Marc LeBreux

Control of the ledge thickness in high-temperature metallurgical reactors using a virtual sensor

A non-intrusive inverse heat transfer procedure for predicting the time-varying thickness of the phase change ledge on the inside surface of the walls of a high-temperature metallurgical reactor is presented. A Kalman filter, based on a state-space representation of the reactor, is coupled with a recursive least-square estimator in order to estimate online the position of the phase front. The data are collected by a heat flux sensor located inside or outside of the reactor wall. The inverse method, used here as a virtual sensor, is coupled to a classic proportional–integral controller in order to control the ledge thickness by regulating the air cooling applied on the outside surface of the reactor wall. The virtual sensor and the control strategy are thoroughly tested for typical phase change conditions that prevail inside industrial facilities. Results show that a virtual sensor that relies on a heat flux sensor embedded inside the reactor wall provides more accurate and stable information, but at a price of a more complicated installation. In that case, it is shown that the discrepancy between the exact and the estimated ledge thicknesses remains smaller than 3% at all times, and that the control strategy ensures a null steady-state tracking error, a maximum tracking error less than 10%, no overshoot and no significant time lag.

Chemical Characterization and Thermodynamic Investigation of Anode Crust Used in Aluminum Electrolysis Cells

The anode cover material (ACM) is necessary to control the top heat losses of aluminum electrolysis cells, to decrease the fluoride fumes emissions and to prevent air burn of carbon anodes. In order to understand the behavior of the ACM and anode crust at operating conditions, samples of these materials are taken from an industrial electrolysis cell in the side channel, center channel and between anodes. Their chemical compositions are analyzed by X-ray diffraction while the temperatures inside the crust are validated by temperature measurements. The reactions occurring in the ACM and anode crust are then determined by thermodynamic equilibrium calculations. Concentration gradients of chiolite (Na5Al3F14), cryolite (Na3AlF6), Na2Ca3Al2F14, and Al2O3 are observed in the anode crust, implying a variation of the cryolite ratio and melting temperature of the crust. The thermodynamic analysis describes the state and behavior of the anode crust in aluminum electrolysis cells.

Is the performance of a virtual sensor employed for the prediction of the ledge thickness inside a metallurgical reactor affected by the thermal contact resistance

A virtual sensor is developed for predicting the time-varying thickness of the ledge on the inside surface of a wall of a high-temperature metallurgical reactor. The virtual sensor tracks the position of the solid-liquid phase front using thermal measurements taken from a heat flux sensor embedded in the reactor wall. The virtual sensor comprises a state observer coupled to a reduced model of the reactor. It also accounts for the thermal contact resistance of the wall structure. Results indicate that the virtual sensor is increasingly accurate as the magnitude of the thermal contact resistance augments. Moreover, the predictions of the virtual sensor remain accurate even when the contact resistance is poorly known.

Prediction of the Time-Varying Ledge Profile inside a High-Temperature Metallurgical Reactor with an Unscented Kalman Filter-Based Virtual Sensor

A non-intrusive inverse heat transfer procedure for predicting the two-dimensional time-varying profile of the protective phase-change ledge on the inside surface of the walls of a high-temperature metallurgical reactor is presented. The inverse method, used here as a virtual sensor, enables the on-line estimation of the position of the solid-liquid phase front using thermal sensors embedded in the reactor wall. The virtual sensor comprises a state observer coupled to a reduced model of the reactor. Results show that the virtual sensor that yields the best prediction comprises an unscented Kalman filter, a nonlinear state-space model of thereactor, and two heat flux sensors located at the wall/ledge interface.

Impact of Cast Iron Degradation and Cathode Block Erosion on the Current Path in the Cathodic Assembly of Aluminum Production Cells

Carbon-cast iron electrical contact degradation is still considered as one of the main cause for the CVD increase over the lifetime of the electrolysis cell. A thermoelectric finite element model was developed to evaluate the effect of the carbon-cast iron electrical contact degradation and the cathode erosion on the CVD and the current distribution inside the cathodic assembly. Chemical degradation data from laboratory and industrial samples were used to calibrate the cast iron and the contact resistivities. Results demonstrate that the carbon block erosion and the contact degradation at the cast iron interface have a direct impact on the current distribution at the carbon block surface. Both factors increase the CVD when taken separately but the contact degradation outperforms the carbon block wear when taken together.

Modeling and measurements of anode cover behavior inside aluminum electrolysis cells

ABSTRACT A model for predicting anode cover behavior inside aluminum electrolysis cells is presented. The model predicts the transformation of anode cover material into a solid crust, the melting/solidification of the bath and crust, and the heat fluxes escaping the anode cover. The model is validated with experimental data taken on industrial electrolysis cells. The temperature and positions of the top crust, the heat flux escaping the anode cover, and the height of the cavity are presented, along with the model predictions. The effect of bath temperature on the crust formation is further investigated. Results show that the bath temperature can greatly enhance the rate of crust formation.

The Impact of the Cavity on the Top Heat Losses in Aluminum Electrolysis Cells

The primary aluminum producers are continuously raising the line amperage to increase the productivity. Hence, the top heat losses of aluminum electrolysis cells are amplified which leads to higher thermal constraints on the anode cover. A finite element model is developed in order to predict the heat fluxes and temperatures in the top parts of the cell. The model is validated against measurements from the top surfaces of real industrial cells. It considers the cavity above the liquid bath, which is formed during the melting and falling of anode crust during the life of the anode. The heat flux escaping the anode cover increases with the height of the cavity according to the results. Considering the cavity inside the model is mandatory in order to obtain a good correspondence between the measurements and the model predictions.

On the Prediction of the Crust Evolution inside Aluminum Electrolysis Cells

A model for predicting the evolution of the crust inside aluminum electrolysis cells is presented. The model takes into account the effects of heat transfer, solid/liquid phase-change, and chemical transformation of anode covering material (ACM). The model predicts: 1) the temperature field inside the cell, 2) the evolution of the ACM conversion into crust, 3) the melting/solidification processes inside the cell and, 4) the time-varying heat losses at the top of the cell. The model is validated with experimental data taken on an industrial electrolysis cell. Results show that the model captures the essential behavioral features of the industrial cell. However, further experiments must be performed in order to provide reliable data on the crust formation. These experimental data are needed for a thorough validation of the mathematical model.