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You are reading the e-book Milk Powder Technology by Vagn Vestergaard.
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 in Fig. 40a and it is important to refer to the test method when specify-ing the filter efficiency in %.
Common for the different requirements is that:
Air heating system
The drying air can be heated either by indirect heating (steam, oil and gas or hot oil) or direct heating (gas or electricity).
A steam heater is a simple radiator. The steam pressure determines the temperature obtained in ths radiator. Under normal conditions it is possible to reach air temperatures 10 ºC 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 high-pressure 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/ºC x h x 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 gases 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 the tubes. Heaters of this type will in the range of 175-250 ºC have an efficiency of about 85 %. See Fig. 41.
Hot oil liquid phase air heaters are used either 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.
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 2000 ºC. 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/ºC), 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:
CH4 + 2 O2 => 2 H2O + CO2 + Heat
The oxygen for the combustion originates from the atmospheric air with 21 % O2 and 79 % N2.
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:
|Exhaust gas from a car:||2000||ppm|
|Heavy traffic intersection:||900||ppm|
|Natural gas boiler stack:||75||ppm|
|WHO food limit for infants:||45||ppm|
|Spray drying chamber:||2-5||ppm|
|Normal fresh milk:||<1||ppm|
|Normal water supply:||0.1||ppm|
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|
and 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.
The air distribution in spray dryers is one of the most vital parts. There are various systems depending on the plant design and the type of product to be produced.
Different dryer designs
Spray dryer design falls into three categories: co-current, counter-current and mixed flow. However, the co-current dryer design best meets the goal of the dairy industry. The goal is to get the best mixture of the hot incoming air and the concentrate droplets in order to obtain a fast evaporation.
In spray dryers with horizontal chamber, the air disperser is arranged like a plenum chamber, and each nozzle will be surrounded by an air stream. The same system is also seen in vertical cylindrical dryers, see Fig. 42.
However, the most common is that the air disperser is situated on top of the spray dryer ceiling, and the atomizing device is placed in the middle of the air disperser thus ensuring an optimal mixing of the air and the atomized droplets. In cylindrical vertical spray dryers it is also seen that the whole ceiling is perforated thus creating a plug-flow air stream - numerous nozzles are situated in the perforated plate in order to ensure that the air is cooled by the concentrate. This system, however, makes fines return complicated, and the obtainable air velocity/nozzle position is not optimal for an efficient drying. It should be noted that an air disperser should have the ability to guide the air and the atomized droplets in the right direction in order to avoid deposits in the drying chamber.
On big capacity dryers equipped with nozzles, the so-called "multi-neck" air disperser is seen, i.e. the dryer is equipped with 3-5 air dispersers and nozzle units. The center area in the ceiling between the air dispersers is, however, impossible to keep free from deposits, and uniform fines return is difficult.
Two types of air dispersers
Today two different types of air dispersers are used in spray dryers for food and dairy products:
Plug-flow air stream
The air enters radially through one side and is distributed through an adjustable air guiding arrangement, see Fig. 44.
This type of air disperser is used for nozzle atomizers, where a laminar plug-flow air stream is wanted. As for the rotary air disperser cooling air is also used here. As the nozzle rods are placed in the middle of the hot air stream, cooling air is also provided for the nozzles lances to keep the product from over-heating.
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