Humidification of gases

Humidification is the process of increasing the amount of vapor present in a gas stream. It is a process in which the vapor content of a gas stream is enriched when the gas is passed over the liquid. This process takes place by diffusion. At a point of transfer, the gas vapor temperature of the liquid content is at equilibrium with the saturated gas vapor.

Also, the process in which the amount of vapor is reduced is known as dehumidification. These processes are applied in water air systems; Such as   air conditioning and gas drying systems. This is of necessity in the industry for the production of materials. Examples include: dehumidification of chlorine in order to reduce the corrosion rate in steel equipment; the compound used in the production of sulphuric acid (contact process) is removed of vapor content before it is introduced to the converter.

Definition of humidification terms

Saturation humidity: this is the humidity of a gas at a particular temperature of saturation state. It is given as

Hs= MA: MB(PA/P-PAo)

Where MA and MB are molecular weight of the component mixture

PoA and P are the saturated partial vapor pressure of the system

Relative humidity: this is the ratio of partial pressure of vapor to the saturated vapor pressure of the liquid at the temperature of the gas.

HR = (100) PA/PAo

Percentage humidity: this is the ratio of the actual humidity of the gas to the saturation humidity at the particular gas temperature.

%H = (100)H/Hs = (Relative humidity) P- PAo/P-PA

Humid heat: this is the heat energy required to raise one gram of or pound of dry gas and it’s vapor content by one degree and one atmosphere.   S= Cpa+ (H)Cpb

Humid volume: this is the volume occupied by a unit mass of dry gas and vapor at standard atmosphere and gas temperature. It is given by

VH =0.0224T/273 (1/MB +H/MA)

T is measured in Kelvin while humid volume is M3 per gram.

Dew point: this is temperature at which a gas is saturated with vapor; this is at constant humidity. Dew point is the temperature when a cooling vapor drops it’s first liquid. (Vapor mole fraction reduces while liquid mole fraction increases).

Measurement of humidity

Measurements of humidity are various approaches humidity of a gas can be measured. They are listed thus:

  • Direct method: the humidity of a gas can be measured directly by knowing the volume of the gas. This is determined by using the appropriate analytical device.
  • Dew point method: the humidity of a gas can be determined using dew point through the cooling of a system in reference to another material. Example; when a cold plate is placed in a liquid or gas of unknown humidity; through temperature relationship between the plate and the liquid at equilibrium.
  • Psychometric chart: this is the most common way of determining humidity. This is done by determining wet bulb temperature and dry bulb temperature/ tracing the appropriate line on the chart in order to ascertain your reading. This chart is used in air water system. The chart has the following properties:

  1. Humidity
  2. Specific volume of dry gas
  3. The saturated volume
  4. The latent heat of vaporization
  • Measurement of heat of absorption on a surface method
  • Electrolytic hygrometry method
  • Capacitance method
  • Chemical bond method



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