The FRESH-Frac design uses selective distillation to condense clean water from waste vapor while the vast majority of contaminants remain in the gas phase. To accomplish this, dry air enters the nozzle and is heated and accelerated by the addition of low-grade heat. Next, the fast-moving, hot air comes into contact with a vortex generator; this creates a swirl motion in the air. Then, wastewater vapor enters the nozzle and mixes with the swirling air, the wastewater vapor is at a temperature 1°C above the saturation temperature of water. This humid air mixture continues through the converging nozzle. As the velocity of the air increases due to the reduction in cross-sectional area, the temperature of the air will decrease as the thermal energy is converted to kinetic energy. Thus the clean water will condense out of the humid air at 1°C below the saturation temperature, while most contaminants do not. As the water condenses the latent heat of condensation is released. In order to maintain the temperature in the nozzle, the latent heat is rejected to a jacket heat exchanger with cold feed wastewater flowing over the nozzle. After the throat of the nozzle, there is misty flow. The swirling motion pushes the water droplets to the periphery of the nozzle where the clean water is collected by an in-line demister. The dry air and gaseous contaminants pass through the demister and flow through a diffuser to reduce the velocity and increase the temperature of the flow, thus allowing more energy to be recouped from the waste stream via heat exchangers. WET lab is currently focusing separately on the development of demister, thermal ejector and…as discussed below in detail.
Separation Experiments
In order to determine the water quality of the treated water at the end of the FRESH-Frac process, as well as the efficacy of water recovery in the treatment process, a separation experiment was developed. In this experiment contaminated water mixes with a flow of hot air and as the misty flow passes through the condensation section, water condenses and samples are taken along the length of the test section. These samples are analyzed using gas chromatography techniques to determine the amount of contaminants in each sample.
Demister
Treated water is removed from the FRESH-Frac system via an inline swirl tube demister. This component separates the purified water from the contaminated gases by taking advantage of the density difference between the liquid and gaseous phases of the flow, much like a cyclone separator. A swirling flow pattern is generated by an upstream stationary fan to induce a centrifugal force on the liquid water droplets, forcing them to the periphery of the tube. Once enough water droplets have gathered on the inner surface of the enclosure, they coeles into a streamline that can be collected in the demister gap and drained into a clean water collection tank. The contaminated gases exit through the center in the demister and are captured for measurement purposes prior to being released to the environment. Swirl tube demister experiments test the influence of different parameters on the efficiency of the clean water collection as well as the cross contamination of carrier gases passing through the clean water collection chamber. Some of the studied physical parameters include tube diameter and length, the demister gap thickness, and the diameters of both the gas and liquid outlets. Operating conditions such as swirl angle, swirl pitch, and fluid mass flow rates are also investigated. The result of this work will be a design guide for swirl tube demisters that will include a correlation of experimental data that can be used to maximize the purified water collection efficiency. Several rules of thumb developed from experimental observation will also be included to ensure there is proper justification for all swirl tube demister design considerations.
Mixing Nozzle
In the FRESH-Frac system, waste steam is suctioned into the nozzle by the Venturi effect. Depending on the condition in the nozzle, different amounts of steam may be necessary to ensure 100% relative humidity in the nozzle but to avoid condensation before the separation section. Currently, experiments are being done to test the effect of different nozzle geometries on the ratio of steam entering the nozzle to air flowing through the nozzle. The ratio is defined as the suction ratio. Based on current experiments and simulations suction ratios between 0.4 and 4.9 are achievable just by changing the geometry of the nozzle.
