Large-scale commercial desalination plants

Dissertation On MODELING, SIMULATION, AND OPTIMIZATION OF LARGE-SCALE COMMERCIAL DESALINATION PLANTS

ABSTRACT

This dissertation introduces desalination processes in general and multistage flash (MSF) and reverse osmosis (RO) in particular. It presents the fundamental and practical aspects of neural networks and provides an overview of their structures, topology, strengths, and limitations. This study includes the applications to prediction problems of large-scale commercial MSF and RO desalination plants to optimize the process performance. In contrast to several recent studies, this work utilizes actual operating data (not simulated) from a large-scale commercial MSF desalination plant (48 million gallons per day capacity, MGPD) and RO plant (15 MGPD) located in Kuwait and the Kingdom of Saudi Arabia, respectively. We apply Neural Works Professional II/Plus (NeuralWare,1993) and SAS (SAS Institute Inc., 1996) software to accomplish this task. This dissertation demonstrates how to apply modular and equation-solving approaches for steady-state and dynamic simulations of large-scale commercial MSF desalination plants using SPEEDUP (Simulation Program for Evaluation and Evolutionary Design of Unsteady Processes) marketed by Aspen Technology, Cambridge, MA. This work illustrates the development of an optimal operating envelope for achieving a stable operation of a commercial MSF desalination plant using the SPEEDUP model. Finally, this dissertation is unique and significant in that it reports the first comprehensive study of predictive modeling, simulation, and optimization of large-scale commercial desalination plants.

Ø CHAPTER 1

INTRODUCTION

This chapter describes the water crisis in Arab counties and the need for water desalination, which provides the motivation for this research. We summarize the goals, significance and uniqueness of our research on modeling, simulation and optimization of large-scale commercial desalination plants. Desalination Industry is driven by desalination process such as Multi Stage Flash (MSF) and Reverse Osmosis (RO). Operations of these processes and controlling them are very complicated due to many reasons. Many researchers tried to study dynamic behavior of desalination plants to address plant variables during operation .The plant shutdown can lead to considerable affects or effects on the plant economics (see Fig. 1).

The study achievement is believed to be beneficial for desalination community and it helps developing local MENA human resources significantly.

Research Aims and Significance:

This research investigates the following:.

A. Steady-state and dynamic simulations of a large-scale commercial multistage flash (MSF) desalination plant.

B. Performance optimization of a large-scale commercial multistage flash (MSF) desalination plant.

Discussion

This study will address two of the most widely used processes of seawater desalination. El-Dessouky and Ettouney argue that, the thermal processes or membrane separation methods are best methods for desalination processes (2002: 11). Based on this point, we are going to analyze an example of each type, namely MSF and RO.

Multistage Flashing:

MSF is considered to be one of the thermal-based processes to desalinate seawater. Simply put, seawater goes through a process of evaporation followed by that of condensation. Based on this observation, we may conclude that the process at hand imitates the kind of evaporation that occurs in nature. The mechanism in which this takes place is explained by the National Research Council (2004: 76) who points out that:

MSF uses a series of chambers, each with successively lower temperature and pressure, to rapidly vaporize (or "flash") water from bulk liquid .The vapour is than condensed by tubes of the inflowing feed water, thereby recovering energy from the heat of condensation.

As it turns out, heat is the main source of energy needed for this process to take place.

Reverse Osmosis :

RO is viewed as one of the membrane-based processes to desalinate seawater. In this method high pressure during semi-permeable membranes permeates the fresh water of highly concentrated brine solution (El-Dessouky and Ettouney 2002, P. 12). Thus, the efficiency of this technique is mainly dependent on how good the membranes are in separating salts, metals and other materials from water. Unlike other membrane processes, RO relies on the pressure put on seawater against the membrane, the higher the pressure the better. The following flow chart explains this process: (RBF Consulting, 2004)

Three criteria will be considered to investigate the advantages and disadvantages that each method has: The Quantity and Quality of the Produced Water: According to RBF Consulting (2004), the fresh water produced by MSF constitutes around 61.6 % of the desalted water in the world. This is, of course, not strange since this method has been used since the mid 1940s. Specifically, the desalted water produced in winter is more than that in summer. Obviously, this does not comply with what is required. To solve this problem, we might ensure that the plant is provided with high temperatures all the year around. However, this might lead to the gradual corrosion of the plant equipment.

On the other hand, the amount produced by RO comprises around 26.7 % of the overall production as indicated by RBF Consulting (2004). Interestingly, Saudi Arabia is ranked second in the world with approximately 12.9 % of the desalted water produced by RO. However, the amount produced by MSF is almost twice as much as the amount produced by RO. The following table shows the change in the capacity of the desalted water in Saudi Arabia:

Water Desalination in the UK. This is considered to be new in one of the wettest countries in the world. This tendency towards this source of drinkable water can be ascribed to two factors: Firstly, there has been an increasing prediction or fear of drier summers. Secondly, the constant growth of population makes it necessary to quickly find alternatives to water sources. This is actually a point of difference between Saudi Arabia and the UK. Specifically, Saudi Arabia's production of desalted water will necessarily be much bigger than that of the UK, because Saudi Arabia is thought to be one of the driest places in the world.

As for the method adopted in the UK, it has been argued that RO is preferred to MSF as being less expensive. This follows from the reduction in the capital costs of membranes, which eventually leads to the reduction in the operational costs. Indeed, RO is chosen to desalinate water in the Beckton Plant in East London which is meant to take saline water from River Thames. This plant is expected to reach a capacity of 150 MLD that will be sufficient for 400.000 households as indicated by Bennett (2005).

As far as quality is concerned, it is evident that water produced by MSF is very pure. However, the purity of the water produced by RO depends on the efficiency of the membrane. In other words, it relies on the ability of the membrane to capture minerals, salt particles and materials. Thus, we may say that purity is one of the advantages that MSF has over RO.

The Cost of the Produced Water:

The inclination of some countries to adopt RO in seawater desalination although it provides less pure water can be linked with the lower costs required for this process. Dickie (2007: 11) argues that large scale thermal desalination has been completely limited to the rich, energy affluent and water poor countries nearby the Arabian Gulf.

Based on this observation, we may account for the wide use of MSF in Saudi Arabia since this country is rich with energy sources required for this process as discussed before. This also justifies the use of RO in the UK as being less expensive. In fact, RO tends to be the optimal method to desalinate seawater all over the world.

Ø CHAPTER 2

DESALINATION PROCESSES

2.1 Need for Desalination

Water is an important resource for use of mankind. It is essential for agricultural and industrial growth, as well as for supporting growing populations who require a safe drinking water supply. We find 97% of all water in oceans, 2% in glaciers and ice caps, and the rest in lakes, rivers and underground.

2.2 History of Desalination

Desalination as a natural phenomenon has occurred on earth for millions of years. The natural distillation cycle of water evaporating from the sea and then condensing to form pure rain water is probably the most obvious example of this phenomenon.

Ø CHAPTER 3

MODELING AND SIMULATION :

This chapter describes how to apply modular and equation-solving approaches for simulation of large-scale commercial MSF desalination plant using advanced commercial software tools.

Ø CHAPTER 4

PERFORMANCE OPTIMIZATION :

Chapter 4 deals with the steady-state optimization of the operation of an existing MSF desalination plant. It begins with a discussion of the most important process variables and constraints, that directly affect the production and performance ratio of the plant.

Why the Work is Important:

Recent development of process and control methodology of the desalination plant behavior using its operating data. The integration of performance and process control will allow improve controllability the plants even during failures' events.

Addressing some dynamic phenomena in the plants and performance of some plants' components will be covered with utilisation of Simulation. Economic analysis of implement different the control system will be done. Furthermore, this project will help in developing knowledge capacity and human resources in MENA region.

The Team Work:

This project is based on a MSc program in the University of Newcastle and it will establish a promising cooperation between two R&D organizations; Saline Water Conversion Corporation (SWCC) and School of Chemical Engineering and Advanced Materials in the University of Newcastle.

The principle investigator, Nasser Zouli, is a member of SWCC team . The project has evaluated more than 10 desalination plants.

References:

1. El-Dessouky, H., and Ettouney, H., Fundamental of Salt Water Desalination, Elsevier Science Publishers, 2002.

2. Reddy, K., and Ghaffour, N., Overview of the cost of desalinated water and costing methodologies, Desalination 205, 2007.

3. Al-Hengari, S., El-Bousiffi, M., El-Moudir, W., Libyan Petroleum Institute Experience in Evaluation of Desalination Plants in the Libyan Oil Sector, Desalination 206, 2007.

4. Volume 2 (Measurements and Control in Water Desalination), Elsevier, 1986.

5. Loir, N., El-Nashar, A., Sommariva C., and Wilf, M., Automation and Operation Optimization to Reduce Water Costs (Draft Final Report MEDRC Project Number 97-AS-002), MEDRC, 3 August 2006.

6- Dickie, P. (2007) Desalination: Option or Distraction for a Thirsty World. Areport prepared for WWF's Global Freshwater Programme.

7- RBF Consulting (2004). Seawater Desalination "White Paper" Prepared for: California American Water. Retrieved 13th July 2009,from World Wide Web http://www.coastalwaterproject.com/pdf/WhitePaper_SeawaterDesalination.pdf

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