(42d) Efficient Precipitation of Spray Dried Submicron Particles for Pharmaceutical Applications Using a Two-Stage Electrostatic Precipitator

The bioavailability enhancement of poorly water soluble drugs can be achieved by the decrease of the particle size of active pharmaceutical ingredients (API). An innovative process is the spray drying with spray conditioning to produce submicron particles. A resulting challenge is the collection of these dispersed particles from a gas flow. The separation in electrostatic precipitators (ESP) is a common technique for air purification purposes where powder particles can be collected rather independently from particle size. An adapted ESP design is necessary to achieve high separation efficiencies and a robust process.

The ESP design in this work uses the precipitation method of Penney filters which separates charging and collection into two zones. Submicron particles tend to follow the gas flow in appearing turbulences. The setting of a laminar flow in the second zone prevents the appearance of eddies, provides the necessary time for the particles to move to the collection electrode and enhances the efficiency compared to single stage ESPs.

The dimensioning is depending on various assumptions which have to be revised. Its design of perpendicular mounted electro-polished stainless steel pipes allows an easy cleaning and simplifies the setup. The gas flow is induced by a fan which draws the air through the precipitator. Particularly with the use of active pharmaceuticals this formation prevents the API to leak before it gets separated.

Several experiments are conducted with the model substance mannitol (PEARLITOL® 160 C, Roquette Pharma®, Lestrem, France) to examine the ESP and characterize its behaviour in long term tests. The crucial parameters in the charging process are the residence time as well as the electric adjustment. These constraints were examined to enhance the product recovery. Based on these tests it is possible to determine a suitable charging length as well as the dimensions of the precipitation zone.

The efficiency of the ESP is measured with an optical particle counter (1.109, GRIMM AEROSOL Technik, Ainring, Germany). To ensure a characterization of the precipitated material the collected powder is studied in a scanning electron microscope (SEM, H-S4500 FEG, Hitachi High Technologies Europe, Krefeld, Germany).

The separation process is conducted with spray dried mannitol submicron particles at a voltage level of 20 kV and a current density of 7.5 mA/m² at start. The precipitation efficiency decreases after 3 hours due to the dust layer and the dust resistivity which is increasing. To enhance the precipitation time it is necessary to examine the first stage of the ESP. In theory the current flow is transferred mainly in the ionization stage. By cleaning this part of the filter during the separation process, the resistivity can be decreased and particles can be treated with a constant amount of charge. An increasing dust layer leads to a higher resistivity and therefore to a lower current flow. For further investigations a cleaning step of the charging zone was added to enhance the separation duration. The results show a separation efficiency in a continuous experiment of higher than 99% for 10 hours for 500 nm particles. These high efficiencies result in a production rate of 1 g/h. In conclusion the first stage is the limitation in the separation process. If a particle doesn’t receive charge in the first stage it cannot be deflected in the second stage. In an upcoming work the functionalization of the separated particles shall be examined.