On maximum power of reverse osmosis separation processes

doi:10.1016/j.desal.2005.09.004

Desalination

Volume 190, Issues 1–3, 15 April 2006, Pages 212-220

https://doi.org/10.1016/j.desal.2005.09.004 Get rights and content

Abstract

This paper considers the application of finite time thermodynamics (FTT) to reverse osmosis (RO) processes. A new thermodynamically ideal model of the endoreversible RO process is proposed and combined with basic transport equations for the irreversible mass transfer through the membrane. This approach is then applied to a simplified case of RO desalting of a concentrated sodium chloride solution. The results emphasize the trade-off between the permeate flow rate and the recovery rate. It also shows the existence of a maximum value for the power of separation which corresponds to the maximum conversion rate of mechanical exergy into chemical exergy. Finally the environmental and technical significances of the criteria such as power of separation are discussed on an example of the RO process.

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Briefly it consists in circulating a pressurized solution tangentially to a high rejection membrane meanwhile low pressure is applied to the other side. The resultant pressure has to exceed the osmotic pressure to produce a flux of purified water through the membrane and a stock of concentrated solution [25]. However, reverse osmosis can be used only with water wastes with acidic or neutral pH [26].
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A critical review of definitions for exergetic efficiency in reverse osmosis desalination plants
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The Pelton turbine and high-pressure pump accumulate 48% of this amount, despite their adequate performance (85%) with respect to typical operational data. In 2006, Sorin et al. [13], considered the application of finite time thermodynamics to reverse osmosis (RO) processes. The results show the existence of a maximum value for the power of separation which corresponds to the maximum conversion rate of mechanical exergy into chemical exergy.
Different approaches for formulating exergetic efficiency in desalination plants are suggested in literature. In this work these formulations, applied to the reverse osmosis technology, are compared and critically reviewed. As a case study, a reverse osmosis desalination plant in operation has been considered. A key factor is the proper definition of the exergy value of the product and the exergy value of the fuel. In reverse osmosis modules, where chemical separation is carried out, chemical exergy plays also an important role. Another influential issue is the thermodynamic model used in the calculation of the thermodynamic properties. Inappropriate thermodynamic models and ambiguous exergetic efficiency definitions bring confused and contradictory results: negative values of the chemical exergy, exergy production in pumps, or larger irreversibilities in the membranes than in the pumps. The enormous deviations found in the Literature can only be due to different conceptual definitions. In order to clarify these contradictions, this work provides a precise definition for the exergetic efficiency in reverse osmosis desalination plants devices.
The exergetic efficiency as a performance evaluation tool in reverse osmosis desalination plants in operation
2017, Desalination
Exergetic efficiency characterizes the performance of a system or a system component from the second-law of Thermodynamics viewpoint. Although this parameter can be used in the comparison of the operation of similar components working under similar conditions, there are not many articles in the literature dealing with this purpose. In this paper, a desalination plant located in Gran Canaria (Canary Islands, Spain) is considered. Different configurations are possible in the ten reverse osmosis production lines of the plant, depending on the procedure used for the energy recovery, the number of reverse osmosis stages, the technologies applied to the intake and filtration processes, or the components involved in feed water pressurization. Using real data, the exergetic efficiency is assessed as a performance evaluation tool. Through the comparison of the exergetic efficiency of similar devices, though different production lines, the components with operation defects that should be repaired are identified: the most inefficient pelton turbines, intake pumps, high pressure pumps, booster pumps, reverse osmosis membrane modules, and pressure exchanger modules. This way, exergetic efficiency can be successfully used to control and to improve the operation of the plant.
Energy efficiency breakdown of reverse osmosis and its implications on future innovation roadmap for desalination
2015, Desalination
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As an infinitesimal amount of permeate is withdrawn from the system, the piston applies just enough pressure to stay above the osmotic pressure of the resulting feed, so that no energy is wasted in generating unnecessary flow. However in cross flow systems, we don't have the luxury of having infinite number of pumps after an infinitesimal amount of permeate is drawn [28] because modules have to be constructed of finite lengths and installing booster pumps after every stage or will be cost prohibitive. Illustrations in Fig. 5 represent the energy savings achieved with a staged cross flow reverse osmosis operation operating with an ideal semipermeable membrane in a well-mixed frictionless system.
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Exergo-environmental analysis of a reverse osmosis desalination plant in Gran Canaria
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Citation Excerpt :
Both the energetic and the economics of the separation process are based on a quantitative formulation of the second law of thermodynamics in terms of the concept of exergy and its destruction. Sorin [16] considers the application of finite time thermodynamics to reverse osmosis processes. They also show the existence of a maximum value for the power of separation which corresponds to the maximum conversion rate of mechanical exergy into chemical exergy.
In this paper an exergo-environmental analysis of a reverse osmosis desalination plant in Gran Canaria (Canary Islands, Spain) has been performed using real plant operation data. The plant has a nominal capacity of 82,000 m³/day. Different configurations are possible depending on the energy recovery, the reverse osmosis stages, the filtration technology or the feed water pressurization.
The exergo-environmental analysis combines an exergy analysis and a Life cycle assessment in order to determine the environmental impact associated with the process. The analysis is conducted at the component level. The primary locations of exergy destruction are the first stage reverse osmosis membrane module and the high pressure pump. Based on the value of the component-related environmental impact, these components are also the major candidates for improvement. The environmental impact associated with the exergy destruction is the largest contributor to the total environmental impact. This means that the overall environmental impact can be reduced by reducing the exergy destruction within specific components, which are identified for potential performance improvement.
Effectiveness-mass transfer units (ε-MTU) model of a reverse osmosis membrane mass exchanger
2014, Journal of Membrane Science
Citation Excerpt :
Other studies have applied the solution-diffusion model for the design of RO modules such as spiral wound, hollow fiber, and crossflow long channels [25,28–33]. The solution-diffusion model has also been used together with relevant conservation laws to optimize the operation of RO systems and minimize the specific power consumption and cost of a plant [34–44]. Song and Tay [33] developed an analytical model of an RO exchanger based on the solution-diffusion model for transport across the membrane and conservation laws for a crossflow configuration; osmotic pressure was linearized, zero salt passage assumed, and hydraulic losses were neglected.
A strong analogy exists between heat exchangers and osmotic mass exchangers. The effectiveness-number of transfer units (ε-NTU) method is well-known for the sizing and rating of heat exchangers. A similar method, called the effectiveness-mass transfer units (ε-MTU) method, is developed for reverse osmosis (RO) mass exchangers. Governing equations for an RO mass exchanger are nondimensionalized assuming ideal membrane characteristics and a linearized form of the osmotic pressure function for seawater. A closed form solution is found which relates three dimensionless groups: the number of mass transfer units, which is an effective size of the exchanger; a pressure ratio, which relates osmotic and hydraulic pressures; and the recovery ratio, which is the ratio of permeate to inlet feed flow rates. A novel performance parameter, the effectiveness of an RO exchanger, is defined as a ratio of the recovery ratio to the maximum recovery ratio. A one-dimensional numerical model is developed to correct for the effects of feed-side external concentration polarization and nonlinearities in osmotic pressure as a function of salinity. A comparison of model results to experimental data found in the literature resulted in an average error of less than 7.8%. The analytical ε-MTU model can be used for design or performance evaluation of RO membrane mass exchangers.

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On maximum power of reverse osmosis separation processes

Abstract

Revue Générale de Chimie

Chem. Eng. Sci.

Desalination

Desalination

Unit Operations of Chemical Engineering