Imad Alatiqi

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.

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.

Performance of wire mesh mist eliminator

Abstract An experimental study was made to measure the performance of wire mesh mist eliminator as a function of broad ranges of operating and design conditions. The experiments were carried out in an industrial scale layered type demister pad made of 316 L stainless steel wires. The demister performance was evaluated by droplet separation efficiency, vapor pressure drop of wet demister, and flooding and loading velocities. These variables were measured as a function of vapor velocity (0.98–7.5 m/s), packing density (80.317–208.16 kg/m 3 ), pad thickness (100–200 cm), wire diameter (0.2–0.32 mm), and diameter of captured droplets (1–5 mm). All the measurement results lie in ranges where, in practice, the wire mesh mist eliminator predominates. The experimental results indicate that the separation efficiency increases with both the maximum diameter of capture water droplets and the vapor velocity and with the decrease of wire diameter. The pressure drop for the dry demister is relatively low and depends only on the vapor velocity. The pressure drop increases linearly up to the loading point, thereafter; the rate of increase is larger. Beyond the flooding point, the increase rate is significant even for the slightest rise in the vapor velocity. The flooding velocity diminishes with the beef-up of the packing density and with the decrease of the wire diameter. Three empirical correlations were developed as a function of the design and operating parameters for the separation efficiency, pressure drop for the wet demister in the loading range, and the flooding and loading velocities. These correlations are sufficiently accurate for practical calculations and demister design. The temperature depression corresponding to the pressure drop in a wire mesh mist eliminator systems installed in a typical multi stage flash desalination plant was estimated from the developed correlation. A good agreement was obtained between the design values and the correlation predictions.

Model based control for reverse osmosis desalination processes

Abstract Computer simulations were performed using the Dynamic Matrix Control (DMC) algorithm for the control of a reverse osmosis (RO) desalination pilot plant. The plant dynamics were modeled by the use of empirical transfer functions developed at the Kuwait Institute for Scientific Research. The two output variables considered were: production flow rate and product water electrical conductivity as a measure of purity. With the selection of proper tuning constants the DMC approach allows for substantial improvement over proportional-integral (PI) control based on the integral square error (ISE) performance criterion. A setpoint change in production flow rate was considered in the simulations since this type of change is the one most likely to occur in an RO plant. For the same magnitude setpoint change in flowrate, the ISE results for the flowrate were comparable between the two controllers; however, the ISE results for conductivity were substantially better using DMC control. The ISE for conductivity using the PI controller is 1688.92 while for the DMC controller, with the proper tuning parameters the ISE may be much less than 1.0. These results indicate that much more flexibility in the operation of an RO plant is available with DMC control.

System identification and control of reverse osmosis desalination

Abstract Efficient control system is the key point to a successful long-term operation in any industry. In reverse osmosis desalination process, various important parameters such as permeates flux and conductivity must be controlled. In this paper, the latest state of the art techniques are utilized to develop an effective closed loop control system for the above two mentioned parameters in a hollow fine fiber membrane configuration system located at the R&D laboratory at Doha Reverse Osmosis Plant of Kuwait. These techniques include: system identification, relative gain array and controllability test to find best pairings and structure, Zeigler-Nichlos settings to design SISO controllers for open loop systems, and for the multivariable system the biggest log modulus tunning technique is implemented. The manipulated variables are feed pressure and pH which control permeate flux and conductivity, respectively. The control system was simulated and satisfactory performance was obtained for set point tracking.

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.

Advanced control of a reverse osmosis desalination unit

Abstract An experimental investigation of constrained model predictive control (CMPC) for a reverse osmosis (RO) desalination unit has been conducted. For comparison purposes, results with traditional PID-type control have also been obtained. The experimental unit consists of a series of four cellulose acetate membranes. A 486-PC is used as the data acquisition and control computer. It is interfaced to the experimental unit via analogue-to-digital and digital-to-analogue converter boards. The models required for CMPC and PID-type controls are obtained by step testing. The RO system has four outputs and two inputs. The outputs are (1) permeate flow rate, (2) permeate conductivity, indicative of the salt content in the product, (3) trans-membrane pressure, and (4) inlet pH. The inputs are (1) flow rate of reject water and (2) inlet acid flow rate. The production objectives are to produce the specified flow rate of permeate, having the desired salt content, subject to the constraints that the inlet pH and the trans-membrane pressure are within specified bounds. It is shown that CMPC can achieve these goals. It is also demonstrated that CMPC can maximize the throughput subject to the constraints on the other three outputs. A comparison of the results with CMPC and PI control reveals the excellent capability of CMPC for RO desalination plant operations.

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.

Energy retrofit study of an ammonia plant

Detailed energy integration study of the front-end of an existing ammonia plant has been performed, using recent advances in Pinch technology. Utility loads demanded by the existing process were found to be very close to the calculated minimum targets. This indicated that the selected ammonia plant is well integrated, and not much saving is expected through process-to-process energy integration. Alternatively, the retrofit study concentrated on better placement of available utilities. Two promising options have been investigated. The first one studied the flue gas temperature in the convection section of the primary reformer, and proposed new arrangements of the heating coils. Boiler feed water heating for high-pressure steam generation has been also considered as a possible source for reduction in energy consumption. Total benefit claimed amounted to 17.6% reduction in combustion fuel consumption. The overall results of this case study are promising. However, economical evaluation of the proposed modifications is essential. Additional possible reduction in energy consumption is also possible in energy and power integration of the back-end section.