Enrichment and drying technology series II.

Microwave
Drying
(1) Microwave
Heating
Drying Principle
Microwave is an electromagnetic wave with a frequency of 300 MHz to 300 GHz (wavelength 1 m-1 mm) with strong penetration. The material medium is composed of polar and non-polar molecules, which are arranged according to the direction of electric field from random distribution state under the action of electromagnetic field.
microwave generator will microwave radiation to dry material, when the microwave into the material, to promote water and other polar molecules with the frequency of microwave rotation, in the microwave electromagnetic field action, these orientation movements to billions of times per second frequency constantly changing, resulting in the intense movement of molecules and collision friction, thereby generating heat, electric energy directly
transconded
as the medium of thermal energy.
, microwave heating is the medium material itself to loss of electric field energy and heat. The media constant of different medium materials and the media loss angle positive cut value tg is different, so the thermal effect under the action of microwave electromagnetic field is not the same. The substances produced by
polar
can absorb microwave energy better.
water molecules are extremely polar and are the best medium for absorbing microwaves, so any substance containing water molecules must absorb microwaves. The other is composed of non-polar molecules, which basically do not absorb or rarely absorb microwaves, such as Teflon, polypropylene, polyethylene, polyethylene, polyethylene, plastic products and glass, ceramics, etc., they can pass through the microwave, but not absorb microwaves. Such materials can be used as containers or supports for microwave heating, or as sealing materials.
in a microwave electric field, the size P of the medium absorbing microwave power is equal to the square of frequency f, the strength of the electric field E, the dielectric constant, and the positive cut value of the medium loss, tg.
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in addition, in practice, there will be a phenomenon, that is, some heating through, some heating impermeable, there is a penetration capacity and transmission depth problem. Penetrating capacity is the ability of electromagnetic waves to penetrate into the inside of the medium, electromagnetic waves from the surface of the medium into and spread within it, because the energy is constantly absorbed and converted into heat, it carries the energy along with the distance deep into the surface of the medium, in exponential form decay.
transmission depth is defined as the depth at which the internal power density of the material is 1/e or 36.8% of the surface energy density.
can be seen from the upper model, microwave heating depth is much larger than infrared heating, because microwave wavelength is nearly a thousand times the wavelength of infrared wavelength. Infrared heating is only surface heating, microwave is deep internal heating.
currently used in China for industrial heating frequencies of 915 MHz and 2450 MHz. The choice of microwave frequency and power can vary depending on the shape, material and moisture content of the heated material.
(2) microwave drying characteristics
(1) high efficiency, energy saving, heating uniform. Microwaves are penetrating and can penetrate the inside of heated objects for heating, so that both inside and outside are heated at the same time, with high electrical efficiency, uniform heating and small heat loss. Compared with conventional electrothermal drying, it can generally reduce power by 1/3-1/2.
(2) low temperature sterilization effect is good, microwave thermal effect, non-thermal effect dual sterilization effect, compared with conventional methods, with low temperature, fast characteristics.
(3) control convenient and timely, production is not affected by climatic conditions, microwave equipment is ready to use, there is no thermal inertia, microwave power size, transmission speed can be continuously smooth adjustment, can be continuously 24 hours of production.
(4) microwave equipment heating itself without thermal radiation, can improve labor conditions. The amount of microwave leakage conforms to national standards. The equipment is compact and covers a small footprint.
Spray Drying
(1) Spray Drying Principle
Spray drying is a drying method that uses a
atomizer
to disperse the raw material fluid into fog droplets and to dry the spray droplets with hot gases (air, nitrogen, or over-hot water vapor). The raw material solution can be a solution, emulsion, suspension, or molten liquid or paste. Drying products can be made into powdery, granular, hollow balls or pellets as needed.
spray drying of the solution is done in an instant. Therefore, it is necessary to maximize its dispersion, i.e. increase the surface area in the solution per unit volume, in order to speed up the heat transmission and mass transmission process.
e.g. a solution with a volume of 1cm3, if dispersed into small spherical droplets with a diameter of 10 m, the surface area is increased by 1290 times compared to the surface area before and after dispersion, thus greatly increasing the evaporation surface and shortening the drying time. Therefore, the atomization of liquids and the dispersion of material into mist droplets are the key components of spray drying.
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