Spray Dryer Configuration


An Important Aspect of Design

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Atomization
Gas/Spray Mixing
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GAS/SPRAY MIXING

The technique used in spray dryers to mix the hot gas with the atomized spray is critical to the success of the spray drying process, not only for evaporation, but for particle size control, product density, and heat degradation.  The variables that affect how the spray is mixed with the hot gas depends upon the spray dryer configuration and the orientation and velocity of the inlet entry point. 

SPRAY DRYER CONFIGURATION

Co-current Flow

  • Both the spray and gas flow downward through the dryer
  • The feed is sprayed into the hottest gas, increasing the instantaneous rate of drying.
  • Can be used with all atomization techniques.
  • Can be configured for the most turbulent gas/spray mixing, increasing the instantaneous rate of drying.
  • Can be configured for the slowest mixing, which can provide the narrowest of particle size distributions.
  • Performance generally not affected by production rate or product changes, as airflows can be changed with little effect on particle trajectory time.
  • Best choice for heat sensitive products because the driest particles are exposed to the lowest temperatures.

Counter-current Flow

  • The feed sprays down and the gas flows up through the spray dryer.
  • Sometimes used for the production of large particle sizes because the up-flow of air slows the particle “fall time”, allowing for extra drying time.
  • The feed is sprayed into the coolest gas, decreasing the instantaneous rate of drying and directionally producing a higher density product.
  • Can be used only with pressure nozzle or two-fluid nozzle atomization techniques.
  • Performance affected by production rate or product changes if temperature profile is important to product quality.
  • Product degradation or burning can occur because the driest particles are exposed to the highest gas temperatures.
  • Inlet or outlet gas bustle increases fabrication costs.

Mixed Flow (Fountain Flow)

  • The gas flows down and the feed sprays up, then comes down with the gas.
  • Sometimes used for the production of large particle sizes because the particle trajectory is increased, allowing for extra drying time, and decreasing the overall spray dryer height required.
  • The feed is sprayed into the coolest gas, decreasing the instantaneous rate of drying and directionally producing a higher density product.
  • Can be used only with pressure nozzle or two-fluid nozzle atomization techniques.
  • Performance generally not affected by production rate or product changes, as airflows can be changed with little effect on particle trajectory time.
  • Product degradation or burning can occur because the driest particles are exposed to the highest gas temperatures.

MIXING TECHNIQUE

With each spray dryer configuration, the mixing technique can vary from slow, parallel flow to fast, turbulent flow.  Each technique has its merits; the choice is a function of the application objectives.  If evaporation (drying) is the only real objective, a fast mixing technique is used.  If other objectives such as particle size are important, a slow mixing technique should be used. 

Slow Mixing

  • Used when minimization of fines are important (narrow particle size distribution) because the velocity of gas at the point of atomization is lowest.
  • Uses full diameter perforated plates for gas distribution.
  • Higher product density is possible.
  • Generally requires a taller spray dryer
  • Can be used only with pressure or two-fluid nozzle atomization

Fast Mixing

  • Used when instantaneous drying rate is more important than a narrow particle size distribution.
  • Typically uses a vane ring at the gas inlet to induce a cyclonic rotation of the gas.
  • Produces a broader particle size distribution, because the high turbulence further atomizes the droplets.
  • Can be used with all atomization techniques (required for centrifugal atomization).

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