Experimental Investigation of Solar-Thermal Desalination Platform Leveraging Dynamic Flash Evaporation and Swirl Flow Separator

Abstract

Abstract Commercial thermal desalination plants usually leverage static flash evaporation and vapor separation processes that occur separately in large chambers. Depending on the level of purity — the product can be used for potable water (for human consumption), for agriculture or ranching, or as input for industrial processes (such as in injection wells in oil and gas production operations). Currently, static methods such as Multi Stage Flash (MSF) or Multi Effect Distillation (MED) are widely used (in addition to Reverse Osmosis) for desalination. These static methods occupy large land area (large footprint). This in turn drives up the capital and production costs of the resulting purified water obtained in these techniques. Desalination processes that leverage evaporation and vapor separation in the same chamber (dynamically) have smaller form factors which confers lower cost of desalination. Thus, the motivation of our study is to develop a novel apparatus to simultaneously generate vapor by flash evaporation and separate the produced vapor in the same apparatus. The novel apparatus is geared for desalination of sea water, remediation of produced water from process-industries and other sources of saline water (such as brackish water) that are deemed unfit for human consumption. The end goal of the project is to develop a solar-thermal desalination platform by leveraging hot saline water as input from solar ponds. In this experimental study, the thermal-hydraulic performance of a prototype (lab-scale) dynamic vapor generation and swirl flow phase separation apparatus is explored for determining the efficacy of this novel concept. Heated water from a constant temperature supply tank (that is comparable to a solar pond in real life) is passed through injection passages into the flow-separation apparatus. As the water flows through the injection passages, vapor bubbles are generated inside the flow passages due to local superheating of the liquid caused by frictional pressure drop. Conversion of liquid into vapor continues as the liquid-vapor mixture flows through the injector ports and eventually the mixture enters a larger separation tube tangentially. Due to the tangential injection of the two-phase mixture, a centrifugal force acts to separate the water and vapor inside the separation tube. The liquid is pushed to the periphery (i.e., the walls) of the separation tube while the vapor forms a stable core at the center. A vapor retrieval tube is then positioned at the center of the vapor core to extract vapor which is then condensed inside the condenser. The formation of the vapor core is demonstrated for different operating conditions (supply liquid flow rates) and maximum superheat (temperature difference between supply tank and condenser) ranging between 45–52°C. Based on this study, the optimal operating conditions for maximizing the thermal conversion upstream of the test section are presented.