Partial Saturation and Humidity

The fluid in a large tank caught on fire 40 minutes after the start of a blending operation in which one grade of naphtha was being added to another. The fire was soon put out and the naphtha was moved to another tank. The next day blending was resumed in the second tank 40 minutes later another fire started. 

Can you explain the reason for this sequence of events What might be done to prevent such incidents  

Your objectives in studying this section are to be able to  

Define relative saturation (humidity), molal saturation (humidity), absolute saturation (humidity), and humidity by formulas involving partial pressures of the gas components. 

Given the partial saturation in one form, calculate the corresponding values in the other three forms as well as the dew point. 

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When the partial pressure p of water vapor in the air at a given temperature equals the vapor pressure of water p, at the same temperature, the air is saturated and the absolute humidity is designated the saturation humidity H,. 

An important variable that determines the size of a given desiccant bed is the relative saturation of the inlet gas. This variable is the driving force that affects the transfer of water to the adsorbent. If saturated gas (100% relative humidity) is being dried, higher useful capacities can be expected for most desiccants than when drying partially saturated gases. However, in most field gas dehydration installations the inlet gas is samrated with water vapor and this is not a variable that must be considered. 

If equilibrium has not been reached between a mixture of components, the condition is referred to as partial saturation. At partial saturation the gas mixture obeys real gas laws. There are several ways to express the concentration of a vapor in a mixture of gases. Most often, weight or mole fraction is used. Other definitions are relative saturation (relative humidity), molal saturation (molal humidity) and absolute saturation (absolute humidity). 

At saturation with resin 1 fully swollen, the relative humidity P /Pq) is equal to unity whilst the reference resin at the same specific water content cannot be at saturation and exhibits a lower partial pressure or relative humidity. Hence the swelling pressure is found from the relationship ... 

Calculate the absolute humidity, relative saturation, and dew-point temperature of a mixture of benzene-nitrogen at a pressure of 1.5 bar and temperature of 320 K if the partial pressure of benzene in the mixture is 0.2 bar. 

Knowing the saturation and the partial water vapour pressure at each time point and place, the relative humidity RH can be calculated using the following equation ... 

Gas pressures and volumes can be measured for many different temperature and humidity conditions. Three of these are body temperature and pressure, saturated (BTPS) ambient temperature and pressure (ATP) and standard temperature and pressure, dry (STPD). To calculate constituent partial pressures at STPD, total pressure is taken as barometric pressure minus vapor pressure of water in the atmosphere ... 

When the partial pressure of the vapor in the gas equals the vapor pressure of the liquid, an equilibrium is reached and the gas is said to be saturated with vapor. The ideal saturated absolute humidity is then... 

When the mass of dry gas in the vapor-gas mixture is multiplied by the humid volume, the volume of the vapor-gas mixture is obtained. The humid volume at saturation is computed with Y = Fs, and the specific volume of the dry gas can be obtained by substituting F = 0. For partially saturated mixtures, may be interpolated between values for 0% and 100% saturation at the same temperature and pressure. 

Take, for example, the case of a water-soluble substance. If the partial pressure of water vapour in the atmosphere is greater than the vapour pressure that would be exerted by a saturated aqueous solution of the pure substance at that temperature, water will be absorbed by the crystals. If, later, the atmospheric moisture content falls to give a partial pressure below the vapour pressure of the saturated solution, the crystals will dry out and bind together. Small fluctuations in atmospheric temperature and humidity, sufficient to bring about these changes, can occur several times in one day. 

Saturation density of water vapour in air at 20 °C is 17.27 g/m. A measured absolute humidity of 9.39 g/m for example would correspond to a relative humidity of 54 % at this temperature. At 10 °C the saturation content is only 9.39 g/m, and this absolute humidity would result in a relative humidity of 100 %. Figure 9.3 shows the connection between water vapour partial pressure, relative humidity and temperature. 

Humidity is a parameter that helps describe the water content in a gas and water mixture. There are many different methods to express humidity absolute humidity, RH, dew point temperature, or mixing ratio. In PEM fuel cell technology, RH is commonly used. The RH is a measure of the amount of water vapor in the air relative to the maximum amount of water vapor that air can hold at a specific temperature, and is usually expressed as a percentage value. The RH at temperature T (RH(T)) is defined as the ratio of the partial pressure and the saturated vapor pressure of water, and can be expressed as in Eqn (8.1) [12] ... 

A gas in this context is referred to as saturated or 100% relative humidity, when the vapor pressure of water in the air is at the equilibrium with vapor pressure due to (liquid) water water (or ice, if cool enough) will fail to lose mass through evaporation when exposed to saturated air. Because the amount of water vapor in air is small, relative humidity, the ratio of the partial pressure due to the water vapor to the saturated partial vapor pressure, is much more useful. Water vapor pressure above 100% relative humidity is called super-saturated and can occur if air is rapidly cooled, for example, by rising suddenly in an updraft. 

Relative Humidity (rh). Relative humidity is the ratio of the mole fraction of water vapor present in the air to the mole fraction of water vapor present in saturated air at the same temperature and barometric pressure it approximately equals the ratio of the partial pressure (or density) of the water vapor in the air to the saturation pressure (or density) of water vapor at the same temperature. 

Relative humidity is usually considered only in connection with atmospheric air, but since it is unconcerned with the nature of any other components or the total mixture pressure, the term is applicable to vapor content in any problem. The saturated water vapor pressure at a given temperature is always known from steam tables or charts. It is the existing partial vapor pressure which is desired and therefore calculable when the relative humidity is stated. 

When (P - Pwq)/(P — Pw) 1, the percentage relative humidity and the percentage humidity are equal. This condition is approached when the partial pressure of the vapour is only a small proportion of the total pressure or when the gas is almost saturated, that is as Pw -> PiM. 

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