Oxygen Separation and Collection

In parallel to development activities in the materials themselves, significant progress has been made in the architecture of devices. This includes a transition from dense, self-supported membranes to the use of thin films supported on porous substrates, as well as developments in tubular geometries and modified planar geometries. This chapter will provide an outiine of these development activities and provide possible insight into potential future developments. 

We will first consider the development activity which has occurred in the application of oxygen separation and collection. A wide range of materials has been examined in this area including perovskites, brownmillerites, fluorites and composites of various structures. There has also been progress in membrane architec- 

Background for Selection of Materials for Oxygen Separation and Collection 

It is important to understand the forces driving materials development activities within this field. Areas of development have included attempts to increase oxygen-ion conductivity, to increase surface exchange rates, to decrease creep rates, and to increase tolerance to CO2. 

Most oxygen-ion transport membranes operate in the temperature range 750 to 1000 °C. This creates several problems for operation of ceramic membranes. First, the membranes must tolerate a 20 bar pressure differential at temperatures that are conducive to generating creep-induced failure. The membrane materials which have high ionic conductivity also have large numbers of lattice vacancies to facilitate mobility of the ionic species, but lattice vacancies are major contributors to elevated creep rates. 

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When considering materials for oxygen separation and collection, the most important properties are oxygen-ion conductivity, electron conductivity, creep resistance, thermal expansion, chemical stability, chemical expansion, oxygen surface exchange rate, thermal shock resistance, GO2 tolerance, steam tolerance, sin-terability, and absence of phase transitions within the operating temperature range (25-900 °G). 

Automatic sample preparation methods for the determination of oarbon-14 and/or tritium, and carbon-14 and/or sulfur-36 in dual labelled samples by liquid scintillation counting are presented. The sample is burnt in a stream of oxygen, and the combustion products carrying radioisotopes are subsequently separated and collected for radioactivity determination. Tritium is measured as water, carbon-14 as "carbamate" and sulfur-35 as sulfuric acid. The procedures run automatically, they are free of memory effect and cross contamination, and provide quantitative recovery. 

Oxygen in a large operating boiler may corrode steam-water separators and boiler surface components such as the top drum (especially at the waterline) and tubes. Oxygen corrosion also may occur in superheater and reheater tubes, especially in places where moisture can collect, such as in bends and sagging tubes. 

Samples of treated slurry were obtained from laboratory-scale continuous culture reactors (3 15 litres) during a series of treatments studying the effects on residual slurry quality of mean treatment time, reaction temperature, dissolved oxygen level and pH value (27). Some were also collected from a 2.4m3 pilot plant which was operating at 35°C and 7 day residence time and with dissolved oxygen saturation of 0 to 40%. The pilot plant was treating separated stored piggery slurry (TS 21 g/1 COD 26 g/1 ). 

Hydrogen and oxygen can be collected separately and combined again in a fuel cell to obtain electrical energy. 

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