Dryer and Drying Process Flashcards
-defined as the removal of liquid from a material by application of heat, and is accomplished by transfer of a liquid from the surface into an unsaturated vapor phase
DRYING
*2 CLASSIFICATIONS OF DRYING METHODS
- Batch
2. Continuous
removal of liquid from a wet material is achieved by different methods such as expression, extraction, adsorption, absorption, etc.
DRYING (Non-thermal definition)
squeezing of a wetted sponge
EXPRESSION
uses desiccants such as anhydrous calcium chloride to remove water from solvent
ADSORPTION
removal of liquid from solid by use of solvent
EXTRACTION
removal of moisture from gases by passage through sulfuric acid column
ABSORPTION
*APPLICATIONS OF DRYING
- Preservation of drug products
- Preparation of bulk materials
- Improved handling
- Improved characteristics
*TYPES OF DRYERS
- Tray Dryers
- Tunnel Dryers
- Roller or Drum Dryers
- Fluidized Bed Dryers
- Spray Dryers
- Pneumatic Dryers
- Rotary Dryers
- Trough Dryers
- Bin Dryers
- Belt Dryers
- Vacuum Dryers
- Freeze Dryers
- food is spread out, generally quite thinly, on trays in which the drying takes place
- heating may be by an air current sweeping across the trays, by conduction from heated trays or heated shelves on which the trays lie, or by radiation from heated surfaces
TRAY DRYERS
- the food is spread over the surface of a heated drum
- the drum rotates, with the food being applied to the drum at one part of the cycle. The food remains on the drum surface for the greater part of the rotation, during which time the drying takes place, and is then scraped off
Roller or Drum Dryers
in this dryer, food slurry is contacted with a hot, revolving drum to form a thin layer on the surface
DOUBLE DRUM DRYER
- food material is maintained suspended against a gravity in an upward-flowing air stream
- there may also be a horizontal air flow helping to convey the food through the dryer.
- Heat is transferred from the air to the food material by convection
FLUIDIZED BED DRYERS
*VARIOUS FLUID BED DESIGN
- Vibrated Fluid Bed
- Fluid Bed Granulation
- Spouted Bed Dryer
- Mechanically Agitated Fluid-Bed Dryer
- Centrifugal Fluid-Bed Dryers
- Fluidized Spray Dryers
- a long, rectangular, narrow drying chamber is vibrated at 5 to 25 Hz. The air velocity within the dryer can be as low as 20% of the minimum fluidisation velocity. The large particles are transported through the dryer by the vibration of the dryer.
- typically dry such products as milk, whey, cocoa, and coffee
Vibrated Fluid Bed
- a blinding liquid is sprayed into the fluidised bed of granules, causing the particles to agglomerate. The average particle size produced ranges from 0.5 to 2 mm.
- widely used in the pharmaceutical industry
Fluid Bed Granulation
- when the particles to be dried are larger than 5 mm and are not readily fluidized in a conventional fluid-bed dryer, this design is employed
- The drying air enters the drying chamber at the centre of the conical bottom. The particles move in a cyclic fashion through the dryer. As they travel upward in the centre, they are carried by the incoming air stream and fall downward at the periphery of the chamber
- dries sensitive goods such as wheat and peas
Spouted Bed Dryer
- a combination fluid-bed and flash dryer used to wet cakes was developed to conserve energy costs. The wet cake is fed directly via a screw conveyor. Once in the drying chamber, a mechanical agitator breaks up the particles while air is introduced to fluidise the small particles. Dry particles are carried to the exhaust system by the fluidising system
- pastes of pigments and dyes are dries using this method
Mechanically Agitated Fluid-Bed Dryer
- diced, sliced, and shredded vegetables that are difficult to fluidise and too heat sensitive to dry in conveyor dryers are also dried in this dryer
- the cylindrical dryers rotate horizontally while air flows in to the chamber through the perforated wall.
Centrifugal Fluid-Bed Dryers
- another alternative for hygroscopic and thermoplastic foods
- the fluidising air is led to the bottom of the drying chamber. The combination of partially dried and dried products allows agglomeration to take place.
Fluidized Spray Dryers
- liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air.
- drying occurs very rapidly, so that this process is very useful for materials that are damaged by exposure to heat for any appreciable length of time
SPRAY DRYER
*TYPES OF ATOMISER
- Centrifugal Atomiser (Wheel Atomiser)
- Pressure Nozzle Atomiser
- Two Fluid Nozzle Atomiser
- droplets are projected horizontally away at 100 to 200 m/s with angular velocities of 10,000 to 30,000 rpm
- since they are not susceptible to clogging, they are often used for slurries or pastes
- produces a homogeneous spray, and varying rotational speed can control the mean particle diameter.
CENTRIFUGAL OR WHEEL ATOMISER
- liquid is forced at a high pressure (700-2000 kpa) through a small orifice. Nozzles have a maximum flow rate of 1 L/hr
- typical pressures range from 300 to 4000 psig
PRESSURE NOZZLE ATOMISER
- compressed air creates turbulence, which atomises the liquid.
- the operating pressure is lower than the pressure nozzle, but a wider range of droplet sizes is produced
TWO FLUID NOZZLE ATOMISER
- the solid food particles are conveyed rapidly in an air stream, the velocity and turbulence of the stream maintaining the particles in suspension
- heated air accomplishes the drying and often some form of classifying device is included in the equipment. In the classifier, the dried material is separated, the dry material passes out as product and the moist remainder is recirculated for further drying
PNEUMATIC DRYERS
- the foodstuff is contained in a horizontal inclined cylinder through which it travels, being heated either by air flow through the cylinder, or by conduction of heat from the cylinder walls
ROTARY DRYERS
- the materials to be dried are contained in a trough-shaped conveyor belt, made from mesh, and air is blown through the bed of material
TROUGH DRYERS
- the foodstuff is contained in a bin with a perforated bottom through which warm air is blown vertically upwards, passing through the material and so drying it
BIN DRYERS
- the food is spread as a thin layer on horizontal mesh or solid belt and air passes through or over the material
BELT DRYERS
-operate under a vacuum, and heat transfer is largely by conduction or by radiation
VACUUM DRYERS
- basically a group of truck-and-tray batch dryers, operated in a programmed series so as to be quasi-continuous
TUNNEL DRYERS
-the food is frozen before being loaded into the dryer. Heat is transferred to the food by conduction or radiation and the vapour is removed by vacuum pump and then condensed
FREEZE DRYERS
*4 STAGES OF FREEZE DRYING
- Pretreatment
- Freezing
- Primary Drying
- Secondary Drying
*METHODS OF PRETREATMENT
- Freeze Concentration
- Solution Phase Concentration
- Formulation to Preserve Product Appearance
- Formulation to Stabilize Reactive Products
- Formulation to Increase the Surface Area
- Decreasing High Vapor Pressure Solvents
- done by placing the material in a freeze-drying flask and rotating the flask in a bath, called a “shell freezer” which is cooled by mechanical refrigeration, dry ice and methanol, or liquid nitrogen
- to cool the material below its triple point, the lowest temp. at which the soli and liquid phases of the material can coexist
FREEZING
- the pressure is lowered through partial vacuum, and enough heat is supplied to the material for the water to sublime
PRIMARY DRYING
- aims to remove unfrozen water molecules, since the ice was removed in the primary drying phase
- this part of the freeze-drying process is governed by the material’s adsorption isotherms
SECONDARY DRYING
*MAIN COMPONENTS Of Heated Air Dryer
- Drying Bin
- Air Distribution System
- Fan
- Air Heater
- usually consist of scalper that lets through the grain but retains straw, a smaller second screen that removes small stones and other impurities, and/or air aspirator for sucking out dust and light empty grains
Pre-Cleaner
the process of applying energy or heat and airflow directly to water to speed up the phase change to vapor, causing evaporation
DIRECT / CONVECTION THERMAL DRYING
*2 TYPES OF CONVECTION THERMAL DRYING
- Using liquid as air transfer medium (hydronic)
2. Using air as medium (open)
