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|1. What is spray
|3. How does spray
Spray drying is a drying method that has developed tremendously in the last 30-50 years. It presents unique advantages to dry materials while preserving them from degradation. It is a very versatile process that can be used at lab scale or within factories producing dozens of tons an hour. It also presents the advantage to produce a powder whose granulometry can be adjusted within limits and therefore greatly increases the properties and easiness of handling of the dried material.
To define spray drying in a short sentence, one can refer to this quote of Masters, 1991 : Spray drying is the transformation of feed from a fluid state into a dried particulate form by spraying the feed into a hot drying medium.
As mentioned by the definition, the materials to dry need 1st to be handle in a liquid form, conditioned at the right viscosity, temperature and pressure before being pumped into a drying chamber, through a spraying nozzle, where dry air is introduced.
Examples of applications for spray drying are given below :
- Instant milk
- Whey powder
- Instant coffee
- Non dairy creamer
- Detergents, soaps. surface active agents
- Antibiotics, vaccine, vitamins, yeast
The list is only partial but already very long. Many industries use spray drying because it offers a continuous drying technique, with a very short residence time in temperature, thus allowing, if the spray drying system is well tuned, to dry heat sensitive components.
The following drawing is a simplified flowsheet of a common spray drying process with co-current air / product flow in the spray dryer chamber and open air cycle. There are more complicated and performant designs but the representation below somehow constitutes a minimum for an industrial line.
Figure 1 - Flowsheet of a spray dryer
Independently of the degree of complexity of the factory, the spray drying process is made of 5 main steps described thereafter.
Prior to spray drying, the materials to be dried are are in liquid form. The process prior to the spray dryer aims at conditioning the materials in a way that will make the spray drying possible and will optimize its yield. Typical process steps in this part of the line are the following :
- Dissolution / formulation - if the raw material is not available directly in liquid form
- Concentration / evaporation : increasing the total solid will result in a better yield for the spray dryer
- Heating : heating can help to counterbalance the increase of viscosity due to the concentration
- Pumping : the slurry is pumped at high pressure to the spray dryer
In order to ensure a very fast drying it is necessary to atomize the influx of liquid in very small droplets. This increases a lot the exchange surface where humidity can leave the particles.
This atomization is done thanks to 1 or several nozzles. Different nozzle design exist and must be chosen according to the material and the flowrate in order to give the best results :
- Pressure nozzles
- Rotary nozzles
- 2 fluids nozzles
Drying is taking place thanks to dry air contacting the sprayed droplets. The air ability to take up moisture, and thus dry the particles is increased by heating up the air prior it enters the spray drying chamber. It has a low relative humidity at inlet and a high relative humidity, lower temperature at the exit.
Air can either be blown co-currently to the product, from the top of the tower, or counter-current, from the bottom. In many process application counter-current is preferred but for spray drying co-current has a key interest : the air at the higher temperature contacts 1st the particles with the highest humidity which helps protecting the particles against over heating.
Drying is actually happening all along the spray drying chamber following the progression of the solids particles inside. Moisture is removed step by step thanks to a mass and heat transfer in between the particle and air :
- Constant drying rate : the particle is saturated in liquid, the mass transfer goes as fast as possible depending especially on the particle surface area
- Falling rate : once the particle surface is not anymore saturated in liquid, the drying will depend on the capacity of the liquid trapped in the solid particle matrix to diffuse until the surface in order to evaporate. It is a diffusion controlled process.
- Bubble formation : during the falling rate period, the temperature of the particle increases. If it reaches a temperature higher than the boiling point of the liquid, it may vaporize in the particle and expand it creating a bubble.
The powder is collected at the bottom of the spray dryer and pneumatically conveyed to a cyclone where it is separated with the air. The air in the drying chamber is also sent to another cyclone where fine, which may have been carried over by the air are separated and sent back to the main product stream. Humid air is generally rejected but in some case, as it is still hot, it can be further used for other process operations - pre-heating...