Aqueous processes and their limits
Fluid bed technology is characterized by excellent heat and mass transfer. Each individual product particle is suspended by the process gas to achieve perfect mixing and homogeneous temperature distribution. These well-known advantages of convection drying over contact dryers are utilized for temperature-sensitive products such as yeasts, enzymes and other reactive substances. Efficient drying in the fluid bed enables a variety of other processes such as spray granulation of suspensions and solutions, agglomeration of powders or coating of individual particles with functional films. These options are state of the art for aqueous processes. However, extraction, precipitation or crystallization processes often require organic solvents, and the outstanding quality of some functional coatings is based on water-insoluble polymers.
More explosion and environmental protection
The handling of organic solvents requires additional protective measures, which can be categorized as follows:
Avoidance of ignition sources and flammable mixtures
- Utilization of inert gas
- Circuit operation under vacuum conditions
Sufficient strength of the systems
- 12 bar pressure shock resistant design
- Operation in a vacuum with standard systems
Avoidance of emissions
- Condensation of the solvent in circulation operation
- Exhaust air purification for fresh air/exhaust air by scrubber or thermal post-combustion
A safe and economical option is the vacuum fluid bed, which combines the advantages of both types of process, convection and vacuum drying. Keeping below the minimum ignition pressure ensures explosion protection. By dispensing with inert gases, operating costs are reduced and components such as heating coils and condensers can be designed to be significantly smaller. Special features such as the single-substance nozzles used for atomizing binders and the drying behavior of the solvents open new possibilities in product development, as the following findings from case studies under normal pressure and under vacuum demonstrate.
Faster, less residual solvent
It is known from contact drying under vacuum that the evaporation of solvents works much more efficiently at low partial pressure and boiling temperature. Nevertheless, drying times of several hours are common, as the heat supply reaches the product via the heated wall through heat conduction. Compared to conventional drying, the fluid bed has the advantage of convection. Here, under normal pressure, inert gas or air causes the individual particles to float; in a vacuum, the vaporized solvent fulfils this purpose. The mass flow of the circulating vapor is low but provides the necessary pressure difference for turbulence due to a higher inflow velocity. At the same time, the gas flow is used for heat input and transfer. The latter depends on the properties of the solvent, the system pressure and the temperature. To maintain the vacuum, the vaporized solvent is removed from the circuit and condensed – a plus point in terms of sustainability.
A major advantage over a fresh air/exhaust air process or the use of inert gas is the additional driving force created by lowering the system pressure. Depending on the solvent, residual values in the two and low three-digit ppm range can be achieved, and the overall drying time can be significantly reduced.
Improved compressive strength for agglomerates
Another typical fluid bed process is the agglomeration of powders into free-flowing, dust-free granules. To investigate the influence of vacuum on the formation of granules, tests were carried out at the Glatt Technology Center in Weimar under normal pressure (with a nitrogen cycle) and under vacuum conditions. As a model substance, lactose powder was sprayed with a 5.5 % binder solution of Eudragit S 100 and acetone/methanol. Atomization in a vacuum was carried out using a single-substance nozzle, whereby the droplet size was set by adjusting the spray rate. The binder content in the final product was 9 %. The agglomerates (Fig. 1+2) were comparable both in terms of particle size distribution (Fig. 3) and bulk density. However, there was a clear difference in strength: the agglomerates in the vacuum were characterized by more than twice the compressive strength (Fig. 4) – a clear advantage for allergenic products, for example.
Drying of yeast cells
The preservation of living organisms, places special demands on the drying process. Bacteria are therefore preserved by freeze-drying as standard. Depending on the strain, yeasts lose their enzyme activity and their ability to divide at temperatures > 45 °C. The fluid bed has become established for drying baker’s yeast (yeast extrudates with 60 – 70 % moisture): Temperatures of max. 30 °C and very dry process air enable the necessary short drying times. Various drying tests in the vacuum fluidized bed have shown that even aqueous yeast extrudates benefit from the lowering of the boiling point. As a result, the survival rate was significantly higher compared to drying under normal pressure. When comparing different parameter settings in vacuum operation, the temperature and drying speed proved to be decisive. Process gas inlet temperatures below 30 °C and a system pressure of 100 mbar led to the best survival rates. These results are promising for the drying of live cells.
Copyright: DLG-Verlag GmbH
Copyright: Glatt