Stoffaustausch und Stoffanreicherung bei zweiphasigen Gegenstromverfahren

For the continuous steel production numerous procedures have been suggested. However, there are still absent investigations of single process steps to evaluate the total efficiencies and to be able to optimise them. In addition one needs models for the concentration courses of the accompanying materials along the reactors in the stationary state. In the present work the relations were examined in a cascade with counter current reactors and theoretical models were developed. Mathematical and physical models were tested. For the examination the convective as well as the chemical driving forces of the material exchange between both phases had to simulate these. In the physical model the counter flowing phases consisted of water to one of which an ionisable Tracer was added. The phase boundary was simulated by an ionic dialysis membrane. The chemical driving force for the material exchange was substituted by an electric field applied across the membrane, which caused the ion transport. The admissibility of this approach was checked and a quantitative relation was derived for the conversion between model and original. Phenomenologically the exchanged amount of material was supposed proportional to the difference of the concentrations c of the tracers in both phases. For the assumption that both sides have the same standard conditions and that the material transport is near the equilibrium, the driving forces can also be expressed by the difference of the chemical potentials µ or by the difference of the activities a. For electro-chemical processes the difference of the electro-chemical potentials epsilon was used, which have in addition to µ an additive term with the load number z, the Faraday – constant F and the electric potential E. By comparison of both cases the equation of equivalence resulted. Only now after derivation of this relation in the present work, mass transfer with chemical reactions can be modelled by phase transitions with electric potential differences. For the counter current cascade with n reactors a new mathematical model was derived which takes into consideration the local exchange-mass-flow-density in the reactors and avoids the well known assumption that the equilibrium is restored in every cell according to Nernst’s distribution law: This assumption is satisfied only in special cases and is rarely met in the operating practice. The cascade equation arose from the mass balances of the single cells and phenomenological assumptions for the driving forces and the transport resistance for n reactors. For n approx. infinitely the cascade equation became identical with the known equation for the counterstream channel what confirms the correctness of the assumptions and the compatibility of the derivation. The physical cascade model was made of four to twelve cylinder-shaped Perspex cells which contained a cellulose membrane. Acidic fuchsine was selected as a tracer, because it is dissolvable in the ionic state in water, which was used on both sides of the membrane as a model of the fluids. The concentration could be determined quantitatively in a spectroscopical way. The results allowed a simulation of the concentration courses of accompanying materials. On account of the chemical reaction between the materials X and A, as well as the concentration slope of the material A or the electric potential slope along the reactor cascade the concentration curves, which are also to be expected after the mathematical model, appear experimentally. The courses of the curves allow the conclusion that the enrichment occurs on grounds of an internal circulation of the tracer. The same effect was predicted by K. H. Schubert and E. Steinmetz on the basis of theoretical considerations for a counterstream channel; it is confirmed, this work for the first time experimentally for a counter current cascade. The measured values results for the concentration courses in the physical model under typical conditions were compared to the enrichment courses of manganese, vanadium or chrome in iron which resulted from the evaluation of the mathematical models derived in this work. The evaluation of the concentration course as a function initial concentrations and final concentrations, relative slag amounts, conversion figures, oxygen activities, equilibrium constants and exchange areas was made with typical investment values and literature values. Although both curves have similar forms for experiment and theoretical calculation and they reach their maxima at the same place, according to the modelling calculations higher values are obtained for the enrichment in the single steps of the cascade and for the maxima than by the corresponding experiments. This is the consequence of the fact that in the experiments not all parameters could be adapted in full extent. Therefore, mathematical as well as physical models have their importance for the simulation of processes in practice.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2008

Author dc:creator

• Wilhelmi, Arndt M. T.

Contributors dc:contributor

• Steinmetz, Eberhard

• info:eu-repo/classification/ddc/620
• Anreicherung
• Metallanreicherung
• Stoffübergang
• Stoffübertragung
• Ingenieurwissenschaften
• kontinuierliche Stahlerzeugung
• Gegenstrom
• Sauerstoffpotential
• continuous steelmaking
• counter current
• oxygen potential

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

ger

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-112893%22

OAI record oai:identifier

https://publications.rwth-aachen.de/oai2d?verb=GetRecord&metadataPrefix=oai_dc&identifier=oai%3Apublications.rwth-aachen.de%3A50344