Design towers

The propylene fractionator operates at a pressure of 1.8 to 2.0 MPa with more than 160 trays required for a high purity propylene product. Often a two-tower design is employed when polymer grade (99.5%+) is required. A pasteurization section may also be used when high purity is required. The bottoms product contains mainly propane that can be recycled to the cracking heaters or used as fuel. Typical tower dimensions and internals for a 450,000 t/yr ethylene plant with naphtha feed are summarized in Table 7. 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

Investigators of tower packings normally report kcCi values measured at very low inlet-gas concentrations, so that yBM = 1, and at total pressures close to 100 kPa (1 atm). Thus, the correct rate coefficient For use in packed-tower designs involving the use of the driving force y — y /yBM is obtained by multiplying the reported k co values oy the value of pf employed in the actual test unit (e.g., 100 kPa) and not the total pressure of the system to be designed. 

Theoretical possible heat removal per pound of air circulated in a cooling tower depends on the temperature and moisture content of air. An indication of the moisture content of the air is its wet-bulb temperature. Ideally, then, the wet-bulb temperature is the lowest theoretical temperature to which the water can be cooled. Practically, the cold-water temperature approaches but does not equal the air wet-bulb temperature in a coohng tower this is so because it is impossible to contact all the water with fresh air as the water drops through the wetted fill surface to the basin. The magnitude of approach to the wet-bulb temperature is dependent on tower design. Important factors are air-to-water contact time, amount of fill surface, and breakup of water into droplets. In actual practice, cooling towers are seldom designed for approaches closer than 2.8°C (5°F). 

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. 

For example, the packed-tower design equation for a dilute system in which gas-phase reaciant A is being absorbed and reacted with liquid-phase reagent B is... 

For an isothermal absorber involving a dilute system in which a liquid-phase mass-transfer limited first-order irreversible chemic reaction is occurring, the packed-tower design equation is derived as... 

A second Scheibel tower design [Am. Jn.st. Chem. Eng. J., 2, 74 (1956) U.S. Patent 2,850,362, 1958] reduced HETS and permitted more direct scale-up. The impellers are surrounded by stationaiy... 

Kiihni Tower The extraction towers designed at Kiihni [see Mogh and Biihlmann, in Lo, Baird, and Hanson (eds.). Handbook of Solvent Extraction, Wiley-lnterscience, New York, 1983, sec. 13.5]... 

Strigle, R. F., Packed Tower Design andAppl cations, 2nd Ed., Gulf Publishing Co., Houston, Texas, 1994. 

Calculations for determining system makeup rates and chemical treating effective half life are presented in the section on Cooling Tower design. 

Bottom draw nozzle too small. Pump cavitation problem. Raising tower 10 feet did not help. Flooded the bottom of the tower. Design error in original plant. 

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. 

Determine the tower diameter based on the flows of (6) above. See Chapter 9, this volume for packed tower design. 

Assume this value constant for tower design. 

Establish a tower design diameter using the Souders-Brown method or the relation of Hunt, both given previously. 

Differences between the capacity and efficiency of an optimal tray and an optimal packed tower design. 

A typical packed tower design has more than 50% of open tower cross-section, with the void fraction of a packed tower being higher at around 90% of tower volume, resulting in the following ... 

Figure 9-21D. Loading, flooding and pressure drop coireiation (one of earlier versions). Adapted by permission from Leva, M. Tower Packing and Packed Tower Design, 2nd ed. U.S. Stoneware Co.

Figure 9-21F. Strigle s latest generalized pressure drop correlation. Note G = gas mass velocity, Ib/ft -sec. Used by permission of Strigle, R. F. Jr., Packed Tower Design and Applications Random and Structured Packings, 2nd ed. Gulf Publishing Co., (1994) p. 19.

From M. Lera, Tower Packings and Packed Tower Design, 2nd Ed. p. 91, U.S. Stoneware Co. (1953), by permission, now, Norton Chemical Process Products Corp. 

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