There is continuous development in spray drier technology. Subtle changes often make all the difference in plant performance. A few of the latest innovations from CPS are described below. These are the TF (Tall Form) Wide Body Chamber, the Zigzag Pattern Plug Flow Fluid Beds and the Sunflower Air Distributor.
The TF Wide Body Drier
The TF Wide Body Drier Chamber is a concept that offers the benefit of creating an optimised compact spray drier lay out.
The TF Wide Body style chamber is a combination of the well known Wide Body Drier Chamber with the top of a Tall Form Spray Drier. The lower part of the chamber cylinder has the typical wide body upgoing reverse airflow alongside the vertical wall that prevents wet droplets colliding on the wall, thus limiting product deposits. The tall form upper part, which has a much smaller diameter, allows the lay out of the outlet air ducting to be optimised and the drier baghouse and/or cyclones to be situated in the best position in the overall spray drier lay out. This is largely possible because the drier outlet air can be exhausted at almost any desired height of the chamber. The small diameter upper section is subject to minimal fouling. This is as predicted in CFD simulation for particle tracking and has been proven in practice. Hence the upgoing airflow along the "upper" wall in the chamber is not really necessary.
This TF Wide Body chamber combines the advantages of the wide body and the tall form bustle drier. The first commercial size drier with this chamber style will be commissioned by the end of this year on baby foods. This style of drying chamber can be used in conjunction with either an “integrated”, “static external”, or “shaking” fluid bed.
TF Wide Body Chamber benefits:
- Co-current, reversed airflow. This pattern ensures that the up going airflow alongside the wall prevents sticky droplets from sprays colliding on wall surfaces
- The tall form top allows short straight exhaust ducts connected directly to the baghouses and/or cyclones
- Reduced building volume due to compact dryer layout
- Less CIP equipment due to compact design, i.e. short straight ducting
The Sunflower Air Distributor
The air distributor is the unit on top of the spray drier chamber. Hot air is fed evenly into the chamber past where the spray lances and the fines return tube are situated. Previously CPS used a venturi type air distributor. Spray driers require huge building volumes and therefore there is a permanent drive to make spray driers as compact as possible. The venturi distributor is a rather tall unit and this new air distributor is known as the “Sunflower Air Distributor”.
Not only has the height been reduced but also the air distribution has been improved resulting in a 50% reduction in pressure drop over the unit. This equates to substantial energy savings. In practice we have also found significantly lower drier deposits due to the low air turbulence in the unit.
It is possible to retrofit sunflower air distributors in existing spray driers with venturi type distributors. Recently this was successfully done in a tall form bustle drier in Ireland.There are many parameters involved in minimizing drier deposits but experience has shown that a well designed air distributor is fundamental in obtaining optimal performance.
Sunflower Air Distributor benefits:
- Low pressure drop therefore significant energy savings
- Compact design allowing a compact drier head, shorter spray lances and lower building volume
- Low air turbulence in hot air jet resulting in maximum contact of the spray droplets in the hot air jet hence ensuring even drying of all particles. This minimises drier deposits
- Venturi type air distributors can be retrofitted with Sunflowers Air Distributor
Fluid beds are used for the final drying and cooling of powders. Fluid beds can be static, shaking, or vibrating. The shaking or vibration action of the beds improves fluidization at reduced airflows. Fluid beds can further be classified into either “well mixed” or “plug flow”.
Well-mixed fluid beds
Well-mixed fluid beds are chosen for drying and or cooling of powder after the initial drying stage. The well-mixed bed can be either integrated in the chamber or in an external fluid bed. The diagram above shows a shaking external fluid bed with well mixed and plug flow sections.
Plug flow fluid beds
Plug flow beds have the principle: Powder first in – first out. This principle makes this bed type most suitable for final drying and product cooling.
In plug flow fluid beds the gill plates (the actual “bed” in the fluid bed) are used for air distribution. The holes in the sheet introduce air at an angle and this results in an acceleration of the powder being transported. If the acceleration is directed towards the bed outlet it may cause too strong transport resulting ‘dead’ or under fluidized zones before weirs. In this concept the air direction is at a right angle to the bed transport direction. This pattern has shown to have significant benefits in start up and shut down of the plant, mainly the prevention of powder falling though the sheet and no more dead zones before bed weirs.
Zigzag pattern in fluid beds benefits:
- Prevents powder dropping through sheet, important for smp and whey powders
- Powder layer build up from front end and not from exit end as with other designs, quicker to reach steady state and a more homogenous product
- Avoiding dead zones in fluid bed, important for fat filled powders
- Improved control start up and shut down due to slow conveying of powder
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