Once-through Multi-stage Flash Research Articles

Concept and analysis of hybrid reversal multi-stage flash and membrane distillation desalination system

ABSTRACT The concept and analysis of integrating membrane distillations (MD) with reversal once-through Multistage Flash (RV-MSF) desalination is presented. The analysis is based on numerical simulation. The MD vessels are integrated into the terminal ends of the RV-MSF system to leverage the thermal energy associated with these terminal streams. Hybridisation at the last MSF stage, i.e. by replacing the brine cooler, contributes marginally to the overall production rate which amounts to 2%. However, it is found that hybridisation at stage one, i.e. utilising the energy of the MSF reject brine can increase the overall production rate by 65%. For seawater feed temperature of 80 oC and 24 MSF stages, 5 MD vessels in series can be integrated with the RV-MSF process. This ultimate hybridisation helped improve the recovery ratio from 7 to 23%, decreasing the specific cooling water requirement from 23 to 12 kg/kg and reducing the specific energy consumption from 129 to 41 kWh/m3 with respect to the stand-alone RV-MSF system. However, this achievement incurs an additional specific area for heat transfer which increased from 29 to 65 m2/(kg/s). This is because a large number of MD modules are incorporated into the hybridisation.

Dynamic simulation of once-through multistage flash (MSF-OT) desalination process: Effect of seawater temperature on the fouling mechanism in the heat exchangers

Scale formation of carbonates and sulphates is one of the most well-known types of crystallization fouling in heat exchangers. Tackling crystallization fouling in Once-Through Multistage Flash Desalination (MSF-OT) is one of the most challenging tasks in the desalination industry. In this paper, a fouling model is developed and then incorporated into a MSF model to investigate the fouling behaviour under variable seawater temperature. The proposed dynamic model investigates the crystallization of calcium carbonate and magnesium hydroxide at the inside tube surface areas by considering the attachment and removal mechanisms. The results show that the fouling rate is higher at high constant seawater temperature. Overall, the fouling rate is lower at the seasonal variation of the seawater temperature, resulting in a higher performance ratio (PR). The results also show that although the brine heater duty increases in winter due to low seawater temperature, the drop of fouling rate in cold months may save some energy.

Crystallization of calcium carbonate and magnesium hydroxide in the heat exchangers of once-through Multistage Flash (MSF-OT) desalination process

In this paper, a dynamic model of fouling is presented to predict the crystallization of calcium carbonate and magnesium hydroxide inside the condenser tubes of Once-Through Multistage Flash (MSF-OT) desalination process. The model considers the combination of kinetic and mass diffusion rates taking into account the effect of temperature, velocity and salinity of the seawater. The equations for seawater carbonate system are used to calculate the concentration of the seawater species. The effects of salinity and temperature on the solubility of calcium carbonate and magnesium hydroxide are also considered. The results reveal an increase in the fouling inside the tubes caused by crystallization of CaCO3 and Mg(OH)2 with increase in the stage temperature. The intake seawater temperature and the Top Brine Temperature (TBT) are varied to investigate their impact on the fouling process. The results show that the (TBT) has greater impact than the seawater temperature on increasing the fouling.

The effect of increased top brine temperature on the performance and design of OT-MSF using a case study

A mathematical model of a Once-Through Multi-Stage-Flash (OT-MSF) desalination system is developed. This study shows the impact of top brine temperature (TBT) of up to 160°C on both the design and performance characteristics of MSF systems. Such a high TBT can be achieved by nanofiltration pretreatment to remove scale-forming compounds. System performance is evaluated by the thermal performance ratio (PR) and the required specific area (sA). For a fixed brine reject temperature (Tend) and inter-stage temperature drop (ΔT), adding stages results in the TBT increasing by ΔT for each stage added and the PR increases monotonically with the TBT. On the other hand, the required sA decreases and then increases again beyond a certain TBT. The Sirte desalination plant in Libya is taken as a case study. It is found that by increasing the TBT to 161°C from a typical value of 118°C keeping Tend and ΔT fixed; the PR can be increased by 41.5%, reaching a value of 14.6 while the required sA increases by 0.9%. Although there is a penalty in terms of the increased number of stages required to achieve this arrangement, there is a clear advantage in terms of PR, with a relatively small compromise in sA.

A shortcut approach to the design of once-through multi-stage flash desalination systems

A shortcut approach to the design of once-through multi-stage flash desalination systems

A shortcut approach to the design of once-through multi-stage flash desalination systems

A shortcut approach to the design of once-through multi-stage flash desalination systems

The forward osmosis and desalination

Forward osmosis (FO) has emerged as a method for desalting saline water and power production. It utilizes a chemical potential difference, or a salinity gradient to permeate fresh water through membranes. This paper investigates the feasibility of using the FO process for seawater (SW) desalination in terms of consumed energy, capital costs, water recovery, operation & maintenance, water quality, and the final product water cost. The study covers FO by itself, and when combines with other desalting systems such as reverse osmosis, multi stage flash (MSF), and multi effect distillation as pretreatment method. This paper reviews first the principles of fluid and solutes flow in the FO membranes, concentration polarization, the difference between the FO, pressure retarded osmosis processes, draw solutions, and the solutes involved in these draw solutions. Then, the main characteristics of the FO membranes and their commercial availability are presented. Previous experimental work and a commercial plant using FO for desalination are also given. The use of FO as pretreatment for other desalting methods is presented in light of two proposed research projects. The first research project proposes utilizing FO as pretreatment for processing treated wastewater and SW in one system. The second project utilizes FO as pretreatment for a once-through MSF desalting method. The analysis proved energy reduction in the energy consumption of both desalting systems by more than 50 and 18%, respectively.

Solving equations describing the steady-state model of MSF desalination process using Solver Optimization Tool of MATLAB software

AbstractThis work presents an algorithm for solving the large system of non-linear equations describing the steady state model of the two main layouts for the MSF process (the multistage flashing with brine recirculation and the once-through multistage flash desalination (MSF-OT)). The model accounts for the geometry of the stage, the mechanism of heat transfer, and the variation of the thermophysical properties of various fluids with temperature and salinity. In addition, the overall heat transfer coefficient was evaluated at each stage using convective heat transfer coefficient for internal and external flows, tube thermal conductivity and fouling resistance. Furthermore, the model takes into consideration the dependence of thermodynamic losses on stream flow rate, temperature, and the brine salinity. The system of equations was solved through an iterative procedure by using Solver Optimization Tool of MATLAB software. For each stage, the developed model was then used for evaluating the temperatures of ...

Performance evaluation of a once-through multi-stage flash distillation system: Impact of brine heater fouling

Multi-stage flash distillation (MSF) system modeling involves a number of process variables. An estimation of all these process variables requires both analytical solutions and experimental/field analysis. However, the accurate estimate of variables related to the brine heater operation in a MSF system is very important for a reliable operation of the system. For example, steam operating conditions as well as the brine properties including fouling of the brine heater tubes have a significant effect on the heat transfer characteristics of the brine heater, which in turn influence the distillate output from the system. In this study, the effect of various design as well as operating conditions on the performance ratio (PR), brine temperature and salinity as it leaves the last flash stage are investigated in a once-through system. Increasing the number of stages from 24 to 32 has a significant effect on the PR, it ranges between 79% (for Δ T = 1.5) and 327% (for Δ T = 2.3) for a top-brine temperature of 106 °C. This value increase as the top-brine temperature increases. Increasing the stage-to-stage temperature difference increases the water salinity as it leaves the final stage and reduces its temperature that would imply better energy utilization within the plant. Results show that brine side heat exchanger fouling has a significant effect in decreasing the overall heat transfer coefficient, which reduces the production rate as the fouling increases with time. A sensitivity analysis to identify the key parameters, which can have a significant influence on the desalination plant performance, is carried out in an attempt to contribute a better understanding and operation of MSF desalination processes.

Performance characteristics of a once-through multi-stage flash distillation process

Many process variables are involved in the design and operation of a multi-stage flash (MSF) distillation process. An estimation of the effect of these parameters on the plant performance is important to improve the optimum operating conditions. Both analytical solutions and experimental/ field analysis are required to identify the most influential parameters that affect the performance and set proper plans for performance optimization. The accurate estimate of the variables related to the brine heater, selecting proper number of stages and the stage-to-stage temperature drop are of crucial importance. In addition, the thermal properties dependent on the operating conditions may affect the accuracy of numerical results. Moreover, the salinity of the feed seawater has a significant effect on the plant characteristics. In this study, the effect of various operating conditions on the performance ratio, brine temperature and salinity as it leaves the last flash stage are investigated in a once-through system. The up-to-date reliable correlations for calculating brine properties that vary with both temperature and salinity are used. The numerical results obtained by the present investigation are compared with the published data on similar plants. A sensitivity analysis to identify the key parameters that significantly influence the desalination plant performance is carried out in an attempt to contribute a better understanding on modeling and optimum operation of MSF desalination processes.

Optimal design of hybrid RO/MSF desalination plants Part I: Modeling and algorithms

Hybridization of seawater reverse osmosis (S WRO), desalting technology and the multi-stage flash (MSF) has been considered to improve the performance of the latter and reduce the cost of desalted water. Coupling of the two processes could be made on different levels of integration and the resulting water cost will depend on the selected configuration: not only the plant configuration that affects water cost but also the cost of materials of construction, equipment, membranes, steam, energy, chemicals, etc. Despite the advantages ofhybridization, it is noticed that the present number ofhybrid plants is very limited. Many workers have investigated the hybridization problem ofRO and MSF technologies from different angles. The present work is a trial to participate in these efforts to throw more light on the feasibility of the hybridization idea through an optimization study for the prediction and comparison of the minimum water cost of seven different designs of hybrid plants. The selected designs comprise brine recycle and once-through MSF units coupled with a single-stage SWRO unit receiving power and steam from an external source. In addition, the study includes a two-stage SWRO desalting plant and a brine recycle MSF plant, each having the same capacity of the other hybrid plants. This study is made up of three parts: (1) modeling, problem definition and algorithms; (2) presentation and discussion of the optimal designs obtained and the associated water costs and other parameters; and (3) sensitivity analysis of the results to variations in cost elements and operating conditions. The first part is presented here where the descriptive and cost models are outlined, the optimization problem is formulated, and the computational loops repeatedly performed to reach the optimal solution for three case studies are flowcharted.

Performance of the once-through multistage flash desalination process

The brine circulation multistage flash desalination process (MSF) dominates the thermal desalination market, while the once-through multistage flash desalination process (MSF-OT) remains to be found on a limited scale. This is because the MSF-OT process has no control on the temperature of the feed sea water. For non-equatorial regions, where the sea water temperature drops to 5 — 15°C during winter operation, the MSF-OT performance deteriorates unless the volume of the low-temperature stages is drastically increased to allow for reduction in the brine reject temperature to lower values. Another approach to solving this problem is to use the brine mixing (MSF-M) technique to control the feed sea water temperature. Irrespective of this, the MSF-OT process should be considered the optimum choice for large-scale thermal desalination in equatorial regions, where the sea water temperature remains constant throughout the year at 28°C. This study focuses on design and performance features of the MSF-OT process. Results are presented in terms of variations in the process thermal performance ratio, the specific heat transfer area and the conversion ratio as a function of the top brine temperature, the number of flashing stages and the feed sea water temperature. The performance of the MSF-OT process is identical to the MSF process as long as the feed sea water temperature remains above 25°C. Evaluation of the MSF-M system and comparison with the performance of the MSF and MSF-OT processes is presented.