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Spray-Dried Particle Morphologies.

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Dev.Chem. Eng. Mineral Process., 10(3/4), pp. 323-348, 2002.
Spray-Dried Particle Morphologies
D.E. Walton"
Department of Chemical Engineering and Applied Chemistry,
Aston University, Aston Triangle, Birmingham B4 7ET, UK
Particulate powders and particle morphologies produced by spray drying are
discussed, along with factors that afJect powder quality, e.g. process conditions, and
particle properties. Structural categories of spray-dried particles are demonstrated
and related to various drying mechanisms. The methodr of studying particle
morphology are also outlined with particular reference
to
the single droplet drying
method, reportedly analogous to industrial spray drying and possibly other
particulate formation processes. The advantages and disadvantages of the method are
discussed and the importance of particle morphology to powder technology
emphasised.
Particulate Powders from Spray Drying
Particulate powders, as opposed to powders produced by crushing or grinding of a
solid after drying or processing, are beneficial to both the manufacturer and the
consumer alike. From the manufacturers point of view, particulate powders can be
produced continuously, have high throughputs, large surface areas and therefore high
drying rates. They are convenient to transport and package both on and off the
manufacturing site, and are ready to use without further preparation. They can also be
moulded and machined (usually with the incorporation of a binder) into any shape or
form. But most importantly, the specifications of a particulate powder can be rapidly
altered at the manufacturing stage to suit customer taste, fashion or market demand.
* (email: d.e.walton@aston.ac.uk)
323
D.E. Walton
From the consumer’s point of view, powders are also easy to handle and transport.
They are convenient to store, have long shelf lives and are easy to re-dissolve or
disperse. They have a textural quality related to flowability and fiiability and can be
given an aesthetic quality by adding colour and fragrances.
Probably the most widely used dried particle formation process in industry today
is spray drying. An example of convective drying, spray drying is applicable to a wide
range of products and industries and is used for the continuous production of dry
solids in powder, granulate or agglomerate form. The process involves the
atomisation of a liquid feed in the form of a solution, emulsion or pumpable
suspension into a spray of droplets, as shown in Figures 1 to 4. The droplets are
contacted with a hot gas (usually air) in a drying chamber flowing in a co-current,
counter-current or mixed flow direction. Evaporation of moisture from the droplets
proceeds under controlled temperature and airflow conditions to form a dry or semidried powder. The powder is then continuously discharged, generally from the base of
the drying tower for separation or powder after-treatment such as re-wet
agglomeration or instantising. Most spray drying plants are equipped with cyclone
filters for primary separation of the powder from the drying air. If further cleaning is
required, a bag collector, electrostatic precipitator or a wet scrubber system can be
added after the cyclone battery.
The widely varying drying characteristics and quality requirements of many
products, together with safety and emission requirements, determine the selection of
the atomiser, the most suitable airflow pattern, and the drying chamber design. Hence
spray drying is an ideal process where the end product must comply with precise
quality standards, e.g. particle size, particle size distribution, bulk and particle density,
particle fiiability, dispersibility, and of course powder moisture content. See Table 1.
Product characteristics like bulk and particle density are important because they have
a bearing on transport costs, packaging considerations and even the overall colour
density of the product. Moisture content, particle size, particle size distribution and
particle sphericity affect bulk density and powder flowability, whereas particle
strength or ftiability, including powder flowability, are important in terms of powder
handling.
324
Spray Dried Particle Morphologies
Figure 1. A small-scale counter current operation spray dryer with a throughput of 1
to 7 kdh, The dving chamber is electrically heated (4.5 kW) and madefiom stainless
steel with a diameter of apprar. 1.2 m. It has an upper drying temperature of 300°C.
Figure 2. Dryer top showing atomiser in situ. The atomiser is turbine driven and
utilises centrifirgalforce to atomise the feed. Alternatively, nozzle atomisation can be
used, i.e. centrifugal-pressure,pneumatic or sonic atomisationfor viscousf e e d ,
Figure 3. Typical examples of an industrially spray-dried product. Left: Dyestuffs:
Mag. x 16. Initial feed conc. 30% to 40% w/w. Drying air temp., In: 350"C,
Out: 110°C; Right: Wall tile clay: Mag. x 16. Drying air temp., In: 550°C. Out: 90°C.
325
D.E. Walton
Figure 4. Pilot-plant-scale
spray h i n g tower. The
tower is approximately 1.5 m
in diameter and 5.5 m high. It
has a tall-form design and is
operated in a counter-current
mode. Atomisation studies are
possible which attempt to
correlate droplet size with
feed characteristics, atomiser
design, pressure (or rotation
speed)
and
throughput.
Studies of gas flow patterns
and velocities are also
possible, as well as particle
residence times. The ability
to
control
product
characteristics (of the type
listed in Table 1) is a major
advantage of the spray drying
process over other drying
methods, especially when
dealing
with
high
performance (high cost, low
volume)
and
consumer
products.
Such
characteristics help to define
the overall quality of a spraydried powder with respect to
both the manufacturers and
the customer’s demands.
Powder/Darficle rnohture content: depends on the drying temperature, particle
and material properties. The feed concentration, degree of feed aeration, particle size
and particle size distribution, particle drying history, e.g. residence time within the
dryer, and the type of particle structure or morphology formed during drying, are all
important considerations when determining the optimum drying temperature to obtain
the desired particle or powder moisture content. The hygroscopic nature of the
material, retention of volatile flavours, aromas and material bioactivity (if relevant)
must also be a consideration, although a balance must be struck against the thermal
and economic operating efficiency of the dryer.
326
Spray Dried Particle Morphologies
Table 1. List of common product characteristicsor quality aspects associated with
spray-dried powders
o Moisture content.
Q Powderform: Reeflowing, agglomerated or granulated.
o Particle strength orRiability.
o Mechanical properties, and incorporation of bindersfor pressing or machining.
o Bulk powder and particle density.
o Particle size and particle size distribution.
o Powderflowability.
Redispersability,
o Powder colour density.
o Particle morphology.
o Volatile retention:flavours and aromas.
o Bioactivity: relevant to the pharmaceutical and biochemical industries, e.g.
e q m e s , hormones.
o Product after-treatment: the addition or re-addition of volatile constituents after
drying, de-dusting agents, catalysts to promote fluidisation, re-wet agglomeration,
etc.
Virtually all bulk powder and particulate properties are related to particle
morphology, as shown in Figure 5 and discussed below.
I Particle Morphology I
t
r-
Process conditions
Feed specijkation
Method and conditions of atomization
Dryer design
Drying temperature
Powder after-treatment
‘1
Powder/particle
flowability
redispersability
I
Particle size
& distribution
Bioactivity
retention
Figure 5. Powder quality and particle morphology.
32 7
D.E. Walton
Powder flowubilitu: depends upon the degree particle agglomeration, particle size,
size distribution, sphericity, density, moisture content, fiiability, and material
properties. Particle agglomeration is a major factor in determining the flowability of a
powder. The size and size distribution of the particles are determined predominantly
by atomisation conditions. The extent of particle surface asperitation is also
important; these can reduce powder flowability considerably by mechanical
interlocking. The material (solid) density will also affect powder flowability. A high
moisture content can make particle surfaces sticky and soft, this can lead to particle
agglomeration and powder cohesion, particularly on prolonged storage. The
production of fines (on handling, transport or storage) can decrease particle size and
alter particle size distribution. The electrostatic and hygroscopic nature of the particle
material is also important. Particle charge density is related to particle size, shape and
particle moisture content.
Bulkhowder densitv: depends on material (solid) density, particle size, size
distribution, sphericity, particle density, friability and moisture content. Particle nonsphericity can alter bulk particle or powder voidage. Particle vacuolation or hollow
particles will affect particle thus powder density. Similarly, the production of fines on
handling or storage. The moisture content of a powder can change on prolonged
storage, increasing its bulk density.
Powder cofour dens@: depends on particle size, size distribution, moisture
content and opaqueness of particle shell. The amount of voidage in the powder is
important for the penetration of incident light and the transmittance of reflected light.
Therefore, particle shape and particle agglomeration are important considerations.
The opaqueness of a particle shell is related to shell or crust thickness and material
properties. Particle density and the degree of particle vacuolation are also important.
Powder redisuersubilifv:depends on material solubility (including the binder if
present), particle size, size distribution, porosity, sphericity, moisture content and
fiiability. Particle shape is related to surface area and thus solubility. Particles or
flakes would make a difference. The integrity of intra-particle cohesion in solution is
important. This is related to particle porosity (structure) and density. Particle density
328
Spray Dried Particle Morphologies
may also affect powder dispersion. The presence of bound and unbound water can
affect particle or powder solubility.
Particle strenpthlfriabilitv: depends on particle size, density, moisture content,
structure, sphericity, and the type of binder if present. Particle density is related to
particle structure, particularly the degree of particle vacuolation or porosity. If the
particles are hollow, this will have a marked affect on particle strength. Moisture
content affects the rheological properties of the particle skin or crust formed and, if
high enough, can weaken the structural integrity of the particle. Particle structure is
obviously very important; consider whether agglomerate, skin forming or crystalline
morphologies are formed. See later. The particle shell or crust thickness (if hollow)
and whether brittle, hard, soft, sticky or crumbly, are also important. With some
materials particle sphericity may be particularly relevant when in buWpowder form
as surface asperities can weaken particle structure when compressed.
Particle sizddistribution: depends on feed specifications, type and conditions of
atomisation, drying temperature, and the amount of agglomeration. Feed
concentration, temperature, and the degree of feed aeration all affect feed viscosity
(surface tension to a lesser extent) and therefore the atomisation properties of the
feed, thus particle size and particle size distribution. The feeds inability to be pumped
and to be effectively atomised can also affect particle size and morphology, e.g.
ligament formation. Operating pressure or rotation speed (depending on the method of
atomisation) affect both the particle size and size distribution. Particles can inflate,
shrivel or distort during drying, the extent of which can be (made) temperature
dependent. Particle ballooning or puffing results fiom expanded particle vacuolation.
Particle vacuolation is dependent on feed specifications such as the degree of feed
aeratioddeaeration, concentration, temperature and the type of material being spray
dried. Particles can be agglomerated or granulated after the initial drying process, e.g.
using spray dryers with integral fluidised beds or by re-wet agglomeration.
Bioactivitv: depends on particle moisture content and drying temperature. Many
bioactive materials such as enzymes use water to protect the activity of functional
sites, as well as being an integral part of the material’s reaction kinetics. If the drying
temperature is too high, the bioactivity of many materials can rapidly decrease. Cryo-
329
D.E.Walton
or thermo-protectants can be added for storage protection. Trehalose is known to
protect the bioactivity of some biologically active materials and can be used as a
carrier when spray drying some products.
Volatile retention: depends on feed specifications, drying temperature and
material properties. A high feed concentration is very important for initial volatile
retention. The particle must form a barrier to mass transfer as soon as possible. A high
concentration gradient within the particle during the initial stages of drying is also
desirable, as the selective diffusion (Thijssen, 1970) of many flavour and aroma based
materials is far less than that of water under certain process conditions. The type of
morphological structure formed by the particle is also very important. Skin forming
materials have the highest volatile retention properties. High feed concentrations can
also subdue morphological events, which can lead to volatile loss during drying, e.g.
drying cycles of ballooning, blowhole
*(
formation and particle
shrivelling.
Thermal decomposition of the product
must be avoided, see Figures 6 and 7.
Droplets A and B contain the same
material at the same concentration and
are
Figure 6. Statistical variations in
droplet drying behaviour.
being
dried
under
identical
conditions, yet both droplets can have
very different drying histories.
Figure 7 demonstrates how the single droplet drying method can be used to
optimise drying conditions for maximum volatile retention (Walton and Mumford,
July 1999). The beneficial effects of accelerated skin formation at 15OOC are also
demonstrated, which are overridden by droplet inflation and rupture at 2OOOC (see
Figure 6). The points represent the averaged data (best fit shown) fiom 3 droplets
dried per time interval. Including statistical variation, the amount of data scatter is
very dependent on the type of material being dried, feed specifications and drying
conditions. Semi-instant skimmed milk is a skin forming material (see later in
Figure 13).
330
Spray Dried Particle Morphologies
100
"
I
I
1
I
I
20
40
60
80
100
I
0
Droplet drying time ( s )
Figure Z Drying curvefor determination of the optimum drying temperature.
'Filled circles' are 150°C; 'open circles' are 200°C. Drying material is 15 wt%
semi-instant skimmed milk containing a pseudo-volatile ethanol at 10 wt%.
Process Variables
The effect of process variables on spray-dried powder characteristics, particle
morphology in particular, the method and conditions of atomisation, the type of
spray/air contact, drying air temperature, particle residence times, feed parameters
such as concentration, temperature and the degree of feed aeration, are difficult to
assess in general terms. This is partly due to the lack of information within the
literature (probably the result of commercial confidentiality) and partly due to the
specific drying nature of most materials. The latter renders morphological phenomena
difficult to classify with respect to dryer operation as the physical, and to a lesser
extent, chemical nature of the particle skin or crust formed during drying determines
the type of particle drying behaviour. For example, the mechanisms of moisture
movement during the non-saturated surface drying period(s) may cause particles to
inflate, distort or shrink. With some materials a skin may form externally and possibly
internally and with others, depending upon the drying conditions, the particles may
crack, case-harden or even fiacture completely giving shell fiagments or fines. The
33 I
D.E. WaIton
situation is complicated further by multicomponent formulation since different
constituents may migrate within the particle during drying. For example, in detergent
manufacture it is not uncommon for feed slurries to contain up to 12 different
ingredients. Post-drying operations such as mechanical grinding or the transference of
feed to the atomiser by compressed gas (resulting in feed aeration) and the wide
variety of techniques used to determine specific data, also make comparative particle
morphology studies difficult. This is particularly true for bulk and particle density
determination. However, some general trends are discernible fiom the literature.
Bulk and particle density are known to increase with an increase in feed
concentration, whereas an increase in drying temperature, atomisation pressure or
feed temperature can produce a decrease, although the opposite effects have been
observed. For example, Duffie and Marshall (1953) found that whilst the bulk density
of some materials decreased with an increase in drying air and feed temperature, with
others it increased, even though they were spray dried under identical conditions.
Factors such as feed rate, powder temperature and obviously residual moisture
content are also known to influence bulk powder density. Varying parameters such as
feed temperature can alter the level of feed aeration, feed viscosity and solute
solubility. Similarly, an increase in feed concentration will increase feed viscosity.
These factors in turn may influence the atomisation properties of the feed and
consequently particle size, particle size distribution, particle shape and particle
residence time within the dryer. Short residence times are good for drying heat
sensitive materials, particularly in the food and dairy industries; the method of
spray/air contact is therefore very important. Increasing atomisation pressure or
rotational speed generally decreases droplet size. It also increases throughput and
higher drying temperatures may be required to maintain the desired powder moisture
content. Increasing the drying air temperature will generally produce a decrease in
bulk and particle density due to an increase in dried particle size, as there is a greater
tendency for the particles to be hollow. This is caused by particle inflation or
ballooning and is particularly common in skin forming materials. The correct
atomisation conditions, chamber design and particle size can also help to avoid heavy
332
Spray Dried Particle Morphologies
wall deposits or fouling of the dryer wall by wet product, thus minimising downtime
for cleaning, spoiled product and possibly avoiding a potential fire hazard.
Because of the wide range of variables involved in spray drying and their interdependency, modelling the spray drying process is very difficult, even with the aid of
modem CFD techniques. Consequently, spray drying (and drying in general) is still
regarded by many as being more of an art than a science, with most drying expertise
residing in the hands of the practitioner rather than the mathematical modeller.
Morphological studies on dried or drying particles can therefore provide a valuable
insight into the fundamentals of droplet drying as well as the physical and chemical
aspects which govern particle structure. Such studies permit the comparison of drying
rates between different feeds, feed concentrations, products or product formulations,
thus enabling the selection and improvement of process conditions and, within certain
constraints, assist in dryer selection, design and simulation.
Study of Particle Morphology
Several different experimental approaches have been used to investigate the particle
morphologies produced by spray drying. The simplest involves collecting dried
powders directly from a spray dryer and examining them under a microscope. This
method has provided valuable information regarding the effects of dryer conditions
and feed specification upon particle morphology, particularly bulk powder properties
such as density, particle size and particle size distribution. However, the technique has
its limitations as direct observation of particle formation in situ is not possible.
Single droplet drying studies, essentially initiated to study the evaporation of pure
liquid droplets under both natural and forced convection, e.g. Langstroth et al. (1950),
have therefore been extended, particularly by Ranz and Marshall (1952), to solutions,
suspensions and pastes. This allowed direct observation of the formation of particle
morphologies produced under controlled conditions, similar to those found in a spray
dryer. The technique involves suspending a single droplet or stream of droplets in fiee
flight or by physical suspension in an oven or windtunnel in which air temperature,
velocity and humidity are carefully controlled and monitored. Alternatively, droplets
are allowed to free fall.
333
D.E. Walton
Free flight studies achieve droplet suspension in a number of ways. For example,
Miura et al. (1977) and Oteng-Attakora et al. (1994) used the inversion of a gas
velocity profile to keep a single droplet floating in an ascending air current by
matching the air velocity with the terminal velocity of the droplet. Other methods
include the use of electrostatic or more recently ultrasonic fields (Toei and Furuta,
1982) for suspending droplets 1.5 mm to 2.0 mm in diameter, although the use of
ultrasonics in spray drying is not new. Alternatively, Greenwald and King (1982)
deve1oped.a device that formed a single stream of droplets of uniform size, which
were allowed to free fall through a heated column 2.3 m long. They reported a
uniform and controllable temperature profile during drying, albeit of very limited
residence time, i.e. approximately 0.2 s to 0.3 s. Under these conditions the degree of
drying would not be comparable to that found in a practical spray dryer.
Physical suspension techniques, typically those used by Charlesworth and
Marshall (1960) and subsequently Crosby et al. (1958, 1971, 1977) have been
extensively used to investigate the evaporation rates of single droplets and the general
phenomena associated with the evaporation process. Although there are a number of
variations, the method generally involves suspending a single droplet in a controlled
air stream from either the end of a vertical (e.g. Walton and Mumford, 1999, see
Figure 8) or in the centre of a horizontal fine glass filament or filament-thermocouple.
The filaments are usually in the region of 200 pm to 600 pm in diameter, some of
which are rotated to ensure a more even exposure of the droplet surface to the drying
air. Initial droplet sizes range from approximately 0.2 mm to 2.0 mm in diameter.
Glass nozzles (Hassanand Mumford, 1993), hypodermic syringes (Audu and Jeffreys,
1975) and micro-burettes (Ranz and Marshall, 1952) have all been used as suspension
devices. Once in place, measurements of droplet weight, size and temperature are
possible, as well as visual observations of droplet drying behaviour.
Limitations of Single Droplet Drying
The free flight, free fall and the physical suspension methods are all subject to certain
limitations. In particular, droplet sizes can be twice the size of those produced by
spray drying, i.e. atomised droplets can range fiom approx. 10 pm up to (and beyond)
334
Spray Dried Particle Morphologies
I
I
Figure 8. Example of a single droplet drying apparatus. Left: droplet
suspensionhotation device; right: filament suspended droplet (scale = I mm).
1000 pm in diameter. The physical support may act as a restraint, induce droplet nonsphericity, and in some cases create an additional heat transfer path. Large droplets
may also oscillate resulting (theoretically) in increased mass transfer (Gamer and
Skelland, 1954; Ahmadzadeh and Harker, 1974). Furthermore, experiments are
performed under carefully controlled laboratory conditions which do not simulate the
type of time-varying drying histories droplets and particles experience in a real spray
dryer, particularly in terms of the turbulent and often cyclonic airflows found in most
drying chambers. The statistical drying nature displayed by droplets can also
contribute significantly to data scatter when drying a small number of droplets. For
example, the initiation of internal bubble nucleation within the droplet is a statistical
phenomenon. Two droplets containing the same material at the same concentration
and dried under identical conditions can have two very different drying histories, as
illustrated in Figure 6 and reported by Duffie and Marshall (1953). Any variations in
droplet drying behaviour will therefore not be averaged out. In an industrial spray
dryer of course, millions of droplets are being dried every second and it is not
uncommon for some dryers to produce over 200 metric tomes of powder an hour. A
comparison of spray-dried particles with those produced by single droplet drying is
335
D.E. Walton
therefore essential to avoid myopic or skewed data. Studying bulk powder properties
may help to average out these variations.
Obviously one simple way to increase the sensitivity of the method is to increase
the number of droplets dried. However, this may or may not be practical as most
single droplet drying methods can be relatively time consuming, requiring a
reasonable level of skill and dexterity to operate. These limitations can make detailed
quantitative (kinetic) studies difficult. Nevertheless, general trends are certainly
discernible as demonstrated in Figure 7. And, such constraints do not necessarily
apply to qualitative work.
Although single droplet drying studies avoid many of the complex interactions
encountered in production and pilot-plant-scale spray dryers (for this reason alone
they have been criticised for being unrealistic, particularly by Toei, 1982), at present
they offer the only practical and direct means of observing the spray drying process at
the droplet or particulate level. Hence they have found increasing use in industrial
development laboratories during the past 20 years or so. The importance of single
droplet drying techniques was emphasised by Genskow (1988) who, in reviewing the
problems associated with the spray drying of consumer products, particularly in terms
of product quality and performance, outlined the need for new analytical methods and
tools to provide a better insight into the fundamentals of drying. There is a cautionary
note however. The increasing use of CFD in spray dryer and plant design (Oakley,
1997) is tending to overshadow the need for continued empirical data, particularly in
the pharmaceutical industries and the relatively new area of biochemical engineering.
Although CFD has proven itself to be a powerful diagnostic tool, the complexity of
the spray drying process including, e.g. atomisation, particle formation, etc., means
that such techniques may never completely replace pilot-scale testing or the empirical
nature of the design process.
Particle Morphology: A Brief Literature Review
Descriptions of particle morphologies within the literature are scarce. Of particular
interest is the work of Marshall, King, and Thijssen (1970). Most describe droplet-
336
Spray Dried Particle Morphologies
drying behaviour or give mechanisms or models for moisture movement or particle
surface effects such as folding, shrinkage or shrivelling during drying. Many describe
the drying of food or dairy products, reflecting the dominance and importance of the
spray drying process within these areas, particularlyjust after the second world war.
Many of the early important phenomenological studies on particle morphology
(particularly the work of Marshall et al., 1950, 1953, 1958, 1960) were carried out by
simply collecting dried powders directly from a spray dryer and examining them
under a microscope. Marshall and Seltzer (1950) for example, briefly describe the
various particle morphologies produced by spray drying in relation to feed
concentration and the methods and conditions of atomisation. The types of materials
studied include lemon juice, pectin, milk, sodium chloride, coffee and gelatine.
Photomicrographs of the particles show most to be spherical with a large proportion
of agglomerates, although gelatine appears to have produced elongated egg-shaped
particles, probably the result of incomplete atomisation. Spray-dried sodium chloride
particles were composed of individual crystals cemented together, whereas the
morphologies of a number of other materials were dominated by hollow particles. A
possible mechanism was also proposed for the formation of hollow particles dried
from skin forming materials, and the importance of certain morphological features
discussed in terms of dryer design and process operating conditions.
Chu et al. (1951) studied the effects of dryer operating variables on the bulk
density and moisture content of a synthetic detergent. Photomicrographs of the spraydried material reveal little about the particle morphology and post-drying operations
raise some doubts as to the reliability of observations regarding bulk density and
moisture content. A compressed gas was used to transfer the feed from the feed tank
to the atomiser; this may have promoted the formation of hollow particles by the
desorption of absorbed gas within the droplets during drying, as reported by
Greenwald and King (1982). The presence of hollow spherical particles and a mixture
of particles with irregular fragments and agglomerates were also reported.
Buckham and Moulton (1 955) spray dried different concentrations of ammonium
sulphate ranging fiom 0.1 g/cm3 to 0.7 g/cm3. Under the same drying conditions, the
larger particles were reported to have a lower density than the smaller particles,
33 7
D,E. Walton
indicating a greater tendency on the part of the larger particles to be porous or hollow.
Photomicrographs of the samples show mainly spherical particles with a high degree
of agglomeration. They suggest the agglomeration might have occurred due to static
electrical effects, although wet particle surface rheology and particle collision or
impingement during drying where not considered. Many particles were reported to
have expanded on drying with particle expansion increasing with particle size and
feed concentration.
Crosby and Marshall (1958) spray-dried sodium sulphate solution, coffee extract,
and clay slip. The final spray-dried product was oven dried and the particle size
distribution, bulk and particle densities determined. Photomicrographs of the dried
particles show the effects of operating variables on the properties of the particles, and
that particle morphologies were dependent on the type of material being spray dried,
classified as crystalline (sodium sulphate), amorphous (coffee extract) and
agglomerate (clay slip). In all cases, the bulk density of the spray-dried material
seldom approached 50% of the true density of the solid being spray dried, suggesting
that most of the particles were hollow. Crosby and Marshall also report that it was
possible to vary the particle properties by changing air temperature, feed
concentration, and in some cases feed temperature.
Photomicrographs of the sodium sulphate particles appear to be hollow and
composed of multiple crystals concentrated at the particle surface. The effective wall
thickness of the larger particles was comparable to that of the smaller particles,
although the internal structure of the smaller particles was reported to be denser. This
resulted in a higher bulk density. It is uncertain however as to whether the difference
in powder voidage was taken into account. They concluded that once a solid wall had
formed no further change in particle diameter occurred throughout the drying process.
The general particle sphericity and uniform wall thickness suggest that the droplets
have dried symmetrically. They also show the crystalline nature of sodium sulphate,
evident fiom the characteristic funy appearance of the particles. Coffee extract
particles appeared to be hollow and exceptionally thin walled, resulting in a lower
than average comparative bulk density. The shape of the larger particles confirms
expansion and contraction during drying, but no particle collapse was reported. The
most interesting morphologies were those produced by the clay particles.
338
Spray Dried Particle Morphologies
Photomicrographs reveal shrivelled mushroom cap-shaped particles, indicative of
collapse or deformation of the wall structure in some way. It was not clear however,
as to whether all the particles were examined before or after oven drying.
Walton and Mumford (1999) examined samples of industrial and pilot plant spraydried materials obtained from various manufacturers together with details of drying
conditions and feed specifications. The samples were subjected to qualitative and
semi-quantitative examination in order to identify structural and morphological
features. The results were interpreted in terms of bulk physical properties, and related
to the drying conditions. Three distinct categories of particle morphology were
identified, similar to those outlined by Crosby and Marshall, namely crystalline, skin
forming and agglomerate, see Figures 9 to 14. Selected properties such as powder
flowability, particle and bulk density, particle size and particle friability, were shown
to have a direct relationship to morphological structure. Single particles were also
produced in a convective drying process analogous to spray drying (see Figures 15 to
17) in which, different solids or mixtures of solids were dried from solutions, slurries
or pastes as single suspended droplets. The localised chemical and physical structures
were analysed and in some cases the retention of volatiles monitored. The results
were related to experimental conditions, viz. air temperature, initial solids
concentration and the degree of feed aeration. Morphologies of multicomponent
mixtures were found to be complex with the respective migration rates of the solids
being dependent on drying temperature.
Dlouhy and Gauvin (1960) studied the evaporation and drying rate of spray-dried
calcium lignosulphate (1 8% w/w) solutions. Photomicrographs of the spray-dried
particles reveal mainly discrete particles with clear, smooth skins. Drying at different
air temperatures produced a considerable difference in particle size and morphology.
At air temperatures of 52°C to 64"C,the particles appeared to be smaller and more
regular in shape than those dried at 192°C to 215°C. A greater proportion of hollow
particles was also produced at the higher drying temperature, although a gas
pressurised feed tank was used to transfer the feed to the atomiser, possibly promoting
particle vacuolation. Dlouhy and Gauvin (1960) reported no significant falling-rate
period when drying the lignosulphate and suggested this was due to the concentration
339
D.E. Walton
Figure 9. Industrially spray-driedparticles - Agglomerate I.
Lef: ferrite, middle: copper oxychloride (close-up of particle surface), right: tungsten carbide.
Figure 10. Industrially spray-dried particles Agglomerate II.
Lef: lead chromate (agglomeration), middle: lead chromate, right: copper oxychloride.
Figure 11. Industrially spray-driedparticles
- Skin forming
I.
Lef: coffee, middle: yoghurt powder (mass agglomeration), right: yoghurt powder (blowhole).
Figure 12. Industrially spray-dried particles - Skinforming II.
Left: codried egg and skimmed milk (shrivelledparticles),middle: dyestuffs (blowhole),
right: coffee (thin shell wall).
340
Spray Dried Particle Morphologies
Figure 13. Industrially spray-driedparticles - Skinforming 111.
Lefr and middle: skimmed milk (highfeed concentration;particle vacuolation.
right: co-dried egg and skimmed milk (hollowparticle).
Figure 14. Industrially spray-dried particles - Crystalline.
Lefr: tri-sodium orthophoqhat, middle: tri-sodium orthophosphate (close-up),
right: organic material containing a UV active compound. Used in washingpowders.
Figure 15. Particles driedfiom single droplets -Agglomerate (200°C).
Lefr:colloirlnl carbon, middle: silica (23w%). right: sodium chloride (36~1%).
Figure 16. Particles driedfiom single droplets - Skin forming (I5wt%, 200°C).
Lefi: sodium silicate, middle: gehtine, right: semi-instant skimmed milk
341
D.E. Walton
Figure 17. Particles driedfiom single droplets - Crystalline (15wt%, 7VC).
h~?:
sodtumpyrophosphate,miaWe: ammonium diwrogen orthophosphate,right: sodium benzoate,
at which a skin was formed. Skin formation did not occur until the very last stages of
drying, with no evidence of skin formation below a solid content of 85% w/w.
Duflie and Marshall (1953) spray dried a number of inorganic and organic
materials, e.g. potassium nitrate, coffee, and corn syrup, in a small-scale laboratory
dryer. Photomicrographs of the particles show morphological features such as
ballooning, mushroom-cap shaped particles and shrivelled particles. Hollow particles
were reported to be a relatively common feature, although some materials such as
corn syrup produced solid, spherical particles. A detailed description was provided of
the drying conditions and final particle morphologies of all the samples. They also
noted the skin forming properties of sodium silicate, along with its tendencies to
produce hollow particles.
Charlesworth and Marshall (1960) dried single droplets of a number of inorganic
solutions and food substances, e.g. copper sulphate, ammonium nitrate, sodium
chloride, coffee extract, sucrose, and milk. Particle morphologies were reported to be
similar, consisting of hollow, thin walled, nearly spherical particle structures with a
smooth outer surface and rough inner surface. No photomicrographs were presented,
although the drying behaviour of the droplets was expressed in the form of a
generalised drying sequence of morphological events, leading to a classification of
particle morphology.
Examples of mechanistic models describing particle surface effects include those
of Bellows and King (1973) and Alexander and King (1985) who suggested the
following relationship between particle surface tension, viscosity, particle rupture
diameter, and the time required for particle rupture closure, namely:
tT = (pLDi)/crL
342
(1)
Spray Dried Particle Morphologies
where tT is time for rupture closure; Di is rupture diameter; p~ is feed viscosity; crL is
feed surface tension. Here particle rupture refers to the release of internal particle
vapours through the particle skin, crust or shell during drying, similar to that
illustrated in Figure 6. King suggested that viscosity was the dominant property in
determining the time for rupture closure. The model was later extended to describe
the tendency of a particle to form ridges or wrinkles during drying. A folding
parameter (T) was defined which compares the relative rates of fold formation (vF) to
fold smoothing (vs), that is:
T =
VF/V~
= [(-dr/dt) pL] CL
(2)
where r is droplet/particle radius; t is time. The following simple folding criterion was
then expressed as: If T >> 1, there is a large tendency to fold. If T << 1, there is little
or no tendency to fold.
Similar mechanistic models also exist for, e.g. particle inflation and vacuolation
(Sano and Keey, 1982), particles with a receding moisture interface (Cheong et al.,
1986), the selective diffusion of volatile materials within a particle (Thijssen, 1970),
and the characteristics of foamed droplets (Crosby and Weyl, 1977).
Generalised Description of Spray-Dried Particle Morphologies
There is a surprising lack of photomicrographs of spray-dried particles, albeit within a
limited literature. This is probably due to the technical difficulties in obtaining and
presenting clear images of the particles. However, it is possible to give a generalised
description of the types of particle morphology produced by spray drying.
Spray-dried particles have three distinct types of morphological structure. These
can be classified as agglomerate, skin forming and crystalline. Inorganic materials fall
typically into the crystalline and agglomerate category and can be further sub-divided
according to their aqueous solubility, i.e. materials which are readily soluble in water
tend to be crystalline, whereas insoluble or partially-soluble materials tend to be
agglomerate. Organic materials are predominantly skin forming, although general
observations and single droplet drying studies suggest that some inorganic and
organic materials can fit into either group. Sodium silicate for example is a wellknown skin forming material.
343
D.E. Walton
Agglomerate particles (see Figures 9 and 10) are composed of individual grains of
material bound together by a sub-micron dust or cement of the same material and
possibly a binder. The particles tend to be spherical, solid and regular in shape.
Morphological features such as blowholes and cratering (see Figures 10R and 1 IM,
respectively) are relatively uncommon due to the highly porous and open nature of the
particle structure. This allows the unhindered flow of water, water vapour and
dissolved gases fiom the interior of the particle to its surface during drying and would
account for the high degree of sphericity shown by most agglomerate particles.
Agglomeration in a general sense is usually restricted to a small number of particles,
never mass agglomeration as shown with many polymeric or skin forming materials.
Skin forming or polymeric particles (see Figure 11) are composed of a continuous
non-liquid phase, which is very difficult to define from a structural point of view.
With dairy products, e.g. skimmed milk or yoghurt powder, the non-liquid phase may
well be denatured proteins such as casein or albumins. With sodium silicate, the skin
could be a tetrameric or even polymerised species of (Si02[OH]2)2-. Conversely,
materials such as glucose, dextran or calcium stearate for example, have well-ordered
crystalline structures but give the appearance of a skin-like particle morphology. Such
morphologies are analogous to those displayed by clays, where the crystalline or
platelet structure (in the case of clays) is so fine that it appears and acts as a
continuous phase or skin. This restricts heat and mass transfer during drying.
The general structural plasticity of these particles during drying has resulted in a
greater morphological diversity than both agglomerate or crystalline particle
structures. Morphological features such as particle inflation (see Figures 16L and M),
particle shrivelling (see Figures 12L and 15R), blowholes, cratering, agglomeration
(Figure 1 1M) and particle vacuolation (Figures 13L and M), all rely on the
rheological nature of the skin or shell formed. The type of drying mechanism(s)
associated with skin forming materials means the particles are almost certainly hollow
and relatively thin walled when fully dried. However, their strength or fiiability varies
considerably fiom hard and brittle to soft and sticky, depending on the type of
material being spray dried and the particle moisture content. Soft particles with a high
moisture content tend to form agglomerates.
344
Spray Dried Particle Morphologies
Crystalline particles (see Figure 14) are composed of large individual crystal
nuclei bound together by a rigid microcrystalline phase and differ structurally from
the pseudo-skinned crystalline materials mentioned above. Particle morphologies (as
with agglomerate particles) are restricted to a relatively limited range of
morphological features, e.g. particles with cracks, fissures, blowholes and
occasionally cratering. This is mainly due to crust rigidity.
It is also possible to give a generalised description of droplet drying behaviour, as
demonstrated by Charlesworth and Marshall (1960), and Walton and Mumford
(1999).
Examples of Particle Technology
The various particle morpkologies produced by spray drying are constantly being
exploited by the manufacturer. The porosity of agglomerate particles for example,
makes them ideal for the incorporation of binders, giving an essentially weak and
highly fiiable particle structure varying degrees of mechanical strength for pressing,
tableting or machining. A binder can alter particle or powder solubility, density,
particle size and therefore powder flowability. Increased power flowability means
better quality powder handling, which is related to transport and packaging. Similarly,
an increase in particle strength where less fines are produced on physical contact or
storage of the powder.
As most spray-dried particles are hollow or have varying degrees of porosity, they
are ideally suited for the purpose of microencapsulation. Particle pores or shells can
act as reservoirs for the encapsulation of, e.g. chemicals for industrial reactions,
catalyst delivery, or even the delivery of medicines orally, intravenously or by
inhalation. The binder or encapsulation material can be manufactured with varying
degrees of solubility or thickness, thus effecting a time-release system, again for the
delivery of medicines or even the slow release of crop fertilisers. However, overefficient and under-efficient encapsulation must be avoided to ensure effective timerelease of the particle contents (Davies, 2001). The hollow nature of skin forming
particles makes them ideal for the encapsulation of flavours in, e.g. glucose drinks
and sweets, or the encapsulation of fragrances, which when crushed release the
345
D.E.Walton
fragrance or aroma. This has been successfully used in lipsticks and air fresheners.
The encapsulation of moisture can increase particlehulk density, solubility, and be
used to cany biomaterials such as vaccines and enzymes. Some particles are opaque
and give variations in powder colour density, thus altering the aesthetic quality of the
powder.
As one can imagine, the utility of particle technology is truly enormous, but it
does depend almost entirely upon the process technology used to create it.
Conclusions
Particle morphology can have great relevance to powder technology. Studies provide
valuable information, particularly empirical data, regarding the mechanism(s) of, e.g.
powder dilation, and powder strength. These properties, including, e.g. particle size,
particle fiiability, bulk powder density, and volatile retention, are known to be related
-
to particle morphology, and from a commercial point of view, can have a significant
bearing on powder quality. Particle and powder technology is of course transferable.
It may help to determine the mechanism(s) associated with ground liquefaction that
occur during earthquakes for example, or landslides, mudslides or even pyroclastic
flows. The area of micro-encapsulation for instance has almost unlimited application,
ranging from the encapsulation of relatively mundane materials such as, glues and
adhesives, flavours, aromas etc., to the absolute exotic, although not necessarily
associated with spray drying, e.g. the polymer encapsulation of hydrogen for laser
hsion experiments! The medical application of particle technology is of course a very
important area of research and has made great progress in the treatment of, for
example, bronchial diseases with dry powder inhalation systems. Nano-particle
technology is yet another area that may offer the prospect of further drug or antibiotic
delivery systems, either by skin absorption of the particles (e.g. via the stick-on patch)
or by injection directly into the bloodstream. In some cases, the nano-particles
themselves can disrupt bacterial or viral cell walls by adhering to the cell wall and
undergoing self-replication, thus destroying the cell wall or membrane (Mann,200 1).
Delivery of DNA or genetically modified material for genetic therapy may also be
possible in the future using nano-particle technology.
346
Spray Dried Particle Morphologies
The value of the single droplet drying method as an aid to spray drying research
has been confirmed. The similarity between industrial spray-dried particles and those
dried experimentally (see Figures 9 to 17) suggests that, within certain limitations it is
possible using such techniques to reproduce and therefore predict the morphology of
production-dried formulations without recourse to expensive plant trials. Moreover,
such studies permit the rapid comparison of drying rates and drying conditions
between different products, or indeed changes in product formulation. Optimisation of
drying conditions is also possible.
References
Ahmadzadeh, J. and Harker, J.H., 1974. Evaporation from Liquid Droplets in Free Fall. Trans. IChemE., 52,
pp.108-lll.
Alexander, K. and King, C.J., 1985. Factors Governing Surface Morphology of Spray-Dried Amorphous
Substances. Drying Technology, 3(3), pp.32 1-348.
Audu, T.O.K. and Jeffreys, G.V.,1975. The Drying of Drops of Particulate Slurries. Trans. IChemE., 53,
pp. 165-172.
Bellows, R.J. and King, C.J., 1973. Product Collapse During Freeze Drying of Liquid Foods. AIChE Symp.
Series, Vol. 69, No. 132, pp. 33-41.
Buckham, J.A. and Moulton, R.W., 1955. Factors Affecting Gas Recirculation and Particle Expansion in
Spray Drying. Chem. Eng. Prog., 51(3), pp.126-133.
Charlesworth, D.H. and Marshall, W.R., Jr., 1960. Evaporation from Drops Containing Dissolved Solids.
AlChE J., Vol. 6, pp.9-23.
Cheong, H.W., Jeffreys, G.V.and Mumford, C.J., 1986. A Receding Interface Model for the Drying of
Slurry Droplets. AIChE J., 32(8), pp.1334-1346.
Chu, J.C., Stout, L.E. and Busche, R.M., 1951. Spray Drying of Santomerse. Chem. Eng. Prog., 47(1),
pp.29-38.
Crosby, E.J. and Marshall, W.R. Jr., 1958. Effects of Drying Conditions on the Properties of Spray-Dried
Particles. Chem. Eng. Prog., 54(7), pp.56-63.
Crosby, E.J. and Weyl, R.W., 1977. Foam Spray Drying: General Principles. AlChE Symp. Series, 73, 163,
pp. 82-94.
Davies, E., 2001. Savour the Flavour. Chemistry in Britain, Vo1.37, No.3, March, pp.37-39.
Dlouhy, J. and Gauvin, W.H., 1960. Heat and Mass Transfer in Spray Drying. AlChE J., 6(1), pp.29-34.
DutXe, J.A. and Marshall, W.R., Jr., 1953. Factors Influencing the Properties of Spray-Dried Materials.
Chem. Eng. Prog., 49(1), pp.480-486.
El-Sayed, T.M., Wallack, D.A. and King, C.J., 1990. Changes in Particle Morphology During the Drying
of Droplets of Carbohydrate Solutions and Food Liquids. Ind. Eng. Chem. Res., 29(12), pp. 2346-2357.
Frey, D.D. and King, C.J., 1986. Experimental and Theoretical Investigation of Foam-Spray Drying:
Mathematical Model for the Drying of Foams in the Constant-Rate Period. Ind. Eng. Chem. Fund., 25,
pp.723-730.
Gamer, F.H. and Skelland, A.H.P.,1954. Mechanism of Solute Transfer from Droplets. Ind. Eng. Chem.,
Vol. 46, NO. I, pp.1255-1264.
34 7
D.E. Walton
Genskow, L.R., 1988. Considerations in Drying Consumer Products. 6"' International Drying Symposium,
IDS'88, Versailles, September, KL 39-46. Drying'89, Eds: M. Roques and AS. Mujumdar, London,
Hemisphere Pub. Corp.
Greenwald, C.G. and King, C.J., 1982. The Mechanism of Particle Expansion in Spray Drying of Foods.
AIChE J., Symp. Series, 78,218, pp.101-110.
Hassan, H.M. and Mumford, C.J., 1993. Mechanisms of Drying of Skin-Forming Materials: Parts 1 to 3.
Drying Technology, I1(7), pp.1713-1782.
Langstroth, G.O., Diel, C.H.H. and Winhold, E., 1950. Can. J. Research, Vol. 2 8 4 pp.580.
Mann, J., 2001. Frontiers: Killer nanotubes. Chemistry in Britain, Vol. 37, No. 11, November, pp.22.
Marshall, W.R., Jr. and Seltzer, E., 1950. Part 1: Fundamentals of Spray-Dryer Operation. Part 2: Elements
of Spray-Dryer Design. Chem. Eng. Prog.,Part 1,46(10), pp.501-508; Part 2,46(1 l), pp.575-584.
Miura, K., Miura,T. and Ohtani,S., 1977. Heat and Mass Transfer to and from Droplets. AIChE J., Symp.
Series, Vol. 73, Nos. 163-164, pp.95-102.
Oakley, D.E., 1997. Produce Uniform Particles by Spray Drying. Chem. Eng. Prog., Vol. 43, pp.48-54.
Oteng-Attakora, G., Walton, D.E. and Mumford, C.J., 1994. Enhanced Heat and Mass Transfer of
Oscillating Droplets Under Forced Convection. IChemE Research Event, University College London,
5-6 January, Vol. 2, pp. 1056-1061.
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Part 1, pp.141-146; Part2, pp.173-180.
Sano, Y.and Keey, R.B., 1982. The Drying of a Spherical Particle Containing Colloidal Material into a
Hollow Sphere. Chem. Eng. Sci., 37(6), pp.881-889.
Sunkel, J.M. and King, C.J., 1993. Influence of the Development of Particle Morphology upon Rates of
Loss of Volatile Solutes During Drying of Drops. Ind. Eng. Chem. Res., 32(10), pp.2357-2364.
Thijssen, H.A.C., 1970. The Effect Of Process Variables on Aroma Retention in Drying Coffee Extract.
Colloq. Inst. Chim. Cafes Verts, Torrefies Leurs Deviv, 4, pp.108-117.
Toei, R. and Furuta, T., 1982. Drying of a Droplet in a Non-Supported State. AIChE J., Symp. Series, Vol.
78, No. 218, pp.111-117.
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During Drying of Drops. Drying Technology, 10(4), pp.875-891.
Walton, D.E. and Mumford, C.J., 1999. Spray-Dried Products: Characterisation of Particle Morphology.
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Walton, D.E. and Mumford, C.J., 1999. The Morphology of Spray-Dried Particles: The Effect of Process
Variables Upon the Morphology of Spray-Dried Particles. Trans. IChemE, Vo1.77, Part A, July,
pp.442-460.
Received 5 April 200 1;Accepted after revision: 3 July 200 1.
348
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