Scale Operations

However, chromatographic processes stiH have a considerable appHcabiHty (106) (see Analytical methods). For instance, in small-scale operations, the greater simplicity of the chromatograph may more than compensate economically for the larger adsorbent inventory and desorbent usage. 

Commercial-scale operations are conducted in batch, fed-batch, or continuous culture systems. Fermentation vessels include the conventional baffled aerated tank, with or without impeller agitation, and the ak-lift tower fermentors in which ak is sparged into an annular space between the... 

There are large-scale operations using direct-heat resistance furnaces. These are mainly in melting bulk materials where the Hquid material serves as a uniform resistor. The material is contained in a cmcible of fixed dimensions which, coupled with a given resistivity of the material, fixes the total resistance within reasonable limits. The most common appHcation for this type of direct-heat electric resistance furnace is the melting of glass (qv) and arc furnaces for the melting of steel (qv). 

New furnace concepts in evolutionary stages include fluidized-bed furnaces (25), coal gasification furnaces (26), and MHD furnaces (27,28). Of these technologies, fluidized-bed combustion has reached commercial-scale operations (Fig. 11). 

A development in the 1960s was that of on-line elemental analysis of slurries using x-ray fluorescence. These have become the industry standard. Both in-stream probes and centralized analyzers are available. The latter is used in large-scale operations. The success of the analyzer depends on how representative the sample is and how accurate the caUbration standards are. Neutron activation analyzers are also available (45,51). These are especially suitable for light element analysis. On-stream analyzers are used extensively in base metal flotation plants as well as in coal plants for ash analysis. Although elemental analysis provides important data, it does not provide information on mineral composition which is most cmcial for all separation processes. Devices that can give mineral composition are under development. 

Scale-up is the process of developing a plant design from experimental data obtained from a unit many orders of magnitude smaller. This activity is considered successful if the commercial plant produces the product at plaimed rates, for plaimed costs, and of desired quaUty. This step from pilot plant to full-scale operation is perhaps the most precarious of all the phases of developing a new process because the highest expenses ate committed at the stages when the greatest risks occur. 

Process probably not competitive for small-scale operations (>10 20 t/d product gas). 

Suspended Particle Techniques. In these methods of size enlargement, granular soHds are produced direcdy from a Hquid or semiliquid phase by dispersion in a gas to allow solidification through heat and/or mass transfer. The feed Hquid, which may be a solution, gel, paste, emulsion, slurry, or melt, must be pumpable and dispersible. Equipment used includes spray dryers, prilling towers, spouted and fluidized beds, and pneumatic conveying dryers, all of which are amenable to continuous, automated, large-scale operation. Because attrition and fines carryover are common problems with this technique, provision must be made for recovery and recycling. 

The BPT is defined as the level of treatment that has been proven to be successful for a specific industrial category and that is currendy in fuU-scale operation. Sufficient data exist for this level of treatment so that it can be designed and operated to achieve a level of treatment consistendy and with rehabUity. For example, in the pulp and paper industry, BPT has been defined as biological treatment using the aerated lagoon or the activated sludge process with appropriate pretreatment. 

The BAT is defined as the level of treatment beyond BPCTCA that has been proven feasible in laboratory and pUot studies and that is, in some cases, in fuU-scale operation. BAT in the pulp and paper industry may include such processes as filtration, coagulation for color removal, and improved in-plant control to reduce the wasteload constituents. 

The first commercial shipment of diatomite ia the United States was made ia 1893 and consisted of material from a small quarry operation ia the vast deposit near Lompoc, California. It went to San Francisco to be used for pipe iasulation. Small-scale operation of parts of the Lompoc deposit continued until it was acquired by the Kieselguhr Co. of America, which later became the CeHte Co. (4). Siace that first work, the iadustry has grown immensely, and diatomite products are used ia almost every country. 

Batch vs Continuous Distillation. The mode of operation also influences the economics of distiUation. Batch distiUation is generaUy limited to smaU-scale operations where the equipment serves several different disflUations. 

Through-ckculation compartments employ perforated or screen bottom trays and suitable flow baffles so gas is forced through the material. If material is not inherently pervious to gas flow, it may be mechanically shaped iato noodles, pellets, or briquettes. These dryers are used ia small-scale operations to dry explosives, foods, and pigments. Dryer efficiency is 50—70%. Based on tray area, water vaporization rates are 1—10 kg/(h-m ). 

Tank Cells. A direct extension of laboratory beaker cells is represented in the use of plate electrodes immersed into a lined, rectangular tank, which may be fitted with a cover for gas collection or vapor control. The tank cell, which is usually undivided, is used in batch or semibatch operations. The tank cell has the attraction of being both simple to design and usually inexpensive. However, it is not the most suitable for large-scale operation or where forced convection is needed. Rotating cylinders or rotating disks have been used to overcome mass-transfer problems in tank cells. An example for electroorganic synthesis is available (46). 

Reports of bench-scale electroorganic reactions date back to the nineteenth century (18). Although the Nalco and Monsanto processes are the only two really large-scale operations, there has been significant growth in processes on a small scale. At least 60 processes appear to be commercial woddwide (4). A listing of some processes can be found in Table 5 further details and a listing of more processes are also available (4,5,25,88, and 99—104). 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

Size Reduction Combined with Size Classification Grinding systems are batch or continuous in operation (Fig. 20-11). Most large-scale operations are continuous batch ball or pebble mills are used... 

Control of Crushers Lower-grade raw materials, higher energy costs, larger-scale operations, and more complex, capital-intensive plants make automatic control of size-reduction equipment more important (Suominen, 21st International Symposium—Applications of Computers and Operations Research in the Mineral Industry, 1011-1018). Benefits are increased productivity, process stability and safety, improved recoveiy of mineral values, and reduced costs [Horst and Enochs, Engineering Mining J., 181(6), 69-171 (1980)]. 

These features and the low cost of the mills make them suitable for medium-scale operations where complicated circuits cannot be justified. The maximum feed size is 20 cm (8 in), and the product size may be as fine as 325 mesh. 

With the addition of appropriate additives as needed, the flotation of refineiy wastewaters reduced their oil content to less than 10 mg/L in pilot-plant operation [Steiner, Bennett, Mohler, and Clere, Chem. Eng. Frog., 74(12), 39 (1978)] and full-scale operation (Simouseu, Hydrocaih. Froce.ss. Fet. Refiner, 41(5), 145, 1962]. Experiments with a cationic collector to remove oils reportedly confirmed theoiy [Angehdon, Keskavarz, Richardson, and Jameson, Jnd. Eng. Chem. Frocess Des. Dev., 16, 436 (1977)]. 

Physical methods such as osmotic shock, in which the cells are exposed to high salt concentrations to generate an osmotic pressure difference across the membrane, can lead to cell-wall disruption. Similar disruption can be obtained by subjecting the cells to freeze/thaw cycles, or by pressuriziug the cells with an inert gas (e.g., nitrogen) followed by a rapid depressurization. These methods are not typically used for large-scale operations. 

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