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1、第八章第八章 干燥干燥 Chapter 8Chapter 8第一节第一节基本概念基本概念 Basic ConceptsBasic Concepts8 81 Importance and 1 Importance and Purpose of dryingPurpose of drying（重要性和目的）重要性和目的） Significance of Drying Process as an Unit Operation It is widely used in every walk of life. E.g. Food stuff, wood processing, plastics indu
2、stry, paper making etc. It is one of the oldest methods of preserving food. Primitive societies practiced the drying of meat and fish in the sun long before recorded history. Today the drying of foods is still important as a method of preservation. Dried foods can be stored for long periods without
3、deterioration occurring. The principal reasons for this are that ? The microorganisms which cause food spoilage and decay are unable to grow and multiply in the absence of sufficient water . ? Many of the enzymes which promote undesired changes in the chemical composition of the food cannot function
4、 without water. (Without moisture or water, the enzyme activity is inactivated)The Purpose of DryingThe Purpose of Drying Easy for preservation or store Easy for preservation or store Drying makes food or raw food materials have long Drying makes food or raw food materials have long shelf life. ( in
5、 terms of microorganism and enzyme shelf life. ( in terms of microorganism and enzyme action).action). Need small space of storehouseNeed small space of storehouse Convenient for transportationConvenient for transportation Weight and volume are greatly reduced,improves Weight and volume are greatly
6、reduced,improves significantly the efficiency of transportation In terms significantly the efficiency of transportation In terms of loading and unloadingof loading and unloading Convenient for consumer useConvenient for consumer use E.g. milk powder,dried vegetables etc.E.g. milk powder,dried vegeta
7、bles etc.8 82 Classification, Definition and 2 Classification, Definition and Contained Processes for DryingContained Processes for Drying8.2.1 Dehydration and Drying8.2.1 Dehydration and DryingDehydration By Mechanical Method Dehydration By Physical-Chemical MethodDehydration By Heating Method （Rem
8、oving Solvent from solids by heating is called dryingThe definition of drying）Dehydration by Dehydration by Mechanical MethodMechanical Method The moisture is removed by filtration , The moisture is removed by filtration , Express, Suction, and Centrifugation. Express, Suction, and Centrifugation. T
9、hese methods are used only in the These methods are used only in the case that the moisture dose not need case that the moisture dose not need to be totally removed. to be totally removed. 即通过过滤、压榨、抽吸和离心分离等方法除即通过过滤、压榨、抽吸和离心分离等方法除 去湿分，这些方法应用于溶剂无需完全除尽的去湿分，这些方法应用于溶剂无需完全除尽的 情况，已在前面讲述。情况，已在前面讲述。Dehydrati
10、on PhysicalDehydration Physical- - Chemical Method Chemical Method The moisture is absorbed by the The moisture is absorbed by the materials that have absorptive materials that have absorptive capacity to the moisture. capacity to the moisture. E.g. lime, potassium calcium and so on.E.g. lime, potas
11、sium calcium and so on. 用吸湿性物料如石灰、无水氯化钙等吸收用吸湿性物料如石灰、无水氯化钙等吸收 水分。因这种方法费用高，操作麻烦，故水分。因这种方法费用高，操作麻烦，故 只适用于小批量固体物料的去湿，或用于只适用于小批量固体物料的去湿，或用于 除去气体中的水分。除去气体中的水分。8.2.2 Heat Dehydration 8.2.2 Heat Dehydration and Drying Definitionand Drying Definition By means of heating, the solvent is By means of heating, t
12、he solvent is evaporated from the materials ,and the evaporated from the materials ,and the vapor is expelled in time.vapor is expelled in time. 即借热能使溶剂从物料中溶化即借热能使溶剂从物料中溶化, ,并排除所生成的并排除所生成的 蒸气来除去湿分。蒸气来除去湿分。 Drying Drying is the operation that by means of is the operation that by means of heating the
13、water and other moisture kept heating the water and other moisture kept by the solid materials are evaporated and by the solid materials are evaporated and the vapor is removed in time.the vapor is removed in time. 用加热的方法使水分或其它溶剂汽化，藉此来用加热的方法使水分或其它溶剂汽化，藉此来 除去固体物料中湿分的操作，称为固体的除去固体物料中湿分的操作，称为固体的干燥干燥。Way
14、s of Classification Ways of Classification for Drying Processfor Drying ProcessClassified by PressureClassified by Pressure Atmospheric pressure dryingAtmospheric pressure drying Vacuum DryingVacuum Drying Classified by Operation MethodClassified by Operation Method Batch dryingBatch drying Continuo
15、us DryingContinuous Drying Classified by Mode of Heat TransferClassified by Mode of Heat Transfer ConductionConduction Convection Convection RadiationRadiation Dielectric dryingDielectric drying（高频干燥）高频干燥）8.2.3 Two Processes are 8.2.3 Two Processes are Contained in the Drying Contained in the Drying
16、 ProcessProcess Drying of foods implies the removal of water from the foodstuff. In most cases, drying is accomplished by vaporizing the water that is contained in the food, and to do this the latent heat of vaporization must be supplied. There are, thus, two important process-controlling factors which enter into the unit operation of drying: the transfer of heat to provide
