Page 82 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

56 2 Basic Properties of Gases

This equation indicates that the overall mass transfer coefficient (K y ) can be
calculated from the single phase mass transfer coefficients (k y and k x ). They are
usually determined experimentally.
To understand Eq. (2.90), the mass transfer can be expressed in terms of driving
potential (see Fig. 5.4) and the corresponding resistance

y y y y
¼ ð2:91Þ
R R x þ R y
where R, R x and R y are overall resistance, liquid phase resistance, and gas phase
resistance to mass transfer, respectively,

1 1 H
R ¼ ; R y ¼ ; and R x ¼ ð2:92Þ
K y k y k x
When the value of H is small, there is a great solubility of the target gas in the
liquid phase; it indicates a low liquid phase film resistance and that the gas phase
resistance is dominating (or gas phase mas transfer is the bottleneck). In this case,
only a small amount of water is needed to hold certain amount of gases. In an
engineering application, this is preferred in favor of low liquid consumption. On the
other hand, when the value of Henry’s law constant is large, it means a low solubility
of the target gas in the liquid. Then, the mass transfer resistance is primarily
attributed to the liquid phase. A large amount of liquid is required to hold the gas.

2.4 Practice Problems

1. Estimate the mass of a single molecule of hydrogen, CO 2 and ethane (C 2 H 6 ).
2. Calculate the root-mean-square speed and kinetic energy of 1 mol of the fol-
lowing gases: He, N 2 ,O 2 ,Cl 2 , and CH 4 at 300 K.
3. Which gas has the highest root-mean-square speed: Helium (He), Neon (Ne),
Argon (Ar), Krypton (Kr), and Xenon (Xe)?
4. Name the two gases of the following: N 2 ,O 2 , CO, CO 2 ,CH 4 , and SO 2 that
have the same kinetic energy and root-mean-square speed at 293 K.
5. Which gas has a lowest root-mean-square speed: Fluorine (F 2 ), Chlorine (Cl 2 ),
SO 2 , Krypton (Kr), and Nitrogen dioxide (NO 2 )?
6. Explain why nitrogen (N 2 ) and carbon monoxide (CO) have the same molar
kinetic energy at 293 K.
7. Compute the kinetic energy of one cubic meter of air, and CO 2 under standard
condition.
8. At what temperature will the root-mean-square speed of CO 2 be the same as
that of CH 4 at 300 K?
9. Which has the greater root-mean-square speed at 25 °C, NH 3 (g) or HCl (g)?