Multiple-effect Distillation Unit Research Articles

Thermodynamic analysis and energy efficiency of thermal desalination processes

This thermodynamic study examines the principles governing energy efficiency and specific energy requirement intrinsic to thermal desalination processes. The practical performances of desalination technologies are investigated and related to the fundamental physical limitations of the processes. The energy efficiency of any thermal desalination process fulfils a limitation similar to the well-known Carnot law for heat engines. The efficiency of a single-effect distiller is limited by a function of the boiling point elevation of the solution. Further analysis shows that, although this can appear as paradoxical, a higher energy efficiency is obtained by a solution with higher boiling point elevation. For multiple-effect distillation, the limit also depends on the temperature drops across the heat exchangers. Comparison with empirical data indicates that these limits are actually approached in real plants. Our study discusses the thermodynamic framework for understanding the performances of thermally-driven desalination technologies, emphasizing the role of the boiling point elevation. The results can be also seen as rules-of-thumb for designing and evaluating the performances of a multiple-effect distillation unit, based only on the properties of the solution to be distilled, in particular, the boiling point elevation.

A transient model for forward and parallel feed MED

The aim of this paper is to investigate the performance of a multiple effect distillation (MED) unit potentially coupled to a concentrated solar power plant and validate the results with the predictions of a dynamic model that was developed for this purpose. A small-scale (10 kWthermal) four effect distillation system was designed and built to demonstrate proof of principle of the concentrating solar power–desalinating sea water system integration. In order to fully characterize this small-scale MED unit, an understanding of the performance for steady state and transient conditions is required. Initially experiments were performed in a steady state situation, various parameters were examined and the experimental findings have already been published. After the initial experimental findings of the steady state operation, the performance of this unit was also investigated for transient conditions. The experimental procedure followed was identical as in the steady state conditions, with the main difference being the variation in the heat input supply to the system as a function of time. For the present study, the heat input supplied to the unit varied between 5 and 10 kWthermal. At the same time, a dynamic model was also developed in order to predict the performance of this unit in consecutive time steps of operation. The performance was calculated in terms of performance ratio (the ratio of the distillate product flow rate to the feeding steam flow rate) and the model results were validated against the experimental findings. The results showed that there is a really good match between the experimental data and the predicted ones from the model.

Simulation and economic evaluation of small-scale SOFC-GT-MED

AbstractThermal nature of multiple effect distillation (MED) process causes the integration of a desalination unit with a high-temperature power cycle like gas turbine in combination with the solid oxide fuel cell. The improvement in energy efficiency is achieved as a result of this combination. This configuration can be introduced as an effective system for the electrical energy–freshwater production. The electrical efficiency of solid oxide fuel cell-gas turbine (SOFC-GT) is reported about 60%. Small-scale MED unit is presented as a solution for heat recovery in the 300–1,000 kW range of (size of SOFC) SOFC-GT power cycle. The exhausted heat of SOFC-GT power cycle is used in heat recovery steam generator to produce a required motive steam for the desalination unit. Simulation, parametric studies, and economic analysis of proposed system are carried out to investigate the system performance. Economic analyses are applied based on the annualized cost of system method. Results show that the combination of ...

Experimental evaluation of a multiple-effect distillation unit in low seawater flow conditions

In this work, we evaluate experimentally the performance of a multiple-effect distillation (MED) unit in low seawater flow conditions and the potential of its integration with a concentrated solar power system. The innovation of this MED unit is the introduction of a flow distributor within the parallel plates of the falling film heat exchanger, designed to improve the system performance and efficiency under low seawater flow conditions. The main parameters examined were the thermal input power and the flow rate of the inlet seawater to each effect and the inlet seawater temperature of the single unit. Furthermore, the experimental results were compared with a control volume energy conservation model. The results showed that lower heat input load results to a higher value of the performance ratio (PR) of the unit and also under constant heat load, a higher temperature of the seawater lead to higher distillate product for the single effect unit. As the number of effects is increased the PR of the unit also increases approximately by 0.7 per effect. This maximum value for each effect is always observed in a constant ratio of seawater to steam flow rate.

Evaluation of a Multiple-Effect Distillation Unit under Partial Load Operating Conditions

The design of a multiple-effect distillation (MED) system is presented, and the results for partial load operation of a single-effect distillation unit are presented. The MED is designed to be driven by solar energy, and thus the dynamic performance and partial load operation production are of interest. Two operating modes are considered in the analysis, with and without the use of a flow distributor. Various tests were performed varying the heating steam flow rate and the intake seawater flow rate. Results are presented as a function of the performance ratio, representing the amount of distillate produced per unit mass of steam input. Results indicate that a higher performance is obtained with the use of the flow distributor.

Transient and dynamic performance of solar driven multiple effect distillation

Transient and dynamic performance of solar driven multiple-effect distillation (MED) system without thermal energy storage (storing of water rather than energy) are studied and presented. A typical 25,900 m3/day operational multiple effect distillation with thermal vapor compression (MED-TVC) is used for the transient and dynamic simulation. Two scenarios are studied for the transient performance to; i) pump the feed water flow and ii) spray feed water onto the horizontal tubes bundle in each effect. The first scenario uses a constant flow rate of feed water along most of operating hours. The second scenario assumes a sinusoidal trend for the feed water flow rate during the operating hours (simulating direct PV electrical generation with no electric energy storage). The dynamic performance is due to a disturbance of a rectangular pulse forcing function on the seawater flow for duration of 50 s, which increased 25% for 5 s. The response of that disturbance on the behavior of the MED process parameters, such as the distributions of the temperatures, mass flow rates, and physical properties is presented. The physical model of each effect is divided into seven compartments including brine pool, vapor space, tubes bundles and feed water pre-heater. Transient and dynamic equations for each compartment and other system components are formulated mathematically. Mass, salt and energy balance equations are solved dynamically taking into consideration the inter-related effect of each compartment onto the other. The transient resultant flow rates of each stream (vapor production, TVC, brine disposal, etc) are presented in details and compared to conventional steady operation. When using the feed water scenario of sinusoidal feed to operate the solar MED unit, the results indicates that the salt precipitation can increase above the designed value. The results show also that the dynamic behavior of feeding water coming into the effects is flows the same trend the disturbance that took place in the main source of seawater supply. The flow rates of feed water are affected by the applied disturbance and increases gradually from normal operation to a peak value then decreases again to the normal operation state value. The system undergoes this disturbance shows to be still stable in operation as the change in its behavior is just bounded around the disturbance time, and regain, in a very short time, to the initial (normal operation) value of starting point. Key words: Solar energy, desalination, dynamic behavior, MED-TVC.

Transient and dynamic performance of solar driven multiple effect distillation

Transient and dynamic performance of solar driven multiple-effect distillation (MED) system without thermal energy storage (storing of water rather than energy) are studied and presented. A typical 25,900 m3/day operational multiple effect distillation with thermal vapor compression (MED-TVC) is used for the transient and dynamic simulation. Two scenarios are studied for the transient performance to; i) pump the feed water flow and ii) spray feed water onto the horizontal tubes bundle in each effect. The first scenario uses a constant flow rate of feed water along most of operating hours. The second scenario assumes a sinusoidal trend for the feed water flow rate during the operating hours (simulating direct PV electrical generation with no electric energy storage). The dynamic performance is due to a disturbance of a rectangular pulse forcing function on the seawater flow for duration of 50 s, which increased 25% for 5 s. The response of that disturbance on the behavior of the MED process parameters, such as the distributions of the temperatures, mass flow rates, and physical properties is presented. The physical model of each effect is divided into seven compartments including brine pool, vapor space, tubes bundles and feed water pre-heater. Transient and dynamic equations for each compartment and other system components are formulated mathematically. Mass, salt and energy balance equations are solved dynamically taking into consideration the inter-related effect of each compartment onto the other. The transient resultant flow rates of each stream (vapor production, TVC, brine disposal, etc) are presented in details and compared to conventional steady operation. When using the feed water scenario of sinusoidal feed to operate the solar MED unit, the results indicates that the salt precipitation can increase above the designed value. The results show also that the dynamic behavior of feeding water coming into the effects is flows the same trend the disturbance that took place in the main source of seawater supply. The flow rates of feed water are affected by the applied disturbance and increases gradually from normal operation to a peak value then decreases again to the normal operation state value. The system undergoes this disturbance shows to be still stable in operation as the change in its behavior is just bounded around the disturbance time, and regain, in a very short time, to the initial (normal operation) value of starting point. Key words: Solar energy, desalination, dynamic behavior, MED-TVC.

Experimental and numerical performance of a multi-effect condensation–evaporation solar water distillation system

The main objective of the work is to demonstrate experimentally and numerically the performance of a simple solar distillation unit that is based on the multiple condensation–evaporation cycle. The pilot plant was designed, fabricated, tested and simulated at the solar energy laboratory, Mattarria Faculty of Engineering, Cairo, Egypt. The distillation chamber consists of a humidifier (evaporator) and a dehumidifier (condenser) units. The circulation of air in the two units is maintained by natural convection. The cold salt water is preheated inside the distillation unit before exchanging heat with the solar collector loop. This plant has a flat-plate collector field area of 3.1 m 2, it constitutes a closed loop with its own storage tank. The research is then carried out to evaluate the unit performance of such design and to estimate the fraction factor of the solar system to the load. A numerical simulation was developed for the system being considered. A detailed annual performance of the system is presented. The annual variation of the temperatures and useful heat gain were estimated for the system components. In addition, the optimum collector area by which the system has the maximum life-cycle savings and solar fraction was obtained. The comparison between the numerical and experimental results are accepted. The multiple-effect distillation unit that is considered in the study produces 24 l/day of distilled water. The system performance can be accepted according to the previous edited results.

Effect of dust deposition on the performance of a solar desalination plant operating in an arid desert area

The performance of a solar desalination plant (whether using thermal or photovoltaic collectors) is influenced by the ability of the glazing system to transmit solar radiation to the collector absorption surface. This ability is influenced by such factors as the intensity of solar radiation, the transmittance of the collector glazing, the tilt angle of the absorbing surface, the operating parameters of the plant, the properties of the materials of construction, etc. This paper discusses the influence of dust deposition on the evacuated tube collector field on the operating performance of the solar desalination plant at Abu Dhabi, UAE. This plant has a collector field area of 1864 m 2 of absorber surface and an MED (multiple effect distillation) unit for seawater desalination with a capacity of 120 m 3/day of distilled water. The reduction in transmittance due to dust deposition on the amount of heat collected has been measured and its influence on the distillate production has been estimated using the computer simulation program SOLDES which has been verified previously as an effective tool for predicting the operating performance of similar plant designs. The frequency of high-pressure water jet cleaning on the performance of the plant was also investigated. It was found that dust deposition and its effect on plant performance depend strongly on the season of the year and the frequency of jet cleaning should be adjusted accordingly.

New aspects in solar powered seawater distillation

This paper deals with the cost reductions which are becoming possible in the field of potable water production by solar powered seawater distillation. It describes a plant having a production capacity of 600 tons of potable water per day. Heat is largely supplied by low cost solar heat collectors, forming a part of a combined heating system which includes conventional heat sources. The plant also includes a multiple-effect distillation unit incorporating optimized design parameters. It can be shown that by using this combined system water production costs can be obtained which are competitive with those of conventionally powered seawater distillation plants.