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Drying

Assoc.Prof.Jomjai Peerapattana

Faculty of Pharmaceutical Sciences

Khon Kaen University13/10/58 2

Scope

DefinitionPurposeTheory of dryingDrying of solids Loss on drying Moisture content Behavior of solids during drying

Classification of dryers Static bed dryers

Tray and truck dryers Tunnel and conveyor dryers

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Moving bed systems Turbo tray dryers

Pan dryers

Fluidized bed systems

Pneumatic systems Spray dryers

Specialized drying methods Freeze dryers

Microwave drying

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Definition

Removal of liquid from material by application of heat, by transfer of liquid from surface into unsaturated vapor phase

Removal of small amount of water from moisture-bearing table salt as well as to recovery of salt from sea by evaporation

Drying and evaporation are distinguishable by relative quantities of liquid removed from the solid

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Nonthermal methods of drying expression of solid to remove liquid (squeezing of a

wetted sponge)

extraction of liquid from solid by use of solvent

adsorption of water from solvent by use of desiccants (such as anhydrous CaCl2)

absorption of moisture from gases by passage through sulfuric acid column

desiccation of moisture from solid by placing it in sealed container with moisture-removing material (silica gel in bottle)

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Purpose

Unit process in preparation of granules

Processing of materials, e.g., preparation of dried aluminum hydroxide, spray drying of lactose, and preparation of powdered extracts

Reduce bulk and weight, lowering cost of transportation and storage

Preservation of animal and vegetable drugs by minimizing mold and bacterial growth in moisture-laden material

Facilitating comminution, dried substance more friable than original

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Theory of Drying

Drying involves both heat and mass transferoperations

Heat transferred to material to supply latent heat required for vaporization of moisture

Mass transfer involved in diffusion of water through material to surface, evaporation of water from surface, and diffusion of vapor into passing air stream

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Rate of evaporation of liquid film at material surface related to rate of heat transfer

Driving force is humidity differential, for heat transfer, it is temp differential

Coefficient of mass transfer is varies with velocity of air stream passing over surface

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Rate of drying may be accelerated by

Increasing air flow rate and raising inlet air temp

Introducing a high-temp radiating heat source into drying chamber

Reducing thickness of material and allowing it to come in contact with raised-temp surfaces

Increasing air velocity

Dehumidifying inlet air

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Foregoing discussion holds true as long as there is a film of moisture on surface of material

When surface becomes partially or completely dry, heat and mass transfer equations become more complex

Rate of drying is controlled by rate of diffusion of moisture from interior of material

This diffusion is greatly influenced by molecular and capillary structure of solid

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When drying surface causes a shrinkage of solid, process becomes complicated This phenomenon can cause blocking and distortion

of capillary structure and interfere transfer of internal water to material surface

This is so-called “case hardening” phenomenon, in which solid surface becomes harder than interior and less permeable to transmission of interior moisture

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Drying of Solids

Loss on Drying (LOD) Moisture in a solid can be expressed on wet or dry-

weight basis

On wet-weight basis, water content is calculated as percentage of weight of wet solid

On dry-weight basis, water is expressed as percentage of weight of dry solid

In pharmacy, LOD, is expression of MC on a wet-weight basis

% LOD = (wt of water in sample/total wt of wet sample) x 100 (6)

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Moisture Content (MC) Calculated on dry-weight basis

Referred to as MC :

% MC = (wt of water in sample/wt of dry sample) x 100 (7)

5 g of moist solid is brought to constant dry weight of 3 g :

MC = (5-3)/3 x 100 = 66.7%

LOD = (5-3)/5 x 100 = 40%

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LOD values can vary from slightly above 0% to slightly below 100%

MC values can change from slightly above 0% and approach infinity

Small change in LOD value, from 80% to 83%, represents and increase in MC of 88%, or a 22% increase in the amount of water that must be evaporated per pound of dry product

% MC is far more realistic value than LOD in determination of dryer load capacity

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Behavior of Solids During Drying

Rate of drying can be determined by suspending wet material on a balance in drying cabinet and measuring weight of sample as it dries as a function of time

Plotted drying rate as MC vs time

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The changes may be easily seen if plotted rate of drying against MC

When a wet solid is first placed in drying oven, it begins to absorb heat and increases in temp

At the same time, moisture begins evaporating and thus tends to cool the drying solid

After a period of initial adjustment, rates of heating and cooling become equal and temp of the material stabilizes

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This period of initial adjustment is shown as segment AB

At point B, temp is stabilized and remains constant as long as there is a film of moisture remaining at surface of solid

Between points B and C, moisture evaporating from surface is replaced by water diffusing from interior of solid at a rate equal to rate of evaporation

Rate of drying is constant, and time BC is constant rate period

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At point C, surface water is no longer replaced at a rate fast enough to maintain continuous film

Dry spots appear, and rate of drying begins to fall off

MC at this point is CMC

Between points C and D, number and area of dry spots continue to grow, and rate of drying falls steadily

Time CD is first falling rate period or period of unsaturated surface drying

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At point D, film of surface water is completely evaporated, and rate of drying depends on rate of diffusion of moisture to solid surface

Point D is second critical point

Between points D and E, rate of drying falls more rapidly than first falling rate, and time DE is called second falling rate period

When drying rate equal to zero, starting at point E, EM period begins, and solid is in equilibrium with its surroundings, i.e., its temp and MC remain constant

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Continued drying after this point is waste of time and energy

Equilibrium Moisture Content (EMC) MC of material that is in equilibrium with

atmosphere

MC at which material exerts water vapor pressure equal to vapor pressure of atmosphere; no driving force for mass transfer

EMC values of various materials may differ greatly under same conditions, despite that they are in equilibrium with their environment

These differences are due to the manner in which water is held by material

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Equilibrium Relative Humidity (ERH)

RH surrounding material at which material neither gains nor loses moisture is called ERH

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Classification of Dryers

Two useful classifications are based on either method of heat transfer or method of solids handling

Type of heat transfer is important in demonstrating gross differences in dryer design, operation, & energy requirements

Method of solids handling is suitable when special attention must be given to nature of material to be dried

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major criterion is presence or absence of agitation

Excessive agitation is contraindicated when the dried material is friable and subject to attrition

drying time can be reduced, if dried product is intended to be pulverized, and process made more efficient, by use of a dryer that produces intense agitation during drying cycle

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Classification based on method of solids handling

Static-bed dryers

No relative movement among solid particles,

May be bulk motion of entire mass

Only fraction of total number of particles is directly exposed to heat sources

Exposed surface can be increased by decreasing thickness of the bed and allowing drying air to flow through it

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Static-Bed Systems

Tray and Truck Dryers

Most commonly used in pharmaceutical plant operations

Called shelf, cabinet, or compartment dryers

Cabinet in which material is spread on tiers of trays

Number of trays varies with size of dryer

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Truck dryer: trays are loaded on trucks (racks equipped with wheels), which can be rolled into and out of drying cabinet

In plant operations, truck dryer is preferred over tray dryer because of greater convenience in loading and unloading

Batch procedure

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Batch drying is used extensively in manufacture of pharmaceuticals: Each batch of material can be handled as separate entity

Batch sizes of pharmaceutical industry are small (500 or less pounds per batch) compared with chemical industry (2000 or more pounds per hour)

Same equipment is readily adjusted for use in drying a wide variety of materials

Tray dryers may be classified as direct or indirect

Most tray dryers are of direct type, heating is accomplished by forced circulation of large volumes of heated air

Indirect tray dryers utilize heated shelves or radiant heat sources inside drying chamber to evaporate moisture, which is then removed by either a vacuum pump or small amount of circulated gas

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Tunnel and Conveyor Dryers

Tunnel dryers are adaptations of truck dryer

Trucks are moved progressively through drying tunnel by moving chain

Trucks are loaded on one side of dryer, allowed to reside in heating chamber for a time sufficiently long to effect desired drying, and then discharged at exit

Operation may be described as semicontinuous, because each truck requires individual loading and unloading before and after drying cycle

Conveyor dryers are an improvement over tunnel dryers because they are truly continuous

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Individual trucks of the tunnel are replaced with an endless belt or screen that carries wet material through drying tunnel

Moving-bed dryers Drying particles are partially separated so that they

flow over each other

Motion may be induced by gravity or mechanical agitation

The resultant separation of particles and continuous exposure of new surfaces allow more rapid heat and mass transfer than in static beds

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Pan Dryers Moving-bed dryers of indirect type

Operate under atmospheric pressure or vacuum

Generally used to dry small batches of pastes or slurries

Shallow, circular jacketed pan with flat bottom and vertical sides

Heat is supplied by steam or hot water

Set of rotating plows in the pan that revolve slowly, scraping moisture-laden mass from the walls and exposing new surfaces to contact with heated sides and bottom

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Atmospheric pan drying allows moisture to escape, whereas in vacuum dryers, solvents are recoverable if evacuated vapors are passed through condenser

Dried material is discharged through a door on bottom of pan

Fluidized-bed dryers Solid particles are partially suspended in an upward-

moving gas stream

Particles are lifted and fall back in a random manner so that the mixture of solid and gas acts like a boiling liquid

Gas-solid contact is excellent and results in better heat and mass transfer than in static and moving beds

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Fluidized-Bed Systems Gas is allowed to flow upward through a bed

of particulate solids at a velocity greater than velocity of particles and less than velocity for pneumatic conveying, solids are buoyed up and become partially suspended in gas stream

Mixture of solids and gas behaves like liquid, and solids are said to be fluidized

Solid particles are continually caught up in eddies and fall back in a random boiling motion

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Efficient for drying of granular solids, because each particle is completely surrounded by drying gas

Intense mixing between solids and gas results in uniform conditions of temp, composition, particle size distribution

Only requirements are granules are not so wet that they stick together on drying, and dried product is not so friable as to produce excessive amounts of fine particles through attrition

Twofold to sixfold advantage in thermal efficiency over tray dryer

Faster in drying and handling time than tray dryer

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Types of Fluidized-Bed Dryers Vertical and horizontal

Fluidizing air stream is induced by a fan mounted in upper part of apparatus

Air is heated to required temp and flows upward through wet material, which is in drying chamber fitted with a wire mesh support at bottom

Bag collector filter is at the top of drying chamber to prevent carryover of fine particles

The unit is batch-type dryer, and drying chamber is removed from the unit to permit charging and dumping

Dryer capacities: 5 kg to 200 kg and average drying time is 20 to 40 min

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Because of the short drying time and excellent mixing action of the dryer, no hot spots are produced, and higher drying temps can be employed than are used in conventional tray and truck dryers

Designed for direct preparation of tablet granulations,drying of conventionally produced wet granulations

As a granulator, dry ingredients are placed in chamber and fluidized while granulating liquid is sprayed into the bed, causing particles to agglomerate into granules

At the end of granulating cycle, granules are dried by heating the fluidizing air

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A continuous dryer is more suitable than a batch type for the drying of larger volumes of materials

A fluidized-bed dryer of this type, is a horizontal vibrating conveyor dryer

The heated air is introduced into a chamber below the vibrating conveying deck and passes up through the perforated or louvered conveying surface, through the fluidized bed of solids, and into an exhaust hood

A fluidized bed of uniform density and thickness is maintained in any given drying zone by the vibration

Residence time in any drying zone is controlled by the length of the zone, the frequency and amplitude of the vibration, and the use of dams

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Dryer can be divided into several different zones with independent control of airflow and temp, so that drying can take place at the maximum desirable rate in each stage without sacrificing efficiency or damaging heat-sensitive materials

Dryer capacity is limited only by retention time produced by conveying speeds

In pharmaceutical operations, capacities range as high as 1 to 2 tons per hour.

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Pneumatic dryer

Drying particles are entrained and conveyed in high-velocity gas stream

Pneumatic systems further improve on fluidized beds

Each particle is completely surrounded by an envelope of drying gas

Heat and mass transfer are extremely rapid; drying times are short

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Pneumatic Systems

Spray Dryers

Handle only fluid materials such as solutions, slurries, and thin pastes

Fluid is dispersed as fine droplets into a moving stream of hot gas, where they evaporate rapidly before reaching the wall of drying chamber

The product dries into a fine powder, which is carried by the gas current and gravity flow into a collection system

When the liquid droplets come into contact with hot gas, they quickly reach a temp slightly above temp of the gas

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Surface liquid is quickly evaporated, and a tough shell of solids may form

As drying proceeds, liquid in interior of droplet must diffuse through this shell

Diffusion of liquid occurs at much slower rate than transfer of heat through the shell to interior of droplet

The resultant buildup of heat causes the liquid below the shell to evaporate at greater rate than it can diffuse to surface

The internal pressure causes the droplet to swell, and the shell becomes thinner, allowing faster diffusion

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If the shell is nonelastic or impermeable, it ruptures, producing either fragments or budlike forms on the original sphere

Thus, spray-dried material consists of intact spheres, spheres with buds, ruptured hollow spheres, or sphere fragments

Rate of feed is adjusted so that each droplet of sprayed liquid is completely dried before it comes in contact with the walls of drying chamber, and the dried powder is not overheated in the drying process

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Proper feed rate is determined by observation of outlet air temp and visual inspection of the walls of drying chamber

If inlet air temp is kept constant, a drop in liquid feed rate is reflected by a rise in outlet temp

Excessive feed rates produce a lowering of outlet temp, and ultimately, a buildup of material on the walls of chamber

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Separation of solid product from effluent gas is by means of cyclone separator (as primary collector) cyclone product

Product that reaches the walls of drying chamber, chamber product, is removed at bottom of chamber coarser in size and subjected to heat longer (because of

increased retention time) than cyclone product

Final dried product is a mixture of both chamber and cyclone products

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Spray Drying and Spray Congealing of Pharmaceuticals

Spray drying finds great utility in pharmaceutical industry because of rapidity of drying and unique form of final product

Three major uses : drying heat-sensitive materials

changing the physical form of materials for use in tablet and capsule manufacture

encapsulating solid and liquid particles

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Specialized Drying Methods

Freeze Dryers Many products lose their viability in liquid state and

readily deteriorate if dried in air at normal atmospheric pressures

These materials may be heat-sensitive or may react with oxygen, in order to be stabilized, they must be dehydrated to a solid state

Material is first frozen and then subjected under a high vacuum to heat (supplied by conduction or radiation, or both) so that frozen liquid sublimes leaving only solid, dried components

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Blood serum, plasma, antibiotics, hormones, bacterial cultures, vaccines, and many foodstuffs are dehydrated by freeze drying, also referred to as lyophilization, gelsiccation or drying by sublimation

Dried product can be readily redissolved or resuspended by addition of water prior to use, reconstitution

Freeze drying depends on phenomenon of sublimation, whereby water passes directly from solid state (ice) to vapor state without passing through liquid state

Schematic pressure-temp diagram for water, sublimation can take place at pressures and temps below triple point, 4.579 mm Hg absolute (4579 microns) and 0.0099°C

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The water contains dissolved solids, resulting in a different pressure-temp relationship for each solute pressure and temp at which the frozen solid vaporizes

without conversion to liquid is referred to as eutectic point

Freeze drying is carried out at temps and pressures well below this point to prevent frozen water from melting, which would result in frothing, as liquid and frozen solid vaporize simultaneously

In actual practice, freeze drying is carried out at temps of - 10°C to - 40°C, and at pressures of 2000 to 100 microns

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Microwave Drying.

The application of microwave energy to the drying of solids represents a radical departure from conventional means of drying

Instead of applying heat externally to a material, energy in form of microwaves is converted into internal heat by interaction with material itself

This permits extremely rapid heat transfer throughout material, which in turn can lead to rapid drying

The heating effect is produced by interaction of a rapidly oscillating electric field (915 or 2450 megahertz) with polarized molecules and ions in material

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The field imposes order on otherwise randomly oriented molecules

As the field reverses polarity, it relaxes and allows molecules to return to their random orientation, giving up stored potential energy as random kinetic energy, or heat

Interaction of alternating field with ions causes billiard ball-like collisions with un-ionized molecules, and impact energy is converted into heat

In microwave drying, mass transfer is primarily result of a pressure gradient due to rapid vapor generation inside material, that is, most of internal moisture is vaporized before leaving the sample

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Thus, moisture is mobilized as vapor rather than liquid, and its movement to the surface can be extremely rapid because it does not depend on mass concentration gradients or on slow liquid diffusion rates

Industrial microwave dryers are usually of static bed continuous type

Materials are placed on conveyor belts and conveyed through the microwave applicator

Generally, a stream of hot air is used simultaneously with microwaves to sweep away moisture evolving from surface of material

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Often, the microwave treatment is used in the last stages of hot air drying (second falling rate period) to remove last remaining portion of solvent, reducing total drying time by 50% or more

Microwave drying can be used for drying of pharmaceutical materials at low ambient temps, avoiding high surface temps, case hardening, and solute migration.

Microwave vacuum drying at low pressure (1 to 20 mm Hg) and moderate temp (30 to 40°C) can be used

for drying thermolabile materials such as vitamins, enzymes, proteins, and flavors

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The rising cost of energy has generated a great deal of interest in microwave drying

microwaves couple directly into solvent, and no energy is used to heat air, walls of dryer, conveyor, or trays extremely efficient energy utilization, and energy savings of

as much as 70% have been realized in industrial installations

References

The theory and practice of industrial pharmacy. Third edition. Chapter 3 drying.

Pharmaceutics the science of dosage form design. Chapter 38 drying.

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