Unit operations, classification

TGL 25000 Chemical engineering unit operations - classification (1974) Thayer, R.H. Software engineering project management A top-down view. In Thayer, R.H. (ed.) Tutorial Software Engineering Project Management, pp. 15-54. IEEE Computer Society Press, Los Alamitos (1988)... 

Each basic operation can be divided into one or more unit operations. Size reduction involves cnishing and grinding depending on the size of material handled, and these may be carried out in stages. Separations can be either soHds from soHds, based on size or mineral composition, or soHds from Hquids, ie, dewatering (qv). Size separation or classification is an integral part of any flow sheet, not only to meet product size specifications, but also to ensure a narrow size distribution for subsequent minerals separation circuits and to decrease the load and improve the efficiency of size reduction units which are energy intensive. 

Processing costs include those for size reduction, size classification, minerals concentration and separations, soHd—Hquid separation (dewatering), materials handling and transportation, and tailings disposal. Size reduction, one of the most expensive unit operations in minerals processing, could account for as much as 50% of the total energy consumed. This cost varies considerably from deposit to deposit and quite often from one area of a deposit to another. Ore bodies are extremely heterogeneous and the associated minerals Hberation, complex. 

Processes and/or unit operations that fall under this classification include adsorption, ion exchange, stripping, chemical oxidation, and membrane separations. All of these are more expensive than biological treatment but are used for removal of pollutants that are not easily removed by biomass. Often these are utilized in series with biologic treatment but sometimes they are used as stand-alone processes. 

The values of the factors are determined on the basis of the Dow s Fire Explosion Hazard Index Classification Guide (Dow, 1987). The Guide includes rules and tables, which cover well the most chemical substances and unit operations. 

Each chapter presents several detailed studies illustrating the application of various optimization techniques. The following matrix shows the classification of the examples with respect to specific techniques. Truly optimal design of process plants cannot be performed by considering each unit operation separately. Hence, in Chapter 15 we discuss the optimization of large-scale plants, including those represented by flowsheet simulators. 

A huge number of publications meanwhile have included the term micro reactors and their applications [9] and some attempts were made to maintain order in the field, e.g. by sorting from the point of design and manufacturing issues [5, 13], by a classification of unit operations [14] or by sorting of the chemical reactions performed [15]. The latter becomes more common than the others, but a generic look seems to be necessary. 

Sedimentation techniques are widely used for partiele size analysis since the determined size distribution relates to unit operations such as classification. The distribution also relates to many end-use properties... 

Classification or size separation is the unit operation that separates the particles according to their size. In general, it can be carried out dry or wet. The fraction of the feed having size less than the size of the screen aperture is the undersize, while that of greater size is the oversize. The fraction of the undersize that flows through the apertures is called underflow (or passing), while the particles that do not pass are called overflow (or remaining). In solid-waste treatment, classification is carried out almost always on screens and in dry, because the particles to be classified are relatively coarse and wet processes are undesirable. 

Air concentration is the unit operation used to separate light from heavy waste components, based on their density difference, with the aid of air stream. Instead of air concentration, the term air classification is usually used, incorrectly, because classification denotes separation according to particle size. There are mainly two designs for air concentrators (1) with horizontal and (2) with vertical airflow. 

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