The Key to Successful Spray Drying

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Producing droplets of specific size and surface area by atomization is the most critical step in the spray drying process.  The degree of atomization, under a set of spray drying conditions, controls the drying rate, and therefore the required particle residence time, and therefore the spray dryer size.  All of the atomizing techniques can give good average particle size control, but there are major differences in the particle size distribution created.  The most commonly employed atomization techniques are:

Pressure Nozzle Atomization

Pressure Nozzle

  • A spray is created by forcing the fluid through an orifice.  The energy required to overcome the pressure drop is supplied by the spray dryer feed pump.
  • The narrowest particle size distribution is possible with this technique.  Must be used when minimization of “fines” is important to the product.
  • The average particle size produced, for a given feed, is a function of the flow per nozzle and the spraying pressure.
  • Spraying pressure depends on feed characteristics and desired particle size, and can range from 300 to 3,000 psig.
  • The most energy efficient of the atomization techniques.
  • Requires a positive displacement, high pressure feed pump, such as a plunger pump or a piston/diaphragm pump.
  • Requires routine changing of the nozzle internal pieces, usually made of tungsten carbide.  Changing schedule depends upon the application.
  • Requires a minimum of approximately 0.10 GPM feed rate, depending upon the size of un-dissolved particles in the dryer feed, due to potential plugging with the small orifice required.
  • With multiple nozzle spray dryers, a problem with one nozzle does not shut operations down.
  • Control of spray dryer wall buildup can be achieved through variations of the spray angle.

Two-Fluid Nozzle Atomization

Two-Fluid Nozzle

  • A spray is created by contacting two fluids, the feed and a compressed gas.  The atomization energy is provided by the compressed gas, usually air.  The contact can be internal or external to the nozzle.
  • A broader particle size distribution is generated.
  • The average particle size produced for a given spray dryer feed is primarily a function of the feed flow per nozzle, and the compressed gas rate and pressure.
  • The least energy efficient of the atomization techniques.
  • Useful for making extremely fine particles (10-30 micron) because of relatively high wear resistance.  Also for the small flow rates typically found in pilot scale dryers.
  • Requires periodic changing of the air and liquid caps.
  • Can typically use any type of spray dryer feed pump.
  • Control of the spray angle is limited.

Centrifugal Atomization

Komline-Sanderson Rotary Atomizer and Wheel

  • A spray is created by passing the fluid across or through a rotating wheel or disk.  The energy required for atomization is supplied by the atomizer motor.
  • A broader particle size distribution is typically generated.
  • The average particle size for most products is limited to under 100 micron due to wall build up issues.
  • The average particle size produced for a given feed is primarily a function of the diameter and RPM of the wheel.
  • Requires relatively high gas inlet velocity to control wall buildup, which can increase the  amount of fines produced.
  • Can generally be run for longer periods of time without routine maintenance.
  • Usually the most resistant to wear.  Requires periodic changing of wheel inserts, usually made of tungsten carbide.
  • Control of wall buildup is minimal, due to direction of spray (horizontal) and broad particle size distribution, forcing the dryer to be relatively large in diameter.
  • Capital cost of the atomizer is typically high.  Comparatively larger diameter spray dryer can increase capital cost.  As with any high speed rotating machine, maintenance costs are high.  Design of dryer roof and atomizer support add to fabrication cost.
  • A problem with the atomizer will shut down spray drying operations.

Particle Size Distribution Comparison

Atomization Energy Example Comparison

(Based on Atomizing 10 GPM of Feed to 70 Micron Droplets)

  1. Pressure Nozzle: Feed Pump @ 1200 psig = 10 HP Total
  2. Centrifugal Atomizer @ 9000 RPM @ 25 HP, Feed Pump @ 30 psig @ 3 HP = 28 HP Total
  3. Two-fluid Nozzle: 180 SCFM Compressed Air @ 80 psig @ 30 HP, Feed Pump @ 80 psig @ 5 HP = 35 HP Total

Mesh to Micron Conversion This document is in Adobe PDF format.

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