In fluid bed drying technology and fluid bed dryers, GEA Niro have set the standard for many years. Both batch and continuous fluid bed drying systems can be supplied in a wide range of sizes. GEA Niro's fluid bed dryers process any particulate matter between 20 and 5000 microns, with source feeds that include dry or damp powders, filter cake or centrifuge cake.
Open and closed fluid bed spray drying systems can be supplied depending upon product, volatiles, operational safety and environmental requirements.
Open fluid bed drying system
Atmospheric air is used in a once-through system in systems where water is to be removed. Normally a push-pull system is used to balance the pressure to be slightly negative in the free board of the fluid bed. Depending upon the product and available heat source, direct or indirect heating may be applied. The exhaust air is cleaned by, e.g., bag filter, cyclone with or without wet scrubber.
In cases where products pose a dust explosion risk, open cycle systems feature pressure shock resistant components or alternatively semi-closed cycle, self-inertizing layouts can be considered.
Closed cycle fluid bed drying system
This type of fluid bed drying operates with a medium of inert gas (usually nitrogen), which is also recycled within the system. Closed cycled fluid bed spray drying must be used for drying feedstocks containing organic volatiles or where the product must not contact oxygen during drying. These systems are gastight, and addition of the inert gas is controlled by monitoring the oxygen content of the drying gas and the system pressure, which is kept positive. The evaporated volatiles are recovered in a condenser.
Fluid bed spray granulation
This is applied in cases were processing requires that the solids are in particulate form prior to entering the fluid bed dryer. In the special case of fluid bed spray granulation, particles are built up in the fluid bed dryer by spraying a liquid into the fluidized layer particles. Particulate solids ideal for fluid bed drying may be formed in an independent process step such as crystallization, coagulation, or polymerization, followed by mechanical dewatering. Particles may also be formed by spray drying. In other cases, dry or wet solid particles may have to undergo a degree of agglomeration or granulation prior to entering the fluid bed.
GEA Niro's Rewet Agglomerator™ is used to increase particle size to a predetermined size. Fine powders are conveyed to an agglomeration chamber where the powder is dispersed in air and contacted with a spray of liquid. Moist, porous agglomerates are formed and subsequently dried under lenient fluidization conditions in a VIBRO-FLUIDIZERŪ. After cooling, the powder is discharged, classified, and fines recycled for further agglomeration together with fines from the exhaust air dust collector. Rewet agglomeration systems are typically used for food products like coffee, cocoa-sugar, where instant properties and coarse particles are required.
Fluid bed dryer nozzle configurations:
Atomization is achieved by feeding liquid into a high-speed wheel. The rotary atomizer is placed in the ceiling air disperser and operates with a vaned atomizer wheel for non-abrasive feeds, and with a carbide bushing wheel for abrasive feeds. Other designs are available for special applications. Powders produced with this atomizer mode have a narrow particle size distribution with a mean size in the range of 20-50 microns.
Two-Fluid Nozzle, Co-Current Mode
Atomization is achieved by using compressed air to atomize the liquid feed. The nozzle placed in the ceiling air disperser is ideal for heat sensitive feeds and has the added advantage of handling both low and high viscosity feeds. Powders produced with the co-current two-fluid nozzle have a mean particle size in the range of 10-40 microns.