Investigation on sea water desalination plants in Spain

DesaiiMtion, 28 (1979) 109-116 @ Elsevier Scientific Pubiihing Company, Amsterdam - Printed in The Netheriands

INVESTIGATION ON SEA WATER DESALINATION PLANTS IN SPAIN*

SANTIAGO MUROa, ROBERT STERNERb AND JAIME UGARTJ3=

‘=?Junta de Energia Nuclear and bi3abcock & Wilcox EspaEoh, S.A. Madrid (Spain)

(Received September 20.1978)

SUMMARY

The main desalination research and development programs and the demand for fresh water on the Spanish coasts are outlined. The experimental plants of the Koncloa (Madrid) (MSF, vapor compression, reverse osmosis and others) and their computer programs are described as are also the pilot plants of Lanzarote (Canary Islands). The close collaboration between private enterprise and the Spanish Government is discussed.

INTRODUCTION

In the 1950s Babcock & Wilcox EspaZola (BWE), a pioneer in desalination in Spain, supplied various MSF plants built with foreign technology. Re- search on sea water desalination in Spain began in the sixties when the Spanish Government and private enterprise collaborated to develop a technology of their own and established the basis for future t.echnoIogical investigations in this field. The Spanish Nuclear Energy Board (JEN) to which the State entrusted the mission of developing its own desalination technology, and BWE began some research work and established the co- operative agreements which permitted the development and construction of as many installations as necessary to achieve good technology and know- how in desalination, especially in d.istillation processes -multi-stage flash, vapor compression and long vertical tubes, but also in reverse osmosis, ion exchange and other processes.

In addition, JEN signed an agreement for Spain within the framework of the Cooperation Europt?enne dans le domaine de la recherche scientifique et technique (COST) for putting into operation a coordinated European program in the field of metallurgy, materials, and desalination plants which

* Presented at the Sixth International Symposium on Fresh Water from the Sea, Las Palmas, September 17-22.1978.

110 S. MURO, R. STERNER AND J. UGARTE

groups various research projects on the development of inexpensive alloys with adequate resistance to corrosion by hot and degasified sea water and hot fresh water, as the cupronickel alloy. The Spanish work concentrated on finding the best concrete material for construction of the MSF plants.

THE DEMAND FOR FRESH WATER ON THE SPANISH COASTS

More than half the Spanish coastline suffers from thirst. This problem extends to almost the whole of the Mediterranean coast of the Peninsula, the whole of the insular province of Las Palmas, the islands of Ibiza and Formentera, and the cities of Ceuta and Melilla, and other isolated points of the different coasts of the Spanish shore.

The desalination of sea water, or at least desalination by distillation processes, is not the solution in the majority of these cases. However, it is the best method of ensuring the supply of fresh water to the population, industry and even to agriculture, where the collection of water in dams and reservoirs is impossible, and the transport of water is too unreliable and expensive.

The reverse osmosis or distillation process (multi-stage flash or vapor compression) is chosen according to the quality of water to be desalted. The zones discussed are indicated on the map (Pig. 1).

O-NSF

A- RO

q - vc

SPAIN

Fig. 1. Desalination plants in Spain.

GENERAL LINES OF THE DESALINATION RESEARCH PROGRAMS 111

The sea water desalination research programs in Spain were planned from the beginning with the general objective of obtaining the technology and know-how necessary to rationalize the processes that are considered more useful in order to solve the problem of supplying fresh water on the Spanish coastline. The greatest efforts have thus been placed in multi-stage flash, vapor compression and reverse osmosis processes. MSF was chosen as the suitable process for desalting sea water on a huge scale, VC for small production, RO for brackish waters and as a possible future choice of sea water treatment, and there are also studies on electrodialysis and freezing.

The development of the processes was carried out in two phases: a) Construction of small desalination units on the premises of the JEN

(Madrid) to study thermohydraulic behavior of the process for design, construction and erection of pilot pIants with close to industrial capacities.

b) Construction and experimentation with pilot plants on locations on the coast that permit the study of the technology of each process, especially in connection with corrosion and scale phenomena.

THE EXPERIMENTAL PLANTS OF THE MONCLOA (MADRID)

The plants of the Moncloa were built to obtain data on heat transfer coefficients, hydrodynamics, control, operation, etc. for design of pilot plants with greater precision and safety. The experimental plants in Madrid were the following:

Multi-stage flash plod (JEN-BWE) Work began in the Nuclear Energy Board in 1965 on the development of

MSF with the construction and operation of an experimental installation in the Moncloa (Madrid). The basic objective of this installation is to obtain project data on the hydraulic variables and heat factors of the process; flows, heat transfer coefficients, instrumentation, etc.

This instahation now comprises twelve stages of which ten are of heat recovery, working in closed circuit. The water which must make the flash is heated in the heat exchanger fed by a steam boiler, and in the stages of heat rejection, the amount of heat contributed is extracted by means of a refrigeration tower manufactured on the basis of cellulose.

In order to achieve a correct functioning of all and of every one of the stages, it was necessary to measure and control flows, temperatures, pressures, Ievels, etc. In addition, and in view of the fact that the conditions between the stages are very close, the instruments must be sufficiently sensitive to show small differences, mainly in temperature and pressure.

The most significant results are the following: 1) Dependence of the overall heat transfer coefficient on the tempera-


ture of the process in general, which can be represented by that of the refrigeration fluid, by that of the steam, etc., as well as by the recycling velocity of the refrigerating fluid.

2) Influence of the different types of venting on the performance of the plant.

3) Standardization of procedures for measuring temperatures and pressures.

4) Investigations on the influence of the geometry and thermohyclraulic variables on the orifice coefficients.

5) Training of personnel in the operation of multi-stage plants.

Vapor compression plant (JEN, BWE, Research Commission) This plant, located on the premises of the Nuclear Energy Board, consists

of a rectangular evaporator of carbon steel, with 100 stainless steel pipes of one inch diameter and 2.5 meters in length, forming a distribution of 4 x 25 pipes. It is single-pass and the pipes have a slope of 2” in order to facilitate the circulation of the water condensed inside them. Eight of these pipes are instrumented on the outside with 50 thermal resistance elements, with the object of determinin g the heat transfer coefficients and temperature variations along the pipes and in different pipes situated at different heights.

The spray nozzles are placed in the upper part of the evaporator, two at each end of the pipes, which throw brine with a very long oval impression. The height and slope of these nozzles is variable in order to study the best distribution of the brine.

The evaporator is supplied with a connection between the condensing chamber and the brine part to study the effects which produce a residual velocity of steam inside the pipes, on the specific consumption; the amount of steam which can be made to pass through this connection is from 0 to a maximum of 25% of the total.

The blower used is of Rootes type, with positive vertical displacement with descending draft; the speed is variable in order to study different working conditions.

The plant has worked from 40” to 118°C and the average production at 80°C was about 10 m3 /day. Drinking water has always been used and data was obtained on coefficients of heat transfer, on ratios of brine recirculation, specific consumptions, the behaviour of the production at different speeds of the blower and the influence of vapor recirculation on the specific consumption.

The plant is joined to a system for the extraction of non-condensables, an auxiliary steam source for start-up and a refrigeration system which consists of a heat exchanger and a refrigeration tower, necessary for its functioni.ng.

SEA WATER DESALINATION PLANTS IN SPAIN 113

Reverse osmosis, etectrodialysis and solar distillation plants Reverse osmosis program (JEN and Sener) 1) Development of the pumps. On the basis of a new system of rotative

pumps with high pressure and small flow, two types of pump were developed - the STV-V and STVS_ The water in this pump is cenfxifhged onto a rotating casing and collected by a pitot which transforms its velocity energy into pressure.

2) Development of motor-pump units to recover the energy by inversion of the functioning of the STV-S of the water pumped and not filtered through the membranes.

3) Study of the problem of the limit Iayer in reverse osmosis which is very important in connection with the yield. The phenomenon was studied in connection with the turbulent current inside the tubular membrane and the corresponding mathematical model has been developed.

4) Deveiopment of filtering membranes with good characteristics. Starting from a sheet of diacetylated cellophane, a device for continuous and eon- trolled acetylation was designed and methods were perfected for the deter- mination of permeability.

Eiectrodialysis JEN made a laboratory scale model to study optimization of the process_

The electrochemical behaviour of the membranes was studied, and in the development of the components, various models of spacers for membrane piles have been designed.

multi-stage solar disti~~~tio~ A device was made in JEN based on an idea of the engineer Mazarrasa. ln

essence, this is a system of condensation-evaporation plates with a heat exchanger and a distributor. The salar heat exchanger is a dihedral laminar structure crossed by collecting pipes and with interspaces full of water in a closed circuit. In addition to observing the behavior of the solar model, the behavior of the different capturing laminas was also studied.

Computer programs (JEN-B WE) Based on the results in the experimental plants of MSF and VC in the JEN

(Madrid), computer programs were developed as a basis for design of large plants_ The most significant were Odisea, Tromba and Compre prepared in collaboration by JEN and BWE.

The Odiseu program solves the optimization and thermohydrauhc design of a MSF plant. Its most important characteristics are considered in detail, the cost of the most important parts of the plant, such as tubes, plates, water boxes, evaporators, demisters, etc. The program is able to optimize long tube plants as well as cross tube plants and is very versatiIe in the choice of different materials for the different parts. The variable factors to


be optimized are ratio of production, number of recovery stages, recirculation, product ratio, diameter of tube, and brine speed into the tubes. Optimizing of the cost of fresh water per cubic meter is made by the direct search method.

The Tromba program tries to partly alleviate the shortage of data on condensation in the interior of the horizontal tubes and the ebullition in films formed on the outside of the horizontal tubes. For the estimation of the theoretical heat transfer coefficients, the correlation of Akers, Rosson and Schulenberg are applied for condensation, calculating the interior wall temperatures previously, and rejecting the difference of pressures between the entry and exit from the tubes. For vaporization, the correlations of Chun and Sebas are applied directly for vertical pipes and with a correction of 43% similar to that found by Schulenberg for condensation. The overall coefficients are obtained by combining the two condensation ones and the two vaporization ones obtained previously.

In addition to this data, ail complementary information for a theoretical study of the subject is obtained.

Compre is an optimization program for vapor compression plants which always use plate exchangers and blowers of Rootes type. At first, it was considered for a fixed production, but later on, the production was included as a variable factor, leaving, in addition to this, the temperature and the speed of the blower as starting data. In addition to providing the model for each component, the program gives the approximate price of the installation and the price/m3 of water produced.

THE LANZAROTE PILOT PLANTS

Since the production of drinking water expected from the two pilot installations is 1,000 m3 /day, for that of the multi-stage flash and 100 m3 /day for the steam compression unit, a production which is considered of interest for any small population nucleus with a shortage of drinking water, the installation of these in some place on the coast with these needs was considered. The location chosen was that of the island of Lanzarote, a place which combines very interesting conditions for the installation of the plants, since its need for drinking water is combined with the possibility of supplying steam and electricity from the dual 5,000 m3 /day plant belonging to the local authorities of Lanzarote, designed and built by Babcock & Wilcox Espaflola. The interministerial agreements enable the client, the Ministry of Public Works, to specify the steam and electricity supply. These plants were installed in a complex with other plants belonging to the local authorities and to private bodies.

The multi-stage flash plant (JEN-BWE) This plant will comprise 24 heat recovery stages and 2 heat rejection

SEA WATER DESALINATION PLANTS IN SPAIN 115

stages, distributed in five evaporators. The first three evaporators wiU oe built of crossed tubes, while the other two will be of the longitudinal type.

The 3rd, 14th and 23rd stages will be specially instrumented for measuring the heat transfer coefficient and to break down the coefficient into its different components. Likewise, in these stages, it is intended to study the problems of the thermaf eq~b~~ of the brine, for high specific flows, which is achieved by reducing the passage width.

The plant can function with the two usual types of treatment: acid and by means of anti-incrusting additives. Corrosion studies are considered very important, and suitable instrumentation will be placed at various points of the plant.

Among the general problems to be studied, the following stand out: 1) Testing of corrosion of various materials. For this purpose five loops

are placed in the recycling and in the sea water intake systems, where gravi- metric and electrochemical measures will be made, Also the behavior of several materials for tube bundles will be studied in various places in the especially instrumented stages.

2) Behavior of the different anti-scale additives, study of their efficiency and optimal dosage.

3) The effect of the venting on the performance of the plant, with the measuring of flow and concentration of vents, especially in instrumented stages.

4) Study of the losses due to lack of heat balance of the brine. 5) Study of the flashing devices, dumps, splash plates, etc. 6) Study of the coefficient of heat transfer by condensation. 7) Study of the demisters, with regard to their position and yiefd. 8) Study of the influence on the performance of the decarbonator and

degasifier of the following parameters: speed of steam scavenge, temperature of the brine, brine distribution, pressure and height of filling.

Vapor compression pilot plant @WE-Government Research Commission) In order to compare the data obfained in the experimental plant, a pilot

plant is to be installed in Lanzarote which, while permitting certain flexi- bility in the process, wili be a genuine commercial plant. Production of 100 m3 /day has been chosen which requires not too excessive an investment.

The plant is equipped with a Rootes blower which has three speeds. It has a vertical descending draft for automatic draiuing. The evaporator is a rectangular single-pass, built of carbon steel, has 810, 90/10 cupronickel 3.35 meter tubes with one inch internal diameter. The tubular sheets and support sheets are also of 90/10 cupronickel and the whole tube bundle can be extracted easily for observation and cleaning. The evaporator is equipped with 9 spray nozzles with the same characteristics as those of the experimental plant and their height, slope and brine flov are adjustable_ The steam recirculation fIow can be varied from between 0 and 15% of the total steam handled by the blower.


The instrumentation is comprehensive in order to study all the process parameters and compare them with those obtained in the experimental plant. A thorough study of corrosion and scale phenomena is planned, a task which is not carried out in the experimental plant since this uses drinking water.

The extraction system of the non-condensable is formed by an ejector which uses part of the water feed. The heat recovery is made by means of two plate exchangers and has four pumps for the recirculation of brine, product, feed and sea water intake, this equipment being normal in this type of installation.

It is planned that the plant will work at between 105” and 9O”C, a range which has given the best yields in the experimental plant and which permits acid treatment.

CONCLUSIONS

Spain is participating extensively in desalination studies through govem- ment and private enterprise in order to solve the problem of scarcity of fresh water on the Spanish coasts. The main objectives of desalination technology in Spain are the creation of Spanish patents and distillation processes for immediate use, and reverse osmosis is seen as a hopeful solution for the not too distant future.

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Investigation on sea water desalination plants in Spain