Hisham Ettouney

Analysis of single-effect evaporator desalination systems combined with vapor compression heat pumps

A comparison was performed for four different types of single-effect evaporator desalination systems. The systems are driven by vapor compression heat pumps including thermal (TVC), mechanical (MVC), absorption (ABVC), and adsorption (ADVC). The study includes the development of mathematical models for the four systems. The models include equations for energy and mass conservation. In addition, design equations are used to determine the heat transfer areas in the evaporator and the condenser. The analysis is based on comparison of the performance ratio, specific power consumption, specific heat transfer area, and specific cooling water flow rate. For the four systems the specific surface area for the evaporator and the condenser is decreased upon the increase of the boiling temperature. The performance ratio for the thermal vapor compression system is decreased as the boiling temperature and pressure of motive steam are increased. In the MVC system, the specific power consumption is found to decrease with the increase of the boiling temperature and its difference with the temperature of the compressed vapor. The ABVC and ADVC systems have higher potential than the other two configurations. This is because of the higher performance ratio found in both systems and the generation of hot utility water.

Steady‐State Analysis of the Multiple Effect Evaporation Desalination Process

desalting system. The algorithm consists of 10 calculation blocks and 6 logical blocks. The algorithm is implemented using L-A-S computer aided language. Results show that the heat transfer coefficients increase with the boiling temperature. Also, the heat transfer coefficient in the evaporator is always higher than that in the feed preheater at the same boiling temperature. The plant thermal performance ratio is nearly independent of the top brine temperature and strongly related to the number of effects. The specific heat transfer area increases by raising the number of effects and reducing the top brine temperature. The effect of the top brine temperature on the specific heat transfer area is more pronounced with a larger number of effects. The required specific heat transfer areas at a top brine temperature of 100 ∞C are 30.3% and 26% of that required at 60 ∞C when the number of effects are 6 and 12, respectively. The specific flow rate of cooling water is nearly constant at different values of top brine temperature and tapers off at a high rate as the number of effects is increased. Two correlations are developed to relate the heat transfer coefficients in the preheater and the evaporator to the boiling temperature. Design correlations are also developed to describe variations in the plant thermal performance, the specific heat transfer area, and the specific flow rate of cooling water in terms of the top brine temperature and the number of effects.

Evaluation of steam jet ejectors

Abstract Steam jet ejectors are an essential part in refrigeration and air conditioning, desalination, petroleum refining, petrochemical and chemical industries. The ejectors form an integral part of distillation columns, condensers and other heat exchange processes. In this study, semi-empirical models are developed for design and rating of steam jet ejectors. The model gives the entrainment ratio as a function of the expansion ratio and the pressures of the entrained vapor, motive steam and compressed vapor. Also, correlations are developed for the motive steam pressure at the nozzle exit as a function of the evaporator and condenser pressures and the area ratios as a function of the entrainment ratio and the stream pressures. This allows for full design of the ejector, where defining the ejector load and the pressures of the motive steam, evaporator and condenser gives the entrainment ratio, the motive steam pressure at the nozzle outlet and the cross section areas of the diffuser and the nozzle. The developed correlations are based on large database that includes manufacturer design data and experimental data. The model includes correlations for the choked flow with compression ratios above 1.8. In addition, a correlation is provided for the non-choked flow with compression ratios below 1.8. The values of the coefficient of determination ( R 2 ) are 0.85 and 0.78 for the choked and non-choked flow correlations, respectively. As for the correlations for the motive steam pressure at the nozzle outlet and the area ratios, all have R 2 values above 0.99.

Performance of parallel feed multiple effect evaporation system for seawater desalination

Abstract Performance analysis is presented for the parallel feed multiple effect evaporation system. Two operating modes are considered in the analysis, which includes the parallel and the parallel/cross flow systems. Analysis is performed as a function of the heating steam temperature, salinity of the intake seawater, and number of effects. Results are presented as a function of parameters controlling the unit product cost, which includes the specific heat transfer area, the thermal performance ratio, the conversion ratio, and the specific flow rate of the cooling water. Results indicate that better performance is obtained for the parallel/cross flow system. However, the parallel feed system has similar characteristics and simpler design and operation procedures. Performance of both systems is consistent with literature data. Comparison of the two parallel feed systems versus conventional multistage flash desalination and the forward feed multiple effect evaporation schemes show that the forward feed system has better performance characteristics than the other three systems.

Multiple-effect evaporation desalination systems. thermal analysis

Abstract Seawater desalination by parallel feed multiple-effect evaporation has a simple layout in comparison with other multiple-effect or multistage desalination systems. Several operating configurations are analyzed, including the parallel flow (MEE-P), the parallel/cross flow (MEE-PC), and systems combined with thermal (TVC) or mechanical (MVC) vapor compression. All models take into account dependence of the stream physical properties on temperature and salinity, thermodynamic losses, temperature depression in the vapor stream caused by pressure losses and the presence of nondashcondensable gases, and presence of the flashing ☐es. Analysis was performed as a function of the number of effects, the heating steam temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapor condensate and the brine blowdown. Results are presented as a function of parameters controlling the unit product cost, which include the specific heat transfer area, the thermal performance ratio, the specific power consumption, the conversion ratio, and the specific flow rate of the cooling water. The thermal performance ratio of the TVC and specific power consumption of the MVC are found to decrease at higher heating steam temperatures. Also, an increase of the heating steam temperature drastically reduces the specific heat transfer area. Results indicate better performance for the MEE-PC system; however, the MEE-P has a similar thermal performance ratio and simpler design and operating characteristics. The conversion ratio is found to depend on the brine flow configuration and to be independent of the vapor compression mode.

Process control in water desalination industry: an overview☆

Abstract Process control is an essential part of the desalination industry that requires for operation at the optimum operating conditions an increase in the lifetime of the plant and reduction of the unit product cost. A review is presented for the commonly used and newly developed control and instrumentation in MSF and RO plants. Process control may be as simple as an on/off valve that is triggered upon offset of the system measured parameters from the desired set-point. More classical and commonly used controllers have combined proportional, integral, and derivative (PID) systems. Also, proportional/integral (PI) controls are used in industry. Controls selection aims at fast response, high stability, and minimum disturbance to the system. Less common controllers include fuzzy logic-based systems: Early testing of such systems shows the need for mathematical analysis of various control loops within the plant, development of control rules, and development and testing on industrial scale. Supervisory systems such as model predictive control are also considered to obtain an integrated control systems of the whole plant. It should be stressed that although the desalination plants are highly complex, accurate and detailed mathematical models for steady state and process of various desalination processes are found in the literature. Such models are necessary for studying plant performance, various control strategies, and forms an essential part for any serious analysis and development of novel and new control systems.

Heat Transfer During Phase Change of Paraffin Wax Stored in Spherical Shells

This study concerns experimental evaluation of heat transfer during energy storage and release for the phase change of paraffin wax in spherical shells. Measurements are made using air as the heat transfer fluid (HTF), copper spheres with diameters of 2, 3, 4, and 6 cm. A detailed temperature field is obtained within the spheres using 10 thermocouple wires. Values of the air velocity and temperature used in the experiments are 4-10 m/s and 60-90°C, respectively. Measured times for melting and solidification varied over a range of 5-15 and 2-5 minutes, respectively. Calculations show that the Nusselt number in the phase change material (PCM) during melting is one order of magnitude higher than during solidification. Results indicate that the Nusselt number for melting has a strong dependence on the sphere diameter, lower dependence on the air temperature, and a negligible dependence on the air velocity. Variations in the Fourier number for melting and solidification show similar trends. An increase in the Nusselt number for a larger sphere diameter is attributed to increase in natural convection cells in the PCM inside the spheres. The larger volume allows for the free motion for the descent and rise of cooler and hotter molten wax. During the solidification process, the solid wax is evenly formed through the sphere, starting from the outer surface and moving inward. As the solidification proceeds, the melt volume decreases with a simultaneous decrease in the magnitude of natural convection within the melt. The higher values of Fourier number for melting indicate the consumption of a large part of the HTF energy in heating the molten wax rather than melting of the solid wax.

Multi-stage flash desalination: present and future outlook

Abstract Multi-stage flash desalination (MSF) is currently the workhorse of the desalination industry with a market share close to 60% of the total world production capacity. As the turning point of the new millennium nears, the process faces many challenges dictated by industrial demands and public needs. The conservative nature of the desalination owner, as well as the strategic characteristics of the product, makes the MSF process favored over other competitive thermal desalination methods. In addition, the process has several merits, which include a large production capacity, proven reliability and well-developed construction and operation experience. This study offers an overview of the present and future developments in the MSF process, which aims to reduce the production cost. Special attention is given to the process fundamentals, which are the key elements for any serious and physically sound development of the MSF process. Also, a summary of the novel (MSF-M) configuration is given, which has recently been proposed by the authors. The process is based on the modification of operational MSF plants as well as the concept of once-through MSF. The modification involves removal of the heat rejection section and the addition of a mixing tank for the feed stream and the unevaporated brine recycle. This eliminates the amount of energy rejected in the cooling seawater stream and reduces the amount of energy rejected in the brine blowdown stream. Analysis of the MSF-M process shows an increase in the thermal performance ratio by a factor of 2–3 over conventional MSF.

Multiple effect evaporation-vapour compression desalination processes

A performance analysis is presented for the vapour compression parallel feed multiple effect evaporation water desalination system. The systems include mechanical (MVC) and thermal (TVC) vapour compression. The system models take into account the dependence of the stream physical properties on temperature and salinity, thermodynamic losses, temperature depression in the vapour stream caused by pressure losses and non-condensable gases, flashing within the effects, and the presence of flashing boxes. The analysis is performed as a function of the brine distribution configuration (parallel or parallel/cross flow), the top brine temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapour condensate and the brine blowdown. The analysis is focused on variations in the parameters that control the product cost, which includes the specific heat transfer area, the thermal performance ratio, the specific power consumption, the conversion ratio, and the specific flow rate of the cooling water. Results show consistent behaviour with industrial practice, where the thermal performance ratio of the TVC system decreases at higher top brine temperatures, while the specific power consumption of the MVC systems decreases at higher temperatures. Also, the specific heat transfer area for all configurations decreases at higher operating temperatures. The conversion ratio is found to depend on the brine flow configuration and to be independent of the vapour compression mode. For the parallel flow configuration, the conversion ratio decreases with the increase of the operating temperature. On the other hand, the conversion ratio for the parallel/cross flow system decreases with the increase of the brine blowdown temperature. Predictions of both models show good agreement with field data.

A NOVEL AIR CONDITIONING SYSTEM Membrane Air Drying and Evaporative Cooling

Anovel system for air conditioning is proposed which combines membrane air-drying and an indirect/direct evaporative cooling (M/ID) system. This combination extends the operating range of the evaporative cooler for small differences of dry and wet bulb temperatures. The study investigates the feasibility of operating the proposed system for the cooling of ambient air to an outlet temperature of 19°C and a relative humidity of 90%. The analysis is performed for the summer weather data of Kuwait, which varies from extremely hot and dry conditions (50°C and less than 20% relative humidity) to warm and humid conditions (35°C and more than 60% relative humidity). System analysis shows limitations imposed on air cooling by the direct evaporative cooler (DEC), the indirect evaporative cooler (IEC), and the indirect/direct evaporative cooler (ID). For ambient temperatures above 35°C, operation of the ID system requires relative humidity values below 30%. Operation of the DEC or the IEC systems is limited to temperatures below 30°C and relative humidity below 50%. The ID system operates at temperatures above 45°C and relative humidity below 50%. The M/ID operation covers a relative humidity range between 30–100% and a temperature range between 25–45°C. Energy consumption for various cooling combinations, including mechanical vapour compression (MVC), is evaluated by the energy efficiency rating (EER). The M/ID system shows savings of up to 86.2% of the energy consumed by the stand-alone MVC system. Also, the combined systems of MVC/IEC and the MVC/ID show savings of 49.8 and 58.9% over the conventional MVC.