One-stage drying is defined as the spray drying process where the product
is dried to the final moisture content in the spray drying chamber, see Fig.
67. However, the fundamental theory about the droplet formation and the
evaporation of the initial moisture is the same in this and the following
processes and therefore discussed here.
The initial velocity of the droplets from the rotary atomizer is about 150
m/sec. Most of the drying takes place while the droplets are decelerated by
their friction to the air. Droplets with a diameter of 100 microns have a
deceleration path of less than 1 m, and for droplets with a 10 micron diameter
it is only a few centimetres. The main temperature drop of the drying air, due
to the evaporation of the water from the concentrate, takes place during this
period. An enormous heat and mass transfer therefore takes place in the
particles during an extremely short period of time, and the product quality may
be seriously harmed, if the factors promoting degradation are not known, or are
disregarded.
During the removal of water from the droplets a
considerable reduction in weight, volume, and diameter of the particle takes
place. Under ideal drying conditions the weight will decrease to about 50%, the
volume to about 40%, and the diameter to about 75% of the created droplet from
the atomizer.
However, the ideal droplet creation and drying technique have not yet been
developed. There will always be some incorporation of air in the concentrate
during pumping from the evaporator, and especially when the concentrate is
pumped into the feed tank due to splashing. But also during the atomization a
lot of air is incorporated into the concentrate in the rotary atomizer, where
the wheel besides atomizing the concentrate is acting as a fan sucking in air
and whipping it into the concentrate. Specially designed wheels will, however,
counteract the incorporation of air in the concentrate. In the curved vane
wheel (the so-called high bulk density wheel), see Fig. 69, the air is partly
separated from the concentrate again due to the centrifugal force, whereas in
the steam-swept wheel, see Fig. 70, the problem is partly overcome by replacing
the liquid/air interface with a liquid/steam interface. It was generally
believed that the nozzles during the atomization incorporated no or very little
air into the concentrate. However, it has been found that some air
incorporation takes place during the very early stage of atomization, both
outside and inside the spray cone due to the air friction prior to the droplet
formation. The higher the capacity of the nozzle (kg/h) the more air will be
whipped into the concentrate.
The ability of a concentrate to
incorporate air (or its foaming ability) is depending on the composition,
temperature and the solids content. It has been found that concentrate with a
low solids content has an extensive foaming ability which even increases with
the temperature. Concentrate with a high solids content has considerably lower
foaming ability which is even further reduced by increasing the temperature.
Generally the foaming ability is less in whole milk concentrate than in skim
milk concentrate. Determination of air in concentrate is described on page
189.
The amount of air in the droplets (present in form of small air bubbles) is
therefore one of the decisive factors as to how far the shrinkage will continue
during the drying. Another factor even more important is the drying conditions,
i.e. the surrounding air temperature. As mentioned, a lot of heat has to be
transferred from the drying air to the droplets and much water vapour the other
way. Therefore, there is a temperature and concentration gradient in the
particle, and the whole process becomes very complex and not fully understood.
Droplets of pure water (water activity 100%) will, when exposed to air at a
higher temperature, evaporate keeping wet bulb temperature until completely
evaporated, while solids containing products dried to the extreme (i.e. with a
water activity approaching zero) are heated to the temperature of the
surrounding air at the end of the drying, which in a spray dryer means the
temperature of the outgoing air.
Not only from the centre to the
surface is there a concentration gradient, but also from one point of the
surface to another resulting in different water concentrations and thus
different temperatures between different regions on the surface. The overall
gradient intensity is bigger, the bigger the particle diameter, due to the
smaller surface/mass ratio. Thus small particles dry in a more uniform way.
During the drying the solids content naturally increases due to the
removal of water - and so does the viscosity and surface tension. This means
that the diffusion coefficient, i.e. the water-vapour diffusion/time and area,
becomes smaller and overheating occurs due to the slower evaporation rate. In
extreme cases the so-called case hardening will take place, which is the
formation of a hard crust on the surface through which the remaining
water-vapour or occluded air will diffuse very slowly. If case hardening
occurs, it is usually at a residual moisture content of 10-30% in the particle,
at which stage the proteins, especially the caseins, are very sensitive to heat
and easily denature resulting in a powder with poor solubility properties.
Moreover, the amorphous lactose will become hard and almost impenetrable to
water vapour, and the particle temperature increases further as the evaporation
rate, i.e. diffusion coefficient, approaches zero.
As there will be
more water vapour and air bubbles in the particle this will now get
superheated, if the surrounding air temperature is high enough resulting in the
vapour and air to expand. The pressure will increase, and the particle will
blow up to a completely round ball with a smooth surface. The particle will
have a lot of vacuoles inside. If the surrounding air temperature is high
enough the particle may even explode, but even if it does not, the particle
will have a very thin crust, about 1 micron, and it will not survive the
mechanical treatment in the cyclones or in the conveying system and thus leave
the dryer with the exhaust air.
If there is only a small content of air
bubbles in the particle the expansion will, in spite of the overheating, not be
too extensive. The overheating as a result of case hardening will, however,
have a detrimental effect on the caseins resulting in bad solubility.
If the surrounding temperature, i.e. the outlet temperature, is kept
low during the drying, the particle temperature will be equally low.
The outlet temperature is determined by many factors of which the most
important ones are:
- Moisture content in the final powder
- Temperature and moisture content of the drying air
- Solids content in the concentrate
- Atomization
- Viscosity of the concentrate
MOISTURE CONTENT IN THE FINAL POWDER
The first and foremost factor is the moisture content in the final powder.
The lower the residual moisture content wanted, the lower the relative humidity
in the outlet air, and that means higher outlet temperature and with that
higher particle temperature.
TEMPERATURE AND MOISTURE CONTEN T OF THE DRYING AIR
As the moisture content is in direct relation to the relative humidity of
the outgoing air, an increase of the inlet air will necessitate a slight
increase in the outlet air due to the higher amount of moisture in the air
resulting from the increased evaporation. Also the initial moisture content in
the drying air plays a big role, and if that is high the outlet temperature has
to be increased to compensate for the extra moisture.
SOLIDS CONTENT IN THE CONCENTRATE
An increase in the solids content will require an increase in the outlet
temperature, as the evaporation becomes slower (average diffusion coefficient
smaller) and a bigger temperature difference (driving force) between the
particle and surrounding air is necessary.
ATOMIZATION
Any attempt to improve the atomization and create a finer spray will result
in a lower outlet temperature, as the specific surface/mass ratio of the
particles becomes bigger. The evaporation will therefore be easier and a
smaller driving force is required.
VISCOSITY OF THE CONCENTRATE
The atomization is influenced by the viscosity. The viscosity
increases with increased content of proteins, crystallized lactose and overall
solids content. Heating the concentrate (beware of age-thickening) and
increasing the atomizer speed or nozzle pressure can remedy the problem.
The overall drying efficiency is expressed in the following
approximated formula:
ξ =Ti - To / Ti - Ta (17)
where:
Ti = air inlet temperature
To = air outlet
temperature
Ta = ambient temperature
It is thus obvious that the
only possibility of increasing the efficiency of spray drying operation is by
increasing ambient temperature by preheating (see page 169), f.inst. using
condensate from the evaporator, or by increasing the inlet temperature or
decreasing the outlet temperature.
The relation ξ is at the same time a
good indication of the dryer performance, as the outlet temperature is
determined by the residual moisture content which has to fulfil certain
standards. A high outlet temperature will indicate that the drying air is not
utilized in an optimal way due to various reasons such as bad atomization, bad
air distribution, high viscosity, etc.
The ξ will in normal spray
dryers operated on skim milk (Ti = 200ºC, To = 95ºC) be around 0.56.
The drying technology discussed so far has been related to a plant with
pneumatic conveying and cooling system, where the powder when leaving the base
of the chamber is dried to the wanted moisture content. The powder will at this
stage be warm and consist of particles stuck together with very weak bindings
in big loose agglomerates due to the primary agglomeration taking place in the
atomizer cloud, where particles of different diameter will obtain different
speed and therefore collide. However, when passed through the pneumatic
conveying system, where the agglomerates are exposed to mechanical forces, the
powder will break up in single particles. This kind of powder, can be
characterized as follows: