drying
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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.