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    Drying: Dr. Khaled Ben Issa

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    asmam3702

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    Drying: Dr. Khaled Ben Issa

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    asmam3702
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    © All Rights Reserved
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    7 views24 pages

    Drying: Dr. Khaled Ben Issa

    Uploaded by

    asmam3702

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    of 24

    Drying

    Dr. Khaled Ben Issa


     Definition

     Objectives

     Theory of drying

     Humidity and humidity measurement

     Drying of wet solids

     Classification of dryer

     Batch or continuous drying

     Normal or vacuum drying

     Static or moving bed dryers

     Convective, conductive or radiation drying


     Equipment's:

    Tray and truck dryers,

    tunnel and convey dryers,

    rotary or tumbler dryer,

    vertical or horizontal fluidized bed dryer,

    vacuum tumbling dryer,

    dryer for dilute solutions and suspensions,

    drum dryer, spray dryer, flash dryer, freeze drying.

     Factors affecting the drying rate


    Introduction
     Drying is an important operation in primary pharmaceutical
    manufacture (i.e. the synthesis of active pharmaceutical
    ingredients or excipients).

     It is usually the last stage of manufacturing before


    packaging.

     Drying is defined as the removal of all or most of the liquid


    associated with a wet pharmaceutical product.

     A process in which the liquid is removed from a material by


    application of heat & is accomplished by the transfer of
    liquid from a surface into an unsaturated vapour phase.
    NEED OF DRYING

     To make products stable.

     Preparation of powder and granules.

     Reducing the bulk and weight.

     Dried before extraction to facilitate grinding and to


    avoid deterioration.

     Preparation of certain products like dried Aluminum


    hydroxide, spray dried lactose & powder extracts.
    Objective

     Drying APIs is an important operation for the production


    of consistent, stable, free-flowing materials for
    formulation, packaging, storage and transport
     Drying can be described by three processes operating
    simultaneously:

    1. Energy transfer from an external source to the water or


    organic solvent

     Direct or Indirect Heat Transfer

    2. Phase transformation of water/solvent from a liquid- like


    state to a vapour state

     Mass Transfer (solid characteristics)

    3. Transfer vapour generated away from the API and out of


    the drying equipment
     Drying to the ‘dry’ solid state, starting with either a wet
    solid or a solution or suspension.

     The former is usually achieved by exposing the wet solid


    to moving, relatively dry air (elevated temperatures to
    accelerate the process are common).

     The latter is possible with equipment such as the spray


    dryer that is capable of producing a dry product from a
    solution or suspension in one operation.
     Most pharmaceutical materials are not completely free
    from moisture (i.e. they are not ‘bone dry’)

     They contain some residual water, the amount of which


    may vary with the temperature and humidity of the
    ambient air to which they are exposed.

     All drying processes of relevance to pharmaceutical


    manufacturing involve evaporation or sublimation of the
    liquid phase and the removal of the subsequent vapour.
    The process must provide the latent heat for these
    processes without a significant temperature rise.

     Naturally the temperature rise will enhance the potential of


    thermal degradation of the product.
     In the majority of cases the ‘liquid’ will be water but also
    more volatile organic solvents, such as isopropanol, may
    need to be removed in a drying process.

     volatile solvents are normally recovered by condensation


    rather than being vented into the atmosphere.

     This is for environmental and economic, the toxicity and


    flammability of organic solvents pose additional safety and
    process considerations.
    Drying of wet solids Fundamental
    properties and interrelationships

     Moisture content of wet solids

    The moisture content of a wet solid is expressed as kg of


    moisture associated with 1 kg of the moisture free or ‘bone-dry’
    solid.

    Thus, a moisture content of 0.4 means that:

    0.4 kg of water is present per kg of the ‘bone-dry’ solid that will


    remain after complete drying.

    It is sometimes calculated as percentage moisture content;


    thus this example would be quoted as 40% moisture content.
    Total moisture content

     It is the total amount of liquid associated with a wet solid.

     Some of water can be easily removed by the simple


    evaporative processes and some cannot.

     The amount of easily removable water ( unbound water )


    is known as the free moisture content.

     The moisture content of the water that is more difficult to


    remove in practice ( bound water ) is the equilibrium
    moisture content.
    Unbound water

     The unbound water associated with a wet solid exists as a


    liquid and it exerts its full vapour pressure. It can be
    removed readily by evaporation.

     Unbound water (During a drying process) is readily lost


    but the resulting solid will not be completely free from
    water molecules as it remains in contact with
    atmospheric air that inevitably contains dissolved water.

     Resulting solid is often known as air dry


    Equilibrium moisture content

     The moisture content of a solid under steady-state


    ambient conditions.

     Its value will change with the temperature and humidity


    of the air, and with the nature of the solid.
    Bound water.

     Part of the moisture present in a wet solid may be adsorbed


    on surfaces of the solid or be absorbed within its structure to
    such an extent that it is prevented from developing its full
    vapour pressure and therefore from being easily removed
    by evaporation.

     It is more difficult to remove than unbound water.

     Adsorbed water is attached to the surface of the solid as


    individual water molecules which may form a mono(or bi-)
    layer on the solid surface.

     Absorbed bound water exists as a liquid but is trapped in


    capillaries within the solid by surface tension.
    Moisture content (MC).

     The moisture content of air is expressed as kg of water


    per kg of ‘bone-dry’ (water-free) dry air.

     An added complication to the drying process.

     Drying air also contains moisture.

     Many pharmaceutical have air-conditioning systems to


    reduce the humidity of the incoming process air

     Removing water from air is a very expensive process


    MC

    The moisture content of air is altered by changes:

     Temperature

     The amount of moisture taken up by the air


    Relative humidity (RH)

     Ambient air is a simple solution of water in a mixture of


    gases and as such follows the rules of most solutions –
    such as increased water solubility with increasing
    temperature, a maximum solubility at a particular
    temperature (saturation) and precipitation of the solute
    on cooling (condensation, rain!).
    • Lower relative humidities can be quantified in terms
    of percentage relative humidity

    • This is approximately equal to the percentage saturation,


    which is:
    Percentage saturation
     It is the more fundamental measure but the expression
    ‘relative humidity’ is most commonly used.

     This is because the amount of water required to saturate air


    is itself dependent on temperature.

     In ambient air, water is in solution in the air gases and in this


    case its solubility increases with increasing temperature.

     If the temperature of the air is raised whilst its moisture


    content remains constant, the air will theoretically be
    capable of taking up more moisture and therefore its
    relative humidity falls.

     Difference between moisture content and relative humidity


    of air
    An additional complication during a drying
    process
     Both the temperature and moisture content of the drying
    air (and therefore its relative humidity) could change
    significantly.

     This arises from two separate factors:

    • Uptake into the drying air of evaporated water vapour from


    the drying solid. If evaporation is high and vapour removal
    inefficient, the drying efficiency will rapidly fall

    • The cooling of the supply air (and consequently the


    product) as the air transfers latent heat to the wet solid.

    This phenomenon is known as evaporative cooling.


     If the cooling is excessive the temperature of the air may
    fall to a value known as the dew point.

     Here the solubility of water in the cooler air is reduced to


    such a point that it is exceeded and liquid water will
    condense and be deposited.
    Thank You For Your Attention !

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