Drying

What is drying ?

Drying is a mass transfer process consisting of the removal of water or another

solvent by evaporation from a solid, semi-solid or liquid. This process is often

used as a final production step before selling or packaging products. To be

considered "dried", the final product must be solid, in the form of a continuous

sheet (e.g. paper), long pieces (e.g. wood), particles (e.g. cereal grains or corn

flakes) or powder (e.g. sand, salt, washing powder, milk powder). A source of

heat, and an agent to remove the vapor produced by the process are necessary. In

bioproducts like food, grains, and pharmaceuticals like vaccines, the solvent to be

removed is almost invariably water.

In the most common case, a gas stream, e.g., air, applies the heat by convection

and carries away the vapor as humidity. Other possibilities are vacwuum drying,

where heat is supplied by conduction or radiation (or microwaves) hile the vapor

thus produced is removed by the vacuum system. Another indirect technique is

drum drying (used, for instance, for manufacturing potato flakes), where a heated

surface is used to provide the energy and aspirators draw the vapor outside the

room. In turn, the mechanical extraction of the solvent, e.g., water, by

centrifugation, is not considered "drying".

Drying Mechanism

In some products having a relatively high initial moisture content, an initial linear

reduction of the average product moisture content as a function of time may be

observed for a limited time, often known as "constant drying rate period".

Usually, in this period, it is surface moisture outside individual particles which is

being removed. If drying is continued, the slope of the curve, the drying rate,

becomes less steep (falling rate period) and eventually tends to the horizontal at

very long times. The product moisture content is then constant at the

"equilibrium moisture content", where it is in dynamic equilibrium with the

dehydrating medium. In the falling rate period, water migration from the product

interior to the surface is mostly by molecular diffusion, i,e. the water flux is

proportional to the moisture content gradient. This means that water moves from

zones with higher moisture content to zones with lower values, a phenomenon

explained by the second law of thermodynamics. If water removal is

considerable, products usually undergo shrinkage and deformation, except in a

well-designed freeze-drying process.

Methods of drying

Application of hot air (convective or direct drying). Air heating increases

the driving force for heat transfer and accelerates drying. It also reduces air

relative humidity, further increasing the driving force for drying. In the falling

rate period, as moisture content falls, the solids heat up and the higher

temperatures speed up diffusion of water from the interior of the solid to the

surface. However, product quality considerations limit the applicable rise to

air temperature. Excessively hot air can almost completely dehydrate the solid

surface, so that its pores shrink and almost close, leading to crust formation or

"case hardening", which is usually undesirable. For instance in wood (timber)

drying, air is heated (which speeds up drying) though some steam is also

added to it (which hinders drying rate to a certain extent) in order to avoid

excessive surface dehydration and product deformation owing to high

moisture gradients across timber thickness. Spray drying belongs in this

category

In a typical phase diagram, the boundary between gas and liquid

runs from the triple point to the critical point. Regular drying is the

green arrow, while supercritical drying is the red arrow and freeze

drying is the blue.

.

Indirect or contact drying (heating through a hot wall), as drum drying,

vacuum drying. Again, higher wall temperatures will speed up drying but this

is limited by product degradation or case-hardening. Drum drying belongs in

this category.

Dielectric drying (radiofrequency or microwaves being absorbed inside the

material) is the focus of intense research nowadays. It may be used to assist

air drying or vacuum drying. Researchers have found that microwave finish

drying speeds up the otherwise very low drying rate at the end of the classical

drying methods.

Freeze drying or lyophilization is a drying method where the solvent is

frozen prior to drying and is then sublimed, i.e., passed to the gas phase

directly from the solid phase, below the melting point of the solvent. It is

increasingly applied to dry foods, beyond its already classical pharmaceutical

or medical applications. It keeps biological properties of proteins, and retains

vitamins and bioactive compounds. Pressure can be reduced by a high

vacuum pump (though freeze drying at atmospheric pressure is possible in

dry air). If using a vacuum pump, the vapor produced by sublimation is

removed from the system by converting it into ice in a condenser, operating at

very low temperatures, outside the freeze drying chamber.

Supercritical drying (superheated steam drying) involves steam drying of

products containing water. This process is feasible because water in the

product is boiled off, and joined with the drying medium, increasing its flow.

It is usually employed in closed circuit and allows a proportion of latent heat

to be recovered by recompression, a feature which is not possible with

conventional air drying, for instance. The process has potential for use in

foods if carried out at reduced pressure, to lower the boiling point.

Natural air drying takes place when materials are dried with unheated

forced air, taking advantage of its natural drying potential. The process is

slow and weather-dependent, so a wise strategy "fan off-fan on" must be

devised considering the following conditions: Air temperature, relative

humidity and moisture content and temperature of the material being dried.

Grains are increasingly dried with this technique, and the total time (including

fan off and on periods) may last from one week to various months, if a winter

rest can be tolerated in cold areas.

Applications of drying

Drying of fish in Lofoten in the production of stockfish

Foods are dried to inhibit microbial development and quality decay. However, the

extent of drying depends on product end-use. Cereals and oilseeds are dried after

harvest to the moisture content that allows microbial stability during storage.

Vegetables are blanched before drying to avoid rapid darkening, and drying is not

only carried out to inhibit microbial growth, but also to avoid browning during

storage. Concerning dried fruits, the reduction of moisture acts in combination

with its acid and sugar contents to provide protection against microbial growth.

Products such as milk powder must be dried to very low moisture contents in order

to ensure flowability and avoid caking. This moisture is lower than that required to

ensure inhibition to microbial development. Other products as crackers are dried

beyond the microbial growth threshold to confer a crispy texture, which is liked by

consumers.

Among Non-food products, those that require considerable drying are wood (as

part of Timber processing), paper and washing powder. The first two, owing to

their organic origins, may develop mold if insufficiently dried. Another benefit of

drying is a reduction in volume and weight…

Drum drying :-

Drum drying is a method used for drying out liquids; for

example, milk is applied as a thin film to the surface of a heated drum, and the

dried milk solids are then scraped off with a knife. Powdered milk made by drum

drying tends to have a cooked flavor, due to caramelization caused by greater

heat exposure.

Compared to spray drying, drum drying is a more intense heat treatment which

results in more denatured proteins. The powder is less soluble as a result. The

temperature uniformity of the heated roller/drum is poor so spray drying results

in better quality milk powder.

Other products where drumdrying can be used are for example starches,

breakfast cereals, baby food, instant mashed potatoes to make them cold water

soluble.

Image of Drum Dryer

Belt dryer

A belt dryer is an apparatus which is used for continuous drying and cooling of

pellets, pastes, moulded compounds and panels using air, inert gas, or flue gas.

Working principle

additional parameter for setting of drying time. The cells can be heated or cooled

directly or indirectly, and all heating media, such as oil, steam, hot water or hot

gas can be A Belt dryer / Belt cooler is a device designed for the particularly

gentle thermal treatment of product. The wet product is continuously and evenly

applied through an infeed chamber onto a perforated belt. The belt,

predominantly in horizontal position, carries the product through the drying area

which is divided into several sections. In these cells drying gas flows through or

over the wet product and dries it. Each cell can be equipped with a ventilating fan

and a heat exchanger. This modular design allows the drying and cooling

temperatures to be controlled separately in the different sections. Thus, each

dryer cell can be individually controlled and the drying / cooling air flow can be

varied in each cell. In addition, the speed of the conveyor belt can be varied what

gives an used.

Design features

Belt dryers / Belt coolers are designed in modular system. Each belt dryer

consists of infeed head, conveyor belt and discharge end. Different kinds of

dryers are possible to construct, e.g.

Single-belt dryer

Multi-stage dryer

Multi-level dryer

Multi-belt dryer

Ventilation options

In general there are two ways of gas flow pattern. The drying air can flow,

according to the treatment process, either through or over the product.

Exemplary conveyer options Chain guided wire mesh conveyor

Chain guided hinge slat conveyor

Chain guided steel plate conveyor

Chainless wire mesh conveyor

Feeding variations Granulating mill – filter cake or amorphous and paste-like products respectively

Slewing belt conveyor – sensitive and free flowing products

Distribution spiral

Rotatable arm feeding device – stable products

Plates feeding

Typical applications

Belt dryers are predominantly used in the following industries:

Chemical Industry

Pharmaceutical industry

Food and feeding-stuff industry

Non-metallic minerals industry

Plastics industry

Wood industry

Ceramics Industry

Product examples

Veneers, wood fiber insulating boards, paints, molding materials, synthetic

rubber, superabsorbent polymer, stearate, catalysts, coke, fruits, vegetables,

cereals.

Tray dryer

Freeze-drying (also known as lyophilisation, lyophilization or

cryodesiccation) is a dehydration process typically used to preserve a

perishable material or make the material more convenient for transport.

Freeze-drying works by freezing the material and then reducing the

surrounding pressure to allow the frozen water in the material to sublimate

directly from the solid phase to the gas phase.

The origins of freeze drying

Freeze-drying was first actively developed during WWII. Serum being sent to

Europe for medical treatment of the wounded required refrigeration. Due to the

lack of available refrigeration, many serum supplies were spoiling before

reaching the intended recipients. The freeze-drying process was developed as a

commercial technique that enabled serum to be rendered chemically stable and

viable without having to be refrigerated. Shortly thereafter, the freeze dry process

was applied to penicillin and bone, and lyophilization became recognized as an

important technique for preservation of biologicals. Since that time, freeze-

drying has been used as a preservation or processing technique for a wide variety

of products. Some of the applications include the processing of food,

pharmaceuticals, diagnostic kits, restoration of water damaged documents, river

bottom sludge prepared for hydrocarbon analysis, ceramics used in the

semiconductor industry, viral or bacterial cultures, tissues prepared for analysis,

the production of synthetic skins and restoration of historic/reclaimed boat hulls.

The freeze-drying process

There are four stages in the complete drying process: pretreatment, freezing,

primary drying, and secondary drying.

Pretreatment

Pretreatment includes any method of treating the product prior to freezing. This may

include concentrating the product, formulation revision (i.e., addition of components to

increase stability and/or improve processing), decreasing a high vapor pressure solvent

or increasing the surface area. In many instances the decision to pretreat a product is

based on theoretical knowledge of freeze-drying and its requirements, or is demanded

by cycle time or product quality considerations. Methods of pretreatment include:

Freeze concentration, Solution phase concentration, Formulation to Preserve Product

Appearance, Formulation to Stabilize Reactive Products, Formulation to Increase the

Surface Area, and Decreasing High Vapor Pressure Solvents.

Freezing

In a lab, this is often done by placing the material in a freeze-drying flask and rotating

the flask in a bath, called a shell freezer, which is cooled by mechanical refrigeration,

dry ice and methanol, or liquid nitrogen. On a larger scale, freezing is usually done

using a freeze-drying machine. In this step, it is important to cool the material below its

triple point, the lowest temperature at which the solid and liquid phases of the material

can coexist. This ensures that sublimation rather than melting will occur in the

following steps. Larger crystals are easier to freeze-dry. To produce larger crystals, the

product should be frozen slowly or can be cycled up and down in temperature. This

cycling process is called annealing. However, in the case of food, or objects with

formerly-living cells, large ice crystals will break the cell walls (a problem discovered,

and solved, by Clarence Birdseye), resulting in the destruction of more cells, which can

result in increasingly poor texture and nutritive content. In this case, the freezing is done

rapidly, in order to lower the material to below its eutectic point quickly, thus avoiding

the formation of ice crystals. Usually, the freezing temperatures are between −50 °C and

−80 °C. The freezing phase is the most critical in the whole freeze-drying process,

because the product can be spoiled if badly done.

Amorphous materials do not have a eutectic point, but they do have a critical

point, below which the product must be maintained to prevent melt-back or

collapse during primary and secondary drying.

Image of tray drayer

Rotary dryer

The rotary dryer is a type of industrial dryer employed to reduce or minimize

the liquid moisture content of the material it is handling by bringing it into direct

contact with a heated gas. The dryer is made up of a large, rotating cylindrical

tube, usually supported by concrete columns or steel beams. The dryer slopes

slightly so that the discharge end is lower than the material feed end in order to

convey the material through the dryer under gravity. Material to be dried enters

the dryer, and as the dryer rotates, the material is lifted up by a series of internal

fins lining the inner wall of the dryer. When the material gets high enough to roll

back off the fins, it falls back down to the bottom of the dryer, passing through

the hot gas stream as it falls. This gas stream can either be moving toward the

discharge end from the feed end (known as co-current flow), or toward the feed

end from the discharge end (known as counter-current flow). The gas stream can

be made up of a mixture of air and combustion gases from a burner, in which

case the dryer is called a direct heated dryer. Alternatively, the gas stream may

consist of air or another (sometimes inert) gas that is preheated. When the gas

stream is preheated by some means where burner combustion gases do not enter

the dryer, the dryer known as an indirect-heated type. Often, indirect heated

dryers are used when product contamination is a concern. In some cases, a

combination of direct-indirect heated rotary dryers are also available to improve

the overall efficiency.

Image or rotary dryer

Spray drying

Laboratory-scale spray dryer.

A=Solution or suspension to be dried in, B=Atomization gas in,

1= Drying gas in, 2=Heating of drying gas, 3=Spraying of

solution or suspension, 4=Drying chamber, 5=Part between

drying chamber and cyclone, 6=Cyclone, 7=Drying gas is taken

away, 8=Collection vessel of product, arrows mean that this is co-

current lab-spraydryer

Spray drying is a method of producing a dry powder from a liquid or slurry by

rapidly drying with a hot gas. This is the preferred method of drying of many

thermally-sensitive materials such as foods and pharmaceuticals. A consistent

particle size distribution is a reason for spray drying some industrial products

such as catalysts. Air is the heated drying media; however, if the liquid is a

flammable solvent such as ethanol or the product is oxygen-sensitive then

nitrogen is used.

All spray dryers use some type of atomizer or spray nozzle to disperse the liquid

or slurry into a controlled drop size spray. The most common of these are rotary

disks and single-fluid high pressure swirl nozzles. Alternatively, for some

applications two-fluid or ultrasonic nozzles are used. Depending on the process

needs drop sizes from 10 to 500 µm can be achieved with the appropriate

choices. The most common applications are in the 100 to 200 µm diameter range.

The dry powder is often free-flowing.

The most common spray dryers are called single effect as there is only one

drying air on the top of the drying chamber (see n°4 on the scheme). In most

cases the air is blown in co-current of the sprayed liquid. The powders obtained

with such type of dryers are fine with a lot of dusts and a poor flowability. In

order to reduce the dusts and increase the flowability of the powders, there is

since over 20 years a new generation of spray dryers called multiple effect spray

dryers. Instead of drying the liquid in one stage, the drying is done through two

steps: one at the top (as per single effect) and one or an integrated static bed at

the bottom of the chamber. The integration of this fluidized bed allows, by

fluidizing the powder inside a humid atmosphere, to agglomerate the fine

particles and to obtain granules having commonly a medium particle size within

a range of 100 to 300 µm. Because of this large particle size, these powders are

free-flowing.

The fines generated by the first stage drying can be recycled in continuous flow

either at the top of the chamber (around the sprayed liquid) or at the bottom

inside the integrated fluidized bed. The drying of the powder can be finalized on

an external vibrating fluidized bed.

The hot drying gas can be passed as a co-current or counter-current flow to the

atomiser direction. The co-current flow enables the particles to have a lower

residence time within the system and the particle separator (typically a cyclone

device) operates more efficiently. The counter-current flow method enables a

greater residence time of the particles in the chamber and usually is paired with a

fluidized bed system.

Alternatives to spray dryers are:

1. Freeze dryer : a more-expensive batch process for products that degrade in

spray drying. Dry product is not free-flowing.

2. Drum dryer : a less-expensive continuous process for low-value products;

creates flakes instead of free-flowing powder.

3. Pulse combustion dryer: A less-expensive continuous process that can handle

higher viscosities and solids loading than a spray dryer, and that sometimes gives a

freeze-dry quality powder that is free-flowing.

Schematic illustration of spray drying process.

A spray dryer is a device used in spray drying. It takes a liquid stream and

separates the solute or suspension as a solid and the solvent into a vapor. The

solid is usually collected in a drum or cyclone. The liquid input stream is sprayed

through a nozzle into a hot vapor stream and vaporised. Solids form as moisture

quickly leaves the droplets. A nozzle is usually used to make the droplets as

small as possible, maximising heat transfer and the rate of water vaporisation.

Droplet sizes can range from 20 to 180 μm depending on the nozzle. There are

two main types of nozzles: high pressure single fluid nozzle (50 to 300 bars) and

two-fluid nozzles: one fluid is the liquid to dry and the second is compressed gas

(generally air at 1 to 7 bars).

Spray dryers can dry a product very quickly compared to other methods of

drying. They also turn a solution, or slurry into a dried powder in a single step,

which can be advantageous for profit maximization and process simplification.

Micro-encapsulation

Spray drying often is used as an encapsulation technique by the food and other

industries. A substance to be encapsulated (the load) and an amphipathic carrier

(usually some sort of modified starch) are homogenized as a suspension in water

(the slurry). The slurry is then fed into a spray drier, usually a tower heated to

temperatures well over the boiling point of water.

As the slurry enters the tower, it is atomized. Partly because of the high surface

tension of water and partly because of the hydrophobic/hydrophilic interactions

between the amphipathic carrier, the water, and the load, the atomized slurry

forms micelles. The small size of the drops (averaging 100 micrometers in

diameter) results in a relatively large surface area which dries quickly. As the

water dries, the carrier forms a hardened shell around the load.

Load loss is usually a function of molecular weight. That is, lighter molecules

tend to boil off in larger quantities at the processing temperatures. Loss is

minimized industrially by spraying into taller towers. A larger volume of air has

a lower average humidity as the process proceeds. By the osmosis principle,

water will be encouraged by its difference in fugacities in the vapor and liquid

phases to leave the micelles and enter the air. Therefore, the same percentage of

water can be dried out of the particles at lower temperatures if larger towers are

used. Alternatively, the slurry can be sprayed into a partial vacuum. Since the

boiling point of a solvent is the temperature at which the vapor pressure of the

solvent is equal to the ambient pressure, reducing pressure in the tower has the

effect of lowering the boiling point of the solvent.

The application of the spray drying encapsulation technique is to prepare

"dehydrated" powders of substances which do not have any water to dehydrate.

For example, instant drink mixes are spray dries of the various chemicals which

make up the beverage. The technique was once used to remove water from food

products; for instance, in the preparation of dehydrated milk. Because the milk

was not being encapsulated and because spray drying causes thermal

degradation, milk dehydration and similar processes have been replaced by other

dehydration techniques. Skim milk powders are still widely produced using spray

drying technology around the world, typically at high solids concentration for

maximum drying efficiency. Thermal degradation of products can be overcome

by using lower operating temperatures and larger chamber sizes for increased

residence times.

Recent research is now suggesting that the use of spray-drying techniques may

be an alternative method for crystallization of amorphous powders during the

drying process since the temperature effects on the amorphous powders may be

significant depending on drying residence times.

Spray drying applications

Food: milk powder, coffee, tea, eggs, cereal, spices, flavorings, starch and starch

derivatives, vitamins, enzymes, colourings...

Pharmaceutical: antibiotics, medical ingredients, additives

Industrial: paint pigments, ceramic materials, catalyst supports

Nano spray dryer

The nano spray dryer offers new possibilities in the field of spray drying. It

allows to produce particles in the range of 300 nm to 5 μm with a narrow size

distribution. High yields are produced up to 90% and the minimal sample amount

is 1 mL.