Sea water desalination

Abstract

Despite the lack of water in some areas of the world in one hand, and the increasing consumption of it in some others make it essential for scientists and engineers to search for practical solutions for this world crisis. This paper closely examined the sea water desalination to be one of the most widespread methods that provide the world with a considerable amount of drinkable water in terms of quality, quantity and cost. Desalination of Seawater is an approach used (and may be the only method) to overcome shortage of fresh water from natural resources in many regions in the world. In recent years, many desalination processes become technically and economically affordable. The production of one cubic meter of desalinated water becomes very reasonably to be less than $1/m3 for many new installations.

Introduction

Desalination Industry is driven by desalination process such as Multi Stage Flash (MSF) and Reveres 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.

Influence of Plant Factor[1] on Plant Economics .

In many researches and new installations, attempts to develop much better and reliable control policy have been investigated and many more are still on going under research . The effective performance and evaluation of different control schemes is important and possibility of implementing new control policies can open new windows for operation of desalination plants for near future.

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At the same time, this research also aims to improve our understanding of controllability and operability of the desalination plants for better, safer and economic plant operations with maximizing productivity, availability and profitability. The study achievement is believed to be beneficial for desalination community and it helps developing local MENA human resources significantly.

Aims and Objectives:

• Study dynamic behaviors with controllability of MSFs and R.O processes. Emphasis on unique characteristics of desalination processes such as physical, chemical and fluid dynamics will be considered,
• Simulation of the two methods will be developed on — to evaluate the response of a classical desalination system to different operation scenarios based on conventional control schemes.
• Addressing desalination system components in control policy such as ventilation performance in capturing non-condensable gases, fouling rate formation in condenser tubes, and brine flow hydraulics
  variations,
• Computational Fluid Dynamics will be used to describe some complex desalination plant dynamic behavior and phenomena such as brine entrainments to distillate trays and mist eliminator efficiency,
  and brine level change with orifice configuration,
• Economic analysis justification for implement iterative Learning control in thermal desalination plants.

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.

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.

Multistage flash. SOURCE: Buros et al. (1980); Buros (2000).

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)

Seawater Reverse Osmosis Plant SOURCE: 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.

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 taskforce team of project on performance of desalination plants in the (SWCC). The project has evaluated more than 20 desalination plants.