MED Process Research Articles

Thermo-economic assessment of forward osmosis as pretreatment to boost the performance and sustainability of multi-effect distillation for seawater desalination

This work presents the thermodynamic and techno-economic assessment of a forward osmosis – multi-effect distillation (FO-MED) system able to improve the thermal performance of the MED seawater desalination process and reduce its environmental impact. Energy, exergy and techno-economic analyses have been carried out to identify the best boundary conditions that enhance the process efficiency and water cost, predicting the scale formation with the Ryznar index. Results show that using FO pretreatment in a 100 m3/d MED plant of 8 effects at 65 °C allows increasing the heating steam temperature up to 100 °C and the number of effects to 16 without increasing the risk of scaling. This results in 44 % reduction of the specific thermal energy consumption and 21 % decrease in the specific heat transfer area. The volume of saline water rejected to the sea is reduced 40 % and the water footprint 36 %. The exergy analysis reveals that the MED is the component with the highest exergy destruction. Finally, the levelized cost of water of the FO-MED is higher (7.6 $/m3) than the standalone MED process (4.6 $/m3). If high-performing and cost-effective membranes are used, the product cost could be reduced 33 % (5.1 $/m3).

Electro-kinetically upgraded sustainable approach and process optimization for synthesis of dual nutrient potassium fertilizer from ammoniacal filtrate wastewater model

Traditional methods for wastewater repurposing and value-added product recovery are associated with operational shortcomings viz. high energy requirements, poor conversion efficiency, purity and sustainability, thus it is highly desirable to upgrade the same via. zero-emission sustainable processes. Herein, electro-kinetically upgraded sustainable approach “metathesis electrodialysis” (mED) is developed and meticulously optimized for synthesis of high value potassium nitrate (KNO3) using low cost KCl and ammoniacal filtrate NH4NO3 electrolyte model as reactants. mED experiments have been carried out using indigenously designed interpolymer based cation and anion exchange membranes with high counter-ion transport selectivity. In order to optimize suitable potential for mED process, the influence of varied applied potential from 1.5 V/cell pair to 2.5 V/cell pair and its effect on KCl to KNO3 conversion, KNO3 purity, and mED process efficiency have been investigated. In particular, 2.0 V/cell pair applied potential has been found optimum with 97.67% KNO3 purity and 61.87% energy efficiency of mED process. Further, effect of initial feed concentration of KCl and NH4NO3 have been investigated in greater details. Up to 94% KCl to KNO3 conversion and 96.71% product purity has been achieved at optimized applied potential of 2.0 V/cell pair using 0.8 M initial feed concentrations of KCl and NH4NO3. Under these operating parameters, mED showed moderate energy consumption of 1.25 kWhkg-1 with 91.73% energy efficiency. The product purity was evaluated in comparison to commercial grade KNO3 using FT-IR and XRD analysis. Moreover, KNO3 purity exceeded 98% when mED operated using 0.2 M initial feed concentration. It is interesting to note that electro-osmotic water transport and solute back diffusion have not been realized during mED process. Finally, based on above results we anticipate that mED has great potential application in chloride free potassium fertilizer synthesis, especially KNO3.

Dynamic Modeling and Simulation of the Forward Feed MED-TVC Desalination Plant

The sudden disturbances in the operating parameters of desalination plants are common, especially when these plants are integrated with concentrated solar power (CSP). These disturbances damage the plants over time and may lead to a complete shutdown. This paper aims to investigate the performance of a forward feed multi-effect distillation plant with a thermal vapor compressor (FF-MED-TVC) which has a high potential for integration with concentrated solar power (CSP). However, the fluctuation and instability of solar energy require the development of dynamic model for the MED process to analyze the transient behavior. Few papers were published in the transient state, especially the forward feed (FF) configuration, as most of the papers focused on the parallel feed (PF) and parallel cross feed (PCF) configuration. Accordingly, a mathematical model has been developed using Engineering Equation Solver (EES) software and validated against data reported from two previous models in the literature, where a perfect agreement was obtained. Then the proposed model was employed to predict the system's response to the most important parameters that may change suddenly in the real operating environment, such as the variation in the motive steam pressure, feed seawater mass flow rate, and temperature. For the same percentage (10%) of these three disturbances, the results indicated that the needed time to return to 95 % of steady state is the longest in the case of increasing the seawater mass flow rate (about 350 seconds after the turbulence is removed), while it is the shortest in the case of increasing the pressure of motive steam (about 50 seconds after the turbulence is removed). In this paper, a comprehensive analysis of the transient performance was performed and a clearer view of the dynamic response was given which enables to optimize the control strategy and improve the stability.

Enhancement of Feed Forward Multi Effect Evaporator Performance for Water Desalination Using PI Control

The increasing of population needs safe, reliable and consistent supply of water had made many manufacturing industries and water treatment plants to look for efficient desalination plants.There are two major types of desalination technologies around the world, namely membrane desalination and thermal desalination . MEE is one of the types of thermal desalination. MED process operates in a series of evaporator condenser vessels called effects and uses the principle of reducing the ambient pressurein the various effects. A novel algorithm to solve the steady state analysis problem of three effects feed forward multi effect evaporator (FF MEE) for water desalination is investigated. A dynamic model is derived for MEE. FF MEE dynamic model is controlled using a Proportional-Integral (PI) controller that designed to improve its performance against the variation of the cold water temperature. Simulation results show the effectiveness of the proposed design and calculation has been presented using MATLAB®.

A parametric simulation on the effect of the rejected brine temperature on the performance of multieffect distillation with thermal vapour compression desalination process and its environmental impacts

AbstractMultieffect distillation with thermal vapour compression (MED–TVC) is one of the most attractive thermal desalination technologies for the production of freshwater. Several mathematical models were presented in the open literature to analyse the steady‐state performance of such process. However, these models have several limitations and assumptions. Therefore, there remains the challenge of having a reliable model to accurately predict the performance of the MED process. Thus, this research attempts to resolve this challenge by rectifying the shortcomings of the models found in the literature and create a new one. The robustness of the developed model is evaluated against the actual data of Umm Al‐Nar commercial plant situated in UAE. In seawater desalinisation, a large amount of high‐salinity stream (brine) is rejected back into the sea. This paper investigates the influence of the rejected (exit) brine temperature on the system performance parameters of MED–TVC process. Specifically, these parameters are considered as total heat consumption, gain output ratio, freshwater production, heat transfer area and performance ratio. Also, the particular parameters of TVC section of the entrainment ratio, compression ratio and expansion ratio are also addressed. Moreover, a critical evaluation of the influence of the rejected brine temperature on the seawater is also embedded.

The Effect of Feed Steam on MED Dealing with High-Salinity Wastewater

The treatment of high salinity wastewater treatment in refinery has been widely noticed due to more and more stringent laws and regulations. In the paper, in order to deal with the high salinity wastewater efficiently, a MED process was performed, and mathematical modeling was established to discuss the effect of feed steam on MED dealing with high-salinity wastewater. The results indicated that the temperature of steam produced by different low-temperature heat and pressures of the steam could all affect the MED system obviously. For example, the mass of pure water increased slightly with the increase of feed steam temperature in 3 effects system. With the same pressure feed steam, more mass of pure water was produced by fewer effects system, etc. This paper would provide a very useful reference for the high-salinity wastewater reduction via MED process in petroleum refinery.

Utilisation of the exergy method for the cost evaluation of integrated nuclear desalination systems

At present, the DEEP code, developed by the IAEA is being used internationally for the cost evaluation of integrated nuclear desalination systems. However, economic models in DEEP are often criticised because they are based on the power credit method in which the allocation of the benefits from the sale of one product (electricity) are arbitrarily attributed to the second product (desalted water). To determine the costs of the two products in an integrated nuclear desalination system, we thus used the exergy method which determines the useful work done by the two products in a more equitable manner, using the second and the third laws of thermodynamics. This paper summarises the first results of an economic evaluation of three integrated systems based on the utilisation of the French PWR900, the AP-600 (Westinghouse) and the gas turbine, combined cycle plant (CC900), all coupled to an MED process, and operating in a co-generation mode, producing about 288,000 m3/day of desalted water. These results are co...

Steam driven large multi effect MVC (SD MVC) desalination process for lower energy consumption and desalination costs

It has been published and demonstrated in operating plants that the low-temperature multi effect desalination process (LT MED) when operated on turbine backpressure steam requires minimal energy consumption yielding specific desalination energy costs approaching values of optimal RO energy costs. However due to variations in electricity demand, the operation of large steam turbines at base load is in most cases impractical. As a result, in order to allow variations in electricity supply and full capacity of the desalination extraction, steam extracted from the turbines at higher pressures than the back pressure is commonly used. The extraction steam pressure of these turbines is in the range of 1.5–4.5 barg. (Typically as required for process steam at refineries and for MSF desalination plants). The LT MED process when utilizing such extraction steam incorporates thermo compressors (steam ejectors) in order to take advantage of the excessive extraction steam pressure, thereby increasing the economy ratio of the MED (decreasing energy consumption). Thermo compressors (ejectors) as a rule have a relatively low adiabatic efficiency, thus limiting the potential of the MED to increase the economy ratio even more. In this paper an approach of using a novel large centrifugal compressor driven by an auxiliary steam turbine utilizing the extraction steam is incorporated instead of the thermo compressor for large LT MED plants. The centrifugal compressor and auxiliary turbine having a much higher efficiency than the thermo compressor results in significant energy savings, thus lowering the desalination costs. This paper will also describe the thermodynamic advantages, the specific investment and desalination costs comparison, and the resulting desalination cost reduction potential of the steam-driven multi effect MVC process.

Development of a computer-package for MED plant dynamics

This paper describes a computer package (SIMED) that we have developed to simulate the dynamics of different Multi-Effect Evaporators systems. SIMED is intended to serve as an educational tool to study in particular the MED coupling to nuclear reactors such as GT-MHR, PBMR (high temperature reactors), PWRs, or to fossile energy sources such as the gas turbine combined cycle or the simple oil or gas burners. It is designed to improve the understanding of the MED process, develop a control strategy for plant dynamics and optimise process operating conditions. SIMED supports mainly two flow configurations—depending on the seawater feed system architecture: (1) typical MED, in which sea water is preheated in the final condenser then fed to all effect in parallel. (2) MED with sea water preheat in the final condenser (and other MED effects) then fed to first effect. For each configuration the heat source could be either hot water, provided by an intermediate loop, interface between the nuclear reactor and the desalination process, or the steam, provided by a flash tank, a device used to produce steam trough the depressurisation of hot water based on an intermediate loop. SIMED runs under the Windows 9x/2000/XP operating system. It offers a user–friendly graphical user interface consisting of two modules; the Simulation Wizard and the Post Processor. Simulation Wizard’s role is to guide the user through the steps required to create data files for simulation. These steps include the choice of system architecture, components characteristics, scenarios, and calculation options. Processor is built around tools for the display of simulation resystem flow sheets, variation of important parameters etc. The physical models of SIMED derive from basic mass, energy and momentum conservation equations and supplementary correlation for heat transfer and physical properties. Conservation laws are applied separately to fluid phases (brine pool, vapour space) in system components such as MED effects, condensers, coolers, pumps and ducts. A mathematical model of the whole process is generated automatically depending on user choices concerning system architecture. The models, and the results of their application, in particular to a PWR+MED system, show very promising results.

Experimental study of a three-effect plate evaporator: seawater tests in La Spezia

EasyMED, a new MED process constructed with plates has shown good performances for desalination of salted water in laboratory with only one effect [1]. In order to prove the reliability of this process in real conditions, a three effects evaporator was recently built in La Spezia Italy (see Fig. 1). This unit is composed of three effects in series and five cells in parallel and is heated by hot water at about 75°C. 3 m 3/d of high quality distillate with a conductivity lower than 20 μS/cm was obtained. The evaporation ratio ranged between 8 and 13%. To avoid large consumption of energy to preheat seawater up to its boiling point, about 80% of the brine is recycled to the evaporator inlet. The overall heat transfer coefficients between heating water and seawater falling film reached 1400 W·m −2·K −1 whereas the coefficient between condensing vapour and evaporating seawater is about 4000 W·m −2·K −1. Temperature difference between the heating cells and the first evaporation cells is around 11°C. For the rest of the unit, the temperature difference between condensation cells and evaporation cells of the next effect is between 3 and 6°C. The first results concerning the influence of the heating fluid temperature and the heating fluid flowrate are presented.

Economic competitiveness of nuclear desalination in Libya

Libya is a very arid country with poor water resources that relies mostly on rainfall and underground water. It has been suffering from water shortage problems for the last three decades due to lack of rainfall, which has led to the over-exploitation of ground water, and an increase in population. The water deficit is expected to reach about 4700 mm 3/y. Since 85% of the population is concentrated along the coastal area, desalting of seawater is essential to meet the demand for potable water. Desalination is an energy-intensive process, thereby the required energy to desalt such large amounts of seawater is very considerable. Selecting the proper energy source will have a positive effect on the product water cost. A comparison of different energy sources coupled to commercially available desalination technologies was performed to assess economic competitiveness of nuclear desalination in Libya for two selected sites: the Tripoli site (Site 1) and the Sirt site (Site 11), with a production capacity of 250, 000 m 3/d and 40, 000 m 3/d, respectively. The results of comparing the product water cost for the three major desalination processes MSF, MED and RO, show ranges of values from a minimum of 0.87$/m 3 for the MED process coupled with a steam boiler at site 1, to a maximum of 1.78$/m 3 for the MSF process coupled with a SMART-type nuclear reactor at Site 11.

Desalinated water production optimisation using a high temperature reactor in a cogeneration mode

Production of desalinated water requires energy whether it is heat in evaporating processes or electricity for pumping at high pressure in reserve osmosis processes. In any of these processes, energy represents a substantial share of the final soft water cost, in the range of 50% for the MED process, the remaining part being the amortisation of the desalination unit and its maintenance and operation costs. Whilst efforts to improve the efficiency of the desalination unit and to decrease its operating and maintenance costs help decrease the water cost, a major effort in lowering water cost should come from the energy part. This is where the specificity of nuclear power and in particular of high temperature gas cooled reactors (HTRs) appears interesting. Nuclear plants are characterised by a high capital cost and low fuel cost; therefore, it is advantageous to base load them. Nevertheless, since electricity cannot be stored on a large scale, the plants must load follow, which means that during low consumption hours, electricity is potentially available at the marginal fuel cost which is very low. We will consider these facts in detail in the following, particularly specific HTR characteristics which permit lowering energy costs.

Forty-year design life: the next target Material selection and operating conditions in thermal desalination plants

The operational experience on the first generation of large MSF desalination plant has demonstrated that the original expected life of these units has been largely exceeded. Several contracts for the rehabilitation and upgrading of desalination units installed 20 years ago have been recently awarded, aiming at extending the life of these units by a further 15 years. Developments in materials technology have resulted in the adoption of nobler materials, and it is expected that the second generation of large MSF desalination plants installed in the last 10 years will last for more than 30 years with minimum maintenance and minor overhauling. On this basis, it is assumed that a 40- to 50-year design life is a reasonable target which can be obtained if the material selection is optimized in respect of the operating conditions. The gradual emergence of the MED process in the market portion previously belonging to MSF technology suggests that an evaluation of the operating conditions and material selection for MED plants can also grant an expected life of 50 years. The sharp influence of material selection on plant cost has previously been demonstrated; therefore, the choice of “where” and “how” to invest in upgrading materials and the evaluation of the financial revenue, in term of extension of plant life and reduction of maintenance, are key technical aspects for the future of desalination. By comparing the various operating conditions occurring in the desalination units and the impact on corrosion/erosion of the materials used, this paper aims at giving guidelines that will allow material selection to be optimized with respect to the plant costs.

Design of the maximum energy efficiency desalination plant (PAME)

This report is the result of work carried out by the IDAE with the objective of defining the most suitable technique from an energy point of view for desalinating water using movable plants, which can be transferred from one site to another. Since the energy consumed in technologies available for desalinating seawater is electricity alone in reverse osmosis, and heat and electricity in distillation technologies (the proportion of which varies according to whether the VVC or MED process is employed), there is an opportunity to make simultaneous use of the electricity and heat produced in all electricity generation, although they can only be applied in distillation technologies and where the plant is designed to operate with a careful combination of the RO and distillation technologies. This optimization of energy can be achieved in all types of plants (permanent or movable), but it is of particular interest in the case of the movable plants because it makes the plant independent with regard to its energy consumption. In short, the plant does not depend on the national electricity network system. This independence offers advantages which will be studied later on. Indeed, the aim of this report is to describe the movable maximum energy efficiency plant and to analyze the costs of the same in comparison with other alternatives.

Optimizing and planning of seawater desalination

Many islands in the Caribbean depend on seawater as the only source for producing drinking water. As a result of a growing tourist industry, the need for desalination capacity is steadily growing. However, the cost of seawater desalination is high. Optimizing and planning of seawater desalination capacity can reduce total costs for drinking water. Studies have been conducted for Aruba, Bonaire and Curaçao. Co-generation systems for the combined production of electricity and water are in use on these islands. It is shown that the optimum size of evaporation (MSF, MED or VC) desalination units can be calculated in relation to the maximum required production capacity and storage capacity. Once the optimum combination of unit size and storage capacity is known, planning of extensions in production capacity can be calculated based on the forecast of water demand, available storage capacity and maintenance or breakdown periods. There is no optimum unit size for reverse osmosis (RO), the alternative desalination process. Because of its modular concept, extension with RO can be planned in any desired capacity. RO is shown to be a more economical system compared to evaporation desalination. Future extensions by RO therefore will further reduce costs for seawater desalination. The low-temperature MED process is shown to be economic in comparison with both MSF and RO in case of high energy costs and/or long expected lifetimes and a low rate of interest.

One year of operation of Israel Desalination Engineering's multi-effect distillation v.s. virgin islands

Three MED plants for 1.25 MGD were installed in St. Thomas and St. Croix by Israel Desalination Engineering. The Low Temperature MED plants made of horizontal aluminium tubes exceeded the acceptance criteria and continuously supply drinking water to the islands with purity of 16 ppm TDS. Since late 1981 the availability has exceeded 97% and the performance ratio is 12.4 or better. Annual average production was 1.3, 1.3 and 1.27 MGD. The most notable advantages of the MED plants are: 1. a. The easy start up and operation of the MED process and short period between start up and full capacity. 2. b. Specific Power Consumption does not change markedly when operating on partial load. 3. c. Low temperature operation results in nearly scale free plant and allows for the use of less expensive construction materials.

Minimum entropy deconvolution

Minimum Entropy Deconvolution (MED—Western Geological Company of America Service Mark) was developed to aid the extraction of detailed reflectivity information from amplitude anomalies in reflection seismic recordings. Interpreting such reflectivity information depends on accurate compensation for the combined effects of the source wavelet and earth filtering. MED approaches this problem by finding a linear operator that maximizes the “spike-like” character of a representative set of traces. Unlike predictive deconvolution, the MED process makes no assumptions about the phase characteristics of the “seismic wavelet”, nor does it assume that the reflection series is white—certainly a poor assumption over short-time windows where “bright spots” dominate. Rather than seeking to whiten data, the MED process seeks the smallest number of large spikes that is consistent with the data. Synthetic data examples show that the method can determine effective operators by exploiting quite small differential moveouts of events across unstacked traces. While maximizing the spikiness of output traces, the derived MED operator selectively suppresses frequencies over which the ratio of coherent signal to random noise is low. This noise suppression characteristic makes unstacked, output traces particularly suited for analysis of offset-dependent variations in reflectivity across bright spots.

The design, construction, testing and operation of a large scale prototype desalting plant in Israel

The various aspects of the joint U.S.-Israel desalting project for the design, construction and operation of a 10 mgd multiple-effect dual purpose plant are described in the paper. The background, the plan and the scope of the project are detailed as well as the characteristic features, advantages and disadvantages of the IDE process which is incorporated in the project. A forecast of the future activity in Israel concerning the use of this MED Process following this project is briefly outlined. Finally, a normalized cost evaluation of desalted water produced in similar plants is presented.