Pressure Exchanger
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A pressure exchanger transfers pressure
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
from a high pressure fluid stream to a low pressure fluid stream. Many industrial processes operate at elevated pressures and have high pressure waste streams. One way of providing a high pressure fluid to such a process is to transfer the waste pressure to a low pressure stream using a pressure exchanger. One particularly efficient type of pressure exchanger is a rotary pressure exchanger. This device uses a
cylindrical A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infini ...
rotor with longitudinal ducts parallel to its rotational axis. The rotor spins inside a sleeve between two end covers. Pressure energy is transferred directly from the
high pressure In science and engineering the study of high pressure examines its effects on materials and the design and construction of devices, such as a diamond anvil cell, which can create high pressure. By ''high pressure'' is usually meant pressures of th ...
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 firing high pressure bullets and it is continuously refilled with new
fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...
cartridges. The ducts of the rotor charge and discharge as the pressure transfer process repeats itself. 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 volumes and pressures. Efficiency is a function of the pressure differentials and the volumetric losses (leakage) through the device computed with the following equation: \eta=\frac=\frac\qquad\qquad(1) 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 volumes to the device.


Reverse osmosis

One application in which pressure exchangers are widely used is reverse osmosis (RO). In an RO system, pressure exchangers are used as
energy recovery Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but mos ...
devices (ERDs). As illustrated, high-pressure concentrate from the membranes is directed to the ERD The ERD uses this high-pressure concentrate stream to pressurize the low-pressure seawater stream (stream becomes stream , which it then merges (with the aid of a circulation pump into the highest-pressure seawater stream created by the high-pressure pump This combined stream feeds the membranes The concentrate leaves the ERD at low pressure expelled by the incoming feedwater flow Pressure exchangers save energy in these systems by reducing the load on the high pressure
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...
. 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.


Applications

Seawater
desalination Desalination is a process that takes away mineral components from saline water. More generally, desalination refers to the removal of salts and minerals from a target substance, as in Soil salinity control, soil desalination, which is an issue f ...
plants have produced potable water for many years. However, until recently desalination had been used only in special circumstances because of the 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 Multi-stage flash distillation (MSF) is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers. Current MSF facilities may hav ...
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 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 Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
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 plant A reverse osmosis plant is a manufacturing plant where the process of reverse osmosis takes place. Reverse osmosis is a common process to purify or desalinate contaminated water by forcing water through a membrane. Water produced by reverse osmo ...
s 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.Reverse Osmosis System
/ref> 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 Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pre ...
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.


See also

*
Richard Stover Richard Lindsay Stover, Ph.D., pioneered the development of the PX Pressure Exchanger energy recovery device Energy recovery that is currently in use in most seawater reverse osmosis desalination plants in existence today. Stover is a water ind ...
, pioneered the development of an energy recovery device currently in use in most seawater reverse osmosis desalination plants


References


Energy Recovery Device Performance Analysis by Richard L. Stover Ph. D.

Ghalilah SWRO Plant by Richard L. Stover Ph. D.


*http://www.energyrecovery.com/news/documents/ERDsforSWRO.pdf *http://www.energyrecovery.com/news/pdf/eri_launches_advanced_swro.doc{{dead link, date=January 2018 , bot=InternetArchiveBot , fix-attempted=yes *https://archive.today/20130421173348/http://www.patentstorm.us/patents/7306437-description.html Fluid dynamics Mechanical engineering Membrane technology