Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Strictly speaking, the result obtained from Eq. 16–22 for the mole fraction

of dissolved gas is valid for the liquid layer just beneath the interface,

but not necessarily the entire liquid. The latter will be the case only when

thermodynamic phase equilibrium is established throughout the entire liq-

uid body.

We mentioned earlier that the use of Henry’s law is limited to dilute

gas–liquid solutions, that is, liquids with a small amount of gas dissolved in

them. Then the question that arises naturally is, what do we do when the gas

is highly soluble in the liquid (or solid), such as ammonia in water? In this

case, the linear relationship of Henry’s law does not apply, and the mole

fraction of a gas dissolved in the liquid (or solid) is usually expressed as a

function of the partial pressure of the gas in the gas phase and the tempera-

ture. An approximate relation in this case for the mole fractionsof a species

on the liquidand gas sidesof the interface is given by Raoult’s lawas

(16–23)

where Pi,sat(T) is the saturation pressureof the species iat the interface tem-

perature and Ptotalis the total pressureon the gas phase side. Tabular data

are available in chemical handbooks for common solutions such as

the ammonia–water solution that is widely used in absorption-refrigeration

systems.

Gases may also dissolve in solids,but the diffusion process in this case

can be very complicated. The dissolution of a gas may be independent of

the structure of the solid, or it may depend strongly on its porosity. Some

dissolution processes (such as the dissolution of hydrogen in titanium, simi-

lar to the dissolution of CO 2 in water) are reversible,and thus maintaining

the gas content in the solid requires constant contact of the solid with a

reservoir of that gas. Some other dissolution processes are irreversible.For

example, oxygen gas dissolving in titanium forms TiO 2 on the surface, and

the process does not reverse itself.

The molar density of the gas species i in the solid at the interface

is proportional to the partial pressureof the species iin the gas

Pi,gas sideon the gas side of the interface and is expressed as

ri,solid side 
Pi,gas side¬¬ 1 kmol>m^32 (16–24)

ri,solid side

Pi,gas sideyi,gas side Ptotalyi,liquid side Pi,sat 1 T 2

812 | Thermodynamics

TABLE 16–2

Henry’s constant H(in bars) for selected gases in water at low to moderate

pressures (for gas i, HPi,gas side/yi,water side) (from Mills, Table A.21, p. 874)

Solute 290 K 300 K 310 K 320 K 330 K 340 K

H 2 S 440 560 700 830 980 1140

CO 2 1,280 1,710 2,170 2,720 3,220 —

O 2 38,000 45,000 52,000 57,000 61,000 65,000

H 2 67,000 72,000 75,000 76,000 77,000 76,000

CO 51,000 60,000 67,000 74,000 80,000 84,000

Air 62,000 74,000 84,000 92,000 99,000 104,000

N 2 76,000 89,000 101,000 110,000 118,000 124,000

yA,gas side yA,liquid side

yA,gas side

yA,liquid side

or

yA,liquid side

or

PA,gas side = HyA,liquid side

Gas: A

Liquid: B

Gas A

PA,gas side

————P

FIGURE 16–23

Dissolved gases in a liquid can be

driven off by heating the liquid.

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