General and Economic Importance

Industrially interesting alkali silicates (sodium and potassium silicates) are characterized either by the SiOi alkali oxide weight ratio or by the Si02 alkali oxide molar ratio. The latter is obtained by multiplication of the weight ratio by 1.032 for sodium silicates and 1.568 for potassium silicates. 

Alkali silicates can be divided according to their molar SiO/metal oxide ratio into two groups  

The total worldwide production of alkali silicates in 1995 as solids was estimated to be ca. 4 10 t/a, to which Western Europe, Japan and the USA each contributed 0.5 10 t/a. In Western Europe and the USA ca. 30% thereof is utilized in detergents, ca. 8% for paper, ca. 5% for the treatment of TiO, and ca. 3% for water treatment. 

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Wear is the progressive loss of material caused by contact and relative motion. In spite of the technical and economic importance, there is no general theory to describe wear, and engineers still have to rely on their extensive empirical knowledge to construct machines with low wear. 

The second bibliography is that of Mueller (54)- Somewhat more general than the Watrous, it covers the history, cultivation, use, manufacture, and economic importance of cacao. It roughly divides the literature chronologically into 1500 to 1900 a.d. and 1900 to 1950, and presents an index by subject. Although the latter part numbers 2700 references alone, certain notable omissions of recent literature will be corrected in later editions. The student of early cacao history will find Mueller most helpful, even though many of the references are unavailable except in the rare book rooms of the world s foremost libraries. 

Biological and economic importance. There is no general explanation for the biological significance of A. Protection against consumption of the plant by animals has been proven in a few cases, but the fact that most insects are limited to one or a few food plants may actually be due to the alkaloid content of these particular species. Many A. have a strong and very specific effect on certain centers of the nervous system (e.g. opiates, see Endorphins). Therefore A. are widely used clinically, as combinations of pure compounds, as extracts of total alkaloids, or as synthetic analogs. However, their use is often accompanied by side effects, mainly toxicity and narcosis. 

As is apparent from the tables, most of the y-glutamyl derivatives have been found in seeds, although some have been identified in fresh plant parts. This is due partly to the fact that seeds have been more thoroughly investigated because of their availability and economic importance, but it also reflects the general tendency for non-protein amino acids and y-glutamyl derivatives to occur in high concentrations in seeds but in low concentrations or not at all in fresh plant parts (cf. Section VII. 5). 

Deactivated catalysts are reused after adequate regeneration processes either on-site or off-site, when possible and economical. Important examples are catalytic cracking (FCC), naphtha-reforming and HDS catalysts. The generation process to be employed differs depending on the kind of catalyst and cause of deactivation. In general carbon deposits are burnt off by air and metal deposits are removed by chemical treatment such as washing with sulfuric acid. Redispersion of sintered metal particles on support is sometimes possible. 

A very important but rather complex application of surface chemistry is to the separation of various types of solid particles from each other by what is known as flotation. The general method is of enormous importance to the mining industry it permits large-scale and economic processing of crushed ores whereby the desired mineral is separated from the gangue or non-mineral-containing material. Originally applied only to certain sulfide and oxide ores. 

The use of supercritical and hot water as a solvent is still largely experimental. Because supercritical technology is well known in the power industry, this use of water is likely to increase in the future. Corrosion control may be an important limiting consideration. General process economics are the second potential limit (see SUPERCRITICAL FLUIDS). 

Sulfur [7704-34-9] S, a nonmetallic element, is the second element of Group 16 (VIA) of the Periodic Table, coming below oxygen and above selenium. In massive elemental form, sulfur is often referred to as brimstone. Sulfur is one of the most important taw materials of the chemical industry. It is of prime importance to the fertilizer industry (see Fertilizers) and its consumption is generally regarded as one of the best measures of a nation s industrial development and economic activity (see Sulfur compounds Sulfurremoval and recovery Sulfuric acid and sulfur trioxide). 

D. B. McKeever and K. C. Jackson, "Economic Importance of the Timber Processing Industdes," ia R. W. Haynes, coordinator. An. Analysis of the Timber Situation in the United States, 1989—2040, General Technical Report RM-199, U.S. Department of Agriculture, Porest Service, Rocky Mountaia Porest and Range Experiment Station, Port Collins, Colo., 1990, Chapt. 4, pp. 59—78. 

Basic to establishing whether power recovery is even feasible, let alone economical, are considerations of the flowing-fluid capacity available, the differential pressure available for the power recovery, and corrosive or erosive properties of the fluid stream. A further important consideration in feasibihty and economics is the probable physical location, with respect to each other, of fluid source, power-production point, and final fluid destination. In general, the tendency has been to locate the power-recoveiy driver and its driven unit where dictated by the driven-unit requirement and pipe the power-recoveiy fluid to and away from the driver. While early installations were in noncorrosive, nonerosive services such as rich-hydrocarbon absorption oil, the trend has been to put units into mildly severe seiwices such as amine plants, hot-carbonate units, and hydrocracker letdown. 

The economic factors must be considered in every application. It is important to find a technique that will meet both the technical and economical requirements. In short, pollution control costs depend on the system characteristics and the application. Some cost equations that generalize the economics of the managing systems are available in the literature. Most of these equations give rough estimates and have an accuracy of only about 30% to 50%. For a comprehensive cost comparison of different units, a detailed cost analysis based on the equipment tender proposals and the special characteristics of the project is necessary. 

An overall economic evaluation must be made to ensure that the contemplated project for petroleum development will recover sufficient capital to pay for the total cost of development, installation and assembly effort of the company. Further, the capital return must yield a financial return consistent with the overall company risk involved. Thus, it is very important that the petroleum engineer designing a recovery system understand the company s evaluation criteria that will be used by management in judging whether a new project is to go forward. Chapter 7 (Petroleum Economics) has detailed discussions of general engineering economics and product (or project) evaluation criteria. 

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