Spray Drying.

 

Definition

  • Drying is defined as the removal of liquid from a product usually with application of heat.

  • Equipment used for the purpose of drying are called “Dryers”.

Spray Drying.

  • Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. 

  • This commonly used method of drying of many thermally-sensitive materials such as foods and pharmaceuticals or materials which may require extremely consistent, fine, particle size. 

  • Air is the heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the product is oxygen-sensitive then nitrogen is used.

Principle of Spray Dryer:

  • Spray dryer operates on the mechanism of “Convection”.

  •  In a continuous process, the liquid feedstock is dispersed as a mist of fine droplets into a stream of hot drying gas, all within a cylindrical drying chamber.

  • As the drying is so rapid, materials are dried in a fraction of a second which helps to make the process useful for thermal sensitive materials such as foods, extracts and other organic substances.

Construction of Spray Dryer:

  • The main components of spray dryer are as follows,

    •  Drying Chamber.

    • Hot Air Supply System.

    • Feed Supply System.

    • Atomizing device.

    • Powder / Fines recovery system.

  1. Drying Chamber:

    • Made up of stainless steel.

    • Consists of a large cylindrical chamber with a short conical bottom.

  2. Hot Air Supply System:

    • Placed on the roof of the drying chamber.

    • It consists of,

      1. Supply Fan.

      2. Air Filters.

      3. Air heaters.

      4. Air dispenser.

    • It supplies heated air for drying.

    • Air is prefiltered and then heated.

    • Air is dispensed via a tangential inlet.

  3. Feed Supply System:

    • Placed on top of the drying chamber.

    • It consists of the following components,

      1. Feed Tank: Contains the feed.

      2. Feed pump: Supplies feed to the atomizing device.

      3. Filter: Filters the concentrate feed.

      4. Feed line: transfers the feed.

  4. Atomizing Device:

    • Transforms the feed into large no. of droplets of predefined size.

    • They are of following types,

      1. Spinning Disc (Centrifugal) Atomizers.

      2. Pneumatic Atomizers.

      3. Pressure nozzle Atomizers.

  5. Powder / Fines Recovery System:

    • The short bottom of the drying chamber is fitted with this assembly.

    • The main purpose of the assembly is to separate powder particles as per size and fine recovery.

    • The assembly contains,

      1. Cyclone separator.

      2. Bag filter.

      3. Etc.

Working:

  • The drying in a spray dryer involves following stages,

    1. Atomization of the feed solution.

    2. Contact of sprayed droplets with hot air.

    3. Evaporation of moisture from droplets.

    4. Particle separation.

  1. Atomization of the feed solution:

    • The feed concentrate from the feed tank is conveyed to the atomizer with help of a pump which results in formation of the droplets of predefined size.

    • The droplets from spray provide enormous rise in surface area resulting in a quick drying.

    • Various types of atomizers are employed as per the product requirement.

  2. Contact of sprayed droplets with hot air:

    • The formed droplets come in contact with the hot air inside the drying chamber.

    • The moisture from the droplet evaporates quickly leaving behind solid particles.

    • If the liquid used is inflammable then air can be replaced with Nitrogen.

  3. Evaporation of moisture from droplets:

    • Evaporation of moisture from the droplet quickly leaves behind solid particles.

    • This step is crucial as it determines the morphology of final product.

    • The vapors generated during evaporation build pressure inside the droplet.

    • The effect of the pressure depends on nature of the dried surface layer as,

      • If the dried surface layer is porous and rigid there will be no effect on the particles.

      • If the dried surface layer is less porous and rigid particles will be broken.

      • If the dried surface layer is non porous and plastic there will be expansion of particles. (Formation of expanded and hollow particles is a main feature of Spray Drying).

  4. Particle Separation:

    • Separation of particles is done by employing cyclone separator.

    • The fines can be collected by using a bag filter.

Uses:

  1. Improved compatibility of excipients e.g. Spray dried lactose.

  2. Improved flow properties.

  3. Improved compressibility of substances e.g. Spray dried crystals of Acetazolamide show excellent compressibility than original crystals.

  4. Useful for doing coating and Microencapsulation.

  5. Size of particles can be predetermined and hence also improves solubility and dissolution profiles.

  6. Very useful for drying of thermolabile substances as heating time is very very less.

  7. Useful for drying of heat sensitive materials

Advantages:

  1. Very less drying time.

  2. Best for thermolabile substances.

  3. Continuous process.

  4. Applicable for a variety of products like thermolabile, corrosive, explosive etc.

  5. Particle size can be predetermined.

  6. Very fast process.

  7. Final product has better solubility.

  8. Final product has better flow properties.

  9. Scalable processes can be used at lab scale as well at an industrial scale.

  10. Labor costs are less.

Disadvantages:

  1. Higher investment.

  2. Higher maintenance cost.

  3. Skilled operators needed.

  4. Bulky assembly.

  5. Final product is hygroscopic, requires special conditions for storage.

Commonly Asked Questions.

  1. With a well labelled diagram describe construction and working of Spray Dryers.

  2. Write a short note on the Spray Drying.

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