Optimization of Spray Drying Processes for Improved Powder Properties

Spray drying is a widely used technique in the food, pharmaceutical, and materials industries and is used to convert liquids into dry powders with controlled morphologies and moisture content. The uniformity of the resulting powder directly affects downstream processing, product stability, and even end-use performance. In a laboratory setting, achieving consistent and predictable powder properties requires a complete examination of process parameters, feed formulation, and equipment design.

Feed Preparation and Characterisation

The quality of the feed solution or suspension is one of the most influential factors in determining the properties of the final powder. Laboratories need to first charac- terise the feed in relation to viscosity, solid content, and surface tension. High-performance viscometers, ten- siometers, and refractometers are used to generate this data. In spray drying, consistency in feed compo- sition directly impacts atomisation droplet formation. Suspensions need to be homogenised using ultrasonic or high-shear mixers to prevent agglomeration or sedimentation, which can form irregular particle structures.

For thermally sensitive materials, encapsulating carriers such as maltodextrin or gum arabic are blended into the feed to protect labile actives during drying. The solids level must be optimised, not simply maxi- mised. When viscosity climbs above ~300 mPa·s the atomiser produces larger droplets and a thick crust forms early, both of which slow down moisture diffusion. Paradoxically, therefore, feeds that start with less water can leave the tower with higher residual moisture if the solids content is pushed too high. Pilot trials should map viscosity versus solids and set an upper limit that still delivers target moisture at the chosen residence time.

Athanasia M. Goula and Konstantinos G. Adamopoulos demonstrated this paradox while spray-drying tomato pulp on a pilot Büchi B-191: raising the feed concentration from 38 °Brix (≈ 25 wt % solids) to 45 °Brix (≈ 35 wt %) reduced the water load by 29 %, yet the powder’s residual moisture increased from 2.0 % to 3.7 % w/w. Feed viscosity rose from 120 mPa·s to 360 mPa·s, producing larger droplets (D₃,₂ 38 µm → 55 µm) and an early crust that trapped internal water.

Atomisation and Droplet Formation

Atomisation converts the liquid feed into a cloud of droplets whose initial size distribution largely determines the eventual particle‐size profile of the powder. Laboratory spray dryers typically employ rotary atomisers (spinning-disk or cup) when a broad, continuous throughput range is needed, and two-fluid nozzles when finer control of droplet size is required or when processing heat-sensitive formulations at lower feed rates.

Because installing optical probes inside a hot, dust-laden chamber is impractical, droplet size is monitored in the spray plume immediately outside the nozzle through a side window or a bypass cell. In-line laser-diffraction or phase-Doppler analy- sers capture real-time droplet size distribution (DSD) data that are assumed representative of the population entering the dryer.

These measurements guide adjustments to:

• Atomising energy – raising the rotary disk speed or the two-fluid atomising-air pressure produces smaller, more uniform droplets;
• Nozzle geometry – switching to a smaller orifice or a differently grooved disk narrows the DSD;
• Feed delivery rate and viscosity – higher flow or higher viscosity shifts the DSD towards larger droplets, lengthening drying time.

By iteratively tuning these variables, the operator can achieve a narrow droplet distribution (e.g., span < 1.2) that minimises variability in drying kinetics, reduces the chance of hollow “blow-holes,” and prevents the formation of oversized or irregular particles that might otherwise be lost in the cyclone or require downstream milling.

Drying Kinetics, Stickiness Mapping and Process Control

The drying chamber environment needs to be monitored to maintain uniform thermal exposure across all droplets. Inlet temperature, outlet temperature, drying-gas flow rate and residence time determine how quickly moisture is removed. Labora-tory dryers are equipped with thermocouples and humidity probes so these variables can be adjusted in real time.

Inlet temperatures are set between 150 °C and 220 °C, depending on material sensitivity. Higher tempera- tures accelerate drying but may cause thermal degradation or surface hardening that traps internal moi-sture; too-low temperatures risk incomplete drying.
Residence time, influenced by chamber height and airflow patterns, must therefore be calibrated to match the drying kinetics of each formulation.

Stickiness avoidance

Two distinct mechanisms create tacky particles and wall build-up:

1. Thermal stickiness – when the particle surface temperature approaches the glass-transition or critical stickiness temperature (T_g/T*). Laboratories profile T_g versus moisture (DSC or fluidised-bed probes) and overlay the data on a Mollier (psychrometric) diagram; plotting the dryer’s inlet/outlet points then ensures the surface stays below T* during its residence time.
2. Moisture-induced stickiness without a T_g shift. Many carbohydrate and amorphous salt powders can absorb water vapour at low temperatures and still turn tacky. Using a modified Powder Rheometer (Delft Solids Solutions method, published in KONA Powder and Particle Journal, 2023) we measure the shear work increase as a function of relative humidity and predict the humidity band in which the spray dryer must not operate. Combining this humidity-stickiness map with the Mollier plot gives operators a complete operating window—temperature and moisture envelopes that keep the powder free-flowing while maximising yield.
   
   Practical control levers include moderating the inlet temperature, increasing disk speed or atomising-air flow to shorten residence time, and diluting the drying gas with low-dew-point make-up air.

Powder Collection and Post-Drying Characterisation

The powder is collected via cyclone separators or filters. Any post-drying agglomeration or deposition may indicate airflow design issues or electrostatic build-up. Laboratory cyclones are cleaned between batches to prevent cross-conta- mination and loss of collection effi- ciency.

Final powder properties are then assessed:

Including porosity and attrition helps predict tabletting behaviour (pharma) or reconstitution kinetics (foods) and guards against fines loss in pneumatic transport.

Uniform Powders: Putting it all together

Achieving uniform powder proper- ties, consistent size, morphology, porosity, density and moisture demands, tight control over every stage from feed preparation through atomisation and drying to collection. Laboratory-scale testing provides the platform for such controlled experimentation with minimal material use. By combining real-time analytical tools with stickiness mapping on the modified Powder Rheometer, Delft Solids Solutions identifies critical parameters and develops spray-drying protocols that yield more consistent, higher-performing powders.
The approach enhances product quality, supports scalability and meets GMP validation requirements where applicable.