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 often involved. In bioproducts like food, grains, and
pharmaceuticals like vaccines, the solvent to be removed is almost
Desiccation may be synonymous with drying or
considered an extreme form of drying.
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 vacuum drying, where heat is supplied by conduction or radiation
(or microwaves), while 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 contrast, the mechanical extraction of the solvent, e.g.,
water, by filtration or centrifugation, is not considered "drying" but
2 Methods of drying
3 Applications of drying
3.2 Non-food products
3.3 Sludges and fecal materials from sanitation processes
4 See also
7 External links
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 a
"constant drying rate period". Usually, in this period, it is surface
moisture outside individual particles that is being removed. The
drying rate during this period is mostly dependent on the rate of heat
transfer to the material being dried. Therefore, the maximum
achievable drying rate is considered to be heat-transfer limited. If
drying is continued, the slope of the curve, the drying rate, becomes
less steep (falling rate period) and eventually tends to nearly
horizontal at very long times. The product moisture content is then
constant at the "equilibrium moisture content", where it is, in
practice, in 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, the products usually undergo shrinkage
and deformation, except in a well-designed freeze-drying process. The
drying rate in the falling-rate period is controlled by the rate of
removal of moisture or solvent from the interior of the solid being
dried and is referred to as being "mass-transfer limited". This is
widely noticed in hygroscopic products such as fruits and vegetables,
where drying occurs in the falling rate period with the constant
drying rate period said to be negligible.
Methods of drying
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.
The following are some general methods of drying:
Application of hot air (convective or direct drying). Air heating
increases the drying 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.
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
Among non-food products, some of those that require considerable
drying are wood (as part of timber processing), paper, flax, 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.
Sludges and fecal materials from sanitation processes
In the area of sanitation, drying of sewage sludge from sewage
treatment plants, fecal sludge or feces collected in urine-diverting
dry toilets (UDDT) is a common method to achieve pathogen kill, as
pathogens can only tolerate a certain dryness level. In addition,
drying is required as a process step if the excreta based materials
are meant to be incinerated.
Greensmith, M. (1998). Practical Dehydration. Woodhead Publishing,
Genskow, L.R.; Beimesch,W.E.; Hecht, J.P.; Kemp,I.C.; Langrish,T.;
Schwartzbach, C.; Smith, (F).L. (2007). Chemical Engineers' Handbook.
Mc Graw Hill Professional. pp. Chapter 12 (Evaporative Cooling
and Solids Drying).
A.S., Mujumdar (1998). Handbook of Industrial Drying. Boca Ratón: CRC
^ "drying - definition of drying by the Free Online Dictionary,
Thesaurus and Encyclopedia". Farlex. Retrieved 23 April 2011.
^ "Modeling the thin-layer drying of fruits and vegetables: A review".
Comprehensive Reviews in Food Science and Food Safety.
Wiley-Blackwell. 15: 599–618. 2016-02-04.
doi:10.1111/1541-4337.12196. Retrieved 2016-02-08.
^ Strande, L., Ronteltap, M., Brdjanovic, D. (eds.) (2014). Faecal
Sludge Management (FSM) book - Systems Approach for Implementation and
Operation. IWA Publishing, UK (ISBN 9781780404738)
Machinery for drying solid materials
Dissolved air flotation
Solid phase extraction
API oil-water separator
Rapid sand filter
Rotary vacuum-drum filter
Vacuum ceramic filter
Aqueous two-phase system