Two-Phase Gas Liquid Systems (Saturation, Condensation, and Vaporization)

Basic principle II Second class Dr.
Arkan Jasim Hadi
CHAPTER 17
TWO-PHASE GAS LIQUID

SYSTEMS (SATURATION,
CONDENSATION, AND
VAPORIZATION)
17.1 Saturation 510

17.2 Condensation 514
17.3 Vaporization 525
17.1 Saturation
When any noncondensable gas (or a gaseous mixture) comes in contact with a liquid, the gas
will acquire molecules from the liquid. If contact is maintained for a sufficient period of
time, vaporization continues until equilibrium is attained, at which time the partial
pressure of the vapor in the gas will equal the vapor pressure of the liquid at the
temperature of the system. At equilibrium, the rate of vaporization is equal to the rate of
condensation; therefore, the amount of liquid and the amount of vapor remain constant.
As a result, regardless of the duration of contact between the liquid and gas, after
equilibrium is reached no more net liquid will vaporize into the gas phase. The gas is then
said to be saturated with the particular vapor at the given temperature. We also say that
the gas mixture is at its dew point. The dew point for the mixture of pure vapor and
noncondensable gas means the temperature at which the vapor just starts to condense.
At the dew point the partial pressure of the vapor is the vapor pressure.
17.2 Condensation
Condensation is the change of vapor (in a noncondensable gas) to liquid. Some typical ways
of condensing a vapor in a gas are:
1. Cool it at constant system total pressure (the volume changes, of course).

2. Cool it at constant total system volume (the pressure changes).
3. Compress it isothermally (the volume changes).
As an example of condensation let's look at cooling a system constant total pressure for
a mixture of air and 10% water vapor. Pick the air—water vapor mixture as the system.
If the mixture is cooled at constant total pressure from 51°C and 750 mm Hg absolute
(point A for the water vapor in Figure 17.2), how low can the tem¬perature go before
condensation starts (at point B, the same as point C, in Figure 17.2a, but a different
point in Figure 17.2b)? You can cool the mixture until the temperature reaches the dew
point associated with the partial pressure of water of
Basic principle II Second class Dr. Arkan Jasim Hadi
From the steam tables you can find that the corresponding temperature is T = 46°C (points B and C in
Figure 17.2a on the vapor pressure curve). After reaching P* = 75 mm Hg at point B, if the
condensation process continued, it would continue at constant pressure (75 mm Hg) and constant
temperature (46°C) until all of the water vapor had been condensed to liquid (point C in Figure 17 2b)
Further cooling would reduce the temperature of the liquid water below 46°C.
Condensation can also occur when the pressure on a vapor—gas mixture is increased. If a pound
of saturated air at 75°F is isothermally compressed (with a reduction in volume, of course), liquid
water will be deposited out of the air just like water being squeezed out of a wet sponge (see
Figure 17.3).
For example, if a pound of saturated air at 75°F and I atm (the vapor pressure of water is 0.43
psia at 75°F) is compressed isothermally to 4 atm (58.8 psia), almost three-fourths of the original
content of water vapor now will be in the form of liquid, and the air still has a dew point of 75°F.
Remove the liquid water, expand the air isothermally back to I atm, and you will find that
the dew point has been lowered to about 36°F. Here is how to make the calculations. Let
1 = state at 1 atm and 4 = state at 4 atm with z = 1.00 for both components.
Basic principle II Second class Dr. Arkan Jasim Hadi
17.3 Vaporization
Vaporization is the reverse of condensation, namely the transformation of a liquid into

vapor (in a noncondensable gas). You can vaporize a liquid into a noncondensable gas, and raise the
partial pressure of the vapor in the gas until the saturation pressure (vapor pressure) is reached at
equilibrium. Figure 17.4 shows how the partial pressure of water and air change with time as
water evaporates into initially dry

Two-Phase Gas Liquid Systems (Saturation, Condensation, and Vaporization)

Uploaded by

Copyright:

Available Formats

Two-Phase Gas Liquid Systems (Saturation, Condensation, and Vaporization)

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Two-Phase Gas Liquid Systems (Saturation, Condensation, and Vaporization)

Uploaded by

Copyright:

Available Formats

Basic principle II Second class Dr.

Arkan Jasim Hadi

TWO-PHASE GAS LIQUID

17.1 Saturation 510

1. Cool it at constant system total pressure (the volume changes, of course).

Vaporization is the reverse of condensation, namely the transformation of a liquid into

You might also like