficiency is improved
Rimozione di tannini mediante elettrocoagulazione: analisi e simulazione del processo in continuo.
No full textBenché l’idoneità dei processi elettrochimici per il trattamento di reflui sia largamente documentata in letteratura, la loro applicabilità è fortemente condizionata dai significativi consumi energetici che li caratterizzano. Incoraggianti prospettive in questo settore sono rappresentate dai processi combinati di ossidazione/coagulazione/precipitazione che permettono da un lato di ottenere elevate rese di abbattimento e dall’altro di contenere i costi associati al trattamento.
Studi precedenti, condotti dagli stessi Autori su apparecchiature da laboratorio, avevano evidenziato l’efficacia del processo di elettrocoagulazione per la rimozione di tannini.
Nel presente lavoro, ai fini della realizzazione di tale processo a livello industriale, ne viene studiato il comportamento su impianto pilota. Sono state dapprima condotte prove in discontinuo con apparecchiature da laboratorio, per individuare le condizioni operative ottimali. Quindi, si è passati allo studio del processo in continuo su impianto pilota, appositamente progettato e realizzato, che opera in ciclo chiuso fino al raggiungimento delle condizioni ottimali di lavoro e successivamente come sistema aperto, in condizioni di stazionarietà.
Modelli matematici formulati per le diverse apparecchiature hanno permesso di simulare l’impianto nelle diverse modalità di funzionamento. Il confronto fra i dati sperimentali ottenuti in condizioni di riciclo totale e quelli teorici hanno confermato l’ottima capacità di previsione dei modelli sviluppati
Removal of vegetal tannins from wastewater by electroprecipitation combined with electrogenerated Fenton oxidation
No full textSynthetic solutions containing up to 2000 ppm of gallotannic acid and real wastewater from vegetal tanning processes with values of chemical oxygen demand (COD) exceeding 100 000 ppm were decontaminated by electrolysis using a sacrifical iron anode coupled to either a titanium-platinised or an O2-diffusion cathode. Experiments were performed in the presence of oxidants and oxidation promoters such as air, oxygen and hydrogenperoxide, the latter being directly added to the solution or electrogenerated by the O2-diffusion cathode. COD and UV-visible absorbance evolution showed that tannins are removed from electrolysed solutions down to relatively low values, permitting more than 94% elimination. Partial oxididation of the mother compound generates short-chain by-products (mostly carboxylic acids) responsible for the remaining low COD values. Contaminants (tannins and non-tannins) containedin industrial wastewater were removed by combining electroprecipitation with a Fenton-assisted process; a final oxidation step, carried out on a boron-doped diamond electrode, was performed in order to decerase the COD to very low final values
Electro-Fenton_based treatments of real effluents from tanning processes and landfills
No full textLeather industry uses high quantities of water, from 50 to 200 m3 per day, the whole amount of which is usually sent to depuration. At the same time it is noteworthy that tanning processes use large quantities of tannin-based organic compounds, which with all the protein, lipids, greases and hairs, constitute an overcharge of COD (Chemical Oxygen Demand) and BOD (Biological Oxygen Demand) in the tanning effluents. These effluents contain many biorefractory compounds, so low biodegradability index, with COD charge varying from 5000 to 120,000 ppm. This, undoubtedly, causes troubles in the depuration processes. Effective treatment with recycling option of such large quantities of technological waters would: (i) ensure a high diminution in the costs of the entire tanning process (as for that of water consumption and that of salt expenses), (ii) allow to easily depurate the issuing waters with lower costs and (iii) eliminate progressively the negative environmental impact of the final rejects in rivers by reducing considerably (or eliminating) the remaining COD and the quantities of sludge at the end of the depuration process. Depuration sludges are usually stocked in specially conceived landfills. Percolating waters from such landfills contain also high quantities of contaminants, COD from 5000-15000 ppm, and are currently sent to the top of the depuration. Periodical recirculation of the same is done, which, in fact, does not always suffer subsequent treatment. The main process which is presented in this work, Assisted Electroprecipitation, combines physico-chemical aspects of the treatment of water effluents with (electro-)Fenton-based electrochemical oxidation and partial coagulation/precipitation of the ending carbonic compounds. This treatment allows complete COD removal from the effluents. The issued technology is suitable to industrial application specially for the recycling tanning effluents and landfills percolates
Realizzazione e caratterizzazione fluido-dinamica di una nuova cella per elettrodi a diffusione di ossigeno
No full textPrecedenti lavori hanno mostrato come la produzione di acqua ossigenata su elettrodi a diffusione di ossigeno presenta, dopo alcune ore di attività, un decadimento della resa del processo. Per ripristinare le condizioni iniziali di efficienza e per confrontare le prestazioni di elettrodi diversi è quindi necessario prevedere frequenti operazioni di montaggio/smontaggio e manutenzione. Viene presentata la progettazione e realizzazione di una cella elettrochimica “tipo filtropressa” da utilizzare in un impianto pilota che opera in continuo in condizioni di riciclo totale o parziale. Le caratteristiche di modularità degli elementi che la costituiscono e la semplicità degli alloggiamenti degli elettrodi permettono l’utilizzo di elettrodi piani, a rete e porosi di materiali diversi e di operare con correnti fluide liquide e gassose. I comparti anodico e catodico, indivisi o separarti da membrane semipermeabili, possono essere smontati singolarmente senza rendere necessario il completo disassemblaggio della cella. L’ispezione della superficie dell’elettrodo può essere agevolmente condotta attraverso un oblò posto in corrispondenza del comparto catodico. La cella è collocata su di un supporto basculante in modo che il controllo puntuale dei battenti idrostatici sulle due superficie dell’elettrodo a diffusione, a contatto con le fasi gassosa e liquida, può essere ottenuto variando a piacere l’angolo di inclinazione dalla posizione verticale a quella completamente orizzontale. Trattandosi di un prototipo completamente realizzato dal gruppo di ricerca si è ritenuto opportuno procedere alla caratterizzazione fluidodinamica attraverso serie di prove sperimentali condotte utilizzando la tecnica ad impulsi al variare della portata della fase liquida. Gli andamenti della concentrazione di tracciante in uscita dalla cella vengono confrontati con i risultati della simulazione condotta attraverso un modello matematico già testato in un precedente lavoro su elettrodi simili. Il coefficiente di dispersione così ottenuto per ciascuna prova, viene posto in forma adimensionale ottenendo i parametri della relazione funzionale che ne descrive il comportamento nel campo di indagine considerato. I primi test nella produzione di acqua ossigenata hanno confermato i dati di efficienza ottenuti con gli stessi elettrodi su impianti sperimentali diversi
Electrochemical removal of tannins from aqueous solutions
No full textThe applicability of electrochemical methods to remove tannins from wastewater was investigated. Gallotannic acid was used as the reference substance. Electrochemical experiments were performed using platinum electrodes. Macroscale potentiostatic or galvanostatic electrolyses were carried out with sodium sulfate or sodium chloride as supporting electrolytes, to analyze direct and indirect oxidation processes. Operating variables such as pH and chloride concentration were considered to determine their influence on the efficiency and energy consumption of the process. The simulation of a pilot plant was carried out with a mathematical model, the parameters of which were determined by fitting of experimental profiles. The results of a preliminary investigation on the oxidation−coagulation process using sacrificial electrodes are also reported
A small-scale pilot plant using an oxygen-reducing gas-diffusion electrode for hydrogen peroxide electrosynthesis
No full textA pilot plant for electro-generation of hydrogen peroxide using an oxygen-reducing gas-diffusion electrode (GDE) was built and a mathematical model was developed to simulate the overall system.
The electrode was made of a web coated with layers of VULCAN XC-72 Carbon catalyst on both sides of the assembly and a coating of SAB (Shawinigan Acetylene Black) on the gas-side. Electrolyses were carried out in galvanostatic mode, using catholyte consisting of 0.07 M NaCl solution, pH 2.7 and adopting a batch recycle mode of operation.
Mathematical analysis was based on a three-phase model, and simulated the evolution of hydrogen peroxide concentration profiles in liquid-filled pores. Various models were developed to represent the behaviour of the cathode compartment in the electrochemical cell. The result is a tool which shows how hydrodynamics affect the performance of the overall system. Hydrogen peroxide production achieved during electrolysis can be calculated by means of this tool. Comparison of numerical simulations against experimental data validated the model developed in this study
Electrochemical removal of gallic acid from aqueous solutions
No full textRemoval of gallic acid from aqueous solutions of different concentrations has been performed by electroprecipitation using a sacrificial iron anode, by indirect electrochemical oxidation carried out via electro- and photoelectro-Fenton processes using an oxygen-diffusion cathode, and by a combination of the first two methods (peroxicoagulation process). In all cases, chromatographic analyses have shown a very quick disappearance of gallic acid and its aromatic by-products within 30–90 min of electrolysis, depending on the method. A pseudo first-order kinetic decay of gallic acid was always observed under galvanostatic conditions. A decay of TOC and COD close to 90 and 95% is observed with electroprecipitation and peroxicoagulation processes, respectively, after electrolysis time lower than 2 h. The specific charge utilised in these two processes was about half of that theoretically required for the complete direct oxidation process (mineralisation). During electrolyses some carboxylic acids have been detected as main intermediates, which completely disappear at the end of the process, except oxalic acid in the case of electro-Fenton method
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