A Study on the Effect of Classifier Diameter on the Performance of a Rotary Flash Dryer

 I. Overview

The rotary flash dryer is a continuously operating convection drying device and is currently one of the preferred pieces of equipment used in industrial production for drying filter cake-like materials. Its operating principle is shown in Figure 1. The main unit can be broadly divided into three sections:

  the air collection chamber and crushing/fluidization section;

  the middle drying section;

  the top recovery section.

Filter cake-like material enters the dryer via a screw feeder and is fed into the middle of the drying section, where it falls in a loose state into the crushing and fluidization section. Hot air enters the air collection chamber tangentially; the cross-section of the air collection chamber tapers spirally. The hot air inside the chamber flows through the gap between the air collection chamber and the crushing and fluidization section, forming a high-speed rotating flow as it enters the crushing and fluidization section. The loose, lumpy material falling from the feeder is broken by the impact of rotating blades and suspended by the rotating airflow. Collisions and friction occur between the material lumps and between the lumps and the rotating blades. As moisture evaporates from the material, particles of varying sizes are formed, which move upward with the rotating airflow into the middle drying section. Due to their varying particle sizes, the particles follow different trajectories, dispersing throughout the drying section to facilitate thorough mass and heat transfer with the hot airflow, resulting in continuous moisture evaporation. Larger particles follow trajectories with larger radii, staying close to the cylinder wall, while smaller particles follow trajectories with smaller radii. A classifier is installed at the top of the middle drying section. Larger particles, due to their higher moisture content and the failure to promptly eliminate agglomeration between particles, are blocked by the classifier and fall back into the crushing and fluidization section at a higher settling velocity, where the above process is repeated. Fine particles pass through the classifier with the airflow into the recovery section and, together with the airflow, enter the fine powder recovery unit to continuously produce the dried product. The process flow of the rotary flash dryer system is shown in Figure 2.

Currently, common issues with rotary flash dryers include high initial capital investment, high energy consumption, relatively low output, and time-consuming and labor-intensive commissioning. Even for drying systems already in operation, it is often unclear whether they are operating at optimal conditions. Consequently, economic efficiency is suboptimal, leading to waste. There are two main reasons for this. First is the proper configuration of the system, such as performing mass and heat balance calculations tailored to different materials and production requirements to determine heat input, airflow, temperature, and other parameters. Second is how to improve the design of the main unit to accommodate different materials and system configurations. Resolving these two issues is expected to further increase production capacity, thereby improving efficiency and achieving energy savings.

While the rational configuration of the system has been discussed in previous articles, there have been no reports on how to improve the main unit and reduce unnecessary disassembly and modifications during on-site commissioning to ensure it operates at its optimal state. In the dryer, the velocity at which material particles are introduced into the rotating upward airflow is variable. The rotating airflow enters the crushing and fluidization section through the annular gap at the bottom of that section, and due to the agitation caused by the rotating blades, the direction and velocity of the airflow are not uniform. Consequently, the material particles do not follow a fixed, predetermined trajectory. Currently, it is impossible to determine the diameter of the classifier at the top of the central drying section. However, manufacturers of this type of dryer today adopt a one-size-fits-all approach in design and production, using a small classifier diameter regardless of the material type. After installation, they check whether the dryer can achieve the user’s required throughput. If it falls short, they attempt to adjust the ratio of the system’s blower and induced draft fan; if that still fails, they are forced to repeatedly disassemble and reassemble the equipment, blindly enlarging the classifier diameter. Whether the classifier is too large or too small, the dryer cannot operate under ideal conditions. This approach is not only time-consuming and labor-intensive but also delays the equipment’s commissioning and results in energy waste. Determining the optimal classifier diameter for different materials through theoretical calculations would undoubtedly be a practical and valuable endeavor.

II. Analysis of Material Particle Motion and Forces During the Drying Process

Analysis of Particle Motion

Inside the dryer, bulk material entering the crushing and fluidization section is dispersed by the impact and collision of rotating blades, and as the moisture it carries evaporates, agglomeration between particles gradually dissipates. Lumps and clumps of material transform into a group of particles of varying diameters, which, under the influence of the rotating upward airflow, generate a rotating upward flow. Since the microscopic motion of the particle clusters during this process is highly complex and the particle sizes are very small, it is assumed that the particle velocity in the circumferential direction is equal to the velocity of the gas flow; In the radial direction, particle motion is subject to the combined effects of centrifugal force and drag, while the influence of other factors is neglected; in the axial direction, since the study focuses on the classifier’s diameter, axial particle motion is temporarily disregarded; and interactions between particles and the fluid are also neglected. That is, only the radial and circumferential motions of the particles are considered to examine the motion of a single particle within the dryer.

Analysis of Forces Acting on Particles

Assuming the particles are spherical, and since the dryer operates under a slight negative pressure where the gas density ρ varies only slightly, it is assumed to be a constant. The total force ∑F acting on a single particle in the crushing and fluidization section of the dryer is given by:

  ∑F = mg + m·

  Where: m — particle mass

  — gravitational acceleration;

  — centrifugal acceleration;

  — drag force exerted on the particle by the fluid;

  — buoyancy force exerted on the particle in the fluid;

  — interparticle interaction force;

  — force exerted on the particle by the blades.

Since particles are subjected to multiple forces in the fluid, among which gravity and buoyancy have little effect on the particles’ radial motion, and assuming that there is no interaction between particles, the interparticle forces can be neglected. Additionally, since the blades exert no radial force on the particles, the radial forces acting on the particles can be simplified to:

  ∑F = Fd + m·

That is, the radial motion of a single particle in the crushing and fluidization section and the drying section of the dryer is primarily governed by the drag force of the fluid and the centrifugal force. By analyzing the particle’s state of mechanical equilibrium, the particle’s radial motion can be determined under ideal mechanical equilibrium conditions. If the centrifugal force acting on the particle exceeds the drag force, the particle moves toward the outer cylinder wall; if the drag force exceeds the centrifugal force, the particle moves toward the central region. The particles enter the recovery section via the classifier. Through ideal force equilibrium, the maximum particle diameter d within the acceptable product range can be determined, where the particle is in a state of radial force equilibrium in the drying section.

  ‑F = 0, that is,

  where: Fd — the radial drag force component acting on the particle.

III. Calculation of the Classifier Diameter

Based on the above analysis of the mechanical equilibrium of a single particle, let vp be the maximum difference in radial velocity between the gas and the stationary particle. Assuming this relative velocity equals the radial velocity of the gas flow in the flow field, an approximate calculation can be performed.

  Where: Q — flow rate; S1 — annular gap area.

  Let v be the tangential velocity of the particle, and assume the tangential velocity of the gas is vg, for an approximate calculation:

  Where: S2 — cross-sectional area of the air collection chamber.

  Let R be the radius of rotation of the particle, and the mass of the particle

  ρs·π

  Where: ds — particle diameter; ρs — particle density.

  π·Cd·d³s/4·ρ·v²ρ/2 = π·Cd·d³s·v²ρ

  Where: ρ — gas density; Cd — drag coefficient.

  Then, Equation (3):

  πρsd3s)/6·v2g/R=k·(π·Cd·d3s·v2ρ

  Where: k — particle shape factor.

  Then:

  ·ρs·ds·v2g)/(3·k·ρ·Cd·v2ρ

IV. Conclusion

Based on different material properties, determining the classifier diameter through theoretical calculations can avoid unnecessary disassembly and reassembly during the commissioning process. At the same time, it maximizes the dryer’s potential capacity, ensuring it operates under optimal conditions and preventing energy waste. This provides theoretical guidance for the design and practical commissioning of dryers.

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