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4.

Components of a spray drying installation

The main components of a modern spray dryer, powder handling and storage as shown in Fig. 4.1. are: 
 
Drying chamber
Hot air supply system
  • Supply fan
  • Air filters
  • Air heater
  • Air disperser
Feed supply system
  • Feed tank(s)
  • Feed pump/supply pump
  • Concentrate heater
  • Filter
  • Homogenizer/high pressure pump
  • Feed line
Atomizing device
  • Rotary atomizer
  • Pressure nozzle atomizer
  • Two-fluid nozzle atomizer
Powder/fines recovery system
  • Cyclone
  • Bag filter
  • Wet scrubber
  • Combinations of the above
Fines return system
Powder after-treatment system
  • Pneumatic transport and cooling
  • Fluid bed after dryer/cooler
  • Lecithin treatment System
  • Powder sieve
Final product conveying, storage and bagging off system instrumentation

In the following, the main components of a spray dryer are discussed in details.
Main components of a modern spray dryer, powder handling and storage
Fig.4.1. Main components of a modern spray dryer, powder handling and storage
4.1.

Drying chamber

The shape of the drying chamber, the location of the air disperser, atomizing device, exhaust air outlet, powder discharge and after treatment system determine the air flow pattern, product flow, product structure and quality. Various drying chamber types are applied for drying milk and have the following characteristics (Fig. 4.2.).
Types of spray drying chamber
Fig.4.2. Types of spray drying chamber
A) Chamber shape:

a) wide body

b) tall form

c) horizontal box type

B) Product flow:

a) leaving the drying chamber with the exhaust air

b) partially separated from the exhaust air in drying chamber

C) Product discharge:

a) by gravity (conical bottom)

b) mechanically (flat bottom)

D) Air flow pattern:

a) rotary downward

b) straight downward

c) straight horizontal

E) Air/spray mixing:

a) concurrent

b) counter-current

F) Powder after-treatment:

a) none

b) pneumatic transport system

c) external fluid bed

d) integrated fluid bed

e) integrated belt

Table. 4.1. Components of a spray drying installation
Referring to the air flow conventional drying chambers are distinguished by their vertical or horizontal design. The vertical chamber is formed by a vertical cylinder of wide body or tall form shape. The ceiling of a cylindrical tower is usually flat. Recently, however, to comply with the safety requirements on mechanical strength in connection with pressure shock resistance, a conical shaped ceiling, convex or concave, is becoming more and more common. Below the cylindrical part is a cone section of 40 to 60° angle, enabling powder discharge by gravity or a flat bottom (possibly also slightly concave or convex conical), requiring a mechanical device to bring the powder to the discharge opening, placed in the centre.
 
The horizontal chambers are often referred to as box dryers, which is very well descriptive for their shape. The bottom of a box dryer is either flat or trough-formed requiring a mechanical device, a scraper, or a screw conveyor for removing the powder.
 
Drying chambers are equipped with service doors, inspection windows, light sources, air sweep doors, wall sweep ports, hammers, overpressure vents and fire extinguishing water nozzles. The drying chamber is usually insulated with 80-100 mm mineral wool to reduce radiation loss, clad with stainless steel, plastic coated steel or aluminium plates.
 
Today overpressure vents, fire extinguishing and overpressure suppressing equipment, complying with national or international standards are required by the authorities in practically all countries, whereas all unnecessary components such as inspection windows, built-in illumination sources and even service doors etc. and, generally all components affecting the smoothness of the chamber inner wall and creating dead pockets where accumulation of powder or washing water are gradually being eliminated for hygienic reasons. 
 
From the very same reason even mineral wool chamber wall insulation is now considered undesirable being a potential danger area for bacterial infection, since cracks in chamber walls eventually occur over the years of operation. A disadvantage of an insulation-free chamber is high heat loss, resulting in about 10% evaporation capacity loss and high temperature in the drying room. Removable air insulation panels are now being introduced and are already being used successfully (see Photo Fig. 4.3.).
Removable insulation panels
Fig.4.3. Removable insulation panels
The duct for the exhaust air in the old chamber types is a continuation of the conical base, and in this case the exhaust air carries also all the powder out of the chamber. However, in most modern dryers, the powder is separated as much as possible from the exhaust air already in the chamber. To achieve this, the exhaust air is drawn either from the upper part of the cylinder periphery, sometimes through the ceiling, or by a duct projecting into the cone with a slight downward slope. The tall form dryers are often equipped with an enlarged conical section (bustle) from which the exhaust air is withdrawn.
 
The development of the spray drying technology achieved within the last four decades established the advantages of the two stage drying method. The principle of two stage drying (discussed in chapter 3.4.) requires discharging of moist powder from the chamber. Therefore, modern dryers are based on chamber types enabling powder discharge without mechanical means and partial separation from the exhaust air.
 
The most up-to-date drying installations i.e. drying chambers with integrated fluid beds or belts accommodate the second drying stage inside the spray drying chamber by means of a static fluid bed such as the GEA Niro MSDTM, or a conveying belt assembly located at the chamber bottom such as the GEA Niro FILTERMATTM.
 
One of the most important factors when designing a spray drying chamber is that no ducts, air-brooms or the like are placed inside the chamber thus obstructing the air flow as that will give reasons of powder deposits with frequent cleaning and/or burned deposits as a result.
4.2.

Hot air supply system

In a spray dryer, the hot air supply system consists of
  • Air Supply Fan
  • Air Filters
  • Air Heater
  • Air Distribution.
4.2.1.

Air supply fan

The process air for the main drying chamber is supplied by a centrifugal air supply fan direct driven by a motor the speed of which is controlled by a frequency converter. This way there will be no energy loss due to belt transmission or dampers controlling the air flow.
4.2.2.

Air filters

The drying air is usually supplied into the system from outside the building. It is normally pre-filtered by a coarse filter. However, it must be realized that dust laden air can cause faster fouling of the filters, and contribute possibly to bacteriological problems or even fire hazard if the filters become too dirty or if they are damaged.
 
When using atmospheric air, the intake filters should be placed on the windward side of the factory, reasonably high above the terrain and far enough from known sources of dust (busy
roads, chimneys, exhaust stacks from other dryers etc.).
 
Until a few years ago no special requirements were given as to the filtration of the process air for the spray drying process. Today, however, very strict requirements are presented by local authorities in order to ensure a cleaner operation. Filter standards are referred to below, and it is important to refer to the test method when specifying the filter efficiency in %. Common for the different requirements is that:
  • The air should be pre-filtered and supplied by a separate fan to the fan/filter/heater room. This room must be under pressure to avoid unfiltered air to enter.
  • Filtration degree and filter position depend on the final temperature of the process air as follows:
- For main drying air to be heated above 120°C, only coarse filtration up to 90% is needed. The filter should be placed on the pressure side of the fan.
- For secondary air to be heated below 120°C or not heated at all, the filtration must be 90-95%, and the filter must be placed after the heater/cooler. Some countries and customers have even stricter requirements demanding a filtration of up to 99.995%, corresponding to EU13/14 (or H13/14).
 
Current practice is as follows:
 
Dairy-like products, equal to or better than 3A: Test method:

Pre-filtration EU4 (or G4)

Main air filtration EU7 (or F7

Secondary air filtration EU7 (or F7)

35% Dust-spot efficiency

90% Dust-spot efficiency

90% Dust-spot efficiency

Baby food products, equal to or better than IDF:

Pre-filtration EU6 (or F6)

Main air filtration EU7 (or F7)

Secondary air filtration EU9 (or F9)

70% Dust-spot efficiency

90% Dust-spot efficiency

>95% Dust-spot efficiency

4.2.3.

Air heater

The drying air can be heated in different ways:

  • Indirect: Steam / Oil / Gas / Hot oil
  • Direct: Gas / Electricity
4.2.3.1.

Indirect: Gas / Electricity

A steam heater is a simple radiator. The temperature to be obtained depends on the steam pressure available. Under normal conditions it is possible to obtain air temperatures 10°C lower than the corresponding saturation enthalpy of the steam.
 
Modern steam heaters are divided in sections, so that the cold air first meets the condensate section, then a section with low steam pressure (which is usually the biggest one in order to utilize as much low-pressure steam as possible), and then the air finally enters the highpressure steam section. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends upon the heat transfer properties of the tubes and fins and is usually about 50 Kcal/°C / h / m3 for an air velocity of 5 m/sec. Steam-heated air heaters will usually have an efficiency of 98-99%. As the steam boiler is usually placed at some distance from the air heater, 2-3 bar g extra pressure on the boiler should be anticipated due to pressure loss in the steam pipe and over the regulating valve. To avoid corrosion of the tubes in the air heater it is recommended to use stainless steel.
 
In indirect oil and gas heaters drying air and combustion gases have separate flow passage. The combustion gasses pass through galvanized tubes that act as heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. The end cover of the heater should be removable for cleaning of tubes. Heaters of this type will in the range of 175- 250°C have an efficiency of about 85%. See Fig. 4.4.
Indirect oil-fired air heater
Fig.4.4. Indirect oil-fired air heater
Hot oil liquid phase air heaters are used both alone, or when high inlet drying air temperatures are required, and the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components a special food-grade oil or heat transfer fluid, which does not crack at high temperatures, is circulated at high speed. The main advantage of hot oil liquid phase is the open pressure-less
system.
4.2.3.2.

Direct heater

Direct gas heaters are only used when the combustion gas can be allowed to come into contact with the product. They are therefore not common in the food and dairy industries. The direct gas heater is cheap, it has a high efficiency, and the obtainable temperature can be as high as 1650°C. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapour resulting from the combustion (44 mg/kg dry air/°C), as this will increase the humidity in the drying air. The outlet temperature has therefore to be increased in order to compensate for this increase in the humidity and to maintain the relative humidity. Combustion of natural gas (methane) takes place according to the following stoichiometric reaction formula:
[4,1]

The oxygen for the combustion originates from the atmospheric air with 21% O2 and 79% N2. All combustion yields small quantities of oxides of nitrogen as a result of the reaction of nitrogen and oxygen at elevated temperatures. Subsequent nitrogen oxide NO and nitrogen dioxide NO2 formation occurs and is referred to as the sum (NOx) of the two. It should be noted that high combustion temperatures, high heat transfer rates, high excess air, and low residence time in the combustion chamber are all factors increasing the formation of NOx

For comparison the following approximate NOx concentration prevails:

Cigarette smoke:

Exhaust gas from a car:

Heavy traffic intersection:

Natural gas boiler stack:

WHO food limits for infants:

Spray drying chamber:

Normal fresh milk:

Normal water supply:

4000 ppm

2000 ppm

900 ppm

75 ppm

45 ppm

2-5 ppm

<1

0.1

The level of NOx in the process air after the direct fired natural gas air heater will depend on many variable factors; however, with a well-adjusted air heater it should be limited to the above. Only about 2% of the NOx formed will be absorbed in the milk powder.

The level of NOx in milk powder depends not only on the method used for heating the process air, but also on the type of food used for the cows, as well as on the type of fertilizer and soil used for producing the food.

The NOx level in milk powder is:

  • Indirect heating: Traces - 2 ppm
  • Direct heating: 1 - 3.5 ppm

The level of nitrates (NO3) is in the order of 5-10 times the level of nitrites (NO2).

Electric air heaters are common on laboratory and pilot plant spray dryers. The heater has low investment costs, but is expensive in operation and therefore not used in industrial size plants.

4.2.4.

Air dispersers

A good mixing of the hot drying air with the spray of droplets and the control of the air and particle flow are essential for the whole process and has a decisive influence on the endproduct part of the whole system. There are two basic types of air dispersers:
 
  • Air disperser creating rotary air flow see Fig 4.5 a). and used in vertical wide body chambers. This type of air disperser operates usually with a rotary atomizer but is also suitable for pressure nozzles.
Air dispersers for straight downward air flow and rotary air flow
Fig.4.5. Air dispersers for straight downward air flow and rotary air flow
  • Air disperser creating straight air flow see Fig 4.5. b) and used exclusively with pressure nozzles for vertical downward air flow (for instance in the tall form dryer, multi stage dryer and the integrated filter dryer). Depending on the type of dryer one chamber can accommodate either just one or several air dispersers of this type (arranged symmetrically in the ceiling). The common goal is to have an air distribution and nozzle assembly that minimizes powder deposits in the drying chamber, and that the nozzles are interchangeable during the production to allow for continuous operation for weeks without stop. To secure a straight downward air flow, this type of air disperser is typically equipped with a number of perforated plates through which the long nozzle lances protrude. This results in a high pressure drop of the drying air (high energy consumption) and a difficult nozzle adjustment to obtain an optional agglomeration.
  • Today a new type of air disperser has therefore been developed. See Fig. 4.6. It operates still with a straight downward airflow but without perforated plates i.e. the pressure loss is low. It is even possible to obtain a rotation of the drying air to utilize the drying air best possible. The nozzle lances are short and operator friendly, and it is easy to adjust the nozzle position - also during operation - to obtain the degree and type of agglomeration as wanted.
The nozzle lances can of course also be changed during operation, so the plant can operate continuously for weeks so only the feed system needs to be cleaned every 20 hours, all depending on product composition.
Air disperser DDD for downward and rotating air flow
Fig.4.6. Air disperser DDD for downward and rotating air flow
4.3.

Feed supply system

The duty of the feed supply system is to deliver feed to the spray dryer via the atomizing device. It is actually a link between the evaporator and the spray dryer, and must compensate also for the capacity fluctuations of both units. The components of the feed supply system, shown on Fig. 4.1. are:
4.3.1.

Feed tank

Feed tanks act as a buffer compensating capacity variations. Usually, two feed tanks are installed to enable change-over from one tank to the other after several hours’ operation for intermediate washing of the emptied tank. This is important for bacteriological reasons as the temperature of 45-50°C with which the concentrate leaves the evaporator is ideal for the growth of thermophile bacteria. The size of a feed tank must be in relation to the plant capacity corresponding to about 5-10 minutes operation. In the case of an emergency, i.e. if the dryer has to be stopped suddenly, the surplus of the concentrate has to be transferred into a so-called escape tank, which is a part of the evaporator.
 
Closed feed tanks with inspection covers, maximum and minimum level transmitters, water connection(s) and CIPnozzles are today common requirements.
Feed tank
Fig.4.7. Feed tank
4.3.2.

Feed pump

Supplies the concentrate to the atomizing device and therefore the type of pump depends on atomizer type. For low pressure systems, as in case of wheel atomizers, almost any type of pump can be used, however preferably a positive pump as e.g. a mono-pump. For high pressure nozzles, a high pressure pump has to be used. If the pump has to process whole milk or other fat containing products, it can be combined with a homogenizer, preferably two stages. In such cases, it must be designed for a total pressure corresponding to the required homogenizing pressure over both stages plus atomizing pressure, including a safety factor. The homogenizer can be used as a feed pump also for wheel atomizer. It is then advantageous to install a second feed pump for non-fat products e.g. a mono-pump. A nonpositive displacement pump delivering the feed to the inlet side of the high-pressure pump with a pressure of a few bars has in any case to be incorporated into the system.
 
The feed pump is in fact a dosing pump, supplying the required amount of concentrate to the dryer. The amount is controlled by the outlet air temperature of the dryer by means of variable speed drive. For low pressure positive pumps and for high pressure pump frequency converters are used to control the feed flow to the atomizer.
4.4.

Concentrate heater

Is essential for high pressure nozzle operation and is highly recommended also for other systems. From various heat exchanger types available, a plate heat exchanger is less suitable due to fouling. This gives a limited time for continuous operation with liquids having properties and viscosities as normal milk concentrates. The most common concentrate heater is today a counter current corrugated tubular “tube in tube” system, where warm water is used as heating medium. With a maximum temperature difference of 5oC between the product and water, this type of pre-heater can operate 20 hours at 80oC product temperature. A filter is placed after the preheater to protect against mechanical impurities and possibly also some lumps created in the evaporator or heater. Usually a twin filter arrangement is used with changes over at regular intervals. The concentrate heater and filter is usually placed prior to the high pressure/homogenizer feed pump.
Concentrate heater type TCM
Fig.4.8. Concentrate heater type TCM
However, spiral tube heaters can be made also in a high pressure execution and be placed in the feed line close to the nozzles. It has the advantage that the residence time of the concentrate after it has been heated is short thus preventing increase of viscosity or agethickening.
 
The pressure drop over a spiral heater is often 20-50 bars, which has to be taken into account for specifying the pump.
 
A low cost method of heating concentrate is direct steam injection. The injector is installed on the feed line with low pressure systems or prior to high pressure pump with pressure nozzles. Food grade steam should be used as the heating medium. The condensing steam dilutes somewhat the concentrate. However it does not cause any reduction of capacity. The diluting medium is supplied into the system as steam (i.e. inclusive the heat of evaporation) and thus it does not consume any heat from the drying air. On the contrary there is some capacity increase due to the reduction of viscosity of the concentrate allowing a reduction of the outlet temperature while retaining the same powder moisture content without heating.
4.4.1.

Filter

An in-line double filter is always incorporated after the pre-heating system to avoid lumps etc. to pass through to the atomizing system.
4.4.2.

Homogenizer/High-pressure pump

If whole milk powder or other fat-containing products should be produced, it is recommended to incorporate a homogenizer in order to reduce the free fat content in the final powder. A two-stage homogenizer is preferred. First stage is operated at 70-100 bar g, and the second stage at 25-50 bar g, usually the homogenizer and feed pump are combined in one unit. If nozzle atomization is used then a higher pressure (up to 250 bar g for the nozzles + 150 bar g for homogenizing) is required, and a combined homogenizer/high-pressure pump is chosen to save cost. A variable speed drive for controlling the feed flow and thereby the outlet temperature is preferred, as a return valve tends to give uncontrollable holding time resulting in viscosity problems.
Homogenizer/High-pressure pump
Fig.4.9. Homogenizer/High-pressure pump
4.4.3.

Feed line

The feed line should naturally be of stainless steel and of course of the high-pressure type, if the atomization is to be carried out by means of nozzles. The dimension should be so that the feed velocity is approx. 1.5 m/sec. In a feed system a return pipe and a device to clean the rotary atomizer, incl. the wheel, as well as the nozzle lances, should also be included for the cleaning solution, so that the entire equipment can be thoroughly cleaned.
4.5.

Atomizing device

The purpose of the atomizing device is to transform the feed into a large number of droplets of well-defined size distribution. The atomization increases tremendously the surface area of the milk concentrate which is then exposed to the hot drying air. The rate of evaporation is then directly proportional to the surface area and thus a fine atomization has positive influence on many properties. The effect of droplet size on the number of droplets and their total surface area when atomizing one litre of concentrate, supposing totally homogeneous sprays is shown in Table 4.2.
Droplet size Number of droplets (x 10) Total surface area m2
1000     1.9 6
500 15.3 12
100 1909.8 60
50 15279.5 120
10 1909859.0 600
5 15278874.5 1200
Table. 4.2. Characteristics of homogenous sprays
The spray of droplets is characterized by a mean droplet size and droplet size distribution. Both depend on the type of atomizing device, operating conditions and the properties of the atomized liquid (concentrate viscosity, surface tension and density). It is common for all types of atomizers that increasing the amount of energy available for atomization results in smaller droplet size. With the same device and same amount of energy, the viscosity of the atomized liquid appears to influence the mean droplet size by 0.2 direct power relationship whereas surface tension has much less significant influence. The particle size distribution of the dried product is to a great extent influenced by the droplet size distribution of the spray, but not necessarily directly proportional. There are many additional factors influencing particle size distribution of the powder. Incorporation of air during the formation of droplets and their expansion or deflation during drying has greater influence on large droplets than on smaller ones. The amount of feed introduced to a single atomizing device or location of atomizing devices close to each other, increases the particle size of the powder through collisions of droplets with primary particles resulting in agglomerates. Such non-intentional agglomeration which takes place in a spray cloud without forced introduction of already dry powder is called primary agglomeration. The selection of the atomization device for a given duty depends mainly on the desired characteristics of the final product. There are three types of atomizers as described below. A photo of a rotary wheel atomizer is shown on Fig. 4.10.
4.5.1.

Rotary wheel atomizer

Rotary atomizer
Fig.4.10. Rotary atomizer
From the design point of view, the wheel atomizer consists of circular horizontal top and bottom plates with radial vanes or bushings between them. The feed enters close to the centre, accelerates across the wheel surface (vanes) and achieves the wheel’s peripheral speed. In a wheel atomizer centrifugal energy is utilized for atomization. A thin film of liquid is formed, as the liquid moves across the vanes or through the bushings, and this film readily disintegrates into droplets when thrown off the wheel edge. Wheel atomizers applied for milk drying operate with peripheral speeds in the range 100-200 m/s, however mostly in the higher speed range. Some types of wheel are shown on Fig.4.11.
Atomizing wheels
Fig.4.11. Atomizing wheels
The atomization effect depends mainly on the peripheral speed. However it was found, that at a given peripheral speed, a wheel of smaller diameter produces a finer spray than a larger wheel. Further influencing factors include the liquid loading and the number, height and design of vanes.
 
Efforts have been made to predict the mean droplet diameter by mathematical expressions. However, it is still difficult to do it with reasonably confidence and universal validity. The various factors and their influence on mean droplet size, reported by Masters [2], are shown in Table 4.3.
Factor Influence expressed in the power range of
Wheel speed - 0.6 to - 0.8
Wheel diameter - 0.2 (to - 0.85)
Peripheral speed - 0.54 to - 0.83
Feed rate 0.17 to 0.24
Vane height - 0.10 to - 0.12
Vane number - 0.10 to - 0.12
Feed viscosity 0. 10 to 0.20
Feed density - 0.5
Feed surface tension  0.1
Table. 4.3. The Influence of various factors on mean particle size.
The wheel atomizer has important advantages in comparison with other atomizing devices. Under practical circumstances, i.e. when considering a given installation, the wheel can operate with all types of dairy products and handle a wide range of capacities. Droplet size is insensitive to the feed rate fluctuations.
 
In comparison with pressure nozzles (see 4.4.2.) the wheel is also insensitive to fluctuations in concentration (resulting in a change of viscosity and of feed rate) and it is able to operate with higher feed viscosities, i.e. also concentrations. Furthermore, as it operates with low pressure feed system, there is a minimum risk of blocking, and it is therefore more suitable to handle feeds containing dispersed, possibly abrasive solids (for instance lactose crystals). Finally, much higher capacities can be handled in a single atomizing device. Generally, wheel atomizers offer a more flexible operation.
 
One disadvantage of wheel atomizer is that it works also as a fan pumping air through the vanes during wheel rotation. Due to this, an amount of air is incorporated into the droplets as bubbles, and these result in occluded air in the final dried particles. This effect depends upon the properties of the feed. It is stronger with liquids forming stable foams, i.e. products of high protein content. The presence of air in the dried particles increases the powder volume. This is undesirable if high bulk density powder is required. A special wheel developed for milk products has curve formed vanes accomplishing the removal of air bubbles from the milk film by centrifugal force as it flows over the vanes. Powders produced by this wheel, (see Fig. 4.12.) other conditions being the same, have lower contents of occluded air and higher bulk densities than powders from radial vane wheels. More about this phenomenon is discussed in chapter 10. Bulk Density. 
An important part of the rotary atomizer is the liquid distributor, which ensures a uniform distribution of the feed into the vanes in the wheel. If the feed distribution is not uniform it will result in a non-optimal drying process with deposits in one side of the drying chamber and excessive hot air in the opposite side. Also heavy vibrations on the spindle and wheel with bend and/or cracked spindles are the result of a non-optimal liquid distribution. 
Rotary atomizer wheel with curved vanes
Fig.4.12. Rotary atomizer wheel with curved vanes
The liquid distributor is positioned above the atomizer wheel. The feed is led through the atomizer via the feed pipe down into the distributor.
Screw distributor
Fig.4.13. Screw distributor
VOLUTE™ distributor
Fig.4.14. VOLUTE™ distributor
Several types of liquid distributors have been used through the time. First the hole-distributor was used. The hole-distributor consists of a ring with holes close to each other, located just above the wheel and placed around the drive shaft on which the wheel is mounted. To ensure good distribution a slight positive pressure (to compensate for the suction effect of the wheel and hydrostatic pressure of the liquid) must be kept in the feed line. This is achieved by adjusting the total surface area of the holes to the liquid flow rate. The flow velocity through the holes should be about 5 m/s. Then the screw liquid distributor, see Fig. 4.13., was introduced in the late 70’s. It operates in fact on the same principle as the grooved core insert in a pressure nozzle (see Fig. 4.15.A), where the feed is brought into rotation to form a conically shaped film. In the 90´s the volute distributor, see Fig. 4.12, became the standard distributor due to deposit and blocking problems in the screw distributor. However the volute distributor has turned out to be less functional on sticky and viscous liquids and it has led to a construction of a combined “volute-hole” distributor. Today a new rotary atomizer may be supplied with more than one type of liquid distributor, each designed to the product to be dried. 
 
Wheels have also been developed which can simultaneously atomize two liquids. A special liquid distributor ensures the feeding of two different feed flows separately into the two-tier wheel.
4.5.2.

Pressure nozzle atomizer

In pressure nozzles, the pressure energy applied by the high pressure pump to the liquid is converted into kinetic energy of thin liquid films, which are at the same time brought into rotary motion. The liquid film has the shape of a hollow cone. The film thickness decreases with the distance from the nozzle orifice. The disintegration of the film into droplets depends on the physical properties of the liquid and is assisted by the frictional effects of the surrounding air.
Types of pressure nozzles
Fig.4.15. Types of pressure nozzles
Pressure nozzles used in the dairy industry are of two types. See Fig. 4.15.:
 
  • Pressure nozzle with grooved core insert,
  • Pressure nozzle with swirl chamber.
Both types operate on the same principle and are often called centrifugal pressure nozzles. It is the grooved core and the swirl chamber which brings the liquid into rotary motion. For assembling a nozzle for a given duty, there are a number of grooved cores, swirl chambers and orifices available. The combinations of grooved core or swirl chamber with orifices (nozzle internals), define the two main characteristics of that combination, i.e. the nozzle capacity and the spraying angle at given pressure. The liquid flow rate is directly proportional to the square root of the pressure. Increase of the pressure increases the capacity but decreases slightly the spraying angle. The influence of the liquid physical properties on the droplet size is similar to that in wheel atomization. Increase of viscosity decreases also the spraying angle. The droplet size depends mainly on the atomization pressure and spraying angle. The higher the pressure and the wider the angle, the smaller is the mean droplet size. However the effect of spraying angle on the droplet size decreases with increasing pressure. Due to the influence of the operational variables on pressure nozzles performance it is essential to choose carefully the combination of grooved core/swirl chamber and orifice and to keep the operational variables as constant as possible.
 
Nozzles for milk drying operate usually in the range of 150-250 bar (80-500 bar in exceptional cases).
 
The correct choice of nozzle internals is normally based on information supplied by the nozzle manufacturer. The following equation applies:
[4,2]
where: 
Q = the flow rate in kg/h
ρf= the density of the feed in g/ml
F = a factor based on the feed properties, 0.8-1.0, for milk close to 0.9
n = number of nozzles installed
P = the pressure in bar
K = capacity constant (the flow rate of water through one nozzle at 1 bar pressure)
 
The most important advantage of the pressure nozzle in comparison with wheel atomizers is the formation of almost air-free droplets thus achieving powders of low content of occluded air and of high bulk densities. Another advantage is good flowability of the final powder. Furthermore it is easy to direct the spray cloud in any direction. This means that in multinozzle dryers, one can combine the individual nozzles close to or apart from each other and let them spray towards or away from each other depending on whether primary agglomeration is required or should be avoided. The disadvantages have been more or less expressed by the advantages of wheel atomizers as discussed in the previous section.
4.5.3.

Two-fluid nozzle atomizer

In this type of atomizer, also called pneumatic nozzle the feed is atomized by high velocity air. There are two types, i.e. with internal or external mixing. See Fig. 4.16. However it is seldom used in the dairy industry. Otherwise, two-fluid nozzles with internal mixing are used only in small pilot dryers and for special duties, where small amounts of liquid must be atomized into very fine sprays (for instance spraying lecithin/oil mixture on powder to produce an instant whole milk powder).
Two-fluid nozzles
Fig.4.16. Two-fluid nozzles
4.6.

Powder recovery system

The exhaust air from some old types of single stage spray dryers carries the total powder production to the cyclones. However in more modern systems partial separation of the powder from the drying air takes place already in the drying chamber so that the exhaust air contains much less powder. The actual amount depends on the product; spray dryer type and operating conditions. Factors contributing to low powder carry-over to cyclones are high feed total solids content, high fat content, high moisture content of the powder leaving the chamber, degree of agglomeration and low protein content. Under such conditions carry-over is usually less than 10 %. On the other hand the carry-over can be more than 50% especially for high-protein low-lactose products dried from low total solids content feeds. The fraction of the powder carried over by the exhaust air is referred to as fines, as it consists of the smallest particles.
 
Generally, a milk spray dryer utilizes - depending on operating conditions - 15-30 Nm3 of drying air per kg dry product. The air leaving the chamber contains - depending on product type - higher (protein rich powders) or lower (high-fat powders) amounts of powder. Before the drying air is discharged to atmosphere, the powder must be separated from the exhaust air as completely as possible, because even a small amount of powder will represent a noticeable economical loss and cause environmental problems. In EU and most other countries, the general requirement for maximum powder emission is 10 mg/Nm3.
 
There are various types and constructions of powder separation equipment, but due to hygienic and safety requirements, only three types are acceptable for milk powder production, i.e.:
  • Cyclone separator
  • Bag filter
  • Wet scrubber
  • Combinations of the above.
4.6.1.

Cyclone separator

The cyclone has some obvious advantages if it is constructed properly, it is easily maintained as there are no moving parts, and, furthermore, it is easy to clean, if it is a fully welded construction. But it does not live up to today’s strict emission requirements as it - depending on the product and operation of the dryer - may reach 250 - 400 mg/Nm3
Cyclone separator
Fig.4.17. Cyclone separator
The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particle to move away from the cyclone axis towards the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is predominant, a separation takes place. Powder and air pass tangentially into the cyclone at equal velocities. Powder and air swirl in a spiral form down to the base of the cyclone separating the powder out to the cyclone wall.
 
Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upwards along the centre axis of the cyclone and passes out at the top. See Fig. 4.17. The centrifugal force each particle is exposed to can be seen in this equation:
[4,3]
Where:
C = centrifugal force
m = mass of particle
Vt = tangential air velocity
r = radial distance to the wall from any given point
 
From this equation it can be concluded that the higher particle mass, the better efficiency. The shorter way the particle has to travel the better efficiency, and the closer the particle is to the wall the better efficiency, because the velocity is highest and the radial distance is short. However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone. From above equation it is evident that small cyclones (diameter less than 1 m) will have the highest efficiency, a fact generally accepted. 
 
However, as the cyclone(s) alone - irregardless of the diameter - cannot clean the exhaust air sufficiently, the spray dryers - especially in the Baby-Food industry - today are equipped with cyclones of up to 4 m diameter followed by a bag filter (a “police filter”) to fulfil requirements from the authorities. Each cyclone has an outlet for powder in form of a rotary valve see Fig. 4.18., or directly into a “blow-through” valve connected to a fines return system.
A rotary valve with conical rotor
Fig.4.18. A rotary valve with conical rotor
4.6.2.

Bag filter

Since 2007 the EU has required “best available technology” to minimize powder loss from spray drying plants being 10 - 15 mg/Nm3 in the exhaust air. Final cleaning of the air is therefore necessary if cyclones are used as the primary separators. Bag filters consist of numerous bags or filters arranged in a filter house so that each bag receives equal quantities of air. The dust loaded air should enter the filter house tangentially to minimize wear and tear of the bags. The direction of the air to the filter bags is from outside in through the filter material to the inner part of the bag from where the cleaned air enters a clean air plenum, from where it is led to the atmosphere via an exhaust fan. With a correct selection of filter material high efficiencies can be obtained and collection of 1 micron particles is reported from the manufacturers. The collected powder is automatically pulsed off by blowing compressed air into each bag at predecided intervals depending on the product. This is done via a specially designed reverse air nozzle positioned above each bag. The powder is collected at the bottom via a rotary valve. If the bag filter is placed after the cyclone, the amount of collected powder is minimal, and it consists of very fine powder particles that flow only with difficulty. The powder may therefore stick to the conical part of the filter house and become discoloured due to the exposure to high temperatures. The filter fraction is therefore considered as a second class powder as it may also have high mould content.
CIP-able bag filter
Fig.4.19. CIP-able bag filter
The bag filter may also replace the cyclones, a solution often chosen for one-stage dryers for whey protein powder or egg white. To prevent condensation, especially on the conical part of the filter housing, warm air or heat tracing is established.
 
When a bag filter is installed after a cyclone, the total pressure loss over the exhaust system – including air ducts – will be 300 to 400 mm WG, equal to a high energy consumption. Designers of spray dryers – including GEA Niro – have therefore developed bag filters designed for CIP. The GEA Niro SANICIP™ CIP-able bag filter is replacing the cyclones and is of the reverse jet type. It consists of cylindrical bag housing with top air inlet, clean air plenum on top, and a conical bottom with fluidized powder discharge. During operation the product collected on the outside of the filter material is removed by a compressed air jet stream blown into each bag via a special reverse jet air nozzle positioned above each bag, see Fig.4.17. A jet is formed which draws air from the clean air plenum into the bag as well, thereby saving compressed air. This is an efficient and sanitary solution. The reverse air jet nozzle has furthermore a dualpurpose during CIP as described below.
 
The bags are clean-blown individually or 4 together, resulting in a very even discharge of powder and using higher air-to-cloth ratios. The frequency and duration of the cleaning sequence can be adjusted to suit actual running conditions. A supply air system for the fluidizing bottom and heating of the cone secures that the fine powder is easily discharged from the filter and that the conical section is kept warm to avoid powder deposits. During standstill the cone heating system is used for heating of the cone alone. Condensation and risk of mould growth is therefore avoided.
 
The filter bags are made from an FDA approved polyester material. This material is fully CIPable with NaOH and HNO3 in 1 to 2% solutions at 75°C and 60°C, respectively. It is heattreated to give a special dust-releasing surface. Each bag is supported on a stainless steel cage and is easy to dismantle.
 
The CIP of the bag filter is divided into the following main sequences:
 
  • The internal bag CIP cleans the bag from the inside towards the dirty side (outside). CIP liquid running on the inside of the bags is forced out through the filter material by the compressed air pulse by the reversed jet nozzle. Powder that has penetrated into the bag material is thus forced out towards the dirty side.
  • The clean air plenum CIP cleans the clean air plenum of the bag filter above the whole plate. No recirculation of CIP liquid in this area.
  • The hole plate CIP cleans the bottom side of the hole plate and the snap ring area of the bag using a specially designed nozzle. This nozzle is positioned on the bottom part of the hole plate between the bags, and it also cleans the outside of the filter bags. The nozzle has a dual purpose as well, as it during the drying process is purged with warm air to keep the hole plate free of deposits. Discolouring/denaturation are thereby minimized. The CIP liquid is recirculated.
  • The shell CIP is performed by means of standard retractable CIP nozzles. The CIP water is recirculated.

Advantages of the SANICIP™ filter:
  • Low pressure loss across the bag filter and thus the entire exhaust system i.e. reduced energy consumption and noise emission
  • Designed for optimum air-to-cloth ratio and powder load (due to one to four bag(s) being blown at the time)
  • Better utilization of raw materials due to no second grade products
  • Design with 4 or 6m bags to suit specific building requirements.
4.6.3.

Wet scrubber

In a wet scrubber powder particles are collected by a washing liquid which is sprayed into the powder-laden air. It is essential to bring the laden air into intimate contact with the spray. This is carried out in a venturi type tube (Fig. 4.20.). The washing liquid is separated from the clean air in a cyclonic type separator.
Wet scrubbers can operate with liquid milk or whey as washing liquid whereby the evaporator feed, before entering the evaporator is used as washing medium in a single pass operation. The advantage of this system is
complete recovery of the powder into product and some pre-concentration of the milk. Due to evaporation, both the temperature of the washing liquid and of the air closely approaches the wet bulb temperature. This is at usual milk drying conditions about 40-45°C which is a temperature range favourable for bacterial growth. To minimize such growth the whole unit is designed for minimum liquid holding volume to ensure a sharp border-line between the dry and wet zones. Good bacteriological quality of incoming milk is of utmost importance for long continuous operation without jeopardizing the bacterial quality of the final powder. Besides, it might be necessary to apply an intermediate CIP of the scrubber in frequent intervals, about 8 hours. Another problem encountered may be excessive foaming. For all these reasons wet scrubbers, using evaporator feed as washing liquid, are usually applied for products for non-human consumption only and exclusively as secondary separators after cyclones. The separation efficiency of a wet scrubber is just as high as a bag filter.
Wet scrubber
Fig.4.20. Wet scrubber
If a wet scrubber is used it is more common to operate it with water in recirculation. Washing water has to be replaced in regular intervals so that the total solids content remains about 10%. Often it is discharged as effluent. Some factories, however, have full control of the bacteriological problems. The washing water is heat treated, cooled down and either spray dried or more often delivered in liquid form for animal feeding. 
 
The above obvious drawbacks of the wet scrubber mean that it is practically not used any longer in modern spray dryer installations.
4.6.4.

Combinations

In the below Table 4.4., a comparison of the different combinations of powder separators is given. Which one to select depends entirely of the product produced and how the collected product – if a bag filter or wet scrubber is used in combination with a cyclone – can be disposed of.
Cyclone Cyclone + Bag Filter Cyclone + Wet Scrubber SANICIP™
Emission 20-400 mg/Nm3 5-20 mg/Nm3 max. 20 mg/Nm3 5-20 mg/Nm3
Pressure loss Exhaust system (incl. ducts etc.) 280 mm WG 340 mm WG 340 mm WG170  mm WG
Auxiliaries none     compressed air liquid circulating system compressed air
Cleaning suitable for CIP difficult suitable for CI suitable for CI
Hygroscopic products insensitive sensitive insensitive insensitive*)
Use of separated product first grade first and second grade not recommended first grade
Maintenance minimal     service of compressed air system and change of bags minimal service of compressed air system and change of bags
Sanitary conditions good relatively good less good good
Table. 4.4. Comparison of powder separation systems
*)watch out for permeate, if the humidity in the outgoing air is too high
4.7.

Fines return system

The fines collected from dry powder separators - be it cyclones and/or bag filters - have to be collected at one point and returned to the process. This can be done in several ways, all depending on the wanted final powder structure. Most common today is a blow-through valve fit directly to a cyclone(s) and/or the CIP-able bag filter(s) product discharge point. The collected powder is then fed into a blow line system and can now be directed back into the process at any given point.
 
For high fat powders gravity fall tube is used leading the fines directly into a cooling bed. A blow line usually has several branches connected to the main line by flow diversion valves. Fines can be directed:
  • into the atomizing zone to form agglomerates,
  • into the rear section of a vibrating fluid bed to produce non-agglomerated powders,
  • into the end of vibrating fluid bed when emptying the system at the end of the production run.
In order to achieve good agglomeration the fines must be brought into the wet atomizing zone to achieve intimate contact with the spray of droplets and fines. There are numerous methods of reintroducing the fines depending on the type of dryer and atomizing device.
Fines powder return arrangement
Fig.4.21. Fines powder return arrangement
Generally speaking it is easier to bring the fines to a pressure nozzle assembly than to a wheel atomizer. Several methods have been developed and used:
4.7.1.

For wheel atomizer

a) through the atomizer above the wheel,
b) below the wheel (Fig. 4.22. a)),
c) through the air disperser above the wheel (Fig. 4.22. b))
Fines return
Fig.4.22. Fines return
4.7.2.

For pressure nozzles

a) around one nozzle,
b) in the centre of three or more nozzles,
c) into a rotating air stream between several nozzles.
Fines return to pressure nozzles
Fig.4.23. Fines return to pressure nozzles
Returning of fines into the atomizer cloud is an important step when producing agglomerated powders. There are, however often vital factors decisive for agglomeration, and these will be discussed in chapter 10. The fines return system can also be used with advantage for introducing additional components (as sugar, cocoa, vitamins etc.) to the dried powder. In such case, a supply silo with dosing equipment (for instance a screw feeder) is connected by a separate blow-through valve to the transport blow line.
In the Baby-Food industry - where the powders have high carbohydrate content, the powder agglomerates very easily, and sometimes too much - it has been a common practice to return only a fraction of the fines to the atomizer and the rest to the fluid bed. The degree of agglomeration and also the bulk density can thus be controlled.
Pressure nozzle assembly with fines return
Fig.4.24. Pressure nozzle assembly with fines return
4.8.

Powder after-treatment system

The drying conditions together with the method of the treatment of the product leaving the chamber determine the structure and overall quality of the produced powder. Powder after-treatment method can be either a pneumatic conveying system or fluid bed system. The latter may be further combined with fines return system and lecithin treatment.
4.8.1.

Pneumatic conveying system

Powder is discharged from the spray drying installation usually at two points, i.e. from the bottom of the spray drying chamber and from the cyclone(s) and or bag filter(s). A pneumatic conveying system is the cheapest way to collect the powder at these points, to cool it down and to transport it to a bagging-off point. The air velocity for pneumatic transport is in the order 20 m/s and the air/powder ratio has to be at least 5:1. The powder is finally separated from the transport air in a cyclone (bagging-off cyclone).
 
Combining a drying chamber with a pneumatic conveying system forms the most simple of spray drying installations, is inexpensive in investment and easy to operate. On the other hand it permits production of only so-called regular or ordinary powders (which mean nonagglomerated powders consisting of single particles). Consequently these powders have relatively high bulk density, are dusty, have poor flowability and are difficult to reconstitute in water. The drying process is single stage drying (see chapter 3) requiring relatively high exhaust air temperature to complete the drying. It is therefore not too gentle towards thepowder and it has low thermal efficiency. It cannot be used for products with high fat content.
4.8.2.

Fluid bed system

Fluid bed after-treatment can be used just for cooling or for after-drying with subsequent cooling. Fluid beds can be either stationary or integrated at the bottom of the drying chamber or they can be vibrating. Typically a combination of the two is used in modern dryers.
 
A vibrating fluid bed (Fig. 4.25.) consists of an oblong housing which features a perforated horizontal plate that separates the lower air plenum section from the upper powder plenum section. The process air is supplied below the perforated plate and exhausted above. The powder enters the fluid bed at the inlet, moves across the surface of the perforated plate and leaves at the discharge. Simultaneously the air passes through the holes of the perforated plate and upwards through the moving powder layer. Powder is brought into fluidization which is a turbulent movement resembled boiling. Powder moves under plug-flow conditions coming all the time into contact with air, i.e. powder receives successive treatment. In order to achieve the desired drying or cooling effect, a relatively high quantity of air has to be used. The air passes through the holes at the velocity of about 20 m/s. This is necessary to avoid powder particles falling through the holes. However the average air velocity above the plate, called fluidizing velocity must be considerably lower to avoid blowing-off the powder with exhaust. The fluidizing velocity is therefore a very important technological parameter which must be adjusted according to the type of product processed. This velocity is typically 0.1-0.2 m/s for caseinates and similar powders, 0.2-0.3 m/s for skim milk, 0.25-0.4 m/s for whole milk and 0.3-0.5 m/s for high fat milk products and for agglomerated whey products. The degree of perforation of the perforated plate expressed in percent of the total area is between 0.5 to 2 %. This enables the required range of fluidization velocities to be achieved. A photo of a modern vibrating fluid bed is shown on Fig. 4.26.
Vibrating fluid bed VIBROFLUIDIZERTM
Fig.4.25. Vibrating fluid bed VIBROFLUIDIZERTM
When fluidizing with high fluidizing velocities and high powder layers powders behave as liquids. Such conditions are difficult to obtain with dry milk products. These so-called dead powders are not easy to fluidize and tend to create channelling effects. To overcome this problem, the whole unit must be vibrated or shaken. The vibration helps to achieve good fluidization even below the critical fluidizing velocity and powder layer.
A modern VIBRO-FLUIDIZERTM
Fig.4.26. A modern VIBRO-FLUIDIZERTM
Perforated plates exist in a number of designs allowing horizontal movement. Such movement, resulting in a fast transport of the powder from the inlet to the outlet, is advantageous for emptying of the unit at the end of the operation, but it inhibits the build-up of a powder layer and reduces residence time. Therefore the fluid beds with such a type of plate are provided by an overflow weir at the outlet end. Such an overflow weir secures the desired bed depth and residence time.
Weir regulator
Fig.4.27. Weir regulator
Various types of perforated plates are shown on Fig. 4.28. The perforated plate can be mounted between flanges. This is very seldom done today, but it has the advantage that different plates can be used for different products. Besides the plate can be taken out for washing outside the spray dryer hall if for sanitary reasons wet cleaning is undesirable.
Various types of perforated plates
Fig.4.28. Various types of perforated plates
Integrated fluid beds are of the back mix type where the residence time is undefined. To improve on this, GEA Niro´s Multi Stage Dryer, MSD™ is equipped with a perforated plate - with directional air flow - welded together in sections to direct the powder onto the powder outlet. Modern sanitary fluid beds have their perforated plate fully welded. More details on fluid bed operation can be found in chapter 5.
4.8.3.

Lecithin treatment system

The background for the philosophy of lecithin treatment of fat containing powders is explained in section 10.4. The lecithin treatment or lecithination refers to the coating of powders by a wetting agent consisting of lecithin dissolved in butter oil or another low melting oil. The concentration of lecithin in fat is between 10 to 60% and the wetting agent is added in such amounts as to achieve lecithin content in the powder of 0.2-0.5%.
 
The wetting agent is sprayed by means of a two fluid nozzle either directly on the powder under vigorous fluidization in a vibrating or static fluid bed or it falls through a funnel feeding the inlet of a fluid bed. The lecithination equipment (Fig. 4.29.) includes:
Lecithination equipment
Fig.4.29. Lecithination equipment
a) preparation tank with agitator and heating jacket for dissolving powdered lecithin in fat or oil,
b) transfer pump, which is a centrifugal pump, for pumping the wetting agent from the preparation tank to the supply tank,
c) supply tank with agitator and heating jacket,
d) dosing pump with variable speed control,
e) electric heater for compressed air,
f) two fluid nozzle placed in a powder flow,
g) heat traced pipes for both lecithin and compressed air,
h) special 4-way valve allowing combinations of lecithin and compressed air flow:
- both lecithin and compressed air to the nozzle,
- lecithin to recirculation to the supply tank and compressed
air to both passages of two fluid nozzle.
i) Vegetable oil tank for rinsing the supply pump and pipeline.
 
For effective lecithination, the temperature of wetting agent must be 50-60°C and compressed air 60-80°C. The spraying angle must be adjusted to 70-90° and the powder kept in vigorous fluidization by warm air also after applying lecithin so as to maintain the final powder temperature at the powder outlet at least 45°C.
 
The lecithination nowadays is provided as an integrated part of the spray drying installation. It can however be an independent unit, consisting of two fluid beds with lecithination spraying conducted in between. The duty of the first fluid bed is to heat the powder prior to lecithination to about 45°C; the second ensures good mixing for uniform distribution of lecithin within the treated powder. Powder is usually fed from a silo by means of dosing equipment. Separate lecithination is advantageous especially when shipping non-lecithinated powder in bulk from the area of production to the point of market distribution. However, for obtaining the best powder quality, filling the lecithinated powder into tins should be linked directly with lecithination process.
4.8.4.

Powder sieve

The last component of a spray drying installation is usually a sifter which is a shaking or vibrating mesh or a cylindrical static mesh with rotating arms. The aim of sifting is to separate the oversize agglomerates or powder lumps, which can occasionally occur. Sometimes the sifter contains a second smaller mesh for separating out the finest fraction which can then via the fines return system be returned to the process.
4.9.

Final product conveying, storage and bagging-off system

After the sifting, the powder is either (a) directly bagged-off into paper bags, (b) filled into transport containers (tote-bins, big bags, etc.) or (c) blown into storage silos before final bagging-off or filling into tins or other retail packages. See various photos Fig. 4.30.
Powder handling and storage equipment
Fig.4.30. Powder handling and storage equipment
Agglomerated products can also be transported by air, in modern dense phase transport systems to silos - using only a small amount of air and low transport velocity. Vacuum systems are usually used from the silos to the bagging off line combined with pre-gassing of the powder before it is bagged off in 25 kg bags. Residual oxygen of 1% or less in the bag is reported from the suppliers. On modern spray dryer installations a start-up silo may be installed. The very first powder leaving the dryer is often “out of spec.”, and this powder is then led to a separate silo from where it is slowly fed back to the dryer via the fines return system when it is in full balance.
 
There are many kinds of silo storage systems, bagging-off systems and retail packaging systems available ranging from the most simple to fully automated systems requiring little or no manpower.
4.10.

Instrumentation and automation

In order to control the drying process and at any time to be able to record the drying parameters the installation should include instrumentation and control equipment. This is done using field instruments and a PLC with monitor, which is placed in a separate control room, (see Fig. 4.31.) partly to keep it dry, but also because the operators here can be in a place with reduced noise level.
Control room
Fig.4.31. Control room
Instrumentation, (see Fig. 4.32.), for a modern spray dryer should include all relevant processing parameters, incl. inlet drying air temperature for the main chamber and fluid beds, as well as outlet air temperature. All temperatures are recorded enabling the operator to see the trend of the temperature development, and also to go back and find the reason why a powder has been downgraded in the laboratory. An hour counter for the atomizer or high-pressure pump is also necessary, as it tells when oil should be changed. A feed pressure gauge should also be included, if the atomization is carried out with nozzles. In order to check the pressure in the chamber which is usually operated under a vacuum of 5-10 mm WG frequency converters on the inlet and outlet air fans should be provided. These can of course be operated manually, but in most cases they are automatically controlled. Automatic start/stop of the plant is therefore possible.
 
The inlet temperature can be automatically controlled by regulating either the steam pressure or the amount of oil or gas to the air heater. The outlet temperature should always be automatically controlled to ensure a powder with constant residual moisture content. If the atomization takes place by means of a rotary atomizer the regulation of the outlet temperature is done by changing the revolution of the feed pump. Another system, which, however, is not very often used - and then only for nozzle atomization - is with a constant supply of feed to the atomizer and then keep the outlet temperature constant by changing the inlet temperature. If the atomization is done by means of nozzles the outlet temperature may be kept constant by changing the revolution of the high-pressure pump. This will naturally have an influence on the nozzle pressure which again will have an influence on the mean particle size and the particle distribution. However, once the right nozzle combination has been found, only marginal changes are seen.
Instrumentation diagram
Fig.4.32. Instrumentation diagram
A drying installation, however, is not only the spray dryer. There is also the evaporator. As the raw milk solids can vary from tank to tank and fouling (micro-thin deposits in the tubes which will alter the K value) may occur after a certain running time, the evaporating capacity and therefore the amount of concentrate will not be constant. It is of course possible to counteract this by manual regulation on the evaporator or spray dryer, but it is also possible to do the regulation automatically. The most common system is to delete the feed tanks and let the last stage of the evaporator or a special vacuum tank take over this function. Level transmitters are then built into the last effect calandria in the evaporator. The level in the evaporator is now controlled by the feed flow and/or steam pressure to the thermo-compressor.
 
During the last years, the development of PLC’s has resulted in equipment for process control, which is attractive both with regard to price and intelligence. The PLC has many advantages, also when we are talking about traditional and uncomplicated controls consisting of simple isolated loops trying to maintain a given parameter at the set point disregarding other parameters, which might well have an influence on the selected set point. This means that the operator’s knowledge of the process has less influence on the plant operation and therefore on the product quality, which can then be closer to the specifications.
 
The PLC also offers a perfect tool for start-up or shut-down of the entire plant. This means that non-productive running time can be avoided. Also sequence control of valves and pumps during CIP of the plant is controlled by the PLC. Data-logging is possible by computing mean values of any selected parameter, and trend curves over for example one hour can be monitored and printed as a hard copy.

Table of contents

  1. 1.Introduction
  2. 2.Evaporation
    1. 2.1. Basic principles
    2. 2.2. Main components of the evaporator
    3. 2.2.1. Heat exchanger for preheating
    4. 2.2.1.1. Spiral-tube preheaters
    5. 2.2.1.2. Straight-tube preheaters
    6. 2.2.1.3. Preheaters to prevent growth of spore forming bacteria
    7. 2.2.1.3.1. Direct contact regenerative preheaters
    8. 2.2.1.3.2. Duplex preheating system
    9. 2.2.1.3.3. Preheating by direct steam injection
    10. 2.2.1.4. Other means to solve presence of spore forming bacteria
    11. 2.2.1.4.1. Mid-run cleaning
    12. 2.2.1.4.2. UHT treatment
    13. 2.2.2. Pasteurizing system including holding
    14. 2.2.2.1. Indirect pasteurization
    15. 2.2.2.2. Direct pasteurization
    16. 2.2.2.3. Holding tubes
    17. 2.2.3. Product distribution system
    18. 2.2.3.1. Dynamic distribution system
    19. 2.2.3.2. Static distribution system
    20. 2.2.4. Calandria(s) with boiling tubes
    21. 2.2.5. Separator
    22. 2.2.5.1. Separators with tangential vapour inlet
    23. 2.2.5.2. Wrap-around separator
    24. 2.2.6. Vapour recompression systems
    25. 2.2.6.1. Thermal Vapour Recompression – TVR
    26. 2.2.6.2. Mechanical Vapour Recompression - MVR
    27. 2.2.7. Condensation equipment
    28. 2.2.7.1. Mixing condenser
    29. 2.2.7.2. Surface condenser
    30. 2.2.8. Vacuum equipment
    31. 2.2.8.1. Vacuum pump
    32. 2.2.8.2. Steam jet vacuum unit
    33. 2.2.9. Flash coolers
    34. 2.2.10. Sealing water equipment
    35. 2.2.11. Cooling towers
    36. 2.3. Evaporator design parameters
    37. 2.3.1. Determination of heating surface
    38. 2.3.2. Heat transfer coefficient
    39. 2.3.3. Coverage coefficient
    40. 2.3.4. Boiling temperature
    41. 2.4. Evaporation parameters and its influrence on powder properties
    42. 2.4.1. Effect of pasteurization
    43. 2.4.1.1. Bacteriological requirements
    44. 2.4.1.2. Functional properties of dried products
    45. 2.4.1.2.1. Heat classified skim milk powders
    46. 2.4.1.2.2. High-Heat Heat-Stable milk powders
    47. 2.4.1.2.3. Keeping quality of whole milk powders
    48. 2.4.1.2.4. Coffee stability of whole milk powders
    49. 2.4.2. Concentrate properties
  3. 3.Fundamentals of spray drying
    1. 3.1. Principle and terms
    2. 3.1.1. Drying air characteristics
    3. 3.1.2. Terms and definitions
    4. 3.1.3. Psychrometric chart
    5. 3.2. Drying of milk droplets
    6. 3.2.1. Particle size distribution
    7. 3.2.2. Mean particle size
    8. 3.2.3. Droplet temperature and rate of drying
    9. 3.2.4. Particle volume and incorporation of air
    10. 3.3. Single-stage drying
    11. 3.4. Two-stage drying
    12. 3.5. Expansion of air bubbles during drying
    13. 3.6. Extended Two-stage drying
    14. 3.7. Fluid bed drying
  4. 4.Components of a spray drying installation
    1. 4.1. Drying chamber
    2. 4.2. Hot air supply system
    3. 4.2.1. Air supply fan
    4. 4.2.2. Air filters
    5. 4.2.3. Air heater
    6. 4.2.3.1. Indirect: Gas / Electricity
    7. 4.2.3.2. Direct heater
    8. 4.2.4. Air dispersers
    9. 4.3. Feed supply system
    10. 4.3.1. Feed tank
    11. 4.3.2. Feed pump
    12. 4.4. Concentrate heater
    13. 4.4.1. Filter
    14. 4.4.2. Homogenizer/High-pressure pump
    15. 4.4.3. Feed line
    16. 4.5. Atomizing device
    17. 4.5.1. Rotary wheel atomizer
    18. 4.5.2. Pressure nozzle atomizer
    19. 4.5.3. Two-fluid nozzle atomizer
    20. 4.6. Powder recovery system
    21. 4.6.1. Cyclone separator
    22. 4.6.2. Bag filter
    23. 4.6.3. Wet scrubber
    24. 4.6.4. Combinations
    25. 4.7. Fines return system
    26. 4.7.1. For wheel atomizer
    27. 4.7.2. For pressure nozzles
    28. 4.8. Powder after-treatment system
    29. 4.8.1. Pneumatic conveying system
    30. 4.8.2. Fluid bed system
    31. 4.8.3. Lecithin treatment system
    32. 4.8.4. Powder sieve
    33. 4.9. Final product conveying, storage and bagging-off system
    34. 4.10. Instrumentation and automation
  5. 5.Types of spray drying installations
    1. 5.1. Single stage systems
    2. 5.1.1. Spray dryers without any after-treatment system
    3. 5.1.2. Spray dryers with pneumatic conveying system
    4. 5.1.3. Spray dryers with cooling bed system
    5. 5.2. Two stage drying systems
    6. 5.2.1. Spray dryers with fluid bed after-drying systems
    7. 5.2.2. TALL FORM DRYER™
    8. 5.2.3. Spray dryers with Integrated Fluid Bed
    9. 5.3. Three stage drying systems
    10. 5.3.1. COMPACT DRYER™ type CDI (GEA Niro)
    11. 5.3.2. Multi Stage Dryer MSD™ type
    12. 5.3.3. Spray drying plant with Integrated Filters and Fluid Beds - IFD™
    13. 5.3.4. Multi Stage Dryer MSD™-PF
    14. 5.3.5. FILTERMAT™ (FMD) integrated belt dryer
    15. 5.4. Spray dryer with after-crystallization belt
    16. 5.5. TIXOTHERM™
    17. 5.6. Choosing a spray drying installation
  6. 6.Technical calculations
    1. 6.1. Evaporation and product output
    2. 6.2. Heating of atmospheric air
    3. 6.3. Mixing of two air stream
    4. 6.4. Dry air rate, water vapour rate and air density
    5. 6.5. Air velocity in ducts
    6. 6.6. Air flow measurements
    7. 6.7. Barometric distribution law
    8. 6.8. The heat balance of a spray dryer
  7. 7.Principles of industrial production
    1. 7.1. Commissioning of a new plant
    2. 7.2. Causes for trouble-shooting
    3. 7.3. Production documentation
    4. 7.3.1. Production log sheets
    5. 7.3.2. General maintenance log book
    6. 7.3.3. Product quality specification
    7. 7.3.4. Operational parameter specification
    8. 7.4. Product quality control
    9. 7.4.1. Process quality control
    10. 7.4.2. Final quality control
  8. 8.Dried milk products
    1. 8.1. Regular milk powders
    2. 8.1.1. Regular skim milk powder
    3. 8.1.2. Regular whole milk powder
    4. 8.1.3. Whole milk powder with high free fat content
    5. 8.1.4. Butter milk powder
    6. 8.1.4.1. Sweet butter milk powder
    7. 8.1.4.2. Acid butter milk powder
    8. 8.1.5. Fat filled milk powder
    9. 8.2. Agglomerated milk powders
    10. 8.2.1. Agglomerated skim milk powder
    11. 8.2.2. Agglomerated whole milk powder
    12. 8.2.3. Instant whole milk powder
    13. 8.2.4. Agglomerated fat filled milk powder
    14. 8.2.5. Instant fat filled milk powder
    15. 8.3. Whey and whey related products
    16. 8.3.1. Ordinary sweet whey powder
    17. 8.3.2. Ordinary acid whey powder
    18. 8.3.3. Non-caking sweet whey powder
    19. 8.3.4. Non-caking acid whey powder
    20. 8.3.5. Fat filled whey powder
    21. 8.3.6. Hydrolysed whey powder
    22. 8.3.7. Whey protein powder
    23. 8.3.8. Permeate powders
    24. 8.3.9. Mother liquor
    25. 8.4. Other Dried Milk Products
    26. 8.5. Baby food
    27. 8.6. Caseinate powder
    28. 8.6.1. Coffee whitener
    29. 8.6.2. Cocoa-milk-sugar powder
    30. 8.6.3. Cheese powder
    31. 8.6.4. Butter powder
  9. 9.The composition and properties of milk
    1. 9.1. Raw milk quality
    2. 9.2. Milk composition
    3. 9.3. Components of milk solids
    4. 9.3.1. Milk proteins
    5. 9.3.2. Milk fat
    6. 9.3.3. Milk sugar
    7. 9.3.4. Minerals of milk
    8. 9.4. Physical properties of milk
    9. 9.4.1. Viscosity
    10. 9.4.2. Density
    11. 9.4.3. Boiling point
    12. 9.4.4. Acidity
    13. 9.4.5. Redox potential
    14. 9.4.6. Crystallization of lactose
    15. 9.4.7. Water activity
    16. 9.4.8. Stickiness and glass transition
  10. 10.Achieving product properties
    1. 10.1. Moisture content
    2. 10.2. Insolubility index
    3. 10.3. Bulk density, particle density, occluded air
    4. 10.4. Agglomeration
    5. 10.5. Flowability
    6. 10.6. Free fat content
    7. 10.7. Instant properties
    8. 10.7.1. Wettability
    9. 10.7.2. Dispersibility
    10. 10.7.3. Sludge
    11. 10.7.4. Heat stability
    12. 10.7.5. Slowly dispersible particles
    13. 10.7.6. Hot water test and coffee test
    14. 10.7.7. White Flecks Number (WFN)
    15. 10.8. Hygroscopicity, sticking and caking properties
    16. 10.9. Whey Protein Nitrogen Index (WPNI)
    17. 10.10. Shelf life
  11. 11.Analytical methods
    1. 11.1. Moisture content
    2. 11.1.1. Standard oven drying method (IDF Standard No.26-1964 [32])
    3. 11.1.2. Free moisture
    4. 11.1.3. Total moisture
    5. 11.1.4. Water of crystallization
    6. 11.2. Insolubility index
    7. 11.3. Bulk density
    8. 11.4. Particle density
    9. 11.5. Scorched particles
    10. 11.6. Wettability
    11. 11.7. Dispersibility
    12. 11.8. Other methods for determination of instant properties
    13. 11.8.1. Sludge
    14. 11.8.2. Slowly dispersible particles
    15. 11.8.3. Hot water sediment
    16. 11.8.4. Coffee test
    17. 11.8.5. White flecks number
    18. 11.9. Total fat content
    19. 11.10. Free fat content
    20. 11.11. Particle size distribution
    21. 11.12. Mechanical stability
    22. 11.13. Hygroscopicity
    23. 11.14. Degree of caking
    24. 11.15. Total lactose and α-lactose content
    25. 11.16. Titratable acidity
    26. 11.17. Whey Protein Nitrogen Index (WPNI)
    27. 11.18. Flowability (GEA Niro [31])
    28. 11.19. Lecithin content
    29. 11.20. Analytical methods for milk concentrates
    30. 11.20.1. Total solids
    31. 11.20.2. Insolubility index
    32. 11.20.3. Viscosity
    33. 11.20.4. Degree of crystallization
  12. 12.Troubleshooting operations
    1. 12.1. Lack of capacity
    2. 12.2. Product quality
    3. 12.3. Deposits in the system
    4. 12.4. Fire precaution
    5. 12.5. Principles of good manufacturing practice
    6. 12.6. The use of computer for quality control and trouble-shooting
  13. References
Reference: Schlünder,E.U.:Dissertation Techn.Hochschule Darmstadt D 17, 1962.