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Drying of processes materials1UNDERSTAND THE DRYING PRINCIPLES, DYING METHODS

DESCRIBE TYPE OF DRYERS STUDENTS SHOULD BE ABLE TO :DEFINITION3DRYING :REMOVAL SMALL AMOUNT OF WATER/OTHER LIQUID FROM MATERIAL- WATER IS REMOVED AS A VAPOR BY AIR

EVAPORATION: REMOVAL LARGE AMOUNT OF WATER/OTHER LIQUID FROM MATERIAL- WATER IS REMOVED AS VAPOUR AT ITS BOILING POINTDRYING METHOD4HOW CAN WATER/OTHERS LIQUIDS BE REMOVED FROM SOLIDS?MECHANICAL-PRESSES,CENTRIFUGE THERMAL -DRYINGCOSTDRIED SUBSTANCE MOISTURE CONTENT DEPEND ON TYPE OF PRODUCT5BONE-DRYPRODUCT CONTAIN NO LIQUIDDRIED TABLE SALT 0.5 % H2ODRIED COAL 4 % H2ODRIED CASEIN (milk protein) 8 % H2O

DRIED SUBSTANCE6Drying as a preservation technique (biological materials, esp food)< 10 % wt water contentMicroorganisms are not activeTypically < 5% wt water content to preserve flavor & nutrition

SOLID PARTICLES TO BE DRIED7MANY DIFFERENT FORMS :FLAKES, GRANULES, CRYSTALS, POWDERS, SLABS, CONTINUOUS SHEETSLIQUID (SUSPENDED SOLID)SOLUTION

GENERAL DRYING METHOD8BatchMaterial inserted into the drying equipment and drying proceeds for a given time period of timeContinuousMaterial continuously added to the dryer Dried material continuously removedGENERAL DRYING METHOD9Physical condition used to add heat & remove water vapor :Direct contact with heated air at atmospheric pressureVacuum drying (low pressure) & heat flow indirectly (in contact with metal wall or radiation)Freeze drying, water is sublimed from frozen material

CLASSIFICATION OF DRYERS10CONTINUOUS OR BATCHAGITATION/UNAGITATIONSOLID IS EXPOSED TO :1. DIRECT CONTACT WITH HOT GAS2. EXTERNAL MEDIUM (CONDENSING STEAM)3. HEATED BY ELECTRIC/RADIANT/MICROWAVECLASSIFICATION OF DRYERS111. ADIABATIC OR DIRECT DRYERS2. NON-ADIABATIC OR INDIRECT DRYERSSOLIDS ARE EXPOSED TO HOT GASHEAT IS TRANSFERRED FROM EXTERNAL MEDIUMSOLID HANDLING IN DRYER12Gas flow across a static bed of solidsGas passing through a bed of preformed solidsShowering action in a rotary dryerSOLID HANDLING IN DRYER13Fluidized solids bedCocurrent gas-solid flow in a pneumatic-conveyer flash dryerDRYING EQUIPMENT14

DRYING EQUIPMENT15

1. Tray Dryers16In tray dryers, the 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. Most tray dryers are heated by air, which also removes the moist vapours.

2. Continuous Tunnel Dryers17Developments of the tray dryer : the trays on trolleys move through a tunnel where the heat is applied and the vapours removed. In most cases, air is used in tunnel drying and the material can move through the dryer either parallel or counter current to the air flow. Sometimes the dryers are compartmented, and cross-flow may also be used.

3. Roller or Drum Dryers

18In these 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. Drum drying may be regarded as conduction drying.

4. Spray Dryers19Liquid or fine solid material in a slurry is sprayed in the form of a fine droplet dispersion into a current of heated air. Air and solids may move in parallel or counterflow. 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. The dryer body is large so that the particles can settle, as they dry, without touching the walls on which they might otherwise stick. Commercial dryers can be very large of the order of 10 m diameter and 20 m high.

FIND THE INFORMATION20

Analyse the advantages & disadvantages of the following type of dryers :Tray driersRotary driersSpray DriersFreeze dryerREFERENCES21Books :1.Geankoplis C. J., Transport Processes and Separation Process Principles, 4th Edition, Prentice Hall, 2003.2. McCabe W. M., Smith J. C. and Harriott P., Unit Operations of Chemical Engineering, 7th Ed., McGraw Hill, 2005.22THANK YOU.