One particularly efficient type of pressure exchanger is a rotary pressure exchanger. This device uses a cylindrical rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside a sleeve between two end covers as seen in Figure 1. Pressure energy is transferred directly from the high pressure stream to the low pressure stream in the ducts of the rotor. Some fluid that remains in the ducts serves as a barrier that inhibits mixing between the streams. This rotational action is similar to that of an old fashioned machine gun (see Figure 2) firing high pressure bullets and it is continuously refilled with new fluid cartridges. The ducts of the rotor charge and discharge as the pressure transfer process repeats itself.
|Figure 1||Figure 2|
|-|| 3D rendering of how pressure energy is transferred|
directly from a high pressure stream
to a low pressure stream.
|Gatling Rapid-Fire Gun Model 1862 Type II .58 .|
The performance of a pressure exchanger is measured by the efficiency of the energy transfer process and by the degree of mixing between the streams. The energy of the streams is the product of their flow rates and pressures. Efficiency is a function of the pressure differentials and the volumetric losses (leakage) through the device computed with the following equation:
where Q is flow, P is pressure, L is leakage flow, HDP is high pressure differential, LDP is low pressure differential, the subscript B refers to the low pressure feed to the device and the subscript G refers to the high pressure feed to the device. Mixing is a function of the concentrations of the species in the inlet streams and the ratio of flow rates to the device. Equation 2 is an expression for volumetric mixing that was derived by mass balance.
Where C is the concentration of a dissolved species and the subscript D refers to the high-pressure outlet of the device. Reverse Osmosis with Pressure Exchangers One application in which pressure exchangers are widely used is reverse osmosis (RO). In an RO system, pressure exchangers are used as energy recovery devices (ERDs). As illustrated in Figure 3, high pressure membrane concentrate from the membranes is directed to the ERD. Pressure transfers from the high pressure concentrate stream [G] to a low pressure feedwater stream [B]. Pressurized feedwater flows from the ERD [D], driven by a circulation pump. This stream merges with the output of a high pressure pump [C] to form the membrane feed stream [E]. The concentrate leaves the ERD at low pressure [H], expelled by the incoming feedwater flow [B]. Figure 3 – Schematic Diagram of an RO Process with Pressure Exchanger Energy Recovery Devices Pressure exchangers save energy in these systems by reducing the load on the high pressure pump. In a seawater RO system operating at a 40% membrane water recovery rate, the ERD supplies 60% of the membrane feed flow. Energy is consumed by the circulation pump, however, because this pump merely circulates and does not pressurize water, its energy consumption is almost negligible: less than 3% of the energy consumed by the high pressure pump. Therefore, nearly 60% of the membrane feed flow is pressurized with almost no energy input.
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.