Desalination in the development of Libya problems of implementation and impact on environment

In Libya, many desalination plants of different working principles and various manufacturers are being installed. Practically all important plants are dual purpose installations, the total desalting capacity being about 140,000 m³/day for sea water and 21,000 m³/ day for brackish water.

The paper points out the main problems faced by the different manufacturers during the erection and maintenance of their plants, particular attention being given to those related to Libyan specific conditions. Suggestions are made in order to decrease the acuteness of these problems in the future.

Also, the impact on the environment of the installation of such desalting plants is considered in all its aspects (social, economic, . . . ), particularly those related to a developing nation.     

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.     

Solar multistage flash evaporation (smsf) as a solar energy application on desalination processes

The application of solar energy on desalination constitutes one of the activities of the research and development program of digases. Within this philosophy a project is being developed in cooperation with the Federal Republic of Germany. The objective of the project is to test and prove the feasibility of the solar energy utilization as the source of the thermal energy required for a multistage flash evaporation plant.

The plant is composed of three systems:

1. Desalination system. Multistage flash evaporation plant with 10m³/d production capacity, 5.0 in performance ratio and 10 stages.

2. Solar energy captation. Composed of one low temperature subsystem of double pipe flat collectors with 670m² of effective captation surface, and a high temperature subsystem composed of parabolic concentrators with 320m² of captation surface.

3. Storage of energy. System designed with such capacity which allows continuous operation of desalination unit.

The poject is separated into two phases. The first includes the design manufacturing and tests for the three systems. The second phase comprises the installation and integration of the systems which compose the plant, the start-up operation and evaluation of plant performance.

At the present time, the first phase of the project is under development and it is expected to finish it at the end of June. The second phase will start in August with system start-up scheduled for the end of October.     

Operating experiences of a 15,000 cubic meter per day municipal desalting plant at Corfu, Greece

The initial eighteen month period of operation of the 15,000 m³/day municipal plant on the Island of Corfu, Greece, is described.

The 15,000 m³/day desalting plant employing the electrodialysis reversal process (EDR) produces potable water (500 ppm) from a blend of brackish sources with salinities up to 2000 ppm TDS. The paper describes the plant and integration of the plant into the municipal system. The unique system employed to segregate the treatment of the different brackish waters employing only the highest salinity water for blowdown water is also described.

The plant was started up in the fall of 1977 and provided water to the municipal system during the 1978 season.

The paper presents operating cost data for this period and compares these costs with projected costs which form part of the contractual agreement, with the Municipality.     

Implementation of ISO 14001:2004 (environmental management system standard) for reverse osmosis desalination plants for the first time in Iran

Implementation of ISO 14001:2004 (environmental management system) has been executed for reverse osmosis desalination plants for the first time in Iran at Noor Vijeh Company (N.V. Co), a water and wastewater firm based in Tehran. The scope of work was the activities and product of company's BWRO desalination plant in the city of Qom (3000 m³ per day) and SWRO desalination plant in Assaluyeh, Pars Special Economic and Energy Zone, Iran. The aims of this project were in line with company’s approach to sustainable development and its direction towards conducting environmental friendly activities and production characteristics.

Initially the famous PDCA (Plan, Do, Check, Act) was used to identify the aspects and evaluate their effects. The significant aspects of each plant affecting the environment are then identified and preventive control measures and reducing their probabilities of loss are anticipated. These cover the normal activities within each plant and those aspects arising from emergency conditions such as earthquakes, fire, etc.

Elements of this system are

• Environmental policy and its targets and programmes

• Practical methods for environmental management system processes

• Executive manuals for implementing special activities

• Tables and indexes for environmental aspects for each plant

• Organizational charts, positions, qualifications, and necessary trainings for the involved personnel.

Environmental aspects are evaluated through their interaction with and effects over, releases to water, emissions to air, land contamination, waste management, energy use, and use of natural resources and raw material.     

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.     

Sea water desalination by reverse osmosis in chigasaki laboratory

For the purpose of constructing a reverse osmosis [RO) sea water desalination plant of 800 m³/day capacity, a series of tests on the following themes have been carried out in the Chigasaki Laboratory:

1. Performance and durability of 8B modules made in Japan

2. Simplification of pretreatment system

3. Establishment of energy recovery system.

Domestic modules showed good and stable performance during long term operation, and water recovery ratio of these modules have been raised to 40%.

In-line coagulation and filtration system has been established for the pretreatment of feed sea water, instead of coagulation, sedimentation and filtration system.

The energy recovery equipment is consisted of a high-pressure pump, a motor and a hydraulic turbine on a common base. Recovered energy from pressurized brine is used for the auxiliary motive power of the high-pressure pump. The experimental data show that about 20% of required power for the pump was recovered.     

Comparative economics of current and advanced desalting systems

A comparative investigation of the economics of desalting based on current and projected technology has been made. Current operating cost of various plant types operating in Israel are reported. These costs range from less than $.4/m³ for membrane plants desalting brackish water to more than three times as much for thermal plants desalting seawater. For new systems, two plant sizes were evaluated: 4,000 m³/day plants applying current technology and 100,000 m³/day plants applying projected technology. The water costs obtained for the various plant types and applied economic parameters, especially energy prices, range between $.2/m³ and $.6/m³ for brackish water desalting and from $.5/m³ to $2.4/m³ for seawater desalting.     

Measurements of dynamic behavior of a multistage flash water desalination system

This study focuses on measuring the process dynamics for the multistage flash desalination process (MSF) in an industrial unit with a capacity of 4546 m³/d. This is a novel addition to the literature because previous studies are limited to theoretical analysis of process dynamics or controller tuning, as well as conceptual design of conventional or advanced control systems. The measurements evaluate the performance of seven control loops, which include the pressure, temperature, and flow rate of the heating steam; the pressure of the vacuum ejector; and flow rates of the brine recycle, make-up seawater, and cooling seawater. All measurements start from steady-state conditions. The system is then set on manual where all control units are disengaged. Subsequently, only one control valve is adjusted by ± 15% of its steady-state setting. A total of 14 experiments were performed involving simultaneous measurements of the system variables. Measurements showed non-linear behavior where increasing or decreasing the valve settings did not provide similar trends. Analysis of results shows that one of the most sensitive variables is the distillate level in the last stage: the distillate trays either were flooded or became dry. The brine level in the last flashing stage was also found to be sensitive to valve settings where level increase resulted in higher product salinity. The results and analysis presented provide a better understanding in system fault analysis which could be caused by improper operating conditions. These data are essential to propose, design, and evaluate advanced/comprehensive control systems for the MSF process.     

ABSORPTION OF SO2 AND H2S IN SMALL-SCALE VENTURI SCRUBBERS

Small-scale venturi scrubbers having geometries of typical large-scale units were constructed from glass and were operated in a laboratory system. Carrier gas streams consisting of air or nitrogen and up to 30% (by volume) CO₂ were mixed with SO₂ or H₂S to give SO₂ or H₂S concentrations in the range of 1890-4400 ppm. These gas mixtures were then scrubbed in the Venturis using various injected liquids (plain water, NaOH solutions, NH₄OH solutions, and NaOH/NaHCO₃ solutions) at L/G ratios of 0.0004-0.0040 m³ liquid per standard (1 atm, 60°F)m³ of gas (3-30 gallons liquid per 1000 standard ft³ of gas). Absorption percentages for SO₂ and H₂S were determined as functions of the L/G ratio, initial liquid pH, and liquid composition. The effects of venturi throat length, gas velocity, and the presence of CO₂ in the gas stream were also determined.     

Maintenance problems of the electrodialysis plants in Libya

Many electrodialysis plants have been installed in the last seven years in Libya. In this paper we shall analyse the operation features, the difficulties occurring in pretreatment and degasification, the locally adopted solutions to the maintenance problems encountered and last but not least why the operational costs are far beyond the tolerable allowance margin.The two plants studied extensively are Benina and Dahra—Benina because it was intended to be the world's largest ED plant with an installed capacity of 19,200m³/day (5.1mgd), and Dahra because of its unusually brackish water supply. Other electrodialysis plants are also considered and in each case the effects of the deteriorating input of brackish water, the pretreatment, the design gaps and sometimes faults are indicated together with the solutions developed on-site.Finally, using Libyan field experience with electrodialysis, some recommendations are presented for developing countries with ambitious desalination programs.     

Present status of electrodialysis process in Japan

This paper describes the present activities of the electrodialysis process in Japan. The ion-exchange membranes and the electrodialyzers are supplied mostly by the three companies. The physical and chemical properties of the membrenes have good performance. The electrodialyzer units which have 50 to 1,800 pairs of 0.03 to 1.6 m² membranes are used for the desalination. Many commercial plants which have the capacities of 1 to 12,000 m³/day are in operation for the desalination of brackish water or sea water, and a few test plants have been operated in order to the desalination of municipal or industrial waste water for reuse.     

重庆市核心城区污水处理厂原址提标扩能运行实例

重庆市主城区核心地段某污水处理厂面对急需提标扩能以及日益突出的“厂群”矛盾等多重难题,在无法迁建扩建的情况下,利用重庆市山地地形特点,改造为半地下式污水处理厂,在不增加用地的情况下,实现了原址提一级A标准和扩能1.2×10⁴ m³/d,并在上部修建市民广场和综合办公大楼,吨水用地为0.36 m²/(m³·d),远低于国家和行业同规模污水厂用地的指标[0.95～1.20 m²/(m³·d)]。臭气集中收集处理后沿综合办公大楼排放管进行100 m高空排放,实现了空间资源利用和环境友好最优化。文中介绍了前置高效沉淀池和前置反硝化脱氮和硝化功能的两级曝气生物滤池工艺的选择与设计,以及主要设计参数,通过环保验收后1整年的运行,出水稳定达到并优于《城镇污水处理厂污染物排放标准》(GB 18918—2002)一级A标准,为类似工程案例提供参考。     

Design and construction of 4 × 1440 t/day MSF plants at Brindisi's power station (Italy): Acid / antiscale additive treatment for 100°C top brine temperature.

This paper reports the main parameters of the project, made for ENEL (Italian Electrical Power Government Company), regarding the design and supply of four MSF desalination plants at Brindisi's Power Station.The evaporators will be operated with anti-scale additive but are designed both for sulphuric acid and polymer antiscale treatment to operate at 100°C max. as top brine temperature.The distillate production (60 t/h each unit) will be used as high pressure boiler water make-up, then the distillate, mixed with the condensate coming from each unit, is demineralized by mixed bed ion exchange resin system; for this purpose, three mixed bed units are supplied.The paper reports the main design data, characteristics and materials of the above units.     

An online cleaning system to reduce demister fouling in MSF Sidi Krir Desalination Plant, 2 × 5000 m/day

This paper discusses the demister scaling issue in (MSF) 2 × 5000 m³/day Sidi Krir Desalination Unit’s site, and how like this issue affects directly in the performance of this system and in production rate too, which decreases gradually till 50%, in addition the unscheduled outage costs for acid cleaning or replacement of these demister sheets by new ones.

The fixing of an online cleaning system during operation was proposed using the flow of condensate brine heater discharge pumps through two headers are fixed at the both lateral side of those stages in which the scaled demisters were recorded. The proposed system will maintain the performance and saves the costs of chemical cleaning and the unscheduled outage of the system. The technical and economical advantages of this proposal are highlighted.     

中试平台SCR低温催化剂测试与氨逃逸分布特征

目前燃煤机组低负荷运行的情况越来越多,研究机组低负荷运行时NO_x控制技术可有效降低燃煤机组的NO_x排放浓度。本文利用已建成的20000m³/h的实际燃煤烟气的污染物脱除试验平台,安装低温段脱硝催化剂整体模块,测试该催化剂的性能指标,现场取样并重点分析氨逃逸和转化率指标,得出实际烟气条件下氨逃逸分布特征。结果表明：该低温脱硝催化剂的低温段在290~250℃范围内,脱硝效率可达到80%,脱硝出口NO_x可控制在50mg/m³（标准）以内,甚至20mg/m³以内,氨逃逸最高为1.68mg/m³,小于标准要求值2.28mg/m³,SO₂/SO₃转化率最高为0.73%,小于标准要求值1%,满足运行要求。文中进一步研究了氨逃逸浓度沿程分布情况,经测试表明,290℃时沿程氨逃逸浓度不断降低,除尘器前降低32.2%,除尘器后降低65.5%。     

VOLATILE ORGANIC COMPOUND (VOC) REMOVAL BY ADSORPTION ONTO ACTIVATED CARBON FIBER CLOTH AND ELECTROTHERMAL DESORPTION: AN INDUSTRIAL APPLICATION

Previous studies have shown good adsorption of volatile organic compounds (VOC) present in air onto activated carbon fiber cloths (ACFCs). In a chemical plant, a treatment process was carried out to control a gaseous emission (flow close to 100 m³ h^-1) loaded with methylene chloride at concentrations ranging between 3 and 30 g m^-3. A final mass flow lower than 100 g h^-1 was required in the outlet emission. Two identical cylindrical ACFC modules worked alternately in an adsorption-desorption cycle. The outlet pollutant mass flow was found to be less than 4 g h^-1. After saturation of the adsorbent, the module was electrothermally regenerated. Desorbed methylene chloride was recovered in a cryogenic trap and reused in the chemical processes of the plant. This system worked continuously for more than 18 months (24 h per day) with no operating problems and giving good performance in terms of VOC outlet emission concentrations.     

基于环三磷腈磷氮阻燃剂的合成及其在聚氨酯泡沫的应用

以六氯环三磷腈、对羟基苯甲醛、苯胺及亚磷酸二乙酯等原料,成功合成阻燃剂六（4-苯胺基次甲基苯氧基-亚磷酸二乙酯基）环三磷腈（HADPPCP）,用于阻燃基于苹果酸多元醇的聚氨酯硬泡。HADPPCP具有良好的热稳定性和成炭性,氮气气氛下的初始分解温度为191.9℃,700℃时的残炭量高到46.8%（质量）。HADPPCP的加入可以改善聚氨酯硬泡的热稳定性、阻燃性能和燃烧行为。添加25%(质量)的HADPPCP可以将聚氨酯泡沫的氧指数从18%提高到25%,最大热释放速率和总热释放量分别从230 kW/m²和20.1 MJ/m²降低至213 kW/m²和16.6 MJ/m²,总产烟量从10.5 m²下降到5.3 m²。     

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.     

Energy production at the Dead Sea by pressure-retarded osmosis: challenge or chimera?

In recent years two types of very large-scale plants have been proposed for handling seawater brought to the Dead Sea, both processes taking advantage of the 400 m drop to Dead Sea level and both sized to replenish the 3,000,000 m³/d evaporation rate of the Dead Sea. Pressure-retarded osmosis (PRO), the process discussed herein, would use the replenishment stream to produce an appreciable amount of benign and renewable electric power. If the seawater plant prior to PRO would be reverse osmosis (RO), handling 5,000,000 m³/d to produce 2,000,000 m³/d of fresh water, PRO could produce 48,000 kW from the RO-concentrated seawater feed at a capital cost for power of about $4,000 per kilowatt and a PRO plant cost of $190,000,000. The electrical energy would be produced at a cost of about $0.07/kWh. The PRO plant would use a DuPont B-9 type or similar hollow fiber modified to have 110 and 320 micron internal and outer diameters (instead of 40 and 90). Osmotic permeation of half of the 3,000,000 m³/d RO reject brine into Dead Sea brine would produce 35 atmospheres of hydrostatic pressure relieved by passage of an equivalent volumetric rate of diluted Dead Sea brine through a hydroturbine/generator set. The second type of plant prior to PRO would use 3,000,000 m³/d of seawater to produce hydropower, estimated at about 130,000 kW. The permeation rate in PRO could then be 2,000,000 m³/d enabling power production in PRO of 70,000 kW at a capital cost for power of $3,300 per kilowatt and a PRO plant cost of $230,000,000. The cost of produced energy in PRO would be $0.058/kWh. It is believed that the Great Salt Lake should also be examined as a site for PRO.