Basic priciple of Spray drying

SPRAY DRYING

Principle

  • Spraydrying can be described as the instantaneous removal of moisture from aliquid. To achieve this, the liquid isconverted into a fog-like mist (atomised), whereby it is given a large surface. The atomised liquid is exposed to a flow ofhot air in a drying chamber. The air hasthe function of supplying heat for the evaporation and, in addition, it acts ascarrier for the vapour and the powder. When the atomised product is in contact with the hot air, the moistureevaporates quickly and the solids are recovered as a powder consisting of finehollow spherical particles with some occluded air. Exhaust air is removed on the side and passesthrough cyclones and filters (where entrained powder particles are separatedand recovered).

Flow diagram of spraydrying system

Spray drying system

Classification of spray dryers

  • Hydraulic pressure jet
  • Pneumatic (compressed air)
  • Centrifugal disc

Methods of furnishing heat

  • Steam
  • Gas
  • Fuel oil
  • Electricity

Methods of heating air

  • Direct (gas or fuel)
  • Indirect (utilizing plate heat exchangers orcoils)

Position of drying chamber

  • Vertical
  • Horizontal

Direction of airflow in relation to product flow

  • Counter-current
  • Parallel
  • Right-angle

Pressure in drier

  • Atmospheric
  • Vacuum

Method of separation of powder from air

  • Cyclone
  • Multi-cyclone
  • Bag filter
  • Liquid dust collector
  • Electrical dust collector

Removal of powder from drying chamber

  • Conveyor
  • Vibrator
  • Sweep conveyor
  • Air conveyed to cyclone

SPRAY DRYING METHOD

Receiving milk

  • Only high-grade milk should be accepted.

Cooling

  • Topreserve the quality of milk.

Standardization

  • This is done to adjust the ratio of fat andsolids-not-fat in raw milk to meet the legal standards for composition in driedproduct. Raw milk is standardized byadding to it a calculated amount of skim milk or cream. The fat: SNF ratio here is standardized to1:2.769.

Pre-heating

  • For efficient filtration/clarification milk ispr-heated to 71ºC.

Filtration/clarification

  • The chief object of this is to remove extraneousmatter. This operation can also be donebefore standardization.

Homogenization

  • It is being done to improve the keeping qualityand prevent fat churning while reconstitution. A pressure of 3000 psi (2500 psi on the first stage and 500 psi on thesecond) at 63 to 74ºC.

Heat

  • This main objective is to prolong the shelf lifeof the dried product by inactivation of lipase. The pre-heating should also ensure pasteurization, thus reducing theviable microorganisms. A combination of82 ºC for 15 minutes is used in practice.

Condensing

  • Normally a concentration of 35-40 per cent totalsolids is produced, and the concentrate is continuously removed from theevaporator with the help of a densitytester.

Pumping

  • Thepreheated concentrate at 71º C is forced through the atomizer at a pressure of2500 psi.

Spray drying

  • Theconcentrated milk is dried with inlet air at 143-232º C and the exit air at 74to 93º C depending on the product characteristics.

Cooling

  • The dry product should be removed promptly fromthe hot air stream to maintain better flavour and body characteristics and alsokeeping quality. The higher thetemperature and the longer the time the product is above the melting point offat, the greater the amount of free fat obtained thus adversely affectingkeeping quality. Hence it is essentialto cool the powder immediately to a temperature below the melting point.

Sifting

  • A12-mesh screen is used for sifting dry whole milk.

Packaging

  • Thepowder-packaging unit should be isolated to reduce the spread of powder dust tothe drier building. Of the severalpacking material used, fiberboard carton with an over wrap of foil laminated topaper, or with an inner lining of foil laminated to paper. A polythene bag inside the fibreboard cartonis also used. Plastic coated paper bagsare the latest. Powder packing should becarried out in a dry atmosphere, sealed promptly hermetically in moisture proofpackage to prevent caking of lactose and fat decomposition leading tooff-flavours. Usually, in packaging of milk powders, the headspaceis reduced followed by removal of oxygen, by injecting vacuum and thenreplacing it with nitrogen.

Storage

  • Shouldbe stored at temperatures lower than, 24º C, in a cool, dry place.
Last modified: Monday, 16 April 2012, 5:48 AM