Modelling the absorption of binary mixtures in plate heat exchangers

Modelling the absorption of binary mixtures in plate heat exchangers

Nuijten, M.P.

Infante Ferreira, C.A. (mentor)
Vlugt, T.J.H. (mentor)
Kapteijn, F. (mentor)
Kirkenier, J.A. (mentor)

Mechanical, Maritime and Materials Engineering

Process and Energy

Chemical Engineering

2015-08-31

Ocean thermal energy conversion (OTEC) is currently developed as a clean and sustainable energy technology for offshore application in tropical areas. The technology shows promising possibilities in combination with the Kalina cycle, a thermodynamic cycle for the generation of energy from low temperature difference heat sources using a working fluid mixture. Due to the differences in boiling point between the components in a mixture and the resulting temperature glide, this allows more heat to be exchanged and thus a higher thermal effectiveness of the overall cycle. Ammonia-water is identified to be a suitable candidate for application in the cycle, but still relatively little is understood about the performance of different components in the cycle when this mixture is used. The condenser in the cycle exchanges heat between the working fluid and cold deep ocean water. In case a mixture is used, the process in this heat exchanger is described as absorption. In this study, an attempt is made to model the relevant hydrodynamics and heat and mass transfer processes in the heat exchanger, given typical operating conditions and a specified heat exchanger geometry. A plate heat exchanger is used for the absorber, due to specific advantages of this type of heat exchanger. A review of the used thermodynamic cycle, working fluids and their properties, and the heat exchanger type is included in this study. Also, literature on the hydrodynamics, pressure drop, heat transfer and mass transfer is discussed. A review of modelling approaches is provided before an extensive description of the implemented model is given. The model is stable, the predicted parameters converge to constant values during the iterative procedure, and the predictions show reasonable agreement with other work. The heat transfer may be overestimated as a result of the assumed hydrodynamics in the model. Besides, jumps in parameter profiles result from full condensation and the corresponding change in flow pattern. The modelling approach shows promise to be extended to make predictions for different operating conditions, working fluids and heat exchanger geometries. In the future, the model could be developed into a tool for optimization of the absorber performance. A preliminary experiment was performed for validation of the model. Comparison of the numerical results with the experiment and previously developed numerical models points out that the prediction of the model for the heat exchanger performance is promising. More experiments of sufficient accuracy and reliability are required for a better validation of the model, and to allow predictions of the heat exchanger performance for a wider range of conditions.

modelling
binary mixtures
absorption
plate heat exchangers
OTEC
Ocean Thermal Energy Conversion
two-phase flow
flow patterns
low-grade heat

http://resolver.tudelft.nl/uuid:d13450ab-fc3c-47a8-8023-55557319d30c

Student theses

master thesis

(c) 2015 Nuijten, M.P.