Principle and terms
Spray drying is an industrial process for dehydration of a liquid feed containing dissolved and/or dispersed solids, by transforming that liquid into a spray of small droplets and exposing these droplets to a flow of hot air. The very large surface area of the spray droplets causes evaporation of the water to take place very quickly, converting the droplets into dry powder particles.
Drying air characteristics
- Dry bulb temperature (td) is the temperature of air, which is not saturated with water vapour, as measured by an ordinary thermometer. In practice, the dry bulb is just referred as air temperature and is expressed either in °C (t) or as the absolute temperature in °K (T) whereby T = t + 273.15.
- Wet bulb temperature (twb) (or more precisely Adiabatic saturation temperature) is a characteristic of moist air of a given dry bulb temperature, expressing the saturation temperature of that air with the same enthalpy, i.e. obtained by evaporation of 0°C water under adiabatic conditions. The difference between dry and wet bulb temperatures is a measure of drying capability (driving force). It is the temperature to which the air of dry bulb temperature (t) will drop, when evaporating water in an isolated air-water system until saturation condition occur (supposing that the temperature of water to be evaporated is 0°C). The enthalpy of the air during this evaporation remains unchanged, as the heat from the air is utilized for evaporation only. It can be also expressed as the temperature a droplet of water will obtain when exposed to a flow of air of temperature (t). Measuring wet bulb temperature is based on the same principle, i.e. the thermometer bulb is kept wet by a thin film of water and exposed to a flow of air. The relative humidity of the air at wet bulb temperature is 1.
- Dew point temperature (tdp) is the temperature where condensation of vapour will commence, if the air is cooled down at constant absolute humidity. The relative humidity of the air at the dew point temperature is 1 and its enthalpy is lower than that of the same air at its dry bulb temperature and wet bulb temperature.
- Air absolute humidity (y) is the ratio of the amount of water vapour (mv) to the amount of dry air (ma). Usually it is expressed in kg of water vapour per kg of dry air.


- Air relative humidity (Φ or %RH) is the ratio of partial pressure of water vapour (pv) to the water vapour pressure at the saturation point (ps) at the same temperature.


- Saturation point is the air temperature at which any further temperature drop will result in condensation. Saturated air has equal dry bulb, wet bulb and dew point temperatures.
- Drying air rate (Aa) is usually expressed as the mass flow of ambient air per hour (kg/h) and includes both the amount of dry air (Ad) and water vapour (Av) which can be calculated using equations:



- Heat capacity is the amount of heat necessary to heat 1 kg of a substance by 1°C and is a function of the temperature.
- Heat capacity of the air (ca) is the amount of heat necessary to heat 1 kg of dry air by 1°C. It is expressed in J/kg/°C and is temperature dependent as shown below, where T is temperature in K:

To get ca in kcal/kg/°C, equation [3,8] is divided by 4186.
For routine technical calculations a constant value 0.245 kcal/kg/°C or 1.026 kJ/kg/°C may be used.
The amount of heat (Q) necessary to heat a given amount of dry air (Ad) from t1 to t2 °C is:

- Heat capacity of water (cw) is approximately 1.0 kcal/kg/°C or 4.186 kJ/kg/°C.
- Heat capacity of water vapour in J/kg/°C is:

- Latent heat of evaporation (r) or vaporization is the amount of heat necessary to transform a liquid to vapour at constant temperature. The reverse process i.e. transforming a vapour to liquid requires a release of the same quantity of heat and is called heat of condensation. The latent heat of water is 597.3 kcal/kg or 2500 kJ/kg at the temperature 0°C and barometric pressure 760 mm Hg.
- Enthalpy (h) of air is the thermal energy of that air expressed as sum of heat necessary to evaporate its moisture content at 0°C and to heat both the water vapour and dry air to its actual temperature, as expressed by the equation:



Terms and definitions
- Ambient air is the atmospheric air supplied to the system from the surroundings of ambient temperature (ta) and ambient humidity (ya)
- Inlet air temperature (t1) is the temperature of the air after heating or cooling at the inlet of a processing system having an inlet air absolute humidity (y1)
- Outlet air temperature (t2) and outlet air humidity (y2) express the same for air leaving the system
- Water content of the feed (milk concentrate) or product (final powder) can be expressed in several ways:



- Density or specific gravity of milk products, both concentrates and powders can be calculated using following formula:

Component | Density at 20°C,g/ml |
---|---|
Non-fat milk solids | 1.52 |
Milk fat | 0.94 |
Amorphous lactose | 1.52 |
Alpha-lactose monohydrate | 1.545 |
Whey solids | 1.58 |
Milk proteins | 1.39 |
Sugar (sucrose) | 1.589 |
Water | 1.00 |
- Heat capacity of milk solids is also a function of temperature. However, for practical purposes it is sufficient to reckon with constants, as given in Table 3.2.
Component | Heat capacity kcal/kg/°C |
---|---|
Non-fat milk solids | 0.3 |
Milk fat | 0.5 |
Water | 1.0 |
Psychrometric chart

Drying of milk droplets
Particle size distribution
Mean particle size
- Most frequent diameter, which can be seen directly from tabularized results or as the highest point of the frequency curve, possibly as an inflection point on the cumulative curve.
- Arithmetic mean diameter, defined as the sum of the diameters of separate particles/droplets, divided by their number. This mean diameter is most significant when the size distribution is not overbalanced by either very large or very small elements.
- Geometric mean diameter, defined as the n-th root of the product of the diameters of the n particles analysed. It has the highest frequency in the log-normal distribution.
- Median diameter, which is the diameter corresponding to 50% of the number, weight or volume of the droplets /particles.



Droplet temperature and rate of drying
- The temperature of the droplet during the whole evaporation process lies between the temperature of the surrounding air and its wet bulb temperature. The droplet moisture determines the water activity of the droplet/particle. This, together with the relative humidity of the surrounding air decides where - between these two limiting points - is the actual droplet/particle temperature.
- Droplets of water (having the water activity aw = 1) will attain the wet bulb temperature regardless of the feed temperature once the first contact is made with the drying air. This temperature will be retained until evaporation is completed.
- Droplets of milk concentrate at the beginning of the drying process will attain a temperature somewhat higher than the wet bulb temperature because the water activity of the concentrate is somewhat lower than 1 (about 0.85 - 0.90).
- As water evaporates, the water activity (aw) gradually decreases. This results in a gradual rise of particle temperature towards the surrounding air temperature.
- When equilibrium is achieved between the drying air and a particle, the particle water activity is equal to the relative humidity of the surrounding air and consequently the particle temperature is equal to the surrounding air temperature, i.e. aw =Φ and tp = t2.


Particle volume and incorporation of air

Single-stage drying
Two-stage drying
Expansion of air bubbles during drying



Extended Two-stage drying
Fluid bed drying
Component | Heat capacity kcal/kg/°C |
---|---|
Non-fat milk solids | 0.3 |
Milk fat | 0.5 |
Water | 1.0 |




Índice
-
1.Introduction
-
2.Evaporation
- 2.1. Basic principles
- 2.2. Main components of the evaporator
- 2.2.1. Heat exchanger for preheating
- 2.2.1.1. Spiral-tube preheaters
- 2.2.1.2. Straight-tube preheaters
- 2.2.1.3. Preheaters to prevent growth of spore forming bacteria
- 2.2.1.3.1. Direct contact regenerative preheaters
- 2.2.1.3.2. Duplex preheating system
- 2.2.1.3.3. Preheating by direct steam injection
- 2.2.1.4. Other means to solve presence of spore forming bacteria
- 2.2.1.4.1. Mid-run cleaning
- 2.2.1.4.2. UHT treatment
- 2.2.2. Pasteurizing system including holding
- 2.2.2.1. Indirect pasteurization
- 2.2.2.2. Direct pasteurization
- 2.2.2.3. Holding tubes
- 2.2.3. Product distribution system
- 2.2.3.1. Dynamic distribution system
- 2.2.3.2. Static distribution system
- 2.2.4. Calandria(s) with boiling tubes
- 2.2.5. Separator
- 2.2.5.1. Separators with tangential vapour inlet
- 2.2.5.2. Wrap-around separator
- 2.2.6. Vapour recompression systems
- 2.2.6.1. Thermal Vapour Recompression – TVR
- 2.2.6.2. Mechanical Vapour Recompression - MVR
- 2.2.7. Condensation equipment
- 2.2.7.1. Mixing condenser
- 2.2.7.2. Surface condenser
- 2.2.8. Vacuum equipment
- 2.2.8.1. Vacuum pump
- 2.2.8.2. Steam jet vacuum unit
- 2.2.9. Flash coolers
- 2.2.10. Sealing water equipment
- 2.2.11. Cooling towers
- 2.3. Evaporator design parameters
- 2.3.1. Determination of heating surface
- 2.3.2. Heat transfer coefficient
- 2.3.3. Coverage coefficient
- 2.3.4. Boiling temperature
- 2.4. Evaporation parameters and its influrence on powder properties
- 2.4.1. Effect of pasteurization
- 2.4.1.1. Bacteriological requirements
- 2.4.1.2. Functional properties of dried products
- 2.4.1.2.1. Heat classified skim milk powders
- 2.4.1.2.2. High-Heat Heat-Stable milk powders
- 2.4.1.2.3. Keeping quality of whole milk powders
- 2.4.1.2.4. Coffee stability of whole milk powders
- 2.4.2. Concentrate properties
-
3.Fundamentals of spray drying
- 3.1. Principle and terms
- 3.1.1. Drying air characteristics
- 3.1.2. Terms and definitions
- 3.1.3. Psychrometric chart
- 3.2. Drying of milk droplets
- 3.2.1. Particle size distribution
- 3.2.2. Mean particle size
- 3.2.3. Droplet temperature and rate of drying
- 3.2.4. Particle volume and incorporation of air
- 3.3. Single-stage drying
- 3.4. Two-stage drying
- 3.5. Expansion of air bubbles during drying
- 3.6. Extended Two-stage drying
- 3.7. Fluid bed drying
-
4.Components of a spray drying installation
- 4.1. Drying chamber
- 4.2. Hot air supply system
- 4.2.1. Air supply fan
- 4.2.2. Air filters
- 4.2.3. Air heater
- 4.2.3.1. Indirect: Gas / Electricity
- 4.2.3.2. Direct heater
- 4.2.4. Air dispersers
- 4.3. Feed supply system
- 4.3.1. Feed tank
- 4.3.2. Feed pump
- 4.4. Concentrate heater
- 4.4.1. Filter
- 4.4.2. Homogenizer/High-pressure pump
- 4.4.3. Feed line
- 4.5. Atomizing device
- 4.5.1. Rotary wheel atomizer
- 4.5.2. Pressure nozzle atomizer
- 4.5.3. Two-fluid nozzle atomizer
- 4.6. Powder recovery system
- 4.6.1. Cyclone separator
- 4.6.2. Bag filter
- 4.6.3. Wet scrubber
- 4.6.4. Combinations
- 4.7. Fines return system
- 4.7.1. For wheel atomizer
- 4.7.2. For pressure nozzles
- 4.8. Powder after-treatment system
- 4.8.1. Pneumatic conveying system
- 4.8.2. Fluid bed system
- 4.8.3. Lecithin treatment system
- 4.8.4. Powder sieve
- 4.9. Final product conveying, storage and bagging-off system
- 4.10. Instrumentation and automation
-
5.Types of spray drying installations
- 5.1. Single stage systems
- 5.1.1. Spray dryers without any after-treatment system
- 5.1.2. Spray dryers with pneumatic conveying system
- 5.1.3. Spray dryers with cooling bed system
- 5.2. Two stage drying systems
- 5.2.1. Spray dryers with fluid bed after-drying systems
- 5.2.2. TALL FORM DRYER™
- 5.2.3. Spray dryers with Integrated Fluid Bed
- 5.3. Three stage drying systems
- 5.3.1. COMPACT DRYER™ type CDI (GEA Niro)
- 5.3.2. Multi Stage Dryer MSD™ type
- 5.3.3. Spray drying plant with Integrated Filters and Fluid Beds - IFD™
- 5.3.4. Multi Stage Dryer MSD™-PF
- 5.3.5. FILTERMAT™ (FMD) integrated belt dryer
- 5.4. Spray dryer with after-crystallization belt
- 5.5. TIXOTHERM™
- 5.6. Choosing a spray drying installation
- 6.Technical calculations
-
7.Principles of industrial production
- 7.1. Commissioning of a new plant
- 7.2. Causes for trouble-shooting
- 7.3. Production documentation
- 7.3.1. Production log sheets
- 7.3.2. General maintenance log book
- 7.3.3. Product quality specification
- 7.3.4. Operational parameter specification
- 7.4. Product quality control
- 7.4.1. Process quality control
- 7.4.2. Final quality control
-
8.Dried milk products
- 8.1. Regular milk powders
- 8.1.1. Regular skim milk powder
- 8.1.2. Regular whole milk powder
- 8.1.3. Whole milk powder with high free fat content
- 8.1.4. Butter milk powder
- 8.1.4.1. Sweet butter milk powder
- 8.1.4.2. Acid butter milk powder
- 8.1.5. Fat filled milk powder
- 8.2. Agglomerated milk powders
- 8.2.1. Agglomerated skim milk powder
- 8.2.2. Agglomerated whole milk powder
- 8.2.3. Instant whole milk powder
- 8.2.4. Agglomerated fat filled milk powder
- 8.2.5. Instant fat filled milk powder
- 8.3. Whey and whey related products
- 8.3.1. Ordinary sweet whey powder
- 8.3.2. Ordinary acid whey powder
- 8.3.3. Non-caking sweet whey powder
- 8.3.4. Non-caking acid whey powder
- 8.3.5. Fat filled whey powder
- 8.3.6. Hydrolysed whey powder
- 8.3.7. Whey protein powder
- 8.3.8. Permeate powders
- 8.3.9. Mother liquor
- 8.4. Other Dried Milk Products
- 8.5. Baby food
- 8.6. Caseinate powder
- 8.6.1. Coffee whitener
- 8.6.2. Cocoa-milk-sugar powder
- 8.6.3. Cheese powder
- 8.6.4. Butter powder
-
9.The composition and properties of milk
- 9.1. Raw milk quality
- 9.2. Milk composition
- 9.3. Components of milk solids
- 9.3.1. Milk proteins
- 9.3.2. Milk fat
- 9.3.3. Milk sugar
- 9.3.4. Minerals of milk
- 9.4. Physical properties of milk
- 9.4.1. Viscosity
- 9.4.2. Density
- 9.4.3. Boiling point
- 9.4.4. Acidity
- 9.4.5. Redox potential
- 9.4.6. Crystallization of lactose
- 9.4.7. Water activity
- 9.4.8. Stickiness and glass transition
-
10.Achieving product properties
- 10.1. Moisture content
- 10.2. Insolubility index
- 10.3. Bulk density, particle density, occluded air
- 10.4. Agglomeration
- 10.5. Flowability
- 10.6. Free fat content
- 10.7. Instant properties
- 10.7.1. Wettability
- 10.7.2. Dispersibility
- 10.7.3. Sludge
- 10.7.4. Heat stability
- 10.7.5. Slowly dispersible particles
- 10.7.6. Hot water test and coffee test
- 10.7.7. White Flecks Number (WFN)
- 10.8. Hygroscopicity, sticking and caking properties
- 10.9. Whey Protein Nitrogen Index (WPNI)
- 10.10. Shelf life
-
11.Analytical methods
- 11.1. Moisture content
- 11.1.1. Standard oven drying method (IDF Standard No.26-1964 [32])
- 11.1.2. Free moisture
- 11.1.3. Total moisture
- 11.1.4. Water of crystallization
- 11.2. Insolubility index
- 11.3. Bulk density
- 11.4. Particle density
- 11.5. Scorched particles
- 11.6. Wettability
- 11.7. Dispersibility
- 11.8. Other methods for determination of instant properties
- 11.8.1. Sludge
- 11.8.2. Slowly dispersible particles
- 11.8.3. Hot water sediment
- 11.8.4. Coffee test
- 11.8.5. White flecks number
- 11.9. Total fat content
- 11.10. Free fat content
- 11.11. Particle size distribution
- 11.12. Mechanical stability
- 11.13. Hygroscopicity
- 11.14. Degree of caking
- 11.15. Total lactose and α-lactose content
- 11.16. Titratable acidity
- 11.17. Whey Protein Nitrogen Index (WPNI)
- 11.18. Flowability (GEA Niro [31])
- 11.19. Lecithin content
- 11.20. Analytical methods for milk concentrates
- 11.20.1. Total solids
- 11.20.2. Insolubility index
- 11.20.3. Viscosity
- 11.20.4. Degree of crystallization
- 12.Troubleshooting operations
-
References