Cost analysis tower design

For prehminary screening and easibility studies or for rough cost estimates, one may wish to employ a version of the isothermal method which assumes that the liquid temperatures in the tower are everywhere equal to the inlet-liquid temperature. In their analysis of packed-tower designs, von Stockar and Wilke [Ind. Eng. Chem. Fun-dam. 16, 89 (1977)] showed that the isothermal method tended to underestimate the reqmred depth of packing by a factor of as much as 1.5 to 2. Thus, for rough estimates one may wish to employ the assumption that the temperature is equal to the inlet-liquid temperature and then apply a design fac tor to the result. 

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. 

The best inemals and the optimum values of pressure, vapor velocity, and reboil vapor ratio are those that permit production of heavy water at minimum cost. The initial cost of the plant depends on a number of factors including the total number of towers, the total amount of reboiler and condenser surface, and the total volume of tower internals. The principal operating cost is for power, which is proportional to total loss in availability of steam as it flows through the towers. A complete minimum-cost analysis requires knowledge of the unit cost of all the important cost components and is beyond the scope of this book. Design for minimum volume of tower internals or minimum loss in availability due to tower pressure drop and for minimum cost of these two important contributors to total cost can be carried out without complete unit-cost data and will be discussed. Because the same choice of reboil vapor ratio minimizes the number of towers, their volume, and the loss of availability within them, this reboil vapor ratio is close to that which leads to minimum production cost. An equation for this optimum reboil vapor ratio will now be derived, and expressions will be developed for the total volume of towers and the total loss in availability in towers designed for the optimum ratio. 

Billet, R., 1989, Packed Tower Analysis and Design, Ruhr Univ ly. Bochum, Germany. Blecker, H. G., and Nichols, T. M., 1973, Capital and Operating Costs of Pollution Control Equipment Modules, Data Mamud, Vol. 2., EPA-R5-73-023b, July, PB-224S36. 

Thns, for a valne of G/L of 4.0, the valnes of Z/HTU for XJX = 0.1 are 1.49 for 85°F and 1.95 for 75°F. The height of the tower at 85°F is 1.48 x 23.5 = 34.8 ft, whereas at 75°F it becomes 1.95 x 23.5 = 45.8 ft. This shows that setting the GIL ratio to 4.0 instead of 2.0 wonld resnlt in a shorter tower. The amount of air requirecf, however, is not doubled 34.8 times 2 divided by 49.5 = 1.40. Thus, at this higher air loading rate, only 40% more air is reqnired. To find the optimum GJL ratio, the entire design mnst be priced and the minimnm cost tower selected. This requires repetitive calculations using a computer and incorporating reasonably accurate cost data as well as mass transfer, enthalpy transfer, and pressure drop characteristics on the detailed analysis. 

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