In vacuum membrane distillation (VMD), a saline liquid stream is vaporized and driven across a hydrophobic porous membrane by lowering the pressure on the vapor side of the membrane. The desalinated vapor is condensed and cooled to recover the latent heat of vaporization. VMD systems are attractive because their relatively low operating temperatures and pressures result in lower capital cost. However, existing VMD systems suffer from poor thermal efficiency (Gained Output Ratio, GOR<1) as drawing a higher vacuum on the vapor side of the membrane to increase throughput results in a lower condensation temperature and lower energy recovery effectiveness. Multi-stage flash (MSF) desalination systems offer superior thermal efficiency (3<GOR<7), but the high temperatures and pressures required for these systems limit their feasibility in small-scale or off-grid applications.
The novel desalination cycle described in this invention addresses these problems by achieving throughput and thermal efficiency comparable to MSF systems while retaining the capital cost advantage of VMD systems.