M.C. Williams

Electrode Performance in Reversible Solid Oxide Fuel Cells

Electrolysis has long been used to dissociate water into its constituents of oxygen and hydrogen. Various electrolyzers have been developed and are commercially available today, including those based on proton exchange membranes, molten carbonate, phosphoric acid, alkaline, and solid oxide technology. 1-5 Some of these are reversible systems capable of operating both as a fuel cell and as an electrolyzer, although fuel cell and electrolyzer functions are carried out in separate subsystems. A reversible fuel cell can take advantage of excess electrical grid capacity during off-peak hours to produce hydrogen fuel, to be utilized later during periods of high electrical demand. The power unit fuel cell is sized for the peaking load in a practical reversible fuel cell, whereas the electrolyzer is rated at a power that can produce sufficient hydrogen to recharge the hydrogen storage capacity over the remaining hours of the day. If energy conversion, electrical to chemical and chemical to electrical, can occur in the same device with reasonable efficiencies, there could be significant overall cost benefits. For solid oxide electrolysis cells SOEC to be of commercial interest, the cost of the hydrogen produced must be competitive with that of other means of production. The cost of electricity is a significant factor in steam electrolysis, comprising 75% to 95% of that of electrolysis-derived hydrogen according to performance and cost

Characterization of the wettability of solid particles by film flotation 1. Experimental investigation

Film flotation is a technique that can be used for assessing the wetting characteristics of particulates. In film flotation, particles placed onto the surface of a liquid are imbibed into the liquid only when their critical wetting surface tension is equal to or higher than the surface tension of the wetting liquid. The efficacy of the method is demonstrated by film floating homogeneous and hydrophobic particles of sulfur, silver iodide, methylated glass beads and quartz, paraffin wax-coated coal and surfactant-coated pyrite. Other factors such as particle size, particle density, film flotation time and the nature of the wetting liquid have a negligible effect on the results of film flotation in the range of conditions tested. Film flotation is sensitive mainly to the surface hydrophobicity and the heterogeneity of particles.

Dynamic model of flotation cell banks — circuit analysis

Abstract While all mineral industry flotation circuits are stable, they are sensitive to low-frequency perturbations in the feedrate. In both countercurrent and cocurrent circuits, the lead cell is more sensitive to feed variations. The frequency response predicts the amount of extra cell capacity needed to handle the maximum feed due to a sinusoidal forcing function. Feedback loops are more significant than sump delays. The countercurrent 4 × 4 circuit floating quartz, with a 200-sec retention time, requires 75 minutes for the concentration of quartz in the input to the first cell to reach 95% of its steady-state value. Countercurrent circuits were found superior to cocurrent circuits in all respects.

Circuit analysis—General product equations for multifeed, multistage circuits containing variable selectivity functions

Abstract The general, steady-state product equations for single and multiple feed input countercurrent, concurrent, scavenger series, and cleaner series mineral processing circuits with variable selectivity functions have been developed. These operations are essential to understanding and predicting the performance of, not only common mineral processing circuits used in the Occident, but also the complex, multiple feed, ramification circuits recently introduced from China. Finally, this proposed approach has greater application to the chemical industry than to the mineral industry.

On the definition and separation of fundamental process functions

Abstract The three fundamental process functions performed by the unit operations in processing separation circuits — roughing, cleaning and scavenging — can be accurately defined. In our age of engineering specialization, the trend is to separate process functions from one another since different functions require specialized operating and design conditions. Separating different functions allows the engineer to control more exactly the operating conditions in each unit. Seven combinations of the three functions are possible in any unit operation. A circuit analysis function separation methodology has been developed for readily redesigning circuits to separate the three functions when they are combined in a single unit operation. Function separation was found to lead to lower unit operation feed loadings which in most instances can be expected to lead to improve circuit performance. This methodology may become a significant new tool in mineral circuit design.

Feasible designs for separation networks: a selection technique

Abstract Presented is a non-mathematical, non-computer technique for obtaining feasible designs for simple separation networks. Once the transfer function of the unit operations of a network are known and the desired separation factor and beneficiation ratio specified, a set of graphs approximating network performance is used to design the network. The technique results in networks which closely meet target specification. These graphs specify: (1) the total number of network stages; (2) the stage at which the feed should enter the network; and (3) the specific arrangement of the stages, thereby specifying the entire network. An unexpected finding of this effort was the acknowledgement that the number of stages the recycle steam is fed back may be greater than one stage. This network designing method appears to be an excellent tool for teaching network design.

A graph-theoretic approach to process plant design

Abstract Somewhere between the almost mindless computer simulation of process plants and the tedious circuit analysis of process plants lies the graph-theoretic approach to process plant design. This approach permits a visual understanding of process plants while providing for sufficient mathematical rigor to obtain realistic plant designs. The graph-theoretic approach, unlike some computer simulation programs, involves the designer in the intelligent decision-making activities of process plant design. This approach offers the advantage of being able to be used with sophisticated optimization strategies, of being able to be computerized when designing large process plants, and of also offering a very rapid non-algebraic solution and sensitivity analysis when working on smaller process problems.

The recedence of thick aqueous films in the dewetting of solid surfaces

Abstract The recedence of relatively thick aqueous films from flat solid surfaces was investigated experimentally for two different substrates: paraffin wax and polished Pyrex glass. The wettability of Pyrex was controlled by the addition of dodecylammonium acetate. Using the conservation of energy principle, a model was developed to analyze steady state recedence velocity in terms of the surface free energy change, kinetic energy of the receding liquid, friction loss, kinetic energy loss and gravitational potential energy of the system. A numerical solution of the model was utilized and compared with the experimental data. The model indicates an insensitivity to viscous disspation and corroborates not only the thermodynamic theory for film recedence, but also the general conceptions of other workers on film movement.

The effect of non-equilibrium adsorption at the solid—vapor interface on the recedence of aqueous films and dewetting of solid surfaces

Abstract Phenomena involving the recedence of relatively thick films were investigated in the paraffin—water and Pyrex—dodecylammoniumacetate—water systems. Non-equilibrium adsorption was found to enhance the dewetting or recedence velocity of aqueous films from the surface of Pyrex glass. An explanation, based on the enhanced zipper-like adsorption at the solid—vapor interface as the liquid—vapor interface is destroyed, is presented to explain the existence of a maximum recedence velocity. These effects are absent in the paraffin—water system. This work reflects recedence behavior in practical non-quiescent systems.

A model for the spreading of thick liquid films on solid surfaces

Abstract A model has been developed for studying the spreading rates of thick liquid films on solid surfaces. This conservation of energy model is in agreement with the available experimental data for thicker films. The model permits the independent assessment of the potential, kinetic, viscous and surface energy contributions, and possesses great flexibility in modeling the spreading phenomenon. The testing of the model is discussed.