PX for RO

Attended a seminar yesterday on Sea Water Desalination, jointly organized by Energy Recovery Inc (ERI) and Dow Water and Process Solutions. Seawater desalination is very energy intensive and to bring down the energy cost, the hydraulic energy of high pressure rejects has to be recovered. This is conventionally done through a turbine (Francis Turbine or Pelton Wheel) that is directly coupled to the feed pump. But the inefficiencies of turbine and pump limit the energy recovery to approximately 65%. ERI has introduced a revolutionary (literally!) device called the PX Exchanger with claims of 98% energy recovery. This consists of a spinning cartridge made from ceramic, where momentum of the high pressure reject fluid is transferred to the low pressure feed through actual contact. The contact time is however so small, of the order of 0.05 seconds, that the increase in salinity of the feed due to mixing with the concentrated rejects is only to the extent of 2.5%., which corresponds to an osmotic pressure increase of 1.3 bar. Since this increase in salinity is a function of the recovery in the RO module, ERI needs to work in close co-operation with the RO solution provider. The ERI website has a Power Model to carry out this preliminary calculation. PX devices are available in modular constructions with capacities ranging from 4.5 to 50 m3/hr. Adopting PX for energy recovery, the power required for seawater RO can be brought down to 3.5 KWH/m3.