Operation of the RO Kinetic® energy recovery system: Description and real experiences☆

Along with the older style centrifugal energy recovery devices (ERDs), there has been a recent proliferation of ERDs that employ positive displacement mechanisms. These “pressure-equalising” or isobaric ERDs transfer the energy from the membrane reject stream directly to the membrane feed stream. This direct, positive displacement approach results in a net transfer efficiency up to 97% [R.L. Stover, Seawater reverse osmosis with isobaric energy recovery devices. Desalination, 203 (2007) 168–175]. A number of devices have been developed to recover pressure energy from the brine reject stream. These ERDs fall into two general categories: centrifugal and isobaric devices. In order to avoid the efficiency losses associated with the energy-conversion steps inherent in centrifugal devices, in the 1980s engineers developed positive-displacement (PD) isobaric devices for seawater reverse osmosis (SWRO) plants. These devices place the SWRO reject and raw feed in contact in pressure-equalising or isobaric chambers. Current manufacturers of isobaric devices include Calder (DWEERTM) [2], KSB (SalTec DT) ,[3], ERI® (PX) ,[4] and RO Kinetic® (RO Kinetic® is a patent of the Canary Islands engineer Manuel Barreto). The RO Kinetic® is a novel energy savings system for RO desalination plants that implies an enormous advance in the specific energy consumption (SEC) reduction in this type of facilities. With this device, we are able to obtain a SEC of slightly higher than 2.20 kWh/m3, which approaches the theoretical minimum limit for energy consumption in seawater reverse osmosis processes [5]. The physical principle that serves as the foundation for the RO Kinetic® system is the “incompressible property of liquids”, making it possible to submit a mass of water to a given pressure without expending any energy. Pressure exchangers are used to put this principle into practice. This article describes the state of the art of the RO Kinetic® energy recovery device; it explains the way it works and the advantages of using this type of ERD in SWRO plants. Furthermore it shows some operational data of different SWRO desalination plants with the RO Kinetic®. © 2009 Elsevier B.V. All rights reserved.

Energy recovery devices have become essential to all SWRO operations, primarily because they significantly reduce energy consumption in these systems. Quantifying device performance involves employing relatively straight-forward hydraulic calculations. ERD selection, like all other aspects of engineering design, involves some degree of compromise. Isobaric ERDs deliver higher efficiency than centrifugal devices, but centrifugal devices are generally better characterised and are easier to maintain and operate. The RO Kinetic® energy recovery device provides high energy transfer efficiency, practically no maintenance, great ease of operation. In the early days of seawater desalination, the energy consumption was about 20 kWh/m3 [8]. The yields improvements in the plants components (especially in the membranes), made possible to reduce gradually the specific energy consumption of the process, that with systems like RO Kinetic® reaches values around 2.20 kWh/ m3. From experience gained in full-scale plants we can conclude that we have an energy recovery device able to reduce considerably the specific energy consumption and therefore the operational costs, approaching the minimum theoretical limits in the energy consumption values of reverse osmosis seawater desalination process. The system adaptability allows a very versatile installation, being able to locate the RO Kinetic ® closed loops in very diverse configurations. The device valves are concentrated occupying a reduced space. These advantages are also transferred to the fact that they do not affect to other facility components such as membranes, valves, pipes, etc. Due to its extremely slow operating regime, it becomes a reliable device with very simple maintenance. Since 2002, once surpassed the initial experiences, RO Kinetic® becomes a technology accessible to any operator or manufacturer of this type of facilities. The attainable values of specific energy consumption with the RO Kinetic® system are close to the theoretical minimums, investigation in the future should concentrate on other aspects of desalination.