Ask the Experts

How much mixing of brine into seawater occurs in a PX device?

In all high-efficiency positive displacement (isobaric) energy recovery devices (ERDs), some contact between the brine and seawater streams occurs. As a result, these streams mix slightly. The PX rotor contains no pistons or barriers; however, mixing between the brine and seawater streams is limited because the contact time between them is so short – less than 1/20 of a second. The PX rotor is designed so that the interface between the brine and seawater never reaches the end of the rotor, preventing brine from contaminating the membrane feed.

How does the amount of mixing vary if multiple PX units are operated in parallel, if the PX is not operated at an optimal design point or if a PX is damaged or worn out?

In an ERI PX device operating with equal high- and low-pressure feed flows, mixing results in a salinity increase at the membrane feed of less than 3% at a 45% water recovery rate. This increase in salinity, negligible in many RO desalination applications, is the same when operating with a single PX device or with many. It is constant over the operating flow range of most PX models and does not change over the lifetime of a device. Mixing can be reduced or eliminated with excess seawater fed to the PX array. ERI routinely guarantees low mixing performance.