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Desalination: The optimization opportunity

Dr. Corrado Sommariva, Mott Mcdonald, Abu Dhabi, UAE

The Middle East’s large installed base of desalination plants is ageing. Upgrading and refurbishing these assets can result in extended lifetimes of 15-20 years, and represents an economic way to meet rising demand.

After forty years of industrial operation on a large scale, the installed desalination capacity in the Middle East has steadily grown and has become a considerable asset. However, these assets are now ageing and the management and maintenance of these plants may become a real burden. This is particularly true for the first generation of multi-stage flash (MSF) plant largely employing carbon steel as a material for construction.

Figure 1. Replacement of incondensable gas extraction point and vent baffles
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However, it has been found that the operational life of these plants can be substantially longer than originally anticipated, and as a result, several rehabilitation and refurbishment projects have been launched to extend the life of a number of plants by 15-20 years.

Developments in materials technology for the modern MSF plant have resulted in the adoption of nobler materials and it is expected that the second generation of large MSF desalination plant installed in the last ten years will last for more than 30 years with minimum maintenance and minor overhauling.

Upgrading existing desalination plants therefore represents a major opportunity to optimize ” at low cost ” assets which in the majority of the cases have already been amortized.

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Table 1. Recent examples of rehabilitation and upgrading projects tendered or awarded

Engineering change

The rehabilitation and upgrading of existing desalination plants is a market with huge potential. As described in Table 1, the volume and number of contracts tendered and awarded in the field of rehabilitation is increasing every year.

Over time, the overall turnover of the rehabilitation business has increased while the engineering activities connected to the refurbishment have become more sophisticated. From initial activities covering re-tubing and patching works, rehabilitation has developed into more focused activities involving process and structural engineering. These activities are aimed at removing the causes of operational problems and optimizing operation and maintenance (O&M) costs such as:

  • Redesign of the incondensable gas vent baffles and extraction pipes
  • Redesign of make-up distribution system
  • Verification and restoration of desalination plant structural strength
  • Increase of demister areas
  • Redesign of insulation
  • Material selection upgrade.

Perhaps the most delicate upgrading activity is the redesign and replacement of the incondensable extraction baffles and pipes. These activities in fact involve both process and structural design. Furthermore, a detailed knowledge of the evaporator conditions is needed in order to achieve proper access to the parts to be replaced without the need for invasive activities.

Long term corrosion

The first generation of desalination plants, installed in the Gulf in the 1960s-1980s, largely employed carbon steel as a material for the evaporator shell and internals. Carbon steel is relatively inexpensive, readily available and possesses engineering properties that have been understood and used for decades.

Another feature of carbon steel that is largely understood is its tendency to corrode and allowance has been made for this in the past by increasing the thickness and hence the weight of the components that are subject to a corrosive environment.

Operational experience on large scale MSF plants has brought about an understanding of the corrosion phenomena induced by the high concentration of CO2, O2 bromamine and incondensable gases and the non-suitability of carbon steel as a material for incondensable gas extraction.

The effect of long term corrosion in the tube bundle has been a gradual loss of material in the incondensable vent channel and thinning of the incondensable extraction pipe. This will in turn bring about a gradual loss in the venting efficiency and the formation of stagnant pockets of incondensable gases which cause in turn a decrease of the external heat transfer coefficient and an increase in corrosion in the tube bundle.

In this case, the number of vent extraction points is increased and the extraction nozzles are distributed across the entire tube bundle. In the original MSF design it was believed

sufficient to extract incondensable gases only in the lowest temperature point of the evaporator. However for the first stages where a very high concentration of CO2 occurs it is important to increase the number of venting points according to the tube bundle length.

As a result of the modification carried out to the venting system, an increase of the thermodynamic performances over and above the original design is observed. This optimization is due to a better heat transfer coefficient achieved in the first stages. Furthermore the need for retubing in the vent extraction areas of the evaporator becomes less frequent as corrosion is prevented.

Uprating performance

Several existing MSF desalination plants have increased heat transfer surface available compared to modern desalination plant. This is due to various factors that influenced the first generation of MSF desalination. These include:

  • Increased margin in the heat transfer coefficient
  • Less effective anti-scalant adopted
  • Higher design fouling factor.

A comparative analysis of new and old generation MSF evaporators can immediately provide an idea on how redundant the heat transfer surface can be.

Figure 2. Calculating pay back for an uprating project
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As can be seen from Table 2, a 7.2 million imperial gallon per day (MIGD) MSF plant installed in recent years requires less heat transfer surface than a 6 MIGD MSF plant installed 20 years ago. This extra heat transfer surface is an asset which can bring extra revenue, and therefore upgrading and uprating existing MSF desalination plant can play a key role in the framework of an intelligent rehabilitation work which ensures comparatively fast investment recovery times and immediate benefits in plant output and management.

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In many cases, the heat transfer surfaces of an existing first generation MSF plant can allow in principle an increase in the distillate water production of up to 20-25 per cent. An optimization of the plant’s performance ratio can also be achieved through to the variation of the following parameters:

  • Increase in the top brine temperature
  • Variations in the process parameters in order to keep the salt concentration at a lower level in the brine recirculation flow
  • Increase in the brine recirculation flow rate by a different tuning of the control valves or modification of existing pump hydraulics
  • Increase in the demister areas
  • Increase in the brine blowdown flow rate
  • Increase in the vacuum system capacity

Figure 3. Break-even analysis
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Breaking even

Uprating an existing desalination plant can bring several advantages for the plant’s operator, including:

  • Obtain an increase in water capacity with relatively low cost impact
  • Lower specific anti-scale consumption
  • Lower specific electric consumption
  • Lower specific steam consumption.

Compared to the development of a new plant, uprating requires only a small investment and allows a quick amortization of the investment. A break-even analysis indicates that the investment needed to uprate a MSF desalination plant would return the capital employed at least five times before the end of the plant’s commercial life.

Further Reading

C. Sommariva, D. Pinciroli, E. Tolle, R. Adinolfi, “Optimization of material selection for evaporative desalination plants in order to achieve the highest cost-benefit ratio”, Desalination 124, 99-103 (1996), Elsevier Science Publ., Amsterdam, 1996.

C. Sommariva, H. Hogg, K. Callister “Forty years design life: the next target material selection and operating conditions in thermal desalination plants”, Desalination 136, 169-176 (2001), Elsevier Science Publ., Amsterdam, 2001.