Countercurrent cooling tower operation

Figure 5.12 Important design parameters for the countercurrent cooling tower operation.

A cooling tower operates in the countercurrent mode as illustrated by Figure 5.13. Entering air has a 5% wet-bulb temperature of 65°F. Hot process water enters the tower at 118°F and cold water leaves at a 15° approach to the wet-bulb (i.e., at 80°F). The cross-sectional area of the tower is 676 ft2. Determine the number of transfer units (Ntu ) required to meet the process requirements. Air is supplied to the tower by a blower having a capacity of 250,000 cfm and the water loading is 1500 lb/(hr)(ft2). 

The exact solution of the equations that describe both the heat and mass transfer processes is rather complicated, although both must be considered as just described. However, for the most common case of an air/water system the ratio of the heat transfer coefficient to the product of the mass transfer coefficient and gas-phase humid heat capacity is unity. In this case, humidity and temperature potentials are combined into enthalpy driving forces that control the rate of transfer [3]. Equation 6-7 represents the operating line of a countercurrent flow water cooling tower. The equilibrium curve is a plot of the enthalpy of saturated air (in Btu/lb BDG) against the temperature of water in thermal equilibrium with the air. 

A plant manufacturing dry ice will bum coke in air to produce a flue gas which when cleaned and cooled will contain 15% C02, 6% 02, and 79% N2. The gas will be blown into a bubble-cap tower scrubber at 1.2 atm and 298 K, to be scrubbed countercurrently with a 30 wt% monoethanolamine (C2H ON) aqueous solution entering at 298 K. The scrubbing liquid, which is recycled from a stripper, will contain 0.058 mol C02/mol solution. The gas leaving the scrubber is to contain 2% C02. A liquid-to-gas ratio of 1.2 times the minimum is specified. Assume isothermal operation. At 298 K and 1.2 atm, the equilibrium mole fraction of carbon dioxide over aqueous solutions of monoethanolamine (30 wt%) is given by... 

Benzene vapor in the gaseous effluent of an industrial process is scrubbed with a wash oil in a countercurrent packed absorber. The resulting benzene-wash oil solution is then heated to 398 K and stripped in a tray tower, using steam as the stripping medium. The stripped wash oil is then cooled and recycled to the absorber. Some data relative to the operation follow ... 

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