UC Davis

Desalination is used in over 120 countries worldwide and is a viable option for providing fresh water for many regions worldwide. The chief approaches to desalination are Thermal Desalination (TD) and Reverse Osmosis (RO). While there have been significant improvements in membrane technology to enhance the efficiency of the RO process, TD research has been relatively less developed. In this dissertation, the prospect of improving TD is discussed with a specific focus on Multistage Flash (MSF) desalination. In MSF, the brine flows through a bank of tubes in multiple stages, in which steam flashes from the preheated brine. The steam condenses on the outer surface of the tubes and is collected as fresh distillate water. Condensers and the stages are made of exotic alloys to withstand harsh operating conditions and are capital intensive. MSF desalination plants have heat transfer bottlenecks related to phase change heat transfer external to the tubes and internal flow within the tubes. In this work, the potential to reduce the footprint of the tube condenser, and hence the size of the stages, by use of multi-port minichannel condensers is explored. The condenser footprint reduction is achieved by two methods: by tightly packing condenser plates and enhancing the overall heat transfer coefficient. While the former reduces the condenser footprint, the latter directly reduces the condenser heat exchange area and hence the condenser material cost. An analytical model is developed to predict the performance of a baseline tube bank condenser and proposed multi-port minichannel condensers. A parametric study using the model is undertaken to assess the impact of variations in geometrical parameters on the multi-port condenser performance, results of which are used to optimize the condenser architecture. The results of the parametric study point to the possibility of a 90% size reduction corresponding to a 17% material cost reduction. A numerical model is developed to test the effect of plate spacing on condensation. From the results, it is concluded that for the geometry of the proposed condenser plate spacing of 6 mm does not adversely affect condensation. An experimental study on condensation using multi-port mini-channels is performed, and the results validate the model within a mean average error of 7 percent. Improvements in the performance of the condenser are observed using hydrophobic coatings. Preliminary long-term salt deposition experiments are performed for CaCO_3 to quantify the potential adverse effects of fouling on condenser performance. These initial results do not show any change in pressure drop over 48 hours of testing at 60 °C. Overall, a potential path to a reduction in the size of MSF condensers and stages is demonstrated in this work.