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Drying Research - Current State and Future Trends.

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Dev. Chem. Eng. Mineral Process., 10(3/4), pp. 225-246, 2002.
Drying Research - Current State and
Future Trends
Arun S. Mujumdar*
Dept of Mechanical Engineering, National University of Singapore,
I0 Kent Ridge Crescent, Singapore I 19260, Singapore
Research and development activity in the inter- and cross-disciplinary area of thermal
drying has been escalating rapidly over the past two decades. It is clearly motivated
by economic incentive to improve the operation to produce a better quality product at
a lower cost in terms of resources, energy consumption as well as environmental
impact. The focus to date has been on improving the engineering aspects of design
and operation to produce “engineered”products of desired characteristics. Much
remains to be achieved in relation to the material science aspects of drying, e.g. the
prediction of quality parameters which are product-dependent. What is needed is a
generalised drying theory that incorporates the transient transport phenomena
involved in drying along with appropriate models to account for the morphological
changes, as well as chemical processes that may occur during drying.
This paper will focus on the current developments in drying as well as recent
trends. Some areas where more R&D is needed and which provide opportunities to
make definitive contributions will be identified. For example, sophisticated analytical
measurements at the micro-scale are needed in order to examine the movement of
moisture during drying, and also detect any changes in the physical structure as well
as solute transport that can occur in drying of foodstuffs or solids containing a
dissolved solid in the liquid phase. Even today, there is need to devise more efficient
and compact dryers. Whether we will ever need “micro-scale”dryers is a speculative
question but certainly there will be the need to design dryers with high “volumetric”
eflcienqy, unlike most dryers built today. Finally, there will be further developments
and extended applications for superheated steam dryers, heat pump-assisted hybrid
dryers and multi-stage dryers using different dryer types in each stage. Mathematical
modelsfor drying as well as for dryers will continue to be in demand to allow reliable
design and scale-up, as well as the control of industrial dtyers.
* (email: nrpwwiia nits.C J ~ I ~ . . Y ~
A.S. Mujumdar
In the past two decades there has been a nearly exponential rise in R&D in the interand multi-disciplinary field of drying. Drying of solids is a complex operation
involving transient transport phenomena coupled with physical/chemical/biochemical
transformations, which, in turn, may lead to changes in heat and mass transfer
mechanisms. Our understanding of drying at the microscopic level is still rudimentary
while our knowledge base with regard to dryers has continued to expand over the
years. Scale-up of most of the dryer types continues to be complex and empirical, and
is often equipment- and product-specific because of the highly non-linear nature of
the governing conservation equations of the transport processes.
Many commonly used drying technologies have matured and perhaps reached
their respective inherent asymptotic limits of performance. New products, new
processes, higher production rates, more stringent environmental regulations,
increased safety concerns, etc., often demand better performance levels at lower costs
than is possible with traditional dryers. This need has led to some innovation in
drying technologies. Many of the innovative concepts are still at the pilot stage while
several have reached successful commercialisation. Due to the lower risk involved
and their shorter gestation periods, many of the successful innovations are
evolutionary in nature; revolutionary innovations involve greater risks, are more
difficult to scale-up because of lack of experience, and hence are less readily
embraced by industry.
Conventional versus New Drying Technologies
Kudra and Mujumdar (1995) have classified and discussed various novel dryers,
ranging from laboratory-scale curiosities (e.g. acoustic drying, drying of slurries by
impinging sprays over a hot surface) to pilot-scale demonstrations (e.g. pulse
combustion dryers, ultrasonic spray dryers, impinging stream dryers) to full-scale
commercial dryers (e.g. pulsed fluid beds, superheated steam fluid bedflash dryers,
rotary dryers with drying air injected into the rolling bed). A full discussion of the
truly bewildering variety of non-conventional dryers is beyond the scope of this
paper. The interested readers may refer to the new book by Kudra and Mujumdar
Drying Research - Current State and Future Trenh
(2001) for a comprehensive coverage of the numerous new drying concepts and
Table 1 summarizes the key features of the newer dryers as compared to those of
conventional ones for drying of various physical forms of the wet feed material. Note
that the new designs are not necessarily better than the traditional ones for all
products, but they do offer some advantages that may make them a better choice in
some applications. Some of them are intelligent combinations of conventional dryers.
Table 2 compares some key features of the newer or emerging drying technologies
with those of the more commonly used conventional techniques. In terms of the
sources of energy there is no difference. However, in terms of how this energy is
delivered and transferred to the wet solid there are some significant differences.
In the chemical industry the most common drying application involves the
production of dry particulates from pumpable liquids (solutions, suspensions, or
slurries), thin or thick pastes (including sludges), or granular solids. Spray and drum
dryers are used most commonly for such applications. Spray dryers today no longer
just convert a pumpable liquid to a powder but can be used to product “engineered”
powders with specific particulate size, as well as structure (e.g. agglomerates,
granules, or large mono-sized spherical particles). Personnel safety on and around
dryers, prevention of environmental pollution, and emphasis on production of a highquality product at minimum cost are paramount considerations in the design of spray
dryers today. With the help of computer simulations, better designs of the dryer
chamber and airflows within the dryer have led to minimal wall deposit problems in
spray dryers. A new spray dryer concept even uses a flexible canvas cone instead of
the usual metallic one.
The development of in-place cleanable bag filters makes it possible to retain the
particulates within the dryer chamber; this is achieved by mounting the filter elements
in the roof of the spray dryer chamber. No external cyclones are needed in this case.
This technology, coupled with the popular fluidised-spray dryer featuring a fluid bed
dryer integrated into the base of the spray dryer chamber, allows efficient production
of dust-free agglomerated or granulated products at substantially lower product
temperatures than those found in conventional spray dryers.
A.S. Mujumdar
Table 1. Conventional versus innovative drying techniques.
Feed Type
Liquid suspension
Fluidised bed (hot air
or combustion gas)
Continuous sheets
(coated paper, paper,
contact dryers;
Impingement dryers
New Techniques*
Fluidfspouted beds of inerts
SprayMuid bed combination
Vacuum belt dryers
Pulse combustion dryers
Spouted bed of inert particles
Fluid bed (FB) (withsolids
Superheated steam dryers
Superheated steam FBD
Vibrated bed
Ring dryer
Pulsated fluid bed
Jet-zone dryer
Yamato rotary dryer
Combined impingement/
radiation dryers
Combined impingement and
through dryers (textiles, low
basis weight paper)
Impingement and microwave
or radio fiequency
(RF) or radiation dryers
New dryrs do not necessarily offer better techno-economicpe irmancefor all products.
For drying granular or particulate solids, the most common dryers in use today
are cascading rotary dryers with or without internal steam tubes, conveyor dryers, and
continuous tray dryers (e.g. turbo or plate dryers), which must compete with fluidisedbed dryers (with or without internal exchangers) and vibrated bed dryers, among
others. At least 20 variants exist of the fluidised-bed dryer alone. For larger particles,
a spouted bed dryer is preferable to the conventional fluidised-bed-dryer. For
difficult-to-fluidize, sticky particles or feed stocks with a wide particle size
distribution, the vibrated bed dryer offers advantages over the conventional fluid bed
because it allows use of low drying air velocities while mechanical vibration assists in
pseudo-fluidizing the solids. Consult Devahastin (2000) for a thorough review.
Fluidised-bed dryers have been operated successfblly using superheated steam as
the drying medium both at near atmospheric pressures (e.g. drying of pulverized
lignite) and at elevated pressures (3-5 bars for drying of beet pulp). In addition to
Drying Research - Current State and Future Trenak
eliminating potential fire and explosion hazards, use of superheated steam permits
utilization of the exhaust steam by condensation, reheating, or compression. Of
course, such steam is often contaminated and must be cleaned before re-utilization
depending on the application. Net energy consumption in superheated steam dryers
may be as low as 700 to 1000 Wkg water evaporated, which is five to ten times lower
than many conventional convective air drying systems consume. Mujumdar (2000)
has discussed the principles, practice and potential of the rapidly emerging
superheated steam drying technologies in various industrial sectors, principally for
drying paper, wood, some processed foods, sludges, etc. Numerous relevant literature
sources are also cited. It is commonly recognized as the drying technology of the
Use of volumetric heating by microwave (MW)and radio frequency (RF) fields
has yet to make major inroads in the chemical industry. It is well established that
MW/RF-assisted drying is faster, but the energy costs are also significantly higher and
scale-up for large production capacities is much more difficult. Such dryers are
expected to find some niche applications. RF drying under vacuum has been applied
successfilly on a commercial scale by a Canadian company ,for drying of timber and
thick veneer with future applications anticipated for drying of chemicals, polymers,
and foods. An RF dryer, in conjunction with impingement with hot air jets, is already
a commercial process for drying of coated paper. The efficiency of conversion of line
power into electromagnetic energy and the cost of electricity are major impediments
in commercialisation of this technology.
Selected Examples of Recent R&D Activities
Despite the explosive growth of the technicaUtechnologica1literature on drying, the
scientific literature has consistently lagged behind. In recent years one can discern a
new trend, one can now see a number of scientific papers dealing with specific quality
attributes of specific products, particularly in the emerging areas of biotechnology and
nanotechnology as well as the so-called mature fields of food, agriculture, paper and
wood drying. The following sections detail a few selected examples. The list and
citation of specific literature is arbitrary and for the purpose of illustration only.
A.S. Mujumdar
Interested readers can find numerous similar examples with an in-depth review of the
current literature.
Table 2. Comparison of conventional versus emerging dving technologies.
Energy (heat source)
Fossil fuel combustion
Mode of heat transfer
Drying medium
(convective dryers)
Number of stages
Dryer control
Natural gas
Electricity (MWRF)
Waste heat
Emerging Trends
Convection (>85%)
Radiation (4%)
frequency (<O.1%)
Hot air
Flue gases
Hybrid modes
Non-adiabatic dryers
I One - most common
Two or three - same
dryer type
Pulse combustion
Superheated steam
Hot air + superheated
I steam mixture or two-stage
I Multi-staging with
different dryer types
Fuzzy logic
Model based control
Artificial neural nets
“Smart dryer” (?)
Transport in porous media
Whitaker (1980) has developed the volume-averaged transport equations for
heterogeneous porous media drying. Many materials being dried are non-isotropic and
deformable. For deformable materials one needs the constitutive equations relating
stress to strain as well as the relationships of stresses on mass transport. Perre and
Turner (1996) have given a comprehensive account of modelling the transport of
water through porous networks and drying.
Most models require a number of parameters that can only be obtained via
experiments. In some cases, it is easier to cany out the experiment rather than
Drying Research - Current State and Future Trends
simulate it numerically as the latter involves much more time, effort and cost than the
former. Also, it is typically not possible to predict the product quality a priori, except
in special cases when only stress-generated cracking of the material is the paramount
quality parameter of interest, e.g. in drying of certain ceramics. Attempts have been
made with limited success to predict the drying kinetics and deformation of certain
products (e.g. wood). More effort needs to be devoted to deformable solids as most
materials do undergo significant physical transformations during drying which affect
the product quality rather strongly.
Transport in porous media is of interest in many diverse fields fiom which
researchers in drying can draw useful results. For example, much research is
published in adsorption, heterogeneous catalysis, enhanced oil recovery, etc., that has
relevance to modelling of transport in drying materials. New instrumental analysis
techniques that can give detailed measurements at the micro-scale will shed more
light onto the transport of moisture in solids. Also, new theoretically based
approaches are needed to allow prediction of effective diffisivity in drying, e.g. the
use of both the binary fiiction model and micro-pore diffision theory by Silva et al.
Steam drying of foods
Recently Tang, Cenkowski and Muir (2000) and Tang and Cenkowski (2000) studied
the drying characteristics of sugar-beet pulp and potatoes in both hot air and in
superheated steam at temperatures up to 165°C. Their results confirmed earlier
results, which show higher drying rates in steam at higher temperatures. For potato
they observed a constant rate only at steam temperatures under 145°C and an
“inversion temperature” between 145-165°C for moistures above 2.6 kg/kg (d.b.) and
between 125-145°C for the second stage below 2.6 kgkg (d.b.). The mechanisms
responsible for these two inversion temperatures are different. Above 2.6 k a g (d.b.)
it is the surface moisture removal rate that defines the inversion temperature; below
this value it is the interval moisture removal rate, i.e. higher rate of moisture diffusion
in steam drying probably because of the higher diffisivities at higher temperatures,
and the fact that there is no resistance to mass transfer in the absence of air.
23 I
A.S. Mujumdar
For sugar-beet the critical moisture content seems to be lower in steam drying
which reduces the overall drying time. No data were given regarding the quality of the
dried product. In the absence of oxygen of course, there are no oxidation reactions
and enzymatic browning of the product. The products are more porous and easy to rehydrate. Case-hardening is also absent in steam drying, Note that the better dried
product quality can fetch higher value in the market. Although most direct dryers can
be operated with superheated steam, the cost of retrofitting is often not justifiable
A nanotechnology application
Maenosono et al. (1999) have reported a study of the growth process of an array of
colloidal nanoparticles (4 to 6 nm diam.) of CdS and CdSe/CdS - suspended in
pyridine and water, respectively. A drop of this suspension is placed on a solid plate
and dried in a nitrogen atmosphere. A ring-form multi-layer is formed around the
drop periphery. They developed a mathematical model for the growth of the ring. This
idea and model are expected to provide a new direction for the preparation of more
sophisticated and ordered arrays of semi-conducting nanoparticles whose optical
properties can be utilized in light-emitting and light-converting devices. The most
significant property of the semi-conducting particles is that by varying the particle
size one can change the wavelength of light emitted by particles in a wide spectral
Maenosono et al. (1999) note that a hurdle to practical application of
nanoparticles of well-defined properties is the preparation of large-scale ordered
particle arrays on a substrate. A number of techniques are used to produce layers of
nanoparticles without binding or anchoring methods. The process of ring formation
appears to be one of the simplest. Very small rings are formed in the interior of the
drying suspension film of certain metal nanoparticles. The nature of the solvent can
affect the ring formation process and the evaporation rate. For example, organic
solvents yield a different growth rate pattern than does water. .
Saraf et al. (1998) examined another interesting problem involving drying as
applied to microelectronics, paint and coating or polymers. Effects of solvent drying
Drying Research - Current State and Future Trendr
on the morphology of organic thin films formed with several polymers are of great
interest to the aforementioned industries. Surface roughness, adhesion and clarity of
the film are related to the drying characteristics. The dynamics of phase separation
during drying is of paramount importance. Saraf et al. (1998) used AFM (atomic force
microscopy) to measure the size of the discrete phase in the top layer. They used a
Digital Instruments Nanoscope 111 model at a contact force of -50 nN range at a scan
rate of 1 Hz. The final thickness of the film after drying is only 100-1000 nm. This is
an example of a highly scientific approach to study the effects of drying at the nanoscale. Such studies are expected to yield new products and processes not yet
Drying of seeds
In agricultural processing the drying of seed grade grains is a critically important
operation since the viability or germination potential of the grains is affected by the
drying conditions. Generally milder drying conditions are used to obtain seed-grade
products. Perdomo and Burris (1998) have reported a very basic study of the
histochemical, physiological and ultra-structural changes in the maize (corn) embryo
during drying. They found that when drying high moisture (0.5 kg waterkg dry grain)
seeds, a preconditioning (PC) period of drying at low temperatures prevents damage
due to a subsequent high temperature drying. They measured reserve stability,
enzyme activity, respiration and ultra-structural changes in the radical meristem.
Higher temperature (35OOC) and high relative humidity (90%) PC resulted in
degradation of starch. During PC, they observed migration of lipids towards cell wall
and formation of protein bodies within vacuoles. This was observed with a
transmission electron microscope. Both seeds resulted from PC for 48 hours at 35°C
and RH values below 35%. Similar effects may be expected for other grains. Perdono
and Burris (1 998) conclude that this effect results from a balance between metabolic
and morphological effects of PC, which give a higher tolerance for high temperature
drying needed for cost-effective drying of seeds.
A.S. Mujumdar
Soil science
Water movement in soils is very important indeed. Some of the initial work on
mathematical modelling of liquid transport during drying in porous media was carried
out by soil scientists. Nonthermal dewatering of clays and clay soils using electro-
kinetic techniques to stabilize wet soil or for in-situ decontamination of soils is well
known. More fundamental studies are being reported in the recent literature. For
example, Simonsson, Berggren and Gustafsson (1999) have examined experimentally
the effect of drying and freezing on the solubility of aluminium and silica in a specific
soil. Drying can cause changes in pH and in the solubility of aluminium (slight) and
silica (by 200%). Fissures in the organic material of the soil caused by drying are
suggested as the reason for the big change in solubility of silica; drying may expose
previously occluded binding sites for adsorption.
Drying using supercritical COz
Use of supercritical C02 to remove water has the advantage that drying can occur
without the effects of surface tension which can cause capillary collapse. Thus, this
technique is useful to dry cakes of very small particles which tend to agglomerate
under normal drying conditions. Also, this technique is used to dry extremely porous
materials when one also needs to maintain the porosity in the dry state. Products can
be over 99 per cent porous after drying. Recently, Iwai et al. (1998) used this idea to
dry three types of coal at 3 13K and 14.8 MPa. The “drying” times are quite long (3060 minutes). The cost of the process is clearly prohibitive for coal but may be justified
for production of some catalysts.
An interesting twist to the use of supercritical (SC) or near-critical carbon
dioxide, was proposed and tested by Sievers et al. (2000). They mixed SC C02with
an aqueous solution and decompressed the mixture through a flow restrictor to
produce a dense aerosol, which can be fast dried at 2595°C.At 7.5 MPa (1 100 psi)
and 25OC up to 2 mol% of C02 dissolve in water. This mixture is ejected from a 50130 pm tube ( 5 cm long) to produce fine droplets and micro-bubbles.
Using a drying chamber less than 1 m long they were able to produce a powder,
which is nearly spherical in shape. With some substances like salt, mannitol and
Drying Research - Current State and Future Trends
tobrarnycin, hollow spheres were obtained when dried around the boiling point. With
others like lactose, solid spherical particles were obtained (0.5-5 pm). Further,
depending on the solute and drying conditions, both crystalline and amorphous
particles could be produced. This is an interesting technique with potential application
in the pharmaceutical and advanced materials areas. Morphologies of the product are
usually spherical but authors report particles also resembling doughnuts, lifeboats and
crumpled ping-pong balls. Solvent treatment in ethanol is found to induce
crystallization if the crystalline form is slightly soluble in ethanol.
In the micro-bubble spray drying technique reported by Sievers et al. (2000),
differences in drying conditions yield major differences in morphologies thus
allowing a selection of desired forms in the 0.5-3pm range. If cromolyn sodium
solution is bubble-dried by the dynamic method by mixing with dry nitrogen at
temperatures between 75 and 95"C, these authors found that lifeboat-shaped particles
were obtained. Alternatively, drying near room temperature by passage of the aerosol
plume over a deccicant (concentrated sulphuric acid) yielded nearly spherical
particles. Another interesting observation by Sievers et al. (2000) is that larger
crystalline needles of cromolyn sodium (a drug) can be obtained from amorphous
spray-dried particles by suspension in ethanol. No physical explanation has been
given for this observation.
Enhanced quality by drying
Drying is often associated with some degradation of quality, which is tolerated by the
consumer because of other offsetting advantages. However, there is a special case
where the converse is true. It has been widely reported in the folklore of Japan and
Korea that dried (and rehydrated) shiitake mushrooms (some 26000 tons are
consumed annually in Japan alone) are much more nutritious than the fresh ones.
Recent studies have shown that this is so. In fact, the D2vitamin levels in sun-dried
shiitake mushrooms can be up to 50,000 times higher than in fresh ones. With modem
convective drying technologies the vitamin content of artificially dried shiitake is
quite low. It is now known that the W light in solar radiation is instrumental in the
conversion of ergosterol (provitamin D2) into vitamin D2. Ergosterol is found in many
A S . Mujumdar
fimgi metabolites and is used as an inexpensive raw material in the synthesis of
vitamin D2 (ergocalciferol), which is the form of vitamin D2used for pharmaceutical
applications and as food supplement. Thus it is clear that an optimal drying
technology for such a product must also consider a part of the drying by radiation heat
transfer. Depending on the chemical kinetics the degree of exposure to UV radiation
could be short or long.
Wood drying and VOC emissions
Drying of wood has a direct impact on its quality and hence its market value. New
technologies such as vacuum steam drying, RF assisted drying, etc., have proven their
significant advantages over the classical hot air kilns. More recently there is a focus
on the environmental issues of wood drying, e.g. emissions of VOCs (volatile organic
compounds). This problem is aggravated in recent years with the popularity of
oriented strand board (OSB),which can replace the expensive plywood boards in
some applications. VOC emissions from softwoods exceed those fiom hardwoods by
an order of magnitude. Banerjee et al. (1997) have conducted a comprehensive
experimental study of VOC emissions from OSB flakes and sawduct; the main
furnish was loblolly pine. They correlated the VOC emission levels to the drying
characteristics, which in turn are related to the temperature levels of the drying
material. For example, they show that the VOC level rises when the drying rate
begins to fall, which signals a rise in the wood temperature. They conclude that
uniform reduction of moisture will reduce VOC emissions. They also recommend
avoidance of “dry‘’spots, which are at high temperatures and hence provide enhanced
VOC emission. However, this is difficult to avoid when the material is inherently
nonhomogeneous with a very wide size and possibly moisture distribution. Lower
VOCs are associated with lower temperature and superheated steam drying both of
which avoid development of “dry” layers during the drying process. Karlsson (2001)
has given a thorough review of the problem of terpene release during drying of wood
flakes for particleboard manufacture.
Drying Research - Current State and Future Trends
Drying of coated polymerfilms
Drying of solvent-coated polymer films is important in industries such as adhesives,
recording tapes, photographic films, etc. Models for binary solutions utilizing simple
unsteady diffusion have been developed since the 1960s. Most recent efforts have
focussed on more realistic models including such phenomena as film shrinkage (or
swelling), diffision-induced convection, multi-component solvents, multi-layered
films, etc. Theories for diffusion of solvents in polymers have been applied to
estimate the mutual binary diffisivities in polymer-solvent systems. The resulting
mathematical formulations can be solved using finite element or finite difference
methods. Accurate prediction of the effects of temperature and concentration on
diffusion coefficients is needed for good model predictions. If multi-component
systems are to be modelled it is necessary to check experimentally the predictions of
multi-component diffusivities. Further, some coatings are visco-elastic, which means
the models must take into account coupling between relaxation and mass transfer - a
very complex problem. The problem becomes relatively simpler, but by no means
simple, for polymer systems above their glass transition temperature. Adhesives, for
example, have very low glass transition temperatures so that there is no need to be
concerned with polymer relaxation phenomena.
Furthermore, some coatings are reactive so that a chemical kinetics model must
be included in the species continuity equation. The relative rates of drying and the
chemical reactions can, in principle, yield mass transfer phenomena affected by
vapor-liquid equilibria, chemical kinetics and diffisivities in coatings. Further, one
can consider radiative heat transfer along with convection as well as nonplanar film
The heating may be continuous or intermittent.
As yet quite
undeveloped is the modelling of latex film formation and drying. However, the
mechanisms of latex film formation are still not agreed upon conclusively.
For the design of the drying ovens, product formulation and defect analysis, it is
usefil to have a reliable mathematical model of the drying process of single or multilayer coatings. Alsoy (1 998a, 1998b) has presented a sophisticated model for drying
of concurrent coatings of multiple layers of polymer films. Drying of multi-layer
films presents more complications than drying of single-layer films due to the fact
23 7
A S . Mujumdar
that they have liquid-liquid interfaces at which equilibrium behaviour can play an
important role in solvent transport. In multi-layer drying it is possible to have
concentration jumps (discontinuities), back diffusion, shrinkage and swelling of each
layer. Also the location of the polymer-gas and liquid-liquid interfaces is time
dependent. The solvent in each layer can also be different.
Polymer B + Solvent A
x = xz (t)
x = x,(t)
t = time
Polymer A + Solvent A
Schematic of drying geometry in multi-layer drying.
Heat pumps for dryers
Mechanical heat pumps have long been found to be cost-effective in several major
drying applications. Chua et al. (2000) have classified the numerous variants of
mechanical heat pump dryers already in use, as well as some types that could be
implemented with some R&D for appropriate applications. In particular, those
utilizing multiple modes of heat input (continuously or intermittently) to enhance
product quality while reducing the net energy consumption. Such technology is
particularly attractive for drying heat-sensitive materials. A more novel idea in this
regard has been proposed and tested analytically and via exploratory 'experimental
studies by Ogura and Mujumdar (2000). They proposed coupling a chemical heat
pump (CHP) to a batch or continuous convective dryer to enhance energy savings.
They considered a CaO/Ca(OH)2 reversible hydratioddehydration reaction to release
and store heat for the CHP.Preliminary results are encouraging but numerous hurdles
remain for implementation of this novel technology for drying. Indeed, it should be
possible to use a CHP, probably even more effective, for indirect vacuum drying or
Dtying Research - Current State and Future Trendr
for combined convective-contact drying.
New reactions, reactor designs and
enhancement techniques for heat and mass transfer rates in the reactant bed are
needed to make this technology commercially attractive.
Advanced analytical instrumentsfor drying research
Recently Balasko, Korosi and Farkas (2002) reported on the application of the
Dynamic Neutron Radiography (DNR) technique to visualize the dewatering paths
and patterns in apple slices of three thicknesses and also to describe the surface
changes during dewatering. Earlier a few reports have appeared in the literature on
the applications of NMR, MRI images, X-ray and gamma ray measurements,
computational tomography and image analysis, as well the use of advanced scanning
electron microscopy to study drying at the micro-scale. While such experimental
techniques are indeed very valuable in drying research, it is important to know what
type of water is being measured, and also if the experimental conditions are such that
they do reflect the real drying environment. For example, measurements at very low
drying rates may not apply at the higher drying rates found in practice, both in terms
of the mechanisms of moisture transfer and the effect of drying conditions on product
Drying of polymorph products
When polymorphic forms of a material being dried exist, drying conditions such as
temperature and humidity can affect the quality of the product. Laurent, Couture and
Roques (1998) report on a pharmaceutical drug that has two polymorphs. Two
hydration states are obtained: a tetrahydrate and a monohydrate. The monohydrate
has a therapeutic form, and a non-therapeutic form that is both stable and is formed
irreversibly during drying. A careful study of the sorption and drying kinetics is
needed in this case. The authors, for example, recommended for this specific drug
(proprietary) the need to reduce water activity faster than the temperature increase to
avoid the formation of the non-therapeutic form. Lower external heat and mass
transfer rates coupled with faster removal of water vapor led to desirable products
being formed.
A.S. Mujumdar
Numerous interesting and challenging drying problems exist in various industrial
sectors but are often not reported in the open literature. Some of these can be found
by searching Internet websites. For example, R&D at Purdue University, USA made
the important discovery that the drying method used and the operating conditions
employed can affect the solid state of the crystalline material that is to be dried.
When drying hydrates or solvates it is possible that different polymorphs can be
formed, which makes batch-to-batch integrity problematic especially for
pharmaceutical materials, Scale-up of laboratory-scale data becomes extremely
difficult as well.
Drying of pentamidine isetionate (PI, an HIV drug) and
phenobarbitol gives a polymorph mixture of hydrates. Freeze drying PI yields a
mixture of tri- and anhydrate in a non-reproducible fashion. Vacuum drying, on the
other hand, gives three different crystal structures. Phenobarbitol is reported to
crystallize in thirteen modifications, of which only four have pharmaceutic value.
Clearly, this is a critical case of the significance of choosing the right drying
technique and conditions. Fortunately, this is not encountered very frequently in
In some cases technology developed for one product may find a more viable
application in an entirely different industry. For example, the impulse drying
technique still under development for drying paper has found successful, costeffective commercial application in the drying of waste sludges.
R&D Needs
In view of the tremendous diversity of drying equipment as well as materials that
need to be dried, it is impossible to summarize or even list in a short presentation the
numerous opportunities for R&D in this truly inter-disciplinary field. Here I choose
spray drying as an example to illustrate the diverse set of problems that still need
research attention. Spray drying is a key operation in numerous industrial sectors.
Over 20,000 units are estimated to be in operation around the world. Despite a lot of
attention this unit operation has attracted because of its industrial significance, design
and scale-up of spray dryers remain a major challenge. The following is a wish list of
research topics in spray drying. Similar lists can be drawn for almost all the major
Drying Research - Current State and Future Trendr
dryer types as well as for drying of several commodity products, e.g. paper, textiles,
wood, etc.
Understanding of the atomizer zone remains a daunting problem. It is generally
preferable to have a monodisperse spray. Also, prediction of drop size distributions
using empirical correlations is not always reliable since most correlations are based
on data using a common Newtonian fluid, viz. water. Extension to viscous or viscoelastic non-Newtonian liquids - which are the norm when atomising suspensions or
gels - is at best dubious. Recently, Tsai and Luu (2000) have shown that viscoelastic
liquids are indeed different from Newtonian ones. Fortunately, they are not
necessarily harder to atomize. Indeed, they found that the aerodynamic conditions
needed to form uniform drops are less severe for some visco-elastic liquids than for
Newtonian liquids. Using ultrasound-modulated two-fluid atomisation at 54 kHz,
they produced 40 pm diameter uniform drops of gel-forming xantham gum solutions,
but not for Newtonian liquids. Solutions of lower molecular weight polymers behaved
like Newtonian fluids.
As noted earlier the air-spray interaction is crucial to understanding the heat
and mass transfer behaviour of sprays in the atomiser zone. New measurement
techniques such as the Phase-Doppler-Anemometer (PDA) allow the measurement of
air and liquid velocities. Schelling and Reh (1999)have reported such measurements
for a number of atomiser designs with coaxial gas flow. They provided a simple
model to permit estimation of the entrained flow in confined co-axial sprays (such as
those found in spray dryers) from free-spray data. They observed that narrow high
velocity sprays lead to higher induced airflow rates, but low entrained airflow rate
while having wider sprays had much higher air entrainment. Their data, however,
covered a droplet velocity range lower than that found in actual spray dryers. Also the
PDA technique did not properly measure the smaller droplets due to a limitation on
laser power. Future R&D in this area will yield results of direct use for design as well
as analysis using powerful CFD codes. Kieviet (1997)has pointed out the advantages
and weaknesses of the CFD approach for modelling quality in spray drying. He
claims CFD is especiaIly useful for “what if’ studies. In future, it is expected to be a
powerful technique for design and analysis of spray dryers.
A.S. Mujumdar
It is also of interest in spray drying to know the chemical composition of the
surface of the dried product. Not much is reported in the literature on this aspect.
Faldt et al. (1998) used ESCA (Electron Spectroscopy) to show a large overrepresentation of surface-active protein in spray-dried solutions of dextran, lactose,
glycine and sodium caseinate.
Spray-dried milk powders produced under very
different drying conditions were found to have the same surface composition.
Solubility is known to be the most significant parameter responsible for crustformation due to precipitation on the surface of the droplet. These ESCA results show
that surface-active components such as proteins may transport preferentially to the
surface and may participate in selective drying.
Generation and drying of sprays are important in spray casting processes. Much
research, both experimental and analytical, has been devoted to this subject. Recently
Liang et al. (2001) provided an analysis of the constant rate period of spray drying of
slurries. Their model can predict the morphology of the dried particles as their
experiments showed for water-based slurries of monodisperse silica particles. They
show, for example, that large primary particles yield dense solid granules. The
morphology of granules depends on the spray drying conditions, e.g. droplet size,
initial concentration of solids, liquid viscosity and operating conditions. As a
challenging future research area one needs to consider polydisperse primary particles,
possible transport of the fines to the particle surface during drying as well as the effect
of the falling rate period. Even for monodisperse silica particle-water suspension,
Liang et al. (2001) show that a stratified structure fiom surface to center develops on
drying. From a survey of the literature it seems there is still scope for extending our
knowledge of atomisation by systematic experimental studies of atomisation. The
following are some suggested areas:
Newer atomisation techniques (e.g. a noncircular single-fluid or two-fluid
Atomization of viscous/elastic non-Newtonian slurries in various
Study of the flow field generated by various atomizers, which will serve as
a boundary condition in CFD models of spray dryers.
Drying Research - Current State and Future Trenah
Study of the interaction of sprays with a gas-particle flow in the spray
chamber, including possible agglomeration effects.
Examination of adhesion phenomenon causing deposits in spray dryers.
Examination of novel designs for the spray chamber, locations of
atomizers, supplementary drying air/cooling air distributions, etc.
Study of steam drying in spray chambers; effects on quality of various
Models of morphological changes during spray drying; surface
morphology of dried particles.
Spray drying at reduced pressures in hot air or superheated steam.
Selective drying in spray chambers; solute migration within droplets,
Clearly, spray drying is not yet a hlly mature technology. Many formidable
R&D problems remain to be tackled. Eventually we will see some true innovative
designs based on fundamental scientific knowledge, both at the micro- and macroscales. If one looks at the developments in industrial spray drying technology, it is
difficult to identify clearly any true innovation; most improvements are marginal and
made empirically. New atomizer concepts and designs, and new spray chamber
geometries with more complex drying air configurations may result fiom a
hdamental understanding of the spray drying operation. In particular, it is necessary
to increase the volumetric hold-up of particles/droplets in the drying chamber so as to
reduce the size (and hence footprint, capital and operating costs) of the spray dryer for
a given, production rate. Whether this is feasible fiom the fluid mechanical viewpoint
is still not clear. This will certainly influence the drying kinetics as well as the quality
of the product.
Closing Remarks
Although drying technologies are considered mature they are not sufficiently well
understood to ensure reliable design and off-design analysis. There is potential to
improve existing technologies and to design intelligent combinations of current
technologies that will lead to better quality product, smaller equipment size, greater
A.S. Mujumdar
reliability, safer operation, lower energy consumption, and reduced environmental
impact while reducing the overall cost of drying. Further R&D is needed coupled with
close interaction between industry and university researchers to enhance our
understanding of the fundamental drying processes so that it may be applied to better
design, optimise, and operate the wide assortment of dryers in use today. Mujumdar
(1995) and Mujumdar and Passos (2000) have discussed various aspects of innovation
and innovative drying technologies to which the interested reader is referred for
additional details.
In view of the direct relationship between the massive energy needs of drying
operations in all industrial sectors and the emissions of greenhouse gases as well as
noxious combustion products, it is obvious that there is a need to develop and operate
environmentally fiiendly drying processes. Pulse combustion technology can help
improve the energy eficiency in drying and can even provide some advantages in
drying certain materials, see Mujumdar (1995). Also, development of “smart” or
“intelligentyydryers will help improve the quality of products as well as enhance the
energy efficiency if it is possible to use model-based or fUzy logic control systems.
As use of renewable energy sources for industrial drying will remain limited,
there is a need to devise more efficient combustors as well as the drying equipment to
obtain high-quality products with the least consumption of resources. Active
collaboration between industry and researchers is needed to meet this goal in a
reasonable time. Also, development of intelligent or smart dryers, that will use
advanced real time sensors to adjust the local drying conditions to assure desired
product quality, will become popular within the next decade. Although already
feasible, most industrial dryers still do not employ model-based control or fuzzy
control strategies, hopefully this too will become commonplace within the decade.
Heat pump drying will also become a more accepted technology.
With the rapid advances made in computer technology, material science, and our
understanding of the underlying transport phenomena in drying of solids, there is
scope for rapid development of more efficient drying technologies that could be
deployed in both the developed and developing parts of the world. It is also unlikely
that micro-scale dryers will ever be highly relevant to industrial drying in view of the
Drying Research - Current State and Future Trena!s
physical dimensions and volumes of products that are dried. However micro-scale
dryers could be usehl for pharmaceutical applications where “scale-up by
replication” has distinct advantages, and when confident scale-up techniques are
devised to design industrial size equipment based on micro-scale experiments.
Finally, superheated steam at near-atmospheric and at low pressures will become
more popular as its use becomes widely known and appreciated. Vendors will
develop such technologies for a host of industrial products ranging from foods and
agro-products to paper to wood and waste sludges, see Kudra and Mujumdar (2001),
and Mujumdar (1998).
The author gratehlly acknowledges the assistance of Purnima Mujumdar in the
preparation of this manuscript.
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Received: 9 April 200 1; Accepted ajier revision: 15 May 200 1.
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