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Spray Drying

Following you find a short process description of spray drying

 

  1. The Spray Drying Process
  2. Atomization
    1. One Fluid Nozzle Pressure Nozzle
    2. Two Fluid Nozzle Outside Mixing
    3. Two Fluid Nozzle Inside Mixing
    4. Disc Atomizer
    5. Ultrasonicatomizer
    6. Other Atomizers
  3. Drying
  4. Product Separation
  5. Utility Equipment
  6. Explosionsprotection
  7. Control System
  8. Energyconsumption

 

1.  Spray Drying Process

The process transforms a solution, suspension, emulsion with a certain solidcontent into a powder of the solid in one step. The spraydryer belongs to the class of convective dryers, where the energy for evaporation of the solvent (in most cases water) is transported by heatconduction and convection from the drying gas to the product. The drying takes place after an intimate mixing of the sprayed liquid with the drying gas. The drying gas has a low relative humidity and therefore the solvent evaporates from the product. The drying gas may be in most cases air or an inertgas or a waste gas from an other process. In a special type, the steamspraydryer, the evaporated solvent itself may be the drying gas. The spraydryer is a short time dryer. Drying is finished within seconds.

2.  Atomization

To increase the drying rate the liquid to be dryed will be atomized into very small droplets.

Example:
A sphere of 1 kg, a density of 1000 kg/m3 and a surface of 4.8 x 10-2 m2 gives by atomization into 20 micrometer spheres 2.4 x 1011 droplets and a total surface of 300 m2.

This enormous increase in specific surface area, which is the heat- and masstransfer area too, allows short drying times even at very low temperatures and within seconds. Its necessary, because the dryed product must not stick to the dryer wall.

The droplet size after atomization depends on several factors:

  • the liquid properties viscosity, surface tension and density,
  • the applied atomizer and its parameters.

The determining viscosity is that at high shear rates, because in the atomizers the liquid will experience high shear rates. There are three types of liquids known:

  • linear Newtonian liquid (most of the solutions belong to this group)
  • structural viscous liquids, with viscosity increase at high shear rate (example starchsuspension)
  • under shear decreasing viscosity showing liquids (example: ceramicsuspension with liquifier)

We find the following common used atomizers:

  • Onefluidnozzle, hollowcone nozzle,pressure nozzle, airless nozzle
  • Twofluidnozzle outside mixing, pneumatic nozzle
  • Twofluidnozzle inside mixing
  • Discatomizer
  • Ultrasonicatomizer
  • Exotic atomizers

2.1 One Fluid Nozzle, Hollow Cone Nozzle, Pressure Nozzle

In this nozzle type the liquid rotates and will be accelerated to a high outlet velocity with following ligament break up. Therefore these nozzles need a high liquid pressure from 5 to 200 bar. The twisted flow inside of the nozzle creates the hollow cone spray, which breaks up into a rather narrow droplet distribution.

Advantages

  • dropletsize 50 to 400 micrometer (dustless endproducts produced in the spraydryer in an one stage process)
  • low energy consumption for atomization
  • narrow droplet distribution even at low pressures from 10 bar on,
  • cheap nozzle construction
  • control of dropletsize with prepressure and/or number of nozzles and/or size of nozzles
  • no moving parts (see explosionprotection)

Disadvantages

  • may plug due to particles in the liquid
  • abrasion at the nozzle mouth

2.2   Twofluidnozzle, outside mixing

The atomization occurs outside of the nozzle by acceleration of the liquid through an expanding gas. The liquid will be supplied in most cases at low pressure or due to the reversed water jet pump effect at slight underpressure. The outlet velocity of the liquid is low and around 1 m/sec. The atomizing gas instead will be accelerated at the outlet tip to the local sound velocity. Common pressures for the atomizing gas range from 1.5 to 5 bar. On the liquidside common diameters range from 1 to 10 mm. Therefore this nozzle type is unsensitive to particles in the liquid to be atomized.

Advantages:

  • abrasionfree
  • unsensitive against plugging
  • liquids with high viscosity (up to 20.000 mPasec = centipoise) atomizable
  • narrow droplet distribution  at condition of fine spray (D50 from 10 to 80 micrometer)
  • pressureless liquid feed
  • very good control of droplet size by variation of the atomizing gas pressure and/or the liquid throughput
  • no moving parts (see explosionprotection)

Disadvantages

  • broad droplet distribution at condition of coarse spray (D50 from 80 to 400 micrometer)
  • atomizing energy needed as compressed gas

2.3 Two Fluid Nozzle Inside Mixing

This nozzle type mixes the liquid and atomizing gas inside of the nozzle. The mixture flows out from the nozzle as a two phase mixture. The droplet formation principle is the same as at the inside mixing two fluid nozzle.

Advantages

  • liquids with high viscosity (up to 20.000 mPasec = centipoise) atomizable
  • narrow droplet distribution  at condition of fine spray (D50 from 10 to 80 micrometer)
  • smaller energy consumption then outside mixing two fluid nozzle
  • very good control of droplet size by variation of the atomizing gas pressure and/or the liquid throughput
  • no moving parts (see explosionsprotection)

Disadvantages

  • abrasion
  • possibility of plugging due to dry out of liquid inside of the nozzle
  • broad droplet distribution at condition of coarse spray (D50 from 80 to 400 micrometer)
  • liquid supply under pressure
  • atomizing energy needed as compressed gas

2.4   Discatomizer

The atomizing principle is acceleration of the liquid in the centrifugal force field of a rotating disc and following jet breakup. At the perimeter of the disc velocities of up to 300 m/sec will be used.

Advantages

  • liquids with high viscosity (up to 20.000 mPasec = centipoise) atomizable
  • narrow droplet distribution  at condition of fine spray (D50 from 10 to 50 micrometer)
  • smaller energyconsumption for atomization then inside mixing nozzle
  • good control of droplet size by variation of the rotation speed  and/or the liquid throughput

Disadvantages

  • abrasion
  • intensive maintenance required
  • much more expensive then nozzleatomizers
  • moving part (see explosionprotection)
  • broad droplet distribution at condition of coarse spray (D50 from 80 to 400 micrometer)

2.5   Ultrasonicatomizer

This atomizer induces in the liquid an oscillation at the eigenfrequency, which breaks up the liquid to uniform droplets. The droplet distribution  compared to the atomizers above is very narrow. Atomizers with and without contact to the liquid to be atomized exist. The latter have the advantage, that no cavitation at the oscillatorsurface may occur. In a cavitating bubble at the surface some solid may be deposited, which results in plugging or malfunction. In spraydrying processes ultrasonic atomization is not yet used commonly, because the atomizers are very expensive and sensitive and not suitable for very high throughputs.

2.6   Other Atomizers

There exist other atomizing processes:

  • drop from a sieveplate
  • between two moving cylinders stretching of liquid to filaments and break up

The dropping process is in use for melted metal and for prilling. The cylinderatomizer has not yet found an application for bigger throughputs.

Many other atomizing processes have been invented, but have not found an economical application.

3.   Drying

As soon as the droplets have been formed, they have to mix with enough hot drying gas. The mixing is done with atomizers 2.1 to 2.4 already because of the high velocities of the droplets at the dryer entrance. For one fluid pressure nozzles one can measure an entrained gasmassflow, which is around ten times bigger then the atomized liquid massflow. One observes around a spray a kind of reversed water jet pump effect. The atomizer 2.5 and 2.6 have problems due to the low dropletvelocities.

The design of the dryer top has to be adopted to the atomizertype. For nozzleatomizers a rather low drying gas entrance velocity is needed. High entrance velocities are needed for the discatomizer, to tend down the horizontal spray and to avoid built up at the atomizer level. Dryers for discatomizing have for the same reason a more quadratical form. Instead nozzleatomizer dryers are more narrow and high.

If the dryed particles reach the wall at the end of the dryer, they have to be dry and not sticky. For this reason the dryer itself has to be big enough, otherwise the built problem will be present. In case of a big enough dryer the operation safety is good, if for any reason the recept of the liquid or the product has to be changed.

For certain difficult products additional equipment like finesreturn, additional cooling, integrated or external fluidized bed or other process extensions are applicable.

4.   Product Separation

Spraydryers normally are equipped with cyclone and/or bagfilters. To reach the required residual dust content bagfilters are common. In case of frequent product changes cyclon-bagfilter or cyclone-scrubber are used. CIP cleanable bagfilters are the latest state of the art, but costly.

The design of the filters should be so, that even at changed productspecifications a safe operation is possible.

5.   Utility Equipment

The drying gas has to be moved through the plant with fans. The energy for evaporation of the solvent shall be provided by heating up the drying gas.
Heatertypes:

  • direct heating with flue gas of a burner (natural gas, liquified gas, oil)
  • direct heating with flue gas from an other process. Condition: no high solvent content in the gas
  • indirect heating with heat exchanger
    • gas or oilfired
    • steam heated
    • heat carrier heated
    • electrical heater

The drying gas may be filtered to avoid contamination of the product.

6.   Explosionprotection

For dustexplosive products and for explosive mixtures of solvent and oxigen (if both conditions apply this is called a hybride mixture) explosionprotection is needed. A list of protection possibilities:

  • avoidance of explosive athmosphere
  • avoidance of igniters (i.e. no moving parts or velocities less then 1 m/sec)
  • pressure pulse resistant design with
    • pressurerelief flaps
    • rupture discs
    • explosionsuppressionsystem
  • explosionpressure resistant design
  • closed circuit with inert drying gas
  • closed circuit with self inertization with a burner

To avoid damage from fire sometimes fireextinguishers will be used.

7.   Control System

The control system of a dryer is mostly made with a digital control system or for smaller plants in conventional technique. The control system may be designed for fully automatic operation. This is desired for a constant product quality. In a fault condition the dryer will go to a safe condition.

8.   Energyconsumption

The energyconsumption of the spraydryer we determine with a simplified energybalance:

    Q = m * hv * (XE - XA)   (1)

with

Q   energy for evaporation kJ
m   mass dry  kg
XA  initial humidity content based on dry
XE  final humidity based on dry
hv    heat of evaporation solvent  kJ/kg

Because the spraydryer is a convective dryer, the energy for evaporation will be transferred as  sensible heat of the drying gas:

    Q=mg * cpg * (TA - TE)   (2)

with

Q    transferred heat kJ
mg  drying gas mass  kg
cpg  specific heat drying gas  kJ/(kg*K)
TA    initial temperature drying gas  C
TE    final temperature drying gas  C

The drying gas has to be heated from ambient to the initial temperature condition:

    Qg=mg * cpg * (TA - TU)

with

Qg   energy to heat up the drying gas  kJ
mg  drying gas mass  kg
cpg  specific heat drying gas  kJ/(kg*K)
TA    initial temperature drying gas  C
TU   ambient temperature dryer  C (here we use the annual mean temperature)

The plantmanager is interested for the energyconsumption based on a kg of the final dryed product. With the equations above we calculate:

Qg/m=  hv * (XE - XA) *(TA - TU)/(TA - TE)

If the price of the energy is known, we know now the energetical production costs.

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