【Daily Question】Chemical Engineering Principles 261: Drying Rate (December 12)

What is the drying rate? The amount of water evaporated per hour per square meter of dry surface area. Into which stages is the drying process divided? Constant-rate drying stage and decelerating drying stage. What controls each one? The influencing factors include the condition of the drying medium, flow rate and the way it comes into contact with the material, the structure and size of the material as well as the way it contacts the drying medium, and the temperature of the material itself.

The mass of water that can vaporize from the material being dried per unit time per unit dry area is referred to as the \"drying rate\". The drying process can be divided into a constant-rate drying stage and a decreasing-rate drying stage, which are controlled by surface vaporization and internal diffusion respectively.

What is the drying rate? Into which stages is the drying process divided? What controls each one? Drying rate: The amount of water evaporated per hour per square meter of dried surface. The process is divided into a constant-rate drying stage and a decreasing-rate drying stage, which are controlled respectively by surface vaporization and internal diffusion

The mass of water that can vaporize from the material being dried per unit time per unit dry area is referred to as the “drying rate”. Factors affecting it: 1: Properties and shape of the material: the physical structure, chemical composition, shape and size of wet materials, the thickness of the material layer, and the way in which moisture is bound to it. 2: Temperature of the material itself: The higher the temperature of the material, the greater the drying rate ； 3: Moisture content of the material: The initial, final, and critical moisture contents of the material determine the duration required for each drying stage. 4: Temperature and humidity of the drying medium: The higher the temperature of the medium and the lower its humidity, the better ； Then the rate during the constant-speed drying stage is greater. However, for heat-sensitive materials, an appropriate temperature should be chosen. 5: Flow rate of the drying medium: During the constant-speed drying stage, increasing the gas flow rate can boost the drying rate; during the decelerating drying stage, the gas flow rate and direction have little impact on the drying rate. 6: Structure of the dryer. 3 The drying process can be divided into a constant-rate drying stage and a decelerating drying stage, which are respectively controlled by surface vaporization and internal diffusion.

The drying rate refers to the amount of water evaporated per hour per square meter of drying area. kg. Water / h·m². The drying process is divided into: the constant-rate drying stage and the decreasing-rate drying stage. Factors affecting it: heat transfer rate, external diffusion rate, internal diffusion rate.

The drying rate is the amount of water vaporized per unit time per unit drying area. Drying is divided into a constant-rate drying stage and a decelerating drying stage. The constant-speed drying stage is controlled by the surface vaporization rate, while the decelerating drying stage is controlled by internal diffusion.

What is the drying rate? Into which stages is the drying process divided? What controls each one? The drying rate is the amount of water vaporized per unit time per unit drying area. Drying is divided into a constant-rate drying stage and a decelerating drying stage. The constant-speed drying stage is controlled by the surface vaporization rate, while the decelerating drying stage is controlled by internal diffusion.

The drying rate is the amount of water evaporated per hour per square meter of dry surface area, expressed in kg of water/(h·m2). Sometimes, the amount of water evaporated per hour per kilogram of anhydrous material is also referred to as the mass drying rate, in units of kg of water/(h·kg of anhydrous material). The factors that affect the drying rate include the heat transfer rate, external diffusion rate, and internal diffusion rate. Constant-rate drying stage and decelerating drying stage. It is controlled by surface vaporization and internal diffusion, respectively.

The drying rate refers to the amount of water that evaporates per hour per square meter of dry surface area. Sometimes, the amount of water that evaporates per hour per kilogram of anhydrous material is also referred to as the mass drying rate. The factors that affect the drying rate include the heat transfer rate, external diffusion rate, and internal diffusion rate. 　　(1) Accelerating the heat transfer rate To accelerate the heat transfer rate, the following measures should be taken: ① Raise the temperature of the drying medium, such as increasing the temperature of the hot gas in the drying kiln or using hot air furnaces; however, it is important not to raise the surface temperature of the green body too rapidly to avoid cracking. ② Increase the heat transfer area: for example, switch from single-sided drying to double-sided drying, stack the green bodies in layers or reduce the number of layers, thereby increasing the surface area in contact with the hot gas. ③ Increase the convective heat transfer coefficient. 　　(II) Increasing the external diffusion rate: When drying is in the constant-rate drying stage, the resistance to external diffusion becomes the main factor affecting the overall drying rate; therefore, reducing this resistance and increasing the external diffusion rate has the greatest impact on shortening the entire drying cycle. The external diffusion resistance mainly occurs in the boundary layer; therefore, it is necessary to: ① increase the flow velocity of the medium and reduce the thickness of the boundary layer, thereby enhancing the convective heat transfer coefficient. It can also increase the convective mass transfer coefficient, which helps to boost the drying speed; ② reducing the water vapor concentration in the medium and increasing the mass transfer area can likewise increase the drying speed. 　　(III) Increasing the internal diffusion rate of water The internal diffusion rate of water is determined by the combined effects of wet diffusion and thermal diffusion. Moisture diffusion is the movement of water in a material due to a humidity gradient, while heat diffusion is the movement of water caused by a temperature gradient in physics. To increase the rate of internal diffusion, the following measures should be taken: ① Ensure that heat diffusion and moisture diffusion occur in the same direction, that is, try to make the temperature at the center of the material higher than the surface temperature, using methods such as far-infrared heating or microwave heating; ② When heat diffusion and moisture diffusion are in the same direction, enhance heat transfer to increase the temperature gradient within the material. When these two processes occur in opposite directions, increasing the temperature gradient may increase the resistance to heat diffusion, but it can still boost heat transfer, raise the material’s temperature, and thereby increase moisture diffusion, thus accelerating drying; ③ Reduce the thickness of the material slab, changing from single-sided drying to double-sided drying; ④ Lower the total pressure of the medium, as this helps to increase the moisture diffusion coefficient and thus the rate of moisture diffusion; ⑤ Other factors related to the properties and shape of the material slab

What is the drying rate? The mass of water that can vaporize from the material being dried per unit time per unit dry area is referred to as the \"drying rate\". Into which stages is the drying process divided? The drying process can be divided into: the constant-rate drying stage and the decelerating drying stage. It is controlled by surface vaporization and internal diffusion respectively. What controls each one? It is controlled by surface vaporization and internal diffusion respectively.