1. Department of Chemical Engineering, University of Patras and FORTH-ICE/HT, Patras GR26504, Greece
2. Department of Mathematics and Statistics, University of Cyprus, P. O. Box 20537, 1678 Nicosia, Cyprus
3. Particle Technology Laboratory, Department of Mechanical and Process Engineering, Institute of Process Engineering, ETH Zurich, Sonneggstrasse 3, CH-8092 Zürich, Switzerland