1. Department of Chemical and Pharmaceutical Engineering, Faculty of Chemistry and Pharmacy, University of Sofia, 1 J. Bourchier Ave., 1164 Sofia, Bulgaria
2. Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 8, 1113 Sofia, Bulgaria
3. Faculty of Mathematics and Informatics, University of Sofia, 5 J. Bourchier Ave., 1164 Sofia, Bulgaria
4. School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom