Towers design considerations

When acetylene is recovered, absorption—desorption towers are used. In the first tower, acetylene is absorbed in acetone, dimethylformarnide, or methylpyroUidinone (66,67). In the second tower, absorbed ethylene and ethane are rejected. In the third tower, acetylene is desorbed. Since acetylene decomposition can result at certain conditions of temperature, pressure, and composition, for safety reasons, the design of this unit is critical. The handling of pure acetylene streams requires specific design considerations such as the use of flame arrestors. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

Bolles and Fair [129] present an analysis of considerable data in developing a mass-transfer model for packed tower design however, there is too much detail to present here. 

Some of the problems that concern the proper methods for consideration of several different objectives in reservoir planning are discussed. Classical systems analysis approach to decision making for multiple objective problems is outlined and the inherent difficulties associated with multiple objectives and subjective estimates are identified. Techniques used in reservoir design and operation are reviewed. An alternate technique for considering noncommensurate, objectives, which relates the objectives in terms of real trade-off costs and eliminates the need for a priori estimates of objective worth is presented. The method is illustrated with three examples, including a reservoir operation problem and a cooling tower design problem. 31 refs, cited. 

This part of a plant survey is, unfortunately, seldom carried out, as it may be considered that there is little, in practice, that can be done to change the plant design or operating conditions. Nevertheless, it can often be a useful exercise to undertake, especially if it is suspected that the tower is considerably undersized or oversized, or if there is evidence of cooling tower fouling. 

Typical design considerations include feed water flow rates, water and air temperatures, tower feed and discharge systems (gravity feed or type and location of pumps), and influent contaminant concentrations. In addition, requirements for effluent water contaminant concentrations and restrictions on air emission should be considered according to the various federal and state regulations. 

For an efficient absorption tower design, the plant should emit tail gases at less than 1000 ppm of nitrogen oxides. This level is about half the current emissions limit in Western Australia. Should emissions exceed this figure, then consideration must be given to the installation of a catalytic tail-gas combustor which enables emission levels to be lowered below 400 ppm. The plant does not normally produce any liquid waste streams (see Section 5.4.7 Environmental Impact Analysis). 

The design of a packed tower requires consideration of mechanical factors, such as pressure drop, flow capacities, and foundation load. In addition, consideration must be given to the factors that influence the effectiveness of contact between the fluid phases. A satisfactory packing should have the following properties ... 

Because of liquid-dispersion difficulties in packed towers, the design of plate towers is considerably more reliable and requires less safety factor when the ratio of liquid mass velocity to gas mass velocity is low. 

The headframe foundation design required additional design consideration as many of the new headframe columns were within the confines of the existing headframe. A construction plan, known as The early works program , was developed to allow as much of the construction to be performed prior to removal of the existing headframe tower. The operators/power room to the north and part of the west collar house foundation was constructed, thus greatly reducing construction schedule. The west collar house was also removed to allow access to construct the new collar house and headframe to the west of the shaft. The bin house foundation was... 

In both these cases, the area ABCD should remain constant—actually it decreases about 2 percent for every 10°F (5.6°C) increase in hot-water temperature. The cooling tower designers take this into consideration in their initial design by applying a hot-water temperature correction to design calculations when the design hot-water temperature exceeds 110°F (43.5°C). See Fig. 3.21. 

Tray towers These are very effective but are not commonly used in humidification, dehumidification, or gas-cooling operations for reasons of cost and relatively high pressure drop, except under special circumstances. Design considerations are available [1,4]. 

In gas absorption, the new phase consists of an inert nonvolatile solvent (absorption) or an inert nonsoluble gas (stripping), and normally no reflux is involved. The following paragraphs discuss some of the considerations peculiar to gas-absorption calculations for plate towers and some of the approximate design methods that can be employed when simplifying assumptions are vahd. 

Overview One of the most important considerations involved in designing gas-absorption towers is to determine whether or not temperatures will vaiy along the length of the tower because of heat effects, since the sohibility of the solute gas normally depends strongly upon the temperature. When heat effects can be neglected, computation of the tower dimensions and required flows is relatively straight-... 

Principles of Rigorous Absorber Design Danckwerts and Alper [Trans. Tn.st. Chem. Eng., 53, 34 (1975)] have shown that when adequate data are available for the Idnetic-reaciion-rate coefficients, the mass-transfer coefficients fcc and /c , the effective interfacial area per unit volume a, the physical solubility or Henry s-law constants, and the effective diffusivities of the various reactants, then the design of a packed tower can be calculated from first principles with considerable precision. 

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