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Understanding How Droplets Become Particles
In food, chemical, and pharmaceutical production, powder particle size and its distribution influence flowability, dissolution rate, bulk density, and downstream processing performance.
Spray drying first atomizes a liquid feed into droplets, then uses hot air to evaporate water or solvent and form particles. Controlling the final particle size therefore requires attention to two stages: the size of the droplets produced during atomization, and how those droplets shrink, form shells, or agglomerate during drying.
Droplet size and final particle size are closely related, but they are not identical. The following five factors deserve particular attention when optimizing particle size.
Adjusting atomization conditions is usually the most direct way to control droplet size. However, different atomization methods require different control parameters.
When atomizer wheel design, feed rate, and feed properties remain approximately constant, increasing rotational speed generally produces finer droplets and often finer powder.
However, rotational speed alone is insufficient for comparing different systems. Wheel diameter, flow passage design, and wear also affect atomization performance. When comparing different models, the peripheral speed of the wheel must also be considered.
With the same nozzle and similar feed conditions, increasing liquid pressure generally helps reduce droplet size. However, nozzle orifice size, internal geometry, and wear also influence the spray pattern and particle size distribution.
Changes in pressure often affect feed flow as well, so throughput and drying load must be checked at the same time.
For nozzles that use compressed air or another gas to atomize the feed, increasing the effective atomizing gas flow and gas-to-liquid ratio generally produces finer droplets when the liquid feed rate remains approximately constant.
Solids content and viscosity are often related, but they influence particle size through different mechanisms.
When initial droplet size and final particle structure are similar, a higher solids content means that each droplet contains more solid material. The resulting dried particles are therefore generally larger.
This does not necessarily mean that the particles will be denser. Final density also depends on shell formation, shrinkage, pore formation, and formulation properties.
Under otherwise similar conditions, higher viscosity generally makes the liquid more difficult to break up. This may produce larger droplets or a broader droplet size distribution. Excessive viscosity can also affect pumping and atomization stability.
Viscosity should therefore be measured at the actual feed temperature, rather than relying solely on room-temperature data.
For the same dry solids output, increasing feed solids content generally reduces the amount of water entering the drying chamber and lowers the evaporation load.
However, upstream concentration energy, pumping requirements, and atomization performance must also be considered. Solids content should be selected to balance stable feeding, effective atomization, and product quality.
Feed rate affects both atomization and drying.
For rotary atomization, increasing the feed rate while keeping rotational speed and feed properties approximately constant may produce a thicker liquid film and larger droplets. This does not necessarily indicate equipment overload; it reflects a change in atomization conditions.
For pneumatic atomization, increasing liquid flow while holding atomizing gas flow constant reduces the gas-to-liquid ratio, often producing coarser droplets.
Pressure nozzles must be assessed in terms of pressure, orifice size, and nozzle quantity. Feed rate and pressure generally cannot be adjusted entirely independently.
In addition, increasing the feed rate under unchanged hot air conditions generally increases the evaporation load and may lower the outlet air temperature. If drying becomes insufficient, moist particles may stick together, increasing the apparent particle size while also causing higher moisture content or wall deposition.
When adjusting feed rate, monitor outlet temperature, final moisture content, wall deposition, and powder recovery at the same time.
Inlet air temperature affects drying rate, but its effect on final particle size does not follow a universal trend.
Under faster drying conditions, some materials form a surface shell earlier. This can limit further shrinkage and produce hollow or porous particles. Other materials may experience shell collapse, breakage, or increased fines.
It is therefore inaccurate to assume that higher temperatures always produce larger particles. Lowering the temperature also does not guarantee dense, solid particles.
Process optimization should consider the following together:
Inlet air temperature: Influences the initial driving force for drying.
Outlet air temperature: Provides an important operating indicator of the system’s heat and moisture balance.
Drying airflow and air humidity: Affect heat and moisture transfer.
Residence time: Influences whether particles can reach the target moisture content.
For materials prone to stickiness, the relationship between particle temperature, moisture content, and sticky behavior also requires attention. Agglomeration may increase the measured particle size without meeting the intended product requirements.
Surface tension is an important liquid property in atomization. Under otherwise similar conditions, higher surface tension generally resists liquid breakup and may produce larger droplets.
However, formulation changes often affect surface tension, viscosity, and film formation during drying simultaneously. Their overall effects must be confirmed through testing.
For some products, emulsifiers or other formulation adjustments can improve atomization. However, surfactants should not be added solely to reduce particle size without evaluating their broader effects. Additives may also change foaming, hygroscopicity, product purity, stability, and application performance.
For suspensions, the particle size and dispersion state of the feed solids also matter. Agglomerates in the feed may affect nozzle passages, atomization stability, and the final powder particle size distribution.
Adjustment | Common Trend | Additional Considerations |
Increase rotary atomizer speed | Droplets generally become finer | Wheel design, feed load, and permissible speed |
Increase liquid pressure at a pressure nozzle | Droplets generally become finer with the same nozzle | Flow changes, wear, and spray pattern |
Increase the gas-to-liquid ratio at a pneumatic nozzle | Droplets generally become finer | Actual gas flow, nozzle design, and powder recovery |
Increase feed solids content | Particles generally become larger at similar initial droplet sizes | Viscosity, atomization stability, and particle structure |
Increase feed rate | Droplets may become coarser under some conditions | Atomization method, outlet temperature, and moisture content |
Change inlet air temperature | The direction of particle size change depends on the material | Shell formation, shrinkage, breakage, and agglomeration |
Reduce surface tension | May promote finer droplets under some conditions | Formulation suitability and changes in other product properties |
These are common trends rather than fixed rules. Actual results depend on feed properties, equipment design, and interactions between operating parameters.
Comparing only the average particle size may conceal excessive fines or an oversized coarse fraction. Compare particle size metrics such as D10, D50, and D90 using the same measurement method and dispersion conditions. Check moisture content, bulk density, flowability, and powder recovery at the same time.
It is also important to specify whether the measurement represents dispersed particles or powder with its agglomerates intact. Different dispersion conditions can produce different results for the same sample.
During testing, begin with a fixed formulation and measurement method, then adjust the main parameters within a controlled range. After initial screening, evaluate interactions between parameters.
Particle size optimization must support product quality, stable operation, and effective powder recovery. Finer powder is not always better. The goal is a particle size distribution that meets downstream application requirements and can be produced consistently.
Our LPG-5 spray dryer features interchangeable rotary atomizer and spray nozzle configurations, allowing users to select a suitable atomization method according to feed properties and testing objectives, and compare powder formation and product performance.
If you need to address excessively fine powder, a broad particle size distribution, or unsatisfactory flowability, please share your material name, solids content, feed viscosity, current operating conditions, and target particle size. Our technical team can help evaluate equipment configurations and testing options.
Material testing and equipment selection enquiry, please contact: info@machtechdryer.com
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