Fabrice Patisson

A non-isothermal, non-equimolar transient kinetic model for gas-solid reactions

A numerical model is presented, designed to simulate the kinetic and thermal behaviour of a porous pellet in which any gas-solid reaction is taking place. Its novelty consists in the fact that it can deal with reactions whether they are exothermic or endothermic, whether they are equimolar or not, whether they are reversible or irreversible, and further reactions in the transient regime and even the possible presence of inert gases and solids can be treated. The numerical scheme is based on the finite volume method in an implicit formulation, with a specific treatment of the thermal source term for strongly exothermic reactions. The model was validated by comparison with analytical and numerical solutions from the literature and was used to simulate the exothermic reaction involved in the oxidation of zinc sulphide.

Study of moisture transfer during the strand sintering process

Moisture transfer during the strand sintering operation was studied both experimentally and using a mathematical model. The drying of iron ore pellets was found to occur in two distinct periods: one at a constant drying rate and the other at a decreasing drying rate, whereas the drying of zinc ore pellets always occurs at a decreasing drying rate. Characteristic drying curves were determined for both materials. The moisture transfer mechanisms during the sintering process were demonstrated in detail, including the recondensation of water in the cold layers of the bed and the formation of an inert, overmoistened zone. The mathematical model presented simulates all of these phenomena and is used to calculate the variables related to moisture transfer. The model is adaptable to other processes where a hot gas passes through a moist packed bed.

Modelling a new, low CO2 emissions, hydrogen steelmaking process

In an effort to develop breakthrough technologies that enable drastic reduction in CO2 emissions from steel industry (ULCOS project), the reduction of iron ore by pure hydrogen in a direct reduction shaft furnace was investigated. After experimental and modelling studies, a 2D, axisymmetrical steady-state model called REDUCTOR was developed to simulate a counter-current moving bed reactor in which hematite pellets are reduced by pure hydrogen. This model is based on the numerical solution of the local mass, energy and momentum balances of the gas and solid species by the finite volume method. A single pellet sub-model was included in the global furnace model to simulate the successive reactions (hematite->magnetite ->wustite->iron) involved in the process, using the concept of additive reaction times. The different steps of mass transfer and possible iron sintering at the grain scale were accounted for. The kinetic parameters were derived from reduction experiments carried out in a thermobalance furnace, at different conditions, using small hematite cubes shaped from industrial pellets. Solid characterizations were also performed to further understand the microstrutural evolution. First results have shown that the use of hydrogen accelerates the reduction in comparison to CO reaction, making it possible to design a hydrogen-operated shaft reactor quite smaller than current MIDREX and HYL. Globally, the hydrogen steelmaking route based on this new process is technically and environmentally attractive. CO2 emissions would be reduced by more than 80%. Its future is linked to the emergence of the hydrogen economy.

Physicochemical and thermal modelling of the reaction between a porous pellet and a gas

A mathematical model has been developed to simulate the kinetic and thermal behaviour of a porous solid pellet undergoing chemical reaction with a gas. The model describes the chemical reaction itself, the transfer of the gaseous species between the external gas and the pellet surface, the transport of these species to the inside of the pellet through the pores and through the layer of solid reaction products, the generation (or consumption) of heat due to the reaction and the associated heat transfer processes, together with the structural changes produced in the solid by the reaction. The model has been validated by comparison with experimental results and data from the literature. Simulation results are presented for two reactions: the exothermal oxidation of zinc sulphide and the hydrofluorination of uranium dioxide.

A thermogravimetric study of the kinetics of hydrofluorination of uranium dioxide

The kinetics of the transformation of uranium dioxide to uranium tetrafluoride by hydrofluorination have been studied with the aid of thermogravimetric experiments performed at temperatures from 220 to 450°C in a nitrogen-diluted hydrogen fluoride atmosphere. Using the Grainy Pellet Model coupled with an optimization program, the rate controlling mechanisms and the characteristic kinetic parameters were determined. The reaction was found to be first order with respect to hydrogen fluoride, with an activation energy of 25 kJ/mol. It follows a chemical or intermediate regime, depending on the pellet diameter. Diffusion in the bulk of the small grains comprising the pellets is not rate-controlling. It is shown that the ideal pellet size in the industrial process is of the order of 1 mm.

Modelling of a moving bed furnace for the production of uranium tetrafluoride. Part 2: Application of the model

Abstract The French nuclear fuel making route uses, prior to enrichment, uranium tetrafluoride UF 4 obtained from the reduction, followed by hydrofluorination of uranium trioxide UO 3 . These two steps are carried out in a specific reactor known as a moving bed furnace. We developed a steady-state numerical model of the moving bed furnace, described in Part 1. In the Part 2, calculation results for a reference set of operating parameters of the furnace are presented in term of temperature, reaction rates, solid and gas compositions. Results analysis enlightens the detail of the furnace behaviour in its different zones. Unknown features have been revealed, such as thermodynamic limitation of the hydrofluorination reaction in the hot core of the moving bed. A sensibility study of various operating parameters shows how some can influence the UF 4 quality and underlines the strong coupling between the different zones of the furnace. Finally, the model is applied to define an optimal temperature progression in the furnace and suggests geometrical modifications. Besides, the validity of using the law of additive reaction times for calculating the reaction rates in such a reactor model has been checked for the first time against a numerical grain model.

Mathematical modelling of uranium dioxide conversion in a moving bed furnace

Abstract We present a mathematical model for simulating the steady-state behaviour of a moving bed furnace used in the French nuclear fuel making route. This furnace is a counterflow gas–solid reactor which converts uranium trioxide UO3 into uranium dioxide UO2, then into uranium tetrafluoride UF4. The model describes most of the physico-chemical phenomena occurring into the reactor by coupling a mechanistic approach (for the vertical part of the furnace) and a systemic one (for the horizontal part). The equations are solved by a numerical code specifically developed. The first results obtained with an industrial reference set of operating parameters are discussed. Then, the influence of some of those operating parameters is studied with the help of the model.

Experimental and theoretical analysis of zinc updraft sintering

The present work aims to improve the understanding of the fundamental physicochemical phenomena involved in the zinc updraft sinter-roasting process and to develop a mathematical simulation model. Laboratory and pilot pan experiments have been performed to investigate some of the phenomena important to the process, such as gas flow through the sintering bed, moisture removal, roasting reaction kinetics, melting, and solidification. The mathematical model of the sinter-roasting operation has been developed based on the results of these specific studies and the coupling of the physical phenomena. The basic principle of the model is to solve dif-ferential balances of heat, mass, and momentum transport (by an implicit finite difference method) using parameters specific to each zone of the sintering bed. From input data concerning the operating conditions and the raw materials characteristics, the model calculates the temperatures and compositions of the solids and gases throughout the bed. Simulated and experimental results obtained for an actual pilot pan are presented and compared. Manuscript submitted May 10, 1991.

Optimization of the Iron Ore Direct Reduction Process through Multiscale Process Modeling

Iron ore direct reduction is an attractive alternative steelmaking process in the context of greenhouse gas mitigation. To simulate the process and explore possible optimization, we developed a systemic, multiscale process model. The reduction of the iron ore pellets is described using a specific grain model, reflecting the transformations from hematite to iron. The shaft furnace is modeled as a set of interconnected one-dimensional zones into which the principal chemical reactions (3-step reduction, methane reforming, Boudouard and water gas shift) are accounted for with their kinetics. The previous models are finally integrated in a global, plant-scale, model using the Aspen Plus software. The reformer, scrubber, and heat exchanger are included. Results at the shaft furnace scale enlighten the role of the different zones according to the physico-chemical phenomena occurring. At the plant scale, we demonstrate the capabilities of the model to investigate new operating conditions leading to lower CO2 emissions.

Hydrazinium lanthanide oxalates: synthesis, structure and thermal reactivity of N2H5[Ln2(C2O4)4(N2H5)]·4H2O, Ln = Ce, Nd.

New hydrazinium lanthanide oxalates N2H5[Ln2(C2O4)4(N2H5)]·4H2O, Ln = Ce (Ce-HyOx) and Nd (Nd-HyOx), were synthesized by hydrothermal reaction at 150 °C between lanthanide nitrate, oxalic acid and hydrazine solutions. The structure of the Nd compound was determined from single-crystal X-ray diffraction data, space group P2₁/c with a = 16.315(4), b = 12.127(3), c = 11.430(2) Å, β = 116.638(4)°, V = 2021.4(7) Å(3), Z = 4, and R1 = 0.0313 for 4231 independent reflections. Two distinct neodymium polyhedra are formed, NdO9 and NdO8N, an oxygen of one monodentate oxalate in the former being replaced by a nitrogen atom of a coordinated hydrazinium ion in the latter. The infrared absorption band at 1005 cm(-1) confirms the coordination of N2H5(+) to the metal. These polyhedra are connected through μ2 and μ3 oxalate ions to form an anionic three-dimensional neodymium-oxalate arrangement. A non-coordinated charge-compensating hydrazinium ion occupies, with water molecules, the resulting tunnels. The N-N stretching frequencies of the infrared spectra demonstrate the existence of the two types of hydrazine ions. Thermal reactivity of these hydrazinium oxalates and of the mixed isotypic Ce/Nd (CeNd-HyOx) oxalate were studied by using thermogravimetric and differential thermal analyses coupled with gas analyzers, and high temperature X-ray diffraction. Under air, fine particles of CeO2 and Ce(0.5)Nd(0.5)O(1.75) are formed at low temperature from Ce-HyOx and CeNd-HyOx, respectively, thanks to a decomposition/oxidation process. Under argon flow, dioxymonocyanamides Ln2O2CN2 are formed.