Following you find a short process description of spray drying
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.
To increase the drying rate the liquid to be dryed will be atomized into very small droplets.
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 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:
We find the following common used atomizers:
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.
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.
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.
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.
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.
There exist other atomizing processes:
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.
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.
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.
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.
The drying gas may be filtered to avoid contamination of the product.
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:
To avoid damage from fire sometimes fireextinguishers will be used.
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.
The energyconsumption of the spraydryer we determine with a simplified energybalance:
Q = m * hv * (XE - XA) (1)
Q energy for evaporation kJ
Q=mg * cpg * (TA - TE) (2)
Q transferred heat kJ
The drying gas has to be heated from ambient to the initial temperature condition:
Qg=mg * cpg * (TA - TU)
Qg energy to heat up the drying gas kJ
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.