Exergy analysis of a reverse osmosis desalination plant in California

The exergy analysis of a 7250 m³/d reverse osmosis (RO) desalination plant in California was conducted by using actual plant operation data, and an alternative design was investigated to improve its performance. The RO plant is described in detail, and the exergies across the major components of the plant are calculated and illustrated using exergy flow diagrams in an attempt to assess the exergy destruction distribution. The primary locations of exergy destruction were the membrane modules in which the saline water is separated into the brine and the permeate, and the throttling valves where the pressure of liquid is reduced, pressure drops through various process components, and the mixing chamber where the permeate and blend are mixed. The largest exergy destruction occurred in the membrane modules, and this amounted to 74.07% of the total exergy input. The smallest exergy destruction occurred in the mixing chamber. The mixing accounted for 0.67% of the total exergy input and presents a relatively small fraction. The second law of efficiency of the plant was calculated to be 4.3%, which seems to be low. The analysis of the alternative design was based on the exergy analysis. It is shown that the second law of efficiency can be increased to 4.9% by introducing a pressure exchanger with two throttling valves on the brine stream, and this saved 19.8 kW electricity by reducing the pumping power of the incoming saline water.     

Exergy analysis of a seawater reverse osmosis plant

Exergy analysis is a powerful tool to determine how inefficiencies of the processes influence system performance. The exergy analysis of a seawater reverse osmosis desalination plant with 21,000 m³/d of nominal capacity located in Tenerife (Canary Islands, Spain) was studied. Once defined, the flow chart of the process, the exergy rate and exergy cost of flows were determined as well as the exergy destruction rate in equipment. The main results indicate that 80% of the exergy destruction is placed on core processes (high pressure pumping and valve regulation, reverse osmosis separation and energy recovery), 29% extra exergy is necessary to obtain the unit of feed exergy from previous stages (seawater pumping and pretreatment) and extra exergy of 1.06 kJ is needed to generate 1 kJ of final product exergy (exergy performance about 50%). In addition, the moderate fluctuations of seawater environmental conditions in the Santa Cruz de Tenerife metropolitan area (and Canary Islands as a whole) establish that environmental parameters present a less important influence on exergy analysis.     

Economic aspects of membrane processes

Care must be exercised, in predicting operating costs for membrane desalination systems for electricity, labor, chemicals, etc. The assumptions made in establishing unit costs can greatly affect the predicted operating cost and hence the ultimate investment decision. The use of data estimating the quantities of chemicals, power, etc., expected to be used per unit of production can assist in predicting costs, as then site-specific cost data can be used. The fact that 0.2 pounds of acid per kgal of permeate is required in a Florida RO plant is more useful for predicting operating costs of an RO plant on the Island of Anegata than is the fact that acid costs at the Florida plant are $0.03/kgal of water produced. Data based on units rather than costs are presented for seven RO facilities and one ED facility in this paper for use in this type of analysis.     

满足不同需求的水热电联产系统的模型和设计简

In order to improve the energy efficiency, reduce the CO2 emission and decrease the cost, a cogenera- tion system for desalination water, heat and power production was studied in this paper. The superstructure of the cogeneration system consisted of a coal-based thermal power plant （TPP）, a multi-stage flash desalination （MSF） module and reverse osmosis desalination （RO） module. For different demands of water, heat and power production, the corresponding optimal production structure was different. After reasonable simplification, the process model ot each unit was built. The economical model, including the unit investment, and operation and maintenance cost, was presented. By solving this non-linear programming （NLP） model, whose objective is to minimize the annual cost, an optimal cogeneration system can be obtained. Compared to separate production systems, the optimal system can reduce 16.1%-21.7% of the total annual cost. showing this design method was effective.     

Technical and economic analysis for the integration of small reverse osmosis desalination plants into MAST gas turbine cycles for power generation

A technical and economic analysis concerning the integration of small reverse osmosis (RO) desalination plants into mixed air steam turbine (MAST) technologies for power generation was carried out. The simulation tool used is the computer aid reverse osmosis calculations optimization algorithm. This user-friendly software takes into account the capital cost, fuel cost and operation and maintenance requirements of each candidate RO desalination plan scheme and calculates the least-cost configuration. The results indicate that the integration of a RO desalination plant into MAST gas turbines has a minor effect on the final operating cost of the power plant.     

Feed water pretreatment for desalination plants

Effective membrane pretreatment is a crucial requirement for trouble free and cost effective RO desalination. Pretreatment construction costs are significant and may reach as much as 10%-20% of the total desalination plant capital costs. This paper presents a general review of the central factors involved in assessing raw water quality. The main foulant categories are described and the various unit operations commonly applied are reviewed. Pretreatment aspects of desalination offer fertile grounds for advancing desalination technologies. Promising research directions are highlighted.     

Nitrate removal with reverse osmosis in a rural area in South Africa

The nitrate-nitrogen concentration (>6 mg/l) and the salinity (>1000 mg/l TDS) of many borehole waters in rural areas in South Africa are too high for human consumption. Therefore, an urgent need for water denitrification and water desalination exists in these areas. Reverse osmosis (RO), electrodialysis (ED), ion-exchange (IX) and certain biological technologies can be very effectively applied for water denitrification. Each of these technologies, however, has its own advantages and disadvantages. Reverse osmosis technology, however, has been selected for this study because the technology is well known in South Africa and because it can be very effectively applied for water desalination. The objectives of this study were: (a) to transfer RO technology through process demonstration performance for water denitrification and water desalination to people living in rural areas; (b) to build capacity regarding the operation and maintenance of an RO application in a rural area; (c) to produce a preliminary operational and maintenance manual for the operation of an RO unit in a rural environment; (d) to train local operators to operate and maintain an RO plant in a rural environment; (e) to evaluate stock watering as brine disposal option; and (f) to determine the preliminary economics ofthe process. The following conclusions were drawn. It was demonstratedthatthe RO process could be very effectively applied for water denitrification and water desalination in a rural area. Nitrate-nitrogen was reduced from 42.5 mg/l in the RO feed to only 0.9 mg/l in the RO product water. The TDS of the RO feed was reduced from 1292 mg/l to 24 mg/l in the RO permeate. Therefore, an excellent quality water could be produced for potable purposes. The RO brine at approximately 50% water recovery should be suitable for stock watering if the conditions for stock watering are met in terms of nitrate-nitrogen concentration, TDS and other constituent concentrations. The capital cost for an approximately 50 m³/d output RO plant is approximately US $29,900. Preliminary cost estimates have shown that the operational cost for water denitrification is approximately US $0.50/m³. This cost, however, should be significantly reduced by optimisation of the chemical dosing and by blending borehole water with RO product water.     

Exergy analysis of desalination by solar-powered membrane distillation units

There has been an increasing interest in using exergy as a potential tool for analysis and performance evaluation of desalination processes where the optimal use of energy is considered an important issue. Unlike energy, exergy is consumed or destroyed due to irreversibilies in any real process and thus provides deeper insight into process analysis. Exergy analysis method was employed to evaluate the exergy efficiency of the “compact” and “large” solardriven MD desalination units. The exergy efficiency of the compact and large units with reference to the exergy collected by the solar collector was about 0.3% and 0.5% but was 0.01% and 0.05%, respectively, when referenced to the exergy of solar irradiance. The exergy efficiency of the flat plate solar collectors in both units varied diurnally and the maxima was 6.5% ad 3% for the compact and large units, respectively. The highest exergy destruction was found to occur within the membrane distillation module.     

An exergetic analysis of a membrane desalination system

In this paper, the exergetic analysis of a seawater membrane-based desalination plant has been carried out. The desalination plant has been described in detail, then the exergy of the various saline water streams has been determined and a comprehensive analysis towards the exergy distribution of the major process components has been conducted. The examination of the exergy losses throughout the plant revealed that exergy destruction was mainly due to pressure drops in the membrane modules, valves and brine lines. Moreover, 12.9% of the exergy input to the system was supplied by the heater. Therefore, the most reasonable way to reduce power input to the plant, thus improving its performance and cost, has been shown (i) to be replacing the valves on the reverse osmosis brine stream by an energy recovery system, and (ii) to have thermal energy available in the plant. With the identified technical changes, energy consumption decreased from 18.3 to 2.05 kWh/m³, resulting in an annual saving of 0.17$/m³.     

Pilot studies on high recovery BWRO-EDR for near zero liquid discharge approach

A hybrid process combining reverse osmosis and electrodialysis has been shown to be effective in recovering 97-98% of brackish water as product water with chloride levels of 200 mg/L or less. Potential for scaling on the brine side of the electrodialysis unit was prevented by acidification, operating the electrodialysis in a reversal mode (EDR), and a side loop crystallizer which prevented buildup of scaling components. Settlers, inline microfiltration, and side-loop ultrafiltration kept suspended solids from returning to the EDR unit. This process was demonstrated in a series of more than eighty batch experiments of 1.5 to 1.8 m³ of RO concentrate of raw brackish groundwater from the Negev Highland, Israel. Each batch could be processed within a single day. The feed-water was concentrated from 0.3% to over 10% TDS super-concentrate while producing the water that could be recycled to the RO permeate. This super-concentrate from the EDR unit was further concentrated in a wind powered WAIV unit that brought final brine TDS to > 30%, and showed promise as a method to recover mineral byproducts such as magnesium salt. Initial economic estimates show that this hybrid process is competitive with conventional RO and other enhanced recovery processes for inland desalination requiring use of evaporation ponds.     

变负荷反渗透海水淡化技术研究

为适应风电、太阳能等新能源的波动性,设计了可变负荷的反渗透(RO)海水淡化装置,用电动阀替代常规浓水调节阀,变频器控制高压泵电机,通过PLC控制变频器的输出频率和电动阀的开度使RO海水淡化的功率可调;利用溶解扩散模型分析了变负荷海水淡化的特性,并通过试验验证了相关结论。在维持膜壳内压力稳定的工况下,低功率运行时可以增加系统回收率,降低单位产水能耗,可以降低运行成本。     

Operational water cost and productivity improvements for small-size RO desalination plants

This detailed study was carried out for the determination of the water cost in small size RO desalination plants in remote areas. Data from desalination plants in three Greek islands were used for a period of three years. The actual expenses for these plants were examined thoroughly and as a result the real cost per cubic meter was estimated. An evaluation of the efficiency of two different energy recovery systems was conducted by using factual operating data. The introduction of new technologies, automation, data acquisition and remote operation can be used to reduce the labor and maintenance cost for small size RO desalination plants. The increase in labor productivity has proved to be the direct result of the introduction of a low cost SCADA system to the RO plant.     

Performance study of RO systems in Saudi Arabia

Desalination of water is one of the most important engineering tasks facing Saudi Arabia today. Lacking unlimited supplies of potable Water, the Kingdom must depend upon the desalination process of brackish well water or sea water to satisfy the ever increasing demands. These demands are expected to reach the 430 MGD figure in 1980 and 700 MGD in 1985.

Abstract

until quite recently, the reverse osmosis played a rather modest role in the country's desalination program and was almost exclusively limited to brackish water applications and plants not exceeding a capacity of 1 MGD. Within the last year, significant changes took place in favor of these processes. The estimated total output of RO units to be commissioned this year will be about 70 MGD. This figure includes the 3.2 MGD plant for desalting the sea water in Jeddah and the 55 MGD plant in Riyahd from brackish well water.

This paper deals primarily with the performance of RO systems, their cost of operation/maintenance and the factors affecting the cost. These factors may be divided into three groups as related to feed water, system design and system operation. The quality of feed water and good pretreatment plays an extremely important role in performance of a RO system. This study pays special attention to these parameters in expectation that more cost effectiveness will evolve in the future either through a better understanding of the processes or an improved process design match to materials employed.     

Implications of desalination for water resources in China — an economic perspective

China is a country with severe water shortages. Water is becoming scarcer due to population growth, industrialization and urbanization. Recent studies show that by the next 50 years water resources per capita will go down to around 1700 m³, which is the threshold of severe water scarcity. Especially in North China, water shortage has become a critical constraint factor for socioeconomic development in the long run. To solve or eliminate water shortage problems, seawater desalination draws more and more attention as an alternative water supply source. The objective of the study is to assess the potential of desalination as a viable alternate water source for China through analysis of the costs of desalination, the water demand and supply situation as well as water pricing practices in China. Based on the investment costs and estimated operation and maintenance costs, an economic appraisal for the costs of desalination for two main processes, MSF and RO, has been conducted. The study shows that there is a decline of unit cost of desalination over time and the average unit cost of the RO process was lower than that of the MSF process. A unit cost of 0.6 $/m³ for desalting brackish water and 1.0 $/m³ for seawater are suggested to be appropriate for the potential application of desalination in China. Future trends and challenges associated with water shortages and water prices are discussed, leading to conclusions and recommendations regarding the role of desalination as a feasible source of water for the future.     

Characterization of uncertainties in the operation and economics of the proposed seawater desalination plant in the Gaza Strip

In the Gaza Strip, the available freshwater sources are severely polluted and overused. Desalination of seawater through reverse osmosis (RO) has become the most realistic option to meet a rapidly growing water demand. It is estimated that the Gaza Strip will need to develop a seawater desalination capacity of about 120,000 m³/d by the year 2008, and an additional 30,000 m³/d by the year 2016 in order to maintain a fresh water balance in the coastal aquifer and to fulfill the water demand for different uses in a sustainable manner. Cost and reliability of a large RO facility are still subject to much uncertainty. The cost of seawater desalination by RO systems varies with facility size and lifetime, financing conditions, intake type and pre-treatment requirements, power requirements, recovery rate, chemicals cost, spare parts cost, and membrane replacement cost. The permeate salinity is a function of feed water temperature, recovery rate, and permeate flux. The quantity of water produced depends mainly on plant size, recovery rate, and operating load factor. Many of these parameters are subject to a great deal of uncertainty. The objective of this work is to develop a probabilistic model for the simulation of seawater reverse osmosis processes using a Bayesian belief network (BBN) approach. This model represents a new application of probabilistic modeling tools to a large-scale complex system. The model is used to: (1) characterize the different uncertainties involved in the RO process; (2) optimize the RO process reliability and cost; and (3) study how uncertainty in unit capital cost, unit operation and maintenance (O&M) cost, and permeate quality is related to different input variables. The model utilizes information from journal articles, books, expert opinions, and technical reports related to the study area, and can be used to support operators and decision makers in the design of RO systems and formulation of operational policies. The structure of the model is not specific to the Gaza Strip and can be easily populated with data from any large-scale RO plant in any part of the world.     

Membrane processes for water supply and reuse - a question of energy consumption and cost

This paper is limited primarily to reverse osmosis which is the dominating membrane process in commercial plants. Desalination of brackish water and seawater with reverse osmosis, with special emphasis on costs and energy consumption, is the primary subject discussed in the paper. Some aspects of and development trends in industrial and domestic applications of membrane processes are also taken up, particularly with regard to by-product recovery and water reuse in connection with advanced wastewater treatment.The first RO plant to be brought into operation in Riyadh, Saudi Arabia, is located at Salbukh. The investment and total operation costs for this plant have been calculated in the paper. The water cost is at least twice as high as in a continental U.S. location. The main reason for this is the very high cost of civil and local works in Saudi Arabia. A similar calculation has been made for RO seawater desalination.Increased energy costs during the last decade have directed research and development work for all desalination methods towards reducing energy consumption. It is shown in the paper that energy recovery in connection with RO seawater desalination is particularly feasible. Different methods for energy recovery have been investigated and reported, the preferred methods depending on the size of the RO plant. A large underground RO plant for energy recovery, based on utilization of the static pressure instead of high pressure pumps, has also been studied.Another possible energy-saving, but also water quality improving method has been proposed, viz . a combined MSF-RO dual purpose plant. Excess power for reverse osmosis seems to be more and more available in Saudi Arabia due to the high power/water ratio in MSF dual purpose plants compared to the real demand for power and water.     

Distillation vs. membrane filtration: overview of process evolutions in seawater desalination

The worldwide need for fresh water requires more and more plants for the treatment of non-conventional water sources. During the last decades, seawater has become an important source of fresh water in many arid regions. The traditional desalination processes [reverse osmosis (RO), multi stage flash (MSF), multi effect distillation (MED), electrodialysis (ED)] have evoluated to reliable and established processes; current research focuses on process improvements in view of a lower cost and a more environmentally friendly operation. This paper provides an overview of recent process improvements in seawater desalination using RO, MSF, MED and ED. Important topics that are discussed include the use of alternative energy sources (wind energy, solar energy, nuclear energy) for RO or distillation processes, and the impact of the different desalination process on the environment; the implementation of hybrid processes in seawater desalination; pretreatment of desalination plants by pressure driven membrane processes (microfiltration, ultrafiltration and nanofiltration) compared to chemical pretreatment; new materials to prevent corrosion in distillation processes; and the prevention of fouling in reverse osmosis units. These improvements contribute to the cost effectiveness of the desalination process, and ensure a sustainable production of drinking water on long terms in regions with limited reserves of fresh water.     

基于经济目标和分解协调策略的换热网络最优设计

分析了换热网络在过程系统中的功能 ,从火用分析和火用经济学的角度提出了换热网络最优合成设计的实用火用经济目标函数 ,它包含了网络的流动火用损费 ,且可分解为各个换热单元的总费用之和 .在换热网络目标函数可分解性的基础上 ,本文又进一步提出了换热网络分解协调优化的研究策略 ,它是在各个匹配单元 (子系统 )和整个换热网络结构 (复杂系统 )之间进行优化协调 ,并获得换热网络的最优结构和各匹配单元的最优操作参数 .最后 ,通过一换热网络的最优合成 ,说明了该方法的应用前景     

基于经济目标和分解协调策略的换热网络最优设计

分析了换热网络在过程系统中的功能 ,从火用分析和火用经济学的角度提出了换热网络最优合成设计的实用火用经济目标函数 ,它包含了网络的流动火用损费 ,且可分解为各个换热单元的总费用之和 .在换热网络目标函数可分解性的基础上 ,本文又进一步提出了换热网络分解协调优化的研究策略 ,它是在各个匹配单元 (子系统 )和整个换热网络结构 (复杂系统 )之间进行优化协调 ,并获得换热网络的最优结构和各匹配单元的最优操作参数 .最后 ,通过一换热网络的最优合成 ,说明了该方法的应用前景