Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

Relative Saturation (Relative Humidity)

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Relative saturation is defined as the partial pressure of the vapor divided by the vapor pressure of the vapor at the temperature of the gas

where subscript 1 denotes the vapor

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

The relative saturation represents the fractional approach to the total saturation.

For example, relative humidity is 60%, therefore:

Zero percent relative saturation means no vapor in the gas whereas 100% means

that the partial pressure of the vapor in the gas is the same as the vapor pressure

of the substance.

Example:

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

The partial pressure of water vapor in the air from the given percent relative

humidity

PH20 = (1.61 in Hg)(0.43) = 0.692 in. Hg

Basis: 1 mi3 water vapor at 94°F and 0.692 in. Hg

a) Amount of water vapor:

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

b) Dew point :

From the steam table, the temperature is about 68 – 69 oF.

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

Molal Saturation

Is another way to express vapor concentration in a gas by using the ratio of the moles of vapor to the moles of vapor-free gas:

If subscripts 1 and 2 represent the vapor and the dry gas, respectively, then for a binary system,

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

Humidity (Specific Humidity)

The special term humidity refers to the mass of water vapor per mass of bone-dry air:

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

Example: The humidity of air at 30°C (86°P) and a total pressure of 750 mm Hg absolute (100 kPa) is 0.0055. Calculate (a) the percent relative humidity, (b) the molal humidity, and (c) the partial pressure of the water vapor in the air. What is the dew point of the air-vapor mixture?

Solution

Data from the steam tables are:

P*H20 at 30°C = 1.253 in. Hg = 31.8 mm Hg = 4.242 kPa

(c) To get the partial pressure of the water vapor in the air, start with the value of the humidity of 0.0055 lb water/lb dry air.

Dr Saad Al-ShahraniChE 201: Introduction to Chemical Engineering

Two-Phase Gas-Liquid Systems (Partial Saturation and Humidity)

from which you can get PH20 = 6.71 mm Hg.

(a) To get the relative humidity,

(b) The molal humidity is