Energy Recovery Devices
The Energy Recovery Devices (from now on ERD) turn up as a consequence of the energy waste from the Reverse Osmosis process since the inlet water enters the membranes until the brine (rejected pressurized water) is driven back to the sea.Booster Pressure to equalize the two streams after leaving the membranes to be returned to the membranes again.
The following chart, shows the process above mentioned:
At the beginning of the 80's, the desalination plant concept was associated to one High pressure pump that fed to a rack of pressure vessels containig six or seven membranes each. The energy consumption was lower (practically half or a third than using a MED process) but in terms to be considerably reduced to fewer figures.
It was achieved with the inclusion of reverse pumps, but it was when the Pelton Turbines appeared when the Energy Consumption reached around 4.5 kWh/m3.
After almost ten years, no one could ever imagine that fewer values could be achieved. The thing is not only were these figures reached but also they have been largely improved. The goal was highly surpassed with the ERDs whose functioning is shown in Figure 1 above.
There are two main kinds of ERD:
1. Centrifugal: They have a rotating element similar to a gun trigger where sea water involves its spin and this movement is accomplished by the recycled brine inlet. The main example of this kind of ERDs is the ERI.
One of the main problems of them is their noise which is a little bit high.
2. Pistons: The movement that causes the Energy Consumption Reduction is created by a piston that pressurizes the sea water helping its inlet to a rack. Its name is DWEER (Double Working Energy Exchange Recover).
One of the main problems of them is they need a very large room to be installed within the facility and the assembling time is rather high in comparison with the centrifugal ones. Anyway the values that can be reached using either one or another are almost the same. An example is shown in the picture below where the central column can be easily appreciated.
Similar devices were manufactured by the Lyng, Ltd and KSB (Recuperator) but unfortunately they have the same problems as DWEER.
The room they need is much higher than the required for the ERI, although this one is much noisier. (at around 94 dB(A))
Lately, with the construction of large desalination plants (more than 100,000 m3/day) the energy consumption reduced until a figure such as 4.0 kWh/m3 but it was very difficult reaching lower values.This impossibility was achieved when the ERD turned up.
With these fittings the Energy Consumption has been stablished even under 2.0 kWh/m3.so that nowadays, a desalination plant is not conceived without these kind of tools.
The streams move under a combined action of both the High Pressure Pump and the Pressure Energy from the concentrated brine.
3. RO Kinetics: This kind of ERD is one of the pioneers and this is the reason why it is mentioned apart.
These kind if systems take advantage of the residual pressure from the brine to drive it back in a closed loop by means of a recirculation pump that help the high pressure pump work easily.
The fluid movement is spontaneously originated, once initiated. This is generated by the vacuum intake stream of the sea water intake together with the recirculation pump and the ERD. Honestly speaking, I do not like the Booster concept as it was used during the first years obrine from the first stage to the second one. This is the reason why I prefer call it recirculation pump as it works in a closed loop and equalizes the pressure drop from the reverse osmosis process. These concepts will be explained in a future article within this series.
The situation like this, the fact is a desalination plant design is not linked any more to an only High Pressure Pump per rack of membrane but a High Pressure Pump for a group of three or four racks depending on the capacity of the plant. This is the beginning of a new concept called Pressure Center that will be treated in a future article.
About Author:Alejandro Núñezis an Industrial Engineer specialized in Water Treatment. His career has been linked to both Sea Water and Sewage Water Treatmentsince 1997 until now. You can read his other interesting articleshere.