Hisham El-Dessouky

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.

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.

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.

Process synthesis: The multi-stage flash desalination system

The process flow diagram for the multi-stage flash (MSF) desalination process is quiet complex, where it includes several stages for brine flashing, preheaters for feed seawater, two sections for heat recovery and rejection, cooling water stream, and brine recycle stream. The process fundamentals are analyzed in order to have a better understanding of the functions and relations for various elements in the process. The analysis is based on performance characteristics for a number of simplified configurations. These characteristics include the amount of product water per unit mass of heating steam, the specific heat transfer area, the specific flow rates of the cooling and feed seawater, and the salinity, temperature, and specific flow rate of brine recycle and blow down. These characteristics are affected by limitations imposed on the number of stages, the stage temperature drop, and terminal temperature difference in the preheaters. The configurations considered in the analysis include a single-stage flashing unit, a once-through multi-stage flashing system, and configurations with brine recycle. The brine recycle systems include a simple mixer for feed seawater and recycle brine as well as one, two, and three flashing stages in the heat rejection section. A summary of the results show that a single-stage flashing unit has a thermal performance ratio less than one and the once-through system has a very large specific flow rate for the feed seawater. In addition, use of the simple brine recycle mixer results in a high temperature of the rejected brine. The single-stage heat rejection cannot be applied because pinching of the temperature profiles of the feed seawater and condensing vapor. The two-stage heat rejection section is not economical because of the small temperature driving force, which results in large heat transfer areas. This analysis leads to the conventional MSF system, which includes three stages or more in the heat rejection section.

Plastic/compact heat exchangers for single-effect desalination systems

Abstract Use of plastic materials and compact heat exchangers remains to be found on a limited scale in the desalination industry. This work focuses on performance evaluation of plastic and compact heat exchangers in the single-effect mechanical vapor compression desalination system. The analysis considers a number of tubing materials that includes PTFE plastic, high steel alloys, Cu Ni 90 10 and Cu Ni 70 30 , and titanium. The analysis includes determination of the specific power consumption and the evaporator and preheaters specific heat transfer area for various materials. Analysis is made as a function of variations in the condensate and the brine boiling temperatures. The specific cost is evaluated for various construction materials. Results show that the specific power consumption is independent of the construction material and depends only on the compression range. Also, the lowest specific heat transfer area is calculated for Cu Ni 90 10 . However, the lowest cost is obtained for the PTFE system because of the much lower cost of the material. Corrosion considerations and the associated reduction in the cost of corrosion inhibitors give an added adge for the use of PTFE plastic. Other factors that favor the use of plastic evaporators and preheaters include ease of construction and machining, lower installation and erection cost, ability to operate at higher top brine temperatures without fear of the effects of scale formation, use of acid cleaning, and virtual elimination of in-leakage problems. These merits of the plastic and compact heat exchangers and the analysis results should encourage the industry to take active steps in adopting these systems.

Single-Effect Thermal Vapor-Compression Desalination Process: Thermal Analysis

A mathematical model is developed to analyze a single-effect thermal vapor-compression (TVC) desalination process. The model considers the effect of various thermodynamic losses on the system preformance ratio, the specific heat transfer area, and the specific flow rate of cooling water. The losses contemplated are the boiling-point elevation, the nonequilibrium allowance, and the temperature depression corresponding to the pressure drop in the demister and during the vapor-condensation process. The model takes into consideration the dependence of the physical properties of the seawater on temperature and salt concentration. In addition, the model considers the effects of the fouling factors and the presence of noncondensable gases on the heat transfer coefficients in the evaporator and the condenser. The system performance is analyzed in terms of parameters controlling the cost of product water, which include the performance ratio, the specific heat transfer area, and the specific flow rate of cooling wa...

Performance of Evaporative Condensers

Experimental investigation is conducted to study the performance of evaporative condensers/coolers. The analysis includes development of correlations for the external heat transfer coefficient and the system efficiency. The evaporative condenser includes two finned-tube heat exchangers. The system is designed to allow for operation of a single condenser, two condensers in parallel, and two condensers in series. The analysis is performed as a function of the water-to-air mass flow rate ratio (L/G) and the steam temperature. Also, comparison is made between the performance of the evaporative condenser and same device as an air-cooled condenser. Analysis of the collected data shows that the system efficiency increases at lower L/G ratios and higher steam temperatures. The system efficiency for various configurations for the evaporative condenser varies between 97% and 99%. Lower efficiencies are obtained for the air-cooled condenser, with values between 88% and 92%. The highest efficiency is found for the two condensers in series, followed by two condensers in parallel and then the single condenser. The parallel condenser configuration can handle a larger amount of inlet steam and can provide the required system efficiency and degree of subcooling. The correlation for the system efficiency gives a simple tool for preliminary system design. The correlation developed for the external heat transfer coefficient is found to be consistent with the available literature data.

Heat transfer in vertically aligned phase change energy storage systems

Convection effects on heat transfer are analyzed in low temperature and vertically aligned phase change energy storage systems. This is performed by detailed temperature measurements in the phase change material (PCM) in eighteen locations forming a grid of six radial and three axial positions. The system constitutes a double pipe configuration, where commercial grade paraffin wax is stored in the annular space between the two pipes and water flows inside the inner pipe. Vertical alignment of the system allowed for reverse of the flow direction of the heat transfer fluid (HTF), which is water. Therefore, the PCM is heated from the bottom for HTF flow from bottom to top and from the top as the HTF flow direction is reversed. For the former case, natural convection affects the melting process. Collected data are used to study variations in the transient temperature distribution at axial and radial positions as well as for the two-dimensional temperature field. The data is used to calculate the PCM heat transfer coefficient and to develop correlations for the melting Fourier number. Results indicate that the PCM heat transfer coefficient is higher for the case of PCM heating from bottom to top. Nusselt number correlations are developed as a function of Rayleigh, Stefan, and Fourier numbers for the HTF flow from bottom to top and as a function of Stefan and Fourier numbers for HTF flow from top to bottom. The enhancement ratio for heat transfer caused by natural convection increases and then levels off as the inlet temperature of the HTF is increased.

Qualifying of manpower for the desalination industry

Rapid expansion and growth expected to occur in the desalination industry in the next two decades necessitates preplanning of a comprehensive program for education, research, and training. The proposed education programs include an engineering subprogram in desalination, a two-year technical degree in desalination technology, graduate programs, and continuing and cooperative education. The graduate education programs include research studies leading to a one-year diploma in desalination as well as MSc and PhD degrees in desalination research. The continuing education courses include introductory to more advanced level topics. Such classes would provide the general public with a realistic view of the industry, needs for water conservation, environmental impact, and the strategic importance of the desalination industry. The proposed cooperative engineering studies would be supported in part by the desalination industry. Field training programs are proposed for entry-level and practicing engineers and technicians. Desalination research centers should be established to address design and operational problems of the industry and to develop novel and more efficient desalination systems.

Computer simulation of the horizontal falling film desalination plant

Abstract In this paper two models are developed; the first one is for the horizontal falling film desalination process and the second one is for a modified combined thermal vapour compression and horizontal falling desalination process. The present models assume a constant heat transfer area in all effects rather than constant temperature drop between the effects. The computed results show that the rate of decrease of the specific heat transfer area of the horizontal falling film process is higher than that of the combined thermal vapour compression and the horizontal falling film processes. After certain maximum allowable operating temperature, depending on the number of effects, the magnitude of the specific heat transfer area of the horizontal falling film process is always smaller than that in the combined thermal vapour compression and the horizontal falling film process. The thermal performance ratio corresponding to the thermal vapour copmpression-horizontal falling film process are much greater than the value corresponding to the horizontal falling film for the same number of effects. The combined thermal vapour compression and the horizontal falling film presented in this paper gives values for the thermal performance ratio higher than the values obtained in the previous conventional combined thermal vapour compression and horizontal falling film desalination process.