Carbon impurity content

Carbon Impurity content Oxidation rate Reference... 

Experience in air separation plant operations and other ciyogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubihty limit. For this reason, the carbon dioxide content of the feed gas sub-jec t to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

Usually it is difficult to separate the effect of ciystallite size on carbon reactivity from the effects of crystallite orientation and impurity content. However, Armington (62) attempted to do so by reacting a series of graphi-tized carbon blacks with oxygen and carbon dioxide, as discus.sed earlier in this article. Assuming that upon graphitization all the carbon blacks are converted to polyhedral particles with the surface composed almost completely of basal plane structure, it is possible to eliminate crystallite orientation as a variable. Spectroscopically, the total impurity content of all the graphitized carbon blacks is quite low and to a first approximation, the analyses of the individual constituents are similar. 

As a follow-up to this work. Walker and Baumbach (148) investigated the effect of heat treatment on the reactivities of carbons produced from 20 different coal tar pitches and one delayed petroleum coke. Heat treatment again produced a marked increase in crystallite size, a marked decrease in impurity content, and only a minor change in surface area. They use the... 

Neutral emission of species other than atomic Mg from MgO is a strong function of impurity content or microstructure (4). Samples with a cloudy appearance due to the presence of brucite (Mg(OH)2) precipitates, displayed especially high NE intensities of O2, CO, H2O, and CH4. The cloudy MgO is nominally as pure as the clear with respect to metallic impurities. Single crystals of MgO are often grown in an arc furnace, with water and carbon as minor impurities. Small amounts of brucite can precipitate in portions of the crystal mass. The precipitate/MgO interface can serve as a sink for many species which are then emitted as these interfaces are exposed in fracture. The cloudy MgO is more typical of geologic materials than clear MgO, implying that NE from geologic materials can be quite intense and rich. 

There are activated carbons of high purity available on the market. The general rule is that synthetic-based activated carbons have much lower impurity content than natural-based activated carbons. They are nevertheless not widely used because their prices are 4-10 times much more expensive. 

Because of the increased sulfur and impurity levels in crudes currently being processed, refiners in recent years have been considering residue desulfurization units upstream of the delayed coker. In addition to the reduction in sulfur content, residue desulfurization units also lower the metals and carbon residue contents. Due to the reduction in the carbon residue, the liquid product yield is increased and the coke yield reduced. In addition, the coke produced from a desulfurized residue may be suitable for use as anode grade coke. Table I shows the yields and product properties after coking Medium Arabian vacuum residue, with and without upstream residue desulfurization. 

Normally, for the partial oxidation processes, only a high-temperature shift conversion is used. This results in a carbon monoxide content of the gas after shift conversion of 3 vol % or sometimes somewhat higher. Liquid nitrogen wash [754] - [756] delivers a gas to the synthesis loop that is free of all impurities, including inert gases and is also the means for adding some or all of the nitrogen required for synthesis. 

The rate of degradation of YBCO depends substantially on the impurity content and the sample s prehistory, as well as on the porosity, grain size, and other macro-structural factors [468]. Surface contamination by carbon-containing particles substantially accelerates intractions with water [469]. Inner stresses in the samples also have a certain effect on these processes [470]. As a rule, high-quality HTSC films are more stable to degradation than the corresponding ceramics [453]. 

Iron-chromium oxide catalysts, reduced with hydrogen-containing in the conversion plants, permit reactions temperatures of 350 to 380°C (high temperature conversion), the carbon monoxide content in the reaction gas is thereby reduced to ca. 3 to 4% by volume. Since, these catalysts are sensitive to impurities, cobalt- and molybdenum-(sulfide)-containing catalysts are used for gas mixtures with high sulfur contents. With copper oxide/zinc oxide catalysts the reaction proceeds at 200 to 250°C (low temperature conversion) and carbon monoxide contents of below 0.3% by volume are attained. This catalyst, in contrast to the iron oxide/chromium oxide high temperature conversion catalyst, is, however, very sensitive to sulfur compounds, which must be present in concentrations of less than 0.1 ppm. 

As for many solid state reactions, the properties of any particular oxide preparation may be influenced by its method of synthesis [11]. Oxides are often products of thermal treatment. Such heating may influence the surface area, impurity content (e.g. strongly-retained traces of water from hydroxide dehydration, oxidized species retained from the decomposition of a nitrate, carbonate etc.) and concentrations and distribution of defects (e.g. vacancies and non-stoichiometry arising during oxidation of a metal). Thus the preparative method exerts significant control over the numbers... 

The impurity content of a WC powder is the result of three main factors (1) the purity content of the starting W powder (2) the impurity content of the carbon black or gaphite, and (3) the carburization temperature (the higher the temperature, the more die trace impurities are evaporated during carburization). A typical analysis is shown in Table 9.1. 

Graphite has also been described as a surprising acylating catalyst whereas carbon black is inactive [31] again no information was given about the exact composition and metal impurity content, and the results should be considered with caution. 

The thermal conductivity of thorium metal is given in Table 6.7. The electrical conductivity of thorium metal is very dependent on its impurity content. Chiotti [C3] found that at room temperature the resistivity of thorium metal containing 0.2 w/o (weight percent) carbon was 37 X 10 fl cm, and that of metal containing 0.03 w/o carbon was 18 X 10 fi cm. An extrapolated value for carbon-free thorium metal is 13 to 15 X 10 f2 cm. The temperature coefficient of resistivity is 3.6 to 4.0 X 10 per °C. 

Because of the growing importance of carbon dioxide sequestration, there is currently a lively debate as to whether future coal-fired power stations should be conventional pulverized fuel, oxy-fuel or gasification designs. This is by no means a straightforward choice and involves considerations of overall fuel efficiency, engineering complexity and capital and operating costs. In addition, there are many types of coal (anthracite, bituminous coal, brown coal) with possibly dissimilar impurity contents, each of which may dictate a different plant design. The jury is still out on whether future coal-fired power stations will employ post-combustion or pre-combustion capture of carbon dioxide this is a crucial issue to decide as the plants have a life of 40—50 years. 

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