CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 31
ATOMIZATION

31.1 Introduction

Atomization is aimed at forming droplets fine enough to dry quickly, but not so fine as to escape with the outlet air after having been dried. Moreover, a very fine powder has undesirable properties because it is hard to dissolve. Thus, t he objective of atomizing is to reduce the milk to a particle size so small that due to the tremendously increased surface area, the resulting mist of milk projected into the current of heated air, surrenders its moisture nearly instantly. The minute particles of milk are dried before they reach the side walls or floor of the drying chamber. The average particle size of the milk fog provided by efficient atomization has a diameter of ~ 50µ. It is estimated that 0.5 lit of milk so atomized contains approximately 5625431140 particles, which represent a surface area of about 24586 m². The milk must be sprayed into the hot air in form of very small particles as nearly uniform in size as possible because

(1) In a mixture of sizes, the smallest particles might be prematurely dried and overheated whilst the largest might be only partially dried

(2) In addition, uniformity assists toward better control of the initial mixing with the stream and easier separation of the powder from it afterwards.

(3) The individual particles should also contain a minimum of trapped air.

In practice, completely uniform particle size has not yet been obtained but individual atomizers produce a majority of particles within a particular size range. Atomizer is a unit which distributes milk in form of very small droplets.

31.2 Advantages of Atomization

(1) Rapid evaporation

(2) Less effect on the quality of the product.

31.3 Types of Atomizers

(1) Pressure jet type

(2) Pneumatic type

(3) Centrifugal type

a. Hanging-Bowl type

b. Disk type

c. Ring jet type

1) Hydraulic Pressure Jet Type

In this type, the original form of atomizer forces the milk at a high pressure ~ 141-211 kg/cm² through a very small orifice, which may be only 0.125 mm in diameter, in a hard steel disk. Modern types impart swirl velocity in addition to axial velocity by tangential entry of the milk to a swirl chamber preceding the orifice.

The atomizer produces a hollow cone spray with the droplets dispersed in an annular ring around the air cone and with a decreasing particle size towards the outer edge. Although drop size is not uniform, it is possible to produce a range of size. Drop size is reduced by increasing the pressure, reducing the nozzle diameter, or reducing the liquid velocity, but this type of atomizer is unsuitable for producing a large proportion of drops below 30µ diameter. The viscosity of the concentrated milk influences the possible pressure with a given orifice, and in general a high degree of concentration is not satisfactory.

Advantages

(1) Its construction is simple,

(2) It is possible to adjust the angle of the cone-shaped spray of the atomized liquid thereby allowing a relatively small diameter of the drier and low vacuole content in the powder particles.

(3) When drying milk, the fat globules are disrupted into much smaller ones, about as they would be in a homogenizer as the pressures applied are comparable.

Disadvantages

(1) Choking of the jet orifice

(2) Alteration of the spray due to an increase in the orifice diameter as a result of erosion, spare nozzles must always be available

(3) This atomizer is not suitable for handling special mixtures containing milk with other solids in suspension

(4) High pressure pumps and milk lines are necessary.

2) Pneumatic type of Atomizer

A stream of compressed air is used to disintegrate a jet of milk. The air impinges at high velocity onto a film of liquid projected at right angle to it, or alternatively several air jets impinge on the milk stream from an angle. In this method

(1) It does not require high pressure pumping of the milk

(2) Erosion of jet orifices is reduced

(3) A more concentrated liquid can be sprayed as compared with the pressure jet type.

A very high velocity is achieved and the spray travels a long distance before breaking. This atomizer is the best type for producing very small particles and is not satisfactory when the size range is required to exceed 50 µ. More energy is required per unit of surface area created than with the other two types.

3) Centrifugal type Atomizer

No pressure is applied to the milk, the velocity being imparted by centrifugal force in a rotating device. The milk leaves the periphery of the atomizer as either in thin sheet or filaments which break up into spherical droplets in an umbrella-shape spray. There are several types of disks but, essentially, the liquid falls on a disk and is flung away at a high speed, e.g., 100 m . s ^{− 1} . Different types are:

(a) Hanging inverted bowl: The concentrated milk is fed to the underside of the bowl, which rotates at a high speed. The milk is thrown outwards by centrifugal force, spreads evenly round the circumference, and leaves the rim as a film of uniform thickness.

(b) Flat wheel shaped hollow disk: It is having openings around the periphery or a hollow slotted basket. The concentrated milk is fed to the centre of the rotation device and is discharged through the opening around the circumference. The speed of rotation is usually high varying between 200 to 300 revolutions per second according to the diameter of the bowl or disk and the peripheral velocity desired.

The drop size depends upon the velocity (decreasing with increasing velocity) and can therefore be varied by altering the speed of rotation or the disk diameter. This permits the production, within certain limits, of powders of different bulk densities. In general however, centrifugal atomizers tend to produce powders of larger average particle size than do jet types. The normal range is from 50 to 250 µ.

The centrifugal atomizer is usually incorporated in modern plants because of the following characteristics:

(a) There are no small orifices to become chocked and, when necessary, materials containing suspended solids can be handled.

(b) High pressure pumps and pipes are eliminated; the atomizer is often supplied by a simple gravity feed, although a timing pump is an advantage.

(c) Highly concentrated milk can be sprayed.

(d) In addition to the thermal economy which results, the bulk density of the powder particles increased whilst the volume of entrapped air is reduced as more concentrated milk is sprayed. This is of considerable importance in connection with both packaging and keeping quality of dried milk.

(e) Moreover, powder containing, a high proportion of large particles - readily obtainable with centrifugal atomizers tend to dissolve in water the most easily.

(f) It has also been claimed and later disputed that a greater degree of uniformity of particle size is obtainable.

(g) Disk atomization is still practicable at high viscosity; highly evaporated milk can thus be processed.

Disadvantages:

(a) Many vacuoles are formed in the particles

(b) The droplets are flung away perpendicularly to the axis of the disk and, accordingly, the chamber has to be wide to prevent the droplets from reaching the wall. Roughly speaking, the distance covered by the droplets in a horizontal radial direction is at least 104 times the droplet diameter.

The modern preference is for the milk to leave the atomizer in definite streams (i.e. from opening in a disk or special channels in an inverted bowl.) rather than as a flat sheet.

(c) Ring – jet Atomizer: A "ring-jet" atomizer operates in two stages which combine centrifugal force and a blast of cold air. The concentrated milk is fed to the centre of a centrifugal spray disk which rotates inside an air nozzle, arranged so that a ring space is formed between the nozzle outlet and the periphery of the disk. The disk rotates at a relatively low speed (3000 rpm with a direct driven motor) and spreads a jet of milk into a fine film and coarse spray with an average droplet size of about 300µ. The ring-shaped blast of cold air then shatters the drops around the periphery of the disk and reduces their size to about 50 to 100µ. A close control of the droplet size is possible; the latter varies with the intensity of the air blast which is governed by the air supply pressure. Moreover, the air content of the particles is greatly reduced. The ring jet is arranged to give n downward spray and the hot drying air also passes vertically down the drying chamber.

It is possible to produce heavy powders containing little trapped air and regulating the particle size to about 100µ. Powders of very good wettability can be obtained.

1. Droplet Size Distribution

Determination of the size distribution of the formed droplets is difficult and none of the methods available for particle-size determination is fully reliable. Usually, the produced powder is taken as a basis, but that involves several uncertainties like the uneven shrinking of droplets, presence of vacuoles, and possible agglomeration of the powder particles.

To obtain smaller drops, the concentrate is often heated to decrease its viscosity. However, this should be done immediately before atomization because viscosity increases rapidly within a minute at a high temperature, especially for concentrated skim milk.

2. Vacuoles

During the atomizing of a liquid, some air is always trapped in the droplets. This generally concerns some 10 to 100 air bubbles per droplet when a disk is used, whereas the number is far less when applying a nozzle, often 0 or 1 air cell per droplet. During the drying of the droplets, water vapor enters the air bubbles, causing them to expand. This is because the water vapor can more easily diffuse to the vacuoles than across the external layer of the drying droplets, which has already been concentrated and has become rigid. Therefore, the vacuoles are only partly filled with air. Raising the drying temperature increasingly expands the vacuoles and enlarges the vacuole volume. Fig. 31.1 shows the morphology of powder particles with vacuoles.

31.4 Change of State of the Drying Air

The temperature to which the drying air may be cooled primarily depends on the corresponding water activity. Higher inlet temperature gives higher efficiency of drying. However, there is an upper limit with respect to the inlet temperature, partly because of damage to the product caused by heating. Moreover, the powder may catch fire in a drying chamber if it stays for a long time at a high temperature (this concerns powder deposited anywhere in the machinery). Ignition may already occur at 140°C and at 220°C, the time needed for spontaneous ignition is about 5 min. Therefore to control the moisture of the powder, following relationships should be followed:

1. If the percentage of the water in the powder has to be increased, the a _w value of the outlet air should be increased; hence, outlet temperature should be lowered.

2. If the dry-matter content of the concentrate entering the dryer is increased or its temperature decreased, the atomization results in larger droplets, primarily by the increased viscosity. To keep the water content of the powder the same, the a _w value of the outlet air should be lower; hence, outlet temperature should be higher.

It is common practice to control the water content of the powder, which varies due to small fluctuations in the evaporating and drying process, by regulating the concentrate supply in such a way that outlet temperature is kept constant. If outlet temperature increases, the concentrate flux is increased, and vice versa. However, the points just mentioned indicate that this is not fully correct. More sophisticated methods using computer programs have been developed to improve control of the drying process.