14.0 Spray Drying and Spray Dryers
14.1 Principles of Operation. Spray drying has become the most important method for dehydration of fluid foods in the Western world. The development of the process has been intimately associated with the dairy industry and the demand for drying of milk powders. However, the technology has been expanded to cover a large food group which now is being successfully spray dried.
|Spray Drying Applications|
|Bananas||Egg (whole)||Proteins (animals)|
|Blood||Egg (white)||Proteins (milk)|
|Cake mixes||Egg (yolk)||Proteins (plants)|
|Citrus Juice||Fish concentrates||Shortening (bakery)|
|Coffee||Infant formulas||Starch derivatives|
|Corn syrup||Milk (whole)||Tea|
|Cream||Milk (skim)||Tomato puree|
|Creamers (coffee)||Milk (replacers)||Yeast|
14.3 Unit Operations. Spray drying consists of the following unit operations:
Pre-concentration of liquid
Atomization (creation of droplets)
Drying in stream of hot, dry air
Separation of powder from moist air
Packaging of product
Relatively high temperatures are needed for spray drying operations. However, heat damage to products is generally only slight, because of an evaporative cooling effect during the critical drying period and because the subsequent time of exposure to high temperatures of the dry material may be very short. The typical surface temperature of a particle during the constant drying zone is 45-50 C. For this reason, it is possible to spray dry some bacterial suspensions without destruction of the organisms. The physical properties of the products are intimately associated with the powder structure which is generated during spray drying. It is possible to control many of the factors which influence powder structure in order to obtain the desired properties.
14.4 Typical Spray Drying Systems. The diagram shows a schematic representation of a typical spray drying system for milk powder. For spray drying, it is usual to pump a concentrate of the liquid product to the atomizing device where it is broken into small droplets. These droplets meet a stream of hot air and they loose their moisture very rapidly while still suspended in the drying air. The dry powder is separated from the moist air in cyclones by centrifugal action. The centrifugal action is caused by the great increase in air speed when the mixture of particles and air enters the cyclone system. The dense powder particles are forced toward the cyclone walls while the lighter, moist air is directed away through the exhaust pipes. The powder settles to the bottom of the cyclone where it is removed through a discharging device. Sometimes the air-conveying ducts for the dry powder are connected with cooling systems which admit cold air for transport of the product through conveying pipes. Cyclone dryers, such as shown here have been designed for large production schedules capable of drying ton-lots of powder per hour.
14.4.1 Cyclone Spray Dryer. The following is a diagram of a typical spray drying operation utilizing a centrifugal atomizer and a cyclone separator.
14.4.2 Box Spray Dryer. A different system, known as the 'Rogers Process' is shown in the next diagram. This design has been popular for relatively small production schedules and for physically sensitive powder material which may not withstand the friction generated by the cyclone action. In this process, filtered air is preheated with steam or gas and is blown into ductwork (A) and then into the specially proportioned distributing head (B), passing through air inlets to the drying chamber (C). The hot air absorbs the moisture from the finely divided, atomized liquid spray and passes under air baffle (D) to cloth filter bags (E) where 100% of any remaining powder is trapped from the exhaust air. The filter bags are intermittently shaken by a mechanical device to release any adhering powder which then drops to the floor and blends in with the remaining powder.
14.4.3 Rogers Spray Drying Process.
The moist air is then exhausted to the atmosphere through the exhaust fan (F). Liquid to be dried is preheated in heater (G), passing to a high pressure pump (H), through the high pressure line (I) to spray nozzles located in air inlets (C). The atomized liquid droplets are dried while suspended in the chamber and the powder settles to the floor where it is conveyed by a reciprocating scraper (J) to the screw conveyor (K). The powder moves to the chamber outlet (L) where it is may be removed manually or picked up by an optional pneumatic system to a cyclone separator (M) which cools the powder with fresh air. A sifter is located below the cyclone discharge.
14.5 Pre-Concentration of Liquid Feed. For operation of a spray dryer it is usual practice to pre-concentrate the liquid as much as possible. There are several reasons for this :
Economy of operation (evaporation is less expensive)
Increased capacity (amount of water evaporation is constant)
Increase of particle size (each droplet contains more solids)
Increase of particle density (reduction of vacuole size)
More efficient powder separation (related to increased density)
Improved dispersibility of product (reduction in surface area)
14.5.1 First, it must be recognized that water removal in a vacuum evaporator and in a spray dryer are two very different processes. Evaporation under vacuum is a process which takes place at a much lower temperature than spraydrying. Generally, the temperature of the first stage is only Å 65 C and subsequent stages even less. For this reason, vacuum evaporation in multiple stages permits the use of low-cost energy and regeneration of the energy contained in the vapor removed from the product. In principle, very little heat energy is used or lost during vacuum evaporation.
In contrast, spray drying takes place at atmospheric pressure; therefore, the drying air needs to be heated to high temperatures, generally around 150-175 C. This requires high-cost fuel in the form of gas or oil. Besides, there is almost no opportunity to regenerate the energy from the vapor phase. Thus, for efficient industrial spray drying operation, it is usual to combine the two processes.
14.5.2 Next, it must be recognized that the performance of a spray dryer is rated according to the maximum amount of water which can be removed per hour by that system. For example, a spray dryer rated at 1000 kg/hr water evaporation will produce only Å111 kg/hr of bone-dry powder from a liquid of 10% total solids. If that liquid is concentrated to 45% total solids, the powder production increases to Å818 kg/hr of bone-dry powder.
14.5.3 Finally, the powder structure and, therefore, the physical properties of a powder is very dependent upon the total solids concentration of the liquid which is being dried. If the droplets are maintained at a constant size, then, the amount of solids will affect both the size and the density of the dry particles. The structure of a spray-dried particle is a hollow sphere, with the solids being a shell which surrounds a central vacuole. As the total solids of the feed increases, the shell becomes thicker and, as a consequence, the particle does not shrink as much during drying. Similarly, as the air-filled vacuole decreases in size, the particle density increases. The increase in particle density has a pronounced influence on the efficiency of powder separation/collection by the cyclones, because these operate on the principle of a difference in the buoyant density difference between air and particles. It is well-known in the spray-drying industry that drying a liquid of low solids content is the cause of very fine particles which are difficult to collect. This results in product losses as well as environmental pollution when they are discharged into the atmosphere.
14.5.4 Limitations on Pre-Concentration. The limit on the extent of pre-concentration of the feed is dictated by the viscosity of the liquid, which must not be so high, that the product cannot be pumped or atomized. For milk powder manufacture, it is common to pre-concentrate the milk (9% total solids in skim milk; 13% total solids in whole milk) to 45% in an evaporator. For many protein isolates, such a high concentration cannot be used, because most protein solutions are very viscous. In this case, spray drying must be done with a concentrate of about 25% total solids concentration. This practice, however, causes the powder particles to have a lower density. Therefore, these products are typically very light and fluffy and the unit cost of operation increases dramatically.
14.6 Atomization. The size and uniformity of droplets are determined by the atomization. Karel (1975) has described this operation as the most important feature of a spray dryer. Two different principles are illustrated in the following diagrams.
14.6.1 Centrifugal Atomizer. This is a spinning disk assembly with radial or curved vanes which rotates at high velocities (2000-20,000 rpm). The feed is delivered near the center and spreads between the two plates and is accelerated to high linear velocities before it is thrown off the disk in the form of thin sheets, ligaments or elongated ellipsoids. However, the subdivided liquid immediately attains a spherical shape under the influence of surface tension. The atomizing effect is dependent upon centrifugal force but also must depend upon the frictional influence of the external air. Centrifugal atomizers have the great advantage of less tendency to become clogged. For this reason, they are preferred for spray drying of non-homogeneous foods.
Centrifugal Atomizer for Cyclone Spray Dryers.
A. Top view. B. Side view.
|14.7 High Pressure Spray Jets. High pressure jets are alternative atomizing systems in which a fluid acquires a high-velocity tangential motion while being forced through the nozzle orifice. The fluid emerges with a swirling motion in a cone shaped sheet, which breaks up into droplets.||
The nozzle shown in the diagram is an atomizer with internal mixing of gas and liquid. The design permits air or steam to break up the stream of liquid into a mist of fine droplets. The action of many jet-spray nozzles is similar to the spray formed by a common garden hose; only the design and construction are made with greater care.
14.8. Heat and Mass Transfer in the Dryer:
There are only few details known about actual heat- and mass-transfer processes in the drying chamber. However the history of drying in a single droplet can be constructed
14.8.1 Drying of a Droplet.
The drying time for a single droplet may be estimated by the following equation:
t = [r2dLÆHV ] x [mi-mf] / [3h(ÆT)] x [1+mi] , where:
t= time (hr); r = radius of droplet; dL = density of liquid (lb/ft3); ÆHV = latent heat of vaporization (Btu/lb); mi = initial moisture content (lb H2O/lb dry food); mf = final moisture content (lb H2O/lb dry food);
h = film coefficient for heat transfer (Btu/ft2/hr/°F); ÆT = temperature difference between initial and final stages (°F).
The typical drying time for an average milk droplet of 40 µ is only a fraction of a second. However, because of the great initial velocity, the particle will have traveled a considerable distance from the atomizer before it is dry (13.5 cm for average conditions). It should be noted, that the drying time is proportional to the square of the radius; thus, for larger droplets the drying time may become so long that the droplet reaches the wall of the dryer while still wet. This problem is often encountered in small scale dryers. The above equation also stresses that the drying time can be shortened by reducing the initial moisture content by pre-concentration of the liquid.
14.9 Separation of Dry Particles Separation is carried out partly within the drying chamber itself and partly in secondary separation equipment. In general, it is easy to remove 90% or more of the powder, but removal of the remainder becomes problematic. Cyclone separators operate on the 'momentum separation' principle (centrifugal action) and are extensively used in large scale dryers for removal of fines.
Charm (1971) has given an equation which relates the dimensions of a cyclone to the smallest particle (Dp) which can be separated:
Dp2 = (3.6 Ai D0 µ )/( ¹ZDV0ds ), where :
Dp = diameter of particle; Ai = inlet cross sectional area of cyclone; D0 = diameter of outlet of cyclone; µ = viscosity of the fluid; Z = depth of the separator; D = diameter of the separator;
V0 = velocity of air/powder mixture entering the cyclone; and ds = density of the particle.
From the equation it appears that in designing a cyclone the depth and diameter should be as large as possible. Increasing the air velocity is also important. Industrial experience has shown that efficiency is also affected by the powder concentration in the air stream. For this reason, it is better to use several cyclones in parallel than just one single separator. Since cyclones do not always allow complete separation of 'fines' other systems are also in use, including filters, scrubbers, or electrostatic precipitation equipment. Cloth-bag filters are very effective systems but are expensive in labor cost to maintain. Besides, the fabric is weakened by high temperatures.
14.9 Diagram of a Cyclone Separator.
14.10 Structure of Spray Dried Powder Particles. The structural features of a spray-dried particle is illustrated in the following diagram. Buma (1971) has used scanning electron microscopy to study the particle structure and the distribution of free fat in the particles. He has shown that spray drying results in hollow particles, the shells of which have a glassy structure, primarily of amorphous (non-crystalline) lactose. Pure lactose dries into spherical particles with no dents or folds, whereas skim milk, caseinates and other proteins always give rise to surface folds. Buma and Henstra (1971) have explained that proteins, unlike lactose and other sugars, shrink unevenly during drying.
The physical properties of spray dried powders is related to the characteristics of the matrix of the shells and to the size of the vacuole. The presence of the vacuole causes the particles to have a lower density than the solid itself (0.33g/cc versus 1.6 g/cc). Generally, powders with low density are fluffy and do not wet or sink readily when brought in contact with water. Sugars, such as lactose in milk powder, do not crystallize during drying but are present in an amorphous state. Such amorphous sugars are very hygroscopic and readily absorb moisture and eventually recrystallize. This is the cause for many powders caking during storage. Recrystallization is usually accompanied by changes in color and development of off-flavors.
14.11 Dispersion Characteristics of Spray Dried Powders. Many spray-dried powders do not disperse readily in water. The difficulties are associated with the exterior of the powder absorbing water very rapidly and forming lumps, dry on the inside but covered with a viscous layer through which water penetrates very slowly. The problem arises particularly in products which contain soluble proteins, such as milk powder and flour. Karel (1975) has discussed the requirements for rapid dispersibility of powders in cold water. The following properties are desired:
A large wettable surface
Sinkability (must not float on surface)
Resistance to sedimentation
The wettability is crucial and depends upon the total surface area of the powder and on the surface properties of the powder particles. The spray drying industry in Western countries have improved the dispersibility characteristics of their products by a combination of surface treatments (for example addition of lecithin) and 'instantizing' (agglomeration). Instantizing is an aggregation process which is intended to prevent powder particles from sticking together and becoming lumpy during rehydration.
14.12 Instantized Powders. 'Instant' powders are usually produced by processes in which the spray-dried powder is first wetted and then redried. The degree of rewetting is closely controlled (Å15%) to permit the particles to stick together and form aggregates before redrying. In these products the small particles are fused together but the points of contact are so few that practically all of the surfaces are available for wetting. The aggregates, however, are sufficiently stable to prevent lumping when stirred into water. During the rewetting procedure, lactose will partly crystallize. Therefore, instant powder is less hygroscopic. The following is a flow diagram of the instantizing process and a schematic representation of particle agglomeration.
14.12.1 Instantizing Process
14.12.2 Particle Agglomeration after Instantizing Process. In the following diagram, A is a representation of regular powder. B represents aggregates from drying a rewetted powder
14.13 Selection of Operating Conditions The most important responsibility for an operator of a spray drier is to maintain a constant moisture content of the powder. This is required to meet legal standards and for maintaining a uniform quality. Average operating conditions for spray drying of milk powder will vary somewhat depending upon the dryer system used and must be adjusted to produce the desired uniform moisture content. It is important to understand how the final moisture content can be controlled by changing the conditions. But first, it should be noted that the final moisture content is controlled by the relative humidity of the outlet air. If that value is too high, then the powder particles will absorb moisture rather than give moisture away. The primary conditions which may be controlled directly by the operator are:
Among other operating conditions, outlet temperature and relative humidity of the outlet air are particularly important and need careful attention. However these can only be indirectly controlled by adjusting the primary conditions.
For outlet temperature, the condition is dependent upon liquid feed intake. If the feed intake is increased, the outlet temperature will drop. If the intake is reduced, the outlet temperature will increase and approach the inlet temperature. The outlet temperature will also be affected by the air flow rate. For a constant inlet temperature and constant feed intake, an increase in the air flow will raise the outlet temperature.
14.14 Average Spray Drying Conditions for Milk
Temperature of air (ambient) 25.0 C
Temperature of feed: 60.0 C
Temperature of inlet air: 150.0 C
Temperature of outlet air: 82.0 C
Temperature of drop surface (const. zone) 45.0 C
Relative humidity (ambient air) 55.0 %
(continued next page)
14.14 Average spray drying conditions for milk (continued)
Relative humidity (inlet air, psychrom.) 0.3 %
Relative humidity (outlet air, psychrom.) 12.0 %
Moisture content of milk: 87.0 %
Moisture content of concentrate: 45.0 %
Moisture content of powder: 4-5.0 %
Moisture zone (constant rate): 90<>30.0 %
Moisture zone (falling rate): 30<> 5.0 %
Droplet size (initial), av. diameter: 40.0 µ
Particle size (final) , av. diameter: 20.0 µ
Density of milk 1.33 g/ccm
Density of milk powder (bulk): .33 g/ccm Velocity of air: 61.0 meters/sec
Velocity of droplet(initial): 17,000.0 cm/sec
Velocity of droplet (free fall): Å 1.0 cm/sec
Drying time (constant rate zone): .0023 sec
Drying time (falling rate zone): .0014 sec
Drying time (total): .0037 sec
Travel distance for drying: 13.5 cm
For relative humidity of outlet air, the value is dependent upon the psychrometric relationship between the conditions of the ambient air and the conditions in the dryer. It is controlled by establishing the correct outlet temperature.
14.15 Study Questions
1. Name some products which may be successfully spray dried.
2. What are the essential parts of a spray drier?
3. A fluid contains 10% total solids. How much water must be removed from 1000 kg to make a spray dried powder with 4% moisture? If the fluid was first evaporated to 50% total solids how much concentrate must now be spray dried and how much water must be removed from the concentrate to make a powder with 4% moisture?
4. What are the benefits of preconcentration of milk prior to spray drying?
5. Make a flow diagram showing the essential steps for making NFDM (skimmilk powder) from reception of raw milk to finished product.
6. What is understood in spray drying technology by the term: "atomizing"?
7. Approximately how long does it take for a droplet to become a dry powder particle in a spray dryer? How are the particles collected?
8. What is understood by the "instantizing" process; why is it done and how is it done?
9. What is the recommended moisture content of NFDM?
10. Complete the following calculations using the humidity chart at the front cover of this book:
Given: Temperature of ambient air 35.0 C
Relative humidity of ambient air 80.0%
Temperature of inlet air 110.0 C
Relative humidity of outlet air 20%
Verify: Temperature of outlet air 72°C
Absolute moisture content of outlet air 0.050 kg/kg dry air
Absolute moisture content of inlet air 0.030 kg/kg dry air
Moisture carrying capacity 0.020 kg/kg dry air
Problem: Some of the conditions are changed as follows:
Temperature of ambient air 20.0 C
Relative humidity of ambient air 40.0%
Temperature of inlet air 120.0 C
Relative humidity of outlet air 20%
Determine: Temperature of outlet air? ºC
Absolute moisture content of outlet air? kg/kg dry air
Absolute moisture content of inlet air? kg/kg dry air
Moisture carrying capacity? kg/kg dry air
Estimate: Approximately by how much must the feed intake be increased or re- duced (in percent) to maintain a steady state under the new conditions?
(1) Buma, T. J. 1971. Fat in spray-dried whole milk. 4. Significance of free fat for other properties of practical importance. Neth. Milk Dairy J. 25: 88-105. 5. Cohesion; determination, influence of particle size, moisture contentand free-fat content. Neth. Milk Dairy J. 25: 107-122.8. The relation between free-fat content and particle porosity ofspray-dried whole milk. Neth. Milk Dairy J. 25: 123-140.
(2) Buma, T. J. and S. Henstra. 1971. Particle structure of spray-dried caseinate and spray-dried lactose as observed by a scanning electron microscope. Neth. Milk Dairy J. 25: 278-281.
(3) Charm, S.E. 1971. Fundamentals of Food Engineering. The AVI Publishing Company, Westport, Connecticut (pp 362-65).
(4) Hansen, P.M.T. 1963. Manufacture of butter powder. Australian Journal of Dairy Technology. 18(2) : 79-86.
(5) Hansen, P.M.T. 1963. The baking performance of butter powder. Australian Journal of Dairy Technology. 18(2) : 86-91.
(6) Karel, M., O.R. Fennema, and D.B. Lund. 1975. Principles of Food Science : Part II. Physical Principles of Food Preservation. Marcel Dekker, Inc. New York. (Chapter 10: Dehydration of foods. pp 342-357.)
(7) King, N. 1965. The physical structure of of dried milk. Dairy Science Abstracts 27 (3): 91-104.
(8) Picecky, J. Bulletin S-217. Milk droplets: Their creation and drying. Niro Atomizer Ltd. Copenhagen, 305 Gladsaxevej. DK-2860, Søborg Denmark. Bull. S-217.
(9) Verhey, J.G.P. 1972 and 1973. Vacuole formation in spray powder particles. 1. Air incorporation and bubble expansion. Neth. Milk Dairy J. 26: 187-202. 3. Atomization and droplet formation. Neth. Milk Dairy J. 27: 3-18.