dryer[1]It refers to a kind of mechanical equipment that vaporizes and escapes the moisture (generally refers to moisture or other volatile liquid components) in materials through heating to obtain solid materials with specified moisture content. It is also a kind of mechanical equipment that realizes the drying process of materials.
Modern dryers began to operate intermittentlyfixed bedType dryer.In the middle of the 19th century,Tunnel drierThe use of the dryer marks the development of the dryer from intermittent operation to continuous operation.Most industrial products need drying treatment at a certain stage of production, and materials need specific moisture content for processing, molding or granulation.Dryers can be classified according to different characteristics such as operating process, operating pressure, heating mode, wet material movement mode or structure.
Since ancient times, people have been accustomed to drying materials with natural heat source and natural ventilation, which are completely restricted by natural conditions and have low production capacity.With the development of production, they are gradually replaced by manually controllable heat sources and mechanical ventilation dehumidification means.
Modern dryers began to operate intermittentlyfixed bedType dryer.In the middle of the 19th century,Tunnel drierThe use of the dryer marks the development of the dryer from intermittent operation to continuous operation.The rotary drum dryer can better realize the agitation of granular materials, and the drying capacity and strength can be improved.Some industries have developed continuous operation dryers to meet the requirements of the industry, such as textile and paper industriesDrum dryer。
In the early 20th century, dairy production began to applySpray dryerIt provides a powerful tool for large-scale drying of liquid materials.Since the 1940s, with the development of fluidization technologyFluidized bedandPneumatic dryerAppear one after another.The freeze sublimation, radiation and dielectric dryers provide new means to meet special requirements.In the 1960s, far-infrared andmicrowave drying Device.
Dryer Overview
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Characteristics of dried materials
shape[3]: plate, block, sheet, needle, fiber, granule, powder, paste or even liquid
Structure: porous and loose, compact
Heat resistance: heat sensitivity
Caking: wet materials that are easy to be caked can be gradually dispersed during drying, and wet materials with good dispersibility may be severely caked during drying
Requirements for products
Degree of dryness: remove surface moisture, combine moisture and even crystal moisture.Average moisture content and drying uniformity required.Requirements for dust and product recovery, maximum allowable drying temperature.
Appearance: the particle size distribution of the product, certain crystal shape and luster, no cracking and deformation, etc.
Drying time: seconds, hours and days
Selection of dryer
The following factors shall be considered in the selection of dryer:
(1) Ensure the drying quality of materials, dry evenly, do not deteriorate, keep the crystal shape intact, and do not crack or deform;
(2) Fast drying rate, short drying time, large amount of vaporized water per unit volume of dryer, which can achieve large production of small equipment;
(3) Low energy consumption, high thermal efficiency and low power consumption;
(4) The drying process is simple, the equipment investment is small, the operation is stable, the control is flexible, the working conditions are good, and the pollution is small.
classification
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Dryers can be classified according to different characteristics such as operating process, operating pressure, heating mode, wet material movement mode or structure.
Classification by operation process
According to the operation process, the dryer is divided into intermittent (batch operation) and continuous type.
Classification by operating pressure
According to the operating pressure, the dryer is divided intoAtmospheric dryerandVacuum dryerTwo types, operating under vacuum can reduce the partial pressure of wet partial vapor in the space to accelerate the drying process, and can reduce the boiling point of wet partial vapor and the drying temperature of materials. The steam is not easy to leak out. Therefore, the vacuum dryer is suitable for drying heat sensitive, oxidizable, explosive and toxic materials, and the wet partial vapor needs to be recycled.
Classification by heating method
According to the heating mode, the dryer is divided into convection type, conduction type, radiation type, dielectric type, etc.Convective dryer, also known as direct dryer, uses hot drying medium to directly contact with wet materials to transfer heat by convection and take away the generated steam;Conductive dryer, also known as indirect dryer, uses conduction mode to transfer heat from heat source to wet materials through metal partition wall. The generated wet partial steam can be sucked under reduced pressure, a small amount of purge gas can be introduced, or at a separately set low temperaturecondenserRemove by surface condensation and other methods.This kind of dryer does not use drying medium, so its thermal efficiency is high and the product is not polluted. However, its drying capacity is limited by the heat transfer area of the metal wall, and its structure is complex, so it is often operated under vacuum;The radiation dryer uses various radiators to emit electromagnetic waves of a certain wavelength range, which are selectively absorbed by the surface of wet materials and then converted into heat for drying;The dielectric dryer makes use of the high-frequency electric field to dry the wet materials with thermal effect inside.
Classification according to movement mode of wet materials
According to the movement mode of wet materials, dryers can be divided into fixed bed type, stirred type, spray type and combined type;According to the structure, the dryer can be divided intoChamber dryerConveyor dryer, drum dryer, vertical dryer, mechanically stirred dryer, rotary dryerfluidized bedType dryerPneumatic dryer, vibrating dryer, spray dryer and combined dryer.
Classification according to heating mode
Equipment for drying wet materials.The materials that need to be dried in various production processes are diverse, and the requirements for drying are also different. Therefore, there are many types of dryers. According to the heating mode, there are four types:
1 Convection dryer
The most widely used type of dryer, includingFluidization dryingDeviceAir dryingDeviceChamber dryer、Spray dryer, tunnel dryer, etc.The main characteristics of this kind of dryer are: ① The hot gas flow is in direct contact with the solid, and the heat isConvective heat transferMode: The hot gas flows to the wet solid, and the generated water vapor is taken away by the air flow; ②The temperature of hot air flow can be raised to the maximum temperature (about 730 ℃) that ordinary metal materials can withstandRadiant heat transferIt will become the main heat transfer mode and can achieve high heat utilization rate; ③The humidity of the air flow has an impact on the drying rate and the final water content of the product; ④When using low temperature air flow, it is usually necessary toDehumidificationHandling; ⑤The energy consumption of vaporizing unit mass moisture is higher than that of the conduction dryer, especially when the water content of the final product is low; ⑥A large amount of hot air flow is required to ensure the heat required for water vaporization. If the particle size of the dried material is very smallDust removal deviceLarge and expensive; ⑦It should be operated under near normal pressure.
Including spiral conveying dryerDrum dryer, vacuum rake dryer, freeze dryer, etc. The main characteristics of this type of dryer are: ① Heat passes through the wall (usually metal wall)heat conductionMode to wet materials; ②The surface temperature of materials can range from below freezing point (during freeze drying) to 330 ℃; ③It is easy to operate under reduced pressure and inert atmosphere, and the volatile solvent can be recovered.It is commonly used for drying materials that are easy to oxidize and decompose, as well as for handling powdery materials.
adoptRadiant heat transfer, the wet material is heated for drying.The electric heating radiation dryer uses an infrared bulb to illuminate the dried material, so that the temperature of the material will rise and dry.The gas heating dryer burns gas to heat the metal or ceramic radiant plate to 400~500 ℃ to generate infrared ray for heating the dried materials.The radiation dryer has high production intensity, compact equipment and flexible use, but its energy consumption is high.It is suitable for drying materials with large and thin surfaces, such as plastic, cloth, wood, paint products, etc.
4 Dielectric dryer
Put the dried materials in the high-frequency electric field, and use the alternating effect of the high-frequency electric field to heat the objects for drying.The heating feature is that the higher the water content in the material, the more heat is obtained.Since the water content inside the material is higher than that on the surface, more energy is obtained inside the material, and the internal temperature of the material is higher than the surface temperature, so that the temperature gradient and the direction of water diffusion are consistent, which can speed up the vaporization of water and shorten the drying time. This dryer is especially suitable for materials that are easy to crust during the drying process and are difficult to remove the internal moisture (such as leather).Dielectric heating and drying consumes a lot of electric energy, which is mainly used in food and light industry production.For the design and calculation of the dryer, the appropriate dryer type must be selected first.The selection of dryer is also very empirical, and the following aspects should be taken into consideration: ① the characteristics of materials and products, such as the form of materials (such as slurry, paste, powder, block, flake, etc.), the particle size and strength of solid particles, the initial water content and the existing form of water, whether the materials are toxic, flammable, and easy to oxidize, and the final water content required by the product,Whether the product is allowed to be slightly polluted, whether the shape is allowed to be slightly changed, the maximum allowable temperature of the product and the price of the product. ②Conditions related to the production process, such as the amount of materials to be treated, pre-treatment and post-treatment of drying, volatile solvents, whether to recover, etc. ③Operating performance and economic indicators of the dryer.After comprehensive consideration of the above aspects, after comparison and screening of various dryers, there are generally only a few dryers left, and then small tests are carried out to find the most appropriate operating parameters and structural parameters. Finally, according to the equipment price and small tests, the type of dryer is determined.
Convective Dryer
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Chamber dryer
Chamber dryer
Small ones are called ovens, and large ones are called ovens[4]。
Intermittently operate hot air under normal pressure or vacuum to pass through the wet material surface to achieve the purpose of drying.
The exhaust gas partial circulation method is commonly used.Multilayer rectangular trays are stacked on the frame, and the thickness of wet materials in the tray is usually 10-100mm.
Advantages: It has strong adaptability to materials, and is suitable for small-scale and multi variety occasions with large changes in drying conditions.
Disadvantages: low thermal efficiency and uneven product quality.
Hot air speed, fan air volume: increasing the speed of hot air can improve the heat transfer coefficientα，However, the wind speed should be less than the material removal speed.
Rotary dryer
Rotary dryer
Rotary dryer[3](Rotary dryer)
The main body is a slightly inclined and rotatable cylinder with a number of boards installed along the axial direction.
Advantages: ① Large production capacity, continuous operation; ②Simple structure and convenient operation; ③Few failures and low maintenance costs; ④Wide application range (such as filter cake drying); ⑤Large operating flexibility; ⑥Easy cleaning.
Disadvantages: ① huge equipment; ②Difficulty in installation and disassembly; ③Small thermal capacity coefficient and low thermal efficiency; ④The residence time of materials in the dryer is long, and the residence time difference between material particles is large.
Spray dryer
Spray dryer
A atomizer is used to disperse the dilute feed liquid into droplets, which settle freely and evaporate rapidly in the hot gas flow (air, nitrogen or superheated water vapor), and are finally dried into solid particles and separated from the gas flow.The droplet diameter is 10~60um, and each liter of solution has an evaporation area of 100~600m2.The drying time is generally 3~10 s[5]。
Advantages: short residence time of materials, suitable for heat sensitive materials;The obtained product is hollow particle with stable operation;Continuous and automatic production;The powder product is directly obtained from the feed liquid, and the evaporation, crystallization, separation and crushing operations are omitted.
Disadvantages: low heat transfer coefficient;Large equipment volume;The operation flexibility is small, the heat utilization rate is low, and the energy consumption is large.
Nozzle: centrifugal sprayer (circumferential speed 90~160m/s), pressure sprayer, air flow sprayer (compressed air or steam ≥ 300m/s).
Airflow dryer
Airflow dryer
The hot medium is in direct contact with the solid particles to be dried, and the solid particles to be dried are suspended in the fluid, so the two phase contact area is large, which strengthens the heat and mass transfer process, and is widely used in the drying of bulk materials.It is the application of dilute phase transportation in solid fluidization in drying[6]。
advantage:
(1) Fast drying speed, mainly used for drying crystals and small particles. Materials are dispersed and suspended in the air stream, with large heat and mass transfer area.The inlet velocity of hot gas is high (10-40m/s), and the relative velocity between gas and solid (especially in the acceleration section) is large. The average heat transfer coefficient is several to dozens of times higher than other types of dryers, and the size is small
(2) The gas-solid concurrent flow operation conforms to the basic law of drying. High temperature gas can be used as the drying medium without burning the material.For a 10m long air flow pipe, 80% of the water is evaporated and dried in the acceleration section about 2m long.
(3) The drying time is short (second level), and the whole drying process does not exceed 2 seconds. It is especially suitable for drying materials that are heat sensitive, easy to oxidize, and not suitable for crushing,
(4) The wet content of the product is uniform.
(5) Simple structure, less equipment investment, small floor area, convenient operation, stable performance and small maintenance.
Fluidized bed dryer
Fluidized bed drying
Fluidized bed drying is the application of fluidization technology in drying operation[7]。
characteristic:
(1) Retention time ratio of particlesAirflow dryerInternal length, the retention time of particles in the dryer can be adjusted at will.
(2) Low operating gas speed, light wear, low pressure drop
(3) It is suitable for processing powder materials with particle size of 6-30mm.Compact equipment.
monolayerFluidized bed dryerDisadvantages: The residence time distribution of materials in the fluidized bed is uneven, so the humidity of the dried products is uneven.
Selection of dryer
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Selection considerations
(1) When dealing with liquid materials, the selected equipment is usually limited toSpray dryer, drum dryer and mixing intermittentlyVacuum dryer[1]。
(2) Vacuum operation is preferable when solvent recovery, flammability, toxic hazard or temperature limitation is required.
(3) For hygroscopic materials or materials difficult to dry with high critical moisture content, dryers with long drying time should be selected, while for materials easy to dry with low critical moisture content and heat sensitive materials sensitive to temperature, dryers with short drying time can be selected, such asAirflow dryerSpray dryer.
(4) Due to small processing capacity, it is better to use the box type dryer and other intermittent operation dryers, while the continuous dryer with large processing capacity is more suitable.
Dryer selection steps
First, drying experiments are carried out based on the shape of wet materials, drying characteristics, product requirements, processing capacity and heat source used.Determine drying kinetics and transfer characteristics, and determine the process size of drying equipment.
According to the environmental requirements, select the appropriate dryer type.
If several dryers are applicable at the same time, cost accounting and scheme comparison shall be conducted to select the best one.
influence factor
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temperature
Drying temperature and heat are the key to open the resultant force between water molecules and hygroscopic polymers.When it is higher than a certain temperature, the attraction between water molecules and polymer chains will be greatly reduced, and the water vapor will be taken away by dry air[8]。
the dew point
In the dryer, wet air is first removed to contain very low residual moisture (dew point).Then, reduce its relative humidity by heating the air.At this time, the vapor pressure of dry air is low.By heating, the water molecules inside the particles get rid of the binding force and diffuse to the air around the particles.
time
In the air around the particles, the absorption of heat and the diffusion of water molecules to the particle surface need a certain time.Therefore, the resin supplier shall specify the time required for a material to be effectively dried at an appropriate temperature and dew point.
airflow
The dry hot air transfers the heat to the particles in the drying silo, removes the moisture on the particle surface, and then sends the moisture back to the dryer.Therefore, there must be enough air flow to heat the resin to the drying temperature and maintain this temperature for a certain time.
matters needing attention
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(1) Do not put too much desiccant to avoid contaminating the bottom of the crucible.
(2) When moving the dryer, hold it with both hands and press the cover tightly with your thumb.
(3) When opening the dryer, the cover cannot be lifted up. The left hand should be used to hold the dryer, and the right hand should be used to carefully push the cover open slightly. After the cold air slowly enters, the cover can be completely pushed open. The cover must be placed on the table.
(4) Do not put too hot objects into the dryer.
(5) Sometimes, after a hot object is put into the dryer, the air will expand when heated, which will lift the lid. In order to prevent the lid from being overturned, you should press it with your hand and push the lid slightly open from time to time.
(6) Crucibles and precipitates after burning or drying should not be placed in the dryer for too long, otherwise the mass will increase slightly due to absorption of some moisture.
(7) The color changing silica gel is blue when dry and pink when wet.The wet silica gel can be dried at 120 ℃ and reused after it turns blue until it is broken and unusable.[8]
application
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The drying process needs to consume a lot of heat energy. In order to save energy, some materials with high moisture content, suspensions or solutions containing solid substances are generally mechanically dehydrated or heated and evaporated before drying in a dryer to obtain dry solids[9]。
The purpose of drying is to meet the needs of material use or further processing.For example, the drying of wood before making wood molds and woodwork can prevent products from deformation, and the drying of ceramic blanks before calcination can prevent finished products from cracking.In addition, the dried materials are also convenient for transportation and storage, such as drying the harvested grain to a certain moisture content to prevent mildew.Since natural drying is far from meeting the needs of production development, various mechanized dryers are increasingly used.
During the drying process, heat and mass (moisture) transfer shall be completed at the same time to ensure that the moisture partial pressure (concentration) on the material surface is higher than the moisture partial pressure in the external space, and ensure that the heat source temperature is higher than the material temperature.
desiccant
Heat is transferred to wet materials from high temperature heat source in various ways, so that the moisture on the surface of the materials evaporates and escapes to the external space, resulting in the difference of moisture content on the surface and inside of the materials.The internal moisture diffuses to the surface and vaporizes, so that the moisture content of the material is constantly reduced and the drying of the whole material is gradually completed.
The drying rate of materials depends on the surface vaporization rate and the internal moisture diffusion rate.Generally, the drying rate in the early stage of drying is controlled by the surface vaporization rate;Then, as long as the external drying conditions remain unchanged, the drying rate and surface temperature of the material will remain stable. This stage is called the constant speed drying stage;When the moisture content of the material decreases to a certain extent, the diffusion rate of the internal wet split surface decreases and is smaller than the surface vaporization rate, the drying rate is mainly determined by the internal diffusion rate, and decreases with the decrease of the moisture content. This stage is called the rate reduction drying stage.
Future development
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The future development of dryer will be further studieddryerDevelop and improve the dryer under the optimal operating conditions for different materials;In addition, large-scale, high strength, high economy, and improving the adaptability to raw materials and product quality are the basic trends of dryer development;At the same time, we will further research and develop new and efficient dryers that meet special requirements, such as combined dryersMicrowave dryerHeyuanInfrared dryerEtc.
The development of dryers should also pay attention to energy conservation and comprehensive utilization of energy, such as adopting various combined heating methods, transplanting heat pump and heat pipe technology, and developingSolar dryingApparatus, etc;We should also develop the automatic control technology of the dryer to ensure the realization of the optimal operating conditions;In addition, as people attach importance to environmental protection, improving the environmental protection measures of dryers to reduce the leakage of dust and waste gas will also be the direction of in-depth research.
