G

Equation (14-182) says that the backcalculated NG is 2:

For diffusing gases of similar molecular weight, the properties that control heat transfer follow the same rules as those that control mass transfer. As a result, the NH3 scrubbing and gas cooling processes achieve similar approaches to equilibrium.

For an entry temperature of 120°C and an adiabatic saturation temperature of 70°C, the expected outlet temperature would be

This looks like a powerful concept, but its value is limited due to uncertainty in estimating hGa. Both hG and a are difficult to estimate due to dependence on power dissipation as discussed below. The primary value of the NG concept is in estimating an expected change from baseline data as in the comparison of Example 19 with Example 20.

Example 20: A Contactor That Is Twice as Long, No Bypassing If we double the length of the pipeline contactor, double the effective contact area, and double the number of transfer units to 4, what do we expect for performance? For NG = 4,

E = 1 - e-4 = 0.982 The NH3 in the exit gas would be expected to drop to (1 - 0.982)(1000) = 18 ppm and the expected outlet temperature would be

If we double the length again, we increase the number of transfer units to 8 and achieve an approach of

E = 1 - e-8 = 0.9997 The outlet temperature would be

70 + (1 - 0.9997)(120 - 70) = 70.015°C Similarly the NH3 in the exit gas would be

(1 - 0.9997)(1000) = 0.3 ppm Note that this approximates the exit condition of Example 17.

Transfer Coefficient—Impact of Droplet Size The transfer coefficients increase as the size of droplets decreases. This is so because the transfer process is easier if it only has to move mass or heat a shorter distance (i.e., as the bubble or droplet gets smaller).

In the limiting case of quiescent small bubbles or droplets, the transfer coefficients vary inversely with average bubble or droplet diameter. For example, in heat transfer from a droplet interface to a gas, the minimum value is hG,min = heat transfer coefficient from interface to gas = 2kG/D

where kG = gas thermal conductivity and D = droplet diameter.

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