Spray drying
The spray drying (including spray drying ) is a method from the process technology for drying of solutions , suspensions or emulsions . The material to be dried is introduced into a stream of hot gas by means of an atomizer, which dries it to a fine powder in a very short time (a few seconds to fractions of a second) .
The concept was probably developed in the dairy industry in the 19th century. In 1872 a first patent was filed by Samuel R. Percy (not Perry as often quoted) for spray drying. Spray drying did not find widespread commercial use until the 20th century.
Basic functionality
This is achieved by means of a pressure atomizer (also single- fluid nozzle, typically 50 to 250 bar depending on the design), a pneumatic atomizer (also two-fluid nozzle, depending on the design typically 1 to 10 bar) or a rotary atomizer (depending on the design typically 4,000 to 50,000 rpm) material to be dried atomized. This increases the total surface of the liquid enormously.
The product is sprayed into a stream of hot gas, whereby the liquid evaporates in a very short time due to the large surface area and the moist material dries to a fine powder . Since the energy for evaporation is provided by the hot gas, spray drying is what is known as convection drying. The hot gas is mostly air; However, inert gases are also used for goods that are sensitive to oxidation or for highly flammable solvents . The inlet temperature of the hot gas is usually below 250 ° C, depending on the application, but temperatures of up to 600 ° C are used in the chemical industry.
The powdery material to be dried is usually separated from the air flow by a cyclone separator . Spray dryers are operated continuously on an industrial scale as long as no discontinuous batch process is required (e.g. in the pharmaceutical industry).
Types
There are spray dryers from the laboratory scale (evaporation capacity below 1 kg / h) to the pilot scale (depending on the industrial sector, evaporation capacity from 1 kg / h to well over 10 kg / h) to the production scale (depending on the industrial sector, evaporation capacity up to over 10 t / h) .
Depending on the design or intended use, the hot gas can flow in the direction of the spray jet or against the spray jet (co-current, counter-current principle ). In newer spray dryers, several gas streams with different temperatures are sometimes introduced in order to improve drying and temperature control.
Most dryers have the spray device in the upper part of the tower. Exceptions here include dryers for powder detergents; here the spray device is located in the lower part of the dryer and is operated like a fountain.
Powder properties
Depending on the atomization technology and the solids content of the liquid, primary particles between 5 and 500 µm form. Depending on the drying behavior of the liquid and the ambient temperature, different primary particle structures are formed, which are mostly homogeneous particles, hollow bodies or transitional shapes.
In many spray dryers, the primary particles are agglomerated in different ways to improve the powder properties (e.g. powder flowability, sinking behavior, tendency to dust). For example, fine material is returned to the area of the atomizer in order to promote agglomeration there.
Due to the large specific surface , spray-dried powders of many products dissolve quickly.
safety
Due to the potential presence of all of the factors necessary for dust explosions , spray dryers are generally at risk of explosion:
- Fuel (if the material to be dried is combustible, usually the case)
- fine distribution of fuel and large exchange surface
- Oxygen (if not operated with inert gas )
- closed process room
- Sources of ignition
Factors 1 and 2 are fundamentally unavoidable. Since operation with inert gas is costly, the factor 3 can only be influenced in a few processes. The effect of factor 4 can be reduced by pressure relief devices in the event of an explosion. In addition to other suppression mechanisms (e.g. extinguishing systems), the risk is usually minimized by a factor of 5 through different design rules for active prevention.
Dust escaping from the system can also pose a risk of explosion in the vicinity of a spray dryer.
application areas
Spray drying is widely used in the chemical industry as well as in the pharmaceutical and food industries . It is also used in the exhaust gas cleaning of waste incineration plants in order to cool the flue gas and enable waste water- free operation.
For sensitive substances such as hormones , proteins , vitamins and essential oils , spray drying is the method of choice. Spray drying also serves as a process for microencapsulating essential oils or other substances that are sensitive to oxidation . Further concrete examples for the application are milk powder (dry milk), breast milk substitute , egg powder , soluble coffee (powder coffee), powder glue , detergent powder , fertilizers and ceramic particles. In addition to gentle drying, the good handling properties of spray-dried powders, such as good solubility, are also decisive for the choice of process.
literature
- B. Bhandari, KC Patel, XD Chen: Spray drying of food materials - process and product characteristics in XD Chen, AJ Mujumdar: Drying Technologies in Food Processing , 2008, ISBN 1-4051-5763-1
- I. Filková, LX Huang, AS Mujumdar: Industrial Spray Drying Systems in AS Mujumdar: Handbook of Industrial Drying , 2015, ISBN 978-1-4665-9666-5
See also
Web links
Individual evidence
- ↑ US125406, Samuel R. Percy, Improvement in drying and concentrating liquid substances by atomizing, 1872
- ↑ VDI 2263 sheet 7: 2010-07 dust fires and dust explosions; Hazards - assessment - protective measures; Fire and explosion protection in spray drying systems (Dust fires and dust explosions; Hazards - assessment - protective measures; Dust fires and explosion protection in spraying and drying integrated equipment). Beuth Verlag, Berlin, p. 22.
- ↑ Joachim Binnig: State of the art in the dedusting of waste incineration plants. In: Hazardous substances - cleanliness. Air . 69, No. 5, 2009, pp. 175-179.
- ↑ VDI 3460 sheet 1 : 2014-02 emission reduction; Thermal waste treatment; Basics (Emission control; Thermal waste treatment; Fundamentals). Beuth Verlag, Berlin. Pp. 131, 161-162.