Multi-stage Flash Desalination Plant Research Articles

Modelling and simulation of a multistage flash (MSF) desalination plant

The paper describes the work done on modelling and simulation of a multistage flash desalination plant containing 15 recovery and 3 rejection stages. Both the steady-state and dynamic simulations have been carried out through the use of SPEEDUP package. In addition, a specifically written FORTRAN program is used for the steady-state simulation based on tridiagonal matrix formulation. Good agreement is achieved by comparing with the vendor supplied as well as actual plant data.

Towards improved automation for desalination processes, Part II: Intelligent control

The frontiers of automatic control are expanding to keep abreast of the state-of-the-art technology. Innovative concepts in control systems have relegated what was considered a satisfactory solution a decade ago to obsolescence today. One novel feature is artificial intelligence (AI) which has ability to cope with uncertainties that we encounter in today's complex process control. Intelligent control systems are becoming almost commonplace worldwide incorporating special features such as expert systems, self-tuning, self-diagnosis, life management, equipment health monitoring, modelling and simulation. Thus, a trend towards larger desalination plants necessitates more sophisticated automation technology. In view of the benefits that can be realized from the application of artificial intelligence in the control field of desalination processes, intelligent control is the application of AI in control, employing methods like expert systems, neural networks, fuzzy logic and pattern recognition. Thereby, learning and decision making are the most important features of intelligent control. The application of intelligent control is necessary due to computational complexity, nonlinear behaviour with many degrees of freedom, and the presence of uncertainty in control environment. Intelligent control can be used at different levels of control systems. On the highest level, monitoring of data, selection of algorithms, selection of objective functions, and assigning of values for the setpoints are the main aims. On the level of advanced regulatory control, process identification can be supported by intelligent control, e.g. selecting of structures by expert systems, implementing neural networks for parameter estimation etc. Expert systems, assisted by pattern recognition, can be used at this level for tuning of adaptive controllers. Adaptation of controller parameters according to an intelligent strategy can also be provided by neural networks. Also, expert systems can be used to assist the design of advanced control systems. In the large field of design of conventional control systems, artificial intelligence methods can be used to provide intelligent user interface, to select the best algorithms, to consider design heuristics for optimization of feedback control loops etc. The varying and uncertain conditions in a multistage flash (MSF) desalination plant during its operation in the annual cycle of seasons under different situations of loads and disturbances, point out that no single fixed strategy, no matter how advanced it may be, is likely to be either valid or possible at all times under all conditions in all the sub-systems related to the plant. This paper is addressed to some aspects of artificial intelligence (AI) which aims at equipping the process with the capability for continual analysis and assessment of the situations that arise during the course of operation of the plant leading to the choice and implementation of an appropriate course of action in process control. At the outset MSF plant conditions calling for consideration of advanced control strategies with AI support are highlighted. Then some aspects of AI applicable to plant control are reviewed, finally, suggestions towards intelligent control of MSF plants are made pointing also to the possibility of an integrated system for Process Control and Care.

Corrosion of 316L austenitic stainless steel under desalination plant conditions

The AISI type 316L stainless steel liner plates in the flash chambers of a multistage flash desalination plant, located on the Arabian Gulf coast, developed severe corrosion within 6 years of operation. The mode and causes of corrosion and the effect of heating (during annealing or welding) and temperature of salt solution on this attack were investigated. Specimens of liner plate were studied in the as received condition and after being heat treated at 900°C in air and air cooled to room temperature. Potentiodynamic polarisation techniques were used to determine corrosion rates. It was found that commercial grade 316L is susceptible to pitting, crevice, and grain boundary corrosion under the operating conditions in the desalination plant. The heat affected zone had larger grains and corroded more severely than other parts of the liner plates. Randomly distributed inclusions containing titanium, chromium, molybdenum, manganese, and sulphur were found in the liner plates. The as received specimens showed the lowest corrosion rate, followed in ascending order by heat treated and heat affected zone specimens.

Experience of using the neural network approach for identification of MSF desalination plants

Mathematical models of physical and engineering systems are frequently of high dimension, possessing interacting dynamic phenomena. Information processing requirements for experimenting with these models for control purposes are excessive. It is therefore natural to techniques that reduce the computational effort. In this paper, the neural network approach for nonlinear identification of the multistage flash desalination plant (large scale system) is presented through the manipulation of collected input-output data from the plant. Results of utilizing such an approach are also discussed.

An overview of modern control strategies for optimizing thermal desalination plants

Water is the essence of life in arid Middle East Countries. The implementation of robust control strategies enhance the efficiency, safety and reliability of the desalination process. A higher degree of automation is essential for today's large scale complex desalination plants. To achieve optimal design and operation of these plants, careful control system design is required. Even when an acceptable design is completed, the question remains as to whether a better design could be found. Conventional techniques provide for analysis and design of the systems which more or less satisfy arbitrary set of performance requirements. However modern control techniques provide an optimum solution based on specified performance index. In this paper an exposition of merits and demerits of conventional and modern control strategies is outlined. A number of optimal procedures that are used for system identification are highlighted. A brief description of Multi-Stage Flash (MSF) desalination process and controls is given. Novel control strategies for a number of important loops of a large MSF desalination plant are presented.

Evaluation of energy recovery in reverse osmosis desalination plants

Desalination plants consume considerable amounts of energy for producing fresh water from seawater. Multistage flash (MSF) desalination plants require as high as 193–290 KJ per Kg of fresh water produced depending on the maximum operationg temerature. Reverse osmosis (RO) for seawater only consume about 43 KJ per Kg of fresh water produced. This energy requirements of RO plants can be reduced approximately by 40% if the energy of the pressurized energy recovery in RO plants. These include Pelton turbine, reaction turbine, hydraulic power recovery turbine and reverse running centrifugal pump. This paper will evaluate the feasibility of using these systems in RO plants for obtaining lower water production cost.

Second law analysis of multieffect and multistage flash desalination plants

Although the first law of thermodynamics indicates the overall performance of the desalting plants, it does not show the actual losses from each component (i.e. process) of these plants. These can be revealed by an analysis where the second law of thermodynamics is applied to the multieffect, once through multistage flash, and recirculation multistage flash desalting plants. The exergy losses from the different processes in each plant are determined for each stage or effect. The proceses of major exergy are the vapor condensation (condenser exergy loss), vapor flashing (flashing exergy loss) and feed heating in the brine heater (heating exergy loss). The other losses such as heat transfer through walls, pressure drop through demister and friction flow in pipes, etc. are of minor magnitudes. For each plant, four parameters are interrelated (i.e. number of stages or effects N, gain ratio D/S, top brine temperature TBT, and heat transfer area A t). Now, as two parameters are kept constant, the exergy losses are plotted against the third or the fourth parameter. Finallly a comparison between the exergy losses is conducted for the three types of plants and for the same gain ratio. The major specific exergy losses are compared for the three plants. It is found that the multieffect plant is the least exergy wasting plant.

Performance of austenitic stainless steels in MSF desalination plant flash chambers in the arabian gulf

The corrosion behavior of AISI type 304L (304L (304L SS) and 316L (316L SS) stainless steels in a multi-stage flash (MSF) desalination plant located on the Arabian Gulf coast was investigated. The 304L and 316L stainless steel liner plates in the flash chambers of the plant developed severe localized corrosion within six years of operation. Liner plate samples retrieved from the floors of flash chambers were studied using optical and scanning electron microscopes. The study showed that both 304L and 316L SS liner plates suffered severe intergranular, pitting and crevice corrosion. Corrosion was more severe at welds and heat affected zones (HAZs). The main causes of corrosion were identified as (1) exposure of the liner plates to stagnant high chloride brine, and (2) grain growth and probably sensitization of the stainless steels at HAZ during welding and stress relieving heat treatment.

Potentiometric determination of fluoride and iodide in natural and processed waters of Bahrain using ion-selective electrodes

To assess the natural availability of fluoride and iodide in Bahrain, natural and processed waters which included ground water, tap waters, Adari Lake water and streams and waters from reverse osmosis (RO), ion-exchange (IE), multistage flash desalination (MSF) and solar desalination (SD) plants, in addition to blended waters, were monitored for 3 months (Oct.–Dec. 1985). The fluoride ion concentration ranges from 0.45 to 1.51 ± 0.26 mg dm −3 in natural waters; 0.003 to 0.33 ± 0.016 mg dm −3 in the processed waters and 0.15 to 0.24 ± 0.095 mg dm −3 in the blended waters. As for the iodide ion, concentration ranges from 0.02 to 1.08 ± 0.07 mg dm −3 in natural waters; 0.01 to 0.02 ± 0.002 mg dm −3 in both processed and blended water.

Optimum selection of cogeneration systems for power and desalination

In this paper, the different parameters affecting the optimum selection of cogeneration plants for electricity and water are postulated. For a given electrical and desalination capacity expansion, the question of the optimum sizing of the component plants was addressed. The objective was to select the plant arrangement which yields the minimum annual costs. Three different plant arrangements were selected, each being potentially capable of supplying the demands for electricity and heat. These arrangements are: (1) a combination of black-pressure and condensing steam turbines with a multi-stage flash (MSF) desalination plant; (2) a combination of extraction—condensing and purely condensing steam turbines with MSF desalination; and (3) a combination of a purely condensing steam plant with reverse osmosis desalination. The annual costs for each plant arrangement were postulated as a function of the basic operating and design parameters. These objective functions were then minimized subject to several constraints for a demonstration case having a power generation of 400 MW and a fresh water production of 181,800 mm 3/d (40 MIGD).

Optimum design for a hybrid desalting plant

Dual purpose multistage flash desalination plant provides fresh water with low cost at high desalting capacity, but requires high installment cost. Two stage reverse osmosis desalination plant requires only half of the multistage flash installment cost while producing water with comparable price. By combining sea water reverse osmosis plant with the dual purpose multistage flash plant, the capital and operating cost can be reduced and the excess power can be efficiently utilized. The design parameters for such a hybrid plant will be the applied pressure and the recovery of the reverse osmosis plant and the number of stages and heat transfer areas for the multistage flash plant. The objective is to minimize the cost of water satisfying maximum total dissolved salt. Different cost scenarios are suggested and their effect on the optimum parameters are investigated. It is concluded from this study that the savings obtained from scaling-up is more than that obtained from hydridization.

Formulas for calculating the approach to equilibrium in open channel flash evaporators for saline water

The evaporator used in multistage flash desalination plants, and considered for open cycle ocean-thermal energy conversion (OTEC) plants is the open-channel type, in which an essentially-horizontal stream of seawater is passed through an open channel in which it is exposed to a pressure lower than the saturation pressure which corresponds to its temperature. This paper reviews the equations used at present to determine the nonequilibrium allowance Δ′ which indicates how far from equilibrium the seawater stream is as it leaves the evaporator. Flash evaporation in open channels is used in most of the water desalination plants, and the existing equations for Δ′ have been developed for that application. The equations are typically empirical correlations, each developed for one type of geometry and range of parameters. The equations are compared graphically, plotted side-by-side to express Δ′ as a function of the major parameters: stage saturation temperature, stage flash-down, flashing seawater flow rate, stage length, and flashing seawater depth. It was found for water desalination applications that there exists a large spread between the non-equilibrium fraction values calculated by the different equations, of about one order of magnitude. The situation is even worse for OTEC conditions. Consequently, it was concluded that no general method exists for the adequately accurate prediction of Δ′, i.e. of the approach to equilibrium of flashing free stream channel flows.

A tridiagonal matrix model for multistage flash desalination plants

An efficient and reliable technique has been developed to solve the large system of non-linear equations which describe the behaviour of multistage flash (MSF) desalination plants. The new method depends on decomposition of the equations, after linearization, into subsets which are grouped by type rather than by stage, followed by formulation of the enthalpy balance equations into a tridiagonal matrix (TDM) form which is solved by the Thomas algorithm. The computer program developed in this work converged rapidly over a wide range of conditions, showing significant reduction in run times relative to those required by previous stage-by-stage approaches. Both recirculatory and once-through type plants have been simulated for different practical operating situations. The development of the linearized equations is discussed in detail, all physical property correlation are documented and full details of a numerical example given.

MSF (Multi Stage Flash) desalination plants: Copper Alloy Tubes, their economic aspects for overall cost reduction

MSF (Multi Stage Flash) desalination plants: Copper Alloy Tubes, their economic aspects for overall cost reduction

Operational strategies for Kuwait's 100 kW e/0.7MW th solar power plant

In March 1981 a 0.7 MW th solar thermal power plant was commissioned at Kuwait's Sulaibiya Solar Complex. The cogeneration of solar power plant was designed to be the main energy supplier for an agricultural desert settlement 35 km south-west of Kuwait City. The power plant produces both the electric and thermal energy needs for its own internal uses and those of the food/water/power complex. The electric users, outside the power plant's own needs, include water pumping from a 110 metre deep water well, an outdoor irrigation network, four desert greenhouses, a walk-in cooler, air conditioning, a reverse osmosis (R.O.) desalination plant, as well as the electric power needs for a multistage flash (M.S.F.) desalination plant, offices, workshop, data acquisition and lighting. The reject thermal energy from the power plant is utilized to power an M.S.F. desalination plant, and the domestic hot water needs. The power plant operational strategies are aimed at satisfying the energy needs for this food/water/power complex under prevailing solar radiation conditions while minimizing the inconvenience to the user (the complex) and maximizing the percent of the total energy derived from the sun (solar fraction). Surplus energy is stored as electric energy, thermal energy, or used to desalinate additional volumes of brackish water which can be stored in strategic water reservoirs. During periods of low solar radiation the power plant may be operated at partial load to supply the essential electric energy needs, charge the thermal storage, or provide the thermal energy needs for the MSF desalination system. An energy cost accounting system was developed to encourage the user to minimize his electric consumption during periods of low solar radiation. A mathematical energy model for the power plant was utilized to predict its output and suggest the optimum operational strategy according to the user's priorities, and predict surpluses or shortages that have to be accommodated by the emergency secondary energy source.

Dynamic stability of MSF desalination plants

In this paper a model is developed which describes the dynamic stability of large multi-stage flash (MSF) desalination plants. In this model, different types of thermodynamic losses in the flashing units and empirical correlations are used in describing the plant characteristics. The differential equations describing the plant are solved with the help of a special computer code. This code can calculate the effect of changing any operating variables on the stability of the plant. Calculated examples are given and compared with the experimental data available.

Construction and commissioning of 6 × 23,000 T/D MSF plant, Al Jobail phase 1

This paper outlines the construction and operational experience with 6 × 23,000 tons per day multi-stage flash desalination plants which were installed at Al-Jobail, Kingdom of Saudi Arabia. Module transportation and construction was adopted on this project to get the high quality of evaporator manufacture at shop and to shorten the construction period at site. During the reliability test of each evaporator unit, various types of anti-scale chemicals were applied to compare the performance results. Extended operation was also done to obtain long term performance results which might be applied as design criteria for other MSF evaporators.

Utilisation of hydraulic coupling for brine recirculation pumps

Due to steadily increased unit sizes for multi-stage-flash (MSF) sea water desalination plants using brine recirculation the requirement for brine flow regulation has gained more and more importance and this for technical as well as economical reasons. Aspects of flow regulation by hydraulic coupling are briefly reviewed and compared technically and economically with the usually used method of flow regulation by throttling. The influence on mechanical and hydraulic design criteria for brine recirculation pumps is investigated. Utilisation of hydraulic coupling for Brine Recirculation Pumps in a 2 × 10 000 m 3/d MSF desalination plant in the Ruwais Utilities Complex in Abu Dhabi, U.A.E. is introduced.

Computer code for optimizing MSF desalination plant

Egypt faces the prospect of population growth pressing on available agricultural land resources. A very large areas of coastal desert could be helpful in population density redistribution and of great economical interest, with the installation of a suitable water supply. Of the many desalination processes, the Multi-Stage Flash (MSF) desalination process is still the most developed and reliable technology for large scale conversion of sea water to fresh water. In this paper, a computer code was developed for optimizing a large scale MSF plant. This code can calculate the optimum design for the plant, and the minimum cost of product fresh water, as a function of both design and operating variables. Sample results obtained using this code for the Egyptian conditions are presented. The effect of, energy and material costs, the chemical treatment types, and the feed water salinity on the optimum performance ratio and the minimum fresh water cost are drawn.

Dynamic stability of MSF desalination plants

In this paper a model is developed which describes the dynamic stability of large multi-stage flash (MSF) desalination plants. In this model, different types of themodynamic losses in the flashing units and empirical correlations are used in describing the plant characteristics. The differential equations describing the plant are solved with the help of a special computer code. This code can calculate the effect of changing any operating variables on the stability of the plant. Calculated examples are given and compared with the experimental data available.