Impurities and Their Effects

Impurities always decrease the cell efficiency, hence the clean-up system is required for it. The most common impurities are sulphur compounds, halides, nitrogen compounds etc. The sulphur tolerance of MCFC is influenced by temperature, pressure, cell components, gas composition etc. The anode has relative tolerance for 10 ppm of H2S, whereas 1 ppm SO2 is also acceptable. For increasing pressures, these concentration limits decrease, whereas for increasing temperatures these limits tend to increase. Sulphur poisons the catalytic reaction sites for the water gas shift reaction and tends to block the electrochemical sites by chemisorption of Ni surface. The following set of reactions takes place when nickel combines with H2S  

In reaction (1.39), the anode is sulphided and then gets reduced to nickel. The poisoning due to H2S interferes with the water gas shift reaction equilibrium. Hence, chromium is used in the anodes as it acts as a sulphur-tolerant catalyst. As CO2 is required at the cathodes, it is supplied by anode gas recychng. Hence, the sulphur may also contaminate the cathode as it may be present in the exhaust stream of the anode. The sulphur upon entering the cathode may react with the carbonate ions to produce alkali sulphates which are transported towards the anode via the electrolyte. On reaching the anode, the sulphate S04 is reduced to S.  

Halide impurities are also disadvantageous for the functioning of MCFC. When halides react with molten carbonates it leads to the formation of alkali halides, CO2 and water. Due to the formation of alkali halides (LiCl/KCl/NaCl, etc.), the vapour pressure increases and the rate of electrolyte loss also increases. For tolerance purpose, the HCl level should be kept below 1 ppm in the fuel gas. The nitrogen compounds such as NH3 and HCN may also react with the electrolyte to form nitrate salts. 

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Water soluble impurities and their effect can be easily included in equation (1-4), through which they are going to directly affect the particle nucleation rate, f(t). If one assumes a first order reaction of an active radical with a water soluble impurity (WSI) to give a stable non-reactive intermediate, then one simply has to add another term in the denominator of equation (1-4), of the form kwsr[WSI](t)-kv, and to account for the concentration of WSI with a differential equation as follows ... 

In this Sect, we describe the starting material impurities and their effect on the processing and cure reactions of TGDDM-DDS epoxies. The cure reactions are characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) studies. The BF3 amine catalysts used to accelerate the cure of TGDDM-DDS epoxies are characterized by nuclear magnetic resonance (NMR) spectroscopy studies. 

Water impurities and their effect 5.5.5.1 Suspended particles... 

Electronic States of Impurities and Their Effect on Material Properties... 

The level of impurities in the sulphur used in sulphonation operations should normally not exceed 0.5% and a target minimum sulphur content of 99.7% is preferred. The types of impurity and their effect on sulphonation operations are summarised below. Generally, sulphur analysis is performed very infrequently but in cases where new suppliers appear or processing problems occur, it is advisable to have the sulphur quality checked to ensure that die specification is being met. Because of the low specified levels of some of the impurities, the expertise available in specialised laboratories should be employed for this analysis. 

Van Driessche, A.E.S. et al. (2009) Direct observation of adsorption sites of protein impurities and their effects on step advancement of protein crystals. Cryst. Growth Des., 9 (7), 3062-3071. 

The uncertain effects of impurities are avoided by periodic or continuous electrolysis of the solution at low current densities to remove metallic contaminants and by filtration through active carbon to remove organic substances. A concise review of the effects of impurities and their removal is given by Greenall and Whittington". 

D. R. Fosnacht andT. J. O Keefe, The Effects of Certain Impurities and their Interactions on Zinc Electrowinning, Metal Trans. B, Vol. 14B, p. 645,1983. 

Thomas, M. D., R. H. Hendricks, and G. R. Hill. Some impurities in the air and their effects on plants, pp. 41-47. In Air Pollution. Proceedings of the United States Technical Conference on Air Pollution. New Yoik McGraw-Hill Book Company, 1952. 

Detection of impurities and their estimation without separation, e.g. by their effect on colligative properties, or by some kind of spectroscopy, or an electroanalytical technique such as polarography. 

Known Variables - Uncontrollable or Controllable Within Limits A long series of experiments sometimes involves situations where variation due to changes in one or more factors is known to exist, but where these factors cannot be completely controlled. An example is raw-material quality. Different lots of raw material or catalyst used in an experimental programme may sometimes vary in chemical composition, impurities, activity, etc. Since these items may not be within the control of the user and may not easily be worked into a balanced experimental design, the differences should at least be recorded and their effect taken into consideration during the analysis... 

Impurity and Aperiodicity Effects in Polymers.—The presence of various impurity centres (cations and water in DNA, halogens in polyacetylenes, etc.) contributes basically to the physics of polymeric materials. Many polymers (like proteins or DNA) are, however, by their very nature aperiodic. The inclusion of these effects considerably complicates the electronic structure investigations both from the conceptual and computational points of view. We briefly mentioned earlier the theoretical possibilities of accounting for such effects. Apart from the simplest ones, periodic cluster calculations, virtual crystal approximation, and Dean s method in its simplest form, the application of these theoretical methods [the coherent potential approximation (CPA),103 Dean s method in its SCF form,51 the Hartree-Fock Green s matrix (resolvent) method, etc.] is a tedious work, usually necessitating more computational effort than the periodic calculations... 

The previous discussion has been concerned with impurities In feed streams and their effects on acid make-up requirement. 

The chemical aspect of the Hecker effect was analyzed by Melrose et al. [17]. Kuhn and Argoul [18,19] found that the growth morphology is very sensitive to small chemical perturbations, for example, pH changes, the presence of small amounts of oxygen, and minute quantities of alkali metal ions. Fleury et al. [20] studied the role of impurities and their dynamic behavior. 

Determine the fate of feed gas impurities (i.e., CO2, methane [CH4], etc.) in the product gas, and their effect on reactor performance, especially with respect to hydrogen yield. 

There are two types of defects associated with phosphors. One involves controlled point defects in which a foreign activator cation is incorporated in the solid in defined amounts. The other involves line and point defects inadvertently formed in the solid structure because of impurity and entropy effects. This chapter will define and characterize the nature of all of these point defects in the soUd, their thermod3mamics and equilibria. It will become apparent that the type of defect present will depend upon the nature of the solid in which they are incorporated. That is. the characteristics of the point defects in a given phosphor will depend upon its chemical composition. Of necessity, this chapter is not intended to be exhaustive, and the reader is referred to the many treatises concerned with the point defect. 

Further work is required to determine the location and composition of the A1 containing phases and their effect on the resistivity of the material. One of the possibilities of diminishing the influence of the Si and A1 impurities is to reduce their amount in the Cr rich metallic alloys. The most promising metallic alloy which is present on the market at the moment is Crofer APU alloy with a basic composition of Fe, Cr (22wt%), Mn (0.4wt%), Ti (0.05wt%), La (0.09wt%). The amount of Si and A1 can be as low as 0.005 and 0.007, respectively. 

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