Tower Economics

An important factor that should be considered in the economic analysis is the potential savings of water costs and sewer taxes that could be realized by choosing one system over another. Table 7.2 outlines a good procedure for estimating potential water cost savings. 

Cheremisinoff, N. P., and P. N. Cheremisinoff. Fiberglass-Reinforced Plastics Deskbook (Ann Arbor, MI Ann Arbor Science Publishers, Inc., 1978). 

Cheremisinoff, N. P. Applied Fluid Flow Measurement-Fundamentals and Technology (New York Marcel Dekker/Inc., 1979). 

Strauss, S. Guide to Evaluate Cooling Tower Performance, Power (October 1975). 

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Rabb, A. Hydrocarbon Process., 47(2) (1968) 122. Arc dry cooling towers economical ... 

For Steam Turbine Drives. . . Are Dry Cooling Towers Economical Rabb, A. 

Kohli, J.P. Design Best Cooling Water System. Hydrocarbon Processing, December 1968, p. 108. Rabb, A. For Steam Turbine Drives—Are Dry Cooling Towers Economical. Hydrocarbon Processing, February 1968, p. 122. 

This secondary reaction starts at about 180°C, but the mass must be heated to 350—400°C to bring the reaction to completion and produce a nitrate-free product. The off-gases are extremely corrosive and poisonous, and considerable attention and expense is required for equipment maintenance and caustic-wash absorption towers. Treatment of the alkaline wash Hquor for removal of mercury is required both for economic reasons and to comply with governmental regulations pertaining to mercury ia plant effluents. 

Environmental Protection. Fumes resulting from exposure of anhydrous aluminum chloride to moisture are corrosive and acidic. Collection systems should be provided to conduct aluminum chloride dusts or gases to a scmbbing device. The choice of equipment, usually one of economics, ranges from simple packed-tower scmbbers to sophisticated high energy devices such as those of a Venturi design (11). 

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Gas leaving the economizer flows to a packed tower where SO is absorbed. Most plants do not produce oleum and need only one tower. Concentrated sulfuric acid circulates in the tower and cools the gas to about the acid inlet temperature. The typical acid inlet temperature for 98.5% sulfuric acid absorption towers is 70—80°C. The 98.5% sulfuric acid exits the absorption tower at 100—125°C, depending on acid circulation rate. Acid temperature rise within the tower comes from the heat of hydration of sulfur trioxide and sensible heat of the process gas. The hot product acid leaving the tower is cooled in heat exchangers before being recirculated or pumped into storage tanks. 

Continuous chlorination of a cooling water system often seems most pmdent for microbial slime control. However, it is economically difficult to maintain a continuous free residual in some systems, especially those with process leaks. In some high demand systems it is often impossible to achieve a free residual, and a combined residual must be accepted. In addition, high chlorine feed rates, with or without high residuals, can increase system metal corrosion and tower wood decay. Supplementing with nonoxidizing antimicrobials is preferable to high chlorination rates. 

Ultimately, the economic choice between counterflow and cross-flow is determined by the effectiveness of the fill, design conditions, and the costs of tower manufacture. 

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

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. 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

FIG. 14-79 Cost of trays in plate towers. Price includes tray deck, bubble caps, risers, downcomers, and structural-steel parts. The stainless steel designated is type 410 Peters and Timmerhaus, Plant Design and Economics for Cbemical Engineers, 4th ed., McGraw-Hill, New York, 1.9.91). 

FIG. 14-81 Fabricated costs and installation time of towers. Costs are for shell with two beads and sldrt, but without trays, packing, or connections. (Peters and Timmerhatis, Plant Design and Economics for Chemical Engineers, 4th ed., McGraw-Hill, New Yoik, 1991. )... 

Solid adsorbents must also be structurally capable of being packed into a tower, resistant to fracturing, and capable of being regenerated and reused after saturation with gas molecules. Although some small units use throwaway canisters or charges, the majority of industrial adsorbers regenerate the adsorbent to recover not only the adsorbent but also the adsorbate, which usually has some economic value. 

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