Factors Affecting the Collection Efficiency of Spray Drying

Atomization mode

Pressure atomization produces droplets with a broad particle size distribution and a high fraction of fine powder, posing challenges to powder collection. Two-fluid pneumatic atomization features better controllability, while centrifugal atomization generates droplets with relatively uniform particle sizes.

Atomization pressure / Rotational speed

Excessively high pressure leads to ultra-fine droplets and a sharp increase in ultra-fine powder content, which can hardly be captured by cyclone separators.

Nozzle orifice size

A larger nozzle orifice forms bigger droplets, which increases the drying load. Meanwhile, it reduces the proportion of fine powder and accordingly improves collection efficiency.

Cone angle

An excessively large cone angle causes airflow short-circuit and reduces separation efficiency. A semi-cone angle of 10°–15° is adopted in conventional designs.

Diameter-to-length ratio (D/L)

An improper ratio between the cylinder diameter and length disrupts the uniform distribution of the centrifugal force field.

Inlet airflow velocity

The optimal velocity range is 15–25 m/s. Insufficient velocity results in inadequate centrifugal force, while excessive velocity intensifies turbulence and lowers separation efficiency conversely.

Insertion depth of exhaust pipe

An over-deep exhaust pipe re-entrains the separated powder; an overly shallow one allows particles to escape via airflow short-circuit.

Cut size

The cut size shall be customized according to material properties. In general, particles with a diameter of 2–5 μm and above are required to be effectively trapped.

Solid content

A higher solid content produces larger droplets and powder particles, which helps raise collection efficiency. However, an excessively high solid content will impair atomization quality, so a reasonable balance must be maintained.

Glass transition temperature (Tg)

When the outlet air temperature exceeds the material’s Tg, the material becomes sticky and adheres to the equipment wall, resulting in material loss. Materials rich in sugar or fat feature a low Tg and usually deliver a relatively low collection efficiency.

Hygroscopicity

Highly hygroscopic materials (e.g., traditional Chinese medicine extracts) tend to absorb moisture and agglomerate in collection vessels, leading to a substantial increase in material loss.

Electrostatic effect

In organic solvent systems or low-humidity environments, powder carries static electricity and adheres to equipment walls, thereby reducing collection efficiency.

Inlet air temperature

An excessively high inlet temperature causes rapid crust formation on droplet surfaces, encapsulating internal moisture. The resultant low-density particles are easily carried away by airflow.

Outlet air temperature

A temperature higher than the material’s Tg leads to wall adhesion; an excessively low temperature results in incomplete drying and damp powder agglomeration.

Feed rate

An overly fast feed rate causes insufficient drying and wall adhesion of wet powder; an overly slow feed rate leads to over-drying and a higher proportion of fine powder.

Air volume / Air velocity

Excessively high air volume or velocity carries away fine powder before it settles, resulting in a sharp drop in collection efficiency.

Baghouse dust collector

Attention shall be paid to filter bag materials (PTFE membrane coating / anti-static treatment), dust cleaning frequency and filtration velocity (typically 0.8–1.2 m/min).

Wet scrubber

Applicable to organic solvent systems, with a recovery rate of over 99%. It is not feasible if the powder is soluble in the solvent.

Electrostatic precipitator

Highly efficient for particles smaller than 1 μm, yet it comes with high costs and is rarely equipped on laboratory-scale equipment.

Cone angle of drying tower

The cone angle at the tower bottom must be larger than the material’s angle of repose. Otherwise, powder will accumulate and form bridging, failing to be discharged and eventually suffering thermal degradation.

Pipe elbows

Excessive elbows cause powder deposition and collision inside pipelines, leading to cumulative material loss.

Rapping / Air hammer device

Regularly rapping the tower walls and pipelines reduces powder retention caused by wall adhesion.

System tightness

The system operates under negative pressure. Air leakage at all joints disturbs airflow and undermines separation efficiency.