Spray drying is a continuous single-step process that converts a liquid feed into a dry particulate solid by atomising it into a stream of heated gas. In pharmaceutical manufacturing its dominant application is the production of amorphous solid dispersions (ASDs), where a poorly soluble API is kinetically trapped in a non-crystalline state within a polymer matrix, raising apparent solubility and dissolution rate by an order of magnitude or more.
Roughly 40 percent of marketed drugs and up to 90 percent of development pipeline candidates have solubility-limited absorption. Spray drying is the most widely deployed answer to that problem, and its appeal is that solidification happens in milliseconds, faster than the API can nucleate and crystallise.
The process in five stages
1. Feed preparation. API and polymer are dissolved in a common volatile solvent, typically acetone, methanol, ethanol, dichloromethane, or a binary mix such as acetone and water. Solids loading usually sits between 5 and 20 percent weight by weight. The key requirement is a genuinely molecular solution: any undissolved API acts as a crystallisation seed in the final product.
2. Atomisation. The feed is broken into droplets, and droplet size determines almost everything downstream. Three technologies dominate:
- Two-fluid (pneumatic) nozzles: compressed gas shears the liquid. Fine droplets (5 to 50 micron), ideal for lab scale, high gas consumption at scale.
- Pressure (hydraulic) nozzles: liquid forced through a small orifice at 20 to 200 bar. The workhorse for commercial pharmaceutical spray drying, giving droplets in the 20 to 150 micron range.
- Rotary atomisers: a high-speed disc throws the liquid outward. High throughput, but the wide radial spray pattern demands a large-diameter chamber and is less common in pharma.
3. Drying. Droplets meet the hot gas stream (nitrogen for organic solvents, to keep oxygen below the limiting concentration and avoid an explosive atmosphere). Solvent evaporates from the droplet surface, the surface solute concentration rises, viscosity climbs, and the droplet vitrifies. Contact time in the chamber is typically only 5 to 30 seconds.
4. Separation. A cyclone recovers the bulk of the product, with a bag filter or scrubber capturing fines. Cyclone efficiency drops sharply for particles below 5 micron, which is a real yield issue for fine dispersions.
5. Secondary drying. Spray dried powder leaves the cyclone with residual solvent well above ICH Q3C limits, often 2 to 5 percent. A tray or fluid bed vacuum secondary dry brings this to specification. Solvent held in an amorphous matrix acts as a plasticiser, lowering the glass transition temperature and destabilising the dispersion, so this step is a stability requirement, not a cosmetic one.
Critical process parameters
| Parameter | Typical range | Primarily controls |
| Inlet gas temperature | 70 to 200 °C | Available drying energy |
| Outlet gas temperature | 35 to 70 °C | Residual solvent, product Tg margin, morphology |
| Feed rate | Scale dependent | Throughput and outlet temperature |
| Atomisation gas to feed ratio | 0.5 to 5 (two-fluid) | Droplet size, therefore particle size |
| Solids loading | 5 to 20 % w/w | Particle density, yield, chamber deposition |
| Drying gas flow | Scale dependent | Residence time and thermal capacity |
Outlet temperature is the master variable. It is not set directly but emerges from the balance of inlet temperature, feed rate, and gas flow. The governing rule is that outlet temperature must sit comfortably below the glass transition temperature of the wet, partially solvated particle. Run too close and particles soften, stick to the chamber wall, and the batch fuses. Run too cold and residual solvent stays high.
Designing the dispersion
Polymer selection determines whether an ASD survives its shelf life.
- HPMCAS (L, M, H grades): the current default for spray dried dispersions. High glass transition temperature, low hygroscopicity, and pH-dependent solubility that also suppresses precipitation in the intestine.
- PVP and PVP-VA (copovidone): excellent solubilisation and strong hydrogen bonding with many APIs, but hygroscopic, and moisture uptake depresses Tg towards ambient.
- HPMC: effective crystallisation inhibitor, but solution viscosity limits achievable solids loading.
- Methacrylic acid copolymers: useful where a pH trigger is needed alongside amorphous stabilisation.
Drug loading is typically 10 to 50 percent. Above that, the API contribution to the blend Tg and the reduced polymer separation between drug molecules both raise recrystallisation risk. The practical stability guideline is to maintain a blend Tg at least 50 degrees Celsius above the intended storage temperature, assessed at the relevant relative humidity rather than dry.
Scale-up
Spray drying scales badly if approached as a set of equipment settings. It scales well if approached as a set of conserved physical conditions.
Hold outlet temperature and relative saturation constant. These two define the thermodynamic environment the droplet experiences. Match them across scales and the drying trajectory is broadly preserved even though gas flows may increase a hundredfold.
Match droplet size, not nozzle settings. Lab work on a two-fluid nozzle rarely transfers directly to a commercial pressure nozzle. The transferable quantity is the droplet size distribution, so nozzle selection at scale should be driven by measured or modelled droplet size.
Expect morphology to shift. Particle formation depends on the ratio of solvent evaporation rate to solute diffusion rate within the droplet (the Peclet number). Fast evaporation relative to diffusion produces hollow, low-density shells. Slow evaporation produces denser spheres. Longer residence times at commercial scale commonly shift particles denser, changing bulk density and downstream compression behaviour.
Plan the densification step. Spray dried powder often has a bulk density of 0.1 to 0.3 g/mL and poor flow. Roller compaction or granulation before tableting is usually required, and should be built into the development plan rather than discovered during tech transfer.
Chamber geometry matters. Wall deposition is the most common scale-up surprise. Larger chambers change the spray envelope and the point at which droplets have dried enough to survive wall contact.
Spray drying versus hot melt extrusion
Both produce amorphous dispersions. Spray drying suits thermally labile APIs, high-melting compounds, and cases where fine particle morphology is needed, and it accepts most polymers. Hot melt extrusion avoids solvents entirely, removing residual solvent, solvent recovery, and explosion-proofing burdens, but requires the API to survive brief exposure at 120 to 180 degrees Celsius and a polymer that is processable in the melt. Where an API is soluble in a volatile solvent and heat sensitive, spray drying is generally the lower-risk route.
Frequently asked questions
What particle size does spray drying produce? Typically 5 to 100 micron for pharmaceutical applications, controlled principally by droplet size and solids loading.
Why nitrogen instead of air? Organic solvent vapours in a hot gas stream form an explosive mixture with oxygen. Closed-loop nitrogen systems with condenser-based solvent recovery keep oxygen below the limiting concentration and allow solvent reuse.
How is an amorphous dispersion confirmed? Powder X-ray diffraction (absence of Bragg peaks) plus modulated DSC (a single blend glass transition, no melting endotherm) is the standard pairing, supported by polarised light microscopy.
What is the main stability risk? Recrystallisation. It is driven by moisture uptake, residual solvent, storage above the effective Tg, and mechanical stress during downstream processing.
Disclaimer:
This article is for informational and educational purposes for life sciences professionals. It is not medical advice, formulation guidance, or a regulatory reference. Technical values are indicative and must be verified against current pharmacopoeial and regulatory requirements for your product and jurisdiction. Patients should consult a qualified healthcare professional regarding any medication.