Optimal design of hybrid RO/MSF desalination plants Part III: Sensitivity analysis

This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics ofthe conventional MSF process. The research project involved an optimization study where the water cost perunit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.     

On the reduction of desalting energy and its cost in Kuwait

All seawater desalting processes, multi-stage flash (MSF), multi-effect boiling (MEB), mechanical vapor compression (MVC) and seawater reverse osmosis (SWRO) consume significant amounts of energy. The recent increase of fuel oil cost raises the cost of energy consumed for desalting water and the final water cost, and creates more interest in using more energy efficient desalting systems.

The most used desalting systems by distillation (MSF and MEB) are usually combined with power plants in what is called co-generation power desalting plants, CPDP. Fuel is supplied to the CPDP to produce both desalted water D and power W, and the fuel cost is shared between D and W. Exergy analysis and equivalent work are among the methods used to determine the fuel energy charged to each product. When desalting systems, such as SWRO and MVC, are not combined with a power plant, the fuel energy can be directly determined from its electrical power consumption.

In this paper, the fuel energy cost charged to desalting seawater in the presently used CPDP in Kuwait is calculated based on exergy analysis. The MSF, known by its high energy consumption, is the only desalting method used in Kuwait. The MSF units consume 258 kJ/kg thermal energy by steam supplied to the brine heater BH, 16 kJ/kg by steam supplied to steam ejectors, and 4 kWh/m3 mechanical energy for pumping. These MSF units are operated either by:

(1) Steam extracted from extraction/condensing steam turbines EC/ST as in as in Doha West, Azzour, and Sabbiya CPDP. This practice is used in most Gulf area.

(2) Steam supplied directly from boilers as occurred in single purpose desalting plants as Al Shuwaikh plant; or in winter time when no steam turbines are in operation in the CPDP to supply steam to the desalting units.

The CPDP have limited water to power production ratio. While they can cope with the increase of power demand, it cannot satisfy the water demand, which is increasing with higher pace than the power demand.

The case of steam CPDP used in Kuwait is presented in this paper as a reference plant to evaluate the amount of fuel energy consumed to desalt water in MJ/m3, its cost in $/m3. The resulted high fuel cost calls for some modifications in the reference CPDP to lower the energy cost, and to increase its water to power ratio. The modifications include the use of an auxiliary back-pressure steam turbine ABPST supplied with the steam presently extracted to the MSF units. The power output of the ABPST operates MVC or SWRO desalting units; while the ABPST discharged steam operates LT-MEB desalting unit. The desalting fuel energy costs when applying these modifications are also calculated by the exergy analysis and compared with that present situation.

It is also suggested to increase desalted water output by using separate SWRO desalting units operated by the existing power plants of typical η_c = 0.388, or by new combined gas/steam turbines power cycle GT/ST-CC of typical η_c = 0.54 under construction. The SWRO with energy recovery is assumed to consume typical 5.2 kWh/m³ electric energy.     

Once-through and brine recirculation MSF designs — a comparative study

This study investigated the feasibility of the once-through (OT) MSF design for constructing large-capacity desalting plants rather than the conventional brine recycle (BR) MSF configuration. This was to explore the possibility of improving MSF process economics through the application of a simpler process design. It is known that the OT design is characterized by its simplicity and elimination of the brine recirculation pump as well as the rejection section. These features are expected to reduce capital and maintenance costs. The current study is based on comparing the two MSF plant configurations, OT and BR, from the standpoint of minimum heat transfer area, which is a major element in capital investment. Alternative tube layouts, long-tube (LT) and cross-tube (CT) arrangements, were considered. Design calculations were based on a plant capacity of 20 MGD, a gained output ratio of 10, a top brine temperature of 120°C, a feed temperature and concentration of 35°C and 48,000 ppm, respectively. The total number of stages was varied stepwise between 20 and 40. A rigorous mathematical model was developed to solve the optimization problem taking into consideration the nonlinearity of the thermo-physical properties of seawater and steam. The Solver tool of Microsoft Excel was used to determine the optimal solutions. Substantial savings in the heat transfer area can be realized through the application of the LT-OT design when the number of stages goes up to 40. However, comparison of the different MSF designs with the optimal number of stages and minimum heat transfer area tells us that the use of the OT designs is not likely to save more than 1% in heat transfer area relative to the conventional CT-BR configuration. Findings related to the specific chemical consumption are not in favor of OT plants where the consumption ratio varies between 1.7 and 1.9 relative to the CT-BR plants. Of course, excessive chemical consumption penalizes the operating cost of the OT plants. However, the increase in operating costs has to be weighed against the savings in capital cost and lower power consumption due to the elimination of the recycle pump and the heat rejection section. This will determine the use of the OT configuration for building MSF plants in the future. In this event, a detailed cost analysis will be needed.     

Integrating hybrid systems with existing thermal desalinationplants

In Gulf countries, most power plants are co-generation power desalting plants (CPDP) that generate electric energy and also produce fresh water through the desalination of seawater. This paper provides detailed technical and economical analyses to evaluate a new generation of dual purpose technology that includes the integration of reverse osmosis (RO) processes with existing thermal desalination processes and power generation (triple hybrid system) at Layyah plant, Sharjah, UAE. Hybridization of sweater reverse osmosis (SWRO) and the multi-stage flash (MSF) technology was considered to improve the performance of latter and reduce the cost of the produced water. Moreover, “idle” power in winter (seasonal surplus of unused power) was mainly utilized by RO to further reduce the cost of the hybrid system for six months of the year. Spinning reserve was also used to further reduce the cost of the proposed hybrid system. Integration ofthe three processes of MSF, MED, and RO desalination technologies could be made at different levels through which the resulting of water cost will depend on the selected configuration and the cost of materials of construction, equipment, membrane, energy, etc. Thus, the capital and annual operating costs were calculated for all potential alternatives for various plant capacities. It was found that for all plant capacities, integrated hybrid systems resulted in most cost effective solution. For example, at a capacity of 50 MIGD, the present worth of the cost was calculated to be 588.7, 443.2, and 380 million US$ for MSF, MED, and hybrid RO systems, respectively.     

Optimal design of hybrid RO/MSF desalination plants Part II: Results and discussion

This paper presents the results of an optimization study, based on minimum water cost, to explore the feasibility of the hybridization of RO and MSF processes. The study explores the possible improvement of MSF process economics. Nine different scenarios for the production of the same capacity of desalted water are presented and compared from the standpoint of minimum water cost, specific capital cost and water recovery. The process and cost models, formulation of the optimization problem and solution outlines were previously presented in the first part of this study. In this work, results show that RO technology is recommended when building new desalination plants. RO technology becomes preferable at low feed concentrations and for brackish water desalination. Although they come in second position after the RO process, some hybrid plants economically exceed by far the MSF process. Computations gave a water cost of 1.1 $/m³ for the brine recycle MSF process against 0.75 $/m³ for the two-stage RO process. Water cost of the MSF process can be reduced by 17 to 24% through hybridization with RO technology.     

Myth and reality of the hybrid desalination process

The paper discusses some misconceptions that have contributed to the continued use of thermal desalination processes and promotion of the hybrid desalination process for new plants being built or considered at Middle East locations. The misconceptions are examined both on the basis of fundamental thermodynamic principles and in terms of practical engineering parameters. The analysis shows that there is no economic or performance advantage in the installation of greenfield hybrid power/thermal desalination/ seawater reverse osmosis (SWRO) plants in preference to power/SWRO plants, because the latter would produce water more cheaply under all conditions and at all fuel costs, and would provide more operational flexibility than the former. The paper identifies situations where the hybrid desalination process can be fully justified: in existing power/desalination plants, where aging boilers and multistage flash (MSF) units need to be repaired or replaced, through retrofitting and repowering. In such situations, abandonment of the MSF process would result in a reduction in the power output of the plant. The paper refers to previous work which showed that the repowering of a typical existing power/desalination station with refurbishment/replacement of the MSF units, together with the addition of SWRO units, would result in a several-fold increase in the water and power output and a dramatic improvement in the fuel efficiency, without any additions to the existing seawater intake system. The paper emphasizes the importance of test stations/demonstration plants at existing power/desalination stations in the Middle East in order to obtain data and make improvements in the technology of higher temperature SWRO, with the feed obtained from the cooling water returning from the power plant condenser and the thermal desalination plant. The paper shows that the potential benefits would easily justify the investment in research and development required to validate this concept.     

海水淡化的现状与未来

本文介绍了近来主要海水淡化方法－ＳＷＲＯ、ＭＳＦ、ＭＥＤ的某些新进展。海水淡化市场的激烈竞争和海水淡化技术进步,尤其是ＳＷＲＯ的进展,使淡化的能耗下降了近一半,产水的出厂价也几乎下降了一半。     

A new look at dual purpose, water and power, plants - economy and design features

In light of rapidly rising equipment and fuel costs, recent studies have shown some important results in the economy and optimum designs for dual purpose power/desalting complexes. The desalination cycle chosen for- detailed comparisons in this study is the well known multistage flash (MSF) evaporator, which uses brine recirculation and polyphosphate scale prevention. The multi- stage flash evaporator has found wide spread application in large plants throughout the world. Single units of up to 1400 m3/hr (9.5 MGD), acid dosing, have been built and are now in operation. In general due to corrosion problems, polyphosphate plants are now preferred over the acid one.

A number of schemes for the combined production of power and. water were chosen, mainly a combination of MSF Unit with each of the following power cycle:

• 1- Automatic extraction steam turbine

• 2- Simple gas turbine with waste heat boiler

• 3- Back pressure steam turbineThis paper presents a study of the economic aspects, thermodynamic features and optimization analysis of each of these combined power and water production plants.

The optimum value of the performance ratio and its effect on reducing the cost of water in each of the above mentioned schemes would be discussed in this paper.     

某焦化厂供水网络多目标建模及优化

对某焦化厂的供水网络进行了详细的分析,在用水供需平衡满足生产要求的前提下,以新鲜水供给量最小以及新鲜水供给成本与循环水及除盐水的浪费成本之和最小作为两个优化目标,建立了多目标混合整数非线性规划(MINLP)的数学模型.利用文献提出的Pareto规则下的多目标改进文化差分进化算法对模型进行了求解.所得的计算结果与现场实时操作数据相比,模型及算法实现的优化操作减少了新鲜水消耗量,降低了新鲜水供给成本与循环水及除盐水的浪费成本,达到了很好的优化效果.     

Energy and water in Kuwait: A sustainability viewpoint, Part II

In Kuwait, the daily consumption per capita of electric power is 14,000 kWh, and of desalted water is 600 L. These are among the highest in the world, and the total consumption of each is almost doubled every 10 years. The cogeneration power desalting plants CPDP producing these two commodities consumed about 54% of the total 150 millions barrels of fuel consumed in the year 2005. If these consumption and production patterns prevail, the fuel oil produced in the country can be fully consumed locally in 30 years, with nothing left for export, the main source of income. The picture can be changed if better desalted water and power production methods are used. These include changing the desalting method from multistage flash MSF known by its high energy consumption to the more energy efficient seawater reverse osmosis SWRO; and power production method of steam or gas turbine cycles to combined gas/steam turbine combined cycle known of its high efficiency. The energy consumed by the air conditioning AC systems should be reduced by using better codes of building insulation and more efficient AC systems. Other conservation methods to reduce water consumption and the energy consumed by transportation are outlined in this paper.     

Analysis of water and power costs by various methods,based on Jeddah IV

The Jeddah IV Power and Desalination Plant is the biggest, dual-purpose plant in the world. Moreover, it is perhaps the first plant of this kind for which authentic cost and operational data are available. As such, it provides a reliable means of assessing the present status of desalination technology and its influence on the cost of power and water.The well-known theoretical methods of cost allocation are used in deriving the product cost from the Jeddah IV plant. The influence of local factors and contractual constraints is discussed. Extrapolations are made to determine the projected cost of water on similar plants but with different performance ratios. Recommendations are made on cost optimization and the most favourable combination for dual-purpose power/desalination plants based on steam turbines and MSF process.     

热膜耦合海水淡化系统的优化设计

采用混合节点和分配节点的概念,建立了多级闪蒸(MSF)和反渗透(RO)海水淡化集成系统的超结构模型,以年度总费用最小为目标,引入产水比的概念,并将该参数作为集成系统的一个优化变量,采用改进的遗传算法进行求解,获得了集成系统的最优结构及相应的操作条件。实例结果表明:集成海水淡化系统的淡水成本比独立运行的RO和MSF低,产水比为0.45时集成系统的费用最小,流程结构为MSF-RO。     

全流程卷式反渗透海水淡化系统操作优化

在对反渗透海水淡化系统流程和实际应用分析的基础上,提出了一种旨在降低总体操作费用的全流程反渗透海水淡化系统优化方法。首先根据系统变参数特点并充分利用蓄水池的缓冲能力,建立了反渗透过程机理模型、蓄水池动态过程模型以及变参数方程模型,实现了整个流程的方程描述。然后根据工艺流程和操作过程费用组成情况建立了总的操作费用模型,得到了全流程单位产水费用指标。在此基础上建立了以总体操作费用最低为目标、以开放方程描述的各模型方程为约束、以设备和产品质量限制为边界的优化命题,采用联立求解技术将该微分代数方程组成的优化（DAOP）问题转化为NLP问题后进行求解。最后对某海水淡化系统进行了实例研究。优化求解结果不仅表明本优化方法可以大幅降低实际操作费用,而且通过求解还可得到各种变参数条件下最优操作费用组成,以及实现费用最低的最优操作压力和流量变化曲线。本研究对优化海水淡化系统操作、降低总体操作费用具有重要意义。     

Assessment of energy consumption in desalination of sea water on the basis of conceptual designs and surveys

Energy equivalent of sea water desalination are quoted as the most suitable measure in the phase discussing here from various survey reports in Japan which include conceptual designs of desalting plants such as dual purpose systems and MSF processes to utilize industrial waste heats.These systems are alloted for supplementing the shortage of drinking water from rivers which may dependent on the rainfall. Then, a diagram is proposed to indicate preferable region for construction of desalting plants or river development from view points of both energy consumption and economyThese ideas were applied to the survey for introducting desalting plant in an industrial city and the necessary energy to produce fresh water from the sea was estimated for the case of 1/10 arid year. The amount of the order of a little less than 1% to the total energy consumption of the city should be evaluated from the view point of total human activities     

MSF海水淡化系统热经济学优化分析

热经济学理论和方法在能量系统分析中占有重要地位。本文以年度化平均淡水成本为目标函数,对ＭＳＦ海水淡系统建立了较完善的优化模型。并就国产化日产３０００吨ＭＳＦ海水淡化系统进行了较详细的浮动价格结构下的优化设计分析。     

考虑水需求的水电联产海水淡化系统的优化设计

水电联产不仅能缓解淡水资源不足的问题,而且可有效降低能耗和淡化成本。建立了水电联产系统数学模型,将优化设计描述为一个混合整数非线性规划(MINLP)问题,并采用混合编码的遗传算法进行求解,结果表明,以水定电模式下水电联产系统最优操作模式为发电、多级闪蒸(MSF)和反渗透(RO)三者的集成,且MSF和RO的产水比存在最优值,发电采用背压式蒸汽轮机。随着淡水需求量的增加,联产系统的淡水成本逐渐降低,MSF与RO的产水比呈现出逐渐降低的趋势。     

MEDA - a new thermal concept for desalting plants

The MEDA project comprises a highly efficient, self-contained, VTFE-VC-MSF sea water distillation plant with a capacity of 5000 tones of fresh water per day at a fuel-use performance ratio (i.e., a performance ratio which includes the electrical and steam consumption of all auxiliaries associated, with the desalination plant) of 11.8 kg/1000 kJ heat content of fuel.The main aim of the MEDA project is to demonstrate advanced desalting technology with a plant of sufficient capacity in various parts of the world under- varying seawater and climatic conditions. For this purpose the desalting plant is on a barge which can be towed to different locations where a demonstration is desired or requested.Advanced features installed on the barge are: continuous ion exchange pretreatment for removal of calcium and regeneration by brine-blowdown, high heat-transfer rates due to enhanced tubing and foamy up-flow, spirally-indented horizontal tubes in the MSF section, surfactant recovery, turbine driven steam compressor with topping effects, and reverse osmosis system with polishing unit for providing boliler make-up for plant start-up. The calcium removal by ion-exchange permits plant operation with a maximum brine temperature of 150°C. Product is redistilled in the MSF cooler to produce high purity distillate for desuperheater injection water and boiler make-up.R and D work is currently underway with a view to demonstrate reliability of operation, to verify projected performance and to evaluate corrosion behaviour of the various materials used in the plant.     

考虑不同水质输入的水电联产优化设计

结合已有的反渗透/多级闪蒸混产系统和发电-多级闪蒸联产系统,设计了一个新的包括热力发电系统、反渗透海水淡化系统和多级闪蒸海水淡化系统的水电联产超结构,对以不同盐度苦咸水、海水为原水的水电联产系统进行了优化设计,通过求解系统以年费用最小为目标函数的非线性数学模型,得到不同盐度下联产系统优化的生产结构. 结果表明,在低盐度(≤25000 mg/L)下采用冷凝式发电和一级反渗透产水,高盐度下采用抽汽冷凝式发电和热膜混合产水,可降低联产系统的年费用,获得较低成本的淡化水. 在本工作所定的生产规模下,优化联产方案的年费用可降低23%~36%.     

Systems analysis of dual-purpose nuclear power and desalting plants Part I. Optimization

The optimal process design of a dual purpose plant for producing power and water is investigated. A nuclear reactor and steam turbine power generator for steam and power production is coupled with two water plants, a multi-stage flash plant and a reverse osmosis plant. The total system cost for producing given levels of power and water is minimized. Optimal designs are presented for several combinations of water and external power demands ranging from 25 mgd to 150 mgd and from 50 MWe to 200 MWe.     

The Curacao KAE-LT-MED and auxiliary steam turbine project: A model for dual purpose MSF plants replacement

KAE, the water and power company of Curacao, has contracted to replace its older and less efficient MSF plants with a 10,000 m³/day Low Temperature, Horizontal Tube, Falling Film, Multi Effect Distillation (LT-MED) plant.The plant operates at a top brine temperature of 70 degrees C and requires steam at only 0.34 bar a.However, motive steam is available, as extraction from the power station's turbines, at 2.55 bar a, 155 °C, as required by the higher temperature MSF plant. This excess pressure potential is used to generate 3.1 MW of power in an auxiliary low pressure steam turbo-generator, thereby improving overall economy and lowering desalted water cost to a level unequalled by any other system. The paper describes the power station turbine, auxiliary low pressure turbo generator and LT-MED plant combined scheme, its operational features and economics.