a desalination plant using solar heat as a heat supply, not affecting the environment with chemicals

ELSEVIER Desalination 122 (1999) 225-234

DESALINATION

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A desalination plant using solar heat as a heat supply, not affecting the environment with chemicals

G. Caruso*, A. Naviglio University of Rome "La Sapienza"- DINCE, Corso I~. Emanuele 11, 244, 00186 Rome, Italy

Tel. +39 (06) 686-8095; Fax +39 (06) 686-8489

Received 3 December 1998; accepted 15 March 1999

Abstract

The project concerns the design, optimization, conslruction, assembling, start-up and extensive monitoring of a full- titanium desalinator. The operational tests took place at the site of the solar pond of the University of Ancona (Italy). Data collected during manufacturing tests and, in particular, during the start-up and the operation of the plant under various conditions, are being utilized for improving expertise on heat recovery with highly corrosive fluids, on cogeneration plants aimed at producing electricity and fresh water, and on desalination fed by solar energy. One major output of the project is the assessment of data which permits an evaluation of the additional cost of the full-titanium desalinator with respect to a traditional technology, with the added benefits of(a) better heat transmission through tube bundles, which means better performance; (b) a reduced need for chemicals and maintenance activities; and (c) improved plant reliability and duration. The research program has three main aims: (1) the improvement of the economics of solar desalination, namely desalination of water through operation of solar ponds; (2) the demonstration of thermal performance, maintenance and chemical requirements, reliability and overall costs of a full-titanium desalination plant through operation of a plant of meaningful size in order to disseminate the technology of full-titanium desalination plants in the electric-energy production industry for use in co-generation units; and (3) the improvement of knowledge regarding industrial-size use of heat recovery from highly corrosive fluids.

Keywords." Thermal processes; Low-temperature processes; Multiple effect; Thermocompression; Solar-powered desalination; Solar ponds; Titanium

1. Introduction

The demand for a steady, economical supply of water is constantly increasing all around the world,

*Corresponding author.

and supply often does not equal the present needs. This problem will become more difficult to solve in the future. One solution to the problem of insufficient water availability throughout the world is certainly desalting. This consists of distilling seawater by various processes, with the result that

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226 G. Caruso, A. Naviglio / Desalination 122 (1999) 225-234

large quantities of water are produced that are suitable for selected final uses.

One drawback of this solution is the high cost involved. A desalination plant requires an initial disbursement for its construction and additional running costs. In spite of this, however, there are many desalination plants in use throughout the world. Their cost is fully compensated by their advantages in terms of amounts of fresh water provided, so necessary for all living beings.

Being able to rely on the availability of drinking water is of primary importance in guaran- teeing a proper life and adequate health for a population. This aspect, added to the growth of industrialization and the growing requests for desalinated water for irrigation purposes, points to the need for having increasing quantities of desalinated water in areas of adverse climatic and geological conditions. Desalination is an excellent solution to the problem, provided that there are sufficient quantities of salt water to meet the needs of the inhabitants. The main reason for attempting to develop a desalination technology is to reduce the cost of eliminating the salt content in the water without running into environmental problems.

Quite apart from the type of process utilized for this aim, toxic residues may be present, and these have to be eliminated in accordance with specific regulations.

The potable water needs in arid countries, such as in developing countries, are partially satisfied with desalted water, but water accessibility in remote areas is very limited and expensive. Since these lands have a very high solar insulation, solar desalination can be seriously considered to satisfy water needs in these areas. Salt gradient solar ponds may be an economically viable method of collecting and storing solar energy to supply heat to several types of desalination systems.

Among the various desalination technologies in use, only those based on a thermal principle of operation must be considered in combination with solar ponds. A low-temperature multi-effect

process is very suitable to be combined with a solar pond for the following reasons: • The temperature of the heat source supplied by

the solar pond (60-75 °C) matches that required for low-temperature multi-effect (ME) desalination plants operating at a top brine temperature of 50-60°C.

• The ME desalting system is very flexible to changes in energy supply and operates in stable conditions under variable heat supply conditions (temperature and flow rate).

• The ME process is economically suitable for limited production.

• The ME plant may be operated as a vapor thermocompression (TC) plant if improve- ments are included during the design phase.

In the project described herein, a special ME-TC desalination plant was designed to be coupled with a solar pond.

2. ME.-TC desalination system

The main part of a ME-TC desalination plant (Fig. 1) is a multi-effect evaporator, with its auxili- aries and ancillaries. An ejector vapor compressor also enables the plant to operate according to the thermocompression process.

The advantages of this plant are the following: • simplicity and compact construction • operation without recirculation • low pumping power • high performance ratio/unit of installed heat

transfer surface area • stable operation • low operating labor cost.

Some of the main features of the process selected are [ 1 ]: • Various stages are kept in depression and

vacuum packed. Vacuum is maintained through an ejector which sucks from the last stage and from the down-flow which is pre-heated in the

G. Caruso, A. Naviglio / Desalination 122 (1999) 225-234 227

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different stages. Appropriate communications with the last stage eliminate the incondensibles in the various stages.

• As the vapor is not forcefully released, it carries only a few drops of salt water, allowing the use of rather simple filters. Moreover, the protective films on the surface of the tubes are not damaged.

• The plant is very stable, even when running at a reduced load.

• The plant works with low fluid velocities, thus reducing corrosion problems and allowing the use of less expensive materials.

• The concentration of salt in the seawater decreases when the temperature increases since there is no recirculation.

• The low running temperature (typical of multi- effect distillation plants), quite below the saturation levels of the calcium sulfate and of magnesium hydroxide, reduces the formation of scale, leading to the decrease of pre- treatment requirements and costs; as a consequence, the corrosion velocity and maintenance expenditures are also low.

The problems connected to the formation of scale and fouling, and the consequent reduction of the plant life, also suggest the use of non- conventional materials.

1. High quality metallic materials such as high level stainless steel or titanium. This solution could meet the demand for purer water produced in these plants (the European Community recommends for drinking water levels of 0.05 ppm of A1, 0.05 ppm of Fe and 0.1 ppm of Cu). More- over, the use of these rather expensive materials should not greatly affect the production cost. In fact, both higher specific costs and lower thermal conductivity of these materials should be fully counterbalanced by the possibility of reducing the wall thickness (for example, 0.3 mm titanium tubes could be used).

2. Plastic materials (polymeric). The low running temperature of these plants enables the use of these materials which are not very expensive and are very resistant to scale.

In this project a full-titanium desalination unit was specifically designed. It was conceived after the experience gained with a preceding plant (three stages, 10ma/d of distillate production) [2,3]. It was designed and built in the framework of a project partially financed by the EC (contract Thermie, No. SE 303/94 IT), concerning the design, optimization, construction, assembling, start-up and extensive monitoring of a full-titanium desalination plant, with a production capacity of 30 m3/d. The operation tests are still taking place at the site of the solar pond of the Department of Energy of the University of Ancona, Italy. Data collected during the manufacturing tests and most of all during the start-up and the operation of the plant under various conditions and alignments are being used for the improvement of the know-how on (1) heat recovery with highly corrosive media, (2) cogeneration plants aimed at producing electricity and fresh water, and (3) desalination fed by solar energy.

One of main targets of the project is the assessment of data which will allow a comparison of the additional cost of the full-titanium desalination plant with respect to traditional technology, with the benefits of (1) better heat transmission through tube bundles, which means better performance; (2) a reduced requirement of chemicals and maintenance activities; and (3) improved plant reliability and duration.

The multi-effect process is used when low temperature heat is available (60--90°C) from a solar pond or from a steam turbine. Thermo- compression configuration needs medium-pressure steam (9-16 bar) to operate; this steam is produced by a steam boiler fed by conventional fuel or a solar system. In Table 1 the main process parameters of the desalination plant are reported.


Table 1 Process parameters of the 30 m3/d desalination plant

Multi- Vapor effect compression

Production, t/h 1.25 1.25 No. of effects 4 4 Seawater flow, t/h 22 11 Make-up flow rate, t/h 4 4 Heat input, kW at 65°C 240 154.7 Heat performance, 690 444

kJ/kg water Efficiency, kg dist./ NA 5.73 kg steam

Inlet vapor temp., °C 55 NA Distilled temp., °C 41.4 41.4 Seawater inlet temp., °C 25 25 Drain outlet temp., °C 33 33 Solar pond exchange 43.2 NA make-up temp., *C

Thermo-ejector driving NA 218 vapor a, kg/h

Ejector outlet temp., °C NA 60 First effect temp., °C 49.6 49.6 Fourth effect temp., °C 34.6 34.6 Blow-down temp., *C 35 35

ap=10 bar; T=I80°C.

The entire plant has been designed for simplicity of control and stability in operation. The plant is capable of operation at a reduced capacity from 100% to 50%.

The feed water to each effect is sprayed over the tube bundles by spray nozzles. Their locations on the distribution headers ensure a uniform flow distribution over the tube bundles to avoid areas of low flow or drying out and the consequent formation of scale on the tube surface. The tube bundle in each effect is horizontal.

3. Use of titanium

The proposed desalinator has been manu-

factured totally in titanium. This is a completely new feature because the international experience so far regards the construction and operation of desalination plants with only some tube bundles in titanium. In fact, titanium has until this time been very expensive, which has limited its use to special duties. Additionally, it is harder to work with titanium than with traditional materials. Titanium component manufacturing and assembling are difficult tasks, and for this reason manufacturers have previously not usually chosen titanium. Nevertheless, titanium has one characteristic which makes it particularly interesting for use in desalination units: resistance to chemical corrosion by salt water, which means a longer life, less possibility of fouling, lower requirements for chemical treatment of seawater, and less periodic maintenance of the tube exchangers and the boxes. To better understand the above statement, one must remember that titanium exhibits a passive state because of the TiO 2 presence on its surface. In this state titanium is subject to quite a non- significant speed of corrosion (<< 1 mm /1000y), it can be subject to a very high flow speed, and it shows a high resistance to chlorine attack.

Because of its own exceptional corrosion resistance, titanium can induce corrosion in other metallic materials, not so good from the corrosion point of view: in a fully titanium manufactured unit, all corrosion problems can be solved easily. Since the fouling of tubes (particularly bio- fouling) depends on many parameters such as temperature, flow speed, oxygen concentration and the surface "status", in a fully titanium manufactured unit we can choose, on the basis of a good design, the better value of each parameter so as to decrease the fouling at a very low level, without problems in other parts of the plant.

This issue was thoroughly analyzed during the conference on "Titanium in Seawater" held in Genova (I) on October 22, 1993: the reduction of fouling may substantially affect the overall heat transfer coefficient of the tubes averaged during the operation of the plant.


The decision to use titanium for the whole desalinator is based on:

1. the cost of titanium, which has dropped in price since the 1980s, which probably will open new opportunities for the use of this material for the substitution of stainless steel and copper-based alloys in desalinators; and

2. the increasing international attention to environmental protection, which pushes the study of process alternatives or component alternatives to reduce the use of pollutants (chemicals) into rivers, underground water and the sea for operation and/or maintenance of plants.

The construction of a completely titanium desalinator requires the solution of the following issues: • a very careful selection of an electrochemical

corrosion protection system • optimization of the thickness of all compo-

nents due to the high cost of the material (the philosophy of design is opposite with respect to traditional desalinators where material used is mainly low in cost, with the possibility of corrosion, thus causing no thickness optimization)

• sophisticated fluid dynamic analysis and design of internals in order to allow a proper distribution of fluid through the bundles, enhancing the overall heat transfer coefficient and reducing stagnation and fouling

• optimization of manufacturing techniques already tested for individual titanium bundles, to be extended and adapted to full-size titanium equipment.

average collecting area of 625 m 2 and the depth is 3.5 m, with the storage zone starting 2 m below the surface level. The heat extraction system from the thermal storage zone consists of four separated cross-linked polyethylene spiral-wound pipes submerged below the interface between the convective and the salt gradient zones, each one fixed so as to affect one quarter of pond area. This kind of pipe shape permits uniform heat removal from the storage zone, reducing the horizontal temperature gradients and the hazard of erosion of the salt gradient region.

The site is quite far from the sea, and the process water feed to the desalting system is provided through two 20 m 3 fiberglass tanks where a salt solution is prepared at a controlled concentration; a third tank of 5 m 3 collects the distillate (Figs. 2 and 3). In the TC mode of operation, the heat source of the desalination plant is a steam generator fed by fossil fuel.

Several thermocouples, pressure and vacuum transducers, flow meters, conductivity meters and alarms are located in relevant components, and the measured data are collected by an acquisition system and a personal computer for control and future analysis and elaboration [5].

In Fig. 4, the desalination plant during the installation is shown.

4. Experimental plant

The experimental plant has been erected in the open area of the laboratory of the Department of Energy of the University of Ancona, Italy, near the existing solar pond. The solar pond [4] has an Fig. 2. Experimental site and solar pond.


f

Fig. 3. Experimental plant. 1 solar pond, 2 boiler, 3 desalinator, 4 heat exchanger, 5 "cold" tank, 6 "warm" tank, 7 product tank, 8 vacuum ejector, 9 fan coils, 10 solar collectors.

Fig. 4. Desalination plant.

232 G, Caruso, A. Naviglio / Desalination 122 (1999) 225-234

5. Tests performed

Several tests have been performed during the experimental phase, aimed at verifying the performance of components and equipment.

After the commissioning tests, performed in April and May 1997, the start-up of the plant occurred in the first days of June 1997. First tests were performed in thermocompression mode until August 1997. From September 1997 to May 1998, the plant was operated in the multi-effect mode, simulating the heat from the solar pond using steam from a boiler because of the unavailability of the solar pond. From June to August 1998, tests using the heat provided by the solar pond were carried out. Generally, the desalination plant has been operated a week each month from June 1997 to August 1998.

Some problems connected to vacuum loss through some leaks in the titanium welding were revealed and solved. The vacuum system was extensively tested and its operation is now satisfactory. Further problems related to hydraulics in the distillate line and in the feed water line were also faced. Some modifications in the piping were studied and performed. Other problems related to the vapor thermocompressor arose in some tests and some modifications were needed.

Only a limited number of tests has been performed so far with the solar pond due to the problems encountered by the University of Ancona in reaching high temperatures inside the pond, because of either unfavorable weather conditions or for technical problems due to the pond maintenance. In the tests performed, the solar pond/desalinator heat exchanger seemed to operate successfully. Some minor problems occurred in the liquid pipeline (down-comer) where the concentrated solution produced in the exchanger is driven only through gravity into the evaporator shell of the first stage. This was due to an insufficient difference of height between the two nozzles, imposed by lay-out requirements.

In Table 2, some averaged values obtained in

the three operating modes are reported. In Figs. 5 and 6, the specific electric consumption and heat performance (thermal heat for unit of distillate

Table 2 Averaged values consumption

of production and thermal energy

Operation H e a t Production, Heat mode input, kg/h performance,

kJ/h kJ/kg

ME ( h e a t 360,000 540 667 from a boiler)

TC 360,000 480 750 ME (heat from 420,000 600 700 solar pond)

0] 71 SJ

,r" E 5- > 4. [3 .

2- 1.

Specific electric consumption

ME (boiler)

Q:

ME (S.P.) TC

Fig. 5. Specific electric consumption during the experimental study compared to the design value (100%).

1000

8O0

6O0

400-

200-

Heat performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ME (boiler) ME (S.P.) TC

I o o% []so*/, I • 66% ] [ ] (100%) I

Fig. 6. Heat performance during the experimental study compared to the design value (100%).


produced) are reported. The values corresponding to 100% are design values, not tested during the experimental stage due to the limited thermal power available. As for the solar pond tests, only 30% and 50% of the design power was tested due to the unavailability previously mentioned of the solar pond. The specific electric consumption values reported in Fig. 5 do not include the contribution of the vacuum system. This additional consumption, due to the additional water flow rate through the high pressure main pump for the vacuum hydro-ejector, may be assumed about 3 kW for the production of 1 m3/h of distillate.

6. Corrosion aspects

The corrosion situation of the desalination unit was first examined without any corrosion protection anodes. Then, sacrificial anodes (made of zinc, aluminium and iron) were installed in turn, and the protection against corrosion was examined again.

The theoretical lifespan of anodes for the measured average currents (in days) are: zinc, 15-20; aluminum, 365; iron, 28.

A set of zinc anodes was installed and connected after the measurement to protect water boxes for about 1 month, during which the analysis of the collected data proceeded.

The behavior of titanium near the tube sheet of the final condenser as well as at the bottom of the evaporators does not show any risk of corrosion. The stainless steel water boxes and tube sheet of the condenser in the absence of protection are exposed to localized corrosion. The risk is increased by the coupling with titanium (and thus near titanium). The stainless steel water boxes and tube sheet of the condenser can be fully protected using a moderate cathodic device. The cathodic protection devices tested performed well in terms of potential attained and of protection. However, the lifetime of the small zinc anodes installed is limited (see Fig. 7).

Fig. 7. Sacrificial zinc anodes after 1 month of operation.

A new set of aluminum anodes with an acceptable lifetime (about 1 year) has been installed. If the consumption of the sacrificial anodes poses a problem for lifespan or for the pollution of the installation, future commercial units could use a protection system with impressed current and uncorrodable anodes.

7. Environmental impact

The impact of the project on the environment is very beneficial for two reasons, in addition to that of the improvement in heat performance (lower primary fuel consumption, if traditional fuels are used).

The first reason is the less amount of chemicals which is expected for the operation of the full- titanium desalinator (a traditional desalinator requires chemical additives in the amount of 4- 10 g ofpolyphosphates/m 3 of raw seawater and of 100-300 g of acid (HCI or H2SO4)/m 3 of raw seawater [1]. From the first results obtained, it is possible to foresee that the innovative desalinator


will require many fewer chemicals. Quantitative data on chemical requirements will be obtained through long-lasting tests.

The second reason is the material of the desalinator, which is much less susceptible to corrosion and to the release into the environment of corrosive salts, oxides and ions.

8. Conclusions

From the technical point of view, the project involving the design, manufacturing, and prelimi- nary testing of the innovative full-titanium desalinator coupled with a solar pond, may be judged as satisfactory. The performance of the plant has been extensively confirmed during a 1-year experimental study. Further experimental studies and following the development of the design should lead to quite interesting commercial opportunities for this technology, with the possibility of economic profitability in remote areas, also coupled with renewable energy source conversion systems.

Most of the initial targets of the activity have been successfully reached. Further tests will be carried out in order to provide complete comprehension of fouling performance and the resistance of titanium.

Unfortunately, problems connected with the unavailability of a large-size, fully operating solar pond have not yet permitted an extensive experimental study of the performance of a combined solar pond/desalinator system. Never- theless, the information collected seems sufficient to provides useful guidelines to improve the design of the plant and to allow a more suitable selection of some critical components (extraction pumps).

References

[1] A.H. Khan, Desalination Processes and Multistage Flash Distillation Pra~ice, Elsevier, Amsterdam, 1986.

[2] M. Calra, G. Caruso, L. Gramiccia and A. Naviglio, 45th ATI National Congress, Cagliari, 1990, pp. 27- 35.

[3] (3. Caruso, A. Moriconi and A. Naviglio, Proc., 2nd Int. Conf. Progress in Solar Pond, Rome, 1990, pp. 491-497.

[4] F. Principi, R. Ricci and E. Ruffini, 51st ATI National Congress, Udine, 1996, pp. 1249-1259.

[5] G. Caruso, A. Naviglio and P. Principi, Use of solar energy for seawater desalination: a solar pond assisted ME-TC desalition plant, ITEEC '97, Marrakesh, Morocco, 1997, pp. 837-842.

a desalination plant using solar heat as a heat supply, not affecting the environment with chemicals

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