Dehumidification and Humidity control

 

Syllabus:

Basic concepts and definition, wet-bulb and adiabatic saturation temperature, psychrometric chart and measurement of humidity, application of humidity measurement in pharmacy, equipments of dehumidification operations.

 

INTRODUCTION

Humidity is concerned with the physical properties of mixtures of air and water vapor.

Absolute Humidity:

Absolute Humidity is defined as the pounds (lb) of water vapor carried by 1 lb of dry air under any given set of conditions. This quantity is also called the “humidity ratio”

Saturated air

It is the air in which the water vapour is in equilibrium with liquid water at the given temperature and pressure.

In such a mixture, the partial pressure of water-vapor in the water-air mixture is equal to the vapor-pressure of pure water at that temperature.

If H_s is the saturation humidity and  the vapor pressure of the liquid,

                             

Relative humidity, H_R

It is defined as the ratio of the partial pressure of the vapor (p) to the vapor-pressure of the water (p’) (when saturated) at the air-temperature.

It is usually expressed on a percentage basis, so

H_R = 100              Þ saturated air

H_R = 0                   Þ vapor free air

By definition:                   

Percentage humidity

It is the ratio of the actual humidity H to the saturation humidity H_S, at the air temperature, also expressed on a percentage basis, or

Humid heat, Cs

It is the heat energy necessary to increase the temperature of unit mass (1 g or 1 lb or 1kg) of dry air plus whatever vapor it may contain by 1⁰C or 1⁰F at constant pressure. Thus

Cs = C_pA  +  C_pWH

Where C_pA and C_pW  are the specific heats of gas and vapor, respectively (at constant pressure) and is humidity.

Unit of  

C_pA = 0.24  Btu/^oF/lb = 1.0 kJ/kg . K

C_pw = 0.45  Btu/^oF/lb =  1.9 kJ/kg . K           C_s  = 0.24 + 0.45 H

Enthalpy (E) of an air-water vapor mixture is the enthalpy of 1 lb of dry air plus the enthalpy of its accompanying water vapor.           Total enthalpy, E =           sensible heat of the vapor

                                                                     +    the latent heat of the liquid at 32⁰F

                                                                     +    the sensible heat of the dry air.

Unit:      It is expressed as Btu / lb of dry air.

Humid volume, v_H is the total volume of a unit mass of dry air plus whatever vapor it may contain at 1 atm pressure and at air temperature.

Unit:      cm³/g     or           ft³/lb     or           m³/kg

Dew point is the temperature at which a mixture of air and water must be cooled (at constant humidity) in order to become saturated (i.e. to be in equilibrium with liquid water at the dew point).

Unit It has unit of temperature (^oF) or (^oC).

Dry bulb-temperature

It is the temperature of moist air when it is measured at rest by any instrument, which is not affected by the moisture content of air or by radiation.

Unit: ⁰C or ⁰F.

Wet-bulb temperature

When a wet surface is exposed to humid air under adiabatic conditions (i.e. no heat is received or given to environment) the temperature of the surface will reach at equilibrium. This temperature is called wet-bulb temperature.

Theory: A thermometer bulb is covered with a small piece of cloth. The lower end of the cloth is dipped inside a water container. Due to wicking action the water is drawn up and the cloth remains wet. This wet-cloth provides the wet-surface. The temperature of the wet-surface is measured from the thermometer. This thermometer is called wet-bulb thermometer.

Initially the temperature of the wet surface is that of the air (i.e. dry bulb temperature). The water evaporates from the surface and, latent heat is taken, surface is cooled, temperature of the surface becomes lower than the air temperature. Heat flows from air to wet-surface. Temperature of wet-surface is raised. Ultimately a steady state will be reached at which the loss of heat due to evaporation is exactly balanced by the heat passing from the heat from air to wet-surface. Thus the temperature of the wet-surface will become constant. This temperature is called wet-bulb temperature.

HUMIDITY CHART or PSYCHROMETRIC CHART

The humidity characteristics of air are best shown graphically in a psychrometric or humidity chart. Such charts can be found in various handbooks. The basic curves are shown in the humidity chart provided with this class note.

·        Humidity vs. temperature curves: In this chart, humidity (expressed as pounds of moisture per pound of dry air)  is plotted on y-axis and temperature (in ⁰F) is plotted on x-axis under atmospheric pressure. The plots are  curved and each represents a definite humidity value. The following information may be obtained from the chart:

(a)    Any point on the chart represents the temperature and humidity of the sample of air.

(b)   The curve line marked 100% gives the humidity of saturated air at various temperatures.

(c)    To the left of the saturation curve (100% RH line), mixtures of air and water vapor cannot exist.

(d)   Curved lines on the right of the 100% RH line represent various percents of humidity, namely 60%, 40% etc.

Useful for designing air-conditioners.

·        Humid heat vs. Humidity: This line is plotted by taking humidity on the right-hand edge of y-axis and humid heat (BTU per deg F per lb of dry air) along the top of the chart (x-axis).

Useful for designing air-conditioners.

·        Specific volume vs. Temperature: The line for specific volume of dry air (and for the saturated volume) is plotted with temperature on x-axis and cu.ft. per lb of dry air along the left edge of chart on y-axis. The humid volume of a sample of air at a given temperature and humidity can be fiound by linear interpolation to the humidity-temperature curve.

·        Adiabatic cooling lines: The groups of straight lines, which are right side of the saturation curve, are the adiabatic cooling lines.

Used to determine humidity from the knowledge of wet-bulb and dry-bulb temperatures.

These lines shows the changes in humidity during drying under adiabatic conditions.

Measurement of humidity

The humidity of a stream of air may be determined by the following three methods:

Method-1: Dew point method:

If a cooled, polished disk is inserted into an air of unknown humidity and the temperature of the disk gradually lowered, the disk reaches a temperature at which mist condenses on the polished surface. The temperature at which this mist just forms is the temperature of equilibrium between the vapor in the air and the liquid phase. This temperature is therefore the dew point  of that air.

A check on the reading is obtained by slowly increasing the disk temperature and noting the temperature at which the mist just disappears. From the average of the temperatures of mist formation and disappearance, the dew point of that air sample is determined. From the dew-point the humidity can be read from the humidity chart as per the following example:

Let the dew point is 60⁰F. If a vertical line is drawn towards the saturation (100% humidity) line then it will cut at point C. At point C the humidity can be read from the humidity axis (e.g. H₁).]

Method-2: Psychrometric chart method

A very common method of measuring the humidity is to determine simultaneously the wet-bulb temperature and dry-bulb temperatures.

Step-1: The dry-bulb temperature is selected at the x-axis.

Step-2: The adiabatic cooling line corresponding to wet-bulb temperature is selected.

Step-3: The vertical line from the dry-bulb temperature on the x-axis intersects the adiabatic cooling line (corresponding to the wet-bulb temperature). From this intersection point the humidity is determined from the right-hand y-axis.

For example let us assume a wet-bulb temperature of 54⁰F and a dry-bulb temperature of 60⁰F.

Step-1: Dry bulb temperature of 60⁰F is selected on the bottom x-axis.

Step-2: Adiabatic cooling line corresponding to 54⁰F is selected.

Step-3: The vertical line from 60⁰F and the adiabatic cooling line of 54⁰F intersects at point F. The humidity H3 corresponding to point F is determined from the right hand y-axis.

Direct methods.

The vapour content of a gas can be determined by direct analysis, in which a known volume (V) of gas is drawn through a desiccating agent.

Weight of the water vapor in the air sample (w)

= Final weight of the desiccating agent – Initial weight of the desiccating agent.

Therefore, humidity =

APPLICATION OF HUMIDITY IN PHARMACY

1. Preservations of pharmaceuticals:

The effect of humidity on the materials is as follows:

(a)    Hygroscopic materials have a tendency of absorbing moisture. E.g. digitalis leaves, sodium sulfate, glycerin, ethanol and concentrated sulphuric acid.

(b)   Deliquescent materials have a natural tendency to absorb water and itself dissolves in it. E.g. calcium chloride, potassium carbonate.

(c)    Efflorescent materials have a natural tendency to loose water. E.g. borax, caffeine, quinidine sulfate, etc.

(d)   Some substances absorb moisture and undergo chemical degradation such as hydrolysis when they are exposed to high humidity.

(e)    Physical stability of drugs and dosage forms may be affected. E.g. gelatin capsules absorbs moisture and become soft and sticky. Suppositories and creams become liquid.

(f)    In low humidity condition hard gelatin capsules loose water and become dry and brittle.

2. Evaluation of dosage forms:

For determining stability of a product it has to be kept in a humidity chamber of controlled humidity.The test chamber offers reproducible temperature and humidity conditions.

3. Processing conditions:

In the granulation section, air-conditions are 45% RH at 22⁰C. In the tablet-punching section the air condition is less than 20% RH and 22⁰C. Without dry condition the tablets may stick to the die and punch surface. This problem will aggravate if some hygroscopic material like sugar is added to the formulation.

During capsule manufacturing the hard gelatin capsules may stick to machine parts in the humidity level is high. So the humidity of the manufacturing zone is kept at a controlled level.

In soft gelatin processing area the condition is 40% RH at 22⁰C. In drying area the condition is 30% RH at 20⁰C.

4. Maintenance of animals and equipments:

The animal house should be air-conditioned to keep the metabolic rate, rate of perspiration and temperature of the body of the animals under control.

Sophisticated analytical instruments line UV-Visible Spectrophotometer and IR- Spectrophotometer should be kept in very dry condition for correct result.

DEHUMIDIFIERS

Dehumidification means removal of humidity or moisture from the air.

Method-1:

Sometimes warm saturated air is required to be dehumidified for instance, dehumidifying the air discharged from a dryer so that it may be reused.

The moist air is brought in contact to cold liquid or surface. The temperature of the cold surface will be below the dew point of the air. At low temperature the air will be saturated and the extra water will be condensed as liquid. The liquid is removed. After dehumidification the gas can be reheated to its original dry-bulb temperature.

 

Instrument-1

Condensation on a cold surface with no water spray. This is accomplished by passing a cold fluid through the inside of finned tubes arranged in banks through which air is blown. The outside surface of the metal tubes must be below the dew point so that water will condense out of the air. Fig-1.

Instrument-2

The moist air is passed through cold water sprays and is adiabatically cooled and humidified. The pumps that takes water from the reservoir below the sprays and discharges back to the sprays may also deliver water to a pipe T that is slotted so as to give a curtain of water to eliminate most of the entrained water-droplets before the air enters a set of mist eliminator baffles P, where the last entrained water is removed. In the pump discharge there is a heater to adjust the air temperature.

A fan R draws the air through the apparatus and discharges it to the point of use.

Fig. 1

Fig.2

Method-2

Humidity can be reduced by compressing the air. During compression the partial pressure of the vapor increases. As soon as it reaches the saturation value, condensation takes place. The water gets liquefied and drained off. The air remained becomes dehumidified.

Method-3

Moist air is passed over a solid absorbent surface (e.g. anhydrous silica gel, activated alumina etc.) or through a liquid absorbent (e.g. brine i.e. concentrated NaCl solution, concentrated solution of lithium chloride, ethylene glycol etc.). The water is absorbed in the absorbent the dry air is released from the instrument.

The moist absorbent materials may be regenerated by drying.

DOWN LOAD---ORIGINALL-----DEHUMODIFICATION  AND HUMIDITY CONTROL 

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