Vacuum degassing of aqueous tetrafluroethane (R134a) solution during seawater desalination utilizing gas hydrate

Abstract

Seawater desalination technology utilizing gas hydrates consists of three primary processes: the formation of hydrate slurry from seawater and treatment gas, the removal of concentrated saltwater from the hydrate slurry, and the decomposition of dewatered hydrate back into treatment gas and potable water. Tetrafluroethane (R134a) was found to be a favored treatment gas, since it can form clathrate hydrate under relatively low pressure at near deep seawater temperature of around 275 K. Thorough recoveries of treatment gas dissolved in removed concentrated brine, in hydrate decomposed water, and in waste salt water are crucial to reduce the release of greenhouse gas, and to improve the economic feasibility. The present study investigated the influences of mechanical agitation and ultrasonic stimulation upon vacuum degassing of decomposed hydrate water which is saturated with R134a at 1 bar pressure. The experimental results indicate that the target concentration of 500 ppm can be achieved within 30 s of degassing. Energy economy analyses were also performed under different combinations of agitation and ultrasound intensities to provide the basic knowledge needed to design degassing reactors for seawater desalination pilot plant producing 2 tons of potable water per day. If the degassing time exceeds 87 s, the energy consumption is estimated to cost more than the value of being recovered R134a.