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|1. What is spray
|3. How does spray
|4. Spray dryer manufacturer
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. A spray drying process 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 :
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 spray drying technologies 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 :
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 spray dryer nozzle design exist and must be chosen according to the material and the flowrate in order to give the best results :
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 :
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...
Industrial spray dryers are able to reach several tons / h and have typically large dyring chambers and several nozzles. However, there is also on the market laboratory spray dryers (mini spray dryers) that are very useful for research work, or simply to get a 1st idea of a product and its easiness of drying before scaling up to the industrial line.
There are many spray dryer manufacturers in USA, Europe and also countries producing a lot of milk like New Zealand as spray dryers are very much used to produce milk powder. Big players are GEA spray dryers, NIRO spray dryers, Tetra Pak spray dryers, SPX anhydro spray dryer... or Buchi spray dryers for lab scale (Buchi mini spray dryer). When chosing a supplier, it is very important to check their references, ask questions (for example with the help of the explanations given on powderprocess.net website) to assess the degree of expertise of the company, and get in touch with more than one company to carry out a competitive bidding among spray drying companies.