Pressure Exchanger - Energy Recovery and Pressure Exchange Systems

Energy Recovery and Pressure Exchange Systems

Seawater desalination plants have produced potable water for many years. However, until recently desalination had been used only in extreme circumstances because of the very high energy consumption of the process.

Early designs for desalination plants made use of various evaporation technologies. The most advanced are the multi-stage flash distillation seawater evaporation desalinators, which make use of multiple stages and have an energy consumption of over 9 kWh per cubic meter of potable water produced. For this reason large seawater desalinators were initially constructed in locations with very low energy costs, such as the Middle East, or next to process plants with available waste heat.

In the 1970s the seawater reverse osmosis (SWRO) process was developed which made potable water from seawater by forcing it under high pressure through a tight membrane thus filtering out salts and impurities. These salts and impurities are discharged from the SWRO device as a concentrated brine solution in a continuous stream, which contains a large amount of high-pressure energy. Most of this energy can be recovered with a suitable device. Many early SWRO plants built in the 1970s and early 1980s had an energy consumption of over 6.0 kWh per cubic meter of potable water produced, due to low membrane performance, pressure drop limitations and the absence of energy recovery devices.

An example where a pressure exchange engine finds application is in the production of potable water using the reverse osmosis membrane process. In this process, a feed saline solution is pumped into a membrane array at high pressure. The input saline solution is then divided by the membrane array into super saline solution (brine) at high pressure and potable water at low pressure. While the high pressure brine is no longer useful in this process as a fluid, the pressure energy that it contains has high value. A pressure exchange engine is employed to recover the pressure energy in the brine and transfer it to feed saline solution. After transfer of the pressure energy in the brine flow, the brine is expelled at low pressure to drain.

Nearly all reverse osmosis plants operated for the desalination of sea water in order to produce drinking water in industrial scale are equipped with an energy recovery system based on turbines. These are activated by the concentrate (brine) leaving the plant and transfer the energy contained in the high pressure of this concentrate usually mechanically to the high-pressure pump. In the pressure exchanger the energy contained in the brine is transferred hydraulically and with an efficiency of approximately 98% to the feed. This reduces the energy demand for the desalination process significantly and thus the operating costs. Therefrom results an economic energy recovery, amortization times for such systems varying between 2 and 4 years depending on the place of operation.

Reduced energy and capital costs mean that for the first time ever it is possible to produce potable water from seawater at a cost below $1 per cubic meter in many locations worldwide. Although the cost may be a bit higher on islands with high power costs, the PE has the potential to rapidly expand the market for seawater desalination.

By means of the application of a pressure exchange system, which is already used in other domains, a considerably higher efficiency of energy recovery of reverse osmosis systems may be achieved than with the use of reverse running pumps or turbines. The pressure exchange system is suited, above all, for bigger plants i.e. approx. ≥ 2000 m3/d permeate production.