Supervisor:
Associate Professor Kim Sørensen

Co-Supervisor:
Associate Professor Jakob Hærvig

Assessment Committee:
Associate Professor Vincenzo Liso, AAU Energy (Chair)
Associate Professor Antonio Buffo, Department of Applied Science and Technology, Institute of Chemical Engineering, Politecnico di Torino, Italy
Dirk Lucas, Computational Fluid Dynamics Division, Institute of Fluid Dynamics, Technical University of Dresden, Dresden, Germany

Moderator:
Associate Professor Vincenzo Liso

Abstract:

To comply with the sulphur regulations introduced by the International Maritime
Organisation in January 2020, ship owners can install exhaust gas cleaning
equipment onboard vessels such as a wet scrubber. The wet scrubber
removes sulphur from the exhaust gas by washing it with seawater or fresh
water. Several countries and regions worldwide have prohibited the use of
scrubbers operating in open-loop mode, which limits the use of scrubbers to
closed-loop operations. During closed-loop operation the freshwater is recirculated over the scrubber and the concentration of pollutants continuously
increases. To comply with the discharge legislation, a high-speed centrifugal
separator is used to sediment the pollutants in an efficientway. To increase the
efficiency of the high-speed separator, a pre-treatment system, consisting of a
pipe flocculator is installed to aggregate the micron-sized particles.
The aggregation and breakage of micron-sized particles are studied using a
multiphase computational fluid dynamics (CFD) model where the continuous
and dispersed phases are described using the two-fluid Eulerian approach.
The number density function of the dispersed phase is tracked using the population balance equation (PBE) which is solved using the class method. Interfacial momentum transfer between the two phases is included in the study to
achieve a two-way coupled model. High-fidelity, large-eddy simulations are
carried out to analyse the decay of secondary motion and the downstream turbulence characteristics of the flow through a 180◦ bend. The particle strength
parameter is validated against an experimentally measured particle size distribution measured in the Alfa Laval Test and Training Centre.
Results show that the most critical geometrical parameter that influences
the particle size distribution is the pipe bend radius. Furthermore, it is shown
that when the Reynolds number is significantly large (Red > 30, 000), the
influence of the bend radius becomes negligible as the naturally occurring
turbulence breaks the largest aggregates.
This PhD project has contributed to achieving a better understanding of
inline flocculation systems with a dilute disperse phase. The proposed modelling
approach allows an in-depth study of the aggregation and breakage of
micron-sized particles.