1、 Wind speed parameter: the "basic switch" that controls the suspension state of particles

The inlet wind speed is the core variable that determines the fluidization state of materials, directly affecting the particle collision frequency and drying rate. If the wind speed is too low, materials are prone to accumulate under the bed, causing local particles to excessively bond and form large particles; If the wind speed is too high, it will cause excessive entrainment of fine powder, and at the same time, strong collisions between particles can easily produce broken particles.

Adjustment technique: The wind speed range should be dynamically set according to the material density - for lightweight materials with a density of 0.3-0.5g/cm ³, the initial wind speed should be controlled at 1.2-1.8m/s, and gradually reduced to 0.8-1.2m/s after the material forms a stable fluidized layer; For heavy materials with a bulk density greater than 0.8g/cm ³, the initial wind speed can be increased to 2.0-2.5m/s to avoid material deposition. In practice, it can be judged by observing the height of the "boiling layer" inside the sight glass: in an ideal fluidized state, the material should be in a uniform rolling "avalanche like" state, without obvious local agglomeration or voids.

2、 Spray parameters: "key variable" for precise regulation of particle growth

The atomization pressure, spray rate and nozzle height of the spray system jointly determine the dispersion and adhesion efficiency of droplets in the fluidized bed, which is the core control link of particle size growth.

Atomization pressure: If the pressure is too low, it can cause the droplet diameter to be too large (>100 μ m), which can easily form a "liquid bridge" on the particle surface and cause local agglomeration; If the pressure is too high (>0.4MPa), the droplets will be too fine (<30 μ m), which is easily carried away by hot air or quickly dried on the surface of particles, making it difficult to form effective bonding. Usually, the atomization pressure needs to be matched with the material characteristics. For example, for Chinese herbal extracts with strong viscosity, the pressure should be controlled at 0.25-0.35 MPa to ensure that the droplets and particles are fully combined.

Spray rate: It needs to be balanced with drying efficiency. If the rate is too fast, it can easily lead to a sudden increase in humidity in the bed, causing particles to bond with each other and form large particles; If the speed is too slow, the particle growth will be slow and the proportion of fine powder will increase. It is recommended to use a "gradient increasing" setting: in the initial stage, spray the liquid at a maximum rate of 20-30%. After the material forms initial particles of 30-50 μ m, gradually increase the rate to 50-70%. At the same time, adjust the exhaust temperature (usually controlled at 45-60 ℃) in real time to ensure that the relative humidity of the exhaust is stable at 30-45%.

Nozzle height: If the nozzle is too close to the material bed surface (<150mm), the droplets may not be fully dispersed and may come into contact with particles, which can easily cause local agglomeration; If it is too high (>300mm), the droplets will dry excessively on the way, and the bonding efficiency will decrease. It is generally recommended to set the nozzle height to 1.2-1.5 times the bed height, and dynamically adjust it according to the particle growth situation to ensure that the droplets evenly cover all fluidized particles.

3、 Temperature parameter: the "environmental barrier" that ensures stable growth of particles

The inlet air temperature and bed temperature directly affect the drying rate and bonding strength of materials. If the inlet air temperature is too low (<50 ℃), it will result in delayed drying and easy particle adhesion and agglomeration; If the temperature is too high (>80 ℃), the surface of the particles will quickly solidify, and the internal moisture will be difficult to discharge, which can easily form "hollow particles" or brittle particles.

Control strategy: Adopt a "segmented temperature control" mode - during the initial granulation stage (0-30 minutes), set the inlet air temperature to 65-75 ℃, quickly remove surface moisture from the material, and avoid initial bonding; Reduce to 55-65 ℃ in the mid-term (30-90 minutes) to provide a suitable bonding environment for particle growth; In the later stage (from 90 minutes to the end), the temperature rises to 65-70 ℃ to enhance the drying effect and ensure that the moisture content of the particles remains stable at 1.5-3.0%. At the same time, real-time monitoring of the bed temperature sensor is required to ensure that the fluctuation of bed temperature does not exceed ± 2 ℃, and to avoid particle size fluctuations caused by sudden temperature changes.

4、 Practical verification and optimization: building a closed-loop control system

After parameter adjustment, multidimensional verification is required to ensure particle size uniformity: a laser particle size analyzer is used to detect particle size distribution, and the relative standard deviation (RSD) of D10, D50, and D90 is required to be less than 15%; Measure particle flowability using a compaction density meter to ensure that the coefficient of variation of compaction density is less than 8%; Simultaneously sampling and observing particle morphology to avoid problems such as adhesion, hollowing, or excessive fine powder.

If there is uneven particle size distribution, the following logic can be used for troubleshooting: if there are too many large particles, priority should be given to reducing the spray rate or increasing the atomization pressure; If the proportion of fine powder is too high, the spray rate can be appropriately increased or the wind speed can be reduced; If the particle size fluctuates greatly, it is necessary to check the stability of the temperature control system to avoid abnormal exhaust humidity. Long term stable control of particle size uniformity can be achieved through closed-loop optimization of parameter adjustment, detection and verification, and problem troubleshooting.