Acknowledgments
The challenge of developing a better expression model was given to a team of PhD students from Dutch Physics Departments in the context of a week-long Physics for Industry event organized by the Dutch research foundation FOM in Fall 2015. The arrangements made by FOM are highly appreciated. In this week, the basis was laid for the model described in this paper which resulted from the MSc final year research project of the second author. The authors acknowledge the practical guidance in lab and pilot plant trials by Richard van Leeuwen, Johan Draisma and Hanung Sonneveld, all FrieslandCampina.