DRYING is an important operation in DRYING is an important operation in pharmaceutical practice, since it is pharmaceutical practice, since it is commonly the last stage of the process commonly the last stage of the process before packaging and has a considerable before packaging and has a considerable effect on the properties of the product.effect on the properties of the product.

Correct drying can prevent deterioration Correct drying can prevent deterioration or can ensure that a product is readily or can ensure that a product is readily soluble or is freesoluble or is free­­flowing.flowing.

Drying is defined as the final removal of Drying is defined as the final removal of liquid from solids by thermal liquid from solids by thermal vaporization, leaving a `dry' solid. vaporization, leaving a `dry' solid.

Dryers for Dilute Solutions Dryers for Dilute Solutions and Suspensionsand Suspensions

The objective of these dryers is to The objective of these dryers is to spread the liquid to a large surface area spread the liquid to a large surface area for heat and mass transfer and to for heat and mass transfer and to provide an effective means of collecting provide an effective means of collecting the dry solid. the dry solid.

Two main types are used, the first Two main types are used, the first spreading the liquid to a thin film spreading the liquid to a thin film and the second and the second dispersing the liquid dispersing the liquid to a spray of small dropletsto a spray of small droplets..

Drum Dryer (Film Dryer)Drum Dryer (Film Dryer) The The drum dryer drum dryer consists of consists of

a drum heated internally, a drum heated internally, usually by steam, and usually by steam, and rotated on its longitudinal rotated on its longitudinal axis.axis.

The liquid is applied to the The liquid is applied to the surface and spread to a surface and spread to a film; this may be done in film; this may be done in various ways, but the various ways, but the simplest method is that simplest method is that shown in the diagram, shown in the diagram, where the drum dips into where the drum dips into a a feed pan.feed pan. Drying rate is controlled by using a suitable speed of rotation and

drum temperature. The product is scraped from the surface of the drum by means of a doctor knife.

AdvantagesAdvantages(a) The method gives (a) The method gives rapid dryingrapid drying, the thin , the thin

film spread over a large area resulting in film spread over a large area resulting in rapid heat and mass transfer.rapid heat and mass transfer.

(b) The equipment is (b) The equipment is compactcompact, occupying , occupying much less space.much less space.

(c) Heating (c) Heating time is shorttime is short, being only a few , being only a few seconds.seconds.

(d) The drum can be enclosed in a vacuum (d) The drum can be enclosed in a vacuum jacket, enabling the temperature of drying jacket, enabling the temperature of drying to be reduced.to be reduced.

(e) The product is obtained in flake form, (e) The product is obtained in flake form, which is convenient for many purposes.which is convenient for many purposes.

The only The only disadvantage disadvantage is that operating is that operating conditions are critical and it is necessary to conditions are critical and it is necessary to impose careful control on feed rate, film impose careful control on feed rate, film thickness, speed of rotation and thickness, speed of rotation and temperature difference.temperature difference.

The drum dryer can handle a variety of The drum dryer can handle a variety of materials, either as solutions or materials, either as solutions or suspensions; substances that are dried by suspensions; substances that are dried by this method include this method include milk products, starch milk products, starch products, ferrous salts, and suspensions of products, ferrous salts, and suspensions of kaolin or zinc oxide.kaolin or zinc oxide.

SpraySpray DryerDryer The spray dryer provides a The spray dryer provides a

large surface area for heat and large surface area for heat and mass transfer by atomizing the mass transfer by atomizing the liquid to small droplets. These liquid to small droplets. These are sprayed into a stream of hot are sprayed into a stream of hot air, so that each droplet dries to air, so that each droplet dries to a solid particle.a solid particle.

There are many forms of spray There are many forms of spray dryer and Fig. is a typical dryer and Fig. is a typical design, in which the drying design, in which the drying chamber resembles the cyclone chamber resembles the cyclone ensuring good circulation of air, ensuring good circulation of air, to facilitate heat and mass to facilitate heat and mass transfer, and that dried transfer, and that dried particles are separated by the particles are separated by the centrifugal action.centrifugal action.

The character of the particles is controlled by the The character of the particles is controlled by the droplet form; hence the type of atomizer is droplet form; hence the type of atomizer is important.important.

Jet atomizers are easily blocked and the droplet size Jet atomizers are easily blocked and the droplet size is likely to vary, but this is not so with rotary types.is likely to vary, but this is not so with rotary types.

Liquid is fed on to the disc, which is rotated at high Liquid is fed on to the disc, which is rotated at high speed ; a film is formed and spreads from the small speed ; a film is formed and spreads from the small disc to a larger, inverted, hemi-spherical bowl, disc to a larger, inverted, hemi-spherical bowl, becoming thinner, and eventually being dispersed becoming thinner, and eventually being dispersed from the edge in a fine, uniform spray.from the edge in a fine, uniform spray.

In addition, the rotary atomizer has the In addition, the rotary atomizer has the advantage of being equally effective with advantage of being equally effective with suspensions of solids and it can operate suspensions of solids and it can operate efficiently at various feed rates.efficiently at various feed rates.

1. Feed Tank2. Centrifugal

Atomizer3. Drying Chamber4. Inlet Air Filter5. Air Supply Fan6. Air Heater7. Triple Inlet Duct8. Adjustable Air

Disperser9. Cooling Air Fan10. Chamber

Product Collector

11. Cyclone Product Collector

12. Exhaust Fan

3

1

4 5 6

9

7

10

28

11

12

Hot AirLiquid FeedDry Powder + Hot Air

Spray Dryer

Advantages of the Spray Drying Advantages of the Spray Drying ProcessProcess

(a) The droplets are small, giving a large (a) The droplets are small, giving a large surface area for heat and mass transfer, so surface area for heat and mass transfer, so that evaporation is very rapid.that evaporation is very rapid.

(b) Because evaporation is very rapid, the (b) Because evaporation is very rapid, the droplets do not attain a high temperature, droplets do not attain a high temperature, most of the heat being used as latent heat of most of the heat being used as latent heat of vaporization.vaporization.

(c) The characteristic particle form gives the (c) The characteristic particle form gives the product a high bulk density and, in turn, ready product a high bulk density and, in turn, ready solubility.solubility.

(d) Provided that a suitable atomizer is (d) Provided that a suitable atomizer is used, the powder will have a uniform used, the powder will have a uniform and controllable particle size.and controllable particle size.

(e) The product is free-flowing, with (e) The product is free-flowing, with almost spherical particles, and is almost spherical particles, and is especially convenient for tablet especially convenient for tablet manufacture.manufacture.

(f) Labour costs are low, the process (f) Labour costs are low, the process yielding a dry, free-flowing powder from yielding a dry, free-flowing powder from a dilute solution, in a single operation a dilute solution, in a single operation with no handling.with no handling.

DisadvantagesDisadvantages

(a) The equipment is very bulky and (a) The equipment is very bulky and with the auxillary equipment (fans, with the auxillary equipment (fans, heaters, separators, etc.) is heaters, separators, etc.) is expensive. expensive.

(b) The thermal efficiency is rather (b) The thermal efficiency is rather low, since the air must still be hot low, since the air must still be hot enough when it leaves the dryer to enough when it leaves the dryer to avoid condensation of moisture.avoid condensation of moisture.

The spray dryer can be used for drying The spray dryer can be used for drying almost any substance, in solution or in almost any substance, in solution or in suspension. It is most useful for suspension. It is most useful for thermolabile materialsthermolabile materials..

Examples of substances that are spray Examples of substances that are spray dried include borax, citric acid, hexamine, dried include borax, citric acid, hexamine, sodium phosphate, gelatin, acacia and sodium phosphate, gelatin, acacia and extracts, while starch, barium sulphate and extracts, while starch, barium sulphate and calcium phosphate are typical of insoluble calcium phosphate are typical of insoluble materials.materials.

In addition, the method is widely used for In addition, the method is widely used for products of indirect pharmaceutical products of indirect pharmaceutical interest, such as milk, soap, and interest, such as milk, soap, and detergents.detergents.

Dryers for Solid MaterialsDryers for Solid Materials

GENERAL PRINCIPLESGENERAL PRINCIPLES

Equilibrium Moisture ContentEquilibrium Moisture Content

Exposure to air at a definite temperature and Exposure to air at a definite temperature and humidity will cause a material to lose or gain humidity will cause a material to lose or gain moisture until an moisture until an equilibrium moisture content is equilibrium moisture content is attained.attained.

The values will vary, depending on the The values will vary, depending on the temperature and humidity of the air and on the temperature and humidity of the air and on the properties of the material, being low with nonproperties of the material, being low with non­­porous solids, but higher and variable with fibrous porous solids, but higher and variable with fibrous or colloidal organic substances.or colloidal organic substances.

Ordinary Ordinary atmospheric atmospheric conditions are of the conditions are of the order of 20°C and 70 order of 20°C and 70 to 75 percent relative to 75 percent relative humidity.humidity.

So that, if exposed to So that, if exposed to the atmosphere, a the atmosphere, a substance such as substance such as kaolin will contain kaolin will contain about 1 percent about 1 percent moisture; textile moisture; textile fibers, as in surgical fibers, as in surgical dressings, will dressings, will contain about 15 contain about 15 percent, while drugs, percent, while drugs, such as leaves, may such as leaves, may have as much as 25 have as much as 25 to 30 per cent.to 30 per cent.

Typical equilibrium moisture contentsat 20°C (1) Drugs, such as leaves.(2) Textile fibers.(3) Inorganic substance, such as kaolin.

The effect of this variation of equilibrium The effect of this variation of equilibrium moisture content is that:moisture content is that:

When a material leaves a dryer it may or may When a material leaves a dryer it may or may not be not be bone dry, bone dry, that is, completely free from that is, completely free from moisture, depending on the temperature and moisture, depending on the temperature and humidity of the air at the dryer outlethumidity of the air at the dryer outlet

Even if a material is reduced to the bone-dry Even if a material is reduced to the bone-dry state, there will be state, there will be a moisture regain a moisture regain if it is if it is exposed to the atmosphere.exposed to the atmosphere.

'Over-drying' should be avoided; that is, drying 'Over-drying' should be avoided; that is, drying should be stopped when the moisture content should be stopped when the moisture content has reached a level equivalent to the has reached a level equivalent to the equilibrium moisture content under the equilibrium moisture content under the conditions to which the material is to be conditions to which the material is to be exposed.exposed.

Moisture ContentMoisture Content The moisture present in a solid may exist The moisture present in a solid may exist

in more than one physical condition:in more than one physical condition:

BOUND MOISTUREBOUND MOISTURE

Bound moisture in a solidBound moisture in a solid is liquid that is liquid that exerts a vapour pressure less than that of exerts a vapour pressure less than that of the pure liquid at the same temperature. the pure liquid at the same temperature. This reduction of vapour pressure may be This reduction of vapour pressure may be because the liquid is in small capillaries, because the liquid is in small capillaries, in solutions, in cells or is combined, for in solutions, in cells or is combined, for example, by adsorption to a surface.example, by adsorption to a surface.

Bound MoistureFree Moisture

FREE MOISTUREFREE MOISTURE

Free moistureFree moisture at a particular temperature and at a particular temperature and humidity is the liquid in excess of the equilibrium humidity is the liquid in excess of the equilibrium moisture content.moisture content.

EXPRESSION OF MOISTURE EXPRESSION OF MOISTURE CONTENTCONTENT

The moisture content is expressed as The moisture content is expressed as percentage percentage moisture content, dry basis, moisture content, dry basis, that is, the number of that is, the number of parts by weight of moisture per hundred parts of parts by weight of moisture per hundred parts of bone dry solid. bone dry solid.

Thus, 50 per cent moisture content, dry basis, Thus, 50 per cent moisture content, dry basis, represents 50 kg moisture per 100 kg dry solid. represents 50 kg moisture per 100 kg dry solid. Alternatively, the statement may be made as parts Alternatively, the statement may be made as parts per part and the previous quantity would be per part and the previous quantity would be expressed as 0.5 kg/kg.expressed as 0.5 kg/kg.

Rate of DryingRate of Drying The rate at which drying occurs The rate at which drying occurs

has been found to show certain has been found to show certain phases, which are illustrated in phases, which are illustrated in Fig.,Fig.,

From From AA to to BB the relationship is the relationship is linear, which is known as linear, which is known as thethe constant rate period.constant rate period.

From From BB to to CC the rate of loss of the rate of loss of moisture decreases and is moisture decreases and is known as known as the the falling rate falling rate periodperiod. .

The end of the constant rate The end of the constant rate period, B, is referred to as period, B, is referred to as the the critical moisture content.critical moisture content.

Fig. Drying curveFig. Drying curveCMC = CMC = critical moisture content critical moisture content EMC EMC = equilibrium moisture content= equilibrium moisture content

CONSTANT RATE PERIODCONSTANT RATE PERIOD For given conditions of For given conditions of

temperature and humidity, most temperature and humidity, most substances dry at a similar rate substances dry at a similar rate in the constant rate period.in the constant rate period.

It is found that the evaporation It is found that the evaporation rate is similar to that from a rate is similar to that from a free liquid surface under the free liquid surface under the same conditions, indicating that same conditions, indicating that the evaporation takes place the evaporation takes place from the wet surface of the from the wet surface of the solid and that the liquid is solid and that the liquid is replaced from below as fast as replaced from below as fast as it is vaporized.it is vaporized.

Controlling factors in this Controlling factors in this period are the rate at which period are the rate at which heatheat can be can be transferredtransferred and and the the removal of the vapour.removal of the vapour.

Heat TransferHeat Transfer The general principles affecting heat transfer The general principles affecting heat transfer

apply, so that in all cases a large area is apply, so that in all cases a large area is necessary. If convection is the principal method of necessary. If convection is the principal method of heat transfer, a suitable air flow is required with heat transfer, a suitable air flow is required with an appropriate temperature gradient, the former an appropriate temperature gradient, the former being of greater importance.being of greater importance.

Rapid drying may lead to the formation of a Rapid drying may lead to the formation of a surface `skin' that will hinder mass transfer and surface `skin' that will hinder mass transfer and so reduce the drying rate.so reduce the drying rate.

Conduction dryers demand good contact Conduction dryers demand good contact between the heating surface and the between the heating surface and the solid. With radiant heat transfer, the solid. With radiant heat transfer, the efficiency is very high and, provided the efficiency is very high and, provided the emitters can `see' the surface of the solid, emitters can `see' the surface of the solid, the heat transfer occurs at the surface the heat transfer occurs at the surface where the drying is taking place.where the drying is taking place.

Removal of VapourRemoval of Vapour Drying in the constant rate period depends on Drying in the constant rate period depends on

mass transfer through the air boundary layers.mass transfer through the air boundary layers.

Turbulent flow conditions are required, which Turbulent flow conditions are required, which will lower the partial vapour pressure in the will lower the partial vapour pressure in the atmosphere and reduce the thickness of the air atmosphere and reduce the thickness of the air boundary layers. boundary layers.

The effect of both of these factors is to increase The effect of both of these factors is to increase the partial pressure gradient, leading, in turn, the partial pressure gradient, leading, in turn, to a higher mass transfer coefficient.to a higher mass transfer coefficient.

FIRST FALLING RATE FIRST FALLING RATE PERIODPERIOD

As moisture is removed from the surface, a point As moisture is removed from the surface, a point will be reached when the rate of vaporization is will be reached when the rate of vaporization is insufficient to insufficient to saturate saturate the air in contact with the the air in contact with the surface. Under these conditions, the rate of drying surface. Under these conditions, the rate of drying will be limited by the transfer of liquid to the will be limited by the transfer of liquid to the surface and, since this becomes increasingly surface and, since this becomes increasingly difficult, the rate decreases continuously.difficult, the rate decreases continuously.

During drying, moisture will be During drying, moisture will be drawn from the lower pores, drawn from the lower pores, moving to smaller pores near the moving to smaller pores near the surface that have been emptied, or surface that have been emptied, or to the surface, according to the to the surface, according to the relative values of the suction relative values of the suction potential. potential.

SECOND FALLING RATE SECOND FALLING RATE PERIODPERIOD

Any moisture that Any moisture that remains at the end of the remains at the end of the first falling rate period is first falling rate period is unable to move, so that unable to move, so that drying cannot take place drying cannot take place on the surface.on the surface.

The plane of vaporization The plane of vaporization retreats from the surface retreats from the surface into the body of the into the body of the solid, and the drying rate solid, and the drying rate depends on the depends on the movement of the vapour movement of the vapour through the pores to the through the pores to the surface, in general, by surface, in general, by molecular diffusion.molecular diffusion.

Types of DryerTypes of Dryer

•Dryers­ may­ be­ classifying­ according­ to­the­ principal method­ of­ heat­ transfer:­

convection,­conduction,­or­radiation­.

Heat Transfer Mechanisms Convection Conduction Radiation

Mass Transfer MechanismsDiffusion

vaporliquid

Capillary FlowShrinkage/Pressure GradientsGravityVaporization/Condensation

CONVECTION­DRYERSCONVECTION­DRYERS

Compartment (or Tray or Shelf) Dryers

• The­ simplest­ form­ of­ dryer­ in­this­category­is­a­cabinet­with­a­heater,­usually­at­ the­bottom­to­assist­ convection,­ but­ it­ is­ of­limited­value,­giving­virtually­no­control­ of­ heat­ transfer­ or­humidity.­

• The­ situation­ can­ be­ improved­ by­ including­ a­ fan,­ so­that­forced­convection­takes­place,­with­increased­heat­transfer­ and­ reduced­ local­ vapour­ concentrations,­ but­control­is­still­inadequate.

• The­best­type­in­this­category­is­the­directed circulation form,­ in­which­air­ is­heated­and­ is­directed­across­ the­material­in­a­controlled­flow.­

Directed-circulation compartmentDirected-circulation compartment dryerdryer

• The­arrangement­of­the­shelves­causes­air­flows­in­the­directions­shown­by­the­arrows.

• Heaters­are­positioned­as­shown,­so­ that­ the­air­ is­ re-heated­ before­ passing­ over­ each­ shelf,­ which­ is­ an­advantage­since­the­air­temperature­is­minimized.­

• As­the­air­passes­over­each­shelf,­a­certain­amount­of­heat­is­given­up­to­provide­latent­heat­of­vaporization.­

• The­compartment­dryer­is­very­flexible­and­versatile,­with­ good­ control­ of­ heat­ and­ humidity,­ if­ designed­correctly.

• The­ output,­ however,­ is­ limited­ due­ to­ size­ and­ to­batch­ operation.­ Further,­ unless­ the­ material­ is­moved­ (This­ difficulty can­ be­ overcome­ in­ a­ simple­manner­ if­ the­ material­ is­ on­ trays;­ at­ intervals,­ the­tray­ nearest­ to­ the­ air­ inlet­ is­ removed,­ all­ others­moved­ one­ position­ nearer­ to­ the­ inlet,­ and­ a­ fresh­tray­ of­ moist­ material­ inserted­ into­ the­ last­ position­before­the­air­outlet).­

• The­ effect­ is­ to­ obtain­semi-continuous­operation­ with­ greater­efficiency­ by­ having­counter­current­movement­ of­ air­ and­drying­solid.

• Uses­ include­ the­ drying­of­ crude­ drugs,­chemicals,­ powders,­tablet­granules,­or­ items­of­equipment.­

Tunnel DryersTunnel Dryers• Such­ counter-current­ movement­ of­ air­ and­

drying­ solids­ is­ arranged­ much­ more­conveniently­ in­ the­ tunnel dryer, where­ the­drying­ method­ resembles­ the­ compartment­dryer,­but­takes­the­form­of­a­long­tunnel,­with­heated­ air­ entering­ at­ one­ end­ and­ some­means­ of­ moving­ the­ material­ to­ be­ dried­ at­the­opposite­end.­

• Compared­ with­ the­ compartment­ dryer,­ the­tunnel­dryer­has­ the­advantage­of­being­semi­continuous­ or­ continuous­ in­ operation;­applications­ are­ similar,­ but­ more­ suitable­ for­large-scale­production.

Rotary DryerRotary Dryer

• The­rotary dryer is­a­modified­form­ of­ the­ tunnel­ dryer­ in­which­ the­ particles­ are­passed­ through­ a­ rotating­cylinder,­ counter-current­ to­ a­stream­of­heated­air.­

• Due­ to­ the­ rotation­ of­ the­cylinder,­the­material­is­turned­over­ and­ drying­ takes­ place­from­ individual­ particles­ and­not­from­a­static­bed.

• The­ cylindrical­ shell­ is­ mounted­with­ a­ slight­ slope,­ so­ that­ the­material­ will­ move­ through­ the­shell­as­it­is­slowly­rotated.

• To­ improve­ contact,­ the­ shell­contains­ baffles­ or­ flights which­lift­ the­ solids­ and­ spill­ the­particles­through­the­air­stream.­

• The­ rotary­ dryer­ is­ used­ for­continuous­ drying­ on­ a­ large-scale­ of­ any­ powdered­ or­granular­solid.

Classical Bed Dryer

Exhaust Outlet

Adjustable Louvers

Fan Screen

Fresh Air

Inlet

Heater

Fluidized Bed DryerFluidized Bed Dryer• Another­ method­ of­ obtaining­

good­contact­between­hot­air­and­particles­ is­ used­ in­ the­ fluidized bed dryer

• Fluidization­ occurs­ when­ a­ flow­of­fluid­upwards­through­a­bed­of­particles­ reaches­ sufficient­velocity­ to­ support­ the­ particles­without­carrying­them­away­in­the­fluid­stream.

• The­ bed­ of­ particles­ then­assumes­ the­ characteristics­ of­ a­boiling­ liquid,­ hence­ the­ term­fluidization.

• The­ fluid­ responsible­ for­ fluidization­may­be­a­gas­or­a­liquid,­the­choice­of­which­ will­ confer­ different­ properties­on­ the­ fluidizing­ system.­ This­ will,­ in­turn,­ affect­ the­ choice­ of­ processes­that­may­be­used.

• At­low­fluid­velocities­the­particles­will­simply­ remain­ in­ the­ loosely­ packed­state­in­the­bed.

• At­ intermediate­ velocities,­ individual­particles­ will­ become­ suspended­ in­the­fluid,­ flowing­while­the­bed­on­the­whole­ remains­ motionless­ relative­ to­the­column­walls;­the­bed­is­now­said­to­be­fluidized

Vertical Fluid Bed Dryer

27

6

1

34

5

1. Fresh Air Inlet Filter

2. Steam or Electric Heater

3. Air Circulating Fan4. Air Volume

Damper5. Air Diffuser6. Vessel Containing

Material7. Nylon Filter Bags8. Air Exhaust

8

Conveyor Fluidized Bed Dryer

Surge Hopper

Vibrating Conveyor

Conveying and air distributing surface

Dry Material

Exhaust Air Duct

Dust Collector

Exhaust Blower

Pilot-scale pulsed fluid-bed dryer: 1- dryer, 2- rotating valve distributor, 3- forced draft fan, 4- induced

draft fan, 5- feeder, 6- hopper, 7- bag filter, 8- electric heater and gas burner, 9- control panel.

AdvantagesAdvantages

• (a)­ Efficient­ heat­ and­ mass­ transfer­ give­high­drying­rates.

• (b)­ The­ fluidized­ state­ of­ the­ bed­ gives­drying­ from­ individual­ particles­ and­ not­from­ the­ entire­ bed.­ Hence,­ most­ of­ the­drying­ will­ be­ at­ constant­ rate­ and­ the­ -falling­ rate­ period­ (when­ the­ danger­ of­overheating­is­greatest)­is­very­short.

• (c)­ The­ temperature­ of­ a­ fluidized­ bed­ is­ uniform­ and­can­be­controlled­precisely.

• (d)­The­fluidized­state­produces­a­free-flowing­product.

• (e)­The­free­movement­of­individual­particles­eliminates­the­risk­of­soluble­materials­migrating.

• (J)­ The­ containers­ can­ be­ mobile,­ making­ handling­simple,­and­reducing­labour­costs.

• (g)­ Short­ drying­ times­ mean­ that­ the­ unit­ has­ a­ high­output­from­a­small­floor­space.

DisadvantagesDisadvantages• (a)­ The­ turbulence­ of­ the­ fluidized­ state­ may­ cause­

attrition­of­some­materials,­with­the­production­of­fines.

• (b)­Fine­particles­may­become­entrained­and­must­be­collected­by­bag­filters,­with­care­ to­avoid­segregation­and­loss­of­fines.

• (c)­ The­ vigorous­ movement­ of­ particles­ in­ hot­ dry­ air­can­ lead­ to­ the­ generation­ of­ charges­ of­ static­electricity,­and­suitable­precautions­must­be­taken.

• The­ fluidized­ bed­ dryer­ can­ be­ used­ for­ drying­ any­powdered­ material;­ a­ special­ application­ is­ the­ drying­of­tablet­granules,­for­which­it­is­an­excellent­method.

CONDUCTION­DRYERSCONDUCTION­DRYERS

Vacuum OvenVacuum Oven

Vacuum dryers are substantially the same as Vacuum dryers are substantially the same as tray dryers, except that they operate under a tray dryers, except that they operate under a vacuum, and heat transfer is largely by vacuum, and heat transfer is largely by conduction or by radiation. The trays are conduction or by radiation. The trays are enclosed in a large cabinet, which is evacuated.enclosed in a large cabinet, which is evacuated.

The water vapour produced is generally The water vapour produced is generally condensed, so that the vacuum pumps have only condensed, so that the vacuum pumps have only to deal with non-condensible gases. Another type to deal with non-condensible gases. Another type consists of an evacuated chamber containing a consists of an evacuated chamber containing a roller dryerroller dryer

A form of vacuum dryer commonly used in A form of vacuum dryer commonly used in pharmaceutical practice is the pharmaceutical practice is the vacuum oven.vacuum oven.

AdvantagesAdvantages

(a) The 'method is especially suitable for (a) The 'method is especially suitable for unstable materials; drying takes place at low unstable materials; drying takes place at low temperature, minimizing damage to temperature, minimizing damage to thermolabile materials, and vacuum operation thermolabile materials, and vacuum operation reduces the risk of oxidation during drying.reduces the risk of oxidation during drying.

(b) If used for manufacturing a dry extract, the (b) If used for manufacturing a dry extract, the product is porous and friable, so that it is product is porous and friable, so that it is especially useful for tabletting. especially useful for tabletting.

(c) The use of a condenser enables solvents to (c) The use of a condenser enables solvents to be recovered, which is useful with expensive or be recovered, which is useful with expensive or inflammable solvents.inflammable solvents.

DisadvantagesDisadvantages (a) (a) Heat transfer coefficients are low.Heat transfer coefficients are low.

(b) The vacuum oven is of limited (b) The vacuum oven is of limited capacity.capacity.

(c) Labour and running costs are high.(c) Labour and running costs are high.

(d) There is a danger of an excessive (d) There is a danger of an excessive temperature gradient, especially as the temperature gradient, especially as the coefficient of thermal conductivity of the coefficient of thermal conductivity of the material will decrease as it dries, leading material will decrease as it dries, leading to over-heating and to decomposition.to over-heating and to decomposition.

Vacuum drying is of specialized Vacuum drying is of specialized application and its uses include the application and its uses include the drying of: drying of:

• thermo-labile materials, such as penicillin.thermo-labile materials, such as penicillin.

• products where the porous form is useful, products where the porous form is useful, such as dry extracts.such as dry extracts.

• removal of liquids other than water, where the removal of liquids other than water, where the solvent recovery is advantageous, ethanol solvent recovery is advantageous, ethanol extractives, for example.extractives, for example.

Freeze DryingFreeze Drying

In a typical phase diagram, the In a typical phase diagram, the boundary between gas and boundary between gas and liquid runs from the triple point liquid runs from the triple point to the critical point. Freeze to the critical point. Freeze drying (drying (blue arrowblue arrow) brings the ) brings the system around the triple point, system around the triple point, avoiding the direct liquid-gas avoiding the direct liquid-gas transition seen in ordinary transition seen in ordinary drying (drying (green arrowgreen arrow).).

Freeze drying (also known as lyophilization) Freeze drying (also known as lyophilization) is a dehydration process typically used to is a dehydration process typically used to preserve a perishable material or make the preserve a perishable material or make the material more convenient for transport. material more convenient for transport.

Freeze drying works by freezing the material Freeze drying works by freezing the material and then reducing the surrounding pressure and then reducing the surrounding pressure and adding enough heat to allow the frozen and adding enough heat to allow the frozen water in the material to sublime directly from water in the material to sublime directly from the solid phase to gas.the solid phase to gas.

A simplified freeze-drying machine

Pharmaceutical companies often Pharmaceutical companies often use freeze drying to increase the use freeze drying to increase the shelf life of products, such as shelf life of products, such as vaccines and other injectables.vaccines and other injectables.

By removing the water from the By removing the water from the material and sealing the material material and sealing the material in a vial, the material can be in a vial, the material can be easily stored, shipped and later easily stored, shipped and later reconstituted to its original form reconstituted to its original form for injection.for injection.

The freeze drying processThe freeze drying process

There are three stages in the complete There are three stages in the complete freeze-drying process: freezing, primary freeze-drying process: freezing, primary drying, and secondary drying.drying, and secondary drying.

1)1) FreezingFreezing

The freezing process consists of freezing the The freezing process consists of freezing the materialmaterial. .

2)2) Primary dryingPrimary drying

During the primary drying phase the pressure is During the primary drying phase the pressure is lowered and enough heat is supplied to the lowered and enough heat is supplied to the material for the water to sublimate.material for the water to sublimate.

3)3) Secondary dryingSecondary drying

The secondary drying phase aims to sublimate The secondary drying phase aims to sublimate the water molecules that are adsorbed during the the water molecules that are adsorbed during the freezing process, since the mobile water freezing process, since the mobile water molecules were sublimated in the primary molecules were sublimated in the primary drying phase. drying phase.

AdvantagesA. Drying takes place at very low temperatures, so that

enzyme action is inhibited, and decomposition, particularly hydrolysis, is minimized.

B. The solution is frozen, so that the final dry product is a network of solid occupying the same volume as the original solution. Thus, there is no case-hardening and the product is light and porous.

C. The porous form of the product gives ready solubility.

D. There is no concentration of the solution prior to drying. Hence, salts do not concentrate and denature proteins as occurs with other drying methods.

E. Under high vacuum, there is no contact with the air, and oxidation is minimized.

DisadvantagesDisadvantages

There are two main disadvantages of freeze drying:There are two main disadvantages of freeze drying:

(a) The porosity, ready solubility, and complete (a) The porosity, ready solubility, and complete dryness yield a very hygroscopic product. Unless dryness yield a very hygroscopic product. Unless dried in the final container and sealed dried in the final container and sealed in situ, in situ, packaging requires special conditions.packaging requires special conditions.

(b) The process is very slow and uses complicated (b) The process is very slow and uses complicated plant, which is very expensive. It is not a general plant, which is very expensive. It is not a general method of drying, therefore, but is limited to certain method of drying, therefore, but is limited to certain types of valuable products that cannot be dried by types of valuable products that cannot be dried by any other means.any other means.

Uses of Freeze DryingUses of Freeze Drying

The method is applied only to biological The method is applied only to biological products; for example, antibiotics (other than products; for example, antibiotics (other than penicillin), blood products, vaccines (such as penicillin), blood products, vaccines (such as BCG, yellow fever, smallpox), enzyme BCG, yellow fever, smallpox), enzyme preparations (such as hyaluronidase) and preparations (such as hyaluronidase) and microbiological cultures.microbiological cultures.

RADIANT HEAT DRYERS Sources of the radiant energy may be:

Lamps-so called infra-red lamps; usually the operating temperature is too high, giving a total emission of energy that is too great and that causes overheating.

Low temperature sources-electric rods or electric or gas-heated panels are used. They may operate at red heat or down to `black' heat (to about 500°C). Such sources give better heating control and are to be preferred.

Uses of Radiant Heat Drying

Dryers adapted for radiant heat transfer may be used for most materials, solid or liquid in thin layers.

The method is very effective for thin liquid films, as on drum dryers, and powders or granules may be dried quicker by radiant heat than by convection heating.

Care must be taken to avoid surface overheating.