Some Refractory Compounds

Gray with pink tinge Dark gray Brick-red Dark brown 

BlacK with violet tinge Brown Gray 

Grayish brown Blue-violet Violet Azure green Grayish brown Blue-violet Blue-gray 

Phase Color in the disperse state (powder Phase Color in the disperse state (powder  

BsaNa AIN Light gray Y2O2S Grayish white 

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One method of sulfur removal from refinery streams is by hydrodesulfurization (HDS) in the refineries. This step also directly impacts the characteristics of low sulfur diesel fuels, such as density, aromatics content, cetane number, and cloud point. The magnitude of these changes will depend upon the type and setup of refinery HDS units. However, in the end some refractory compounds in fuel, e.g., 4,6-diraethyl dibenzothiophene, are very resistant to desulfurization, owing to the inaccessibility of the organically bound sulfur atom. Lower pressure HDS units which can work satisfactorily at 350 rag/kg sulfur levels, may have difficulty achieving reduction to 50 or 10 rag/kg sulfur level. 

Figure 1 The coefficient 7 in the electronic heat capacity = jT plotted versus the average number of valence electrons per atom for some refractory compounds, j is obtained through Eq. (3) from the electron density of states N E) in ah initio electron structure calculations for 3d metal compounds (5) and for NbN andTaN (2) i.e., without the enhancement factor (1 + that is present only at low temperatures (approximately for T < 0d/3).

Figure 11 correlates the entropy-related force constant ks with the hardness of some refractory compounds. Because the hardness is measured in the unit of pressure (i.e., having the dimension of force constant per length), we correlate hardness with ks/a, where a is the measured lattice parameter. 

Figure 11 The hardness of some refractory compounds plotted versus k la, where is the force constant obtained from 0s through experimental vibrational entropy data. Adopted from Ref. 30.

Chapter IV, dealing with optical properties, contains information on the color of some refractory compounds, emission coefficients, and absorption spectra in the infrared region. The color of the compounds naturally depends on their composition, especially their degree of dispersion, etc. The data given in the corresponding table provide a qualitative idea of the color of a number of refractory compounds in the disperse state, and in the author s opinion may be useful. 

Most hafnium compounds have been of slight commercial interest aside from intermediates in the production of hafnium metal. However, hafnium oxide, hafnium carbide, and hafnium nitride are quite refractory and have received considerable study as the most refractory compounds of the Group 4 (IVB) elements. Physical properties of some of the hafnium compounds are shown in Table 4. 

Refractory Compounds. Refractory compounds resemble oxides, carbides, nitrides, borides, and sulfides in that they have a very high melting point. In some cases, they form extensive defect stmctures, ie, they exist over a wide stoichiometric range. For example, in TiC, the C Ti ratio can vary from 0.5 to I.O, which demonstrates a wide range of vacant carbon lattice sites. 

Details of some inducible P450 forms that play key roles in the metabolism of xenobiotics are shown in Table 2.4. P450s belonging to family lA are induced by various lipophilic planar compounds including PAHs, coplanar PCBs, TCDD and other dioxins, and beta naphthoflavone (Monod 1997). As noted earlier, such planar compounds are also substrates for P450 lA. In many cases, the compounds induce the enzymes that will catalyze their own metabolism. Exceptions are refractory compounds such as 2,3,7,8-TCDD, which is a powerful inducer for P450 lA but a poor substrate. 

The most refractory compounds in the resids may be the PAHs. Those present in residues consists of fused rings of more than six cycles, with molecular weight above 600. Some of the complex larger PAHs isolated and identified are collected in Table 10. These examples probably correspond to the smallest PAH compounds in a resid cut, but worth to set the lowest limit to the level of complexity, from where the reacting mixture starts. 

The problems associated with direct reaction calorimetry are mainly associated with (1) the temperature at which reaction can occur (2) reaction of the sample with its surroundings and (3) the rate of reaction which usually takes place in an uncontrolled matmer. For low melting elements such as Zn, Pb, etc., reaction may take place quite readily below S00°C. Therefore, the materials used to construct the calorimeter are not subjected to particularly high temperatures and it is easy to select a suitably non-reactive metal to encase the sample. However, for materials such as carbides, borides and many intermetallic compounds these temperatures are insufficient to instigate reaction between the components of the compound and the materials of construction must be able to withstand high temperatures. It seems simple to construct the calorimeter from some refractory material. However, problems may arise if its thermal conductivity is very low. It is then difficult to control the heat flow within the calorimeter if some form of adiabatic or isothermal condition needs to be maintained, which is further exacerbated if the reaction rates are fast. 

Among the more important catalysts are metals, which may be promoted by other metals, or by oxides and oxides, which are usually rendered more effective by mixing with other oxides. It is usual to distinguish between supported catalysts, generally metals in a finely divided condition on the surface of silicate minerals, and promoted catalysts, where an oxide, or occasionally some other compound, is mixed with the metal the mixture being sometimes also supported on an inert refractory support. The distinction is not, however, absolutely sharp. 

In water-wall incinerators. The internal walls of the combustion chamber are lined with boiler tubes that are arranged vertically and welded together in continuous sections. When water walls are employed in place of refractory materials, they are not only useful for the recovery of steam but also extremely effective in controlling furnace temperature without introducing excess air however, they are subject to corrosion by the hydrochloric acid produced from the burning of some plastic compounds and the molten ash containing salts (chlorides and sulfates) that attach to the tubes. 

Some complexes (e.g., [Fe(III)EDTAJ) can undergo total photolysis in a sunny day within hours, while others are only slightly affected (e.g., [Mn(II)EDTA], [Co(III)EDTA]), or not affected by light at all. The ability of [Fe(III)EDTA] to undergo photolysis is very fortunate because EDTA is a refractory compound, and thus the natural photolytic pathway provides a means for its destruction. (From these cases it can also be deduced that some metal complex-containing samples to be analyzed for environmental purposes must be isolated from light immediately after collection). 

In some cases, several refractory compounds can result from two or more parallel reactions occurring simultaneously in the combustion wave. A typical example of this type is the Ti-C-B system, where both the Ti+C and Ti-I-2B reactions affect the combustion synthesis and structure formation processes (Shcherbakov and Pityulin, 1983). By adjusting the contents of carbon and boron powders in the reactant mixture, either carbide- or boride-based ceramics can be obtained. 

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