Analysis and Modelling of Desalination Brines

University dissertation from Lund University

Abstract: Seawater desalination constitutes an important water supply to the population bordering the Arabian Gulf, the Mediterranean Sea and the Red Sea. The three regions represent about 11.8% of the world land area and the countries hosted approximately 9% of the world population in 1950 and 2008 and are also projected to do so in 2050. The results obtained for desalination capacity in the study area were 62%, 58% and 60% of the world capacity for 1996, 2008, and 2050, respectively. The increase in the recovery ratio is considered an important factor in this study. In 1996 this ratio was about 30 to 35%, and in 2008 it was 40 to 45%, although in some plants it reached up to 50%. The gulf desalination capacities were obtained as 50, 40 and 45% of total world capacity at the end of 1996, 2008 and 2050 (prognosis) respectively. All Middle Eastern countries suffer from a shortage of water along with increasing demand due to high population growth. Desalination can be a cost-effective way to produce fresh water and possibly electricity. A suggested solution for Sinai and the Gaza Strip involves the building of a joint power and desalination plant, located in Egypt close to the border with Gaza. This joint Egypt-Palestine project would increase the water supply by 500,000 m3/d and the power supply by 500MW. The result emerged from field work and the two experiments can also be applied to the design of this project. Water and salt mass balances for the Dead Sea were modelled by including and excluding the water from the proposed Red Sea Dead Sea Canal project, RSDSC. Precipitation, evaporation, river discharges, ground water flows, input/output from potash companies and salt production in addition to brine discharge were included in the models. The mixing time in the Dead Sea was modelled using a single-layer (well-mixed) and a two-layer (stratified) system. An efficient method for increasing the dilution rate of brine water discharged into the sea is an inclined negatively buoyant jet of a single port or multi-diffuser. Two small-scale experimental studies were conducted to investigate the behaviour of a dense jet discharged into lighter ambient water. The first lab-scale experiment concerned the benefit of the initial angle of inclined dense jets, where the slope increased for the maximum levels as a function of this angle. An angle of 60 deg. led to a better result than 30 or 45 deg. An empirical prediction was found based on five geometric quantities to be considered in the future plan. Field work measurements have been conducted in Cyprus, where the brine from a desalination plant and the Mediterranean Sea coastline have been investigated at the Eastern Mediterranean University. The result from the measured data demonstrates the need for more than one outfall (a series of outfalls) to the sea to avoid or minimize environmental impact on the coastline. The result also agreed well with simple, two dimensional mathematical models assuming Gaussian distribution. The calculated bottom slope is about 7.4%, which can explain the pollution that appears at the coast close to the discharge point. Thus small slope could be one reason. The second lab-scale experiment studied the near and intermediate fields of negatively buoyant jets. The dilution along the flow was increased by about 10% and 40% with bottom slope and bottom slope together with a 30 degree jet inclination, respectively. This method can be applied in brine discharge outlets to recipients to minimize concentration and facilitate faster and greater dilution. Over 16% bottom slope and more field work is needed for comparison with this result. It was found that an inclination of 30 degrees with 16 % bottom slope was more sustainable in designing brine discharge outfall. A Matlab code can be used to describe the lateral spreading and centerline dilution of buoyant jets and plumes in near and intermediate fields.

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