Problems with Solids

The problem with solid—solid reactions is that of sufficiently bringing the reagent molecules together. If the product is liquid or partially liquid, this problem is, to a great extent, obviated, as transport can take place through the liquid phase. The reactions in the rotary Portland cement kiln are an example of this. Adequate mechanical working of the mixture to spread the liquid over the surface of the solids is beneficial. 

Most problems with solid fat content determination arise from inconsistencies in sample preparation. As noted above, the solids content is dependent on the temperature history of the sample, so deviations from the tempering protocols can lead to inaccuracies. The time required to measure a sample is not expected to lead to extensive crystallization or melting, but care should be taken to start the measurement as soon as possible after the oil is removed from the tempering block. Several instruments can be fitted with a thermostatic control for the sample in the coil, and this should be used if it is reliable. It is recommended that the NMR tubes be capped, particularly for the long tempering times required for stabilizing fats. 

Based on our current designs, the gasoline directly from the ZSM-5 reactors could contain 4 to 7 wt% durene. These concentrations of durene could lead to problems with solids build-up in carburetors during cold starts. While durene is present in conventional gasoline, the typical concentrations are low enough not to cause problems. 

EXAMPLE 11.15. a) Calculate the concentration of Zr + produced when excess solid zinc is treated with 100.0 mL of 6.000 M HCl. Assume no change in volume, (b) Repeat the problem with solid aluminum. 

The fouling of heat exchanger surfaces can be a major problem with solid biomass fuels, especially straws and herbaceous residues. Fouling occurs because of formation in the conversion zone of low-fusion point alkali metal salt... 

Polyesters, polyetherimide, melamine-formaldehyde, polyurethanes, polyurethane-ureas and polyamides (nylons) are examples of condensation polymers prepared by REX. " Because a small molecule is produced in condensation reactions, vent ports are employed. Between the ports are melt sealing screw sections, to prevent back mixing of volatilizing melt. Sealing screw sections are constructed by a right handed-left handed sequence of screw elements. Another chemistry feature of step-growth reactions is their sensitivity to errors in stoichiometric feed proportions. This poses problems with solid feed materials, which are normally converted to liquid form for more... 

Film formation is similar to adsorption in involving formation of a film on the electrode surface from insoluble electrolysis products. This process also produces changes in wave shape and may even passivate an electrode surface through formation of a thick, insulating film. This is more of a problem with solid electrodes than with the dme, which continuously produces a new, clean surface. 

Another problem with solid surfaces is the lack of data on surface tension as a function of temperature. Most data available are for higher temperatures. Although it can often be assumed that the surface tension decreases with increasing temperature, the behaviour over larger temperature ranges is usually unknown. The surface tension of a solid also depends on the crystallographic orientation and on the crystallite size, which determine the influence of curvature on surface tension. Both parameters further increase the difficulty in obtaining reliable surface-tension data. 

As noted above, these authors also proved the general validity, for the Rayleigh problem, of the principle of exchange of stabilities. Further, by formulating the problem in terms of a variational principle, Pellew and South-well devised a technique which led to a very rapid and accurate approximation for the critical Rayleigh number. Later, a second variational principle was presented by Chandrasekhar (C3). A review by Reid and Harris (R2) also includes other approximate methods for handling the Benard problem with solid boundaries. 

One of the main problems with solid adsorbents is that of obtaining satisfactorily low and reproducible blanks. Even if adsorbents can be adequately cleaned in the laboratory, very elaborate precautions must be adopted to prevent recontamination (and in the case of some synthetic resins, degradation) before deployment. Further problems may be encountered with in situ systems where the extended deployment times leave the sample (and adsorbent) vulnerable to bacterial attack. 

As the electronics and the radio communication industries developed, it became apparent that there would be a need for human-made diodes to replace the natural crystals that were used in a trial-and-error manner. Two development paths were followed solid-state diodes and vacuum tube diodes. By the middle of the twentieth century, inexpensive germanium-based diodes had been developed as solid-state devices. The problem with solid-state diodes was that they lacked the ability to handle large currents, so for high-current applications, vacuum tube diodes, or thermionic diodes, were developed. In the twenty-first century, most diodes are semiconductor devices, with thermionic diodes existing only for the rare very high-power applications. 

The examination of the solid-water interface can sometimes be approximated by measurements at the solid-air interface [23-25]. The wetting properties of a solid are defined as the extent to which liquids spread over the solid surface [11]. However, this can be a particular problem with solid polymers, which rapidly reorient to minimize their interfacial energy with air or water. The analysis of the surface chemistry of a hydrated polymer is not a trivial experiment. Several groups have been working in this area of research. Andrade et al. [26] utilized water-... 

Another alternative solid state electrolyte under study is the use of solid-state proton conductors such as heteropoly acid (HPA) electrolytes. The two most common HPAs are H4SiWi2O40 (SiWA) and H3PW12O40 (PWA) [121]. The HPA materials have high proton conductivities at room temperature (solid form of pure SiWA = 27 mS.cm i). The traditional problem with solid state proton conductors is their poor film making properties that make forming a separator difficult. 

Although solid-phase synthesis is frequently linked to combinatorial chemistry, this is not a requirement. Other synthetic methodologies, such as solution-phase synthesis and soluble polymer-supported synthesis, have also been used to effect the combinatorial synthesis process. However, solid-phase synthesis allows the most efficient combinatorial synthesis. The advantages and problems with solid-supported synthesis are described in later chapters. Thus, combinatorial chemistry is not solid-phase chemistry, albeit combinatorial chemistry can be advantageously performed on the solid phase. 

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