Conduction drying, also known as contact drying, is very adaptable to moist particles, and the conduction dryer has a higher thermal efficiency. Evaporated water vapor or organic solvent is extracted by vacuum or discharged with a small amount of air flow. The air flow is the main carrier of moisture, and vacuum-operated operation is recommended for heat-sensitive granular materials. In conductive dryers, paddle dryers are used to dry paste materials. Rotary dryers with internal flow tubes have now been designed and put into use.
The vacuum drying operation is expensive, but vacuum drying is recommended only when the material must be dried at a low temperature or in an oxygen deficient state, or when the material is deteriorated by heating in a heating medium and at a high temperature. For a certain evaporation efficiency, the use of high temperature operation is more effective, which can reduce the gas flow rate and reduce the equipment volume. For low-temperature drying operations, an appropriate low-temperature waste heat or solar collector can be selected as the heat source, but the dryer has a relatively large volume.
Freeze-drying is a special case of drying under vacuum conditions. Here, the temperature is below the triple point of water, and the water (or ice) sublimates directly into water vapor. Although the heat required for sublimation is several times lower than that of evaporation, the power consumption of other aspects is large, so the cost of vacuum drying is still expensive.
In conductive heat transfer, heat is supplied to the dry material by jackets, agitation, heat transfer tubes, etc., using a heat medium instead of hot air. A typical device is a trough type cylinder dryer. Cone stirring dryer, jacket and built-in heating tube rotary dryer, drum dryer, rake dryer, vacuum belt dryer, vacuum freeze dryer and so on. In hot air drying, the temperature during constant-rate drying corresponds to the wet bulb temperature. In conducting heat transfer drying, it is necessary to artificially create a heating surface so that the heat transfer area is small and the heat transfer coefficient is increased. In this way, the structure is more complicated and the equipment investment is larger than the hot air dryer.
The conductive drying method transfers the heat required for drying indirectly through metal and other surfaces. The drying rate is lower than the direct drying method. The temperature of the product during constant speed drying is not related to the temperature of the heating source and is generally the same as the saturation temperature of the gas pressure in the device. In order to increase the drying rate and prevent uneven drying, mechanical agitation is usually used or the container itself is rotated to increase or constantly update the heat transfer surface of the material. Therefore, it is necessary to further study the adhesion problem of the heat transfer mechanism. The drying device itself is expensive, but it is characterized by a small load on the dust collecting system, a high thermal efficiency, easy recovery of the solvent, and a total cost much less than the direct drying method.
(1) Hollow paddle dryer (blade dryer)
Hollow blade dryer, also known as blade dryer, is mainly composed of W-shaped shell with jacket and two hollow blade shafts and transmission device. A hollow blade is arranged on the shaft, and the shaft end is equipped with a rotating medium introduced by a rotary joint. The heat required to dry the moisture is conducted to the material by the inner wall of the jacket with W-shaped grooves and the hollow blade wall. During the drying process of the material, the hollow shaft with the hollow blade stirs the material while heating the material, thereby updating the heating surface. It is a continuous conduction heating dryer.
The heating medium is steam, hot water or heat transfer oil. The heating medium is introduced into the jacket of the housing and in the two hollow blade shafts, and the material is heated and dried in a conductive heating manner. Different material hollow blade shaft structures are different.
The machine is suitable for processing various paste-like, granular, powdery and other materials with good thermal stability. Under special conditions, the heat-sensitive material can also be dried and the solvent can be recovered during the drying process. Commonly used for drying or cooling of carbon black, light calcium carbonate, titanium dioxide, barium carbonate, nitrile uric acid, gypsum, clay, manganese dioxide, nylon and polyester chips, polyethylene, polypropylene (recovery solvent) and other materials.
(2) Vacuum rake dryer
The dry material is added from the middle of the upper part of the shell of the vacuum rake dryer. Under the stirring of the continuously rotating rake, when the material comes into contact with the shell wall, the surface is continuously updated, and the dried material is subjected to steam (or hot water, heat transfer oil). Indirect heating, but the material moisture vaporization, vaporized water pumped away in time by the vacuum pump. The material to be dried is indirectly heated by the heat carrier, so that the moisture in the material is vaporized, and the vaporized water is discharged in time by the vacuum pump. Due to the high operating vacuum, generally in the range of 400-700mmHg, the surface water vapor pressure of the dried material is much greater than the vapor pressure of the evaporation space in the dryer shell. It is beneficial to the discharge of moisture and surface moisture inside the dried material, which is beneficial to the movement of the water molecules of the dried material and achieves the purpose of drying. The vacuum rake dryer is suitable for heat-sensitive materials that are easily oxidized at high temperatures or materials that tend to harden when dried, and materials that must be recycled when the steam is discharged during drying. Typical dry materials are sodium acrylate, CMC, phthalocyanine blue, dye intermediates, carboxymethyl starch, maltodextrin, indene sulfonic acid, and the like.
(3) Belt dryer
The belt dryer consists of several individual unit sections. Each unit segment includes a circulation fan, a heating device, a separate or common fresh air intake system and an exhaust gas discharge system. The operating parameters of the quantity of drying medium, temperature, humidity and exhaust gas circulation amount can be controlled independently, so as to ensure the reliability of the dry machine operation and the optimization of the operating conditions. The belt dryer has the advantages of flexible operation, wet material feeding, and drying process in a completely sealed box. The working conditions are good and the leakage of dust is avoided. It is especially suitable for the drying of sheets, strips and granular materials with good air permeability, especially for dehydrated vegetables, Chinese Herbal Pieces, etc., which have high water content and do not allow material temperature to be high.
(4) Roller Scraper Dryer
Roller squeegee dryer is a kind of continuous operation equipment that is used to heat the liquid phase material or strip-shaped material attached to the outer wall of the cylinder through a rotating cylinder and is dried by heat conduction. The liquid to be dried is fed from the high tank to the receiving tank of the drum dryer. The drying drum is driven by the transmission and rotates at a specified speed. The material is formed by a cloth film device to form a material film on the wall surface of the drum. The heating medium is continuously fed into the cylinder, the cylinder body is heated, the heat of the material film is vaporized by the heat transfer from the cylinder wall, and then the material required for drying is scraped off by a scraper, transported to the storage tank by screwing, and packaged. The moisture removed by evaporation can be introduced into the corresponding treatment device depending on the nature of the moisture through a closed hood; generally water vapor can be directly discharged from the exhaust pipe at the top of the hood to the atmosphere. The machine is mainly used for processing liquid materials, and it can be heated and dried by steam, hot water or hot oil, and can also be cooled by cold water. According to different material properties and process requirements, immersed, spray-type, grind-assisted and other feeding methods can be adopted.
(5) Double cone rotary vacuum dryer
The double-cone rotary vacuum dryer is slightly olive-like, with two ends covered and two shafts in the middle to support the body. The body has a jacket for heating, the body can be rotated when dry, so that the material and the wall are often replaced contact, overcome the shortcomings of the material in the vacuum oven mainly rely on the heating tube conduction and low thermal efficiency. Rotary vacuum dryers have been widely used in fine chemicals, medicines, etc., and are not suitable for materials with high viscosity or strong adhesion during rotation. The equipment is mainly used for the concentration, mixing and drying of powdery, granular and fibrous materials in the pharmaceutical, chemical and food industries, and materials that require low-temperature drying (such as biochemical products, etc.), and is more suitable for easy oxidation, volatile, and heat-sensitive. Strongly irritating, toxic materials and materials that are not allowed to damage the crystals are dried.
(6) Vacuum oven
Since the decompression, the evaporation temperature of the volatile matter contained in the material evaporation can be reduced, which is suitable for the drying of various heat-sensitive and easily oxidized materials. This device is often a cylinder or other vacuum-operated housing. It is heated by electric or hot water and heat-conducting oil through a heating plate or heating tube. It is suitable for small batch batch production.
The oven is a universal drying equipment with a wide range of applications. It is a tray-type intermittent drying equipment used in pharmaceuticals, chemicals, and foods. Light industry, heavy industry and other industries heat curing, drying and dehydration of materials and products. Such as raw materials, crude drugs, Chinese Herbal Medicine, extracts, powders, granules, granules, water pills, bottles, pigment dyes, dehydrated vegetables, dried fruits, sausages, plastic resins, electrical components, baking and so on.
(7) Freeze dryer
Freeze-drying machine is to freeze and then freeze the liquid, and then sublimate the ice under reduced pressure to obtain dry material. Since the entire process is performed below the freezing point, it is commonly used for the drying of heat-sensitive articles. Due to the difficulty of heating the frozen material under vacuum, and the need to condense and sublimate the water vapor under reduced pressure, a relatively large refrigeration system is required, and thus the drying cost is large. The moisture in the highly heat-sensitive material is frozen and will be maintained under a high vacuum to a temperature below the freezing point. Sublimates moisture and separates it from the material. The loss of active ingredients in the material is small, but the drying rate is low.
