1. Helsinki University of Technology, Industrial Chemistry, P. O. Box 6100, FIN-02015 HUT, Finland, Microchemistry Ltd., P. O. Box 132, FIN-02631 Espoo, Finland, Fortum Oil and Gas Scientific Services, Technology Center, P. O. Box 310, FIN-06100 Porvoo, Finland, and Fortum Oil and Gas Oy, P. O. Box 110, FIN-00048 Fortum, Finland